ECE 476/ECE 501C/CS Wireless Communication Systems Winter Lecture 1: Introduction

Transcription

1 ECE 476/ECE 501C/CS Wireless Communication Systems Winter 2004 Lecture 1: Introduction Chapter 1 - Introduction to Wireless Communication Systems It is dangerous to put limits on wireless. Guglielmo Marconi, 1932 There has been tremendous growth in wireless in the past 10 years Even more in Europe and Asia than North America Driven by technological advances digital and RF circuit fabrication improvements, large scale circuit integration, miniaturization technologies, digital switching Driven by business investment although overinvestment has created bad profitability recently. Driven by consumer demand Regardless of current business profitability, the growth rate in terms of numbers of customers is substantial. The ability to communicate wirelessly is of obvious benefit to many. What are some of the benefits? Lecture 1, Page 1 of 18

2 I. History Wired Communications 1834 Gauss and Weber build telegraph system in Germany 1844 Morse connects Baltimore and Washington by telegraph 1858 First transatlantic telegraph cable laid 1876 Alexander Bell demonstrates telephone 1911 New York can telephone Denver Wireless Communications Not so new Had slow growth at first compared to other inventions. But now is growing very rapidly Marconi sends first radio message across Atlantic 1905 Hulsmeyer detects ships with radar 1927 US & Europe telephones linked by HF radio 1934 AM mobile police radios for public safety widely used 1935 Edwin Armstrong demonstrates FM radio system, became the primary modulation technique. Lecture 1, Page 2 of 18

3 II. Frequencies 1940 First microwave radar 1965 First commercial communication satellite 1968 AT&T proposes cellular phone system to Federal Communications Commission (FCC) 1983 FCC allocates spectrum for analog cellular service (AMPS) 1990 GSM digital cellular service introduced in Europe 1995 FCC auctions new Personal Communication Service (PCS) licenses in U.S. for digital services million cellular phone users in U.S In some countries, mobile users outnumber conventional wireline customers million subscribers worldwide (as compared to 1 billion wired phone lines Over 1% of worldwide wireless subscribers have abandoned wired telephone service for home use. RF Radio Frequency 1 MHz to 1 GHz general classification, not absolute 100 MHz to 1 GHz more widely used definition Microwave 1 GHz to 300 GHz general 1 GHz to 100 GHz more widely used Trends towards use of higher frequencies greater signal bandwidth (BW) per channel more users and/or higher data rates but more difficult to design! more $$, more engineering required Lecture 1, Page 3 of 18

4 III. Wireless Applications Mature Home Appliances - What devices are used that are wireless? Communications fixed microwave (point-to-point or Line Of Sight) nearly 20,000 in U.S satellite to fixed ground stations (TV, phone, defense, etc.) analog cellular AMPS (FM) since 1980's paging Emerging WLAN: Wireless Local Area Networks Mobile computers/ Wireless Local Loop (WLL) local phone service via wireless connection prominent in non-industrialized nations cheaper to install than wired lines new IEEE standard has been developed for WLL. Wireless-enabled Personal Digital Assistants (PDAs) Lecture 1, Page 4 of 18

5 Wireless Device Connectivity between computer peripherals (printers, monitors, keyboards, etc.) - Bluetooth Satellite to mobile ground units Land Mobile Satellite (LMS) Motorola/Iridium Digital Cellular/PCS PCS = Personal Communication Services Several types of services and capabilities are offered (see homework) Radio Frequency Identification Tags (RFID s) on merchandize in warehouses and stores. Sensor networks small devices wirelessly communicating among themselves to monitor environments using a variety of sensors. IV. U.S. Mobile Radio Spectrum Two entities have a tremendous impact on wireless systems, services, and technologies. 1. the services they offer, where they offer them, the technologies they choose to use, the prices they charge, and how they choose to differentiate themselves from their competitors. Since wireless requires huge investments in infrastructure (towers, switching, etc.), wireless service offerings are limited by those who can provide them. 2. They arbitrate the fair sharing of spectrum, and wield a lot of power because of that. controls all usable Radio Spectrum - allocates specific frequency bands for specific uses Military Public safety and public service - Police, fire, utilities, medical Commercial - To customers, between commercial mobiles Lecture 1, Page 5 of 18

6 Unlicensed Amateur Etc. Fig , pg. 592 SMR Bands Specialized Mobile Radio Three 20 MHz bands from MHz Large number of licenses nationwide paging/messaging voice dispatch taxi, Police/Fire/Ambulance data (UPS/Fedex) Extended SMR Nextel/Motorola partnership Nationwide coverage providing digital cellular/data service Created by buying SMR licenses from a large number of private radio service providers Lecture 1, Page 6 of 18

7 ISM Bands Industrial/Scientific/Medical MHz, MHz, & MHz receivers Garbage bands spread spectrum modulation Transmit (Tx) power level < 1 W Remote meter reading Wireless medical monitors Digital cordless telephones Big new application: Wireless Local Area Networks (WLAN s) Cellular Phone AMPS: Advanced Mobile Phone System MHz Reverse Channel: Transmit from mobile to fixed base station MHz Forward Channel: Transmit from base station to mobile FCC mandated duopoly in Major Trading Areas (MTAs) MTA = 51 largest U.S. cities two providers per MTA PCS Band GHz FCC Spectrum Auctions $10 Billion!! 1 st time spectrum sold for $$ in U.S. It is has been hard for companies to recover this investment (and the cost of towers). A & B blocks for Major Trading Areas (MTAs) duopoly like AMPS C, D, E, & F blocks Basic Trading Areas (BTAs) BTA = 492 large rural areas (includes MTAs) Digital cellular phone service + PCS PCS = special services like messaging, caller ID, voice mail, FAX, data, etc. Lecture 1, Page 7 of 18

8 Compete with analog cellular and SMR services combined More on PCS in Chapter 2 V. Mobile Radiotelephony in the U.S. The focus of this course: mobile wireless communications in the U.S. Our predominant focus will be on mobile cellular communications Historically voice communications, but also incorporating data into newer generation systems. In the U.S. Wireless is certainly big all around the world We will focus on the U.S. just because of its particular frequency allocations and standards. But we will also take a substantial look at Wireless LAN s Have grown quickly over the past couple of years. Are an important replacement opportunity for traditional wiring in buildings. Some are trying to make them a competitor to cellular for data communications. Cellular can or will provide 10 s to 100 s of kilobits per second. But if one can connect to a Wireless LAN (either at home/office or in public hot spot areas), 10 s of Megabits per second are readily available. And below all of these technologies are important radio transmission issues we need to study Radio signal propagation signal strengths varies with distance from the transmitter, but may also vary by large amounts over a few centimeters (called ) Digital modulation putting data on analog wireless signals. Compensation for fading making channels more reliable Frequency reuse and sharing (multiple access techniques) making best use of spectrum to support multiple users. Sharing in the time, frequency, and code domains. Lecture 1, Page 8 of 18

9 For you personally, what are issues of most curiosity to you about wireless communications? These can be big issues or just small technical issues. Now to focus more on mobile telephone, what are the biggest challenges to providing mobile telephone service? Lecture 1, Page 9 of 18

10 Early mobile phone systems used a high power Tx to cover a large spatial area (R = 50 km) Half-duplex (HDX) operation two-way communication using same radio channel transmit or receive only at a given time (HDX) push-to-talk system CB radio Allocated spectrum determines maximum # of simultaneous users e.g., 10 MHz allocated BW with 100 khz channels = 100 simultaneous users/market Demand was great in large cities and this led to poor service (many blocked calls) Spectrally inefficient system allocated spectrum supports small # of users In 1976, Bell Mobile Phone Service only had 12 channels for all of New York City (10 million people), which could acceptably only support 543 customers. Acceptable service - Certain probability of not being able to make a call (i.e., be "blocked") Given the # of customers, average calling load per customer, # of channels can compute blocking Cellular Concept break coverage area (market) into many small (many transmitters) where each non-adjacent cell will reuse different portions (not all) of allocated spectrum Increase spectrum efficiency many users share same channels Increase in required system infrastructure (base stations) more capital costs to provide adequate coverage Advanced Mobile Phone System (AMPS) spectrum allocated by FCC in 1983 Full duplex (FDX) operation : simultaneous two-way communication two 30 khz channels (forward & reverse) Two providers for each market duopoly limited competition Lecture 1, Page 10 of 18

11 Analog frequency modulation (FM) used exclusively Frequency Division Multiple Access (FDMA) one channel per pair of users USDC: U.S. Digital Cellular proposed in 1991 (D AMPS or IS 54) Replace single user analog channel with digital channels that support 3 users/30 khz channel BW User capacity is 3 times greater than AMPS more provider revenue ($$) Digital modulation & speech coding allow Time Division Multiple Access (TDMA) 3 users share one channel by using different time slots This service is provided under the title "Digital Cellular" VI. Mobile Radio Terminology Table 1.4, pg. 10 everyone is responsible for these definitions Mobile: high speed motion (e.g. cell phone in car) Portable: low speed motion (cordless phone in home, walking) Mobile Unit = subscriber unit = user communication device Transmitter (Tx) and Receiver (Rx) Base Station: Tx/Rx on tower at center of cell that provides service to group of mobile users Forward/Reverse Channels (a.k.a. downlink/uplink) Forward: From base station to mobile Reverse: From mobile to base station Simplex (SX), Half Duplex (HDX), & Full Duplex (FDX) Frequency Division Duplexing (FDD) - Using two separate frequency bands to provide both sides of the duplex operation Example: AMPS uses MHz for reverse channel and MHz for forward channel PSTN: Public Switched Telephone Network Lecture 1, Page 11 of 18

12 VII. Paging Systems One-way communication (SX) Send short message to mobile unit (pager) Wide area coverage Page broadcast from many base stations simultaneously to remote units no information as to user location Lecture 1, Page 12 of 18

13 Reliable communication everywhere (need good Signal to Noise performance) requires large Tx power and low data rate (~ 2 8 kbps) Noise has less of an effect when the data rate is lower. Coverage needed even inside buildings w/ db signal attenuation Needs an extensive network of transmitters to transmit the signal everywhere VIII. Cordless Telephone Systems Primarily in-home use Use ISM bands 900 MHz most popular for a while, now 2.4 GHz is common and 5.8 GHz is available. Why do you think 2.4 GHz has become more common? Low power, limited range (~ 100 m) and coverage, and limited mobility Lecture 1, Page 13 of 18

14 IX. Cellular Telephone Systems Large geographic coverage Limited frequency spectrum a surprisingly low amount of spectrum has been allocated for a service with such popularity. High user mobility High system capacity Large # of simultaneous users obtained by limiting coverage of each base station to small area (cell) frequency spectrum can be reused by other non-adjacent cells in network Lecture 1, Page 14 of 18

15 Base station serves mobile users in each cell bridge between mobile unit and MSC connected to MSC via phone line (for example, T1 of 24 channels or T3 of 672 channels) or Line of Sight microwave link MSC: Mobile Switching Center brains of cellular network controls base stations, call initiation & routing, handoffs, etc. connects cellular system to Public Switched Telephone Network (PSTN) cellular network brains: call initiation/setup base station handoffs controlling power levels in mobile units billing information roaming user ID and verification Typically handles 5000 simultaneous calls supporting 100,000 cellular subscribers Common Air Interface (CAI) Standard mechanism used by all mobiles. Defines 4 different channels to be used by a mobile unit Forward/reverse voice channels FVC/RVC Full Duplex communication Forward/reverse control channels FCC/RCC call initiation & setup makes up 5% of total # of available channels One cell contains 10 to 60 voice channels and only 1 to 3 control channel pairs (F+R) MSC broadcasts call request from PSTN over all FCC's of all base stations to find the mobile user Lecture 1, Page 15 of 18

16 There are two ways to keep mobiles connected to the best base station 1. Mobile unit monitors FCC's looking for strongest base station (closest) and incoming call if FCC signal < acceptable level mobile looks for another base station neighboring base stations must use frequencies for FCC/RCC handoff from one base station to another occurs when FCC signal is less than an acceptable level 2. Base stations (current server + adjacent stations) monitor RVC and report mobile unit signal strength to MSC (analog AMPS system) if RVC signal < acceptable level MSC initiates handoff to one of neighboring base stations neighboring base stations must use different frequencies for FVC/RVC Cellular Phone Call Timing Mobile Identification Number (MIN) is the subscriber's telephone number Electronic Serial Number (ESN) is device identifier. Station Class Mark (SCM) identifies the class of the device, based on its maximum transmit power level. Lecture 1, Page 16 of 18

17 Initiated by landline (PSTN) user Fig. 1.6, pg. 16 Lecture 1, Page 17 of 18

18 Initiated by mobile user Fig. 1.7, pg. 17 Roaming: Ability for a user to operate in a service area other than the one from which the service is subscribed. And also frequently with a different technology (for example, using analog instead of digital cellular) Next lecture: Latest trends in Wireless Communications (Chapter 2) Lecture 1, Page 18 of 18