Cellular Systems Basics
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Transcript of 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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Dynamic Channel "ssignment
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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Dynamic Channel "ssignment
*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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Prioriti,ing +andoffs
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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Prioriti,ing +andoffs
>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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Prioriti,ing +andoffs
>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!