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learn_06_qpsk.asv
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% 研究QPSK调制与解调
Fs = 1000; % 采样率1000Hz
fs = 40; % 信号频率40Hz
t1 = 0; t2 = 0.4; % 仿真时间0-0.2s
baud = 20; % 波特率20bps
codes = [0 1 0 1 0 0 1 0 1]; % 原始信息
t = t1:1/Fs:t2-1/Fs;
baud_iq = baud/2;
i_codes = zeros(1, ceil(length(codes)/2));
q_codes = zeros(1, ceil(length(codes)/2));
bin_signal = zeros(1, length(t));
i_signal = zeros(1, length(t));
q_signal = zeros(1, length(t));
% 生成数字信号序列（仅作展示）
for i = 1:length(t)
bin_signal(i) = codes( floor(t(i)*baud)+1 );
end
% 生成IQ序列
for i = 1:2:length(codes)
% 如果不足1组，补零
if (i>length(codes)-1)
codes(length(codes)+1) = 0;
end
% 硬编码实现格雷码
if ( codes(i)==0 && codes(i+1)==0 )
i_codes(ceil(i/2)) = 1;
q_codes(ceil(i/2)) = 1;
else
if ( codes(i)==0 && codes(i+1)==1 )
i_codes(ceil(i/2)) = -1;
q_codes(ceil(i/2)) = 1;
else
if ( codes(i)==1 && codes(i+1)==1 )
i_codes(ceil(i/2)) = -1;
q_codes(ceil(i/2)) = -1;
else
i_codes(ceil(i/2)) = 1;
q_codes(ceil(i/2)) = -1;
end
end
end
end
% 生成IQ信号序列
for i = 1:length(t)
i_signal(i) = i_codes( floor(t(i).*baud_iq)+1 );
q_signal(i) = q_codes( floor(t(i).*baud_iq)+1 );
end
% QPSK调制
c1 = cos( 2*pi*fs*t );
c2 = sin( 2*pi*fs*t );
t1 = i_signal.*c1;
t2 = q_signal.*c2;
ta = 1/sqrt(2)*(t1-t2); % 天线发送，假设理想传输
% QPSK解调
% 同步解调
r1 = c1.*ta;
r2 = -c2.*ta;
% 积分
sum_i = 0;
sum_q = 0;
ri = zeros(1, length(t)); % i,q通道积分数据
rq = zeros(1, length(t));
for i = 1:length(t)
sum_i = sum_i + r1(i);
sum_q = sum_q + r2(i);
ri(i) = sum_i;
rq(i) = sum_q;
if ( mod(i,Fs/baud_iq)==0 ) % 接收每个码元后归零
sum_i = 0;
sum_q = 0;
end
end
% 模拟判决（反面例子）
% 注意！这里直接通过积分值判决，但实际情况不能这么用，
% 因为如果信道不理想，两个通道值可能不能同时跳变，形成毛刺，
% 实际情况中应该使用统一的时钟来抽样判决。
% last_i = 0;
% last_q = 0;
% ji = zeros(1, length(t));
% jq = zeros(1, length(t));
% for i = 1:length(t)
% ji(i) = last_i;
% jq(i) = last_q;
% if ( ri(i)>10 )
% last_i = 1;
% else
% if ( ri(i)<-10 )
% last_i = 0;
% end
% end
%
% if ( rq(i)>10 )
% last_q = 1;
% else
% if ( rq(i)<-10 )
% last_q = 0;
% end
% end
% end
% 模拟IQ通道抽样时钟
decoded = zeros(1,length(codes));
p_decoded = 1;
for i=1:length(t)
% 码元中点
if ( mod(i,Fs/baud_iq)==Fs/baud_iq/2 )
% 判决，解格雷码，输出结果
if( ri(i)>0 && rq(i)>0 )
decoded(p_decoded) = 0;
decoded(p_decoded+1) = 0;
else
if( ri(i)<0 && rq(i)>0 )
decoded(p_decoded) = 0;
decoded(p_decoded+1) = 1;
else
if( ri(i)<0 && rq(i)<0 )
decoded(p_decoded) = 1;
decoded(p_decoded+1) = 1;
else
decoded(p_decoded) = 1;
decoded(p_decoded+1) = 0;
end
end
end
p_decoded = p_decoded + 2;
end
end
% 结果，IQ格雷码解码
for i=1:length(t)
if ( ji(i)==1 && jq(i)==1 )
% 绘图
figure(1),
subplot(8,2,1),plot(bin_signal),title('原始信号');
subplot(8,2,3),plot(i_signal),title('I路信号');
subplot(8,2,4),plot(q_signal),title('Q路信号');
subplot(8,2,5),plot(ta),title('载波');
subplot(8,2,7),plot(r1),title('解调I');
subplot(8,2,8),plot(r2),title('解调Q');
subplot(8,2,9),plot(ri),title('积分I');
subplot(8,2,10),plot(rq),title('积分Q');
subplot(8,2,11),plot(ji),title('判决I');
subplot(8,2,12),plot(jq),title('判决Q');