The use of software controlled passive Reconfigurable Intelligent Surface (RIS) in wireless communications has attracted many researchers in recent years. RIS has a certain degree of control over the scattering and reflection characteristics of the electromagnetic waves, compared to the conventional communications in which the received signal is degraded due to the uncontrollable scattering of the transmitted signal and its interaction with the objects in propagating medium. Further, in RIS assisted communications, the phases of the multiple incoming signals can be controlled to enable constructive addition of multiple signals from different channel paths to improve Signal to Noise Ratio (SNR). On the other hand, Non-Orthogonal Multiple Access (NOMA) provides massive connectivity and low latency. The power domain variant NOMA uses superposition coded symbols with different powers for different user symbols. In this paper, a novel RIS assisted downlink NOMA system is proposed by combining the merits of both RIS and NOMA to improve the reliability of the system. Analytical expressions are derived for the Bit Error Rate (BER) performance of the proposed RIS assisted power domain NOMA system. The BER performance of the proposed system is analyzed using the numerical simulation results. It is observed that the proposed system has better performance than the conventional NOMA system.
In this article, the bit error rate (BER) performance of a tightly packed reconfigurable intelligent surface (RIS) assisted communication system is analyzed in terms of the level of spatial correlation. In conventional models, channels are assumed to be independent and identically distributed and continuous phase shifts are available for phase compensation. However, in practice, as RIS reflecting units are arranged in more compact form (less than 0.5λ$$ \lambda $$) due to space constraints, correlation effects among the reflecting elements are to be considered. Further, discrete phase shifters are widely used to meet practical deployment limitations. These practical constraints severely affect the error performance of the system. It necessitates that the impact of spatial correlation and discrete phase shifts on the BER performance of the system is to be analyzed. The instantaneous signal to noise ratio is the square of the sum of correlated random variables. In order to analyze the BER performance accurately, the exact statistical distribution for the sum of spatially correlated random variables is to be chosen. In this work, appropriate distribution fitting is chosen based on the level of spatial correlation. Numerical BER expressions are derived with the chosen statistical distributions. The analysis on the impact of spatial correlation and discrete phase shifter on BER performance would be more beneficial for design and practicing engineers in the field of 6G wireless communications.
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