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detect: refactor receiver for large frequency drifts (~0.1%)

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Roman Zeyde
2015-01-15 18:14:16 +02:00
parent 5401206178
commit 1da258ebf8
5 changed files with 74 additions and 53 deletions
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51
amodem/detect.py
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@@ -20,13 +20,14 @@ class Detector(object):
TIMEOUT = 10.0 # [seconds]
def __init__(self, config):
def __init__(self, config, pylab):
self.freq = config.Fc
self.omega = 2 * np.pi * self.freq / config.Fs
self.Nsym = config.Nsym
self.Tsym = config.Tsym
self.maxlen = config.baud # 1 second of symbols
self.max_offset = self.TIMEOUT * config.Fs
self.plt = pylab
def _wait(self, samples):
counter = 0
@@ -71,16 +72,46 @@ class Detector(object):
bufs.append(np.array(trailing))
buf = np.concatenate(bufs)
offset = self.find_start(buf, self.CARRIER_DURATION*self.Nsym)
offset = self.find_start(buf, duration=self.CARRIER_DURATION)
start_time += (offset / self.Nsym - self.SEARCH_WINDOW) * self.Tsym
log.debug('Carrier starts at %.3f ms', start_time * 1e3)
return itertools.chain(buf[offset:], samples), amplitude
buf = buf[offset:]
def find_start(self, buf, length):
N = len(buf)
carrier = dsp.exp_iwt(self.omega, N)
z = np.cumsum(buf * carrier)
z = np.concatenate([[0], z])
correlations = np.abs(z[length:] - z[:-length])
return np.argmax(correlations)
prefix_length = self.CARRIER_DURATION * self.Nsym
amplitude, freq_err = self.estimate(buf[:prefix_length])
return itertools.chain(buf, samples), amplitude, freq_err
def find_start(self, buf, duration):
filt = dsp.FIR(dsp.exp_iwt(self.omega, self.Nsym))
p = np.abs(list(filt(buf))) ** 2
p = np.cumsum(p)[self.Nsym-1:]
p = np.concatenate([[0], p])
length = (duration - 1) * self.Nsym
correlations = np.abs(p[length:] - p[:-length])
offset = np.argmax(correlations)
return offset
def estimate(self, buf, skip=5):
filt = dsp.exp_iwt(-self.omega, self.Nsym) / (0.5 * self.Nsym)
frames = common.iterate(buf, self.Nsym)
symbols = [np.dot(filt, frame) for frame in frames]
symbols = np.array(symbols[skip:-skip])
amplitude = np.mean(np.abs(symbols))
log.debug('Carrier symbols amplitude : %.3f', amplitude)
phase = np.unwrap(np.angle(symbols)) / (2 * np.pi)
indices = np.arange(len(phase))
a, b = dsp.linear_regression(indices, phase)
self.plt.figure()
self.plt.plot(indices, phase, ':')
self.plt.plot(indices, a * indices + b)
freq_err = a / (self.Tsym * self.freq)
last_phase = a * indices[-1] + b
log.debug('Current phase on carrier: %.3f', last_phase)
log.debug('Frequency error: %.2f ppm', freq_err * 1e6)
self.plt.title('Frequency drift: {0:.3f} ppm'.format(freq_err * 1e6))
return amplitude, freq_err