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Title: An investigation into the application of companding to wireless OFDM systems
Author: Vallavaraj, Athinarayanan
Awarding Body: Glasgow Caledonian University
Current Institution: Glasgow Caledonian University
Date of Award: 2008
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Abstract:
The everyday increase in the demand for high speed mobile multimedia communications requires transmission techniques that are robust to channel hostilities and yet providing higher data rates. Orthogonal Frequency Division Multiplexing (OFDM) promises to be a key modulation technique that caters to these requirements; however, it suffers from a very high peak to average power ratio (PAPR) resulting in other associated problems. This thesis provides a review of various PAPR reduction techniques available in the published literature and then presents a novel approach to PAPR using the less explored technique of companding. For the first time, a complete evaluation and quantification of PAPR and BER for companding in relation to OFDM is carried out in terms of different companding profiles, number of carriers and type of modulation employed. In addition, a new parameter related to the companding profile called the peak ratio (PR) is defined. It is demonstrated that PR can reduce further the PAPR of OFDM signals and further improve the BER performance. Typically, an OFDM signal companded with a PR =2 has shown a channel EtlNo gain of 5.7 dB for a BER of 0.0001, and a PAPR reduction of 5.88 dB in relation to an uncompanded OFDM signal. A thorough investigation of High Power Amplifier (HPA) nonlinearity on companded OFDM is also carried out in terms of evaluating and quantifying the BER performance and signal splatter as a function of the OFDM carriers, modulation, companding and HPA nonlinearity parameters. The most significant result is that application of companding to OFDM is demonstrated to provide robustness of improved PAPR and BER when nonlinear amplifier characteristics are present. Typically, while a 64-QAM OFDM signal under the influence of a nonlinear HPA cannot produce a BER lower than 0.001 for any value of channel EtlNo greater than 20 dB, the companded OFDM signal produces a BER of about 0.00001 for a channel EtlNo of around 19 dB. The companding technique has also been compared for performance in terms of PAPR reduction and BER with the more popular PAPR reduction techniques of clipping, peak-windowing, phasing and selected mapping. It is shown that companding surpasses these techniques for certain particular companding profiles. Typically, companding with a PR =1'.4 results in an improvement of about 2.5 dB over general OFDM and a'n improvement of about 12 dB over clipped OFDM with a clipping ratio of 1.4. The novel approach of combining companding with these popular PAPR reduction techniques has also been investigated for the first time with the results demonstrating a further dramatic improvement in PAPR reduction and BER performance for OFDM transmissions. Further, combining a particular phasing technique with companding can reduce the PAPR to as low as 0.5 dB. This research work has also resulted in some further new insights. Firstly, for multi-tone signals, standard phasing techniques can be further refined for improvement of PAPR; secondly a modified phasing scheme for QPSK is possible to reduce further PAPR, though like other PAPR reduction techniques, requires additional overhead to be transmitted; and thirdly alternative means of simple encoding techniques are possible to improve upon present implementations of selected mapping PAPR reduction methods. The results of this thesis therefore have significant implications for the practical applications of broadband OFDM transmissions in terms of improved PAPR and BER performance.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.486485  DOI: Not available
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