function[p]=cm_sm34(snr_in_dB)
%[p]=cm_sm34(snr_in_dB)
%     CM_SM34求出以dB為單位的給定信噪比的比特誤碼率和符號(hào)誤碼率
N=5000;
E=1;                      %每符號(hào)能量
snr=10^(snr_in_dB/10);    %信噪比
sgma=sqrt(E/(4*snr));     %噪聲方差
%隨后為數(shù)據(jù)源的產(chǎn)生
for i=1:2*N,
    temp=rand;            %在區(qū)間(0,1)間的一個(gè)均勻隨機(jī)變量
    if (temp<0.5),
       dsource(i)=0;
    else
       dsource(i)=1; 
    end;
end;
%隨后為數(shù)據(jù)源的差分編碼
mapping=[0 1 3 2];
M=4;
[diff_enc_output]=cm_dpske(E,M,mapping,dsource);
%接收信號(hào)
for i=1:N
   [n(1) n(2)]=gngauss(sgma);
   r(i,:)=diff_enc_output(i,:)+n;
end;
%判決、誤碼率的計(jì)算
numoferr=0;
prev_theta=0;
for i=1:N,
  theta=angle(r(i,1)+j*r(i,2));
  delta_theta=mod(theta-prev_theta,2*pi);
  if ((delta_theta<pi/4) | (delta_theta>7*pi/4)),
    deris=[0 0];
  elseif (delta_theta<3*pi/4),
    deris=[0 1];
  elseif (delta_theta<5*pi/4),
    deris=[1 1];
  else
    deris=[1 0];
  end;
  prev_theta=theta;
  %如果判決不對(duì)，計(jì)錯(cuò)器加一
  if ((deris(1)~=dsource(2*i-1)) | (deris(2)~=dsource(2*i))),
     numoferr=numoferr+1;
  end;
end;
p=numoferr/N;