Broadband Wireless Networking Lab

School of Electrical and Computer Engineering Georgia Institute of Technology

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Small Cells for Beyond 4G Cellular Systems

Project Description

Introduction

The proliferation of wireless mobile devices including smartphones and tablets has tremendously increased the need for high-speed broadband services. Cisco has recently forecasted an 18-fold traffic increase (10.8 exabytes per month) by 2016. In this context, cellular networks are faced with the challenges of providing enormous system capacity and achieving superior cellular coverage. Studies also show that more than 50% of voice traffic and 70% of cellular data traffic originate from indoor and enterprise environments. According to small cell forum, the capacity gain achieved in cellular networks using different techniques is indicated in Table 1.

Table 1 - Capacity gain for cellular networks using different techniques.
Source: Small Cell Forum, Release One, Feb 2012.

Technique	Capacity Gain
Frequency Division	5
Modulation Techniques	5
Access to wider range of spectrum	25
Frequency reuse using more cell sites	1600

Among the different approches in Table 1, it is clear that spatial frequency reuse using more cell sites offers the greatest capacity gain. Therefore, to meet such a demand, small cells, planned or uncoordinated cellular systems of smaller coverage based on the concept of frequency reuse, are utilized by wireless carriers as one of the primary solutions to improve signal quality, cellular coverage and capacity at the edge of macrocells and in indoor environment. The whole small cell market value surpassed $200 million in 2012 with more than 5 million small cells already deployed and forecasts showing that the deployments of small cells will grow exponentially and reach over 90 million units by 2016.

Small cells offer several benefits. These include i) over two orders of magnitude increase in overall capacity, ii) cost-effective coverage extension, and iii) green radio solution. Small cells vary between each other in several ways including coverage area, physical size and backhaul technology and can be utilized for both indoor and outdoor deployments. In general, there are four common deployment configurations for small cells:

• Residential deployment: Typically, for this case, small cells operating with low-power and short range such as femtocell base stations are utilized. The major attributes of this type of small cells is their internet-based backhaul, low power consumption, self-organization and optimization and robust deployment.
  
  
• Enterprise deployment: This constitutes the deployments within large office buildings or in public indoor areas such as airports. Both femtocell and picocell base stations are adopted for enterprise deployments. When femtocells are utilized, it is common to deploy a number of femtocells with self-organizing features. In this case, the deployment within the enterprise may be planned but it need not require careful RF planning with the rest of the cellular network. However, picocells which have a relatively larger coverage area than the femtocells need careful radio frequency planning before they are deployed. The incorporation of self-organizing features in picocells can facilitate flexible deployment. In addition, the picocells typically feature dedicated backhaul (wired or wireless) unlike the IP backhaul utilized by femtocells.
  
  
• Outdoor urban deployment: For the particular case of outdoor urban or metropolitan areas, metrocells are utilized to provide capacity enhancements as well as coverage to areas that experience large losses from macrocells due to shadowing. Metrocells are typically mounted on building walls, lamp posts, etc.
  
  
• Outdoor rural deployment: For rural areas, the key objective is to provide coverage over large areas not served by the macrocells. In this case, microcells are utilized which have a substantially large coverage area compared to the picocells and femtocells. However, the lack of infrastructure may result in incorporating LOS/NLOS wireless backhauls for these microcells. However, microcells may also be present in large cities for addressing the capacity needs.

Figure 1 showcases a scenario with macrocells underlaid with different small cell types.

Figure 1 - Next generation cellular systems with small cells

Inspite of their potential benefits, small cells introduces several challenges and calls for a profound rethinking of several existing approaches for interference management, mobility management, energy savings, backhaul management, massive MIMO, and new carrier types, among others.

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