MC Unit 1

CS 455 Mobile CommunicationsText Book Referred Mobile Communications 2edByProf. Dr.-Ing. Jochen Schiller

Unit 1

Introduction to wireless Networks Applications History of mobile communication Simplified Reference ModelWireless TransmissionMobile Communications CS455Unit 1

Computers for the next decades?Computers are integratedsmall, cheap, portable, replaceable - no more separate devicesTechnology is in the backgroundcomputer are aware of their environment and adapt (location awareness)computer recognize the location of the user and react appropriately (e.g., call forwarding, fax forwarding, context awareness))Advances in technologymore computing power in smaller devicesflat, lightweight displays with low power consumptionnew user interfaces due to small dimensionsmore bandwidth per cubic metermultiple wireless interfaces: wireless LANs, wireless WANs, regional wireless telecommunication networks etc. (overlay networks)

Mobility and WirelessAll are interested in staying connected while on the go.In the present scenario a plethora of devices and applications are keen in communicating while they are mobile.Mobility may be of two types namely User mobility the user can move and the services follow the user.Device portability the device moves and the mechanism ensures the connectivity.A mobile device accesses the network wirelessly.i.e a wire is replaced by electro magnetic waves

Communication device exhibit the following characteristicsFixed and wired: stationary computer / desktop computer the device is fixed for performance reasonsMobile and wired: Laptops. Communication access through telephone network and modem or ether net card.Fixed and wireless: Established in historical buildings or in places where the network is setup very fast.Mobile and wireless: No cable restriction to the user. The user roam between networks.

Mobile communicationTwo aspects of mobility:user mobility: users communicate (wireless) anytime, anywhere, with anyonedevice portability: devices can be connected anytime, anywhere to the networkWireless vs. mobile Examples stationary computer (Fixed and wired) notebook in a hotel (Mobile and wired) wireless LANs in historic buildings (Fixed & wireless) Personal Digital Assistant (PDA) (Mobile & wireless)The demand for mobile communication created the need for integration of wireless networks into existing fixed networks:local area networks: standardization of IEEE 802.11, ETSI (HIPERLAN)Internet: Mobile IP extension of the internet protocol IPwide area networks: e.g., internetworking of GSM and ISDN

Applications IVehiclestransmission of news, road condition, weather, music via Digital Audio Broadcasting (DAB)personal communication using Global System for Mobile (GSM)position via Global Positioning System (GPS)local ad-hoc network with vehicles close-by to prevent accidents, guidance system, redundancy vehicle data (e.g., from buses, high-speed trains) can be transmitted in advance for maintenance and for logistics purpose.Fleet management improves the organization by saving time and money. EmergenciesHigh quality wireless connection to the hospital sends vital information about the patient to the hospital from the scene of accident helps in early diagnosis with the specialist.replacement of a fixed infrastructure in case of earthquakes, hurricanes, fire etc.crisis, war, ...

Typical application: road trafficad hocUMTS, WLAN,DAB, GSM, cdma2000, TETRA, ...Personal Travel Assistant,DAB, PDA, laptop, GSM, UMTS, WLAN, Bluetooth, ...

Mobile and wireless services Always Best ConnectedUMTS,DECT2 Mbit/sUMTS, GSM384 kbit/sLAN100 Mbit/s,WLAN54 Mbit/sUMTS, GSM115 kbit/sGSM 115 kbit/s,WLAN 11 Mbit/sGSM 53 kbit/sBluetooth 500 kbit/sGSM/EDGE 384 kbit/s,WLAN 780 kbit/sLAN, WLAN780 kbit/s

Applications IITravelling salesmendirect access to customer files stored in a central locationconsistent databases for all agentsmobile officeReplacement of fixed networksremote sensors, e.g., weather, earth activitiesflexibility for trade showsLANs in historic buildingsEntertainment, education, ...outdoor Internet access intelligent travel guide with up-to-date location dependent informationad-hoc networks for multi user games HistoryInfo

Location dependent servicesLocation aware serviceswhat services, e.g., printer, fax, phone, server etc. exist in the local environmentFollow-on servicesautomatic call-forwarding, transmission of the actual workspace to the current locationInformation servicespush: e.g., current special offers in the supermarketpull: e.g., where is the Black Forrest Cherry Cake?Support servicescaches, intermediate results, state information etc. follow the mobile device through the fixed networkPrivacywho should gain knowledge about the location

Mobile devicesperformancePager receive only tiny displays simple text messagesMobile phones voice, data simple graphical displaysPDA simpler graphical displays character recognition simplified WWWPalmtop tiny keyboard simple versions of standard applicationsLaptop fully functional standard applicationsSensors,embeddedcontrollers

Effects of device portabilityPower consumptionlimited computing power, low quality displays, small disks due to limited battery capacityCPU: power consumption ~ CV2fC: internal capacity, reduced by integrationV: supply voltage, can be reduced to a certain limitf: clock frequency, can be reduced temporallyLoss of datahigher probability, has to be included in advance into the design (e.g., defects, theft)Limited user interfacescompromise between size of fingers and portabilityintegration of character/voice recognition, abstract symbolsLimited memorylimited value of mass memories with moving partsflash-memory or ? as alternative

Wireless networks in comparison to fixed networksHigher loss-rates due to interferenceemissions of, e.g., engines, lightningRestrictive regulations of frequenciesfrequencies have to be coordinated, useful frequencies are almost all occupiedLow transmission rateslocal some Mbit/s, regional currently, e.g., 9.6kbit/s with GSMHigher delays, higher jitterconnection setup time with GSM in the second range, several hundred milliseconds for other wireless systemsLower security, simpler active attackingradio interface accessible for everyone, base station can be simulated, thus attracting calls from mobile phonesAlways shared mediumsecure access mechanisms important

Many people in history used light for communicationheliographs, flags (semaphore), ...150 BC smoke signals for communication; (Polybius, Greece)1794, optical telegraph, Claude ChappeHere electromagnetic waves are of special importance:1831 Faraday demonstrates electromagnetic inductionJ. Maxwell (1831-79): theory of electromagnetic Fields, wave equations (1864)H. Hertz (1857-94): demonstrates with an experiment the wave character of electrical transmission through space (1888, in Karlsruhe, Germany, at the location of todays University of Karlsruhe)Early history of wireless communication

History of wireless communication I1895Guglielmo Marconifirst demonstration of wireless telegraphy (digital!)long wave transmission, high transmission power necessary (> 200kw)1907Commercial transatlantic connectionshuge base stations (30 100m high antennas)1915Wireless voice transmission New York - San Francisco1920Discovery of short waves by Marconireflection at the ionospheresmaller sender and receiver, possible due to the invention of the vacuum tube (1906, Lee DeForest and Robert von Lieben)1926Train-phone on the line Hamburg - Berlinwires parallel to the railroad track

History of wireless communication II1928 many TV broadcast trials (across Atlantic, color TV, TV news)1933 Frequency modulation (E. H. Armstrong)1958A-Netz in Germanyanalog, 160MHz, connection setup only from the mobile station, no handover, 80% coverage, 1971 11000 customers1972B-Netz in Germanyanalog, 160MHz, connection setup from the fixed network too (but location of the mobile station has to be known)available also in A, NL and LUX, 1979 13000 customer in D1979NMT at 450MHz (Scandinavian countries)1982Start of GSM-specificationgoal: pan-European digital mobile phone system with roaming1983Start of the American AMPS (Advanced Mobile Phone System, analog) 1984CT-1 standard (Europe) for cordless telephones

History of wireless communication III1986C-Netz in Germanyanalog voice transmission, 450MHz, hand-over possible, digital signaling, automatic location of mobile deviceWas in use until 2000, services: FAX, modem, X.25, e-mail, 98% coverage1991Specification of DECTDigital European Cordless Telephone (today: Digital Enhanced Cordless Telecommunications)1880-1900MHz, ~100-500m range, 120 duplex channels, 1.2Mbit/s data transmission, voice encryption, authentication, up to several 10000 user/km2, used in more than 50 countries1992Start of GSMin D as D1 and D2, fully digital, 900MHz, 124 channelsautomatic location, hand-over, cellularroaming in Europe - now worldwide in more than 170 countriesservices: data with 9.6kbit/s, FAX, voice, ...

History of wireless communication IV1994E-Netz in GermanyGSM with 1800MHz, smaller cellsAs Eplus in D (1997 98% coverage of the population)1996HiperLAN (High Performance Radio Local Area Network)ETSI, standardization of type 1: 5.15 - 5.30GHz, 23.5Mbit/srecommendations for type 2 and 3 (both 5GHz) and 4 (17GHz) as wireless ATM-networks (up to 155Mbit/s)1997Wireless LAN - IEEE802.11IEEE standard, 2.4 - 2.5GHz and infrared, 2Mbit/salready many (proprietary) products available in the beginning1998Specification of GSM successorsfor UMTS (Universal Mobile Telecommunication System) as European proposals for IMT-2000Iridium66 satellites (+6 spare), 1.6GHz to the mobile phone

Simple Reference ModelMediumData LinkPhysicalData LinkPhysicalRadio

Physical LayerConverts the stream of bits into signals or vice versa (in wired)Responsible for frequency selection, generation of the carrier frequency, signal detection, modulation of data and encryption.Data Link LayerAccessing the mediaMultiplexing of different data streamsCorrection of transmission errorsSynchronization (i.e., detection of data frame)Altogether responsible for reliable point to point connection two devices or point to multipoint connection between one sender and several receivers.Network LayerRouting packets through a network or establishing connection between two entities over many other intermediate systems.AddressingRoutingDevice location Handover between different networks

Transport LayerEstablishes end to end connection.Takes care of quality of service &Flow and congestion controlApplication LayerService locationSupport for multimedia applicationsAdaptive applications that can handle large variations in transmission characteristicsWireless access to the web using portable deviceDemanding applications are video (high data rate) and interactive gaming.

Frequencies Signals Antenna & Signal propagation

Multiplexing Spread spectrum Modulation Cellular systems

Frequencies for communicationVLF = Very Low FrequencyUHF = Ultra High FrequencyLF = Low Frequency SHF = Super High FrequencyMF = Medium Frequency EHF = Extra High FrequencyHF = High Frequency UV = Ultraviolet LightVHF = Very High Frequency

Frequency and wave length: = c/f wave length , speed of light c 3x108m/s, frequency f1 Mm300 Hz10 km30 kHz100 m3 MHz1 m300 MHz10 mm30 GHz100 m3 THz1 m300 THzvisible lightVLFLFMFHFVHFUHFSHFEHFinfraredUVoptical transmissioncoax cabletwisted pair

VLF Very Low Frequency LF Low Frequency.Long waves, can penetrate water and follows earths surface.Used by submarinesUsed by some radio stations in GermanyMF Medium Frequency HF High FrequencyTypical transmission frequencies for radio stations either as Amplitude Modulation (AM) or Short Wave (SW) or Frequency Modulation(FM)SW is used by amateur radio transmissionVHF Very High Frequency UHF Ultra High Frequency.TV station uses this frequency for transmission.Digital Audio Broadcasting (DAB) uses this frequency.UHF is used byMobile phones with analog technology.Digital GSM uses 890 960 MHz , 1710 1880 MHzDECT standard uses 1880 1900 MHz3G cellular system uses 1900 1980 MHz, 2020 2025 MHz, 2110 - 2190 MHz4G cellular System uses 2 - 8 GHzSHF Super High Frequency.Used by directed Microwave links

Frequencies and regulationsITU-R holds auctions for new frequencies, manages frequency bands worldwide (WRC, World Radio Conferences)

Europe

USA

Japan

Cellular

Phones

GSM 450-457, 479-486/460-467,489-496, 890-915/935-960,

1710-1785/1805-1880

UMTS (FDD) 1920-1980, 2110-2190

UMTS (TDD) 1900-1920, 2020-2025

AMPS, TDMA, CDMA 824-849, 869-894TDMA, CDMA, GSM 1850-1910,1930-1990

PDC 810-826, 940-956,1429-1465, 1477-1513

Cordless

Phones

CT1+ 885-887, 930-932

CT2

864-868

DECT

1880-1900

PACS 1850-1910, 1930-1990

PACS-UB 1910-1930

PHS 1895-1918

JCT

254-380

Wireless LANs

IEEE 802.11

2400-2483

HIPERLAN 2

5150-5350, 5470-5725

902-928

IEEE 802.11

2400-2483

5150-5350, 5725-5825

IEEE 802.11 2471-2497

5150-5250

Others

RF-Control

27, 128, 418, 433, 868

RF-Control

315, 915

RF-Control

426, 868

ITU R International Telecommunication Union Regulations located at Geneva is responsible for coordination of telecommunication worldwide.Divides the globe into following 3 regions Region 1 Europe, middle east, countries of former Soviet Union, and AfricaRegion 2 Greenland, North and South AmericaRegion 3 Far East, Australia and New Zealand.These regions have national agencies for further regulations.Regions have national agencies for further regulationsEg. Federal Communication Commission (FCC) in USAEuropean Conference for Posts and Telecommunication (CEPT)ITU R holds world Radio Conference (WRC) periodically to discuss and decide the frequency allocations for all the three regions.

Signals IPhysical representation of dataFunction of time and locationSignal parameters represent the data valuesClassificationContinuous time/discrete timeContinuous values/discrete valuesAnalog signal = continuous time and continuous valuesDigital signal = discrete time and discrete valuesSignal parameters of periodic signals: period T, frequency f=1/T, amplitude A, phase shift sine wave a special periodic signal for a carrier: s(t) = At sin(2 ft t + t)

Fourier representation of periodic signals1010ttideal periodic signalreal composition(based on harmonics)

Different representations of signals amplitude (amplitude domain)frequency spectrum (frequency domain)phase state diagram (amplitude M and phase in polar coordinates) aka signal constellation diagram.

Amplitude domain Frequency domain Phase domainSignals II

SignalsRepresentation in Time domain is problematic if there are many different frequencies. In this case the better representation is the Frequency domain.Here the amplitude of a certain frequency part is shown versus the frequency.Arbitrary periodic functions has many peaks known as frequency spectrum. Tool used is spectrum analyzer.In phase domain M represents the amplitude of a signal and the phase in polar coordinates.

Antennas couple electromagnetic energy to and from space to and from a wire or coaxial cable.Antenna is an isotropic radiator.Isotropic radiator: equal radiation in all directions (three dimensional) - only a theoretical reference antennaReal antennas always have directive effects (vertically and/or horizontally) i.e., the intensity of radiation is not the same in all direction. Radiation pattern: measurement of radiation around an antenna

Antennas: isotropic radiatoridealisotropicradiator

Antennas: simple dipolesReal antennas are not isotropic radiators, e.g., dipoles with lengths /4 on car roofs or /2 as Hertzian dipole shape of antenna proportional to wavelength

Example: Radiation pattern of a simple Hertzian dipole

Gain: maximum power in the direction of the main lobe compared to the power of an isotropic radiator (with the same average power)

side view (xy-plane)xyside view (yz-plane)zytop view (xz-plane)xzsimpledipole/4/2

Antennas: directed and sectorizedside view (xy-plane)xyside view (yz-plane)zytop view (xz-plane)xztop view, 3 sectorxztop view, 6 sectorxzOften used for microwave connections or base stations for mobile phones (e.g., radio coverage of a valley)directedantennasectorizedantenna

Directional antennas positioned in valleys or between buildings are useful. They have a fixed preferential transmission and reception direction.The main lobe is in the direction of x axis.Sectorized antennas several directed antennas are combined on a single pole to construct a Sectorized antenna.Multi element antenna arrays two or more antennas are combined to improve reception by counteracting the negative effects of multi path propagation.

Antennas: diversityGrouping of 2 or more antennasmulti-element antenna arraysAntenna diversityswitched diversity, selection diversityreceiver chooses antenna with largest outputdiversity combiningcombine output power to produce gaincophasing needed to avoid cancellation

Signal propagation ranges (Wireless)distancesendertransmissiondetectioninterferenceTransmission rangecommunication possiblelow error rateDetection rangedetection of the signal possibleno communication possibleInterference rangesignal may not be detected signal adds to the background noise

Signal propagationPropagation in free space always like light (straight line)Receiving power proportional to 1/d (d = distance between sender and receiver)Receiving power additionally influenced byfading (frequency dependent)shadowingreflection at large obstaclesrefraction depending on the density of a mediumscattering at small obstaclesdiffraction at edgesreflectionscatteringdiffractionshadowingrefraction

Real world example

Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction

Time dispersion: signal is dispersed over time interference with neighbor symbols, Inter Symbol Interference (ISI)The signal reaches a receiver directly and phase shifted distorted signal depending on the phases of the different partsSignals travelling along different paths with different lengths arrive at the receiver at different times. This effect is called delay spread. (the original signal is spread due to different delays of parts of the signal)Multipath propagationsignal at sendersignal at receiverLOS pulsesmultipathpulses

Effects of mobilityChannel characteristics change over time and location signal paths changedifferent delay variations of different signal partsdifferent phases of signal parts quick changes in the power received (short term fading)

Additional changes indistance to senderobstacles further away slow changes in the average power received (long term fading)short term fadinglong termfadingtpower

Multiplexing in 4 dimensionsspace (si)time (t)frequency (f)code (c)

Goal: multiple use of a shared medium

Important: guard spaces needed!s2s3s1Multiplexingftck2k3k4k5k6k1ftcftcchannels ki

Frequency multiplexSeparation of the whole spectrum into smaller frequency bandsA channel gets a certain band of the spectrum for the whole timeAdvantages:no dynamic coordination necessaryworks also for analog signals

Disadvantages:waste of bandwidth if the traffic is distributed unevenlyinflexibleguard spaces

k2k3k4k5k6k1ftc

ftck2k3k4k5k6k1Time multiplexA channel gets the whole spectrum for a certain amount of time

Advantages:only one carrier in the medium at any timethroughput high even for many users

Disadvantages:precise synchronization necessary

fTime and frequency multiplexCombination of both methodsA channel gets a certain frequency band for a certain amount of timeExample: GSM Advantages:better protection against tappingprotection against frequency selective interferencehigher data rates compared to code multiplexbut: precise coordination requiredtck2k3k4k5k6k1

Code multiplexEach channel has a unique code

All channels use the same spectrum at the same timeAdvantages:bandwidth efficientno coordination and synchronization necessarygood protection against interference and tappingDisadvantages:lower user data ratesmore complex signal regenerationImplemented using spread spectrum technologyk2k3k4k5k6k1ftc

ModulationDigital modulationdigital data is translated into an analog signal (baseband)ASK, FSK, PSK - main focus in this chapterdifferences in spectral efficiency, power efficiency, robustnessAnalog modulationshifts center frequency of baseband signal up to the radio carrierMotivationsmaller antennas (e.g., /4)Frequency Division Multiplexingmedium characteristicsBasic schemesAmplitude Modulation (AM)Frequency Modulation (FM)Phase Modulation (PM)

Modulation and demodulationsynchronizationdecisiondigitaldataanalogdemodulationradiocarrieranalogbasebandsignal101101001radio receiverdigitalmodulationdigitaldataanalogmodulationradiocarrieranalogbasebandsignal101101001radio transmitter

Digital modulationModulation of digital signals known as Shift KeyingAmplitude Shift Keying (ASK):very simplelow bandwidth requirementsvery susceptible to interference Frequency Shift Keying (FSK):needs larger bandwidth

Phase Shift Keying (PSK):more complexrobust against interference101t101t101t

Advanced Frequency Shift Keyingbandwidth needed for FSK depends on the distance between the carrier frequenciesspecial pre-computation avoids sudden phase shifts MSK (Minimum Shift Keying)bit separated into even and odd bits, the duration of each bit is doubled depending on the bit values (even, odd) the higher or lower frequency, original or inverted is chosenthe frequency of one carrier is twice the frequency of the otherEquivalent to offset QPSK

even higher bandwidth efficiency using a Gaussian low-pass filter GMSK (Gaussian MSK), used in GSM

Example of MSKdataeven bitsodd bits1111000tlow frequencyhigh frequencyMSKsignalbiteven0 1 0 1odd0 0 1 1signalh n n h value- - + +h: high frequencyn: low frequency+: original signal-: inverted signalNo phase shifts!

Advanced Phase Shift KeyingBPSK (Binary Phase Shift Keying):bit value 0: sine wavebit value 1: inverted sine wavevery simple PSKlow spectral efficiencyrobust, used e.g. in satellite systemsQPSK (Quadrature Phase Shift Keying):2 bits coded as one symbolsymbol determines shift of sine waveneeds less bandwidth compared to BPSKmore complexOften also transmission of relative, not absolute phase shift: DQPSK - Differential QPSK (IS-136, PHS)

11100001At

Quadrature Amplitude ModulationQuadrature Amplitude Modulation (QAM): combines amplitude and phase modulationit is possible to code n bits using one symbol2n discrete levels, n=2 identical to QPSKbit error rate increases with n, but less errors compared to comparable PSK schemesExample: 16-QAM (4 bits = 1 symbol)Symbols 0011 and 0001 have the same phase, but different amplitude. 0000 and 1000 have different phase, but same amplitude. used in standard 9600 bit/s modems

Hierarchical ModulationDVB-T modulates two separate data streams onto a single DVB-T streamHigh Priority (HP) embedded within a Low Priority (LP) streamMulti carrier system, about 2000 or 8000 carriersQPSK, 16 QAM, 64QAMExample: 64QAMgood reception: resolve the entire 64QAM constellationpoor reception, mobile reception: resolve only QPSK portion6 bit per QAM symbol, 2 most significant determine QPSKHP service coded in QPSK (2 bit), LP uses remaining 4 bitQI0010000010010101

Spread spectrum technologyProblem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interferenceSolution: spread the narrow band signal into a broad band signal using a special codeprotection against narrow band interference

protection against narrowband interferenceSide effects:coexistence of several signals without dynamic coordinationtap-proofAlternatives: Direct Sequence, Frequency Hopping

detection atreceiverinterferencespread signalsignalspreadinterferenceffpowerpower

Effects of spreading and interferencedP/dffi)dP/dffii)senderdP/dffiii)dP/dffiv)receiverfv)user signalbroadband interferencenarrowband interferencedP/df

Spreading and frequency selective fadingfrequencychannel quality123456narrow band signalguard spacenarrowband channelsspread spectrum channels

DSSS (Direct Sequence Spread Spectrum) IXOR of the signal with pseudo-random number (chipping sequence)many chips per bit (e.g., 128) result in higher bandwidth of the signalAdvantagesreduces frequency selective fadingin cellular networks base stations can use the same frequency rangeseveral base stations can detect and recover the signalsoft handoverDisadvantagesprecise power control necessaryuser datachipping sequenceresultingsignal0101101010100111XOR01100101101001=tbtctb: bit periodtc: chip period

DSSS (Direct Sequence Spread Spectrum) IIXuser datachippingsequencemodulatorradiocarrierspreadspectrumsignaltransmitsignaltransmitterdemodulatorreceivedsignalradiocarrierXchippingsequencelowpassfilteredsignalreceiverintegratorproductsdecisiondatasampledsumscorrelator

FHSS (Frequency Hopping Spread Spectrum) IDiscrete changes of carrier frequencysequence of frequency changes determined via pseudo random number sequenceTwo versionsFast Hopping: several frequencies per user bitSlow Hopping: several user bits per frequencyAdvantagesfrequency selective fading and interference limited to short periodsimple implementationuses only small portion of spectrum at any timeDisadvantagesnot as robust as DSSSsimpler to detect

FHSS (Frequency Hopping Spread Spectrum) IIuser dataslowhopping(3 bits/hop)fasthopping(3 hops/bit)01tb011tff1f2f3ttdff1f2f3ttdtb: bit periodtd: dwell time

FHSS (Frequency Hopping Spread Spectrum) IIImodulatoruser datahoppingsequencemodulatornarrowbandsignalspreadtransmitsignaltransmitterreceivedsignalreceiverdemodulatordatafrequencysynthesizerhoppingsequencedemodulatorfrequencysynthesizernarrowbandsignal

Cell structureImplements space division multiplex: base station covers a certain 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. locallyProblems:fixed network needed for the base stationshandover (changing from one cell to another) necessaryinterference with other cellsCell sizes from some 100 m in cities to, e.g., 35 km on the country side (GSM) - even less for higher frequencies

FHSS (Frequency Hopping Spread Spectrum) IIImodulatoruser datahoppingsequencemodulatornarrowbandsignalspreadtransmitsignaltransmitterreceivedsignalreceiverdemodulatordatafrequencysynthesizerhoppingsequencedemodulatorfrequencysynthesizernarrowbandsignal

Cell structureImplements space division multiplex: base station covers a certain 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. locallyProblems:fixed network needed for the base stationshandover (changing from one cell to another) necessaryinterference with other cellsCell sizes from some 100 m in cities to, e.g., 35 km on the country side (GSM) - even less for higher frequencies

Frequency planning IFrequency reuse only with a certain distance between the base stationsStandard model using 7 frequencies:

Fixed frequency assignment:certain frequencies are assigned to a certain cellproblem: different traffic load in different cellsDynamic frequency assignment:base station chooses frequencies depending on the frequencies already used in neighbor cellsmore capacity in cells with more trafficassignment can also be based on interference measurements

Frequency planning IIf4f5f1f3f2f6f7f3f2f4f5f1f3f5f6f7f2f23 cell cluster7 cell cluster3 cell clusterwith 3 sector antennas

Cell breathingCDM systems: cell size depends on current loadAdditional traffic appears as noise to other usersIf the noise level is too high users drop out of cells

**Universitt KarlsruheInstitut fr TelematikMobilkommunikationSS 1998Prof. Dr. Dr. h.c. G. KrgerE. Dorner / Dr. J. Schiller*Representation in Time domain is problematic if there are many different frequencies. In this case the better representation is the Frequency domain.Here the amplitude of a certain frequency part is shown versus the frequency.Arbitrary periodic functions has many peaks known as frequency spectrum. Tool used is spectrum analyzer.In phase domain M represents the amplitude of a signal and the phase in polar coordinates.Universitt KarlsruheInstitut fr TelematikMobilkommunikationSS 1998Prof. Dr. Dr. h.c. G. KrgerE. Dorner / Dr. J. Schiller*Universitt KarlsruheInstitut fr TelematikMobilkommunikationSS 1998Prof. Dr. Dr. h.c. G. KrgerE. Dorner / Dr. J. Schiller*