MATLAB: Does PRACH preamble 4 not show any data for TDD duplexing

LTE Toolbox

I would like to modify the example given in the 'lteprach' documentation page:

https://www.mathworks.com/help/releases/R2020a/lte/ref/lteprach.html

such that it uses preamble format 4 and TDD duplexing mode. However, when I try simply setting:

chs.Format = 4;

ue.DuplexMode = 'TDD';

I get nothing but zeros. Why is this the case?

Best Answer

On the 'Random Access Channel' documentation page, you can see details regarding each of the preamble formats:

https://www.mathworks.com/help/releases/R2020a/lte/ug/random-access-channel.html#bt1028e-1

There, you can see the following note which specifically addresses TDD in combination with preamble format 4:

"Note that Preamble Format 4 is only applicable for TDD in special subframes (subframe 1 or 6) and with Special Subframe Configuration that results in UpPTS with 2 symbols duration i.e. the Preamble Format 4 PRACH sits in UpPTS"

If you take that information and navigate to the 'FDD and TDD Duplexing' doc page, you can get a more complete picture of what parameters need to be set:

https://www.mathworks.com/help/releases/R2020a/lte/ug/fdd-and-tdd-duplexing.html#btw_se1-1

On that page, there is a table which shows that only SSC values 5 and above result in UpPTS with 2 symbols:

Based on the note addressing the limitations for Preamble Format 4, you must make sure that the SSC is set to a value between 5 and 9 rather than the default value of '0' which is used in the example. Otherwise, our UpPTS will not be of symbol length 2.

Then, you must set the subframe number to one that corresponds to a special subframe. This means 1 (for all TDD configurations) or 6 (for 0,1,2, or 6) as per the following table.

In summary, you can set the following values: (for example)

ue.DuplexMode = 'TDD';
ue.TDDConfig = 1;
chs.Format = 4;
ue.NSubframe = 1; 
ue.SSC = 6;

which will generate a valid PRACH.

To apply this to the example on the page:

ue.NULRB = 6;
chs.HighSpeed = 0;
chs.CyclicShiftIdx = 0;
chs.FreqOffset = 0;
chs.SeqIdx = 0;
chs.PreambleIdx = [ 0 ];
ue.DuplexMode = 'TDD';
ue.TDDConfig = 1;
ue.SSC = 6;
chs.Format = 4;
ue.NSubframe = 1;
[prachSym,prachInfo] = ltePRACH(ue,chs);
figure; plot(abs(prachSym))

Related Solutions

MATLAB: Am I getting an error during function call to lteDMRSIndices, when I an trying to modify an example in lteDMRS function to display DMRS for R.50 reference channel

The lteDMRSIndices function expect the resource block set allocation to be a vector, not a cell array.

For "R.50", this parameter is set to a cell array, as the resource allocation is dynamic (it changes depending on the subframe number).

The way to use the lteDMRSIndices function is then to input the setting for each subframe, one at a time:

(note the for subframeNr loop added in the code below)

enb = lteRMCDL('R.50');
enb.PDSCH.TxScheme = 'Port7-14';
enb.PDSCH.NLayers = 4;
ntxants = 8;
enb.PDSCH.W = lteCSICodebook(enb.PDSCH.NLayers,ntxants,[0 0]).';
cfg = enb;
for subframeNr = 0:9
    cfg.NSubframe = subframeNr;
    cfg.PDSCH.PRBSet = enb.PDSCH.PRBSet{subframeNr+1};
    subframe = zeros(lteResourceGridSize(cfg,ntxants));
    cfg.PDSCH.NTxAnts = size(cfg.PDSCH.W,2);
    dmrsInd = lteDMRSIndices(cfg,cfg.PDSCH);
    dmrs = lteDMRS(cfg,cfg.PDSCH);
    subframe(dmrsInd) = dmrs;
end

MATLAB: How to generate LTE15 PRACH subframes

Hello Reza,

It is not possible to directly generate the waveform at a different sampling rate, but it should be very easy to perform resampling using the MATLAB resample() function as follows:

>> ue.NULRB = 75;
>> prach.Format = 0;
>> waveform = ltePRACH(ue,prach);
>> waveform = resample(waveform,23040,30720);
>> size(waveform)
ans =
       23040           1

For information, the reason we use 30.72 Ms/s is for compatibility with our normal uplink waveforms created using lteSCFDMAModulate.

Regards,

Iain.

Best Answer

Related Solutions

MATLAB: Am I getting an error during function call to lteDMRSIndices, when I an trying to modify an example in lteDMRS function to display DMRS for R.50 reference channel

MATLAB: How to generate LTE15 PRACH subframes

Related Question