Easy Small Cell Backhaul Feb 2012

7/29/2019 Easy Small Cell Backhaul Feb 2012

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Easy small cell backhaul February 2012 1

Contents

Authors .................................................................................................. 1

1 Executive summary ............................................................................ 2

2 Market overview .................................................................................. 3

3 The industry challenge ........................................................................ 5

3.1 Capacity is the principal driver ..................................................... 6

3.2 Coverage: Where will the small cells be? .................................... 7

3.3 Physical design and installation ................................................... 7

3.4 Quality of service ......................................................................... 8

4 Types of small cell backhaul solution .................................................. 9

5 Economics of small cell backhaul ..................................................... 13

5.1 Total Cost of Ownership (TCO) factors ...................................... 14

6 Cambridge Broadband Networks vision for small cell backhaul........ 16

7 Conclusion ........................................................................................ 18

Authors

Julius Robson, Leader of the NGMN Small Cell Backhaul Group

Lance Hiley, VP Marketing, Cambridge Broadband Networks Limited


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1 Executive summary

It is widely accepted that future demands for mobile broadband will not be solely satisfiedby increases in spectrum and the more efficient 4G technologies alone, and that network

densification will also be needed. Analysys Mason predicts that operators in Western

Europe could see their Radio Access Network (RAN) costs rise to USD40 billion per year

by 2016, compared to USD5 billion per year in 2011, to meet the growing demand for data

traffic by deploying more base stations1.

This evolution translates into more radio stations of different shapes and formats and

subsequently results in more, and smaller, cells. The term small cells, which is much

debated today, comes from this and may include, but is not limited to Femtocells,

Picocells, Distributed Antenna Systems (DAS) and traditional base stations gettingsmaller, etc.

What is not absolutely clear is how all these new small cell sites will be backhauled.

In this paper we look at the fundamental requirements for a small cell backhaul solution in

terms of capacity, coverage and cost, as well as important implementation aspects such

as form factor, quality of service and time to deployment.

A wide variety of different solutions are then considered and compared against these

requirements.

Non-line of sight (NLoS) wireless solutions might seem like a great way to reach the newoutdoor small cells, but there is a distinct lack of suitable spectrum at low enough

frequencies to deliver sufficient capacity. Any harmonised spectrum in licensed bands is in

high demand for mobile access, and the unlicensed (Wi-Fi) bands are already heavily

congested in areas of high demand. Worse, what works today might collapse tomorrow as

even more unlicensed devices are added to the environment.

It is also clear that Point-to-Point (PtP) microwave will be too expensive and too

cumbersome for the continuously evolving small cell deployment. Multipoint microwave

occupies the ideal middle-ground between these extremes, having enough capacity to

backhaul tens of small cells, and a much lower Total Cost of Ownership (TCO) andspeed-to-deploy than a PtP solution. Multipoint microwave will be an essential component

in an operators toolbox of small cell backhaul solutions.

1The case for Wi-Fi offload: the costs and benefits of Wi-Fi as a capacity overlay in mobile networks,

Analysys MasonsWireless Networksresearch programme, January 2012
http://www.analysysmason.com/Research/Content/Reports/RRN06_WiFi_offload_Dec2011/http://www.analysysmason.com/Research/Content/Reports/RRN06_WiFi_offload_Dec2011/http://www.analysysmason.com/Research/Content/Reports/RRN06_WiFi_offload_Dec2011/http://www.analysysmason.com/What-we-offer/Research/#%21/Network-technologies/Programmes/Wireless-Networkshttp://www.analysysmason.com/What-we-offer/Research/#%21/Network-technologies/Programmes/Wireless-Networkshttp://www.analysysmason.com/What-we-offer/Research/#%21/Network-technologies/Programmes/Wireless-Networkshttp://www.analysysmason.com/What-we-offer/Research/#%21/Network-technologies/Programmes/Wireless-Networkshttp://www.analysysmason.com/Research/Content/Reports/RRN06_WiFi_offload_Dec2011/


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2 Market overview

Small cells are comingRapid adoption of mobile data services triggered by the availability of dongles,

smartphones, and tablets is placing heavy traffic demands on cellular networks and

making them congested. Analysys Mason forecasts the number of consumer Internet-

connectable devices (excluding computers and mobile phones) worldwide will increase

from 1.2 billion in 2010 to 15.7 billion by 2020. Operators can capitalise on this trend by

promoting the use of wireless, not just Wi-Fi, connectivity on most of these devices, which

increases the utility of the devices for users and helps operators to earn some additional

revenue. This will drive the growth in global mobile traffic demand over the next five years

which is forecast to be around 26X, equivalent to 80 per cent CAGR

2

.

In order to supply these demands, operators will need to increase capacity with the three

key ingredients of a wireless network: spectrum, spectral efficiency (a property of the

technology) and cell density. Combining these three factors gives us capacity density,

measured in Mbps per km2. Looking ahead, new spectrum is currently being made

available for next generation mobile broadband services in many regions; however this is

unlikely to more than double existing allocations for mobile voice and data. Despite much

hype around headline peak rates of over 300Mbps, the net spectral efficiency

improvements from 4G3 technologies such as WiMAX and LTE are only expected to be

around 1.5 - 2X

4

over the same five year period. Combining spectrum and spectralefficiency increases provides only 4X capacity gain, which implies cell densification by a

factor of around 8X will be needed in order to supply a capacity density increase of the

aforementioned 26X.

Macrocellular networks based on high-power base stations can be densified to a good

extent with both cell splitting and additional sites. However, the large form factors required

limit the range of locations where macrocells can be deployed. Limitations on roof space

for antennas and cabinets impose density limits of around five macrosites per km2

[Holma5], equivalent to a cell spacing of approximately 500m. Densification beyond this

level requires smaller form factor equipment to be able to be deployed in availablelocations which include sides of buildings and street furniture. Reducing the size of a base

station also limits space for power supplies and cooling, which in turn reduces the

2Cisco Visual Networking Index: Global Mobile Data Traffic Forecast Update, 2009-2014, February 2010.

Downloaded from

http://www.cisco.com/en/US/solutions/collateral/ns341/ns525/ns537/ns705/ns827/white_paper_c11-

520862.html

3We note that the first releases of LTE and WiMAX are not IMT-Advanced candidates, which many consider

to be the true definition of 4G

4"4G Capacity Gains", Real Wireless, January 2011,http://stakeholders.ofcom.org.uk/market-data-

research/technology-research/2011/4G-Capacity-Gains/

5LTE for UMTS: Evolution to LTE Advanced, Harri Holma, Antti Toskala, 2nd ed., Wiley 2010
http://www.cisco.com/en/US/solutions/collateral/ns341/ns525/ns537/ns705/ns827/white_paper_c11-520862.htmlhttp://www.cisco.com/en/US/solutions/collateral/ns341/ns525/ns537/ns705/ns827/white_paper_c11-520862.htmlhttp://www.cisco.com/en/US/solutions/collateral/ns341/ns525/ns537/ns705/ns827/white_paper_c11-520862.htmlhttp://stakeholders.ofcom.org.uk/market-data-research/technology-research/2011/4G-Capacity-Gains/http://stakeholders.ofcom.org.uk/market-data-research/technology-research/2011/4G-Capacity-Gains/http://stakeholders.ofcom.org.uk/market-data-research/technology-research/2011/4G-Capacity-Gains/http://stakeholders.ofcom.org.uk/market-data-research/technology-research/2011/4G-Capacity-Gains/http://stakeholders.ofcom.org.uk/market-data-research/technology-research/2011/4G-Capacity-Gains/http://stakeholders.ofcom.org.uk/market-data-research/technology-research/2011/4G-Capacity-Gains/http://www.cisco.com/en/US/solutions/collateral/ns341/ns525/ns537/ns705/ns827/white_paper_c11-520862.htmlhttp://www.cisco.com/en/US/solutions/collateral/ns341/ns525/ns537/ns705/ns827/white_paper_c11-520862.html


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maximum Radio Frequency (RF) transmit power. Low-power base stations will have

shorter range and will therefore need to be deployed closer to the users they will serve.

Outdoor users will typically be at street level, and so low-power small cell base stations

will need to be mounted near to them on sides of buildings or on street furniture, as

opposed to the rooftop locations suitable for higher power macrocells6,7.

Alternatives to small cells are Cloud RAN solutions where the baseband is in a

centralised location with only the radio part on site either in the form of remote radio units

plus antenna(s), active antennas or radio embedded antennas. However, all of these

options still require a large number of antennas deployed on roofs (which impacts site

rental costs) plus the active antenna and radio embedded options are large and heavy

which impacts installation and maintenance costs.

In summary, over the next few years, operators will need to deploy dense networks of low-

power small cells located near to users, in order to supply sufficient capacity density high-

quality contiguous/ubiquitous coverage (5bars) to meet the ever growing demands for

mobile broadband services.

In this paper we consider the implications of small cells for backhaul networks. We start by

outlining requirements for backhaul systems intended to provide high-capacity

connectivity to street level small cells. We then consider the benefits and limitations of

different technology solutions, such as fibre, NLoS, multipoint etc. and consider a case

study for a small cell deployment in the centre of London which considers a continual

process of densification. We show how multipoint microwave can provide both high

capacity and excellent street level coverage at the lowest cost.

6

Small Cell Solutions: Outdoor Metrocells Leveraging 3G, 4G and W-Fi, Belair Networks,http://www.belairnetworks.com/applications/index.cfm/intSolutionID/2/intApplicationID/9

7Microcells, May 2011,http://www.bssbook.com/index.php/microcells
http://www.belairnetworks.com/applications/index.cfm/intSolutionID/2/intApplicationID/9http://www.belairnetworks.com/applications/index.cfm/intSolutionID/2/intApplicationID/9http://www.bssbook.com/index.php/microcellshttp://www.bssbook.com/index.php/microcellshttp://www.bssbook.com/index.php/microcellshttp://www.bssbook.com/index.php/microcellshttp://www.belairnetworks.com/applications/index.cfm/intSolutionID/2/intApplicationID/9


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3 The industry challenge

Small cell backhaul requirements

Figure 1: Backhaul connects many small cells with a Point of Presence (PoP) with sufficient

capacity

The fundamental function of backhaul is to provide connectivity between large numbers of

small cell sites back to Points of Presence (PoP) with connectivity to the core network.

Backhaul aggregates traffic from many small cells back to a single PoP. Requirements for

backhaul systems broadly fall into two categories, what it does, and how it does it, as

illustrated in Figure 2.

Figure 2: How requirements for small cell backhaul drive the form of the solution

PoP

Small Cells


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Fundamental requirements of this backhaul connectivity are:

1. Coverage: Backhaul solution must be able to reach the small cells in difficult

locations

2. Capacity: Backhauling ten small cells requires a system with significantly greater

capacity than each small cell

3. Cost: Small cell backhaul needs to have many more connections than a macrocell

network. Cost per connection needs to be lower

Of equal importance to what the backhaul does is how it does it. Small cell backhaul

solutions will need to be fast, cheap and easy to deploy. At the same time, operators are

indicating that quality of service cannot be compromised.

3.1 Capacity is the principal driverIts all about capacity: As described earlier, the main reason to deploy small cells is to

ease congestion on macrocells by adding capacity. It therefore follows that small cell

capacities must not be limited in any way by the backhaul. Backhaul solutions providing

basic connectivity might be suitable to get coverage to not-spots, where there is currently

no coverage, but high-capacity backhaul is needed to serve the busy hot-spots.

Data is different: The backdrop to the small cell phenomenon is the mass adoption of

mobile data services, which not only increase the volume of capacity required, but

completely change its characteristics too. A detailed analysis by the NGMN8 shows that

very high peaks in backhaul traffic from an LTE base station occur when the network is

lightly loaded. This is the opposite of legacy voice-only networks where peaks occur

simultaneously on all base stations during busy hours.

Smaller means burstier: Small cells and higher-order RAN protocols like LTE

exacerbate the burstiness of backhaul traffic, as they typically cover fewer users than a

macrocell and only support one omnidirectional cell per site. Averaging traffic over fewer

users and cells means lower traffic on average, but the peaks stay the same as they are

limited only by the maximum speed of the deployed technology. Table 1 summarises our

view of small cell provisioning for peak and loaded conditions.

Backhauling many small cells each with bursty traffic is best done by pooling resources

between them with a multipoint solution. A case study of measured traffic on a data-rich

HSPA+ network shows that a multipoint topology achieved 50per cent improvement in

spectral efficiency compared to a Point-to-Point topology9. The vast majority of solutions

proposed for small cell backhaul are multipoint.

8

"Guidelines for LTE Backhaul Traffic Estimation", NGMN Alliance, July 2011,http://www.ngmn.org/uploads/media/NGMN_Whitepaper_Guideline_for_LTE_Backhaul_Traffic_Estimation.pdf

9The Effect of System Architecture on Net Spectral Efficiency for Fixed Services, John Naylon, Nov 2011

http://cbnl.com/resources/effect-system-architecture-net-spectral-efficiency-fixed-services
http://www.ngmn.org/uploads/media/NGMN_Whitepaper_Guideline_for_LTE_Backhaul_Traffic_Estimation.pdfhttp://www.ngmn.org/uploads/media/NGMN_Whitepaper_Guideline_for_LTE_Backhaul_Traffic_Estimation.pdfhttp://cbnl.com/resources/effect-system-architecture-net-spectral-efficiency-fixed-serviceshttp://cbnl.com/resources/effect-system-architecture-net-spectral-efficiency-fixed-serviceshttp://cbnl.com/resources/effect-system-architecture-net-spectral-efficiency-fixed-serviceshttp://www.ngmn.org/uploads/media/NGMN_Whitepaper_Guideline_for_LTE_Backhaul_Traffic_Estimation.pdf


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Small cell technology Loaded, Mbps Peak, Mbps

HSDPA 2x2 64 QAM 6 42

DC HSDPA 2x2 64 QAM 12 84

LTE 10MHz 2x2 18 75

LTE 20MHz 2x2 34 150

Table 1: Backhaul provisioning for small cells. Sources: 3GPP10

, NGMN8CBNL Analysis

Operators dont want to micromanage last mile capacity: Traffic demand will

increase rapidly and continuously. However, the exact locations of the demand will not be

known in advance. Small cells will need to be installed in the capacity hotspots as they

appear. Operators want to be able connect up the new site to the backhaul without having

to re-plan the whole network. This plays to a further strength of multipoint solutions, where

capacity is automatically shared dynamically amongst many small cells and thus can be

managed at a much higher level.

3.2 Coverage: Where will the small cells be?

Small cells are expected to be mounted at 4-6m above street level, rather than on

rooftops6,7,11. Doing so greatly improves coverage to consumers both outdoors and

indoors, as the low-level small cells can see better into buildings.

Whilst this is good news for the RAN, it pushes the rooftop-to-street-level connectivityproblem into the backhaul arena. Good line of sight coverage for high-capacity

connectivity is possible down the urban canyons, but reaching around corners is more of

a challenge. Last mile extensions are a group of solutions that get around the issue of

corners with multi-hops, non-line of sight extensions or plain old Cat. 5 cable.

3.3 Physical design and installation

Small cell equipment and technologies need to be optimised for rapid low-cost

deployment. Both the small cell and backhaul hardware need to be small and light enough

to be easily mounted on building sides and street furniture, such as lampposts, without

needing special skills. Any alignment of antennas should be made as simple as possible.

Close integration of the two units is desirable, as is a single power connection for both.

For example, a power over Ethernet solution could be used to supply the small cell from

the backhaul unit, or vice versa, reducing the number of power supplies and connections.

10"Further advancements for E-UTRA physical layer aspects", 3GPP TR 36.814 V9.0.0 (2010-03)

11NGMN Alliance Optimised backhaul solutions for LTE, challenges of Small Cell deployment and Co-

ordinated QoS, NGMN Alliance, Layer 123 LTE/EPC & Converged Mobile Backhaul, December 2011


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3.4 Quality of service

Although quality not quantity might be an appropriate description for a macrocell network,

the reverse is not necessarily true for small cells. Certainly from a data rates perspective,

the principal reason to deploy small cells is to relieve congestion and improve user

experience. One of the design aims for 4G RAN technologies such as LTE was to reduce

dependency of RAN nodes on backhaul performance: it should be the end user services

which dictate quality parameters such as latency, jitter and interruption times, rather than

RAN equipment signalling itself. A low-quality backhaul network may be acceptable for

best effort data offload, but operators wishing to offer interactive services such as voice,

gaming and cloud access will need to ensure their small cell network also has high-quality

backhaul connections.

Regarding availability and redundancy, overall network performance will be less sensitive

to single small cell outages compared to those in macrocells. In a heterogeneous

deployment of mixed cell sizes, the macrocell layer can be considered a safety net as

described in NGMN Alliance Optimised backhaul solutions for LTE, challenges of Small

Cell deployment and Co-ordinated QoS11. Opportunities to reduce backhaul costs for the

small cells then exist through reduced levels of redundancy and extended ranges where

less than five nines of availability is acceptable. Taking the low-availability concept to

the extreme, it has been suggested that small cells may be switched off altogether to save

power during times of low traffic demand12,13.

In summary, a challenging mix of requirements.

Figure 3: Comparison of key attributes for macrocells and metrocells

12

A mixed up future for LTE, Alcatel Lucent, April 2011,http://www.wilson-street.com/2011/04/a-mixed-up-future-for-lte/

13Design of Energy-Aware Networking and Cooperation Mechanisms, COoperative aNd Self growing

Energy awaRe Networks CONSERN, 28 February 2011,http://goo.gl/RYSNI
http://www.wilson-street.com/2011/04/a-mixed-up-future-for-lte/http://www.wilson-street.com/2011/04/a-mixed-up-future-for-lte/http://www.wilson-street.com/2011/04/a-mixed-up-future-for-lte/http://www.wilson-street.com/2011/04/a-mixed-up-future-for-lte/http://goo.gl/RYSNIhttp://goo.gl/RYSNIhttp://goo.gl/RYSNIhttp://goo.gl/RYSNIhttp://www.wilson-street.com/2011/04/a-mixed-up-future-for-lte/http://www.wilson-street.com/2011/04/a-mixed-up-future-for-lte/


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4 Types of small cell backhaul solution

Small cell solutions are now emerging and the industry has not yet discovered whichsolutions best fit the bill. It is most likely that operators will tackle the problem with a

toolbox of solutions while they develop an understanding of which tools suit which

scenarios.

Fibre: Great where already available, otherwise slow and costly to install. Fibre

provides a very high-capacity and low-latency connection, however it is neither fast to

install nor cheap to do so. Fibre will be needed to provide connectivity at PoPs, and will be

present at an increasing number of indoor locations. However, given the need to keep up

with evolving traffic demand, fibre is not likely to be a cost-effective way of connecting up

a continuously evolving set of outdoor small cells mounted on street furniture.DSL: Like fibre, but without the performance. DSL is more widely deployed than fibre

today and, where it is already installed, can provide a basic level of connectivity. DSL data

rates should satisfy the provisioning requirements for the loaded HSPA and potentially

10MHz LTE, but it will not provide the peaks needed for a good end user quality of

experience. Where DSL is not already installed, it would make more sense to install fibre.

Non-line of sight (NLoS) wireless: Good for coverage, but capacity limited by

available spectrum. NLoS wireless backhaul would be the perfect solution were it not for

the small cells and Wi-Fi hotspots already using the entire low frequency spectrum

available. NLoS propagation requires low carrier frequencies of less than a few GHzwhich are highly prized for mobile access itself, as shown in Figure 4. The height of the

bar indicates the MHz of bandwidth for uplink and downlink traffic. As a general rule, the

bandwidth available for backhaul needs to be at least as much as that for access. Some

claim that the spectral efficiency of the backhaul will be higher to compensate, but this

seems unlikely given that access and backhaul are operating in very similar (NLoS)

propagation conditions and with interference from nearby co-channel transmitters.

Figure 4: Usage of spectrum suitable for non-line of sight backhaul (UK example shown)


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Unpaired TDD spectrum could potentially be used for NLoS backhaul, but as the graph

shows, the quantity of this is small compared to the LTE and HSPA bands it will have to

backhaul. The 3.5GHz band is large and underused, however 3GPP is currently

incorporating this into both UMTS (HSPA) and LTE specifications14. One operator has

already announced a deployment here15.

Free unlicensed spectrum worth every penny: The ISM bands at 2.4 and 5.8GHz

provide a large amount of freely available bandwidth. However, in the hotspots where

additional data capacity is needed most, ISM spectrum is likely to be already heavily used

by Wi-Fi for data off-load, Bluetooth and other equipment. All these other transmissions

represent unwanted interference which will reduce signal quality, throughput and general

quality of solutions using these bands. Niche opportunities exist where interference can be

reduced in isolated locations or with smart antenna technologies. Backhaul in unlicensed

spectrum represents a hostage to fortune: it may work on the day of installation, but could

be fatally disrupted by a consumers new device tomorrow.

NLoS backhaul is built into the LTE standards: A feature rather like NLoS backhaul,

called in-band relay, is included in the LTE advanced standard16 where a base station

can use half of the access spectrum to backhaul signals to a connected donor cell site.

Whilst this is good for extending coverage for early deployments, spectral efficiency for

end-user traffic is effectively halved, so it is not a capacity-enhancing solution needed to

meet increasing demand.

Microwave plenty of spectrum, a mature technology for fixed links: Large amounts

of bandwidth are available at microwave17 frequencies from 10-60GHz, which in turn

means lots of capacity. These frequencies are already widely used for high-capacity fixed

communication links with Point-to-Point and multipoint topologies. The small wavelength

at these frequencies brings a mix of benefits and challenges. On the plus side, high-gain,

compact antennas are easy to build which improves link budgets, however such antennas

need to be carefully aligned to the other end of the link. The short wavelength also means

that effectively line of sight is the only option as diffraction and penetration around or

through buildings and trees incurs high losses. This can be turned to an advantage as the

high attenuation helps reduce interference from nearby links wishing to re-use the same

frequencies. Given the maturity of technologies and availability of spectrum for high-

capacity backhaul, microwave looks to be the mainstay of small cell backhaul solutions.

14UMTS-LTE 3500MHz Work Item Technical Report, 3GPP TR 37.801 3rd Generation Partnership Project;

Technical Specification Group Radio Access Networks

15UK Broadband looking to enable LTE by 2012, Think Broadband, June 2011,http://goo.gl/MCOZI

16"Further advancements for E-UTRA physical layer aspects", 3GPP TR 36.814 V9.0.0 (2010-03), 9, p17

17Microwave is a general term where the wavelength is small compared to the circuit or environment under

consideration. Here we consider frequencies from around 5GHz and up
http://goo.gl/MCOZIhttp://goo.gl/MCOZIhttp://goo.gl/MCOZIhttp://goo.gl/MCOZI


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This should not be a surprise; microwave backhaul has historically been used more for

backhaul links than all the other technologies put together.

E-band: Around 10GHz of spectrum is available between 71-76 and 81GHz, a window

between peaks of high atmospheric absorption. Regulators have made this spectrum

available under light licensing conditions to encourage innovation. Although background

attenuation is several times higher here than in microwave bands, short high-capacity

links of over 1km are possible18. New technologies are emerging to exploit the wide

bandwidths available to produce high-capacity Point-to-Point links. High-gain and highly-

directional compact antennas are easily made given the very short wavelengths, and in

fact are needed to make the link budget work. Wider beam widths for easier alignment or

multipoint topologies are a challenge.

Network topologies: The topology of the network affects the characteristics of

connectivity and capacity between the PoP and the small cells.

Figure 5: Topologies for backhaul networks

Point-to-Point uses one or more links to connect cells to a PoP. Each link requires an

antenna and radio at each end, and so the PoP site can easily become crowded with too

many antennas. The solution to this is to create a tree structure of intermediate nodes.

Provisioning capacity across the tree requires consideration of the number of downstream

small cells, so adding in new small cells may require a re-planning of the network.

Furthermore, since frequency allocations are often managed on a per link basis, building

out such networks can be time consuming. This has not prevented widespread use of

18Multigigabit wireless technology at 70GHz, 80GHz and 90GHz, RF Design, May 2006,http://goo.gl/ZKt8h
http://goo.gl/ZKt8hhttp://goo.gl/ZKt8hhttp://goo.gl/ZKt8hhttp://goo.gl/ZKt8h


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microwave Point-to-Point for macrocell sites, but for the rapid rollout and continuous

evolution required for small cell networks, it is likely to be untenable.

Ring topologies are formed when you close a chain of links on a Point-to-Point network. It

offers better protection without resorting to redundant links. A disadvantage of the ring

topology is that it takes more radio hops to reach distant sites. Statistical multiplexing is

possible in a ring topology, as the total capacity of the links in a ring is generally greater

than that of the links in the branches of a tree. Provisioning capacity against a backdrop of

increasing demand is difficult and potentially expensive as each link requires an identical

upgrade. One way to increase ring capacity is to evolve it into a mesh by adding crossing

links, and breaking the ring into smaller rings as described next.

Mesh topologies include many redundant links to provide multiple connectivity paths

between nodes in the network. They are favoured for military communications as they are

tolerant to loss of nodes and links, making them robust in a battlefield environment. More

recently, Wi-Fi mesh networks have been proposed for municipal areas, as the variety of

connectivity path options helps to improve coverage over a Point-to-Point topology.

Latency can become an issue for nodes several hops away from the PoP, an issue

exacerbated if intermediate nodes have only a single radio and have to store and forward

the through traffic. Capacity-wise, links near to the PoP where traffic concentrates can

present bottlenecks and so ranges should be planned accordingly with shorter links

nearer the PoP. It is also worth considering the antenna issues faced at higher

frequencies and the challenges that this will present when installing each node.

Multipoint topology is what has always been used in the access network itself: a central

hub shares its capacity amongst a number of terminals. Since traffic demands are bursty,

it is not sensible to permanently allocate a fixed resource to each small cell that may only

need it occasionally. It is more efficient to pool resources across a larger number of cell

sites and average out any difference in traffic demand at different times of day. The result

is a much higher utilisation of the spectral resource, and ultimately more useable capacity

per Hz of spectrum. It is not only the capacity that is shared, but the equipment too. Like

access networks, hub sites use sector antennas which provide coverage over a large

surrounding area, each serving many small cells. This has the added benefit of halving

installation costs, since only the small cell end of the link needs to be visited to connect up

the backhaul to the hub.


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5 Economics of small cell backhaul

A Total Cost of Ownership (TCO) analysis undertaken by analysts Senza Fili

19

shows thatmultipoint microwave is particularly cost-effective compared to PtP microwave in high-

density environments for two main reasons: (1) it efficiently leverages gains from traffic

aggregation; and (2) it allows operators to buy, install, and operate less equipment as the

hub equipment is shared across multiple RAN locations. Multipoint gives operators a 49

per cent cost saving over PtP microwave in an LTE macrocell deployment over five years.

The cost savings over E-band are a comparable 47 per cent. A leased fibre solution may

cost seven times as much as a multipoint solution depending on where the network is

being deployed due to the high recurring costs for leased circuits.

However, another interesting dynamic is the rising cost of transporting data on a cost permegabyte basis over small cells. As they begin planning for small cell deployments,

mobile operators need to understand the business case. Installing a small cell is easier

and cheaper than a macrocell, but operators will have to install many more small cells

than macrocells, and that changes both the cost dynamics and the operational processes.

Figure 6 shows the cost per Mbps of capacity for LTE macrocells and small cells, for

different technologies. While in both scenarios - macro and small cells - multipoint

solutions are less expensive because of the more efficient use of equipment and

spectrum, the cost on a per Mbps basis is much higher for small cells. This is because the

combined capex and opex for each cell site is comparable for small and macrocells, butthe capacity requirements for small cells are lower.

Figure 6: Comparison of backhaul costs over a five year period for a macrocell (three

sectors) and a small cell (one sector) LTE deployment (Source: Senza Fili)

19Crucial economics for mobile data backhaul, Senza Fili, December 2011 http://cbnl.com/resources/crucial-

economics-mobile-data-backhaul
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5.1 Total Cost of Ownership (TCO) factors

To undertake a TCO analysis of a small cell network prior to the business case being well

understood for street-level deployment, a series of assumptions need to be made

regarding key operating expenditures.

The factors that affect TCO for a backhaul network to the greatest degree are:

Capex

1. Equipment numbers and purchase value

2. Installation cost

3. Site acquisition planning and preparation costs

Opex

1. Spectrum cost

2. Energy costs

3. Site space rentals (antenna space rental, cabinet rental)

For lower-capacity solutions the ability of the backhaul architecture to aggregate traffic

can have significant impact to reduce transport cost.

The significant reduction in the quantity of the equipment deployed in a multipoint

backhaul network reduces equipment purchase and installation costs (capex). It also

affects the site rental costs although the impact of this may vary somewhat with some

street furniture deployment business models.

The business case for outdoor, street-level deployment of small cells is still in its infancy.

Several models appear to be emerging including:

Rental per piece of equipment mounted

Rental per site

Free rental of the site in exchange for the provision of free services (e.g. metro

Wi-Fi)

Self-build where operators acquire space for and then build their own piece of

street furniture (dedicated mast solely for the purposes of building a small cell

site) Replacement of existing street furniture (lamppost) with a new one designed to

incorporate a small cell. This is usually paid for by the operator

Operators have been following the self-build approach for several years now with mini-

cells but it is time consuming and costly. Interest in using street furniture for deployment is

very strong with the main attraction being availability and the simplicity of dealing with a

reduced number oflandlords. Regardless of what business model is used for acquiring

sites, the architecture of the backhaul solution deployed is crucial with the time to deploy,

the energy required to run the sites and the spectrum required for the backhaul solution

key opex considerations.

Multipoint also reduces the amount of spectrum required through its superior ability to

aggregate (statistically multiplex) packet data traffic9. What is more, the benefit increases


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as the number of sites multiplexed together goes up, giving more opportunities for a peak

from one site to cancel a trough from another. So unlike any other form of microwave

backhaul, multipoint microwave becomes more efficient as networks become denser.

This latter characteristic also impacts the last critical parameter: the cost to carry a

megabyte of data. Many operators and vendors are anticipating that the initial capacity of

LTE small cells will be lower than the average LTE macrocells deployment (single sector

vs. multi-sector deployment). With the capacity of the small cell lower than a macrocell,

the cost to carry a megabyte of traffic tends to be higher decreasing as the capacity of

the small cells increases. This can be seen in Figure 7.

As traffic builds on a small cell network, cost of transport drops with all solutions. The

trend-line shows how a multipoint architecture delivers better value sooner, from the

moment of installation and scales better as traffic grows through its aggregation benefits.

From an investment point of view, the Net Present Value (NPV) or measurement of

whether an investment is worthwhile is superior for multipoint backhaul because it delivers

a better business case over the network lifetime.

Figure 7: Small cell cost per Mbps of traffic carried on a five year TCO basis

The superior capital and operating expense characteristics and aggregation benefits of

multipoint microwave architectures ensure that the cost per Mbps carried is lowest across

the widest range of data rates; something not easily matched by PtP architectures.


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6 Cambridge Broadband Networks vision for small cell

backhaulCambridge Broadband Networks Limited (CBNL) is a supplier of multipoint microwave

mobile backhaul. This is a mature technology that has been refined over ten years of

deployment across the world. Today there are over 50 deployments of CBNLs VectaStar

product backhauling live networks.

Multipoint microwave is fundamentally well suited to small cell backhaul

High-capacity

Wide bandwidths are available at microwave frequencies, and capacity is pooled

between a large number of bursty small cells

Rapid rollout

Multipoint spectrum is normally area licensed so does not require regulator

frequency assignment for every new link

New small cell sites can be added without a visit to the hub site

Low-cost

Multipoint topology improves spectral utilisation, reducing bandwidth needed for a

given number of small cells at a given level of performance

Shared antennas and radios at the hub site means costs can be amortised over

the many small cells supported.

Line of sight (LoS) coverage in urban environments

As with all microwave based solutions, LoS is required for links to function with the

desired quality of service. Figure 8 shows the coverage to small cells at street level that

can be achieved with a LoS from two hub sites in a dense urban environment. This plot is

based on a ray tracing on a high-resolution 3D building database, which accurately

predicts LoS conditions. Streets radiating out from the hub are well covered with the

highest modulation.

Figure 8: Street level coverage with line of sight only (the ubiquitous rooftop coverage is not

shown for clarity)


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Optimised physical design for small cell deployment

Form factor is much more important for street level small cell equipment than it is for roof-

top macrosites. The equipment has to be small and light for rapid unskilled installation as

well as unobtrusive from a planning perspective.

Zero footprint designs will be essential for small cell site locations on street furniture.

Figure 9 shows the Cambridge Broadband Networks VectaStar Metro small cell backhaul

solution and Figure 10 illustrates how this could be deployed as part of a small cells

strategy in a dense urban location. The VectaStar Metro is a new range of outdoor units

designed specifically for street coverage installation on lampposts or other urban

structures.

Figure 9: Small cell backhaul unit

flexible and convenient to deploy

Figure 10: Computer generated illustration of how small cell backhaul units could be

deployed as part of a small cells strategy in a dense urban location


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7 Conclusion

The decision by operators to construct a small cell network is clearly going to need carefulconsideration of the backhaul strategy employed. The sheer number of sites considered

could cause the cost of backhaul to escalate rapidly unless the right architecture

combining efficient use of resources, low recurring costs, high-capacity and rapid flexible

deployment is selected. The obvious selections may not yield the best return on

investment and the cost to carry a Mbps of traffic is a crucial parameter if data services

are to maintain profitability. The best approach to small cell wireless backhaul may be

delivered by developing new strategies for the coexistence of multiple backhaul solutions;

fibre where practical and cost-effective, high-capacity multipoint microwave wireless

everywhere else using an effective selection process for the best suited solution at each

small cell deployment.

For more information on Cambridge Broadband Networks small cell backhaul

solutions and planning tools, please contact:

Cambridge Broadband Networks Limited

Selwyn House

Cambridge Business Park

Cowley Road

Cambridge

CB4 0WZ

United Kingdom

T +44 1223 703000

F +44 1223 703001

[email protected]

Wwww.cbnl.com
mailto:[email protected]:[email protected]:[email protected]://www.cbnl.com/http://www.cbnl.com/http://www.cbnl.com/http://www.cbnl.com/mailto:[email protected]

Easy Small Cell Backhaul Feb 2012

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