ACS: Use weighted average for 2.4 GHz adjacent channel interference

The interference factors for adjacent 2.4 GHz channels were summed
together without doing any kind of weighted average on them. This
resulted in the channels at the band edges getting undue preference due
to only including interference factors from three channels vs. five for
the channels in the middle of the band.

While it is somewhat unclear whether the design here was supposed to
count overlapping channels together in this way or whether that is
already covered in channel survey results, it is clear that this summing
of three to five values together and then comparing the sum rather than
average of some kind gives too much preference to the channels at the
edges of the band by assuming that there is no interference whatsoever
outside the band.

Use weighted average of the interference factors rather than a sum from
different number of values. For now, the adjacent 2.4 GHz channels get
weight of 0.85 (1.0 for the main channel itself) and the neighboring
channels to those adjacent ones get 0.55 weight. Band-edge channels are
handled in a way that takes average over the channels that were actually
considered instead of assuming zero interference from neighboring bands.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>

src/ap/acs.c

@@ -517,6 +517,14 @@ static struct hostapd_channel_data *acs_find_chan(struct hostapd_iface *iface,
 }
 
 
+#ifndef ACS_ADJ_WEIGHT
+#define ACS_ADJ_WEIGHT 0.85
+#endif /* ACS_ADJ_WEIGHT */
+
+#ifndef ACS_NEXT_ADJ_WEIGHT
+#define ACS_NEXT_ADJ_WEIGHT 0.55
+#endif /* ACS_NEXT_ADJ_WEIGHT */
+
 /*
  * At this point it's assumed chan->interface_factor has been computed.
  * This function should be reusable regardless of interference computation
@@ -557,6 +565,8 @@ acs_find_ideal_chan(struct hostapd_iface *iface)
 		   -1);
 
 	for (i = 0; i < iface->current_mode->num_channels; i++) {
+		double total_weight;
+
 		chan = &iface->current_mode->channels[i];
 
 		if (chan->flag & HOSTAPD_CHAN_DISABLED)
@@ -588,14 +598,17 @@ acs_find_ideal_chan(struct hostapd_iface *iface)
 		factor = 0;
 		if (acs_usable_chan(chan))
 			factor = chan->interference_factor;
+		total_weight = 1;
 
 		for (j = 1; j < n_chans; j++) {
 			adj_chan = acs_find_chan(iface, chan->freq + (j * 20));
 			if (!adj_chan)
 				break;
 
-			if (acs_usable_chan(adj_chan))
+			if (acs_usable_chan(adj_chan)) {
 				factor += adj_chan->interference_factor;
+				total_weight += 1;
+			}
 		}
 
 		if (j != n_chans) {
@@ -609,31 +622,42 @@ acs_find_ideal_chan(struct hostapd_iface *iface)
 		if (iface->current_mode->mode == HOSTAPD_MODE_IEEE80211B ||
 		    iface->current_mode->mode == HOSTAPD_MODE_IEEE80211G) {
 			for (j = 0; j < n_chans; j++) {
-				/* TODO: perhaps a multiplier should be used
-				 * here? */
-
 				adj_chan = acs_find_chan(iface, chan->freq +
 							 (j * 20) - 5);
-				if (adj_chan && acs_usable_chan(adj_chan))
-					factor += adj_chan->interference_factor;
+				if (adj_chan && acs_usable_chan(adj_chan)) {
+					factor += ACS_ADJ_WEIGHT *
+						adj_chan->interference_factor;
+					total_weight += ACS_ADJ_WEIGHT;
+				}
 
 				adj_chan = acs_find_chan(iface, chan->freq +
 							 (j * 20) - 10);
-				if (adj_chan && acs_usable_chan(adj_chan))
-					factor += adj_chan->interference_factor;
+				if (adj_chan && acs_usable_chan(adj_chan)) {
+					factor += ACS_NEXT_ADJ_WEIGHT *
+						adj_chan->interference_factor;
+					total_weight += ACS_NEXT_ADJ_WEIGHT;
+				}
 
 				adj_chan = acs_find_chan(iface, chan->freq +
 							 (j * 20) + 5);
-				if (adj_chan && acs_usable_chan(adj_chan))
-					factor += adj_chan->interference_factor;
+				if (adj_chan && acs_usable_chan(adj_chan)) {
+					factor += ACS_ADJ_WEIGHT *
+						adj_chan->interference_factor;
+					total_weight += ACS_ADJ_WEIGHT;
+				}
 
 				adj_chan = acs_find_chan(iface, chan->freq +
 							 (j * 20) + 10);
-				if (adj_chan && acs_usable_chan(adj_chan))
-					factor += adj_chan->interference_factor;
+				if (adj_chan && acs_usable_chan(adj_chan)) {
+					factor += ACS_NEXT_ADJ_WEIGHT *
+						adj_chan->interference_factor;
+					total_weight += ACS_NEXT_ADJ_WEIGHT;
+				}
 			}
 		}
 
+		factor /= total_weight;
+
 		wpa_printf(MSG_DEBUG, "ACS:  * channel %d: total interference = %Lg",
 			   chan->chan, factor);