Double-IRS Aided MIMO Communication under LoS Channels: Capacity Maximization and Scaling

Abstract

Intelligent reflecting surface (IRS) is a promising technology to extend the wireless signal coverage and support the high performance communication. By intelligently adjusting the reflection coefficients of a large number of passive reflecting elements, the IRS can modify the wireless propagation environment in favour of signal transmission. Different from most of the prior works which did not consider any cooperation between IRSs, in this work we propose and study a cooperative double-IRS aided multipleinput multiple-output (MIMO) communication system under the line-of-sight (LoS) propagation channels. We investigate the capacity maximization problem by jointly optimizing the transmit covariance matrix and the passive beamforming matrices of the two cooperative IRSs. Although the above problem is non-convex and difficult to solve, we transform and simplify the original problem by exploiting a tractable characterization of the LoS channels. Then we develop a novel low-complexity algorithm whose complexity is independent of the number of IRS elements. Moreover, we analyze the capacity scaling orders of the double-IRS aided MIMO system with respect to an asymptotically large number of IRS elements or transmit power, which significantly outperform those of the conventional single-IRS aided MIMO system, thanks to the cooperative passive beamforming gain brought by the double-reflection link and the spatial multiplexing gain harvested from the two single-reflection links. Extensive numerical results are provided to show that by exploiting the LoS channel properties, our proposed algorithm can achieve a desirable performance with low computational time. Also, our capacity scaling analysis is validated, and the double-IRS system is shown to achieve a much higher rate than its single-IRS counterpart as long as the number of IRS elements or the transmit power is not small.

Performance Analysis of Large Intelligent Reflecting Surface Aided Moderate MIMO for 5G Communication

“…So, the customer receives the channel directly from the AP at the same time, just as the IRS does. The author in [13], propose a twofold IRSassisted multiple input multiple output (MIMO) communication framework for LOS channels. Advance the transmit co-variance network and the inactive beam forming grids of the two agreeable IRSs to investigate the limit amplification issue.…”

Intelligent Reflecting Surface-Aided LEO Satellite Communication: Cooperative Passive Beamforming and Distributed Channel Estimation

“…Similar to [51], we reasonably consider that the link distance between the GN and IRS 1 is large enough (e.g., considerably larger than the array sizes at the GN and IRS 1) such that 4) can be well approximated as a rank-one LoS channel matrix given by…”

“…Furthermore, most of the prior works on IRS have considered the wireless system aided by one or more non-cooperative IRSs (i.e., each independently serving its associated users) [38]- [44]; while the inter-IRS channel was usually ignored for simplicity. Only recently, the great potential of the cooperative passive beamforming (CPB) over inter-IRS channels has been unveiled in the double-/multi-IRS aided communication system [45]- [51], which was shown to achieve a much higher-order passive beamforming gain than its single-IRS counterpart. In [45], the authors presented an initial study to reveal the CPB gain in a double-IRS aided communication system, where a single-antenna BS communicates with a single-antenna user over the double-reflection link established by two distributed IRSs placed near the BS and user, respectively.…”