Adjust trellis to work in smaller send chunks

Smaller send chunks can cause the trellis to not go through all the
depth. Causing the bits to not be decoded and ending in errors.

Author: eSoares

commpy/channelcoding/convcode.py

@@ -695,12 +695,13 @@ def viterbi_decode(coded_bits, trellis, tb_depth=None, decoding_type='hard'):
     rate = k/n
     total_memory = trellis.total_memory
 
-    if tb_depth is None:
-        tb_depth = 5*total_memory
-
     # Number of message bits after decoding
     L = int(len(coded_bits)*rate)
 
+    if tb_depth is None:
+        tb_depth = min(5 * total_memory, L)
+
+
     path_metrics = np.full((trellis.number_states, 2), np.inf)
     path_metrics[0][0] = 0
     paths = np.empty((trellis.number_states, tb_depth), 'int')