Coded modulation schemes for wireless channels

Abstract

In this thesis coded modulation schemes designed for transmission over mobile wireless fading channels are proposed and investigated. Specifically, coded modulation is a bandwidth efficient scheme, where the redundancy introduced by the channel coding scheme used does not expand the required bandwidth, since the parity bits are absorbed by the extended modulated signal constellation. Coded modulation schemes were designed for transmission over both Additive White Gaussian Noise (AWGN) channels and narrowband fading channels. However, typical mobile wireless channels are dispersive, where the employment of the conventional coded modulation scheme alone may be insufficient for achieving a coding gain. In this thesis, the employment of coded modulation in the context of conventional Decision Feedback Equalisers (DFE), Radial Basis Function (RBF) based equalisers and Orthogonal Frequency Division Multiplexing (OFDM) is investigated. Furthermore, coded modulation is also being investigated in a Code-Division Multiple Access (CDMA) environment, in the context of both DFE based Multi-User Detection (MUD) as well as Genetic Algorithm (GA) assisted MUD.

Another means of mitigating the effect of wideband fading channels is employing adaptive modulation techniques. More specifically, a higher-order modulation mode is employed, when the instantaneous estimated channel quality is high in order to increase the number of bits per symbol transmitted and, conversely, a more robust lower-order modulation mode is used when the instantaneous channel quality is low, in order to improve the mean Bit Error Rate (BER) performance. In this thesis, adaptive coded modulation schemes are investigated in the context of both conventional DFE schemes and DFE based MUD aided CDMA schemes. Turbo equalisation (TEQ) is another technique of mitigating the effects of wideband fading channels. Specifically, TEQ is a joint channel equalisation and channel decoding scheme, where the equaliser is fed by both the channel outputs and by the soft decisions provided by the channel decoder. This process is then invoked in a number of iterations. In this study coded modulation schemes are also amalgamated with the proposed RBF-based TEQ and with an RBF-based reduced complexity In-phase(I)/Quadrature-phase(Q) TEQ.

Finally, coded modulation is proposed for increasing the achievable diversity gain when com­municating over fading channels. Specifically, IQ-interleaved coded modulation is introduced and investigated in the context of Space Time Block Coding (STBC) schemes as well as in Rake receiver based CDMA schemes. Explicitly, IQ-interleaved coded modulation is capable of achieving a novel type of diversity, namely IQ diversity, while STBC is well known for attaining both transmit and time diversity, while the Rake receiver employed is useful for achieving multipath diversity.

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