C46. Robust beamforming in multi-users cognitive radio system

In cognitive radio (CR) networks with multi-antenna links, secondary users (SUs) can transmit over the same spectrum as primary users (PUs) with the condition that the SUs interference must not exceed a certain level on PUs. Our target is to provide the SUs with a minimum acceptable quality-of-service (QoS), while keeping the interference to the PUs below a given threshold. However, it is not easy to know the channel state information on the links to PUs, thus uncertainty should be considered on those links. In this paper we present a study of the optimal transmit beamforming weights of the SUs network in the presence of uncertainty on multiple-input multiple-output (MIMO) PU links that is modeled by two different methods. Moreover the two models of the problem are solved using semidefinite programming (SDP) optimization technique and they are both compared to a previous model using computer simulations.