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Backhaul for Urban Small CellsA Topic Brief

June 2015

095.05.1.03

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Report title: Backhaul for urban small cells – a topic briefIssue date: June 2015Version: 095.05.1.3

ABOUT THE MEF

The MEF is a global industry alliance comprising more than 220 organizations including telecommunications service providers, cable MSOs, network equipment/software manufacturers, semiconductor vendors and testing organizations. The MEF’s mission is to accelerate the worldwide adoption of carrier-class Ethernet networks and services for business and mobile backhaul applications. The MEF is the defi ning industry body for Carrier Ethernet, developing technical specifi cations and implementation agreements, and educational work to promote interoperability, certifi cation and deployment of Carrier Ethernet worldwide. For more information about the Forum, including a complete listing of all current MEF members, please visit www.MetroEthernetForum.org

THE MEF, CARRIER ETHERNET AND MOBILE BACKHAUL

Ethernet adoption has been accepted by the vast majority. The MEF’s Carrier Ethernet 2.0 for Mobile Backhaul brings answers to the challenges associated with managing rapid backhaul data growth while scaling costs to new revenues. MEF Mobile Backhaul Phase 2 Specifi cation (MEF 22.1 with MEF 22.1.1 amendment) covering use of Carrier Ethernet services, synchronization 4G/LTE Deployment and Small Cell Introduction. The MEF also publishes business, technical and best practices papers and provides presentations on optimizing MBH with multiple classes of service, packet synchronization, resiliency, performance objectives, microwave technologies and converged wireless/wireline backhaul.

Report title: Backhaul for urban small cells Issue date: 09 June 2015 Version: 095.05.1.03

Scope

This document details backhaul for small cells deployed in an urban scenario: ‘Urban’ refers to small cells that offer capacity for dense environments, which may be outdoor (e.g., city centres and parks), indoor (e.g., transport hubs), but are clearly urban. The primary driver is capacity, but indoor and outdoor coverage are both important. There will usually be interaction with the macro layer. Urban small cell models are designed for high traffic areas, these are engineered into robust enclosures suitable for deployment in unsupervised areas. Although capable of high traffic capacity and tens to hundreds of concurrent users, these may not require significantly higher RF power because they target a relatively short range.


Executive summary

This paper helps operators, backhaul and service providers to understand the particular needs of urban small cells from a backhaul perspective. It summarises key aspects that must be considered when designing and deploying the transport network, and points to sources of further information.

• In our operator survey in [1], backhaul was perceived as one of the main barriers to urban small cell deployment. We provide here both technical and financial analysis which shows that feasible and cost effective solutions are available to address their concerns.

• Our network architectures paper [2] shows that small numbers of small cells can be connected into the existing macro transport network with minimal complexity in initial deployments. For longer term scalability, dedicated small cell core network nodes may be deployed, enabling the use of alternative backhauling solutions by utilizing functions specific to the small cells including transport security termination, traffic aggregation, synchronization, element management, SON, QoS support etc.

• Backhaul performance requirements vary with the operator’s motivation to deploy, and relaxations on hard requirements are possible in different deployment scenarios:

• For capacity driven deployments, backhaul should be provisioned to match the capabilities of the small cells, for which data is provided for dedicated carrier small cells. Where small cells share carriers with the macro in a het-net, we expect reduced backhaul capacity requirements compared to the dedicated carrier case.

• For coverage driven deployments, backhaul can be provisioned according to end use demand rather than the limitations of the small cell RAN.

• The MEF specify in their MEF22.1 implementation agreement the transport performance required to support mobile backhaul.

• We note that adding small cells which offload users from the macro network can increase the efficiency of the macro and thus its backhaul capacity requirement.

• Hetnet interference co-ordination may require small and macro cells to be tightly synchronized. Achieving this with packet sync techniques such as 1588v2 drives delay performance characteristics for the last mile backhaul more so than the control plane co-ordination signaling over X2, which is more delay tolerant. [3] contains a detailed analysis of small cell synchronization, and [4] has details of X2 interface signaling in HetNets.

• Last mile backhaul to the small cell site is particularly challenging, and a range of wireless and wireline solutions are available to address this. Our backhaul solutions paper [5] details individual solution types, and finds that together they address the varying requirements across the use cases envisaged.

• The choice of backhaul technology largely determines the transport topology – the way individual links are combined to provide the end-end connectivity. We define two general topologies:

• Street launched: Wireline solutions typically provide connectivity to street-level cabinets. This may be trenched to reach the nearest small cells site, and then a wireless solution used to extend connectivity to nearby sites.


• Macro launched: operator’s existing macro rooftop backhaul is extended down to the street level small cell sites using wireless links.

• Operators need to develop guidelines to simplify the wide range of backhaul options down to a simple toolkit with accompanying rules to facilitate planning of specific deployments. Guidelines must take into account operators available assets and holdings, as well as performance and economic targets.

• A business case analysis for urban small cells in [1], including a detailed analysis backhaul Total Cost of Ownership, confirms that an evolving mix of technologies, and thus topologies, will be needed to keep backhaul costs to a minimum. The analysis considers the tradeoff between positioning at the ‘benefit maximizing location’ of the small cell in the center of the hotspot, versus the additional cost of extending connectivity out to that point. In the case study, value was optimized where small cells were typically offset 5m from the hotspot center (governed by street cabinet locations), although in general the business case did not appear sensitive to the offset.

• The business case analysis found positive value for operators to deploy and operate small cells compared to an alternative method of accommodating the increasing traffic using only a macro only approach. A range of commercial models could be applied to this value chain, where business entities such as site shares, neutral hosts or leased capacity may be able to bring economies of scale by providing sites, backhaul or other aspects to multiple operators.

Overall we conclude that there are a range of backhaul solutions which together meet both technical requirements of the use cases envisages, and have low enough TCO to provide a positive business case for urban small cells overall. Backhaul is not a barrier to small cell deployment.


Contents

1. Introduction .....................................................................1 2. Network architectures for urban small cells .....................3 3. Developing backhaul requirements for urban small cells ..6 3.1 Understanding the deployment scenario ................................. 6 3.2 Impact of deployment motivation on backhaul requirements ..... 7 3.3 Small cell backhaul capacity provisioning ................................ 8 3.4 Transport security ............................................................. 10 3.5 Backhaul requirements to support co-ordinated HetNets ........ 11 4. Backhaul topologies ....................................................... 12 4.1 Last mile topologies ........................................................... 12 4.2 Redundant backhaul .......................................................... 13 5. Backhaul technologies .................................................... 14 6. Backhaul TCO and commercial models ........................... 16 6.1 Backhaul TCO and business case ......................................... 16 6.2 Commercial models ........................................................... 19 7. Backhaul selection, planning and deployment ................ 21 7.1 Overview .......................................................................... 21 7.2 Developing guidelines ........................................................ 22 7.3 Planning ........................................................................... 24 7.3.1 Planning non line of sight backhaul ...................................... 26 7.3.2 Line of sight planning ......................................................... 27 7.3.3 Wireline backhaul planning ................................................. 29 8. MEF implementation agreements for backhaul services . 32 8.1 MEF references ................................................................. 33 9. Synchronisation for urban small cells ............................. 34 References ................................................................................ 37

Tables Table 3-1 Small cell backhaul requirements and variations across use cases ......... 8 Table 5-1 Last mile small cell backhaul technologies ......................................... 14 Table 9-1 Radio technology synchronization requirements ................................. 35 Figures Figure 1-1 Related documents within the urban theme ........................................ 2 Figure 2-1 Reference network architecture for urban small cells ........................... 3 Figure 2-2 Transport segments in the urban small cell network ............................ 4 Figure 2-3 Components and functions in the end to end transport network ............ 4


Figure 3-1 City centre and transport hub deployment scenarios ........................... 7 Figure 3-2 Backhaul Traffic generated by various small cell configurations in a

dedicated carrier ............................................................................. 9 Figure 3-3 Increases to median user throughputs when adding small cells are

indicative of increases to backhaul capacity. ...................................... 10 Figure 4-1 ‘Macro launched’ small cell backhaul scenarios .................................. 12 Figure 4-2 ‘Street launched’ small cell backhaul scenarios .................................. 13 Figure 4-3 ‘Macro-launched’ wireless with redundant topology ............................ 13 Figure 6-1 Anatomy of urban small cells and their backhaul ................................ 16 Figure 6-2 Four types of backhaul nodes used in the TCO analysis....................... 17 Figure 6-3 Changing proportion of macro down and street up throughout the

study Period .................................................................................. 18 Figure 6-4 Numbers of different types of backhaul link throughout the capacity

driven case study ........................................................................... 18 Figure 6-5 Commercial models for urban small cells .......................................... 19 Figure 7-1 Process for planning and deploying small cells and their backhaul ........ 21 Figure 7-2 Inputs and outputs of the small cell backhaul planning process ............ 24 Figure 7-3 Assignment of small cells to backhaul solution ‘layers’ during planning . 25 Figure 7-4 Area planning – coverage prediction example (RSSI) ......................... 27 Figure 7-5 Area planning – downlink and uplink carrier to interference ratio

(CINR) ......................................................................................... 27 Figure 7-6 Sample graphical topographic data representation (Infovista) ............. 28 Figure 7-7 Example LoS planning process in automatic planning tool (Infovista) ... 29 Figure 7-8 Broadband services for small cell transport ....................................... 30 Figure 7-9 Ethernet services for small cell transport .......................................... 31 Figure 8-1 Carrier Ethernet Mobile Backhaul over Different Access Technologies

(MEF) ........................................................................................... 32 Figure 8-2 MEF mobile backhaul terminology .................................................... 33

Report title: Backhaul for urban small cells Issue date: 09 June 2015 Version: 095.05.1.03 1

1. Introduction

‘Urban’ small cells – also referred to as Outdoor picocells, microcells and metrocells – are seen as a key tool for operators to increase capacity and coverage depth in areas of high traffic demand density. We have seen that residential and enterprise small cells can be self-deployed to provide indoor coverage where consumers or companies need it. Urban small cells on the other hand are operator deployed and managed to enhance mobile broadband connectivity and Quality of Experience in public spaces, both outdoor and indoors.

Our market drivers paper in [6] provides a detailed look at operators’ motivations as well as their perceived barriers to rollout of urban small cells. Of these, providing cost effective backhaul is considered one of the key challenges, and is the subject of this topic brief.

The small cell backhaul challenge can be summarized as providing carrier grade connectivity to hard-to-reach small cell sites down amongst the urban clutter, all at a fraction of the cost of macro cell backhaul.

This document is an urban focused topic brief, which accompanies our detailed paper on backhaul use cases, requirements and solutions [5]. In this paper we discuss the following considerations:

• Reference network architectures: Defines the interfaces between network nodes which backhaul must support.

• Backhaul requirements: Describes performance and features required from the transport connections to meet the needs of network interfaces

• Backhaul technologies: Summarises the different types of last mile backhaul solution that can be used.

• Backhaul topologies: How different link technologies can be combined together and the impact on end-end performance

• Backhaul TCO and commercial models: Describes the different commercial entities which may be involved in the provision of backhaul, and which parts of the value chain they occupy. Summarises the backhaul TCO elements of the business case analysis in [1].

• Backhaul selection, planning and deployment: Considers the operators process of planning and deployment, including development of planning guidelines in the form of a toolbox of solutions and rules governing their selection.

• Backhaul services: outlines the work of the Metro Ethernet Forum in specifying Carrier Ethernet profiles describing connectivity performance requirements for backhaul.

• Synchronization for urban small cells. Summarises requirements and architectures to distribute synchronization to urban small cells, based on our detailed paper [3].


Figure 1-1 Related documents within the urban theme

Visit www.scf.io to download.

http://www.scf.io/


2. Network architectures for urban small cells

A reference architecture typical of Urban Small Cell deployments is shown below in Figure 2-1. This covers all options associated with Small Cell RAN (SC-RAN), transport and Small Cell Core Networks (SC-CN). Optional elements / functions are shown with dotted outlines.

Figure 2-1 Reference network architecture for urban small cells

Notes: 1. SC-AP may be 3G or LTE or both. The SC-APs may correspond to differing

base station classes, including Home, local area and medium range. 2. The optional SC-Gateway may be the 3G-SC-GW (i.e., 3GPP HNB-GW) or the

LTE-SC-GW (i.e., 3GPP HeNB-GW) or a combination. Furthermore, an LTE-SC-GW may be realized only for the control plane or for both data and control planes.

3. SON-C and SON-D refer to centralized and distributed SON functions respectively.

4. Signaling between the small cell and SC-management System, SC-timing server and SC-SON elements may optionally bypass the SeGW but be secured via transport level security schemes.

5. Interface between SC-APs: Iurh for 3G and X2 for LTE; 6. Interface between SC-AP and macro eNB: Only for LTE-SC-AP (and optional)

using X2, which for the X2 control plane may be either direct or via an optional X2 gateway

7. If backhaul is trusted, use of IPsec and associated SeGWs is optional. 8. Boxes indicating logical function nodes and not necessarily physical nodes. In

practice, a physical node may realize one or more Logical Functional Nodes. 9. Dotted boxes & lines are optional

The transport network includes backhaul functions over either “trusted” or “untrusted” backhaul. Trusted backhaul networks are generally managed by the mobile carrier directly, and employ suitable security functionality including encryption and device authentication. With such backhaul networks, the need for an additional small cell core network can be avoided and the small cells interfaced directly with an existing macro core network. This is the more common case when deploying outdoor open small cells in urban environments.


The Urban backhaul network differs significantly from the residential and enterprise network architectures, comprising of a last mile backhaul network and a middle mile backhaul network, as shown below in Figure 2-2. The middle mile network towards the operator’s core network will typically leverage the same infrastructure as is used to support the RAN transport for the macro-cellular network. The new challenge for the deployment of urban small cells is then largely related to the last mile backhaul network between the street-level small cells and a local aggregation point-of-presence (PoP) [5].

Figure 2-2 Transport segments in the urban small cell network

Figure 2-3 Components and functions in the end to end transport network

QoS support is an important function to be supported over the end to end transport network. In capacity driven small cell deployments the last mile backhaul is often shared and contended, driving the need for sophisticated QoS management in both backhaul and RAN. User plane, control plane and management plane traffic must be handled accordingly, and S1 & X2 traffic needs to be routed with appropriate latency. . X2 traffic is ideally routed locally across an aggregation POP which puts particular bridging requirements on the last mile backhaul. The interface to the backhaul networks should ideally be in accordance with industry defined demarcation points such as the MEF carrier Ethernet UNI definitions. DSCP markings over S1 and X2 can be mapped to CE UNI profiles to provide consistent QoS handling across RAN, Transport and Core Networks. More integrated QoS schemes are outlined in our Urban network architectures paper [2] with small backhaul solutions emerging with closer integration with the LTE system for improved end to end performance with various points of contention in both RAN and transport networks.


Further details of small cell backhaul architecture requirements can be found in the [2] which covers topics such as VLAN requirements, QoS and SON.


3. Developing backhaul requirements for urban small cells

In any design process, it is always good practice to first consider the requirements before considering the potential solutions. Ideally, we could set requirements and then chose a solution to match. However, we find that for small cell backhaul there is no one-size-fits-all solution, and so we must return to the requirements and consider different deployment scenarios and where certain of the requirements can be relaxed. In general we find there is a tradeoff between capacity, coverage and cost to be made.

3.1 Understanding the deployment scenario

Here we consider the different types of urban small cell deployment and factors which impact the backhaul:

• Operator’s principal driver to deploy urban small cells:

• Hotspot capacity • Wide area capacity for enhanced user QoE • Indoor coverage depth

• Access technology: 3G, LTE, WiFi + combinations thereof • Access spectrum: only total MHz of bandwidth impacts backhaul capacity

requirements, there is little sensitivity to the RAN carrier frequency itself, when considering the frequency ranges currently utilised for mobile RANs.

• Het net co-ordination: dedicated or shared carrier for small cells. eICIC, carrier aggregation etc.

Further detail on motivations can be found in our papers on market drivers for urban small cells [6] and the associated business case analysis [1]. In these analyses we identify three key deployment scenarios that result in differing approaches and thus backhaul requirements:

Urban Small Cell Scenarios analysed in the business case study [1]

1. Capacity-dominated small cell deployment in urban city centre 2. Coverage-dominated small cell deployment in urban city centre 3. Transport Hub: train station, airport etc.

http://www.scf.io/document/087



Figure 3-1 City centre and transport hub deployment scenarios

3.2 Impact of deployment motivation on backhaul requirements

Backhaul technologies for small cells [5] provides a detailed set of performance and functional requirements relating to the generic use cases – or motivations – for small cell deployment, which are principally coverage or capacity. These are summarized in Table 3-1 below. From a backhaul perspective, we see relaxations can be made depending on the use case: For a capacity driven small cell deployment backhaul capacity should not limiting the throughput of the small cell.. In the capacity case, existing macro coverage is assumed, and so there is potential for relaxed availability for the small cells. Overlapping macro - small cell coverage may require co-ordination for handover and potentially on resource usage, which drives tighter synchronization and in turn tighter backhaul delay performance.




Use Cases

Requirement

Capacity Coverage

Hotspot Peppered Outdoor Indoor

Backhaul coverage To hotspot Flexible To not-spot To building

Capacity provisioning

Backhaul should not limit throughput of small cell

Backhaul capacity can be relaxed

Delay/jitter Potential relaxation if service-specific offload Same as macro

Synchronization Frequency and phase sync

needed for shared carrier co-ordination

Phase sync not necessary for isolated cells

Availability Can be relaxed (99-99.9%) Same as macro (99.9-

99.99%)

Security IPsec support for all use cases

QoS / CoS support 2-4 CoS levels for all use cases

Physical design Predominantly outdoor urban designs Rural designs Indoor

design

Management Self-organising and scalable management for all use cases

Table 3-1 Small cell backhaul requirements and variations across use cases

Source: [5]

Further detail and quantification on each of the above transport requirements can be found in our backhaul paper [5].

3.3 Small cell backhaul capacity provisioning

Backhaul capacity is a key design decision and, where outsourced, an important part of the SLA. Different approaches are needed for coverage driven versus capacity driven deployments:

In a coverage limited scenario typical of enterprise small cells, the small cells are not running at full capacity, and so provisioning can be determined by end user traffic demands. Considerations in dimensioning for small cells based on consumer demand are given in our enterprise backhaul document [7].

In a capacity limited case, more typical of the main driver for the urban case, we assume demand exceeds the capability of the small cell, and backhaul capacity can be determined by the limitations of the small cells themselves. Figure 3-2 provides figures for the uplink and downlink traffic that given configurations of Rel 8 LTE or






HSPA small cells can generate, based an analysis by the NGMN [8]. This includes various overheads for the transport protocol and X2 traffic (in the case of LTE). Details can be found in [8]. Figures are given for both peak rates (single user in ideal channel conditions) and busy time loaded conditions where there are many users of varying channel conditions sharing the resource.

Figure 3-2 Backhaul Traffic generated by various small cell configurations in a dedicated carrier

Source: NGMN [8]

Dedicated vs shared carrier From our business case analysis we see that urban small cells are likely to be deployed where an operator has limited spectrum with which to increase macro capacity, hence sharing of the RAN spectrum between small cells and macro is likely. In the shared carrier case, resource co-ordination such as eICIC or CS CoMP is used to balance loading on macro and small cell layers. Whilst the overall effect of co-ordination to increase the total combined capacity of macro & small cells, evaluating the capacity needed for each layer separately requires consideration of several mechanisms:

Considering the capacity needed by small cells in a shared carrier, we might view the NGMN’s dedicated carrier figures as an upper bound as the small cell can use the entire carrier to serve user plane traffic. Restrictions on resource usage needed for co-ordination will reduce user plane capacity and therefore backhaul traffic compared to the dedicated carrier where no co-ordination is needed. The degree of reduction depends on the level of co-ordination needed, which in turn depends on how the small cell capacity is distributed relative to the demand. Clearly defined demand hotspots served by small cells at the ‘benefit maximising location’ need less co-ordination and thus can deliver capacity closer to the dedicated carrier case.

The presence of small cells may also impact the backhaul capacity needed by the ‘parent’ macro, although mechanisms are at play which have opposing effects: As discussed above het net co-ordination techniques reduce resources and thus layer throughputs compared to the dedicated carrier case. On the other hand, field experience of one forum operator member found macro capacity actually increased where small cells were deployed to offload the macro cell edge. Small cells located around the macro cell edges captured previously inefficient users. Offloading these brought up the average spectral efficiency of the rest of the macrocell and allowed it to deliver more traffic.


Figure 3-3 Increases to median user throughputs when adding small cells are indicative of increases to backhaul capacity.

Provisioning for peak rates and impact of under-provisioning Figure 3-2 above shows that a 20MHz LTE small cell can generate peaks of nearly 190Mbps of backhaul traffic. Furthermore we see reports in the press of LTE-Advanced configurations developing peaks up to 300Mbps [9] when aggregating multiple 20MHz carriers. Although it is desirable to maximise the capacity carrying capability of all sites, small cell RAN configurations will be constrained by power and size which may preclude the very high capacity configurations with high order MIMO and multiple aggregated carriers. Restrictions on resource usage imposed by co-ordination will further reduce the peak rates of small cells, as discussed above.

Whether to provision for the high peak rates is an operator decision, and may be driven by marketing objectives as much as by capacity dimensioning. In the absence of a better metric for comparison, consumers might use advertised ‘up to xx Mbps’ figures as guidance for selecting the best performing network. From a dimensioning perspective, an analysis presented in [5] shows that backhaul provisioning below the peak rate has very little impact to the user throughput distributions under medium and heavy loading conditions, reducing only the rates experienced by the best case users. Provisioning backhaul below the ‘busy time loaded’ figure impacts network capacity and is not recommended for the capacity driven deployment scenario.

3.4 Transport security

The traffic carried over small cell transport should be protected against unauthorized intrusion and tampering. Some types of small cells including 3G and Home-(e)NodeB classes have mandatory encryption on their backhaul interfaces, and so are protected from these by default.

For eNodeB classes of small cell, 3GPP state that transport encryption is only required for transport segments not considered ‘trusted’ by the operator. Transport which is inherently secure can therefore avoid the bandwidth overhead of IPsec [10]. 3GPP do not themselves provide a formal definition for ‘trusted’, but it should be tamperproof and protected against unauthorised intrusion. The NGMN provide guidelines for trusted/untrusted in their whitepaper on backhaul security [11].

Trusted backhaul is generally considered to be that which is owned and managed by the operator. Urban small cell networks may comprise segments outsourced to service providers, such as fibre or cable asset owners, which might not be considered trusted from an operator perspective.

IPsec encryption terminates in the small cell at one end and in a security gateway at the other– which may reside in the small cell core network shown in the reference




network architecture in Figure 2-1. It should be noted that, when the backhaul is being used to distribute synchronization and/or time, security measures to protect these packet flows should be in place.

3.5 Backhaul requirements to support co-ordinated HetNets

Time domain co-ordination between small cells and macro cells is needed for TDD networks and for het-net co-ordination. These require phase synchronisation between co-ordinated cells, as well as signalling between cells over the backhaul to setup and adjust the co-ordination parameters. Of these, it is the phase synchronisation which may drive stringent backhaul delay performance requirements, where packet synchronisation techniques are used (as opposed to GPS). Further detail on synchronisation is given in section 9.

In the case of LTE-A eICIC, X2 signalling can be used to co-ordinate which almost blank subframe (ABS) patterns will be used. Whilst the standard enables these patterns to be changed every 40 ms, it is typically expected that these patterns will not be changing more frequently than every minute or so [REF: SCF 059], so there is no requirement for low latency X2 in this case. CS CoMP may though drive tight x2 delay performance.


4. Backhaul topologies

Backhaul has to provide connectivity between small cells, macro cells, core networks and potentially various gateway nodes in between. These various end points are interconnected by a network of physical links, each with differing characteristics in terms of capacity, latency, availability etc. The transport topology describes how these different physical links are combined in order to meet desired end to end service levels. For example, a physical link transporting traffic from many cells must have sufficient capacity for the aggregate of all their traffic, as well as increased reliability compared to a link carrying traffic for only one small cell.

4.1 Last mile topologies

One of the key challenges for urban small cells is to provide the last mile backhaul to the small cells themselves. Connectivity is required not only between the core network and the small cells, but also from small cell to small cell and from small cell to macrocell. This latter connection may be needed to enable co-ordination across the different types of cells in the heterogeneous network.

We broadly consider two types of last mile small cell backhaul: macro launched and street launched, as illustrated in Figure 4-1 and Figure 4-2, respectively.

Figure 4-1 ‘Macro launched’ small cell backhaul scenarios

Macro launched SCBH:

• Extends existing macro backhaul connectivity to down to street level • May need to upgrade capacity of macro backhaul:

• Joint het-net traffic depends on # small cells, sharing of spectrum, co-ordination, etc.

• Sharing small cell and macro should bring increased multiplexing gains over dedicated small cell backhaul

• The position of the small cell within the macrocell coverage area impacts macro capacity depending on spectral efficiency of UEs offloaded – as discussed earlier under capacity requirements


Figure 4-2 ‘Street launched’ small cell backhaul scenarios

Dedicated wireline + street launched wireless extension:

• Small cell connected directly with wireline backhaul • Where capacity available, wireline connection may then be extended using

‘street launched’ wireless • Wireless technology options largely the same as for macro launched,

although performance may differ when used in the street launched mode. • Wireless solution positioned on street level lighting poles, must be able to

cope with pole’s sway.

4.2 Redundant backhaul

Small cell planning and deployments will take into considerations possible backhaul options, based on available infrastructure, and adopt the best possible solution for each location. Mobile backhaul planners will be given rules regarding how many small/macro cells can be cascaded without redundancy (‘max. cascade with no redundancy’). Any additional cascading will result in a need for redundant topology. As the mobile network continuously evolves, the backhaul gear installed at the time of the first small cell deployments will need to support cascading, even if this is not required at day one.

Figure 4-3 ‘Macro-launched’ wireless with redundant topology


5. Backhaul technologies

Small cell backhaul technologies are detailed in our backhaul solutions paper [SCF049] and fall into two main categories:

• Wireless backhaul • Wired backhaul

The following technologies are detailed in the paper:

Wireless solutions Wired solutions

Millimetre 70-80GHz Direct fibre Millimetre 60GHz Digital subscriber line (xDSL) Microwave point-to-point FTTx Microwave point-to-multipoint Hybrid fibre-coax (HFC) and DOCSIS® 3.0 Sub 6GHz Licensed

Sub 6GHz Unlicensed TVWS Satellite Table 5-1 Last mile small cell backhaul technologies

Wireless solutions are primarily grouped according to the carrier frequency at which they operate. This in turn dictates key characteristics such as whether non-line-of-sight propagation is supported, the amount of spectrum available and its licensing arrangement. Another key differentiator is whether connectivity is point-to-point or point-to-multipoint. Wired solutions are categorised according to whether connectivity uses fibre, copper (or a combination of the two) is used.

Each solution category is described in detail and indicative performance ranges given for the quantifiable requirements such as capacity and latency. Qualitative statements are also provided to explain the types of deployment use case to which the solution is best suited.

Overall, although no single solution category is superior in all scenarios, we find that together the range of options can address all of the use cases envisaged. Total cost of ownership will determine the choice when multiple options are available.

An operator’s small cell network is in most cases likely to comprise of various deployment cases. An operator deploying outdoor small cells for urban access is likely to also have indoor small cells for public or enterprise use. A mixture of indoor, outdoor, street pole based, building based small cell deployments will drive the need for a variety of small cell backhaul solutions. The SCF has provided an overview of available small cell backhaul solutions which individually suit different deployment types included wired and wireless. This choice of backhaul technology, and inevitable use of multiple types in a single network presents an operator with a challenge. The basic challenge is how to manage the backhaul network. Provisioning of individual backhaul connections with a multitude of mediums will require complex operational processes. This complexity could be avoided if all backhaul solutions were to fit within a common management framework. Furthermore, if services (QoS) can be provisioned in a common way over these multiple backhaul solutions, a unified and very efficient service activation process could be established, hence reducing the TCO of the small cell network. A scenario where the small cell backhaul architecture changes through



the lifecycle of the network should also be considered. As the demands on the small cell network increase and user density increases to a point where fibre backhaul can economically displace wireless, the mix of backhaul types will be seen to change. With a common backhaul management and service provisioning framework, this ongoing adaptation to changing small cell backhaul architecture will be easier for the operator to handle. Low cost wireless backhaul solutions can be a key enabler for small cells but, equally, an adaptable small cell backhaul management framework can be the key to the small cell network keeping up with user demand. Within a densifying small cell network, the possible routes to the Mobile Core from any one node can multiply. The use of secondary complementary backhaul connections to boost capacity and/or redundancy lead to the need to manage traffic routing to ensure the backhaul is used as efficiently as possible and user QoE is optimised.


6. Backhaul TCO and commercial models

6.1 Backhaul TCO and business case

The Small Cell Forum commissioned a Business Case Analysis for urban small cell deployments which has been completed by Real Wireless [1]. The study set out to establish prime drivers for urban small cells and shows a positive business case for the deployment of small cells across a range of urban deployment scenarios. It covers a wide range of aspects of costs and benefits from urban small cells, but places a particular focus on cost and performance of small cell backhaul approaches as this can be a major contributor to the overall small cell TCO. City center deployments are considered in the study by comparing costs against network benefits, and how the balance of these changes according to differing existing macrocell network and backhaul availability. Capacity and Coverage driven urban case studies are considered.

Figure 6-1 Anatomy of urban small cells and their backhaul

Source: Real Wireless [1]

Simplified backhaul topologies are considered by the study consisting of both Wireline and Wireless small cell backhaul technologies. The study does not attempt to compare individual small cell backhaul technologies, but takes costs from a representative cross section of wireline and wireless technologies (including PTP, PTMP, NLOS, LOS, Licensed Spectrum and Unlicensed Spectrum) in order to derive TCO associated with a condensed set (four types) of backhaul deployment models, show in Figure 6-2.






Figure 6-2 Four types of backhaul nodes used in the TCO analysis


• Type A: These are small cells supported by a wireless connection back to an urban macrocell. The connection from the macrocell site can be point to point (P-P) or point to multipoint (P-MP), from an urban macrocell (with an existing high capacity backhaul link) to a small cell mounted on street furniture or the side of a building. The small cell can either be integrated within the backhaul unit, or can be a separate device.

• Type B: These are small cells with a wireless backhaul connection which is provided via the type-A unit backhaul capability. These tend to be from street furniture to other items of street furniture but could be on the side of a building.

• Type C: These are small cells with a backhaul connection provided to a network termination point (NTP) by a wireline backhaul solution (this can be either direct fibre, FTTx, Hybrid fibre or xDSL). Some additional digging may be required to extend the wireline connectivity to the base of the small cell.

• Type D: These are small cells with a wireless connection which extends the link from type C to an additional node in an analogous manner that type B extends type A.

The study considers that type-A and type-B are ‘macro-down’ solutions, and type-C and type-D are ‘street-level-up’. Due to the different CAPEX and OPEX profiles of macro-fed versus street-fed deployments, the study concludes that the business case is enhanced through changing the ratio of the two through lifecycle of the deployment, as shown in Figure 6-3 . The study considers a six year period over which small cells are deployed in order to add capacity (or coverage), over which the ratio of macro-down versus street-up installations vary in accordance with the network capacity demand.




Figure 6-3 Changing proportion of macro down and street up throughout the study Period


Further to this, the study concludes that, for an optimal business case, a mix of backhaul installation cases (and technologies) will be used throughout the lifecycle, and the mix changes as the network matures – tending towards higher capacity, more expensive street-up installations.

Figure 6-4 Numbers of different types of backhaul link throughout the capacity driven case study







6.2 Commercial models

Urban business models can take several forms, and encompass different use cases, from operator installed, owned, and maintained, to variants where customers manage part of the activities to models where a third party provides all activities on behalf of multiple operators and customers.

It is not the intention of this paper to describe the commercial benefits of such models but to highlight the deployment differences in terms of the people, tools and processes that enact the design/planning, building and operating of the Small Cell network.

We identify four general types of commercial models for urban small cells as follows:

• Classic: Operator self-deployed • Operator site share • 3rd party neutral host • 3rd party leased (wholesale) capacity

Figure 6-5 Commercial models for urban small cells

Operator self-deployed represents the approach where an operator plans, designs, builds and operates the network. In this case, the operator is responsible for site acquisition, permitting, installation, integration, backhaul provisioning, optimization and operations.

Operator site share is characterized by multiple operators working together to share common infrastructure. A possible scenario in this case is that one Operator will obtain access to a vertical structure for installation of the small cell or small cell antennas and share access with additional Operators. Typically, each Operator will provide their own installation, integration, backhaul provisioning, optimization and operations support. The level of technological complexity will dictate the degree to which assets can be shared.


Third party neutral host encompasses a single organisation providing services for a number of operators wishing to place infrastructure at that location. In this model, the 3rd party neutral host interfaces with multiple operators to provide a solution that is amenable to the operators and is municipally friendly. A common scenario is for the 3rd party neutral host to provide access to the vertical asset, permitting, installation, power and, in some cases, backhaul connection. The Operator, or small cell hardware vendor, is responsible for integration into the macro network, optimization and monitoring.

Third party leased (wholesale) capacity business model, for the purpose of understanding the impacts on the deployment topic, are similar to an extensive neutral host proposition to operators, to the extent that all activities from design/plan build and operate are undertaken by the third party rather than a pick and mix of activities which could be solution for neutral host.

Further details on commercial models and the people, processes and tools needed to deploy urban small cells can be found in our deployment issues paper [12].


7. Backhaul selection, planning and deployment

7.1 Overview

Here we consider the process needed to select and deploy small cells and their backhaul. By small cell backhaul (SCB) we mean the transport between the small cell itself and a point of presence (PoP) which has a connection the operator’s core network. In general SCB will comprise a mix of wireline connectivity and a number of wireless technologies. Since there is no one SCB solution that suits all conditions, operators will need to use a toolkit. Here we look at how such a toolkit selected and the development of planning guidelines to decide which tool to use where. Figure 7-1 illustrates the key steps and their various inputs and outputs. We focus here on the backhaul aspects, but note there will be other considerations to factor in.

Figure 7-1 Process for planning and deploying small cells and their backhaul

Developing guidelines provides rules to simplify the subsequent planning of individual networks, and is referred to as “capability design” in our deployment paper [12]. Development considers the range of conditions in the markets in which an operator deploys, the backhaul technology options and factors such budget, lifespan and zoning restrictions. A wide range of possibilities is reduced down to a toolkit of backhaul solutions, with rules to select the appropriate tool for a given set of conditions. Examples might include:

• Minimum and maximum limits on number of backhaul extensions attached to a PoP to ensure sufficient amortisation costs without limiting the small cell’s performance.

• Plugging in operator and region specific TCO data for each backhaul option (e.g. costs for fibre leasing, spectrum, deployment services, etc.)

• Development of flow diagrams to facilitate decision making: IF X>Y then select solution Z




Planning applies the guidelines to a specific city or market in order to identify the set of small cell sites needed to meet coverage and capacity requirements, and the type of backhaul to be used for each. Market specific input data needed for planning will include a spatial forecast of unserved demand, a list of candidate sites and locations of Points of Presence (PoPs). Planning will involve using CAD tools to design different components of the network. We later provide examples from such tools for NLOS and LOS backhaul solutions. Adherence to the guidelines should result in the desired network improvement being achieved within the TCO budget. The plan may incorporate a phased deployment in step with forecast growth in demand. Refinement or waiver of certain rules will likely be needed during early planning and for special situations.

Deployment is then the process of installing and commissioning at the chosen sites according to the plan. Acceptance testing revealing the actual achieved network and backhaul performance might point to shortfalls to be addressed in updates of the plan. Operation then maintains the network to deliver the target service levels, with ongoing maintenance and planned upgrades.

This is a continuous cycle rather than a one-shot process, with updates needed to refine processes and keep up with changes in market conditions. However it is also desirable to develop guidelines and plans with a reasonable shelf-life, such that teams involved with subsequent stages are able to benefit from a period of stability in which to refine supporting tools and processes.

In the following sections we provide further detail on the development of guidelines and planning process. A detailed description of the deployment and operation process is provided in document [12]. In addition, our urban case studies document [13].

7.2 Developing guidelines

The development of guidelines requires a consideration of the operators strategy for the roll out of small cell backhaul. It does not make choices on individual technologies, but creates an economic based approach to leveraging available carrier class backhaul, and if necessary, how this can be complemented by other lower cost SCB technologies to minimize TCO and optimize small cell capacity (or end user QoE) as well as keeping up with capacity demand increases through the projected life cycle of the small cell network. The development of guidelines is referred to ‘capability design’ in our deployment paper [12].

The choice of wireline technology is usually based on what's available to the operator in the area where the small cells are being deployed. When not owned by the operator, the ongoing leasing costs associated with carrier class wireline POPs can dominate the small cell TCO. These costs usually determine the operators SCB strategy and choices can be made to, for example, initiate the deployment with minimal wireline POPs complemented by relatively low cost wireless SCB, and increase the proportion of wireline to wireless as the network capacity demand increases and the deployment is densified. The choice of wireless SCB, or tool kit of wireless SCB technologies, depends on the operator’s motivation for using wireless as well as the physical deployment environment.

Inputs to determining a SCB strategy include:

• What existing carrier class backhaul can be leveraged for small cells (macro, street level, operator owned, third party)






• Costs for each solution implementation: fiber trenching to nearest wireline POP (related to each location), various wireless technologies, truck roll cost for each type of solution.

• Accessible RF spectrum assets and associated licensing constraints for wireless SCB.

• Network capacity year on year ramp projections across small cell deployment life cycle.

• Year on year cumulative TCO limit across the small cell deployment lifecycle (in order to satisfy target ROI).

The target outputs of the SCB strategy are:

• A network wide maximum ratio of POPs to wireless SCB based on target TCO per deployment phase.

• A toolkit (short list) of SCB (wired and wireless) solutions appropriate to the deployment.

These outputs form the basis on which the detailed SCB network planning is performed. Capex and OPEX of the available wireline backhaul connections can be calculated per small cell. This can be further divided into existing and new POPs.

From the RF spectrum assets, a short-list of suitable wireless SCB solutions can be established. These can be costed in a similar way to the wireline options. At this stage, it's recommended that the operator select both LOS and NLOS solutions to carry forward into the network planning process. For the purposes of setting strategy, the individual wireless TCOs may be averaged - the more options given to the network planners, the more likely they are to hit the average TCO. Selection processes of individual SCB solutions depends on operator’s evaluation and vendor selection process. The suitability of an individual solution for inclusion in the operator’s tool kit will largely depend on the mobile service being offered.

The costs derived above can be used in a TCO model to determine a maximum ratio of wireline to wireless backhaul technologies which satisfies the year on year targets. When combined with a small cell count projection (through the small cell access planning process), the maximum number of allowable wireline POPs can be determined. This is then set as a target for the network planners.

Questions such as "will the backhaul deliver adequate capacity and coverage" are deferred to the planning phase. Once the basic TCO constraints are established along with available assets, the framework for the network planning can be set. Without this basic economic analysis, there remains significant risk of the network plan failing to meet operator ROI targets.

TCO period considerations: Densification with small cells and their backhaul network increases the number of assets an operator has to manage. Unlike visits to macro cells which are located on rooftop or dedicated cellular towers, that require one or two technicians’ only and minimal pre-coordination with 3rd party, visit to small cell at street level is planned with participation of : cellular technicians, city council lamp-post technician, an potentially traffic management.. Truck roll to install/upgrade/visit a small may comprise a significant part in the project’s cost. Considering above issues, small cell project management should consider the impact of the planning lifetime on TCO: ”Minimal touch” or “deploy & forget” approaches will change the CAPEX/OPEX balance compared to ‘pay as you grow’ type strategies and thus impact TCO depending on operators’ planning lifecycle duration. Reliability (MTBF), capacity and automated installation process are important considerations.


7.3 Planning

During the Capability Design Phase, the backhaul strategy has created a financial framework for the network planners to work within. This is essentially based on a projection of the number of small cells required, and the maximum ratio of wireline to wireless backhaul connections that can be used whilst staying within the maximum TCO. It also presents a toolkit of backhaul solutions (and their associated costs) from which the network planner can base his or her detailed plans. The objective of the backhaul network planning phase is to create a network design which, by using an optimum mix of backhaul technologies, provides required coverage and capacity to the small cells within the TCO bounded by the strategy. Due to the sensitivity of backhaul equipment and installation cost on the overall small cell business case, it may be beneficial to create phased deployment plans, where the ratio of backhaul types changes per phase due to evolving needs for capacity and coverage.

Figure 7-2 Inputs and outputs of the small cell backhaul planning process

SCB planning is performed during the network physical design phase and starts by plotting locations of each existing wireline POP. The available wireless backhaul technologies will be used to extend the existing POPs, or share them between a cluster of surrounding small cells. The reach or coverage area around each POP for each available backhaul option can be plotted through propagation analysis planning. This should utilise planning tools which take account local clutter in three dimensions. Planning parameters and suitable propagation models are technology specific and advice is usually given by the vendor or experienced wireless system integrator. The following sections provide examples of processes used for NLOS and LOS backhaul.

The coverage predictions per backhaul type can be used to create a geospatial matrix of "available" backhaul connections. It should be noted that some wireless backhaul technologies support multiple hops to extend their coverage. This should be accounted for in the modeled coverage. Multiple backhaul connections to a single small cell site are also supported by some solutions. E.g. some multi-hop solutions enable redundancy with ring topologies that can also deliver up to double capacity. These can be considered within the same general planning process. The number of wireline POPs can be minimised at this stage. Each POP is rated based on cost and whether or not it is co-located with a target small cell location. The rating can be used to reduce the number of wireline POPs used at the phase of the network deployment being planned. The number of planned POPs should not exceed the maximum set during the backhaul


strategy planning. The target small cell locations can then be overlayed and an automatic cell planning (ACP) scoring analysis made to find the most cost effective backhaul option per small cell. The target small cell location may not be a single location, but could be multiple locations or area polygons. In this case, the same principle can be applied to establish the cost optimal combination of small cell location and backhaul type.

When wireline solution in not possible, based on the SC location, the planning tool (or process) takes into account LoS probability between adjacent locations and/or closet POP. This process provides the possible options for LoS and NLoS solutions.

Once each small cell is tagged with its preferred backhaul type, the backhaul systems can be planned in detail according to vendor recommendations. Each backhaul type can be considered as a unique backhaul layer. During the detailed planning of each layer, capacity modeling and frequency planning takes place. If a layer is overloaded, small cells can be moved to a second choice layer based on the scoring from the previous planning phase.

Figure 7-3 Assignment of small cells to backhaul solution ‘layers’ during planning


The above process is cyclical and should conclude with a final check of TCO against the boundaries set per deployment phase in the strategy planning process.

The following sections provide further detail on solution specific planning for different types of backhaul.

7.3.1 Planning non line of sight backhaul

An operator knows the desired small cell sites and also the potential aggregation points, which may be macrosites or street cabinets. An important element in planning is desired link availability and target traffic load per each small cell site (peak, busy time). Pending spectrum availability, equipment can be deployed either in frequency reuse of 1 scenario in which case interference analysis is critical, or in lower frequency reuse schemes like 2 or 3 where interference is not as severe. Even in lower frequency reuse deployment, interference analysis is important as morphology (reflections and/or canyon effect) will heavily influence interference map.

Workflow: 1. Compute link pathloss (including adequate fade margin for desired

availability) from each hub site location to any small cell site – remote backhaul module (RBM)

2. Compute Tx/Rx antenna gain for each hub beam from each hub site location to any RBM for a set of pointing directions in both horizontal and elevation planes

3. Select frequency (assignment) reuse scheme for the specific topology 4. Determine clustering of RBMs (small cell sites) that can be served by each

hub given the capacity requirements 5. Select hub beam assignment for each RBM (if multiple beam antennas are

used) 6. Analyse different interference management schemes for each cluster 7. Compute DL SINR and hence busy and peak throughput of an RBM for its

assigned resource slot(s). The peak time throughput is computed when an RBM is assigned to all the radio resources. The busy time throughput in case of PmP, is computed when RBMs share the resource slots equally in the same cluster.

8. Compute UL SINR and hence busy and peak throughput of an RBM for its assigned resource slot(s)

9. If design is not satisfactory , go back to step 4


Examples of data provided by planning report :

Figure 7-4 Area planning – coverage prediction example (RSSI)

The signal strength map verifies the hubs’ coverage by the predicted signal levels. The color of the map can be adjusted to show the contours of specific signal levels. In the example, contours of the signal level at -55dBm, -65dBm and -75dBm are shown.

The hub height, tilt and azimuth of antennas can be adjusted to ensure the hubs only light up their intended coverage area without over-shooting RF energy to their neighboring hubs.

Figure 7-5 Area planning – downlink and uplink carrier to interference ratio (CINR)

7.3.2 Line of sight planning

Planning and design of point-to-point and point-to-multipoint LOS radio transmission links is commonly addressed by commercially available software tools. Those tools collect various types of data in order to enable optimal, interference free, wireless links design and deployments. Input parameters include the following:

• Frequency band, channel bandwidth, modulation type • Transmit power, L1 radio capacity, receiver sensitivity threshold • System gain, receiver overload • Co and adjacent channel C/I


• Antenna beam width • Automatic transmitter power calibration range • DFM – dispersive fade margin • The relevant standard (in most cases ETSI or FCC) as base frame for the

potential capabilities and allowed boundaries. • System in use specifications (vendor specific). As most carrier-grade

systems deploy multiple modulations schemes, the specifications are given per each modulation mode.

One time loading of above parameters is performed per backhaul system vendor and model, and then the data is being used repeatedly for the ongoing link planning.

Geographic and topographic planning tools The geographic and topographic data are essential inputs for long-range LOS links planning. The Planning tools are being loaded with both. A sample graphical view of a typical LOS input data is shown in Figure 7-6 (taken from “Mentum Ellipse” by Infovista)

Figure 7-6 Sample graphical topographic data representation (Infovista)

As small cells are deployed with relatively short distances between hops, the LOS planning tool is expected to be in intensive use in large scale deployments. For initial project rollouts, site surveys will be sufficient.

Frequency related parameters: The selected frequency band being deployed for each of the LOS links, has several effects on the link performance, and thus is taken into account by the planning tools. Gaseous absorption – oxygen and vapor are factors that should be taken into account, as well as rain fades. All are strongly frequency dependent. Attenuation is measured in dB/km and takes into account also temperature and the relative humidity in the air. Further details on the how different wireless backhaul solutions are designed to exploit the different propagation effects from sub 6GHz to beyond 80GHz can be found in our backhaul solutions document [5].

Fresnel zone clearance: “Line of sight” propagation requires a certain clearance around the direct path between transmitter and receiver. Elliptical shaped areas to be kept clear are called Fresnel zones, and their size and shape depends on the link range




and frequency of operation. 3D Topographic modelling tools take the required clearance zone into account when planning individual links, as shown in Figure 7-7.

Figure 7-7 Example LoS planning process in automatic planning tool (Infovista)

7.3.3 Wireline backhaul planning

The carrier planner’s goal is to identify the best and most economic transport solution to backhaul the small cell traffic to the network. This starts with a forecast of the bandwidth demand of the small cell backhaul, which is dependent on the small cell topology (for example, a single cell on an outdoor pole or a building full of small cells connected by a local LAN).

Impact of backhaul capacity constraints on small cells For maximal spectrum efficiency of the small cells, unconstrained backhaul capacity is the best solution. However, the economics of small cell provision may drive towards a lower cost constrained backhaul solution. The key to success is to balance performance with a cost effective constrained backhaul solution. There are risks with constrained backhaul as noted below:

• Increased buffer times, playback re-buffering, and dropped frames for streaming services

• Slower file transfers on both UL and DL • Increased time to first byte, decreased page download speed, increased page

download failures • Excessive PERs for UDP sessions without retransmissions when the burst rate

is greater than the provisioned, constrained backhaul

Guidelines for transport services The following demonstrates the guidelines for determining the best transport options:


Consumer broadband access for backhaul Consumer or enterprise broadband services provide several advantages to small cell deployments. Typically, these services provide a fast and lower cost alternative to more traditional transport solutions. In addition, several choices for transport are available, such as xDSL, cable, fiber and IP over Ethernet.

Figure 7-8 Broadband services for small cell transport

Ethernet services for transport A more traditional approach is to incorporate Ethernet options into the transport design. The advantages are straightforward: better performance, better security, and


the ability to leverage on an existing macro backhaul infrastructure. Backhaul redundancy performance can be improved without a major increase in transport costs.

Figure 7-9 Ethernet services for small cell transport


8. MEF implementation agreements for backhaul services

The Metro Ethernet Forum (MEF) has developed an Implementation Agreement for Mobile Backhaul that describes the requirements of Carrier Ethernet services for mobile backhaul. Known as MEF 22.1, the mobile backhaul implementation agreement is a “tool box” for mobile backhaul based on MEF service types, i.e., E-Line, E-LAN and E-Tree (MEF 6.2), and the related Ethernet Service Attributes (MEF 10.3) and Circuit Emulation Service (MEF 8). The implementation agreement supports all 3GPP generations from 2G to 4G/LTE over Ethernet backhaul. The Implementation agreement includes one amendment (MEF-22.1.1) that describes specific use cases for small cell backhaul.

Ethernet services are becoming increasingly available, even at sites with access to legacy circuits. LTE and LTE Advanced mobile equipment (including small cells) utilize Ethernet interfaces for transport; therefore Ethernet based services are most suitable for backhauling mobile traffic. Carrier Ethernet services provide the connectivity in the mobile backhaul network, and allow for convergence of services with traditional fixed business and residential services (where they already dominate all other services). MEF Carrier Ethernet services can be supported over any transport (referred to as the TRAN layer in MEF 4) as shown in Figure 8-1. These definitions aim to support a wide range of mobile network topologies (including small cells backhaul topologies).

Figure 8-1 Carrier Ethernet Mobile Backhaul over Different Access Technologies (MEF)

MEF 22.1 defines the role of a mobile operator (Subscriber or Customer who purchases the Ethernet backhaul service) and Carrier Ethernet Network (CEN) operators (backhaul or service provider). A mobile operator (with a RAN network) purchases a Carrier Ethernet backhaul service from a CEN operator that is demarked at a UNI, as shown in Figure 8-2. These roles can also be applied for business units


within the same operator, for e.g., where a wireless business unit might obtain the MEF service from the same operator’s transport business unit.

Figure 8-2 MEF mobile backhaul terminology

The mobile backhaul may consist of more than one segment provided by different CEN operators to achieve connectivity between the base station sites and network controller/serving gateway sites.

The mobile operator is not constrained to using Carrier Ethernet services end to end as they may only require service for a portion of the mobile backhaul if, for instance, they own some portion of the backhaul.

A mobile operator can also choose to use Carrier Ethernet services from a CEN operator for some network segments of the mobile backhaul and use non MEF services for other portions of the network for example where an IP VPN service is available and desirable. When combinations of MEF and non-MEF services are used, the mobile operator is responsible for the end-to-end performance across the different segments.

8.1 MEF references

[14], [15], [16], [17], [18], [19]

RAN Radio access network

RAN BS RAN base station

RAN NC RAN network controller

RAN CE RAN customer edge –mobile network node/site


9. Synchronisation for urban small cells

The general synchronization requirements identified in [3] for both frequency and phase and time are listed in Table 9-1 below:

Radio technolog

y BTS type

Frequency

accuracy

Phase differenc

e

Time accurac

y

Technical specificatio

n Notes

GSM

Macro BTS 50ppb 3GPP TS 45.010 [

Clause 5.1

Frequency accuracy at the air interface Pico BTS 100ppb

All ±3.69µs (optional)

3GPP TS 45.010

Clause 5.2

Optional BTS alignment of 1 symbol period

CDMA2000

Macro 50ppb 3GPP2

C.S0010 Clause 4.1

Frequency accuracy at the air interface

Pico/Femto 100ppb

All

±3µs (norm)

±10µs (max)

3GPP2 C.S0010

Clause 4.2

Pilot time alignment error to CDMA system time

WCDMA-FDD

Wide Area 50ppb

3GPP TS 25.104

Clause 6.3.1

Frequency accuracy at the air interface, over one timeslot period (0.67ms)

Med. Range 100ppb

Local Area 100ppb

Home 250ppb

WCDMA-TDD

(including TD-

SCDMA)

Wide Area 50ppb

3GPP TS 25.105

Clause 6.3.1

Frequency accuracy at the air interface, over one timeslot period (0.67ms)

Local Area 100ppb

Home 250ppb

All ±3µs 3GPP TS 25.123

Clause 7.2

Maximum deviation in frame start times at the air interface




All ±2.5µs

3GPP TS 25.402 Clause 6.1.2.1

Relative phase difference at the synchronisation input

WCDMA MBSFN ±12.8us

3GPP TS 25.346 Clause

7.1B.2.1

Optional feature - Release 8 onwards

LTE (FDD and

TDD)

Wide Area 50ppb

3GPP TS 36.104

Clause 6.5.1

Frequency accuracy at the air interface, over one sub-frame period (1ms)

Med. Range 100ppb

Local Area 100ppb

Home 250ppb

LTE-TDD

Wide area, >3km radius

±10µs

3GPP TS 36.133

Clause 7.4.2

Maximum deviation in frame start times at the air interface

(for cells on the same frequency with overlapping coverage areas)

Wide area, ≤3km radius

±3µs

Home BS, >500m

rad. ±1.33 +

Tprop µs1

Home BS, ≤500m

rad. ±3µs

LTE handoff to CDMA200

0 (if req'd.)

±10µs 3GPP TS 36.133

Clause 7.5.2

Maximum time difference between eNodeB frame boundaries and CDMA system time

Table 9-1 Radio technology synchronization requirements

Source: [5]

In the urban environment, additional functionalities such as enhanced Inter-cell interference coordination eICIC may be required in order to manage interference between macro-eNobeB and pico-eNobeBs. eICIC, even for FDD radio technologies, 1 Tprop is the propagation delay between the home BS and the cell selected as the network listening synchronisation source




imposes a 1-5 usec phase synchronization requirements between the slaves on the macro-eNodeBs and the pico-eNobeBs.

In indoor environments, the global navigation satellite systems (GNSS) signal may be too weak to penetrate buildings. Therefore, in those environments, packet based synchronization solutions such as PTP is one of the most practical and generic option to synchronize both in phase and frequency the macro-eNobeBs and the pico-eNobeBs since the PTP grand master GNSS antenna location is independent from the locations of the pico-eNobeBs. The grand master GNSS antenna can be placed at a location where it has the best GNSS satellite reception. Moreover, a small grand master at the edge of the network could be deployed to serve a cluster of 20 pico-eNobeBs, which represents substantial saving in term of GPS antenna installation costs.

Note that it is also important to place the PTP grand master as close as possible to the pico-eNobeBs in order to reduce asymmetry and accumulated time error. PTP features such as PTP boundary clocks and/or additional synchronization mechanisms such as synchronous Ethernet (SyncE) could also be deployed to improve the robustness of the design.

A PTP based solution can also be applied in outdoor environments. A GPS directly on the pico-eNobeBs is an alternative solution in those environments.

As stated earlier one of the key challenges for urban small cells is the choice of the last mile backhaul. This choice is also of paramount importance for the synchronization performance that can be achieved. [3] describes the challenges presented by the different network access technologies. It is important to take into account those challenges in the choice of the backhaul transport technology.

A more detailed analysis of synchronisation requirements, solutions and deployment architectures can be found in our white paper LTE Synchronisation [3].






References

1 [SCF087] ‘Business case for urban small cells’, http://scf.io/documents/087 2 [SCF088] ‘Urban small cell network architectures’, http://scf.io/documents/088 3 [SCF075] ‘Synchronisation for LTE small cells’, http://scf.io/documents/075 4 [SCF059] ‘X2 interoperability in multi-vendor X2 HetNets’,

http://scf.io/documents/059 5 [SCF049] ‘Backhaul technologies for small cells’, http://scf.io/documents/049 6 [SCF086] ‘Market drivers for urban small cells’, http://scf.io/documents/086 7 [SCF078] ‘Backhaul for enterprise small cells: A topic brief’,

http://scf.io/documents/078 8 ‘Small cell backhaul requirements’, NGMN Alliance, v1.0, June 2012 9 ‘Telstra demos 300Mbps LTE-A’, Telecomsasia.net, Dec 2013,

http://goo.gl/BfpPXV 10 ‘Guidelines for LTE Backhaul Traffic Estimation’, NGMN, July 2011,

http://goo.gl/EWQQg 11 ‘Security in LTE backhauling’, NGMN Alliance, February 2012, http://goo.gl/lrbdd 12 [SCF096] ‘Deployment issues for urban small cells’, http://scf.io/documents/096 13 [SCF090] ‘Small cell services in the urban environment’,

http://scf.io/documents/090 14 ‘Mobile Backhaul Implementation Agreement Phase 2’, MEF 22.1 15 ‘Mobile Backhaul Phase 2, Amendment 1 – Small Cells’, MEF 22.1.1 16 ‘Metro Ethernet Services Definitions Phase 3’, MEF 6.2 17 ‘Ethernet Services Attributes Phase 3’, MEF 10.3 18 ‘Implementation Agreement for the Emulation of PDH Circuits over Metro Ethernet

Networks’, MEF 8 19 ‘Metro Ethernet Network Architecture Framework Part 1: Generic Framework’,

MEF 4

http://scf.io/documents/087









http://goo.gl/BfpPXV

http://goo.gl/EWQQg

http://goo.gl/lrbdd





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