Multicarrier Block-Spread CDMA for Broadband Cellular Downlink
To cope with the Multi-User Interference (MUI) in the downlink of a Direct-Sequence Code Division Multiple Access (DS-CDMA) system like UMTS, chip equalization suppresses the Inter-Chip Interference (ICI) and hence restores the orthogonality of the user signals. However, effective ICI suppression calls for multi-channel reception at the mobile through receive diversity. Antenna diversity usually requires large antenna spacings for maximum diversity gain, and is therefore seldom implemented. As we will show, polarization diversity solves this spacing problem, while keeping the same benefits as antenna diversity. Assuming perfect Channel State Information (CSI), the ideal Minimum Mean Squared Error (MMSE) multi-channel chip equalizer realizes a significant performance improvement compared to the conventional RAKE receiver. However, in practice, CSI needs to be acquired or updated before the actual equalization and detection can take place. To tackle this CSI acquisition/updating problem, we propose two methods for direct chip equalizer design. The first method, coined pilottrained, only exploits the presence of a code-multiplexed pilot. The second method, coined semi-blind, additionally exploits the knowledge of the unused spreading codes. For both methods, we devise Recursive Least Squares (RLS) as well as Least Mean Squares (LMS) type of adaptive algorithms. The different algorithms are compared both in terms of performance and complexity.