Overview of CMC Slides

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Cellular Wireless Networks



Cellular Network OrganizationUse multiple low-power transmitters (100 W or less)Areas divided into cells

Each served by its own antennaServed by base station consisting of transmitter,receiver, and control unitBand of frequencies allocatedCells set up such that antennas of all neighbors areequidistant (hexagonal pattern)



Cells







Cell size and capacityCell size determines number of cellsavailable to cover geographic area and(with frequency reuse) the totalcapacity available to all usersCapacity within cell limited by availablebandwidth and operationalrequirementsEach network operator has to size cellsto handle expected traffic demand



Cell structureImplements space division multiplex: base station covers acertain transmission area (cell)Mobile stations communicate only via the base station

Advantages of cell structures:

higher capacity, higher number of usersless transmission power neededmore robust, decentralizedbase station deals with interference, transmission area etc.locally

Problems:fixed network needed for the base stationshandover (changing from one cell to another) necessaryinterference with other cells

Cell sizes from some 100 m in cities to, e.g., 35 km on thecountry side (GSM) - even less for higher frequencies



Capacity of a Cellular SystemFrequency R e-Use DistanceThe K factor or the cluster size

Cellular coverage or Signal tointerference ratioSectoring



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Frequency re-use distance is based on the cluster size K

The cluster size is specified in terms of the offset of the center of a cluster fromthe center of the adjacent cluster

K = i2 + ij + j2

K = 2 2 + 2*1 + 1 2

K = 4 + 2 + 1

K = 7

D = D 3K * R

D = 4.58R

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The K factor and Frequency Re-Use Distance



K = i2 + ij + j2

K = 2 2 + 2*0 + 0 2

K = 4 + 0 + 0

K = 4D = D 3K * R

D = 3 .46R i

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The Frequency Re-Use for K = 4



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The Cell Structure for K = 7



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Cell Structure for K = 4



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Cell Structure for K = 12



Increasing cellular systemcapacity

Cell sectoringDirectional antennas subdivide cell into 3or 6 sectorsMight also increase cell capacity by factorof 3 or 6



Increasing cellular systemcapacity

Cell splittingDecrease transmission power in base andmobileR esults in more and smaller cellsR euse frequencies in non-contiguous cellgroups

Example: ½ cell radius leads 4 foldcapacity increase



Tri-Sector antenna for a cell



Highway

TownSuburb

R ural

Cell Distribution in a Network



Optimum use of frequencyspectrum

Operator bandwidth of 7.2MHz (36 freq of 200 kHz)TDMA 8 traffic channels per carrier

K factor = 12What are the number of traffic channelsavailable within its area for these three cases

Without cell splitting

With 72 cellsWith 246 cells



One Cell = 288 traffic channels

72 Cell = 1728 traffic channels

246 Cell = 5904 traffic channels

R e-use of the frequency

8 X 36 = 288

8 X (72/12 X 36) = 1728



F requency ReuseAdjacent cells assigned different frequencies toavoid interference or crosstalk Objective is to reuse frequency in nearby cells

10 to 50 frequencies assigned to each cellTransmission power controlled to limit power at thatfrequency escaping to adjacent cellsThe issue is to determine how many cells must

intervene between two cells using the same frequency



Approaches to Cope withIncreasing Capacity

Adding new channelsF requency borrowing ± frequencies are taken fromadjacent cells by congested cells

Cell splitting ± cells in areas of high usage can besplit into smaller cellsCell sectoring ± cells are divided into a number of wedge-shaped sectors, each with their own set of

channelsMicrocells ± antennas move to buildings, hills,and lamp posts



Cellular System Overview



Cellular Systems TermsBase Station (BS) ± includes an antenna, acontroller, and a number of receiversMobile telecommunications switching office

(MTSO) ± connects calls between mobile unitsTwo types of channels available between mobileunit and BS

Control channels ± used to exchange informationhaving to do with setting up and maintaining callsTraffic channels ± carry voice or data connection

between users



Steps in an MTSO ControlledCall between Mobile Users

Mobile unit initializationMobile-originated call

PagingCall acceptedOngoing call

Handoff



Additional F unctions in anMTSO Controlled Call

Call blockingCall termination

Call dropCalls to/from fixed and remote mobilesubscriber



Mobile Radio PropagationEffects

Signal strengthMust be strong enough between base station and mobileunit to maintain signal quality at the receiver

Must not be so strong as to create too much cochannelinterference with channels in another cell using thesame frequency band

F ading

Signal propagation effects may disrupt the signal andcause errors



Handoff Performance MetricsCell blocking probability ± probability of a newcall being blockedCall dropping probability ± probability that a call

is terminated due to a handoff Call completion probability ± probability that anadmitted call is not dropped before it terminatesProbability of unsuccessful handoff ± probability

that a handoff is executed while the receptionconditions are inadequate



Handoff Performance MetricsHandoff blocking probability ± probability that ahandoff cannot be successfully completedHandoff probability ± probability that a handoff occurs before call terminationRate of handoff ± number of handoffs per unittimeInterruption duration ± duration of time during ahandoff in which a mobile is not connected toeither base stationHandoff delay ± distance the mobile moves fromthe point at which the handoff should occur to the

point at which it does occur



Handoff Strategies Used toDetermine Instant of Handoff

Relative signal strengthRelative signal strength with threshold

Relative signal strength with hysteresisRelative signal strength with hysteresis andthreshold

Prediction techniques



Power ControlDesign issues making it desirable to includedynamic power control in a cellular system

Received power must be sufficiently above the

background noise for effective communicationDesirable to minimize power in the transmitted signalfrom the mobile

Reduce cochannel interference, alleviate health concerns, save battery power

In SS systems using CDMA, it¶s desirable to equalizethe received power level from all mobile units at the BS



Types of Power ControlOpen-loop power control

Depends solely on mobile unit No feedback from BS

Not as accurate as closed-loop, but can react quicker tofluctuations in signal strength

Closed-loop power controlAdjusts signal strength in reverse channel based onmetric of performanceBS makes power adjustment decision andcommunicates to mobile on control channel



Traffic EngineeringIdeally, available channels would equalnumber of subscribers active at one timeIn practice, not feasible to have capacityhandle all possible loadF or N simultaneous user capacity and L

subscribers L < N

± nonblocking system L > N ± blocking system



Blocking System PerformanceQuestions

Probability that call request is blocked?What capacity is needed to achieve a certainupper bound on probability of blocking?What is the average delay?What capacity is needed to achieve a certainaverage delay?



Traffic Intensity

Load presented to a system:

P = mean rate of calls attempted per unit timeh = mean holding time per successful callA = average number of calls arriving during averageholding period, for normalized P

h A P!





F irst-Generation Analog

Advanced Mobile Phone Service (AMPS)In North America, two 25-MHz bands allocatedto AMPS

One for transmission from base to mobile unitOne for transmission from mobile unit to base

Each band split in two to encouragecompetitionF requency reuse exploited



AMPS OperationSubscriber initiates call by keying in phonenumber and presses send keyMTSO verifies number and authorizes user

MTSO issues message to user¶s cell phoneindicating send and receive traffic channelsMTSO sends ringing signal to called partyParty answers; MTSO establishes circuit and

initiates billing informationEither party hangs up; MTSO releases circuit,frees channels, completes billing



Differences Between F irst andSecond Generation Systems

Digital traffic channels ± first-generation systemsare almost purely analog; second-generationsystems are digital

Encryption ± all second generation systems provide encryption to prevent eavesdroppingError detection and correction ± second-generationdigital traffic allows for detection and correction,giving clear voice receptionChannel access ± second-generation systems allowchannels to be dynamically shared by a number of users



Mobile Wireless TDMA DesignConsiderations

Number of logical channels (number of time slotsin TDMA frame): 8Maximum cell radius (R): 35 km

F requency: region around 900 MHzMaximum vehicle speed ( V m ):250 km/hr Maximum coding delay: approx. 20 msMaximum delay spread ( ( m ): 10 QsBandwidth: Not to exceed 200 kHz (25 kHz per channel)





GSM Network Architecture



Mobile StationMobile station communicates across Um interface(air interface) with base station transceiver insame cell as mobile unit

Mobile equipment (ME) ± physical terminal, suchas a telephone or PCSME includes radio transceiver, digital signal processorsand subscriber identity module (SIM)

GSM subscriber units are generic until SIM isinserted

SIMs roam, not necessarily the subscriber devices





Network Subsystem (NS) NS provides link between cellular network and public switched telecommunications networks

Controls handoffs between cells in different BSSs

Authenticates users and validates accountsEnables worldwide roaming of mobile users

Central element of NS is the mobile switchingcenter (MSC)



Mobile Switching Center (MSC)Databases

Home location register (HLR) database ± storesinformation about each subscriber that belongs toitVisitor location register (VLR) database ± maintains information about subscribers currently

physically in the regionAuthentication center database (AuC) ± used for authentication activities, holds encryption keysEquipment identity register database (EIR) ± keeps track of the type of equipment that exists atthe mobile station



TDMA F ormat ± Time SlotF ields

Trail bits ± allow synchronization of transmissionsfrom mobile unitsEncrypted bits ± encrypted data

Stealing bit - indicates whether block containsdata or is "stolen"Training sequence ± used to adapt parameters of receiver to the current path propagationcharacteristics

Strongest signal selected in case of multipath propagation

Guard bits ± used to avoid overlapping with other bursts



GSM Speech Signal Processing

l l



GSM Signaling ProtocolArchitecture



F unctions Provided by ProtocolsProtocols above the link layer of the GSMsignaling protocol architecture providespecific functions:

Radio resource managementMobility managementConnection managementMobile application part (MAP)BTS management



Advantages of CDMA Cellular F requency diversity ± frequency-dependenttransmission impairments have less effect onsignal

Multipath resistance ± chipping codes used for CDMA exhibit low cross correlation and lowautocorrelationPrivacy ± privacy is inherent since spreadspectrum is obtained by use of noise-like signalsGraceful degradation ± system only graduallydegrades as more users access the system



Drawbacks of CDMA Cellular Self-jamming ± arriving transmissions frommultiple users not aligned on chip boundariesunless users are perfectly synchronized

Near-far problem ± signals closer to the receiver are received with less attenuation than signalsfarther awaySoft handoff ± requires that the mobile acquiresthe new cell before it relinquishes the old; this ismore complex than hard handoff used in F DMAand TDMA schemes

M bil Wi l CDMA D i



Mobile Wireless CDMA DesignConsiderations

RAKE receiver ± when multiple versions of asignal arrive more than one chip interval apart,RAKE receiver attempts to recover signals frommultiple paths and combine them

This method achieves better performance than simplyrecovering dominant signal and treating remainingsignals as noise

Soft Handoff ± mobile station temporarily

connected to more than one base stationsimultaneously



Principle of RAKE Receiver

T f Ch l S t d b



Types of Channels Supported byF orward Link

Pilot (channel 0) - allows the mobile unit toacquire timing information, provides phasereference and provides means for signal strengthcomparisonSynchronization (channel 32) - used by mobilestation to obtain identification information aboutcellular systemPaging (channels 1 to 7) - contain messages for one or more mobile stationsTraffic (channels 8 to 31 and 33 to 63) ± theforward channel supports 55 traffic channels

F d T ffi Ch l



F orward Traffic ChannelProcessing Steps

Speech is encoded at a rate of 8550 bpsAdditional bits added for error detectionData transmitted in 2-ms blocks with forwarderror correction provided by a convolutionalencoder Data interleaved in blocks to reduce effects of errorsData bits are scrambled, serving as a privacy mask

F d T ffi Ch l



F orward Traffic ChannelProcessing Steps (cont.)

Power control information inserted into trafficchannelDS-SS function spreads the 19.2 kbps to a rate of

1.2288 Mbps using one row of 64 x 64 WalshmatrixDigital bit stream modulated onto the carrier usingQPSK modulation scheme



Third-Generation CapabilitiesVoice quality comparable to the public switchedtelephone network 144 kbps data rate available to users in high-speedmotor vehicles over large areas384 kbps available to pedestrians standing or moving slowly over small areasSupport for 2.048 Mbps for office use

Symmetrical / asymmetrical data transmissionratesSupport for both packet switched and circuitswitched data services



Third-Generation CapabilitiesAn adaptive interface to the Internet to reflectefficiently the common asymmetry betweeninbound and outbound traffic

More efficient use of the available spectrum ingeneralSupport for a wide variety of mobile equipmentF lexibility to allow the introduction of newservices and technologies



Alternative Interfaces



CDMA Design ConsiderationsBandwidth ± limit channel usage to 5 MHzChip rate ± depends on desired data rate, need for error control, and bandwidth limitations; 3 Mcps

or more is reasonableMultirate ± advantage is that the system canflexibly support multiple simultaneousapplications from a given user and can efficientlyuse available capacity by only providing thecapacity required for each service

Documents

Overview of CMC Slides