Strange Received Signal for 2x1 MISO OFDM (sometimes!)

Hi,

I am trying to transmit 2x1 MISO OFDM. The two transmit USRPs are
connected by MIMO cable, and the USRP sink section of the .py file is:

    self.uhd_usrp_sink_0_0_0 = uhd.usrp_sink(
        device_addr="addr0=192.168.10.2,addr1=192.168.10.7",
        stream_args=uhd.stream_args(
            cpu_format="fc32",
            otw_format="sc16",
            channels=range(2),
        ),
    )
    self.uhd_usrp_sink_0_0_0.set_clock_source("mimo", 1)
    self.uhd_usrp_sink_0_0_0.set_time_source("mimo", 1)
    self.uhd_usrp_sink_0_0_0.set_samp_rate(samp_rate)
    self.uhd_usrp_sink_0_0_0.set_center_freq(uhd.tune_request(center_freq, 

rf_freq=(center_freq +
lo_offset),rf_freq_policy=uhd.tune_request.POLICY_MANUAL), 0)
self.uhd_usrp_sink_0_0_0.set_gain(tx_gain, 0)
self.uhd_usrp_sink_0_0_0.set_antenna(tx_ant, 0)
self.uhd_usrp_sink_0_0_0.set_center_freq(uhd.tune_request(center_freq,
rf_freq=(center_freq +
lo_offset),rf_freq_policy=uhd.tune_request.POLICY_MANUAL), 1)
self.uhd_usrp_sink_0_0_0.set_gain(tx_gain, 1)
self.uhd_usrp_sink_0_0_0.set_antenna(tx_ant, 1)

At the receiver I sometimes get a very strange response - see attached
picture.

If I use an external PPS signal, and set sync to ‘Unknown PPS’, the
occurrence of this strange signal - where some parts of the signal seem
to just be a noisy sinusoid, is dramatically reduced - but not avoided
always. Using ‘PC Clock’ does not improve the situation.

Is there something obviously wrong with the way I am trying to transmit
over two USRPs simultaneously? I am not clearly why the problem is
intermittent. All 3 USRPs are connected two one PC, they are N210s with
XCVR2450s, tx gain = 0, rx gain = 0, centre freq = 2.4GHz, samp rate =
1M.

Regards,

David


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