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Re: [Discuss-gnuradio] OFDM spectrum with power rolloff?

From: George Nychis
Subject: Re: [Discuss-gnuradio] OFDM spectrum with power rolloff?
Date: Fri, 23 Jul 2010 09:38:12 -0700

Thanks a bunch for your response, Matt!


Since you are using an interpolation is a multiple of 4, you won't see CIC rolloff, you get a nice flat passband.

Got ya! 

There are likely a couple of issues causing the spectrum to look bad. First, successive symbols coming out of the FFT will have amplitude discontinuities.  This causes out of band emissions unless you smoothly transition between the symbols.  This is normally done with windowing. The 802.11g spec is a good example.

I will check the spec for this, thanks for the pointer there.  I will see how they do the windowing.

Second, you may be clipping.  OFDM has a very high peak to average ratio, so you need to make sure the peaks are below clipping.

Clipping nailed it.  I get a very clean spectrum now :) Super awesome!

One last question still, do you know where I am missing a factor of two here:

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.

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