Aspects of Fixed-point Implementation of Multiuser Detctors in WCDMA

Licentiate thesis, 2005

Wideband code-division multiple-access (WCDMA) has been selected as the radio technique standard for the 3G mobile communication systems. However, it is well known that the conventional single-user detector, which is commonly used in CDMA systems, suffers from multiple-access interference (MAI). A possible remedy is to introduce a multiuser detector (MUD). Research on MUDs in the past two decades has proved that MUD can provide significant capacity improvement at the cost of high computational complexity. With the development of digital signal processors (DSPs), implementing a multiuser detector for commercial use in a WCDMA system has become feasible, although the trade-off between performance gain and complexity still has to be taken into account. It has been proposed that high speed superconducting digital technology based on rapid single flux quantum (RSFQ) logic is suitable for implementing a successive interference canceller (SIC). The work in this thesis determines the minimum precision needed for a fixed-point SIC, such that the system can achieve a satisfactory performance. A fixed-point design procedure was implemented to obtain the necessary precision. The results offer a useful guide for designing an SIC with RSFQ. A model was developed for the fixed-point SIC. The advantage of this model is that it can be analyzed. Thus the performance of a fixed-point SIC of given precision can be predicted. With the help of this model, the implementation of a fixed-point SIC was simplified compared to using the fixed-point design procedure. The analysis on this model has a good agreement with simulations for a moderate loaded system, especially for the high precision, which is of most interest. The gain of a sub-optimum multiuser detector (a decorrelator) in a convolutionally coded WCDMA system was also studied. The system applies the decorrelator separately from the channel decoder (i.e., no feedback in the detection). Simulations show that the single-user Rake receiver outperforms the decorrelator in a range of low signal-to-noise ratio (SNR) values. This SNR range increases with the error correction capability of the channel code. It was concluded that a decorrelator offers no gain at low SNR values for a convolutionally coded WCDMA system. When targeting at a reasonable bit-error rate (BER) level, the single-user detector is superior to the decorrelator.