Merged r9564:9568 from jcorgan/bert into trunk. Adds BPSK bit error testing examples and support blocks.

git-svn-id: http://gnuradio.org/svn/gnuradio/trunk@9569 221aa14e-8319-0410-a670-987f0aec2ac5

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+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 an M&M 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 mean symbol power,
+the average noise variance, 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.