Blind decentralized projection receiver for asynchronous CDMA in multipathchannels

An asynchronous direct sequence code division multiple access (DS-CDMA) sys tem employing periodic sequences is considered to be operating in a frequen cy selective channel. The cyclostationary spread signal is received at mult iple sensors and/or is sampled multiple times per chip (oversampling), lead ing to a stationary vector-valued received signal. Hence, such a model repr esents a very particular multi-input multi-output (MIMO) system with plenti ful side information in terms of distinct spreading waveforms for the input signals. Depending on the finite impulse response (FIR) length of the prop agation channel, and the processing gain, the channel of a certain user spa ns a certain number of symbol periods, thus inducing memory or intersymbol interference (ISI) in the received signal in addition to the multiple-acces s interference (MAI) contributed by concurrent users. The desired user's mu ltipath channel estimate is obtained by means of a new blind technique whic h exploits the spreading sequence of the user and the second-order statisti cs of the received signal. The blind minimum mean square error-zero forcing (MMSE-ZF) receiver or projection receiver is subsequently obtained. This r eceiver represents the proper generalization of the anchored MOE receiver [ 1] to the asynchronous case with delay spread. Classification of linear rec eivers obtained by various criteria is provided and the MMSE-ZF receiver is shown to be obtainable in a decentralized fashion by proper implementation of the unbiased minimum output energy (MOE) receiver, leading to the minim um variance distortionless response (MVDR) receiver for the signal of the d esired user. This MVDR receiver is then adapted blindly by applying Capon's principle. A channel impulse response is obtained as a by-product. Lower b ounds on the receiver filter length are derived, giving a measure of the IS I and MAI tolerable by the receiver and ensuring its identifiability.