Pros and cons of small cells

With mobile data traffic expected to double annually, small cell base stations are set to play an important role in expanding the capacity of wireless networks. Mobile operators are realizing that to meet the demands for data, video and application access caused by smart phones and other devices, there is a real beauty to going small.

Small cells provide flexibility and increased QoS capabilities at an attractive cost. Implementing a small cell infrastructure is also more environmentally friendly as it will reduce the number of cell towers (maybe even eventually eliminate them) and it provides a cleaner signal with less power.

But going small is the answer to just one part of the equation. The ability to interact with today’s macro networks as well as effectively backhaul small cells is another.

Compact base stations (C-BTS) have been referred to by several nomenclatures such as micro and pico base stations. They are small size, fully integrated base stations that include baseband processing and radio module in one physical unit. They are relatively light in weight (e.g. a few kilograms) and are easy to deploy and maintain. They come with varying output power ranging from a half-watt to a few watts.

They are provisioned to support a limited number of subscribers that range in the tens of subscribers. These features separate them from larger ‘macro’ base stations that typically have a split or all-indoor architecture in addition to larger capabilities in terms of output power and number of supported subscribers. Thus, it is common to deploy compact base stations at relatively low height (e.g. 10-15 meters) to cover a limited area (e.g. 100 meter) to provide capacity to a hot spot or coverage in a dead zone. Macro base stations on the other hand are mounted higher at 30-45 meters, for example, and used to provide wider coverage (e.g. 500 meters or more).

Despite the differences in architecture and form-factor, the data rate that a large or small base station can support is the same. This is the link-level data rate that is measured in a lab environment, in the absence of interference. The link-level rate is determined by the capability of the wireless access technology such as WCDMA or LTE.

However, what matters to wireless network operators and subscribers alike is the actual performance of a wireless network. Hence, network-level performance is the real measure with link-level performance being an upper-bound that’s only reached in ideal scenarios seldom present in a real deployment.

Small cell base stations provide higher capacity than macro cells because of the deployment scenario. Because these base stations are mounted low above ground, they are less susceptible to interference. This translates directly into higher capacity.

Higher signal quality leads to better throughput because it allows the system to use a more spectrally efficient transmission scheme where more bits can be transmitted at the same time.  Therefore, the areas where users can transmit and receive at higher data rate are larger in a small cell than a macro cell. This is significant because it leads directly to a large increase in overall capacity: the capacity of small cell base station is about 89 per cent higher than that of a macro cell.

Additional benefits include:

1-    Lower delay: users will experience lower latency for data services and will enjoy faster download and upload time.

2-    In-building coverage: small cells provide better outdoor-to-indoor coverage. Considering that 40 per cent of mobile traffic originates from home and 25 per cent from work, this can represent a significant source of revenue for network operators.

3-    Better cell-edge coverage: Macro base stations provide poor service at the cell edge which includes a large percentage of the cell area. Small cells provide better cell-edge performance, particularly for the uplink than large cells.

The fact that small cells provide almost double the capacity of a macro cell is why they are set to become an important part in addressing the capacity crunch in wireless networks.

However, small cell base stations have met with challenges from a business case perspective even though they can be made at a fraction of the cost of larger base stations. The issue is how to backhaul such base stations and the cost of the backhaul. As these base stations are mounted low above ground, typical microwave systems are not technically effective because they require a clear line-of-sight between the two nodes of the microwave link – which is difficult to provide in urban areas where small cells are deployed below building rooflines. Alternatively, fiber is expensive to lay as it may not be available at the spot where the small cell is required in addition to large monthly fees that make it unattractive from an economical perspective.

Frank Rayal is VP of Product Management at backhaul supplier BLiNQ Networks

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