Rs

A channel can be considered as a medium through which some information is transmitted, where as a signal has a mathematical importance and it is, most of the times generated at the physical layer itself.

Cell specific reference signals are available for all UEs in a cell. Cell specific signals enable the UE to determine the phase reference for demodulating the downlink control channels and downlink data. Cell-specific reference signals shall be transmitted in all downlink subframes in a cell supporting non-MBSFN transmission.

UE specific reference signals which may be embedded in the data for specific UEs. UE shall receive UE specific reference signals in addition to cell specific reference signals if it is configured by higher layers. The UE-specific RSs are embedded only in the RBs to which the PDSCH is mapped for those UEs. If UE specific reference signals are transmitted, the UE is expected to use them to derive the channel estimate for demodulating the data in the corresponding PDSCH RBs.A typical usage of the UE-specific RSs is to enable beamforming of the data transmissions to specific UEs.

One of the motivations for the UE specific reference signals is the use of precoding wherein the UE specific reference signals are also precoded in the same manner as the data.

Reference signals DL

The downlink LTE reference signals are discussed in the following sections.

Cell specific Reference Signal (C-RS)

The Cell-specific Reference Signal (abbreviated C-RS or just RS) is transmitted on resource

elements spread throughout the frame in specific locations as defined by the standard. The

Cell-specific RS is used by the UE's to compensate the downlink frame for channel frequency

response and cross-channel effects so that the signal can be demodulated.

Antenna ports 0-3 each have unique C-RS locations. Antenna ports do not transmit on

resource elements allocated for RS on other antenna ports.

UE-specific Reference Signal (UE-RS)

In addition to the Cell-specific RS, the base station may transmit UE-specific RS in RBs

allocated to the PDSCH of a UE. In LTE Release 8, UE-specific RS is used to accomplish single-

layer beamforming and is transmitted on antenna port 5.

In Release 9, UE-RS for single-layer beamforming can be transmitted on antenna port 5, 7, or

8. UE-RS for dual-layer (spatial multiplexing) beamforming is transmitted on antenna ports 7

and 8. See 3GPP TS 36.211, Section 6.10.3 for more information.

The LTE demodulator only supports viewing beam patterns for linear antenna arrays. The number of

elements in an antenna group and spacing between elements is specified by the Antenna Group

parameters.

Positioning Reference Signal (P-RS)

The Positioning RS is used to enhance UE geolocation accuracy. The P-RS is transmitted

periodically in certain frames and occupies certain resource elements within a rectangular

area in the frame (RBs x SFs) as defined by the P-RS parameters.

Multicast/Broadcast Single Frequency Network Reference Signal (MBSFN-RS)

The MBSFN-RS is used to compensate the downlink channel effects on the Physical Multicast

Channel (PMCH), which contains the multicast/broadcast data, and is only transmitted

during MBSFN subframes.

Reference signals UL

There are two types of uplink reference signals, the Demodulation Reference Signal (DMRS)

and the Sounding Reference Signal (SRS). Reference signals are used for channel estimation

or equalization.

DMRS

The Demodulation Reference Signal (DMRS) is used by the base station to equalize and

demodulate the UE's transmissions.

The PUSCH demodulation reference signal is a Zadoff-Chu sequence, which results in

constellation points on a circle centered about the origin.

The PUCCH demodulation reference signal, however, is a reference sequence transmitted on a

rotated QPSK constellation. The amount of rotation is determined by cyclic shift () as defined

in the standard.

Each uplink user transmits a Demodulation Reference Signal during certain symbols in each

resource block allocated to the user. DMRS is transmitted on all subcarriers allocated to the

user during the symbols listed in Table 5.5.2.2.2-1 in 3GPP TS 36.211.

SRS

The sounding reference signal (SRS) is transmitted separately from PUCCH and PUSCH. SRS

can be transmitted on any number of subcarriers in the last symbol in an uplink subframe

whether or not the subcarriers are assigned to another channel. The exception is that PRACH

transmissions and PUCCH Format 1 and 2/2a/2b transmissions take precedence over SRS

transmissions.

SRS is transmitted by a UE to give the base station an idea of the channel characteristics for

that UE. The base station can use the information to assign good uplink allocations for the UE

to transmit on.

At the time of handover, the UE needs to acquire the UL synchrnization through Timing alignment instruction from enodeB ( achieved through contention/ non contention based RACH).

Cell-specific RSs, UE-specific RSs, MBSFN-specific RSs, Positioning RSs, from Release 9 onwards , Channel State Information (CSI) RSs, which are introduced in Release 10 . These RS are gold sequence based with length of 31, where each RS in initialized differently. For cell specific refernce signal which all UE measures in the serving cell and neighbour cells, are initialized with Cell id ( PCI- Physical cell id) at the start of each OFDM symbol. So cell specific

RS carries one of the 504 Cell ids , in other words scrambled with one of the unique 504 cell ids. In addition to above there is a cell specific  frequency shift is applied to the patterns of reference symbols  given by NcellID mod6. This shift helps to avoid time-frequency collisions between cell-specific RSs from up to six adjacent cells

Cell specific reference signals are however still transmitted for the transmission of common control signaling, mobility measurements and downlink channel quality measurements. In order to save the resources occupied by UE specific reference signals

MIMO Feedback In LTE Rel 8CSI feedback allows downlink transmission to be adaptively optimized

based on the instantaneous DL channel, so that closed loop beamforming

and adaptive link adaptation can be enabled to optimize the system

performance.

The DL reference signal used for CSI measurement is different in Rel-8 and

Rel-10. In Rel-8, CSI measurement is based on CRS, which is also used for

data demodulation. In contrast, CSI measurement in Rel-10 is based on a

set of newly introduced CSI-RS signals, which is low-duty cycle and low-

density, and allows a higher reuse factor than Rel-8 CRS. The feedback

mechanisms of LTE Rel-8 and LTE-Advanced Rel-10 are both based on the

implicit feedback framework that has been well established and tested since

early 3GPP releases.

In brief, UE measures the DL channel through measurement reference

signals and feeds back the channel state information (CSI) in the form of

recommended transmission formats.

This includes: Rank indicator (RI): number of layers recommended for SU-MIMO transmission

Precoding matrix indicator (PMI): index of the recommended SU-MIMO precoding matrix in the

feedback/precoding codebook, corresponding to the RI

Channel quality indicator (CQI): indication of the channel quality corresponding to the reported

RI/PMI in LTE, CQI is defined as a set of transport block sizes, each of which translates to a

maximum code rate and quadrature amplitude modulation (QAM) order that can be received by the

UE at a certain block error rate (BLER).

As a criterion for testing the CQI report accuracy, when the reported code

rate and QAM order is used for actual data transmission, the UE must be

able to decode the data with a BLER below 10 percent.

3GPP UMTS Long Term Evolution (LTE) requires the calculation of three different feedback values such

as Precoding Matrix Indicator (PMI), Rank Indicator (RI) and Channel Quality Indicator (CQI). Our novel

method proposes, these three different feedback values are applying to the wireless multihop relaying

network for improving the efficiency of the MIMO. To reduce the computational burden for the user

equipment, our method decomposes the problem into three separate steps, one of jointly evaluating the

PMI and RI based on a mutual information metric and one of choosing the CQI value to achieve a given

target Block Error Ratio (BLER) constraint and these feedback values are applying into the Wireless

multihop relaying. Wireless multihop relaying can increase the aggregate network data capacity and

improve coverage of wireless systems by reducing path loss, mitigating shadowing, and enabling spatial

reuse. The influence of estimated channel knowledge on the feedback choice is investigated for BLER,

maximum likelihood Estimator.

Note that PMI and RI jointly represent the spatial directions of the MIMO

channel, while CQI indicates the strength of the corresponding spatial

directions. It is easy to see that the same feedback mechanism (RI/PMI/CQI)

can be applicable and followed in LTE/LTE-Advanced.

This is independent of the exact measurement reference signals (CRS in

Rel-8 and CSI-RS in Rel-10) that are used for CSI feedback, as long as they

reflect the un-precoded antenna signals. More important, this commonality

ensures backward compatibility so that eNB and UE of different releases

can operate together seamlessly.

Benefits of this implicit PMI/CQI/RI framework include those listed here.

Overhead: It is well known in the literature that codebook-based feedback is an effective means to

achieve reasonably accurate CSI quantization with manageable overhead.

UE receiver transparency: UE receiver implementation (usually proprietary) is implicitly

reflected in the CSI report and therefore can stay transparent. For example, UE with an advanced

interference rejection combining (IRC) receiver may report a higher CQI value than other UE with

a simple minimum mean square error (MMSE) receiver. UE vendors are therefore encouraged to

differentiate their products through advanced receiver implementation, offering better user

experiences.

Testability: Interoperability is an essential part of any multivendor ecosystem, including LTE/LTE-

Advanced. The implicit feedback framework (e.g., reported RI/PMI/CQI must pass 10 percent BLER

test when used for data transmission) has a proven track of testability to ensure that the CSI report

is reliable.

The downlink signals are broadly classified into two,

1. Reference Signals

2. Synchronization signals

Reference Signals

The reference signal, as the name says, it is a reference to some one, who can predict

certain other things based on the quality of these reference signals received. The LTE

downlink reference signals are again classified into 2,

1. Cell specific reference signals (CSRS)

2. UE specific reference signals (UeSRS)

The CSRS is cell specific, which means that, these do not depend/change per user but

remain same for all the users and entire system, once configured. These reference

signals are used by the UE to estimate the downlink channel and do a relative

equalization to remove the channel effect over the signal. Hence the UE will generate

the CSRS on his side and do a comparison of the generated and received CSRS to get

an estimate of channel effect. The CSRS is transmitted with some specific power,

which the UE must know, to calculated the multipath effect and this power is conveyed

to the UE using SIB messages. The CSRS is mapped onto symbol 0, 4, 7 ,11 of all

downlink subframes in FDD. The CSRS is mapped to every sixth subcarrier in these

symbols, the start index is determined by the physical cell ID using the below formula,

Most of the channels (e.g, DPSCH, DPCCH, PBCH etc) is for carrying a special information (a sequence of bits) and they have some higher layer channel connected to them, but Reference Signal is a special signal that exists only at PHY layer. This is not for delivering any specific information. The purpose of this Reference Signal is to deliver the reference point for the downlink power.

 When UE try to figure out DL power (i.e, the power of the signal from a eNode B), it measure the power of this reference signal and take it as downlink cell power.

 These reference signal are carried by multiples of specific Resource Elements in each slots and the location of the resource elements are specifically determined by antenna configuration.

 As LTE gets evolved into higher version, we are getting more and more reference signal which is mapped to a specific antenna port. And we are getting more and more confused as a result -:)

 Following shows the reference signals supported by each 3GPP version.

 3GPP Reference Signal (Antenna Ports)

36.211 V8.9.0 (2010-01) - Section 6.10 p0,p1,p2,p3,p4,p5

36 211 V9.1.0 (2010-04) - Section 6.10 p0,p1,p2,p3,p4,p5,p6,p7,p8

36.211 V10.7.0 (2013-04) - Section 6.10 p0,p1,p2,p3,p4,p5,p6,p7,p8,p9,p10,p15,p16,p17,p18,p19,p20,p21,p22

36 211 V11.4.0 (2013-10) - Section 6.10 p0,p1,p2,p3,p4,p5,p6,p7,p8,p9,p10,p15,p16,p17,p18,p19,p20,p21,p22

  For the exact Resource Element locations of each reference signal, refer to following pages.

  RS (Reference Signal) - Cell Specific (Antenna port 0,1,2,3) RS (Reference Signal ) - MBSFN (Antenna Port 4) RS (Reference Signal ) - UE Specific (Antenna Port 5,7,8,9,10) RS (Reference Signal ) - Positioning (Antenna Port 6) RS (Reference Signal ) - CSI (Antenna Port 15,16,17,18,19,20,21,22)

 To implement this signal, you need to go through two steps - signal generation and resource allocation.

 Signal generation is done by the following procedure. You would notice that Cell ID is a key parameter for the sequence and you would guess the sequence will be unique for each Cell ID.

CSRS start position = Cell ID % 6

The below diagram shows 2 examples of CSRS mapping for 2 different cell ID 12 and

8. For Cell ID 12 since the above formula results in 0, the CSRS mapping starts at 0th

subcarrier in 0th RB and continues to map every 6th subcarrier till end of the

bandwidth. Similarly for the second case of cell ID 8, the formula results in 2 and the

CSRS mapping starts at 3rd subcarrier (Since the subcarrier count starts from 0 and

not 1) and continues to map.

Channel aware scheduling across wide system bandwidth is one of the key techniques

enabling this goal. The Sounding Reference Signal (SRS) in uplink comes in support of

this feature as its main purpose is to allow the LTE Base Station (eNodeB) estimating the

UL channel of the users across the scheduling bandwidth. Therefore, SRS channel and

channel gain estimators are important functions that can drive the overall performance of

the system. In this paper, we study such estimation algorithms and show that SRS noise

variance estimation is necessary to deliver a non-biased channel gain estimation. We

suggest to provision a cyclic shift of the SRS for this purpose and show simulation results

validating the performance of the proposed scheme.

TM No of CW No of Layers No of Tx No of Rx DCI

Format

Reference Signal (Antenna Port)

Control CH PDSCH CSI Meas

TM1 1 1 1 1 1, 1A p0 p0 p0

TM2 1 22 2 1, 1A p0,p1 p0,p1 p0,p1

4 2 1, 1A p0,p1,p2,p3 p0,p1,p2,p3 p0,p1,p2,p3

TM3

12 2 2 1A p0,p1 p0,p1 p0,p1

4 4 2 1A p0,p1,p2,p3 p0,p1,p2,p3 p0,p1,p2,p3

22 2 2 2A p0,p1 p0,p1 p0,p1

4 4 2 2A p0,p1,p2,p3 p0,p1,p2,p3 p0,p1,p2,p3

TM41

1 1 1 1A p0 p0 p0

2 2 2 2 p0,p1 p0,p1 p0,p1

4 4 2 2 p0,p1,p2,p3 p0,p1,p2,p3 p0,p1,p2,p3

2 2 2 2 2 p0,p1 p0,p1 p0,p1

4 4 2 2 p0,p1,p2,p3 p0,p1,p2,p3 p0,p1,p2,p3

TM5 1

1 2 2 1D p0,p1 p0,p1 p0,p1

2 2 2 1A p0,p1 p0,p1 p0,p1

1 4 2 1A,1D p0,p1,p2,p3 p0,p1,p2,p3 p0,p1,p2,p3

4 4 2 1A,1D p0,p1,p2,p3 p0,p1,p2,p3 p0,p1,p2,p3

TM6 12 2 2 1A,1B p0,p1 p0,p1 p0,p1

4 4 2 1A,1B p0,p1,p2,p3 p0,p1,p2,p3 p0,p1,p2,p3

TM7 1

1 1 1 1 p0 p5 ?

1 2 2 1 p0,p1 p5 ?

1 4 2 1 p0,p1,p3,p4 p5 ?

2 2 2 1A p0,p1 p0,p1 p0,p1

4 4 2 1A p0,p1,p3,p4 p0,p1,p3,p4 p0,p1,p3,p4

TM8

1

1 2 2 2B p0 p7 or p8 ?

2 2 2 1A p0,p1 p7 or p8 ?

4 4 2 1A p0,p1,p3,p4 p7 or p8 ?

22 2 2 2B p0,p1 p7,p8 ?

4 4 2 2B p0,p1,p3,p4 p7,p8 ?

TM9

1 2 2 2 1A p0,p1 p0,p1 p0,p1

2

2 2 2 2C p0,p1 p7,p8 p15,p16

4 4 2 2C p0,p1 p7,p8 p15,p16,p17,p18

8 8 2 2C p0,p1,p3,p4 p7,p8p15,p16,p17,p18

p19,p20,p21,p22

Reference Signal (Antenna Port Number) vs Transmission Mode

Reference Signals are used for various purpose and the type of reference signal being used varies depending on transmission mode. Some of the possible combination of

refernece signal and transmission mode are as follows.

Note : ',' indicate "AND". (E.g, p0, p1 means that p0 AND p1 are used)

Note : UE Specific means "UE Specific Reference Signal, UE Specific Antenna ports" or is called "DMRS (Demodulation Reference Signal) as well.

Note : "No of Tx" means the number of Tx antenna on eNodeB and "No of Rx" means number of Rx antenna on UE

Note : TM9 can have much more combinations, but I listed only on the combination I have seen until now (Jun 2014)

Note 1 : The DL Reference Signal (Cell Specific Reference Signal) is mainly determined by Physical Cell ID.Note 2 : The resource element locations for DL reference signal gets different according to Physical Cell ID, but there is possibility that the reference signal location with two different physical cell ID can be same if (PCI1 mod 6) == (PCI2 mod 6). (PCI stands for Physical Cell ID). It means that you should be careful when you allocate the physical cell ID for multiple cells in a specific area. Following is some of examples of Reference Signal Location with different physical cell IDs.

Sounding Reference SignalLTE defines an optional sounding reference signal (SRS) in the UL. What is it for? UL channel quality, timing advance, and more. SRS is transmitted by the UE using a known sequence, similar to UL demodulation reference signal (DM RS), so the eNB can use it to estimate the UL channel quality. You may have a question: UL DM RS is already there and the eNB can decode the UL information with its assistance, what's special about SRS? Well, UL DM RS is transmitted together with the UL data and both locate exactly in the same RBs, so the channel quality information the eNB extracts from the UL DM RS is for that transmission. In terms of SRS, it may be transmitted periodically in a wider bandwidth (beyond PUSCH RBs allocated for UL data transmission) and when there is no UL data transmission, so the channel information obtained from SRS is a good input to UL scheduler. It's like CQI report from UE for DL scheduler. Also, since SRS can be transmitted periodically, the eNB can use it to check the UE timing alignment status and send time alignment command to the UE accordingly.

Where is SRS located in a UL PHY frame? It is transmitted in the last symbol of a subframe if scheduled.

Figure 1: Illustration of SRS location in UL PHY frame

In the time domain, SRS can transmit once or periodically based on eNB scheduling. If periodically, the UE-specific periodicity can be 2 / 5 /10 / 20 / 40 / 80 / 160 / 320 ms as defined in TS36.213 section 8.2. The eNB defines the SRS transmit time instance (specific subframes) in TS 36.211 section 5.5.3.3, which is shared by all UEs.

In the frequency domain, the SRS from a UE is transmitted in the unit of 4RBs, defined in TS36.211 section 5.5.3.2-1/2/3/4. For example, with system bandwidth of 50 RBs, we should check Table 5.5.3.2-2. There are 8 possible configurations. Here we show the configuration 1 on the left and the configuration 2 on the right. With CSRS = 1, the SRS for a UE can occupy 4 RBs, 8 RBs, 16 RBs, or up to 48 RBs. It's a tree-based structure and the allocation scheme would be similar to OVSF allocation in WCDMA. So, multiple UEs share the RBs in frequency domain.

Figure 2: Example of SRS bandwidth configuration

You may notice that the same RBs are shared by two UEs. Why? It's because SRS uses Interleaved SC-FDMA (IFDMA) where consecutive subcarriers can be allocated to several UEs alternatively. LTE decides to multiplex two UEs in one RB as shown in Figure 3, i.e., subcarriers in one RB are allocated to two UEs alternatively. This structure provides comb-like effect and expands the SRS channel bandwidth of a UE. Frequency hopping can also be used to help cell edge UE (with power limitation) spread sounding channel bandwidth by using different frequency locations in different subframes/transmit opportunities.

Figure 3: Multiplexing of two UEs in one RB (12 subcarriers over one slot)

Next, what is content of the SRS? SRS uses the same Zadoff-Chu sequence as UL demodulation reference signal (DM RS). Since the cyclic shift versions of the Zadoff-Chu sequence are orthogonal, several UEs (up to 8) can transmit using different cyclic shifts on the same physical radio resource. 

In summary, SRS can be used to estimate UL channel quality and provide input to UL scheduler, as well as help timing advance. SRS is transmitted in the last symbol of a subframe periodically and occupies one comb leg of several RBs (multiple of 4). The content of SRS is Zadoff-Chu sequence. Multiple levels of UE multiplexing are supported: subframe (time), RBs (frequency/ RB level), comb leg (frequency/ subcarrier level), cyclic shift (code).

Demodulation reference signal :This facilitates coherent demodulation and associated with transmission of PUSCH or PUCCH. It is transmitted in the fourth SC-FDMA symbol of the slot and is the same size as the 

Multiple antennas in LTE may also be used to transmit the same signal

appropriately weighted for each antenna element such that the effect is to

focus the transmitted beam in the direction of the receiver and away from

interference, thereby improving the received SINR.

The beam-forming weight vector should increase the antenna gain in the

direction of the desired user while simultaneously minimizing the gain in

the directions of interferers.

Beamforming can provide significant improvement in the coverage

range, capacity, and reliability. To perform transmit beamforming, the

transmitter needs to have accurate knowledge of the channel, which in the

case of TDD is easily available owing to channel reciprocity but for FDD

requires a feedback channel to learn the channel characteristics so it is not

implemented in LTE Release 8 or 9 yet.

LTE Quick Reference - BeamForming         Go Back To Index                  Home : www.sharetechnote.com  What is BeamForming ? It means just as it sounds. BeamForming is a techology to 'Form' a 'beam'.Then what does it mean by 'beam' in this context ? I would say it means 'electromagnatic wave radiation pattern(propagation pattern) for a set of antenna system'. Simply put, BeamForming is a technic that constuct the antenna radation pattern as shown in < Case 2 > of the following illustration. High level meaning is simple like this.. but real implementation would be very complicated and is out of my understanding. So I would just give you only a big picture of this technology.  Motivation (Why we need BeamForming ?) Why we need beamforming ?It is simple. Let's look at the two illustrations as shown below. There are two antenna system and let's assume that the two antenna is transmitting the exactly same amount of total energy.In case 1, the antenna system is radiating the energy in almost same amount in all direction. The three UEs around the antenna would receive almost same amount of the energy but a large portions of energy not directed to those UEs is wasted.In case 2, the signal strength of the radiation pattern ('beam') is specially 'formed' in such a way that the radiated energy in direction to UEs are much stroger than the other parts which is not directed to UEs. 

  Technology for BeamForming There are several different ways to implement the beamforming. Followings are couple of techniques most commonly used (As I mentioned, the details of implementation is out of my understanding). For a little bit further details, see All Beamforming Solutions Are Not Equal. This is mainly for WLAN, but can be a good introduction. Switched Array Antenna : This is the technique that change the beam pattern (radiation form) by switching on/off antenna selectively from the array of a antenna system. DSP Based Phase Manipulation : This is the technique that change the beam pattern (radiation form) by changing the phase of the signal going through each antenna. Using DSP, you can change the signal phase for each antenna port differently to form a specific beam pattern that is best fit for one or multiple specific UEs. Beamforming by Precoding : This is the technique that change the beam pattern (radiation form) by applying a specific precoding matrix. This is the technique used in LTE. In LTE, following transmission mode is implemeting 'BeamForming' implictely or explicitely.

TM 6 - Closed loop spatial multiplexing using a single transmission layer. TM 7 - Beamforming (Antenna port 5) TM 8 - Dual Layer Beamforming (Antenna ports 7 and 8)

 CQI/PMI Feedback Type

 There are two reporting modes in the time domain

Periodic reporting: the UE reports CQI, PMI, and RI with reporting periods configured by the higher layer. PUCCH is used for this.

Aperiodic reporting: can be used to provide large and more detail reporting in a single reporting instance via PUSCH. Report Timing is triggered by DCI.

For subbandCQI feedback In periodic reporting : The UE cycles through different subbandsfrom one reporting

instance to the next, to reduce overhead In aperiodic reporting :

o Higher layer configured subbandreporting: UE reports the subbandCQI for each band in a single feedback report.

o UE selected subband report: UE reports the subbandCQI for the ‘M’ bands with the highest CQI values

 What kind of Physical Channel does a UE has to use to report CSI (CQI, RI, PMI) ?Following table from 36.213 gives you the answer. In short, UE use PUSCH to report CSI in aperiodic manner and use PUCCH for periodic report.

In downlink, LTE specifies transmission modes (TM). TM 7 and 8 are dedicated to beamforming but can TM 5 and 6 use beamforming as well?

When beamforming is used at the eNB in downlink, is it also used in uplink ? with the same radiation pattern (same weights) or specific to UL?

Could you provide references on beamforming in LTE and current status of trials and deployments if any.

Beamforming is made possible by weighting the magnitude and/or phase of

the signal

at the individual antennas: where w is the weight vector. The signals are

weighted so that they can be added constructively in the direction of an

intended transmitter/receiver, and destructively in the direction of

interferers

As of today, beam forming is specific only to LTE TDD and can operate

either under 4×4 or 8×2 configurations.

Beamforming in LTE (continued)

Signal processing for TM7 & TM8

A summary of the defined downlink signal processing flow for transmission

modes 7 & 8 is

shown in Figure 8.

Beamforming in LTE (continued)

Signal processing for TM7 & TM8

A summary of the defined downlink signal processing flow for transmission

modes 7 & 8 is

shown in Figure 8.

As with other transmission modes, the PDSCH data transport block

information is channel

encoded and rate matching applied, producing either 1 or 2 codewords,

which are then

mapped onto layers.

But note that for transmission modes 7 & 8 the precoding block is non-

codebook based.

And so it is left up to the base station to determine the optimum

beamforming precoding to

apply. This can be derived by the base station from direct measurement of

the received UL

Sounding Reference signal, plus can include the use of any configured UE

channel feedback

(CQI / PMI / RI) information. Also note that beamforming precoding can be

dynamic, and

vary on a per subframe and resource block basis to adapt to changing

channel conditions.

For demodulation purposes, transmission TM7 and TM8 include the

mapping of

UE-specific reference signal (UE-specific RS), also known as demodulation

reference

signals (DM-RS) in each PDSCH resource block.

It is important to note that the UE-specific RS undergo the same

beamforming precoding

as the associated PDSCH. This concept is shown in Figure 8. The

beamforming precoding

is primarily calculated to maximize the SINR observed by the target UE

device, but will

also attempt to minimize interference to other UE within the serving or

adjacent cells.

As well as producing user specific beam patterns, the base station also has

the capability

of shaping a desired sector wide broadcast beam pattern for common

control channel

content, which is received by all user devices within the cell. This is possible

when the

number of beamforming antenna elements is greater than the number of

configured

Cell-specific RS Ports. This concept is also shown in Figure 8.

As with other transmission modes, the PDSCH data transport block

information is channel

encoded and rate matching applied, producing either 1 or 2 codewords,

which are then

mapped onto layers.

But note that for transmission modes 7 & 8 the precoding block is non-

codebook based.

And so it is left up to the base station to determine the optimum

beamforming precoding to

apply. This can be derived by the base station from direct measurement of

the received UL

Sounding Reference signal, plus can include the use of any configured UE

channel feedback

(CQI / PMI / RI) information. Also note that beamforming precoding can be

dynamic, and

vary on a per subframe and resource block basis to adapt to changing

channel conditions.

For demodulation purposes, transmission TM7 and TM8 include the

mapping of

UE-specific reference signal (UE-specific RS), also known as demodulation

reference

signals (DM-RS) in each PDSCH resource block.

It is important to note that the UE-specific RS undergo the same

beamforming precoding

as the associated PDSCH. This concept is shown in Figure 8. The

beamforming precoding

is primarily calculated to maximize the SINR observed by the target UE

device, but will

also attempt to minimize interference to other UE within the serving or

adjacent cells.

As well as producing user specific beam patterns, the base station also has

the capability

of shaping a desired sector wide broadcast beam pattern for common

control channel

content, which is received by all user devices within the cell. This is possible

when the

number of beamforming antenna elements is greater than the number of

configured

Cell-specific RS Ports. This concept is also shown in Figure 8.

One popular beam-forming algorithm is based on Direction of Arrival where

the incoming signals to a receiver may consist of desired energy and

interference energy—for example, from other users or from multipath

reflections.

What is a Rank Indication in LTE?

Rank Indication is one of the important input to eNB , in selection of the transmission layer in

downlink data transmission. Even though the system is configured in transmission mode 3 (or open

loop spatial multiplexing) for a particular UE and if the same UE report the Rank Indication value 1 to

eNB, eNB will start sending the data in Tx diversity mode to UE . If UE report Rank Indication 2 ,

eNB will start sending the downlink data in MIMO mode (Transmission Mode 

Precoding matrix indicator (PMI)

The precoding matrix determines how the individual data streams (called layers in LTE) are mapped to the antennas. Skillfully selecting this matrix yields a maximum number of data bits, which the UE can receive together across all layers. However, this requires knowledge of the channel quality for

each antenna in the downlink, which the UE can determine through measurements. If the UE knows what the allowed precoding matrices are, it can send a PMI report to the BS and suggest a suitable matrix. The UE can use the PMI reporting to recommend a downlink precoding matrix to the BS that will achieve the highest data throughput for the given channel state. T

The rank (of the channel matrix) defines the number of linearly independent rows or columns in H. It indicates how many independent data streams (layers) can be transmitted simultaneously.

Beamforming uses multiple antennas to control the direction of a wavefront by appropriately weighting the magnitude and phase of individual antenna signals (transmit beamforming). For example this makes it possible to provide better coverage to specific areas along the edges of cells. Because every single antenna in the array makes a contribution to the steered signal, an array gain (also called beamforming

gain) is achieved.

CQI, PMI and RI are the three types of feedback in LTE system. The configuration of the reports is

defined by eNB and depends on the transmission mode, particular UE situation (e.g. UE speed) as

well as cell load condition (i.e. allowed feedback overhead).

CQI is the basic form of feedback (used in all types of antenna configuration – i.e. SISO, Tx

Diversity, MU-MIMO, SU-MIMO) indicating what’s the actual status of the channel quality.

PMI and RI are however transmitted in multiantenna scenarios. PMI indicates the best-matched

precoding matrix to be used by the eNB from the predefined codebook for a current transmission in

case of SU-MIMO or MU-MIMO. RI defines, how many spatial layers the UE is able to decode in SU-

MIMO mode (rank 2, 3, 4) or to switch between SU-MIMO (rank 2 or larger) and TxDiversity/SISO

(rank 1).

PMI and RI are independent, since e.g. for 4×4 MIMO scheme there are 16 different PMIs (related to

16 different precoding matrices), while the RI for 4×4 MIMO can have a value of 2, 3 or 4, i.e.

independently on the reported RI, the same PMI may be reported and used.

CQI and RI are also independent in terms of values – i.e. RI is a separate measure and CQI is a

separate measure. However, it is important to notify, that the way the CQI is calculated, may be

different for each transmission mode, i.e. SISO, Tx Diversity or SU-MIMO according to UE

calculation algorithm. So because the RI also may correspond to switch between SU-MIMO or

SISO / TxDiversity – i.e. based on the reported RI, the eNB may switch the transmission mode – the

calculated CQI value may be different depending on this mode.

channel state information (CSI) refers to known channel properties of a communication link. This information describes how a signalpropagates from the transmitter to the receiver and represents the combined effect of, for example, scattering, fading, and power decay with distance. The CSI makes it possible to adapt transmissions to current channel conditions,

A closed-loop MIMO system utilizes Channel State Information (CSI) at the transmitter

CQI (Channel Quality Indicator), reported by UE to eNB. UE indicates modulation scheme and coding scheme to eNB , if used I would be able to demodulate and decode the transmitted downlink  data with maximum block error rate 10%. To predict the downlink channel condition, CQI feedback by the UE is an input. CQI reporting can be based on PMI and RI. Higher the CQI value (from 0 to 15) reported by UE, higher the modulation scheme (from QPSK to 64QAM ) and higher the coding rate will be used by eNB to achieve higher efficiency. 

PMI (Precoding Matrix Indicator), UE indicates to eNB , which precoding matrix should be used for downlink transmission which is determined by RI.

How is the PMI used by multi-antenna UE in LTE-A ?

Answer:

The reason for the PMI in DL MIMO is twofold:

UE uses PMI, that is provided in DCI with other allocation information for DL MIMO

operation to “deprecode” the signal. In that case the eNB precodes the

transmitted PDSCH data for that UE with the use of selected PMI and tells the UE

which one it has selected;

UE calculates the best suited PMI and tells the eNB which one to use for following

TTIs and reports within a feedback message.

RI (Rank Indicator), UE indicates to eNB, the number of layers that should be used for downlink transmission to the UE.

RI and PMI can be configured to support MIMO operation (closed loop and open loop spatial multiplexing). These both transmission modes use precoding from a well defined codebook (the lookup table of cross coupling factors used for precoding shared between UE and eNB) to form the transmission layers. In case of transmit diversity PMI and RI need not to be reported to eNB.

In wideband CQI reporting UE report one wideband CQI for the full system bandwidth region.  However, UE can also report CQI value for sub band also.

Now, what about periodicity of CQI, PMI and RI and its values. Yes these can be periodic and aperiodic .

eNB configure type of CQI reporting by RRC signaling. Aperiodic reporting is on request based (by eNB ), which always go with PUSCH. 

Periodic CQI reporting can go on both PUCCH and PUSCH (along with data).  The minimum periodicity could be 2 ms. Periodicity are defined in 36.213 for different values of CQI-PMI-ConfigIndex (Table 7.2.2-1A for FDD). The range of CQI-PMI-ConfigIndex is 0 to 1023. Also the periodicity of RI is based on riconfig-index (Table 7.2.2-1B for FDD) and periodicity of CQI-PMI. The range of riconfig-index is 0 to 1023. 

Example: From Table 7.2.2-1A of 36.213, for the value of CQI-PMI-ConfigIndex “17” the periodicity of CQI reporting is 20 ms (say X).  From Table 7.2.2-1B of 36.213, for the value of riconfig-index “483” the Y is 8 and the periodicity of RI will be 8 times of X (20ms) =160ms.

What about if CQI/PMI/RI collides with either ACK/NACK or SR on the same subframe? If CQI/PMI/RI collides with positive SR the CQI/PMI/RI will be dropped. If CQI/PMI/RI collides with ACK/NACK and simultaneousACKNACKandCQI is false CQI/PMI/RI will be dropped otherwise CQI/PMI/RI will be multiplexed with ACK/NACK. 

It is only the eNB which decide the time and frequency on which UE can transmit the CQI, PMI and RI.  

Difference between SRS and DMRS

There are two types of reference signals used in LTE uplink, to estimate uplink channel quality. Which allow eNB to take smart decisions for resource allocation for uplink transmission, link adaptation and to decode transmitted data from UE .

So to take first smart decision by eNB Sounding Reference Signal  (SRS) is being used. SRS is being transmitted by UE on the last symbol of subframe (in which subframe will come to know later). This SRS report the channel quality of over all bandwidth and using this information eNB assign the resource (to UE for uplink transmission )has better channel quality comparing to  other bandwidth  region.

So is SRS optional in LTE? Yes. SRS is configurable and infact we do not need SRS at all in case eNB assign all resource block or full bandwidth or have no choice.

Now on the basis of configuration and node wise there are two types of SRS (refer 36.211), cell specific (Common SRS) and UE specific (Dedicated SRS).  eNB notify UE about the configuration of SRS parameter by RRC messages. 

There are two types of SRS on the basis of periodicity. Periodic and Aperiodic (In Rel. 10 LTE Advance). The minimum periodicity of SRS is 2ms (1ms=1subframe) and the maximum is 320ms (it is even more than 320ms which is reserved according to specs 36.213).

Now you might be thinking what if all UEs transmit the SRS with same interval and periodicity or in other words how eNB distinguish the UE specific SRS in case of overlapped SRS transmission. Well in that case using transmission_comb and cyclic shift parameters present in RRC Connection setup and RRC Connection Reconfiguration, eNB distinguish and decode different UE specific SRS.

Demodulation reference signal (DMRS) in uplink transmission is used for channel estimation and for coherent demodulation which comes along with PUSCH and PUCCH. If DMRS is bad or by some reason not decoded properly by base station , PUSCH or PUCCH will be not decoded as well. Hence DMRS is not optional like SRS.

DMRS only state channel quality of frequency region in which PUSCH or PUCCH is being transmitted. So what about positioning of DMRS in resource grid, is this fixed ? Answer is Yes and No both. So, when DMRS sent by UE with PUCCH, position of reference signal vary according to PUCCH format indicator. But in case of PUSCH it is always the center symbol of a slot (3rd symbol of slot0 and 10th symbol of slot1).

To support a large number of UEs (User terminal), a large number of DMRS sequences needed and it is achieved by cyclic shifts of a base sequence. As we know in LTE -Advance we will have concept of MIMO in uplink as well, hence DMRS have to enhance for MIMO transmission and each UE will use different DMRS sequences.

DMRS is always mapped to PUSCH in multiple of 12 sub-carriers , however  DMRS mapped to PUCCH  is always in terms of 12 sub-carriers only.

The only similarity in between SRS and DMRS is both uses Constant Amplitude Zero Autocorrelation (CAZAC) sequences.

You may observe less throughput in case of SRS enabled data transmission , because to report SRS during  uplink data transmission , eNB schedule some RBs to UE which could have been used for actual data. 

What is Rank Indicaton in LTE

Rank Indication is one of the important input to eNB , in selection of the transmission layer in downlink data transmission. Even though the system is configured in transmission mode 3 (or open loop spatial multiplexing) for a particular UE and if the same UE report the Rank

Indication value 1 to eNB, eNB will start sending the data in Tx diversity mode to UE . If UE report Rank Indication 2 , eNB will start sending the downlink data in MIMO mode (Transmission Mode 3).

Why we need this RI in LTE concept? When UE experience bad SNR and it would be difficult (error prone) to decode transmitted downlink data  it gives early warning to eNB by stating Rank Indication value as 1. When UE experience good SNR it pass this information to eNB by indicating rank value as 2.

Because of this reason, you might have observed that some time data transmitted by eNB is in Tx diversity mode, though MIMO was configured and hence you may have observed less downlink throughput than expected one. 

However, it is not necessary that eNB will always change the transmission mode based on RI value, it could be implementation specific decision.  

The total received power of the carrier RSSI includes the power from co-channel serving & non-serving cells, adjacent channel interference, thermal noise, etc. Total measured over 12-subcarriers including RS from Serving Cell, Traffic in the Serving Cell

RSSI - Received Signal Strength Indicator:The carrier RSSI (Receive Strength Signal Indicator) measures the average total received power observed only in OFDM symbols containing reference symbols for antenna port 0 (i.e., OFDM symbol 0 & 4 in a slot) in the measurement bandwidth over N resource blocks.

The total received power of the carrier RSSI includes the power from co-channel serving & non-serving cells, adjacent channel interference, thermal noise, etc. Total measured over 12-subcarriers including RS from Serving Cell, Traffic in the Serving Cell

RSRP - Reference Signal Received Power: RSRP is a RSSI type of measurement, as follows there are some definition of it and some details as well.

It is the power of the LTE Reference Signals spread over the full bandwidth and narrowband. A minimum of -20 dB SINR (of the S-Synch channel) is needed to detect RSRP/RSRQ

RSRQ - Reference Signal Received Quality: Quality considering also RSSI and the number of used Resource Blocks (N) RSRQ = (N * RSRP) / RSSI measured over the same bandwidth. RSRQ is a C/I type of measurement and it indicates the quality of the received reference signal. The RSRQ measurement provides additional information when RSRP is not sufficient to make a reliable handover or cell reselection decision. 

In the procedure of handover, the LTE specification provides the flexibility of using RSRP, RSRQ, or both. 

It must to be measured over the same bandwidth:

oo Narrowband N = 62 Sub Carriers (6 Resource Blocks)o Wideband N = full bandwidth (up to 100 Resource Blocks / 20 MHz)

 RSRP 3GPP Definition

Reference Signal Received Power (RSRP), is defined as the linear average over the power contributions (in [W]) of the resource elements that carry cell-specific reference signals within the considered measurement frequency bandwidth.For RSRP determination the cell-specific reference signals R0 according TS 36.211 [3] shall be used. If the UE can reliably detect that R1 is available it may use R1 in addition to R0 to determine RSRP.The reference point for the RSRP shall be the antenna connector of the UE.

Or, even better, Reference Signal Received Quality is defined as the ratio N×RSRP/(E-UTRA carrier RSSI), where N is the number of RB’s of the E-UTRA carrier RSSI measurement bandwidth. The measurements in the numerator and denominator shall be made over the same set of resource blocks.

If receiver diversity is in use by the UE, the reported value shall not be lower than the corresponding RSRP of any of the individual diversity branches.

Applicable for: RRC_IDLE intra-frequency, RRC_IDLE inter-frequency, RRC_CONNECTED intra-frequency, RRC_CONNECTED inter-frequency

Note1: The number of resource elements within the considered measurement frequency bandwidth and within the measurement period that are used by the UE to determine RSRP is left up to the UE implementation with the limitation that corresponding measurement accuracy requirements have to be fulfilled.

Note 2: The power per resource element is determined from the energy received during the useful part of the symbol, excluding the CP.

RSRP

In other words RSRP (Reference Signal Receive Power) is the average power of Resource Elements (RE) that carry cell specific Reference Signals (RS) over the entire bandwidth, so RSRP is only measured in the symbols carrying RS. 

ooo RSRP is the average received power of a single RS resource element.o UE measures the power of multiple resource elements used to transfer the reference signal

but then takes an average of them rather than summing them.o Reporting range -44…-140 dBm

RSRP does a better job of measuring signal power from a specific sector while potentially excluding noise and interference from other sectors.

RSRP levels for usable signal typically range from about -75 dBm close in to an LTE cell site to -120 dBm at the edge of LTE coverage.

RSRP mapping 3GPP TS 36.133 V8.9.0 (2010-03)

The reporting range of RSRP is defined from -140 dBm to - 44 dBm with 1 dB resolution.

The mapping of measured quantity is defined in the table.

RSRQ 3GPP Definition

Reference Signal Received Quality (RSRQ) is defined as the ratio N×RSRP/(E-UTRA carrier RSSI), where N is the number of RB’s of the E-UTRA carrier RSSI measurement bandwidth. The measurements in the numerator and denominator shall be made over the same set of resource blocks.

E-UTRA Carrier Received Signal Strength Indicator (RSSI), comprises the linear average of the total received power (in [W]) observed only in OFDM symbols containing reference symbols for antenna port 0, in the measurement bandwidth, over N number of resource blocks by the UE from all sources, including co-channel serving and non-serving cells, adjacent channel interference, thermal noise etc. 

The reference point for the RSRQ shall be the antenna connector of the UE. If receiver diversity is in use by the UE, the reported value shall not be lower than the corresponding RSRQ of any of the individual diversity branches.

Applicable for: RRC_CONNECTED intra-frequency, RRC_CONNECTED inter-frequency

RSRQIn formula:

RSRQ = N x RSRP / RSSI

oo

oo N is the number of Physical Resource Blocks (PRBs) over which the RSSI is measured,

typically equal to system bandwidtho RSSI is pure wide band power measurement, including intracell power, interference and

noiseo The reporting range of RSRP is defined from -3…-19.5dB

Reference Signals recap: OFDMA Channel Estimation

In simple terms the Reference Signal (RS) is mapped to Resource Elements (RE). This mapping follows a specific pattern (see to below).

So at any point in time the UE will measure all the REs that carry the RS and average the measurements to obtain an RSRP reading.

oo

o Channel estimation in LTE is based on reference signals (like CPICH functionality in WCDMA)o Reference signals position in time domain is fixed (0 and 4 for Type 1 Frame) whereas in

frequency domain it depends on the Cell IDo In case more than one antenna is used (e.g. MIMO) the Resource elements allocated to

reference signals on one antenna are DTX on the other antennaso Reference signals are modulated to identify the cell to which they belong.

RSSI (Received Signal Strength Indicator) is a parameter which provides information about total received wide-band power (measure in all symbols) including all interference and thermal noise.

RSSI is not reported to eNodeB by UE. It can simply be computed from RSRQ and RSRP that are, instead, reported by UE.

RSSI = wideband power = noise + serving cell power + interference power

So, without noise and interference, we have that 100% DL PRB activity: RSSI=12*N*RSRP

Where:

ooo RSRP is the received power of 1 RE (3GPP definition) average of power levels received

across all Reference Signal symbols within the considered measurement frequency bandwidtho RSSI is measured over the entire bandwidtho N: number of RBs across the RSSI is measured and depends on the BW

Based on the above, under full load and high:

SNR: RSRP (dBm)= RSSI (dBm) -10*log (12*N)

So we have:

RSRQ = RSRP / (RSSI/N)

ooo N = Number of PRBs (Physical Resource Blocks)o RSSI = noise + serving cell power + interference power during RS symbolo So we have that RSRQ depends on serving cell power and the number of Tx antennas

Impact of serving cell power to RSRQ:

Example for noise limited case (no interference): If all resource elements are active and are transmitted with equal power, then

RSRQ = N / 12N = -10.8 dB for 1Tx

RSRQ = N / 20N = -13 dB for 2Tx taking DTX into account

(because RSRP is measured over 1 resource element and RSSI per resource block is measured over 12 resource elements).

Remember that RSSI is only measured at those symbol times during which RS REs are transmitted - We do not have to take into the count DTx!!!

So, when there is no traffic, and assuming only the reference symbols are transmitted (there are 2 of them within the same symbol of a resource block) from a single Tx antenna then the RSSI is generated by only the 2 reference symbols so the result becomes

RSRQ = N / 2N = -3 dB for 1Tx

RSRQ = -6dB for 2Tx

 SINR Definition

SINR is the reference value used in the system simulation and can be defined:

1.1.

1. Wide band SINR2. SINR for a specific subcarriers (or for a specific resource elements)

SINR = S/(I+N), all measured over the same bandwidth

SNR vs. RSRP

RSRQ to SINR

RSRQ depends on own cell traffic load, but SINR doesn’t depend on own cell load.

Used Resource Elements per Resource Block (RE/RB) in serving cell is an input parameter for RSRQ -> SINR mapping

If we take as assumption that RSRP doesn’t contain noise power we will have: 

Precoding

Next, the layers are precoded using a precoding matrix defined by Section 6.3.4.2 in 3GPP TS

36.211. The result of precoding is a set of modulation symbols that are to be mapped directly

onto the subcarriers. Precoding involves multiplying the layers matrix with a precoding matrix

which creates the antenna port subcarrier values that are sent to the OFDMA mapper and

then to the antenna ports.

For the single-antenna case, precoding is just a direct assignment of the layer contents to the

antenna port data for the current physical channel.

For multiple-antennas, there are two types of precoding: transmit diversity precoding and

spatial multiplexing precoding. In addition, spatial multiplexing can be

either Without CDD (cyclic delay diversity) or with Large Delay CDD.

Spatial Multiplexing precoding reduces correlation between the layers. This makes it easier for

the antenna port signals to be separated using MIMO techniques once they are received.

Instead of precoding, beamforming can be used. Multi-layer spatial multiplexing can also be

done using UE-RS antenna ports and each layer can be beamformed differently from the

others.

OFDMA Mapping

After layer data for the physical-layer channels is precoded to create C-RS antenna port data,

the OFDMA mapper combines the precoded values from physical-layer channels together with

the reference signal and sync signals and places the subcarrier values into the appropriate

locations in an OFDM symbol.

This OFDMA symbol mapping is performed separately for each antenna port. For more

information about antenna ports and their respective contents, see the Antenna Ports and

Transmit-Receive Pathstopic.

OFDM Modulation

After values have been assigned for all subcarriers in an OFDM symbol for an antenna port

(including the reference signal and control channels), the symbol is sent through an IFFT,

which converts the symbol into time data. A cyclic prefix is appended and the time data is

transmitted. The following image illustrates the entire TX chain for the single antenna case,

which performs OFDM modulation directly after scrambling and modulation (no precoding).

The difference when precoding is used is that the subcarrier values in the image below are

replaced with the complex antenna port values that are a combination of the subcarrier values

from each layer.