What’s All This Small Cell Stuff, Anyhow?
You have heard the old saying “splitting hairs” referring to arguing about very small differences or trivial details. There are splitting hairs and then there are “splitting cells” – cell splitting to be exact. This is occurring all over the world as wireless service providers attempt to add ubiquitous coverage and more traffic capacity with ever smaller cells in the face of exploding mobile data requirements. Industry experts see a near doubling of capacity required year-over-year! To cope, the small cell universe has grown to include femtocells, picocells, metrocells and microcells – generally increasing in size from femtocells (the smallest) to microcells (the largest). Any or all of these small cells may be based on femtocell technology.
Ever since the first days of AMPS analog cellular service, wireless site design was based on macrocells; relatively few regional three sector cells employing frequency reuse such as could provide a coverage range of 10 to 15 kilometers or more. In the 1990s so-called “microcells” appeared. These access points, often simple outdoor repeaters, typically had service radii of less than a kilometer or so to add coverage to weak spots. Subsequently, the concept of distributed antenna systems (DAS), involving the chaining or grouping of small cells, emerged. Thus arose the notion of “picocells”, becoming popular for in-building systems. Picocells, by contrast to microcells, typically cover areas less than 100 meters. Notwithstanding, both pico and micro cells are “RF equivalent” cell sites placed under the macrocell umbrella, generally for capacity fill-in and expansion use, at least for now.
Femtocells are the smallest of the small cell family, with coverage on the order of single rooms or offices. In 2007, AT&T sold the first shoebox size “femtocell” for home and office use. Unlike many passive DAS systems who are backhauled “over-the-air” to a macro cell, an AT&T femtocell has a unique IP address and traffic is passed through the subscriber’s broadband connection to an access gateway installed in the AT&T network.
Small cells deployed in metropolitan areas are often referred to as metrocells – compact and discrete micro base stations mounted on lampposts, positioned on the sides of buildings or found indoors in stadiums, transport hubs and other public areas. Small cells typically can cover up to 200 meters for some applications. “Small cells” therefore is nothing new, technically speaking but they do present deployment challenges much different from “greenfield” sites. The discussion within the wireless industry about small cells and Heterogeneous Networks (HetNets is the combination of both macro and smaller cell sites) is reminiscent of the mid 1990s when there was talk about underlay-overlay techniques and macro versus micro base station.
One can say that small cells are in the orbit of Distributed Antenna Systems or “DAS”. I speculate that small cells will be the future because of spectrum demands; tower sites will be too high and thus a small cell “drop-in” will be appealing to the wireless carriers. The idea is for a small cell “shoebox” to contain integrated 3G/4G/Wi-Fi, but most manufacturers are still in the product development stage for that topology.
Wireless providers are and will continue to use small cells and some are now trialing them in strategic locations. Where DAS has been king for coverage fill-in and capacity growth, experts estimate that small cell equipment growth should catch up to DAS by late 2016. Small cells and DAS should complement each other. ABI Research estimates that one-fourth of DAS will eventually be fed by small cells due to smaller equipment and may be more inexpensive than feeding DAS with more expensive macro site. Because small cells are completely IP-based, a standard Internet service over a virtual private network can be used to connect to the carrier’s core network.
Small cells are designed to increase overall sector density or capacity, but can present challenges when it comes to acquiring sites and integrating them into the macro wireless network. Some of the challenges faced by the macro network are similar to todays’ small cell deployment challenges, including the availability and cost of backhaul and commercial power. If carriers do not achieve significant economies of scale on small cell equipment, site costs could be more than expected and result in a negative NPV and no return on investment. In many urban and suburban areas, because the cost of acquiring a cell site can be greater than the cost of installing equipment, carriers are planning to deploy lower power and smaller antenna cells (or mini-cells) that occupy less space.
An often very frustrating consideration involves acquiring approvals from local regulatory and zoning bodies to permit installation of small cell sites. Regulatory processes can be time and resource consuming; thus the cost to acquire site access may exceed the cost to install the equipment. Concealment and aesthetics also require attention for these dense urban small cell sites. Many municipalities and permitting bodies require specific aesthetic designs and unappealing sites can face additional costs.
In addition to reducing the visual impact of the pico/micro cell equipment, carriers must address engineering challenges such as interference with adjacent sectors and providing adequate backhaul to reduce transport/backhaul congestion. Without self-optimizing network (SON) capabilities, interference and coordination may be unmanageable and impractical. Furthermore, to achieve maximum data throughput in areas with high data usage by customers, improving the signal level and reducing noise and interference so that all data packets can be successfully received is critically important. Performance degrades with distance from the cell, so antenna pattern shaping and signal-to-interference control needs to be optimum.
In summary, key considerations in the deployment of small cells will include: 1) development of flexible all-outdoor non-traditional structures, 2) meeting strict zoning/permitting requirements by blending into the urban environment using compact design and mini-antennas, 3) engineering systems to be simple to deploy, maintain, and operate with maximum integration, and 4) meeting microcell backhaul needs where fiber may not be available.
With all these challenges the big question is how to utilize the small cell for coverage and capacity enhancements without “splitting hairs” even further!