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Introduction

As data capacity in networks continues to rise at an exponential rate, mobile operatorsare looking at new architectures that can help them reduce cost, simplify networks andshare resources to match the dynamic nature of mobile networks. Power and space are ascarce resource at cell sites and the total power bills for these operators are considerable.They therefore look for every possibility to reduce these ongoing power costs and spacerequirements and enable more dynamic use of network resources and spectrum.

One recent trend by mobile operators to address these requirements is the move to Consolidated-RAN or Cloud-RAN, (both

C-RAN), architectures. C-RAN involves moving some parts of the radio network control function from being co-located

with the antenna at the cell site, to locations deeper in the network and introduces a new transmission network into the

overall mobile network infrastructure Mobile Fronthaul. This new C-RAN architecture provides great benets in controlling

ongoing operational costs and also greatly

increases the exibility of the network.

This application note will briey look at the

C-RAN architecture and the challenges this

brings in the transmission network. It will thenconsider some options from Transmode and

the application of these choices in C-RAN and

Mobile Fronthaul networks.

Transmodes strengths in synchronization

and transparent data transport bring unique

advantages to mobile and wholesale operators

rolling out C-RAN and fronthaul networks,

with multiple options to address all networking

requirements.

Mobile FronthaulTransmodes unique passive, semi-passive and active options enable

mobile operators to migrate to Cloud-RAN architectures

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Mobile operator challenges and themigration to C-RAN

As the demand for network traffic grows, network capacityhas to grow to match this demand. Newer technologies

allow greater capacity per cell site but this increase comesat a cost beyond the capital cost of the new equipment.Power consumption in cell sites is becoming a greater andgreater portion of the overall network cost and operatorsare looking for methods to address this growing cost.

Data published by China Mobile shows that over a 7 yearperiod, 60% of the total cell site cost is OPEX costs versus40% for initial CAPEX. Power costs are approximately1/3rd of this OPEX cost, therefore about 20% of the overallcell site cost over that 7 year period. Also, the publisheddata from China Mobile shows that this cell site powerrequirement is over 70% of the overall network power

requirement. Overall, cell site power consumption is aserious factor that needs addressing.

The migration to ber interconnected antennashas key valuesOne approach that operators have taken to addressthis challenge is to migrate from coax to ber basedinterconnections between the Baseband Unit (BBU), whichperforms signal processing functions and creates the radiosignal and the Remote Radio Head (RRH), which convertsthe radio signal into an RF signal.

With traditional copper interconnection, the BBU and RRHare collocated within a cabinet in the cell site and a coaxcable is used to connect the RRH to the antenna at thetop of the cell site. With ber based interconnection theRRH is collocated with the antenna at the top of the cellsite and interconnected to the BBU in the cabinet using aDigital Radio over Fiber (D-RoF) protocol such as either theCommon Public Radio Interface (CPRI) or the Open BaseStation Architecture Initiative (OBSAI) protocols. The useof an optical interface is a much lower power consumptionapproach, especially at higher data rates within the cell.

One key benet that the migration to ber basedinterconnection between the BBU and RRH is that it allowsfor the possibility of longer reach optics and then alsomoving the BBU from the cell site back into the network,centralizing stacked BBUs at a central office location. Thishas advantages for the network operator.

First, it further reduces OPEX by reducing the overall powerrequirement of the network and also reduces the spacerequirement within the cell site, which becomes increasinglyimportant as more and more antennas are added to existingcell site locations.

Second, from a networking perspective, the move tocollocated BBUs greatly simplies the X2 interface betweenBBUs in LTE networks and also increases security over theBBU to RRH link, removing the need for IPsec.

This move to centralized BBUs creates a new domain withinthe mobile network. The network between the BBU and thecore network is still the mobile backhaul network and thenew network between the BBU and the RRH in the cell site isreferred to as the fronthaul network.

A further step that some operators are planning orimplementing is to combine the stacked BBUs at the centraloffice into a single larger BBU with load-balancing. Thisenables a true cloud-RAN where capacity/spectrum canbe load balanced across a number of antennas to allowresources to closely match demand at differing times of theday at the varying locations covered by these antennas.

This also brings further advantages to the network operatorsuch as simplifying mobility management as users movebetween cells. It also further lowers the cost base of the

network with less total BBU capacity requirements andalso better optimized backhaul with less backhaul capacityrequirements.

Making Fronthaul a network

Both the options above where the BBU moves from the cellsite to a central office require a fronthaul network. This cantake many forms. The simplest option is a dedicated berper RRH running the CPRI or OBSAI protocol.

The CPRI and OBSAI protocols are dened with a rangeof speeds from 600 Mbit/s to 10 Gbit/s. While 3 of these

rates closely match the 1G, 2.5G and 10G line rates of WDMoptics, recent advances in optics options now enable the

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Fig. 1 Fiber based antennas allow the Remote Radio Head (RRH) to

be placed with the antenna, bringing advantages such as less powerconsumption at cell sites.

Fig. 2 The Mobile Fronthaul and Mobile Backhaul within a mobile

network.

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full range of protocol rates to be supported over WDM. Thisenables the possibility of adding WDM technology to thefronthaul network. This allows for better use of the availableber and adds networking capabilities such as manageabilityand protection, which become increasingly more importantas these networks grow in capacity and reach.

However, the fronthaul network and the CPRI/OBSAIprotocols have very stringent requirements that need veryspecial consideration. These protocols are extremely latencysensitive and this often is the overall limiting factor in howfar the network can extend. This is particularly the case withthe higher speed CPRI/OBSAI options that are required fortodays high capacity mobile networks.

Fronthaul networks also require that the signalsynchronization is transferred transparently which is aparticular consideration if active WDM systems are used.Additionally, consideration is required for the space and

power requirements of any networking solution as space incell sites is extremely limited and a large driver behind thewhole change is reduction in power consumption.

Fronthaul networks may appear to be relatively simple,but due to the need to meet these strict latency andsynchronization requirements and to support the CPRI andOBSAI protocols, these networks are actually quite complexand consequently the networking options available in themarket are limited.

Transmodes Mobile Fronthaul solution

Transmodes packet-optical solutions are built on twokey technologies; Ethernet and WDM. Transmodes WDMsolutions have specic strengths from a heritage in metroand regional networking which are highly applicable infronthaul networks. These include compact and low powersolutions, passive and active WDM options, ultra-lowlatency and superior synchronization performance.

Fig. 3 Transmodes Mobile Fronthaul solution presents key valuesin low or even no power consumption, superior synchronizationcapabilities and extremely low latency.

These capabilities allow Transmode to offer four mainoptions to mobile operators or wholesalers looking to buildfronthaul networks. These include a passive WDM option,a semi-passive WDM option and two unique active WDMoptions; one based on transparent transponders and onebased on framed muxponders allowing for multiplexingof lower speed CPRI/OBSAI and additional managementcapabilities.

Fig. 4 Transmodes Mobile Fronthaul solution offer four options; a passive WDM options, a semi-passive WDM option and two differentactive WDM options ; one based on transparent transponders, theother on framed muxponders. All options ties smoothly into theMobile Backhaul solution.

Option 1 - Passive WDM; simple networking withzero power consumptionPassive WDM is the simplest way to add WDM networkingto a fronthaul network. Transmodes TG-Series is a widelydeployed passive WDM platform that has characteristicsthat make it a leading solution in this area of networking.

The TG-Series has an extremely broad range of networkingoptions including CWDM or DWDM, single ber or ber pair,point-to-point or ring architectures using terminal or add/drop lters and scalable capacity up to 80 wavelengths.

The platform also offers outstanding optical performancewith excellent optical specication (insertion loss etc),upgrade ports allowing simple hitless capacity expansionwithout loss of spectrum/channels and monitor ports toassist fault nding.

The TG-Series supports an extended temperature range anda range of mounting options for racks, street cabinets orsplice chambers, allowing deployment outside traditionaltelco-environment locations.

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Administration and Maintenance (OA&M) capabilities,Transmode has extended the passive option to includeactive monitoring capabilities.

These capabilities address three main areas of functionality;resource monitoring, optical channel monitoring and link

status monitoring. All these performance statistics can bemonitored in Transmodes multi-layer management suiteEnlighten.

Resource monitoring uses the inventory capabilities andchannel use information to allow the operator to quicklyand simply understand the usage of the network and addnew services.

Optical channel monitoring uses the TM-Series OpticalChannel Monitor (OCM) to monitor the optical power levelsof individual wavelengths. Offset and degradation alarmscan then be set to alert the operator to problems in theoptical BBU to RRH connection that is running over thesemi-passive network.

Link status monitoring uses a spare wavelength to loop awavelength from the BBU site to the remote cell site andback to monitor the status of the link. This wavelengthcan either be one of the unused C/DWDM wavelengthsor 1310/1625 nm ports that are available on the TG-Serieslters.

The wavelength is then connected to a TM-Series controlunit and the wavelength status is monitored by themanagement system. This enables the operator to bealerted in the case of a ber cut or some other network

issues if the link status wavelength is lost.

Fig. 6 Transmodes semi-passive Mobile Fronthaul providing activemonitoring capabilities to allow more advanced OA&M.

One important advantage of this approach is that as theremote equipment is all passive, it is possible to detect otherexternal fault conditions such as cell site power failure. Inthis scenario, the remote cell site equipment may all failcausing alarms in those systems and in the Transmode OCMleading operators to assume that perhaps the ber has beencut. But here the link status monitor wavelength wouldremain up, showing the operator that the ber isnt cutand another issue has caused the loss of all systems, suchas cell site power failure or an environmental issue such asooding.

For mobile fronthaul, 16 channel CWDM or higher capacityDWDM can be used depending on the specic requirementsof the network. In this case C/DWDM SFPs supporting theCPRI/OBSAI protocol are used directly in the RRH to providethe necessary WDM wavelength signal. These also need tosupport the broader Industrial Temperature (I-Temp) rangedue to the range of temperatures that can be experiencedin the antenna locations. The use of these SFPs necessitatesthat the RRH vendor provides these as an option, or allowsthe use of third party SFP units.

Once the RHH is equipped with WDM optics then theTG-Series components can be used to provide the bestnetworking options. Furthermore, it optimizes the availableber with multiple RHH to BBU connections sharing thesame ber(s) in either ring or point-to-point architectures tosave on the amount of bers needed in the network.

All these options are extremely compact and are totally

passive requiring no power, thus helping the networkoperator with their cost reduction goals. Passive WDMnetworks can support optical paths of up to approximately80 km and are therefore highly attractive for mobilefronthaul networks.

Fig. 5 Transmodes passive Mobile Fronthaul featuring the TG-Series based passive WDM option enabling low or even no powerconsumption.

As the RHH to BBU connection is now over a long distanceof outside plant ber, consideration should be taken toprotect this against ber cuts and other faults. Some purelypassive WDM systems just comprise of optical lters andare not able to address this challenge. But the TG-Series canaddress this challenge with the Fiber Protection Unit.

This unit splits the signal into two identical signals which

can then be routed around diverse routes around thenetwork to the far end where a second unit recombines thesignals and switches between them in case of a networkfailure such as a ber cut. This can be used on both point-to-point and ring architectures.

Option 2 - Semi-Passive WDM; adding monitoringcapabilitiesOne potential drawback of a purely passive approach isthat by the passive nature of the solution there is limitedmanagement capabilities beyond inventory managementthrough the Enlighten management suite.

To address the requirement for more advanced Operations,

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The practicalities of deployment; mixing up fronthaul and backhaul

Transmodes passive TG-Series and active TM-Seriesplatforms have many factors that make them particularly

suitable for mobile fronthaul networks but deployment offronthaul is often not as simple as looking at the fronthaulin total isolation within the wider network.

When looking at a specic geographic area that requiresmobile fronthaul, there will be traffic over the same regionthat is traditional mobile backhaul traffic.

Some cell sites will maintain the traditional RRH/BBUcolocation while others migrate to a fronthaul architecture.Some will be macro cells while others are small cells oraggregation points for small cell traffic. Even if a completenetwork migrates to a fronthaul architecture the cell sitesfurthest away from the core nodes will have a fronthaulnetwork to a central office. This fronthaul network is thenbackhauled over the same network as the fronthaul trafficfor those nodes that are closer to the core.

Of course it is possible to totally separate fronthaul andbackhaul networks with completely separate infrastructure.But this is unnecessary and more costly.

This application note will not discuss Transmodes mobilebackhaul solutions in detail as they are well documentedelsewhere (see further reading). Both the mobile fronthauland the mobile backhaul solutions are managed withthe same management system; Transmodes multi-layer

management suite Enlighten.One key advantage of the packet-optical approach used inTransmodes Ethernet based mobile backhaul solution isthat there is no expensive centralized switching fabric. Thenetwork is built up one wavelength at a time and there is nodependency or cost implication of mixing different traffictypes within the same infrastructure. One wavelength canbe carrying Layer 2 Ethernet mobile backhaul traffic, anotherLayer 2 Ethernet Switched Video Transport traffic while athird could be carrying mobile fronthaul.

Fig. 9 Mobile Fronthaul and Mobile Backhaul networks overlapping.Transmode has solutions in both areas, providing synergy benets.

Transmodes Mobile Fronthaul solution can carry legacymobile backhaul traffic as alien wavelengths.

In conclusion

Cloud-RAN and mobile fronthaul are emerging technologytrends that have a great potential in helping address theongoing rapid growth in mobile traffic. Optical networkingand Transmodes solutions in particular can play animportant role in meeting the goals of these networkarchitecture initiatives. However, the technical challenges inmeeting the strict latency and synchronization requirementsof these networks makes this difficult to achieve for manysystems vendors.

Transmodes strengths in signal and synchronizationtransparency and broad passive and active WDM productrange provides network operators with a robust toolkit toaddress the challenges presented by these networks.

Transmodes solutions provide passive, semi-passive andactive options each with unique advantages to the networkoperator:

TM-Series based active platform enhanced networkingand management capabilities, superior sync design andultra/low-latency options.

TG-Series based passive and semi-passive options simpleand compact no/low power design, totally transparent,traffic protection option, scalable with ring or point-to-point architectures with enhanced OA&M capabilities viathe semi-passive option.

And nally, once an option is chosen, it can be easily mixedwith Transmode or legacy mobile backhaul if required.

For further reading: Ethernet Mobile Backhaul Application Note

Flexible Optical Networking Application Note TM-Series web page TG-Series web page

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The specications and information within this document are subject to change without further notice.All statements, information and recommendations are believed to be accurate but are presentedwithout warranty of any kind. Contact Transmode for more details.

www.transmode.com

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An Mobile Fronthaul D