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In this paper, we study user selection (US) strategies for a multiple-input–multiple-output (MIMO) cognitive radio (CR) downlink network, where the r-antenna underlay CR secondary users (SUs) coexist with a primary user (PU), and all terminals are equipped with multiple antennas. Two main scenarios are considered: 1) The t-antenna cognitive base station (CBS) has perfect or partial channel state information at the transmitter (CSIT) from the CBS to the PU receiver (RX), and 2) the CBS has absolutely no PU CSIT. For these scenarios, we propose and evaluate multiple SU selection schemes that are applicable to both best-effort PU interference mitigation and hard interference temperature (IT) constraints. The computational complexity of the proposed schemes can be significantly smaller than that of an exhaustive search with negligible performance degradation. For the selection of C SUs out of K, whereas an exhaustive search is on the order of \mathcal{O}(\binom{K}{C}C^4r^3). When t$ and r$ are of the same order, the computational complexity of the proposed scheme can be \binom{K}{C}C^4/K times smaller. Mathematical complexity analysis and numerical simulations are provided to show the advantage of our schemes. [ABSTRACT FROM PUBLISHER]

This paper considers a two-way amplify-and-forward (AF) multiple-input multiple-output (MIMO) relay network consisting of two sources (each with multiple antennas) and multiple physically dispersed single-antenna relays. A central station finds the optimum relay amplifying matrices and transmit/receive beamforming vectors explicitly and iteratively under the transmit power constraints at the sources and the relays. The minimum-mean-square-error criterion is used. Numerical results show that the proposed scheme outperforms the existing two-way AF MIMO network schemes proposed by Arti et al. and Wang et al. [ABSTRACT FROM PUBLISHER]

This paper presents a high-efficiency ferrite meander antenna (HEMA), which can be used to realize a 2 x 2 multiple-input-multiple-output (MIMO) communication system when it is used at both the transmitter and the receiver ends. This antenna is designed to operate at 2.45 GHz center frequency (fc). It consists of two spatially separated half-cycle microstrip meander structures. Ferrite material is not used for the entire substrate, only beneath each meander structure. A standard FR-4 substrate is utilized as a system board. Impedance bandwidth and radiation patterns of the fabricated antenna are measured and compared with those of the simulation results. The -10 dB impedance bandwidth of the fabricated antenna is 262 MHz, whereas the simulated bandwidth is 235 MHz. According to the simulations, the gain and efficiency of the antenna are 2.2 dB and 81%, respectively. The efficiency of the antenna is confirmed by measurements. By using the simulated radiation patterns, correlation between the radiation patterns is calculated and employed in the generation of the channel matrix. Mutual impedance of the antennas and antenna efficiency are also included in the channel matrix, which in turn is used in bit error rate (BER) and ergodic capacity simulations. BER and ergodic capacity are utilized as performance metrics. The effect of antenna efficiency, mutual impedance of the antennas, and correlation between radiation patterns on system performance are presented. [ABSTRACT FROM AUTHOR]

This current paper addresses the noncooperative distributed wireless relay networks under a jamming environment. The amplify- and-forward single-input multiple-output (SIMO) and multiple-input single-output (MISO) schemes are considered with multiple N-relay nodes. The main contribution of this paper is to determine the optimal relay precoder vectors by minimizing the minimum mean square error (MSE) criterion. Using the derived optimal relay precoder vectors, the impacts on the jamming environment in the AF SIMO/MISO wireless relay systems are investigated using bit error rate (BER). Results show that the diversity order of a system decreases under a jamming environment. It is also seen that the system BER performance enhances as the number of relay nodes N increases. [ABSTRACT FROM AUTHOR]