LTE System Toolbox

LTE Waveform Modeling Using Downlink Transport and Physical Channels

This example shows how to generate a time domain waveform containing a Physical Downlink Shared Channel (PDSCH), corresponding Physical Downlink Control Channel (PDCCH) transmission and the Physical Control Format Indicator Channel (PCFICH), for one subframe.

Introduction

This example demonstrates how to generate a complete Downlink Shared Channel (DL-SCH) transmission for 6 resource blocks, 4 antenna transmit diversity using the functions from the LTE System Toolbox™. The following physical channels are modeled:

  • Physical Downlink Shared Channel (PDSCH)

  • Physical Downlink Control Channel (PDCCH)

  • Physical Downlink Control Format Indicator Channel (PCFICH)

This example generates a time domain (post OFDM modulation) for all 4 antenna ports. A single subframe (number 0) is considered in this example.

Note: The recommended way to generate RMC waveforms is using lteRMCDLToollteRMCDLTool, this example shows how a waveform can be built by creating and combining individual physical channels, as happens in an LTE system.

Cell-wide Settings

eNodeB settings are configured with a structure.

enbConfig.NDLRB = 6;                % No of Downlink Resource Blocks(DL-RB)
enbConfig.CyclicPrefix = 'Normal';  % CP length
enbConfig.PHICHDuration = 'Normal'; % Normal PHICH duration
enbConfig.DuplexMode = 'FDD';       % FDD duplex mode
enbConfig.CFI = 3;                  % 4 PDCCH symbols
enbConfig.Ng = 'Sixth';             % HICH groups
enbConfig.CellRefP = 4;             % 4-antenna ports
enbConfig.NCellID = 10;             % Cell id
enbConfig.NSubframe = 0;            % Subframe number 0

Subframe Resource Grid Generation

A resource grid can easily be created using lteDLResourceGridlteDLResourceGrid. This creates an empty resource grid for one subframe. The subframe is a 3 dimensional matrix. The number of rows represents the number of subcarriers available, this is equal to 12*enbConfig.NDLRB since there are 12 subcarriers per resource block. The number of columns equals the number of OFDM symbols in a subframe, i.e. 7*2, since we have 7 OFDM symbols per slot for normal cyclic prefix and there are 2 slots in a subframe. The number of planes (3rd dimension) in subframe is 4 corresponding to the 4 antenna ports as specified in enbConfig.CellRefP.

subframe = lteDLResourceGrid(enbConfig);

DL-SCH Channel Coding

We now generate the DL-SCH bits and apply channel coding. This includes CRC calculation, code block segmentation and CRC insertion, turbo coding, rate matching and code block concatenation. It can be performed using lteDLSCHlteDLSCH.

DL-SCH transport block size is chosen according to rules in TS36.101, Annex A.2.1.2 [ 1 ] "Determination of payload size" with target code rate $R=1/3$ and number of bits per subframe given by codedTrBlkSize.

codedTrBlkSize = 480;  % Coded transport block size

% Transmission mode configuration for PDSCH
pdschConfig.NLayers = 4;               % No of layers
pdschConfig.TxScheme = 'TxDiversity';  % Transmission scheme
pdschConfig.Modulation = 'QPSK';       % Modulation scheme
pdschConfig.RNTI = 1;                  % 16-bit UE-specific mask
pdschConfig.RV = 0;                    % Redundancy Version

transportBlkSize = 152;                % Transport block size
dlschTransportBlk = randi([0 1], transportBlkSize, 1);

% Perform Channel Coding
codedTrBlock = lteDLSCH(enbConfig, pdschConfig, codedTrBlkSize, ...
               dlschTransportBlk);

PDSCH Complex Symbols Generation

The following operations are applied to the coded transport block to generate the Physical Downlink Shared Channel complex symbols: scrambling, modulation, layer mapping and precoding. This can be achieved using ltePDSCHltePDSCH. As well as some of the cell-wide settings specified in enbConfig this function also requires other parameters related to the modulation and channel transmission configuration, pdschConfig. The resulting matrix pdschSymbols has 4 columns. Each column contains the complex symbols to map to each antenna port.

pdschSymbols = ltePDSCH(enbConfig, pdschConfig, codedTrBlock);

PDSCH Mapping Indices Generation and Mapping

The indices to map the PDSCH complex symbols to the subframe resource grid are generated using ltePDSCHIndicesltePDSCHIndices. The parameters required by this function include some of the cell-wide settings in enbConfig, channel transmission configuration pdsch and the physical resource blocks (PRBs). The latter indicates the resource allocation for transmission of the PDSCH. In this example we have assumed all resource blocks are allocated to the PDSCH. This is specified using a column vector as shown below.

These indices are made '1based' for direct mapping on the resource grid as MATLAB® uses 1 based indexing. In this case we have assumed that both slots in the subframe share the same resource allocation. It is possible to have different allocations for each slot by specifying a two column matrix as allocation, where each column will refer to each slot in the subframe.

The resulting matrix pdschIndices has 4 columns, each column contains a set of indices in linear style pointing to the resource elements to be used for the PDSCH in each antenna port. Note that this function returns indices avoiding resource elements allocated to reference signals, the control region, broadcast channels and synchronization signals.

The generated indices are represented in 1-base format as used by MATLAB but can be made standard specific 0-based using string options '0based' instead of '1based'. If this string option is not specified the default is 1-based index generation.

prbs = (0:enbConfig.NDLRB-1).';  % Subframe resource allocation
pdschIndices = ltePDSCHIndices(enbConfig, pdschConfig, prbs, {'1based'});

% Map PDSCH symbols on resource grid
subframe(pdschIndices) = pdschSymbols;

DCI message configuration

Downlink Control Information (DCI), conveys information about the DL-SCH resource allocation, transport format, and information related to the DL-SCH hybrid ARQ. lteDCIlteDCI can be used to generate a DCI message to be mapped to the Physical Downlink Control Channel (PDCCH). These parameters include the number of downlink Resource Blocks (RBs), the DCI format and the Resource Indication Value (RIV). The RIV of 26 correspond to full bandwidth assignment. lteDCIlteDCI returns a structure dciMessage and a vector containing the DCI message bits dciMessageBits. Both contain the same information; the structure is more readable, while the serialized DCI message is a more suitable format to send to the channel coding stages.

dciConfig.DCIFormat = 'Format1A';  % DCI message format
dciConfig.Allocation.RIV = 26;     % Resource indication value

[dciMessage, dciMessageBits] = lteDCI(enbConfig, dciConfig); % DCI message

DCI Channel Coding

The DCI message bits are channel coded. This includes the following operations: CRC insertion, tail-biting convolutional coding and rate matching. The field PDCCHFormat indicates that one Control Channel Element (CCE) is used for the transmission of PDCCH, where a CCE is composed of 36 useful resource elements.

pdcchConfig.NDLRB = enbConfig.NDLRB;  % Number of DL-RB in total BW
pdcchConfig.RNTI = pdschConfig.RNTI;  % 16-bit value number
pdcchConfig.PDCCHFormat = 0;          % 1-CCE of aggregation level 1

% Performing DCI message bits coding to form coded DCI bits
codedDciBits = lteDCIEncode(pdcchConfig, dciMessageBits);

PDCCH Bits Generation

The capacity of the control region depends on the bandwidth, the Control Format Indicator (CFI), the number of antenna ports and the PHICH groups. The total number of resources available for PDCCH can be calculated using ltePDCCHInfoltePDCCHInfo. This returns a structure pdcchInfo where the different fields express the resources available to the PDCCH in different units: bits, CCEs, Resource Elements (REs) and Resource Elements Groups (REGs). The total number of bits available in the PDCCH region can be found in the field pdcchInfo.MTot. This allows us to build a vector with the appropriate number of elements. Not all the available bits in the PDCCH region are necessarily used. Therefore the convention adopted is to set unused bits to -1, while bit locations with values 0 or 1 are used.

Note that we have initialized all elements in pdcchBits to -1, indicating that initially all the bits are unused. The elements of codedDciBits are mapped to the appropriate locations in pdcchBits.

Only a subset of all the bits in pdcchBits may be used, these are called the candidate bits. Indices to these can be calculated using ltePDCCHSpaceltePDCCHSpace. This returns a two column matrix. Each row indicates the available candidate locations for the provided cell-wide settings enbConfig and PDCCH configuration structure pdcchConfig. The first and second columns contain the indices of the first and last locations respectively of each group of candidates. In this case these indices are 1-based and refer to bits, hence they can be used to access locations in pdcchBits. The vector pdcchBits has 664 elements. The 72 bits of codedDciBits are mapped to the chosen candidate in pdcchBits. Therefore out of 664 elements, 72 will take 0 and 1 values, while the rest remain set to -1. ltePDCCHltePDCCH will interpret these locations as unused and will only consider those with 1s and 0s.

pdcchInfo = ltePDCCHInfo(enbConfig);    % Get the total resources for PDCCH
pdcchBits = -1*ones(pdcchInfo.MTot, 1); % Initialized with -1

% Performing search space for UE-specific control channel candidates
candidates = ltePDCCHSpace(enbConfig, pdcchConfig, {'bits','1based'});

% Mapping PDCCH payload on available UE-specific candidate. In this example
% the first available candidate is used to map the coded DCI bits.
pdcchBits( candidates(1, 1) : candidates(1, 2) ) = codedDciBits;

PDCCH Complex Symbol Generation

From the set of bits used in pdcchBits (values not set to -1) PDCCH complex symbols are generated. The following operations are required: scrambling, QPSK modulation, layer mapping and precoding.

ltePDCCHltePDCCH takes a set of PDCCH bits and generates complex-valued PDCCH symbols performing the operations mentioned above. In this case pdcchSymbols is a 4 column matrix, each corresponding to each antenna port.

pdcchSymbols = ltePDCCH(enbConfig, pdcchBits);

PDCCH Mapping Indices Generation and Resource Grid Mapping

PDCCH indices are generated for symbol mapping on resource grid. pdcchIndices is a matrix with 4 columns, one column per antenna port. The rows contain the indices in linear form for mapping the PDCCH symbols to the subframe resource grid.

pdcchIndices = ltePDCCHIndices(enbConfig, {'1based'});

% The complex PDCCH symbols are easily mapped to each of the resource grids
% for each antenna port
subframe(pdcchIndices) = pdcchSymbols;

CFI Channel Coding

The number of OFDM symbols in a subframe is linked to the Control Format Indicator (CFI) value. Cell-wide settings enbConfig specifies a CFI value of 3, which means that 4 OFDM symbols are used for the control region in the case of 6 downlink resource blocks. The CFI is channel coded using lteCFIlteCFI. The resulting set of coded bits is a 32 element vector.

cfiBits = lteCFI(enbConfig);

PCFICH Complex Symbol Generation

The CFI coded bits are then scrambled, QPSK modulated, mapped to layers and precoded to form the PCFICH complex symbols. The pcfichSymbols is a matrix having 4 columns where each column contains the PCFICH complex symbols that map to each of the antenna ports.

pcfichSymbols = ltePCFICH(enbConfig, cfiBits);

PCFICH Indices Generation and Resource Grid Mapping

The PCFICH complex symbols are mapped to the subframe resource grid using the appropriate mapping indices. These are generated using ltePCFICHIndicesltePCFICHIndices and will be used to map the PCFICH symbol quadruplets to resource element groups in the first OFDM symbol in a subframe. All antenna ports are considered, and resource elements used by Reference Signals (RSs) are avoided. Note that the resulting matrix has 4 columns; each column contains the indices in linear form for each antenna port. These indices are 1-based, however they can also be generated using 0-based. The linear indexing style used makes the resource grid mapping process straight forward. The resulting matrix contains the complex symbols in pcfichSymbols in the locations specified by pcfichIndices.

pcfichIndices = ltePCFICHIndices(enbConfig);

% Map PCFICH symbols to resource grid
subframe(pcfichIndices) = pcfichSymbols;

OFDM Modulation

Time domain mapping by performing OFDM modulation for downlink symbols. The resulting matrix has 4 columns; each column contains the samples for each antenna port.

[timeDomainMapped, timeDomainInfo] = lteOFDMModulate(enbConfig, subframe);

Selected Bibliography

  1. 3GPP TS 36.101