cell planning

Cellular Mobile Systems and Services (TCOM1010) 2009-June

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

Cell Planning

1 WHAT IS CELLULAR MOBILE SYSTEMS................................................................................................................2 2 CELL PLANNING PRELIMINARIES...........................................................................................................................3

2.1 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 = i2 + ij + j2, 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), cellclusterFRD RND .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)



2.1 Co-channel Interference (CCI) Smaller cell means higher reuse factor, low power but higher handover (that processing) and higher co-channel interference. The co-channel interference is the interference due to neighboring cells which are operating on the same carrier frequency. The co-channel interference is presented as Carrier-to-Cochannel Interference (C/I) ratio, which is the ratio of the signal power to the total interference power.

�=

== K

kkI

C

I

C

P

PPTotal

PIC

1,

_/ , where K = number of interfering co-channels

– Consider

� A fully developed cellular system (that is, NCluster ≥ 7) � Only the interference from the first tier � All the first tier co-channels are active (maximum interference)

=> K = 6

– Consider all the cells are equal in size and shape and transmitting identical power

I

K

kkI PKPCCITotal ._

1, ==�

=

, where PI is the interference per co-channel

– Radio signal gets attenuated proportional to square of the distance (d) in free space (that is P ∝ 2−d ). Taking

multipath factor into account this can be as high as P ∝ 5−d . For general case P ∝ γ−d , where γ = 2 to 5.

The lowest signal power (that is, signal power at the cell boundary), λ−∝ CellMinC RP ,

Interference power (average) λ−∝ FRDI DP (Note: the mobile station can be at any point in the cell. One of them is the closest and another is the furthest from the interfering base-station. The average of them is the distance between two base stations => DFRD)

Considering the above facts the maximum co-channel interference (C/I)Max is:

( )γ

γ

γ

��

��

�==

��

��

�= −

−

=� Cell

RFD

FRD

cellK

kk

MinCMax R

DDR

IMax

PIC .

61

6/

1

,

The ratio Clustercell

FRD NRD

3= is called Co-channel Interference Reduction Factor (q). When the value of q increases

the C/I improves (that is total I decreases).



3 Cell Planning Criteria Cell 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 Capacity Let 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

Erlangs 60QT

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 can simultaneously 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 NN

SizeClusterCellsofNumberTotal

F ==_

___

� 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, ChannelsTrafficofNumber

sSubscriberofNumberGtrunking ___

__= is called Trunking-Gain.

cell planning

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