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Mobile Telephony: What Makes it Mobile?

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This chapter is from the book
What makes mobile telephony work? Basestations and mobile switching centers, for starters. Read on to learn about frequency reuse, air interfaces, and the specific inner workings of an actual cellular phone.

In this chapter

  • A World of Choices
  • The Cellular Concept
  • Underlying Technology
  • CDMA Explained
  • Cellular Evolution

Is there any artifact more indicative of the wireless revolution than the mobile phone? I think not. Which is why I have devoted an entire chapter to it. (Heck, you'll probably get interrupted by a call on one while you are reading this chapter. Wouldn't that be ironic?)

There are many different mobile telephone systems worldwide. There are different generations, different technologies, and different frequency bands. If you live in the U.S. and think it is the only place with cellular phone service, you are in for a big surprise. Not only is there mobile phone service outside the United States, but a case could be made that the U.S. trails the other two leading regions (Japan and Europe) in technology deployed and services available. The reason why will soon become apparent.

In any event, it is good to understand the underlying technology of this lifestyle-altering wireless service. This chapter gives you a top-down view of mobile telephony, which includes a discussion of basestations, mobile switching centers, and what makes it mobile. But you will not be spared the details. You will also learn about frequency reuse, air interfaces, and the specific inner workings of an actual cellular phone.

CDMA, which is a popular air interface (and getting more popular all the time), is a mystery to most people. You may know what CDMA stands for, but it is doubtful that you understand how it works. Well all that is about to change. In recognition of its growing importance in the world of mobile telephony, an entire section of this chapter is devoted to explaining how CDMA (and spread spectrum) can cram more phone calls into a given bandwidth than any other air interface.

Finally, this chapter concludes with an abbreviated discussion of the migration paths to 3G (third generation cellular service). As much as I would like to tell you that this chapter will clear up the mess that is 3G, I'm just not that good. The reality is that the paths to 3G nirvana (circa 2001) are a free-for-all. Different technologies using different frequencies (some not yet allocated) in different parts of the world are all trying to accomplish the same thing: make a lot of money for the service providers. Because of all these incompatible approaches, the one truly noble goal of 3G—international uniformity—is not likely to happen any time soon. What will be the outcome? If I knew that, I'd have to charge a lot more for the book. Stay tuned.

A World of Choices

Differentiators

There are many choices for mobile telephone service in the United States. Each of these systems has one or more distinguishing characteristics that differentiate it from the others.

One of the most prominent ways that these mobile telephone services differentiate themselves is by frequency. Each service is allocated a different frequency band in which to operate. The first mobile service offered in the U.S., and the one that is most commonly referred to as "cellular," operates in the 900 MHz band. The newer mobile service in the U.S., dubbed Personal Communications Service (or PCS), operates in the 1900 MHz band. In some cases, the only thing that separates "cellular" from PCS is the frequency band of operation. All other aspects of the technology are identical. (Of course the marketing people at the PCS companies don't want you to know that.) Table 7–1 is a summary of the frequency band allocations for some mobile services in the United States.

Table 7–1 Allocated Frequency Bands in the U.S.

System

Mobile to Basestation

Basestation to Mobile

Cellular

824–849 MHz

869–894 MHz

PCS

1850–1910 MHz

1930–1990 MHz

SMR

806–824 MHz

851–869 MHz


Another way these services differentiate themselves is by the technology used to transport the voice signal. The earliest mobile phone systems used analog voice signals, while the newer ones use digital. (In the not-too-distant future, I predict there will be no more analog systems.)

In the case of the first cellular systems, the upgrade to digital technology used much of the existing infrastructure, including the assigned frequency bands. These new digital systems differentiate themselves by the modulation they use to encode the digital information onto the RF carrier. Most use a form of phase modulation or QAM (discussed in Chapter 5). Mobile telephone services also differentiate themselves by something called air interface, which you will learn about shortly.

Some of these services differentiate themselves by offering additional features compared to "standard" mobile service. One of the lesser known mobile services available is something called Specialized Mobile Radio or SMR. SMR, which operates in two different frequency bands in the 800 MHz range in the U.S., was originally intended for use as a wireless dispatch service (think taxi cabs). Today, it has evolved into a combination dispatch and mobile phone service. This combination service distinguishes SMR from all the other mobile phone services available. Not only can the service be used to make "ordinary" mobile calls in the interconnected mode, it can also be used to conduct wireless teleconferencing in dispatch mode. In this mode, several people using the service can hold a conversation simultaneously. As such, SMR is popular with teams of mobile salespeople who need to conduct spontaneous sales meetings.

Worldwide Systems

Just so you do not get the wrong idea, the United States is far from being the only place with mobile telephony. Table 7–2 shows some of the world's major mobile telephone systems. The first thing to notice is that there are a lot of different analog and digital technologies that have evolved over the years and none of them talk to each other.

Table 7–2 Worldwide Mobile Telephone Systems

Acronym

System

Where First Deployed

Technology

AMPS

Advanced Mobile Telephone Service

United States

Analog

CDMA

Code Division Multiple Access

United States

Digital

D-AMPS

Digital Advanced Mobile Telephone Service

United States

Digital

DCS1800

Digital Communication Service

Germany & England

Digital

GSM

Group Special Mobile

80 European countries

Digital

JTACS

Japan Total Access Communications System

Japan

Analog

NADC

North American Digital Cellular

United States

Digital

NMT

Nordic Mobile Telephone

Scandinavian countries

Analog

PCS1900

Personal Communications Services

United States

Digital

PDC

Personal Digital Cellular

Japan

Digital

SMR

Specialized Mobile Radio

United States

Both

TACS

Total Access Communications System

England

Analog


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