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   1 #
   2 # Copyright 2008 Free Software Foundation, Inc.
   3 #
   4 # This file is part of GNU Radio
   5 #
   6 # GNU Radio is free software; you can redistribute it and/or modify
   7 # it under the terms of the GNU General Public License as published by
   8 # the Free Software Foundation; either version 3, or (at your option)
   9 # any later version.
  10 #
  11 # GNU Radio is distributed in the hope that it will be useful,
  12 # but WITHOUT ANY WARRANTY; without even the implied warranty of
  13 # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14 # GNU General Public License for more details.
  15 #
  16 # You should have received a copy of the GNU General Public License
  17 # along with GNU Radio; see the file COPYING.  If not, write to
  18 # the Free Software Foundation, Inc., 51 Franklin Street,
  19 # Boston, MA 02110-1301, USA.
  20 #
  21
  22 from gnuradio import gr, eng_notation
  23 import math
  24
  25 n2s = eng_notation.num_to_str
  26
  27 class receive_path(gr.hier_block2):
  28     def __init__(self,
  29                  if_rate,        # Incoming sample rate
  30                  symbol_rate,    # Original symbol rate
  31                  excess_bw,      # RRC excess bandwidth, typically 0.35-0.5
  32                  costas_alpha,   # Costas loop 1st order gain, typically 0.01-0.2
  33                  costas_beta,    # Costas loop 2nd order gain, typically alpha^2/4.0
  34                  costas_max,     # Costas loop max frequency offset in radians/sample
  35                  mm_gain_mu,     # M&M loop 1st order gain, typically 0.001-0.2
  36                  mm_gain_omega,  # M&M loop 2nd order gain, typically alpha^2/4.0
  37                  mm_omega_limit, # M&M loop max timing error
  38                  ):
  39
  40         gr.hier_block2.__init__(self, "receive_path",
  41                                 gr.io_signature(1, 1, gr.sizeof_gr_complex), # Input signature
  42                                 gr.io_signature(0, 0, 0))                    # Output signature
  43
  44         self._if_rate = if_rate
  45         self._sps = int(self._if_rate/symbol_rate)
  46         print "IF sample rate:", n2s(self._if_rate)
  47         print "Symbol rate:", n2s(symbol_rate)
  48         print "Samples/symbol:", self._sps
  49         print "RRC bandwidth:", excess_bw
  50
  51         # Create AGC to scale input to unity
  52         self._agc = gr.agc_cc(1e-5, 1.0, 1.0, 1.0)
  53
  54         # Create RRC with specified excess bandwidth
  55         taps = gr.firdes.root_raised_cosine(1.0,          # Gain
  56                                             self._sps,    # Sampling rate
  57                                             1.0,          # Symbol rate
  58                                             excess_bw,    # Roll-off factor
  59                                             11*self._sps) # Number of taps
  60
  61         self._rrc = gr.fir_filter_ccf(1, taps)
  62
  63         # Create a Costas loop frequency/phase recovery block
  64
  65         print "Costas alpha:", costas_alpha
  66         print "Costas beta:", costas_beta
  67         print "Costas max:", costas_max
  68
  69         self._costas = gr.costas_loop_cc(costas_alpha,  # PLL first order gain
  70                                          costas_beta,   # PLL second order gain
  71                                          costas_max,    # Max frequency offset rad/sample
  72                                          -costas_max,   # Min frequency offset rad/sample
  73                                          2)             # BPSK
  74
  75         # Create a M&M bit synchronization retiming block
  76         mm_mu = 0.5
  77         mm_omega = self._sps
  78
  79         print "MM gain mu:", mm_gain_mu
  80         print "MM gain omega:", mm_gain_omega
  81         print "MM omega limit:", mm_omega_limit
  82
  83         self._mm = gr.clock_recovery_mm_cc(mm_omega,       # Initial samples/symbol
  84                                            mm_gain_omega,  # Second order gain
  85                                            mm_mu,          # Initial symbol phase
  86                                            mm_gain_mu,     # First order gain
  87                                            mm_omega_limit) # Maximum timing offset
  88
  89         # Add an SNR probe on the demodulated constellation
  90         self._snr_probe = gr.probe_mpsk_snr_c(10.0/symbol_rate)
  91         self.connect(self._mm, self._snr_probe)
  92
  93         # Slice the resulting constellation into bits.
  94         # Get inphase channel and make decision about 0
  95         self._c2r = gr.complex_to_real()
  96         self._slicer = gr.binary_slicer_fb()
  97
  98         # Descramble BERT sequence.  A channel error will create 3 incorrect bits
  99         self._descrambler = gr.descrambler_bb(0x8A, 0x7F, 7) # CCSDS 7-bit descrambler
 100
 101         # Measure BER by the density of 0s in the stream
 102         self._ber = gr.probe_density_b(1.0/symbol_rate)
 103
 104         self.connect(self, self._agc, self._rrc, self._costas, self._mm,
 105                      self._c2r, self._slicer, self._descrambler, self._ber)
 106
 107     def frequency_offset(self):
 108         return self._costas.freq()*self._if_rate/(2*math.pi)
 109
 110     def timing_offset(self):
 111         return self._mm.omega()/self._sps-1.0
 112
 113     def snr(self):
 114         return self._snr_probe.snr()
 115
 116     def ber(self):
 117         return (1.0-self._ber.density())/3.0
 118