--- /dev/null
+This is a work-in-progress implementation of a m-sequence based channel
+sounder for GNU Radio and the USRP.
+
+In typical use, the user would run the sounder as a transmitter on one
+USRP, and a receiver on another at a different location. The receiver
+will determine the impulse response of the RF channel in between.
+
+The sounder uses a custom FPGA bitstream that is able to generate and
+receive a sounder waveform across a full 32 MHz wide swath of RF spectrum;
+the waveform generation and impulse response processing occur in logic in
+the USRP FPGA and not in the host PC. This avoids the USB throughput
+bottleneck entirely. Unfortunately, there is still roll-off in the AD9862
+digital up-converter interpolation filter that impacts the outer 20% of
+bandwidth, but this can be compensated for by measuring and subtracting
+out this response during calibration.
+
+The sounder is based on sending a maximal-length PN code modulated as BPSK
+with the supplied center frequency, with a chip-rate of 32 MHz. The
+receiver correlates the received signal across all phases of the PN code
+and outputs an impulse response vector. As auto-correlation of an m-sequence
+is near zero for any relative phase shift, the actual measured energy at a
+particular phase shift is related to the impulse response for that time delay.
+This is the same principle used in spread-spectrum RAKE receivers such as are
+used with GPS and CDMA.
+
+The transmitter is designed to work only with the board in side A. The
+receiver may be in side A or side B. The boards may be standalone LFTX/LFRXs
+or RFX daughterboards.
+
+To use, the following script is installed into $prefix/bin:
+
+Usage: usrp_sounder.py [options]
+
+Options:
+ -h, --help show this help message and exit
+ -R RX_SUBDEV_SPEC, --rx-subdev-spec=RX_SUBDEV_SPEC
+ select USRP Rx side A or B
+ -f FREQ, --frequency=FREQ
+ set frequency to FREQ in Hz, default is 0.0
+ -d DEGREE, --degree=DEGREE
+ set sounding sequence degree (2-12), default is 12,
+ -t, --transmit enable sounding transmitter
+ -r, --receive enable sounding receiver
+ -l, --loopback enable digital loopback, default is disabled
+ -v, --verbose enable verbose output, default is disabled
+ -D, --debug enable debugging output, default is disabled
+ -F FILENAME, --filename=FILENAME
+ log received impulse responses to file
+
+To use with an LFTX board, set the center frequency to 16M:
+
+$ usrp_sounder.py -f 16M -t
+
+The sounder receiver command line is:
+
+$ usrp_sounder.py -f 16M -r -F output.dat
+
+You can vary the m-sequence degree between 2 and 12, which will create
+sequence lengths between 3 and 4095 (128 us). This will affect
+how frequently the receiver can calculate impulse response vectors.
+
+The correlator uses an O(N^2) algorithm, by using an entire PN period
+of the received signal to correlate at each lag value. Thus, using a
+degree 12 PN code of length 4095, it takes 4095*4095/32e6 seconds to
+calculate a single impulse response vector, about a half a second. One
+can reduce this time by a factor of 4 for each decrement in PN code
+degree, but this also reduces the inherent processing gain by 6 dB as
+well.
+
+The impulse response vectors are written to a file in complex float
+format, and consist of the actual impulse response with a noise floor
+dependent on the PN code degree in use.
+
+There is a loopback test mode that causes the sounding waveform to be
+routed back to the receiver inside the USRP:
+
+$ usrp_sounder.py -r -t -l -F output.dat
+
+The resulting impulse response will be a spike followed by a near zero
+value for the rest of the period.
+
+Synchronization at the receiver is not yet implemented, so the actual
+impulse response may be time shifted an arbitrary value within the the
+impulse response vector. If one assumes the first to arrive signal is
+the strongest, then one can circularly rotate the vector until the peak
+is at time zero.
+
+Johnathan Corgan
+Corgan Enterprises LLC
+jcorgan@corganenterprises.com
+5/28/07
projects / debian / gnuradio / blobdiff