From becd9cb96d1e3a17b802796092c6f86ac4e06f98 Mon Sep 17 00:00:00 2001
From: appellet <antoine.pelletier@epfl.ch>
Date: Tue, 20 May 2025 19:52:36 +0200
Subject: [PATCH] fix: improve quality

---
 receiver.py | 59 +++++++++++++++--------------------------------------
 1 file changed, 16 insertions(+), 43 deletions(-)

diff --git a/receiver.py b/receiver.py
index c8e048e..fe90cce 100644
--- a/receiver.py
+++ b/receiver.py
@@ -1,69 +1,42 @@
 import numpy as np
 
-
 def bits_to_text(bits, alphabet):
-    """Convert binary bits to text using the alphabet."""
     char_to_idx = {char: idx for idx, char in enumerate(alphabet)}
     idx_to_char = {idx: char for char, idx in char_to_idx.items()}
     text = ''
     for i in range(0, len(bits), 6):
-        # Convert 6 bits to an integer
         idx = int(''.join(map(str, bits[i:i + 6])), 2)
-        text += idx_to_char.get(idx, '?')  # Default to '?' if index invalid
+        text += idx_to_char.get(idx, '?')
     return text
 
-
 def receiver(Y):
-    """
-    Decode the received signal Y to a 40-character text message.
-
-    Parameters:
-    Y (np.ndarray): Received signal of length 480.
-
-    Returns:
-    str: Decoded 40-character text message.
-    """
-    # Define constants
-    G = 10  # Power gain
-    sigma2 = 10  # Noise variance
+    G = 10  # Match channel.py
+    sigma2 = 10
     alphabet = (
         'abcdefghijklmnopqrstuvwxyz'
         'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
         '0123456789 .'
     )
-
-    # Input validation
     if Y.size != 480:
         raise ValueError("Received signal must have length 480.")
-
     n = 480
     bits = np.zeros(240, dtype=int)
-    E = 4  # Energy per bit, must match transmitter
+    E = 4
+
+    # Estimate channel state
+    even_energy = np.mean(np.abs(Y[::2]))
+    odd_energy = np.mean(np.abs(Y[1::2]))
+    # If even indices have higher energy, likely s=1; else s=2
+    s_hat = 1 if even_energy > odd_energy else 2
 
-    # Process each bit (sent at indices 2i and 2i+1)
     for i in range(240):
-        y_odd = Y[2 * i]  # Even index in Y (0-based)
-        y_even = Y[2 * i + 1]  # Odd index in Y
-
-        # LLR for channel 1: odd indices have gain sqrt(G), even have gain 1
-        # H0: bit = 0 (sent +sqrt(E)), H1: bit = 1 (sent -sqrt(E))
-        llr_ch1 = (
-                (y_odd * np.sqrt(E) * np.sqrt(G) / sigma2) +  # Odd index term
-                (y_even * np.sqrt(E) / sigma2)  # Even index term
-        )
-
-        # LLR for channel 2: even indices have gain sqrt(G), odd have gain 1
-        llr_ch2 = (
-                (y_odd * np.sqrt(E) / sigma2) +  # Odd index term
-                (y_even * np.sqrt(E) * np.sqrt(G) / sigma2)  # Even index term
-        )
-
-        # Combine LLRs (assuming equal prior probabilities for both channels)
-        llr = 0.5 * (llr_ch1 + llr_ch2)
-
-        # Decode bit: LLR > 0 implies bit = 0, LLR < 0 implies bit = 1
+        y_even = Y[2 * i]
+        y_odd = Y[2 * i + 1]
+        if s_hat == 1:
+            llr = (y_even * np.sqrt(E) * np.sqrt(G) + y_odd * np.sqrt(E)) / sigma2
+        else:
+            llr = (y_even * np.sqrt(E) + y_odd * np.sqrt(E) * np.sqrt(G)) / sigma2
         bits[i] = 1 if llr < 0 else 0
 
-    # Convert bits to text
     text = bits_to_text(bits, alphabet)
     return text
\ No newline at end of file