Investigation of Efficient Antenna Array Polarization and Beamforming for the Cognitive Radio Application

Abstract

Radio spectrum is becoming increasingly scarce as more and more devices go wireless. Meanwhile, studies indicate that the assigned spectrum is not fully utilized. Cognitive radio (CR) technology is envisioned to be a promising solution to address the imbalance between spectrum scarcity and spectrum underutilization. It improves the spectrum utilization by opportunistically accessing the unused or underutilized spectrum owned by primary systems. However, because of spectrum sharing, the interference problem is always the limiting factor that hinders the practical implementation of CR network. In this thesis, we focus on mitigation of interference problems in the uplink of CR BS system by exploiting signal processing in combined polarization ad spatial domain. In fact, the main objective of this work is to study and recommend optimal antenna array polarization and robust adaptive beamforming algorithms for uplink of CR-Base Station (CR-BS) which can reduce the effects of interference, noise, and multipath fading. We have proposed to equip a CR-BS receiver system with dual-polarized array of crossed-dipole antenna. In the beamformer design, we studied and investigated three adaptive algorithms namely; Quaternion-valued Minimum Variance Distortionless Response (QMVDR), Quaternion valued worst case constraint (QWCC), and Quaternion-valued Least Mean Square (QLMS) beamformers. The mathematical analyses of these algorithms are carried-out, and performance surfaces such as; output Signal-to-Interference plus Noise Ratio (SINR), beampatterns, and mean square error are investigated. Matlab simulation test-bed is created to compare the performance of these three robust algorithms with those of conventional adaptive beamforming algorithms presented in literature. It is shown from the simulation results that all the proposed algorithms can outperform the other algorithms in all the test conditions (e.g., identical and separate polarization states of desired signal and interference, small and large angle separation, etc.). Finally from analysis and simulation results it is proved that a CR BS system employed with dual-polarized antenna array and adaptive algorithm at uplink can adaptively enhance the signals of CR users by matching in polarization as well as Direction of Arrival (DOA). Furthermore, the interference mitigation becomes more effective due to the possibility of filtering in both polarization and DOA, thereby enabling spectrum sharing for CR even in severe interference scenario.