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 cellular 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.
|Type||Power||Coverage Radius||Capacity||Primarily Used|
|Metrocell||5 Watts||up to 1,000 ft||up to 200 users||Outdoors|
|Picocell||1 Watt||up to 750 ft||up to 64 users||Indoors|
|Femtocell||0.1 Watts||up to 60 ft||up to 6 users||Indoors|
Regardless of the type chosen, for a wireless call to be completed 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.
Being satellite-based, the 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.
|Provider||Manufacturer||Name||Part No.||Year||Technology||Coverage (ft2)||Min Internet speed||Price|
|Samsung||4G LTE Network Extender||SLS-BU103||2016||4G LTE only||7,500||10 Mbps||$249.99|
|Samsung||Network Extender||SCS-2U011||2009||2G CDMA/3G EV-DO||5,000||3.1 Mbps||$249.99|
|Samsung||Network Extender||SCS-2U031002||2011||2G CDMA/3G EV-DO||7,500||3.5 Mbps||$399.99|
|Samsung||4G LTE Network Extender for Enterprise||SLS-BU1022||106||4G LTE||31,500||20 Mbps||$3000|
|Alcatel Lucent||Metrocell 9363||9363||2016||3G/HSPA+||15,000||10 Mbps||$5,000|
|Alcatel Lucent||Metrocell 9962||9962||2016||3G/LTE/HSPA+||15,000||20 Mbps||$5,000|
|Airvana||Airave 2.5||C1-600-RT||2010||CDMA||5,000||3 Mbps||discontinued|
|Commscope||S1000||S1000||2017||2.5GHz LTE||20 Mbps|
|Alcatel Lucent||4G LTE Cellspot||9961||2015||3G/4G UMTS/4G LTE||3,000||5 Mbps|
AT&T has sold 3 different models of their Cisco consumer picocells, which they call a “microcell." All three models (the DPH-151, DPH-153, and DPH-154) support only 2G and 3G, but not 4G LTE. As a result they don’t support AT&T’s new “HD Voice” features. If you’re interested in purchasing one of these, we highly recommend checking out Otto Pylot’s excellent Microcell Technical Guide.
One big limitation of AT&T’s Microcells is that calls do not “hand-off” from the macro cellular network to the devices. So if you walk into your home or office while on a call, it will likely be dropped instead of transitioning to your Metrocell’s signal.
The three models AT&T have sold are functionally identical, covering up to 2,500 square feet each - the main differentiator is that the newer DPH-154 does not include an external GPS antenna port. Since the GPS antenna port can be very useful, we recommend finding an older, white model to purchase rather than buying the newer DPH-154 model.
Recommendation: Purchase the older DPH-151/DPH-153, which includes an external GPS antenna port. If you need LTE data or HD Voice, consider an AT&T signal booster instead.
AT&T’s two Alcatel-Lucent “Metrocell” devices are really “femtocell”-class small cells. The two models are priced identically at $5,000 each, but the newer 9962 model includes support for up to 32 3G devices and up to 32 LTE devices, while the older 9363 model supports only 32 LTE devices. A maximum of 3 Metrocells can be provisioned in a single building. User’s phones will hand-off to one another, and each unit will cover up to 15,000 square feet.
Recommendation: Purchase the newer Alcatel-Lucent 9962 model, which also supports 3G in addition to LTE.
Verizon sells only one consumer-grade device: the Samsung SCS-2U01. The device broadcasts 2G CDMA voice signal and 3G EVDO data signal, but no LTE signal, meaning that it won’t provide “HD Voice” coverage. Verizon claims that the device will cover up to 5,000 square feet, though in reality you should plan to cover no more than 2,500 square feet with the device.
The device supports up to 6 simultaneous voice calls, up to 4 simultaneous data sessions, and a maximum of 6 combined voice and data sessions.
Similar to the AT&T unit, the Verizon Network Extender will hand-off to the macro cell network, but will not hand-off from the macro cell network. So if you walk into a building with a Verizon Network Extender installed, your call will be dropped.
Recommendation: A great unit as long as you are happy with non-LTE signal and don’t mind dropped calls walking into your home or office. If you need LTE data or HD Voice, consider the 4G LTE version below instead.
The “Network Extender for Business” is a slightly upgraded version of their SCS-2U01 picocell. The unit will cover up to 7,500 square feet (compared to 5,000 square feet for the consumer device), and up to 6 simultaneous voice calls, 8 simultaneous data sessions, and up to 7 combined voice and data sessions.
The big advantage of the business version is that up to 5 units can be linked together to form a single cell covering up to 37,500 square feet and up to 35 combined voice and data sessions. However, there is no support for LTE or HD Voice.
Recommendation: A great solution as long as you are happy with non-LTE signal and don’t mind dropped calls walking into your office. If you need LTE data or HD Voice, consider a Verizon signal booster or the larger Enterprise LTE Network Extender (below) instead.
The new “4G LTE Network Extender" SLS-BU103 is an updated version of Verizon's SCS-2U01 picocell that supports only 4G LTE service. That means that anyone who has an older phone that doesn't support "HD Voice" won't be able to connect to the device. The SLS-BU103 covers up to 7,500 square feet and up to 7 simultaneous voice calls.
Unfortunately there is no way to limit who can access the device, so if you live in an apartment and there are other users within range of the device, they can connect and use the extender's signal.
Recommendation: If you're happy with just 4G LTE and no 1x CDMA signal and will have no more than 7 active callers at any one time, Verizon's new 4G LTE Network Extender is an excellent choice. If you need more than just LTE signal or have more users, you may want to consider a Verizon signal booster instead.
The Verizon 4G LTE Network Extender for Enterprise supports 4G LTE only, for up to 42 simultaneous active users, and up to 200 idle users. Each individual unit will cover up to 30,000 square feet, but an unlimited number of devices can be installed within a building to provide coverage over a much larger area.
The Enterprise unit doesn’t support 2G voice or 3G data, which means that unless your devices support Voice over LTE, the generated signal won’t be usable for calls.
Recommendation: A great solution as long as you are happy with LTE only signal. If you need both 2G/3G and 4G, consider a Verizon signal booster or combine the device with Verizon’s smaller Network Extender for Business (above).
T-Mobile only offers one unit, the Alcatel-Lucent LTE Cellspot, which supports up to 8 simultaneous 2G/3G users and 8 simultaneous 4G LTE users. The device will cover up to 3,000 square feet, and includes an external GPS antenna port.
Similar to AT&T and Verizon’s picocells, calls will not transfer from the macro network to the LTE Cellspot. So if you walk into your home or office while on a call, the call will be dropped instead of handing off to the device. However, the device will attempt to hand off calls to the macro network as you walk out of the building.
Recommendation: A great unit as long as you don’t mind dropped calls walking into your home or office. If that’s an issue, consider a T-Mobile signal booster instead.
Sprint’s original Samsung Airave device supported 2G CDMA voice only for 3 simultaneous users, but has now been discontinued.
Sprint’s Airave 2.5 unit is built by Airvana, which was acquired by Commscope. The device supports 2G CDMA voice and 3G EVDO data and covers up to 5,000 square feet and up to 6 simultaneous voice or data users.
Unfortunately Sprint’s device will not hand-off in either direction, so calls will drop both as you enter and leave the building where the device is installed.
Recommendation: The lack of hand-off to the macro cellular network can be very frustrating for users, and it appears that Sprint is in the process of discontinuining this device. Consider a Sprint signal booster instead.
Sprint is currently in the process of rolling out there new Sprint S1000 device that runs on the 2.5GHz TDD frequency band. The device is only available in some markets, and since Sprint has not yet rolled out Voice over LTE, the device does not currently support voice calls.
Recommendation: Lack of support for voice calls means that the unit is useful if you require LTE signal only, and not voice calls. Consider a Sprint signal booster instead.