function [next_out, next_state, last_out, last_state] = trellis(g)% copyright Nov. 1998 Yufei Wu% MPRG lab, Virginia Tech% for academic use only% set up the trellis given code generator g% g given in binary matrix form. e.g. g = [ 1 1 1; 1 0 1 ];% next_out(i,1:2): trellis next_out (systematic bit; parity bit) when input = 0, state = i; next_out(i,j) = -1 or 1% next_out(i,3:4): trellis next_out  (systematic bit; parity bit) when input = 1, state = i;% next_state(i,1): next state when input = 0, state = i; next_state(i,i) = 1,...2^m% next_state(i,2): next state when input = 1, state = i;% last_out(i,1:2): trellis last_out (systematic bit; parity bit) when input = 0, state = i; last_out(i,j) = -1 or 1% last_out(i,3:4): trellis last_out  (systematic bit; parity bit) when input = 1, state = i;% last_state(i,1): previous state that comes to state i when info. bit = 0;% last_state(i,2): previous state that comes to state i when info. bit = 1;[n,K] = size(g);m = K - 1;max_state = 2^m;% set up next_out and next_state matrices for systematic codefor state=1:max_state   state_vector = bin_state( state-1, m );      % when receive a 0   d_k = 0;   a_k = rem( g(1,:)*[0 state_vector]', 2 );   [out_0, state_0] = encode_bit(g, a_k, state_vector);   out_0(1) = 0;     % when receive a 1   d_k = 1;   a_k = rem( g(1,:)*[1 state_vector]', 2 );   [out_1, state_1] = encode_bit(g, a_k, state_vector);   out_1(1) = 1;   next_out(state,:) = 2*[out_0 out_1]-1;   next_state(state,:) = [(int_state(state_0)+1) (int_state(state_1)+1)];end% find out which two previous states can come to present statelast_state = zeros(max_state,2);for bit=0:1   for state=1:max_state      last_state(next_state(state,bit+1), bit+1)=state;      last_out(next_state(state, bit+1), bit*2+1:bit*2+2) ...         = next_out(state, bit*2+1:bit*2+2);   end end