Downlink Beamforming for WCDMA based on Uplink Channel Parameters Christopher Brunner , 1, 2 Michael Joham, 2 Wolfgang Utschick, 2 Martin Haardt, 1 and Josef A. Nossek 2 1. Siemens AG, ICN CA CTO 71 2. Institute for Network Theory and Circuit Design Hofmannstr. 51, D-81359 Munich, Germany Munich Univ. of Technology, D-80290 Munich, Germany Phone / Fax: +49 (89) 722-29480 / -44958 Phone / Fax: +49 (89) 289-28511 / -28504 E-Mail: Martin.Haardt@icn.siemens.de E-Mail: Christopher.Brunner@ei.tum.de Abstract – The downlink spectral efficiency of third generation mobile radio systems is especially important since several services will be asymmetric, i.e., on the av- erage the downlink data rates will be higher than on the uplink. We propose to utilize adaptive antennas at the base stations because spatial interference suppres- sion is able to reduce the near-far effect caused by high data rate connections in the downlink of single-user de- tection DS-CDMA systems. The algorithm that calcu- lates the downlink beamforming vectors takes into ac- count the correlation properties of the spreading and scrambling codes. It is also based on estimates of the downlink channel parameters in terms of the domi- nant directions of arrival, corresponding delays, and corresponding medium-term average path losses. A non-linear minimization problem with non-linear con- straints is set up, where the total transmit power is min- imized while each mobile is provided with the required signal to interference and noise ratio (SINR) at the out- put of its rake receiver. 1 Introduction Future mobile communication systems require a signif- icant increase in capacity to accommodate the growing number of users and to allow new services with higher data rates and a variety of quality of service requirements. The proposed concepts for third generation mobile radio sys- tems allow an easy and flexible implementation of new and more sophisticated services. Recently, ETSI SMG selected the TD-CDMA concept for time-division duplex (TDD) systems and the WCDMA concept for frequency-division duplex (FDD) systems 1 [5]. Adaptive antennas exploit the inherent spatial diversity of the mobile radio channel and perform spatial interference suppression. Therefore, they are an important technology to meet the high spectral effi- ciency and quality requirements. We have investigated the uplink data detection in WCDMA utilizing adaptive anten- nas at the base station (BS) in [2, 3]. In this paper, we focus on the downlink of WCDMA. In general, high data rate connections on the downlink of WCDMA must be transmitted with more power than low 1 This solution has been contributed to the International Telecommu- nication Union - as the European proposal for IMT-2000 transmission technology. HF HF HF + + PA PA PA DAC DAC DAC + Figure 1: Illustration of downlink beamforming for =3 users and antenna elements. data rate connections in order to compensate for the lower processing gain. If the spreading factors differ signifi- cantly, the near-far effect may degrade the performance of the low data rate mobiles significantly. Downlink beam- forming, cf. Figure 1, leads to spatial interference suppres- sion and, therefore, reduces the near-far effect. Moreover, fast fading can be mitigated by exploiting the spatial trans- mit diversity. In the sequel, we assume that the BS is en- hanced with an antenna array. The mobiles are equipped with a single antenna and a conventional maximum ratio combining rake receiver [11]. Notice that the simplicity of the mobile is very important from an economic point of view. Depending on the service, each mobile requires a certain transmission rate and bit error ratio. These parameters set the target SINR required at the output of the mobile maximum ratio combining rake receiver. In [7], a down- link beamforming approach is introduced which provides each user with a given SINR. To this end, a complex non- linear constrained optimization problem is set up and sev- eral approximations are discussed. However, the users in [7] are separated by space only, whereas for WCDMA, separation takes place in the space and the code domain. Therefore, the calculation of the downlink beamforming vectors for WCDMA should also consider the auto- and cross-correlation properties of the spreading and scram- bling codes in addition to the (medium-term) downlink