Cellular Systems Basics

8/12/2019 Cellular Systems Basics

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Definitions


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definitions


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Definitions


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basic cellular systemMobile Switching Center (MSC)


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MSC

The MSC coordinates the activities of all of the base

stations and connects the entire cellular system to

the PST! " ty#ical MSC handles $%%&%%% cellular

subscribers arid '&%%% simultaneous conversations ata time& and accommodates all billing and system

maintenance functions& as well! n large cities&

several MSCs are used by a single carrier!


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Ty#es of channels

forward voice channels (FC)

reverse voice channels (*C)!

forward control channels (FCC)

reverse control channels (*CC)!

Control channels are often called setu# channels

because they are only involved in setting u# a

call and moving it to an unused voice channel!


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Since the control channels are standardi,ed andare identical throughout different mar-ets withinthe country or continent& every #hone& scans thesame channels while idle!

.hen a tele#hone call is #laced to a mobile user&the MSC dis#atches the re/uest to all basestations in the cellular system!


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The mobile identification number (M)& which is thesubscriber0s tele#hone number& is then broadcastas a #aging message over all of the forward controlchannels throughout the cellular system!

The mobile receives the #aging message sent bythe base station which it monitors& and res#onds byidentifying itself over the reverse control channel!


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The base station relays the

ac-nowledgment sent by the mobile and

informs the MSC of the handsha-e! Then&

the MSC instructs the base station to

move the call to an unused voice channel

within the cell (ty#ically& between ten to

si1ty voice channels and 2ust one controlchannel are used in each cell0s base

station)!


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"t this #oint the base station signals the

mobile to change fre/uencies to an

unused forward and reverse voice channel

#air& at which #oint another data message

(called an alert) is transmitted over the

forward voice channel to instruct the

mobile tele#hone to ring!


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.hen a mobile originates a call

" call initiation re/uest is sent on thereverse control channel!

.ith this re/uest the mobile unit transmits

its tele#hone number (M)& electronicserial number (3S)& and the tele#honenumber of the called #arty!

The mobile also transmits a station classmar- (SCM) which indicates what thema1imum transmitter #ower level is for the#articular user!


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The cell base station receives this data and

sends it to the MSC! The MSC validates

the re/uest& ma-es connection to the called

#arty through the PST& and instructs thebase station and mobile user to move to an

unused forward and reverse voice channel

#air to allow the conversation to begin!


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Channel "ssignment Strategies

ncreasing ca#acity and minimi,ing

interference

Fi1ed and Dynamic

The choice of channel assignment

strategy im#acts the #erformance of the

system& #articularly as to how calls are

managed when a mobile user is handed

off from one cell to another


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*oaming

"ccom#lished over the FCC& since each

roamer is cam#ed on to a FCC at all

times! 3very several minutes& the MSC

issues a global command over each FCCin the system& as-ing for all mobiles which

are #reviously unregistered to re#ort their

M and 3S over the *CC!


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Fi1ed channel assignment

3ach cell is allocated a #redetermined set of voice

channels!

f all the channels in that cell are occu#ied& the call is

bloc-ed4 one a##roach& called the borrowing strategy& a cell is

allowed to borrow channels from a neighboring cell if all of

its own channels are already occu#ied!


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Fi1ed channel assignment

(borrowing strategy)

The mobile switching center (MSC)

su#ervises such borrowing #rocedures and

ensures that the borrowing of a channel does

not disru#t or interfere with any of the calls in

#rogress in the donor cell!


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Dynamic Channel "ssignment

3ach time a call re/uest is made& theserving base station re/uests a channelfrom the MSC!

The switch then allocates a channel to there/uested cell following an algorithm thatconsiders54

6i-elihood of fi1ture bloc-ing within the cell4 Fre/uency of use of the candidate channel&

4 *euse distance of the channel

4 7ther cost functions!


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The MSC only allocates a given fre/uency

if that fre/uency is not #resently in use in

the cell or any other cell which falls within

the minimum restricted distance offre/uency reuse to avoid co8channel

interference!


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*educes the li-elihood of bloc-ing& which increases thetrun-ing ca#acity of the system& since all the availablechannels in a mar-et are accessible to all of the cells!

re/uires the MSC to collect real8time data on channel

occu#ancy& traffic distribution& and radio signal strengthindications (*SS) of all channels on a continuousbasis!


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+andoff Strategies

.hen a mobile moves into a different cell while aconversation is in #rogress& the MSCautomatically transfers the call to a new channelbelonging to the new base station!

This handoff o#eration not only involves a newbase station& but also re/uires that the voice andcontrol signals be allocated to channelsassociated with the new base station!


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+andoff

7nce a #articular signal level is s#ecifiedas the minimum usable signal foracce#table voice /uality at the base

station receiver (normally ta-en asbetween 9:% d;m and 9$%% d;m)& aslightly stronger signal level is used as athreshold at which a handoff is made!

This margin cannot be too large or toosmall!


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+andoff margin

f the margin is too large& unnecessary

handoffs which burden the MSC may

occur& and if he margin is too small& there

may be insufficient time to com#lete ahandoff before a call is lost due to wea-

signal conditions


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Pro#er and m#ro#er handoff


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+andoff

" dro##ed call event can ha##en when there is an

e1cessive delay by the MSC in assigning a handoff&

or when the threshold (margin) is set too small for the

handoff time in the system!

31cessive delays may occur during high traffic

conditions due to com#utational loading at the MSC

or due to the fact that no channels are available on

any of the nearby base stations (thus forcing the MSC

to wait until a channel in a nearby cell becomes free)!


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+andoff

n deciding when to handoff& it is im#ortant

to ensure that the dro# in the measured

signal level is not due to momentary fading

and that the mobile is actually moving awayfrom the serving base station! n order to

ensure this& the base station monitors the

signal level for a certain #eriod of timebefore a handoff is initiated!


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+andoff First


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+andoff First


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+andoff Second


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+andoff Second


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+andoff Second


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;loc-ing handoff= bloc-ing call

From the user0s #oint of view& having a call

abru#tly terminated while in the middle of

a conversation is more annoying than

being bloc-ed occasionally on a new callattem#t!


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Prioriti,ing +andoffs

guard channel conce#t

4 " fraction of the total available channels in a

cell is reserved e1clusively for handoff

re/uests from ongoing calls which may behanded off into the cell!

4 This method has the disadvantage of

reducing the total carried traffic& as fewer

channels are allocated to originating calls!


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guard channel conce#t (advantage)

4 7ffer efficient s#ectrum utili,ation when

dynamic channel assignment strategies are

used& which minimi,e the number of re/uiredguard channels by efficient demand8based

allocation!


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>ueuing conce#t4 Tradeoff between the decrease in #robability

of forced termination and total carried traffic!

4 >ueuing of handoffs is #ossible since there is

a finite time interval between the time thereceived signal level dro#s below the handoffthreshold and the time the call is terminateddue to insufficient signal level!

4 The delay time and si,e of the /ueue isdetermined from the traffic #attern of the#articular service area!


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>ueuing conce#t

4 t should be noted that /ueuing does not

guarantee a ,ero #robability of forced

termination& since large delays will cause thereceived signal level to dro# below the

minimum re/uired level to maintain

communication and hence lead to forced

termination!


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Practical +andoff Considerations

Problems arise when attem#ting to design for a

wide range of mobile velocities!

4 +igh s#eed vehicles (/uic-ly through a cell)

4 Pedestrian (may never need handoff) .ith microcells& the MSC can /uic-ly become

burdened if high s#eed users are constantly

being #assed between very small cells!

The idea is to have schemes to minimi,e the

handoff intervention from the MSC with

simultaneous lows#eed and highs#eed users!


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The umbrella8cell a##roach

Different antenna heights and different

#ower levels #rovide ?large? and ?small?

cells which are co8located at a single

location!

Provides large area coverage to high

s#eed users while #roviding small area

coverage to users traveling at low s#eeds!


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The umbrella8cell a##roach

Minimi,es the number of handoffs!

The s#eed of each user may be estimated by the base

station or MSC by evaluating how ra#idly the short8

term average signal strength on the *C changes over

time!

+ d ff id ti


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+andoff considerations

(cell8dragging #roblem)

Pedestrian users that #rovide a very strongsignal to the base station (67S urban

environment)

+andoff may not be made even when the user

has traveled well beyond the designed range ofthe cell (dee# into a neighburing cell)

Potential interference and traffic management

#roblem! +andoff thresholds and radio coverage

#arameters must be ad2usted carefully!


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+andoff ty#ical features

First generation analog cellular systems4 the ty#ical time to ma-e a handoff& once the

signal level is deemed to be below the

handoff threshold& is about $% seconds! This

re/uires margin between @ d; to $A d;!


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+ard handoff vs! Soft handoff

+ard handoff!

4 Channeli,ed wireless systems that assign

different radio channels during a handoff

Soft +andoff! (S#read s#ectrum mobilesshare the same channel in every cell)

4 means that a different base station handles

the radio communication tas- 4 not a #hysicalchange in the assigned channel!

f h d ff


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Soft handoff

Simultaneously evaluates the received signals

from a single subscriber at several neighboringbase stations!

MSC may actually decide which version of theuser0s signal is best at any moment in time!

This techni/ue e1#loits macrosco#ic s#acediversity #rovided by the different #hysicallocations of the base stations and allows theMSC to ma-e a ?soft? decision as to whichversion of the user0s signal to #ass along to thePST at any instance!

The ability to select between the instantaneousreceived signals from a variety of base stationsis called soft handoff!

Documents

Cellular Systems Basics