the role of small cells in heterogeneous networks

Author:Marcel NoordmanEngagement Consultant, Mobile Broadband,Ericsson Latin America and Caribbean

Global smartphone uptake continues to show strong momentum. About one-third of all handsets sold in 2011 were smartphones, compared with about one-fifth in 2010 [1]. Total smartphone subscriptions reached around 700 million in 2011 and are expected to reach around 3 billionin 2017 [3]. The number of tablets sold worldwide is expected to rise from about 18 million in 2010 to more than 326 million by 2015 [2]. Subscribers are using these connected devices not only to access the internet, but also to access applications and cloud-based services, including video and other bandwidth-intensive content.

As a result of these trends, mobile data traffic generated by smartphones almost doubled between Q1 2011 and Q1 2012 [3] and is expected to keep growing at an annual rate of 60% globally. Currently, the average smartphone user generates approximately 500MB of traffic per month [1] – and this number is rising. Overall, mobile data traffic is expected to grow fifteen-fold by 2017 [3].

1 July 2012 – The role of smalls cells in Heterogeneous Networks

As the Networked Society takes shape, providing the right user experience is a top priority for operators. Bandwidth hungry applications, common on smartphones, tablets and other connected devices, are driving data traffic growth. For the networks to keep pace with demand, operators can improve and densify them as well as adding small cells in what is known as heterogeneous networks.

Small cells are complementary to macro sites and a great way to solve capacity and coverage problems in hotspots such as busy city squares, commercial streets, railway stations, hotels, shopping malls, offices and airports. Close coordination with the rest of the network is needed to maximize reuse of scarce spectrum and ensure quality of service.

Making small cells work effectively as part of an integrated heterogeneous network needs careful planning and precise location in the network. This puts requirements on site acquisition, power connection and the availability of (carrier grade) backhaul. Furthermore total network cost typically scales with number of sites (large and small). Keeping operations and maintenance cost under control can become an issue.



Delivering quality user experience end-to-endUsers are increasingly aware of the connection speed, data rate, coverage and availability of their mobile broadband services. To ensure that subscribers remain satisfied, operators must deliver a consistent, high-quality and seamless mobile broadband experience that meets or exceeds their expectations. As Figure 1 shows, achieving subscriber satisfaction will require improved data performance overall and at cell edges, especially indoors where about 70 percent of today’s data traffic is generated.

Although macro cells have been proven to be cost effective for most scenarios, meeting the demand for mobile broadband is now increasingly challenging in certain scenarios, such as:

• Large outdoor hotspots, such as town squares and commercial streets with high traffic demand and an already dense macro network – areas where interference is high;

• Large, isolated indoor hotspots, such as businesses and hotels that may be difficult to reach from an outdoor macro network;

• Large indoor hotspots, such as shopping malls, airports and subway stations, where mobility demands and interference are high;

• Localized, indoor hotspots or minor coverage holes, such as small offices, restaurants and retail outlets that challenge the deployment and cost structure of conventional cellular networks.

In each of these locations, heterogeneous networks, in which small cells complement macro cells, can help to meet the growing demand for mobile broadband. However, to make this happen, a number of challenges need to be overcome.

As small cells complement the improved and densified macro cells, and the number of radio nodes increases, backhaul becomes more important. Backhaul performance not only affects the data throughput available to users, but also the overall performance of the radio-access network. Internet grade backhaul provides sufficient performance for non- or loosely-integrated small cells.

However, for more tightly coordination of small cells, needed for optimized usage of scarce spectrum, a high performance backhaul with low latency is required. Networks with a large number of (small) cell sites require backhaul solutions that can use a selection of physical transmission media, including microwave, fiber, copper lines and wireless connectivity. For operators, there is a clear trade-off between using infrastructure resources that are already available – perhaps not at exactly the needed location – and investing in dedicated backhaul transmission that results in higher overall network performance. This decision also affects installation costs and the time needed for site acquisition and installation.

To obtain maximum value from the radio spectrum, operators will need flexible base-station site solutions that allow for ideal placement of the radio site. Operators may need to consider alternatives for site location by cooperating with new partners such as municipalities, retailers and external agencies rather than traditional deals made with landlords and tower-approval committees. In metropolitan areas, complementing an already-dense macro network with additional small cells at street level needs to be implemented using small antennas in such a way that equipment is almost invisible.

The sitechallenge

The backhaulchallenge

Figure 3: A combined approach to maximizing the performance of heterogeneous networks: (1) Improve: better overall cell-site performance; (2) Density: enhanced cell-edge data rates: and (3) Add: increase indoor data rates.


Deployments that involve a significant number of small radio base stations change the economics of mobile network rollouts. The time and cost of planning, installing and configuring a conventional macrocell site is difficult to justify for small cells that may serve a limited number of subscribers. Holistic planning and phased migration will help operators scale up in a given network area. From a radio network perspective, the complexity of a heterogeneous network composed of multiple layers and radio technologies could easily become unmanageable unless it is carefully designed. The number of neighbor relations in a network of different cell sizes can grow substantially, and manual handling of cell identity and neighbor cell lists will become labor intensive and costly. The need to move seamlessly from one radio-access technology to another to maintain maximum coverage and resource utilization calls for effective inter-working between macro cells and small cells, as well as across radio-access technologies.

For most operators, radio spectrum is a limited resource and one of the most strategic and important investments. This leads to a demand for spectrum to be used as efficiently as possible – especially in densely populated areas. In a heterogeneous network, the coordination between macro cells and small cells has a positive impact on the performance of the radio network and consequently on the overall user experience.

Coordinated, embedded small cells improve performance through frequency reuse, increasing both network data capacity and throughput without the need to split the available spectrum. The highest coordination gain is achieved when using a dedicated high-bandwidth, low-latency link among several radios provided by the same baseband.

Coordination reduces the number of required small cells by 50% to 70% and increase user bit-rates for devices limited by transmission power or interference by a factor of two to ten [4].

This reduces TCO of a small cell layer by up to 50% [4] due to reduced infrastructure (fewer cells), rollout, operation and maintenance costs.

Delivering quality user experience end-to-end

Figure 2: Cell selection in a heterogeneous network.

The spectrum challenge

The scalabilitychallenge


MAKING THE RIGHT HETEROGENEOUS NETWORK CHOICESDesigning a heterogeneous network in the most effective way involves improving, densifying and adding to the mobile broadband infrastructure:

• Improve existing macro cell sites – by enhancing macro cells with more spectrum, advanced antennas, increased order of diversity on the receiver and/or the transmitter, and greater baseband processing capacity within and between nodes. Continued evolution of HSPA and LTE technology will drive macro network efficiency through specialized features, such as higher-order modulation, higher sectorization, multi-carrier and multi-antenna solutions, as well as spectrum refarming using hybrid radio solutions. Increasing capacity and data rates in this way eliminates the need for new sites.

• Densify the macro network – the capacity and data rates achieved by enhancing the macro network alone will eventually prove insufficient to meet demand. A simple way to densify a network could be a cell-split, which enables a site to transition from a three-sector site to a six-sector site. Another option is to add new macro sites in strategic locations.

• Add small cells – complement macro cells with small cells and dedicated indoor solutions based on the 3GPP standard. This approach can include the use of micro cells, pico cells or low-power remote radio units (RRUs), as well as Wi-Fi. It delivers high per-user capacity and rate coverage in areas covered by the small cells, with the potential to improve performance in the macro network by offloading traffic generated in hotspots. The degree of integration that can be achieved throughout the heterogeneous networks will determine the overall network performance.

As shown in Figure 3, the key is to find the right mix – in other words where to improve, densify andadd to meet future capacity and coverage demands. How and when to use each tool depends on the existing networks (macro site density), the availability of backhaul (whether owned or leased), the availability of spectrum (whether licensed or unlicensed), estimated traffic volumes, and required datarates, as well as the technical and economic feasibility of each individual approach.

Figure 3: A combined approach to maximizing the performance of heterogeneous networks: (1) Improve: better overall cell-site performance; (2) Density: enhanced cell-edge data rates: and (3) Add: increase indoor data rates.


Common misunderstandings about small cells

As is clear from the discussion above the question is not macro or small cells but rather a smart combination of the two. It is easy to get confused and there are a few common misunderstands about the topic:

> Misunderstanding 1:

“Small cells are the only way to expand capacity to meet growing mobile data traffic.” Small cells are part of a much larger array of options that mobile operators have to expand their network capacity and improve service quality. Before adding small cells, operators should improve and densify the current macro network by e.g. adding spectrum and carriers, upgrading to higher modulation schemes and data speeds, increase receiver and transmitter diversity, radio network optimization, sectorization and deploying LTE in new spectrum on the same grid. After maxing out on these options, operators can deploy supporting small cells in hot spot areas to provide further coverage and capacity. By 2017, over 30 percent of the world’s population are expected to live in metro and urban areas. These areas represent less than one percent of the Earth’s total land area, yet are set to generate around 60 percent of mobile traffic by 2017. In these areas, heterogeneous networks will complement macro network improvements, serving the traffic and providing good coverage and high quality user experience. By 2017, Ericsson expects that in metro and urban areas, each macro cell will on average have three supporting small cells in a heterogeneous architecture. This is roughly equivalent to one small cell per macro sector.


“Small cells are less complex than macro cells.” Spectrum is a scarce and valuable resource to network operators and needs to be managed accordingly. This requires a well-planned network that is coordinated and optimized across its different layers. Such a network puts strict requirements on placement of small cells within the existing network. Optimizing the network in this way poses challenges with regards to the availability of space (no suitable site to mount the equipment), power and backhaul (no carrier grade connection available). Self Optimizing Network (SON) solutions will help by automating regular planning activities but provide no substitute for proper design, planning and optimization expertise. A more opportunistic approach to

small cells networks without coordination and dedicated spectrum will inevitably lead to suboptimal networks, lower utilization of scarce resources and ultimately; higher cost for operators, end users and the economy at large.


“Small cells are cheaper than macro cells.” As a rule of thumb, cost for building and especially running a mobile network grows proportional to the number of sites (large and small). Each site needs a location, power, a connection to the rest of the network, monitoring, maintenance and sometimes repair. The economics of telecom networks is one of economies of scale and as such, when including all actual costs, macro sites will always have a lower cost per unit of capacity than smaller cells.


“Small cells are better from a public RF exposure perspective than macro cells.” All telecom equipment and installations are compliant with national and international safety rules for Radio Frequency (RF) exposure. The applicable limits have been set with large safety margins, and the exposure levels in public areas are typically far below these limits. RF exposure levels are broadly speaking dependent on the output power from the antenna and the distance to the antenna; the exposure diminishes rapidly with the distance. Macro sites have higher output power but are much further away from the public while small cells have lower output power but are regularly on street level. Therefore, the public RF exposure levels from different site types are comparable, and always below established limits. Consequently, there is no difference between small cells and macro cells from a public health perspective.

References

1. Ericsson, Fourth Quarter Report, January 2012.http://hugin.info/1061/R/1579912/493212.pdf

2. Gartner, Gartner Identifies Top 10 Commercial Business Applications for Tablet Devices,November 15, 2011.http://www.gartner.com/it/page.jsp?id=1849621

3. Ericsson, Traffic and Market Data Report, June 2012.http://www.ericsson.com/res/docs/2012/traffic_and_market_report_june_2012.pdf

4. Landström, S., Furuskär, A., Johansson, K., Falconetti, L., Kronstedt. F., (2011) “Heterogeneousnetworks – increasing cellular capacity and coverage,” Ericsson Review 1/2011, pp. 4-9.http://www.ericsson.com/news/110211_hetnets_244188811_c

About the autor

Marcel Noordman is engagement manager for mobile broadband business with Ericsson Telecommunications in Latin America. In this role he works with senior management of operators to develop their business and turn challenges into business opportunities. He has a special interest in Business Planning and Strategy, financial modeling and analysis and sustainability.

[email protected]

Telefonaktiebolaget LM EricssonSE-126 25 Stockholm, SwedenTelephone +46 8 719 0000Fax +46 8 18 40 85www.ericsson.com

ERC-12:000342 Uen© Ericsson AB 2012

the role of small cells in heterogeneous networks

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