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Currently I am working with the Ettus Research's N310 on the implementation of different PSK modulation schemes. I am interested on measuring the Bit Error Rate -BER- for each scheme when I transmit data between two USRPs located one beside the other. Therefore, I am employing GNU Radio Companion for the SW development. In the case of the BPSK transceiver, I am using a standard configuration of a vector source and a constellation modulator to create the PSK symbols, which are transmitted at 2.45GHz using the VERT2450 antennas. These antennas work in the frequency range from 2.4 - 2.5 GHz and from 4.9 - 5.9 GHz. Since I have a desktop computer with only one ethernet port, I am using the NetGear GS108 switch, which has 16Gbps bandwidth and a forwarding rate of 10Mbps port. The current SW setup is shown in the following figure: enter image description here

I am using as input a vector of only zeros since I am interested to probe that my transceiver detects correctly one constellation. However, I am having continious jumps between the constellation points as you can see from the picture in the left side. I have several questions about my setup:

  1. What is the correct baud rate for each modulation scheme? It means how many symbols per second should I use for BPSK, QPSK, 8PSK and 16 QAM.
  2. Since the USRP N310 has a default sample rate of 125MSamples/second, and my desktop machine can only deal with 5MSamples/second, then I have a decimation rate of 25(sample_rate_usrp/sample_rate_desktop). What is the value for the sps -samples per symbol- parameter that I should assign in each block of the transceiver?
  3. When is the CMA equalizer necessary? Since the USRPs have a static position, then there is no frequency changes due to the Doppler effect. Consequently, an equalizer should not be necessary. Why is this reasoning not correct? I suppressed the equalizer and the constellation diagram is presented as a circle.
  4. Does the Polyphase Clock Synchonization really synchronize the received signal with the transmitted signal or can I supprime it and replace it with an equalizer?

I would really appreciate if someone could help me to bring some light to all of this questions.

Thanks in advance

Marcus Müller
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Bruce McLaren
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2 Answers2

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See my response at https://lists.gnu.org/archive/html/discuss-gnuradio/2020-08/msg00172.html

  1. The 'correct' baud rate is anything you want to use.
  2. You need to check the minimum sample rate for the N310.
  3. The CMA Equalizer is optional under your conditions. I left it out of the BPSK to simplify the flowgraph.
  4. The Polyphase Clock Sync Block recovers the timing of the received signal. The equalization is for fading and is a separate function.
Barry Duggan KV4FV
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It looks like you're modulating with 8 SPS but then demodulating 16 SPS -- 8:1 on the PCSync and 2:1 on the CMA eq.

Seth
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