Multistage Blind Interference Cancellation and Channel Estimation for DS-CDMA Systems 1

In this paper we propose a blind inter- ference cancellation and channel estimation receiver for direct-sequence code-division multiple-access (DS- CDMA) systems, which assumes knowledge of only the nominal desired user signature sequence. It is a multistage nonlinear blind interference cancellation and channel estimation receiver that uses higher or- der statistics (second and fourth moments) of the re- ceived baseband signal. Simulation results show that unlike linear receivers, the scheme is exceptionally ef- cien t in systems with strong and highly correlated interferers, and is also able to jointly estimate chan- nel response and perform interference cancellation. I. Introduction Modern wireless DS-CDMA communications systems use vari- able spreading gain in order to accommodate dieren t data rates, and in general, dieren t quality of service, while main- taining the same chip rate (1). To achieve higher data rates, channelization codes (signature sequences) with lower spread- ing gains are to be extensively used (e.g., in W-CDMA sys- tems spreading gains of 4; 8; 16 and 32 are of prime inter- est for higher data rates). Because of lower spreading gains, these cases are more aected by multiple-access interference (MAI), than in the cases of higher spreading gains. Even though it is possible to design good signature sequences, asyn- chronous transmissions, overloading (more users than signal dimensions) and dispersive environments make it very di- cult to guarantee low signature crosscorrelations. Therefore, in order to provide required bit-error rates (BER), interfer- ence cancellation (IC) and/or multiuser detection (MUD) are to be applied in the cases of lower spreading gains. A number of dieren t solutions, assuming knowledge of all users' param- eters in the system, perform dieren t centralized IC or MUD schemes (e.g., (2, 3, 4, 5)). In this paper we consider a DS-CDMA system where the signature sequences are short (i.e., for a user, the same sig- nature sequence is used for each symbol period). This as- sumption allows a receiver to adaptively learn the structure of MAI and the channel response and apply the results for IC and detection. Unlike adaptive receivers that use a predened training sequence (e.g., see (4, 6, 7)), we present a fully decen- tralized (i.e. blind) solution that assumes only knowledge of the nominal (i.e., transmitted) signature sequence. This work 1 This work is supported in part by the New Jersey Commis- sion on Science and Technology under the New Jersey Center for Wireless Communication Technologies represents an extension of the algorithm that is originally pre- sented in (8). In (8) the focus was on blind interference cancel- lation, but here we present a solution for joint blind channel estimation and interference cancellation. It is well known that linear receivers (centralized and decentralized) do not perform well in the case of systems with strong and highly correlated interferers (with respect to desired user signature sequence), as may be the case in overloaded DS-CDMA systems and/or dispersive environments where multipath components intro- duce signican t interference. In this paper we propose a blind interference cancellation and channel estimation receiver. It is a multistage nonlinear blind interference cancellation and channel estimation receiver (BICCE) that uses higher order statistics of the received base- band signal. Specically , we use the second and fourth mo- ments of the received signal to determine a component of the received vector that has signican t mean energy and low vari- ability of the energy which are favorable characteristics for application in an interference cancellation scheme that uses hard decisions. The structure of the receiver is multidimen- sional and can be viewed as a matrix of receivers. Each row in the matrix consists of receivers that perform (hard deci- sions) cancellation of successive components that have signif- icant mean energy and low variability of the energy and, at the same time, estimates the channel response for the desired user. The columns of the matrix essentially resemble multi- stage receivers that iteratively rene performance from earlier stages. Simulation results show that unlike linear receivers, the BICCE is exceptionally ecien t in systems with strong and highly correlated interferers, and is also able to jointly estimate channel response and perform interference cancella- tion.