Downlink Capacity Enhancement by Employing SDMA in GSM

In this paper we investigate the downlink capacity enhancement in GSM by employing an antenna array at the base station. The Space Division Multiple Access (SDMA) approach allows multiple users to be served on the same frequency and timeslot. The proposed two-dimensional channel model reflects a realistic spatio-temporal scenario of scatters and is compatible with GSM specifications. The downlink beamforming scheme employed, based on the estimation of spatial covariance matrices, maximises the average carrier-to-interference ratio (CIR), but may suffer from the non-observability of the downlink channel in FDD. To overcome this problem, a frequency transposition technique is used to approximate the downlink from the uplink channel second order statistics. Furthermore, in order to ensure that CIR for each user is kept above a required level and to optimise the trade-off between capacity and CIR performance, two spatial channel allocation strategies are applied. The resulting capacity enhancement is evaluated via simulations in terms of several system parameters.