Utilizing reconfigurable intelligent surfaces (RIS) equipped with numerous passive reflecting elements to control wireless communication environment has garnered significant interest from both academic and industrial circles. In this paper, we study the performance of RIS-aided wireless communication systems based on the path loss model in lower frequency band and mmWave band. Based on the proposed model, it is difficult to directly study and obtain the closed-form expressions for the achievable capacity, since that millimeter- wave (mmWave) band may lead to the near-field communication (NFC) environment. Consequently, the upper bound of the achievable capacity are analysed with Hölder inequality in NFC. Simulation and analytical results indicate the correctness of our derived expressions, which shows that the RIS elements number, the RIS elements length, and the angle of signals transmitting from RIS to user can affect the upper bound of achievable capacity for our proposed system.

The digital representation of various signals allows, at the subsequent stages of their transmission, to apply correction codes that provide protection against possible errors arising from the action of interference in the communication channel. At the same time, it is important that, with the required correcting ability, these codes have the maximum possible speed. The article presents the results of calculations for linear codes, showing their really achievable limiting capabilities.

This paper considers a hybrid relay network consisting of the source, the amplify-and-forward (AF) relay, the decode-and-forward (DF) relay, and the destination. We propose the optimal power allocation schemes between two different relays which maximize the achievable rate under a sum relay power constraint for given channel gains and transmit power from source. By solving the optimization problem to maximize the achievable rate for each relay network, the transmit power values in closed-form are derived. When the channel gains are the same, the optimal power allocation scheme for AF-DF relay network proves that a more power should be allocated at the first relay to maximize the achievable rate. In case of the DF-AF relay network, we derive the optimal power allocation scheme for the possible four cases. Under the same SNR condition at the first hop, we show that the achievable rate of AF-DF relay network is greater than that of DF-AF relay network when the channel gain between two relays is higher than that between the second relay and destination. Simulation results show that the proposed power allocation schemes provide a higher achievable rate than the equal power allocation schemes.