ABSTRACT

The wireless industry has been experiencing an explosion of data traffic usage in recent years and is now facing an even bigger challenge, an astounding 1000-fold data traffic increase in a decade. The required traffic increase is in bits per second per square kilometer, which is equivalent to bits per second per Hertz per cell yen; Hertz yen; cell per square kilometer. The innovations through higher utilization of the spectrum (bits per second per Hertz per cell) and utilization of more bandwidth (Hertz) are quite limited: spectral efficiency of a point-to-point link is very close to the theoretical limits, and utilization of more bandwidth is a very costly solution in general. Hyper-dense deployment of heterogeneous and small cell networks (HetSNets) that increase cells per square kilometer by deploying more cells in a given area is a very promising technique as it would provide a huge capacity gain by bringing small base stations closer to mobile devices. This article presents a holistic view on hyperdense HetSNets, which include fundamental preference in future wireless systems, and technical challenges and recent technological breakthroughs made in such networks. Advancements in modeling and analysis tools for hyper-dense HetSNets are also introduced with some additional interference mitigation and higher spectrum utilization techniques. This article ends with a promising view on the hyper-dense HetSNets to meet the upcoming 1000yen; data challenge.

INTRODUCTION

Mobile data traffic has just started exploding: the amount of traffic usage has been doubling each year during the last few years with the increasing popularity of smart phones and new types of mobile computing devices. Now the wireless industry is preparing for an even bigger challenge, an astounding 1000-fold increase in data traffic expected in this decade [1]. A set of new radio access technologies are required to satisfy future requirements of 1000yen; capacity. The required capacity is in bits per second per square kilomiter, which is equivalent to bits per second per Hertz per cell yen; Hertz yen; cell per square kilometer. Higher utilization of spectrum (bits per second per Hertz per cell) in given frequency resources per cell is quite saturated; recent results show that at least point-to-point link throughput is very close to the theoretical limits. Utilization of more bandwidth (Hertz) is a very costly solution, unless devices can utilize additional radio access technologies for unlicensed bands with seamless aggregation and offloading. The final and probably one of the most promising frontiers to achieve the goal is to increase cells per square kilometer by deploying more cells of different types/technologies in a given area. Heterogeneous and small cell networks (HetSNets), whose goal is to maximize the utilization of existing spectrum by deploying more cells, are thus expected to be important to challenge the future of cellular networks [2]. HetSNets are networks deployed with a mix of traditional high-power macrocells and lowpower smaller cells such as pico, femto, and/or relay nodes. We assume that HetSNets include wireless local area network (WLAN) and wireless personal area network (WPAN) technologies as well, so HetSNets are networks consisting of multiple radio technologies. In theory, the overall capacity scales with the number of small cells deployed in a unit area: shrinking the radius of each cell and packing more cells in a given area would actually offer more capacity and more spectrum reuse. In reality, however, as cells get closer, the hyper-densification of HetSNets is challenged in many ways. In a hyperdense deployment, not only desired signal strength but also interference from other cells increase. Increasing other-cell interference needs to be mitigated, and a better mobility management mechanism is required as the mobile users see cell edges more frequently. Furthermore, as some privately owned small cells implement restricted access schemes, they can generate/ receive strong uncoordinated interference to/ from external cells sharing the same radio resources [3]. The deployment of small cells are mostly unplanned, so a network self-organizing mechanism needs to be developed. The promising performance gain by deploying more cells can only be achieved by successfully addressing these problems. Despite the barriers, there have been many technical breakthroughs in HetSNets in recent years. The development of further enhanced intercell interference coordination in Third Generation Partnership Project (3GPP) Long Term Evolution (LTE)-Advanced systems enables the mobile stations to coordinate/mitigate intercell interference and successfully set up connectionswith small cells. Self-interference cancellation (SELIC) enables the coexistence of heterogeneous radios in the same mobile device. Expanding SELIC further to even enable full duplex radios is also being actively investigated [4]. Intelligent incentive schemes can motivate privately owned small cells to open up for wider access. Thanks to those advanced interference management and self organizing network (SON) techniques, the overall capacity enabled by HetSNets is expected to continue grow as the densification of small cell deployment increases. In this regard, several mathematical models for analyzing the hyper-dense HetSNets have recently been proposed [5–7]. The main goal of this article is to provide a holistic view of the hyper-dense deployment of HetSNets. We first compare the two approaches for future wireless systems, massive antenna and dense deployment, and present the fundamental preference for dense deployment. We review the technical challenges in HetSNets, which are due to other-cell and in-device interference, user mobility, privately owned small cells, and difficulties in performance analysis. We then provide recent technical breakthroughs to overcome those barriers. We introduce additional techniques that can be use

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