Cellular Systems Basics

Cellular Systems Design Foundation

Definitions

Definitions

definitions

Definitions

basic cellular system Mobile Switching Center (MSC)

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 at a time& and accommodates all billing and system maintenance functions& as well! n large cities& several MSCs are used by a single carrier!

Ty#es of channels • forward voice channels (FC) • 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!

+ow a Cellular Tele#hone Call is Made • First scans the grou# of forward control channels to determine the one with the strongest signal! • Then monitors that control channel until the signal dro#s below a usable level! • "t this #oint it again scans the control channels in search of the strongest base station signal!

• Since the control channels are standardi,ed and are identical throughout different mar-ets within the country or continent& every #hone& scans the same channels while idle! • .hen a tele#hone call is #laced to a mobile user& the MSC dis#atches the re/uest to all base stations in the cellular system!

• The mobile identification number (M)& which is the subscriber0s tele#hone number& is then broadcast as a #aging message over all of the forward control channels throughout the cellular system! • The mobile receives the #aging message sent by the base station which it monitors& and res#onds by identifying itself over the reverse control channel!

• 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 control channel are used in each cell0s base station)!

• "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!

.hen a mobile originates a call • " call initiation re/uest is sent on the reverse control channel! • .ith this re/uest the mobile unit transmits its tele#hone number (M)& electronic serial number (3S)& and the tele#hone number of the called #arty! • The mobile also transmits a station class mar- (SCM) which indicates what the ma1imum transmitter #ower level is for the #articular user!

• 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 the base station and mobile user to move to an unused forward and reverse voice channel #air to allow the conversation to begin!

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

*oaming • Subscribers can o#erate in service areas other than the one from which service is subscribed! • .hen a mobile enters a city or geogra#hic area that is different from its home service area& it is registered as a roamer in the new service area!

*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 FCC in the system& as-ing for all mobiles which are #reviously unregistered to re#ort their M and 3S over the *CC!

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-ed 4 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!

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!

Dynamic Channel "ssignment • 3ach time a call re/uest is made& the serving base station re/uests a channel from the MSC! • The switch then allocates a channel to the re/uested cell following an algorithm that considers5 4 6i-elihood of fi1ture bloc-ing within the cell 4 Fre/uency of use of the candidate channel& 4 *euse distance of the channel 4 7ther cost functions!

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 of fre/uency reuse to avoid co8channel interference!

Dynamic Channel "ssignment • *educes the li-elihood of bloc-ing& which increases the trun-ing ca#acity of the system& since all the available channels 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 strength indications (*SS) of all channels on a continuous basis!

+andoff Strategies • .hen a mobile moves into a different cell while a conversation is in #rogress& the MSC automatically transfers the call to a new channel belonging to the new base station! • This handoff o#eration not only involves a new base station& but also re/uires that the voice and control signals be allocated to channels associated with the new base station!

*e/uirements for successful handoff • "s infre/uently as #ossible • m#erce#tible to the users • n order to meet these re/uirements& system designers must s#ecify an o#timum signal level at which to initiate a handoff!

+andoff • 7nce a #articular signal level is s#ecified as the minimum usable signal for acce#table voice /uality at the base station receiver (normally ta-en as between 9:% d;m and 9$%% d;m)& a slightly stronger signal level is used as a threshold at which a handoff is made! • This margin cannot be too large or too small!

+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 a handoff before a call is lost due to weasignal conditions

Pro#er and m#ro#er handoff

+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)!

+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 away from the serving base station! n order to ensure this& the base station monitors the signal level for a certain #eriod of time before a handoff is initiated!

Dwell Time • The time over which a call may be maintained within a cell& without handoff& is called the dwell time! The dwell time of a #articular user is governed by a number of factors& which include #ro#agation& interference& distance between the subscriber and the base station& and other time varying effects!

+andoff First
+andoff First
+andoff Second
+andoff Second
+andoff Second
;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 call attem#t!

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 be handed off into the cell! 4 This method has the disadvantage of reducing the total carried traffic& as fewer channels are allocated to originating calls!

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/uired guard channels by efficient demand8based allocation!

Prioriti,ing +andoffs • >ueuing conce#t 4 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 the received signal level dro#s below the handoff threshold and the time the call is terminated due to insufficient signal level! 4 The delay time and si,e of the /ueue is determined from the traffic #attern of the #articular service area!

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 the received signal level to dro# below the minimum re/uired level to maintain communication and hence lead to forced termination!

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!

+andoff Considerations • n #ractice it is difficult for cellular service #roviders to obtain new #hysical cell site locations in urban areas! • it is often more attractive for a cellular #rovider to install additional channels and base stations at the same #hysical location of an e1isting cell!

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!

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!

+andoff considerations (cell8dragging #roblem) • Pedestrian users that #rovide a very strong signal to the base station (67S urban environment) • +andoff may not be made even when the user has traveled well beyond the designed range of the cell (dee# into a neighburing cell) • Potential interference and traffic management #roblem! • +andoff thresholds and radio coverage #arameters must be ad2usted carefully!

+andoff ty#ical features • First generation analog cellular systems 4 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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Modern +andoff features • ewer cellular systems can ma-e handoff decisions based on a wide range of metrics other than signal strength! 4 The cochannel and ad2acent channel interference levels may be measured at the base station or the mobile& 4 This information may be used with conventional signal strength data to #rovide a multi8dimensional algorithm for determining  when a handoff is needed!

+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 mobiles share the same channel in every cell) 4 means that a different base station handles the radio communication tas- 4 not a #hysical change in the assigned channel!