# Note: This task will only work if the basic (hard-decision decoding) Viterbi
# decoder from PS3_viterbi does.
class SoftViterbiDecoder(ViterbiDecoder):
# Override the default branch metric with a soft decision metric:
# the square of the Euclidian distance between the
# expected and received voltages.
def branch_metric(self,expected,received):
diff = 0
for i in xrange(len(expected)):
diff += (expected[i] - received[i])**2
return diff
if __name__=='__main__':
# Test whether branch metrics are as expected
PS3_tests.test_soft_metrics(SoftViterbiDecoder)
# This test for soft decoding produces a random message of
# nbits=1000 in length, from a fixed random seed (so each time the
# program is run, the same behavior will occur. We encode the
# message and then use your soft decoder (code above) to test it.
# If the decoded stream is the same as what we expect from our
# encoder, we consider the test case to have passed. Note, it is
# possible (though not likely) that your soft decoder may not pass
# this test case, but is in fact correct. That's why we have not
# made the test case part of the verification before checkoff.
K, glist = 3, (7, 5)
soft = SoftViterbiDecoder(K, glist)
nbits = 1000
sigma = 0.5 # noise defined as 2*sigma^2 = 1/8
numpy.random.seed(617617617)
class SoftViterbiDecoder(ViterbiDecoder):
# Override the default branch metric with a soft decision metric:
# the square of the Euclidian distance between the
# expected and received voltages.
def branch_metric(self, expected, received):
#your code here
bm = 0
for i in range(len(expected)):
bm = bm + (expected[i] - received[i]) * (expected[i] - received[i])
return bm
if __name__ == '__main__':
# Test whether branch metrics are as expected
PS3_tests.test_soft_metrics(SoftViterbiDecoder)
# This test for soft decoding produces a random message of
# nbits=1000 in length, from a fixed random seed (so each time the
# program is run, the same behavior will occur. We encode the
# message and then use your soft decoder (code above) to test it.
# If the decoded stream is the same as what we expect from our
# encoder, we consider the test case to have passed. Note, it is
# possible (though not likely) that your soft decoder may not pass
# this test case, but is in fact correct. That's why we have not
# made the test case part of the verification before checkoff.
K, glist = 3, (7, 5)
soft = SoftViterbiDecoder(K, glist)
nbits = 1000
sigma = 0.5 # noise defined as 2*sigma^2 = 1/8
numpy.random.seed(617617617)
