CDMA Capacity and Quality Optimization.pdf

Source: CDMA CAPACITY AND QUALITY OPTIMIZATION

Part

Key Radio Concepts

Part 1 of this text, “Key Radio Concepts,” is provided for readers who

are not already familiar with the engineering principles of radio and

how they apply to cellular systems. It also will benefit radio experts

in two ways. First of all, it will help our readers explain these

concepts to people in other fields and to businesspeople. Second,

today’s code division multiple access (CDMA) wireless technology is

built on a series of developments going back over 30 years. It is easy

to be expert in a system and not to know where it came from or to

have an in-depth knowledge of how it works. However, in

understanding the history of our field and the challenges faced by

our predecessors, we gain a deeper expertise, improving our ability to

handle the problems we face today.

Chapter 1, “Radio Engineering Concepts,” defines the

fundamentals of radio, including frequency, amplitude and power,

and modulation. It also includes explanations of multiple access and

modulation, a description of how a radio signal is altered by an

antenna and by the space between the transmitter and receiver, and

how we calculate signal power through those changes.

In Chapter 2, “Radio Signal Quality,” we discuss impairments to

the radio signal, such as noise, interference, distortion, and

multipath. Chapter 2 also covers the measurement of radio signals,

errors in those measurements, and the measurement of both analog

and digital radio signals.

Chapters 3 and 4 describe the components at the two sides of the

radio-air interface, the user terminal and the base station. Chapter 3,

“The User Terminal,” describes the components of a user terminal,

commonly known as a cell phone. Chapter 4, “The Base Station,”

describes the cellular base station: the antennas that receive the

signal through the tower and cable, the power amplifier, the receiver,

the components that transmit cellular signals, those which send

telephone calls through the link to the mobile switching center, and

the base-station controller that manages the operations of the base

station. There is also a discussion of component reliability modeling.

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Key Radio Concepts

Chapter 5, “Basic Wireless Telephony,” provides a picture of how

all the parts of a cellular network work together to create the wireless

signal path and how the whole cellular system is laid out, i.e., its

architecture.

In Chapters 6 and 7 we describe the early analog and digital

cellular radio technologies. In Chapter 6, “Analog Wireless

Telephony (AMPS),” we describe the original Advanced Mobile Phone

Service (AMPS) analog cellular technology that pioneered the

cellular architecture as the first radio system relying on managed

interference. In Chapter 7, “TDMA Wireless Telephony (GSM),” we

introduce the world’s first and largest digital cellular system,

Europe’s Global System for Mobility (GSM) time division multiple

access (TDMA) technology, which is serving about 700 million users

worldwide in 2002.

Having built a solid background in the fundamentals of radio and

the evolution of cellular telephony, we turn to code division multiple

access (CDMA) in Chapter 8. In Chapter 8, “The CDMA Principle,”

we discuss the underlying concept of CDMA, called spread spectrum,

the mathematical derivation of the CDMA method of managed same-

cell interference, and the principles of key CDMA components such as

the rake filter and power control. We also describe how CDMA

operates in both the forward and reverse directions and how it

performs handoffs as subscribers move from cell to cell.

The CDMA cellular networks our readers support embody both

concepts developed for the first analog cellular systems and also the

latest digital chipsets and technologies. With the background

provided in Part I, “Key Radio Concepts,” cellular engineers will be

well prepared to understand the latest CDMA technology so that we

can design and optimize today’s CDMA networks.

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Source: CDMA CAPACITY AND QUALITY OPTIMIZATION

Radio Engineering Concepts

Chapter

We all know what radio is, at least enough to get by. This chapter is for our readers who

came to cellular from landline telephony or information technology and for those who

want a refresher in the basics of radio engineering.

1.1 Radio

Radio is electromagnetic radiation, a changing electric field accompanied by a similarly

changing magnetic field that propagates at high speed, as illustrated in Fig. 1.1. A ra-

dio wave is transmitted by creating an electrical voltage in a conducting antenna, by

putting a metal object in the air and sending pulses of electricity that become radio

waves. Similarly, a radio signal is received by measuring electrical voltage changes in

an antenna, by putting another metal object in the air and detecting the very tiny

pulses of electricity generated by the varying electrical field of the radio waves.

The technology of radio transmission is developing the ability to transmit a radio

signal containing some desired information and developing a receiver to pick up just

that particular signal and to extract that desired information. One of the latest tech-

nologies to do this is code division multiple access (CDMA), a long way from the dit-dah

Morse code transmission of the earliest wireless equipment. Both Morse code and

CDMA, however, are digital radio technologies.

We use radio to get some kind of information, a signal, from one place to another us-

ing a radio wave. We put that signal onto the radio medium, the carrier we call it, with

some kind of scheme that we call modulation. The Morse code sender uses the simple

modulation scheme of a short transmission burst as a dit and a longer transmission

burst as a dah. The demodulation scheme does the reverse: The telegraph receiver

makes audible noise during a radio burst, and the listener hears short and long bursts

of noise as dits and dahs. Morse code is simple and elegant, and it used the technology

of its day efficiently.

All the components of the process of radio communications were already present in the

telegraph. There is a meaningful message to be sent that was coded into a specific for-

mat, the letters of the alphabet. The formatted message, the signal, is then modulated

by the telegraph operator into a radio message that is then demodulated into something

that looks like the original signal. The receiver restores the message’s meaning, reading

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Radio Engineering Concepts

Key Radio Concepts

Electric

Field

Magnetic

Field

Wave Motion

Figure 1.1 An electromagnetic radio wave.

the letters to form words. In this case, the formatted message is a sequence of letters, a

digital signal.

The meaningful message in telephony is primarily spoken voice. The formatting

stage is done with a microphone and amplifiers to form an electrical voltage over time

that represents the speech, an analog signal in this case. If this signal is fed into an

amplifier and a speaker, then we hear meaningful voice output.

1.2 Frequency

In addition to their magnitude, analog signals such as audio, electricity, and radio also

have the attribute of frequency. We all know frequency as the pitch of a sound or the

station numbers on a car radio, but frequency is a deep, basic, fundamental, primal

mathematical concept that deserves some attention.

The simplest view of frequency is that it is the number of waves that pass a given

point at a given time. In this simplified view, wavelength is in inverse ratio to fre-

quency, with the speed of transmission as the constant.

We measure frequency in cycles per second, or hertz (Hz). 1 Frequencies we use in real

life vary considerably. We have the very low 50 or 60 Hz of electrical power from the

wall outlet. Sound we hear is air pressure waves varying from 20 Hz to 20 kHz. 2 Our

AM radio stations operate from 500 kHz to 1.6 MHz, and FM and older broadcast

f .

2 While almost everybody reading this knows that kHz stands for kilohertz, 1000 hertz, some of

the other prefixes may be more obscure. The entire list is in the “Physical Units” section at the

end of this book.

for radian frequency values, where

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1 More mathematical texts often measure frequencies in radians per second and usually use the

Greek letter omega

Radio Engineering Concepts

television stations are in the very high frequency (VHF) band around 100 MHz. Air-

craft radios operate in the VHF band as well.

Electromagnetic radiation propagates at the speed of light c , which is about 3 10 10 cm/s.

Therefore, Advanced Mobile Phone Service (AMPS), the U.S. analog cellular system, at

900 MHz frequency, has a wavelength of about 33 cm, and the primary cdmaOne fre-

quency, 1.9 GHz, has a wavelength of about 16 cm. Wavelength is a major element in

determining the types of attentuation that we will need to manage. For example, in the

upper microwave bands used for satellite transmission, wavelength is a fraction of a

centimeter, and raindrops can cause attenuation. However, rain is not a problem for

cellular systems. Attenuation tends to occur when the intervening objects are of a size

about equal to one-half the transmission wavelength. 3 To put radio frequencies into

perspective, the visible red laser light used in fiber optic cable is around 500 THz,

500,000,000,000,000 cycles per second, with a wavelength of about 0.00006 cm.

In a sound wave, there is some atmospheric pressure at every instant of time, so we

can say that the atmospheric pressure is a function of time, and we can describe that

function as the time response of the sound wave. In our human experience, sound usu-

ally comes in periodic waves, and the number of waves per unit time determines the

pitch, the frequency of the sound. Normal sound is a mixture of many frequencies, and

its frequency response is often more informative than its time response. 4

A radio wave has a voltage at every instant of time, so its time response is voltage

rather than air pressure. Radio also is usually transmitted in periodic waves with an

associated frequency. As in the case of sound, radio waves usually contain many fre-

quencies, and their frequency response is important.

A function can be represented as f ( x ). In the case of electrical voltage over time, we

can represent the voltage v at each time t as v ( t ). The mathematical concept of a func-

tion tells us that there is one f ( x ) for each x or, in our electrical case, one specific volt-

age v ( t ) for each time t .

Fourier analysis tells us that we can think of the same v ( t ) in another form as V ( s ),

where s is one particular frequency rather than an instant of time. The function V ( s ) is

a little more complicated than v ( t ) because it contains not only the amplitude of fre-

quency s but also its phase. The relationship between time response v ( t ) and frequency

response V ( s ) is a pair of integrals from college calculus.

∞

V ( s )

v ( t ) e ist dt

(1.1)

t ∞

∞

v ( t )

V ( s ) e its ds

(1.2)

∞

3 The coauthor who lives in Texas notes that this could mean that 3-in hail interferes in the

CDMA band. Frankly, we’re a lot more concerned about equipment damage than about radio in-

terference when the hail is the size of tennis balls.

4 The audible difference between an oboe and a violin playing the same steady note B

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is in their

response at higher frequencies. Those higher frequencies are called harmonics. The audible dif-

ference between an oboe and a piano, on the other hand, is not only their frequency response but

the percussive time response of the piano.

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