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Adaptive Techniques for Multiuser OFDM
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Thesis submitted by Eric
Lawrey in Dec 2001 in partial fulfilment of the requirements for the
degree of Doctor of Philosophy, School of Engineering at James Cook
University. |
Abstract
The demand for high-speed mobile wireless
communications is rapidly growing. OFDM technology promises to be a key
technique for achieving the high data capacity and spectral efficiency
requirements for wireless communication systems of the near future.
This thesis presents an investigation into methods
for maximising the spectral efficiency of Orthogonal
Frequency Division Multiplexing (OFDM) systems. As
part of this, an investigation of detrimental effects on OFDM is
presented, showing the effect of: band pass filtering, the use of a
raised cosine guard period, clipping distortion, Additive White
Gaussian Noise (AWGN) on modulation BER rate, time synchronisation
error, and frequency offset errors.
An investigation of two adaptive techniques is
also presented. These techniques utilise knowledge
obtained by dynamically tracking the radio channel
response, to optimise the user frequency, and
subcarrier modulation. Adaptive modulation independently optimises the
modulation scheme applied to each subcarrier so that the spectral
efficiency is maximised, while maintaining a target Bit Error Rate
(BER). For a fading channel, adaptive modulation results in an
improvement of 12 - 16 dB in the Signal to Noise Ratio (SNR) required
to maintain a given BER, as compared with fixed modulation. Adaptive
user allocation exploits the difference in frequency selective fading
between users, to optimise user subcarrier allocation. In a multipath
environment the fading experienced on each subcarrier varies from user
to user, thus by utilising user/subcarrier combinations that suffer
the least fading, the overall performance is maximised. Adaptive user
allocation results in an additional average signal power improvement of
3 - 5 dB.
The simulated performance of the adaptive
techniques is presented using a set of measured wide
bandwidth (70 MHz) frequency response measurements
taken at 1 GHz. These measurements show the changes in the frequency
selective fading with small increments in space, allowing the effects
of different tracking rates for the adaptive allocations schemes to be
investigated.
This thesis also presents a method for maximising
the signal strength within buildings, by using
transmission repeaters. This is similar to the
Single Frequency Networks used in DAB and DVB
systems, except applied to small-scale bi-directional communications.
Using multiple repeaters causes multipath problems in most conventional
systems, however OFDM has a sufficiently high multipath tolerance to
combine the multipath energy. It was found to decrease the path loss by
7 dB for an indoor system with two repeaters, and up to 20 dB for eight
repeaters.
In addition, two techniques are presented for
reducing the Crest Factor (peak to average power
ratio of the RF signal envelope) of OFDM signals.
The first technique is a phasing scheme for OFDM
pilot symbols, which uses genetic algorithms to
optimise the phase angle of each subcarrier. This technique achieves a
lower CF than any previously published techniques, obtaining a CF as
low as 0.65 dB, which is 2 dB lower than commonly used techniques. The
second technique reduces the CF of data carrying symbols, by including
additional subcarriers that are optimised in amplitude and phase to
cancel out the peaks in the overall OFDM symbol. This was found to
produce a net improvement of 4 dB to the worst-case symbol CF.
Code
The matlab code developed using version 5.3 and is provided as is with no support.
The code is split into two parts depending on what area you are interested in.
- Part 1: (2 MB) Main code, everything except diagrams in Chapter 4 Multiuser OFDM
- Part 2: (7.4 MB) Code for Chapter 4 including fading measurement data.
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