IT 802DMobile Communications

Debasis Das

Recommended BooksJ. Schiller, Mobile Communications, Addison –

Wesley, 2003

2. T. S. Rapport, Wireless Communications,

Principle and Practices

3. Forouzan, Data Communications and

Networking, TMH

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Introduction

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Module TopicsGeneral Overview: History, Transmission

Medium,

Need, Advantages, Disadvantages

Different Standards. AMPS, GSM, GPRS, 3G.

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Usage ScenariosFixed & wired

Desktops connected to a wired set upMobile & wired

Laptops get connected to wired networks in hotels

Fixed & wirelessTemporary arrangements, places where fixed

wiring may not be possibleMobile & wireless

Topic of this course, users are completely free to move around

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Mobile Devices That Need to

CommunicateCellular handsetsWalkie-talkiesPagerPDA/Pocket computerCordless phonesLaptops on wireless LANsTablet computerseReaders

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What Do We Need!A means of carrying information from one

party to another

A means of modifying the carrier to convey

information

Standardized formats for communication

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RF Can Do the JobElectromagnetic waves in RF frequencies

can travel long distances

The carrier frequency can be modulated to

carry information

The carrier needs to be changed in some way

to be proportional to the input information

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RF BandsVery Low Frequency

VLF 9 kHz - 30 kHz 33 km - 10 km

Low Frequency LF 30 kHz - 300 kHz 10 km - 1 km

Medium Frequency MF 300 kHz - 3 MHz 1 km - 100 m

High Frequency HF 3 MHz - 30 MHz 100 m - 10 m

Very High Frequency

VHF 30 MHz - 300 MHz 10 m - 1 m

Ultra High Frequency

UHF 300 MHz - 3 GHz 1 m - 100 mm

Super High Frequency

SHF 3 GHz - 30 GHz 100 mm - 10 mm

Extremely High Frequency

EHF 30 GHz - 300 GHz 10 mm - 1 mm

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Electromagnetic BandsUp to 300 GHz

RF band

Up to 300 THz

IR band

Above 300 THz

Visible light, UV

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Wireless Communication Scenarios- 1

Lambda=c/f wavelengthC=3*10^8 meters/s

LF – submarine communications, penetrates water and other obstacles

MF & HF - radio broadcastsAM band; 520 kHz to 1605.5 kHzSW band; 5.8 MHz to 26.1 MHzFM band; 87.5 MHz to 108 MHz

UHF & VHF – TV broadcasts 174 MHz to 230 MHz & 470 to 790 MHz

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Wireless Communication Scenarios- 2

VHF & UHF; Digital audio 232 to 230 MHz, 1452 to 1472 MHzDigital TV 470 to 862 MHzAnalog mobile phone 450-465 MHzDigital GSM 490-960 MHz, 1710-1880 MHzDECT cordless 1880-1900 MHz3G cellular, UMTS standard 1900-1980 MHz,

2020-2025 MHz, 2110-2190 MHz ….

Most wireless mobile communication use the VHF and UHF bands. Small antennas.

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Basic RF Communication

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Transmitter Receiver

Propagation Basics

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Transmitter

Receivers

Transmission range

Detection range

Interference range

Various Range in TransmissionTransmission range : The zone around the

transmitter where communication is possible.

Receiver receives enough signal so that error

rates are very low

Detection range : transmitted power can be

detected, error rate is high however

Interference zone : not high enough to be

detected but adds to the interference with

other signals

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Radiation Pattern

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Omni-directional Directional

Transmitters

Antenna IssuesTheoretical isotropic antenna is one that

radiates equal energy in all directions.Practical antenna is a dipole, length

lambda/2, centre fed, two halves of lambda/4Dipoles are Omni-directionalSectorized antennas are multiple directional

antenna on a single poleMultiple element antennas help combat

fading effects

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MultiplexingSpace division multiplexing (SDM)

Frequency division multiplexing (FDM)

Time division multiplexing (TDM)

Code division multiplexing (CDM)

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Channel AllocationBorrowing channel allocation (BCA)

Borrow unused channel frequencies from

neighboring cells

Fixed channel allocation (FCA)

Channel allocations per cell is fixed

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Inverse Square LawRadio energy available at any point some

distance away from the transmitter

Is inversely proportional to the square of the

distance

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d

Surface area= 4.pi.d^2

Modes of PropagationGround wave

Sky wave

Space wave

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Ground Wave propagationRadio waves in the low frequency ranges (<

2 MHz)

Radio energy travels along the ground mainly

Enclosed between the ground and the Ionosphere

Radio transmissions in medium wave are examples04/12/2023 Debasis Das Mallabhum Institute of Technology 22

Sky Wave PropagationThere is a ionized belt around the earth( 2 to

30 MHz)

Radio wave is reflected in this ionosphere

and travels long distance

Wavelengths are shorter than the frequency

band that uses the ground wave mode

Radio shortwave transmissions are examples04/12/2023 Debasis Das Mallabhum Institute of Technology 23

Space Wave PropagationWavelengths are short enough to penetrate

the ionosphere (> 30 MHz)

Energy needs to travel in straight line to the

receiver

TV transmissions are an example

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Other Signal Propagation Effects

Blocking/shadowing

Reflection

Refraction

Scatter

Diffraction

Multi path propagation

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Multipath Situation

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Car

Skyscraper

Obstacle

Transmitter

Multipath + Channel Characteristics

Multipath causes delay spreadEffect= Inter-symbol interference short term fading

(It can help if an estimate can be made of these multi path delays, at least the main path. Sender transmits a “training sequence : known to receiver, programs an equalizer to compensate

Long term fading, due varying distance from sender, compensate by raising/ lowering power transmitted so that received power is within specified range

Doppler shift

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Mobile Communication Schemes

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(A)

(B)

CDMA

Cellular

North American Systems-1

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Standard Type Year Multipleaccess

Frequencyband

Modulation

AMPS Cellular 1983 FDMA 824-894 FM

NAMS Cellular 1992 FDMA 824-894 FM

USDC Cellular 1991 TDMA 824-894 Pi/4DQPSK

CDPD Cellular 1993 PH/Packet 824-894 GMSK

IS-95 Cellular 1993 CDMA 824-8941.8-2.0

QPSK/BPSK

GSC Paging 1970s Simplex Several FSK

North American Systems-2

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Standard Type Year MultipleAccess

Frequencyband

Modulation

POCSAG Paging 1970s Simplex Several PSK

FLEX Paging 1993 Simplex Several 4-PSK

DCS-1900(GSM)

PCS 1994 TDMA 1.85-1.99 GMSK

PACS Cordless/PCS

1994 TDMA/FDMA

1.85-1.99 Pi/4-DQPSK

MIRS SMR/PCS 1994 TDMA Several 16 QAM

IDen SMR/PCS 1995 TDMA Several 16 QAM

European Systems

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Standard Type Year MultipleAccess

Frequencyband

Modulation

ETACS Cellular 1985 FDMA 900 FM

NMT-450 Cellular 1981 FDMA 450-470 FM

NMT-900 Cellular 1986 FDMA 890-960 FM

GSM Cellular/PCS

1990 TDMA 890-960 GMSK

C-450 Cellular 1985 FDMA 450-465 FM

ERMES Paging 1993 FDMA Several 4-PSK

CT2 Cordless 1989 FDMA 864-868 GPSK

DECT Cordless 1993 TDMA 1880-1900 GPSK

DCS-1800 Cordless/PCS

1993 TDMA 1710-1880 GMSK

Japanese Systems

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Standard Type Year MultipleAccess

Frequency

band

Modulation

JTACS Cellular 1988 FDMA 860-925 FM

PDC Cellular 1993 TDMA 810-1501 Pi/4-DQPSK

NTT Cellular 1979 FDMA 400/800 FM

NTACS Cellular 1993 FDMA 843-925 FM

NTT Paging 1979 FDMA 280 FSK

NEC Paging 1979 FDMA Several FSK

PHS Cordless 1993 TDMA 1895-1907 Pi/4-DQPSK

Basic Modulation Schemes(Digital)

In wireless network analog modulation need to be used, these include following basic schemes

Amplitude Shift Keying ASKFrequency Shift Keying FSKPhase shift keying PSK

The digital input is converted to analog baseband signal before modulation

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Reasons for Going AnalogAntenna

Antenna need to comparable to wavelength of carrier

Frequency Division multiplexingAnalog modulation shifts baseband signal to

different carrier signal. Higher carrier frequency gives you higher bandwidth

Medium CharacteristicsPath loss, penetration of obstacles, reflection,

scattering, diffraction are wavelength dependent

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Modulation in a Transmitter

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Digitalmodulation

Analog Modulation

Digital data

Analog baseband signal

Radio carrier

Modulatedsignal

Demodulation in Receiver

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Analogdemodulation

Synchronizationdecision

Radio carrier

Analog baseband signal

Digitalsignal

ASKSimple scheme, send one level of carrier

amplitude for 1 and another for 0

Needs low bandwidth

But amplitude is affected by multipath ,

noise, path loss etc.

ASK is not used for RF communication

However, is used in optical communication04/12/2023 Debasis Das Mallabhum Institute of Technology 37

FSKUsed often in RF communication, binary FSK

or BFSKDifferent frequencies are send for the two

digital levelsSudden phase changes can generate high

frequenciesFrequency modulation in continuous phase

modulation (CPM) is usedDemodulation can use two fixed frequency

filtersNeeds larger bandwidth, less susceptible to

noise04/12/2023 Debasis Das Mallabhum Institute of Technology 38

Phase Shift KeyingChange phase of carrier by 180 degrees,

every time baseband changes from 1 to 0 or 0 to 1. binary PSK

BPSK can be simply multiplying the carrier by +1 when input is 1 and multiplying by -1 when input is 0

Reception uses phase locked loop for reference

Compared to FSK, PSK is more resistant to interference

Receiver and transmitter are more complex to design

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MSK(Min Phase Shift Keying)Avoids abrupt phase changes

Data bits are separated into odd and even

bits, duration of bits are doubled

Two frequencies f1 & f2, such that f2=2*f1

Phase is shifted based different criteria

Gaussian MSK is a variation, GMSK is used

in most European wireless standards 04/12/2023 Debasis Das Mallabhum Institute of Technology 40

Advanced PSK2 bits are considered together, smaller phase

shifts for each group of 2 bits, 4 phase shifts are used

This is the QPSK schemeCarrier reference is a must, frequent

synchronization requiredDifferential PSK, decides phase shifts with

respect from the last phase, not absoluteDQPSK is one of the most efficient schemesIS-136, PACS of US and Japanese PHS of

Japan are examples04/12/2023 Debasis Das Mallabhum Institute of Technology 41

QAMPhase shift keying and amplitude keying is

combined

QPSK and several levels of amplitude

64QAm for example can combine QPSK and 4

levels of amplitude

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Multi Carrier ModulationMCM, orthogonal frequency division

multiplexing(OFDM), coded OFDM

Used in European digital audio broadcast

(DAB), WLAN standards such as IEEE

802.11, HIPERLAN2

Good ISI tolerance04/12/2023 Debasis Das Mallabhum Institute of Technology 43

Spread SpectrumDirect sequence spread spectrumFrequency hopping spectrum system

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X Modulator

Chippingsequence

Radiocarrier

Userdata

Spread spectrum signal

Transmissionsignal

Demodulator

XIntegrato

rDecision

CarrierData

Correlator

Frequency Hopping Spread Spectrum

Available bandwidth is split into many channels plus guard spaces

Transmitter and receiver stay on a channel for some time and then jump to another, the sequence of this change is the hopping sequence

Slow hopping: transmitter uses one frequency for several bit times.

Fast hopping: transmitter changes even during one bit period

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FHSS System

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Modulator

Modulator

Frequency

synthesizer

Demodulator

Demodulator

Frequency

synthesizer

Hoppingsequence

Hopping sequence

Narrowband signal

Data

Spread spectrumsignal

Data

Narrowband signal

Cellular SystemsOne base station serving a cell (limited area)

Higher the user density, smaller the cell

Frequency re-use is possible

3 and 7 cell clusters are common

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Advantages of Small CellsHigher capacity

Space division multiplexing allows frequency reuse, more users can be supported

Less transmission powerMobile station power need to be limited, smaller

cells allow better communicationLocal interference only

Need to worry about local interference onlyRobustness

More base stations mean that the system would not fail as a whole if some base stations fail

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Disadvantages of Small CellsInfrastructure needed

Larger amount of infrastructure, in terms of base stations, towers and other s will be needed

Handover neededWith mobile users, users moving from one cell

to another, calls will have to be maintainedFrequency planning

Careful frequency planning needed to avoid interference when you have only a handful of frequencies allocated

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