Hi , i'm trying to do iterative decoding using vitdec from matlab, i would like to know how can i get the metrics of the bits to intruce them in the demodulator again, using the function vitdec. The output of this function is the final sequence , but i just want the metrics of the bits. Thank you TA

# Iterative decoding with viterbi decoder

Started by ●December 3, 2003

Reply by ●December 4, 20032003-12-04

you can get the final weighting vectors from that function to estimate the soft metrics, but if you want an actual soft-input soft-output viterbi decoder you will have to write you own, or look in mathworks' library. julius On Wed, 3 Dec 2003, Ta wrote:> Hi , i'm trying to do iterative decoding using vitdec from matlab, i > would like to know how can i get the metrics of the bits to intruce > them in the demodulator again, using the function vitdec. The output > of this function is the final sequence , but i just want the metrics > of the bits. Thank you > TA >-- The most rigorous proofs will be shown by vigorous handwaving. http://www.mit.edu/~kusuma opinion of author is not necessarily of the institute

Reply by ●December 4, 20032003-12-04

thank you , but in fact i haven't understood you. I think that i hev not explained well what i want to know. This is the code that i use to get the sequence without iterative decoding: %BICM simulates a system with a transmitter, channel and receiver using Bit %Interleaved Coded Modulation. %TRANSMITTER frameLength = 1296; N = 1; numSymb = N*frameLength; zeroPad = 6; %Random source generation for i=0:N-1 seed(1) = 654321 + i*frameLength; seed(2) = 123456 + i*frameLength; rand('state', seed(1)); randn('state', seed(2)); msg_orig(i*frameLength+1:(i+1)*frameLength,1) = [randsrc(frameLength-zeroPad,1,[0 1]); zeros(zeroPad,1)]; end %Convolutional encoder constlen = [7]; codegen = [133 171]; codeRate = 1/2; trel = poly2trellis(constlen, codegen); %Interleaver depthIL = frameLength/codeRate; %Modulation labelling = 'Gray'; modulation = 'PSK'; M = 8; Initial_phase = 0; %Sample frequency of convolutionally encoded signal Fd = 1; %Sample frequency of msg_tx Fs = 1; %CHANNEL k = log2(M); SNRpBit = [7]; SNR = SNRpBit + 10*log10(k); SNR= SNR -10*log10(1/codeRate)-10*log10(Fs/Fd); channelType = 'RAYL'; %AWGN or RAYL RaylType = 'uncorr'; %'uncorr' OR 'correl' DoppMax = 10; Tmax = 1;%Choose a logical Tmax, taking into account the Tx rates %we want to work with and the number of symbls we are transmiting %RECEIVER %quantization of metrics Res = 8; %the computed metrics will have 256 levels. %soft-input VITERBI decoder tblen = frameLength; %trace-back of the viterbi decoder %LOOP for TX frame by frame, N different channels and Performance averaged msg_dec = zeros(frameLength*length(SNR), N); for index = 0:N-1 currentFrame = msg_orig(index*frameLength+1:(index+1)*frameLength,1); [msg_enc_bi final_state] = convenc(currentFrame, trel); [msg_IL RandIndexVector]= BitInterleaver(msg_enc_bi, depthIL); %Mapping and transmission %Options QPSK,8-PSK,16-QAM,64-QAM msg_TX = mapping(msg_IL, modulation, M, labelling, Fd, Fs, Initial_phase); %Normalization of the TX signal, to have E[|x|^2] = 1. Due to Interleaving %we can assume equiprobable TX symbols. For PSK M=4 or 8 the TX signal has %always E[|x|2] = 1; for multi-level modulation QAM we have to normalize. if M == 16 msg_TX = msg_TX/3.1623; %/sqrt(10) elseif M ==64 msg_TX = msg_TX/6.4807;%/sqrt(42) end disp(sprintf('%s %d %s %d','Frame TX number',index+1,'of',N)) msg_decTemp = []; %LOOP, for each frame generate a LOOP of transmitted and decoded frames for %different values of SNR at the receiver. for SNRindex = 1:length(SNR) [msg_RX,raylEnv,raylPhase] = channelBICM(msg_TX, channelType, SNR(SNRindex), DoppMax, Tmax, RaylType); msg_demod = demod(msg_RX, modulation, channelType, M, labelling, SNR(SNRindex), Fd, Fs, Initial_phase, raylEnv, raylPhase); %the pseudo-random sequence of Interleaving at the Transmitter and %interleaving depth, length(RandIndexVector), are known at the receiver. msg_deIL = metricsDeinterleaver(msg_demod, RandIndexVector); maxVal = mean(msg_deIL) + 0.4*var(msg_deIL);%max(msg_deIL); %~8;%select linear values for max and min of saturation. minVal = mean(msg_deIL) - 0.4*var(msg_deIL);%min(msg_deIL); %~1.001; %case QAM64, maxVal and minVal are better: %maxVal = mean(abs(msg_deIL))+2*sqrt(var(abs(msg_deIL))) msg_input_dec = qntizer(msg_deIL, Res, maxVal, minVal); msg_decSNR = vitdec(msg_input_dec, trel, tblen, 'term', 'soft', Res); msg_decTemp = cat(1,msg_decTemp,msg_decSNR); clear msg_decSNR; end msg_dec(:,index+1)= msg_decTemp; clear msg_decSNR; end ------------------------------------------------------------------------ I just generate the sequence, poly2trellis , then convenc, then apply an interleaver, mapping, then the channel, ... in the demodulation i get the ratios of the bits, after deinterleaving and making the quantification of the ratios i apply vitdec, then i get the sequence. But i don't know how to make this process iterative, i don't after the vitdec but i should do get the a posteriori probabilities. Could you help me? Julius Kusuma <kusuma@mit.edu> wrote in message news:<Pine.GSO.4.55L.0312041008340.1355@biohazard-cafe.mit.edu>...> you can get the final weighting vectors from that function to estimate the > soft metrics, but if you want an actual soft-input soft-output viterbi > decoder you will have to write you own, or look in mathworks' library. > > julius > > > On Wed, 3 Dec 2003, Ta wrote: > > > Hi , i'm trying to do iterative decoding using vitdec from matlab, i > > would like to know how can i get the metrics of the bits to intruce > > them in the demodulator again, using the function vitdec. The output > > of this function is the final sequence , but i just want the metrics > > of the bits. Thank you > > TA > >

Reply by ●December 5, 20032003-12-05

i'm including the vitdec help information below. what i said was that you can use "finalmetric", "finalstates", and "finalinputs" from this function to get some sort of soft output from the viterbi algorithm. here is the help file: vitdec Convolutionally decode binary data using the Viterbi algorithm Syntax decoded = vitdec(code,trellis,tblen,opmode,dectype); decoded = vitdec(code,trellis,tblen,opmode,'soft',nsdec); decoded = vitdec(...,'cont',...,initmetric,initstates,initinputs); [decoded,finalmetric,finalstates,finalinputs] =... vitdec(...,'cont',...); On Thu, 4 Dec 2003, Ta wrote: [long code snipped] -- The most rigorous proofs will be shown by vigorous handwaving. http://www.mit.edu/~kusuma opinion of author is not necessarily of the institute

Reply by ●December 17, 20032003-12-17

On Thu, 4 Dec 2003 23:27:18 -0500, Julius Kusuma <kusuma@mit.edu> wrote:>i'm including the vitdec help information below.I know also that such kinds of SI-SO algorithms are used in a Turbo-code / a Turbo-equalizer. You can find some decoders for them in IT++ (http://itpp.sourceforge.net) as long as you are also interested in the c++ language. BR, ------ James K. (txdiversity@hotmail.com) - Any remarks, proposal and/or indicator would be greatly respected. - Private opinions: These are not the opinions from my affiliation. [Home] http://home.naver.com/txdiversity