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Hi,
I am trying to implementing a multilevel modulation and demodulation scheme in Warplab environment. I had created functions for the modulation and demodulation in separate m-files. But then I proceeded by modifying the warplab_siso_example_comm.m and before implementing it we tested the qam 64 modulation and demodulation available in matlab. What I did was the following:
1. In modulation I replaced the dpsk function and took in a qammod (from modulation) and scaled it between 1 and -1.
2. In the demodulation I replaced the demodulation function of dpsk and used the standard qam demodulation available in matlab using the sma e scaling for modulation.
We get a phase shift in the received constellation. We do not get the signals properly. Do you have an example for 64 qam modulation and demodulation in WARPLAB?
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You're probably seeing the effects of carrier frequency offset. The two WARP nodes use their own clock signals to generate RF carriers. These clocks are not perfectly synchronized, so the two carrier frequencies are slightly different. This manifests as a time-varying phase (i.e. frequency) offset between the Tx and Rx signals. You can either use a differentially encoded modulation scheme (like dqpsk) or implement a CFO estimation and compensation scheme for regular PSK/QAM.
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