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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 2.0.0.4 (Windows/20070604) |
Shravan,
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 :)
Shrava
Tom
- [Discuss-gnuradio] Re: Regarding OFDM implementation,
Tom Rondeau <=