What is a small cell, really?

Analog Devices: ‘There have been a lot of different uses of the term small cell’

Despite the current emphasis on small cells operating in higher 5G frequencies, this infrastructure is not actually limited, or defined, by frequency. In fact, 4G small cells operate in sub-3 GHz frequencies and some 5G small cells operate in frequencies lower than the 30 GHz to 300 GHz of millimeter wave (mmWave).

What makes a small cell a small cell, then, according to the Federal Communications Commission (FCC), is its height and volume — perhaps not too surprising given its name.

More specifically, per the FCC, a small cell must meet the following specifications:

1. Mounted on structures 50 feet or less in height including their antennas, or mounted on structures no more than 10% taller than other adjacent structures, or does not extend existing structures on which it is located to a height of more than 50 feet or by more than 10%, whichever is greater;
2. Each antenna associated with the deployment, excluding equipment associated with the antenna itself, is no more than three cubic feet in volume
3. All other wireless equipment associated with the structure, including the wireless equipment associated with the antenna and any pre-existing associated equipment on the structure, is no more than 28 cubic feet in volume.

Peadar Forbes, the director of radio platform development at Analog Devices, told RCR Wireless News that while the definition of a small cell “hasn’t changed much over time” for the company, the industry has exercised “a lot of different uses of the term small cell.”

The first small cells were femtocells, a small, low-powered cellular base station designed primarily for use in a home. Forbes said that while femtocells didn’t enjoy the popularity initially expected by the industry, network densification did become a requirement, making small cells more essential. 

“Today, we define small cells as any solution designed for densification efforts and for lower output power than a macro radio,” he said, placing the output power of a small cell within the bounds of from about 250 milliwatts to about 10 watts.

“Within that,” he continued, there are indoor small cells, which are typically 250 milliwatts and outdoor small cells, which are anywhere from one watt to 10 watts.” 

How do does the small cell fit into D-RAN/C-RAN deployments?

Also worth noting is how small cells fit into the larger ecosystem evolution from distributed radio access networks (D-RAN) to centralized radio access networks (C-RAN). Centralizing the RAN allows for network resource virtualization and for services to be deployed on the network edge rather than the core, moving services closer to the user, improving customer experience while reducing the bandwidth needed for backhaul traffic.

C-RAN also allows for BBU pooling, joint processing and cooperative radio sharing. When the base station and network capacity are centralized, an operator doesn’t have to decide on the day of deployment how much capacity to allocate to each cell site; instead, capacity can be used more dynamically, so that it is allocated to different sites as needed.

“Traditionally,” Forbes explained, “small cells and femtocells, in particular, were in the vein of D-RAN, where essentially the entire base station was in a box at the edge of the network. I think the evolution, really, within small cells, is from that distributed model more towards that centralized RAN model.”

However, he added that not all small cell deployments can be expected to be a centralized deployment as each deployment scenario will determine how much an operator wants to centralize versus how much it wants to distribute.

What are the small cell use cases?

As Forbes mentioned, small cells can be deployed indoors or outdoors, but they can also be deployed in licensed, shared or unlicensed spectrum, making them a versatile coverage and capacity solution. They are initially expected to be used in dense urban environments, followed by environments like sport complexes, concert halls and other enclosed venues where thousands of people gather at one time are also promising use cases for small cells, which can provide the extra capacity needed during moments of data influx.

Down the line, use cases within smart cities and cellular vehicle-to-everything (C-V2X) are expected. And while still in the early stages, there is growing interest in the use of small cells for private networks.

“There is a lot of interest in private networks — mining, ports, manufacturing — and 5G brings a lot of benefits to those use cases,” said Forbes. “A lot of these use cases will use small cells. In a factory or a small campus […] the natural thing you would do is deploy small cells.”