The need to increase wireless coverage and capacity within an increasingly crowded ecosystem has led to a variety of alternative solutions and new challenges for the owners and operators of today’s mobile networks. Each new solution forces the industry to reconsider the landscape and assess how it all fits together.
One of the more recent developments has been the use of “small cells” in order to provide coverage and capacity indoors and out.
Whether deployed as standalone networks or integrated with the macro layer to create heterogeneous networks, small cell solutions are being touted for their ability to help operators achieve higher radiodensity and increased capacity. These heterogeneous networks also allow operators to achieve much better fill-in coverage and, by using small cell nodes to off-load traffic from over-burdened macro sites, they are realizing higher data throughput as well.
At the same time, the use of distributed antenna systems (DAS) has exploded as facility owners and operators rush to satisfy the growing demand for seamless high-speed indoor coverage. Today, DAS networks are being deployed in a wide variety of locations, including universities, sports arenas, stadiums, hotels, casinos, corporate campuses, malls, airports, and subways.
There is an often confusing and constantly evolving definition of a small cell network. According to ABI Research, small cells can be characterized as low-powered radio access nodes that operate in a licensed and unlicensed spectrum that have a range of 10 meters to 1 or 2 kilometers. The term “small” refers to the physical footprint of the solution, compared to a traditional macro cell.
On its website, the Small Cell Forum adds a bit more specificity to the definition, stating that: “Small cells” is an umbrella term for operator-controlled, low-powered radio access nodes, including those that operate in licensed spectrum and unlicensed carrier-grade Wi-Fi.”
While this definition adds a bit more clarity, it adds confusion as well. Does the term “operator controlled” suggest that small cell is exclusively a single-operator solution? Must it be owned by the mobile operator or can it be owned and operated by a neutral host provider?
Still, other industry resources and literature characterize small cells as low-powered solutions that have a small physical footprint and are “typically deployed piecemeal to provide coverage or enhance capacity in much smaller areas with a single wireless communications technology for a single wireless carrier.
There is also confusion when identifying the types of technology solutions that fall under the small cell rubric. The four types of small cell solutions listed by the Small Cell Forum are femtocells, picocells, metro cells and microcells and are loosely defined by their general power output and the coverage radius provided by each.
Suffice it to say that there appears to be no concrete agreed-upon definition for exactly what a small cell network is or the technologies it includes. The following is a brief overview of each of the small cell technologies.
A femtocell is characterized as a very low-range, low-power base station, able to be deployed in a home, home office or very small business. The coverage range is usually less than 30 meters and the output power is around 20 dBm. Within the home or small office, the femtocell operates like a miniature macrocell, providing consistent and reliable coverage for a limited number of users. In most cases, the femtocell is owned or leased by the user who operates and manages it. So, from the mobile operator’s perspective, it is an unmanaged asset.
Femtocells operate in the same licensed frequency bands as macro cells and support various mobile air interfaces. For backhaul, the femtocell requires a connection—typically fixed line—to the mobile network operator. When deployed to serve a limited number of pre-approved users, the femtocell provides a good coverage solution. When deployed with any density across a moderately-sized facility, however, interference can quickly become an issue, requiring careful power control in order to manage it. The quality of service can also become an issue as femtocells utilize the network’s broadband connection, limiting the bandwidth available for other broadband applications. There is also the potential for conflicts with service-level agreements if the provider of the broadband service differs from the mobile network provider.
Picocells operate on the same principles as femtocells. A dedicated BTS (Base station) feeds the remote radio heads and antennas, creating a network of very small individual cells. Whereas a femtocell is typically owned and managed by the user, its slightly larger cousin, the picocell, is usually owned, operated and managed by the mobile operator. These solutions are primarily deployed indoors. Typically, a femtocell can serve only somewhere between 4 and 16 simultaneous users, whereas a picocell may be able to handle up to 100 users.
As with femtocells, if the building is small enough to be served by a single picocell, these units are an ideal solution for coverage and capacity. In larger environments, however, deploying multiple cells can create interference problems. Like Wi-Fi access points, femtocells must alternate channels to avoid co-channel interference, and doing this requires carriers to use a lot of frequency in a small area given the relatively small coverage footprint of the solution.
The technology also does not lend itself to supporting multiple carriers. Picocells, as well as femtocells, are generally single-frequency devices, so providing coverage for multiple mobile operators requires the network operator to deploy a separate set of small cells for each frequency to be covered. This scenario quickly becomes cost prohibitive and space prohibitive.
Microcells are among the largest of the small cell solutions, operating at an approximate power output of about 30 dBm and providing a coverage radius of up to 500 meters. These solutions are most often used as part of a heterogeneous network to enhance outdoor coverage in areas where surrounding obstacles prohibit the use of macro cells.
Occasionally they are deployed indoors to add network capacity in areas with very dense phone usage, such as train stations or shopping malls. The technology is also used to increase the capacity of cellular networks to offload usage during peak hours.
Microcells appear to be a technology in search of a clear position. Currently, they are sandwiched between the smaller femtocells and picocells (which are clearly targeted to smaller indoor deployments) and the larger metro cells (which have found a niche in extending outdoor coverage and capacity).
Metrocells are low-power single-sector-channel, independent small cells that can support several hundred users. Combining a small independent BTS and antenna, they are often deployed on lamp-posts or sides of buildings to support mobile data demand in dense metropolitan areas.
Relative to a macro system, metro cells are easy, faster and less expensive to deploy, making them an excellent choice for urban infill, wireless backhaul, and offloading a portion of the macro network traffic.
Operating at about 5 watts of power, metro cells have a coverage radius of about 600 to 1200 feet (approximately 183 to 366 meters). With the exception of very large and open venues such as airports and stadiums, metro cells are typically limited to use outdoors.
Among the various definitions of small cells, DAS is rarely mentioned, despite the fact that the technology has often been referred to as the original small cell.
A typical DAS solution consists of a centrally located radio or headend equipment, remote communications nodes, and a high-capacity transport network—typically fiber—to connect the nodes to the central equipment. The remote nodes are distributed in various IDF forms throughout the facility or area. The antennas are distributed throughout the facility and connect to the remote units via coaxial cable (although options are available over twisted pair/fiber cabling).
Based on the general parameters common to most definitions of small cell—low power output and small physical footprint—DAS fits nicely into the small cell rubric. Like other types of small cells, a DAS network operates at signal power levels that are much lower than macrocells.
A low-power remote typically has a composite output power of 30 dBm at the antenna port, translating to a transmit power of 1 watt. Similarly, the power output to microcell, picocell and femtocell nodes typically range from 20 dBm to 30 dBm. Like other small cell solutions, DAS nodes have a physically compact footprint, making them suitable to use indoors and outside. But, unlike other small cells, DAS also exists as a high-powered solution.
Operating at power levels up to 40 watts (46 dBm), these systems provide larger coverage areas, making them more affordable for some applications.