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Sep 17 2012

Answers to the Top 5 Questions on Metro Cell Deployment

Understanding the why, where, when, and how of metro cell deployment helps mobile network operators (MNOs) develop the right strategy.

Why should I deploy metro cells?

Metro cells — small cells used in public or open access areas — give MNOs a low-cost option to quickly expand network capacity and coverage. More capacity for less money As small, low-power access points, metro cells offer lower TCO than a macro-only solution. To determine the savings metro cells can provide, Alcatel-Lucent Bell Labs compared the TCO of a macro-only solution to that of a macro and metro cell solution, with and without the early introduction of LTE. The Bell Labs study of a large European Metropolitan area showed that over a 5-year period a W-CDMA macro and metro cell solution provides a 38% TCO savings compared to a W-CDMA macro-only solution (Figure 1). When LTE is introduced in Macro and Metro Layer, the 5-year TCO is reduced by 45% compared to deploying W-CDMA macro cells only (Figure 2). The main benefit the metro cell layer is bringing is the reduction in the number of needed new macro cell sites that a macro-only solution would be forced to acquire as data traffic continues to build over the study period.

Capacity and coverage exactly where it’s needed Due to their small form factor, metro cells can be easily mounted on walls, lamp posts, poles or even the side of a building. Because they can be deployed just about anywhere, they can bring capacity and coverage to just about any location, indoors or outdoors. Improve QoE Metro cells bring the base station closer to the user, improving air interface quality. People experience very good voice and data service and enjoy higher throughput and faster, more reliable data connections. Metro cells also offload heavy data users from the macro network. This frees limited resources for subscribers on-the-move and improves quality of experience (QoE) for users on the macro network.

Where should I deploy metro cells?

Metro cells are typically deployed in 3 areas (Figure 3):

  • Outdoor urban hotspots: Metro cells are deployed in high-traffic locations, such as city centers, train stations, airports and shopping centers. They bring massive capacity to places where nomadic or pedestrian users consume a high volume of data.
  • Indoor urban hotspots: Metro cells are deployed in event and convention centers, hotel lobbies and office buildings to improve voice and packet data services where people gather indoors.
  • Rural locations: Metro cells are deployed in small towns and other rural areas to extend voice and packet data services to locations with poor or non-existent macro coverage. MNOs may also use metro cells to deliver a capacity boost to rural areas. For example, W-CDMA metro cells may be used to provide higher-bit-rate data services to rural areas that currently have 2G service, while LTE metro cells may be used in areas that currently have 3G services.

When should I deploy metro cells?

MNOs looking to increase coverage should deploy W-CDMA metro cells now. They can immediately improve QoE in locations that have traditionally been challenging to cover with a macro-only solution.

MNOs looking to increase capacity need to determine when their macro cell sites will become saturated. To better understand when high-density W-CDMA networks will become saturated, Alcatel-Lucent Bell Labs conducted a 5-year network simulation with a tier 1 MNO in Western Europe. The simulation showed that:

  • Without LTE, 70% of macro sites will be saturated by 2016.
  • If LTE is launched in 2013, only 25% of macro sites will be saturated by 2016.

The simulation also showed that approximately 2% of all W-CDMA macro cell sites are already saturated and that the best short term solution for these areas is a selective deployment of metro cells. As traffic continues to grow more and more macro cell sites will need to be complemented by the deployment of metro cells. This is will particularly critical for MNOs planning to delay the launch of LTE and they will need to consider substantial W-CDMA metro cell networks over the period 2013-14 to offload an increasing number of saturated macro cells.

LTE metro cells, on the other hand, should be incorporated into the LTE network design from the onset - especially for dense urban zones such as sports stadiums and business, shopping, and entertainment districts as these locations are expected to saturate quickly after service launch.

How should I integrate metro cells in the network?

MNOs typically incorporate metro cells into their networks by building a metro cell radio access network on top of the existing macro network. W-CDMA network integration

In W-CDMA networks, MNOs can use either the 3GPP-defined macro architecture (Iub) or the Home NodeB (HNB) architecture (Iuh) to integrate metro cells (Figure 4).

  • With Iub, metro cells connect via the Iub interface to a RNC, which could be a metro cell RNC or the same RNC used by the macro cell, provided by the same vendor just like macro NodeBs.
  • With Iuh, metro cells, as well as other small cells such as home and enterprise cells, connect via the Iuh interface to a small cell gateway (SC GW) especially designed for the support of small cells which may be also used to support residential and enterprise femto cells. The SC GW serves to control and manage the metro cells, as well as to aggregate their signaling into a single 3GPP-compliant Iu interface for easy integration with the core network.

Iuh offers many advantages over Iub, including massive scalability, multi vendor support, ease of deployment, relaxed backhaul requirements, efficient metro-to-metro handovers, IP traffic offload, and subscriber group control (see Table 1). Iub on the other hand, supports soft handover (SHO), an essential feature for metro cells supporting Circuit Switched (CS) voice services in an outdoor environment using shared carrier, that is, when the macro and metro cell layers share at least one common radio carrier. However, metro cells that carry only high-capacity traffic, as recommended by Alcatel-Lucent, would not benefit from SHO since these cells are not burdened with low-capacity but high-latency-sensitive voice. In other words, Alcatel-Lucent strongly recommends that voice and very-high-mobility data traffic be assigned to the macro network using a dedicated macro-only carrier, while low-mobility data traffic be kept on the metro cell network. This “assignment” is made possible by the traffic management features supported in the metro cells, which shift the device connection from the metro cell to the macro network in the event of an incoming voice or high-mobility data call or handover. Furthermore, SHO to support CS services is not required once Voice over LTE (VoLTE) is deployed and voice is also supported over HSPA on both macro and metro layers.

LTE network integration In LTE networks, MNOs can use the 3GPP eNodeB or Home Enhanced NodeB (HeNB) architectures to integrate metro cells (Figure 5).

  • With the eNodeB architecture, metro cells are integrated into the network in the same way as macro cells. That is, they use the S1 interface to communicate with the core network and the X2 interface to communicate with nearby macro and metro cells.
  • With the HeNB architecture, a gateway between the metro cells and the core network concentrates the signaling from multiple metro cells onto a single S1 interface back to the core network. It does not use the X2 interface at all.

MNOs can use one or both architectures in their network. The choice is based on the trade-off between the number of interfaces presented to the core network and the benefits of using direct X2 interfaces between the macro and metro cells. The HeNB architecture works best for high density indoor deployments, while the eNodeB architecture is preferable for outdoor deployments.

Should I use dedicated or shared carrier?

While both dedicated and shared carrier solutions can be built, a selective dedicated carrier is always preferred over shared carrier for metro cell deployments, especially in outdoor environments where macro cell signal levels will be higher (Figure 6):

  • With dedicated carrier, the MNO reserves a carrier for the exclusive use of metro cells in geographies that have been specifically targeted as needing extra capacity. In all other areas this same carrier may be used by macro cells. The use of a dedicated carrier avoids interference with the macro cell, which enables metro cells to cover wider areas and to absorb a larger amount of traffic off the macro network - greatly improving the TCO.
  • With shared carrier deployments, metro cells use one of the same carriers assigned to the macro layer, which would normally have one or more additional dedicated carriers. The coverage range and offload effectiveness of metro cells deployed with shared carrier are lower than those using dedicated carriers. Additionally, metro cells cannot be placed too close to high-power macro cells with this type of deployment, as represented by the red exclusion zone in Figure 6.

Turning answers into strategy

Each MNO’s metro cell deployment strategy should be based on their W-CDMA spectrum holdings and LTE launch timing (Table 2).

Spectrum-rich W-CDMA MNO launching LTE early (2011 – 2013) MNOs with rich W-CDMA spectrum holdings and launching LTE early should deploy W-CDMA metro cells to both indoor locations and traffic hotspots using a dedicated carrier, while also deploying LTE metro cells to traffic hotspots. To help offload the macro network in hotspots, the MNO may also consider deploying Wi-Fi-integrated metro cells. Spectrum-rich W-CDMA MNO launching LTE late (2013 – 2015) Operators with rich W-CDMA spectrum holdings, but launching LTE late, should also deploy W-CDMA metro cells to both indoor locations and traffic hotspots, using shared carrier. To help manage interference and avoid handoffs, traffic should also be segmented, so only HSPA data traffic is offloaded to the metro cells in hotspots. These MNOs will also benefit from deploying Wi-Fi access points that are integrated with the metro cells to help further offload the macro network. Spectrum-poor W-CDMA MNO launching LTE early (2011 – 2013) MNOs with poor W-CDMA spectrum holdings and launching LTE early should deploy W-CDMA metro cells in shared carrier to indoor locations only, while also deploying LTE metro cells to traffic hotspots for extra capacity. If the MNO has a Wi-Fi access network, these sites can easily be used for the deployment of metro cells by simply swapping out the legacy Wi-Fi access points for Wi-Fi-integrated metro cells. 2G-only MNO launching LTE early (2011 – 2013) 2G-only MNOs launching LTE early should deploy LTE metro cells as soon as possible in traffic hotspots to supplement the capacity of the macro network, since these MNOs should expect very high traffic as subscribers discover the benefits of LTE’s speed. If MNOs have already invested in a Wi-Fi access network, these same indoor and outdoor sites can be used to deploy metro cells by replacing legacy Wi-Fi access points with Wi-Fi integrated metro cells. For more information about the benefits of metro cells and in-depth answers to the top 5 questions about metro cell deployments, please read our white paper “Metro Cells – The Bigger Picture”. To contact the authors or request additional information, please send an e-mail to

About Azfar Aslam
Azfar is the Director of Bell Labs Advisory Services, EMEA, and a lead Consultant, covering a wide range of Business, Technology and Regulatory Strategy domains. Azfar has the experience of leading a significant number of engagements with Fixed and Mobile service providers covering topics such as spectrum re-farming strategy, regulatory framework development, mobile data strategy, network Evolution strategy, adjacent and New market entry for Telcos, wholesale business strategy, and emerging business models. More recently, Azfar has been working with MNOs on the commercial drivers for LTE introduction, Timing, Spectrum and various deployment strategies. Azfar appears regularly in industry conferences as a subject matter expert in the area of telecommunications business environment. Azfar holds a M.SC. and a Doctorate in Telecommunications, studied at University College London and received MBA training at London Business school.