The inputs to the FFT for each subcarrier are the sine and cosine of the phase
of the signal. This is usually done in a lookup table. However, you're right
that a FFT is inefficient for 9 subcarriers. Each could be generated by an
accumulator and a sine/cosine lookup table. The input to the accumulator is the
frequency of the subcarrier and the phase can be altered by adding to the
output of the accumulator (modulo table size). Receiving would be done by
multiplying the incoming signal by the sine and cosine of each subcarrier
center frequency. The in-phase and quadrature outputs are then accumulated over
the sample period.
The phase change has to be done at the beginning of each sample period. This
actually generates multiple sidebands as the modulating signal is a square
wave. However, all sidebands except the first sideband cancel out if the
subcarriers are evenly spaced.
73,
John
KD6OZH
----- Original Message -----
From: Rud Merriam
To: digitalradio@yahoogroups.com
Sent: Monday, October 22, 2007 15:40 UTC
Subject: RE: [digitalradio] Re: OFDM Proposal
Thanks, John, for this reference and the other responses. I do keep an eye on
Phil's work and comments.
Time for a direct question: How do you modulate an OFDM sub channel using an
IFFT? Or is that not the way to do it in practice?
I have experimented with Excel. Looking at the FFT of a cosine wave I get a
nice solid single frequency bin. Doing a modulation of it with a phase change
in the middle I get a number of bins which looks impractical to implement.
A value in a single bin then running an IFFT generates a nice cosine. I can
kind of make a PSK modulate signal by copying 3 values from the FFT experiment
above but other attempts generate a mess. <g> Again, that does not seem like a
feasible approach.
Musing about it while going to sleep I got thinking about another approach
based on the observation that the sign of a value in the complex number
controls the phase of the start of the curve. The process is:
Generate a "symbol"
If a phase change is needed change the sign in the bin
Generate the next "symbol"
The trick is that the "symbols" are offset by 1/2 the timing period, i.e. the
start of the 2nd symbol is actually the midpoint of the 2nd symbol. This works
because in OFDM the symbol period contains complete cycles of the waves.
Possibly using the IFFT for an HF OFDM signal is inefficient, especially when
working with a 500 Hz bandwidth signal. The 62.5 baud suggestion you made only
using 9 tones so generating them directly would not be a CPU intensive process,
especially using table lookups.
Rud Merriam K5RUD
ARES AEC Montgomery County, TX
http://TheHamNetwork.net
-----Original Message-----
From: digitalradio@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of
John B. Stephensen
Sent: Monday, October 22, 2007 7:40 AM
To: digitalradio@yahoogroups.com
Subject: Re: [digitalradio] Re: OFDM Proposal
Look at KA9Q's web site, especially http://www.ka9q.net/code/fec/, for FEC
software.
73,
John
KD6OZH
