Space-time coded massive (STCM) multiple-input multiple-output (MIMO) system provides superior bit error rate (BER) performance compared with the conventional space-time coding and massive MIMO techniques. The transmitter of the STCM-MIMO system consists of a large antenna array. In a practical system, the self-interference created by the signals transmitted by the elements of this antenna array, known as mutual coupling (MC), degrades the performance of the system. The MC effect is pronounced in communication systems with a large antenna array. On the other hand, increasing the number of transmitting antennas results in improved BER performance. Hence, there is a trade off in selecting the optimum number of transmitting antennas in an STCM-MIMO system. In order to take the impact of MC into account, we have derived an analytical expression for the received signal to accurately model the STCM-MIMO system under the existence of the MC effect. We present an algorithm to select the optimal number of antennas to minimize mutual coupling and the system bit error rate (BER). Through computer simulations, we investigate the BER performance of the STCM-MIMO system for different numbers of array elements.
Millimeter wave (mmWave) non‐orthogonal multiple access (NOMA) is a multiuser transmission scheme that can increase the quality and data rate of wireless mobile networks. In this article, we introduce an mmWave‐NOMA system utilizing a sparse antenna array that not only offers small and lightweight antenna devices in the base station but also improves the outage probability performance of the system. We also introduce a low‐complex computation technique to find the optimum beamforming vector to reduce the outage probability. We provide a proof of convexity for the outage probability of the mobiles' communication links in the proposed system for which a gradient‐based algorithm can be used to find the optimum solution. The simulation results show that utilizing the proposed optimum beamforming vector reduces the outage probability of the system up to 98% in high signal‐to‐noise ratios. Our simulation and analysis results demonstrate that the system equipped with a sparse antenna array applying the optimum beamforming vector outperform conventional mmWave‐NOMA system with uniform linear array antennas in terms of outage probability and sum rates.
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