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