Distinguished Faculty Lecture

Wireless Communications and the Pleasures of Engineering

David M. Pozar Electrical and Computer Engineering December 1, 2003

James Clerk Maxwell (1831 – 1879) Scottish, Professor of physics, King’s College (London) and Cambridge University. Formulated the theory of electromagnetism from 1865 to 1873.

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His work established the theoretical foundation for the development of wireless communications.

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From a very long view of the history of mankind - seen from, say, ten thousand years from now - there can be little doubt that the most significant event of the 19th century will be judged as Maxwell's discovery of the laws of electrodynamics. The American Civil War will fade into provincial insignificance in comparison with this important scientific event of the same decade.

" Richard Feynman, Lectures on Physics, Vol. II

Timeline of Wireless Communications Development . . .

Dave interviews at Bell Labs for Mobile Phone project – “This thing isn’t going anywhere.” Prof. H. Hertz (1857 1894) experimental validation of Maxwell 1886-1888 at Karlsruhe Guglielmo Marconi (1874-1937) development of wireless telegraphy trans-Atlantic 1901 Martin Cooper, Motorola, develops first handheld cellular phone in 1973 1920 1860 1880 Prof. J. Maxwell (1831-1879) theory of electromagnetism developed in 1865 2003 - US cellular subscribers exceed 150M 1900 KDKA Radio -1920 1940 1960 1980 2000 First television broadcast 1928 Two-way mobile radio services 1960s – 1970s 1983 - Cellular AMPS service in Chicago

Wireless Communications Theory 101

1. How does the radiated power density decrease from a transmitting antenna ?

2. What is electrical noise, and what is its effect on wireless communications ?

We need some basic physics and math . . .

• power is measured in Watts • power density is measured in Watts/meter 2 • the surface area of a sphere of radius

R

is 4 

R

2 • the Principle of Energy Conservation

Consider an imaginary sphere of radius

R

total power,

P

.

enclosing an antenna that radiates a The total power passing through this sphere is given by the area of the sphere multiplied by the power density radiated by the antenna*.

From energy conservation, the

total

power radiated must not depend on radius. Thus the power density must vary as, 4 

P R

2 This is known as the

inverse square law: Received power decreases as the inverse square of distance between transmitter and receiver.

R

* assuming an isotropic antenna

Random electrical noise is generated by • thermal energy • electric equipment (motors, vapor lamps, ignition systems, …) • lightning • stellar sources • interstellar background radiation 1.0

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Desired Signal Power 0.4

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Noise Power 2 4 6 8 Distance Between Transmitter and Receiver 10

Cellular Telephone Systems

The need for mobile communications . . .

In 1976 only 545 users in New York City had Bell System mobile telephones, with 3,700 customers on a waiting list. Nationwide, 44,000 users had AT&T mobiles with 20,000 people on five to ten year waiting lists.

An AT&T marketing survey for US cellular telephone market in early 1980s:

“… less than 900,000 users by year 2000

” (actual figure in 1998 was over 60 million)

The technical challenges . . .

• power requirements (talk time, safety, weight) • processing electronics required for base stations • very limited frequency spectrum

Radio Spectrum – US Frequency Allocations . . .

AM Radio TV 2-4 TV 5-6 FM Radio TV 21-36 TV 38-69 Cell TV 7-13

Some current US radio spectrum allocations . . .

Cellular telephone (824-849, 869-894 MHz) PCS (1710-1785, 1805-1880 MHz) GPS (1227, 1575 MHz) FM Radio (88-108 MHz) WLANs (2.400-2.484 GHz) Broadcast TV (54-72, 76-88, 174-216, 470-890 MHz) 50 MHz 150 MHz 41 MHz 20 MHz 84 MHz 492 MHz

The cellular radio concept . . .

• Many small “cells” (1 – 8 mile diameter) with low power transmitters • Each cell has a base station that communicates with users within that cell • Frequency reuse among seven nonadjacent cells (represented by same colors) • Resolves problem of limited radio spectrum • Actual cell coverage does not conform to the ideal plan shown below

Cellular base station and connection to publicly switched telephone network trunk lines PSTN MTSO from other base stations base station cell cellular user

Cellular Telephone Operation (AMPS) . . .

Mobile transmit band: 824-849 MHz, divided into 832 channels, 30 kHz wide Mobile receive band: 869-894 MHz, divided into 832 channels, 30 kHz wide Communication between mobile unit and base station uses four channels: FCC – forward control channel (base to mobile) RCC – reverse control channel (mobile to base) FVC – forward voice channel (base to mobile) RVC – reverse voice channel (mobile to base) Each base station is assigned a single FCC/RCC pair, and 59 FVC/RVC pairs.

Mobile unit scans all possible FCC channels, selects strongest signal.

Mobile unit responds over RCC with Mobile Identification Number (MIN) Mobile requesting a call sends request and number over RCC Base responds by assigning an FVC and RVC to mobile for voice For a call to a mobile, MTSO sends request via base station FCC

Call Handoff . . .

As mobile phone moves from one cell to an adjacent cell, the FCC signal from first cell will decrease, while FCC from second cell is increasing. When FCC of second cell is larger, the call will be “handed off” from first to second cell, with a new assignment of FVC and RVC.

This is one reason why cell phone use is discouraged on airplanes.

Why is cellular coverage sometimes very poor ?

building, vehicle, or other reflector multiple paths (multipath) can lead to cancellation of signal at phone cellular user reflected path base station direct path 1.0

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2 square law with multipath 4 6 8 10 Distance Between Base and Phone 12

Earth-Orbit Communications Satellites orbit for LEO satellites (500 – 2000 km height) orbit for GEO satellites (36,000 km height

photo courtesy of Dr. Fred Dietrich, Loral

Artist’s rendering of Earth-orbit satellites . . .

courtesy of Professor Kurt Manheim, Loyola Law School

Characteristics of Some Recent Satellite Systems Satellite System Iridium Globalstar Teledesic DBS Purpose Parent Company Number of satellites Orbit Cost Completion Date Current Status Voice Voice Data Direct TV Motorola Loral/Qualcomm McCaw/Gates Hughes (and others) 66 48 LEO LEO 840 (1994) 288 (1997) 2 LEO GEO $5B $4B 1998 1999 bankrupt 1999 (1) bankrupt 2002 (2) suspended $9B ?

$175M (each) not completed 1993 $2.1B revenue (2001) (1) Iridium assets acquired for $25M, limited service restarted 2001, DoD primary customer.

(2) Globalstar assets acquired for $55M, continuing with limited service.

Photograph of main mission antenna panel for Iridium satellite.

Each of the 66 Iridium satellites employs three of these antennas, at cost of about $200,000 each.

photo courtesy of Raytheon Company

Communications Satellite Launches - Actual vs. Forecast 90 80 70 60 50 40 30 20 10 0 1996 Actual Liftoffs 1998 2000 1998 Forecast 2000 Forecast 2003 Forecast 2002 2004 Year 2006 2008 2010 2012

abstracted from Wired Magazine, data from FAA

Can extraterrestrials receive our television broadcasts ?

No

Because of the inverse square law, and background noise . . .

Assuming equal noise and signal powers, the distance for which a broadcast signal can be received is given by

R

 16 

t

2

r

 2 Assume TV channel 4 (67 MHz), 1000 W transmit power, 4 MHz bandwidth, 4 dB transmit antenna gain, and consider two cases: Case 1. Low-gain receive antenna (10 dB), T b =3 K Then

R

= 4.4

 10 6 km (distance to Venus is

R

= 4.2

 10 7 km) Case 2.

High-gain receive antenna (134 dB), T b =290 K Then

R

= 7.5

 10 11 km ( distance to Alpha Centauri is

R

= 4.1

 10 13 km)

Wireless systems of the (not too distant) future . . .

• Wireless solutions to the “last mile” problem • High data rate wireless local area networks (Gigabit, with QoS and security) • Ultra Wideband networking (short distances, high data rate) • Wireless Personal Area Networks (PDAs, cameras, printers, …) • Mesh Networks (adaptive routers, sensor networks) • Wireless phone subscribers will continue to outpace land line users • A new generation of GPS satellites with improved accuracy . . . and others, as yet unimagined

Engineering . . .

• From the Latin,

ingeniatorem –

“one who is ingenious at devising” •

Engineering

unfortunately shares the same root as the word

engine

• Application of scientific and mathematical principles to practical problems • Engineering education is broadly-based in math, science, economics, ethics, . . .

• But successful engineering requires intuition about problems and solutions • “The engineering process begins with a desire” - James Adams • Engineering creativity produces original ideas, new approaches, radical designs • Most engineering developments are

incremental

, but some are

disruptive

• Originating a new idea may be glorious, implementing that idea is much harder • Experimentation is often required, and failure is common • It is difficult to foresee how a new technology will be used • Being “ingenious at devising” is a fundamental characteristic of humans . . .

Children are natural-born engineers . . .

Mike Pozar, ingeniously devising a transportation system, circa 1987

The End

Some suggested reading . . .

•

The Science of Radio

, Paul Nahin, 1996 •

The Evolution of Untethered Communications

, National Research Council, 1997 •

The Soul of a New Machine

, Tracey Kidder •

Flying Buttresses, Entropy, and O-Rings – The World of an Engineer

, James Adams •

The Existential Pleasures of Engineering

, Samuel Florman

(some additional slides follow)

Are cellular telephones safe ?

Some background information:

• Radio and microwave radiation (non-ionizing) is a known health hazard • Proven biological hazards of RF radiation are due to thermal effects • Best measure of RF internal exposure is the Specific Absorption Rate (SAR) – W/kg • FCC/FDA limits peak exposure to 1.6 W/kg of tissue, averaged over any 1 gram • European limits are less restrictive, specifying 1.6 W/kg averaged over 10 grams • FCC limits power density at 869 MHz to 0.58 mW/cm 2 • FCC/FDA limits handset power to 600 mW; newer phones run at about 125 mW • Base station power typically 5-10 W • Worst-case exposure from 50’ tower, 50 channels, is about 0.14 mW/cm 2 • Exposure levels decrease quickly with distance (inverse square law) • A proven hazard of cellular phones – using while driving

• Many safety studies of non-thermal RF effects have been performed, and more are ongoing. A recent heavily-referenced review states, “

The epidemiological evidence for an association between RF radiation and cancer is found to be weak and inconsistent, the laboratory studies generally do not suggest that cell phone RF radiation has genotoxic or epigenetic activity, and a cell phone RF radiation–cancer connection is found to be physically implausible. Overall, the existing evidence for a causal relationship between RF radiation from cell phones and cancer is found to be weak to nonexistent.”

from “Cell Phones and Cancer: What Is the Evidence for a Connection?”, J. E. Moulder*,

et al,

RADIATION RESEARCH vol. 151, pp. 513–531, 1999.

* Radiation Oncology, Medical College of Wisconsin

.

• An $800M lawsuit brought by a 43 year-old neurosurgeon against several cell-phone companies, alleging that his brain tumor was caused by cell phone use, was dismissed in US District Court in Maryland in 2002. The Court ruled that there was no reliable scientific evidence linking cell phone use to brain cancer.

Bandwidth vs. Data rate . . .

Contrary to current parlance, these are

not

equivalent.

Data rate (

C

bits/sec) for a given bandwidth (

B

Hz) and signal-to-noise ratio (

S/N

) is given by the Shannon Channel Capacity theorem:

C



B

log 1

2   

S N

  Depending on the signal-to-noise ratio,

S

/

N

, we may have

C



C B,

<

B C > B

(“traditional” radio systems, e.g., 100 MHz  100 Mbps) (GPS, Ultra Wideband radio, e.g., 5 GHz  100 Mbps) (DBS, other high-data-rate systems, e.g., 100 MHz  400 Mbps)

Wireless Network Standards

Enterprise SMB Home / SOHO Phase 3 True enterprise grade Gigabit Ethernet WLAN and back haul solution Gi Fi T M Phase 2 54 Mbps at 2.4 GHz 802.11g

QoS 802.11e

Security 802.11i

Phase 1 802.11a

54 Mbps at 5.7 GHz 802.11b

11 Mbps at 2.4 GHz 802.11

2 Mbps at 2.4 GHz Late 1990s

slide courtesy of Dr. Dev Gupta, Newlans, Inc.

2000 Early 2000s 2005