Kota Kali Krishna supervised by Dr. Praful Mankar received her doctorate in Electronics and Communication Engineering (ECE). Here’s a summary of her research work on Performance Analysis of Reconfigurable Intelligent Surface-aided Communication Systems:

The performance of wireless communication systems is highly dependent on the propagation environment, which is uncertain, volatile, and constantly changing due to factors such as multipath fading, doppler shift, path loss, etc. Legacy solutions for reliable communication, such as multiple input multiple output, relays, etc., have circumvented these issues to a certain extent. Nonetheless, as communication systems are heading towards a future with data rate requirements in the order of gigabits/second and latencies in the order of microseconds, efficient system design is the need of the hour. Especially, solutions that merely circumvent the ill-effects of wireless propagation environments are no longer sufficient. Towards this, a new technology named Reconfigurable Intelligent Surface (RIS) is emerging as a potential solution that has the capability to alter the propagation environment to a large extent, and hence is envisioned to be a part of next-generation cellular standards. RIS is a planar array consisting of many reflecting elements along its surface that collectively re-orient the impinging wave towards the desired direction. Upon configuring these reflecting elements, the signals reflected off of its surface can add up constructively at the receiver enabling the ability to control the environment partially. There are many research directions for the implementation of RIS-aided communication systems, such as RIS hardware design, channel modeling, channel estimation, optimal beamforming, capacity and outage analysis, optimal RIS deployment, etc. These directions must be explored in-depth in order to realize the benefits of such RIS-aided communication systems fully. This thesis aims to explore RIS-aided communication systems design and their performance analysis in terms of capacity and outage, with a special emphasis on proposing low-complexity beamforming solutions and their performance analysis. The main contribution of the thesis is to determine statistically optimal low-complexity beamforming and RIS-phase shift matrix solutions of a RIS-aided downlink multiple input single output (MISO) system while utilizing both the direct link and RIS-assisted indirect link under different propagation/fading environments, namely 1) Rician-Rician, 2) Rician-Rayleigh, and 3) Rayleigh-Rayleigh, along RISassisted indirect link and direct link, respectively. Moreover, we assume scenarios with correlated as well as independent and identically distributed ( i.i.d.) fading along both links. More importantly, this work focuses on providing closed-form expressions for the optimal transmit beamformer and RIS phase shifts to provide a deeper understanding of the exact performance functional dependence of the optimal solutions on key system parameters. Such a statistically optimal beamforming-based system design has significant advantages over the instantaneous channel state information (CSI)-based beamforming as it does not require reliable feedback channels. This is because the reliability of feedback channels decides the efficiency of the estimated channel coefficients. Further, the ergodic capacity and outage probability performance achievable by the proposed optimal beamforming solutions under the aforementioned propagation environments are also derived to provide insights into the performance limits of such a system. For example, the maximum mean signal-to-noise ratio (SNR) is shown to improve linearly/quadratically with the number of RIS elements in the absence/presence of line-of-sight (LoS) component under i.i.d. fading. Further, it has been analytically established that the statistically optimal beamforming performs better under correlated fading compared to i.i.d. case in the absence of LoS paths. It is also numerically shown that correlated fading is advantageous/disadvantageous compared to the i.i.d. case in the absence/presence of LoS paths. Next, the thesis investigates beamforming and outage analysis for a RIS-aided MISO downlink system under Rician fading along both the direct and the RIS-assisted indirect links. Toward this, the focus has been on maximizing the capacity for two transmitter architectures: fully digital (FD) and fully analog (FA). This capacity maximization problem with optimally configured RIS is shown to be L1 norm-maximization with respect to the transmit beamformer. To obtain the optimal FD beamformer, a complex L1-PCA-based algorithm has been proposed that has low computational complexity. Another low-complexity optimal beamforming algorithm has been proposed to obtain the FA beamforming solution. Additionally, analytical upper bounds on the SNR achievable by the proposed algorithms are derived and used to determine the lower bounds on outage probabilities. The derived bounds are numerically shown to be exact for a unit-rank channel matrix. The final part of the thesis focuses on designing a RIS-non orthogonal multiple access (NOMA) aided multi-user downlink system with low-complexity transceiver architecture. Towards this, the FA architecture is proposed for the base station (BS), where all the antenna elements are connected to a single radio frequency (RF) chain. However, this single RF chain limits the BS to single-user transmission. Thus, BS is facilitated with NOMA in order to enable multi-user communication. Additionally, integrating RIS into such low-complexity systems offers two advantages, namely 1) improved receive SNR at each user and 2) propagation environment becomes conducive to maximizing the benefits of NOMA, where the RIS can be configured to enhance successive interference cancellation capability. The thesis investigates the sum rate and energy efficiency performances of the proposed system while ensuring minimum rates for all users. However, these optimization problems are non-convex due to fractional objectives, unit-modulus RIS-matrix constraint, minimum rate constraint, and coupled optimization variables. Consequently, a quadratic transform-based approach is applied to solve both problems efficiently. Finally, upon comparing the proposed RIS-NOMA-aided FA architecture-based system to an FD architecture (configured optimally with a singular value decomposition-based precoder and optimal power allocation using a Water-filling algorithm), it can be observed that the proposed system with a reasonable number of RIS elements performs better than the FD architecture in terms of sum rate in the lower SNR region and in terms of energy efficiency for a wide range of SNR. These results are particularly important as they show that introducing RIS can help reduce transceiver complexity drastically while providing performances better than benchmark schemes.

February 2025