Effective network planning is essential to cope with the increasing number of data subscribers and bandwidth-intensive services competing for limited radio resources. Operators have attempted to meet this challenge by increasing capacity with new radio spectrum, adding multi-antenna techniques and implementing more efficient modulation and coding schemes. However, these measures alone have been insufficient to address the capacity requirements.
One way to expand an existing macro network, while maintaining it as a homogeneous network, is to make it denser by adding more sectors per evolved Node B (eNB) or deploying more macro-eNBs. However, reducing the site-to-site distance in the macro network can be pursued only to a certain extent because finding new macro sites becomes increasingly difficult and can be expensive, especially in crowded areas. A viable alternative is the introduction of small cells through the addition of low-power base stations (eNBs, home eNBs [HeNBs], relay nodes [RNs] and remote radio Heads [RRH]) to the existing macro-eNBs. The result is a heterogeneous network (HetNet), with large macrocells and small cells providing increased bitrates per unit area.
Key advantages of small cells
Small cells help in offloading data from licensed/unlicensed spectrum by using a combination of technologies such as 2G, 3G and long term evolution (LTE) along with carrier-grade Wi-Fi. These are primarily added to increase capacity at hotspots with high user demand and in areas not covered by the macro network. Small cells also improve network performance and service quality by offloading from large macrocells. Moreover, site acquisition is easier and cheaper with these as they take up a considerably smaller area.
Types of small cells
Small cells are typically classified as femtocells, picocells, microcells and metrocells, depending on the base station power and purpose of usage. Femtocells are the smallest of small cells, which are primarily deployed in consumer and enterprise markets. These units are typically single-sector with an omni-directional antenna to improve coverage indoors for 4-32 users; however, there is no strict restriction on the number of users. Moreover, consumer femtocells typically transmit up to 100 mW radiation, while enterprise-grade femtocells may transmit up to 300 mW due to wider coverage requirements. These were first introduced by US-based Sprint Airave in 2008. Since then, several operators have deployed these cells for both the consumer and the enterprise segment.
Picocells usually refer to cells that are deployed for improving coverage in indoor public areas, including shopping malls, train stations and airports, as well as enterprise locations. Picocells are also single-sector and typically transmit less than 4 W radiation, catering to more than 32 users. Compared to femtocells, these are deployed on a smaller scale due to their larger coverage area and smaller target market. Picocells have traditionally been a less intelligent version of femtocells and have acted as typical base stations, although vendors are now including femtocell-developed technologies in these larger units in order to adopt several benefits, including auto-configuration, radio environment awareness and remote support.
Microcells can be regarded as small macrocells and are usually deployed in capacity-constrained urban areas. These are sometimes also deployed in rural areas where the coverage of a macrocell may not make sense due to concentrated population in a limited region. Microcells are typically three-sector, unless deployed in light poles or building walls where they are single-sector. The power transmitted by microcells can be as high as 40 W.
Meanwhile, metrocells are a special type of single-sector microcells that are deployed primarily in capacity-constrained areas. These are deployed as an overlay rather than as the primary cellular network, thereby necessitating advanced features such as self-optimising networks and auto-configuration.
Apart from these categories, there are some additional types of small cells that target specific market segments. For instance, meadow cells have been specifically developed for deployment in rural areas.
Deployment challenges
The sheer volume of small cells (around 10-40 times more than macrocells) and the use of inherently insecure internet backhaul present some deployment challenges pertaining to security and protection that are unique to small cells and must be addressed by the operator. For instance, operators will need an architecture that allows for bootstrapping, that is, the loading of a start-up configuration of small cells and intelligent load balancing of the traffic. This has implications on the capabilities of the core
network and especially the security gateway. The security gateway will need to have the ability to issue
the appropriate certificate to the femtocell after initial authentication and also support a mechanism for session-aware load balancing across security gateways based on each security gateway session. Thus, provisioning, configuration and management routines that may have been cost effective for macrocells become unsustainable for small cells.
Secondly, due to the large number of cells that need to be aggregated, small cells are more likely to create signalling traffic storms, overloading the mobility management element with signalling traffic and creating service outages. Unexpected signalling storms could occur due to power outages, resulting in a large number of cells re-establishing their connections at the same time, dysfunctional behaviour of smartphone applications and misconfigured small cells. To protect the core network from such situations, the security gateway needs to support the ability to frame and monitor a variety of signalling protocols.
Future outlook
Mobile broadband traffic will continue to grow as operators expand 3G and 4G networks and move towards 5G networks. Although initial network deployments will predominantly be macrocells, insatiable subscriber demand will force the operator to densify its network by deploying small cells. Meanwhile, the increased availability of low power devices and the expansion of the internet-of-things platform will further increase the need for small cell solutions.
According to the Small Cell Forum (SCF), around 14 million small cells were shipped as of March 2016. The small office and home segment was the biggest category of small cell users. However, SCF predicts that by 2020, enterprise deployments will grow to become the largest market for small cells, in spite of the continued growth of consumer small cell installations. In the enterprise space, a key market driver will be the ability to create new services and drive revenue opportunities based on small cell connectivity. SCF estimates that around 40 per cent of the enterprise small cells in 2020 will be managed from the cloud, and that the resulting market for mobile cloud services based on small cell infrastructure would be around $4 billion.
Further, as per SCF, in 2014, around 88 per cent of non-residential small cells were deployed in low- or medium- density configurations, that is, less than 25 small cells per square kilometre. However, in 2016, SCF estimates that around half of all small cell deployments will be in much higher density configurations, with as many as 15 per cent in hyper dense configurations of more than 150 small cells per square kilometre.
Adding small cells and integrating these with their macro networks to spread traffic load is likely to remain the key strategy of telecom operators to improve service quality and efficiently utilise their existing spectrum. Going forward, efforts must be made to improve the technology by addressing challenges related to integration and optimisation across the small cell layer, increasing network virtualisation, multiple frequency usage and network security.
