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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: Fri, 23 Jul 2010 00:19:52 -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 07/22/2010 11:51 PM, George Nychis wrote:
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.

I don't know what you mean by the above. The peaking on either end of the spectrum might be caused by this.

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..

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

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.

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.


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