By Eric Lawrey, Copyright 1997-2001
The current status of the research is that
OFDM appears to be a suitable technique as a modulation technique for high
performance wireless telecommunications. An OFDM link has been confirmed
to work by using computer simulations, and some practical tests were performed
on a low bandwidth base-band signal. So far only four main performance
criteria have been tested, which are OFDMís tolerance to multipath delay
spread, channel noise, peak power clipping and start time error. Several
other important factors affecting the performance of OFDM have only been
partly measured. These include the effect of frequency stability errors
on OFDM and impulse noise effects.
OFDM was found to perform very well compared
with CDMA, with it out-performing CDMA in many areas for a single and multicell
environment. OFDM was found to allow up to 2 - 10 times more users than
CDMA in a single cell environment and from 0.7 - 4 times more users in
a multi-cellular environment. The difference in user capacity between OFDM
and CDMA was dependent on whether cell sectorization and voice activity
detection is used.
It was found that CDMA only performs well
in a multi-cellular environment where a single frequency is used in all
cells. This increases the comparative performance against other systems
that require a cellular pattern of frequencies to reduce inter-cellular
One important major area, which hasnít
been investigated, is the problems that may be encountered when OFDM is
used in a multiuser environment. One possible problem is that the receiver
may require a very large dynamic range in order to handle the large signal
strength variation between users.
This thesis has concentrated on OFDM, however
most practical system would use forward error correction to improve the
system performance. Thus more work needs to be done on studying forward
error correction schemes that would be suitable for telephony applications,
and data transmission.
Several modulation techniques for OFDM
were investigated in this thesis including BPSK, QPSK, 16PSK and 256PSK,
however possible system performance gains may be possible by dynamically
choosing the modulation technique based on the type of data being transmitted.
More work could be done on investigating suitable techniques for doing
OFDM promises to be a suitable modulation
technique for high capacity wireless communications and will become increasing
important in the future as wireless networks become more relied on.