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The Transceiver The transceiver consists of three modules: --- transceiver --- radioInterface --- USRPDevice The USRPDevice module is basically a driver that reads/writes packets to a USRP with two RFX900 daughterboards, board A is the Tx chain and board B is the Rx chain. The radioInterface module is basically an interface b/w the transceiver and the USRP. It operates the basestation clock based upon the sample count of received USRP samples. Packets from the USRP are queued and segmented into GSM bursts that are passed up to the transceiver; bursts from the transceiver are passed down to the USRP. Our current implementation includes a polyphase resampler, since there is no 64e6/N sample rate that nicely matches a multiple of the GSM symbol rate of 1.625e6/6. A better implementation would involve running the USRP off of a 13Mhz clock; then the resampler can be discarded altogether. The transceiver basically operates "layer 0" of the GSM stack, performing the modulation, detection, and demodulation of GSM bursts. It communicates with the GSM stack via three UDP sockets, one socket for data, one for control messages, and one socket to pass clocking information. The transceiver contains a priority queue to sort to-be-transmitted bursts, and a filler table to fill in timeslots that do not have bursts in the priority queue. The transceiver tries to stay ahead of the basestation clock, adapting its latency when underruns are reported by the radioInterface/USRP. Received bursts (from the radioInterface) pass through a simple energy detector, a RACH or midamble correlator, and a DFE-based demodulator. NOTE: There's a SWLOOPBACK #define statement, where the USRP is replaced with a memory buffer. In this mode, data written to the USRP is actually stored in a buffer, and read commands to the USRP simply pull data from this buffer. This was very useful in early testing, and still may be useful in testing basic Transceiver and radioInterface functionality.