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physical channel and a physical signal are all together different things. A channel can be consideredas a medium through which some information is transmitted, where as a signal has a mathematicalimportance and it is, most of the times generated at the physical layer itself

Antenna Ports and Transmit-Receive

Paths (LTE)The purpose of this topic is to give an overview and explanation of the relationships between

antenna ports, physical transmit antennas, and receive channels. Included in this discussion is

information about reference signals,PDSCH,usage of antenna ports, and beamforming. Also,

this discussion is not an exhaustive listing of all the variations supported by theLTEstandard,

but is focused around the analysis capabilities of the 89600 VSA.

For exact information about what is possible in LTE, see 3GPPTS36.211 and 36.213. Also,

seeApp Note 5990-9997: Verify and Visualize Your TD-LTE Beamforming Signals.

Antenna Ports

The LTE standard defines what are known as antenna ports. These antenna ports do not

correspond to physical antennas, but rather are logical entities distinguished by their

reference signal sequences. Multiple antenna port signals can be transmitted on a single

transmit antenna (C-RSport 0 andUE-RSport 5, for example). Correspondingly, a single

antenna port can be spread across multiple transmit antennas (UE-RS port 5, for example).

Let us consider antenna ports used for PDSCH allocations since they probably have the most

variations. Initially, the 89600 VSA's LTE demodulator supported only analysis of PDSCH

transmitted on Antenna Ports 0, (0 and 1), (0, 1, 2), or (0, 1, 2, 3). These ports are

considered C-RS antenna ports, and each port has a different arrangement of C-RS resource

elements. Various configurations are defined that use these C-RS antenna ports, including 2-

or 4-port Tx Diversity and 2-, 3-, or 4-port Spatial Multiplexing.

Then beamforming support was added and single-layer PDSCH allocations transmitted on Port

5 could be analyzed. The LTE demodulator has since been enhanced to support the LTE

Release 9 which added Transmission Mode 8--Dual-Layer Beamforming (i.e. beamforming +

spatial multiplexing)--where PDSCH is transmitted on Antenna Ports 7 and 8 (note that

single-layer beamforming in Rel 9 can also use port 7 or port 8 in addition to port 5). In Rel

10 of the standard, the new transmission mode 9 (TM9) added up to 8-layer transmissions

using Ports 7-14. TM9 is supported by the LTE-Advanced demodulator.

As Ports 0-3 are indicated by the existence of C-RS, so Ports 5 and 7-14 are indicated by

theUE-specific Reference Signal (UE-RS). The following is a table that summarizes the

various PDSCH mappings that can be used along with the corresponding reference signal and

antenna ports.
http://cp.literature.keysight.com/litweb/pdf/5990-9997EN.pdfhttp://cp.literature.keysight.com/litweb/pdf/5990-9997EN.pdfhttp://cp.literature.keysight.com/litweb/pdf/5990-9997EN.pdfhttp://cp.literature.keysight.com/litweb/pdf/5990-9997EN.pdfhttp://cp.literature.keysight.com/litweb/pdf/5990-9997EN.pdf


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ReferenceSignal

PDSCHMapping

#layers

# physicalantennas

AntennaPorts

LTERelease

C-RS Single-layer 1 1 0 8

Tx Diversity 2 or 4 2 or 4 0-1 or 0-3 8

Sp Multiplexing 2, 3, or4

2, 3, or 4 0-1, 0-2, or0-3

8

UE-RS Single-layer 1 >= 2 5 8

5, 7, 8 9

Dual-layer 2 >= 2 7-8 9

N-layer, N = N 7-(6+N) 10

In aMIMOor Tx Diversity configuration, each C-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 antennabut changing the phase of the each antenna's signal relative to the others. Since the same

UE-RS sequence is sent from each antenna, the 89600 VSA can compare the received UE-RS

sequence with the reference sequence and calculate the weights that were applied to the

antennas to accomplish the beamforming.

Multi-layer beamforming adds some complexity to beamforming by transmitting as many UE-

RS sequences as there are layers to allow demodulation of each layer's PDSCH data. The UE-


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RS sequence for each antenna port is orthogonal to the others, either in time/frequency

domain or in the code domain. This can be thought of as beamforming of each layer

independently. N-layer beamforming is an extension of dual-layer beamforming and supports

up to 8 data layers with the ability to beamform each layer separately.

For reference, the following table lists the different LTE downlink reference signals and the

antenna ports they use.

Reference Signal Antenna Ports LTE Release

C-RS 0-3 8

MBSFN-RS 4 8

UE-RS 5 8

5, 7, 8 9

5, 7-14 10

P-RS 6 9

CSI-RS 15-22 10

Transmit-Receive Paths

In the case of a single-layer, single-antenna LTE signal (using only C-RS), there would only be

one antenna port signal that could be received over the air, but in general, the reception of an


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LTE signal will contain a combination of multiple transmit antennas, each of which may be

transmitting a combination of multiple antenna ports. The LTE standard does not specify any

particular setup for transmit antennas, but since the C-RS antenna ports are used for most

control channels and PDSCH, the 89600 VSA LTE demodulator uses Cell-specific RS antenna

ports instead of transmit antennas when indicating transmit paths between transmitter and

receiver.

The VSA denotes C-RS antenna ports by the mnemonic C-RSnon the user interface and in the

documentation, where nis the antenna port number. Correspondingly, the receive channel is

denoted by Rxm, where mis the measurement channel number - 1.

Together, these two endpoints constitute an transmit-receive path from transmitter to the

receiver (ultimately the 89600 VSA). A transmit-receive path is denoted by C-RSn/Rxm, so

for example, C-RS2/Rx1 on theMIMO Info Tableshows metrics calculated from the (C-RS)

antenna port 2 signal received on VSA measurement channel 2.

Reference Signal - Downlink

Most of the channels (e.g, DPSCH, DPCCH, PBCH etc) is for carrying a special information (asequence of bits) and they have some higher layer channel connected to them, butReference 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 slotsand the location of the resource elements are specifically determined by antennaconfiguration.

As LTE gets evolved into higher version, we are getting more and more reference signalwhich is mapped to a specific antenna port. And we are getting more and more confused asa 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,p1

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

For the exact Resource Element locations of each reference signal, refer to following pages.
http://rfmw.em.keysight.com/wireless/helpfiles/89600B/WebHelp/subsystems/lte/content/trc_mimo_info_table.htmhttp://rfmw.em.keysight.com/wireless/helpfiles/89600B/WebHelp/subsystems/lte/content/trc_mimo_info_table.htmhttp://rfmw.em.keysight.com/wireless/helpfiles/89600B/WebHelp/subsystems/lte/content/trc_mimo_info_table.htmhttp://rfmw.em.keysight.com/wireless/helpfiles/89600B/WebHelp/subsystems/lte/content/trc_mimo_info_table.htm


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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)

Codewords and Layers and Ports

We throw a lot of terms around when we discuss LTE data transmissions, especially when we get into some of thedetails of sending data over the PDSCH using multiple antenna techniques. We describe transport blocks as holdingthe data we're trying to send, but how do they relate to codewords? Assigning bits to different layers can be used toimprove reliability or throughput, but are layers the same as antenna ports? Let's have a closer look at what's goingon down in the Physical (PHY) Layer.

User data and signaling messages are processed by the PDCP, RLC and MAC layers before being passed down tothe PHY layer to be sent over the air. A lot happens to a data packet before PHY gets it, but for the moment, let's justtreat the MAC PDU (Protocol Data Unit) that PHY receives from MAC as "data". To PHY, it's just a string of bitsanyway. This will be our transport block.

Transport Blocks to Codewords

What does PHY do with a transport block? First, it converts the transport block into a codeword. There are a numberof steps involved in this process, depending on the length of the transport block:

Append a 24 bit checksum (CRC) to the transport block. This CRC is used to determine whether thetransmission was successful or not, and triggers Hybrid ARQ to send an ACK or NACK, as appropriate

Segment the transport block into code blocks. A code block must be between 40 and 6144 bits long. If thetransport block is too small, it is padded up to 40 bits; if the TB is too big, it is divided into smaller pieces,each of which gets an additional 24 bit CRC.

Process each code block with a 1/3 turbo coder

Reassemble the resulting code blocks into a single codeword

A codeword, then, is essentially a transport block with error protection. Note that a UE may be configured to receiveone or two transport blocks (and hence one or two codewords) in a single transmission interval.

Codewords to Layers

PHY then converts each codeword into modulation symbols. For each codeword, PHY must:

Scramble the contents of each codeword, using a sequence based on the UE's C-RNTI and the cell's

Physical Cell ID (PCI) Convert the bit sequences into the corresponding modulation symbols (using QPSK, 16QAM or 64QAM)

Assign the modulation symbols to one or more layers, depending on the specific transmission scheme beingused

In the case of a single transmit antenna, the last step is pretty simple: the contents of the codeword are mapped to asingle layer. For transmit diversity, it's almost as easy: the symbols from the codeword are distributed evenly acrossthe 2 or 4 layers in a round-robin fashion.
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In spatial multiplexing situations, things get a little more complicated, since one or two codewords may be distributedacross 1, 2, 3 or 4 layers. In brief, here's how the mapping is handled:

The number of layers used in any particular transmission depends (at least in part) on the Rank Indication (RI)feedback from the UE, which identifies how many layers the UE can discern.

Layers to Antenna Ports

The final steps apply any required precoding adjustments and assign the modulation symbols to the physicalresources:

Apply the required precoding factors to the modulation symbols in each layer

Map the precoded symbols to the appropriate antenna ports

Assign the modulation symbols to be transmitted on each antenna port to specific resource elements (thesubcarriers and symbols within the resource blocks)

Generate the final time-domain OFDM signal for each antenna port

Note that the number of layers is always less than or equal to the number of antenna ports (transmit antennas). Ifthere's only one antenna port, then it carries just a single layer. In multiple (2 or 4) antenna situations, though, eachantenna port may end up carrying a complicated combination of the symbols from multiple layers. Check out spec36.211, section 6.3.4 if you really want to dig into the details.

What's the answer in a nutshell? One transport block -> one codeword -> one or two layers -> one or more antennaports. Fortunately, the eNodeB and the UE always know what's going on, even if I have trouble keeping it all straightsometimes.

Antenna mappingis the combination of layer mapping and pre-coding, which process the modulation symbolsfor one or two codewords to transmit them on different antenna ports

there are mainly two types of layer mapping:

spatial multiplexing, transmit diversity.


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In case of spatial multiplexing, there may be one or two code-words. But the number of layers is restricted.On one hand, layers should be equal to or more than the number of codewords

On the other hand, the number of layers cannot exceed the number of antenna ports. The most important concept is

layer. The layers in spatial multiplexing have the same meaning as streams. They are used to transmit multiple

data streams in parallel, so the number of layers here is often referred to as the transmission rank. In spatial

multiplexing, the number of layers may be adapted to the transmission rank, by means of the feedback of a Rank

Indicator (RI) to the layer mapping.

In case of transmit diversity, there is only one codeword and the number of layers is equal to the number ofantenna ports. The number of layers in this case is not related to the transmission rank, because transmit-diversity

schemes are always single-rank transmission schemes. The layers in transmit diversity are used to conveniently

carry out the following precoding by some pre-defined matrices.

Transmit diversity for two antenna ports is based on Space Frequency Block Coding (SFBC), and transmit diversity

for four antenna ports is based on a combination of SFBC and Frequency Shift Transmit Diversity (FSTD). According

to the specifications, transmit diversity is implemented by a predefined matrix. It can be seen that comparing the case

of two antenna ports, the four-antenna-port transmission has a reduced bandwidth. Note that unlike spatial

multiplexing,


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Where, RI is the Rank Indicator, for instance, a recommended number of layers by the User Equipment (UE) and PMI

is the Pre-coding Matrix Indicator, and it could be a recommended pre-coder matrix by the UE.

The symbols for codewords, layers and antenna ports can be individually expressed as

An antenna portis defined such that the channel over which a symbol on the antenna

port is conveyed can be inferred from the channel over which another symbol on the same

antenna port is conveyed

each antenna port has its ownreference signal

antenna ports do not correspond to physical antennas, but rather are logical entities distinguished by

their reference signal sequences

Multiple antenna port signals can be transmitted on a single transmit antenna (C-RSport 0 andUE-

RSport 5, for example). Correspondingly, a single antenna port can be spread across multiple

transmit antennas (UE-RS port 5, for example).

Note that the number of layers is always less than or equal to the number of antenna ports (transmit antennas). Ifthere's only one antenna port, then it carries just a single layer. In multiple (2 or 4) antenna situations, though, eachantenna port may end up carrying a complicated combination of the symbols from multiple layers. Check out spec36.211, section 6.3.4 if you really want to dig into the details.

What's the answer in a nutshell? One transport block -> one codeword -> one or two layers -> one or more antennaports. Fortunately, the eNodeB and the UE always know what's going on, even if I have trouble keeping it all straightsometimes.
http://www.sharetechnote.com/html/Handbook_LTE_ReferenceSignal.htmlhttp://www.sharetechnote.com/html/Handbook_LTE_ReferenceSignal.htmlhttp://www.sharetechnote.com/html/Handbook_LTE_ReferenceSignal.htmlhttp://mycommunity.theiet.org/user_content/photos/110/030/1100308141/8555625aae149133d2fce8d00aece947-original-2.pnghttp://mycommunity.theiet.org/user_content/photos/110/030/1100308141/8555625aae149133d2fce8d00aece947-original-2.pnghttp://www.sharetechnote.com/html/Handbook_LTE_ReferenceSignal.html


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The cell-specific reference signals are pilot signals inserted into the downlink signal that are

used by the

UE to perform downlink channel estimation in order to perform coherent demodulation of

the


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information-bearing parts of the downlink signal. These signals are modulated using QPSK

to make them

resilient to noise and errors and they carry one of the 504 different cell identities. They are

also

transmitted in a power boosted way (6dB more than surrounding data symbols) so they areeasily

detected, received and demodulated.

Downlink physical signals:

Reference signal

Synchronization signal (P-SS and S-SS)

Uplink physical signals:

Demodulation reference signal (UL-RS), associated with transmission of PUSCH and PUCCH.

Sounding reference signal (SRS), not associated with transmission of PUSCH and PUCCH.


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signal depends on the number of antenna ports configured for the physical channel or signal

The downlink signals are broadly classified into two,


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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,

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.

LTE Downlink Reference Signals

The CSRS is a QPSK modulated sequence.

Similarly we also have UeSRS, which we will not discuss her, but this is UE specific and used only for

beam forming.

Synchronization Signals

There are 2 synchronization signals in LTE downlink,

1. Primary synchronization signal PSS

2. Secondary synchronization signal SSS

The PSS and SSS are both mapped always in Subframe 0 and 5 for FDD. The PSS is always mapped

to the last symbol of first slot and SSS to the last but one symbol of first slot. Also these are always

mapped to the central 6 RB of the bandwidth, irrespective of any system bandwidth or configuration.
http://ltebasics.files.wordpress.com/2013/06/csrs1.jpg


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This enables the UE to decode these signals, even when it does not know the system bandwidth. The

PSS/SSS detection is a very early procedure that the UE should do, to get the cell ID of the system.

The PSS is a Zadoff-Chu sequence, of length 62 and the SSS is a combination of 2 binary sequences

of length 31. Since the PSS/SSS have only 62 subcarrier valid, the remaining 10 subcarrier are

padded with zeros. 5 zero pads on each side of the sequence.

The PSS/SSS not only convey the cell ID but also the current subframe number, slot boundary,

duplexing mode to the UE. There are 3 different sequences of PSS and 168 different sequences of

SSS.

As part of later release of LTE (Rel 10) they have also introduced a new signal known as the

Positioning Reference Signal, which assists in positioning detection.

We shall discuss the cell ID detection procedure by the UE in a separate article.

How Reference Signals Map In LTE?

For channel estimation different types of reference signals are in use:

Cellspecificreferencesignals.

MBSFNreferencesignals.

UE specific reference signals

CELL-SPECIFIC REFERENCE SIGNALS

These signals are transmitted for all non-MBSFN subframes only where subcarrier spacing is 15 kHz. In MBSFN

subframes only the first 2 OFDM symbols can be used. The 10 ms long pseudo-random sequence is cell-id specific and

additionally 6 different cell-specific frequency shifts are defined.

MBSFN REFERENCE SIGNALS

Only in subframes allocated for MBSFN these signals are present and are used with extended CP only. A pseudo

random sequence depending on MBSFN id will be transmitted on antenna port 4.

UE SPECIFIC REFERENCE SIGNALS

In case of single antenna port (port 5) transmission the reference signals are UE specific. UE takes these signals for

channel demodulation of the PDSCH. The signals are only transmitted on RBs assigned to this specific UE.

Synchronization Of Signals In LTE

504 different physical-layer cell ids are composed of 168 different physical-layer cell id groups each of 3 physical layer

ids within the group

There are

primary synchronization signal based on a frequency domain Zadoff-Chu

sequence indicating the physical layer id,

secondary synchronization signal indicating the physical layer cell id group.


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The mapping to resource elements is

the primary synchronization signal is broadcast in the last OFDM symbol of slot 0 and slot 10. In the frequency it is

located symmetrically in the centre of the configured cell bandwidth. The sequence is 62 symbols long.

the secondary synchronization signal is shifted by one OFDM signal and thus

occupies the second last OFDM symbol of those slots.

5 REs are kept empty at each side of the frequency.HI, CFI, DCI signaling

Hybrid ARQ information (HI), Control format information (CFI) and DL Control

Indicator (DCI) are signaled via PHICH, PCFICH and PDCCH.

HI is used for ACK/NACK info on UL-SCH HARQ. As there is only one TB in UL there is one bit to be signaled.

CFI is cell- and subframe specific and contains two bit where three different values are in use. DCI is existing in

different formats and may combine different types of information.

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