Subscriber Demands and Network Requirements –
the Spectrum Capex trade-off
Hugh Collins
Agenda
Traffic modelling principles Service modelling Data: the growth area Mobile network dimensioning Spectrum Efficiency Tool: modelling the
relation of spectrum, traffic and network dimensioning
Traffic Modelling Principles
Traffic modelling principles
The network must carry the offered traffic! … but carrying all traffic is hard to do – traffic peaks
can be very high– Partly a technical problem – spectrum is limited, so
networks have limited capacity but traffic peaks can be far above average traffic
– Therefore an economic problem also – if the network is built to handle the peaks, then it is very under-used for most of the time
“Grade of Service” – probability of network busy– Calls fail, data transmitted slowly or delayed– Wireless networks usually designed to reject about 2% of
voice calls in the busy hour
For voice use Erlang B; For Data use Erlang C
The Erlang B formula
Erlang B calculates the probability of blocking– The probability that a call arriving at a link or switch (with a
defined capacity) finds the link/switch busy Erlang B is used for low latency traffic such as voice
or video calls
– Pb = Probability of blocking (%)– m = number of servers/ circuits/ links/ lines– E = λh = total amount of traffic offered (Erlangs)
(Arrival rate x average holding time)
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Calculating Erlang B
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0 10 20 30 40 50
Traffi
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Number of lines
5%2%1%
The Erlang C formula Erlang C calculates the probability of waiting in a
queuing system– If all servers are busy when a request arrives, the request is
queued– An unlimited number of requests may be held in the queue
simultaneously Erlang C used for data traffic
– PW = probability of queuing for a time > 0 secs (%)– m = number of servers/ circuits/ links/ lines– E = total amount of traffic offered (Erlangs)
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Calculating Erlang C
Service modelling
Categorising subscriber services Before we can dimension, we need to understand the
services and their traffic requirements Various methods of categorising can be used One potential way is presented in ITU-R Rec M.8161:
– Speech: Toll quality voice (64kb/s on a fixed network, much less than this on a mobile network)
– Simple messaging: User bit rate of 14 kb/s– Switched data: User bit rate of 64kb/s– Asymmetrical multimedia services
Medium multimedia: User bit rate of 64/384 kb/s High multimedia: User bit rate of 128/2000 kb/s
– High interactive multimedia: User bit rate of 128/128kb/s Faster services represented as multiples of this
However service speeds have risen in the past decade!
1 ITU-R Recommendation M.816 - Framework for services supported by International Mobile Telecommunications-2000
Typical service characteristics
Some typical values are shown below, but local data should be used where available
Busy Hour Call Attempts
Call duration (seconds)
Activity factor
Pedestrian Vehicular Pedestrian Vehicular Pedestrian Vehicular
Speech 0.8 0.4 120 120 0.5 0.5
Simple messaging 0.3 0.2 3 3 1 1
Switched data 0.2 0.02 156 156 1 1
Medium multimedia 0.4 0.008 3000 3000 0.003/
0.0150.003/ 0.015
High multimedia 0.06 0.008 3000 3000 0.003/
0.0150.003/ 0.015
High interactive multimedia
0.07 0.011 120 120 1 1
Source: ITU-R Report M.2023 – Spectrum requirements for IMT-2000
Service demands will also vary by location Different areas will provide:
– Different population densities– Different service mixes– Different service demands– Different service time profiles
Consider, for example:Hot spots– Airports– Railway or bus stations– Cafes– Sports stadiumsHot routes– Motorways/ highways – Railway lines
Service demands vary by time Our earlier service characteristics were partly defined
by Busy Hour Call Attempts (BHCA)– But voice and data busy hours are typically different– And data typically has similar use across a number of hours
0%
1%
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3%
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8%
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Perc
enta
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ly tr
affic
Time of day
Voice
Data
Data: the growth area
Data applications
E-mail:– Message 5-10 kbytes– Attachment 20-1000+ kbytes– 10 messages in busy hour?– average 1 Mbyte per
user in busy hour– Symmetrical up and down
Internet browsing:– Download 40 pages in busy
hour– Average 50 kbytes per page– average 2 Mbytes per
user in busy hour– Asymmetrical: more down
than up
Streamed audio:– 128 kb/s– Average say 5 mins in busy
hour– average 4.8 Mbytes per
user in busy hour– Downstream
Streamed video:– 512 kb/s– Average say 5 mins in busy
hour– average 19.2 Mbytes per
user in busy hour– Downstream
Data growth
Global mobile data traffic is growing very fast:– Nearly tripled year-on-year, for the past 3 years!– In March 2010, Ericsson reported that global mobile data
traffic overtook mobile voice traffic
CAGR 92%
Source: Cisco Visual Networking Index 2011
Driven by devices
The introduction of smarter mobile devices drivesdata increases (as well as the applications used!)
Source: Cisco Visual Networking Index 2011
Mobile network dimensioning
Example: A mobile network
Other Networks
GMSCBSC
VLR
MSC
VLR
VLR
MSC
HLR
BSC
BSC
BSC
BSC
BSC
BSC
BSC
BSC
MSC
BTS
BTS
BTS
BTS
BTS
GMSC
GMSC
Radio Layer MSC Layer Transit Layer. May not exist in all
networks
Network components to be dimensioned Radio Access Network:
– eNode-B/ Node-B/ BTS– RNC (Radio Network Controller) or BSC (Base Station Controller)– Access links/ backhaul
Core Network:– Links: for example STM-1, Gigabit Ethernet, 10GE– Routers, Switches– Databases: for example HLR, VLR– Network operations and management
Application Platforms:– Data/ Internet access– Voicemail– MMS/ SMS– etc
Network component capacities
In radio networks, the relevant measures of capacity are:– connected subscribers– voice minutes– megabytes of traffic– erlangs of traffic– service platform usage
Challenges created by traffic growth
There are many! And the whole network is affected Some examples:
–More sites/ smaller site radii– Increase in backhaul capacity
Movement towards high capacity microwave/ fibre–Need for Evolved Packet Core
To facilitate improved session, mobility and QoS management– Improvements in ‘back-office’
For example, the challenges faced in billing to measure ‘caps’ and charging
– Improvements in network monitoring and management To identify and removing bottlenecks To optimising equipment performance and interworking
… additional investment required
Spectrum Efficiency Tool:
modelling the relation of spectrum, traffic and network dimensioning
Main network dimensioning dependencies
RAN siteCapacity
TrafficServices
QoS
Site count /Network cost
Availablespectrum
A typical network dimensioning process
1. Set the objectives, for example:– The technology to be used– The geographic and population coverage– The traffic throughput – The Quality of ServiceWith the spectrum available, these parameters determine the network’s capacity
2. Obtain the geographic and population data– Population by administrative region– Define/ designate and use types; rural, suburban and urban
3. Compute the number of sites required to meet the objectives– Thereby the network design/architecture – Thereby the network cost
Site / spectrum requirements modelling
An engineering model to generate dimensioning of radio network under varying assumptions of:– Subscriber numbers / market share– Services provided / traffic offered– Spectrum available
Illustrates how changing subscriber demands can have a significant impact on the network– The spectrum versus sites trade-off– The cost versus capacity versus QoS trade-off
Developed to examine and optimise spectrum allotment / assignment decisions
Model overview
Traffic/service mix
Spectrum/technology mix
Regionalcoverage
Subscriberpopulation
Calculatorengine
Requiredspectrum,site count& costs
Graph store
Basis for spectrum calculation Based on ITU-R Recommendation M.1390 - Methodology for
the Calculation of IMT-2000 Terrestrial Spectrum requirements
For each service:
where:FTerrestrial= Terrestrial component spectrum requirement (MHz) = Guard band adjustment factor (dimensionless)es = Geographic weighting factor (dimensionless)Tes = Traffic (Mb/ s / cell)Ses = Net system capability (Mb/ s / MHz / cell)
es
eseseseslTerrestria
STαβFαβF
Net system capability Accounts for underlying modulation & multiple access
factors ...– ... as well as radio resource management factors– Such as power control, discontinuous transmission, frequency
reuse pattern, band splitting/grouping, frequency hopping, adaptive antennas
Net system capability for different evolutions of systems, Hideaki Takagi and Bernhard H. Walke (2008), Spectrum Requirement Planning in Wireless Communications, pp56, John Wiley & Sons Ltd
Spectrum requirements calculation overview
Cell area /Population density /
Penetration rate
Busy hour call attempts /
Call duration /Activity factor
Number of users / cell
Offered traffic / user
Offered traffic / group
Blocking probability(delay critical)
Queuing probability(non delay critical)
Channels / group
Offered traffic / cell
Required bit rate / cell
Required spectrum
Guard band adjustment factor
Geographic weighting factor
Channels / cellGroup size
Final total spectrum requirement
Service channel bit rate
Setting accessible population data
Defining how the population in each region is split between each geotype ....
... and then defining what percentage of this population is accessible
Total% # % # #
1 Region A 93.9 2,127,400 6.1 137,700 2,265,100 2 Region B 39.2 107,900 60.8 167,100 275,000 3 Region C 95.1 829,000 4.9 42,600 871,600 4 Region D 82.9 862,800 17.1 178,500 1,041,300 5 Region E 75.8 102,000 24.2 32,500 134,500 6 Region F 62.6 109,900 37.4 65,600 175,500 7 Region G 54.8 60,900 45.2 50,300 111,200 8 Region H 71.2 104,100 28.8 42,200 146,300 9 Region I 85.6 109,200 14.4 18,300 127,500
10 Region J 70.9 58,100 29.1 23,800 81,900 11 Region K 71.8 281,200 28.2 110,700 391,900 12 Region L 34.9 79,600 65.1 148,600 228,200
83 4,832,100 17 1,017,900 5,850,000
ID NamePopulation
Urban Rural
Total
Total% km2 % km2 km2
1 Region A Hilly Large 4.0 303 96.0 7,265 7,569 2 Region B Desert Small 0.6 147 99.4 26,496 26,646 3 Region C Flat Large 2.5 119 97.5 4,662 4,782 4 Region D Flat Large 11.9 186 88.0 1,380 1,568 5 Region E Hilly Small 5.0 21 95.0 393 413 6 Region F Hilly Small 6.7 27 93.3 381 409 7 Region G Desert Small 0.2 58 99.8 32,832 32,894 8 Region H Hilly Small 4.3 40 95.7 893 933 9 Region I Flat Small 0.7 45 99.3 6,840 6,886
10 Region J Hilly Small 1.3 28 98.7 2,178 2,205 11 Region K Hilly Small 6.2 69 93.8 1,040 1,109 12 Region L Hilly Small 2.0 72 98.0 3,548 3,621
1.3 1,115 98.7 87,908 89,035
Rural
Total
LandmassID Name Terrain City Type Urban
Setting administrative area data
Terrain type, city type and geotype define how signals propagate in the link budgets
Setting target coverage levels
Define the target coverage In this example, defined by existing operator coverage levels
Population factor estimates the ratio of population living in the coverage area
Landmass Population% km2 Factor # % km2 Factor # km2 #
1 Region A 87.7 266 0.90 1,818,927 4.0 291 0.50 65,408 557 1,884,335 2 Region B 66.0 97 0.90 92,255 0.5 140 0.50 79,373 237 171,627 3 Region C 92.1 110 0.90 708,795 2.5 116 0.50 20,235 226 729,030 4 Region D 87.9 164 0.90 737,694 11.9 164 0.50 84,788 328 822,482 5 Region E 94.9 20 0.90 87,210 5.0 20 0.50 15,438 40 102,648 6 Region F 93.2 26 0.90 93,965 6.7 26 0.50 31,160 51 125,125 7 Region G 52.0 30 0.90 52,070 0.2 58 0.50 23,893 88 75,962 8 Region H 87.4 35 0.90 89,006 4.3 38 0.50 20,045 73 109,051 9 Region I 81.9 37 0.90 93,366 0.7 45 0.50 8,693 82 102,059
10 Region J 96.3 27 0.90 49,676 1.2 27 0.50 11,305 54 60,981 11 Region K 93.8 64 0.90 240,426 6.2 64 0.50 52,583 128 293,009 12 Region L 92.4 66 0.90 68,058 2.0 70 0.50 70,585 137 138,643
84.4 941 4,131,446 1.2 1,059 483,503 2,001 4,614,948
Target Coverage
ID Name
Total
Urban RuralPopulationLandmass Landmass Population
Total
Calculating cell area
Link budget for most limiting
service
Antenna geometry
Urban geometry
Regiontopography
COST 231 Propagation
Models
Cell area
Cell rangeSectors / site
Cell area together with population, geographic and coverage data enable subscriber densities to be calculated
Setting services and use statistics
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Speech (S) 73 73 0.8 0.4 120 120 16 16 0.5 0.5 0.5 0.5 16 16 0.07 0.07Simple Message (SM) 40 40 0.3 0.2 3 3 14 14 1 1 1 1 14 14 0.125 0.125Switched Data (SD) 13 13 0.2 0.002 156 156 64 64 1 1 1 1 64 64 0.125 0.125Medium Multimedia (MMM) 15 15 0.4 0.008 3000 3000 64 384 0.003 0.015 0.003 0.015 64 384 0.125 0.125High Multimedia (HMM) 15 15 0.006 0.008 3000 3000 128 2000 0.003 0.015 0.003 0.015 128 2000 0.125 0.125High Interactive Multimedia (HIMM) 25 25 0.007 0.011 120 120 128 128 1 1 1 1 128 128 0.125 0.125
Net system capability
(bit/s/Hz/cell)Pedestrian Vehicular
Penetration rate (%)
Busy Hour Call
Attempts
Call Duration (s)
Net user rates (kb/s)
Activity Factors Service channel bit rate* (kb/s)
Define what services are used and how they are used The above example relates to 3G Traffic metrics based on ITU-R Report M.2023 -
Spectrum Requirements for IMT 2000, but real observed traffic figures should be used wherever possible
Calculating spectrum required
The traffic offered by each service can be calculated This can be aggregated and mapped to traffic
channels–Using Erlang B and Erlang C, as appropriate
From this, the amount of required spectrum can be derived (using the ITU-R Rec. M.1390 formula) –To meet demanded traffic, as driven by the subscriber
numbers–Based on calculated site numbers
Spectrum requirements planning
The spectrum calculation is made many times by varying the cell radius factor
The results can then be graphed, and interpreted ...
Maximumcell radius
(link budget)
CellularSite Minimum
cell radius(economic limit)
100%(radius factor)
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More spectrum requiredMore sites required
Typical output: spectrum versus site count
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Site
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Spectrum (MHz)
Spectrum Site Count (Capacity)
10%20%30%40%50%60%70%80%90%100%
Marketshare
Interpreting the curves
Sites
Spectrum
Block steps/packaging representing possible choices
Volatile choice (model viewpoint)
Greedy choice(Poor network design,spectrum hoarding)
Optimum area (purely technical perspective)
Unsatisfactory choice
(operator starved)
Improved capacity of networks, improved economy for operatorsBetter for re-farming, more competition possible
Simple 2-operator example, 36MHz available
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Operator A BTS
Operator B BTS
Total BTS
Thank you
Any questions?