Chapter 14

Optimal Multi-User MMSE Equalizer

Interference1 is said to be the most important performance-limiting factor of modern communication systems. This especially holds true for Wideband Code-Division Multiple Access (WCDMA) networks, where frequency selectivity in the Downlink (DL) channel causes a loss in orthogonality between the spreading codes utilized and imposes restrictive throughput constraints. This issue has already been identified in Chapter 12 for the development of the link-quality model, as well as in the simulation results in Chapter 13. However, besides the unavoidable interference caused by spreading code crosstalk, the question arises as to whether possibilities exist to exploit structural properties in the interference for Multiple-Input Multiple-Output (MIMO) High-Speed Downlink Packet Access (HSDPA). The information theoretic principles for this kind of interference problem are represented by the MIMO broadcast channel [14, 37], which is related to the MIMO multiple-access channel in the Uplink (UL) [36]. Although the capacity regions in the case of fading channels are not known yet, all of the results so far indicate the need for interference mitigation to come close to the upper bound on the sum-capacity [12, 30].

Interference can be combated at the transmitter and/or the receiver side. In particular, in the DL each receiver needs to detect a “single” desired signal, while experiencing two main types of interference. These are caused by the serving ...

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