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

From: Matt Ettus
Subject: Re: [Discuss-gnuradio] OFDM spectrum with power rolloff?
Date: Mon, 26 Jul 2010 10:08:58 -0700
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20100430 Fedora/3.0.4-2.fc12 Thunderbird/3.0.4

On 07/23/2010 09:38 AM, George Nychis wrote:

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

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.

The math looks right to me. Make sure you're using complex signals and a complex FFT. Also, check carefully that you are putting your complex signals in consecutive bins. If you were to put signals in every 2nd bin, the spectrum would look twice as wide and you wouldn't see the holes.


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