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