Abstract
This paper investigates the performance of spacedivision multiple access (SDMA) schemes in cognitive radio systems (CR) with multiple primary users (PUs) and secondary users (SUs) sharing the same spectrum. By adding more constraints to the conventional SDMA schemes and using the optimization of the Lagrange multipliers we developed an iterative weights calculating method to determine the new cognitive beamforming (BF) vectors for the secondary network. The beamformer is designed to maximize the signal-to-interference plus noise ratio (SINR) at the secondary base station, defined as the ratio of the desired cognitive signal power to the total interference power from all other secondary and primary users, while controlling the interference levels to the PUs. Using simulation, the performance of the conventional SDMA schemes is compared to the constrained one. It is noted that at low SNR the symbol error rate (SER) and the system capacity obtained for the CR system are very close to the ones obtained by the traditional schemes. It is also noted that as we increase the SNR of the CR, the gap between the system capacity of the constrained SDMA scheme and that of the traditional one becomes significant.
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