Add AWGN channel models and route IMDD simulation through them

Functions/Channel_structures/awgn_alpha_d_channel.m

@@ -0,0 +1,32 @@
+function signal_out = awgn_alpha_d_channel(signal_in, options)
+%AWGN_ALPHA_D_CHANNEL Apply a 1 + alpha*D FIR channel followed by AWGN.
+
+    arguments
+        signal_in
+        options.alpha (1,1) double = 0
+        options.delay_samples (1,1) double {mustBeInteger, mustBePositive} = 1
+        options.snr_dB (1,1) double = 20
+        options.randkey = []
+    end
+
+    signal_out = signal_in;
+
+    taps = zeros(1, options.delay_samples + 1);
+    taps(1) = 1;
+    taps(end) = options.alpha;
+
+    signal_out.signal = filter(taps, 1, signal_in.signal, [], 1);
+
+    if isa(signal_out, 'Signal')
+        desc = sprintf('1 + alpha*D channel with alpha %.4f and delay %d samples', ...
+            options.alpha, options.delay_samples);
+        meta = struct( ...
+            'alpha', options.alpha, ...
+            'delay_samples', options.delay_samples);
+        signal_out = signal_out.logbookentry(desc, meta);
+    end
+
+    signal_out = awgn_channel(signal_out, ...
+        "snr_dB", options.snr_dB, ...
+        "randkey", options.randkey);
+end