Multicarrier modulation : duplexing design and interference/distortion mitigation
Abstract: Aspects of modern communication systems is the overall theme of this thesis with emphasis placed on multicarrier modulation. The work considers four facets of such systems; namely duplexing design, interference mitigation, channel estimation and multiuser detection. The first area deals with duplexing design for very high bit rate digital subscriber lines (VDSL) using discrete multitone modulation (DMT). We present a novel method based on DMT - the Zipper duplex method. Zipper is proposed for VDSL in different standardization bodies worldwide - International (ITU), North America (ANSI) and in Europe (ETSI) where it also has been accepted as a part of the VDSL standard. Zipper has superior flexibility and spectrum efficiency. This is obtained by freely assigning different subcarriers for the up- and downstream direction. In one design Zipper operates fully network synchronized by using a masterclock. In an asynchronous design Zipper operates without any reference to a masterclock which is a requirement for unbundled networks but reduces some of the flexibility. To obtain highest flexibility in unbundled networks, without using a masterclock, an algorithm is derived that self-synchronizes all Zipper modems. Another area deals with interference- and distortion mitigation. Narrowband interference (NBI) in orthogonal frequency division multiplexing- (OFDM) and DMT-based systems is considered. NBI can be very harmful for both radio- and wireline systems. We introduce two efficient NBI cancellers for OFDM and DMT. One canceller is based on a deterministic polynomial model of the NBI. The other canceller models it as a narrowband stochastic process and use the linear minimum mean square error (LMMSE) criterion for the cancellation. We lower its complexity by using the theory of optimal rank reduction. Impulse noise is a different type of harmful interference that can be encountered in VDSL. In this thesis we study the effects of impulse noise in DMT-based VDSL systems and present a robust generalized likelihood ratio test for detecting impulse noise. It is used for obtaining reliable erasures in a Reed-Solomon decoding scheme which reduces the probability of symbol errors significantly. Pilot symbol assisted modulation (PSAM) can be used in OFDM for tracking the distortion variations in a fading radio channel. We analyze the pilot symbol spacing in PSAM as a trade-off between high effective SNR and good channel tracking capabilities for two channel estimators with different complexities. Code division multiple access (CDMA) is part of the standard for the third generation of mobile phones. In this thesis we present a low complexity multiuser detector for a wireless DS-CDMA system. With a pipelined structure it can produce maximum likelihood sequence detector (MLSD) decisions on many of the received bits by only performing additions after the front end matched filters.
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