GNURadio PSK bit recovery

Question

I have followed the wonderful GNURadio Guided Tutorial PSK Demodulation: https://wiki.gnuradio.org/index.php/Guided_Tutorial_PSK_Demodulation

I've created a very simple DBPSK modulator

I feed in a series of bits that are sliding. So the first byte I feed in is 0x01, the next byte is 0x02, 0x04, 0x08 and so on. This is the output of hd:

00000000  00 00 ac 0e d0 f0 20 40  81 02 04 08 10 00 20 40  |...... @...... @|
00000010  81 02 04 08 10 00 20 40  81 02 04 08 10 00 20 40  |...... @...... @|
*
00015000

The first few bytes are garbage, but then you can see the pattern. Looking at the second line you see: 0x81, 0x02, 0x04, 0x08, 0x10, 0x00, 0x20, 0x40, 0x81

The walking ones is there, but after 0x10, the PSK demodulator receives a 0x00, then a few bytes later is receives a 0x81. It almost seems like the timing recovery is off.

Has anyone else seen something like this?

Answer 1

OK, I figured it out. Below is my DBPSK modulation.

If you let this run, the BER will continue to drop. Some things to keep in mind. The PSK Mod takes an 8-bit value (or perhaps an short or int as well). It grabs the bits and modulates them. Then the PSK Demod does the same. If you save this to a file, you will not get the exact bits out. You will need to shift the bits to align them. I added the Vector Insert block to generate a preamble of sorts.

Then I wrote some Python to find my preamble:

import numpy as np
import matplotlib.pyplot as plt


def findPreamble(preamble, x):
    for i in range(0, len(x) - len(preamble)):
        check = 0
        for j in range(0, len(preamble)):
            check += x[i + j] - preamble[j]
        if (check == 0):
            print("Found a preamble at {0}".format(i))
            x = x[i + len(preamble)::]
            break
    return check == 0, x

def shiftBits(x):
    for i in range (0, len(x) - 1):
        a = x[i]
        a = a << 1
        if x[i + 1] & 0x80:
            a = a | 1

        x[i] = (a & 0xFF)
    return x

f = open('test.bits', 'rb')
x = f.read();
f.close()

preamble = [0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08]
searchForBit = True
x = np.frombuffer(x, dtype='uint8')
x = x.astype('int')
print(x)

while searchForBit:

    x = shiftBits(x)
    print(x)
    found, y = findPreamble(preamble, x)
    if found:
        searchForBit = False
        y = y.astype('uint8')
        f = open('test.bits', 'wb')
        f.write(y)
        f.close()