From 2db5bf68aec5dbaabd32e5b5452e3bb5f6feca1e Mon Sep 17 00:00:00 2001
From: Nao Pross <np@0hm.ch>
Date: Thu, 9 Dec 2021 17:22:24 +0100
Subject: Create phase correction block in gr-fadingui

---
 src/gr-fadingui/python/phasecorrection.py | 124 ++++++++++++++++++++++++++++++
 1 file changed, 124 insertions(+)
 create mode 100644 src/gr-fadingui/python/phasecorrection.py

(limited to 'src/gr-fadingui/python/phasecorrection.py')

diff --git a/src/gr-fadingui/python/phasecorrection.py b/src/gr-fadingui/python/phasecorrection.py
new file mode 100644
index 0000000..7d79529
--- /dev/null
+++ b/src/gr-fadingui/python/phasecorrection.py
@@ -0,0 +1,124 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+
+import pmt
+
+import numpy as np
+from gnuradio import gr
+
+import fadingui.logger
+
+from fadingui.logger import get_logger
+log = get_logger("phasecorrection")
+
+class phasecorrection(gr.sync_block):
+    """
+    Apply phase and frequency correction where there is a correlation peak tag.
+
+    The correlation peak tags are NOT propagated, and instead replaced with a
+    frame_start tag.
+    """
+    def __init__(self):
+        gr.sync_block.__init__(
+            self,
+            name='Phase and Frequency Correction',
+            in_sig=[np.complex64],
+            out_sig=[np.complex64]
+        )
+
+        # tags should not be propagated, we then output our own tags
+        self.set_tag_propagation_policy(gr.TPP_DONT)
+
+        # because we do block processing, we need to keep track of the last tag
+        # of the previous block to correct the first values of the next block
+        self.last = None
+        self.lastfreq = 0
+
+    def block_phase(self, start, end):
+        """
+        Compute a vector for the phase and frequency correction for the samples
+        between two tags (start and end).
+
+        @param start Tag where the samples should start to be corrected
+        @param end   Tag where to stop correcting
+
+        @return A vector of phase values for each sample. To correct the samples
+                the data should be multiplied with np.exp(-1j * phase)
+        """
+        # compute number of samples between tags
+        nsamples = end.offset - start.offset
+
+        # unpack pmt values into start and end phase
+        sphase = pmt.to_python(start.value)
+        ephase = pmt.to_python(end.value)
+
+        # compute frequency offset between start and end
+        phasediff = (ephase - sphase) % (2 * np.pi)
+        freq = phasediff / nsamples
+
+        # save this one for the last block (see variable `end' in self.work)
+        self.lastfreq = freq
+
+        # debugging
+        log.debug(f"Correction for chunk of {nsamples:2d} samples is " \
+              f"sphase={sphase: .4f} rad and freq={freq*1e3: .4f}e-3 rad / sample")
+
+        # compute chunk values
+        return sphase * np.ones(nsamples) + freq * np.arange(0, nsamples)
+
+    def work(self, input_items, output_items):
+        counter = self.nitems_written(0)
+
+        # nicer aliases
+        inp = input_items[0]
+        out = output_items[0]
+
+        # read phase tags
+        is_phase = lambda tag: pmt.to_python(tag.key) == "phase_est"
+        tags = list(filter(is_phase, self.get_tags_in_window(0, 0, len(inp))))
+
+        if not tags:
+            log.warning(f"There were no tags in {len(inp)} samples!")
+            out[:] = inp
+            return len(out)
+
+        # debugging
+        log.debug(f"Processing {len(tags)} tags = {tags[-1].offset - tags[0].offset} " \
+              f"samples out of {len(inp)} input samples")
+
+        # compute "the middle"
+        enough_samples = lambda pair: ((pair[1].offset - pair[0].offset) > 0)
+        pairs = list(filter(enough_samples, zip(tags, tags[1:])))
+        chunks = [ self.block_phase(start, end) for (start, end) in pairs ]
+        middle = np.concatenate(chunks) if chunks else []
+
+        # compute values at the end, we do not have informations about the future
+        # but we can use the frequency of the last tag to approximate
+        nback = len(inp) - (tags[-1].offset - counter)
+        log.debug(f"Processing {nback} samples at the back of the buffer")
+        end = np.ones(nback) * pmt.to_python(tags[-1].value) \
+                + self.lastfreq * np.arange(0, nback)
+
+        # compute the "start", using the last tag from the previous call
+        nfront = tags[0].offset - counter
+        log.debug(f"Processing {nfront} samples at the front of the buffer")
+        start = self.block_phase(self.last, tags[0])[-nfront:] \
+                if self.last and nfront else np.zeros(nfront)
+
+        # compute correction
+        correction = np.exp(-1j * np.concatenate([start, middle, end]))
+        length = len(correction)
+
+        # write outputs
+        out[:length] = inp[:length] * correction
+
+        # save last tag for next call
+        self.last = tags[-1]
+
+        # add tags
+        for tag in tags:
+            self.add_item_tag(0, tag.offset, pmt.intern("frame_start"), pmt.PMT_T)
+
+        # FIXME: should return `length' but then the last sample is not
+        #        included and self.last does something weird
+        return len(out)
\ No newline at end of file
-- 
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