Code design and signal extraction for advanced code-division communications.

摘要

The objective of this work is the solid, complete development of a theoretical framework and practical technology for transmitter and receiver designs for code-division communication networks. Special emphasis is provided to the problems of optimum and adaptive spreading code design as well as to the problem of blind signal extraction.;The research work herein can be divided into three parts. The first part deals with the development of new lower bounds on the total squared correlation (TSC) of quaternary (quadriphase) signature/sequence sets for all lengths L and set sizes K. For all K, L, we design minimum-TSC optimal sets that meet the new bounds with equality. Since TSC is not a good criterion for overloaded code division multiplexing (CDM) applications, we also propose hierarchical criteria for the design of codes which can let the overloaded CDM systems attain satisfactory performance with a simplified maximum-likelihood (SML) detection scheme.;In the second part of this work, the problem of signal eavesdropping on wireless direct-sequence CDMA channels is investigated. We develop a novel iterative least-squares-type algorithm that extracts the information symbols of the concurrent users with no knowledge of the signature waveforms of the active spread spectrum signals in the system, and/or the channel state as well as no availability of pilot signaling (training sequence). We also consider the problem of simultaneous power and code-channel allocation for a secondary (unlicensed) code-division transmission links co-existing with a primary (licensed) CDM system. Other than signal extraction, this eavesdropping scheme is utilized for developing a blind primary-users identification scheme to estimate the binary code sequences (signatures) utilized by primary users. Then, we propose two alternative schemes (of moderate and low computational complexity) that optimize transmitting power and binary code-channel allocation of one secondary link without causing "harmful" interference to the primary users.;Finally in the third part of this work, we focus on steganalysis, which is the countermeasure of steganography. We propose a passive steganalysis algorithm to identify the presence of hidden messages and an active steganalysis algorithm to recover unknown messages hidden in image hosts via multi-signature direct-sequence spread-spectrum embedding. Neither the original host nor the embedding signatures is assumed available.