Published on February 4, 2008
Distinguished Faculty LectureWireless Communications and the Pleasures of EngineeringDavid M. PozarElectrical and Computer EngineeringDecember 1, 2003: Distinguished Faculty Lecture Wireless Communications and the Pleasures of Engineering David M. Pozar Electrical and Computer Engineering December 1, 2003 Slide2: His work established the theoretical foundation for the development of wireless communications. "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 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. Slide3: Timeline of Wireless Communications Development . . . 1900 2000 1880 1860 1980 1960 1940 1920 Martin Cooper, Motorola, develops first handheld cellular phone in 1973 2003 - US cellular subscribers exceed 150M Guglielmo Marconi (1874-1937) development of wireless telegraphy trans-Atlantic 1901 Prof. H. Hertz (1857-1894) experimental validation of Maxwell 1886-1888 at Karlsruhe Prof. J. Maxwell (1831-1879) theory of electromagnetism developed in 1865 First television broadcast -1928 Two-way mobile radio services 1960s – 1970s 1983 - Cellular AMPS service in Chicago KDKA Radio -1920 Slide4: 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 ? Slide5: Consider an imaginary sphere of radius R enclosing an antenna that radiates a total power, P. 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, * assuming an isotropic antenna This is known as the inverse square law: Received power decreases as the inverse square of distance between transmitter and receiver. Slide6: Random electrical noise is generated by thermal energy electric equipment (motors, vapor lamps, ignition systems, …) lightning stellar sources interstellar background radiation Slide7: 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 Cellular Telephone Systems Slide8: Radio Spectrum – US Frequency Allocations . . . AM Radio TV 2-4 FM Radio TV 7-13 TV 21-36 TV 38-69 Cell TV 5-6 Slide9: Some current US radio spectrum allocations . . . Cellular telephone (824-849, 869-894 MHz) 50 MHz PCS (1710-1785, 1805-1880 MHz) 150 MHz GPS (1227, 1575 MHz) 41 MHz FM Radio (88-108 MHz) 20 MHz WLANs (2.400-2.484 GHz) 84 MHz Broadcast TV (54-72, 76-88, 174-216, 470-890 MHz) 492 MHz Slide10: 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 Slide11: Cellular base station and connection to publicly switched telephone network Slide12: 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 Slide13: 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. Slide14: base station cellular user building, vehicle, or other reflector Why is cellular coverage sometimes very poor ? multiple paths (multipath) can lead to cancellation of signal at phone Slide15: Earth-Orbit Communications Satellites photo courtesy of Dr. Fred Dietrich, Loral Slide16: courtesy of Professor Kurt Manheim, Loyola Law School Artist’s rendering of Earth-orbit satellites . . . Slide17: Characteristics of Some Recent Satellite Systems (1) Iridium assets acquired for $25M, limited service restarted 2001, DoD primary customer. (2) Globalstar assets acquired for $55M, continuing with limited service. Slide18: 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 Slide19: Communications Satellite Launches - Actual vs. Forecast abstracted from Wired Magazine, data from FAA Slide20: Can extraterrestrials receive our television broadcasts ? Slide21: No Slide23: 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 Slide24: 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 . . . Slide25: Children are natural-born engineers . . . Mike Pozar, ingeniously devising a transportation system, circa 1987 Slide26: 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 Slide27: (some additional slides follow) Slide28: 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/cm2 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/cm2 Exposure levels decrease quickly with distance (inverse square law) A proven hazard of cellular phones – using while driving Slide29: 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. Slide30: 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: Depending on the signal-to-noise ratio, S/N, we may have C B, (“traditional” radio systems, e.g., 100 MHz 100 Mbps) C < B (GPS, Ultra Wideband radio, e.g., 5 GHz 100 Mbps) C > B (DBS, other high-data-rate systems, e.g., 100 MHz 400 Mbps) Wireless Network Standards: Wireless Network Standards slide courtesy of Dr. Dev Gupta, Newlans, Inc.