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diff --git a/gr-digital/examples/README b/gr-digital/examples/README index 904a64895..1c50ad69b 100644 --- a/gr-digital/examples/README +++ b/gr-digital/examples/README @@ -82,3 +82,72 @@ useful as a diagnostic tool when experimenting with line coding or whitening algorithms. + +********************************************************************** +********************************************************************** + + +BERT testing example scripts + +benchmark_tx.py + +This sets up a BPSK transmitter that is modulated with a pseudorandom +sequence of bits. The PN code is generated by sending an all 1s +sequence through a 7-bit scrambler. The transmitter performs the BPSK +modulation, then passes the complex baseband waveform through a +root-raised-cosine filter and onto the USRP. + +The --sps parameter controls how many baseband samples per symbol +are created and passed through the RRC filter, prior to going to the +USRP over the USB for interpolation to the final DAC rate. + +The baseband bit rate is controlled by -r or --rate. This value, when +multiplied by the --sps parameter, must result in valid interpolation +rate for the USRP. For example, if the baseband rate is 250k bits/sec, +and the samples per symbol is 4, then the final rate is 1M samples/sec, +which results in an interpolation rate of 128. The valid interpolation +rates for the USRP are multiples of 4 between 16 and 512. + +Finally, the RRC excess bandwidth may be specified by --excess-bw. +(See ./benchmark_tx.py -h for additional parameters.) + + +benchmark_rx.py + +This sets up a BPSK receiver to demodulate the received waveform. It +accepts a similar set of parameters as the transmitter, except that one +specifies the USRP decimation rate desired. The resulting sample stream +rate must be an integral number of baseband symbols. For example, the +parameters corresponding to the above transmitter would be to use a +decimation rate of 8 (32 sps), 16 (16 sps), 32 (8 sps), 64, (4 sps), or +128 (2 sps). The lower the USRP decimation, the more CPU is required to +demodulate the signal, so not all valid decimation rates will work. + +The baseband signal from the USRP is first passed through an AGC to +establish an average power of 1.0. It is then passed through a matched +filter (another RRC), a Costas phase-locked loop, and a Mueller and +Muller bit timing recovery loop. The resulting constellation has an SNR +estimation probe attached, and is then sliced into a bit stream. + +The recovered bits are then passed through a 7-bit descrambler. If +there are no channel errors, the all 1s sequence is recovered. In the +event of a channel error, there will be a 0 in the bit stream for each +feedback tap in the descrambler. In this case, the CCSDS descrambler is +using 3 feedback taps. + +Finally, the signal is passed into a bit density measurement probe. The +channel BER is measured by dividing the 0s density by three. This +measurement is inaccurate at high BER rates (>10%) as the error 0s +begin to overlap. + +The benchmark script will, once per second, output the Costas loop +frequency offset, the recovered timing error, the estimated SNR, and the +average BER. + +NOTE: The particular SNR estimator used is inaccurate below about 7dB, +and will report erroneously high values even for random noise. + +There are a variety of Costas and M&M loop parameters one can adjust. +See ./benchmark_rx.py -h for the full set. + + |