Rake receiver analytical study for TD-CDMA downlink evaluation

DMA systems generate intra-cell interference due to multipath propagation and or asynchronous transmission. Whereas interference cancellers or joint detectors are envisaged to reduce the interference in the uplink, CDMA mobile handsets are more limited in complexity. This paper is focused on the evaluation of the RAKE receivers when spreading sequences have short periods in a synchronous downlink transmission with multipath propagation. The bit-error-rate evaluation based on the randomness assumption of spreading codes can not be applied. We show that exact evaluation of the bit-error-rate, exploiting the sequences properties for any multipath channel profile (considering fractionally chip delayed paths) is feasible. This proposed evaluation is applied to the TD-CDMA mode of UMTS. This analytical study can be easily implemented and gives the performances for different scenarios avoiding simulations. It can also be useful to optimize the channel allocation code strategy or to evaluate the equivalent interference factor needed for capacity evaluation.

1. Introduction

In the third generation mobile system, the TDD (Time Division Duplex) mode is based on a combination of TDMA (Time Division Multiple access) and CDMA (Code Division Multiple Access) techniques. The TDD mode is intended for applications in micro and picocell environments and is well suited for asymmetric applications as well as for multirate applications with multi-slot and/or multicode allocation. The TDMA component combined with short spreading codes make the use of the joint detection feasible for the base station reception [1]. Mobile handsets may use a basic RAKE receiver or multi-user Joint detectors according to tradeoffs between performance and complexity. In any case, it must be pointed out that it is interesting to study the performances obtained with the basic RAKE receiver in order to determine the limits of such a receiver and to decide if or not the implementation of more complex receivers has to be done. In a single-user context the RAKE receiver is optimal on a white additive gaussian channel without multipath, since in that case, it corresponds to the matched filter receiver for direct spreading sequence modulations. The matched filter receiver is still optimal in a CDMA multi-user context when the spreading sequences are orthogonal. This is the case of a synchronous downlink transmission without multipath propagation. The RAKE receiver becomes a sub-optimal solution when multipath propagation occurs because it generates intersymbol as well as multiple access interference. The degradation depends on the channel time spreading and on the correlation properties between spreading sequences. The aim of this paper is to evaluate this degradation by evaluating analytically the RAKE receiver performance for the TD-CDMA downlink transmission. This evaluation is compared to the matched filter bound (MFB) in the case of a single code transmission. Indeed, the performances of a CDMA system in a multi-user context is lower bounded by this MFB due to multipath, which can be evaluated for any channel profile and for any signal waveform [2], [3], and in particular for specific signature CDMA spreading waveforms.