正交相位编码MATLAB程序

下面是一个简单的正交相位编码（QPSK）的MATLAB程序：

% 设置参数
N = 1000; % 生成的符号数
fc = 100; % 载波频率
fs = 1000; % 采样频率
T = 1/fs; % 采样周期
SNR = 10; % 信噪比

% 生成随机符号序列
symbols = randi([0 3],1,N);

% 进行映射
I = zeros(1,N);
Q = zeros(1,N);
for i = 1:N
    if symbols(i) == 0
        I(i) = 1;
        Q(i) = 1;
    elseif symbols(i) == 1
        I(i) = -1;
        Q(i) = 1;
    elseif symbols(i) == 2
        I(i) = -1;
        Q(i) = -1;
    else
        I(i) = 1;
        Q(i) = -1;
    end
end

% 生成基带信号
t = (0:N-1)*T;
baseband = I.*cos(2*pi*fc*t) - Q.*sin(2*pi*fc*t);

% 添加高斯白噪声
power = mean(abs(baseband).^2);
noise_power = power/(10^(SNR/10));
noise = sqrt(noise_power)*randn(1,N);
received = baseband + noise;

% 解调
demod_I = received .* cos(2*pi*fc*t);
demod_Q = -received .* sin(2*pi*fc*t);

% 低通滤波
[b,a] = butter(4,2*fc/fs);
filtered_I = filter(b,a,demod_I);
filtered_Q = filter(b,a,demod_Q);

% 判决
demod_symbols = zeros(1,N);
for i = 1:N
    if filtered_I(i) > 0 && filtered_Q(i) > 0
        demod_symbols(i) = 0;
    elseif filtered_I(i) < 0 && filtered_Q(i) > 0
        demod_symbols(i) = 1;
    elseif filtered_I(i) < 0 && filtered_Q(i) < 0
        demod_symbols(i) = 2;
    else
        demod_symbols(i) = 3;
    end
end

% 计算误码率
errors = sum(demod_symbols ~= symbols);
BER = errors/N;

% 绘制信号图像
subplot(2,1,1);
plot(t,I,'b',t,Q,'r');
xlabel('时间 (s)');
ylabel('幅度');
title('调制信号');
legend('I路','Q路');

subplot(2,1,2);
plot(t,received);
xlabel('时间 (s)');
ylabel('幅度');
title(['接收信号（SNR = ',num2str(SNR),' dB）']);

% 显示误码率
disp(['误码率：',num2str(BER)]);

在这个程序中，我们生成了一个长度为N的随机符号序列，然后进行了QPSK调制，把符号映射到I/Q空间中。然后，我们生成了一个基带信号，并添加了高斯白噪声。接着，我们解调了接收信号，并通过低通滤波得到了I/Q分量。最后，我们进行了判决，计算了误码率，并绘制了信号的图像。