From 8c2e6ed4333a9918038ba25a282e6d2009f6338c Mon Sep 17 00:00:00 2001
From: pabr <pabr@pabr.org>
Date: Thu, 13 Jul 2017 11:45:58 +0200
Subject: [PATCH] cstnl_receiver: Handle overflows to improve timing recovery
 at low SNR

---
 src/leansdr/sdr.h | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/src/leansdr/sdr.h b/src/leansdr/sdr.h
index 2f0486c..8a11f93 100644
--- a/src/leansdr/sdr.h
+++ b/src/leansdr/sdr.h
@@ -399,7 +399,20 @@ namespace leansdr {
       s_angle phase_error;
     };
     inline result *lookup(float I, float Q) {
-      // float-to-s8 saturates on some platorms, which is good.
+      // Handling of overflows beyond the lookup table:
+      // - For BPSK/QPSK/8PSK we only care about the phase,
+      //   so the following is fine.
+      // - For amplitude modulations this is not appropriate.
+      //   However, if there is enough noise to cause overflow,
+      //   demodulation would probably fail anyway.
+      //
+      // Comment-out for better throughput at high SNR.
+#if 1
+      while ( I<-128 || I>127 || Q<-128 || Q>127 ) {
+	I *= 0.5;
+	Q *= 0.5;
+      }
+#endif
       return &lut[(u8)(s8)I][(u8)(s8)Q];
     }
     inline result *lookup(int I, int Q) {