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[Discuss-gnuradio] OFDM spectrum with power rolloff?

From: George Nychis
Subject: [Discuss-gnuradio] OFDM spectrum with power rolloff?
Date: Thu, 22 Jul 2010 23:51:26 -0700

Hi all,

I am generating my own "OFDM" waveform which actually does not modulate anything.  Step by step.

I am simulating 1024 subcarriers but first generating 1024 zeros, and then I want to only enable the center 100 subcarriers.  To do this, I map the center frequency at the first index in the array, then the positive frequency subcarriers, then the negative frequency subcarriers.  To enable the center 100 subcarriers, I multiply the first 50 positive frequency subcarriers by: (1.472) * complex(1,1), and do the same with the first 50 negative frequency subcarriers.  Finally, I take the IFFT of the data, with a size of 1024.  I multiply this by sqrt(1024).

If I then take the FFT, to double check what I've done, and plot against the subcarrier index, I get what I expect:

Now, I want to transmit this out of the USRP2 so I write the typical GNU Radio python script to read an 8-byte complex file source and pump it to the USRP2... repeating the complex samples in the file on finish for continuous transmission.

I start up the spectrum analyzer and get a clean noise floor:

Now, when I transmit from the USRP2 using an interpolation rate of 32:

Well, I can certainly see the waveform.  But I have two questions...

1) My calculation of the bandwidth of the 100 active bins, and the actual bandwidth, is off by a factor of two:
(((100 MHz) / 32) / 1024) * 100 = 305.175781 kilohertz, where 32 is the interpolation rate, 1024 was the size of the IFFT, and 100 is the number of active bins.  I see 610KHz, however.

2) I'm not seeing a clean power falloff outside of the active bins.  I am seeing a lot of power rolloff.  Do I need to be applying a low pass and/or high pass filter for this?  Or is this the result of weirdness created by the interp CIC filter and the dual half-band filters? I'm not too familiar at this level of communications..

I'd greatly appreciate any feedback.  I can provide any code, but my guess is that I have a more fundamental misunderstanding than a coding error.


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