added error

channel.py

@@ -2,12 +2,22 @@ import numpy as np
 
 channel_response = np.array([0, 0, 0, 1, 0, 0, 0])
 
+# 15dB seems to be around the minimum for error-free transmission
+snr_db = 15
+
 def sim(in_data):
     out_data = np.ndarray((len(in_data), len(in_data[0])), dtype=np.csingle)
 
+    # noise stuff is straight copied from the DSP illustrations article
     for i in range(len(in_data)):
         convolved = np.convolve(channel_response, in_data[i], mode='same')
-        out_data[i] = convolved
+
+        signal_power = np.mean(abs(convolved**2))
+        noise_power = signal_power * 10**(-snr_db/10)
+
+        noise = np.sqrt(noise_power / 2) * (np.random.randn(*convolved.shape) + 1j*np.random.randn(*convolved.shape))
+
+        out_data[i] = convolved + noise
 
     return out_data
 

main.py

@@ -7,7 +7,7 @@ Hopefully eventually this modem design makes it onto an fpga.
 TODO:
     Add comments for functions
     Explain what the main function is doing
-    Finish doing the channel simulation stuff (add noise, and verify channel response)
+    Add more errors, like a shifted signal
     Add support for 16-QAM, 64-QAM, etc...
     Add channel estimation via pilot carriers
     Add some sort of payload support, i.e. be able to drop the padding at the end
@@ -48,7 +48,7 @@ if __name__ == '__main__':
 
     bytes = bytearray(data, 'utf8')
 
-    parallel = parallelise(16, bytes)
+    parallel = parallelise(64, bytes)
 
     modulated = qam.modulate(parallel)
 
@@ -66,6 +66,6 @@ if __name__ == '__main__':
 
     to_serialise = qam.demodulate(to_decode)
 
-    data = serialise(16, to_serialise)
+    data = serialise(64, to_serialise)
 
     print(data)