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Convolutional decoding



out = lteConvolutionalDecode(softBits) returns out, data recovered by convolutionally decoding softBits, a vector of soft bits.

The decoder uses a soft input wrap-around Viterbi algorithm without any quantization. The algorithm creates training data to append to the start and end of the packet by cyclically extending the packet. The traceback decoding length is 42.


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Generate random bits and convolutionally encode them.

txBits = randi([0 1],1000,1);
codedData = lteConvolutionalEncode(txBits);

QPSK modulate the coded bits and add noise to the received symbols.

txSym = lteSymbolModulate(codedData,'QPSK');
noise = 0.5*complex(randn(size(txSym)),randn(size(txSym)));
rxSym = txSym + noise;

Show rxSymbols constellation, setting txSymbols as the reference constellation.

xylimits = [-2.5 2.5];
cdScope = comm.ConstellationDiagram('ReferenceConstellation',txSym,'XLimits',xylimits ,'YLimits',xylimits);

Figure Constellation Diagram contains an axes and other objects of type uiflowcontainer, uimenu, uitoolbar. The axes contains 2 objects of type line. This object represents Channel 1.

Demodulate the noisy symbols to obtain soft bits, convolutionally decode the soft bits, and display the number of erroneous bits.

softBits = lteSymbolDemodulate(rxSym,'QPSK','Soft');
out = lteConvolutionalDecode(softBits);
disp(sum(out ~= int8(txBits)))

Input Arguments

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Soft bit data, specified as a column vector. The function assumes that input data has been encoded by a tail-biting convolutional code with constraint length 7, coding rate 1/3, and octal polynomials G0 = 133, G1 = 171 and G2 = 165. The function also assumes that the input data vector is structured as three encoded parity streams concatenated block-wise in the form [D0 D1 D2], where D0, D1, and D2 are the separate parity streams resulting from the original encoding with individual polynomials G0, G1, and G2.

Data Types: double

Output Arguments

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Convolutionally decoded data, returned as a column vector. The length of this vector is 1/3 of the length of the softBits input.

Data Types: int8

Introduced in R2014a