_init_rate_match(k, n) gives wrong CRC degree

[catkira]

I want to decode the PBCH. In matlab I can do it like this

    iBIL = false;
    iIL = true;
    crcLen = 24;
    nMax = 9;
    A = 32;
    P = 24;
    K = A+P;
    N = 512;
    decIn = nrRateRecoverPolar(pbchBits_csi,K,N,iBIL);
    decBits = nrPolarDecode(decIn,K,E,polarListLength,nMax,iIL,crcLen);

However when I try it with sionna like this

    from sionna.fec.polar import PolarEncoder, Polar5GEncoder, PolarSCLDecoder, Polar5GDecoder, PolarSCDecoder
    enc = Polar5GEncoder(k=K, n=864)
    dec = Polar5GDecoder(enc, dec_type="SCL", list_size=8)
    dec(np.expand_dims(pbchBits_csi, 0))

I get a wrong (different than matlab) result. I know the matlab result is correct, because the CRC gives 0.
I found something suspicious with enc._init_rate_match(k, n), when I call it with k=54 and n=512 (or n=864) it gives CRC order 11, but it should be 24 (24C CRC polynomial). Is this a bug?

[catkira]

I just read that only uplink is implemented sionna, but what I want to do is decode downlink, is that the problem?

[catkira]

yes. that seems to be the problem. It would be nice if the Polar5GEncoder had another parameter, to set up- or downlink mode. Some things are different for up- and downlink, they are very well summarized in table I of this paper https://arxiv.org/pdf/1804.04389.pdf

[jhoydis]

Yes, Sionna currently only supports the uplink (as stated in the documentation).