3g network design

8/3/2019 3G Network Design

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1. Prerequisite for a 3G network design

At the best of times, designing a cellular network is like doing a puzzle without anyinstructions. With 3G, you have the added complication of both the operator andcustomers holding back some of the pieces and planners having to force pieces to fit

together because the edges are a bit rough!

Getting the required information for a network plan is the most crucial part in building acost effective quality network. Unfortunately, some of the necessary data is confidentialand not only that, it's anybody's guess as to what the 3G mobile service mix and usagewill be. In an environment where operators need comprehensive designs and redesignsin a very short time frame, there's plenty to get jittery about. Other vendors are rumouredto give better coverage with fewer sites, operators claim that other vendor's products aresuperior and vendor sales people seem to be promising everything to get the deal. Ontop of that, timetables keep on changing, but of course, the network launch date remainsfixed. For the vendor, the worst thing is that once you've won the contract, you actuallyhave to build the network you promised!

A lot of different information from various sources is needed for initial network (roll-out)plan. Here is a non-exhaustive list of required data:

Operators business planThis should define what kind of service the operator is planning to provide, how theseservices will be implemented and how much money is needed for the total roll-out.Sometimes this information is public knowledge and sometimes it is a well guardedsecret.

Technical section of business planThis should contain the desired coverage, capacity, quality, features, service mix andcustomer intake plans.

UMTS License agreementThis usually contains the coverage, capacity and service deployment plans as well asrequirements to hire a predetermined amount of employees and perhaps the requiredamount of domestic goods and services that need to be purchased.

Operators funding planThis should give guidelines of how the roll-out should progress. However, usuallyoperators do not want to share this information. Quite a few UMTS networks are vendorfinanced and such information could help network planners estimate the roll-out pace.

Operators risk analysis documentsThese documents show where bottlenecks will be and show the project's critical path.Often site acquisition is in the critical path, which means that site RF planning will haveto compromise some of the desired sites. However, technically the air interface capacityis normally the network limiting capacity factor and so network roll-out planning shouldreally be started from there.


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Consultant reportsOver the past few years, operators have asked a lot of consultants to do 3G roll-outanalysis reports. Even though this information gets old quickly, these reports might havesome helpful facts.

Operator's internal studies of mobile usage

Information on mobile usage is very helpful and can pinpoint where 3G customers andkey corporate clients are likely to be. Also, whether the 3G network will be used as aplatform for other wireless technologies. Mobile usage profiles and customer distributioninformation is required to simulate the network load.

Government statisticsGovernment sources can provide statistics of population type and information such asincome, distribution of wealth, taxation, spending habits etc., which are useful toestimate future mobile usage in different areas.

In a real life all of this information is compressed into the operators request for quote.This typically calls for an estimation of how many base station locations each networkvendor thinks is required to provide a network. The operator normally asks vendors toguarantee the level of coverage for a certain load level, using the minimum amount ofbase stations and cost. Vendors have to commit to these figures even when most of thesites are yet to be acquired and some of the performance parameters will be definedlater. Vendors are expected to reply in a very short period of time with limitedinformation, so it is easy to see why network quality is not the biggest consideration ininitial planning. There is a tendency for operators to use this tactic to get the lowestpossible initial quote from vendors.

An experienced network planner can produce a "quick and dirty" network base stationrequirement figure with only a few parameters. The most crucial parameters for the initialroll-out are:

Capacity requirements - the planned customers and service usage in each area ofthe network (with BTS site capacity calculation) should be known in order to get therequired amount of base stations for capacity.

Coverage requirements - the link budget of high data rate services should becalculated in order to estimate the required base station amount in each network area toget the amount of base stations for coverage.

In each network area, take the larger number of capacity OR coverage base stations forthat area, then add each area together to get a total. To get the final required number of

base stations, the following formula can be used:- Add 10% more quality sites to provide special coverage or a dominant server indifficult or important areas - tunnels, bridges, exhibition and sports venues, shoppingcentres, airports, big hotels, high rise buildings, MD's home and CEO sailing and poloclub!- Add additional 10% more sites to fix holes because not all planned sites can beacquired.- By this time the sales team will tell you that your plan is 30% too expensive, so youneed to cut 30% of your base stations (and 40% of your acquisition budget)


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The total will tell you how many sites you need to build an initial 3G network. Don't tellyour customer you found the formula on the Internet!

2. Link Budget and Coverage

The WCDMA link budget calculations start from the uplink (reverse link) direction. Uplinkinterference (noise from other mobiles) is usually the limiting factor in cdma systems.

The starting point of a link budget calculation is to define the required data rate(s) ineach network areas and Eb/No (Energy per Bit to Noise power density ratio) targets.Usually the operator predefines these, but simulation tools can be used to tailor theEb/No. Simulation can be done by creating a uniform base station and a mobiledistribution plan with defined service profiles. Almost every UMTS vendor has asimulation tool for operators to test their network plan models.

The next step is to gather vendor specific data like a BTS output power and a receiver

noise figure, defined and used cable systems (thicker the cable, more expensive it is toinstall), used antenna types, usage of intelligent antenna systems in specific areas,possible additional line amplifiers, used diversities (like antenna, polarisation, receiver)etc.

For each geographical areas network operator has to define Eb/No, data services, asystem loading factor, estimated mobile speeds, different penetration losses, coveragereliability and a used fade margin. Soft handover area sizes will be addressed later.

Mobile power levels, the chip rate and the process gains are defined by the UMTSstandards. Soft handover gain and the thermal noise density are the same in everyUMTS system. Both parties also have to agree on propagation models after drive tests.

The link budget gives a cell range and from that cell coverage area can be calculated.Cell coverage overlap parameter is usually missing from the calculation as it increasesthe cell count dramatically. Most network planners agree that overlap should be 20-30percent, but that relates directly to build cost. After all that, the base station requirementsfor the each type of areas can to be calculated.


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Cell coverage calculation using link budget.

This link budget is for an example only. The values for the real network may vary a lot!

Notice how sensitive the site count is to variations of the propagation model and celloverlap values. High speed 3G network needs a lot of base stations!


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3. UMTS Capacity Planning

The number of installed transceivers limits the mobile network theoretical capacity. Incdma systems interference, accepted and planned quality and grade of service will

determine the system capacity. Cdma systems also have soft capacity, whichcomplicates the network area capacity estimations. The link budget is used to calculatethe maximum allowed path loss and the maximum range for cell. The link budgetincludes the interference margin, which is the increased noise level caused by greaterload in a cell. So by increasing the cell load, cell coverage area becomes smaller. That'show cell coverage and capacity dimensioning are interlinked.

System capacity planning is divided to two parts:

The first thing is to estimate a single transceiver and site capacity. Calculations howthe noise raises as the cell load increases is out of the scope of this page, but in-cellnoise, Eb/No requirements, planned data rates, coverage probability, air resourcesusage activity factor, target interference margin and processing gains are needed toapproximate the transceiver and site capacity. Depending on the parameter values,planned transceiver capacity is typically from 400 kbits/s to 700 kbits/s per transceiver.

The second part of the process is to estimate how many mobile users each cell canserve. Once the cell capacity and subscriber traffic profiles are known, network areabase station requirements can be calculated. Estimations can be done in Erlangs persubscriber or kilobits per subscriber. Network vendor normally has simulation tools totest system parameters and verify rough estimations. A lot of data is required forcomprehensive network dimensioning; number of subscribers and growth estimations,traffic / user / busy hour / geographic segment and required throughput including service

mixes in geographic segments for example.

Each type of traffic has to be estimated for capacity calculations.

Here is a rough downlink capacity calculation example:

During a busy hour an average user downloads 10 Mbits with 384 kbits/s, 2 Mbits with144 kbits/s and makes one 60-second voice call. Data has to be retransmitted 1.1 timesbecause of network conditions.


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Used kbits/s per user per busy hour downlink only are:

Service Rate Average Rate

(10000 kbits / 3600 sec) x 1.1*) 3.06 kbits/s

(2000 kbits / 3600 sec) x 1.1*) 0.61 kbits/s

(60sec x 12.2 kbits/s) / 3600 sec 0.20 kbits/s

Total 3.87 kbits/s / user / busy hour

If a cell capacity is estimated to be 500 kbits/s, each cell can be dimensioned for about129 users.

This example was simplified, but please remember when you see capacity estimationswith various traffic mixes, that those are just estimations. Notice how sensitive thecapacity is to variations ofdownload amount, retransmit rate and estimated cellcapacity values. Before UMTS networks are on air and customers start to use highspeed services, network capacity calculations are anybody's guess!

Even when the capacity calculations are done in a very beginning, normally the mobilenetworks are initially planned to meet the coverage objectives. Capacity sites andtransceiver upgrades are installed later, once the real traffic load is known. In the early1990s most of capacity requirements of new 2G networks were initially over-estimated,partly because operators needed to present (over-) optimistic business plans to securethe funding and partly because nobody knew how much people would use their phoneon certain price levels. Generally busy hour Erlangs per subscribers were not what wasoriginally anticipated, and this tendency is likely to continue in 3G.
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Above formula is just a rough guide. The correct formula for is Effective traffic = Usertraffic / (1-p),(where p is the loss probability) considering the fact that retransmissions may take placemore than once

4.Common Design Guidelines

After calculating the coverage and capacity requirements in each geographical area, thegreater one of those two values has to be chosen. Ideally requirements match in eachgeographical area, but normally that does not happen. To optimise the used resourcessome readjustments should be made.

If a geographical area is coverage limited, the load on each sector can be reduced untilcoverage and capacity requirements match. Reducing the load will cut the link budgetinterference margin and increase Node B count. If area is capacity limited, transmitter

diversity can be added or amount of transceivers can be increased.

Operators are normally forced to co-locate their 3G base stations with existing sites orselect new site locations only on buildings known to be owned by friendly site owners.This limits the cell planning options and may sacrifice the network quality, but it helps tobuild networks faster. Forced co-location should be taken in to account in initial capacityand coverage planning. All variation to standard configuration may need pilot power,handover, antenna, cable and base station power level modifications.

Some network areas need special attention. Very dense urban area (CBD), openspaces, in-building areas, water surroundings, hot spots and other special coverageareas need well planned approach. Out-of-Cell Interference versus soft-handover cell

overlap has also to be considered. Hierarchical systems work with multi-frequencynetworks, but not with single-frequency systems (like cdma). If multi-layered is planned,separate frequencies are needed for different layers.

There is no need for plan channelisation codes as those are managed by the RNC.Unlike the channelisation codes, the scrambling codes need to be planned. The numberof scrambling codes is so great that the planning is a quite trivial task.

The uplink scrambling code for each user is allocated by the RNC. Every RNC has apool of codes that are unique to it. Allocation of uplink scrambling codes to RNCs is asimple task, due to the huge number of available codes.

Downlink scrambling codes planning is an important issue in the border areas betweencountries. There is a minimum separation that should be used between cells using thesame scrambling code, but in practice the maximum separation will be planned.

Codes Scrambling codes Channelisation codes

Uplink Different users Channels from the same user

Downlink Different Node B's Different users


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UMTS code usages

5. RAN Planning

Planning the UMTS RAN and core network side is basically selecting the desirednetwork layout, future expansion approach, calculating the required hardware, decidingsoftware features and dimensioning all interfaces.

Radio Access Network has several interfaces, which need to be configured anddimensioned. This dimensioning is out of the scope of this page, but these are the RANinterfaces that need to be configured:

Iu: Interface between the RNC and the Core Network (MSC or SGSN). - Iucs: Iu circuit switched (voice from/to MSC) - Iups: Iu packet switched (data from/to SGSN)

Iub: Interface between the RNC and the Node B.

Iur: Interface between two RNCs.

Node B amount is derived form air interface capacity and coverage calculations, butNode Bs also have to be configured. Hardware configuration is vendor specific, but hereis a general list of things that need to be considered when configuring Node Bs:

Call mix of expected trafficType of Node Bs (outdoor vs. indoor)Amount of low capacity Node BsRequired redundancies (e.g. 2N, N+1)Required diversitiesNumber of carriers per sectorNumber of sectors per Node BNumber of usersVoice and data traffic to be carriedNode B software featuresRequired Node B optional featuresRequirements for special antenna systemsRequirements for power and transmission systems

The RNC planning is done after the air interface dimensioning and network interfacesplanning. After those are prepared, the bandwidth of each RNC link is known. RNCdimensioning is to calculate the number RNCs and configuration of RNCs needed tosupport the radio access network requirements. Any network side equipment will havethe trade-offs in configuration selection. Network can be designed for maximising theease of future expansion or for minimising the total cost. Usually RNC locations are fixedbased on network operator's main site locations and transmission costs will determinethe most cost effective RNC configurations. RNC Hardware configuration is also vendorspecific, but here is a general list of things that need to be considered whendimensioning RNCs:

RNC capacity and configuration options


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Total CS traffic (Erlangs)Total PS traffic (Mbps)Total traffic and signalling loadTotal number of Node BsTotal number of cellsTotal number of carriers

Used channel configurationsRNC software featuresRequired RNC optional featuresType of transmission interfacesExpansion possibilities

6. Core Network Planning

Planning the UMTS core network consist of GSN (GPRS Service Node) system design,MSC and registers dimensioning, OMC dimensioning, Core network interface

dimensioning.

The Core Networkhas several interfaces, which need to be configured anddimensioned. This dimensioning is out of the scope of this page, but these are theinterfaces that need to be configured:

Gn: Interface between SGSN and GGSN Gi: Interface between GGSN and external packet data network

Other interfaces are between MSCs, to PSTN, HLR, AUC, EIR, SMS, Billing Centre,Voice Mail, OMC, WAP & Multi Media Servers and other network elements.

The main inputs to dimensioning of GSN system are similar to what is required in airinterface design. Hardware configuration is vendor specific, but here is a general list ofthings that need to be considered when configuring GSN System:

Number of SubscribersNumber of PDP ContextsService Activation RatePeak Traffic amount and overheads (bits/s or packets/s)Number of required linksNumber of RNC in served area

The Core Network hardware configuration is also vendor specific. Network vendors havevery extensive documentation how to design all aspects of core network starting fromthe room environmental requirements up to the post integration system quality audit.Here is a general list of things that need to be considered when dimensioning MSCs:

Number of SubscribersAverage call lengthsCall mixTotal CS traffic (Erlangs)
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System featuresInterconnection to other equipmentMSC software featuresRequired redundancies (e.g. 2N, N+1)Total traffic and signaling loadsIu-cs and other interface dimensioning

Type of transmission interfacesExpansion possibilitiesMSC capacity and configuration optionsMost cost effective deployment methodNumber of RNC in served area

All other network equipment will be designed in a similar way. Once the maincomponents are designed, the rest is easy!

Other core network plans:

Numbering plan

Traffic and interconnectivity routing planSignalling network planSynchronisation planExpansion planLogical and physical node dimentioningPS and CS network planNetwork security planRedundancy and QoS plan

7. Transmission Planning

Most of the 3G network license holders have an existing 2G license and will face greattransmission planning challenges when upgrading a network from 2G to 3G:

Massive capacity growthControlling of the expansion processLimited microwave spectrum availability and related cost issuesLimited space for new equipment and EMR sourcesCost and budget constrainsRequirements of design scalability and flexibilityImplementation of new technologiesIncreased complexity and multivendor environment

Time table, availablility of skilled manpower

Reliability requirements (climate, redundancy, etc)

2G network transmission capacity planning depends on the amount of transceivers, but3G transmission planning depends on the customer phone usage profiles and othercarried traffic. Where 2G backbones are aggregated small pipes, 3G traffic will be incombined large pipes carrying mixed traffic. 3G project transmission planning include alot of issues:


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Before the 3G Roll-outTraffic type and amount estimationUpgrade and future network layout plan (layers, topology, media)Availability, Operation & Maintenance, spare unit planFault, Configuration, Performance, Security management plansNetwork synchronisation planQoS and system performance specificationDeploying flexible upgradeable equipmentsCurrent network inventory, evaluation and status

During the 3G Roll-outNormal roll-out plans; manpower, logistics, HW, time table etc.Deploying; ATM network, upgraded radios, nodes etc.Links: 15GHz, 23GHz,18GHz,7GHz,26GHz,38GHz,28GHz,13GHz,8GHz,6GHz,

11GHz,32GHz,50+ GHzInitial and continuous system testingContinuous training of staff for equipment and technologyRedundancy (link, network, equipment) plan and testingTraffic prioritising planUsage of other carriers capacity and reselling own capacityWork what vendors call "future proofing"; you better ask them ;-)

After the initial 3G Roll-outTraffic analysis and managementUnderstanding the trafficEquipment performance and suitability analysisUpgrades and updatesPreparing for the next roll-out cycle, long term development plan

Comparing the transmission system to competition and other vendor solutions

Future 3G transmission networks will be largely IP based systems. Topology of thenetwork will be a mess mesh; hybrid of point-to-point, star and ring configurations.Transmission options are still microwave, optical fibre, leased line and even copper line.Addition to current high capacity PDH / SDH networks, higher capacity ATM networkswill be implemented to carry the main load. Internet will be integrated to mobile network(or visa versa).

_________________________________IP = Internet ProtocolPDH = Plesiochronous Digital Hierarchy; point-to-point, time division multiplexing, n * 2Mbits/s (Europe)SDH = Synchronous Digital Hierarchy; typically 2/34/140 Mbits/s, n * 155Mbits/s,SONET in the USAATM = Asynchronous Transfer Mode; 155Mbits/s, 622.Mbits/s


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8. The Summary

The goal of the network planning is not only to define the initial network roll-out targets,but also to provide moving targets to the continuous process that takes the whole life time

of the network. Before the 3G network is launched, all the work is focused on estimatinghow the network should look like. After the network launch customer intake andbehaviour will decide the network development direction.

The most demanding tasks are to gather all the required information for the planning

work and making the network roll-out decisions based on all these estimations, operator

demands and assumptions of future costs. Cost estimations are usually very sensitive tothe changes in basic assumptions and it is crucial that all assumptions are recorded to the

network roll-out plan.

Picture below show the issues that vendors and operators face in the beginning of the

network roll-out.


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3g network design

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