Nonlocal Reconfigurable Intelligent Surfaces for Wireless Communication: Modeling and Physical Layer Aspects

Abstract

Delivering wireless ultrahigh-speed access at wider coverage is becoming considerably challenging due to the prohibitive investment costs per user and the necessary shift to range-limited millimeter-wave (mmWave) transmissions. Reconfigurable intelligent surfaces (RIS) are expected to extend the reach of mmWave and TeraHz signals more cost-effectively in situations where fiber backhaul and fronthaul are not accessible or infrastructure densification is costly. This paper investigates some challenges facing this technology, particularly in terms of scalability and the question of what type of RIS configurations would be appropriate for mmWave networks and what design strategies can be adopted to optimize the performance and minimize the signaling overhead. We conclude that RIS configurations for the wireless infrastructure likely need to be nonlocal (i.e., redirective, wavefront-selective) rather than local (i.e., reflective) to support communications and networking tasks such as integrated fronthaul and access (IFA) most efficiently.

Age-Limited Capacity of Massive MIMO

“…wherein γ 0 is a regularization parameter to be tuned. By vectorizing the cost function in (9) and defining C ≜ (B T ⊗ A) ∈ C M N ×K 2 it follows that: 3 Without loss of generality, α is taken to be real valued since any phase can be included in the precoding matrix F. 4 The constraint 7e is needed if an amplifier is introduced within the port connection for better gain [37].…”

“…This appreciably reduces the incurred signaling overhead and operational complexity for RIS-aided systems. In fact, the control overhead scales only logarithmically with the RedRIS size [37] in contrast to the reflective RIS where the control overhead scales linearly with the number of phase shifters [40], [41] and even more so in BD-RIS systems. Although the lens-type RedRIS is a passive network element, it can be integrated with two-port amplifiers contrarily to the reflective RIS which would require reflection-mode amplifiers where stability becomes a major issue [37], [42].…”

“…In fact, the control overhead scales only logarithmically with the RedRIS size [37] in contrast to the reflective RIS where the control overhead scales linearly with the number of phase shifters [40], [41] and even more so in BD-RIS systems. Although the lens-type RedRIS is a passive network element, it can be integrated with two-port amplifiers contrarily to the reflective RIS which would require reflection-mode amplifiers where stability becomes a major issue [37], [42]. To the best of our knowledge, aside from our previous work in [30], [37], the use of lens-type RedRIS for electromagnetic radiation control has not been investigated in the open literature.…”

“…Although the lens-type RedRIS is a passive network element, it can be integrated with two-port amplifiers contrarily to the reflective RIS which would require reflection-mode amplifiers where stability becomes a major issue [37], [42]. To the best of our knowledge, aside from our previous work in [30], [37], the use of lens-type RedRIS for electromagnetic radiation control has not been investigated in the open literature. Such lenstype smart surface is called redirective RIS (RedRIS) due to its capabilities of wirelessly redirecting the impinging waves to their intended destinations in the downlink and uplink modes alike.…”

“…An in-depth study of redirective RIS can be found in[37]. Additionally, the network controlled repeater[38], a standardized 5G component, can also be regarded as a particular type of RedRIS combined with an amplifier within the port interconnection.…”