[Discuss-gnuradio] PSK modulation (along with packed vs unpacked bytes)

[Nathan West]

Twoish questions:

I've done a lot of code reading the last 2 weeks and worked on a custom PSK modulator (basically hierarchical 2/4psk). I've tested it at baseband on everything seems good, so I'm hoping now to use something like benchmark{rx,tx} to test it ota.

I've built a couple of sample flowgraphs. The first one is a BPSK loopback:
> from gnuradio import gr, digital
>
> src_data = (range(0,256))
>
> # blocks
> src = "" src_data )
> mod = digital.bpsk.bpsk_mod()
> demod = digital.bpsk.bpsk_demod()
> repacker = gr.unpacked_to_packed_bb(1, gr.GR_MSB_FIRST)
> dst = gr.vector_sink_b()
>
> # flow graph
> tb = gr.top_block()
> tb.connect(src, mod, demod, repacker, dst)
> tb.run()
> print dst.data()


It's output is *almost* correct:
> (0, 0, 0, 0, 0, 0, 0, 20, 18, 128, 0, 129, 1, 130, 2, 131, 3, 132, 4, 133, 5, 134, 6, 135, 7, 136, 8, 137, 9, 138, 10, 139, 11, 140, 12, 141, 13, 142, 14, 143, 15, 144, 16, 145, 17, 146, 18, 147, 19, 148, 20, 149, 21, 150, 22, 151, 23, 152, 24, 153, 25, 154, 26, 155, 27, 156, 28, 157, 29, 158, 30, 159, 31, 160, 32, 161, 33, 162, 34, 163, 35, 164, 36, 165, 37, 166, 38, 167, 39, 168, 40, 169, 41, 170, 42, 171, 43, 172, 44, 173, 45, 174, 46, 175, 47, 176, 48, 177, 49, 178, 50, 179, 51, 180, 52, 181, 53, 182, 54, 183, 55, 184, 56, 185, 57, 186, 58, 187, 59, 188, 60, 189, 61, 190, 62, 191, 63, 192, 64, 193, 65, 194, 66, 195, 67, 196, 68, 197, 69, 198, 70, 199, 71, 200, 72, 201, 73, 202, 74, 203, 75, 204, 76, 205, 77, 206, 78, 207, 79, 208, 80, 209, 81, 210, 82, 211, 83, 212, 84, 213, 85, 214, 86, 215, 87, 216, 88, 217, 89, 218, 90, 219, 91, 220, 92, 221, 93, 222, 94, 223, 95, 224, 96, 225, 97, 226, 98, 227, 99, 228, 100, 229, 101, 230, 102, 231, 103, 232, 104, 233, 105, 234, 106, 235, 107, 236, 108, 237, 109, 238, 110, 239, 111, 240, 112, 241, 113, 242, 114, 243, 115, 244, 116, 245, 117, 246, 118, 247, 119, 248, 120, 249, 121, 250, 122, 251)

The first few bytes are 0's, which as I understand has to do with the RRC filters, frequency correction, and possibly agc filter. Then it starts at 128, 0 and increments by 1. I think I would expect it it to go from 0..255 but it looks like something else that's also incrementing by 1 is interleaved in there. Looking at the output before the repacker confirms that each byte appears repeated twice, but the first instance has the MSB set to 1.

Doing a similar loopback in GRC with PSK Mod/Demod results in what I would expect (non-repeated bytes with MSB not set to 1). Would this be the proper way of using these blocks?

A related question, where is the packing being done in the benchmark_rx path? The generic_demod does an unpack_k_bits, but doesn't appear to pack them back up. In the benchmark_rx callback it looks like the packet is already full of packed bytes, but I haven't seen the unpacked_to_packed in pkt.py.

Thanks for any help,
-Nathan