[Paper Review] Performance Analysis of Active Large Intelligent Surfaces (LISs): Uplink Spectral Efficiency and Pilot Training

Minchae Jung, Walid Saad+1 more|arXiv (Cornell University)|Mar 31, 2019

Advanced Wireless Communication TechnologiesEngineering48 references35 citations

TL;DR

The paper analyzes uplink spectral efficiency of active LIS systems under a practical uplink frame with pilot training, deriving asymptotic SSE bounds and optimal pilot length under pilot contamination in single- and multi-LIS settings.

ABSTRACT

Large intelligent surfaces (LISs) constitute a new and promising wireless communication paradigm that relies on the integration of a massive number of antenna elements over the entire surfaces of man-made structures. The LIS concept provides many advantages, such as the capability to provide reliable and space-intensive communications by effectively establishing line-of-sight (LOS) channels. In this paper, the system spectral efficiency (SSE) of an uplink LIS system is asymptotically analyzed under a practical LIS environment with a well-defined uplink frame structure. In order to verify the impact on the SSE of pilot contamination, the SSE of a multi-LIS system is asymptotically studied and a theoretical bound on its performance is derived. Given this performance bound, an optimal pilot training length for multi-LIS systems subjected to pilot contamination is characterized and, subsequently, the performance-maximizing number of devices that the LIS system must service is derived. Simulation results show that the derived analyses are in close agreement with the exact mutual information in presence of a large number of antennas, and the achievable SSE is limited by the effect of pilot contamination and intra/inter-LIS interference through the LOS path, even if the LIS is equipped with an infinite number of antennas. Additionally, the SSE obtained with the proposed pilot training length and number of scheduled devices is shown to reach the one obtained via a brute-force search for the optimal solution.

Motivation & Objective

• Motivate LIS as a paradigm for near-field, space-intensive communications using active surfaces.
• Develop an uplink SSE framework for active LISs with a practical 3GPP-like frame structure.
• Characterize the impact of pilot contamination in multi-LIS deployments.
• Derive optimal pilot training length and scheduling strategy to maximize SSE.
• Provide insights into how SSE scales with a large number of LIS antennas.

Proposed method

• Model an uplink LIS system with N LISs, K devices per LIS, and M antennas per LIS unit.
• Use a LOS-dominated LIS channel model for desired links and a Rician/NLOS model for inter-LIS interference, with spatial correlation.
• Adopt a 3GPP-like uplink frame with t pilot symbols and T-t data symbols, and employ LS channel estimation for pilot contamination analysis.
• Derive an SINR expression under LS CSI and MF reception, incorporating intra- and inter-LIS interference and estimation errors.
• Perform asymptotic SSE analysis as M → ∞ to obtain deterministic performance bounds and scaling laws.
• Provide a method to determine the optimal pilot training length t and the optimal number of scheduled devices per LIS to maximize SSE.

Experimental results

Research questions

• RQ1 How does the uplink spectral efficiency of active LISs behave under a practical frame structure with pilot training and contamination?
• RQ2 What bounds and scaling laws describe SSE as the number of LIS antennas grows large in single- and multi-LIS setups?
• RQ3 What is the optimal pilot training length to maximize SSE given pilot contamination and inter-LIS interference?
• RQ4 How should devices be scheduled across LIS units to maximize SSE in multi-LIS environments?

Key findings

• SSE can be approximated deterministically in the large-antenna limit, with performance influenced by pilot contamination and LOS/inter-LIS interference.
• Pilot contamination remains a key limiting factor in multi-LIS systems, even with an infinite number of LIS antennas, due to inter-LIS interference via LOS paths.
• An optimal pilot training length exists and converges to the number of devices within an LIS area as M grows without bound.
• The proposed LS-based channel estimation and MF receiver yield SSE close to brute-force optimum under the studied conditions.
• Simulation results show the derived operating parameters (pilot length and scheduled devices) achieve maximum SSE comparable to exact optimization in single- and multi-LIS scenarios.