The research of the OFDM research group at Monash University covers many
practical and theoretical aspects of OFDM.
Current areas of particular interest are:
Impulse noise and impulse noise mitigation, particularly with application
to digital television broadcasting;
Efficient digital signal processing implementation of OFDM ;
Peak-to-average power reduction in OFDM;
Improved modulation schemes based on OFDM - for example, Polynomial
Cancellation Coded OFDM (PCC-OFDM);
The effect of frequency offset in OFDM;
Multiple input/Multiple output (MIMO) OFDM.
We have published over thirty
papers on these aspect of OFDM in the last few years.
These research areas will now be described in more detail
Impulse Noise
Impulse noise caused by
electrical appliances being turned on and off, or by car ignitions can interfere
with digital television reception and can cause the picture to break up.
We have developed impulse mitigation techniques which substantially reduces
the effect of impulse noise. Digital signal processing is used to identify
high noise peaks and subtract them out before the television picture is decoded.
Digital Signal Processing
Implementation
In many OFDM applications it is
important to reduce the cost or power consumption of the transmitters and
receivers. For example for Wireless LANs used in laptop computers power
consumption must be minimise to reduce the load on the batteries. Research
by the group has resulted in more efficient implementation of some of the
functions within the OFDM transmitter and receiver.
Peak-to-average Power Reduction
One of the disadvantages of OFDM is that it has a high peak-to-average power
ratio (PAPR). This means that the output amplifier at the transmitter has
to be linear over a large dynamic range. Linear amplifiers are not
efficient, and so increase the power requirement of the transmitter. This
is a major disadvantage in high power applications such as broadcast television
and in battery operated applications such as Wireless LANs. Research by
our group has shown that by using a novel clipping and filtering technique the
PAPR can be substantially reduced without increased out of band power. We
have also shown that the distortion caused by clipping techniques has much less
effect on the overall system performance than was previously thought. This
is because of two factors: first the main effect of clipping is to shrink the
overall transmitted constellation, not to add noise; secondly clipping noise is
added at the transmitter and this fades with the signal. It has been shown that
in a multipath fading channel, clipping noise has negligible effect on system
performance.
Improved modulation
schemes including PCC-OFDM
Although OFDM has many advantages and these have led to it being adopted for
many new and emerging broadband communication systems, it also has a number of
disadvantages. One of these is its sensitivity to frequency errors and
phase noise. This is due to the sin(x)/x form of the spectra of the subcarriers.
We have developed a new modulation scheme called polynomial cancellation coded (PCC)
OFDM which is much less sensitive to these impairments.
The effect of frequency
offset in OFDM
OFDM is very sensitive to frequency offset. The exact effect of
frequency offset depends very strongly on details of the receiver design.
Precise understanding of these effects is important in receiver design and in
comparing the performance of different techniques.
Multiple input/multiple
output (MIMO) OFDM
In MIMO systems there are multiple transmit and receive antennae. MIMO
systems offer the possibility of greatly increasing the capacity of radio
channels. Because of this MIMO is a very active research area worldwide.
MIMO combines very naturally with OFDM. Our research group is also working
on MIMO OFDM systems, particularly combining MIMO with some other areas of our
OFDM research.
