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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* **<=**