Downlink Rate Analysis for Virtual-Cell based Large-Scale Distributed Antenna Systems

TL;DR

This paper analyzes how the size of virtual cells in large-scale distributed antenna systems affects user rates, deriving explicit formulas and proposing optimal virtual cell size rules to enhance system performance.

Contribution

It provides a comprehensive analysis of virtual cell size impact on user rates, including explicit rate formulas and strategies for optimal virtual cell sizing in DAS.

Findings

Optimal virtual cell size maximizes average user rate.

Grouping users with overlapping virtual cells improves rates.

Larger virtual cells benefit user rates when users are grouped.

Abstract

Despite substantial rate gains achieved by coordinated transmission from a massive amount of geographically distributed antennas, the resulting computational cost and channel measurement overhead could be unaffordable for a large-scale distributed antenna system (DAS). A scalable signal processing framework is therefore highly desirable, which, as recently demonstrated in \cite{Dai_TWireless}, could be established based on the concept of virtual cell. In a virtual-cell based DAS, each user chooses a few closest base-station (BS) antennas to form its virtual cell, that is, its own serving BS antenna set. In this paper, we focus on a downlink DAS with a large number of users and BS antennas uniformly distributed in a certain area, and aim to study the effect of the virtual cell size on the average user rate. Specifically, by assuming that maximum ratio transmission (MRT) is adopted in each user's virtual cell, the achievable ergodic rate of each user is derived as an explicit function of the large-scale fading coefficients from all the users to their virtual cells, and an upper-bound of the average user rate is established, based on which a rule of thumb is developed for determining the optimal virtual cell size to maximize the average user rate. The analysis is further extended to consider multiple users grouped together and jointly served by their virtual cells using zero-forcing beamforming (ZFBF). In contrast to the no-grouping case where a small virtual cell size is preferred, it is shown that by grouping users with overlapped virtual cells, the average user rate can be significantly improved by increasing the virtual cell size, though at the cost of a higher signal processing complexity.