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#1 2016-Apr-29 03:22:39

David Garcia-Roger
Member
Registered: 2015-Nov-03
Posts: 10

wl_example_siso_ofdm_txrx, why does the interpolation rate must be 2?

Hello all:

I was recently puzzled by line 27 of wl_example_siso_ofdm_txrx.m, specifically the comment that reads "must be 2".

Code:

INTERP_RATE = 2; % Interpolation rate (must be 2)

Indeed the interpolation rate must be 2 when enabling WARPLab-in-the-loop; otherwise the example is not able to receive the waveform satisfactorily. However, INTERP_RATE may be 1 when using a sim-only approach (the example is able to receive the waveform in a pure MATLAB approach).

What is the actual mathematical or hardware reason (I guess hardware) for this behavior? I may have misunderstood something... but could you please help enlightening me? :)

As usual, thanks a lot for your time. :)

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#2 2016-Apr-29 09:31:11

chunter
Administrator
From: Mango Communications
Registered: 2006-Aug-24
Posts: 1212

Re: wl_example_siso_ofdm_txrx, why does the interpolation rate must be 2?

There isn't any hardware reason for that requirement. An earlier version of the code actually supported an INTERP_RATE of 1 (40 MHz bandwidth) or 2 (20 MHz bandwidth) in hardware, but it made the code messy having to deal with switching on that parameter to apply different processing (e.g. selectively applying the interpolation/decimation filter).

We ultimately decided that the INTERP_RATE of 1 not only made the example messy, but also was more generally a bad example of how to do 40 MHz OFDM since the underlying implementation in that example remains a 64-point FFT. Normally, 40 MHz OFDM (like 802.11n/ac) keep the same subcarrier width and increase the FFT size to 128-point. Doing this would be a straightforward extension of our existing example.

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#3 2016-May-02 01:53:17

David Garcia-Roger
Member
Registered: 2015-Nov-03
Posts: 10

Re: wl_example_siso_ofdm_txrx, why does the interpolation rate must be 2?

Oh, now I get it. Thank you so much for the speedy reply. :)

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