Powering Small Cells


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Rosy forecasts of mobile data and internet usage from wireless industry leaders like Ericsson and Cisco are coming true. Surging mobile data and internet access growth is creating huge capacity demand on wireless networks. The carriers’ response is to densify their networks. This means locating antennas closer to mobile devices using lots of small cells. In this scenario, the U.S. installed base of roughly 350,000 cell sites could triple over the next five years. (See, Counting Cell Sites)

Think of small cells as being just like macrocells, only smaller. That’s true to a point. Macrocells serve up to 1,000 simultaneous users over a radius of 1-2 kilometers. They typically are installed on tall towers or rooftops with space for a carrier’s hut or shelter that provides security and air conditioning for baseband and backhaul equipment along with a DC power plant with large batteries for power backup. 

Macrocells can consume over 1 kilowatt (kW) of power. Battery reserve capacity usually is designed to keep the site on the air for up to eight hours when utility AC power fails. By contrast, small cells are mounted on telephone and utility poles or streetlights to serve about 100 users over a radius of roughly 200 meters. A rule-of-thumb density ratio is 10 small cells to a single macrocell. A small cell consumes about 100 watts of power. With antennas at the top of the pole, small cell radio and backhaul electronics are compact enough to fit in an equipment cabinet attached to, or integrated into, the pole. 

Small cells usually are powered from a local utility AC drop but with such limited space, backup power is not included. In a power outage, small cells will go off the air. One could argue that losing a small cell will not have a big impact on the larger network. But with 5G intended to be very much application- and use case-driven, a small cell may deliver specific services to a nearby Enterprise or institution. If the small cell goes down, that customer may lose critical capabilities. In these cases, small cell backup power is imperative.

A common power hub is a viable solution to providing primary and backup power to a cluster of small cells without the need for individual AC power drops. Already, several infrastructure equipment suppliers are promoting centralized power hub schemes that can serve several dozen small cells in an urban or suburban cluster. 

The power hub comprises an AC/DC power plant with rectifiers, controller and batteries all housed in an outside plant (OSP) cabinet that can be located curbside, in a central office or even at a macrocell site. The power plant output capacity and battery reserve hours can be sized according to the number of small cells connected and the distance covered by the cluster. A metered utility AC power line connects to the DC power plant inside the cabinet. DC power is delivered, consistently and reliably, from the hub to small cells over a dedicated paired-wire copper cable strung between sites spaced at their operating radii in the cluster.

Interestingly, this shared architecture creates a unique opportunity to include a dedicated fiber optic feed to each cell site from a distribution point in the access network (see, 5G Fiber-rich Networks). With a fiber terminal installed in the hub, a hybrid fiber-copper cable, similar to that used to connect baseband units (BBUs) and remote radio units (RRUs) at macrocells, can provide signaling and power to each small cell in the cluster in a secure, cost-effective manner. This step helps the wireless carrier avoid the expense of leasing fiber capacity and copper pairs from telephone, cable or fiber companies.

DC power and battery backup represents only a single-digit percentage of the total capital investment at a cell site but is 100 percent critical to keeping on-the-air macrocells and small cells alike.

By John Celentano, Inside Towers Business Editor

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