We borrowed a 30-ft Army surplus AS-2236 mast from a consulting client. These run $200-$400. We also bought a 8'-long 9 dBi marine cellular antenna, new, for $100, to put on top of it.
We scored a dB Products DB4090 cavity duplexer on eBay for $60.
We used our existing USRP, RFX900 daughter cards and an Apple MacBook Pro for the digital radio, GSM stack and Asterisk server. (Those are Nokia 3310 and 5110 handsets on top of the USRP.) That USRP equipment cost about $1,500. Running at full load (7 active lines) the multi-threaded basestation consumed about 50% of the Intel dual-core 2.33 GHz MacBook.
The power amp turned out to be a problem. First, we bought a 50 W Vocom 900 MHz PA off of eBay for about $60.
The problem was that the USRP didn't generate a hot enough signal to drive it. When we looked inside this Vocom amp, we found a perfectly good Motorola UHF module, wrapped with a bunch of other stuff we didn't want or need.
So now we were one week from a site test without a serviceable power amp. We had three choices:
A couple of days later, we picked up the 3318PA and hooked it up to the USRP. We put 23 dBm in, tweaked a few trim pots and got 33 dBm out. Our STA allows 39 dBm ERP. So 33 dBm at the PA, plus 9 dBi antenna gain, minus 3 dB of cable and insertion losses and bam! We are right on the money. We'll bring the 3340PA kit just in case we need it, but time is tight so we're moving on to the next hurdle.
Our "technical team" conferred by e-mail and arrived at a four candidate solutions:
Harvind rewired and retuned the cavities for option 1. He got great transmitter suppression, but the USRP was outputting so much receive-band noise on the transmit side that the overall system performance wasn't any better. The good news, though, the cavities had been mistuned the first time around and we picked up an extra 20 dB of isolation by fixing that. Harvind also made some adjustments to the internal gain stages in the USRP that bought us about 10 dB more useful dynamic range. The system was serviceable again and only needed about 10 dB more isolation for our range to be downlink-limited.
Meanwhile, David built an 840 MHz 2-element filter with a hacksaw and hand drill.
It was ugly, leaky and poorly-matched, but thanks to all those GaAs-FET preamps we had gain to spare and it gave us a another 40 dB of isolation. With this filter in place, our range should be thermal-noise limited, although we were still concerned about crosstalk between the PCBs inside the USRP case. [Note: We later discovered that this crosstalk could be greatly reduced through proper gain settings in the RFX cards.]
We needed a power source that could produce an average of 80 W all week. We looked at solar, but then we read about how windy the Playa is most of the time, so we ordered a Southwestern Windpower Air X turbine for about $500 and picked up an RV battery at Sam's for $80. Here they are getting bench-tested.
(Yes, that's an R&S CMD-57 test set and some classic Tektronics mainframes in the corner. The Tek stuff's going to travel, but the R&S box will probably stay home.)
We built up a power distribution board that included fusing, safety disconnects and a shut-brake for the wind turbine, along with regulated 6-volt and 5-volt buses so that we could the run USRP and network equipment without wasting power through inverters and wall-warts. This took about $400 in parts, with most of that going to regulation for the low-voltage buses. The clever part was the shunt brake, which shorts the turbine generator when it is disconnected from the rest of the system. Normally the shunt brake and disconnect are built together in high-current double-pole switch. What we did instead was get a matched pair of pull-out disconnects and then hide one of the pull-out handles. The probably saved us $200 in electrical hardware.
Another key component of the site would be the Multitech GSM/GPRS modem, which will be used for interference surveys, power monitoring, and to read the SIMs of test subscribers for provisioning.