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Practical Reconfigurable Intelligent Surfaces (RIS)

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Cell-free mmWave massive MIMO network [4].

Short Description

Since the beginning of research for fifth-generation (5G) wireless systems and beyond, providing high data rates for all users was one of the key requirements. From now on, the expectation is to have more than 50 billions connected devices [1] in the network. To meet this demand, technologies that improve energy efficiency are a big trend, so as to promote green and sustainable wireless systems.

Reconfigurable intelligent surfaces (RISs) are a promising solution that has emerged in recent years with the goal of altering electromagnetic fields by controlling the phase, amplitude, frequency and polarization of the incoming signals [2]. With a meta-surface equipped with integrated electronic circuits, RIS has the power to combat adversarial effects of wireless propagation and will substitute the traditional structure of wireless networks to an innovative hybrid one with both active and passive components [2,3].

In [1], a RIS is used in the downlink of a MIMO system to investigate the improvement provided by these devices in terms of energy efficiency. A design focused on this purpose is developed not only for the transmit power allocation but also for the phase shifts of the reflecting elements and predicts to improve the energy efficiency in 300%, compared to regular multi-antenna amplify-and-forward relaying. As opposed to the majority of existing results that consider continuous phase shifts at the reflecting elements, in [4], a finite set of discrete phase shifts is considered at each element, with the objective to minimize the transmit power at each access point (AP). This minimization is performed with a joint optimization of the precoding at the AP and the discrete reflect phase shifts at the RIS, subject to a set of constraints, and claims to maintain squared power gain when compared to continuous phase shifts. Similarly, new problems were solved in [5] to minimize the total transmit power at the AP, also by joint optimization of the transmit beamforming by active antenna array at the AP and reflect beamforming by passive phase shifters at the RIS.

Unlike the scenario in [4], one of the many problems that can be addressed in practical RIS is the transmit precoding and phase shifts optimization with multiple APs and RIS. In realistic scenarios, APs are assisted by multiple RIS and RIS can assist multiple APs. Therefore, in this project, we will explore the potential of RIS for 5G and beyond, considering a distributed network with multiple APs and RIS so as to address the phase selection problem. By developing algorithms to adjust the phase shifts, we are able to boost the desired signal power and mitigate interference, improving the system’s performance, without the need of extra APs [4].


[1] C. Huang, A. Zappone, G. C. Alexandropoulos, M. Debbah and C. Yuen, "Reconfigurable Intelligent Surfaces for Energy Efficiency in Wireless Communication," in IEEE Transactions on Wireless Communications, vol. 18, no. 8, pp. 4157-4170, Aug. 2019, doi: 10.1109/TWC.2019.2922609.

[2] E. Basar, M. Di Renzo, J. De Rosny, M. Debbah, M. Alouini and R. Zhang, "Wireless Communications Through Reconfigurable Intelligent Surfaces," in IEEE Access, vol. 7, pp. 116753-116773, 2019, doi: 10.1109/ACCESS.2019.2935192.

[3] Q. Wu and R. Zhang, "Towards Smart and Reconfigurable Environment: Intelligent Reflecting Surface Aided Wireless Network," in IEEE Communications Magazine, vol. 58, no. 1, pp. 106-112, January 2020, doi: 10.1109/MCOM.001.1900107.

[4] Q. Wu and R. Zhang, "Beamforming Optimization for Wireless Network Aided by Intelligent Reflecting Surface With Discrete Phase Shifts," in IEEE Transactions on Communications, vol. 68, no. 3, pp. 1838-1851, March 2020, doi: 10.1109/TCOMM.2019.2958916.

[5]Q. Wu and R. Zhang, "Intelligent Reflecting Surface Enhanced Wireless Network via Joint Active and Passive Beamforming," in IEEE Transactions on Wireless Communications, vol. 18, no. 11, pp. 5394-5409, Nov. 2019, doi: 10.1109/TWC.2019.2936025.

Status: Available

Looking for 1-2 Semester/Master students
Contact: Victoria Menescal Tupper Palhares


VLSI I (recommended)
Communication Systems (recommended)


20% Literature Research
80% System Development


Christoph Studer

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