Millimeter Wave Beamforming for Wireless Backhaul and Access in Small Cell Networks

Recently, there has been considerable interest in new tiered network cellular architectures, which would likely use many more cell sites than found today. Two major challenges will be i) providing backhaul to all of these cells and ii) finding efficient techniques to leverage higher frequency bands for mobile access and backhaul. This paper proposes the use of outdoor millimeter wave communications for backhaul networking between cells and mobile access within a cell. To overcome the outdoor impairments found in millimeter wave propagation, this paper studies beamforming using large arrays. However, such systems will require narrow beams, increasing sensitivity to movement caused by pole sway and other environmental concerns. To overcome this, we propose an efficient beam alignment technique using adaptive subspace sampling and hierarchical beam codebooks. A wind sway analysis is presented to establish a notion of beam coherence time. This highlights a previously unexplored tradeoff between array size and wind-induced movement. Generally, it is not possible to use larger arrays without risking a corresponding performance loss from wind-induced beam misalignment. The performance of the proposed alignment technique is analyzed and compared with other search and alignment methods. The results show significant performance improvement with reduced search time.

I. Introduction

The exponential growth of demand for mobile multimedia services has motivated extensive research into improved spectrum efficiency using techniques that increase geographic spectrum reusability, such as multi-tier cell deployment (e.g., picocell and femtocell networks) [2]. Picocell networks are expected to be typically deployed to support demand from small, high throughput areas (e.g., urban centers, office buildings, shopping malls, train stations). One of the major impediments to deployment of heterogeneous small cell networks, such as picocell networks, is access to cost effective, reliable, and scalable backhaul networks.