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diff --git a/gr-sounder/README b/gr-sounder/README index ef495f1e8..3c5df21d4 100644 --- a/gr-sounder/README +++ b/gr-sounder/README @@ -1,10 +1,9 @@ This is a work-in-progress implementation of a m-sequence based channel sounder for GNU Radio and the USRP. -At present, only the transmitter is implemented. When completed, you -will be able to place the sounder transmitter at one location, the -sounder receiver at another location, then determine in real-time the -channel impulse and frequency response between them. +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; @@ -17,45 +16,76 @@ 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 (partially implemented at this time but not working yet) 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 +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 and receiver are designed to work only with the board in -side A. This may be a standalone LFTX/LFRX or an RFX daughterboard. +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] +Usage: usrp_sounder.py [options] -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 - -d DEGREE, --degree=DEGREE - set souding sequence degree (len=2^degree-1), default - is 16 - -n SAMPLES, --samples=SAMPLES - number of samples to capture on receive, default is - infinite -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 -You can vary the m-sequence degree between 1 and 16, which will create -sequence lengths between 1 (DC) and 65535 (1.023 us). This will affect -how frequently the receiver can calculate impulse response vectors (to be -documented as the receiver is completed.) +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 -4/25/07 +5/28/07 |