[Discuss-gnuradio] How to select the proper equalizer for 8PSK modulation

[Damindra Bandara]

Hi,

I am trying to send 8PSK modulated signal using Ettus USRP. I designed the application using GNURadio PSK Mod/Demod blocks. The two blocks worked perfectly for BPSK and QPSK modulations. However for 8PSK modulation did not work as expected. The receiver received packets only once in a while and the throughput was less than 1%. Then I decomposed the blocks inside the PSK demod block and I observed that the constellations after the Costas loop looks very strange. A figure is attached herewith. With some readings and playing with the GNURadio channel models I realized this is because I am not using the equalizers properly. Currently I am using "CMA equilizer"  with the following settings: Num of taps=15, modulus=1, Gain=10m, sps=2.

1-Will this problem resolve if I choose the correct equalizer and its settings (or is this some other problem that I do not understand?)? 2-How can I decide which equalizer to use?

 My current flowgraph is,

USRP_source --> Polyphase Clock Sync--> CMA equalizer --> Costas loop --> Packet decoder --> File sink

I appreciate if anyone could help me to resolve this issue and get the 8PSK modulation working.

Thank you very much,

Damindra

Damindra Savithri Bandara,

Ph.D. in Information Technology (Candidate)
George Mason University,
Fairfax,
Virginia

8psk_constellations.png
Description: PNG image