[Discuss-gnuradio] Strange Received Signal for 2x1 MISO OFDM (sometimes!)

[David Halls]

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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2x1_TD_RX.png
Description: 2x1_TD_RX.png