Abstract
A communication link aided by a reconfigurable intelligent surface (RIS) is studied in which the transmitter can control the state of the RIS via a finite-rate control link. Channel state information (CSI) is acquired at the receiver based on pilot-assisted channel estimation, and it may or may not be shared with the transmitter. Considering quasi-static fading channels with imperfect CSI, capacity-achieving signalling is shown to implement joint encoding of the transmitted signal and of the response of the RIS. This demonstrates the information-theoretic optimality of RIS-based modulation, or “single-RF MIMO” systems. In addition, a novel signalling strategy based on separate layered encoding that enables practical successive cancellation-type decoding at the receiver is proposed. Numerical experiments show that the conventional scheme that fixes the reflection pattern of the RIS, irrespective of the transmitted information, as to maximize the achievable rate is strictly suboptimal, and is outperformed by the proposed adaptive coding strategies at all practical signal-to-noise ratio (SNR) levels.
Highlights
In the context of wireless communications, a reconfigurable intelligent surface (RIS) usually acts as an “anomalous mirror” or a “focusing lens” that can be configured to reflect or refract impinging radio waves towards arbitrary angles by applying appropriate phase shifts to the incident signals [1], [2]
The high-signalto-noise ratio (SNR) asymptotic limit in (31) implies that, in the high-SNR regime, capacity is achieved by using independent random codebooks with uniform distribution for the codeword symbols s and the RIS reflection pattern θ, and perfect channel estimation can be obtained by using ≥ pilot sub-blocks
We illustrate and discuss numerical examples with the main aims of (i) comparing the capacity achieved by the proposed joint encoding scheme with the achievable rates attained by the max-SNR and the layered encoding schemes, and (ii) assessing the impact of imperfect Channel state information (CSI)
Summary
In the context of wireless communications, a reconfigurable intelligent surface (RIS) usually acts as an “anomalous mirror” or a “focusing lens” that can be configured to reflect or refract impinging radio waves towards arbitrary angles by applying appropriate phase shifts to the incident signals [1], [2]. The acquisition of CSI is made complicated by the fact that the RIS is a nearly-passive device, and it cannot process and transmit pilot signals To account for this practical constraint, in this paper, the information-theoretic analysis is based on a model in which the CSI is estimated at the receiver via pilot-assisted transmission [14], and it may or may not be shared with the transmitter. Realizations of random variables, vectors, and matrices are denoted by lowercase, boldface lowercase, and boldface uppercase italic-font letters, respectively. The Kronecker product of matrices and is denoted by ⊗
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
