reference signal and use

8/21/2019 Reference Signal and Use

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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 physicallayer itself.

reference signalsis 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 thereference point for the downlink power reference signal are carried by multiples of specific

Resource lements in each slots and the location of the resource elements are specificallydetermined by antenna configuration. !s "T gets evolved into higher version# we are

getting more and more reference signal which is mapped to a specific antenna port. !nd weare getting more and more confused as a result $%

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 &ell

'( is a key parameter for the se)uence and you would guess the se)uence will be uni)ue foreach &ell '(. Reference Signals are used for various purpose and the type of reference

signal being used varies depending on transmission mode

Reference signals UL

Demodulation Reference Signal (DMRS) , Sounding reference signal (SRS)

Reference signals DL

Cell specific Reference Signal (C-RS) , UE-specific Reference Signal (UE-RS)

Positioning Reference Signal (P-RS) ,Channel State nformation Reference Signal(CSI-RS)

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

Cell-specific RSs !"-specific RSs MBSFN-specific RSs #ositioning RSs fro$ Release % onwards C&annel

State 'nfor$ation (CS') RSs w&ic& are introduced in Release * +&ese RS are gold sequence ,ased wit&

lengt& of w&ere eac& RS in initiali.ed differentlyCell specific reference signals are however still

transmitted for the transmission of common control signaling, mobility measurements anddownlink channel quality measurements. In order to save the resources occupied by UE

specific reference signals

RS *Reference Signal+ $ &ell Specific *!ntenna port ,#-##/+

RS *Reference Signal + $ 01S23 *!ntenna Port 4+

RS *Reference Signal + $ 5 Specific *!ntenna Port 6#7#8#9#-,+ RS *Reference Signal + $ Positioning *!ntenna Port :+ RS *Reference Signal + $ &S' *!ntenna Port -6#-:#-7#-8#-9#,#-#+

Cell specific reference signalsare available for all UEs in a cell. Cell specific signals enable the UE todetermine 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-!"#$ transmission. &ell$

specific RS *&$RS+ was designed for use in channel estimation for up to 4$layer spatial multiplexing#

with separate &$RSse)uences for each antenna port *,$/+

#or cell specific refernce signal which all UE measures in the serving cell and neighbour cells, areinitiali%ed with Cell id & 'CI- 'hysical cell id( at the start of each )#* symbol. "o cell specific +" carriesone of the Cell ids , in other words scrambled with one of the unique cell ids. In addition to above
http://www.sharetechnote.com/html/FrameStructure_DL.html#RShttp://www.sharetechnote.com/html/FrameStructure_DL.html#RS_UE_Specifichttp://www.sharetechnote.com/html/FrameStructure_DL.html#RS_UE_Specifichttp://www.sharetechnote.com/html/FrameStructure_DL.html#RS_Positioninghttp://www.sharetechnote.com/html/FrameStructure_DL.html#RS_CSIhttp://rfmw.em.keysight.com/wireless/helpfiles/89600B/WebHelp/subsystems/lte-a/Content/lte_sym_tbl_framesummary_rs.htmhttp://www.sharetechnote.com/html/FrameStructure_DL.html#RShttp://www.sharetechnote.com/html/FrameStructure_DL.html#RS_UE_Specifichttp://www.sharetechnote.com/html/FrameStructure_DL.html#RS_UE_Specifichttp://www.sharetechnote.com/html/FrameStructure_DL.html#RS_Positioninghttp://www.sharetechnote.com/html/FrameStructure_DL.html#RS_CSIhttp://rfmw.em.keysight.com/wireless/helpfiles/89600B/WebHelp/subsystems/lte-a/Content/lte_sym_tbl_framesummary_rs.htm


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there is a cell specific frequency shift is applied to the patterns of reference symbols given by $cellI*mod/. 0his shift helps to avoid time-frequency collisions between cell-specific +"s from up to si1 ad2acent

cells )nce UE knows the 'CI for a given cell, it also knows the location of cell +eference signals asshown in figure &red and black squares(. +eference signals are used in channel estimation, cellselection 3 reselection and handover procedures

!" specific reference signalsw&ic& $ay ,e e$,edded in t&e data for specific !"s.. +&e !"-specific

RSs are e$,edded only in t&e RBs to w&ic& t&e #SC0 is $apped for t&ose !"s* 'f !" specificreference signals are trans$itted t&e !" is e1pected to use t&e$ to deri2e t&e c&annel esti$ate

for de$odulating t&e data in t&e corresponding #SC0 RBs.4 typical usage of the UE-specific +"s is to

enable beamforming of the data transmissions to specific UEs. )ne 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 UE shall receive UE specific reference signals in addition to cell specific reference signals if it is configured

by higher layers

Positioning RSis used to enhance 5 geolocation accuracy. The P$RSis transmitted periodically in

certain frames and occupies certain resource elements within a rectangular area in the frame *R1s x

S2s+ as defined by the P$RS parameters.

01S23$RSis used to compensate the downlink channel effects on the Physical 0ulticast &hannel*P0&H+# which contains the multicast;broadcast data# and is only transmitted during 01S23

subframes

&S'$RSis the &hannel State 'nformation Reference Signal and is used by the 5to estimate the

channel and report channel )uality information *&


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symbols in each resource block allocated to the user. (0RS is transmitted on all subcarriers

allocated to the user during the symbols

The P5S&H demodulation reference signal is a ?adoff$&hu se)uence# which results in

constellation points on a circle centered about the origin.

The P5&&H demodulation reference signal# however# is a reference se)uence transmitted on a

rotated


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U5 data transmission( and when there is no U5 data transmission, so the channel information

obtained from "+" is a good input to U5 scheduler. It:s like C


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The rank(of the channel matrix) denes the number of linearly independent

rows or columns in H. It indicates how many independent data streams (layers) can

be transmitted simultaneously


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Rank indicator (RI): UE indicates to eNB, the number of layers that should be used for downlink

transmission to the UE. number of layers recommended for SU-I! transmission 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). 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

Why we need this RI in LTE concept? When UE experience bad SNR and it would be difficult (error

prone to decode trans!itted downlin" data it #i$es early warnin# to eN% by statin# Ran" Indication

$alue as &' When UE experience #ood SNR it pass this infor!ation to eN% by indicatin# ran" $alue as

"recoding matri# indicator ("I$% index of the recommended SU!I!" #recoding matrix in the

feedback$#recoding codebook% corres#onding to the &I%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.The precoding matrix determines how the individual

data streams (called layers in LT) are mapped to the antennas. !"illfully selecting

this matrix yields a maximum number of data bits# which the $ can receive

together across all layers. However# this re%uires "nowledge of the channel %uality

for

each antenna in the downlin"# which the $ can determine through measurements.

If the $ "nows what the allowed precoding matrices are# it can send a &'I report to

the ! and suggest a suitable matrix. The $ can use the &'I reporting to

recommend a downlin" precoding matrix to the ! that will achieve the highest

data throughput for the given channel state. T

&hannel 'uality indicator (&I$% indication of the channel 'ualit corres#onding to the re#orted &I$!I

in L*E% C+I is defined as a set of trans#ort block si,es% each of -hich translates to a maximum code rate

and 'uadrature am#litude modulation (+!) order that can be recei/ed b the UE at a certain block

error rate (0LE&). CQI (Channel Quality Indicator)) reported by UE to eN%' UE indicates !odulation

sche!e and codin# sche!e to eN% ) if used I would be able to de!odulate and decode the trans!itted

downlin" data with !axi!u! bloc" error rate &*+' To predict the downlin" channel condition) ,-I

feedbac" by the UE is an input' ,-I reportin# can be based on ./I and RI' 0i#her the ,-I $alue (fro! *

to &1 reported by UE) hi#her the !odulation sche!e (fro! -.S2 to 34-5/ and hi#her the codin#

rate will be used by eN% to achie$e hi#her efficiency'

!I and &I 1ointl re#resent the s#atial directions of the !I!" channel% -hile C+I

indicates the strength of the corres#onding s#atial directions.

s a criterion for testing the C+I re#ort accurac% -hen the re#orted code ra te and +! order is used for

actual data transmission% the UE must be able to decode the data -ith a 0LE& belo- 23 #ercent.


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!eam"orming is a technic that constuct the antenna radation pattern signalstrength of the radiation pattern *>beam>+ is specially >formed> in such a way that

the radiated energy in direction to 5s are much stroger than the other parts which

is not directed to 5s.

*he beamforming -eight /ector should increase the antenna gain in the direction of thedesired user -hile simultaneousl minimi,ing the gain in the directions of interferers.

0eamforming is made #ossible b -eighting the magnitude and$or #hase of the signal

at the indi/idual antennas: -here - is the -eight /ector. *he signals are -eighted so that the

can be added constructi/el in the direction of an intended transmitter$recei/er% and

destructi/el in the direction of interferers eamforming uses multiple antennas to

control the direction of a wavefront by appropriately weighting the magnitude and

phase of individual antenna signals (transmit beamforming). or example this

ma"es it possible to provide better coverage to specic areas along the edges of

cells. ecause every single antenna in the array ma"es a contribution to the steeredsignal# an array gain (also called beamforming

gain) is achieved.

This techni%ue is used to control the shape and direction of transmitted or received

signals. It combines signals in antenna elements in such a way# that constructive

interference happens in a certain direction and destructive interference happens in

other directions. eamforming can be used to extend the range of signals in a

certain direction# such as towards a highway# where density of mobile users is more

compared to other directions. In other words# the beam can be steered in the

desired direction.

There are several different ways to implement the beamforming Switched !rray !ntenna#(SP 1ased

Phase 0anipulation# 1eamforming by Precoding


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1eamforming by Precoding% This is the techni)ue that change the beam pattern *radiation form+ by

applying a specific precoding matrix. This is the techni)ue used in "T. 'n "T# following transmission

mode is implemeting >1eam2orming> implictely or explicitely.

T0 : $ &losed loop spatial multiplexing using a single transmission layer.

T0 7 $ 1eamforming *!ntenna port 6+

T0 8 $ (ual "ayer 1eamforming *!ntenna ports 7 and 8+

transport block -= one codeword -= one or two layers -= one or more antenna ports

+ransport Blocks to Codewords

Codewords to 3ayers

3ayers to 4ntenna #orts


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+&e $ain task of t&e rate-$atc&ing is to e1tract t&e e1act set of ,its to ,e trans$itted

wit&in a gi2en ++'. 0he rate-matching for 0urbo coded transport channels is defined for each code block9 thereare three basic steps composing a rate-matching. $amely, sub-block interleaver, bit collection and bit selection.

#inally, after the rate-matching, each individually processed code block has to be concatenated and transferred to a

modulation block &a mapper(. 0he sub-block interleaver is defined for each output stream from 0urbo coding. 0he

streams include a systematic bit stream, a parity bit stream and an interleaved parity stream. 0he bit collection step

concatenates the three bit streams &the systematic bit stream, parity bit stream and interleaved parity stream(

#"DM$ Mapping

!fter layer data for the physical$layer channels is precoded to create &$RS antenna port data# theA2(0! 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 A2(0 symbol.


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This A2(0! symbol mapping is performed separately for each antenna port. 2or more

information about antenna ports and their respective contents# see the !ntenna Ports and

Transmit$Receive Pathstopic.

#"DM Modulation

!fter values have been assigned for all subcarriers in an A2(0 symbol for an antenna port

*including the reference signal and control channels+# the symbol is sent through an '22T#

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

transmitted.

4ntenna $appingis the combination of layer mapping and pre-coding, which process the modulation symbolsfor one or two codewords to transmit them on different antenna ports
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M4C #! (#rotocol ata !nit) t&at #05 recei2es fro$ M4C as 6data6* +o #05 it7s 8ust a string of ,its anyway*

+&is will ,e our transport ,lock*

+ransport Blocks to Codewords

6hat does ';8 do with a transport block7 #irst, it converts the transport block into a codeword. 0here are a number

of steps involved in this process, depending on the length of the transport block9

4ppend a > bit checksum &C+C( to the transport block. 0his C+C is used to determine whether the

transmission was successful or not, and triggers ;ybrid 4+< to send an 4C? or $4C?, as appropriate

"egment the transport block into code blocks. 4 code block must be between and /@ bits long. If the

transport block is too small, it is padded up to bitsA if the 0! is too big, it is divided into smaller pieces,each of which gets an additional > bit C+C.

'rocess each code block with a @3B turbo coder

+eassemble the resulting code blocks into a single codeword

4 codeword, then, is essentially a transport block with error protection. $ote that a UE may be configured to receive

one or two transport blocks &and hence one or two codewords( in a single transmission interval.

Code words to 3ayers

';8 then converts each codeword into modulation symbols. #or each codeword, ';8 must9

"cramble the contents of each codeword, using a sequence based on the UE:s C-+$0I and the cell:s'hysical Cell I* &'CI(

Convert the bit sequences into the corresponding modulation symbols &using


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In spatial multiple1ing situations, things get a little more complicated, since one or two codewords may be distributed

across @, >, B or layers. In brief, here:s how the mapping is handled9

3ayers to 4ntenna #orts

0he final steps apply any required precoding ad2ustments and assign the modulation symbols to the physical

resources9

4pply the required precoding factors to the modulation symbols in each layer

ap the precoded symbols to the appropriate antenna ports

4ssign the modulation symbols to be transmitted on each antenna port to specific resource elements &the

subcarriers and symbols within the resource blocks(

enerate the final time-domain )#* signal for each antenna port

$ote that the number of layers is always less than or equal to the number of antenna ports &transmit antennas(. If

there:s only one antenna port, then it carries 2ust a single layer. In multiple &> or ( antenna situations, though, each

antenna port may end up carrying a complicated combination of the symbols from multiple layers. Check out spec

B/.>@@, section /.B. if you really want to dig into the details.

ntenna !ort antenna ports do not correspond to physical antennas# but rather are

logical entities distinguished by their reference signal se)uences LTE sy!bols that are

trans!itted $ia identical antenna ports are sub6ect to the sa!e channel conditions' In

order to deter!ine the characteristic channel for an antenna port) a UE !ust carry

out a separatechannel esti!ation for each antenna port' Separate reference si#nals (pilot

si#nals that are suitable for esti!atin# the respecti$e channel are defined in the LTE

standard for each antenna port'

!ntenna port is logical concept# not a physical concept *meaning >!ntenna port> is not

the same as >Physical !ntenna>+

ach !ntenna port represents a specific channel model


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The channel that is transmitted by a specific antenna port can be done by using the

reference signal assinged fort the port *This is why each antenna port has its

own reference signal

0ultiple antenna port signals can be transmitted on a single transmit antenna *&$

RSport , and 5$RSport 6# for example+. &orrespondingly# a single antenna port can

be spread across multiple transmit antennas *5$RS port 6# for example+.

"T demodulator supported only analysis of P(S&H transmitted on !ntenna

Ports ,# *, and -+# *,# -# +# or *,# -# # /+. These ports are considered &$RS

antenna ports# and each port has a different arrangement of &$RS resource

elements. Barious configurations are defined that use these &$RS antenna ports#

including $ or 4$port Tx (iversity and $# /$# or 4$port Spatial 0ultiplexing.

Then beamforming support was added and single$layer P(S&H allocations

transmitted on Port 6 could be analy=ed. The "T demodulator has since been

enhanced to support the "T Release 9 which added Transmission 0ode 8$$(ual$

"ayer 1eamforming *i.e. beamforming C spatial multiplexing+$$where P(S&H is

transmitted on !ntenna Ports 7 and 8 *note that single$layer beamforming in Rel

9 can also use port 7 or port 8 in addition to port 6+. 'n Rel -, of the standard#

the new transmission mode 9 *T09+ added up to 8$layer transmissions using

Ports 7$-4. T09 is supported by the "T$!dvanced demodulator.
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*pen Loop Transmissions are congured with minimal feedbac" from the $

+an" %ual to the number of layers in an LT spatial multiplexing

transmission

+I +an" Indicator , indication of the number of layers that can be supported

on a given channel


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@. SM Spatial Multiplexing transmission scheme in which different spatial paths carry

different data streams,Spatial $ultiple1ing&seen abbreviated SMor SM9( is a transmission

technique inI)wireless communicationto transmit independent and separately encoded

data signals, so-called streams, from each of the multiple transmit antennas. 0herefore, the

space dimension is reused, ormultiple1ed, more than one time.

>. If the transmitter is equipped with antennas and the receiver has antennas, the

ma1imum spatial multiple1ing order &the number of streams( is,

B.

. if a linear receiver is used. 0his means that streams can be transmitted in parallel, ideally

leading to an increase of the spectral efficiency&the number of bits per second and per ;%

that can be transmitted over the wireless channel(. 0he practical multiple1ing gain can be limited

by spatial correlation,which means that some of the parallel streams may have very weak

channel gains.

enabling multi-ayer ransmissions

losed Loop Transmissions are congured with detailed feedbac" from the $

hannel correlation The degree to which transmissions on the same channel

appear to the +x to be the same. Low channel correlation indicates the

transmissions can be distinguished# allowing multi-layer transmission.

Spatial multiplexing works by creating separate data streams on

multiple antennas. In spatial multiplexing, the eNodeB divides the

data to be sent to a given U on a given sub!channel into data

streams, called layers. The number o" layers is the same as the rank

o" the transmission. Transmission ran" is determined according to channel

conditions at the $# as well as other considerations such as available

resources at the e/ode. In the simplest case for spatial multiplexing# a

ran"-0 spatial multiplexing transmission on a 0x0 'I'* antenna

conguration will transmit one layer from each Tx. In this case# the paths 1-1

and 1-0 shown in igure 1

represent Layer 1# while paths 0-1 and 0-0 represent Layer 0. ach layer

reaches each +x along a di2erent path. The $ then reconstructs the layers

using information from both antennas
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In open loop operations# the e/ode receives minimal information from the

$3 a +an" Indicator (+I)# the number of layers that can be supported under

the current channel conditions and modulation scheme4 and a hannel

5uality Indicator (5I)# a summary of the channel conditions under thecurrent transmission mode# roughly corresponding to !/+. The e/ode then

uses the 5I to select the correct modulation and coding scheme for the

channel conditions. ombined with this modulation and coding scheme# 5I

can also be converted into an expected throughput.

In closed loop operations# the $ analy6es the channel conditions of each Tx#

including the multipath conditions. The $ provides an +I as well as a

&recoding 'atrix Indicator (&'I)# which determines the optimum precodingmatrix for the current channel conditions. inally# the $ provides a 5I

given the +I and &'I# rather than basing 5I on the current operation mode.

This allows the e/ode to %uic"ly and e2ectively adapt the transmission to

channel conditions. losed loop operations are particularly important for

spatial multiplexing# where 'I'* o2ers the greatest throughput gains.

LT supports up to ran"-0 transmissions for 0x0 or 7x0 antenna

congurations# and up to ran"-7

for 7x7 antenna congurations. Throughput gains from losed-Loop and

*pen-Loop ran"-0 transmissions can be seen in igure 8.


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'n a 0'0Aor Tx (iversity configuration# each &$RS antenna port must be

transmitted on a separate physical antenna to create spatial diversity between the

paths. Single$layer beamforming# on the other hand# is accomplished by sending

the same signal to each antenna but changing the phase of the each antenna>s

signal relative to the others. Since the same 5$RS se)uence is sent from each

antenna# the 89:,, BS! can compare the received 5$RS se)uence with the

reference se)uence and calculate the weights that were applied to the antennas to

accomplish the beamforming.

0ulti$layer beamforming adds some complexity to beamforming by transmitting as

many 5$RS se)uences as there are layers to allow demodulation of each layer>s

P(S&H data. The 5$RS se)uence for each antenna port is orthogonal to the

others# either in time;fre)uency domain or in the code domain. This can be thought

of as beamforming of each layer independently. 3$layer beamforming is an

extension of dual$layer beamforming and supports up to 8 data layers with the


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ability to beamform each layer separately


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'I'* can be sub-divided into three main categories3

(1)&recoding

(0)!patial multiplexing

(8)9iversity coding

Precoding the layers are precoded using a precoding matrix 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 A2(0! mapper and then to the antenna ports.

#recodingis a generali%ation ofbeamformingto support multi-stream &or multi-layer( transmission

in multi-antennawireless communications.

In point-to-point systems, precoding means that multiple data streams are emitted from the transmit

antennas with independent and appropriate weightings such that the link throughput is ma1imi%ed at

the receiver output

"or the single-antenna case, precoding is %ust a direct assignment of the la&er

contents to the antenna port data for the current ph&sical channel'
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"or multiple-antennas, there are to t&pes of precoding transmit di*ersit&

precoding and spatial multiple+ing precoding' n addition, spatial multiple+ing

can e either Without CDD(c&clic dela& di*ersit&) or ith Large Delay CDD'

Spatial Multiple+ing precoding reduces correlation eteen the la&ers' his

ma.es it easier for the antenna port signals to e separated using MM#

techni/ues once the& are recei*ed'

nstead of precoding, eamforming can e used' Multi-la&er spatial

multiple+ing can also e done using UE-RS antenna ports and each la&er can

e eamformed differentl& from the others'

In I)communicationsystems, array gainmeans a power gain of transmitted signals that is

achieved by using multiple-antennas at transmitter and3or receiver, with respect to single-input

single-outputcase. It can be simply calledpower gain. In a broadside array, the array gain is

almost e1actly proportional to the length of the array

spatial multiplexing# a high rate signal is split into multiple lower rate streams and

each stream is transmitted from a di2erent transmit antenna in the same fre%uency

channel. If these signals arrive at the receiver antenna array with su:ciently

di2erent spatial signatures# the receiver can separate these streams into (almost)

parallel channels. !patial multiplexing is a very powerful techni%ue for increasingchannel capacity at higher signal-to-noise ratios (!/+). The maximum number of

spatial streams is limited by the lesser of the number of antennas at the transmitter

or receiver. !patial multiplexing can be used with or without transmit channel

"nowledge. !patial multiplexing can also be used for simultaneous transmission to

multiple receivers# "nown as space-division multiple accessing. The scheduling of

receivers with di2erent spatial signatures allows good separability.

!pace 9ivision 'ultiplexing is used to achieve higher data rates instead of signal

%uality. !o# it wor"s well when !/+ of the channel is high

diversity methods# a single stream (unli"e multiple streams in spatial multiplexing)is transmitted# but the signal is coded using techni%ues called space-time coding.

The signal is emitted from each of the transmit antennas with full or near

orthogonal coding
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reference signal and use

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