Chip-equalised UMTS downlink for fast fading channels

Abstract

The 3/sup rd/ generation of mobile communications networks, such as those based on WCDMA, will require high performance downlink receivers to maximise capacity. The Rake receiver, commonly proposed for WCDMA systems to counteract multipath channel effects, provides poor performance when there are a high number of active users per cell. This is due to loss of orthogonality between user spreading codes, which results in high levels of multiple access interference (MAI). Chip equalisation has been shown to suppress MAI by restoring the orthogonality of user codes prior to despreading. We propose a simple system for chip equalisation of 3/sup rd/ generation UMTS transmission frames in fast fading channels. FIR MMSE equalisation with common pilot channel estimation is used and slot-wise linear interpolation between equaliser tap coefficients provides a simple method for tracking the time-varying channel. The performance of the proposed system over a vehicular multipath time-variant channel is assessed by simulation. The benefits to system performance of UMTS-standard turbo coding and interleaving in the fast fading environment are studied. Performance is compared with the equivalent Rake receiver at each stage. It is shown that, despite increased sensitivity to channel estimation error relative to the Rake receiver, the proposed method can be used to achieve significant performance benefits in highly-loaded cells, even for high mobile terminal speed. Performance benefits are particularly marked where chip equalisation is used in conjunction with turbo coding and interleaving.