Turbo Coding and Power Control for Wcdma Downlink

The main air interface in the 3rd generation cellular mobile communications is based on wideband code-division multiple-access (WCDMA). In this networks the downlink capacity is expected to be more crucial than the capacity of the uplink due to the asymmetric capacity requirements, i.e., the downlink direction should offer higher capacity than the uplink [1]. Therefore, the use of efficient downlink receivers is important. To achieve an efficient performance, multiuser receivers [2] can be used. The channel equalizer has proven to be one of the most promising terminal receivers for WCDMA/FDD downlink. Thus it has drawn attention and inspired numerous publications in the recent years. The equalization at chiplevel has been treated e.g. in [3-6]. Also a large variety of adaptive equalizers suitable for WCDMA downlink have been presented, and for in-depth discussion the reader is instructed to [7], [8] and references therein. The equalization is performed prior to the spreading sequence matched filtering in the aforementioned references, but the equalization can also be done at symbol level, as shown in [9]. Performance of chip-level equalizer with channel coding was presented in [10] but the considered equalizer was an ideal linear equalizer, thus providing performance bounds for adaptive equalizers. In [11] some results were presented with channel coding for the adaptation method introduced in the paper. In [12] some results concerning the usage of the proposed equalizers combined with turbo coding (optional for the third generation) are presented. However, the fast power control for downlink is a very important part of the system. The aim is to keep the received power per bit at a satisfactory level. It is well known that the power control can be very efficient when the receiver velocity is relative small. On the other hand, turbo codes with long interleaver are expected to combat successfully with short deep fades appearing during the fast moving of the receiver. In this paper the results of a simulation study for the simultaneous implementation of turbo coding and power control for WCDMA downlink receiver are presented. The receivers having the new proposed equalizers [7] and the conventional RAKE receiver are considered. The rest of the paper is organized as follows. The system model is defined in Section II. The employed turbo coding as well as the algorithm for power control are briefly explained in Section III. The simulation results are presented in Section IV, followed by concluding remarks in Section V. II. SYSTEM MODEL