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We address the problem of downlink interference rejection in a DS-CDMA system. Periodic orthogonal Walsh-Hadamard sequences spread different users' symbols followed by scrambling by a symbol aperiodic base-station speciJic overlay sequence. This corresponds to the downlink of the European UMTS wideband CDMA proposal. The point to point propagation channel ffom the cellsite to a certain mobile station is the same for all downlink signals (desired user as well as the interference). The composite channel is shorter than a symbol period for some user signals, while other users can have significant IS1 owing to a faster transmission rate. In any case, orthogonality of the underlying Walsh-Hadamard sequences is destroyed by multipath propagation, resulting in multiuser interference if a coherent combiner (the RAKE receiver)is employed. We propose linear zeroforcing (ZF) and minimum mean-squared-error (MMSE) receivers which equalize for the estimated channel, thus rendering the user signals orthogonal again. A simple code matched filter subsequently sufices to cancel the multiple access interference (MAI) ffom intracell users.

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In this paper we address the problem of Weighted Sum Rate (WSR) maximization for a K-user Multiple-Input Single-Output (MISO) cognitive Interference Channel (IFC) with linear transmit beamforming (BF) vectors in an underlay cognitive radio setting. We consider a set of L single-antenna Primary receivers to which the cognitive system can causes a limited amount of interference. We thus propose an iterative algorithm to determine the BF vectors for the secondary transmission. The optimization of the Lagrange multipliers involved in the optimization problem is based on the subgradient method. The expression of the BF vector can be interpreted as dual Uplink (UL) MMSE receiver that takes into account the interference caused by a fictitious link between the primary user and secondary base station. FinallyDeterministic Annealing is applied to make the convergence of the algorithm easier.

SINR duality is shown in a multi-input singleoutput (MISO) downlink (DL) interference channel (IFC) and its uplink (UL) single-input multi-output (SIMO) dual with linear transmit (Tx) beamformers (BF). While UL-DL duality under the sum power constraint is well-established between the broadcast channel (BC) for the SINR balancing (max min SINR) beamforming problem and its (easier to solve) UL multipleaccess (MAC) dual channel, such duality does not at first seem relevant for the IFC. We show that SINR duality under the sum power constraint nevertheless holds in the MISO IFC leading to BF design through similar considerations as the BC-MAC case. We next impose further per-Tx power constraints meaningful for the IFC structure and show continued existence of SINR duality in the MISO IFC and the corresponding UL SIMO dual channel, but this time with an uncertain UL noise scaling. The beamformers, Tx powers and noise variances are found through an iterative algorithm. We extend the solution to a cognitive radio network where this MISO IFC is co-deployed in a underlay fashion with a primary network with interference power constraints on primary single antenna receivers.