6 research outputs found
A Hardware Architecture for Reconfigurable Intelligent Surfaces with Minimal Active Elements for Explicit Channel Estimation
Intelligent surfaces comprising of cost effective, nearly passive, and
reconfigurable unit elements are lately gaining increasing interest due to
their potential in enabling fully programmable wireless environments. They are
envisioned to offer environmental intelligence for diverse communication
objectives, when coated on various objects of the deployment area of interest.
To achieve this overarching goal, the channels where the Reconfigurable
Intelligent Surfaces (RISs) are involved need to be in principle estimated.
However, this is a challenging task with the currently available hardware RIS
architectures requiring lengthy training periods among the network nodes
utilizing RIS-assisted wireless communication. In this paper, we present a
novel RIS architecture comprising of any number of passive reflecting elements,
a simple controller for their adjustable configuration, and a single Radio
Frequency (RF) chain for baseband measurements. Capitalizing on this
architecture and assuming sparse wireless channels in the beamspace domain, we
present an alternating optimization approach for explicit estimation of the
channel gains at the RIS elements attached to the single RF chain.
Representative simulation results demonstrate the channel estimation accuracy
and achievable end-to-end performance for various training lengths and numbers
of reflecting unit elements.Comment: 5 pages, 2 figures, invited/accepted to IEEE ICASSP 202
Reconfigurable Intelligent Surfaces for Energy Efficiency in Wireless Communication
The adoption of a Reconfigurable Intelligent Surface (RIS) for downlink
multi-user communication from a multi-antenna base station is investigated in
this paper. We develop energy-efficient designs for both the transmit power
allocation and the phase shifts of the surface reflecting elements, subject to
individual link budget guarantees for the mobile users. This leads to
non-convex design optimization problems for which to tackle we propose two
computationally affordable approaches, capitalizing on alternating
maximization, gradient descent search, and sequential fractional programming.
Specifically, one algorithm employs gradient descent for obtaining the RIS
phase coefficients, and fractional programming for optimal transmit power
allocation. Instead, the second algorithm employs sequential fractional
programming for the optimization of the RIS phase shifts. In addition, a
realistic power consumption model for RIS-based systems is presented, and the
performance of the proposed methods is analyzed in a realistic outdoor
environment. In particular, our results show that the proposed RIS-based
resource allocation methods are able to provide up to higher energy
efficiency, in comparison with the use of regular multi-antenna
amplify-and-forward relaying.Comment: Accepted by IEEE TWC; additional materials on the topic are included
in the 2018 conference publications at ICASSP
(https://ieeexplore.ieee.org/abstract/document/8461496) and GLOBECOM 2018
(arXiv:1809.05397
Exploiting Constructive Mutual Coupling in P2P MIMO by Analog-Digital Phase Alignment
In this paper, we propose a joint analog-digital (A/D) beamforming scheme for the point-to-point multiple-input-multiple-output system, where we exploit mutual coupling by optimizing the load impedances of the transmit antennas. Contrary to the common conception that mutual coupling strictly harms the system performance, we show that mutual coupling can be beneficial by exploiting the concept of constructive interference. By changing the value of each load impedance for the antenna array based on convex optimization, the mutual coupling effect can be manipulated so that the resulting interference aligns constructively to the useful signal vector. We first prove that the full elimination of mutual coupling effect is not achievable solely by tuning the values of the antenna load impedances. We then introduce the proposed A/D scheme for both PSK and QAM modulations, where performance gains with respect to conventional techniques are obtained. The implementation of the proposed schemes is also discussed, where a lookup table can be built to efficiently apply the calculated load impedances. The numerical results show that the proposed schemes can achieve an improved performance compared to systems with fixed mutual coupling, especially when the antenna spacing is small