Convolution Coding and Viterbi Decoding with Matlab: High BER with code rate of 5 / 8

[Ernte1893]

Hey,

I am building a model for a transceiver and I am struggling atm with the convolution coding and especially its decoding ! I want to use the following code rates:
1/3; 1/2; 5/8; 3/4;

For the first two I use different trellis operators (still standard ones) and since I don't use any noise the BER is obviously 0.
For the 3rd and 4th rate I deleted some samples and added a zero pattern at the end (instead of the deleted samples). Am I doing something wrong ? I get really high BER values for the 3rd and 4th rate (0.2 - 0.5 !).

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Code Matlab M - [expand]
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% FEC Code rate selection: k/n : k = information bits, n = total bits
coderate = 2; % 0 means no puncturing but coding rate of 1/3 = size: 3
              % 1 means puncturing (coding rate of 1/2 )     = size: 2
              % 2 means puncturing (coding rate of 5/8 )     = size: 8
              % 3 means puncturing (coding rate of 3/4 )     = size: 4
rate = [1/3 1/2 5/8 3/4];
 
 
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% QPSK transmitter
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 
%% Create SU payload with FEC
% Create Payload
rand_data = randn(1,data); 
Input = zeros(1,data);
for i = 1:data
        if rand_data(i) >= 0.5
            Input(i) = 1;
        else
            Input(i) = 0;
        end
end
 
% FEC
% Convolution Coding: for each input bit, there are 2 redundant bits concatenated: 
% a -> [ a a a ]; 
trellis = poly2trellis(7,[171 165 133]);
coded_data = convenc(Input,trellis);
% Puncturing: 
% depending on the code rate, the signal has to be changed as followed
switch coderate
    case 0 % Stays the same: 1/3        
    case 1 % a -> [ a a ] : 1/2
        coded_data(2:3:end) = [];
    case 2 % [ a b c d e ] -> [ a a b c c d e e ] : 5/8
        coded_data(3:15:end) = [];
        coded_data(3:14:end) = [];
        coded_data(3:13:end) = [];
        coded_data(6:12:end) = [];        
        coded_data(6:11:end) = [];        
        coded_data(6:10:end) = [];
        coded_data(9:9:end)  = [];
    case 3 % [ a b c d ] -> [ a a b c ] : 3/4
        coded_data(3:9:end) = [];
        coded_data(3:8:end) = [];
        coded_data(3:7:end) = [];
        coded_data(4:6:end) = [];
        coded_data(4:5:end) = [];        
    otherwise
end
 
Signal = coded_data;
 
% According to selected code rate, zeros are added to the signal
decoded = zeros(1,size(Signal,2)*rate(coderate+1)*3);
switch coderate
    case 0 % Stays the same: 1/3        
        decoded = Signal;        
        Output = vitdec(decoded,trellis,12,'trunc','hard');
    case 1 % [ a a ] New trellis operator: 1/2
        trellis2 = poly2trellis(7,[171 133]);
        decoded = Signal;  
        Output = vitdec(decoded,trellis2,12,'trunc','hard');
    case 2 % [ a a b c c d e e ] -> [ a a 0 0 0 b c c 0 0 0 d e e 0 ]  : 5/8
        decoded(1:15:end) = Signal(1:8:end);
        decoded(2:15:end) = Signal(2:8:end);
        decoded(6:15:end) = Signal(3:8:end);
        decoded(7:15:end) = Signal(4:8:end);
        decoded(8:15:end) = Signal(5:8:end);
        decoded(12:15:end) = Signal(6:8:end);
        decoded(13:15:end) = Signal(7:8:end);
        decoded(14:15:end) = Signal(8:8:end);        
        Output = vitdec(decoded,trellis,96,'trunc','hard');
    case 3 %  [ a a b c ] -> [ a a 0 0 0 b 0 0 c ]: 3/4
        decoded(1:9:end) = Signal(1:4:end);
        decoded(2:9:end) = Signal(2:4:end);
        decoded(6:9:end) = Signal(3:4:end);
        decoded(9:9:end) = Signal(4:4:end);          
        Output = vitdec(decoded,trellis,96,'trunc','hard');
    otherwise
end
 
% Viterbi Decoder for convolutianally encoded data
[number,ratio]=biterr(Output,Input); % Bit error rate

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[D.A.(Tony)Stewart]

Raised cosine filtering is important to prevent ISI with symbol RLL span.
I did not look at your code.

 

[Ernte1893]

Raised cosine filtering is important to prevent ISI with symbol RLL span.
I did not look at your code.

Hey,
thanks for the reply, but since both convolution filters are working perfectly fine for both coderates 1/2 and 1/3, the ISI shouldn't be the reason, or what do you think ?