Advanced RAKE receivers for W-CDMA

Abstract

The thesis investigates receiver algorithms and structures developed for mobile terminals in high-speed-data communication over code division multiple access (CDMA) systems. Wideband CDMA (VV-CDMA) for third-generation (3G) mobile communication systems is an air interface designed to support high-data-rate services. It employs variable spreading factors and multi-code schemes to accommodate mobile multimedia communications. The proposed algorithms and structures are capable to suppress interference in high-speed-data transmissions using CDMA based technologies. They are relatively simple in implementation and suitable for the systems with random scrambling and spreading codes. A conventional RAKE receiver with coherent maximum-ratio combining provides effective multipath diversity gain. However, it is well known that the performance of a conventional RAKE receiver suffers greatly from the presence of interpath interference and multi-code interference when low spreading factors and multi-code transmission are used. Advanced receivers such as multiuser detection and interference cancellation can be employed to combat these interference. These receivers perform multiuser demodulation and thus require information of all active users. Hence, they are suitable for base stations, but not for mobile terminals. We present a model for RAKE receivers in the presence of interpath interference. Based on the proposed model, an optimum combining scheme is developed for RAKE receiver to enhance its performance for high-bit-rate, low spreading factor transmissions, e.g. in W-CDMA for 3G. The model is further extended to incorporate the effect of inter-code interference in multi-code transmissions. This leads to a unified view of downlink RAKE reception since multi-code interference and downlink multiple access interference have similar structure. An optimum combining scheme for multi-code downlink is then developed. The proposed optimum combining schemes provide superior performance compared to the maximum-ratio combining (MRC) scheme. Since the structure of intercede interference (ICI) and downlink multiple access interference (MAI) are similar, the proposed optimum scheme is also capable of suppressing MAI at the same time. Additional performance enhancement can be obtained from spatial domain via antenna diversity. A concatenated-RAKE receiver structure is proposed for dual-antenna mobile terminals. The concatenation of the resolved multipath signals from the two antennas effectively doubles the degrees of freedom. This novel receiver structure not only provides spatial diversity gain, but also substantially enhances interference suppression performance. An alternative signal model, which treats IPI as intersymbol interference, is presented. It leads to a two-stage receiver structure. In the first-stage, a modified maximum-ratio combining scheme is used to enhance the combined signal and in the second-stage, maximum likelihood sequence estimation is employed to perform equalization.