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.
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