Day 08 Cell Planning

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Cellular Mobile Systems and Services (TCOM1010) 2009-June

Cell Planning.doc Dr. Monzur Kabir, P.Eng Page 1 of 6

Cell Planning

1 WHAT IS CELLULAR MOBILE SYSTEMS................................................................................................................2

2 CELL PLANNING PRELIMINARIES...........................................................................................................................32.1 CO-CHANNEL INTERFERENCE (CCI) .............................................................................................................................4

3 CELL PLANNING CRITERIA.......................................................................................................................................5

4 NETWORK CAPACITY..................................................................................................................................................6

4.1 FREQUENCY REUSE FACTOR AND TRUNKING GAIN......................................................................................................6

Disclaimer: This document is a draft and intended for the author’s

consumption only. It is by no means complete and error free.





1 What is Cellular Mobile Systems• The access network is wireless but core network is based on standard telephony system (ISDN)

• Coverage area is divided into small cells each of which covered by one antenna station. Smaller coverage area of an

antenna means low transmission power requirement (longer battery life and/or smaller light-weight battery needed).

This is one of the most important advantages of a cellular system.

• Total allocated frequency channel (a licensed band) is divided among a set of channels. In the following figure the

set of cannel includes 7 cells (Cell A, B, C, D, E, F and G). This set is called cluster. All other cluster will reuse the

same set of frequencies. This is one of the most important advantages of a cellular system. The reuse of say 7 times

means 6 times more traffic using no additional frequency band.

• Allows long-haul mobility. To manage the mobility the network has mobility management capability which

performs ‘handover’ a mobile station from one-cell to another in order to continue a call. This is another important

advantage of a cellular system.

• Allows roaming from one network to another (including international roaming). The user database of different

networks can share information as needed basis to let a mobile station of one network use another network for call

connection.





2 Cell Planning Preliminaries

A cell-cluster is a group of adjacent cells, which are allocated all the frequency channels without duplication.Cluster-size (number of cells in a cluster), Ncluster = i

2+ ij + j

2, where i = 0, 1, 2 … and j = 0, 1, 2 …

i= 0 1 2 3 4

j = 0 x 1 4 9 16

1 1 3 7 13 21

2 4 7 12 19 28

3 9 13 19 27 37

4 16 21 28 37

5

Frequency Reuse Distance (FRD), cellcluster FRD R N D .3=

The cell size (in radius) can be tens of meters (pico cell), hundreds of meters (microcell) and tens of kilometers (macro cell)

depending on the design criteria (discussed later)







3 Cell Planning CriteriaCell planning depends on a number of criteria. Some examples are given below. There are some requirements which are

mutually conflicting.

• Co-channel interference limit (it puts a lower limit of cell size)

• Traffic volume per cell together with GOS (Grade of Service) sets the minimum channel requirements

• For a given spectral width (a given set of frequency channels), more channels per cell means smaller cluster size

• Smaller cell means lower transmission power

• Smaller cell => More reuse => more capacity ----- good for city center

• Bigger cell => less number of radio antenna station but less capacity ----- good for rural area

• Bigger cell preferred for high-speed traffic in order to reduce frequent handover

• High-speed traffic through high-call area => overlay cell (more than one cell at a place; one is bigger than the other)





4 Network CapacityLet us calculate capacity and channels per cell

• A Teletraffic engineer estimates calls/hour (Q) for a cell, estimates average call-duration (T in minutes) and calculates

Erlangs60QT A = => Erlang (A) is total call-hours per hour (since it is hour/hour Erlang does not have unit)

Erlang B Formula

http://www.stuffsoftware.com/trafficerlangb.html

Calculator: http://www.erlang.com/calculator/erlb/ Table: http://www.sis.pitt.edu/~dtipper/2720/erlang-table.pdf

When Erlang value and the Grade-of-Service (or blocking probability) is known the number of required channel (N) can be

calculated using Erlang formula calculator.

Example: A GSM network cell s have arrival rate = 2000 calls/hour/cell, average call duration = 1.8 minutes and blocking

probability = 1%. Assume cluster size = 7 cells. Calculate the number of frequencies required for the network.

=> A = 2000*1.8/60 = 60

Using the online calculator with A = 60 and Blocking Probability = 1% ,

Number of traffic Channel, NTraffic = 75

In Time-Division-Multiplexed (TDM) system a frequency-channel is divided into a number of TDM time-slots (for

example, GSM divides a frequency-channel into 8 time-slots) each of which is equivalent to a voice-traffic-channel.

Traffic-channels-per- frequency-channel (Nslot) specifies how many voice-grade-traffic-channels a frequency channel cansimultaneously accommodate.

Number of Frequency Channel per cell, Ncell-f = NTraffic /NSlot

Here, NTraffic = 75, NSlot = 8 => Ncell-f = ceiling(75/8) = 10

If total number of frequency channel per cluster is Ncluster-f , = NCluster × NCell-f

Here, NCell-f = 10, NCluster = number of cells per cluster = 7 => Ncluster-f = 10 × 7 = 70

4.1 Frequency Reuse Factor and Trunking Gain

Frequency Reuse Factor (FRF),

cluster

reuse N

N

SizeCluster

Cellsof Number TotalF ==

_

___

The traffic-channels, which carry actual voice (or subscribers’ data), are not dedicated to but shared by subscribers. They

are dynamically allocated to subscribers on demand. Usually, number of subscriber is many-times larger than total

number of traffic channel. The ratio,ChannelsTrafficof Number

sSubscriber of Number Gtrunking

___

__= is called Trunking-Gain.

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Day 08 Cell Planning