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[Discuss-gnuradio] Re: Regarding OFDM implementation

From: Tom Rondeau
Subject: [Discuss-gnuradio] Re: Regarding OFDM implementation
Date: Thu, 12 Jul 2007 11:11:03 +0100
User-agent: Thunderbird (Windows/20070604)


I copied this to the listserv as I hope others find the discussion useful.

Shravan Rayanchu wrote:
Hi Tom,

I have a few questions regarding the OFDM implementation in gnuradio
(I am a networking guy and I have a little knowledge of DSP, so some
of my questions might be trivial):

1. Can you point me to the version with 16-QAM (or 64-QAM) running
over OFDM on the air?
The code is checked in, but I can't guarantee that it will work (especially the receiver). We have currently separated the modulators into gr_ofdm_<mod>_mapper; there's one for QAM, which was meant to be a generalized QAM modulator, but we just hard-coded 16QAM for now.

Eventually, these will go away into a single gr_ofdm_mapper function that takes a constellation vector as an argument, allowing any type of modulation per subcarrier.

2. Does the argument 'occupied tones' refer to subcarriers? What is
the frequency band occupied by each subcarrier? Can I vary the number
of subcarriers used? What does "cp length" correspond to?
Yes. The fft_length is the total number of subcarriers, and the total number of subcarriers that carry actual data is the occupied_tones value. We currently allocate the middle number of subcarriers to carry data (instead of allowing random subcarries to be turned on and off as in non-contiguous OFDM).

The frequency bands of each subcarrier depends on the interpolation rate you use in the transmitter.

The cp_length value is the number of samples in the cyclic prefix. The total number of samples in time is the fft_length + cp_length per symbol.

3. What PHY data rate does each of the modulation correspond to ? (For
eg. in OFDM, 64-QAM supports PHY data rates of 56Mbps and 48 Mbps)
We were able to get a maximum of 2.4 Mbps using the 16-QAM, so 600 kHz of occupied bandwidth. The limitation in speed is mostly in the receiver complexity, so as the code is optimized, the data rates will increase.

Just a side note, this is a problem I always run into. OFDM is _not_ 802.11g. 802.11g uses OFDM as it's PHY layer modulation, so it's incorrect to say that OFDM at 64QAM gives you 54 Mbps; this is only true in the 802.11g implementation. In our system, at the maximum rate, we would get 3.6 Mbps with 64QAM.

4. Is there any other documentation available regarding the
implementation (I looked at the presentation available on the trac)
Nope, not at this time. I think we've scattered a few references inside the code to papers we used when designing the system.

Thanks much for the help :)



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