Cellular Wireless NetworksCellular Wireless Networks

Chapter 10Chapter 10

C ll l N t k O i tiCellular Network OrganizationU l i l l i (100 WUse multiple low-power transmitters (100 W or less, cell phones under 5 watts, normally mW)Overall coverage area divided into cells

Each cell served by its own antenna system (switched directional)Each cell is served by base station consisting of transmitter, receiver and control unit, the base stations are connected to a MTSO (slide # 8)B d f f i ll t d d l i tiBand of frequencies allocated, duplex communication channels used in a cellular/voice systemCells set up such that antennas of all neighbors areCells set up such that antennas of all neighbors are equidistant (hexagonal pattern)

Cell Pattern (hexagonal)Cell Pattern (hexagonal)

d (3R)1/3d = (3R)1/3

F RFrequency ReuseAdj t ll i d diff t f i t idAdjacent cells assigned different frequencies to avoid interference or crosstalkObjective is to reuse frequency in nearby cells (hiddenObjective is to reuse frequency in nearby cells (hidden transmitter phenomena)

Frequencies assigned to each cell (usually 10 to 50 frequencies)

Transmission power controlled at both ends to limit power at that frequency escaping to adjacent cells (ALE)Th i i t d t i h ll t i tThe issue is to determine how many cells must intervene between two cells that will be using the same frequencies or co-channel cells# of cells in a repetitious pattern where the cells in the pattern use a unique/non-repeating set of frequencies -

hi h i f N i2 +ij + j2Reuse Factor N which must satisfy N = i2 +ij + j2

Frequency Reuse PatternsFrequency Reuse Patterns

Each cell would normallyEach cell would normally have one control channel and a number of voice channels dependent on N

Cells, N Reuse Factor, Clusters Design ConstraintsCells, N Reuse Factor, Clusters Design Constraints

N C ll hi h ll ti l th t f il bl f iN Cells which collectively use the same set of available frequenciesis a Cluster (none of the cells within the cluster use the same frequency)The i and j terms will indicate where the next co-channel cell is located (cells that use the same frequencies)Larger N less co-channel interference as the ratio of the cell radius to distance between co-channel cells decreases A smaller N indicates co-distance between co-channel cells decreases. A smaller N indicates co-channel cells are much closer togetherAs the number of Clusters is replicated within a system, then the total system capacity is increased (more clusters means lower NOverall design is to allocate the total available bandwidth for each cluster (which will be replicated to comprise the total system) and divide up the ( p p y ) pcluster bandwidth amongst the N reuse factor to determine the number of channels per cell. The co-channel interference will dictate the size of N

Approaches to Increasing CapacityApproaches to Increasing Capacity

Adding new channels (have a reserve avaiable)Adding new channels (have a reserve avaiable)Frequency borrowing – frequencies are taken from adjacent cells by congested cellsj y gCell splitting – cells in areas of high usage can be split into smaller cells (more cell equipment per system $$)Cell sectoring – cells are divided into a number of wedge-shaped sectors each with their own set of channels using base station highly directional antennas (120o/60o)base station highly directional antennas (120o/60o)

Microcells – cells become much smaller and antennas move to buildings, hills and lamp posts but still controlled g p pby a single site. Reduced power levels required in the microcells (zone cells)

C ll l S t O iCellular System OverviewDigital (~ T1 Connection)

Cell Tower

MTSO

C ll l S t TCellular Systems TermsBase Station (BS) includes an antenna system aBase Station (BS) – includes an antenna system, a controller and a number of receiversMobile telecommunications switching officeMobile telecommunications switching office (MTSO) – connects calls between mobile unitsTwo types of channels available between mobile ypunit and BS, channels further divided into forward and reverse for duplex communications

C t l h l d t h i f tiControl channels – used to exchange information having to do with setting up and maintaining callsTraffic channels – carry voice or data connection ybetween users

Steps in an MTSO Controlled C ll b t M bil UCall between Mobile Users (Fig 10.6)

Mobile unit initialization (select strongest base station)

bil i i d llMobile-originated call (request for a connection)

Paging (sent from MTSO to other BSs or data base query + verify)Paging (sent from MTSO to other BSs or data base query verify)

Call accepted ( + billing)

Ongoing callHandoff (switching base stations when required)Handoff (switching base stations when required)

Additional Functions in an MTSO C t ll d C llMTSO Controlled Call

ll bl ki ( b b i l )Call blocking (BS busy, busy signal sent)Call termination (releases traffic channels)Call termination (releases traffic channels)Dropped Call (loss of signal strength)Calls to/from fixed and remote mobile subscriber (use of public switched telephone ( p pnetwork – POTS). POTS is also used to connect to a remote MTSOconnect to a remote MTSO

M bil R di P ti Eff tMobile Radio Propagation EffectsSignal strength (microwave frequencies)Signal strength (microwave frequencies)

Must be strong enough between base station and mobile unit to maintain signal quality at the receiverMust not be so strong as to create excessive cochannel interference with channels in another cell using the same frequency band Adaptive control of power verysame frequency band. Adaptive control of power very key to system.

FadingFadingSignal propagation effects may disrupt the signal and cause errorsWireless RF environment extremely tough. Operational aspects don’t help either (small omni-directional antennas in handsets users move around handoffsantennas in handsets, users move around handoffs, low-power, non-optimum/urban locations)

H d ffHandoffs - Performance Metrics for Decision

C ll bl ki b bilit b bilit fCell blocking probability – probability of a new call being blockedC ll d i b bilit b bilit th t llCall dropping probability – probability that a call is terminated due to a handoffC ll l ti b bilit b bilit th tCall completion probability – probability that an admitted call is not dropped before it terminatesP b bilit f f l h d ff b bilitProbability of unsuccessful handoff – probability that a handoff is executed while the reception conditions are inadequateconditions are inadequate

H d ff P f M t iHandoff Performance Metrics (cont)

Handoff blocking probability – probability that aHandoff blocking probability – probability that a handoff cannot be successfully completedHandoff probability – probability that a handoffHandoff probability probability that a handoff occurs before call terminationRate of handoff – number of handoffs per unitRate of handoff number of handoffs per unit timeInterruption duration – duration of time during aInterruption duration duration of time during a handoff in which a mobile is not connected to either base stationHandoff delay – distance the mobile moves from the point at which the handoff should occur to the ppoint at which it does occur

Various Handoff Strategies Used t D t i I t t f H d ffto Determine Instant of Handoff

l i i l hRelative signal strengthRelative signal strength with thresholdRelative signal strength with thresholdRelative signal strength with hysteresis (prevent ‘oscillating’ handoffs)(prevent oscillating handoffs)

Relative signal strength with hysteresis and h h ldthreshold

Prediction techniquesq

P C t lPower ControlDesign issues make it desirable to includeDesign issues make it desirable to include dynamic power control in a cellular system

Received power must be sufficiently above theReceived power must be sufficiently above the background noise for effective communicationDesirable to minimize power in the transmitted signal from the mobilefrom the mobile

Reduce cochannel interference, alleviate health concerns, save battery powerDynamic transmitter power control becoming mandatory inDynamic transmitter power control becoming mandatory in almost all modern RF communications systems

In SS systems using CDMA, it’s desirable to equalize the recei ed po er le el from all mobile nits at the BSthe received power level from all mobile units at the BS so as to maximize # of users (N = 1 in CDMA) with a usable SNR for all the ‘customers’ in each cell

T f P C t lTypes of Power ControlO l lOpen-loop power control

Depends solely on mobile unit (monitor Base Station il i l)pilot signal)

No feedback from BSl d l b i kNot as accurate as closed-loop, but can react quicker to

fluctuations in signal strength

Cl d l lClosed-loop power controlAdjusts signal strength in the reverse channel based on

f t iperformance metricsBS makes power adjustment decision and communicates to mobile on control channel to setcommunicates to mobile on control channel to setthe user’s cell phone power level

T ffi E i iTraffic Engineering (Queueing Theory)

Id ll h b f il bl h l ldIdeally the number of available channels would equal the number of subscribers active at one timeIn practice, not economically feasible to have a capacity to handle all of the possible (maximum) load at all times Traffic EngineeringFor N simultaneous user capacityp yand L subscribers

L < N – nonblocking system (cost not an issue)g y ( )L > N – blocking system (nominal)

Bl ki S P f Q iBlocking System Performance Questions

b bili h ll i bl k dProbability that a call request is blocked?What capacity is needed to achieve a certainWhat capacity is needed to achieve a certain upper bound on probability of blocking?Wh t i th d l ? ( i )What is the average delay? (service queue)What capacity is needed to achieve a certain p yaverage delay?

T ffi I t itTraffic Intensityd dLoad presented to a system:

hA λλ = mean rate of calls attempted per unit time

hA λ=λ mean rate of calls attempted per unit timeh = mean holding time per successful callA = average number of calls arriving during theA average number of calls arriving during the average holding period (a normalized version of λ)A is called Traffic Intensity or Offered Traffic (notA is called Traffic Intensity or Offered Traffic (not

necessarily carried) and is a dimensionless unit in ErlangsThis is the normal start point for Queuing Theoryp Q g y

Intro to Queueing TheoryIntro to Queueing Theory(the definitive reference is Queueing Systems by L. Kleinrock, any good textbook ( Q g y y y gor a good http://www.dcs.ed.ac.uk/home/jeh/Simjava/queueing/)

λ = mean arrival rate in packets/secondsh = 1/µ = average holding time in seconds/packetsh = 1/µ = average holding time in seconds/packets

thus µ = service time in packets/second (more common term as compared to h in Stallings text)

A = λ h = λ / µ Traffic Intensity (dimensionless unit in Erlangs)

also called Offered TrafficA = λ h Traffic Intensity is a critical parameter, network congestion

d l A 1develops as A 1

Traffic Congestion (buffer fills up with messages)Traffic Congestion (buffer fills up with messages)

Traffic Intensity

Q i M d lQueueing Models

Easiest mathematical representation of a queueing model is the M/M/1 Queue, which is aMarkov Arrival/Markov Service Interval/One Server (M/M/1) model that has a finite buffer with a service interval that is modeled with exponentially distributed service times(mathematically one of the subsets of a Markov process)

Various service disciplines and buffer considerations discussed in text, pages 278 -282 and Appendix A –Traffic Analysis (page 511)

M lti Q i M d lMulti-server Queueing Model

M/M/2 finite buffer shown occupied with n packets)

Factors that Determine the Nature of th T ffi M d lthe Traffic Model (different queueing disciplines)

M i hi h bl k d ll h dl dManner in which blocked calls are handledBlocked calls rejected and dropped (no queue, Erlang B System)

Lost calls cleared (LCC) – user waits before another attemptSometimes referred to as Block Calls ClearedLost calls held (LCH) – user repeatedly attempts calling

Lost calls delayed (LCD) – blocked calls put in a queue awaiting a y ( ) p q gfree channel (Erlang C System)

Appendix A – Traffic Analysis (starting on page 511)E l B A tiErlang B Assumptions:

Call requests are memoryless (all users may request channel at any time)All channels fully available unless occupiedThe probability of a user occupying a channel (service time) is exponentially distributed longer calls are less likelyexponentially distributed longer calls are less likely

Traffic ModelsTraffic Models (continued)

Erlang B Mathematical assumptions continued:Erlang B Mathematical assumptions continued:Finite number of channels available in the trunking poolRequests for service are Poisson distributed which implies

ti l di t ib ti ( l t i l )exponential distribution (memoryless triangle)Interarrival times of call requests are independent

Many different types of traffic modelsMany different types of traffic modelsWhether number of users is assumed to be finite or infiniteLost calls cleared or delayed

Grade of Service (GOS) – probability that an attempted call is blocked (lost, no queue) or delayed (queued) which ties into that amount of traffic that can be handled by aties into that amount of traffic that can be handled by a given capacity

Fi t G ti A lFirst-Generation Analogd d bil h i ( )Advanced Mobile Phone Service (AMPS)In North America, two 25-MHz bands allocated ,to AMPS

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

Each band split in two to encourageEach band split in two to encourage competitionFrequency reuse exploitedFrequency reuse exploited

AMPS O tiAMPS OperationS b ib i iti t ll b k i i hSubscriber initiates call by keying in phone number and presses send keyMTSO ifi b d th iMTSO verifies number and authorizes userMTSO issues message to user’s cell phone i di ti d d i t ffi h lindicating 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 First and S d G ti S tSecond Generation Systems

Di it l t ffi h l fi t ti tDigital traffic channels – first-generation systems are almost purely analog; second-generation systems are digitalsystems are digitalEncryption – all second generation systems provide encryption to prevent eavesdroppingprovide encryption to prevent eavesdroppingError detection and correction – second-generation digital traffic allows for detection and correctiondigital traffic allows for detection and correction, giving clear voice receptionChannel access – second-generation systems allowChannel access – second-generation systems allow channels to be dynamically shared by a number of users (TDMA or CDMA)use s ( o C )

Mobile Wireless TDMA Design G i R i tGeneric Requirements

N b f l i l h l ( b f ti l tNumber of logical channels (number of time slots in TDMA frame): 8 (min req for multiplexing considerations)

M i ll di (R) 35 kMaximum cell radius (R): 35 km (for traffic levels)

Frequency: region around 900 MHz (allocated)

Maximum vehicle speed (Vm): 250 km/hr (for trains)

Maximum coding delay: approx. 20 ms( d l f i i b d i di )(max delay for voice conversations based on propagation distance)

Maximum delay spread (Δm): 10 μs (max multipath)

B d id h N d 200 kHBandwidth: Not to exceed 200 kHz(25 kHz per channel; typical channel bandwidth)

Digitize(bit rates)

DigitaU

s

Steps in Design of TDMA Timeslot

(bit rates)

Packetization

Delay

al System D

es

ser Design Co Delay

FEC/redundancyoverhead ~ 50%

Adaptive equalization, set max

sign Constrain

onstraints

Data rate

xmission duration

nts

# of time slots, guard bits

Fig 10.12 page 290

TDMA Time SlotTDMA Time SlotTo correct for changes inTo correct for changes in transmission path characteristics

Global System for Mobile Communications (GSM) Network Architecture (2nd Generation, start Europe)

Generic handsets until SIM card is inserted

M bil St tiMobile StationM bil t ti i t U i t fMobile station communicates across Um interface (air interface) with base station transceiver in same cell as mobile unitsame cell as mobile unitMobile equipment (ME) – physical terminal, such as a telephone or PCSas a telephone or PCS

ME includes radio transceiver, digital signal processors and subscriber identity module (SIM)y ( )

GSM subscriber units are generic until SIM is inserted (smart card)( )

SIMs roam, not necessarily the subscriber devices

B St ti S b t (BSS)Base Station Subsystem (BSS)BSS i t f b t ti t ll dBSS consists of base station controller and one or more base transceiver stations (BTS)Each BTS defines a single cell

Includes radio antenna, radio transceiver and a ,link to a base station controller (BSC)

BSC reserves radio frequencies, managesBSC reserves radio frequencies, manages handoff of mobile unit from one cell to another within BSS and controls paginganother within BSS and controls paging

N t k S b t (NS)Network Subsystem (NS)NS id li k b ll l k dNS provides link between cellular network and public switched telecommunications networks

Controls handoffs between cells in different BSSsAuthenticates users and validates accountsEnables worldwide roaming of mobile usersUser/Equipment Databases

Central element of NS is the mobile switching center (MSC)

Mobile Switching Center (MSC) DatabasesHome location register (HLR) database – storesHome location register (HLR) database – stores information about each subscriber that belongsVisitor location register (VLR) database –Visitor location register (VLR) database maintains information about subscribers currently physically in the regionp y y gAuthentication center database (AuC) – used for authentication activities, holds encryption keys, yp yEquipment identity register database (EIR) –keeps track of the type of equipment that exists at p yp q pthe mobile station

GSM TDMA F Ti Sl Fi ldGSM TDMA Format – Time Slot Fields

T il bi ll h i i f TDMA fTrail bits – allow synchronization of TDMA frames from mobile units at differing distances from baseE d bi d dEncrypted bits – encrypted dataStealing bit - indicates whether block contains data

i " l “ f l i lior is "stolen“ for urgent control signalingTraining sequence – used to adapt parameters of

i h h ireceiver to the current path propagation characteristics

St t i l l t d i f lti th tiStrongest signal selected in case of multipath propagationGuard bits – used to avoid overlapping with other burstsbursts

GSM S h Si l P iPredictive Speech CodingGSM Speech Signal Processing

Gaussian Min Shift Keying (GMSK)

Slow freq hopping to minimize multipathfrequency changed every 4.615 mS

GSM Signaling Protocol A hit tArchitecture

F ti P id d b P t lFunctions Provided by ProtocolsP t l b th li k l f th GSMProtocols above the link layer of the GSM signaling protocol architecture provide

ifi f ispecific functions:Radio resource managementMobility managementConnection managementgMobile application part (MAP)BTS managementBTS management

CDMA Cellular – 2nd Generation SystemCDMA Cellular 2 Generation System

Direct sequence spread spectrum (DSSS)Direct-sequence spread spectrum (DSSS)Frequency diversity – frequency-dependent transmission impairments (e g fading) have lesstransmission impairments (e.g. fading) have less effect on signal which is spread over a large bandwidthMultipath resistance – chipping codes used for CDMA exhibit low cross correlation and low autocorrelationautocorrelationPrivacy – privacy is inherent. For DSSS, each user has a unique code resulting in spreaduser has a unique code resulting in spread spectrum/noise-like signalsGraceful degradation – system only gradually g y y g ydegrades (SNR) as more users access the system

D b k f CDMA C ll lDrawbacks of CDMA CellularS lf j i i i t i i f lti lSelf-jamming – arriving transmissions from multiple users not aligned on chip boundaries unless users are perfectly synchronized, requires very accurate timing sources (GPS y q y g (receiver/discipline clock)Near-far problem – signals closer to the receiver are

i d ith l tt ti th i l f threceived with less attenuation than signals farther away and given lack of complete orthogonality, distant stations more difficult to recover – power control very importantp y pSoft handoff – requires that the mobile acquires the new cell before it relinquishes the old; this is more complex th h d h d ff d i FDMA d TDMA hthan hard handoff used in FDMA and TDMA schemes

Mobile Wireless CDMA Design Considerations

RAKE receiver when multiple versions of aRAKE receiver – when multiple versions of a signal (multipath very common) arrive more than one chip interval apart, RAKE receiver attempts to p p , precover signals from these multiple paths and combine them for performance

Thi th d hi b tt f th i lThis method achieves better performance than simply recovering dominant signal and treating remaining signals as noise

Soft Handoff – mobile station is temporarily connected to more than one base station simultaneously switching center selects bestsimultaneously, switching center selects best

P i i l f RAKE R ia’s areattenuation factors

τ are differingPrinciple of RAKE Receiver gmultipath delays

Weighting factors(a’) estimated fromchannel

IS-95 2G CDMA Scheme - Types of Channels Supported by Forward Link (page 300)

Pil t ( h l 0) ll th bil it tPilot (channel 0) - allows the mobile unit to acquire timing information, provides phase reference and provides means for signal strengthreference and provides means for signal strength comparisonSynchronization (channel 32) - used by mobileSynchronization (channel 32) - used by mobile station to obtain identification information about cellular systemcellular systemPaging (channels 1 to 7) - contain messages for one or more mobile stationso e o o e ob e stat o sTraffic (channels 8 to 31 and 33 to 63) – the forward channel supports 55 traffic channelso w d c e suppo s c c e s

Forward Traffic Channel P i StProcessing Steps

S h i d d f 8550 bSpeech is encoded at a rate of 8550 bpsAdditional bits added for error detectionData transmitted in 2-ms blocks with forward error correction provided by a convolutional p yencoderData interleaved in blocks to reduce effects ofData interleaved in blocks to reduce effects of errorsData bits are scrambled serving as a privacy maskData bits are scrambled, serving as a privacy mask

Forward Traffic Channel P i St ( t )Processing Steps (cont.)

P l i f i i d i ffiPower control information inserted into traffic channelDS-SS function spreads the 19.2 kbps to a rate of 1.2288 Mbps using one row of 64 x 64 Walsh matrixDigital bit stream modulated onto the carrier using g gQPSK modulation schemeIS-95 Reverse Link described on page 303IS 95 Reverse Link described on page 303

ITU’s View of Third-Generation C biliti (3G)Capabilities (3G)

Voice quality comparable to the public switchedVoice quality comparable to the public switched telephone network144 kbps data rate available to users in high speed144 kbps data rate available to users in high-speed motor vehicles over large areas384 kbps available to pedestrians standing or moving384 kbps available to pedestrians standing or moving slowly over small areasSupport for 2 048 Mbps for office useSupport for 2.048 Mbps for office useSymmetrical / asymmetrical data transmission ratesS f b h k i h d d i i i h dSupport for both packet switched and circuit switched data servicesFi 3G i l 1 EV DOFirst 3G wireless system: 1x EV-DO (CDMA high data rate)

ITU’s View of Third-Generation C bilitiCapabilities

A d ti i t f t th I t t t fl tAn adaptive interface to the Internet to reflect efficiently the common asymmetry between inbound and outbound trafficinbound and outbound trafficMore efficient use of the available spectrum in generalgeneralSupport for a wide variety of mobile equipmentFl ibilit t ll th i t d ti fFlexibility to allow the introduction of new services and technologies (video, text messaging, wireless VOIP GPS services etc )wireless VOIP, GPS services, etc.)

Alt ti I t f (IMT 2000)Alternative Interfaces (IMT-2000)Allow smooth transition for 1G and 2G systemsy

CDMA D i C id tiCDMA Design ConsiderationsB d idth li it h l t 5 MH tillBandwidth – limit channel usage to 5 MHz, still adequate to support high data ratesChi t d d d i d d t t d fChip rate – depends on desired data rate, need for error control, and bandwidth limitations; 3 Mcps or more is reasonableor more is reasonableMultirate – advantage is that the system can flexibly support multiple simultaneousflexibly support multiple simultaneous applications from a given user and can efficiently use available capacity by only providing the use ava ab e capac ty by o y p ov d g t ecapacity required for each service

U f l W b Sit f St lliUseful Web Sites from Stallings

Chapter 10 - Cellular Wireless Communications

GSM World: Information about GSM technology and services.

CDMA Development Group: A consortium of wireless companies active in CDMA wireless systems around the world.

Cellular Telecommunications and Internet Association: Another industry yconsortium.

3G Americas: A trade group of Western Hemisphere companies supporting TDMA, GSM, GPRS, EDGE, and UMTS. Includes industry news, white , , , , y ,papers, and other technical information.

3G Today: Contains a variety of information on 3G systems and providers.

http://www.mobilepipeline.com/ Mobile Pipeline email/web Newsletter

http://www.nwc.com/mobile Mobile Observer email/web Newsletter

Chapter 10 ProblemsChapter 10 ProblemsR i Q i (l k k iReview Questions (look over, key topic area of this course))

P bl 10 1 10 6 10 12 d 10 15Problems 10.1, 10.6, 10.12 and 10.15