viterbidecoder.m

卷积码的Viterbi译码算法仿真程序 要求：递归卷积码

function  [LLR,dec_outbits ] =ViterbiDecoder(Pn,rx_bits)
%Viterbi译码器,递归卷积码

global  state_num;
global  memory_len;

global  state_inbits;
global  state_outbits1;
global  state_outbits2;


global alpha;%状态度量
global gama; %幸存路径


L=length(rx_bits);
LLR=zeros(1,L-memory_len);
dec_outbits=zeros(1,L-memory_len);
alpha=zeros(L+1,state_num);
gama=zeros(L,state_num,state_num);
%初始化alpha和beta
alpha(1,1) =1;




%计算alpha值
for k=2:L+1
    for s2=0:state_num-1;
        metric=zeros(1,2);
        state=zeros(1,2);
        index=0;
        for s1=0:state_num-1
                if alpha(k-1,s1+1)==0
                    continue;    
                end
                if state_inbits(s1+1,s2+1)~=0 
                    index=index+1;
                    metric(index)=rx_bits(k-1,1)*state_outbits1(s1+1,s2+1)+rx_bits(k-1,2)*state_outbits2(s1+1,s2+1)+alpha(k-1,s1+1);
                    %metric(index)=(rx_bits(k-1,1)-state_outbits1(s1+1,s2+1))^2+(rx_bits(k-1,2)-state_outbits2(s1+1,s2+1))^2;
                    state(index)=s1;
                end
        end
        if index~=0
            [ma,index]=max(metric);
            alpha(k,s2+1)=ma;
            gama(k-1,state(index)+1,s2+1)=1;
        end
    end
end


%计算LLR
s2=0;
for k=L:-1:1
    for s1=0:state_num-1
        if gama(k,s1+1,s2+1)==1
            if k<=L-memory_len
                dec_outbits(k)=(state_inbits(s1+1,s2+1)+1)/2;
                LLR(k)=alpha(k+1,s2+1)-alpha(k,s1+1);%计算软输出
            end
            s2=s1;%更新状态
            break;
        end
    end
end