Performance Improvement in Ds-spread Spectrum Cdma Systems Using a Pre- and a Post-rake

The Rake receiver is a well known technique in spread spectrum systems, which is used to obtain signal diversity in multipath environments. In TDD systems, a technique known as pre-Rake has been proposed recently, in which the signal-processing is transferred to the transmitter, and it has been shown that this technique provides a capacity increase in the downlink. In this paper we propose a combination of both methods and show that a substantial performance improvement can be reached this way. 1 Introduction Mobile communications systems based on Code Division Multiple Access (CDMA) and direct sequence spread spectrum techniques have been gaining a lot of ground lately. This technique, already being used in current IS95 [1]systems, is also going to be employed in the next generation of mobile communication systems [2,3] currently under standardisation. One of the main advantages of DS-SS transmission is its resilience to multipath propagation, which can be used to obtain signal diversity by employing a Rake receiver[4]. Another technique that has been increasingly popular is Time Division Duplexing (TDD) in which the same carrier is used for both upand downlink in different time slots. This technique is employed in the European cordless system DECT [5] and is likely to be used in part of the frequency range allocated to third generation systems [2,3]. One property of such systems is that, since the same frequency is used, the channel characteristics are nearly the same in both links, provided the channel does not change too rapidly. This is useful for example when smart antennas are used, since the optimised pattern used for uplink reception is also optimal for downlink transmission. This fact also implies that knowledge of the channel impulse response, which is estimated upon reception, can be obtained before transmission. This information can be used to achieve multipath diversity gain using signal processing at the transmitter, instead of employing a Rake receiver at the mobile station. This technique, known as pre-Rake, was first suggested by Esmailzadeh and Nakagawa[6,7]. With a pre-Rake transmitter at the BS, the mobile station needs just a conventional receiver instead of a more complex Rake receiver, and can be kept simple and cheap since the signal processing is transferred to the base station. It can be shown that the single user performance obtained with a pre-Rake is similar to the one obtained with a conventional Rake receiver [6], provided good channel estimates are available before transmission. It was also shown that the pre-Rake improves downlink system capacity in comparison to a Rake receiver [7,8]. The maximisation of the downlink capacity is particularly important if we bear in mind that future communications systems will have to support asymmetric services, like internet browsing or video on demand, and these will probably offer more traffic in the downlink. Capacity enhancing techniques like multi-user detection are however too complex and should be avoided in the downlink if the mobile stations are to be kept cheap and power thrifty. The pre-Rake represents a good simple solution to the capacity problem in the downlink if the necessary conditions (a priori channel knowledge) are satisfied. In this paper we will show that the link performance and downlink system capacity can be even further improved if a combination of a pre-Rake at the transmitter and a Rake receiver, which we call a post-Rake, is used. The post-Rake is basically like a conventional Rake receiver, but it is matched to the combination of pre-Rake and multi-path channel. The post-Rake requires little extra complexity in relation to a conventional Rake receiver and provides a significant capacity gain. In Section 2 a qualitative analysis will be made to show why a performance improvement can be expected with a post-Rake. In Section 3 the post-Rake will be better described. Using a semi-analytical approach, we will calculate the performance of a transmission link with a preand a post-Rake and compare it with the one obtained with a conventional Rake receiver or with a preRake. In Section 4 the multi-user environment will be investigated, and we will show that a significant capacity increase can be achieved in the downlink with the use of a post-Rake. Finally, in Section 5 a brief conclusion will be presented. 2 Motivation The Rake receiver [4] is a well known technique in spread spectrum systems that makes use of the signal diversity inherently provided by multi-path propagation. Suppose we have a channel1 with frequency response H(f) and additive white gaussian noise, then a Rake receiver with maximum ratio combining and ideal channel estimation corresponds to a channel matched filter H*(f) (see Fig. 1), which according to the theory [4] maximises the signal-to-noise ratio. The Rake receiver is hence the optimal technique in channels with white noise if no pre-processing is made at the transmitter. The pre-Rake is also a channel matched filter, but the filtering occurs before transmission, and its performance in the presence of white noise with no further processing at the receiver is the same as with a Rake receiver[6,8], provided ideal channel knowledge is available beforehand. The pre-Rake alone is however not yet optimal, since, as already mentioned, the signal-to-noise ratio can be maximised with a matched filter at the receiver, which has not been considered in the original pre-Rake implementation. However, this filter has to be now matched not to the channel only, as in a conventional Rake receiver, but to the combination of channel and pre-Rake, as shown in Fig. 1. This filtering, which we call a postRake, maximises the signal-to-noise ratio when a preRake is employed. A better performance can be thus obtained in relation to a simple pre-Rake, and consequently in relation to a conventional Rake receiver. The reason for the better performance with a preand a post-Rake is that, with a channel-matched pre-filtering, the signal power is concentrated at the frequency ranges where the channel is most favourable. The principle behind channel matched transmission is analogous to the concept of water-filling [9], i.e., we should not waste signal power where the channel is noisy. Fig. 1 Concept of a pre/post-Rake 3 System description and performance analysis (single user) As channel model we have considered a baseband tapped delay line with tap spacing equal to the chip period. The channel changes slowly in relation to a transmission frame, so that the channel can be assumed to be timeinvariant within a frame. We consider data transmission with pulse shaping in band limited channels, where the shaping filter satisfies the Nyquist criterion. Ideal sampling is also assumed, so that no inter-symbol interference arises due to the filter. We also assume that both transmission and reception filters have unit energy. Suppose the discrete transmitted signal before filtering is s(k), then the sampled signal after the reception filter will be