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Upena Dalal
Wireless
Communication
and Networks
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Cellular Theory
Chapter 2
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Why cellular technology
Cells, clusters, and cell splitting
Frequency reuse concept and reuse distance calculation
Cellular system components
Antennas for base station
Operations of cellular systems and handoff
Channel assignment fixed and dynamic
Cellular interferencesco-channel and adjacent channel
Sectorization Mobile traffic calculation
Spectrum efficiency of cellular systems
Location management
Key Topics
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The concept of trunking (resource sharing) between two
central telephone exchanges was used in conventionallandline telephone systems.
A trunked radio system is a complex, centrally controlled, full
duplex radio system that allows sharing of limited radio
frequency (RF) channels among a large group of users. It usescontrol or signalling channels as well.
Introduction
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The concept of cellular technology encompasses various
aspects of infrastructure, for instance, division of area,
frequency management or allocations, and call handling.
Following are the two major components of mobile
telecommunication system in general:
Central station-- This is common for many subscribers and
includes the switching equipment and an RF transmitter
and receiver.
Mobile telephone-- Every subscriber must have a mobile
telephone that includes a microphone, a speaker, dialling
facility, a radio transmitter, and a receiver.
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In modern cellular telephony, rural and urban regions are divided
into areas according to specific provisioning guidelines orstandard protocols.
Deployment parameters, such as amount of cell splitting and cell
sizes, are determined experimentally in the cellular system
architecture. Provisioning for each region is planned according to an
engineering plan, which includes cells, clusters, frequency reuse,
and handovers.
Today, software tools are also available to plan and designcellular architecture.
Cellular infrastructure
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A cell is the basic geographic unit of a cellular system.
The term cellular comes from the honeycomb (hexagonal)shape of the areas into which a coverage region is divided
theoretically.
Cellwise, one base station provides transmission over a small
geographic area. Cell sizes vary depending on the landscape (topographic
locations such as mountains, valleys, and plains).
Due to the constraints imposed by natural terrain and man-
made structures, practically, the cells are not perfecthexagons.
Cells
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A group of cells form a cluster , the size of which is denoted as N.
Figure illustrates a seven-cell cluster. Each cluster is allocated a setof frequencies. N is restricted by the following equation:
Cluster
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non-negative integers i
andj determine the relative locations of co-channel cells, as shown
in Figure.
i = vertical move, j = slant horizontal move
A relationship exists between the cluster
design equation and frequency reuse as well as
reuse distance.
Reaching to co-channel cell
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The size of a cell depends on the density of subscribers in an
area.
The capacity of a network in a densely populated area can be
improved by reducing the size of the cells or by increasing the
number of cells along with installing low-power base stations.This will effectively increase the number of channels in that
area because of more frequency reuse.
On the basis of their size, cells may be categorized asmacrocells, microcells , picocells, and femtocells .
Cell Size
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Cell Splittingfor maximizing number of users
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A radio channel consists of a pair of frequencies for full duplex
operation. The concept of frequency reuse is based on assigning to each cluster
the same group of radio channels used within a small geographic area.
A set of N different frequency groups {f1, ..., fN} is used for each cluster
of N adjacent cells and shared among the cells almostequally.
The set of frequencies assigned to a cell is completely different from that
assigned to the neighbouring cells.
The coverage area of the cells is called the footprintsurrounded by outer
boundary of the cell.
If there are seven members in a cluster, number of available frequencysets is seven, the frequency reuse factor is 1/7. This means that each
cell uses one-seventh of the available cellular channel for reuse.
Cellular Frequency Reuse
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Let L = total number of duplex channels available for reuse (i.e., frequencies per
cluster)
k = number of duplex channels allocated to each cell of a cluster (k < L)
N = cluster size (in which there are N cells)M = number of times the cluster is repeated
C = total effective number of duplex channels available in the area
Thus, L = k N
C = M L = MkN
Cluster of seven cells
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For Example:
Reuse distance (D) calculation
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Cellular planning can be done byusing various softwares and
planning tools
screenshot of Ericsson TEMS
Real world cells
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There are three types of cellular systems
1. Analog Circuit-switched Cellular System
Mobile Unit or mobile subscriber unit (MSU)
Cell Site or base station Mobile Telecommunication Switching Office
(for connection management and billing)
Cellular system components
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2. Digital Circuit-switched Cellular System
Mobile Station
Base Station or Base Transceiver Station
Base Station Controller
Switching Subsystems 3. Packet-switched Cellular System
It has six elements: MS (user equipment), basestation, radio network controller(RNC), service
support node (SSN), gateway support node (GSN), andcharging gateway function (CGF).
Cellular system components
E l f di it l i it it h d ll l t l
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To be studied in Chapter 11
In detail.
Example of digital circuit switched cellular system example
(GSM system)
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Two main types of antennas are used in the
wireless industry for a BTS omnidirectional
and directional
Omnidirectional Directional with 3 sectors
Antennas for cellular systems
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Omnidirectional antennas are nothing but the dipoles and
they radiate in all lateral directions.
A directional antenna shapes and projects a beam of radioenergy in a specific direction and receives radio energy onlyfrom a specific direction, employing various horizontal
beamwidths.
Here, the term beamwidth refers to the conical size of theradiated beam.
Downtilting---The radiation pattern of a downtilt antenna iselectrically or mechanically tilted downwards at a specifiednumber of degrees to avoid shadowing under and near theantenna zone.
Antennas for cellular systems
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A base station antenna is chosen according to the situation
depending upon many factors, such as the following: Size of the area to be covered
Configurations of the neighbouring cell sites
Type of antenna usedomnidirectional or directional
Antennas beamwidth in case of a directional antenna
Allotted RF spectrum the antenna can utilize
Antennas for cellular systems
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The probable adverse effects of the radiation from the base stations are a
major concern, especially among residents in urban areas. Independent
monitoring of RF exposure levels around mobile phone base stations has been
conducted for many years under the control of government agencies.
Radiation pattern from base station (Lighter shades of grey indicate decreasing power
strength)
Adverse Effects of Base Stations
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Currently, PIFA (planar inverted-F antennaa shorted patch)wideband antennas are used widely and sometimes fractalantennas are also used. PIFA is the modified version of thequarter-wavelength patch antenna.
As the patch is shorted at the end, the current at the end of the
patch antenna is no longer forced to be zero. This results in thesame currentvoltage distribution as that of a half-wave patchantenna. The antenna is resonant at a quarter-wavelength, thusreducing the space needed on the telephone; moreover, ittypically has good specific absorption rate (SAR) properties. Ithas a low profile and an omnidirectional pattern.
Antennas for Mobile Radio Frequency Front End
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Stages for Mobile-originated Call
There are two possibilities in a mobile-originated call:
mobile-to-mobile call and mobile-to-landline call .
As an example, the call set-up sequences for mobile-originated calls in a GSM
system are given in Fig. 11.3 in Chapter 11
1. Mobile-originated Call
Operations of cellular systems
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There are two possibilities:
mobile-to-mobile call and landline-to-mobile call
2. Mobile-Terminated Call
Stages for Mobile-terminated Call
Operations of cellular systems
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Mobile-originated calls are routed to the BTS first and then to the BSC.
The BSC forwards this call to the MSC. The MSC authenticates and routes the call to the called subscriber as
per the dialled digits.
If the called subscriber is in mobility, the process to receive the call on
his mobile device is described in Section 2.5.2
Mobile-terminated calls come to the MSC first, where the HLR/VLR
enquiry is carried out, and as per the information, the MS is paged in
the suitable BSC.
The BSC forwards this page to all BTSs where the actual paging is done.
After a BTS gets a response from the mobile, it allocates a channel forthis call.
On ending the call, the BTS informs the BSC and MSC.
Steps
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A feature called registration is used for landline-originated calls.
A landline user dials a mobile unit number.
The telephone company zone office recognizes that the call is for
a mobile number and forwards it to the MTSO/MSC.
The MTSO/MSC sends a paging message to certain cell sites
based on the mobile unit number and the search algorithm. Eachcell site transmits the page on its own set-up channel.
If the mobile unit is registered, the registered site pages the
mobile. The mobile unit recognizes its own identification
on a strong set-up channel, locks onto it, and responds to the cellsite.
The mobile unit also follows the instruction to tune to an assigned
voice channel and initiates user alert.
Network-originated or Landline-originated Call
ll
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When a mobile user turns off the transmitter, a particular
signalling tone is transmitted to the cell site and the voicechannel is freed by both the sides.
The mobile unit resumes monitoring pages through the strongest
set-up channel.
Call Termination
d ff d
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Handoff is also knownas handover.
It can be categorized
as:
hard handover
soft handoversofter handover
Hard Handover scenario
Handoff Procedure
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Handoff probability It is the probability that a handoff is executedbefore call termination.
Rate of handover It is the number of handovers per unit time.
Interruption duration It is the duration of time during a handoverprocedure in which a mobile is not connected to any base station.
Handoff delay It is the distance between the point at which the handoffshould occur and the point at which it does occur.
Probability of unsuccessful handoff It is the probability that a handoffis executed while the reception conditions are inadequate.
Handoff blocking probability It is the probability that a handovercannot be completed successfully.
Few terms associated with the handoff procedure
H d i
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The handoff occurs only if the new base station is sufficiently stronger than the
current one by handover margin .
Hard handover scheme prevents theping-pong effect.
Hard handover procedure exhibits hysteresis .
Handover strategies The following are the two different strategies for
handover:
(a) Centralized methods (used in GSM)
(b) Decentralized methods [used in DECT (WLL)]
Handover margin
M h d f h d ff
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Mobile-assisted handoff The mobile measures the power receivedfrom the surrounding base stations and continuously reports theresults of the measurements to the serving base station. A handoff isinitiated when the power received from the base station of aneighbouring cell begins to exceed that from the current base stationby a certain level or a certain period of time. This method is currentlybeing used in the mobile systems.
Base station-assisted handoff In the first-generation systems, thestrength measurements are made by the base stations and supervisedby the MTSO. Here, the base station measures the signals from themobiles served by it, as well as from the mobiles in the neighbouringcells and reports to the MTSO. The MTSO decides whether a handoff is
necessary as well as who needs it. Here, the load of the MTSO is morecompared to the mobile-assisted method, and hence handoffs areslower.
Methods of hand-off
S ft H d
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It allows the MSC to make a soft decision regarding the version of
the users signal to pass.
The ability to select between the instantaneous received signals
from a variety of base stations is called soft handover. The
technique is discussed in detail in Chapter 11.
More suitable for CDMA based systems.
Soft Handover
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Figure for Example 2.3
Ch l A i t
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Fixed Channel AssignmentIn the fixed channel assignment method, each cell is allocated apredetermined set of voice channels. Only the unused channels in a particular cell canserve any call attempt within that cell. If all the channels are occupied, then the call will be
blocked and the subscribers will not get any service.
Borrowing StrategyIn the borrowing strategy method, a cell is allowed to borrow achannel from a neighbouring cell, if all of its own channels are occupied. The MSCsupervises the borrowing procedure and ensures that the borrowing of the channel doesnot disrupt or interfere with any of the calls in progress in the donor cell.
Dynamic Channel AssignmentIn the dynamic channel assignment method, voice channelsare not allocated permanently in any of the cell. When a call request is made, the servingbase station requests a channel from the MSC, which then allocates a channel to therequested cell.
Advantages of the dynamic channel assignment --the likelihood of call blocking reduces, co-channel
interference reduces and channel utilization increases.
Disadvantage is that the MSC must be fast and capable of collecting real-time data on channel occupancy,traffic distribution, and radio signal strength indications of all channels on a continuous basis.
Channel Assignment
C ll l i t f
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Two major types of cellular
interferences are co-channel
interference and adjacent
channel
interference.
Cochannel Interference
due to this scenario
Cellular interferences
C h l i t f l l ti
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The co-channel cells surrounding a particular cell exist in a circular (or rather
hexagonal) pattern. There will be maximum interference from the nearest circle of
cells, which are also called first-tier cells and are always six in number. Based on
thatthe signal-to-interference ratio (SIR) for a mobile receiver
where S is the desired signal power from the desired base station and Ii is the
interference power of the base station of the ith interfering co-channel cell
(neglecting noise). n is the path loss exponent. The empirical value ofn ranges from
two to five.
Based on the distance of the interfering base stations to the desired mobile receiver
Di, D/R ratio and cluster size N
Co-channel interference calculations
C h l i t f l l ti
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where k is the proportionality constant, which depends on the power
radiated by the base station at the centre. I represents the power radiated by
the other base stations in the first tier, which are six in number.
Hence,
From above two equations
From the equation it is clear that N should be designed critically for the
desired SIR.
Co-channel interference calculations
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SINR f dj t h l i t f
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Adjacent channel
interference can beminimized through
careful filtering at IF
stages and careful
channel assignments.
SINR for adjacent channel interference
S t i ti
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Sectorized base stations are created by subdividing an omnicell into sectors that
are covered using directional antennas mounted in the same base station
location. Following is the example of 3 sector division and each sector is treatedas different cell.
To sectorize a cell, a
horizontal, equilateral
platform resembling a
triangle is deployed on a
tower. Each side of the
platform is called a face.
Three, four, or six
directional antennas are
installed on the platform,depending on the number
of sectors.
Sectorization
Why Sectorization
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Sectorization facilitates wireless engineering and operations in
the following ways: It minimizes or eliminates co-channel interference.
It optimizes the frequency reuse plan. This is facilitated
through another concept known as thefront-to-back ratio .
At a minimum, it triples the capacity of any given coveragearea when compared to the capacity offered by deploying
omni-antennas.
Why Sectorization
Mobile traffic calculation
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Traffic calculation is essential to both mobile phone and landline
systems. On the basis of the traffic condition, the telephone system must
be designed such that only a minimum number of calls are
blocked and uninterrupted service is provided to the subscribers.
The traffic varies considerably throughout a day, but mostsystems are designed to handle the traffic during the peak busy
hour in a day.
Mobile traffic calculation
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Traffic variation in a small business firm as per normal activities in a day
from 6 a.m. to 10 p.m. is shown in figure by average traffic measured every
hour
In teletraffic engineering, trunk describes any entity that will carry one call. The
number of trunks connecting one MSC with another is the number of voice pairsused in the connection; hence, it is important to determine the number of trunks
required between the MSCs.
Trunking efficiency refers to the decrease in call blocking and increase in coverage
without call drops, at the cost of management overhead.
Some definitions
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Call blocking It is the non-availability of any trunk or call propagation path.
Busy hour or peak busy hour It is the 60-min interval in which the traffic or the number
of call attempts is the highest in a day. It usually varies from day to day or over anumber of days.
Time-consistent busy hourIt is the one hour period starting at the same time each day
for which the average traffic volume or the number of call attempts is the highest over
the days under consideration.
Call completion rate It is the ratio of the number of successful calls to the number of callattempts. It can be represented in percentage.
Busy hour call attempts It is the number of call attempts in the busy hour and it decides
the network capacity.
Average busy hour calls (completed) = Busy hour call attempts call completion rate
Busy hour calling rate It is the average number of calls originated by a subscriber during
the busy hour or the call intensity per traffic path during the busy hour.
Day to busy hour traffic ratio It is the ratio of busy hour calling rate to the average
calling rate for the day. It indicates how much of the days total traffic is carried during
the busy hour.
Some definitions
Contd
Some definitions
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Cont d
Centum call second (CCS) It represents the calltime product and is a traffic measurement
unit. 1 CCS is one call for a duration of 100 s or 100 calls for 1 s duration each or any other
combination. Other measures are call seconds (CS) and call minutes (CM).
Average call arrival rate It is the average number of calls that arrive during the specified
time duration.
Average call holding time It is the average duration of calls that arise within the specified
time duration or the average duration of occupancy of a traffic path by a call.
Erlang It is the unit of traffic that represents the total use of one channel or one call perhour that lasts for one hour.
Set-up time It is the time required to allocate a trunk (or trunked radio) channel.
Blocked call or lost call It is the call that cannot be completed due to lack of channels.
Some definitions
Traffic Characterization
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Traffic intensity, more commonly called the traffic, is defined as the average
number ofcalls in progress. Although this is a dimensionless quantity, the unit of
traffic has been given a nameerlang (abbreviated as E).
In a group of channels, the average number of calls in progress depends on both
the number of calls that arrive and their duration. The duration of a call is often
called its holding time .
The traffic carried by a group of trunks is given by
Where,A is the traffic in erlangs, c is the average number of call arrivals during
time T , and h is the average call holding time.
Traffic Characterization
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Call Handling and Grade of Service
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When all the channels become occupied, the system cannot accept further calls.
This state is known as congestion .
The arrival of a new call can then be handled by either of the following two
methods:
(a) Blocked (for a lost call system based on circuit switching)
(b) Queued (for a delayed system based on message or packet switching)
Traffic carried = Traffic offered - Traffic lost
Call Handling and Grade of Service
The proportion of calls lost or delayed due to congestion is used to measure
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The proportion of calls lost or delayed due to congestion is used to measure
the service quality, called the grade of service (GOS). The GOS B for a lost call
system may be defined as
B = Number of calls lost/Number of calls offered
B = Traffic lost/Traffic offered
= Probability that a call will be lost due to congestion
Thus, if a traffic ofA erlangs is offered to a group of trunks or channels having
a GOS B , the traffic lost is AB and the traffic carried is
A (1 B ) erlangs.
Larger the GOS value, worse will be the service provided.
Erlang B and C tables are readily available in Appendix F
Mathematical Modelling of Traffic
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A simple mathematical model is based on the following assumptions:
Pure chance traffic--If the traffic is pure chance, all call arrivalsand call terminations are independent random events,
therefore, sometimes called memoryless traffic . A commonly
used model for random, mutually independent message (here,
call) arrivals is the Poisson process .
Statistical equilibrium--According to the statistical equilibrium
assumption, the generation of traffic is a stationary random
process; that is, the probabilities do not change during the
period being considered. Consequently, the mean number of
calls in progress remains constant.
Mathematical Modelling of Traffic
Poisson Process
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The probability of two or more arrivals happening during is
negligible compared to the probability of zero or one arrival. The distribution of the number of arrivals in a time interval of t to
t + T is independent of the starting time t . T is the interval between
call arrivals or the interval between two random events.
The probability of the number of call arrivals in a given time has aPoisson distribution given by
Poisson Process
Markov model for a number of occupied channels in a
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P(1), P (2), , P ( N ) are the state probabilities .
P0,1, P1,2,and so on are the probabilities of a state increment.
network wit N channels
Erlang B Formula
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Erlang determined the GOS (i.e., the loss
probability) of a lost call system having N trunkswhen the offered traffic isA . Thesolution/formula was obtained on the basis of the
following assumptions.: Pure chance traffic
Statistical equilibrium
Full availability
Loss of calls encountering congestion
The formula is
Erlang B Formula
Erlang C Formula
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In an Erlang C telephone system, N channels areavailable. New calls are assigned a channeluntilall channels are full. When all the channels areoccupied, a new call is queued until it can beserved. This is in contrast to an Erlang B system in
which new calls are blocked. The assumptions made here are the same as in
an Erlang B system, except the fourth one.
The formula is
Erlang C Formula
Spectrum efficiency of cellular systems
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Let
k = number of channels per cell
N = cluster size
Wchannel = total bandwidth for the cellular net
Wsignal = occupied bandwidth per channel
Then Wchannel = kNWsignal
The spectrum efficiency SE of a cellular net can be defined as the carried traffic per
cell Ac, expressed in erlangs, divided by the bandwidth of the total systemWchannel and the area of the cell Su. Ac is mostly computed from the Erlang B
formula, with Ac equal to the attempted traffic multiplied by the probability of
success (= 1 blocking probability). Spectrum efficiency is expressed in
erlang/MHz/
Spectrum efficiency of cellular systems
Location management
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Oxford University Press 2014. All rights reserved.
The study of location management aims at tracking a subscriber
(with an active mobile unit) on move and at reducing the
overhead incurred in locating that subscriber in a cellular
environment, though he/she moves continuously from cell to cell.
Actually, mobility management in cellular systems consists of two
components:
handover management
location management.
Location management
Location management
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In literature, location update schemes are classified into two main groups:
Static or global scheme Location update is triggered based on thetopology of the network.
Dynamic or local scheme A mobile sends a location update messageaccording to the time elapsed (time-based method), the number of
cells visited (movement-based method), or the distance in terms ofcells travelled (distance-based method).
The following are a few terms related to location management:
Centre cell It is the cell where the last location update occurred.
Residing area It is the area in which the mobile unit can be located. Polling cycle It is the process performed by the network when a call
arrives at a mobile terminal. The network sends a polling signal to thetarget cell in the residing area and waits for the response.
Location management