Application Of Interference Alignment In Cognitive Radio Systems

Interference alignment is a revolutionary strategy that can reduce the impact of interference in wireless communication system. The idea of interference alignment is to coordinate multiple transmitters so that their mutual interference aligns at undesired receivers. Interference alignment is also extended to cognitive radio systems, by which secondary user can communicate with each other by opportunistically utilizing the spectrum originally allocated to primary users. The study of our work is around the interference alignment in cognitive radio systems. Specifically, the main contributions of this thesis are shown as follows:1. Parameter of intrinsic degrees of freedom(IDoF) which is the measure about relative transmission-ability of channel from transmitter to receiver at finite signal to noise power ratio(SNR) are proposed. In cognitive radio system, the number of spatial dimensions which can be allocated to secondary users by primary user is the number of subchannels whose parameters of IDoF are equal to zero. The number of spatial dimensions allocated to interferer which is no more than the maximum number of subchannels associated with the decreasing parameter of IDoF of transmission channel due to interference in multi-user multi-antennas interference channel is given from information-theory perspective.2. Space-alignment(SA) scheme with K primary users in different cell working in the the same frequency band based on K-symbol extension for cognitive transmission in multiple-input multiple-output(MIMO) system is proposed. The main idea is secondary transmitter sends a supersymbol over K time slot, meanwhile, secondary transmitter aligns its interference directions by precoding to the kth primary channel eigenmodes unused at the kth time slot, and at the same time adapts its transmit power to satisfying interference-power constraints at the other K-1 primary receivers. Simulation results show that our proposed scheme achieves a higher performance in comparison with SA scheme with the single primary user and also with the scheme of the direct singular value decomposition(D-SVD) of secondary channel with interference-power constraint.3. A space-alignment(SA) scheme based on primary transmission-outage(TO)for cognitive transmission in MIMO uplink system is proposed. The main idea is secondary transmitter aligns its transmit directions by precoding to unused primary channel eigenmodes where TO event happened, which is denoted by bit error rate(BER)over a target BER which is the typical minimum value required for acceptable system performance. Our proposed scheme achieves a higher performance in comparison with SA scheme of zero outage.4. Achievable degrees of freedom(DoF) for K-user MIMO interference channel through the opportunistic interference alignment(OIA) is given and the feasibility of OIA used in MIMO channel is also given.