Implementing Small Cell Networks

ImplementIng SmAll Cell networkS Strategic White PaPer

the implementation of small cell networks requires many decisions to be

made, some of which can be far reaching and likely long-lasting. Largely, these

decisions revolve around the type of network (3g or 4g) and the nature of the

problem to be solved (for example, improved coverage or increased capacity).

this white paper focuses on those issues related to spectral management and

core network connectivity. additional white papers on small cells address

backhaul and physical considerations, such as mounting, powering, and

operational management.

tAble of ContentS

coverage or capacity / 1

Spectral management / 3

Placement / 4

interference mitigation in Lte networks / 5

integrating small cells with the macro network / 6

3g networks / 6

Lte networks / 9

Summary / 9

Implementing Small Cell NetworksAlcAtel-lucent StrAtegic white pAper

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CoverAge or CApACIty As a coverage solution, metro cells are usually deployed in areas with poor or non-existent macro coverage to deliver superior voice and packet data services. These include both in-building locations, such as event centers, offices, and hotel lobbies, as well as outdoor locations, such as the edge of suburban (coverage limited) cells or small rural towns. Traditionally, coverage holes such as these are very expensive to fill using conventional macro cell sites. The ability to cover them also depends on the band being used, especially for mobile network operators (MNOs) having Long Term Evolution (LTE) in the upper bands. These operators will most likely mount new LTE radios on their legacy Global System for Mobile communications (GSM) and Wideband Code Division Multiple Access (W-CDMA) towers, which are typically planned for the propagation characteristics of 850 MHz, 1900 MHz, or 2100 MHz spectrum.

A propagation analysis of typical mobile networks using a simple “flat earth” model with an assumed minimum requirement of 128 Kb/s upstream suggests that the maximum distance between an LTE base station and its user equipment at 2.6 GHz will be 40 percent of that which can be attained at 700 MHz, and about 70 percent in the AWS band. To be expected, radio signals in the upper bands are also more prone to through building attenuation, leading to poorer indoor penetration and user quality of experience (QoE).

Figure 1. Relative propagation range of common wireless bands

10.0 12.0 14.0 16.08.06.04.02.0

Band 72600 MHz

Band 2PCS (1900 MHz)

Band 31800 MHz

Band 4AWS

Band 17Lower 700 MHz 4.3 km

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DISTANCE IN KM FROM MACRO BTS AT TARGET UPLINK RATE OF 128 KB/S

8.2 km2.6 km

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Rural in carSuburban indoor

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7.6 km2.4 km

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When used as a capacity solution in urban hotspots, metro cells can be expected to be deployed in selected high-traffic locations, such as city centers, train stations, or shopping malls. In these cases, they may be deployed when the local macro layer is reaching spectrum exhaustion, or as a means of attaining higher capacity in an area with weaker coverage.

To better understand the timeline at which urban wireless networks become saturated, Bell Labs conducted a five-year network simulation with a Tier 1 MNO in Western Europe. In their case, 15 MHz of Universal Mobile Telecommunications System (UMTS) spectrum was already held, and they expected to receive 30 MHz of LTE spectrum in both the lower (below 1 GHz) and upper (above 1 GHz) bands. An 18-fold increase in capacity demand between 2012 and 2016 was assumed, which is consistent throughout many corners of the world. The simulation showed that by early 2012, approximately


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2 percent of the provider’s UMTS macro cells would be saturated due to the uneven nature of traffic distribution, but average network utilization would only reach 25 percent. By 2016, however, a macro-only network will no longer meet their capacity demands, and 70 percent of their base station sites will likely become saturated.

Figure 2. Study of network saturation over time for a Tier 1 operator

2016201520142013

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W-CDMA sites saturating without LTE

W-CDMA NETWORK UTILIZATION AND SITE SATURATION WITH AND WITHOUT LTE

W-CDMA average network utilization without LTEW-CDMA sites saturating with LTE upgradeW-CDMA average network utilization with LTE upgrade

Running the same simulation as in Figure 2 using metro cells resulted in a 38 percent cost savings over the macro-only analysis (see Figure 3). Moreover, adding capacity using both LTE and metro cells provided a 45 percent cost savings. The total cost of ownership (TCO) calculations includes the costs associated with having more nodes to manage as well as the need for more backhaul.

Figure 3. TCO savings provided by metro cells

W-CDMAmacro-only

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38% TCO savings 45% TCO savings

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SpeCtrAl mAnAgement The economic return of a metro network is driven by the percentage of traffic it offloads from the macro network, and this percentage is heavily influenced by what frequency band is allocated to the metro layer. Metro cells can be deployed using either a dedicated carrier or a carrier shared by the macro network. In LTE networks, it will likely be the latter, but in 3G networks, it can be either.

With a 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, therefore, 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 a shared carrier are lower than those using dedicated carriers. Also, with this type of deployment, metro cells cannot be placed too close to high-power macro cells because of the negative impact to signal to interference plus noise ratio (SINR) and a much lower efficiency of the metro cell itself due to a reduction in effective range — this is represented by the red exclusion zone in Figure 4.

Figure 4. Coverage area of metro cells deployed with dedicated and shared carrier

Low efficiencyzone

Moderate efficiencyzone

High efficiency zone

Exclusion zone

Coverage area of metro cellsdeployed with shared carrier

Coverage area of metro cellsdeployed with dedicated carrier

To quantify the economic differences between dedicated and shared carrier deployments, Bell Labs ran stringent simulations based on 3GPP models. These simulations demonstrated that, with typical shared carrier implementations, approximately 30 percent to 35 percent of the traffic can be offloaded from the macro layer, whereas with a dedicated carrier, 70 percent of the traffic can be offloaded. The high economic return of using a dedicated carrier for metro cell deployments suggests that, when it comes to metro cells, MNOs may want to rethink how they allocate bandwidth in urban areas with metro cells now on the horizon.


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The advantages of a dedicated carrier notwithstanding, a shared carrier must be considered because of its effectiveness in many situations, especially those in which spectrum is fully assigned to the macro layer. In instances where there is poor coverage from the macro, a metro cell may be deployed with minimal interference limitations. Furthermore, for operators that have low spectrum holdings, a shared carrier is the only option available for the deployment of metro cells.

There are MNOs that have sufficient 3G spectrum to dedicate a carrier to the metro cells, although they have already used all their 3G carriers on the macro network in urban centers. In this situation, they may want to initially deploy metro cells using a shared carrier, and then reduce the macro power in that carrier over time. The metro cells should be placed at the macro cell edge and other poor coverage areas to offload the macro cells, allowing the macro power level and range to be reduced. This, in turn, extends the reach of the metro cells and reduces the size of the exclusion zone, leading to an iterative encroachment of metro cells toward the macro cell center. Eventually, the macro carrier can be switched off, so that the metro cells can provide ubiquitous but higher capacity coverage across the entire macro cell geography.

Either a dedicated or shared carrier can be used, with a dedicated carrier being preferred for 3G, especially for high-density deployments. For shared carrier deployment in 3G networks, it is preferred that metro cells be used only for static or pedestrian high-speed packet access (HSPA) traffic, keeping all high-velocity traffic on the macro network. It should also be considered that metro cells in outdoor urban/suburban environments generally operate at reduced coverage ranges and require more exact placement to mitigate interference.

plACementEffective use of metro cells within an LTE shared carrier environment depends on optimal placement while considering the exclusion zones mentioned above. With optimal placement among concentrated traffic hotspots, capacity gains upwards of 400 percent are achievable using only four metro cells per macro cell. Placing metro cells too close into the macro cell center causes them to compete with the macro and dramatically reduces their effectiveness. Good small cell planning necessitates that hotspots be well understood because these are the best locations to place metro cells and achieve the greatest return on investment (ROI).

Table 1 considers several instances of metro cell placement: One case in which four metro cells are placed within 200 meters of the macro cell base station; one in which they are placed 400 meters away; and one in which they are placed 700 meters away. Important to consider is that metro cells are assumed to be placed in the hotspots of the network in all cases.

table 1. Impact of placement on effectiveness of metro cells (use of four metro cells considered)

metro cell distance from macro cell base station percent gain in median user throughput

200 m 2%

400 m 70%

700 m 467%

Engineered to satisfy hotspot traffic, metro cells provide cell splitting-like gains. Moreover, metro cell placement planning is critical to realize gains of LTE heterogeneous networks with Release 8 and Release 9 user equipment that can take advantage of these.


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InterferenCe mItIgAtIon In lte networkSFor spectral efficiency and engineering simplicity, it is common in LTE for cells and sectors to reuse the same set of frequency resources. This is termed an N = 1 reuse pattern. This helps increase capacity and system throughput at the expense of some complexity in equipment design.

To enable reuse 1 while best utilizing spectrum, LTE technology varies the amount of information transmitted across a number of predefined time slots throughout the assigned spectral band. This spreads the transmission of information across both time and space. This process is managed by the scheduler.

The design of an LTE scheduler can be kept simple by utilizing well-established “weighted round-robin” techniques similar to those found in IP routing networks — picking time slots and physical resource blocks based on a measure of link quality (that is, highest signal to noise ratio [SNR]) and capacity (that is, air link capacity). This technique works well in low-loss, error-free data networks, and it explains why the performance of many systems will score well early on, when networks are lightly loaded. But, this technique will not work well in high-loss, high-error rate wireless networks.

When it is designed correctly, the LTE scheduler should enable RF power to be controlled within the band to mitigate interference and optimize cell edge performance for both lightly loaded and heavily loaded networks. Doing so requires that information (that is, data) be allocated dynamically across both time and space using the physical resource blocks that comprise an LTE orthogonal frequency division multiplexing (OFDM) signal. Two such methods for doing this are as follows:

• FrequencySelectiveScheduling(FSS)providestheabilitytoschedulearoundinterference in surrounding cells based on user equipment (UE) reports of channel quality. The largest benefits are derived in heavily loaded systems. FSS selectively chooses the best locations in frequency in which to schedule a use of a particular time slot to provide maximum performance. For low mobility traffic, which will represent the vast majority of wireless data traffic in a small cells network, the biggest gains in terms of spectral efficiency and cell edge throughput are realized when utilizing FSS. Bell Labs analysis projects and tests have confirmed a 30 percent improvement in spectral efficiency for those users in a low latency mode.

Figure 5. Selective scheduling of LTE bandwidth

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• FrequencyDiverseScheduler(FDS):FDSschedulesusersbasedonpredefinedgroupsof tones that are spread across the full LTE carrier bandwidth. Users with lower velocity benefit from FSS, while users moving at higher velocities (where feedback is usually stale by the time a user is scheduled) can achieve better performance with FDS. This is due to the random nature of interference.

With FSS and FDS, both of which are outlined in 3GPP, Release 5, operators have the tools they need to implement heterogeneous networks (HetNets) successfully. Should metro cells be used in high-velocity networking, enhanced inter-cell interference coordination (eICIC) features in Release 10 can offer some additional gains. However, all of these scheduling functions comprise only part of an end-to-end quality of service (QoS) architecture. As such, operators are advised to treat QoS as an end-to-end topic when trying construct networks in which a mixture of traffic types will be present (for example, voice, video streaming, and best effort).

IntegrAtIng SmAll CellS wIth the mACro networkMNOs will generally implement a HetNet overlay solution in which a metro cell radio access network (RAN) is built on top of the existing macro network (see Figure 6). The metro cells may be controlled and managed by dedicated centralized controllers that link the metro cells back to the core network. Signaling interfaces can be used as an option to link the metro and macro networks, either at the access point or at the controller to help guide the UE between radio networks.

Figure 6. Typical heterogeneous network

Macro cell

Metro cell

BSC/RNC

Controller

Mobile corenetwork

BSC = Base Station ControllerRNC = Radio Network Controller

3g networksOperators can integrate metro cells into their 3G networks by using either the 3GPP-defined macro (Iub) or the Home NodeB (HNB) (Iuh) architecture, as shown in Figure 7.

With Iub, metro cells connect through the Iub interface to a radio network controller (RNC), which could be a metro cell RNC or the same RNC used by the macro cell, provided by the same vendor, just like NodeBs. In this architecture, the metro cell RNC usually supports an Iur interface to the nearby macro cell RNC, enabling soft handovers (SHOs) between the metro and macro cells. Because SHO enables circuit-switched (CS) services, such as voice calls, to be maintained during metro-to-macro cell handovers, SHO is considered to be the main advantage of an Iub architecture.


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Figure 7. 3G Iub and Iuh metro cell network

Macro cell

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With Iuh, metro cells, as well as other small cells such as home and enterprise cells, connect to a purposely-designed small cell gateway (SC GW). 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. The Iuh-based architecture offers MNOs the following advantages:

• Massivescalability.TheIuharchitectureisdesignedtoscalemuchmorecosteffectivelythan a traditional Iub architecture by moving some of the RNC functions down to the access points, thereby flattening the network. This greatly reduces the complexity of the SC GW, compared to the macro RNC function, so the Iuh architecture using SC GWs can scale with lower cost and higher reliability and availability than by trying to scale RNCs. Also, because the SC GW is designed to support mobility across metro cells, it allows large clusters of metro cells to be deployed with minimal impact on the core network. This architecture not only enables MNOs to easily deploy more metro cells as needed to meet rapidly growing capacity demands, but also shields macro RNCs from the potential signaling load caused by the use of chatty applications and the higher incidence of mobility events that are likely to occur in a metro cell network.

• Multivendorsupport.AlthoughtheIubisa3GPP-definedinterface,itisuncommon for NodeBs from one vendor to be used with RNCs from another vendor. This is due to a lack of intervendor interop testing. The use of the SC GW enables MNOs to use any vendor’s metro cells, not only those from the incumbent macro vendor.

• Easeofdeployment.ByusingtheSCGW,theMNOisabletogreatlysimplifythedeployment and management of a large volume of metro cells by leveraging the TR-069 management capabilities and self- organizing/self-optimizing network (SON) features field proven with the large deployment of small cells. TR-069 capabilities automate the provisioning, configuration, and software upgrades of metro cells, while SON enables metro cells to automatically configure themselves to the surrounding network and to periodically monitor, update, and optimize their neighbor-relation lists and handover parameters.

• Relaxedbackhaulrequirements.Originallydesignedtosupportinexpensivesmallcellsusing low-cost backhaul, the SC GW flattened the network by distributing some RNC type functions to the access nodes, making the SC GW significantly less sensitive to latency and jitter on the backhaul than RNCs. Because of this, the SC GW can tolerate lower TCO backhaul, such as digital subscriber line (xDSL) and Wi-Fi®, than the Iub-based architecture that uses RNCs. Access to a wider range of backhaul options also increases the number of potential sites for metro cell deployments.

• Efficientmetro-to-metrohandovers.TheIuharchitectureisabletosupportefficienthandovers from one metro cell to another. There is no impact on the core network and the execution time is faster with 50 percent fewer messages used.


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• IPtrafficoffload.TheIuharchitecturesupportsIPtrafficoffloadusingthewell-defined3GPP Selective IP Traffic Offload (SIPTO) and/or Local IP Access (LIPA) standards. This enables metro cells to offload selective traffic directly to the Internet, bypassing the packet-switched core network altogether to reduce the TCO.

• Subscribergroupcontrol.WiththeIuharchitecture,operatorscancontrolwhichusersmay access the metro cells by configuring them as open, semi-open or closed. This choice gives operators the flexibility to use metro cells to address both public and private markets and even users with specific QoS requirements, opening up potentially creative commercial opportunities.

table 2. key differences between Iub and Iuh architectures

Iub ArChIteCtUre Iuh ArChIteCtUre

Macro and metro cell multi-vendor support a

ease of deployment a

relaxed backhaul requirements a

Massive scalability a

ShO support a

efficient metro and macro handovers a

iP traffic offload a

Subscriber group control a

Table 2 shows that there is only one advantage of the Iub architecture, the support for soft handover, an essential feature for metro cells supporting circuit-switched voice services in an outdoor environment using shared carrier. However, this is a moot point for metro cells used outdoors in a shared carrier to terminate only high-capacity traffic, as these cells will not be burdened with low-capacity, latency-sensitive, high-velocity prone voice. In other words, voice, as well as very high-mobility data traffic should be left on the macro network, using a dedicated macro-only carrier, while low-mobility data traffic is 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 circuit-switched services is not required after voice over LTE (VoLTE) is deployed. Voice is also supported over HSPA on both macro and metro layers.

There are many other advantages of using the Iuh architecture with metro cells deployed to focus on data and capacity:

• Asdescribedearlier,keepingCSvoiceoffoutdoormetrocellsreducesthestringentlatency and jitter requirements needed for voice, leading to a greatly reduced backhaul and metro cell TCO.

• SupportofSHOsplacesanexcessiveprocessingloadonthemacronetworkbyrequiring the provisioning of RNC and NodeB resources to support the metro-to-macro cell handovers.

• WiththeIubarchitecture,metro-to-metrohandoversareslowandgreatlyimpactthecore network due to the large number of messages required; typically, 50 percent or more messages are required than with the Iuh architecture.

• AddingalargenumberofmetrocellswiththeIubarchitectureexhaustsRNCportsand processing power, making metro cell scalability difficult.


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lte networksMetro cells may be integrated into an LTE network using the eNodeB or the Home Enhanced NodeB (HeNB) architectures, both based on 3GPP standards, as shown in Figure 8. With the eNodeB architecture, metro cells are integrated into the network in the same way as macro cells. That is, metro cells use the S1 interface to communicate with the core network and the X2 to communicate with nearby macro cells. The HeNB architecture, on the other hand, uses a SC GW between the metro cells and the core network to concentrate the signaling from multiple metro cells into a single S1 interface back to the core network and does not use the X2 interface at all.

Figure 8. LTE metro cell network

Macro cell

Metro cell

SC GW

S1

S1

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Mobile corenetwork

As LTE metro cells work with both eNodeB and HeNB architectures, MNOs may choose to use one or both architectures in their network. The choice between architectures 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. Alcatel-Lucent recommends that the HeNB architecture be used for high-density indoor deployments and the eNodeB architecture be used for outdoor deployments.

SUmmAryAdding capacity or coverage to a network using small cells requires consideration be paid to spectrum allocation (dedicated or shared), subscriber use (data, voice, or both), and other factors that will define the minimum level of expected network performance (Iub versus Iuh; gateway or without gateway). Some of these will depend on the wireless technology generation being used (3G or 4G). In any event, solid evidence exists to suggest that small cells offer significant savings, making the effort worthwhile.

www.alcatel-lucent.com alcatel, Lucent, alcatel-Lucent and the alcatel-Lucent logo are trademarks of alcatel-Lucent. all other trademarks are the property of their respective owners. the information presented is subject to change without notice. alcatel-Lucent assumes no responsibility for inaccuracies contained herein. copyright © 2013 alcatel-Lucent. all rights reserved. NP2013030453eN (March)

Implementing Small Cell Networks

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