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Two buffers, inner and outer, are used in the transceiver
implementation. The outer buffer interfaces with the device receive
interface to guarantee timestamp aligned and contiguously allocated
sample buffers. The inner buffer absorbs vector size differences between
GSM bursts (156 or 157 samples) and the resampler interface (typically
fixed multiples of 65).
Reimplement the inner buffer with a ring buffer that allows fixed size
segments on the outer (resampler) portion and variable lengths (GSM
side) on the inner side. Compared to the previous stack-like version,
this implementation removes unnecessary copying of buffer contents.
Signed-off-by: Tom Tsou <tom.tsou@ettus.com>
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Setup correlator and detection process similar to the GMSK
receiver chain. Require 4 SPS sampling on both Rx and Tx paths
as 1 SPS sampling adds too much distoration for 8-PSK recovery.
Core receiver operations still run at 1 SPS with the exception
of fractional delay filtering, which runs at the higher rate.
Perform linear equalization to handle the Gaussian pulse
induced ISI. The fixed impulse response used for equalizer tap
calculation consists of combined EDGE pulse shape filter and
effects of the downsampling filter. Note that the non-adaptive
equalizer corrects for modulation induced band limiting and
does not account for or compensate for fading channel effects.
Signed-off-by: Tom Tsou <tom.tsou@ettus.com>
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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>
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