The Complete Guide to Small Cells (Microcells, Metrocells, eFemtos)

While most cell towers are outside, most mobile phone calls today are made inside buildings. This leads to two problems: coverage and capacity.

Wireless signals can have difficulty penetrating building exteriors, especially large office towers, which are made of steel, concrete and often utilize energy-saving windows. Even if there is only one wireless user in the building, they might not be able to make a call due to lack of signal coverage.

But of course, there usually isn't just one person in a building; there may be hundreds or even thousands. Even if there is some wireless coverage within the building from the outside cell towers, it often won't provide enough capacity for everyone in the building.

One way to address in-building wireless coverage is by using cell phone repeaters. But repeaters don't address capacity issues; they simply take the wireless signal available outside the building and make it available indoors.

Once the number of occupants in a building exceeds a certain number, additional capacity is needed to accommodate them. One way to add capacity is by placing a Base Transceiver Station (or BTS, essentially the same as a full cell tower) inside the building and distributing the signal using a Distributed Antenna System (DAS). Since the BTS is supplied by the wireless service provider and requires that they run their own leased fiber to the building, this solution is very expensive.

A less expensive way and much quicker to add additional in-building capacity is by using small cells. Small cells are exactly what you likely think they are: small cellular base stations designed specifically to add additional capacity over small coverage areas. They're often used outdoors to add capacity in high-density areas where a lot of people congregate, such as stadiums and amphitheaters. Small cells are also perfectly suited for use in buildings because they solve both the coverage and capacity issues, and can be deployed quickly and are inexpensively.

Small cells fall into one of three general categories that are dictated by their power level: metrocells, picocells and femtocells. General performance measures for each category are shown in the table below.

Regardless of the type chosen, for a wireless connection to be established the small cell needs to be connected to the cellular provider's network. This is known as backhaul. For outdoor cell towers, this usually involves fiber optic or microwave backhaul.

Since neither of those are available in buildings, small cells connect to the cellular provider's network via what carriers call "untrusted backhaul," or what you refer to as simply "the Internet." The devices typically make a secure IPSec Tunnel to prevent any eavesdropping of calls and SMS messages. That means that a high speed Internet connection is a requirement for any kind of picocell or femtocell deployment. The minimum required bandwidth depends on the type and number of small cells deployed.

Because today's cellular phones use digital modulation, their signals need to be generated with a high degree of timing precision. Since the beginning of digital cellular technology, the accurate, external timing source used has been GPS (Global Positioning System).

The GPS signal used for the small cells is the same satellite-based signal used to determine your location and give you driving directions in Google Maps. While GPS is not perfect, it is readily available and relatively inexpensive to implement.

GPS signals are most readily available outdoors, which make using them for large cell towers very convenient. However, even when cell coverage moves indoors with small cells, they still need an accurate, external timing source.

Small cells on the market today either have a built-in GPS antenna (which means they need to be placed near a window to get GPS signal), or include an external GPS antenna that needs to be positioned so that it's near a window or outdoors.