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IT 2402 Mobile Communication ByDr Gnanasekaran thangavel

UNIT I WIRELESS COMMUNICATIONCellular systems- Frequency Management and ChannelAssignment- types of handoff and their characteristics, Dropped call rates & their evaluation MAC SDMA FDMA TDMA CDMA Cellular Wireless Networks

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Physical Properties of WirelessMakes wireless network different from wired networks.Standard wired networks are connected together using copper cables that carry data around the network in the form of electrical signals.In some circumstances laying cables may be inconvenient or even impossible. The answer may be to implement a wireless network to move the data around.

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Wireless = WavesElectromagnetic radiation.Emitted by sinusoidal current running through a wire (transmitting antenna).Creates propagating sinusoidal magnetic and electric fields according to Maxwells equations:

Fields induce current in receiving antenna

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Wave Propagation Example

electricfield

magneticfield

propagation direction5

Wireless CommunicationsWireless communication is the transfer of information between two or more points that are not connected by an electrical conductor. The most common wireless technologies use radio. Special Note: Wireless can also be interpreted as wire-less, which means to create a network without wires. In this case, a wire-less network may be using infra-red or even lasers to communicate this is much rarer than using radio.

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Mobile Communication A communication network which doesn't depend on any physical connection between two communication entities and have flexibility to be mobile during communication. The current GSM and CDMA technologies offers Mobile Communication.7

Frequency & Public Use BandsPropagating sinusoidal wave with some frequency/wavelengthC (speed of light) = 3x108 m/sName900 Mhz2.4 Ghz5 GhzRange902 - 9282.4 - 2.48355.15 - 5.35Bandwidth26 Mhz83.5 Mhz200 MhzWavelength.33m / 13.1.125m / 4.9.06 m / 2.4

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Free-space Path-loss Power of wireless transmission reduces with square of distance (due to surface area increase of sphere)Reduction also depends on wavelengthLong wave length (low frequency) has less lossShort wave length (high frequency) has more loss

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Multi-path PropagationElectromagnetic waves bounce off of conductive (metal) objectsReflected waves received along with direct wave

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Multi-Path EffectMulti-path components are delayed depending on path length (delay spread)Phase shift causes frequency dependent constructive / destructive interference

AmplitudeTime

AmplitudeFrequency

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ModulationModulation allows the wave to carry information by adjusting its properties in a time varying wayAmplitudeFrequencyPhaseDigital modulation using discrete steps so that information can be recovered despite noise/interference8VSB - US HDTVBFSK - Mote Sensor NetworksQPSK - 2 Mbps 802.11 & CMDA(IS-95)12

Multi-transmitter InterferenceSimilar to multi-pathTwo transmitting stations will constructively/destructively interfere with each other at the receiverReceiver will hear the sum of the two signals, which usually means garbage13

Cellular Network OrganizationUse multiple low-power transmitters (100 W or less)Areas divided into cellsEach 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 are equidistant (hexagonal pattern)14

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Frequency ReuseAdjacent cells assigned different frequencies to avoid interference or crosstalkObjective is to reuse frequency in nearby cells10 to 50 frequencies assigned to each cellTransmission power controlled to limit power at that frequency escaping to adjacent cellsThe issue is to determine how many cells must intervene between two cells using the same frequency16

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The factor N is called the cluster size and is given N=i2+ij+j2

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To find the nearest co-channel neighbor of a particular cell, one must do the following:

move i cells along any chain of hexagons and thenturn 60 degrees counter-clockwise and move j cells.19

i=1, j=2 , N=1+2+4=7 20

Interference

R - the radius of the cellD - the distance between centers of the nearest co-channel cellsQ - the co-channel reuse ratio 21

Approaches to Cope with Increasing CapacityAdding new channelsFrequency borrowing frequencies are taken from adjacent cells by congested cellsCell splitting cells in areas of high usage can be split 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 posts22

Cellular System Overview

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Cellular Systems TermsBase Station (BS) includes an antenna, a controller, and a number of receiversMobile telecommunications switching office (MTSO) connects calls between mobile unitsTwo types of channels available between mobile unit and BSControl channels used to exchange information having to do with setting up and maintaining callsTraffic channels carry voice or data connection between users24

Steps in an MTSO Controlled Call between Mobile UsersMobile unit initializationMobile-originated callPagingCall acceptedOngoing callHandoff25

Additional Functions in an MTSO Controlled CallCall blockingCall terminationCall drop-thedropped-call rate(DCR) is the fraction of thetelephone callswhich, due to technical reasons, were cut off before the speaking parties had finished their conversation and before one of them had hung up (dropped calls) This fraction is usually measured as a percentage of all calls.Calls to/from fixed and remote mobile subscriber26

Mobile Radio Propagation EffectsSignal strengthMust be strong enough between base station and mobile unit to maintain signal quality at the receiverMust not be so strong as to create too much cochannel interference with channels in another cell using the same frequency bandFadingSignal propagation effects may disrupt the signal and cause errors27

Power ControlDesign issues making it desirable to include dynamic power control in a cellular systemReceived power must be sufficiently above the background noise for effective communicationDesirable to minimize power in the transmitted signal from the mobileReduce cochannel interference, alleviate health concerns, save battery powerIn SS systems using CDMA, its desirable to equalize the received power level from all mobile units at the BS28

Types of Power ControlOpen-loop power controlDepends solely on mobile unitNo feedback from BSNot as accurate as closed-loop, but can react quicker to fluctuations in signal strength Closed-loop power controlAdjusts signal strength in reverse channel based on metric of performanceBS makes power adjustment decision and communicates to mobile on control channel29

Traffic EngineeringIdeally, available channels would equal number of subscribers active at one timeIn practice, not feasible to have capacity handle all possible loadFor N simultaneous user capacity and L subscribersL < N nonblocking systemL > N blocking system30

Blocking System Performance QuestionsProbability that call request is blocked?What capacity is needed to achieve a certain upper bound on probability of blocking?What is the average delay?What capacity is needed to achieve a certain average delay?31

Traffic IntensityLoad presented to a system:

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

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Factors that Determine the Nature of the Traffic ModelManner in which blocked calls are handledLost calls delayed (LCD) blocked calls put in a queue awaiting a free channelBlocked calls rejected and droppedLost calls cleared (LCC) user waits before another attemptLost calls held (LCH) user repeatedly attempts callingNumber of traffic sourcesWhether number of users is assumed to be finite or infinite33

HANDOFFS AND DROPPEDCALLS

Dropped Call Rate (DCR) is a term in telecommunications denoting the fraction of the calls which, due to technical reasons, were cut off before the speaking parties had finished their conversation and before one of them had hung up. This fraction is usually measured as a percentage of all calls.35

Handover or Handoff

Handover basically means changing the point of connection while communicating.Old ConceptWhenever Mobile Station is connected to 1 Base Station and there is a need to change to another Base Station, it is known as HANDOVER.

New Concept

When mobile station switches from one set of radio resources to another set, HANDOVER is said to have taken place.

HANDOVER

Handoff Performance MetricsHandoff blocking probability probability that a handoff cannot be successfully completedHandoff probability probability that a handoff occurs before call terminationRate of handoff number of handoffs per unit timeInterruption 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 point at which it does occur39

Handoff Performance MetricsCell blocking probability probability of a new call being blockedCall dropping probability probability that a call is terminated due to a handoffCall completion probability probability that an admitted call is not dropped before it terminatesProbability of unsuccessful handoff probability that a handoff is executed while the reception conditions are inadequate40

Handoff Strategies Used to Determine Instant of HandoffRelative signal strengthRelative signal strength with thresholdRelative signal strength with hysteresisRelative signal strength with hysteresis and thresholdPrediction techniques41

Handoff- requirement Mobiles may move out of coverage area of a cell and into coverage area of a different cell during a callMSC must identify new BS to handle callMSC must seamlessly transfer control of call to new BSMSC must assign call new forward and reverse channels within the channels of new BSSome important performance metrics in handoff:Seamless user should not know handoff occurringMinimum unnecessary Handoff due to short time fadingLow probability of blocking new calls in the new cellHandoff to a good SNR channel so that an admitted call is not dropped

Handoff Main StepsInitiationResource reservationExecutionCompletion

Important handoff parameter:SNRold to initiate handoff based on minimum acceptable qualitySNRnew of the target channel (SNRnew > SNRold )D = SNRnew - SNRold dBIf D too small, unnecessary handoffs occurIf D too large, may be insufficient time to complete handoff before SNRold becomes too weak and signal is lost

HANDOFF DECISIONS

There are numerous methods for performing handoff. From the decision process point of view, one can find at least three different kinds of handoff decisions.

Network-Controlled Handoff

Mobile-Assisted Handoff

Mobile-Controlled Handoff

Network-Controlled HandoffIn a network-controlled handoff protocol, the network makes a handoff decision based on the measurements of the MSs at a number of BSs.In general, the handoff process takes 100200 ms.Network-controlled handoff is used in first-generation analog systems such as AMPS (Advanced Mobile Phone System), TACS(Total Access Communication System), and NMT (Nordic Mobile Telephone).

Mobile-Assisted HandoffIn a mobile-assisted handoff process, the MS makes measurements and the network makes the decision.In the circuit-switched GSM (global system mobile), the BS controller (BSC) is in charge of the radio interface management. This mainly means allocation and release of radio channels and handoff management.The handoff time between handoff decision and execution in a circuit-switched GSM is approximately 1 second.

Mobile-Controlled Handoff

In mobile-controlled handoff, each MS is completely in control of the handoff process.This type of handoff has a short reaction time (in the order of 0.1 second).MS measures the signal strengths from surrounding BSs and interference levels on all channels.A handoff can be initiated if the signal strength of the serving BS is lower than that of another BS by a certain threshold.

TYPES OF HANDOVER

Hard handoff - break-before-make

2. Soft handoff- make-before-break

3. Softer handoff- Softer handover is the situation where one base station receives two user signals from two adjacent sectors it serves.

HARD HANDOVERBREAK BEFORE MAKEOld connection is broken before a new connection is activated

Primarily used in FDMA and TDMA systems (e.g. GSM)

Different frequency ranges used in adjacent cells to minimize the interference

Mechanism of Hard Handover

The base station BS1 on one cell site hands off the mobile station(MS)s call to another cell BS2.The link to the prior base station, BS1 is terminated before the user is transferred to the new cells base station, BS2. The MS is linked to no more than one BS at any given time.

CHARACTERISTICSA Hard handover is relatively cheaper and easier to implement in comparison to other types of Handover.

It is primarily used in FDMA (frequency division multiple access) and TDMA (time division multiple access), where different frequency ranges are used in adjacent channels in order to minimize channel interference.

It is simpler as phone's hardware does not need to be capable of receiving two or more channels in parallel.

SOFT HANDOVERMAKE BEFORE BREAKNew connection is activated before the old is brokenUsed in UMTS to improve the signal qualityUplink and downlink signals may be combined for better signalA mobile may in UMTS spend a large part of the connection time in soft handoverBetter connection reliability

More seamless handover.

MECHANISM OF SOFT HANDOVER

The call is first connected to the new base station BS2 and then it is dropped by the previous base station BS1.

The call will be established only when a reliable connection to the target cell is obtained. The MS is linked to two BS for a brief interval of time. Thus soft handover involves connection to more than one cell.

CHARACTERISTICSIt offers more reliable access continuity in network connection and less chances of a call termination during switching of base stations in comparison to aHard handoff.

It is commonly used in CDMA (Code-division multiple access) systems that enables the overlapping of the repeater coverage zones, so that every cell phone set is always well within range of at least one of the base stations.

Technical implementation of a Soft handoff is more expensive and complex in comparison to a Hard handoff.

It is used in sensitive communication services such as videoconferencing.

SOFTER HANDOVERSofter handover is the situation where one base station receives two user signals from two adjacent sectors it serves.

In the case of softer handover the base station receives 2 separated signals through multi-path propagation. Due to reflections on buildings or natural barriers the signal sent from the mobile stations reaches the base station from two different sectors.

SOFTER HANDOVER

INTER-CELL AND INTRA-CELL HANDOVER

The inter-cell handover switches a call in progress from one cell to another cell,

The intra-cell handover switches a call in progress from one physical channel of a cell to another physical channel of the same cell.

Inter BSC/MSC handoff: This for of GSM handover or GSM handoff occurs when the mobile moves out of the coverage area of one BTS but into another controlled by the same BSC/MSC.

Inter system handoff: If during ongoing call mobile unit moves from one cellular system to a different cellular system which is controlled by different MTSO, a handoff procedure which is used to avoid dropping of call is referred as Inter System Handoff.

Intra system handoff: If during ongoing call mobile unit moves from one cellular system to adjacent cellular system which is controlled by same MTSO, a handoff procedure which is used to avoid dropping of call is referred as Intra System Handoff.

Intra carrier handoffs: CDMA-One carrier frequency to other.

Inter mode handoff:FDD to TDD

HANDOFF FAILURESBecause frequencies cannot be reused in adjacent cells, when a user moves from one cell to another, a new frequency must be allocated for the call.

If a user moves into a cell when all available channels are in use, the users call must be terminated.

Problem of signal interference where adjacent cells overpower each other resulting in receiver desensitization is also there.

Dropped Call RatesThe dropped call is defined as an established call which leaves the system before it is normally terminatedThe Dropped Call Rate (DCR) parameter represents what percentage of all established calls is dropped during a specified time periodThe DCR and voice quality are inversely proportional and high DCR may indicate coverage, handoff, or channels accessibility problems

The perception of dropped call rate by the subscribers can be higher due to:

1. The subscriber unit not functioning properly (needs repair).2. The user operating the portable unit in a vehicle (misused).3. The user not knowing how to get the best reception from a portable unit (needs education).

Relationship Among Capacity, Voice Quality, Dropped Call RateRadio Capacity m is expressed as follows:

BT=Available bandwidth BC= Channel bandwidth

FORMULA OF DROPPED CALL RATEGeneral Formula of Dropped Call RateThe general formula of dropped call rate P in a whole system can be expressed as:

Where And

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Frequency Management and Channel Assignmentchannel allocationschemes are required to allocate bandwidthandcommunication channelsto base stations, access points and terminal equipment. The objective is to achieve maximumsystem spectral efficiencyin bit/s/Hz/site by means offrequency reuse, It assure a certaingrade of service by avoidingco-channel interferenceandadjacent channel interferenceamong nearby cells or networks65

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There are two types of strategiesFixed: FCA, fixed channel allocation: Manually assigned by the network operatorDynamic: In which voice channel are not allocated to cell permanently, instead for every call request base station request channel from MSC. The channel is allocated following an algorithm which accounts likelihood of future blocking within the cell. It requires the MSC to collect real time data on channel occupancy

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Cellular Wireless Networks68

First-Generation Cellular SystemThe first generation systems are voice oriented analog cellular and cordless telephoneFrequency Division Duplex AMPS,ETACS,NMT900,JATACS.Frequency modulation WLAN with ISM bands.Each band split in two to encourage competition.Frequency reuse exploited69

Second-Generation Digital Cellular SystemSecond generation 2G cellular telecom networks were commercially launched on theGSMstandard in 1991.Three primary benefits of 2G networks over their predecessors were that phone conversations were digitally encrypted; 2G systems were significantly more efficient on the spectrum allowing for far greater mobile phone penetration levels; and 2G introduced data services for mobile, starting withSMStext messages. 2G technologies enabled the various mobile phone networks to provide the services such as text messages, picture messages and MMS (multi media messages). All text messages sent over 2G are digitally encrypted, allowing for the transfer of data in such a way that only the intended receiver can receive and read it.70

3G,Thethird generation of mobile telecommunications technology.This is based on a set of standards used for mobile devices and mobile telecommunications use services and networks that comply with theInternational Mobile Telecommunications-2000 (IMT-2000)3Gfinds application in wireless voicetelephony,mobile Internetaccess,fixed wirelessInternet access,video callsandmobile TV.An adaptive interface to the Internet to reflect efficiently the common asymmetry between inbound and outbound trafficMore efficient use of the available spectrum in generalSupport for a wide variety of mobile equipmentFlexibility to allow the introduction of new services and technologies

Third-Generation Digital Cellular System71

Differences Between First and Second Generation SystemsDigital traffic channels first-generation systems are almost purely analog; second-generation systems are digitalEncryption all second generation systems provide encryption to prevent eavesdroppingError detection and correction second-generation digital traffic allows for detection and correction, giving clear voice receptionChannel access second-generation systems allow channels to be dynamically shared by a number of users72

ITUs View of Third-Generation CapabilitiesVoice quality comparable to the public switched telephone network144 kbps data rate available to users in high-speed motor vehicles over large areas384 kbps available to pedestrians standing or moving slowly over small areasSupport for 2.048 Mbps for office useSymmetrical / asymmetrical data transmission ratesSupport for both packet switched and circuit switched data services73

Air InterfaceRadio Transmission TechniquesFDMA TDMACDMAChannelsPhysical channelsLogical channels74

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MULTIPLE ACCESS

Multiple Access methods address the problem of how many users can share the same spectrum resources in an efficient manner. We distinguish betweenMultiple access within one cell, i.e., a fixed assignment of resources in time or bandwidth to specific usersRandom access, i.e., a dynamic assignment of spectrum resources in time or bandwidth to users, according to their needs Frequency reuse, i.e., assignment of spectrum resources considering the location of users and the attenuation of radio signals that travel over sufficiently large distances. 76

MEDIA ACCESS CONTROL(MAC)A channel-access scheme is also based on a multiple access protocol and control mechanism, also known as media access control (MAC). This protocol deals with issues such as addressing, assigning multiplex channels to different users, and avoiding collisions.77

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Frequency Division Multiple Access (FDMA)In an FDMA system, each user has its own frequency channel. This implies that relatively narrow filters are needed in each receiver and transmitter. Most duplex FDMA systems must transmit and receive simultaneously. (Frequency Division Duplex, FDD)79

FDMA

TimeFrequency

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Time Division Multiple Access (TDMA)

In TDMA, a set of N users share the same radio channel, but each user only uses the channel during predetermined slots. A frame consists of N slots, one for each user. Frames are repeated continuously81

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Spread Spectrum Transmission

In Spread Spectrum communication, the bandwidth occupancy of a single transmitted signal is much higher than in systems using conventional modulation methods. This band-spreading is achieved by selecting appropriate transmission waveforms with a wide bandwidth. A very popular method is to multiply the user data signal with a fast code sequence, which mostly is independent of the transmitted data message. In the case that multiple users share the same portion of the radio spectrum but use different codes to distinguish their transmissions, we speak of Code Division Multiple Access (CDMA)83

TDMA

TimeFrequency

01234567

Channel

Time Slot

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CDMA85

CDMA

FrequencyTime

Code

Code 1Code 2Code 386

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ReferencesBook: Wireless Communications and Networks by William Stallings PPT: WilliamStalling.com/StudentsSupport.html.http://www.wirelesscommunication.nl/reference/about.htm

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