The Return of the Small Cell

10 Feb

Not a long time ago, in a galaxy not far away – in fact this one – small cells made their first appearance on the scene. In a 3G network, small cells were primarily used for in building coverage where it was needed. It was an important role, but not a glamorous one.

So what awakening in the force is calling for a return of the small cells now? First, the demand for mobile data took over the world, leaving operators with overloaded networks and scrambling to add both coverage and capacity. Rolling out LTE networks for mobile traffic was the first step, but these networks are already being strained.

Strategy Analytics predicted in its report, Mobile Data Traffic Forecasts 2014-2018,that by 2018 mobile networks will carry 56.2 Exabytes of data, up from 21.3 Exabytes of mobile data traffic in 2014. This growth in traffic is being driven by both strong performance on 4G/LTE networks and rapid growth in smartphone data subscriptions, with future growth driven by tablets, consumer electronics and M2M devices contributing to a larger share of the total data traffic.

There are a number of ways that operators can add capacity to their networks, including acquiring new spectrum, which leads us to our second point – LTE spectrum is both scarce and expensive. Mobile operators are turning to a Heterogeneous Network model – or HetNet – to maximize their spectrum. A HetNet is composed of a combination of macrocells, Wi-Fi, distributed antenna systems (DAS) and small cells. By using a layered network model with deployed small cells, mobile operators not only address coverage concerns between macrocells and in indoor environments, they can also add much needed capacity to the network and improve the overall end user experience.

Provisioning Small Cells

The addition of small cells to the mobile network adds complexity when it comes to RF network planning as compared to a macrocell-only network; therefore, efficient and autonomous coordination between macrocells and small cells is key. Self-organizing Networks (SON) techniques provide the real-time self-configuration, self-optimization and self-healing capabilities that are becoming mandatory features for the HetNet to work as a cohesive network. SON offers the promise of reducing costs in initial rollouts, enabling more effective coordination of time and frequency resources, providing dynamic interference management, and adapting to changing network conditions.

Interference Management and Carrier Aggregation

Mobile operators are deploying LTE-Advanced capabilities such as carrier aggregation and interference management techniques to make their networks even more efficient. Here again, small cells remain critical to operators’ overall strategy as they provide additional capacity in dense indoor environments where a majority of data traffic will be generated.

In a HetNet scenario combining small cells with Wi-Fi and macrocells, mobile operators are rolling out a number of interference management techniques to ensure the network is optimized for capacity and coverage.

  • Enhanced Inter-cell Interference Coordination (eICIC) works as the interference manager for small cells as part of a HetNet. It uses advanced time domain scheduling to reduce radio interference and increase the coordination between network cells to ensure a streamlined flow of information.
  • Multiple Input and Multiple Output (MIMO) is an approach which serves to increase efficiency across the spectrum by leveraging smart antenna technology that analyzes how base stations, antennas and user equipment communicate.
  • Coordinated Multi-Point (CoMP) is a technique that ensures that even greater performance is achieved at the edge of the network, by increasing coordination between small cells, and between small cells and macro cells.
  • Relay Nodes are low-power base stations that reduce the site-to-site distance in the macro network. They were added to the LTE Release 10 specification.

Mobile operators are currently focused primarily on deploying eICIC and MIMO. Relay nodes add additional complexity to the network and will be rolled out at a later date.

Carrier aggregation aggregates mobile operators’ 3G spectrum freed up by LTE roll-outs along with LTE and LTE-Advanced spectrum to add increased throughput to the network. The scarcity of spectrum has led to faster adoption of carrier aggregation by mobile operators as compared to other LTE-Advanced capabilities. Carrier aggregation is also being used for both LTE-FDD and LTE-TDD modes, allowing mobile operators with both network assets to adopt the technology to gain even more performance.

Carrier Aggregation and LTE-Unlicensed

One of the biggest innovations in driving the return of the small cells is LTE-Unlicensed technology, also known as LTE-LAA (LTE-License Assisted Access). Mobile operators are beginning to aggregate unlicensed spectrum in the 5 GHz band with their available licensed spectrum to add even more bandwidth.

As LTE-LAA is an extension of LTE-Advanced and is based on carrier aggregation, it’s no surprise that small cells remain central to its deployment. Leveraging small cells for LTE-LAA provides a localized approach to carrier aggregation that helps mobile operators co-exist with the Wi-Fi community, while being able to further maximize their spectrum to increase capacity and coverage and ease network strain. Small cells are also suited to deployment in the LTE-LAA 5 GHz band as they are better suited to the band’s low power requirements as opposed to macrocells.

LTE-LAA is expected to be fully standardized in 3GPP Release 13, currently planned to be finalized in 2016.

Breaking Down the Numbers

Dell’Oro Group has forecast that small cell RAN revenues will account for 16% of the total RAN market by 2020. According to Pongratz, “the indoor enterprise/public access market improved significantly in 2015, though outdoor revenues still account for the greatest portion of revenue. We expect the indoor segment to grow at a quicker pace and expect the revenue split between indoor and outdoor to be closer to 50/50 by 2020.” And 5G small cells will account for close to 5% of the small cell market by 2020.

Deployments Around the World

A large proportion of the world’s small cells have been deployed in Asia Pacific in Korea and Japan, with volumes picking up in China and India. In dense environments, mobile operators are deploying enterprise and residential small cells in indoor venues to add capacity locally, with the potential for a 1-to-4 ratio of macrocells to small cells.

Outdoor picocells will most likely leverage a Cloud-RAN architecture and will roll out in 2017. Cloud-RAN deployments are now out of the proof of concept stage and are currently in trials. These cloud-based access points, which are also known as virtual base stations or C-RANs, will form the base of a 5G network architecture. They will handle not only the voice and data traffic for consumers, but will also support the M2M and IoT applications that form the ‘connected network’ of the future.

The majority of small cells will be multi-mode, supporting both LTE and Wi-Fi in the 5GHz band, allowing operators to take advantage of cost savings due to leveraging unlicensed spectrum. By supporting the 5GHz band, mobile operators can also deploy LTE-LAA on the same small cell.

Critical Lessons Learned

Each small cell deployment is unique – and what happens in the lab is never replicated exactly in the field. In addition, we learned that focusing on data speeds in trials wasn’t enough. In real-world deployments, it’s more important to make sure that the small cells gel with the network from all angles and not just provide the required data speeds.

Mobile operators also need to plan for more than just mitigating interference between small cells and macrocells. They also need to mitigate issues associated with small cell placement and acquiring the necessary real estate. This needs to happen early in the planning process to ensure a smooth roll-out.

Small cells have returned in a big way. From a role of just filling coverage gaps in 3G networks, small cells now form a major part of mobile operators’ strategy as they contend with exploding mobile data traffic on their networks and chart their path towards 5G. It’s all about adding capacity efficiently and economically. Now that the small cell force has awakened, we can’t wait for the next sequel.




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