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.
Perform Convolutional Decoding
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;
rxSymbols constellation, setting
txSymbols as the reference constellation.
xylimits = [-2.5 2.5]; cdScope = comm.ConstellationDiagram('ReferenceConstellation',txSym,'XLimits',xylimits ,'YLimits',xylimits); cdScope(rxSym)
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)))
softBits — Soft bit data
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
are the separate parity streams resulting from the original encoding with
individual polynomials G0, G1, and
Introduced in R2014a