Add AWGN channel models and route IMDD simulation through them

Functions/Channel_structures/awgn_channel.m

@@ -1,5 +1,43 @@
-function signal_out = awgn_channel(signal_in)
-    
+function signal_out = awgn_channel(signal_in, options)
+%AWGN_CHANNEL Apply additive white Gaussian noise to a signal object.
+
+    arguments
+        signal_in
+        options.snr_dB (1,1) double = 20
+        options.randkey = []
+    end
+
     signal_out = signal_in;
-    
-end
+    x = signal_in.signal;
+
+    signalPower = mean(abs(x).^2, 'all'); %or use signal_in.power
+    if signalPower == 0
+        return
+    end
+
+    snrLinear = 10^(options.snr_dB/10);
+    noisePower = signalPower / snrLinear;
+
+    if isempty(options.randkey)
+        if isreal(x)
+            noise = sqrt(noisePower) * randn(size(x));
+        else
+            noise = sqrt(noisePower / 2) * (randn(size(x)) + 1i * randn(size(x)));
+        end
+    else
+        rs = RandStream('mt19937ar', 'Seed', options.randkey);
+        if isreal(x)
+            noise = sqrt(noisePower) * randn(rs, size(x));
+        else
+            noise = sqrt(noisePower / 2) * (randn(rs, size(x)) + 1i * randn(rs, size(x)));
+        end
+    end
+
+    signal_out.signal = x + noise;
+
+    if isa(signal_out, 'Signal')
+        desc = sprintf('AWGN channel with SNR %.2f dB', options.snr_dB);
+        meta = struct('snr_dB', options.snr_dB, 'randkey', options.randkey);
+        signal_out = signal_out.logbookentry(desc, meta);
+    end
+end