PAPR and ICI Reduction
Techniques for OFDM Based
Satellite Communication Systems
By
Emad Al-Dalakta
A thesis submitted for the degree of
Doctor of Philosophy
June 2012
Abstract
Multi-carrier systems such as orthogonal frequency division multiplexing (OFDM) are
significantly affected by peak-to-average-power ratio (PAPR). Unfortunately, the high
PAPR inherent to OFDM signals envelopes will occasionally drive high power amplifiers
(HPAs) to operate in the nonlinear region of their characteristic curve. The nonlinearity
of the HPA exhibits amplitude and phase distortions, which cause loss of orthogonality
among the subcarriers (SCs), and hence, inter-carrier interference (ICI) is introduced in
the transmitted signal. The ICI power is proportional to the amplitude of the signal at the
amplifier input and it may cause a considerable bit error rate (BER) degradation.
A plethora of research has been devoted to reduce the performance degradation due to the
PAPR problem inherent to OFDM systems. Some of the reported techniques such as
amplitude clipping have low-complexity; on the other hand, they suffer from various
problems such as in-band distortion and out-of-band expansion. Signal companding
methods have low-complexity, good distortion and spectral properties; however, they
have limited PAPR reduction capabilities. Advanced techniques such as coding, partial
transmit sequences (PTS) and selected mapping (SLM) have also been considered for
PAPR reduction. Such techniques are efficient and distortionless, nevertheless, their
computational complexity is high and requires the transmission of several side
information (SI) bits.
In this thesis, a new low-complexity scheme is proposed based on the PTS that employs
two inverse fast Fourier transforms (IFFTs) and two circulant transform matrices, in
order to reduce complexity and improve the system performance. Furthermore, the lowcomplexity scheme is simplified by omitting one of the circulant transform matrices in
order to reduce both the computational complexity and the number of SI bits at the cost
of a small reduction in PAPR and BER performance.
It is well known that, accurate PAPR estimation requires oversampling of the transmitted
signal, which in turn results in increased complexity.
More importantly, minimising the PAPR does not necessarily minimize the distortion
produced by the nonlinearity of the HPA. Therefore, minimizing PAPR does not
necessarily imply that the BER will be minimized too. Efficient and less complex
schemes for BER reduction of OFDM systems in the presence of nonlinear HPA and/or
carrier frequency offset (CFO) are proposed. These proposed techniques are based on
predicting the distortion introduced by the nonlinearity of HPA and/or CFO.
Subsequently, techniques such as the PTS and SLM are invoked to minimise the
distortion and BER. Three distortion metrics are adopted in this thesis: inter-modulation
distortion (IMD), peak interference-to-carrier ratio (PICR) and distortion-to-signal power
ratio (DSR). Monte Carlo simulations will confirm that the DSR and PICR are more
reliable than the PAPR and IMD for selecting the coefficients of the PTS and SLM to
minimise the BER. Furthermore, complexity analyses demonstrate that the proposed
schemes offer significant complexity reduction when compared to standard PAPR-based
methods.
A closed form solution for accurate BER for the OFDM signals perturbed by both the
HPA nonlinearity and CFO was derived. Good agreement between the simulation results
and the theoretical analysis can be obtained for different HPA parameters and CFOs.
Finally, efficient approaches to reduce the impact of nonlinear power amplifiers with
respect to the BER of OFDM systems are proposed. These are approaches based on: the
well-established PAPR schemes, a power amplifier model and a simple single point cross
correlator. The optimum phase sequence within the proposed approaches is selected by
maximizing the correlation between the input and output of the power amplifier
model. Simulation results have confirmed that the BER using the proposed approaches is
almost identical to the DSR, while the complexity is reduced significantly for particular
system configurations.