Lecture 1: Introduction

Chapter 1

Introduction to Wireless Communication Systems

Introduction

 “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 2

 

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?

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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.

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        1899 - 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.

1940 - First microwave radar 1965 - First commercial communication satellite 1968 - AT&T proposes cellular phone system to Federal Communications Commission (FCC) 6

 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   1998 - 40 million cellular phone users in U.S.

2000 - In some countries, mobile users outnumber conventional wireline customers.

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 2001 - 630 million subscribers worldwide (as compared to 1 billion wired phone lines.

  2001 - Over 1% of worldwide wireless subscribers have abandoned wired telephone service for home use.

2005 − Over 130 million cellular phone users in U.S. (out of population of 300 million including children).

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II. Frequencies

   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 9

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 10

 Emerging    WLAN: Wireless Local Area Networks Mobile computers/email Wireless Local Loop (WLL)     local phone service via wireless connection prominent in non-industrialized nations cheaper to install than wired lines new IEEE 802.16 standard has been developed for WLL.

 Wireless-enabled Personal Digital Assistants (PDAs) 11

 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 12

  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.

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 FCC 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 Unlicensed Amateur Etc.

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 SMR Bands - Specialized Mobile Radio  Three 20 MHz bands from 800-900 MHz   Large number of

radio system

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 16

     ISM Bands - Industrial/Scientific/Medical  902-928 MHz, 2400-2484 MHz, & 5725-5850 MHz  “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) 17

 Cellular Phone  AMPS: Advanced Mobile Phone System 

824-849 MHz

 Reverse Channel: Transmit from mobile to fixed base station 

869-894 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 18

 PCS Band  1.8-1.9 GHz  FCC Spectrum Auctions - $10 Billion!!

  1st time spectrum sold for $$ in U.S.

It is has been hard for companies to recover this investment  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.

 Compete with analog cellular and SMR services combined 19

V. Mobile Radiotelephony

 The focus of this course: mobile wireless communications.

 Our predominant focus will be on mobile cellular communications  Historically voice communications, but also incorporating data into newer generation systems.

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 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 21

 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   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.

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 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 23

 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 24

 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 25

 Cellular Concept - break coverage area (market) into many small cell (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 26

BASE STATION

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 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   Analog frequency modulation (FM) used exclusively Frequency Division Multiple Access (FDMA)  one channel per pair of users 28

 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" 29

VI. Mobile Radio Terminology

 Table 1.4, pg. 10 definitions - everyone is responsible for these      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 30

 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 824-849 MHz for reverse channel and 869-894MHz for forward channel  PSTN: Public Switched Telephone Network 31

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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 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/ 20-30 dB signal attenuation  Needs an extensive network of transmitters to transmit the signal everywhere 33

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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.

Low power, limited range (~ 100 m) and coverage, and limited mobility 35

IX. Cellular Telephone Systems

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   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 37

 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 38

 MSC: Mobile Switching Center  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 (at most 5% of subscribers are assumed to be active at anyone time) 39

 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 40

 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

same

frequencies for FCC/RCC 

handoff

from one base station to another occurs when FCC signal is less than an acceptable level 41

2. Base stations (current server + adjacent stations) monitor RCC and report mobile unit signal strength to MSC (analog AMPS system)  if RCC signal < acceptable level - MSC initiates handoff to one of neighboring base stations 

neighboring

base stations must use

different

frequencies for FCC/RCC 42

 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.

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