LTE Air Interface

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LTE Fundamentals Tutorial | Commercial in confidence | 2010-05-25 | Page 1

LTE Air interface & Architecture

Presentation By

Shachi Agarwal

Dhanesh Goel

Shubha Koundinyan


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AGENDA LTE/EPC Introduction LTE Downlink Access LTE Uplink Access Physical Layer Processing Time Frame Structure Multi Antenna Solutions Mobility

LTE AIR INTERFACE and Architecture


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3G LTE release


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3GPP (3rd generation partnership project)› The 3rd generation partnership project is an agreement brings together a no. of telecommunication bodies to produce globally applicable Technical specifications & technical reports.

› The standard development in 3GPP is grouped into two work items, where LTE targets on Radio Access Network only and SAE (System Architecture Evolution) targets on Core Network.

› Common to both is that only Packet Switched Domain is defined.

› The name of actual RAN is E-UTRAN and the name of core network is EPC (Evolved Packet Core)

› E-UTRAN+EPC = EPS


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Architecture


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Simplified network architecture

WCDMA LTE/SAE


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LTE Key Features› LTE radio access

› Advanced antenna solutions

› Spectrum flexibility

› Efficient Time Dispersion Handling

› All-IP architecture

SC-FDMAOFDMA

TX TX

20 MHz1.4 MHz


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LTE Radio Interface

mnm

•The E-UTRAN Standard is based On OFDM/OFDMA downlink operation and SC-FDMA uplink operation•Support great spectrum flexiblity with a no. of possible deployments

from 1.4mhz to 20mhz spectrum allocations.


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LTE Radio Access – DownlinkOFDM - Orthogonal Frequency Division Multiplexing

› OFDM shows very good performance in highly time dispersive environment (i.e many delayed and strong multipath reflections). The data stream is divided in many sub-carriers

› The users in DL separated with OFDMA i.e each user has its own time & frequency resources so called resource Block. Radio transmissions is based on short TTI of 1ms,speeds up the operation n reduce radio interface latency.

time

f = 15 kHz

frequency

User 1User 2User 3


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LTE DL Physical Resources

12 sub-carriers

time

frequency

One Scheduling Block(2 Resource Blocks)180 kHz and 1 ms


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Radio Access Frames› 1SB = 12×14 (14 resource element)› 1RB = 12×7 (7 resource element)› 1SB = 2RB› 1 resource element corresponds to 1 OFDM symbol.› 1Radio frame = 10 SB(10 ms)› 1 Subframe consist of 2 slots 7×.5ms = 1RB 14×1ms =1SB


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Baseband OFDM SystemDiscrete frequency domainEach input controls signal at one frequency

Discrete time domainSamples of modulated &Multiplexed symbols.


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How are OFDM symbols generated?› Parallel data streams are used as inputs to an IFFT› IFFT output is sum of signal samples› IFFT does modulation and multiplexing in one step› Filtering and D/A of samples results in baseband signal.

Typical IFFT output samples.


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LTE Radio Access – UplinkSC-FDMA – Single Carrier FDMA (DFTS-OFDM)

• Single-carrier FDMA• “Single-carrier” Improved power- amplifier efficiency• Reduced terminal power consumption and cost, and improved coverage• FDMA Intra-cell orthogonality in time and frequency domain• Improved uplink coverage and capacity• Can be seen as pre-coded OFDMA, more specifically “DFT-S-OFDM”• Same basic transmission parameter (frame length, “sub-carrier spacing”, …)

tim

efrequency

User 1User 2User 3


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Uplink Transmission


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Uplink Frequency Hopping


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LTE DL peak rate20 MHz and 4x4 MIMO AND 64 QAM

…and UL no MIMO 75Mbps

14 OFDM symbols per 1.0 ms sub frame64QAM = 6 bits per symbol6 x 14 = 84 bits per 1.0 ms sub frame

84bits/1.0ms = 84kbps per sub carrier12 x 84kbps = 1.008Mbps per Scheduling Block100 Scheduling Blocks in 20MHz100 x 1.008Mbps = 100.8Mbps per antenna

4 x 4 MIMO: 403.2Mbps !

BUT in reality approx. 300Mbps


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Channel Structure› Logical channels :- describe which type of data is conveyed whether user plane data and control plane data.

› Transport Channels:- how data is transmitted and are mapped to different physical channels.

› Physical Channels:- describe how physically data is mapped on to frequency channel with required coding and modulation.


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Logical Channels› Control ChannelsBroadcast Control Channel (BCCH) Paging Control Channel (PCCH) Common Control Channel (CCCH) Multicast Control Channel (MCCH) Dedicated Control Channel (DCCH)

› Traffic ChannelsDedicated Traffic Channel (DTCH) Multicast Traffic Channel (MTCH)


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Transport Channels› Downlink:

Broadcast Channel (BCH) Downlink Shared Channel (DL-SCH) Paging Channel (PCH) Multicast channel (MCH)

› Uplink:

Uplink Shared channel (UL-SCH)Random Access Channel (RACH)


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Physical Channels


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Channel Mapping


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Physical layer Processing

UE


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Physical Layer procedures

1. Power Control 1. Downlink

• used on the data channel (PDSCH):- uniformly distributed• used on control channels (PBCH, PDCCH, PCFICH, and PHICH)• relies on CQIs and handover measurements.

2. Uplink•used both on the PUSCH and on the PUCCH •open loop combined with a closed loop mechanism is used

2. Link Adaptation

Time Domain (/user)Modulation schemeChannel coding

Frequency Domain (/SB)Not that common

Data Control Data Control

Channel coding

Modulation scheme

16QAM 64QAMQPSK

OFDM symbol 2bits 4bits 6bits


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Time Domain Structure

FDD means that UL/DL transmits at different frequency. It proves efficient in utilizing bandwidth during switch over from transmitter to receiver.TDD means that UL/DL transmits at same frequency but at diff intervals.


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Time Domain Structure FDD

The fig is valid for both UL/DLOne radio frame of length 10ms consistsOf 10 sub frames of 1ms each.Further a sub frame is divided in two slots,Each consists of 7 OFDM symbols.

Note* The sub frame is typical scheduling unit of LTE


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Time Domain Structure TDD

In TDD ,the complete Radio frame is divided into two half frames each consists of 5 sub frames of 1ms each. The second sub frame within each half frame has special structure (Downlink part, Guard Period, Uplink part), through which DL to UL switching takes place in second sub frame of each half frame.


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TDD special sub frame configurations.

1 subframe,14 symbols


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Multi Antenna Solutions


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SU-MIMO and MU-MIMO SU-MIMO

MU-MIMO

› increases the data rate for a single user by creating several layers for that user

› Used in Downlink.› No. of layers gives the rank of the system.

› different UE are spatially multiplexed› are allocated the same resource blocks › each UE does not experience a data rate multiplication › increases the capacity in the cell and used in uplink.


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LTE Mobility› The no. of states in which UE defined are Detached mode ,Idle mode ,Connected mode.

› In ECM-IDLE, the UE position is known by the network at TA level

› In ECM-CONNECTED ,the UE position is known at the cell level by e-nodeb.

› When UE attached to the network, an IP address is provided to it & remain with it even when UE turns to idle mode but released when UE detached from network.


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LTE STATES

LTE Detached:- when UE is off. When it powers on, it can directly go to LTE_Idle mode if no call request is intercepted on Paging Channel OR can be synchronized with eNB to make UL transmission possible called as IN_Sync or remain OUT_Of_Sync.


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Inter eNB HO executed either by X2 interface or S1 interface.CASE 1: UE mobility within eNBs of same pooling areasIn this case HO takes place through X2 interface only and no further relocation of parameters is required.CASE2: UE mobility within eNBs of different pooling areas.In this case S1 interface comes into picture, also MME needs to relocatefrom one pooling area to another one. S-GW also needs to relocate hand in hand


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Handover preparation

If another cell is identified as better server then HORequest is send to the target node.•Again use either X2 /S1 interface for HO signaling.•Target nodeB performs control functions and if it isable to accept HO,it reserves cell resources required for UE. enode B sends the info to node A for forwarding to UE


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Lte Architecture


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Features of enode-b› Cell control and Mobility Pool support› Mobility control› Control and User plane security› Measurements and Reporting› Scheduling› HARQ› Physical layer functionalities-Beamforming processing,

scrambling› Multiplexing and Mapping-Mapping logical channels to

transport channels


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Functionality of MME› Acts as a controlling node› Idle mode mobility management› Checks authorization of the mobile in the service providers

PLMN and enforces Roaming restrictions› Provides control plane functionality between LTE and other

2G and 3G networks› It is involved in initial authentication of user by interacting

with the HSS.


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Service/packet gateway› Acts as a Interconnection between the different eNodeB’s.› It acts as an anchor between LTE system and other 3GPP

networks like WCDMA› Connecting LTE network to non GPP systems like WiMax

and other 3GPP2 networks like CDMA,EVDO.


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hss› HSS is home subscriber server.› It contains the database of all the subscribers› It also contains their location and subscription details


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Radio Interface protocols-USER plane

IP IP

MAC MAC

RLC RLC

PDCP PDCP

PHY PHY

UE eNodeB GW


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Control Plane

NAS NAS

PDCP PDCP

RRC RRC

RLCRLC

MAC MAC

PHY PHY

UE eNodeB MME


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LTE 3GPP Rel 10(4G)

› Carrier aggregation

› Spectrum aggregation

› DL/UL Multi-Antenna transmission

› Higher peak rates

20 MHz

100 MHz total bandwidth

20 MHz20 MHz

40 MHz total bandwidth

4

8

Peak rates: 3Gbps/1.5Gbps !


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Thanks

LTE Air Interface

Documents

lte fundamentals

x2 interface

paging channel

physical channels

transport

ue position

pooling areas

core network