Differential MMSE: A New Approach for Adaptive Interference Suppression Over Time-Varying Channels
Zhu, Liping Julia
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https://hdl.handle.net/2142/81318
Description
Title
Differential MMSE: A New Approach for Adaptive Interference Suppression Over Time-Varying Channels
Author(s)
Zhu, Liping Julia
Issue Date
1998
Doctoral Committee Chair(s)
Upamanyu Madhow
Department of Study
Electrical Engineering
Discipline
Electrical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Electronics and Electrical
Language
eng
Abstract
The use of the linear minimum mean square error (MMSE) criterion to design coherent adaptive receivers is a classical method for interference suppression, with applications ranging from equalization to adaptive multiuser detection for direct sequence code division multiple access (CDMA) systems. A drawback of this method is that standard MMSE adaptation is unable to track the amplitude and phase variations typical of a wireless fading channel. A classical method for noncoherent communication over such channels is differential modulation and demodulation, which obviates the need to track amplitude and phase by exploiting the fact that channel variations over successive symbols are negligible. However, there are no known methods for adaptive interference suppression for differentially modulated systems. In this thesis, this gap is filled by reformulation of the linear MMSE criterion for differential modulation. This provides the first cohesive framework for addressing the two most serious impairments of the wireless medium: interference and fading. The resulting differential MMSE receiver is obtained as a solution to an optimization problem with quadratic cost function and quadratic constraint, and is amenable to both block and recursive adaptive implementations. When the fading is frequency selective, the differential MMSE receiver provides automatic multipath combining for slow fading rates. For faster fading rates, it is necessary to extend the differential MMSE criterion to provide explicit multipath combining. A number of such extensions are proposed, notable among which is the eigen-rake receiver, which provides interference suppression and diversity combining without requiring any explicit information regarding the desired user's propagation channel. While the application explored in detail in this thesis is adaptive interference suppression for direct sequence CDMA with short spreading sequences, differential MMSE reception is equally applicable to intersymbol and cochannel interference suppression in narrowband fading channels.
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