5G DMRS sequence generation

2 min readFeb 1, 2023

The sequence generation is described in the spec (TS 38.211 5.2.1) and is implemented in the MATLAB 5G Toolbox function nrPRBS. Starting on line 61, there is a block of code:

% If a subsequence was requested, extract the subsequence
if ~isscalar(n)
    prbs = prbsInternal(end-n(2)+1:end);
else
    prbs = prbsInternal;
end

I have two related questions, What is the reason for requesting a subsequence? How does the UE know where to look for this DMRS sequence?

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The function nrPRBS accepts a vector or a scalar to specify the number of elements in the sequence, n.

When n is specified as a two element vector [p m], seq contains m contiguous elements of the PRBS generator, starting at position p (zero-based).

This pseudo-random sequence is used in generating the reference signals which are confined to the bandwidth part (BWP). The bandwidth part can start from any position in the carrier, which might be at an offset to point A. This is scenario where a subsequence can be used directly.

Consider the case of demodulation reference signal (DM-RS) of PDSCH, which is only a part of complete BWP.

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