New Standards for WLANs - commlab.chungbuk.ac.krcommlab.chungbuk.ac.kr/lab/lecture2/lecture2_1/9....

FB 1 –TZI ikom / Kommunikationsnetze

New Standards for WLANs

Wintersemester 2006

Dr.-Ing. Andreas Timm-Gielikom - Universität Bremen

E-Mail: [email protected]: + 49 421 218 9719

www.comnets.uni-bremen.de/~atg

Room S2260 (NW1)

- 2 -


Andreas Timm-Giel – Winter Term 2006

Personal Background• 1988-1994 Student, Electrical Engineering/Information

Technology, Bremen

• 1994-1999 Research Assistant, Microwave Department, University of Bremen, EU-Projects mobile Satellite Communications, Mobile CommunicationsPh.D. 1999: Modelling of Mobile Wideband Radio Channels

• 2000-2002, Project Leader MediaMobil Communication GmbH, Product Manager, Manager Network Operations M2sat Ltd.

• Since Dec. 2002 Senior Researcher, Univ. Bremen, ikom, AG KommunikationsnetzeEU-Projects, Mobile and Wireless Communication Systems, Application and Services, SFB Selbststeuerung in der Logistik

- 3 -



Organisational Issues

Exam:

• Oral Exam (30min) after the end of the term

Questions, Critics etc.:

• always welcome

How to reach me?

• Best by email and ask for an appointment ;-)

Exercises:

• Thursday 12-13, N1350 to deepen understanding,

open discussion, some labs/simulation

- 4 -



Objective of the Lecture

� Understand wireless LAN: how does it work?

� Give an overview on existing and emerging

wireless standards

� Give an idea what is coming up in the future in

Wireless Communications

- 5 -



Course Overview I

Overview:

– Mobile and Wireless Communications Basics (radiopropagation, modulation, coding, multiple access, duplex schemes, access protocols)

– Overview on Wireless Communication Standards (WLAN, PAN, BAN, WMAN, MBWA)

– IEEE 802.11(Wireless LAN)

• Overview 802.11 a, b, g, h...

• Physical Layer

• MAC Layer

• DLC Layer

- 6 -



Course Overview II– IEEE 802.15 - Wireless Personal Area Network

(WPAN): Bluetooth and Zigbee

– IEEE 802.16 - Wireless Metropolitan Area Network(WMAN) - WiMAX

– IEEE 802.20 - Mobile Broadband Wireless Access (MBWA)

– Sensor Networks

– Wireless Protocols:

• Ad Hoc Networking Protocols

– Outlook to the future: mobile and wireless convergence

- 7 -



Literature• See Website – www.comnets.uni-bremen.de – Updated during the

course

• Jochen Schiller – Mobilkommunikation, Pearson, 2003, available in English, from Addison-Wesley.

• Bernhard H. Walke – Mobile Radio Networks, J. Wiley & Sons, 1999• Walke, UMTS – the fundamentals• IEEE Standardisation documents (links on website)

• Many others, like:– K. David/T. Benkner, Digitale Mobilfunksysteme, Teubner, 1996– Kammeyer, Nachrichtenübertragung, Teubner, 1996

• Please also check the website… Some links to pdf and ppt will begiven there…


Chapter 1: First Mobile and Wireless Communication Systems

- 9 -



Definition of Wireless and Mobile

• Wireless

– Communication without wires, can be mobile

and fixed

• Mobile

– Portable devices (laptops, notebooks etc.)

connected at different location to wired

networks (e.g. LAN or PSTN)

– Portable devices (phones, notebooks, PDAs

etc.) connected to wireless networks (UMTS,

GSM, WLAN….)

- 10 -



Early History of Wireless Communications

• In history first light and sound have been used to transmit messages over wide distances

Bilder aus http://www.connected-earth.com

- 11 -



Transmission of electromagnetic Waves

• 1831: Faraday demonstrates magnetic induction

• 1865: Maxwell Theory of electromagnetic fields, wave equation

• 1876 Patent on phone, Alexander Graham Bell (Antonio

Meucci 1849)

• 1888: H. Hertz: demonstratesthe wave character of electrical transmission through space

Fotos from www.wikipedia.org

- 12 -



History of Wireless Communication

• 1895 Guglielmo Marconi

– first demonstration of wireless telegraphy (digital!)

– long wave transmission, high transmission power necessary (> 200kw)

• 1907 Commercial transatlantic connections

– huge base stations (30 100m high antennas)

• 1915 Wireless voice transmission New York - San Francisco

• 1920 Discovery of short waves by Marconi

– reflection at the ionosphere

– smaller sender and receiver, possible due to the invention of the vacuum tube (1906, Lee DeForest and Robert von Lieben)

Foto from www.wikipedia.org

- 13 -



Beginning of Mobile Communications• 1911 mobile transmitter on Zeppelin

• 1926 train (Hamburg – Berlin)

• 1927 first commercial car radio (receive only)

• First Mobile Communication Systems startedin the 40s in the US and in the 50s in Europe.

CONCEPTS:

• Large Areas per Transmitter

• „Mobiles“ large, high power consumption

• Systems low capacity, interference-prone

• Expensive !!!1924

- 14 -



Wireless Communication1984 CT-1 standard (Europe) for cordless

telephones

1992 Specification of DECT– Digital 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 countries

1996 HiperLAN (High Performance Radio Local Area Network)– ETSI, standardization of type 1: 5.15 - 5.30GHz,

23.5Mbit/s– recommendations for type 2 and 3 (both 5GHz) and 4

(17GHz) as wireless ATM-networks (up to 155Mbit/s)

- 15 -



Wireless Communications II

1997 Wireless LAN - IEEE802.11– IEEE standard, 2.4 - 2.5GHz and infrared,

2Mbit/s

– already many (proprietary) products available in the beginning

• Standardization of additional wireless LANs– IEEE standard 802.11b, 2.4 GHz, 11Mbit/s

– IEEE 802.11a (5GHz, 54 Mbit/s), 802.11g (2.4 GHz, 54 Mbit/s)…

– Bluetooth for piconets, 2.4Ghz, <1Mbit/s

– 2003/2004: Zigbee/ IEEE 802.15.4 for Sensor Networks


End Chapter 1


Chapter 2 – Mobile Communications

Definitions & Basics

- 18 -



Overview

• Digital Transmission System

• Frequencies

• Radio Propagation and Radio Channels

• Multiple Access

• Modulation

• OFDM

• Interference

• Bandwidth Requirements

- 19 -



Acknowledgement• Pictures and some slides of this chapter are taken

from:

– B. Walke, P. Seidenberg, M. P. Althof, UMTS: the fundamentals, Wiley

– Schiller: Mobilkommunikation (Mobile Communications), Pearson Studium/AddisonWesley, 2003/2002

– David/Benkner: Digitale Mobilfunksysteme, Teubner 1996

– Proakis/Saleh, Grundlagen der Kommunikationstechnik, Pearson Studium 2004

- 20 -



Structure of digital Transmission System

Digital Source

Sink

- 21 -



Simple Reference Model

Application

Transport

Network

Data Link

Physical

Medium

Data Link

Physical

Application

Transport

Network

Data Link

Physical

Data Link

Physical

Network Network

Radio

- 22 -



– service location

– new applications, multimedia

– adaptive applications

– congestion and flow control

– quality of service

– addressing, routing, device location

– hand-over

– authentication

– media access

– multiplexing

– media access control

– encryption

– modulation

– interference

– attenuation

– frequency

• Application layer

• Transport layer

• Network layer

• Data link layer

• Physical layer

- 23 -



Frequencies for Communication

VLF = Very Low Frequency UHF = Ultra High Frequency

LF = Low Frequency SHF = Super High Frequency

MF = Medium Frequency EHF = Extra High Frequency

HF = High Frequency UV = Ultraviolet Light

VHF = Very High Frequency

Frequency and wave length:

λ = c/f

With wave length λ, speed of light c ≅ 3x108m/s, frequency f

1 Mm300 Hz

10 km30 kHz

100 m3 MHz

1 m300 MHz

10 mm30 GHz

100 µm3 THz

1 µm300 THz

visible lightVLF LF MF HF VHF UHF SHF EHF infrared UV

optical transmissioncoax cabletwisted pair

- 24 -



Frequencies for Mobile Communication

• VHF-/UHF-ranges for mobile radio

– simple, small antenna for cars

– deterministic propagation characteristics, reliable connections

• SHF and higher for directed radio links, satellite communication

– small antenna, focusing

– large bandwidth available

• Wireless LANs use frequencies in UHF to SHF spectrum

– some systems planned up to EHF

– limitations due to absorption by water and oxygen molecules (resonance frequencies)

• weather dependent fading, signal loss caused by heavy rainfall etc.

- 25 -



Frequencies and Regulations

• ITU-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, 869-894 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

- 26 -



ISM-BandISM band

The industrial, scientific, and medical (ISM) radio bands wereoriginally reserved internationally for non-commercial use of RF electromagnetic fields for industrial, scientific and medical purposes.

• The ISM bands are defined by the ITU-R in 5.138 and 5.150 of theRadio Regulations. Individual countries' use of the bands designated in these sections may differ due to variations in national radio regulations.

• In recent years they have also been used for license-free error-tolerant communications applications such as wireless LANs and Bluetooth:– 900 MHz band (33.3 cm wavelength) – 2.4 GHz band (12.2 cm wavelength) – 5.8 GHz band (5.2 cm wavelength)

• IEEE 802.11b/g wireless Ethernet also operates on the 2.4 GHz band

From Wikipedia, the free encyclopedia.

- 27 -



Some typical ISM bands

24 – 24,25 GHz

WLAN5,725 – 5,875 GHz

WLAN/WPAN2,4 – 2,5 GHz

Americas900 – 928 MHz

Europe433 – 464 MHz

40,66 – 40,70 MHz

26,957 – 27,28 MHz

13,553-13,567 MHz

CommentFrequency

ISM Bands

- 28 -



Signal Propagation

reflection scattering diffractionrefraction

• Propagation in free space always like light (straight line)

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

• Sources of distortion– Reflection/refraction – bounce of a surface; enter material

– Scattering – multiple reflections at rough surfaces

– Diffraction – start “new wave” from a sharp edge

– Doppler fading – shift in frequencies (loss of center)

– Attenuation – energy is distributed to larger areas with increasing distance

- 29 -



Attenuation results in path loss• Effect of attenuation: received signal strength is a function

of the distance d between sender and transmitter

• Captured by Friis free-space equation

– Describes signal strength at distance d relative to some reference distance d0 < d for which strength is known

– d0 is far-field distance, depends on antenna technology

- 30 -



Pathloss in Free Space• Received power depends on frequency, transmitted

power, antenna gains, distance and constants only

• EIRP: effective isotropic radiated power:

– EIRP = PTGT

RTTR GGd

PP

=

π

λ

4

56.147log20log20log10log10)(

log10)(

+−−+=

=

dfGGdBL

P

PdBL

RTF

T

RF [Hz]Frequency :

[m] Distance:

[m] Wavelength:

r/ReceiverTransmitteGain Antenna:

Powerreceived/dtransmitte:

/

/

f

d

G

P

RT

RT

λ

- 31 -



Attenuation of different frequencies

� Attenuation depends on the used frequency

�Can result in a frequency-selective channel

� If bandwidth spans frequency ranges with different attenuation properties

- 32 -



Attenuation in Atmosphere

David, Benkner: Digitale Mobilfunksysteme

- 33 -



• 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 sender

signal at receiver

LOS pulsesmultipathpulses

- 34 -



Effects of MobilityChannel characteristics change over time and location

– signal paths change

– different delay variations of different signal parts

– different phases of signal parts

� quick changes in the power received (short term fading)

Additional changes in

– distance to sender

– obstacles further away

� slow changes in the average power received (long term fading)

short term fading

long termfading

t

power

- 35 -



Radio Channel Characteristics• Superposition of

numerous direct and reflected multipathcomponents withdifferent attenuationand phasing

• time variant

• Differentiation of fast and slow fading

A

B

• Fast fading due to superposition of different phase

• Slow fading is due to the change of propagationenvironment

- 36 -



Real world example

- 37 -



Pathloss in Real World

� Received power of a sender is decreasingwith the distance between sender and receiver

� Depends on Frequency

� Many Models, e.g.

�Okumura-Hata

�Walfish-Ikegami

�UMTS 30.03

� Mostly shown in dB (attenuation)

Distance [m]

Pathloss[dB]

0 200 400 600 800 1000

40 60 80 100 120 140 160

UMTS 30.03 VehicularFrom Walke: UMTS the fundamentals

- 38 -



Noise and Interference

• So far: only a single transmitter assumed

– Only disturbance: self-interference of a signal with multi-path “copies” of itself

• In reality, two further disturbances

– Noise – due to effects in receiver electronics, depends on temperature

• Typical model: an additive Gaussian variable, mean 0, no

correlation in time

– Interference from third parties

• Co-channel interference: another sender uses the same spectrum

• Adjacent-channel interference: another sender uses some other part of the radio spectrum, but receiver filters are not good enough to fully suppress it

Effect: Received signal is distorted by channel, corrupted by noise and interference

- 39 -



Channel model

• Simplest model: assume transmission power and

attenuation are constant, noise an uncorrelated

Gaussian variable

– Additive White Gaussian Noise model,

• Situation with no line-of-sight path, but many

indirect paths: Amplitude of resulting signal has a

Rayleigh distribution (Rayleigh fading)

• One dominant line-of-sight plus many indirect

paths: Signal has a Rice distribution (Rice fading)

- 40 -



Principle of Channel Coding

- 41 -



Channel Coding• Forward Error Correction (FEC)

– Detects and corrects errors

• Automatic Repeat Request (ARQ)

– Mainly just detects errors

• Parity Control

– Adding the sum of a modulo 2 operation of each bit to the end

• Block Codes

– Hamming Code, Reed Muller Code

• Convolutional Codes

– Viterbi, Trellis

- 42 -



- 43 -



Channel Coding

- 44 -



• Linear Block Codes

– k-digit information word is transformed into n-digit codeword

• Convolutional Codes

– M-bit word is transformed into n-bit word, code rate m/n

– transformation is a function of last k information symbols, where k is the constraint length of the code.

– Codes symbols are calculated by modulo 2 additions of memory counters

Error Correcting Codes

- 45 -



Properties of Convolutional Codes

- 46 -



Interleaving

- 47 -



Concatenated Codes and Iterative Decoding

1 2 qu c

1 2 D2 D1

Oute r c ode

Inner c od e

� Parallel Concatenatedcodes (Turbo codes)

� Serial Concatenatedcodes

1111

1

21

+−+++

=

qRRR

R

cqcc

c

L cqccc RRRR ⋅⋅⋅= L21

- 48 -



Iterative decoding of parallel concatenated system

• Every decoder has same received channel

observation LLR and a-priori information LLR from

other decoder.

D1

Inte rleaver

Lc hyD2

Le1( )u

La2( )uL1( )u

L2( )u^

^

^

-1

Deinterleaver

Le2( )uLa1( )u ^

Inte rleaver

-

-

- -

- 49 -



EXtrinsic-Information-Transfer-Diagram (EXIT-chart)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Ie2

=Ia1

I e1=

I a2

Iteration trajectory

chracteris

tic of 1

st decoder

chra

cter

istic

of 2

nd dec

oder

Eb/N

0=1dB

- 50 -



ARQ

• David/Benkner p.235

• Return Channel is required

• ARQ protocols

– „Stop and Wait“

• Waits until positive ack. received or timer expires

– „Go back N“

• Continous transmission of data, if NACK received, continues N steps back

– „Selective Repeat“

– „Hybrid ARQ“

- 51 -



Adapative Error Control

- 52 -



ARQ

- 53 -



- 54 -



- 55 -



ARQ Pros & Cons

• Advantages

– Simple protocol

– Quasi adaptive scheme, adapts to channel

properties and can therefore be very efficient

• Disadvantages

– Difficult to guarantee constant end-to-end delay

and constant net (user) bit rate

– If channel quality is very low, retransmission is

not efficient enough (will be retransmitted and

have error again)

- 56 -



- 57 -



Modulation• Analogue Modulation Schemes

– AM, FM

• Digital Modulation Schemes

– QPSK,

– dQPSK

– 8-PSK

– 16-QAM

– GMSK

– …

– Multicarrier Modulation (MCM

- 58 -



Digital Modulation Schemes• 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

- 59 -



Advanced Phase Shift Keying• BPSK (Binary Phase Shift Keying):

– bit value 0: sine wave

– bit value 1: inverted sine wave

– very simple PSK

– low spectral efficiency

– robust, used e.g. in satellite systems

• QPSK (Quadrature Phase Shift Keying):

– 2 bits coded as one symbol

– symbol determines shift of sine wave

– needs less bandwidth compared to BPSK

– more complex

• Often also transmission of relative, not absolute phase shift: DQPSK - Differential QPSK (IS-136, PHS)

11 10 00 01

Q

I01

Q

I

11

01

10

00

A

t

- 60 -



Quadrature Amplitude Modulation

• Quadrature Amplitude Modulation (QAM): combines amplitude and phase modulation

• it is possible to code n bits using one symbol

• 2n discrete levels, n=2 identical to QPSK

• bit error rate increases with n, but less errors compared to comparable PSK schemes

Example: 16-QAM (4 bits = 1 symbol)

• Symbols 0011 and 0001 have the same

phase φ, but different amplitude a. 0000 and 1000 have different phase, but same amplitude.

• � used in standard 9600 bit/s modems

0000

0001

0011

1000

Q

I

0010

φ

a

- 61 -



Hierarchical Modulation• DVB-T modulates two separate data streams onto a single DVB-T

stream

• High Priority (HP) embedded within a Low Priority (LP) stream

• Multi carrier system, about 2000 or 8000 carriers

• QPSK, 16 QAM, 64QAM

• Example: 64QAM

– good reception: resolve the entire64QAM constellation

– poor reception, mobile reception: resolve only QPSK portion

– 6 bit per QAM symbol, 2 mostsignificant determine QPSK

– HP service coded in QPSK (2 bit), LP uses remaining 4 bit

Q

I

00

10

000010 010101

- 62 -



Multicarrier Modulation (MCM)• Idea:

– Use of a number of carriers simultaneously for onesignal/user in order to reduce ISI

• Approach

– If c symbols/s are to be transmitted, distribution over n subcarriers, each with a code rate of c/n symbols/s

– Results in n carriers with lower speed and lessproblems of ISI.

– Frequency selective fading leads to attenuation of singlecarriers only

– Typical guard time between symbols

– It is also possible to send identical symbols over severalcarrieres

- 63 -



Application of OFDM• ADSL and VDSL broadband access via telephone lines

• IEEE 802.11a and 802.11g Wireless LANs

• Digital audio broadcasting systems> EUREKA 147, Digital Radio Mondiale, HD Radio, T-DMB and ISDB-TSB.

• Terrestrial digital TV systems DVB-T, DVB-H, T-DMB and ISDB-T.

• IEEE 802.16 or WiMax Wireless MAN standard.

• IEEE 802.20 or Mobile Broadband Wireless Access (MBWA) standard.

• Flash-OFDM cellular system.

• Some Ultra wideband (UWB) systems.

• Power line communication (PLC)

Source Wikipedia, Nov. 06

- 64 -



MCM - OFDM• OFDM is a special implementation of MCM with orthogonal

subcarriers + very efficient FFT based algorithms for

modulation/demodulation

• Orthogonal subcarriers:

• Symbolrate 1/T for each subcarrier is selected in a way,

that they equal the next carrier with separation ∆f, thus

subcarriers are orthogonal irrespective of φ.

,...2,1,/:with

0)2sin()2sin(

==−

=++∫

nTnff

tftf

jk

T

o

jjkk ϕπϕπ

- 65 -



OFDM Implementation• MC Modulator generates k independent QAM subcarriers,

each with a symbolrate 1/T.

• Simplification: use of iFFT for modulation and FFT fordemodulation

- 66 -



- 67 -



OFDM Key FeaturesAdvantages

– Can easily be adopted to severe channel conditions without complex equalization

– Robust to narrow-band co-channel interference– Robust to inter-symbol interference and fading caused by multipath

propagation– High spectral efficiency– Efficient implementation by FFTs– Low sensitivity to time synchronization errors– Tuned sub-channel receiver filters are not required (unlike in

conventional FDM)– Facilitates Single Frequency Networks, i.e. transmitter

macrodiversity.• Disadvantages

– Sensitive to Doppler shift.– Sensitive to frequency synchronization problems– Inefficient transmitter power consumption, since linear power

amplifier is required.

Source Wikipedia, Nov. 06

- 68 -



OFDM in IEEE 802.11a (and HiperLAN2)

• OFDM with 52 used subcarriers (64 in total)

• 48 data + 4 pilot

• (plus 12 virtual subcarriers)

• 312.5 kHz spacing

subcarrier

number

1 7 21 26-26 -21 -7 -1

channel center frequency

312.5 kHzpilot

From: Schiller, Mobilkommunikation

- 69 -



Multiple Access

• Space Division Multiple Access (SDMA)

• Frequency Division Multiple Access (FDMA)

• Time Division Multiple Access (TDMA)

• Code Division Multiple Access (CDMA)

- 70 -



Frequency Multiplex• Separation of the whole spectrum into smaller frequency bands

• A channel gets a certain band of the spectrum for the whole time

• Advantages:

• no dynamic coordination necessary

• works also for analog signals

• Disadvantages:

• waste of bandwidth if the traffic is distributed unevenly

• inflexible

• guard spaces

k2 k3 k4 k5 k6k1

f

t

c

- 71 -



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 the

medium at any time

• throughput high even

for many users

• Disadvantages:

• precise

synchronization

necessary

- 72 -



f

Time and Frequency Multiplex

t

c

k2 k3 k4 k5 k6k1

• 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

- 73 -



Code Multiplex• Each channel has a unique code

• All channels use the same spectrum at the same time

• Advantages:

– bandwidth efficient

– no coordination and synchronization necessary

– good protection against interference and tapping

• Disadvantages:

– lower user data rates

– more complex signal regeneration

• Implemented using spread spectrum technology

k2 k3 k4 k5 k6k1

f

t

c

- 74 -



Duplex Schemes

• TDD

• FDD

- 75 -



Spread spectrum technology• Problem of radio transmission: frequency dependent fading can wipe

out narrow band signals for duration of the interference

• Solution: spread the narrow band signal into a broad band signal using a special code

• protection against narrow band interference

• protection against narrowband interference

• Side effects:

– coexistence of several signals without dynamic coordination

– tap-proof

• Alternatives: Direct Sequence, Frequency Hopping

detection atreceiver

interference spread signal

signal

spreadinterference

f f

power power

- 76 -



Effects of spreading and interference

dP/df

f

i)

dP/df

f

ii)

sender

dP/df

f

iii)

dP/df

f

iv)

receiverf

v)

user signalbroadband interferencenarrowband interference

dP/df

- 77 -



CDMA Theoretical Approach

- 78 -



Spreading and frequency selective fading

frequency

channelquality

1 2

3

4

5 6

narrow bandsignal

guard space

22

22

2

frequency

channelquality

1

spreadspectrum

narrowband channels

spread spectrum channels

- 79 -



DSSS (Direct Sequence Spread Spectrum) I• XOR of the signal with pseudo-random number (chipping sequence)

– many chips per bit (e.g., 128) result in higher bandwidth of thesignal

• Advantages

– reduces frequency selective fading

– in cellular networks

• base stations can use the same frequency range

• several base stations can detect and recover the signal

• soft handover

• Disadvantages

– precise power control necessary

user data

chipping sequence

resultingsignal

0 1

0 1 1 0 1 0 1 01 0 0 1 11

XOR

0 1 1 0 0 1 0 11 0 1 0 01

=

tb

tc

tb: bit period

tc: chip period

- 80 -



DSSS (Direct Sequence Spread Spectrum) II

X

user data

chippingsequence

modulator

radiocarrier

spreadspectrumsignal

transmitsignal

transmitter

demodulator

receivedsignal

radiocarrier

X

chippingsequence

lowpassfilteredsignal

receiver

integrator

products

decision

data

sampledsums

correlator

- 81 -



FHSS (Frequency Hopping Spread Spectrum) I

• Discrete changes of carrier frequency

– sequence of frequency changes determined via pseudo random number sequence

• Two versions

– Fast Hopping: several frequencies per user bit

– Slow Hopping: several user bits per frequency

• Advantages

– frequency selective fading and interference limited to short period

– simple implementation

– uses only small portion of spectrum at any time

• Disadvantages

– not as robust as DSSS

– simpler to detect

- 82 -



FHSS (Frequency Hopping Spread Spectrum) II

user data

slowhopping(3 bits/hop)

fasthopping(3 hops/bit)

0 1

tb

0 1 1 t

f

f1

f2

f3

t

td

f

f1

f2

f3

t

td

tb: bit period td: dwell time

- 83 -



FHSS (Frequency Hopping Spread Spectrum) III

modulator

user data

hoppingsequence

modulator

narrowbandsignal

spreadtransmitsignal

transmitter

receivedsignal

receiver

demodulator

data

frequencysynthesizer

hoppingsequence

demodulator

frequencysynthesizer

narrowbandsignal

- 84 -



CDMA Example• Sender A

– sends Ad = 1, key Ak = 010011 (assign: „0“= -1, „1“= +1)

– sending signal As = Ad * Ak = (-1, +1, -1, -1, +1, +1)

• Sender B

– sends Bd = 0, key Bk = 110101 (assign: „0“= -1, „1“= +1)

– sending signal Bs = Bd * Bk = (-1, -1, +1, -1, +1, -1)

• Both signals superimpose in space

– interference neglected (noise etc.)

– As + Bs = (-2, 0, 0, -2, +2, 0)

• Receiver wants to receive signal from sender A

– apply key Ak bitwise (inner product)

• Ae = (-2, 0, 0, -2, +2, 0) • Ak = 2 + 0 + 0 + 2 + 2 + 0 = 6

• result greater than 0, therefore, original bit was „1“

– receiving B

• Be = (-2, 0, 0, -2, +2, 0) • Bk = -2 + 0 + 0 - 2 - 2 + 0 = -6, i.e. „0“

- 85 -



CDMA on signal level Idata A

key A

signal A

data ⊕ key

key

sequence A

Real systems use much longer keys resulting in a larger distance

between single code words in code space.

1 0 1

10 0 1 0 0 1 0 0 0 1 0 1 1 0 0 1 1

01 1 0 1 1 1 0 0 0 1 0 0 0 1 1 0 0

Ad

Ak

As

- 86 -



CDMA on signal level III

Ak

(As + Bs)

* Ak

integrator

output

comparator

output

As + Bs

data A

1 0 1

1 0 1 Ad

- 87 -



CDMA on signal level IV

integrator

output

comparator

output

Bk

(As + Bs)

* Bk

As + Bs

data B

1 0 0

1 0 0 Bd

- 88 -



comparator

output

CDMA on signal level V

wrong

key K

integrator

output

(As + Bs)

* K

As + Bs

(0) (0) ?

- 89 -



EbN0

S BN R.=

S N .= GP

spreading gain

data rate

user bandwith

The Eb/N0 is GP times greater than the S/N

Signal-to-Noise Ratio

- 90 -



Interferences

� Interferences by other sender, which are transmitting in the samefrequency band

� in cellular networks inherent existing, because neighboring cellsusing the same frequency

� The strenght of the interference is depending on pathloss betweensender and receiver

� By equal transmitter power of all subsribers the interference is onlydepending on the geometrical constellation

� Interferences by other sender, which are transmitting in the samefrequency band

� in cellular networks inherent existing, because neighboring cellsusing the same frequency

� The strenght of the interference is depending on pathloss betweensender and receiver

� By equal transmitter power of all subsribers the interference is onlydepending on the geometrical constellation

- 91 -



Carrier to Interference Ratio (CIR)

Ratio of Carrier-to Interference-powerat the receiver

C/I = C / ( I + N)

Typical in GSM:C/I=15dB (Factor 32)

I

I

I

C

(Uplink Situation)

- 92 -



Range-limited systems

�Mobile stations are at the border or beyond thecoverage zone

�C at the receiver is toolow, because the pathloss between senderand receiver is too high

C/I is too low

C

- 93 -



Interference-limited systems

�Mobile stations are in thecoverage zone

� C at the receiver issufficient, but too muchinterference

is received at the receiver

C/I is too low

C

I

- 94 -



� Mobile stations are in theilluminated zone

� C at the receiver is sufficient, I is small enough

C/I is sufficient

� No more resources(channels) available

Capacity-limited systems

C

I

- 95 -



Examples

� GSM 1992

�Range-limited systems, because no wide-area coverage isavailable, few users, little interference

� GSM 2000 (European 900 MHz networks)

� Interference-limited system, because many subscribers cause interferences. Interference-limiting countermeasures like Power Control or Frequency Hopping are applied


�Capacity-limited systems, because enough spectrum for large clusters (little interference) is available

� GSM 1992

�Range-limited systems, because no wide-area coverage isavailable, few users, little interference


� Interference-limited system, because many subscribers cause interferences. Interference-limiting countermeasures like Power Control or Frequency Hopping are applied


�Capacity-limited systems, because enough spectrum for large clusters (little interference) is available

- 96 -



For the transmission of symbols with rate r a frequency

bandwith of

f ≥≥≥≥ r

is necessary (with appropriate pulse shaping).

For the transmission of symbols with rate r a frequency

bandwith of

f ≥≥≥≥ r

is necessary (with appropriate pulse shaping).

Frequency Bandwidth

information sourcer

information sink

sender

channel

receiver

- 97 -



short signal

long signal narrow frequency spectrum

wide frequency spectrum

pulse shaping

infinite length signale.g. sine

finite frequency spectrum

finite length signale.g. rectangle (“bit“)

infinite frequency spectrum

- 98 -



Transmission over noisy channels

source sink

sender

channel

interference(e.g. noise)

- 99 -



nT

TP x

„0“ „1“

p(x)

x

~ signal power S

decision thresholdProbability of detecting “0“although “1“ was sent

~ noise power N

Interference by additive Gaussian noise

The greater SS and the smaller NN, the smaller is theerror probability.

- 100 -



Shannon Theorem

The transmission rate is proportional to thefrequency bandwidth (b).

The transmission rate is proportional to thefrequency bandwidth (b).

The error probability is reverse proportional to thesignal-to-noise ratio S/N.

The error probability is reverse proportional to thesignal-to-noise ratio S/N.

The maximum achievable throughput over a noisy channel is:

C = b * log2(1+S/N)C = b * log2(1+S/N) [bit/s]

In many cases S/N >>1, therefore simplified:

C ≈≈≈≈1/3 * b * S/NdBC ≈≈≈≈1/3 * b * S/NdB

- 101 -



Channel Capacity (Shannon)• Bandwidth and S/N are reciproke to each other

• Means:

– With low bandwidth very high data rate ispossible provided S/N is high enough

• Example: higher order modulation schemes

– With high noise (low S/N) data communicationis possible if bandwidth is high enough

• Example: spread spectrum….

– Long time Capacity as theoretical limit, sinceturbo coding (1993) practical systems with 0.5 dB to Shannon channel bandwidth…

- 102 -



Spectral Efficiency• Spectral Efficiency of Mobile Communciation Systems

• Unit: Erlang/MHz/km²,T: Traffic [Erlang]

B: Total Bandwidth (depends on clustersize N)A: Area

AB

TM

⋅=η

- 103 -



Spectral Efficiency

η [bit/s/Hz]

16-QAM8-PSKQPSKBPSKModulation Scheme

]//[ HzsbitB

RS =η

for Modulation Schemes

- 104 -



Coding

• Adding redundant bits to the data stream to detect

and possibly correct errors

– Interleaving

– Channel Coding

– Source Coding

- 105 -



Interleaving

• Diagonal Interleaving

• Block Interleaving

• Convolutional Interleaving

- 106 -



ARQ – „Stop and Wait“• From: David/Benkner, Digitale Mobilfunksysteme

- 107 -



Continous Transmission/Selective Rep.• From: David/Benkner, Digitale Mobilfunksysteme

- 108 -



Media Access Protocols

• Aloha

• Slotted Aloha

• Carrier Sense Multiple Access

• Demand Assigned Multiple Acess (DAMA)

• Packet Reservation Multiple Access (PRMA)

• TDMA with Reservation

• Multiple Access with Collision Avoidance

• Polling

• Inhibit Sense Multiple Access

- 109 -



Motivation: Media Access for Wireless

• Can we apply media access methods from fixed networks?

• Example CSMA/CD

– Carrier Sense Multiple Access with Collision Detection

– Checks if medium is free (CS), if ok, sends data, continues to listen if collision occurs (CD), if yes stops transmission, sends jam signal… (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”

- 110 -



• Hidden terminals

– 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

– B sends to A, C wants to send to another terminal (not A or B)

– C has to wait, CS signals a medium in use

– but A is outside the radio range of C, therefore waiting is not necessary

– C is “exposed” to B

Motivation - hidden and exposed terminals

BA C

- 111 -



• Terminals A and B send, C receives

– signal strength decreases proportional to the square of the distance

– the signal of terminal B therefore drowns out A’s signal

– C 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

• Also severe problem for CDMA-networks - precise power control needed!

Motivation - Near and Far Terminals

A B C

- 112 -



Access methods SDMA/FDMA/TDMA

• SDMA (Space Division Multiple Access)

– segment space into sectors, use directed antennas

– cell structure

• FDMA (Frequency Division Multiple Access)

– assign a certain frequency to a transmission channel between a sender and a receiver

– permanent (e.g., radio broadcast), slow hopping (e.g., GSM), fast hopping (FHSS, Frequency Hopping Spread Spectrum)

• TDMA (Time Division Multiple Access)

– assign the fixed sending frequency to a transmission channel between a sender and a receiver for a certain amount of time

- 113 -



FDD/FDMA - General Scheme, Example GSM

f

t

124

1

124

1

20 MHz

200 kHz

890.2 MHz

935.2 MHz

915 MHz

960 MHz

- 114 -



TDD/TDMA - general scheme, example DECT

1 2 3 11 12 1 2 3 11 12

tdownlink uplink

417 µs

- 115 -



• Mechanism

– random, distributed (no central arbiter), time-multiplex

– Slotted Aloha additionally uses time-slots, sending must always start at slot boundaries

• Aloha

• Slotted Aloha

Aloha/slotted aloha

sender A

sender B

sender C

collision

sender A

sender B

sender C

collision

t

t

- 116 -



DAMA - Demand Assigned Multiple Access

• Channel efficiency only 18% for Aloha, 36% for Slotted Aloha (assuming Poisson distribution for packet arrival and packet length)

• Reservation can increase efficiency to 80%

– a sender reserves a future time-slot

– sending within this reserved time-slot is possible without collision

– reservation also causes higher delays

– typical scheme for satellite links

• Examples for reservation algorithms:

– Explicit Reservation according to Roberts (Reservation-ALOHA)

– Implicit Reservation (PRMA)

– Reservation-TDMA

- 117 -



Carrier Sense Multiple Access (CSMA)

• Use classical ALOHA, but listen to channel beforetransmitting

• Drastic decrease of collision probability

• 1-persistent: each station checks continously channel and starts as soon as free

• Non persistent: checks channel in stochastic intervals onlyand starts transmitting directly after channel detected free

• P-Persistent CSMA: uses slots, if detects slot free, transmits with probability p, with (1-p) waits for next slot, checks again, if free transmits with p…

• CSMA/CA: load dependent waiting time, before listening isallowed after collision; used e.g. in IEEE 802.11

Tanenbaum, Computerneztwerke

- 118 -



Channel Load

Tanenbaum, Computerneztwerke

- 119 -



DAMA: Explicit Reservation

•DAMA Demand Assigned Multiple Access

•Explicit Reservation (Reservation Aloha):

– two modes:

• ALOHA mode for reservation:competition for small reservation slots, collisions possible

• reserved mode for data transmission within successful reserved slots (no collisions possible)

– it is important for all stations to keep the reservation list consistent at any point in time and, therefore, all stations have to synchronize from time to time

Aloha reserved Aloha reserved Aloha reserved Aloha

collision

t

- 120 -



PRMA•Implicit reservation (PRMA - Packet Reservation MA):

– a certain number of slots form a frame, frames are repeated

– stations compete for empty slots according to the slotted aloha principle

– once a station reserves a slot successfully, this slot is automatically assigned to this station in all following frames as long as the station has data to send

– competition for this slots starts again as soon as the slot was empty in the last frame

frame1

frame2

frame3

frame4

frame5

1 2 3 4 5 6 7 8 time-slot

collision at

reservation

attempts

A C D A B A F

A C A B A

A B A F

A B A F D

A C E E B A F Dt

ACDABA-F

ACDABA-F

AC-ABAF-

A---BAFD

ACEEBAFD

reservation

- 121 -



Access method DAMA: Reservation-TDMA

•Reservation Time Division Multiple Access

– every frame consists of N mini-slots and x data-slots

– every station has its own mini-slot and can reserve up to k data-slots using this mini-slot (i.e. x = N * k).

– other stations can send data in unused data-slots according to a round-robin sending scheme (best-effort traffic)

N mini-slots N * k data-slots

reservations

for data-slotsother stations can use free data-slots

based on a round-robin scheme

e.g. N=6, k=2

- 122 -



MACA - collision avoidance• MACA (Multiple Access with Collision Avoidance):

– No base station, no hidden station problem, flexibilty of ALOHA and dynamic reservation

• MACA uses short signaling packets for collision avoidance

– RTS (request to send): a sender request the right to send from areceiver with a short RTS packet before it sends a data packet

– CTS (clear to send): the receiver grants the right to send as soon as it is ready to receive

• Signaling packets contain

– sender address

– receiver address

– packet size

• Variants of this method can be found in IEEE802.11 as DFWMAC (Distributed Foundation Wireless MAC)

- 123 -



• MACA avoids the problem of hidden terminals

– A and C want to send to B

– A sends RTS first

– C waits after receiving CTS from B

• MACA avoids the problem of exposed terminals

– B wants to send to A, C to another terminal

– now C does not have to wait for it cannot receive CTS from A

MACA examples

A B C

RTS

CTSCTS

A B C

RTS

CTS

RTS

- 124 -



MACA variant: DFWMAC in IEEE802.11

idle

wait for the

right to send

wait for ACK

sender receiver

packet ready to send; RTS

time-out;

RTS

CTS; data

ACK

RxBusy

idle

wait for

data

RTS; RxBusy

RTS;

CTS

data;

ACK

time-out ∨data;

NAK

ACK: positive acknowledgement

NAK: negative acknowledgement

RxBusy: receiver busy

time-out ∨NAK;

RTS

- 125 -



Polling Mechanisms• If one terminal can be heard by all others, this “central” terminal

(a.k.a. base station) can poll all other terminals according to a certain scheme

– now all schemes known from fixed networks can be used (typical mainframe - terminal scenario)

• Example: Randomly Addressed Polling

– base station signals readiness to all mobile terminals

– terminals ready to send can now transmit a random number without collision with the help of CDMA or FDMA (the random number can be seen as dynamic address)

– the base station now chooses one address for polling from the list of all random numbers (collision if two terminals choose the same address)

– the base station acknowledges correct packets and continues polling the next terminal

– this cycle starts again after polling all terminals of the list

- 126 -



ISMA (Inhibit Sense Multiple Access)

• Current state of the medium is signaled via a “busy tone”

– the base station signals on the downlink (base station to terminals) if the medium is free or not

– terminals must not send if the medium is busy

– terminals can access the medium as soon as the busy tone stops

– the base station signals collisions and successful transmissions via the busy tone and acknowledgements, respectively (media access is not coordinated within this approach)

– mechanism used, e.g., for CDPD (USA, integrated into AMPS)

- 127 -



Access method CDMA•CDMA (Code Division Multiple Access)

– all terminals send on the same frequency probably at the same time and can use the whole bandwidth of the transmission channel

– each sender has a unique random number, the sender XORs the signal with this random number

– the receiver can “tune” into this signal if it knows the pseudo random number, tuning is done via a correlation function

•Disadvantages:

– higher complexity of a receiver (receiver cannot just listen into the

medium and start receiving if there is a signal)

– all signals should have the same strength at a receiver

•Advantages:

– all terminals can use the same frequency, no planning needed

– huge code space (e.g. 232) compared to frequency space

– interferences (e.g. white noise) is not coded

– forward error correction and encryption can be easily integrated

- 128 -



• Aloha has only a very low efficiency, CDMA needs complex receivers to be able to receive different senders with individual codes at the same time

• Idea: use spread spectrum with only one single code (chipping sequence) for spreading for all senders accessing according to aloha

SAMA - Spread Aloha Multiple Access

1sender A0sender B

0

1

t

narrow

band

send for a

shorter period

with higher power

spread the signal e.g. using the chipping sequence 110101 („CDMA without CD“)

Problem: find a chipping sequence with good characteristics

1

1

collision

- 129 -



Comparison SDMA/TDMA/FDMA/CDMAApproach SDMA TDMA FDMA CDMA

Idea segment space intocells/sectors

segment sendingtime into disjointtime-slots, demanddriven or fixedpatterns

segment thefrequency band intodisjoint sub-bands

spread the spectrumusing orthogonal codes

Terminals only one terminal canbe active in onecell/one sector

all terminals areactive for shortperiods of time onthe same frequency

every terminal has itsown frequency,uninterrupted

all terminals can be activeat the same place at thesame moment,uninterrupted

Signalseparation

cell structure, directedantennas

synchronization inthe time domain

filtering in thefrequency domain

code plus specialreceivers

Advantages very simple, increasescapacity per km²

established, fullydigital, flexible

simple, established,robust

flexible, less frequencyplanning needed, softhandover

Dis-advantages

inflexible, antennastypically fixed

guard spaceneeded (multipathpropagation),synchronizationdifficult

inflexible,frequencies are ascarce resource

complex receivers, needsmore complicated powercontrol for senders

Comment only in combinationwith TDMA, FDMA orCDMA useful

standard in fixednetworks, togetherwith FDMA/SDMAused in manymobile networks

typically combinedwith TDMA(frequency hoppingpatterns) and SDMA(frequency reuse)

still faces some problems,higher complexity,lowered expectations; willbe integrated withTDMA/FDMA


End Chapter 2 – Mobile Communications

Definitions & Basics

- 131 -



Intel liefert Wireless-Chip für 134 MBit/s aus, von Kai Schmerer, ZDNet, 18. April 2005

Hersteller sieht Wimax-Technologie als DSL-Ersatz, Als nach eigenen Angaben erster Hersteller liefert Intel eine Ein-Chip Wimax-Lösung aus. Durch den hochintegrierten Baustein werden laut Hersteller der Entwicklungsprozess von Endgeräten beschleunigt und die Kosten niedrig gehalten. Der Intel Pro Wiresless 5116 verfügt über eine programmierbare Architektur, die es Geräteherstellern ermöglichen soll, innovative Applikationen zu integrieren, um zum Beispiel die Installation der Geräte zu erleichtern. Als erstes Einsatzgebiet sieht Intel DSL-freie Zonen. Drahtlose breitbandige Internetzugänge bieten sich hier zur Vernetzung an, beispielsweise von Gebäuden, die mehrere Kilometer voneinander entfernt sind. Als Standard-basierte Technologie erleichtert Wimax sowohl existierenden als auch neuen Breitbandnutzern den drahtlosen Internetzugang. "Wimax basiert auf einem Standard, der den schnellen, drahtlosen Internetzugang spezifiziert. Daher bietet sich diese Technologie als Basis für die nächste Expansionswelle des Internets an, die Milliarden neuer Nutzer erreichen kann", erklärt Scott Richardson, General Manager der Intel Broadband Wireless Division. In Deutschland gehört Siemens Mobile zu den ersten Herstellern, die den neuen Intel-Chip für entsprechende Endgeräte nutzen wollen. …

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