Polytechnic University, Brooklyn, NY11201 Guest Lecturer...

Guest Lecturer: Professor Elza Erkip

Polytechnic University, Brooklyn, NY11201

Wireless Communications

©Erkip/Goodman EE3414: Wireless Communications 2

Outline

• Cellphone System Overview• Properties of wireless channels

– Fading and multipath• Methods for obtaining diversity, effects of diversity • Interference in wireless• Multiple access methods

– TDMA/ FDMA– Random multiaccess– CDMA

• Overview of cellular (1G, 2G, 3G)• Overview of Wireless LAN, bluetooth, WiMax• Ad-hoc networking• Multimedia over wireless

Cellphone System Overview

• Initiate phone call• Search for the cellphone• Respond to the page message• Assign radio channel• Conversation• Handoff• Network elements• Network architecture• Technology challenges• Technology details

SWITCH

Initialize Cellular Phone

SWITCH

913-7123502-9178

Initiate Phone Call

913-7123

BASEBASE

913-7123

SWITCH

913-7123

Search for Cellular Phone

Respond to Page Message

SWITCH

913-7123

Assign Radio Channels

SWITCH

*channel 103

Conversation

SWITCH

Handoff

SWITCH

Network Elements

TERMINALS (MS)

BASE STATION (BS)

SWITCH (MSC)

DATABASES (HLR, VLR)

Network Architecture

E Ai Di

MSC PSTN ISDN

HLRMSC ACBSMS

EIR VLR VLRG

©Erkip/Goodman EE3414: Wireless Communications

Wireless Communication Challenges

RADIO TRANSMISSIONINTERFERENCEFADINGSECURITY

MOBILITY FIND ME WHEN SOMEONE CALLSMAINTAIN CONNECTION WHEN I MOVE

ENERGYEXTEND BATTERY LIFE

Technology Details

RADIO TRANSMISSION: MULTIPLE ACCESSMODULATION, CHANNEL CODINGSOURCE CODINGANTENNA DIVERSITY

MOBILITY MANAGEMENTREGISTRATIONPAGING

Technology Details

RADIO RESOURCE MANAGEMENTADMISSION CONTROLCHANNEL ALLOCATIONPOWER CONTROLHANDOFF

SECURITYAUTHENTICATIONENCRYPTION

Communication channel model

Channel Filter +

Noise + Interference

Input Output

• Channel filter causes distortions•Wireless channel is time varying!

• Noise in receiver equipment• Interference from the environment• Limited bandwidth and power Goal: Design signals to effectively send information over communication channels.

Wireless Channel

• Reflection from buildings, cars etc: Multipath• Scattering: Fading

Multipath Fading Channel

Transmitted signal s Received signal r

T1 T2 T3

• Amplitudes Ai and delays Ti randomly vary in time

Delay T1

Delay T2s

Flat Fading

• Only one path– Received signal r=αs+n– s is a carrier signal with amplitude A, n is Gaussian noise

with variance σ2

• What is the effect of fading in error probability?– Without fading, received amplitude=sent amplitude=A

– With fading α, received signal amplitude =αΑ

α is randomly varying, bad if α is <<1. Error probability is the average over different α (what the Eα operator means)

)(,/ 22 SNRQPASNR e == σ

{ })( SNRQEPe αα=

αασα given for ),(,/ 222 SNRQPASNR e ==

Effect of Fading

0 5 10 15 20 25 30 3510-4

SNR (dB)

FADING GAUSSIAN

Ex: to obtain Pe=10^-4, need 34-8=26 dB higher PSNR, about 1000 times more transmission energy

Diversity

• Diversity to combat channel variations

• Receive L independently fading signals representing source.– Even if one signal is weak, the others may be stronger

Source Destination

Copy 1 of signal

Copy 2 of signal

Diversity Mechanisms

• Time diversity: Repetition coding, interleaving• Frequency diversity: Frequency hopping• Space diversity: Multiple antennas, multihop (cooperation)

• Multipath diversity: Process all the received paths (Rake receiver)

• The receiver can do– Switching: If current signal is not good, try another– Selection: Choose the strongest signal– Equal gain: Add received signals– Maximal ratio: Add received signals but not equally. Give more

weight to the ones that are more reliable (optimum)

Effect of Diversity on Performance

0 5 10 15 20 25 3010

10−5

10−4

10−3

10−2

10−1

SNR per bit (dB)

AWGN div 1div 2div 3div 4

Interference in Wireless

NoiseT1

Base station receiver

Time Division Multiple Access (TDMA)

Frequency

User 1 User 2 User 3 User 4

• Each user transmits at a separate time slot in the assigned frequency band. • Needs for buffering in transmitter and receiver for voice communications

Frequency Division Multiple Access (FDMA)

FrequencyW Hz

T secUser 1

User 2

User 3User 4

• Each user transmits at a separate frequency slot

Code 2

Code 4

Code 3

Code 1

Transmitted bit

• Each user transmits all the time over all the frequency band, but has a different “ spreading code.”• Code i belongs to user i.• The base station differentiates users based on their codes.

Code Division Multiple Access (CDMA)

Spreading Codes

Transmitted bit

b=1 b=-1

Transmitted signal

• User transmits signal bS(t) where

=0 bit if ,11bit if ,1

Orthogonal Codes

• Spreading codes belonging to different users are orthogonal

• Note S1=(1,1), S2=(1,-1) and

•The base station can easily identify users• Problem: Mobile users asynchronous, and hence they transmit at different times. So it is hard for s1(t) and s2(t) to be orthogonal.

0)()(, 2121 =>==< ∫ dttstsssρ

Interference and Power Control in CDMA

• The base station receives

• To “listen” to user 1

where is the correlation coefficient between s1(t) and s2(t) .

• Signal to noise plus interference ratio (SINR)

• To achieve a certain “quality” need to keep SINR of all users at level

)()()()( 2211 tntsbtsbtr ++=

∫ ++=>==< nbbdttstrsry 2111 )()(, ρ

∫= dttsts )()( 21ρ

21SINR

σρ +=

Random Multiple Access

• Each user transmits its message (called “packet”) when it is ready.

• If there is nobody else transmitting the message goes through.

• Otherwise a collision happens and the message is lost.• In this case the user waits a random period of time and

transmits again.• This is essentially what Wireless LAN uses (known as

carrier sensing multiple access with collision avoidance or CSMA/CA)

• Ethernet uses carrier sensing multiple access with collision detection or CSMA/CD)

Evolution of Wireless Communications

• Pre-electric: Smoke signals• Wireless electrical: Marconi (1897)• Mobile telephone

– Police cars: AM (1934)– AT&T: Commercial radio telephone (1946)

• Uses FM• High power transmitter• Fixed frequency for each user• Connected to public switched telephone network (PSTN)• One base station covers a big geographic area (e.g. one city),

and has only a few channels

Cellular Concept

• Break the coverage area into cells (AT&T, 1960’s)

Advantages/disadvantages of cellular

• Advantages– Received power (and hence SNR) decreases with distance

• Path loss: – Mobile user in one cell is far away from other cells

→ Causes very small interference in other cells. Hence we can reuse the same spectrum.Previous figure illustrates reuse factor K=7.

– Limited coverage area means low power base stations.• Disadvantages

– Need a large number of cells in metropolitan areas• Large number of base stations (expensive)

– Need to contact another base-station (handoff) when a user moves from one cell to another

• Large number of handoffs.

α−= dPP transrec

First Generation Cellular

• First cellular system in Japan (1979), then in Europe.• United States: Advanced Mobile Phone System

(AMPS), 1983• AMPS is analog, uses FM modulation in 800 MHz• Total available bandwidth 20 MHZ, each user 30

kHZ.– Total 666 channels

Second Generation Cellular

• Second generation is digital. Advantages:– Efficient source coding– Digital modulation more immune to noise – Channel coding to increase noise immunity– Easy handling of control information

• Uses TDMA, FDMA or CDMA for multiple access

Second Generation based on TDMA

• In Europe: Global System for Mobile Communications (GSM)– Unified system for Europe– New frequency band: 890-960 MHz

• In USA: North American TDMA Digital Cellular or IS-54 standard– Upgrade over AMPS– New version: IS-136

2G based on CDMA

• cdmaOne or IS-95 CDMA standard– Same band as AMPS 800 MHz– Also on PCS band 1.9 GHz– Supports wideband signaling– Higher capacity

1G versus 2G Cellular

Third Generation Cellular

• Goal: consolidate standards and provide more services• Main accessing scheme: CDMA• Higher data rates (up to 2MBits/sec)• Different classes of users (variable quality of service-

QoS)– Variable spreading– Adaptive coding and modulation– Multiple input multiple output (MIMO) systems for spatial

diversity

Wireless at a glance

Wireless Local Area Network (WLAN)

• Wireless LAN’s, also known as Wi-Fi, provide high data rates in limited geographical areas

• WLAN complements cellular:– Cellular provides seamless coverage and mobility– WLAN provides high speed in select areas

• Terminals “talk” to the access point• Increasing demand for WLAN: Hundreds of WLAN’s

in Manhattan– Poly WLAN– Public WLAN’s: Starbucks– At home

WLAN Standards

• Known as IEEE 802.11• Many variations: a, b, c, d ….• Operates in 2.4 Ghz and 5 GHz bands • Speeds up to 54 Mbits/sec• Uses Orthogonal Frequency Division Multiplexing

(OFDM)– Divides the wide frequency band into narrowband carriers

• Interference from cordless phones, microwaves, bluetooth

Bluetooth

• Low power, short range wireless communications• Range up to 10 meters, output power 1mW• Original goal was to eliminate wires connecting

keyboard, computer, printer etc• Can also be used for devices to detect each other and

form networks– Cell phone, PDA and computer automatically synchronize

Broadband Wireless Access: WiMax

• WiMax attempts to provide high data rates over large areas to a large number of users

• IEEE Standard 802.16 family• Broadband access to homes and businesses• Wireless alternative to cable modems, digital

subscriber lines (DSL) and fiber optic links– Advantage: Quick and low cost deployment, more ubiquitous

broadband access

• IEEE 802.16e: Support mobility up to speeds 70-80 miles/hour

Future of Wireless Networking

• Ad-hoc networking• Cellular and WLAN have centralized control

– Mobile to BS (or access point)/BS to mobile• Instead have decentralized communication

– Terminals form a network and communicate

• Has commercial, military, emergency rescue applications.

Cooperative/MultihopCommunications

• Cooperative/Multihop information routing can be used on top of existing infrastructure

• Extends cell coverage, reduces interference

Challenges for Multimedia over Wireless

• Wireless channels• Unreliable due to signal fading• Bandwidth limited

• Mobile terminals: Power limited• Multimedia applications

• High data rates• Error sensitive• Delay intolerant

• Higher transmission rates and reliability for the wireless channel• End-to-end performance metrics• Power efficient algorithms• Cross-layer optimization

Backbone Network

Wireless LAN

Wireless Cellular

Wired LAN

Wireless LAN

Wireless Cellular

Wired LAN

Ad-hoc Network

Wireless Multimedia Networks

What You Should Know

• Process involved in making a cellular phone call. Key elements of a cell phone system

• Wireless channel properties:– Fading and multiple path– Effect of fading on transmission reliability

• How do multiple users share the wireless channel? TDMA, FDMA, CDMA, random access

• Interference and power control in CDMA• Evolution of cellular systems• Alternatives to cellular: WLAN, bluetooth, WiMax, ad-

hoc networking

References

• N. Chandran and M. C. Valenti, “Three generations of cellular wireless systems.” IEEE Potentials , Volume: 20, Issue: 1, pp. 32 -35, Feb/Mar 2001.

• M. Frodigh, S. Parkvall, C. Roobol, P. Johansson and P. Larsson, “Future-generation wireless networks.” IEEE Personal Communications, Volume: 8, Issue: 5, pp. 10-17, Oct 2001.

• A. Ghosh, D.R. Wolter, J.G. Andrews, R. Chen, “Broadband wireless access with WiMax/802.16: Current performance benchmarks and future potential.” IEEE Communications Magazine, Volume 43, Issue 2, Feb 2005 Page(s):129 - 136

• R. Schneiderman, “Bluetooth's slow dawn.” IEEE Spectrum , Volume: 37, Issue: 11, pp. 61-65, Nov 2000.

• “Wi-Fi hotspot networks sprout like mushrooms.” IEEE Spectrum , Volume: 39, Issue: 9, pp. 18-20, Sep 2002.

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