On the downlink of an SDMA mobile radio system, different users who share the same frequency band and time slot have to be spatially separated by beamforming. In this paper we analyze an approach to jointly calculate weights in such a way that all users receive their signal with a given signal-to-noise-and-interference ratio. The computations are based on spatial covariance matrices which describe the user-specific channels averaged over the fast fading. The presented algorithm succeeds in minimizing the array transmit power while spatially separating the users, no matter which rank their spatial covariance matrices have. But since it needs to solve a complex nonlinear constrained optimization problem, several approximations of this algorithm are presented and compared to each other on the basis of array power, complexity, and convergence.

In SDMA mobile radio systems where only base stations, but no mobiles are equipped with antenna arrays, co-channel interference on the downlink has to be kept down by beamforming. The performance of the beamformer is determined by the directions of arrival (DOAs) of the users operating in the same frequency/time/code slot. Thus the full capacity of an SDMA cell can only be made use of by an effective channel allocation scheme. In this paper, a real time concept for channel allocation is presented. Furthermore, several criteria to quickly evaluate the spatial separability of subsets of users, were defined and then compared to each other by means of simulations.

On the downlink of an SDMA mobile radio system, co-channel interference has to be kept down by beamforming. In this paper two DOA-based beamforming approaches are presented minimizing array signal power while maintaining given signal-to-noise-and-interference ratios for all users. The linear approach is computationally cheap, so that it is suited for channel allocation as well, quickly evaluating the spatial separability of a specific mobile radio scenario. The nonlinear algorithm yields optimum results but only converges quickly, if provided with a good starting point. Therefore, its basic application is burst-to-burst updating of the beamformer.

An approach is proposed to introduce a space division multiple access (SDMA) component into a frequency/time division multiple access (FTDMA) mobile radio system like the European GSM system providing higher frequency reuse and spectral efficiency. Except from signalling aspects the mobiles remain uneffected by the presented SDMA method without any need for antenna diversity or more sophisticated equalizers. Additional hard- and software-demands due to the new SDMA features are only restricted to base stations as they have to be equipped with an adaptive antenna array. This array is used to separate wavefronts in coherent multipath environments by means of a new algorithm. The number of antenna elements needed is significantly lower than the number of wavefronts impinging on the array.