Transceiver52M: Replace convolve and related calls with SSE implementation

This large patch replaced the convolve() call with an SSE vector
enabled version. The lower C and SSE intrinsic based code operates
on fixed and aligned vectors for the filter taps. The storage format
of interleaved I/Q for both complex and real vectors is maintained.

SSE filter tap values must:

  1. Start 16-byte aligned
  2. Number with a multiple of 4 between 4 and 20 for real taps
  3. Number with a multiple of 4 for complex taps

Non-compliant values will fall back to non-SSE usage. Fixed length
iterators mean that head and tail cases may require reallocation of
the input vector, which is automatically handled by the upper C++
interface.

Other calls are affected by these changes and adjusted or rewritten
accordingly. The underlying algorithms, however, are unchanged.

  generateGSMPulse()
  analyzeTrafficBurst()
  detectRACHBurst()

Intel SSE configuration is automatically detected and configured at
build time with Autoconf macros.

Signed-off-by: Thomas Tsou <tom@tsou.cc>
Transceiver52M: Reverse storage of convolution vectors

With convolution changes, we now assume that tap values for
filterting are stored in reverse. Along the same lines,
the tap values used during correlation are no longer stored
in reverse, however, they are still assumed to be conjugated.

Signed-off-by: Thomas Tsou <tom@tsou.cc>
Transceiver52M: Replace resampler with SSE enabled implementation

Replace the polyphase filter and resampler with a separate
implementation using SSE enabled convolution. The USRP2 (including
derived devices N200, N210) are the only supported devices that
require sample rate conversion, so set the default resampling
parameters for the 100 MHz FPGA clock. This changes the previous
resampling ratios.

  270.833 kHz -> 400 kHz      (65 / 96)
  270.833 kHz -> 390.625 kHz  (52 / 75)

The new resampling factor uses a USRP resampling factor of 256
instead of 250. On the device, this allows two halfband filters to
be used rather than one. The end result is reduced distortial and
aliasing effecits from CIC filter rolloff.

B100 and USRP1 will no be supported at 400 ksps with these changes.

Signed-off-by: Thomas Tsou <tom@tsou.cc>
Transceiver52M: Add SSE floating point / integer conversion

Convertions are performed in multiples of 4 or 8. All loads are
considered unaligned.

Signed-off-by: Thomas Tsou <tom@tsou.cc>
Transceiver52M: Disable energy detector

The adaptive energy threshold gating suffers a near-far problem
at certain gain levels. This is due to exponential threshold
raising, but linear decreases. A large signal level followed by
a period low signal level causes (comparatively) weak signals to
go undetected. Additionally, the algorithm performs differently
at multiple RF gain levels.

This patch switches solely to correlation based gating for burst
detection. The main computational load with this approach is
sub-sample width peak interpolation, which we disable for intial
detection and run after threshold passing.

Signed-off-by: Thomas Tsou <tom@tsou.cc>
Transceiver52M: Add 4 samples-per-symbol Laurent pulse shape

When 4 samples-per-symbol operation is selected, replace the
existing pulse approximation, which becomes inaccurate with
non-unit oversampling, with the primary pulse, C0, from the
Laurent linear pulse approximation.

Pierre Laurent, "Exact and Approximate Construction of Digital Phase
  Modulations by Superposition of Amplitude Modulated Pulses", IEEE
  Transactions of Communications, Vol. 34, No. 2, Feb 1986.

Octave pulse generation code for the first three pulses of the
linear approximation are included.

Signed-off-by: Thomas Tsou <tom@tsou.cc>
Transceiver52M: Add UmTRX support

Requires Fairwaves UHD driver.

  https://github.com/chemeris/UHD-Fairwaves.git

Signed-off-by: Thomas Tsou <tom@tsou.cc>
Transceiver52M: Refactor RACH and normal burst detection

Both RACH and normal bursts are detected with the same approach of
midamble correlation combined with peak-to-average ratio. The
difference is the midamble placements and lengths. Thus, there is
no reason to have independent implementations.

This patch creates a common call burstDetect(), while leaving the
correlation window indexing in the original calls.

Signed-off-by: Thomas Tsou <tom@tsou.cc>
Transceiver52M: Increase normal and RACH correlation thresholds

Because we can. New threshold values obtained by from bench testing
with generated noise and modulated PN sequences. Detection still
achievable near noise floor.

Signal source: Rohde & Schwarz SMIQ

Signed-off-by: Thomas Tsou <tom@tsou.cc>
Transceiver52M: Check that sample rates are sane before using

If there is an error in the sample rate determination, noted
by a negative return sample rate value, error directly and
don't try to set the device rate.

Signed-off-by: Thomas Tsou <tom@tsou.cc>
Transceiver52M: Setup dual Laurent pulse shaping filter

Provides substantially improved transmit phase error
performance when enabled. Requires use of 4 samples
per symbol, and is enabled by default when set.

Signed-off-by: Thomas Tsou <tom@tsou.cc>
Transceiver52M: Set 4 samples-per-symbol as default

Aside from embedded systems (e.g. ARM), CPU load for OpenBTS
on the majority of user systems is minimal. Set 4 SPS using
dual pulse Laurent GMSK approximation as the new default.
This allows operation with phase error below 1 degree RMS on
capable RF devices.

Disable support at 2 sps since the phase error improvements
are marginal compared to the dual pulse 4 sps configuration
without the low compuatational benefits of 1 sps.

Signed-off-by: Thomas Tsou <tom@tsou.cc>
Transceiver52M: Add B200 support

Set master clock rate to 52 MHz for B200. Also, we want to avoid
floating point comparison errors on clock rate settings, but we
expect to be able really set the rates we specify. Set the
offset limit to 1 Hz. If we can't set our rates with that level
of precision, then something is wrong.

Signed-off-by: Thomas Tsou <tom@tsou.cc>
Transceiver52M: Widen resampling filter bandwidth

This patch only applies to resampling use at 4 samples-per-symbol.
By extention that means only USRP2 / N2xx devices are affected.
By widening the resampling filter we achieve slightly better total
phase distortion. At 4 samples-per-symbol this gives us below
0.7 degrees of RMS phase error on USRP2 against Agilent E4406A
signal analyzer.

Signed-off-by: Thomas Tsou <tom@tsou.cc>
Transceiver52M: Reduce RACH and TSC correlation windows

Start the correlation search window at 4 symbols before
the expected correlation peak. End the search at 10
symbols and 4 + maximum expected delay for RACH and TSC
bursts respectively.

This change lowers receive side cpu utilization while
maintaining reasonable timing jitter and accuracy tolerance.

Signed-off-by: Thomas Tsou <tom@tsou.cc>
Transceiver52M: Disable equalization

Unsupported at 4 sps, and performance benefits remain
to be proven at 1 sps. Disable until further testing.

Signed-off-by: Thomas Tsou <tom@tsou.cc>
Transceiver52M: Downsample receive path to 1 sample-per-symbol

We don't need to run the full 4 sps receive signal through the
GSM receiver. In fact, we're better off bandlimiting the signal
(because of noise) and downsampling (for efficiency). Run both
through the quarter rate polyphase resampler.

In the absence of interference (external or internal generated)
this allows decoding below -100 dBm on most devices with a
signal generator reference signal.

Signed-off-by: Thomas Tsou <tom@tsou.cc>
Transceiver52M: Update device offset timings

Update empirically determined device Tx/Rx offset settings
for new modulator configuration. These offsets are the
combinations of filter group delays and other device lag
times that occur before and after the sample timing point
in the FPGA.

Signed-off-by: Thomas Tsou <tom@tsou.cc>