An Introduction to Wireless Technologies

Part 1

F. Ricci

Content

Wireless communication standards

Computer Networks

Simple reference model

Frequencies and regulations

Wireless communication technologies

Signal propagation

Signal modulation

Multiplexing

Medium access control

Most of the slides of this lecture come from prof. JochenSchiller’s didactical material for the book “Mobile Communications”, Addison Wesley, 2003.

Wireless systems: overview

cellular phones satellites wireless LANcordlessphones

1992:GSM

1994:DCS 1800

2001:IMT-2000

1987:CT1+

1982:Inmarsat-A

1992:Inmarsat-BInmarsat-M

1998:Iridium

1989:CT 2

1991:DECT 199x:

proprietary

1997:IEEE 802.11

1999:802.11b, Bluetooth

1988:Inmarsat-C

analogue

digital

1991:D-AMPS

1991:CDMA

1981:NMT 450

1986:NMT 900

1980:CT0

1984:CT1

1983:AMPS

1993:PDC

4G – fourth generation: when and how?

2000:GPRS

2000:IEEE 802.11a

200?:Fourth Generation(Internet based)

Nokia N95

Operating Frequency: WCDMA2100 (HSDPA),EGSM900, GSM850/1800/1900 MHz (EGPRS)

Memory: Up to 160 MB internal dynamic memory; memory card slot - microSD memory cards (up to 2 GB)Display: 2.6" QVGA (240 x 320 pixels) TFT –ambient light detector - up to 16 million colors

Data Transfer:WCDMA 2100 (HSDPA) with simultaneous voice and packet data (Packet Switching max speed UL/DL= 384/3.6MB, Circuit Switching max speed 64kbps)Dual Transfer Mode (DTM) support for simultaneous voice and packet data connection in GSM/EDGE networks - max speed DL/UL: 177.6/118.4 kbits/sEGPRS class B, multi slot class 32, max speed DL/UL= 296 / 177.6 kbits/s

Cellular Generations

FirstAnalog, circuit-switched (AMPS, TACS)

SecondDigital, circuit-switched (GSM) 10 Kbps

Advanced secondDigital, circuit switched (HSCSD High-Speed Circuit Switched Data), Internet-enabled (WAP) 10 Kbps

2.5Digital, packet-switched, TDMA (GPRS, EDGE)40-400 Kbps

ThirdDigital, packet-switched, Wideband CDMA(UMTS)0.4 – 2 Mbps

FourthData rate 100 Mbps; achieves “telepresence”

SpeedSpeed

Services

E-mail file10 Kbyte

Web Page9 Kbyte

Text File40 Kbyte

Large Report2 Mbyte

Video Clip4 Mbyte

Film with TVQuality

2G

8 sec

9 sec

33 sec

28 min

48 min

1100 hr

PSTN

3 sec

3 sec

11 sec

9 min

18 min

350 hr

ISDN

1 sec

1 sec

5 sec

4 min

8 min

104 hr

2G+

0.7 sec

0.8 sec

3 sec

2 min

4 min

52 hr

UMTS/3G

0.04 sec

0.04sec

0.2 sec

7 sec

14 sec

>5hr

Source: UMTS Forum

Computer Networks

A computer network is two or more computersconnected together using a telecommunication system for the purpose of communicating and sharing resourcesWhy they are interesting?

Overcome geographic limitsAccess remote dataSeparate clients and server

Goal: Universal Communication (any to any)

Network

Type of Networks

PAN: A personal area network is a computer network (CN) used for communication among computer devices (including telephones and personal digital assistants) close to one person

Technologies: USB and Firewire (wired), IrDA and Bluetooth (wireless)

LAN: A local area network is a CN covering a small geographic area, like a home, office, or group of buildings

Technologies: Ethernet (wired) or Wi-Fi (wireless)MAN: Metropolitan Area Networks are large CNs usually spanning a city

Technologies: Ethernet (wired) or WiMAX (wireless)WAN: Wide Area Network is a CN that covers a broad area, e.g., cross metropolitan, regional, or national boundaries

Examples: Internet Wireless Technologies: HSDPA, EDGE, GPRS, GSM.

Reference Model

Application

Transport

Network

Data Link

Physical

Medium

Data Link

Physical

Application

Transport

Network

Data Link

Physical

Data Link

Physical

Network Network

Radio

Reference model

Physical layer: conversion of stream of bits into signals – carrier generation - frequency selection – signal detection – encryption

Data link layer: accessing the medium –multiplexing - error correction – syncronization

Network layer: routing packets – addressing -handover between networks

Transport layer: establish an end-to-end connection – quality of service – flow and congestion control

Application layer: service location – support multimedia – wireless access to www

Wireless Network

The difference between wired and wireless is the physical layer

Wired network technology is based on wires or fibers

Data transmission in wireless networks take place using electromagnetic waves which propagates through space (scattered, reflected, attenuated)

Data are modulated onto carrier frequencies(amplitude, frequency)

The data link layer (accessing the medium, multiplexing, error correction, syncronization) requires more complex mechanisms

IEEE standard 802.11

mobile terminal

access point

fixedterminal

application

TCP

802.11 PHY

802.11 MAC

IP

802.3 MAC

802.3 PHY

application

TCP

802.3 PHY

802.3 MAC

IP

802.11 MAC

802.11 PHY

LLC

infrastructurenetwork

LLC LLC

Network layer

Transport layer

Data link layer

Physical link l.

Electromagnetic Spectrum

SOURCE: JSC.MIL

SOUND LIGHTRADIO HARMFUL RADIATION

VHF = VERY HIGH FREQUENCYUHF = ULTRA HIGH FREQUENCYSHF = SUPER HIGH FREQUENCY EHF = EXTRA HIGH FREQUENCY

4G CELLULAR56-100 GHz

3G CELLULAR1.5-5.2 GHz

1G, 2G CELLULAR0.4-1.5GHz

UWB3.1-10.6 GHz

Frequencies and regulations

ITU-R (International Telecommunication Union –Radiocommunication) holds auctions for new frequencies, manages frequency bands worldwide

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, 869-894 TDMA, 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

Valu

es in

MH

z

Wireless Telephony

SOURCE: IEC.ORG

AIR LINK

PUBLIC SWITCHEDTELEPHONE NETWORK

WIRED

Mobile Communication Technologies

Local wireless networksWLAN 802.11

802.11a

802.11b802.11i/e/…/w

802.11g

WiFi 802.11h

Personal wireless nwWPAN 802.15

802.15.4

802.15.1 802.15.2Bluetooth

802.15.4a/bZigBee

802.15.3

Wireless distribution networksWMAN 802.16 (Broadband Wireless Access)

802.20 (Mobile Broadband Wireless Access)+ Mobility

WiMAX

802.15.3a/b

802.15.5

Bluetooth

A standard permitting for wireless connection of:Personal computersPrintersMobile phonesHandsfree headsetsLCD projectorsModemsWireless LAN devicesNotebooksDesktop PCsPDAs

Bluetooth Devices

NOKIA 9110 + FUJIDIGITAL CAMERA

ERICSSONCOMMUNICATOR

ERICSSON R520GSM 900/1800/1900

ALCATELOne TouchTM 700

GPRS, WAPERICSSON

BLUETOOTHCELLPHONE

HEADSET

Bluetooth Characteristics

Operates in the 2.4 GHz band - Packet switched

1 milliwatt - as opposed to 500 mW cellphone

Low cost

10m to 100m range

Uses Frequency Hop (FH) spread spectrum, which dividesthe frequency band into a number of hop channels. Duringconnection, devices hop from one channel to another 1600 times per second

Bandwidth 1-2 megabits/second (GPRS is ~50kbits/s)

Supports up to 8 devices in a piconet (= two or more Bluetooth units sharing a channel).

Built-in security

Non line-of-sight transmission through walls and briefcases

Easy integration of TCP/IP for networking.

Wi-Fi

Wi-Fi is a technology for WLAN based on the IEEE 802.11(a, b, g) specifications

Originally developed for PC in WLAN

Increasingly used for more services:

Internet and VoIP phone access, gaming, …

and basic connectivity of consumer electronics such as televisions and DVD players, or digital cameras, …

In the future Wi-Fi will be used by cars in highways in support of an Intelligent Transportation System to increase safety, gather statistics, and enable mobile commerce (IEEE 802.11p)

Wi-Fi supports structured (access point) and ad-hoc networks (a PC and a digital camera).

Wi-Fi

An access point (AP) broadcasts its SSID (Service Set Identifier, "Network name") via packets (beacons) broadcasted every 100 ms at 1 Mbit/sBased on the settings (e.g. the SSID), the client may decide whether to connect to an APWi-Fi transmission, as a non-switched wired Ethernet network, can generate collisionsWi-Fi uses CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) to avoid collisionsCSMA = the sender before transmitting it senses the carrier – if there is another device communicating then it waits a random time an retryCA = the sender before transmitting contacts the receiver and ask for an acknowledgement – if not received the request is repeated after a random time interval.

WiMAX

IEEE 802.16: Broadband Wireless Access / WirelessMAN / WiMax (Worldwide Interoperability for Microwave Access)

Connecting Wi-Fi hotspots with each other and to other parts of the Internet

Providing a wireless alternative to cable and DSL for last mile (last km) broadband access

Providing high-speed mobile data and telecommunications services

Providing Nomadic connectivity

75 Mbit/s up to 50 km LOS, up to 10 km NLOS; 2-5 GHz band

Initial standards without roaming or mobility support

802.16e adds mobility support, allows for roaming at 150 km/h.

Advantages of wireless LANs

very flexible within the reception area

Ad-hoc networks without previous planning possible

(almost) no wiring difficulties (e.g. historic buildings, firewalls)

more robust against disasters like, e.g., earthquakes, fire - or users pulling a plug...

Wireless networks disadvantages

Higher loss-rates due to interferenceemissions of, e.g., engines, lightning

Restrictive regulations of frequenciesfrequencies have to be coordinated, useful frequencies are almost all occupied

Low transmission rateslocal some Mbit/s, regional currently, e.g., 53kbit/s with GSM/GPRS

Higher delays, higher jitterconnection setup time with GSM in the second range, several hundred milliseconds for other wireless systems

Lower security, simpler active attackingradio interface accessible for everyone, base station can be simulated, thus attracting calls from mobile phones

Always shared mediumsecure access mechanisms important

Signals I

Physical representation of dataUsers can exchange data through the transmission of signalsThe Layer 1 is responsible for conversion of data, i.e., bits, into signals and viceversaSignals are a function of time and locationSignal parameters of periodic signals: period T, frequency f=1/T, amplitude A, phase shift ϕ

sine wave as special periodic signal for a carrier:s(t) = At sin(2 π ft t + ϕt)

Sine waves are of special interest as it is possible to construct every periodic signal using only sine and cosine functions (Fourier equation).

Different representations of signals amplitude (amplitude domain)frequency spectrum (frequency domain)

phase state diagram (amplitude M and phase ϕ in polar coordinates)

Composed signals transferred into frequency domain using Fourier transformationDigital signals need:

infinite frequencies for perfect transmission modulation with a carrier frequency for transmission (analog signal!)

Signals II

f [Hz]

A [V]

ϕ

I= M cos ϕ

Q = M sin ϕ

ϕ

A [V]

t[s]

Digital modulation

Modulation of digital signals known as Shift Keying

Amplitude Shift Keying (ASK):

very simple

low bandwidth requirements

very susceptible to interference

Frequency Shift Keying (FSK):

needs larger bandwidth

Phase Shift Keying (PSK):

more complex

robust against interference

1 0 1

t

1 0 1

t

1 0 1

t

Modulation and demodulation

synchronizationdecision

digitaldataanalog

demodulation

radiocarrier

analogbasebandsignal

101101001 radio receiver

digitalmodulation

digitaldata analog

modulation

radiocarrier

analogbasebandsignal

101101001 radio transmitter

Modulation

Digital modulationdigital data is translated into an analog signal (baseband) with: ASK, FSK, PSKdifferences in spectral efficiency, power efficiency, robustness

Analog modulation: shifts center frequency of baseband signal up to the radio carrier

Motivation smaller antennas (e.g., λ/4)Frequency Division Multiplexingmedium characteristics

Basic schemesAmplitude Modulation (AM)Frequency Modulation (FM)Phase Modulation (PM)

Signal in wired networks

There is a sender and a receiver

The wire determine the propagation of the signal (the signal can only propagate through the wire

twisted pair of copper wires (telephone)

or a coaxial cable (TV antenna)

As long as the wire is not interrupted everything is ok and the signal has the same characteristics at each point

For wireless transmission this predictable behavior is true only in a vacuum – without matter between the sender and the receiver.

Signal propagation ranges

distance

sender

transmission

detection

interference

Transmission rangecommunication possiblelow error rate

Detection rangedetection of the signal possibleno communication possible

Interference rangesignal may not be detected signal adds to the background noise

receiver

Path loss of radio signals

In free space radio signal propagates as light does – straight lineEven without matter between the sender and the receiver, there is a free space loss

Receiving power proportional to 1/d² (d = distance between sender and receiver)

If there is matter between sender and receiverThe atmosphere heavily influences transmission over long distanceRain can absorb radiation energyRadio waves can penetrate objects (the lower the frequency the better the penetration –higher frequencies behave like light!)

Signal propagation

In real life we rarely have a line-of-sight between sender and receiverReceiving power additionally influenced by

fading (frequency dependent)shadowingreflection at large obstaclesrefraction depending on the density of a mediumscattering at small obstacles (size in the order of the wavelength)diffraction at edges

reflection scattering diffractionshadowing refraction

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 parts

Multipath propagation

signal at sendersignal at receiver

LOS pulsesmultipathpulses

Multiplexing describes how several users can share a medium with minimum or no interference

Example: lanes in a highway

Cars in different lanes (space division multiplexing)

Cars in a line but at different times (time division multiplexing)

Multiplexing in 4 dimensions

space (s)

time (t)

frequency (f)

code (c)

Important: guard spaces needed!

Multiplexing

Space division multiplexing

Different channels for communications are allocated to different spacesHere only three channels can be separatedExample: each subscriber of an analogue telephone system is given a different wireExample: FM stations can transmit only in a certain regionSDM is the simplest and inefficientUsually associated with other methods.

s2

s3

s1 f

t

c

k2 k3 k4 k5 k6k1

f

t

c

f

t

c

channels ki

Frequency multiplex

Separation 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

k2 k3 k4 k5 k6k1

f

t

c

f

t

c

k2 k3 k4 k5 k6k1

Time multiplex

A channel gets the whole spectrum for a certain amount of time

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

Disadvantages:Precise synchronization necessary (clocks)Guard space

f

Time and frequency multiplex

Combination of both methods

A channel gets a certain frequency band for a certain amount of time

Example: GSM

Advantages:

better protection against tapping

protection against frequency selective interference

higher data rates compared to code multiplex

but: precise coordination required

t

c

k2 k3 k4 k5 k6k1

Code multiplex

Each channel has a unique code:a vector of 1 and -1,These vectors are orthogonal and have a large autocorrelation (norm of the vector) All channels use the same spectrum at the same timeAdvantages:

bandwidth efficientno coordination and synchronization necessarygood protection against interference and tapping

Disadvantages:lower user data ratesmore complex signal regeneration.

k2 k3 k4 k5 k6k1

f

t

c

Medium access control

Medium access control comprises all mechanisms that regulate user access to a medium using SDM, TDM, FDM or CDM

MAC is a sort of traffic regulation (as traffic lights in road traffic)

MAC belongs to layer 2 (OSI Model): data link control layer

The most important methods are TDM

TDM is convenient because the systems stay tuned on a given frequency and the us the frequency only for a certain amount of time (GSM)

Motivation for a Medium Access Control

Can we apply media access methods from fixed networks?

Example CSMA/CD

Carrier Sense Multiple Access with Collision Detection

send as soon as the medium is free, listen into the medium if a collision occurs (original method in IEEE 802.3)

Problems in wireless networks

signal strength decreases proportional to the square of the distance

the sender would apply CS and CD, but the collisions happen at the receiver

it might be the case that a sender cannot “hear” the collision, i.e., CD does not work

furthermore, CS might not work if, e.g., a terminal is “hidden” (too far to be heard).

Hidden terminals: the medium seems free and collisions are not detected

A sends to B, C cannot receive A C wants to send to B, C senses a “free” medium (CS fails)collision at B, A cannot receive the collision (CD fails)A is “hidden” for C

Exposed terminals: the medium seems in use but this will not cause a collision

B sends to A, C wants to send to another terminal (not A or B)C has to wait, CS signals a medium in usebut A is outside the radio range of C, therefore waiting is not necessaryC is “exposed” to B

Motivation - hidden and exposed terminals

BA C

Terminals A and B send, C receivessignal strength decreases proportional to the square of the distancethe signal of terminal B therefore drowns out A’s signalC cannot receive A

If C for example was an arbiter for sending rights, terminal B would drown out terminal A already on the physical layer

Motivation - near and far terminals

A B C