Rf and microwave circuit and component design for wireless systems

This is the first book to provide comprehensive coverage of hardware and circuit design specifically for engineers working in wireless communications. It serves as a reference for practicing engineers and technicians working in the areas of RF, microwaves, communications, solid-state devices, and radar

1 Introduction

1.1 Objectives and Brief History

1.2 Frequency Spectra and Atmospheric Propagation Loss1.3 Wireless Applications

1.4 Organization of This BookReferences

2 General Wireless Systems

2.1 Introduction

2.2 Wireless System Frequency Allocations2.2.1 Cellular Radio Systems

2.2.2 Cordless Telephony2.2.3 Mobile Satellite Systems

2.3 Cellular Systems in the United States, Europe, and Japan2.3.1 Analog Cellular Systems

2.3.2 Digital Cellular Systems

2.3.3 Code Division Multiple Access Systems2.4 Cordless Telephony

2.4.1 Analog Cordless Phones2.4.2 Digital Cordless Telephones2.4.3 WACS/PACS

2.4.4 DCS 1800

2.5 Wireless LAN Systems

2.6 Satellite Communication Systems2.6.1 Iridium System

2.6.2 Globalstar System2.6.3 ICO-P System

2.7 Future Wireless Systems2.7.1 Bluetooth Systems2.7.2 PCN/PCS Systems

2.7.3 Third Generation Cellular Systems2.7.4 MMDS/LMDS

3 Overview of Active Devices and Circuit Technologies

3.1 Active Device Types

3.2 Circuit Types and Their Fabrication3.3 Active Device Operation

3.3.1 Si Bipolar Transistor3.3.2 GaAs MESFET

3.3.3 Heterojunction Field-Effect Transistors3.3.4 Heterojunction Bipolar Transistors3.3.5 Future Trend

3.4 Circuit Manufacturing Technologies3.4.1 Printed Circuit Board

3.4.2 Hybrid Integrated Circuit

3.4.3 Monolithic Integrated Circuit3.4.4 Multichip Modules

3.4.5 Technology Comparison and ChoicesReferences

4 Transmitter and Receiver System Parameters

4.1 Introduction

4.2 Receiver System Considerations4.2.1 Natural Sources of Receiver Noise4.2.2 Receiver Noise Figure

4.2.3 Equivalent Noise Temperature

4.2.4 Dynamic Range, 1dB Compression Point, and Minimum Detectable Signal4.2.5 Third-Order Intercept Point and Intermodulation Products

4.3 Transmitter System Considerations4.3.1 Transmitter Noise

4.3.2 Adjacent Channel Power RatioReferences

5 Transmission Lines and Impedance Matching Techniques

5.1 Introduction

5.2 Transmission Line Equation

5.3 Reflection, Transmission, and Impedance5.4 Voltage Standing-Wave Ratio

5.5 Smith Charts5.6

S -Parameters

5.7 Wires and Twin Lines5.8 Coaxial Lines

5.9 Microstrip Lines5.9.1 Analysis Formulas5.9.2 Synthesis Formulas5.9.3 Effects of Strip Thickness5.9.4 Effects of Enclosure or Shield5.9.5 Graphical Method

5.9.6 Effects of Dispersion5.9.7 Losses

5.10 Other Planar Transmission Lines5.10.1 Striplines

5.10.2 Coplanar Waveguides and Coplanar Striplines5.10.3 Slotlines

5.11 Waveguides5.11.1 TE Modes5.11.2 TE10 Mode5.11.3 TM Modes5.12 Lumped Elements5.12.1 Lumped Resistors5.12.2 Lumped Capacitors5.12.3 Lumped Inductors

5.13 Impedance Matching Networks

6 Filters and Couplers

6.3.1 Lumped Element Filters6.3.2 Coaxial Filters

6.3.3 Microstrip Line Filters6.3.4 SAW Filters

6.4 Couplers

6.4.1 Coupler Parameter Definition

6.4.2 Coupler Requirements for Wireless Applications6.5 Design of Couplers

6.5.1 90° Hybrid6.5.2 Rat-Race Hybrid

6.5.3 Coupled Line Directional Couplers6.5.4 Other Couplers

7.3.1 Multithrow Switches7.3.2 Matrix Switches7.3.3 Diversity Switch7.4 High Isolation Switches

7.5 Broadband Switches7.6 High Power Switches

7.6.1 Impedance Transformation Technique7.6.2 Stacked FETs Method

7.6.3 Resonant Circuit Technique

7.6.4 Power Handling of PIN Diode Switches7.7 Low Distortion Switches

7.8 Switching Speed

7.9 Biasing of Switching Devices7.9.1 Biasing of PIN Diodes7.9.2 Biasing of FETs

7.9.3 Single Positive Power Supply Operation7.9.4 Switches With Integrated ControlReferences

8.5.1 Stability Circles8.5.2 Constant Gain Circles

8.5.3 Constant Noise Figure Circles

8.6 Small Signal Amplifier Design Procedure8.7 Narrow Band Amplifiers

8.7.1 Maximum Power Gain Design8.7.2 Low-Noise Amplifier Design

8.8 Low-Noise Amplifiers for Wireless Applications8.8.1 Silicon Low-Power LNAs

8.8.2 GaAs Low-Power LNAs8.8.3 HBT Low-Noise AmplifiersReferences

9.5 Devices for Mixers

9.5.1 Diode Mixer Theory and Operation9.5.2 FET Mixer Theory and Operation9.5.3 Dual-Gate FET (DGFET) Mixer9.6 Mixer Classifications

9.6.1 Single-Ended Mixers

9.6.2 Balanced Mixers9.6.3 Couplers

9.6.4 Single Balanced Mixers (SB Mixers)9.6.5 Double Balanced Mixers (DB Mixers)9.6.6 Image Reject Mixers

9.6.7 Gilbert Cell Mixers

9.7 Monolithic Low-Power Mixer Examples9.7.1 Introduction

9.7.2 Mixer Implementation in Silicon Technologies9.7.3 GaAs MESFET Mixers

9.7.4 Mixer Implementation in HEMTs9.8 Monolithic Low-Power Downconverters9.8.1 Downconverter Characterization9.8.2 Design Examples

10 Oscillators and Modulation

10.1 Introduction

10.2 Oscillator Characteristics10.3 General Theory

10.4 Oscillators Using Gunn and IMPATT Devices10.5 Small-Signal Design for Transistor Oscillators10.6 Large-Signal Design for Transistor Oscillators10.7 Transistor Oscillator Circuits

10.7.1 Feedback Oscillators10.7.2 Reflection-Type Oscillators

10.7.3 Stable Dielectric Resonator Oscillators10.8 Voltage-Tunable Transistor Oscillators10.9 Noise In Transistor Oscillators

10.10 Crystal Oscillators10.11 Phase-Locked Oscillators10.12 Frequency Synthesizers

10.13 Modulation and Demodulation Techniques10.13.1 Analog Modulation and Demodulation10.13.2 Digital Modulation

11.2 Device Models — Linear and Nonlinear11.2.1 Linear Model

11.2.2 Nonlinear Model11.3 Power Amplifier Design11.3.1 Device Type and Size

11.3.2 Device Models and Loadpull Data11.3.3 Design Methodology

11.3.4 Matching Networks11.3.5 Biasing Power Amplifiers11.3.6 Fabrication Technology11.3.7 Stability Analysis11.3.8 Thermal Design

11.4 Design of Linear Power Amplifiers11.4.1 Predistortion Techniques

11.4.2 Linear Amplifier Design Example11.5 High Efficiency Amplifiers

11.5.1 Class B Design11.5.2 Class E Design11.5.3 Class F Design

11.5.4 Comparison Between Various Classes11.6 Power Combining Techniques

11.7 Measurements

11.7.1 Loadpull Measurements11.7.2 IP3/IM3

11.7.3 ACPR MeasurementReferences

12 Antennas

12.1 Introduction

12.2 Isotropic Radiator, Plane Waves, and Far-Field Region12.3 Antenna Analysis

12.4 Antenna Characteristics and Parameters12.4.1 Input VSWR and Input Impedance12.4.2 Bandwidth

12.4.3 Power Radiation Patterns

12.4.4 Half-Power Beamwidth and Sidelobe Level12.4.5 Directivity, Gain, and Efficiency

12.4.6 Polarization and Cross-Polarization Level12.4.7 Effective Area

12.4.8 Beam Efficiency12.4.9 Back Radiation

12.4.10 Estimation of High-Gain Antennas12.5 Monopole and Dipole Antennas12.5.1 Radiation Patterns and Directivity12.5.2 Input Impedance

12.5.3 Whip, Folded, Sleeve, and Inverted Monopoles12.6 Loop Antennas

12.10 Other Antennas

12.11 Antenna Arrays and Phased Arrays

CHAPTER ONEIntroduction

1.1 OBJECTIVES AND BRIEF HISTORY

The major objective of this book is to present modern RF and microwave technology for wirelessapplications Wireless personal and data communication is expected to be one of the fastest-growing technologies in the next two decades Since the cellular mobile phone system wasintroduced in the early 1980s, the industry has undergone several generations of revolutionarychanges Figure 1.1 shows that the number of cellular phone subscribers in the United States hasincreased from 200,000 in 1985 to 76.3 million in 1999 [1] The increase rates in the last fewyears are more than 25% a year The easy access to a reliable and migratory means ofcommunication has drastically changed our daily lives Several satellite systems are underdevelopment to provide voice and data communication systems Although wirelesscommunication is a major application of RF and microwave technology, the technology also hasextensive applications in radar, sensors, navigation, radio frequency identification (RFID),remote sensing, surveillance, broadcast, smart automobiles and highways, and so on.

The wireless era was started by two European scientists, James Clerk Maxwell and HeinrichRudolf Hertz In 1864, Maxwell presented the Maxwell's equations by combining the works ofLorentz, Faraday, Ampere, and Gauss He predicted the propagation of electromagnetic waves infree space at the speed of light His theory was not well accepted until 20 years later, after Hertzvalidated electromagnetic wave (wireless) propagation Hertz demonstrated RF generation,propagation, and reception in the laboratory Hertz's work remained a laboratory curiosity foralmost two decades until a young Italian, Guglielmo Marconi, envisioned a method fortransmitting and receiving information Marconi commercialized the use of electromagneticwave propagation for wireless telegraphs and allowed the transfer of information from onecontinent to another without a physical connection The telegraph became the means of fastcommunications Distress signals from the S.S Titanic made a great impression on the publicregarding the usefulness of wireless communications.

FIGURE 1.1 Growth of cellular subscribers [1] (used with permission from IEEE, © IEEE).

In the early 1900s, most of the wireless transmission were made at very long wavelengths In the1920s, a one-way broadcast was made to police cars in Detroit The use of radio waves forwireless broadcasting, communications between mobile and land stations, public safety systems,maritime mobile services, and land transportation systems increased drastically During WorldWar II, radio communications became indispensable for military use in battlefields and troopmaneuvering World War II also created an urgent need for radar (standing

for radio detection and ranging) The resolution of a radar (i.e., the minimum object size that can

be detected) is proportional to wavelength Therefore, shorter wavelengths or higher frequencies(i.e., microwave frequencies and above) are required to detect smaller objects such as fighteraircrafts.

Wireless communications methods such as telegraphs, broadcasting, telephones, and point radio links were available before World War II The use of these communication methodsaccelerated, becoming widespread, during and after the war For long-distance wirelesscommunications, relay systems or tropospheric scattering were used After 1960, satellitesystems were emerging as a means of global communications A satellite uses a broadband high-frequency (normally in gigahertz) system that can simultaneously support thousands of telephoneusers, tens or hundreds of TV channels, and many data links Starting in the 1980s, cordlessphones became popular and have enjoyed very rapid growth in the last two decades Recently,personal communications systems (PCS) operating at higher frequencies with wider bandwidthshave emerged, providing a combination of various services such as voice mail, email, video,messaging, data and computer on-line services.

point-to-The RF and microwave wireless applications have enjoyed tremendous growth since the ColdWar ended in 1990 Many books have been published in response to the strong market demands.However, most of the books were written with emphasis on systems The objective of the presentbook is to describe the component and circuit designs for wireless applications All importantcomponents such as transmission lines, matching networks, filters, couplers, switches,amplifiers, mixers, frequency converters, oscillators, modulators, and antennas are covered indetails System applications of these components are also presented.

FIGURE 1.2 Electromagnetic spectrum.

1.2 FREQUENCY SPECTRA AND ATMOSPHERIC PROPAGATION LOSS

The microwave frequency spectrum ranges from 300 MHz to 30 GHz with a correspondingwavelength from 100 to 1 cm Below the microwave spectrum is the RF spectrum, and above isthe millimeter-wave spectrum Above the millimeter-wave spectrum are submillimeter-wave,infrared, and optical spectra Millimeter waves (30–300 GHz), which derive their name from thedimensions of their wavelengths (from 10 to 1 mm), can be classified as microwaves sincemillimeter-wave technology is quite similar to that of microwaves Figure 1.2 shows theelectromagnetic spectrum For convenience, microwave and millimeter-wave specta are furtherdivided into many frequency bands Figure 1.2 shows some of the microwave bands, and Table1.1 shows some of the millimeter-wave bands The RF spectrum is defined loosely One canconsider the frequency spectrum below 300 MHz as the RF spectrum But frequently, literatureuses the RF term for frequencies up to 2 GHz or even higher.

The performance of the wireless system is strongly affected by atmospheric absorption as shownin Figure 1.3 The propagation loss is caused by the absorption of microwave energy by watervapor and molecular oxygen For frequencies below 10 GHz, the atmosphere causes little loss ofRF or microwave signals The attenuation is high at frequencies above 30 GHz Severalresonances occur at 22.2, 60, 120, and 183 GHz These resonances occur when the frequencycoincides with one of the molecular resonances of water or oxygen resulting in maximumabsorption The high attenuation at 60 GHz has applications in secure local area network (LAN)communications and intersatellite communications There are several “windows” at 35, 94, and140 GHz with lower attenuation Some radar or radio applications are centered around thesefrequencies The data shown in Figure 1.3 are for clear weather conditions Rain and storm willincrease the attenuation dramatically.

TABLE 1.1 Millimeter-Wave Band Designation

Q-band 33–50

U-band 40–60

V-band 50–75

E-band 60–90

FrequencyRange(GHz)

1.3 WIRELESS APPLICATIONS

Two of the historically most important RF/microwave applications are communication systemsand radar; but there are many others Currently, the market is driven by the phenomenal growthof personal communication systems, although there is also an increasing demand for satellite-based video, telephone, and data communication systems.

Radio waves and microwaves play an important role in modern life Television signals aretransmitted around the globe by satellites by using microwaves Airliners are guided from takeoffto landing by microwave radar and navigation systems Telephone and data signals aretransmitted using microwave relays The military uses microwaves for surveillance, navigation,guidance and control, communications, and identification in their tanks, ships, and planes.Cellular telephones are everywhere.

RF and microwave wireless technologies have many commercial and military applications Themajor application areas include communications, radar, navigation, remote sensing, RFidentification, broadcasting, automobiles and highways, sensors, surveillance, medical, andastronomy and space exploration The details of these applications are listed below:

Although this book emphasizes communication applications the technologies can be used forother applications as well.

1. Wireless communications: space, long-distance, cordless phones, cellular telephones, mobile,

PCS, LAN, aircraft, marine, vehicle, satellite, global, etc.

2. Radar: airborne, marine, vehicle, collision avoidance, weather, imaging, air defense, traffic

control, police, intrusion detection, ground penetration radar (GPR), weapon guidance,surveillance, etc.

3. Navigation: microwave landing system (MLS), global positioning system (GPS), beacon, terrain

avoidance, imaging radar, collision avoidance, autopilot, aircraft, marine, vehicle, etc.

4. Remote sensing: earth monitoring, meteorology, pollution monitoring, forest, soil moisture,

vegetation, agriculture, fisheries, mining, water, desert, ocean, land surface, clouds,precipitation, wind, flood, snow, iceberg, urban growth, aviation and marine traffic, surveillance,etc.

5. RF Identification: security, antitheft, access control, product tracking, inventory control, keyless

entry, animal tracking, toll collection, automatic checkout, asset management, etc.

6. Broadcasting: AM radio, FM radio, TV, direct broadcast satellite (DBS), universal radio system,

7. Automobiles and highways: collision warning and avoidance, global positioning system (GPS),

blind-spot radar, adaptive cruise control, autonavigation, road-to-vehicle communications,automobile communications, near obstacle detection, radar speed sensors, vehicle RFidentification, intelligent vehicle and highway system (IVHS), automated highway, automatic tollcollection, traffic control, ground penetration radar, structure inspection, road guidance, rangeand speed detection, vehicle detection, etc.

8. Sensors: moisture sensors, temperature sensors, robotics, buried object detection, traffic

monitoring, antitheft, intruder detection, industrial sensors, etc.

9. Surveillance and electronic warfare: spy satellites, signal or radiation monitoring, troop

movement, jamming, antijamming, police radar detectors, intruder detection, etc.

10. Medical: magnetic resonance imaging, microwave imaging, patient monitoring, cancer

treatment, etc.

11. Radio astronomy and space exploration: radio telescopes, deep space probes, space monitoring,

1.4 ORGANIZATION OF THIS BOOK

This book is organized into 12 chapters Chapter 2 presents an account of some general wirelesssystems in cellular, satellite, and personal communications Chapter 3 gives an overview ofvarious active devices and microwave integrated circuit technologies commonly used in theconstruction of RF and microwave wireless systems Chapter 4 describes the system parametersfor a typical receiver or transmitter Chapters 5 and 6 are devoted to passive components andcircuits such as transmission lines, impedance matching networks, couplers, and filters Active orsolid-state circuits such as switches, low-noise amplifiers, mixers, frequency converters,oscillators, modulators, and power amplifiers are presented in Chapters 7 through 11 Finally,every wireless system needs an antenna The antennas and their operating principles arediscussed in Chapter 12.

Another market segment that is emerging very rapidly is in the short-range wireless accessapplications A new wireless system, called the Bluetooth system, is being developed for thisapplication Bluetooth system, was initiated by a special-interest group consisting of four majorcompanies in 1998 A consortium was created in 1999 and it has now over 2500 participants.

This market is expected to be a significant portion of the total wireless market in the next fewyears Many companies are designing products that would integrate cellular phones withBluetooth technology.

While the wireless technology advanced in the terrestrial systems, communication systems thatutilize satellites were also proposed With these satellite systems, voice, digital and video datacould be transmitted from any place in the world at any time to any other place The Iridiumsystem proposed by Motorola was the first of its kind that utilized low-orbiting satellites toprovide instant communication around the world [2].

In this chapter we will discuss the terrestrial and satellite-based wireless systems of differenttypes.

2.2 WIRELESS SYSTEM FREQUENCY ALLOCATIONS

Frequency allocations are controlled in different countries by their respective regulatoryagencies Federal Communications Commission (FCC) is responsible for the regulation offrequency use and allocation in the United States.

2.2.1 Cellular Radio Systems

The first analog cellular radio standard implemented in the United States is known as theadvanced mobile phone service (AMPS) AMPS is also used in Canada, South America, andAustralia Some of the major analog cellular standards used in Europe are TACS (total accesscommunication systems), NMT (Nordic mobile telephones), and Radiocom 2000 In Japan, thefirst analog cellular was introduced in 1979 Table 2.1 shows the summary of analog cellularsystems, its frequency allocation, and the geographical regions where they are used.

The digital cellular system, also known as second-generation cellular system, arose from thesuccess of the first-generation (analog) system and was built on some of the lessons learned inthe technical, commercial, political, and regulatory fields Technically, there was a need toimprove the spectrum efficiency and to adopt digital encoding of the voice and digitalmodulation of the radio bearer Development of the digital speech coding technique made thecompletely digital cellular system viable This second-generation cellular system can supportmore users per base station per megahertz of spectrum A digital cellular system known asGroupe Special Mobile (GSM) was introduced in 1982 to address the need for accommodatingan increasing number of users in Europe GSM system (now known as Global System Mobile)was originally intended for 900 MHz band operation In 1989, UK Department of Trade andIndustry allocated 150 MHz of spectrum in the 1800 MHz band for personal communicationnetworks (PCN) The standard chosen for this application is also GSM and this system is knownas Digital Cellular System 1800 or DCS 1800.

TABLE 2.1 Analog Cellular Phone Standards

To meet the increasing demand for cellular radios in high-density areas in the United States, adigital cellular system was proposed in 1992 The Electronic Industries Association (EIA) andthe Telecommunications Industry Association (TIA) adopted the Interim Standard-54 (IS-54)standard based on TDMA (time division multiple access) [3] for digital cellular radio Thisdigital standard retained the 30 kHz channel spacing of AMPS (to keep up the evolution of theanalog system) The IS-54 standard allowed a dual-mode operation in the sense that the analogand digital operations could coexist The EIA/TIA also adopted another standard known as IS-95based on CDMA (code division multiple access) [4] This standard allows users to share acommon channel for transmission These IS-95-based systems have several benefits includingincreased capacity, flexibility for accommodating different transmission rates, variable ratespeech coding, and power control They can operate in the CDMA mode or in the AMPS mode.In Japan, a digital cellular system, known as personal digital cellular (PDC) system, wasintroduced in 1992 Like IS-54, PDC is based on TDMA technology and has a channel spacing

of 25 kHz; and it employs π/4 DQPSK modulation scheme PDC system operates in two sets of

frequency bands in the 800–900 MHz and 1400–1500 MHz ranges Table 2.2 summarizes thedigital cellular standards.

2.2.2 Cordless Telephony

Analog cordless telephone handset operates in the 46.6–47.0 MHz receive band and 49.6–50.0MHz transmit band Analog FM is used for voice signal, and the effective radiated power is

about 20 μW Even though high-power digital cordless telephones operating in the ISM band

were introduced recently, popularity of the 49 MHz telephones are still very strong A cordlessphone system, known as CT1, and an enhanced version CT1+, have been very successful inEurope These systems operate in the 914–915 MHz transmit and 959–960 MHz receive bands.In Japan, analog cordless phones operated under the allocation of around 254 MHz for transmitand 380 MHz for receive bands.

Digital cordless telephones in North America were operated in the ISM (industrial, scientific,and medical) band In USA and Canada, the ISM band included 902–928, 2400–2483.5, and5725–5820 MHz frequency ranges Cordless telephones, operating in this unlicensed band usingthe frequency hopping or direct sequence spread spectrum technology, were allowed to have upto 1 W of transmitted power Since this is an unlicensed band, there is not much of a regulation,and the manufacturers have considerable design freedom In UK, a digital cordless phonesystem, known as CT2 was introduced to remedy some of the deficiencies of the analog cordlessphones A European standard DECT (digital European cordless telecommunications) was alsointroduced as a flexible interface to provide cost-effective communication services Table2.2 shows a summary of the cordless telephone systems and its frequency allocation Acomparison of important characteristics of digital cellular phone and the cordless telephonesystems are shown in Table 2.3 [5].

TABLE 2.2 Digital Cellular and Cordless Phone Standards

TABLE 2.3 Comparison of Cordless and Digital Cellular Phone Characteristics

2.2.3 Mobile Satellite Systems

Satellite-based systems allow global communication These systems operate under specificfrequency allocation The MSS (Mobile satellite systems) services can be broadly divided intothree categories based on the altitude of the satellites used in the system The geostationarysatellites orbit at an altitude of about 35,800 km The medium Earth orbit satellites are orbiting atan altitude of about 10,000 km The low Earth orbiting satellites are at an altitude of about 1000km.

2.3 CELLULAR SYSTEMS IN THE UNITED STATES, EUROPE, AND JAPAN

Bell Laboratories pioneered cellular system design in the 1970s The success of the cellular radiois the result of the automatic switching capability of present-day mobile telephone network Adetailed discussion of cellular systems can be found in reference 6 In the cellular systemoperation, the service area to be covered is arranged into a network of contiguous radio cells Theidealized cellular network is a hexagonal structure Each cell has a base station and associated setof radio channels to effectively connect to any mobile unit located in the cell Each system has afull-duplex operation and channel search capability Control channels are used to transfer systemcontrol information from the base station to the mobile radio The voice channels provide thelink for the speech and data transmission The base stations are connected by landline cables ormicrowave links to a mobile switching center It is this center that controls the connection ofmobile units to each other.

A geographical area is covered by a pattern of cells; each having been assigned a set of controland voice channels A group of adjacent cells share between them all the available frequencychannels, and the same frequencies can be reused in the same area in another cell cluster A

major advantage of the cellular system is the cell splitting capability It is possible to increase the

capacity of the system by simply splitting the large cells into small cells Such a system canemploy cells of different sizes depending on the density and distribution of traffic.

Another salient feature of the cellular radio is the hand-off capability Handoff allows the mobile

radio user to move between cells without the interruption of service During the telephoneconversation, the base station monitors the level of the received signal from the mobiles and

adjusts its transmit power as required If the received signal falls below the predetermined low

level, adjacent base stations are commanded to monitor the signal strength and the call is handed

off to the base station that received the strongest signal.

2.3.1 Analog Cellular Systems

The first analog cellular standard implemented in the United States is known as the AMPS InAMPS a total of 50 MHz of spectrum, 25 MHz each in the 824–849 MHz band (for transmittingsignals) and 869–894 MHz band (for receiving signals), is allocated for the cellular mobile radioapplications Each of the 25 MHz spectrum is divided into 832 channels, each 30 kHz wide Thespeech coding is accomplished by frequency modulation with 8 kHz of deviation in frequency.The signaling channel uses frequency shift key (FSK) modulation with a bit rate of 10 kbps Anexpanded version of the AMPS systems later introduced is known as EAMPS.

The first analog cellular system deployed in the United Kingdom is known as TACS Severalother systems were introduced in other parts of Europe as well (See Table 2.1) All these systemsuse frequency modulation for speech and FSK for signaling The channel spacing is 25 kHz forTACS, NMT-450, and RTMS, and 10 kHz for NMT 900 and Radiocom 2000 In Japan theanalog phones operated at the transmit frequency of 825–940 MHz and the receive frequency of870–885 MHz.

The cellular phone has proven itself and has justified its usefulness in a world full of alternativemeans of wireless communications The world is covered by cellular radio systems operating indifferent frequency bands and using different protocols The explosive growth of subscribers tothe cellular service in the past decade resulted in the steady expansion of cellular networks Theinstallations and expansion led to greater coverage and capacity But as the number ofsubscribers increased, backlog in cellular capacities increased, especially in metropolitan areaswhere subscriber density and traffic volume are higher Due to the limited availability ofspectrum, new technologies were proposed Among the solutions were narrow band radios,microcells, and time multiplexing digital radio techniques.

Analog radio has the effect of passing the physical disturbances in radio transmission linksdirectly into the audio path of the receive band The disturbances such as fades, interferences,and spurious signals, manifest in the audio channels as statics, hums, hisses, crackling sounds,and fadeouts With digital techniques, the audio signal is not transmitted as such, but transformedinto the digital data patterns These and other regulatory reasons led to the development of thesecond-generation system, which is also known as the digital cellular radio system.

2.3.2 Digital Cellular Systems

The lead in second generation cellular phone was taken by Europe with the creation of the GSMCommittee by the European post and telecommunication operators' organization GSM wasgiven the task of devising a mobile telecommunication system that would operate at the samestandard throughout Europe and allow users to access and use the system irrespective of theuser's location or the equipment used [7].

The approach in the United States was to develop a dual-mode AMPS (DAMPS) standard tomeet the growing need to increase cellular capacity in high-density areas Another standard thatemerged in 1993 is the CDMA system, also known as IS-95 system Frequency allocation of allthe cellular and cordless phones are shown in Table 2.2.

2.3.2.1 Global System Mobile GSM was originally used as an acronym for a committee, Groupe

Special Mobile, that was formed to develop the standard mobile communication systems inEurope In 1982, European telecommunication authorities reserved two frequency bands, 890–915 MHz and 935–960 MHz, primarily for cellular application Before that period, Europeancellular market was characterized by a large number of incompatible analog standards, includingTACS and NMT As the cellular subscribers and their mobility all over Europe increased, theanalog cellular systems that have the territorial limits on services posed a serious inconvenience.This led to the next generation European digital cellular system based on the narrow bandTDMA mode of operation TDMA system has the advantage of offering a much greater varietyof services than analog cellular systems In addition, it has the ISDN capability and thepossibility of channel splitting and advanced speech coding The major operational requirementsdeveloped for the GSM include [8,9]:

GSM uses both TDMA and frequency division multiple access (FDMA) for transmit and receiveinformation A typical TDMA/FDMA frame structure is shown in Figure 2.1 These systems usedata packets at specific times at specific frequencies Therefore, several conversations can takeplace simultaneously at the same frequency at different time slots Since frequency duplex isused in GSM, the transmission and reception are concurrent GSM system has eight time slotsper channel, and the spacing between carriers is 200 kHz The bandwidth of GSM is 25 MHz,which allows 125 carriers each having a bandwidth of 200 kHz (see Tables 2.2 and 2.4) Themodulation method adopted for GSM is Gaussian minimum shift keying (GMSK) Thisfacilitates the use of narrow bandwidth and coherent detection capability In GMSK scheme,rectangular pulses are passed through a Gaussian filter prior to their passing through amodulator The data rate of 270.8 kbps, along with the 200 kHz carrier spacing results in aspectral efficiency of 1.35 b/s/Hz for the GSM system The data rate of 270.8 kbps dividedamong eight users in GSM produces a data rate of 33.85 kbps The speech coder, regular pulseexcitation with long-term predictor (RPE-LTP), converts the speech into 13 kbps In the nearfuture the full-rate coding scheme will be changed to “half-rate” coding resulting in about 7kbps There are five different categories of mobile telephone units specified for the GSM Thespecified power levels are 0.8, 2, 5, 8, and 20 watts with the power level varying capabilitybetween 3.7 mW and 20 W The 8 and 20 W units are either for vehicle mounted or portablestation use.

FIGURE 2.1 A typical TDMA/FDMA frame structure.

TABLE 2.4 Digital Cellular and Cordless Phone Specifications

2.3.2.2 NADC The North American Digital Cellular (NADC) system adopted IS-54 standard

based on TDMA This retained the 30 kHz channel spacing of AMPS to facilitate evolution fromthe analog to digital and in addition provided a raw RF bit rate of 48.6 kb channel This standard

utilized the π/4 DQPSK modulation scheme and each frequency channel had a RF bit rate of 48.6

kbps The capacity is divided among six time slots, two of which were assigned to each user.Each 30 kHz frequency pair can serve three users simultaneously This IS-54 provided triple thecapacity of AMPS systems With the introduction of half-rate speech coders, the capacity couldbe increased six times the analog systems Since this system supports both analog and digitalsystems, it is known as the dual mode AMPS system or DAMPS Important attributes of IS-54systems are shown in Table 2.4

2.3.2.3 Personal Digital Cellular The digital system established in Japan in 1991 is known as

PDC system This is similar to IS-54 systems in many ways While NADC is designed to coexistwith AMPS, the PDC was designed to replace many of the incompatible analog systems inJapan In this respect its implementation is like GSM PDC offers three sets of frequency bands.One set is in the 800–900 MHz band with a duplex offset of 130 MHz, and two other sets are inthe 1500 MHz band with 48 MHz duplex offsets It is TDMA-based and has three slotsmultiplexed into each channel, like IS-54 system The channel spacing is 25 kHz, with a RF bit

rate of 42 kbps This standard, like IS-54 standard, utilizes the π/4 DQPSK modulation scheme.

The unique feature of PDC is the mobile-assisted hand-off that facilitates the use of small cells.Key attributes of PDC are summarized in Tables 2.2 and 2.4.

2.3.3 Code Division Multiple Access Systems

The frequency reuse in cellular systems necessitated the development of several multiple accesstechniques for allowing users to share the same frequency band The two simplest techniques areFDMA, which assigns each user within a geographical region a specific frequency channel; andTDMA, which assigns several users to a common frequency band, but transmit on a rotatingbasis during specifically assigned time slots Another technique is CDMA A system that usesCDMA technique allows many users to transmit at the same time within the same frequencyband Each user is assigned a unique “code” or “signature” sequence within the CDMA system.This code sequence allows the transmitter to generate a signal, which may be uniquelyrecognized by the intended receiver Figure 2.2 gives a pictorial description of FDMA, TDMA,and CDMA.

The CDMA technology, first developed by QUALCOMM, calls for a wideband channel havingbandwidth of 1.23 MHz to be used in every cell, which is equivalent to 42 channels of 30 kHzbandwidth Each channel is shared by many users with different codes Even though there aremany CDMA systems in the world, one of the most widely used is the IS-95 version of CDMA,which is a result of a comprehensive proposal from QUALCOMM, with cooperation fromAT&T, Motorola, and others A single CDMA channel is 1.23 MHz wide and a dozen or sosubscribers typically share the same wide channel simultaneously.

Some of the advantages of a CDMA system are [10]:

FIGURE 2.2 A pictorial description of FDMA, TDMA, and CDMA schemes.

The CDMA system, however, imposes substantial demands on microelectronics circuitry toprocess data.

One of the fundamental assumptions of the CDMA system is that signals for each user arrive atthe receiver with equal power If an interfering signal arrives at the receiver with power higherthan that of the desired signal, it would cause performance degradation As a result, practicalCDMA systems have to incorporate elaborate power control algorithms Another importantproblem is synchronization Timing requirements for CDMA system are much tighter than thatfor other types of communication systems, since receiver must be synchronized to determine theprecise time of arrival of desired signal While there are good techniques available forovercoming these implementation difficulties, most of the solutions require complexmicroelectronics components that increases the handset cost The major challenges of CDMAtechniques are

These challenges have in part facilitated the development of miniaturized microelectronicscomponents having low power consumption and high efficiency.

2.3.3.1 Dual mode operation of CDMA CDMA system does not share any of the existing AMPS

resources other than the fact that it is in the same frequency band When a CDMA phone enters a

system that supports CDMA protocol, it gains access to the CDMA mode If, on the other hand,the servicing system does not support the CDMA protocol, the phone recognizes the missingCDMA signature and becomes a conventional AMPS phone.

2.4 CORDLESS TELEPHONY

Cordless telephone systems offer the user limited spatial mobility When compared with thecellular radio systems, they are characterized by a shorter radio range The typical ranges are 50–300 m depending on the application and the standard implemented On the other hand, theoperating cost for the subscriber is significantly lower than that of the cellular systems Theirmost widespread application is in residential cordless telephony European digital cellular phonesconsisted of CT2, CT2+, and DECT standards In Japan, the cordless telephones are coveredunder the personal handyphone system (PHS) standard.

2.4.1 Analog Cordless Phones

Since 1984, analog cordless telephones in the United States operated in two separate bands Oneband, in the 46.6–47.0 MHz, consisted of 10 frequency ranges for receiving the signals by theportable handset Another band, in the 49.6–50.0 MHz, had another 10 corresponding frequencyranges for transmitting the signals The allowed emission bandwidth was 20 kHz and the

effective radiated power was 20 μW Analog FM was used as voice signal, and digital coding of

the signal was used for security purpose.

Because of the popularity of this type of phones, the existing 10 pairs of frequency have becomeinadequate, particularly in the high-density areas In 1995, FCC allocated additional 15frequency pairs near the 44 MHz band for handset receive and near the 49 MHz band for thehandset transmit Despite the recent introduction of digital cordless phones operating at higherfrequencies, the analog phones operating at 49 MHz still remain very popular due to their lowcost.

The cordless telephones were initially imported to Europe from the United States and the FarEast The first UK standard, similar to the one in the United States, allowed for eight channelpairs The handset receive frequency is near 1.7 MHz and the transmit frequency is near 47.5MHz A similar standard was adopted in France as well This original standard is sometimescalled CT0 standard In the rest of Europe, the demand for cordless phones was addressed bydeveloping a standard known as CT1 This provided for 40 duplex channels of 25 kHz each, inthe frequency bands 914–915/959–960 MHz In the enhanced version, known as CT1+, 80 pairswere allocated in the frequency bands 885–887/930–932 MHz Adoption of a form of dynamicchannel assignment (DCA) allowed the selection of one of the 40 (or 80 for CT1+) duplexfrequency pairs at the beginning of each call.

In Japan, 89 duplex channels near 254 MHz for handset transmit, and near 380 MHz for handsetreceive, were allocated for analog phones The channel spacing is 12.5 kHz and the radiatedpower limit is 10 mW.

2.4.2 Digital Cordless Telephones

2.4.2.1 CT2/CT2+Systems The deficiencies such as limited number of channels and high

blocking probabilities of analog cordless telephone systems have stimulated the development ofan alternative technology known as CT2–Common Air Interface (CT2-CAI) CT2 is based on adigital technology using combined frequency division/time division transmission technique Themodulation scheme is GMSK CT2 spectrum allocation consists of 40 FDMA channels with 100kHz spacing in the 864–868 MHz band The transmit power is 10 mW and the typical range is30–100 m CT2 supports data in addition to voice transmission CT2 was initially introduced astelepoint standard in which telepoint networks use cordless base stations to provide wireless payphone service After the initial success, the user acceptance of CT2 phones decreased in Europe,while the use in Far East increased In Canada an enhanced CT2, known as CT2+ and operatingin 944–948 MHz, was implemented for cordless telephony This was designed to provide someof the missing mobility-management functions In this five carriers are reserved for signaling,and each carrier included 12 common signaling channels using TDMA These channelssupported the local registration, local updating, and paging.

2.4.2.2 DECT System The DECT system was designed as a flexible interface to provide a

cost-effective phone service in the high-density areas using picocells It is suitable for residential,public access, and local loop telephony This system is designed for low cost and flexibleoperation in an uncoordinated environment DECT is closer to a cellular system than a classicalcordless phone system Base station for DECT can therefore handle multiple handsetssimultaneously with a single transceiver DECT also interfaces with GSM, which leads toadditional network capability.

DECT uses TDMA and the time division duplex (TDD) transmission scheme The operatingfrequency for this system is in the 1800–1900 MHz range The band is divided into 10 carriersand each of the carriers is subdivided into 24 time slots, 12 in each direction The speech isencoded with 32 kbps adaptive differential pulse code modulation (ADPCM) modulation with achannel rate of 1.152 kbps over a channel width of 1.728 MHz, which provides a bandwidthefficiency of 0.67 b/s/Hz The speech data rate is much higher than that used in digital cellularstandard and, as a result provides speech quality that is comparable to the wired telephonesystem Details of the frame structure and the time slot of a DECT system are shown in Figure2.3 [11] As shown in the figure, the total frame duration of 10 ms is divided equally betweentwo 5 ms segments for communication in each direction Each 5 ms segment is divided into 12time slots representing 12 channels A total data rate of 1.152 Mbps is provided Compared withCT2 phones, DECT has double the transmission range (20–200 m indoors and up to 300 moutdoors) DECT standard includes a number of security provisions including encryption ofradio transmission and authentication of the portable device.

FIGURE 2.3 Frame structure and time slot of DECT system.

DECT standard is mandatory in Europe The standard also has widespread support in theindustry and the first commercial system was implemented in 1993 DECT is also drawinginterest outside Europe A standard based on DECT is being adopted in US for application in thePCS bands.

2.4.2.3 Personal Handyphone System (PHS) In Japan PHS was launched in 1989 to provide

home, office, and public access capability The PHS allocation consists of 77 channels, 300 kHzin width, in the 1895–1918 MHz band The 1906.1–1918.1 MHz band is designated for publicsystems and the 1895–1906.1 MHz band is used for home/office applications Like DECT, PHSuses TDMA and TDD, but has four duplex channels per frequency rather than 12.

2.4.3 WACS/PACS

In the United States, Bellcore developed an air interface for wireless access communicationsystems (WACS) This interface provides wireless connectivity to local exchange carrier Thetargeted application is low-speed portables Base stations are envisioned as small boxes mountedon the telephone poles and separated by about 600 m WACS uses frequency division duplex

(FDD) instead of the TDD employed in the DECT system Each frequency carries 10 user timeslots The speech coding is 32 kbps and the frame duration was 2 ms The modulation schemeadopted for this is quadrature phase shift keying (QPSK), with coherent detection.

As a part of the standard being established for the 2 GHz personal communication service (PCS)in the United States, attributes of WACS and PHS have been combined to create a new standardknown as personal access communication services (PACS) PACS retains many of the attributesof WACS, with a few exceptions Time slots are reduced from 10 to 8, with the corresponding

reduction in the channel bit rate and bandwidth The modulation scheme has been changed to π/4

QPSK as well.2.4.4 DCS 1800

GSM was originally established to operate in the 900 MHz band Even before the GSM systembecame operational, the UK government licensed three operators to provide communicationservices, using a spectrum allocation of 2 × 75 MHz at around 1800 MHz The operators settledon a system based on GSM Later on this 150 MHz of spectrum allocation near 1800 MHz wasapproved for personal communication network (PCN) all over Europe This system is known asDCS 1800.

The differences between the GSM and DCS 1800 are relatively small Both are based onidentical modulation and can offer the same range of services The significant differences aresummarized in Table 2.5 DCS 1800 has increased spectrum availability, and supports only lowpower handheld units, as opposed to the capability of GSM on a wide range of equipment fromlow-power handheld to high-power mobile units.

2.5 WIRELESS LAN SYSTEMS

Wireless data systems are designed for packet-switched operation, as opposed to theconventional circuit switched operation Operators of wide-area messaging systems use licensedspectrum, and sell services to customers The wireless local-area networks, on the other hand, isprivately owned and operated This provides high-rate data communication over a small area.LANs are unlicensed and operate in the ISM bands.

TABLE 2.5 Major differences between GSM 900 and DCS 1800

Wireless LANs are mostly used for indoor communication at ≥1 Mbps data rate Standards forwireless LAN are covered under the IEEE 802.11 in the United States and under HiperLAN inEurope HyperLAN uses a dedicated band of 5.150–5.3 GHz, whereas the IEEE 802.11 utilizesthe 2.4 GHz ISM band In Japan, two types of wireless LANs have been implemented One is formedium data rates in the 256 kbps to 2 Mbps and the other one for > 10 Mbps The first oneoperates in the 2.4 GHz ISM band, while the second one uses the 18 GHz band.

2.6 SATELLITE COMMUNICATION SYSTEMS

The satellite communication industry has experienced tremendous growth in the last decade.Communication satellite provides a platform to relay radio signals between points on the ground.A satellite is capable of performing the role of a microwave repeater for Earth stations that arelocated within its coverage area There are three basic types of orbit configurations for thesatellites: (a) low-Earth-orbiting systems (LEOS), (b) medium-Earth-orbiting systems (MEOS),and (c) geostationary or geosynchronous orbit systems (GEOS) MEOSs are also known asintermediate circular orbit systems (ICOS) A GEO satellite can cover one-third of the Earth'ssurface with the exception of the polar regions So a minimum of three satellites are required fora universal coverage At present about 18 commercial GEO satellites are in operation From35,800 km above the equator, they dispense many services including TV, distribution toterrestrial broadcasting stations, direct TV and maritime LEOS and MEOS approaches requiremore satellites to cover the surface of the earth Satellites in LEOS are typically 500–1500 kmand those in MEOS are 5000–12,000 km above the Earth Due to the fact that the satellite movesin relation to the Earth, a full complement of satellites, called the constellation, is required toprovide continuous coverage But individual satellites in low orbit systems are smaller, lighter,and less expensive Atmospheric drag and radiation from the inner Van Allen radiation belt areexpected to limit the orbital lifetime of LEO satellites (typically 5–7 years) The MEOS satelliteshave a lifetime of about 12 years The cost for launching an LEO satellite is significantly lowerthan that of the heavier MEO and GEO satellites The path length for the LEOS is the shortestand this leads to the minimum propagation delay The round trip propagation delay for a GEO isabout 260 ms, compared to 10 ms for an LEO satellite system such as Iridium Increased delay ofGEOS and MEOS would cause degradation in quality of service or throughput Table2.6 summarizes the important attributes of the major satellite systems [12].

The uplink and downlink frequencies fall within the bands approved by the internationaltelecommunication union (ITU) Uplinks from phones to the non-GEO satellites are in the 1610–1626.5 MHz band The downlinks are in the 2483.5–2500 MHz.

TABLE 2.6 Comparison of Major Personal Mobile Satellite Systems

2.6.1 Iridium System

The Iridium system, first proposed by Motorola, is regarded as the most ambitious in terms ofsize and complexity Now Iridium system is operated by Iridium LLC, an internationalconsortium of about 20 telecommunication and industrial companies Iridium system seeks toextend cellular service to many more people even in the absence of land-based cellular service.Users of conventional cellular service will be able to switch to satellite service without

interruption It consists of 66 satellites orbiting in six evenly spaced near-polar orbits 780 kmabove the Earth (Iridium was originally proposed as a communication system consisting of 77satellites The system was named Iridium after the element that has atomic number 77 Furtherstudy showed that 66 satellites would be sufficient to provide a global coverage.(However, theoriginal name, Iridium, was still retained) Figure 2.4 shows the constellation of satellites used inthe Iridium system [13] Each satellite is triangular in shape, 4.5 m long and 1 m along its base,shaped so that several can fit on a single launch vehicle An Iridium satellite weighs about 690lbs when it is fully loaded The deployed configuration of an Iridium satellite is shown in Figure2.5 [13]

A call placed by an Iridium subscriber to another subscriber is transmitted directly by satellite toits destination worldwide If the call is from a party with conventional phones, it will beupconverted and transmitted by the system The first set of satellites was launched in May, 1997,and the Iridium telephone network was offered for public use in November, 1998 Motorola andKyocera designed handheld phones for Iridium application A Photograph of a Motorolahandheld phone is shown in Figure 2.6 [13] Table 2.7 summarizes the key characteristics of theIridium systems.

FIGURE 2.4 Satellite constellation of Iridium system.

FIGURE 2.5 Deployed configuration of an Iridium satellite.

In spite of the technological innovation and engineering excellence, the market for Iridium-basedphones did not develop as was originally anticipated The handset was too heavy to carry aroundand it required a line-of-sight connection to satellite to function properly The cost of the handsetand the airtime charge were very high Because of these reasons and the explosive growth oflow-cost cellular phones and other communications means, the demand for Iridium service wasvery low The company, Irdium LLC, filed for bankruptcy in 1999.

2.6.2 Globalstar System

The Globalstar system uses a constellation of 48 satellites They are located in eight circularorbits at 1414 km altitude Another eight satellites serve as spares The orbits are inclined 52° tothe equator and spaced 45° from one another along the earth's greater circle The Globalstarsystem will be capable of providing communication between 70° north and 70° south of theequator But it will not cover the polar regions The first set of satellites for the Globalstar systemwas launched in 1998 and are expected to provide communication services in 1999 Figure2.7 shows the constellation of Globalstar satellites [12] The Globalstar system has neitheronboard processing capability nor intersatellite communication links Instead, many functionsincluding call processing and switching operations are located on the ground A Globalstarsatellite therefore has lesser weight (450 kg) compared to an Iridium satellite A comparison ofthe key characteristics of Globalstar system with those of Iridium is presented in Table 2.7.Globalstar satellites are designed in to a trapezoidal shape and this allows multiple satellitelaunches by using the same rocket Globalstar system does not connect one caller to anotherdirectly through satellites; instead it downlinks calls received by the satellite over feeder links, toa gateway The calls are processed at the gateway and routed through the terrestrial

infrastructure But if the called party is another Globalstar customer, the call will be uplinkedfrom the gateway Globalstar requires more gateways than does Iridium Iridium uses 11gateways, whereas Globalstar is expected to use over 78 gateways Globalstar system startedoperating in 1999 The customer base for this satellite system is very small and the future of thissystem too is uncertain.

FIGURE 2.6 Photograph of Motorola's Iridium handheld phone.

2.6.3 ICO-P System

The ICO-P system is owned by ICO Global Communications Inc It was formed in 1995 as anoffshoot of Inmarsat, the 80-nation consortium that provides mobile satellite communicationservice to maritime terminals ICO system will provide the worldwide communication by

deploying 10 satellites as an MEOS or intermediate circular orbit (ICO) system These satellitewill be orbiting in two orthogonal planes inclined at 45° and 135° with respect to the equator, atan altitude of 10,355 km ICO satellites will be linked to ground stations with multiple antennaand switching capabilities These stations are located around the globe to provide globalcoverage The system is expected to provide partial service in 1999 and full service in year 2000.The satellite is expected to weigh about 2750 kg ICO-P system was also not successful at themarket place and filed for bankruptcy in 1999.

TABLE 2.7 Summary of Key Characteristics of Iridium and Globalstar Satellite Systems

FIGURE 2.7 Satellite constellation of Globalstar system.

2.7 FUTURE WIRELESS SYSTEMS

Wireless personal communications offers many possibilities Part of the challenges in planningthe future wireless communication system is to determine the services that they will be requiredto support One of the thrust areas of the ITU-Radio (ITU-R) is to define future public landmobile telecommunication systems (FPLMTS) 1992 World Administration Radio Conference(WARC '92) allocated frequency spectrum for FPLMTS on an international basis Rapidadvances in the component technology coupled with the developments in service engineeringand network management led to an increased desire to combine the fixed and mobile networks.There is also a great desire to have a handheld unit capable of multiple applications The successof the second-generation cellular technology with its cost-effective solutions raises thesignificant prospect that they will reach an early capacity and service saturation These marketdemands will force a third generation of cellular systems.

2.7.1 Bluetooth Systems

Bluetooth is a global RF-based, short-range connectivity solution for portable personal devices.This system will provide wireless connectivity within three areas: data and voice access points,

cable replacement, and ad hoc networking.

Five companies (Intel, Nokia, IBM, Ericsson, and Toshiba) formed a special interest group (SIG)in 1998 to formulate a protocol for using the Bluetooth architecture A consortium was laterformed to address the creation of a single digital wireless protocol to address the need to