On Monday (day 3), we had made our first coverage estimates, sending SMS to ourselves or to short codes that returned status information. We did this a few more times over the course of the week. We had hoped to use RRLP to automate the coverage estimation process, but so few phones were returning useful RRPL results that we had to do it by hand, with a bicycle, a test phone and a note pad. This is a rough estimate of our -90 dBm coverage within BRC:
"A", "B" and "C" are our three sectors, arranged to maximize coverage over the city and insure coverage at the front gate, the airport and the greeters' station. The "x" at 4:30 marks our camp and tower. The "0" at 3:00 marks Fusion Valley, the first link in our backhaul chain. These coverage estimates match those predicted by the Hata suburban model, the same propagation characteristic observed in BRC in our 2008 test.
Despite the failure of RRLP to produce data, we did get some automatic coverage information from measurements of the timing errors of RACH bursts arriving at each BTS. In a GSM system, the timing error of an arriving RACH burst relative to the signal framing yields a rough estimate of the round-trip propagation time from the BTS to the handset and back. We logged every one of the 2,018,321 validated RACH bursts we received and finally got around to sorting through some of that data late last week.
Analysis of RACH burst timing indicated that most of the arriving RACH bursts came from a distance of about 1 km (mode of 1 km and mean of 1.3 km) and nearly all of them (>99%) came from a distance of 3 km or less. That makes sense given our coverage, our tower location, the geometry of the city and the fact that BRC accounts for >90% of the area's population during the festival.
Outside BRC, though, we predicted that coverage would follow the Hata open rural model, meaning that once a handset got out of the shadow of all of the RVs, shipping containers and metal-framed domes, our range could be 15 km or more. There is some evidence of that in our logs. A small fraction of RACH (~0.5%) bursts arrived on sector A from clustered distances of 11 km and 15 km. Gerlach, NV was 15 km away in the direction of sector A, has a population of about 500 and could account for the cluster of access attempts at that distance. We still don't have a good explanation for the cluster of access attempts at 11 km.