Changing traceback gmsk viterbi demodulator object

Hi,

I want to see how changing the traceback (Tb) of a gmsk demodulator affects the BER curve in GMSK (pulse length=3), but there is no variation in the BER curve when Tb change, except when Tb=1. I tried Tb values from 2 to 100, does anybody have an idea why it occurs?

Thanks in advance.

%%
close all
clear all
clc

%% Modulation parameters
oversamp = 30;
L = 3;
h = 0.5;
BT = 0.5;
fast_sim=0;
 

%% Bit generation and diff encoding
Nbits=10e4; % 12 bits for fixed input
%Nbits=5000;

    bits = randi([0 1],1,Nbits-2);
    bits = [[0 1] , bits];
    encoded_bits = xor(bits(1:end-1),bits(2:end));
    encoded_bits = not(encoded_bits);
    encoded_bits = (encoded_bits*2 - 1)';
    %encoded_bits =[-1 1 -1 -1 1 1 1 1 -1 1 1 -1]';

 %% Objects 

hMod = comm.GMSKModulator(...
       'SamplesPerSymbol',oversamp,...
       'BitInput', false,'InitialPhaseOffset' ,0, 'PulseLength',L);
   
hDeMod = comm.GMSKDemodulator(...
       'BitOutput',false,...
       'SamplesPerSymbol',oversamp, 'InitialPhaseOffset' ,0, 'PulseLength',L,...
        'TracebackDepth',2);  
    
%% Modulation

object_gfsk= step(hMod,encoded_bits);

%% BER calculation for Matlab object  

EbNodB = 0:1:10;
%EbNodB =100;
BER = zeros(1,numel(EbNodB)); 

for index = 1: numel(EbNodB)
    EbNodB(index),
    errors = 0;
    count = 0;
    while(1)
            %% Channel
            hAWGN = comm.AWGNChannel('NoiseMethod','Signal to noise ratio (Eb/No)','EbNo', (EbNodB(index)),'SamplesPerSymbol',oversamp);
            received = step(hAWGN, (object_gfsk));
            reset(hAWGN)
            
            %% Demodulator         
           
           estimatedBits=step(hDeMod, received);
           reset (hDeMod);%OJO NO BORRAR
          
           %% Error estimation  
            
            hError = comm.ErrorRate('ReceiveDelay', hDeMod.TracebackDepth);

            errorStats = step(hError, (encoded_bits(1:end)) , (estimatedBits));
            errorsN = errorStats(2);
            
            errors = errors + errorsN;
            
            count = count + 1;
          
            if(fast_sim == 1)
                if(errors>300 || count > 500)
                    BER(index) = errors / (count*numel(encoded_bits));
                    break;
                end
            else
                if(errors>700 || count > 1200)
                    BER(index) = errors / (count*numel(encoded_bits));
                    break;
                end
            end

    end
end

%% Plotting

semilogy(EbNodB,BER,'k','LineWidth',1.5);
grid on

[gidon schwartz]

hi,

Is it possibble that in GMSK a traceback of 2 is sufficient for decoding ( and therefore no improvement is shown for higher values of TB)?

do you have any new conclusions regarding this question?