Q4 2012 SOUTH ASIAN WIRELESS COMMUNICATIONS 23

The changes in the telecommunicationsconsumer market, mainly the proliferationof smartphones and tablets with

associated requirements for higher speed, aredriving the adoption of new access technologies,such as high-speed 3G (HSPA and CDMA2000)and LTE. The introduction of the iPhone in 2007radically and irreversibly changed the game whiletablets such as the iPad and Android OS havefurther contributed to a dramatic reshuffling of themobile market toward more data-driven traffic1.

This has led to the decline of copper T1(1.55Mbps) and E1 (2Mbps) lines in mobilebackhaul, since their speeds became inadequate for backhauling faster 3G and LTE signals.Stacking multiple T1/E1 lines to increase theaggregate capacity is not an economically viablesolution, and so the replacement of copper inmobile backhaul therefore appears to be imminent.

The steady displacement of copper over the nextfew years is also confirmed in a report publishedby Infonetics. It says that copper will constitute“five per cent” of worldwide backhaul connections

by 2016 – the rest will be almost equally dividedbetween fibre and microwave2. Indeed, these areunanimously considered to be the technologies ofthe future to support an increasing number ofbandwidth-hungry services.

The co-existence of fibre and microwave is drivenby two main considerations. On the one hand, fibreis generally regarded as a preferred medium due tovirtually unlimited capacity (hence complete future-proofing), long reach, and guaranteed serviceavailability. On the other hand, microwave is oftenviewed to be cheaper and faster to deploy, especially

in areas where optical fibre cable deployment is anexpensive and lengthy process. So do theseperceived advantages and limitations of fibre andmicrowave represent the reality?

Capacity

Fibre does indeed provide a very large capacity. Asan example, long-haul commercial optical transportsystems (like those used in the mobile networkbackbone) can provide up to 8.8Tbps of bidirectionalcapacity per fibre pair. In mobile backhaul, thecharacteristics of fibre transceivers are typicallygoverned by Ethernet standards to obtain the desiredcost-effectiveness (USD40-300 for a 1Gbps Ethernettransceiver, and USD500-2,700 for a 10Gbpstransceiver3). Higher speed 40Gbps and 100GbpsEthernet transceivers are also available from somevendors at a higher price. Further increases incapacity can be achieved by lighting multiple fibrepairs in the cable, therefore making fibre a future-proof medium for mobile backhaul.

It must be noted that over the years, tremendous

When it comes to backhauling high-speed cellular networks,should operators choose fibre or microwave? DR. SERGEYMAKOVEJS discusses the pros and cons of each technology.

Wired or wireless?

Dr. SergeyMakovejs,Marketdevelopmentmanager, Corning Optical Fiber

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progress has been achieved in increasing the capacityof point-to-point and point-to-multipoint microwavesystems. While traditional microwave platforms oper-ated at approximately 155Mbps, the latest generationof equipment can reach 1-4Gbps (depending on theconfiguration and the technology used). Althoughthese capacities still lag behind those achievable byfibre, 1-4Gbps is sufficient for backhauling 3G andLTE signals from the cell tower. However, as wemove closer to the backbone network, higher capac-ities may be needed to accommodate the aggregatetraffic from multiple cell towers (see figure 1 below).

The likely path towards increasing microwavesystem capacity is to adopt a higher-density modu-lation scheme (such as 4096-QAM), ‘4x4’ MIMO,and to utilise larger frequency bands. But the fullimpact of applying those techniques is unclear:it is likely that denser modulation will reduce thelink budget, causing reduction in reach, serviceavailability or both; ‘4x4’ MIMO will contribute to the system cost due to the increased number ofantennae; and larger frequency bands may increaseopex due to higher recurring spectrum costs.

Reach and service availability

Fibre backhaul can provide a guaranteed reach of ~100km, which is sufficient for most mobilebackhaul connections. Similarly to the data rate,the reach is determined by transceiver grade (seefigure 2). Fibre also provides guaranteed serviceavailability where the backhaul can be designed toprovide both main and protection routes in orderto provision for a potential cable cut.

Contrary to fibre transmission, microwavetransmission experiences frequency dependentimpairments in the presence of rain, with E-bandspectrum (i.e. 70-80GHz) affected more thantraditional frequencies (6-38GHz). This means thatfor the same capacity and service availability, themicrowave transmission will have a lower reach in areas with frequently occurring monsoons,compared to less rainy regions.

In practice, radio planning engineers prefer to set the required reach and service availability, and let the system adaptively switch the modulationbetween high spectral-efficiency (high capacity) andlow spectral efficiency (low capacity) as the rainintensity increases. Thus, there is no guarantee that

the desired capacity is achievable at all times,potentially leading to disruption of some real-timeapplications such as video transmission.

Costs

For both fibre and microwave backhaul, the overall costs consist of several components. Formicrowave, operators will need to invest in: radioequipment; building a tower to mount the outdoorequipment (around USD50,000); building anaggregation hut in the case of a split-mountequipment configuration (around USD100,000-150,000); and recurring spectrum costs.

For fibre backhaul, the costs include: transceivers(almost negligible); the fibre cable; cable deployment;and building the aggregation hut which is the sameas microwave. The cost of deployment is one of thelargest contributors to the overall outlay, and consistsof right of way costs as well as the physicaldeployment. While the former is governed by theregulations within a specific country (and is typicallyoutside the control of a mobile network operator),the latter could be controlled through the selection of a particular fibre cable deployment technique.

Microtrenching, horizontal directional drilling(HDD) and manual trenching are well suited forurban areas, and cost approximately USD50,USD37 and USD25 per metre respectively. Theterm ‘microtrenching’ is used to emphasise thesmall size of trench in the ground where the cableis deployed, and is typically in the range of 1.5-5cm in width and 20-40cm ‘in depth’. HDD on the other hand, is a completely trenchless cabledeployment method. Here, an inner duct is placedin a drilled path and the cable is subsequentlypulled through. Both techniques are extremelyeffective in areas where significant disruption ofsurface is undesirable. However, if cost is the mainconcern, manual trenching could be used instead.

In rural areas, more cost-effective techniques, such

as direct cable burying (plouging), machine trenching(in areas not suitable for ploughing), and aerialdeployment could be used instead and at a cost ofapproximately USD7, USD14 and USD4 per metre,respectively. The exact cost will depend on othervariables, such as soil condition, type of terrain, etc.

In general however, the cost of fibre versus micro-wave backhaul must be considered on a country-by-country and project-by-project basis. There are manycases when microwave is cheaper to deploy thanfibre and should therefore be used in the backhaulwhen cost is the main concern. However, microwavemay not always be the cheaper option – particularlyin areas where strong rains necessitate the use ofmultiple microwave hops to achieve the desireddistance, thus requiring the build-out of frequentlyspaced backhaul towers at a higher overall cost.

Another aspect that needs to be accounted forwhen building a mobile backhaul network is powerconsumption. Two Ethernet transceivers (requiredfor bidirectional transmission) consume around 2W,compared to a few tens of Watts for an equivalentoutdoor microwave transmit-receive system. Whilethis difference per backhaul link may appear to besmall, it quickly escalates for the whole networkwith hundreds of backhaul connections. Thedifference becomes even more dramatic for an LTE network, where more cells (and hence morebackhaul connections) are needed to cover a giventerritory compared to a 3G network due to thelower coverage radius of LTE cells.

Finally, the presence of fibre in the vicinity mayaffect the operator’s decision to deploy even morecable. For example, fibre could be a preferredmedium for small cell backhauling if it already has a point of presence in a building. In this case,a small cell could be placed on the building wall,so that the fibre infrastructure could be sharedbetween fixed and mobile services.

Overall, the industry consensus is that microwaveand fibre will co-exist in mobile backhaul, at leastover the next several years. There is no single goldenrule that will determine when to use fibre and whento use microwave. Instead, all considerations such ascost, required capacity, distance, service availability,future upgrades and regulatory environment need to be taken into account. However, the growingdemand for even higher bandwidth in the access part of the network is likely be followed by the adoption of next-generation LTE Advancedtechnology, therefore pushing fibre even deeper intoaccess to satisfy the required backhaul capacities. !References1 Internet, mobile and the cloud – a dynamic convergence,Coady Diemar 20122 Mobile backhaul equipment and services, Infonetics 20123 http://datainterfaces.com/Fiber_Transceivers.aspx

24 SOUTH ASIAN WIRELESS COMMUNICATIONS Q4 2012

FFiigguurree 11:: Schematic diagram of a mobile backhaul network. Thick lines illustrate the fact that higherlink capacity is required as more signals are aggregated closer to the backbone network.

FFiigguurree 22:: Cost in USD of fibre Ethernet transceivers for different data rates and different reach (from aparticular distributor, for example)3.

10km rated 40km rated 80km rated 120km rated

1Gbps 39 131 161 275

10Gbps 445 1,255 2,695 -

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LLooww ccaappaacciittyyFibre – yes

Microwave – yes

MMeeddiiuumm ccaappaacciittyyFibre – yes

Microwave – maybe

HHiigghh ccaappaacciittyyFibre – yes

Microwave – no

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