In the near-field of a reconfigurable intelligent surface (RIS), the steering vector of the channel between the RIS and a user depends on both azimuth and distance. As a result, new channel estimation schemes, which are required for configuring the RIS and data demodulation, are needed. We study a low-training-overhead parametric maximum-likelihood (ML) scheme that estimates the directional cosine, distance, and complex gain to reconstruct the RIS-to-user channel gain. We derive novel expressions for the Cramér-Rao lower bound for the mean square error (MSE) of the estimated channel parameters and the RIS-to-user channel gain. Using the asymptotic efficiency of the ML estimate, we present novel, insightful expressions for the statistics of the cascaded channel gain between the access point and the user in the presence of errors in configuring the RIS. These are also essential for building its linear minimum MSE estimator. We then derive an expression for the achievable rate in the presence of estimation errors, which leads to insightful, closed-form expressions for the optimal pilot and data powers. Our approach requires fewer pilots than the number of RIS elements and achieves a lower MSE and a higher rate than several benchmarking schemes.

The steering vector for the channel between a reconfigurable intelligent surface (RIS) and a user depends on the distance and the azimuth of the user relative to the RIS when the user lies in the near-field of the RIS. Hence, techniques for estimating the RIS-user channel’s gain differ from those for conventional far-field estimation. This estimate is necessary to configure the RIS phases. We study a low training overhead parametric maximum likelihood scheme that estimates the directional cosine, distance, and complex gain to reconstruct the RIS-user channel. We present a novel linear minimum mean square error (LMMSE) estimator for the access point (AP)-RIS-user effective channel gain. Estimating this channel is necessary to demodulate data. Our estimator is based on novel expressions for the mean and variance of the effective channel gain. It accounts for the impact of errors in estimating the RIS-user channel, which leads to an imperfect RIS phase configuration, and the noise while estimating the effective channel gain. We present novel expressions of the Cramer-Rao lower bound of the mean square error (MSE) of the estimated channel parameters and the RIS-user channel gain. Our approach achieves a lower MSE and a lower symbol error probability than several benchmarking schemes. Fewer pilots than the number of RIS elements are required to achieve a given NMSE.

The reconfigurable intelligent surface (RIS)-assisted wireless communication system is studied in this paper, where two full-duplex (FD) users simultaneously communicate with each other. The reception at each user is facilitated through a dedicated RIS. Owing to the geometry and placements of meta-atoms, the analysis incorporates spatial-correlation among the channels related to RIS. Using moment-matching approach, a new analytical framework is presented for spatially-correlated channels considering the optimal phase-shifts from RIS meta-atoms. The effects of residual self-interference (RSI), self-reflections and interference from the RIS of transmitting user are considered in the analysis. The expressions are derived for the outage probability and ergodic capacity for infinite block-length transmission, and for finite block-length transmission, the expressions are obtained for block-error rate (BLER), ergodic capacity and system goodput. To gain further insights, the expression for the asymptotic outage probability is also derived. The finding of this paper confirms the ability of RIS-assisted communication to counter the detrimental effects of self-interference in FD scenario, even in presence of spatial-correlation. The derived expressions are validated using the Monte-Carlo simulations.

This paper investigates reconfigurable intelligent surface (RIS)-based multiple-destination downlink communication system under ultra-reliable low-latency communications (URLLC) constraints. A source communicates with multiple destinations with the aid of RIS and direct path transmissions. We consider the effect of spatial correlation arising due to the RIS geometry. Assuming that the source can communicate with only one destination in the given time-slot, we examine several scheduling schemes, such as random (RND), absolute (ABS) channel power based (CPB) and normalized (NRM) CPB. The expressions for the block-error rate (BLER) are derived for all the scheduling schemes and verified using the Monte-Carlo simulations. Effects of various parameters such as, channel uses, number of destinations, number of information bits, correlation are analyzed and the performance for the considered scheduling schemes is compared.

Abstract An early stage detection of Parkinson's disease (PD) is crucial for its appropriate treatment. The quality of life degrades with the advancement of the disease. In this paper, we propose a natural (time) domain technique for the diagnosis of PD. The proposed technique eliminates the need for transformation of the signal to other domains by extracting the feature of electroencephalography signals in the time domain. We hypothesize that two inter‐channel similarity features, correlation coefficients and linear predictive coefficients, are able to detect the PD signals automatically using support vector machines classifier with third degree polynomial kernel. A progressive feature addition analysis is employed using selected features obtained based on the feature ranking and principal component analysis techniques. The proposed approach is able to achieve a maximum accuracy of 99.1±0.1%. The presented computer‐aided diagnosis system can act as an assistive tool to confirm the finding of PD by the clinicians.

The paper explores simultaneous transmitting and reflecting (STAR) reconfigurable intelligent surface (RIS)-aided rate-splitting multiple access (RSMA) communication. A base-station (BS) transmits information to the cell-edge users which are assisted by the STAR RIS in such a way that two users, one placed in front of and other behind the RIS, are served by reflection and transmission, respectively. The energy splitting (ES) and mode switching (MS) configurations of the STAR RIS are considered for analysis. Owing to the close-placement of RIS elements and rectangular geometry of RIS, the Rician distributed RIS channels are assumed to be spatially-correlated. The new expression for the joint moment of spatially-correlated Rician channels is derived. Both, infinite and finite block-length transmissions are analyzed in this work. For infinite block-length transmission, the expressions for outage probability and channel capacity are derived, whereas for finite block-length transmission, the expressions for block error rate (BLER), system goodput and channel capacity are derived. To get further insights, the expression for asymptotic outage probability is also derived. Furthermore, the effects of various parameters, including RSMA power splitting factor, reflection coefficients, inter-element spacing, distances between STAR RIS and users, finite block-length parameters, imperfect channel state information (CSI), etc., are examined.

We consider a full-duplex (FD)-enabled multi-user two-way communication system with adaptive amplify-and-forward (AF)/decode-and-forward (DF) relaying protocol. All the users are assumed to be mobile and to have FD abilities. The effect of mobility, which results in time-selective fading, is modeled using a first-order autoregressive (AR1) process. The fading channel-based approach characterizes the residual self-interference (RSI) at the FD relay, user nodes and is modeled as a Rician distributed random variable. This paper constitutes the most critical use case of 5G, i.e., ultra-reliable low latency communication (URLLC), which adopts short-packets finite blocklength (FB) codes to spin out into the mission-critical applications where strict latency and reliability requirements are highly desirable. The outage performance of the system is studied over independent and non-identically distributed complex Gaussian (Rayleigh envelope) channels with imperfect channel state information (CSI) for with and without URLLC use cases. The closed-form expressions for the outage probability and block error rate (BLER) are derived for the absolute channel power-based scheduling scheme considering the different Doppler power spectra models and the effect of co-channel interference (CCI). The expressions for the asymptotic outage probability are also derived. The presented analysis is compared with baseline schemes, e.g., the results derived with adaptive AF/DF relaying are also compared with both AF and DF relaying, and the performance of the FD transmissions is compared to that of half-duplex (HD) transmissions. The impact of node mobility, RSI, FBL, number of user pairs, and imperfect CSI on the system performance is investigated. Moreover, essential insights are obtained related to the performance gain and region of the superiority of the adaptive AF/DF relaying scheme. The derived analytic results are validated through Monte Carlo simulations. Furthermore, at high transmit power, the outage performance for the adaptive AF/DF protocol approaches the derived asymptotic floor.

A reconfigurable intelligent surface (RIS)-assisted wireless communication system with non-linear energy harvesting (EH) and ultra-reliable low-latency constraints is considered for its possible applications in industrial automation. A distant data-center (DC) communicates with the multiple destination machines with the help of a full-duplex (FD) server machine (SM) and RIS. Assuming the deficiency of enough transmission power at the FD-SM, the SM is considered in the near vicinity of the destinations in the industry to forward the data received from the distant DC. The reception at SM is assisted by the RIS and a non-linear hybrid power-time splitting (PTS) based EH receiver architecture is adopted to extend the lifespan of SM, thus increasing network lifetime. The scheduling of multiple destinations is done by SM based on the considered selection criteria namely, random (RND) scheduling, absolute (ABS) channel-power-based (CPB) scheduling and normalized (NRM) CPB scheduling. The end-to-end performance of the considered FD RIS-assisted network is analyzed, and the expressions for the block error rate (BLER) for all scheduling schemes are derived. Moreover, the effects of number of RIS elements, packet size, channel uses on the system performance are analyzed for the considered ultra-reliable and low-latency communication (URLLC) network. The scheduling fairness of all the scheduling schemes is also analyzed to study the performance-fairness trade-off. The derived analytical results are verified through Monte-Carlo simulations.