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#!/usr/bin/env python
# -*- coding: utf-8 -*-
#
# Copyright 2021 Naoki Pross.
import numpy as np
from numpy_ringbuffer import RingBuffer
from gnuradio import gr
class xor_frame_sync(gr.sync_block):
"""
Performs a frame synchronization by XOR matching a preamble bit sequence
"""
def __init__(self, sync_pattern, buffer_size):
# TODO: buffer size should be in packets
gr.sync_block.__init__(self,
name="xor_frame_sync",
in_sig=[np.byte],
out_sig=[np.byte])
# binary pattern to match
self.pattern = np.unpackbits(np.array(sync_pattern, dtype=np.uint8))[::-1]
self.nbytes = len(sync_pattern)
self.nbits = len(self.pattern)
assert(self.nbits % 8 == 0)
# packed buffer to delay the data
self.delaybuf = RingBuffer(buffer_size, dtype=np.uint8)
self.delay = 0
# unpacked buffer to compute correlation values, initially filled with zeros
self.corrbuf = RingBuffer(self.nbits)
self.corrbuf.extend(np.zeros(self.nbits))
# buffer to store correlation values
self.xcorrs = RingBuffer(buffer_size)
# synchronization state
self.synchronized = False
def xcorrelation(self, v):
"""
Compute the binary correlations between the stored pattern and
correlation buffer, while shifting v into the buffer.
Binary correlation between two bit vectors is just size of the
vector(s) minus the number of bits that differ.
"""
v_arr = np.array(v, dtype=np.uint8)
for b in np.unpackbits(v_arr):
self.corrbuf.appendleft(b)
yield self.nbits - np.sum(np.logical_xor(self.corrbuf, self.pattern))
def work(self, input_items, output_items):
"""
Process the inputs, that means:
- Check that the buffer is synchronized, i.e. there is the sync
pattern appears every k bits, where k is the size of the packet.
- If the buffer is not synchronized, compute a binary cross
correlation to find how much the stream should be delayed.
Notes:
- Even though the block input is of type np.byte, inp is an array
of 255 bytes, probably for performance reasons.
TODO: block processing
"""
inp = input_items[0]
out = output_items[0]
if not self.synchronized:
for v in inp:
# compute the cross correlation
xcs = self.xcorrelation(v)
# add cross correlations to buffer and save value
self.xcorrs.extend(list(xcs))
self.delaybuf.appendleft(v)
peak = np.argmax(self.xcorrs)
self.delay = peak
self.synchronized = True
if self.xcorrs[peak] != self.nbits:
self.synchronized = False
print(f"Warning! XOR correlation did not find a peak (max = {self.xcorrs[peak]} should be {self.nbits})")
# return data with delay
out[:] = self.delaybuf[self.delay]
return len(output_items[0])
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