Hello Fan:
USRPs (or any other general-purpose SDRs) do not do the timing recovery for
you. It is designed to transmit arbitrary signals, so it simply does not have
any idea about the "timing" of your signals. The mismatch in the internal
oscillator clocks will cause phase drift over time.
You can try using external clock inputs (PPS and 10 MHz inputs) of the USRPs to
forcefully synchronize them. This is often not practical in real-world wireless
systems, though. It is great for some (albeit limited) applications and for
debugging jobs.
Something else that you can do is implementing your own synchronization
algorithms. You can either oversample your signals and then use signal
processing frameworks (like GNU Radio) to do the recovery, or implement them on
the FPGA of the B210. Both will give you the same performance, assuming that
your USB port can handle the oversampled signals and the FPGA is sufficiently
large for your purposes.
Regards,
Kyeong Su Shin
________________________________
보낸 사람: fan <fyt201...@163.com>
보낸 날짜: 2022년 7월 14일 목요일 오후 3:12
받는 사람: usrp-users@lists.ettus.com <usrp-users@lists.ettus.com>
제목: [USRP-users] Is it possible to control the sampling position of the
baseband signal on the rx side of the usrp?
Hi，everyone. I have the question: Is it possible to control the
sampling position of the baseband signal on the rx side of the usrp?
The situation is as follows: I use a usrp b210 to transmit a set of
16QAM signals, and receive this signal on the rx side of the same device, and
repeat this several times. However, under the condition that the transmitted
signal, tx gain, rx gain, channel frequency and other settings are unchanged,
there are some differences in amplitude and phase between the received data
each time. I never moved the position of b210, so the channel should be
time-invariant.
I think there are two possible reasons: First, even if the
transmitted signal and transmit settings are the same each time, the signal
actually transmitted by usrp each time is still different; second, the receiver
does not sample at the optimal sampling point , but randomly samples.
Firstly，let's discuss the second possibility. As far as I know,
USRP's dsp module downconverts the received bandpass signal to baseband and
then samples the baseband signal. a gardner loop is usually used to get the
best sampling point from the sampled data (timing recovery), but the
implementation of the gardner rings in 2x2 mimo with 16QAM is more difficult.
Is there a UHD interface that can control the position of the sample points?
Or, the real reason is the first？
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