8/11/2019 LT3602 LTE Air Interface - V3 http://slidepdf.com/reader/full/lt3602-lte-air-interface-v3 1/202 LTE Air Interface Course Code: LT3602 Duration: 2 days Technical Level: 3 LTE courses include LTE/SAE Engineering Overview LTE Air Interface LTE Radio Access Network Cell Planning for LTE Networks LTE Evolved Packet Core Network 4G Air Interface Awareness Understanding Next Generation LTE ...delivering knowledge, maximizing performance... www.wraycastle.com Wray Castle – leading the way in LTE training www.wraycastle.com the essential guide to L TE training
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
The manual you are reading is protected by copyright law. This means that Wray Castle Limited could take you and
your employer to court and claim heavy legal damages.
Apart from fair dealing for the purposes of research or private study, as permitted under the Copyright, Designs andPatents Act 1988, this manual may only be reproduced or transmitted in any form or by any means with the prior
permission in writing of Wray Castle Limited.
All of our paper is sourced from FSC (Forest Stewardship Council) approved suppliers.
LTE (Long Term Evolution) represents the next developmental step for the 3GPP (3rd Generation
Partnership Project) standards group. It provides for a continued evolutionary path from 2G GSM/GPRS,
beyond 3G UMTS/HSPA and ultimately towards a 4G solution.
UMTS (Universal Mobile Telecommunications System) has continued to build on the success of GSM
(Global System for Mobile Communications) and momentum is gathering behind its significantlyincreased capability with the introduction of HSPA (High Speed Packet Access). The classic fixed and
mobile telecommunications business models are undergoing enormous change with the move towards
all-IP switching and a total-communications service profile. Meanwhile, the last decade has seen the
Internet develop into a serious business tool and fixed broadband access is fast becoming a basic
commodity.
This market landscape is ready for a technology that combines broadband capabilities with an efficient
scalable switching infrastructure and a flexible service delivery mechanism. LTE provides just such a
solution and is designed to address growing global demand for anywhere, anytime broadband access
while maintaining efficient provision of traditional telecommunications services and maximizing
compatibility and synergies with other communications systems.
Although LTE most obviously represents an evolutionary path for UMTS networks, it has also been
designed to allow cost-effective upgrade paths from other technology starting points. For example, GSM
operators can now access 3G-like performance through EDGE (Enhanced Data rates for Global
Evolution) Evolution, and this in turn can be used as a direct pathway to LTE. Similarly, the interworking
capabilities of the EPC (Evolved Packet Core) make it possible for CDMA (Code Division Multiple
Access) to migrate radio access from 1x or 1xEV-DO (1x Evolution – Data Only) to LTE.
Evolution beyond LTE has been mapped out by 3GPP with the specification of LTE-Advanced, which
offers the possibility of downlink data rates (to stationary or low mobility users) of 1GBit/s or more.
Further Reading: 3GPP TS36.300 (LTE Radio Access), 23.401 (LTE Core Network)
Wide-area LTE radio access combined with the EPC represents a complete adoption of an all-IP
(Internet Protocol) architecture, offering broadband delivery capability with the potential for bit rates of
several hundred megabits per second and QoS (Quality of Service) management suitable for real-time
operation of high-quality voice and video telephony.
LTE has a very important role in the overall telecommunications service convergence concept. LTE couldprovide a key to unlocking a truly converged fixed/mobile network for the delivery of quadruple play
services. Its potential bandwidth capabilities are sufficient for the support of services ranging from
managed QoS real-time voice or video telephony to high-quality streamed TV. Its flat all-IP architecture
means that it can act as a universal access network for a wide range of core network types.
Since the publication of the first GSM (Global System for Mobile Communications) specifications in the
late 1980s, the technologies and techniques employed by GSM networks have continually evolved and
developed. GSM itself underwent a series of changes, from Phase 1 to Phase 2 and eventually to Phase
2+. Phase 2+ progressed with a series of yearly releases, starting with Release 96.
The UMTS (Universal System for Mobile Communications) was introduced as part of Release 99 andfrom then onwards the 3GPP (3rd Generation Partnership Project) 3G network technology has also been
undergoing a process of evolution. The evolutions that particularly affect the air interface are mainly
contained in Releases 5, 6, 7 and 8. Release 5 and 6 introduced HSPA (High Speed Packet Access) –
HSDPA (High Speed Downlink Packet Access) in R5 and (HSUPA) High Speed Uplink Packet Access,
or Enhanced Uplink, in R6. Release 7 outlines the changes necessary to deliver HSPA+ and Release 8
specifications begin to describe LTE – the Long Term Evolution of UMTS. Specification of LTE, generally
described as 3.9G, was completed in Release 9. Specification of LTE-Advanced, a full 4G solution, is
Tests and evaluations carried out during 2007 led to the publication of the Release 8 36-series of
specifications, which began to detail the technological basis for LTE.
Of the original four candidate air interface technologies, two were chosen for the final version: OFDMA
(Orthogonal Frequency Division Multiple Access) and SC-FDMA (Single Carrier FDMA).
OFDMA is employed on the LTE downlink and is expected eventually to provide peak data rates
approaching 360 Mbit/s in a 20 MHz channel. SC-FDMA is employed on the LTE uplink and may deliver
up to 86 Mbit/s. SC-FDMA is also sometimes known as DFT-FDMA.
In addition to the air interface technologies, LTE simplifies the range of technologies employed in other
parts of the network.
LTE is an ‘all-IP’ environment, meaning that all air interface, backhaul and core network interfaces will
carry only IP-based traffic. The need to support different protocols for different traffic types, as was the
case with R99, is therefore avoided.
In this all-IP environment, layer 4 transport layer functions for signalling connections are performed usingan alternative to the traditional choices, TCP (Transmission Control Protocol) or UDP (User Datagram
Protocol).
SCTP (Stream Control Transmission Protocol) was developed with the needs of IP-based signalling in
mind and is used to manage and protect all LTE signalling services.
The basic building blocks of the E-UTRA access network are the eNB (Evolved Node B) plus backhaul –and nothing else. All layers of the air interface protocol stack, including the elements that previouslyresided in the RNC (Radio Network Controller) – RRC (Radio Resource Control), RLC (Radio LinkControl) and MAC (Medium Access Control) – have been moved out to the base station. As the eNB nowanchors the main backhaul link to the core network, it has also assumed responsibility for managing the
PDCP (Packet Data Convergence Protocol) service, which provides header compression and cipheringfacilities over the air interface.
HSDPA began the process of moving RRM (Radio Resource Management) functions, such as packetscheduling, from the RNC to the Node B. In LTE, all remaining RRC functions are devolved to the eNB,meaning that there is no longer a role for a device such as the RNC.
Among the RRM functions now devolved to the eNB are radio bearer control, radio admission control,connection mobility control and the dynamic allocation (via scheduling) of resources to UEs (User Equipments) in both uplink and downlink directions.
Following on from innovations in R4 and R5 networks, LTE also supports the concept of flexibleassociations between access and core network elements, meaning that each eNB has a choice of MME
(Mobility Management Entity) nodes to which to pass control of each UE. Dynamic selection of an MMEfor each UE as it attaches is therefore also an eNB responsibility. An eNB may be associated with MMEsbelonging to different PLMNs (Public Land Mobile Networks), allowing for the easy creation of multi-operator networks.
The eNB also receives, schedules and transmits control channel information in its cells, including pagingmessages and broadcast system information, both of which are received from the MMEs. It retains manyof the traditional roles associated with base stations, such as bearer management. It is responsible for routing U-plane traffic between each UE and its S-GW (Serving Gateway). The complexity of the eNBand of the decisions it is required to make are therefore much greater than for an R99 Node B
The complexity of the eNB and of the decisions it is required to make are therefore much greater than for an R99 Node B.
The broadening of the range of services offered by the LTE EPS over time has lead to the developmentof several specialised sub-types of eNB. Femtocell services, for example, are provided via HeNBs (HomeeNBs), whilst LTE Relay facilities are offered by Relay Nodes and controlled by DeNBs (Donor eNBs).
Essentially, the EPS offers no services to the user other than connectivity to an external PDN (Packet
Data Network). In this respect the EPS can be considered to function only as an IP-CAN (IP Connectivity
Access Network). Any specific services that the user receives are provided through service platforms that
are implemented in the external PDN.
Connectivity is provided through one or more EPS bearers. The EPS bearer defines a transmission pathwith an associated set of QoS parameters between the UE and the PDN-GW.
The establishment of EPS bearers is managed by the PCRF (Packet Control Resource Function) in
The EPS ARP facility allows the system to handle busy and overload periods. Each QoS class is
associated with an ARP level, which determines the relative priority of the EPS bearers established
according to those classes.
There are fifteen ARP levels ranging from Priority 1 (highest) to 15 (lowest).
The other components of ARP are simple ‘yes’ or ‘no’ parameters: a connection’s Pre-emption
Capability, which defines whether it is able to pre-empt other, lower priority connections; and Pre-emption
Vulnerability, which determines whether it can be pre-empted by higher-priority connections.
During busy periods when there is competition for scarce resources, the EPC will judge which of several
competing EPS bearer requests will be confirmed based on the ARP of the services requested. Those
with a higher ARP priority will be established and those with a low ARP priority will be rejected. In
overload situations the EPC will use the ARP levels of existing bearers to select those that can be
dropped – EPS bearers with a low ARP will be dropped first.
ARP does not have an effect on packet forwarding or prioritization decisions within EPC nodes, where
decisions are made based on a bearer’s QCI, or in the IP transport network, where these considerationsare handled by DiffServ (Differentiated Services).
Further Reading: 3GPP TS 23.401:4.7.3; 23.203:6.1.7
QoS in the EPC is currently defined by three levels: GBR, MBR and AMBR (Aggregate Maximum Bit
Rate).
GBR connections are assigned a guaranteed data rate and are therefore useful for carrying certain types
of real-time and delay-sensitive traffic. MBR connections are non-guaranteed, variable-bit-rate services
with a defined maximum data rate. If a connection’s data rate goes beyond the set maximum the networkmay decide to begin discarding the excess traffic.
GBR and MBR parameters are applied on a ‘per bearer’ basis, whereas AMBR is applied to a group of
bearers; specifically, a group of non-GBR bearers that terminate on the same UE. AMBR allows the EPS
to set a maximum aggregate bit rate for the whole group of bearers that can then be shared between
them.
The APN-AMBR parameter sets the shared bit rate available to a group of non-GBR bearers that
terminate on the same APN and can therefore be seen to be applied on a ‘per PCS’ basis; the UE-AMBR
parameter aggregates all non-GBR bearers associated with one UE.
Dedicated bearers can be established as GBR or non-GBR (i.e. MBR) as required. Default bearers, dueto the probable need to adjust their bandwidth after the initial Attach has taken place, must be non-GBR.
The establishment of a new EPS bearer is triggered from the PCRF. This is likely to be in response to a
service request from a service AF (Application Function) in an external PDN for the establishment of a
UE originated or terminated service or the establishment of the default EPS bearer when a UE registers
with the system.
The PRCF is responsible for PCC (Policy Control and Charging), and as such is responsible for decidingwhen an EPS bearer is required and what the QCI value should be used for the EPS bearer. This is
indicated to the PDN-GW using a PCC Decision Provision message.
The PDN-GW uses the QoS information to establish the appropriate bearer-level QoS parameters and
forwards these, along with the UL-TFT to be used in the UE, to the S-GW in a Create Dedicated Bearer
Request message. The message is then forwarded to the MME along with the S1 bearer ID.
The MME assembles the information in this message along with the EPS bearer ID and sends it to the
eNB in a Bearer Setup Request message. The eNB uses the QCI information to determine the
appropriate rules and configuration to apply for handling data relating to the EPS bearer and the creation
of the radio bearer. It then instructs the UE to set up the radio bearer using the RRC Connection
Reconfiguration message. This message contains all the information that the UE needs to configure theEPS bearer at all layers of the protocol stack.
The main means of identifying EPS subscribers remains the IMSI (International Mobile Subscriber
Identity), which is permanently assigned to a subscriber account. The IMEISV (International Mobile
Equipment Identity and Software Version) and MSISDN (Mobile Station ISDN Number) also remain as
LTE identifiers.
Temporary and anonymous identification of subscribers is provided by the GUTI (Globally UniqueTemporary Identity), which is assigned by the serving MME when a UE has successfully attached and is
reassigned if the UE moves to the control of a new MME. The GUTI is analogous to the legacy TMSI
(Temporary Mobile Subscriber Identity) but with the additional feature that its structure uniquely identifies
not only the subscriber within the MME but also the MME that assigned it.
The GUTI is constructed from the GUMMEI, which consists of the network’s MCC and MNC followed by
a MMEI (MME Identifier), and the M-TMSI (MME Temporary Mobile Subscriber Identity). The M-TMSI is
used to provide anonymous identification of a subscriber within an MME once that subscriber has been
authenticated and attached. As with legacy TMSI use, the MME may elect to reissue the M-TMSI at
periodic intervals and it will be reissued in any case if the UE passes to the control of a different MME.
The M-TMSI allows a subscriber to be uniquely identified within an individual MME, whereas the S-TMSI(SAE TMSI) allows subscribers to be identified within an MME group or pool. To achieve this, the S-TMSI
also includes the one-octet MMEC (MME Code). The MMEC is the MME’s index within its pool.
In order to offer effective service to UEs, the EPS needs to be able to define and keep track of the
availability and reachability of each terminal. It achieves this by maintaining two sets of ‘contexts’ for
each UE – an EMM (EPS Mobility Management) context and an ECM (EPS Connection Management)
context – each of which is handled by ‘state machines’ located in the UE and the MME.
A further state machine operates in the UE and serving eNB to track the terminal’s RRC state, which canbe either RRC-IDLE (which relates to a UE in idle mode) or RRC-CONNECTED (which relates to a UE
with an active traffic bearer).
EMM is analogous to the MM processes undertaken in legacy networks and seeks to ensure that the
MME maintains enough location data to be able to offer service to each UE when required. The two EMM
states maintained by the MME are EMM-DEREGISTERED and EMM-REGISTERED.
The ECM states describe a UE’s current connectivity status with the EPC, e.g. whether an S1 connection
exists between the UE and EPC or not. There are two ECM states, ECM-IDLE and ECM-CONNECTED.
Although the EMM and ECM states are independent of each other, they are related, and any discussion
of a UE’s reachability is best served by viewing these states in a combined fashion. There are three mainphases of UE activity, each of which can be described by a combination of EMM and ECM states. These
are with the UE powered off, with the UE in idle mode and the UE with an active traffic connection.
As part of the Attach process, the EPS will establish at least a default bearer for the UE. Details of a
‘default APN’ to use for the bearer may be stored in the subscriber’s HSS profile or may be selected
dynamically by the EPC.
HSS data may also indicate whether additional dedicated bearers need to be established along with the
default bearer during the Attach.
Each UE, irrespective of the number of EPS bearers it has established or PDN Connectivity Services it is
using, will only be served by one MME and one S-GW at any one time (except for the brief but inevitable
overlap that occurs during a relocation). Connection establishment and rating decisions for each UE will
be handled by the same PCRF in networks that employ dynamic PCC; this ensures that service-related
decisions are made by one device that has all details of the UE’s current service set to hand.
IP address allocation occurs as part of the Default Bearer establishment process; all linked Dedicated
Bearers will share the same IP address.
The UE will generally be assigned an IP address from within the range allocated to the default APN; it is
also possible for the UE to signal that it requires an IP address to be assigned by the external PDN towhich the APN connects. In either case the default IP address allocation method is DHCP (Dynamic Host
Configuration Protocol), although static IP address allocation (based on an address stored in the HSS
Each EPS Bearer will be carried between or controlled by a specific set of devices. For an EPS Bearer
established between the UE’s home E-UTRAN and home EPC, this set of devices will be an eNB, an
MME, an S-GW and a PDN-GW. As each network can be expected to have a number of devices of each
type deployed to it, the methods by which the devices involved in serving a bearer must be clearly stated.
Device selection for EPS connections operates as follows:
The UE selects the eNB to use based on air interface selection and reselection actions.
The eNB selects the MME to use from the MME Pool available based on load balancing principles and
any current overload notifications. Load balancing is managed using the MME ‘weighting factor’, which is
related to the MME’s capacity and is signalled to eNBs using the MME Relative Capacity information
element in the MME Configuration message during S1 set-up. An MME with a capacity of 0 is not
accepting connections; an MME with a capacity of 255 has the highest relative capacity level.
The eNB does not select the MME in the case of MME Relocation, when a target MME is selected by the
source MME. The SGSN is responsible for MME selection in the case of inter-RAT handover.
The MME selects the S-GW to use from the set associated with the UE’s current Tracking Area andtakes any current overload notifications into account. The MME may also take the UE’s current TA List
into account, by selecting an S-GW that serves one or more of the TAs included on the list.
The PDN-GW is selected by the MME based on APN details stored in the user’s HSS subscription data
or on the USIM.
It is common for network operators to ensure service resilience by deploying multiple instances of the
same APN to different PDN-GWs; this ensures that if one PDN-GW fails the APN service can continue.
Further Reading: 3GPP TS 23.401:4.3.8, 36.413:9.2.3
The UE’s objective when performing an attach is to register the subscriber’s identity and location with the
network to enable services to be accessed. During the attach procedure the UE will be assigned a
default EPS bearer to enable always-on connectivity with a PDN. The UE may be provided with details of
a local P-CSCF to enable it to register with the IMS.
A simplified view of the attach process – assuming that it is an initial attach with stored details from arecent previous context for a UE using its H-PLMN (Home Public Land Mobile Network) and accessing
via the Home E-UTRAN – is shown and the stages of the process are described below.
Once a suitable cell has been selected the UE employs the Random Access procedure to request an
RRC connection with the chosen eNB. With that in place an Attach Request message can be
transmitted. If the UE has previously been registered with the PLMN (Public Land Mobile Network), it
may include a previously assigned GUTI in the message, otherwise the Attach Request message
contains the subscriber’s IMSI and some other parameters.
On receipt of the Attach Request the eNB either derives the identity of the previously used MME from the
supplied GUTI or selects an MME from the pool available and forwards the message.
The MME contacts the HSS indicated by the subscriber’s IMSI and in response receives the relevant
elements of the ‘quintuplet’ that allows the EPS-AKA process to take place.
Optionally, at this point the MME may be required to check the identity and status of the UE via the EIR
using the ME Identity Check process. Ciphering may then be invoked over the air interface.
Once the AKA procedures have successfully concluded the MME transmits an Update Location message
to the HSS and receives the Insert Subscriber Data message in response containing the user’s service
profile . An Insert Subscriber Data Ack from the MME is followed by an Update Location Ack from the
A default bearer must then be established and the MME selects the S-GW that will handle it and a
PDN-GW that supports the requested APN. The MME issues a Create Default Bearer Request to the
selected S-GW, which assigns a GTP TEID to the EPS bearer and passes the request to the indicated
PDN-GW. If the network employs dynamic PCC the PDN-GW will query the PCRF assigned to serve
the UE for bearer parameters, otherwise the bearer will be established using local QoS parameters
stored in the PDN-GW.
A Create Default Bearer Response message passes from the PDN-GW to the S-GW, which contains
relevant parameters such as the EPS bearer’s IP address and possibly the IP address or DNS name of
a local IMS P-CSCF. The S-GW creates the bearer as specified and passes the Create Default Bearer
Response message to the MME. The details that define the S1-U service will also have been defined
during this stage.
The MME sends an Initial Context Setup Request/Attach Accept message, which contains the assigned
parameters for the EPS bearer context, to the eNB. That element in turn sends an RRC Connection
Reconfiguration message to the UE to inform it of the bearer details and the changed air interface
parameters. The UE returns an RRC Connection Reconfiguration Complete message to verify that the
radio bearer, which was initially established just to carry the attach message, has been reconfigured tosupport the new parameters. The eNB forwards an Attach Complete message to the MME.
The UE then sends a Direct Transfer message to the eNB, which confirms the details of the EPS
Bearer. Finally, the eNB sends an Attach Complete message to the MME to confirm that both the
Attach and the Default EPS Bearer processes have completed successfully. Uplink and downlink data
If a UE and a network supports CS Fallback, the UE will request CS Fallback registration during the
Attach process. The service will only be available in areas where there is overlapping E-UTRAN and
GERAN/UTRAN coverage.
An Attach Request message with the ‘Attach Type’ set to EPS/IMSI attach and a CS Fallback capable
flag set is sent to the MME. This will trigger the combined attach process in the MME. The MME willderive the number of the VLR responsible for the Location Area that maps to the UE’s current Tracking
Area. This information will form part of the MME’s databuild and will not be dynamically discoverable. The
MME uses the SGs interface, which is an evolved version of the Gs interface that provides connectivity
between legacy SGSNs and MSCs.
The MME forwards a Location Update to the MSC/MSC-Server causing the MSC/MSC-Server to create
an SGs association for the UE. The MSC/MSC-Server confirms the combined Attach with a Location
Update Accept and the MME confirms the Attach to the UE using an Attach Accept message.
A TAU takes place between a UE and the MME with which it is registered and is triggered by the UE
detecting a change in TAI after a cell reselection. A TAU is also used as part of the Initial Attach process
and may additionally be triggered by events such as the expiry of the periodic TAU timer or as part of
MME load balancing or rebalancing.
In the example message flow it is assumed that the UE is connected to its HPLMN and that an S-GWchange and MME relocation are not required. After detecting a change in TAI, the UE transmits a TAU
Request message to the eNB. The TAU Request includes the old GUTI, old TAI and EPS bearer status.
The eNB forwards the TAU Request (plus the new TAI and ECGI) to the MME indicated by the supplied
GUTI. If the MME indicated by the GUTI is not associated with the new eNB, an MME relocation will be
triggered and the base station will select a new MME to pass the TAU Request to. If the message
integrity check is successful the MME may elect not to reauthenticate the UE. If the MME is configured
always to reauthenticate or if the integrity check fails, then the EPS-AKA process must be followed and a
new GUTI (which includes the new M-TMSI) will be issued.
Once the MME is satisfied that the UE/USIM is authentic and assuming that the UE is allowed to roam in
the new TA, it transmits a TAU Accept message to the eNB, which relays it to the UE. The TAU Acceptmessage contains the new GUTI, if one was assigned, plus the current TA List associated with the UE.
The TA List enables the UE to determine the set of TAs within which it can roam without being required
to perform another TAU. The UE responds with a TAU Complete message, which finishes the process.
The main purpose of the TAU process is to ensure that the MME knows roughly where each UE is in the
event that there is inbound traffic to deliver. Paging will usually be triggered by the receipt of an S-GW
Downlink Data Notification at the MME, indicating that data has arrived at the S-GW on the S5/S8 portion
of a parked EPS Bearer.
If it becomes necessary to contact an idle UE (that is, a UE that has entered the ECM-IDLE state), theMME will employ the paging process. With no equivalent node to the RNC, EPS paging is managed
directly between the MME and eNBs. When a Paging message is to be sent, the MME checks the
current TA list stored for the target UE and inserts the paging data into the S1 paging messages sent to
all eNBs in the indicated TAs.
Each eNB inserts the UE’s NAS paging ID (IMSI or S-TMSI can be used) into the appropriate repetitions
of its PCH (Paging Channel). Paging groups may be established to reduce the number of repetitions of
the PCH that each UE is required to monitor; the operation of the paging reduction scheme is controlled
via cell-specific DRX (Discontinuous Reception) functions.
When a UE receives its paging ID on the PCH it initiates the service request process, which ensures that
any ‘parked’ EPS bearers are reactivated ready to carry traffic.
Consider a radio carrier being modulated by a 10 kbit/s bit steam using QPSK (Quadrature Phase Shift
Keying). It could be expected to see a spectral envelope following a (sin x)/ x function, as shown in the
diagram, with the first null located 5 kHz from the centre frequency.
In a classic FDM (Frequency Division Multiplexing) system, other radio carriers would be allocated and
spaced far enough away from the first to ensure minimal adjacent channel interference. The size of theguard band required would depend on the transmitter and receiver characteristics as well as the relative
powers.
However, in such a system it is assumed that there is no synchronization between the potential
interferers. It is this that leads to the need for large frequency spacing between adjacent carriers. In fact,
if there was synchronization between adjacent channels, a much smaller frequency spacing could be
used. The key is to be able to make use of the complex nature of the instantaneously transmitted
spectrum. The modulation envelope is only an artificial way of indicating all possibilities over time; a
snapshot at an instant in time would look different.
Consider a second radio carrier allocated such that its centre frequency coincides exactly with the null in
the first carrier’s envelope. It is using the same modulation scheme and carrying the same data rate.The result is as shown. Note that the carrier spacing of 5 kHz is the same magnitude as the symbol rate
of 5 ksps. The spectra of the two carriers now overlaps, but as long as the carrier frequencies and the
baseband data remain accurately synchronized, both can be demodulated successfully. The reason is
that this relationship between centre frequency offset and symbol rate maintains a high level of
Considering again the two overlapping QPSK radio carriers, it can be seen that there is a relatively large
spectral efficiency gain. If the effective bandwidth of the transmitted signal is considered to be the
frequency separation of the first nulls then a single QPSK carrier modulated with 10 kbit/s would have a
null-to-null bandwidth of 10 kHz.
However, here there are two sub-carriers, each of which is carrying 10 kbit/s using QPSK. Their respective null-to-null spectra overlap by 5 kHz. This gives a collective null-to-null bandwidth for the pair of
sub-carriers of 15 kHz. Thus QPSK is being used to carry 20 kbit/s in a radio bandwidth of 15 kHz. Note
that a single QPSK modulated carrier carrying 20 kbit/s would result in a null-to-null bandwidth of 20 kHz.
The principle of independent reception of orthogonal radio carriers with overlapping spectrum can be
extended by using a large number of narrowband radio carriers within one wideband channel allocation.
This results in a very spectrally efficient channel that can carry high bit rates.
For example, if 1000 orthogonal radio carriers were modulated using QPSK, each carrying 10 kbit/s, the
net throughput for the channel would be 10 Mbit/s. This would require a total channel bandwidth of slightly
more than 5 MHz. Carrying the same bit rate with QPSK modulation onto a single radio carrier would
require a null-to-null bandwidth of 10 MHz. Thus OFDM (Orthogonal Frequency Division Multiplexing)almost doubles the spectral efficiency. Moreover, the resulting OFDM transmission is more resilient to
Spectral efficiency is not the only benefit associated with using OFDM. It also exhibits good tolerance to
the effects of multipath propagation in the channel; both fading and time dispersion.
Because the data rate on individual subcarriers with the channel is very low, the symbol period is
correspondingly long. The resulting symbol period is typically significantly longer than the time dispersion
that occurs in the channel. This means that relatively simple equalization can be used to counteractmultipath even though the net rate in the whole channel is very high.
Furthermore, a guard period can be inserted in every symbol period that covers the expected time
dispersion for the channel. This removes most of the time dispersion distortion from the useful symbol
period.
This guard period is usually created by repeating a copy of the last part of the symbol at the start. In this
Tolerance to multipath fading effects comes from the overall wideband characteristic in the channel. A
narrowband channel tends to exhibit flat fading characteristics; that is to say, the fading characteristics
are coherent across the whole channel bandwidth. The effects of this can be seen in the diagram.
OFDM channels, on the other hand, are usually used to carry very high data rates and therefore require
many subcarriers occupying a relatively large bandwidth. In most cases the bandwidth will exceed thecoherence bandwidth by a large factor, so differing fading characteristics will be seen in different parts of
the channel. In effect, the wide channel provides a degree of frequency diversity with a resulting
improvement in performance.
However, it would be wrong to assume that this benefit for OFDM results solely because the channel
bandwidth is wide. A single carrier system with the same bit rate would also result in a wide radio
channel. Therefore, a single carrier system also benefits from this form of frequency diversity to some
extent.
In the single channel system, energy from each symbol will be spread across the whole radio channel
and each symbol will therefore suffer some distortion from any fading that may occur in any one part of
the channel. In an OFDM system only those symbols transmitted on subcarriers in the part of the channelaffected by a fade will be distorted. Symbols transmitted on other subcarriers will remain unaffected. It is
then possible to adapt the subcarriers in use according to the varying fading characteristics. This means
All OFDM systems use some form of QAM (Quaderature Amplitude Modulation). Note that QPSK is a
special case of QAM where there are just four phase states and one amplitude state.
The diagram shows a basic QPSK modulation train. Serial input data is symbol mapped into two parallel
(I and Q) streams. In this example a logic 1 bit is mapped to a symbol with value –1 and a logic 0 bit is
mapped to a symbol with value +1. One pair of symbols at a time is multiplied onto the respective I and Qversions of the radio carrier. The I and Q signals are then added to produce the resultant QPSK signal
and four-point constellation.
The only modification required to enable the same modulation train to produce a higher level QAM signal
is in the symbol mapping. For example, to produce a 16QAM signal two bits are now mapped into each
of the I and Q streams in each symbol period. Therefore, groups of four bits in total are inputted during
each symbol period; the first and third are mapped into the I stream while the second and fourth are
mapped into the Q stream. The following symbol mapping rule is then used in each of the I and Q
streams:
01 maps to +3
00 maps to +1
10 maps to –1
11 maps to –3
The rest of the modulation train remains the same. These symbols are again multiplied onto the
respective I and Q versions of the radio carrier. Then the I and Q signals are added, this time producing a
resultant 16QAM signal and 16-point constellation. Note that as it is, this new constellation would result in
a higher average power. To avoid this the signal would also be multiplied by an appropriate scaling
The received signal is separated into its I and Q constituent parts by multiplying it by the same I and Q
signals used in the modulator. The result is that on the I branch the cosine component will integrate to
zero and conversely, on the Q branch, the sine component will integrate to zero.
The output of the integration on each branch will therefore be of a magnitude dependent on the
transmitted I and Q symbol mapping. In effect this is amplitude demodulation. A threshold detector isthen used to recover the I and Q symbols. Finally, symbol-to-bit mapping is used to reconstruct the
The diagram shows a block representation of the transmitter that brings together the elements of symbol
mapping for QAM and the application of the IFFT in order to produce an OFDM signal.
The serial data to be carried on the radio link is first passed through a serial-to-parallel conversion process.
The number of parallel streams will be equivalent to the number of data-carrying subcarriers in the system.
This number will usually be a power of two since this makes best use of the efficiencies offered by the IFFT.
Bits on the parallel data streams will also be grouped as appropriate for the symbol constellation of M-ary
QAM scheme in use. For example, for QPSK bits are grouped in pairs; for 16QAM they are grouped in fours
and for 64QAM they are grouped in sixes.
The next process is symbol point mapping for the bit groups on each parallel data stream. The resulting
complex number symbols then form the input to an N-point IFFT where N will be a power of two equivalent
to the number of subcarriers in use.
The output of the IFFT will be a series of complex number digital samples representing the OFDM signal
during each symbol period. At this point the cyclic prefix is added by copying the last samples onto the
beginning of the symbol period. These complex real and imaginary sample values are used to form the Iand Q symbol streams. Next, the I and Q branches are subsequently multiplied onto sine and cosine
representations of the radio carrier. This generates a digital representation of the required multicarrier M-ary
QAM modulated transmit signal.
After digital-to-analogue conversion the resulting signal can be up-converted to the required channel centre
The OFDM cyclic prefix is designed to combat the ISI (Inter Symbol Interference) effects caused by
multipath and other channel impulse response effects. Multipath causes ‘echoes’ of a previous part of the
signal that, having travelled via a longer path than the primary component of the signal, arrive later in
time.
The cyclic prefix eliminates or masks the effects of ISI, as long as the cyclic prefix period is longer thanthe maximum delay spread suffered by the signal.
The cyclic prefix is formed by taking a portion of the ‘useable’ part of each OFDM symbol and copying it
onto the beginning of the symbol period. As can be seen in the diagram the total OFDM symbol period
(Ts) is the sum of the useful symbol period (Td) and the cyclic prefix (Tg).
The copied samples in the cyclic prefix fill what would otherwise have been an empty guard period. This
ensures that any sample window within Ts and equivalent in duration to Td will always contain a whole
number of cycles for each of the received subcarriers, thus maintaining orthogonality.
The cyclic prefix ratio effectively reduces the bit rate carried relative to the total bandwidth required in the
radio channel, which has potentially significant consequences for the bandwidth efficiency of a channel.However, the consequences tend to be outweighed by the benefits in terms of minimized ISI.
Nevertheless, better performance can be obtained if the relative duration of the cyclic prefix and the
useful symbol period can be varied within a fixed total symbol period in sympathy with prevailing channel
There are many interrelated figures that describe the characteristics of any given OFDM system. These
include the overall data rate, the number of subcarriers, the subcarrier spacing, the size of the FFT and
the cyclic prefix ratio. The sampling rate for the receiver is related, either directly or indirectly, to all of
these.
Ultimately, the data rate applied to the subcarriers and the M-ary QAM constellation in use willdetermine the data symbol duration Td. The number of samples required during this period is
determined by N, corresponding to the size of the FFT.
However, the requirement for cyclic prefix must be factored in and this results in a higher sampling rate
requirement. The ratio between the nominal sampling rate and the required sampling rate is generally
referred to as the sampling factor and is an important system design parameter. This is illustrated with
an example in the diagram. In this example an OFDM system is operating in a total channel bandwidth
of 10 MHz with a 512-point FFT. This requires a subcarrier spacing of approximately 19.5 KHz.
The bandwidth of each subcarrier will therefore be 39 kHz with a nominal data symbol period of 51.3 μs.
With a 512-point FFT it would be necessary to sample 512 times in each 51.3 μs symbol period.
Therefore the nominal sampling rate will be 10 MHz. However, this does not account for the inclusion of a cyclic prefix.
In practice the cyclic prefix is included not by extending the symbol period, but by reducing the useful
part of the symbol period Td in order to make space for the cyclic prefix Tg. In the example, the cyclic
prefix is set such that it occupies one eighth of the total symbol period Ts. In order to maintain
orthogonality it is still necessary to take 512 samples in the period Td. This requires a new higher
sampling rate, which is maintained through the cyclic prefix. Since the Tg is one eighth of Ts, it is now
necessary to sample 585 times in the total symbol period Ts. This corresponds to a required sampling
rate of 11.4 MHz.
The ratio of the required sampling rate of 11.4 MHz and the nominal sampling rate of 10 MHz is the
It turns out that a simpler solution for operating in different bandwidths is to adjust the FFT size rather
than the subcarrier symbol rate. This means that most of the key system parameters remain the same for
different bandwidths and gives greater opportunity for taking advantage of channel adaptation as the
overall channel bandwidth is increased.
This approach is often referred to as scalable OFDM. The diagram shows a system operating in a 5 MHzchannel with an FFT size of 256 and then the same system operating in a 10 MHz channel with an FFT
size of 512. The symbol rate and therefore the required subcarrier separation is the same in both cases
The simplest option for multiple access in an OFDM system is to use a form of time multiplexing on the
OFDM radio bearer. This is illustrated in the top part of the diagram. Each user is allocated the full
channel bandwidth and all data subcarriers exclusively for a defined number of symbol periods.
The greatest efficiency can be achieved if dynamic time allocation is applied so that users with higher bit
rate requirements are allocated a greater proportion of time. However, in such a system the minimumresource allocation is one OFDM symbol. Even with dynamic time allocation, such an arrangement can
still become very inefficient when there is strong demand for multiple lower bit rate connections, for
example when multiple voice circuits are active. Consider an OFDM system operating in a 10 MHz
bandwidth, with a 512-point FFT and using 16QAM. Allowing for null and reference subcarriers, such a
system could transfer in the order of 1,600 bits in a single OFDM symbol period. This may seem a
modest resource unit, but delay requirements must also be accounted for. For a real-time service such
as voice it is essential to avoid excessive round-trip delay. To meet the delay requirement for a voice
service, resources may need to be allocated, for example once every 20 ms. This would mean in a
minimum bandwidth allocation to one user of 80 kbit/s (or 120 kbit/s if 64QAM is in use). Even allowing
for the error protection overhead this minimum resource will significantly reduce system efficiency and its
ability to benefit from optimal techniques such as discontinuous transmission and channel adaptation.
Greater efficiency in resource allocation can be gained from the use of subchannelization. This involves
division of resource by time and by frequency. Thus a user may be allocated a subset of the subcarriers
available in the system, as illustrated in the lower part of the diagram. This approach allows much finer
granulation in resource allocation and therefore greater efficiency. OFDM systems that support this are
usually described as OFDMA (Orthogonal Frequency Division Multiple Access) systems.
In practice, subchannelization is generally achieved through the structured grouping of all available
subcarriers into a defined set of subchannels. The smaller the group of subcarriers in each subchannel
the finer the granularity of resource allocation.
In some systems the subchannel definition may be fixed and resource allocation will simply involve the
identification of one or more subchannels for one or more symbol periods. However, system performancecan be enhanced if the subchannel structure can be made flexible.
The most significant aspect of subchannel structure is the way that grouped subcarriers are allocated
from across the channel bandwidth. There are essentially two options: they may be localized or
distributed.
The top part of the diagram shows a subchannelization organization where subcarriers within a
subchannel are localized. Since all the subcarriers in a subchannel are similar in frequency there is little
or no frequency diversity gain. The benefit in this strategy lies in the high correlation in propagation
characteristics within the subcarrier set. This high correlation enables more accurate channel estimation,
and therefore adaptation, to be applied. In particular this approach gives the best performance when
optimal techniques such as MIMO (Multiple Input Multiple Output) antenna systems are used.
The lower part of the diagram shows a subchannelization organization where subcarriers within a
subchannel are distributed across the channel bandwidth. This approach has the significant advantage
that it maximizes frequency diversity gain for the subchannel.
The quality of the radio link is affected by many factors including fading, interference and time dispersion.
Terrestrial mobile radio channels, which are usually assumed to be non-line of site, can be very poor.
Therefore most terrestrial cellular radio systems are designed with robust modulation schemes and large
error protection overheads.
However, close examination of real channel conditions shows them to be very variable in short timeframes, and much of the time any given channel will show good performance. Thus the standard
approach engineers the channel to deal with the worst case, which only occurs for a small amount of
time.
It is clear that if the channel could be adapted at a rate fast enough to track changing channel conditions
then the average performance of a channel could be significantly improved. This is the principle of
channel adaptation. Channel adaptation is a common approach in many broadband radio systems and in
most cases involves the adaptation of the modulation scheme and the error protection overhead applied.
Adaptive scheduling can also be very effective, enabling the cell to make the best use of the pool of
channels allocated to different mobiles, each of which will be varying independently.
The specific operation of Turbo decoding algorithms is extremely complex, but the basic principle is
illustrated in the diagram. Turbo decoding is an iterative process that utilizes confidence information on
each iteration. Each alternate iteration is based on either a constituent coder 1 or coder 2 decode. The
output of each iteration provides reliability information for the subsequent decode.
This process is repeated until there is very little improvement in the reliability of the decode withsuccessive iterations, typically 18 to 20 iterations. Working in this manner provides a considerable
improvement in error correction performance when compared to a single convolutional coder. However,
the decode algorithms are more complex and the multiple iterations must be performed very rapidly to
avoid cumulative delay. Additionally, the improvement in performance is most noticeable for large data
A chief disadvantage of OFDM systems is the poor power efficiency achievable in the power amplifier.
There are two main reasons for this. The first relates to the use of QAM modulation, which necessitates
the use of a highly linear power amplifier in order to meet the ACLR (Adjacent Channel Leakage Ratio)
performance requirements. This would be true for any system using QAM modulation, UMTS for
example, but in OFDM systems this is compounded by a second factor relating to the multi-carrier nature
of the OFDM signal itself.
The OFDM signal is the sum of multiple sinusoids at different frequencies and each carrying different
modulation symbols. The sum of the sinusoids produces a complex waveform with a significantly higher
peak to average ratio than a single carrier signal would have. As different combinations of transmit
symbols occur on the concurrent subcarriers, the summed transmitted signal exhibits the periodic
occurrence of a high PAPR (Peak to Average Power Ratio). The larger the number of subcarriers the
more frequently this condition occurs. The result is that in order to maintain linear operation in the power
amplifier, larger power back-off must be provided. In turn this means either less efficient or more
expensive power amplifier solutions.
There are a number of strategies that can be applied in OFDM systems to mitigate the problems of the
high PAPR. Most relate to power amplifier design and signal conditioning. These techniques are used inthe LTE downlink, which is a pure OFDMA system. However, both cost and power efficiency are very
critical in the UE, so 3GPP has opted to use modification of OFDMA in the uplink known as SC-FDMA
(Single Carrier Frequency Division Multiple Access). As the name suggests this technique gives the
transmitted signal a single carrier characteristic, which in turn reduces the PAPR.
The typical layout of an SC-FDMA transmitter is shown in the diagram. Despite its name, the transmitted
radio signal for SC-FDMA is an orthogonal multicarrier transmission similar to OFDMA. The term ‘single
carrier’ refers to the preprocessing of the baseband data prior to its application to the IFFT.
The data stream is presented in a serial fashion to the radio transmission process. The first stage is
modulation symbol mapping, which produces parallel blocks of ‘M’ complex valued symbols. The number of bits represented by each of the ‘M’ modulation symbols is dependent on the modulation scheme in
use.
Each group of ‘M’ modulation symbols is then presented to an M-point DFT, which produces an output
effectively representing the frequency components of the group of modulation symbols. It is then these
frequency components that are mapped to the allocated inputs of the N-point IFFT, where N is the total
number of subcarriers available. The diagram shows localized mapping, but a distributed mapping to
allocated subcarriers would also be possible.
The output of the IFFT and modulator will be a multi-carrier transmission. However, unlike OFDMA there
is not a direct mapping of each baseband symbol onto individual transmitted subcarriers. Instead, the
frequency components of each baseband symbol are now represented across all the transmittedsubcarriers, it is this that gives the transmitted signal a ‘single carrier’ characteristic. The result is a
transmitted signal with an improved PAPR compared to an equivalent OFDMA transmission.
MIMO (Multiple Input Multiple Output) antenna arrays offer significant performance improvements over
conventional single antenna configurations.
The technique involves placing several uncorrelated antennas at both the receiving and transmitting ends
of the communication link. If there are four uncorrelated antennas at the transmitter and a further four
uncorrelated antennas at the receiver, then there will be 16 possible direct radio paths between thetransmitter and the receiver. Each of these is open to multipath effects, creating even more radio paths
between the transmitter and the receiver. These radio paths can then be constructively combined, thus
producing micro diversity gain at the receiver.
Since the receiver can distinguish between the various uncorrelated antennas, it is possible to transmit
different data streams in different paths. The stream applied to each antenna can be referred to as a
‘layer’ and the number of antennas available at the transmitter and receiver can be referred to as ‘rank’.
For example, a system operating with a 4x4 MIMO antenna array can be described as having four layers
and being of rank four. The way in which data streams are mapped to layers will change the specific
benefits offered by a particular MIMO implementation, and the specification of this is an important part of
system design. Pre-coding may also be used to improve the MIMO system performance. Pre-coding may
be adaptive and as such would be based on some source of channel estimation. This could be derived atthe transmission or the reception end of the link.
It is relatively easy to mount antennas on the base station in an uncorrelated manner. For a 2x2 MIMO
array a single cross-polar panel could be used. A 4x4 MIMO array would require two cross-polar panels
with suitable special separation. This is harder to achieve in a mobile. However, as for the base station,
2x2 MIMO could be achieved with cross polarization, but this could result in some undesirable directivity
MIMO is potentially a complex technology but it can provide very significant benefits in system capability.
There are three key ways in which MIMO improves system performance. Any given MIMO
implementation may make use of all these benefits or may be configured to take particular advantage of
one of them. Ideally, a system should be designed with sufficient flexibility in MIMO implementation to
allow a system operator to choose the most suitable implementation for different environments or system
goals.
Diversity gain arises out of the provision of multiple antennas at the transmitting and/or receiving end of
the radio link. This creates multiple transmission paths with decorrelated fading characteristics. The
result is an overall improvement in channel signal-to-noise ratio leading to increased channel throughput
and reliability.
Array gain refers to the beamforming capability of a multiple antenna array. With suitable signalling of
feedback from the receiver, or with measurements made on a return link, it is possible to direct radiated
energy toward the receiver in a steered beam. The result is improved channel performance and
increased throughput.
Spatial multiplexing gain arises out of the orthogonality between the multiple transmission paths createdby the multiple antenna array. Since the receiver can resolve independent transmission paths it is
possible to map different information streams into the transmission paths, identifiable by their spatial
signature. This results in a direct increase in the channel throughput in proportion to the number of
The basic implementation of MIMO is generally referred to as SU-MIMO (Single-User MIMO).
The SU-MIMO concept can be developed into MU-MIMO (Multi-User MIMO). In this case the spatial
multiplexing capability of MIMO is used to multiplex a link to more than one mobile using the same
time/frequency resource. The order of multiplexing available depends on the number of antennas (or
rank) available at the transmitter and receiver ends of the link. For example, the diagram shows a 2x2MIMO arrangement being used for MU-MIMO with two mobiles. In this case, the rate available to each
mobile would be lower than that potentially available to a single mobile with an SU-MIMO configuration,
but both mobiles are allocated the same time/frequency resource and still have the potential for diversity
and array gain. Thus cell capacity is increased, but the resource can be shared between a larger number
of users. The use of more than one transmitting or receiving station in this way is sometimes called
virtual MIMO.
It is also possible to implement MU-MIMO in one direction only with just single antennas on each of the
mobiles. In this case, array and diversity gain would be reduced, but time/frequency resources can still
be reused in the cell.
MU-MIMO can be further developed into multi-cell MU-MIMO. In this case the data streams are mappedto the combined antenna resources of two or more base stations that provide a combined connection to
multiple mobiles in multiple cells. The scenario in the diagram is in effect 4x4 MIMO but shared between
two connections. Note that spatial diversity will be significant in such a scenario because of the
geographical separation of the base station and of the mobiles.
E-UTRA supports services in a variety of channel bandwidths. In fact, the specification explicitly labels
E-UTRA as ‘bandwidth agnostic’, meaning that it has no rigidly defined or preferred channel bandwidth
and can be scaled to channels of almost any size. Both FDD and TDD modes are supported, as is a
‘half duplex’ mode.
E-UTRA has also been designed to work as the bearer for Multicast and Broadcast Multimedia Services(MBMS) and as such includes support for SFN (Single Frequency Network) operation.
Support for advanced antenna configurations has also been designed into the specification with MIMO
and beam-forming adaptive antennas both being referenced.
E-UTRA/LTE is designed to work in a variety of bandwidths ranging initially from 1.4 MHz to 20 MHz. As
E-UTRA is described as being ‘bandwidth agnostic’, other bandwidths, ones that allow E-UTRA to be
backwards compatible with channel allocations from legacy network types, for example, could be
incorporated in the future.
The version of OFDMA employed by E-UTRA is similar to the versions employed by WiMAX or DVB, butwith a few key differences. In systems such as WiMAX, OFDMA schemes occupying different channel
bandwidths employ different subcarrier spacing, meaning that there is a different set of physical layer
parameters for each version of the system.
The E-UTRA scheme allows for two fixed subcarrier spacing options, 15 kHz in most cases, with an
optional 7.5 kHz spacing scheme, only applicable for TDD operation and intended for in very large cells
in an SFN. Fixing the subcarrier spacing reduces the complexity of a system that can support multiple
channel bandwidths.
Further Reading: 3GPP TS 36.211, 36.101:5.5, 36.104:5.5
There is considerable regional variation in the availability of spectrum for LTE operation and this is
reflected in the standards. Along with flexibility in bandwidth there is considerable flexibility for spectrum
allocation. There are no requirements for minimum band support nor for band combinations. It is
assumed that this is determined by regional requirements
The standards currently identify 19 bands for FDD operation, ranging from frequencies of approximately700 MHz through to frequencies in the range 2.7 GHz. There also eight bands identified for TDD
operation ranging from approximately 1900 MHz to 2.6 GHz. Considerable scope has been left in the
standards to add more frequency bands as global requirements evolve.
Further Reading: 3GPP TS 36.101; 5.5, TS 36.104; 5.5
For both uplink and downlink operation subcarriers are bundled together into groups of 12. This grouping
is referred to as an RB (Resource Block). The RB also has a dimension in time and when this is
combined with the frequency definition it forms the basic unit of resource allocation.
The number of resource blocks available in the system is dependent on channel bandwidth, varying
between 100 for 20 MHz bandwidth to just six for 1.4 MHz channel bandwidth. The nominal spectralbandwidth of an RB is 180 kHz for the standard 15 kHz subcarrier spacing. Note that this means there is
a difference between the stated channel bandwidth and the transmission bandwidth configuration, which
is expressed as n x RB. For example, in a 5 MHz channel bandwidth the transmission bandwidth would
be approximately 4.5 MHz. This difference acts as a guard band.
OFDMA channels are allocated within an operator’s licensed spectrum allocation. The centre frequency
is identified by an EARFCN (E-UTRA Absolute Radio Frequency Channel Number). The precise location
of the EARFCN is an operator decision, but it must be placed on a 100 kHz raster and the transmission
bandwidth must not exceed the operator’s licensed spectrum.
Further Reading: 3GPP TS 36.101:5.6, 5.7; 36.104:5.6, 5.7
64QAM (64-state Quadrature Amplitude Modulation). BPSK is only employed for a limited set of
signalling and reference functions, while 64QAM is optional on the uplink.
The range of error coding options used in E-UTRA devices is far more limited than those available to, for example, a UMTS device. For most channels the only option is one-third rate turbo coding based on
convolutional coding.
Broadcast traffic channels are only permitted to use 1/3 Tail Biting convolutional coding. Various control
channels have been assigned either convolutional coding, block coding or simple repetition as their error
coding options.
In addition to error coding, transport blocks containing user and control traffic may also optionally have a
CRC block attached. Transport blocks on connections that have CRC (Cyclic Redundancy Check)
selected have a 24-bit CRC block appended to the end of the data container.
The familiar UMTS error monitoring levels of Bit Error Rate (BER), derived from the error coding service,and BLER (Block Error Rate), derived from CRC, continue to be available in E-UTRA.
There are two basic frame types employed in E-UTRA, which are common to both uplink and downlink.
Type 1 frames are employed for FDD full- and half-duplex systems, while Type 2 frames are reserved for
TDD operation only.
The Type 1 frame duration is 10 ms and it is divided into 20 slots, each of 0.5 ms duration. More
significantly, however, for most information transmission, two slots are combined to form a subframe.Thus subframe duration is 1 ms, which corresponds to the TTI (Transmission Time Interval) for E-UTRA.
Type 1 slots contain either 7 or 6 symbols, depending upon which cyclic prefix type is in use.
Additionally, the length of the cyclic prefix prefixed applied in a particular symbol within a slot varies, also
dependent on which cyclic prefix length is in use. With the normal CP, symbol 0 in each slot has a CP
equal to 160 x Ts or 5.2 μsec, while the remaining symbols in the slot have slightly shorter CPs of just
144 x Ts or 4.7 μsec. When using the extended CP, all symbols are prefixed with a CP of 512 x Ts or
16.7 μsec.
Scheduling occurs across a subframe period. Up to the first three symbols in the first slot of each
subframe can be defined as a ‘control region’ carrying control and scheduling messages. The remaining
symbols of the first and all symbols in the second slot within the subframe are then available for user traffic.
Type 2 frames are used in TDD (Time Division Duplex) configured systems. They have a structure that is
generally similar to UMTS TDD LCR (Low Chip Rate), sometimes known as TD-CSCDMA. They share
the 10 ms frame structure and 1 ms subframe, but an additional demarcation known as a half-frame is
also defined.
Each half-frame carries five subframes, the second of which contains three specialized fields. DwPTS(Downlink Pilot Time Slot), UpPTS (Uplink Pilot Time Slot) and GP (Guard Period) appear in subframe 1
and optionally also in subframe 6 within a frame.
GP provides the downlink to uplink switching point for TDD operation, thus the system is configurable for
either 5 ms switching or 10 ms switching. The uplink to downlink switching points are variable within
either the 5 ms half-frame or the 10 ms frame, dependent on the configuration selected. Subframes 0
and 5, along with DwPTS, are always used for downlink transmission. UpPTS and the following frame
are always used for uplink transmission. The aim being to provide backward compatibility with UMTS
TDD mode and potentially also with WiMAX.
The terms DwPTS and UpPTS are inherited from UMTS, but in E-UTRA they can be used for normal
uplink or downlink symbol transmission carrying some control functions. Thus they really representfractional slot use leading into and out of a guard period.
In any mobile-radio system it is necessary to provide mobile devices with a means of measuring and
monitoring the strength and quality of the signal they receive and of calibrating their own output to ensure
that the correct frequencies are being employed.
E-UTRA’s version of OFDMA/SC-FDMA employs a physical reference signal, embedded in the main
body of the transmitted signal to provide an opportunity for channel estimation and frequency calibrationon the downlink. On the downlink, three types of downlink reference signals are currently defined: cell-
specific reference signals, MBSFN (Multicast/Broadcast Single Frequency Network) reference signals,
associated with MBSFN transmission, and UE-specific reference signals. In most circumstances only the
first of these reference signal types will be used. The reference signal takes the form of a modulated time
and frequency shifted of symbols generated from a Gold code of length 231–1.
Reference signal symbols are inserted into the transmitted resource grid following a predetermined
sequence, as shown in the diagram for cell-specific SISO (Single Input Single Output) and 2x2 MIMO
(Multiple Input Multiple Output) antenna arrangements and the normal CP. Modifications of this pattern
are also defined for 4x4 MIMO operation, use of the extended CP, and for MBSFN operation.
Cell-specific reference signals, as well as providing a ‘known signal’ upon which to base channelestimations, are modulated to identify the cell to which they belong. The sequence is related to the cell’s
physical layer identity in the set of 504 options.
Reference signals may have an applied power-boost over data symbols of up to 6 dB.
UE-specific reference signals are used in addition to, not in place of, cell-specific reference signals. They
are intended for use when the cell supports beamforming antennas for individual UEs. The UE-specific
reference signals are only transmitted in PRBs that are scheduled to be received by the UE.
When beamforming is used the channel characteristic in the beam will be different than that for general
cell coverage. Additionally, the cell may be based on a MIMO transmission whereas there is only a SISOoption for UE-specific reference signals. Thus the UE-specific reference signals are required for accurate
channel modelling and CQI feedback for a UE with an allocated beam.
The diagram shows the arrangement for UE-specific reference symbols in the resource grid for cell SISO
operation and the normal CP. A second pattern is defined for the extended CP. UE-specific reference
signals are considered to be on port 5, ports 0 to 3 being for normal cell operation up to 4x4 MIMO and
There are two types of reference signal used in the uplink, known as DRS (Demodulation Reference
Signals) and SRS (Sounding Reference Signals). DRS symbols are multiplexed with user data and
control transmissions. DRSs provide the receiving eNB with a ‘known signal’ element upon which to
perform channel estimations and from which it can calculate timing adjustments.
For the PUSCH one DRS symbol is transmitted per slot in the 4th symbol position (symbol number three). For PUCCH there may be either two or three DRS symbols per slot dependent on configuration
(not shown).
Because DRS symbols are multiplexed with user data they will always occupy the same allocated
bandwidth as the user data. This means that the length of the reference symbol sequence needs to be
the same as the number of allocated subcarriers in the transmission bandwidth (and always a multiple of
12). For each possible bandwidth allocation a number of base DRS sequences are defined. This is
organized such that there are 30 base sequences for 1, 2 and 3 resource block allocations and more
than 30, dependent on specific bandwidth, for allocations of more than three resource blocks. Thus there
are multiple DRS sequences in many different lengths. They are organized into 30 ‘sequence groups’.
Each sequence group contains one base DRS sequence of each length up to that suitable for bandwidth
allocations up to five resource blocks, and two base DRS sequences for bandwidth allocations above fiveresource blocks.
Each cell is allocated one sequence group. In addition, multiple orthogonal DRS sequences are then
created from a single base sequence using cyclic shifts; 12 are available for each base sequence. These
orthogonal sequences are used to multiplex signals from different UEs in the same cell.
Channel estimations for received uplink signals are made by the eNB based on measurements taken of
the reference signal symbols embedded in uplink transmissions.
If there is no uplink transmission taking place, however, the eNB cannot take measurements. In these
circumstances a UE may be instructed to perform uplink sounding, which consists of the UE transmitting
a reference signal within an uplink resource allocation specifically set aside for the purpose. Sounding isperformed on the transmission of SRB signals. Resources for SRS are allocated over multiples of four
resource blocks and always transmitted in the last symbol of a subframe. SRS transmissions can be set
as periodic, can be frequency hopping and can have variable bandwidth; the configuration is set using
higher-layer signalling.
UEs may also be instructed to undertake sounding to enable the eNB to perform ‘frequency-specific
scheduling’. This term describes a procedure whereby the eNB measures the sounding signal
transmitted by a UE across some or all subcarriers and then chooses the resource block that consists of
the best performing set of frequencies. This is similar to the downlink process whereby scheduling can be
influenced by the UE’s CQI (Channel Quality Indication) reporting.
Synchronization signals are used for initial synchronization and for synchronization to neighbour cells
during handover measurements. There are two synchronization signals, the PSS (Primary
Synchronization Signal) and the SSS (Secondary Synchronization Signal).
The transmission structure for the PSS and SSS is independent of system bandwidth since they are only
transmitted on the centre 62 subcarriers. These are transmitted in the last two symbols of the first slot inthe first and sixth subframe within every frame. Note that the SSS is transmitted before the PSS. Note
also that the PSS and SSS sequences are of length 62, which means that the outermost subcarriers in
RBs occupied by PSS and SSS are not modulated. The DC subcarrier is also not modulated.
A defined set of Zadoff-Chu sequences is transmitted in the PSS and SSS, which are used to indicate
the cell’s physical layer identity. There are 504 physical layer cell identities divided into 168 groups of
three identities. The SSS identifies the group for the cell and the PSS identifies the identity within the
group. Generally, three cells on one eNB would have three identities from the same group.
The detected positions of the PSS and SSS provide frame and slot alignment for the UE, and since the
symbol positions are different for TDD mode, an FDD/TDD discrimination can also be made. Further, the
symbol duration is also different for the extended CP so normal/extended CP discrimination can bemade.
The SSS sequence is transmitted with two different shift values; one used in the first half of the frame
and the other in the second half. This means that the 10 ms frame alignment can be derived from the
It is essential that the PBCH can be received without the UE having knowledge of the system bandwidth.
Therefore it is transmitted only on the centre 72 subcarriers.
The PBCH occupies the first four symbols of the second slot in each radio frame. However, it will not use
resource elements that are reserved for reference signals on the cell. This means that the cell’s antenna
port configuration affects the way in which the PBCH is encoded. The UE initially has no knowledge of the cell’s antenna port configuration, so blind decoding of the PBCH must be used until the UE identifies
which scheme applies.
The amount of information carried in the PBCH is very small, to reduce overhead. The RRC
MasterInformationBlock message is only 14 bits long and with error protection applied it is transmitted
across PBCH instances in four consecutive frames. Thus it is transmitted every 40 ms. However, the
error protection includes a very large overhead and it is possible that the UE can decode the message
before it has received all four transmissions in the four consecutive frames.
The DC subcarrier is only applicable in the downlink direction. It is always positioned as the centremost
subcarrier and always remains unmodulated. However, as the diagram shows, its position relative to the
RB alignment varies dependent on the configured system bandwidth.
For bandwidths with an even number of RBs (1.4, 10 and 20 MHz) the DC subcarrier is positioned
between the centre pair of RBs such that the highest frequency subcarrier of the lower indexed RB isspaced by two subcarrier steps from the lowest frequency subcarrier in the higher indexed RB. Thus the
placing of PSS, SSS and PBCH is aligned with the structure of the centre six RBs.
For bandwidths with an uneven number of RBs (3, 5 and 15 MHz) the DC subcarrier is positioned centre
of the centre RB such that there are six subcarriers either side of the DC subcarrier within the centre RB.
This RB therefore occupies a radio bandwidth equivalent to 13 subcarriers. Since the location of SSS,
SSS and PBCH is always over the 62 and 72 centre subcarriers, then in this case they will not be aligned
with the RB structure. As can be seen in the diagram they occupy the equivalent of five full RBs and two
The downlink control channels PHICH, PCFICH and PDCCH are mapped into the first symbols of every
subframe in every resource block. Dependent on configuration, this may be one, two or three symbols.
The only function of the PCFICH is to identify which option is configured.
Most of the available control channel capacity is used for PDCCH, but the resource is used to carry
multiple PDCCHs. A PDCCH is used to transmit uplink and downlink resource allocations for the PDSCHand PUSCH channels. Resource allocations are carried in messages called DCIs (Downlink Control
Information). Each PDCCH is mapped into a division of the radio resource called a CCE (Control
Channel Element). Each CCE is made up from nine sets of four resource elements known as REGs
(Resource Element Groups). The eNB may decide to map one PDCCH to one, two, four or eight CCEs,
depending on the channel conditions for the UE to which the DCI is addressed.
The PHICH is used for eNB HARQ ACK/NACK of transmissions in the PUSCH from UEs. The channel is
mapped onto REGs within the downlink control channel allocation in positions dependent on
configuration. Walsh codes are used to provide code multiplexing of multiple ACK/NACK indications
within one PHICH and multiple PHICHs will be configured on one cell.
The PCFICH carries the downlink control channel allocations with a parameter known as CFI (ControlFormat Indicator). Although this could potentially be determined though blind decoding in the UE, the
provision of a channel carrying this information reduces the processing overhead. The PCFICH is
mapped to REGs in the first symbol of each subframe and will not occupy resource elements that may be
allocated as reference signals. The specific positions and scrambling of the PCFICH is related to the
Type 2 allocation has two modes of operation known as localized and distributed . For localized operation
the allocation is defined in terms of an offset indicating the PRB in which the allocation begins and a size
in PRBs. Thus the allocation will always be contiguous, but it can be any size and in any part of the band.
For distributed type 2 allocation the PRBs within the TTI are shared between pairs of UEs. Additionally,
resources are always allocated in pairs of PRBs. The result is that the allocation is frequency hoppedbetween the two slots within the TTI. This method is particularly useful for offering frequency diversity
with very small bandwidth allocations since the paired PRBs can be in any part of the band. The most
The diagram shows an example of a populated downlink FDD frame using the normal CP, 2x2 MIMO
and implemented in a 5 MHz bandwidth channel.
The PBCH is transmitted during subframe 0 of each 10 ms frame and occupies the centremost six
resource blocks. Alongside this and also in the sixth subframe in the frame are the primary and
secondary synchronization signals. Reference signal position for two resource blocks within a singlesubframe are shown for both antenna ports in the 2x2 MIMO system.
The diagram also shows the space allocated for downlink control channels, which includes PDCCH,
PCFICH and PHICH resources. A UE will be required to monitor some proportion of this dependent on
the connectivity state and the cell configuration.
The remainder of the allocation space will be used for scheduled downlink transmission in the PDSCH.
This includes common control signalling (system information and paging), dedicated control signalling
Logical channels are mapped by the MAC layer to transport channels on entry to the physical layer, and
then ultimately to physical channels within the physical layer.
The BCCH (Broadcast Control Channel) is used for system information broadcasting and carries three
RRC message types. The MasterInformationBlock message is mapped to the BCH (Broadcast Channel)
transport channel and then to the PBCH (Physical Broadcast Channel). All other system informationmessages are mapped to the DL-SCH (Downlink Shared Channel) and PDSCH (Physical Downlink
Shared Channel).
The PCCH (Paging Control Channel) carries paging messages and is mapped to the PCH (Paging
Channel) and PDSCH.
The CCCH (Common Control Channel), DCCH (Dedicated Control Channel) and DTCH (Dedicated
Traffic Channel) are all bidirectional channels and will be mapped to the DL-SCH and PDSCH for
downlink flows and UL-SCH (Uplink Shared Channel) and PUSCH (Physical Uplink Shared Channel) for
uplink flows.
The PRACH (Physical Random Access Channel) and RACH (Random Access Channel) are used only inthe uplink for initiating RRC connectivity. The random access process involves an interaction at the
physical layer under the control of MAC. There is no higher layer information in the random access
channels but the process will result in the allocation of resources for higher-layer message exchange.
A PDCP enti ty is created for each SRB and/or DRB on a per-UE basis. Al l PDCP ent it ies are
bidirectional, thus when the AM mode of RLC is being used there is a one-to-one mapping between a
PDCP entity and AM SAP in RLC. However, for the UM mode of RLC one PDCP entity will be associated
with two UM SAPs, one configured for transmit functions and the other configured for receive functions.
Within a PDCP entity sequence numbering is applied for higher layer PDUs. This ensures in-order delivery at the receiving end. In the user plane PDCP control PDUs can be used to indicate missing
PDUs.
In the user plane, only IETF-defined ROCH (Robust Header Compression) is provided. Support for this is
only mandatory for UEs that have VoIP (Voice over IP) capability.
In the control plane, integrity protection is provided for RRC signalling messages.
Ciphering is then applied in both control and user planes, although separate cipher keys are applied for a
In the control plane PDCP will carry RRC signalling messages in the DCCH channel. The resulting PDU
includes a 5-bit PDCP SN (Sequence Number) and a 32-bit I-MAC (Integrity Message Authentication
Code).
In the user plane Data PDU are in one of two similar formats, the difference being the length of the PDCP
SN, which may be configured as seven or 12 bits. In addition the PDU begins with a D/C (Data/Control)bit, which differentiates a Data PDU from a PDCP control PDU. Note that message integrity is not used
for user-plane traffic.
There are two types of PDCP control PDU, both of which are applicable only in the user plane. The first
is used in support of the ROCH process. ROCH feedback confirms the initial static IP header values that
will be removed in compression.
The second PDCP control PDU is known as a status PDU and contains information about missing PDUs.
This takes the form of and FMS (First Missing PDCP SN) field and an optional supporting bitmap. The
status PDU is primarily used in conjunction with lossless handovers.
LTE offers two mechanisms for maintaining data flow to a UE as it moves from one cell coverage area to
another. Once a handover decision has been made based on measurement information the system may
transfer the data flow using either a seamless handover or a lossless handover.
A seamless handover is intended for non-delay-tolerant services such as VoIP. In general, seamless
handover is applied for radio bearers using the UM mode of RLC. In this procedure untransmittedpackets can be transferred from the source eNB to the target eNB, but no acknowledgment status is
supplied. Thus in the example shown the packet transmitted with PDCP sequence number 3, which has
not been acknowledged, will be lost. Only PDCP packet 4 is transferred to the target eNB since it has not
yet been transmitted. When transmission is resumed on the target cell, PDCP and other counters are
reset. This results in a faster handover, but may result in packet loss.
For data services that can tolerate some delay but for which packet loss is not acceptable, a lossless
handover is used. In general, lossless handover is applied for radio bearers using the AM mode of RLC.
In this procedure the PDCP context is transferred from the source eNB to the target eNB and the UE
uses a PDCP Status PDU to indicate missing PDUs. PDCP and other counters continue the existing
sequence. Thus, in the example, PDCP packet 3 is retransmitted on the target eNB since although it had
already been transmitted, it had not yet been acknowledged.
Ciphering is applied in the PDCP layer to both RRC signalling and traffic. The keys, known as K RRCenc and
KUPenc for the control and user planes respectively, are supplied by the RRC layer, which in turn has
derived it from KeNB, itself derived from CK and IK generated by a standard UMTS-based authentication
procedure performed in the NAS.
KRRCenc and KUPenc are truncated to 128 bits and then used in the ciphering algorithm along with a time-variable COUNT value, a direction indication (UL/DL), the radio bearer ID and a length indication
dependent on the PDCP SDU size in use. The output of the algorithm is a key stream block with a length
matching that of the plain text block (PDCP SDU). The key stream block and the plain text block are
modulo 2 added to create the cipher text block. The key stream block is recalculated for each
consecutive plain text block with incrementing COUNT values. The process is repeated at the receiving
RLC provides three levels of service: acknowledged mode, unacknowledged mode and transparent
mode. Radio bearers are mapped through RLC to logical channels and an RLC entity is created for each
active radio bearer.
For the transparent mode and the unacknowledged mode RLC entities are configured as either
transmitting or receiving entities. For acknowledged mode a single entity provides both transmit andreceive functionality for one side of the link. This configuration facilitates retransmission of failed RLC
Unacknowledged mode entities are accessed through a UM-SAP. Unacknowledged mode reorganizes
RLC SDUs into a size requested by the MAC layer. Unacknowledged mode also provides sequence
numbering for in-order deliver to higher layers at the receiving end. Reordering in the RLC layer is used
in support of the HARQ functions provides by the MAC layer.
Reorganization of RLC SDUs is provided by the segmentation and concatenation function. As shown inthe diagram, higher-layer SDUs can be fragmented and reassembled into the RLC PDU payload area to
produce a packet size suitable for scheduling by the MAC layer for transmission over the air interface.
The RLC header enables the receiving entity to reassemble the higher-layer SDU in the correct order.
The application of unacknowledged mode is limited to the user plane, where it would be utilised for
packet traffic flows with low tolerance to delay. The most common example would be VoIP connections.
The acknowledged mode of RLC is applicable in the control plane for RRC signalling messages carried
in DCCH and for user plane traffic carried in DTCH. Acknowledged mode entities are accessed through
an AM-SAP.
General transmission and reception functionality in terms of segmentation, concatenation, buffering and
HARQ reordering for AM mode are similar to those for UM mode. However, AM mode also providesretransmission of failed RLC PDUs. In this respect a number of enhancements in functional architecture
are provided. Firstly, a single entity for transmission and reception is required for interaction between the
transmitting and receiving side. Secondly a retransmission buffer is required in the transmit side. All
transmitted RLC PDU are retained in the transmission buffer until acknowledgement is received.
Additionally, control (status) PDUs are required in addition to data PDUs in order to manage the
retransmission process. These must be multiplexed with data PDUs at the transmission end and
demultiplexed (routed) from data PDUs at the reception end.
There are three types of RLC PDU used for the AM mode of operation in RLC. The general structure of
each is shown in the diagram.
The basic PDU for delivery of RLC SDUs is the AMD PDU. It has a very similar structure to the UM PDU
but with three additional fields. The D/C bit distinguishes this PDU from a Status PDU. The RF
(Resegmentation Flag) distinguishes this PDU from an AMD PDU Segment. The P (Polling) bit is used totrigger the transmission of a Status PDU from its peer entity on the receiving end of the link.
AMD PDU segments are used for the retransmission of failed AMD PDUs where the PDU size indicated
by MAC is smaller than was used for the initial transmission of the AMD PDU. Thus the original AMD
PDU is resegmented into a number of AMD PDU segments. The general structure is the same as the
AMD PDU with two additional fields that are required for the reconstruction of the original AMD PDU.
These are the LSF (Last Segment Flag), used to indicate the last segment of a segmented AMD PDU,
and the SO (Segment Offset), used to indicate the position of the segment within the Segmented AMD
PDU.
Status PDUs are used to acknowledge received PDUs or trigger retransmissions of failed PDUs. They
contain only an RLC header part and a payload part. The header part is made up of a D/C bit, three CPT(Control PDU Type) bits and an E1 (Extension) bit. The Type field indicated the Control PDU type,
although in Release 8 of the standards only the type shown exists. The E1 bit indicates whether or not
any of the optional NACK fields are present in the payload part of the PDU.
The payload part of the contains one ACK_SN field, which indicates the SN of the next expected PDU.
Optionally it many also contain one or more NACK_SN fields, which indicate the SN of PDUs that have
been detected as lost. Each NACK_SN may be supplemented by SOstart and SOend if the NACK_SN
relates to lost segments of a PDU.
Further Reading: 3GPP TS 36.322:6.2.1.4, 6.2.1.5, 6.2.1.6
The RACH procedure is handled by the MAC and the physical layer and operates using a combination of
the PRACH on the uplink and the PDCCH on the downlink. UEs are informed of the range of random
access preambles available in system information, as are the contention management parameters. When
a random access event is required, the UE will perform the following functions:
review and randomly select a preamble
check the BCCH for the current PRACH configuration; this will indicate the location and periodicity
of PRACH resources in uplink subframes
calculate open loop power control parameters – initial transmit power, maximum transmit power
and power step
discover contention management parameters
Once the UE transmits an initial preamble it will wait a specified period of time for a response before
backing off and retrying. Open loop power control ensures that each successive retry will be at a higher
power level.
Upon receipt of a successful uplink PRACH preamble, the eNB will calculate power adjustment andtiming advance parameters for the UE based on the strength and delay of the received signal and
schedule an uplink capacity grant to enable the UE to send further details of its request. This will take the
form of the initial layer 3 message. If necessary, the eNB will also assign a Temporary C-RNTI (Cell
Radio Network Temporary Identifier) for the UE to use for ongoing communication.
Once received, the eNB reflects the initial layer 3 message back to the UE in a subsequent downlink
scheduled resource to enable unambiguous contention resolution. After this point further resource
allocations may be required for signalling or traffic exchanges; these will be addressed to the C-RNTI.
In addition to DRX for UEs in idle mode, E-UTRA supports DRX for UEs in RRC connected mode. This
process is controlled collectively by MAC and RRC. The parameters are set by RRC but it is the MAC
layer the operates the process itself.
The onDurationTimer defines the length of time that the UE is active and monitoring downlink control
channels when DRX is running; in the example in the diagram this is set to two subframes (2 ms). Thisoperates in conjunction with a DRX cycle that defines the amount of time that the UE can be ‘off’. There
are two DRX cycles defined for a UE known as the longDRX-Cycle and the shortDRX-Cycle. As can be
seen in the diagram, the longDRX-Cycle is the default value.
When a period of activity is started through the scheduling of resources for the UE’s C-RNTI, the UE
starts the drx-InactivityTimer . If the UE remains active long enough for the drx-InactivityTimer to expire,
or if it receives a MAC CE on which it may have to act, then, when activity stops, the UE will use the
shortDRX-Cycle period and also start the drxShortCycleTimer . If no further activity takes place before the
drxShortCycleTimer expires then the UE reverts to the longDRX-Cycle period.
As with other E-UTRA protocols, the RRC layer, which previously resided in the RNC, has been
relocated to the eNB. In addition, the functionality and complexity of RRC has been significantly reduced
relative to that in UMTS. The main RRC functions for LTE include creation of BCH (Broadcast Channel)
system information; creation and management of the PCH (Paging Channel); RRC connection
management between eNB and UEs, including generating temporary identifiers such as the C-RNTI;
mobility-related functions such as measurement reporting, inter-cell handover and inter-eNB UE contexthandover; QoS management; and direct transfer of messages from the NAS to the UE.
The RRC is in overall control of radio resources in each cell and is responsible for collating and
managing all relevant information related to the active UEs in its area.
System information provides the main means of advertising the services available in a cell and the
means by which those services can be accessed. For E-UTRA the BCH carries only basic information
and acts as a pointer for broader system information related to the NAS, such as PLMN identity (network
code and country code) and AS details such as cell ID and tracking area identity; all of which is carried in
the downlink dynamically scheduled resource (DL-SCH).
E-UTRA has been designed with network sharing in mind and system information can carry details of upto six sharing PLMNs.
Each eNB is responsible for managing inter-cell handovers between all the cells it controls. When
handover to another cell site is required the eNB will pass details of the current UE context to its
neighbour. This includes details of identities used, historical measurements taken and active EPS
RRC exists only in the control plane of the air interface AS (Access Stratum) protocol stack. RRC
receives information from functional entities in the NAS (Non Access Stratum) in the form of complete
messages for direct transfer, and also in the form of requests, information elements and parameters that
will trigger RRC activity and be used in RRC messages.
For broadcast functions over the air interface RRC messages are mapped directly to logical channels.This includes paging and system information broadcasting using the PCCH (Paging Control Channel)
and BCCH (Broadcast Control Channel) logical channels respectively.
For dedicated signalling functions between a UE and an eNB signalling flows are mapped into an SRB
(Signalling Radio Bearer). When a UE transitions to the RRC connected state a set of SRB instances is
created. SRB 0 is used only for the initial establishment of the RRC connection and is mapped to the
CCCH (Common Control Channel). Once the RRC connection is established the UE will be issued with a
C-RNTI (Cell-Radio Network Temporary Identity) and SRB 1 and optionally SRB 2 will be created. SRB 1
is used for all RRC specific signalling functions. SRB 2 is used for RRC direct transfer of NAS signalling
messages. However, NAS messages may also be piggybacked with RRC signalling in SRB 1. Both SRB
1 and SRB 2 are mapped to DCCH (Dedicated Control Channel) logical channels.
If required, one or more DRB (Data Radio Bearers) may be created during or subsequent to an RRC
connection establishment. These exist in the user plane and carry traffic. However, ‘traffic’ in this context
includes service-related signalling between service applications in higher layers, for example VoIP
connection establishment using the IMS. DRBs are mapped to DTCH (Dedicated Traffic Channel) logical
The primary function of paging is to trigger the establishment of EPS bearers for incoming calls or to
establish or re-establish EPS bearers for new data activity relating to a UE that is already receiving EPS
connectivity service.
Paging requests are generated in the MME. The MME indicates a paging requirement to eNBs across a
tracking area. Each eNB compiles paging requests as one or more PR (Paging Record) informationelement within a single RRC Paging message. The maximum number of PRs in a Paging message is 16.
Paging messages are transmitted via the PCCH/PCH channel combination; however, the UE does not
need to monitor the PCH transport channel all the time since it is carried in the dynamically scheduled
downlink physical resource. This means that there is no paging channel as such at the physical layer and
the UE need only monitor scheduling information for paging occurrences. Scheduling instances for the
transmission of RRC Paging messages are identified with a specifically defined RNTI called the P-RNTI.
In addition UEs will apply a defined DRX cycle for the monitoring of the scheduling information.
Paging also has three secondary functions. Indications for system information change, ETWS and CMAS
are also carried in RRC Paging messages. This removes the need for the Idle mode UEs to monitor
The overall function of RRC is to create, maintain and clear DRBs as required to provide the radio link
segment of one or more EPS bearer relating to one or more EPS connectivity service. RRC receives
instructions on what EPS bearers are required from the NAS. The NAS activity in turn is driven by
instructions from service applications (via the PCRF on the EPC side).
In order to manage DRBs, RRC must exchange signalling with its peer entity and provide direct transfer for NAS signalling exchange. Connectivity for this comes from SRBs. However, signalling relating to
service applications, which are always external to the LTE/EPS, are treated as traffic flows and as such
are carried in DRBs within an EPS bearer. Note that an EPS bearer has only one set of associated QoS
characteristics, so if application signalling were to require different QoS treatment to the traffic that it
facilitates then a second EPS bearer would have to be defined. Multiple EPS bearers may or may not be
part of the same EPS connectivity service dependent on their respective connectivity requirements.
The RRC connection establishment procedure is always initiated from the UE. It begins with the
transmission of the RRCConnectionRequest message containing an identity and a cause value. If the UE
has already registered with the network then it will use the S-TMSI as its identity. If this is a new mobile
needing to perform an initial registration then it will generate and use a 40-bit random value. The
message is carried in the CCCH/UL-SCH channel combination. This requires a scheduled resource
allocation, which is secured using the lower-layer random access procedure and the RACH (Random Access Channel). The lower-layer random access procedure also facilitates the allocation of a C-RNTI at
this stage.
The eNB responds with an RRCConnectionSetup message containing a transaction identifier, used to
relate future messages as part of this signalling sequence, and the radio resource configuration for SRB
1. Note that the exchange of the two messages to this point has involved the use of the implicitly
configured SRB 0.
The final part of this three-way handshake is the confirmation from the UE in the form of the
RRCConnectionSetupComplete message now using the defined SRB 1 and DCCH/UL-SCH
combination. For registered UEs this message contains identities of the PLMN and MME with which it is
registered. In any case the message will also piggyback the initial NAS message that triggered the RRCestablishment procedure, for example, a service request or registration message.
Further Reading: 3GPP TS 36.331:5.3.3, TS36.321:5.1
There are many possibilities for UE capability, both in terms of LTE feature support and the UE’s support
of one or more other RAT (Radio Access Technology). It is therefore important that at some point after
initial RRC connection establishment the eNB determines exactly what the UE capabilities are.
This is achieved through the relatively simple capability enquiry procedure. The diagram shows the
general information that may be contained in the UE’s UECapabilityInformation response message. Atpresent this covers the possibility for multimode operation with GSM/GPRS, UMTS, CDMA2000 1x and
1xEV-DO. It is for future study whether this may also include other RATs such as WiMAX or Wi-Fi. This
does not preclude the support of any other RATs in the device, but does mean direct interworking at the
eNB level is not possible. However, interworking via a GANC would still be possible.
Definitions for RATs other than E-UTRA/LTE are to be found in the relevant documents indicated in the
table. For E-UTRA/LTE the information elements in the UE capability container predominantly cover the
UE’s capability at layer 2 and at the physical layer. It includes the UE’s category, which describes the key
factors determining the maximum throughput that UE can achieve, for example, the UEs MIMO antenna
capability.
Further Reading: 3GPP TS 36.306, TS 36.331:5.6.3,6.3.6
RRC plays a largely intermediary role in the air interface security processes. Its main function is the
control of the available security functions, but the implementation of those functions is performed in lower
layers.
The RRC SecurityModeCommand message is used to start one or all of the three independent air
interface security functions, RRC message integrity, RRC message encryption and data trafficencryption. Each process uses a different key, each of which will have been generated in both network
and UE through NAS signalling interaction between the UE and the MME. Once started, the security
functions themselves, including RRC message integrity, are performed in the PDCP (Packet Data
When a measurement report indicates that an I-RAT handover is required, the eNB cannot negotiate
directly with the target cell. Instead, the mobility procedures are handled by interactions via the MME.
Once suitable resources are allocated on the target cell, handover information is forwarded to the source
eNB, which forwards them to the UE in an RRC MobilityFromEUTRACommand message.
On reception of this message the UE changes RAT mode and implements the new channel asinstructed. Handover acceptance and confirmation after this point is dependent on the RAT concerned.
However, for GSM or UMTS this will involve the transmission of a RR or RRC Handover Complete
Before the UE can obtain service from an LTE system it must perform a cell search procedure.
Essentially this procedure is used to find and then determine timing and frequency parameters that
enable successful transmission and reception from the cell in question.
The cell search procedure is used for initial synchronization when a UE has just been switched on or has
just entered LTE coverage, and it is also used for new neighbour cell identification for the overall mobilitymanagement processes that control both the idle and connected modes of operation. For initial search
and for new neighbour identification the UE is likely to need to be able to search other RATs in addition
to LTE. In this case the cell search and synchronization procedures are as specified for the technology
being searched.
After detecting an LTE signal the UE searches for the primary and secondary synchronization signals on
the cell. Acquisition of the primary synchronization signal provides slot alignment and identifies the cell ID
(one of three) within the cell ID group. The secondary synchronization signal provides frame alignment,
identification of the cell ID group (one of 168) and identification of CP length. This is sufficient information
for the UE to be able to read the MIB (MasterInformationBlock ) message in the PBCH.
In turn, information in the MIB and the cell ID enable the UE to find and decode the downlink signallingchannels. Ultimately scheduling based on the SI-RNTI in the PDCCH enables the UE to find and read all
the system information for the cell, which is transmitted in the PDSCH.
Idle mode represents a state of operation for the UE where it has successfully performed the following:
PLMN selection, cell selection and location registration (by tracking area).
Once in idle mode, the UE will continue to reassess the suitability of its serving cell and, in some
circumstances, its serving network. In order to do this it will implement cell and PLMN reselection
procedures. A UE in idle mode will be monitoring its current serving cell in terms of radio performanceand signalling information. The radio performance measurements are done on the basis of a quality
measure. This is an assessment of radio signal strength and interference level, and it can be made for
both the serving cell and its neighbours. The aim will be to ensure that the UE is always served by the
cell most likely to give the most reliable service should information transfer of any kind be required.
The UE will also be monitoring two key types of signalling from the serving cell system information
messages and paging or notification messages. System information messages convey all the cell and
system parameters. The UE will record changes in these parameters that may affect the service level
provided by the cell, or access rights to the cell. Changes in these parameters could provoke a cell
reselection, or a PLMN reselection. Paging or notification messages will result in connection
establishment.
All of these procedures are performed through communication between the AS and the NAS. In general,
instructions are sent from the NAS to the AS; the AS then performs the requested procedure and returns
a result to the NAS.
If CSG (Closed Subscriber Group) is supported then these procedures are modified such that a cell’s
broadcast CSG ID forms another level of differentiation between cells. CSG is intended for use with
Cell reselection in LTE both reuses many principles that were are well established in legacy technologies
and introduces new strategies. A key addition for LTE is the use of RAT/frequency prioritization. Each
frequency layer that the UE may be required to measure, either E-UTRA or any other RAT, is assigned a
priority. The cell-specific priority information is conveyed to UEs via system information messages.
Additionally, UE-specific values can be supplied in dedicated signalling, in which case they take priority
over the system information values. Any indicated frequency layers that do not have a priority will not beconsidered by the UE for reselection.
In general, the measurement rules are used to reduce unnecessary neighbour cell measurements. The
UE always measures cells on a higher priority E-UTRA inter-frequency or IRAT frequency. The UE will
only measure E-UTRA intra-frequency cells if the Srexlev value for the current selected cell falls below
an indicated threshold (Sintersearch). Similarly, the UE only measures E-UTRA inter-frequency or IRAT
frequency cells on equal or lower priority layers if the Srexlev value for the current selected cell falls
below an indicated threshold (Snonintrasearch).
Measurements are then evaluated for potential reselection. Again, the frequency/RAT priority level is
used along with system-defined threshold for this assessment. A UE will always reselect a cell on a
higher priority frequency if its value of Srxlex exceeds Threshx,high for longer than TreselectionRAT. It willonly select a cell on a lower priority frequency when the Srxlev of the serving cell falls below
Threshserving,low and Srxlev of the neighbour is above Threshx,low for TreselectionRAT and there is no other
alternative. For neighbour cells on intra-frequencies or on equal priority E-UTRA inter-frequencies, the
UE uses a ranking criterion ‘Rs’ for the serving cell and ‘Rn’ for the neighbour cell. Ranking is based on a
comparison of the respective Srxlev values with a hysteresis added to the serving cell value and an offset
added to the neighbour cell value. The UE will select the highest ranked cell if the condition is maintained
for TreselectionRAT.
In addition to all of this, the UE will apply scaling to Treselection, hysteresis values and offset values
dependent on an assessment of its mobility state, which may be high, medium or low. This is based on
When the UE becomes RRC connected, the measurement and reporting process as well as mobility
decisions becomes the responsibility of the eNB. The required measurement and reporting settings are
signalled to the UE in the RRCConnectionReconfiguration message.
The measurement object defines what the UE is to measure. This is defined as a frequency and
measurement bandwidth; optionally it may also contain a list of cells. If it does contain a list of cells thenthey will be indicated as either white list or black list. The UE will measure any cells it detects but will not
report black list cells. Frequency- or cell-specific offsets will also be included in this field.
The reporting configuration sets what quantities the UE is to measure, what quantities the UE is to report
and under what circumstances a measurement report is to be set. Reporting may be set as either trigger-
based, periodic or triggered periodic. This field also defines the other contents of the measurement report
message.
Measurement identities provides a reference number such that some part of this identified measurement
can be modified or removed in future.
The Quantity configuration sets the filtering to be used on the measurements that are taken.
The gap configuration defines periods when the UE can take measurements of neighbour cells.
The trigger events defined for E-UTRA are as follows:
Event A1 – The serving cell becomes better than absolute threshold
Event A2 – The serving cell becomes worse than absolute threshold
Event A3 – A neighbour cell becomes better than an offset relative to the serving cell
Event A4 – A neighbour cell becomes better than absolute thresholdEvent A5 – The serving cell becomes worse than absolute threshold1 and a neighbour cell becomes
better than absolute threshold2
There are also two more events for IRAT mobility:
Event B1 – A neighbour cell becomes better than an absolute threshold
Event B2 – The serving cell becomes worse than absolute threshold1 and a neighbour cell becomes
better than absolute threshold2
Events can be modified with time-to-trigger values and hysteresis values if required. Triggers may be
used to cause the transmission of a single measurement report or may be used to trigger a session of
periodic reporting. This is defined by setting the parameters reportAmount (which includes an infinitevalue) and reportInterval.
Even though uplink transmissions from LTE UEs in a cell are orthogonal, uplink power control is still
important if maximum throughput efficiency is to be achieved for individual UEs and for the cell as a
whole.
The UE calculates the transmit power to be used in each subframe in which it has a resource allocation
according to the formula shown in the diagram. Maximum power is limited by the UE power class, whichwill correspond to 23 dBm. The calculation for power to be used below this level is based on three
elements: a bandwidth-dependent element, a semi-static open-loop operating point and a dynamic
closed-loop offset.
The bandwidth element is based on the number of scheduled RBs in the UE’s uplink transmission.
The semi-static control point is itself made up from two elements. The first, P O_PUSCH( j ) , is a cell-
defined offset between –126 dBm and +23 dBm. The second part is a compensation factor based on the
UE’s estimate of downlink path loss. The value α can be varied between 0 and 1. Variation of
P O_PUSCH( j ) and α provide a trade-off between absolute cell performance and overall system
performance.
The dynamic closed loop offset is based on TPC (Transmit Power Control) commands transmitted to the
UE in the PDCCH and identifies using a TPC-RNTI. The closed loop mode of operation can operate in
two modes, one in which absolute power control commands are sent and one where corrections on a
accumulative value are sent. It is in the latter case that is referenced by the parameter f (i ).
If a UE were to be allocated an uplink bandwidth that resulted in a calculated power higher than 23 dBm,
then the UE would be unable to use the full resource. To avoid this the UE will send power headroom
reports to the eNB. These represent the UE’s estimate or its power control requirements in the current
subframe, and based on this, the eNB will be able to schedule resources efficiently between UEs in a
In order to maintain orthogonality between uplink transmissions from multiples UEs in a cell, timing
adjustment must be applied to compensate for variations in propagation delay.
Initial timing advance is calculated at the eNB from a UE’s preamble transmission on the PRACH. The
timing advance correction is given as an 11-bit value although the range is limited to 0–1282 timing
advance steps. Granularity is in steps of 16Ts (0.52 μs) so timing advance can be varied between 0 and0.67 ms. One timing advance step corresponds to a distance change of c.78 m and is significantly
smaller than the normal CP. The maximum timing advance value corresponds to a range of c.100 km.
The maximum specified speed for a UE relative to an eNB is 500 km/h (139 m/s), which would require
slightly more than one timing advance change every two seconds. Consideration also needs to be given
to the possibility of more extreme changes in the multipath characteristics of a channel, for example the
sudden appearance or disappearance of a strong reflected path from a distant object or delay through a
repeater. However, these are extreme examples and, in any case, timing advance update commands
can indicate up to +/– 16 μs in a single step. Thus the rate at which timing advance commands need to
be sent in practice is typically much less than one every two seconds.
Timing update commands are transmitted to UEs as MAC control messages and as such are included inMAC PDUs carrying data for the UE on the PDSCH. The command itself is a six-bit value giving a
number range from 0–63. Values less than 31 will reduce timing advance and values greater than 31 will
In its first release, LTE is specified with several options for SU-MIMO implementation and a more limited
option for MU-MIMO operation. The specification include descriptions of operation up to rank 4 (4x4
MIMO).
The simplest option is not MIMO, as such, but uses the multi antenna array at an eNB to provide transmit
diversity. The standards allow configuration with up to four antennas at the base station. It is likely thatcross-polar antennas would be used as part of the antenna array, so a two-antenna array could be
implemented using a single cross-polar panel, with a four-antenna array requiring two cross-polar panels.
Transmit diversity involves the transmission of a single data stream to a single UE, but makes use of the
spatial diversity offered by the antenna array. This can increase channel throughput or increase cell
range.
There are also two beamforming options available. These are based on the use of a single layer with
rank one pre-coding but make use of a multi antenna array for beamforming to a single UE. The two
options for this are a closed loop mode, which involves feedback of PMI (Pre-coding Matrix Indicators)
from the UE, and an open loop mode, which involves the transmission of UE-specific reference signals
and the eNB basing the pre-coding for beamforming on uplink measurements.
Full SU-MIMO configurations are available in LTE in the downlink direction with ranks up to four.
However, a maximum of two data streams is used, even when four antenna ports are available. In SU-
MIMO the UE can be configure to provide PMI feedback as well as RI (Rank Indicators), which indicates
the rank that the UE calculates will give the best performance.
In the first release of the LTE specification there is only a limited implementation of MU-MIMO specified.
It is applicable in the uplink direction and allows two UEs to use the same time frequency resource within
one cell.
Further Reading: 3GPP TS 36.211:6.3.3, 6.3.4, 36.213:7.1