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Capacity planning exercise. M.Sc. Mika Husso 9.2.2007. Traffic reviewed …. The unit of traffic is E [erlang] Single line or sever can handle up to 1 E traffic. Offered Traffic (total traffic created by subcribers) A= h = call intensity * mean service time
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Capacity planning exercise M.Sc. Mika Husso 9.2.2007
Traffic reviewed … • The unit of traffic is E [erlang] • Single line or sever can handle up to 1 E traffic. • Offered Traffic (total traffic created by subcribers) • A= h = call intensity * mean service time • Carried Traffic (or served traffic) = Total amount of traffic the network is able to serve • Lost Traffic (or rejected traffic) = Offered Traffic – Carried Traffic • Potential traffic: Offered traffic if there would be no restrictions on the use of the service.
… Traffic reviewed • In practice, it is not feasible for mobile network to have the capacity to handle any possible load at all times. • Fortunately, not all subscribers place calls at the same time and so it is reasonable to size the network to be able to handle some expected level of load. • > planner has to design the network to meet a predefined blocking probability (e.g. 2 %), which depends on the desired GoS
Capacity planning process (TDMA/FDMA) 1. Considering the available resources (number of carriers etc.) and the GoS requirements (blocking prop., ) 2. Estimating the amount of Offered Traffic on each area 3. Estimating how many cells (BSs) and how many traffic channels per cell are needed to serve the offered traffic on the area with the given blocking propability (e.g. 2 %) -> Determining the CAPACITY based cell area (and radius) 4. Checking if also COVERAGE can be granted for the capacity based cell (i.e. can the signal reach the user/BS without attenuating too much?) -> if not the cell radius is decreased so that COVERAGE can be granted
1. Considering the resources • There are available resources • Number of carriers (channels) • Multiplexing (TDMA, FDMA, CDMA) • Duplexing (TDD, FDD) • … • There are also requirements for GoS • Blocking propability • Call dropping propability • …
2. Estimating the traffic … • Offered voice traffic from a user group can be predicted as follows , where • N is the number of persons • C denotes the penetration • T is the average traffic generated by one user • F denotes the area coverage probability (users not on the network coverage area can’t offer traffic)
… Estimating the traffic • The traffic offered by each user is: A = μH Erlangs , where H is the average holding time of a call μ is the average number of calls requested/time unit by the user For example H = 2 minutes and μ = 0.8 calls / hour -> A = 2 * (0.8/60) ≈ 26.7 mErlang
Traffic estimate • Penetration CP = 0.25 • Offered traffic per user: TO,1=TO,2=20 mErlang • Coverage probability: F1=0.8 (Pedestrian), F2=0.95 (Vehicular) • Distance between pedestrians S1=4 m • Distance between vehicles S2=25 m • Number of people in a car 2=1 (1=1) • Number of pedestrians N1 = 1 Ls/S1 = 1•10000/4=2500 • Number of cars N1 = 2 Ls/S2 = 1•10000/25=400 • Traffic offered by pedestrian users T1=F1CP TO,1 N1 = 0.8•0.25•0,02•2500=10 Erlang • Traffic offered by vehicular users T2=F2CP TO,2 N2 = 0.95•0.25•0,02•400=1.9 Erlang • Offered traffic A= T1+T2 = 11.9 Erlang
3. Estimating the number of BSs needed • Assuming we have estimated a total traffic of 20 Erlang on a given area, how many BSs do we need if the smax number of tranceivers (channels) on a BS is 5 and the desired blocking level is 3 %? • Using Erlangs B formula (or table) • We need 27 channels, which can serve a total of 20.31 Erlang (from the Erlang B table) • We need 27 / 5 = 5.4 -> 6 Base Stations (using 5 transceivers in a BS)
4. Can coverage be granted? • Will be dealt with in the coverage planning exercise • Based on the calculation of a link budget • Can the signal be received with adequate power? • If the signal attenuates too much, the maximum distance between BS and user (i.e. cell radius) must be reduced
Example of capacity planning • System parameters • Penetration (all user groups): 25 % • Offered traffic/user (all user groups): 20 mErlang • Coverage probability target: vehicular users 95%, 1 user/car, pedestrian users 80% • Multiple access method: FDMA, 28 TRX/cell • Blocking probability target: 2 % • Service area divided into 4 homogenous Regions with spatially uniformly distributed users • In Region A the vehicular generated traffic is handled by macrocells and pedestrian generated traffic by microcells, in other Regions all traffic is handled by macrocells • Approach: Minimum excess capacity, starting from Region with highest traffic density, cells possibly overlapping to adjacent Regions will reduce the area in these to be covered correspondingly
Geometry of the service area Region types: A: dense city B: city C: suburban D: rural
Parameters of the Regions in the Service Area Basic assumption: Vehicles, pedestrians, and traffic are assumed to be spatially uniformly distributed in each Region.
Choosing the cell structure • An ideal cell would have a circular shape. • To get complete coverage a certain overlapping must be allowed. • Minimum overlapping with hexagonal structure, which is the most common in theoretical investigations • Another possible cell structure giving complete coverage is the quadratic cell structure • In this example the quadratic cell structure gives easier calculations and will be used • FROL = Fractional Overlapping
Estimating the amount of traffic channels and BSs to be used
… Dimensioning cells • When region B is dimensioned, it usually partly overlaps regions A, C and D and therefore also serves some of their offered traffic • > When dimensioning regions A, C and D the traffic already served by region B should not be served again (equipment should be minimized) • Otherwise the dimensioning process is done exactly as for region B • When the next region is dimensioned (in this case D), the traffic served by it in other regions should also not be served again
Dimensioning microcells • The procedure is similar, but • In the city area, buildings cause significant attenuation to the signal • To minimize equipment, the BSs should generally be located at the street crossings • Usually pedestrian originated traffic on the area is served using microcells and vehicular originated using macrocells
