Heterogeneous Cellular Networks with LoS and NLoS Transmissions--The Role of Massive MIMO and Small Cells

TL;DR

This paper develops a stochastic geometry framework to compare the impacts of small cell densification and macro BS antenna expansion on downlink rates in heterogeneous networks, revealing optimal deployment strategies.

Contribution

It introduces a novel analytical framework for evaluating LoS/NLoS transmissions and provides practical guidelines for small cell density and antenna allocation in heterogeneous networks.

Findings

Small cell deployment improves rate faster than macro antenna expansion initially.

Exceeding a critical small cell density reduces network capacity benefits.

Expanding macro antenna arrays benefits capacity until pilot contamination limits are reached.

Abstract

We develop a framework for downlink heterogeneous cellular networks with line-of-sight (LoS) and non-line-of-sight (NLoS) transmissions. Using stochastic geometry, we derive tight approximation of achievable downlink rate that enables us to compare the performance between densifying small cells and expanding BS antenna arrays. Interestingly, we find that adding small cells into the network improves the achievable rate much faster than expanding antenna arrays at the macro BS. However, when the small cell density exceeds a critical threshold, the spacial densification will lose its benefits and further impair the network capacity. To this end, we present the optimal small cell density that maximizes the rate as practical deployment guidance. In contrast, expanding macro BS antenna array can always benefit the capacity until an upper bound caused by pilot contamination, and this bound also surpasses the peak rate obtained from deployment of small cells. Furthermore, we find that allocating part of antennas to distributed small cell BSs works better than centralizing all antennas at the macro BS, and the optimal allocation proportion is also given for practical configuration reference. In summary, this work provides a further understanding on how to leverage small cells and massive MIMO in future heterogeneous cellular networks deployment.