core/conv: Only consider error for non-zero soft values

If the input value is '0' it should not really affect the error
since it's just an indecisive bit. We accept this either an internal
'0' (generated via puncture) or as an external '0' (generated via an
external puncturing scheme). A real received bit should never be '0',
it's always gonna be closer to 1 or the other value ...

(thanks to mad@auth.se on the ML for the idea)

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>

1 files changed, 6 insertions, 3 deletions

diff --git a/src/conv.c b/src/conv.c
index cf820a33..70bdffba 100644
--- a/src/conv.c
+++ b/src/conv.c
@@ -389,9 +389,12 @@ osmo_conv_decode_finish(struct osmo_conv_decoder *decoder,
 			m = 1 << (code->N - 1);		/* mask for 'out' bit selection */
 
 			for (j=0; j<code->N; j++) {
-				ov = (out & m) ? -127 : 127; /* sbit_t value for it */
-				e = ((int)in_sym[j]) - ov;   /* raw error for this bit */
-				nae += (e * e) >> 9;         /* acc the squared/scaled value */
+				int is = (int)in_sym[j];
+				if (is) {
+					ov = (out & m) ? -127 : 127; /* sbit_t value for it */
+					e = is - ov;                 /* raw error for this bit */
+					nae += (e * e) >> 9;         /* acc the squared/scaled value */
+				}
 				m >>= 1;                     /* next mask bit */
 			}