Table Of ContentSource: CDMA CAPACITY AND QUALITY OPTIMIZATION
Part
1
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.
1
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Key Radio Concepts
2 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, “TDMAWireless 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 CDMAPrinciple,”
we discuss the underlying concept of CDMA, called spread spectrum,
the mathematical derivation of the CDMAmethod of managed same-
cell interference, and the principles of key CDMAcomponents 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 CDMAcellular 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 CDMAtechnology so that we
can design and optimize today’s CDMAnetworks.
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Source: CDMA CAPACITY AND QUALITY OPTIMIZATION
Chapter
1
Radio Engineering Concepts
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. Ara-
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 receivedby 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 carrierwe 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 demodulatedinto something
that looks like the original signal. The receiver restores the message’s meaning, reading
3
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Radio Engineering Concepts
4 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).1Frequencies 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.2Our
AM radio stations operate from 500 kHz to 1.6 MHz, and FM and older broadcast
1More mathematical texts often measure frequencies in radiansper second and usually use the
Greek letter omega (cid:1)for radian frequency values, where (cid:1)(cid:2)2(cid:3)f.
2While 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.
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Radio Engineering Concepts
Radio Engineering Concepts 5
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 (cid:4)1010cm/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 functionof time, and we can describe that
function as the time responseof 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
itsfrequency responseis often more informative than its time response.4
Aradio 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.
Afunction can be represented as f(x). In the case of electrical voltage over time, we
can represent the voltage vat each time tasv(t). The mathematical concept of a func-
tiontells us that there is one f(x) for each xor, in our electrical case, one specific volt-
agev(t) for each time t.
Fourier analysis tells us that we can think of the same v(t) in another form as V(s),
wheresis one particular frequencyrather 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-
quencysbut also its phase.The relationship between time response v(t) and frequency
responseV(s) is a pair of integrals from college calculus.
∞
(cid:1)
V(s)(cid:2) v(t)eistdt (1.1)
t(cid:2)(cid:5)∞
∞
(cid:1)
v(t)(cid:2) V(s)e(cid:5)itsds (1.2)
s(cid:2)(cid:5)∞
3The coauthor who lives in Texas notes that this could mean that 3-in hail interferes in the
CDMAband. Frankly, we’re a lot more concerned about equipment damage than about radio in-
terference when the hail is the size of tennis balls.
4The audible difference between an oboe and a violin playing the same steady note B(cid:1)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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Radio Engineering Concepts
6 Key Radio Concepts
While our time and frequency intervals in real life do not go from minus infinity ((cid:5)∞)
to plus infinity ((cid:6)∞), an important message from these two integrals is that frequency
response V(s) depends on the time response v(t) over an extended period of time and,
conversely, that the time response v(t) is determined by knowing enough about the fre-
quency response V(s). Equations (1.1) and (1.2) for time and frequency are nearly sym-
metric, and they tell us that there is a duality in the time-frequency relationship. Asig-
nalv(t) consisting of a single continuous unchanging wave v(t)(cid:2)sin(2(cid:3)ft) has only one
frequencyf, as shown in Fig. 1.2.
An important asymmetry in the time-frequency duality is the notion of phase shift.
Consider the waveforms shown in Fig. 1.3. In each case we have two frequencies, one
twice the other. However, the phase relationship between the two waves is different in
the two cases, and their pictures are quite different.
Frequency is often a more natural representation of our radio world than voltage
amplitude. To put this another way, it is often easy and natural to work with frequency
in radio system design. For example, we can build frequency filters that restrict a re-
ceiver to a certain range of frequency so that the received signal is not affected by ac-
tivity at other frequencies. This all seems very natural today, hardly worth going over,
but the core technology CDMA, the subject of this book, pushes radio technology very
hard and tests these basic ideas. Thus, understanding the fundamental concept of ra-
dio frequency is a prerequisite to having a thorough understanding of CDMA.
Technology has changed, and VHF has become an anachronistic acronym. The ultra-
high frequency (UHF) band is the upper hundreds of megahertz, and it was allocated
in the United States to television stations. As cable television has reduced the need for
70 UHF TV stations, the UHF band has been reallocated to other services, including
the first North American cellular telephone service.
Since then, the competition for the UHF band has become severe, and wireless
telephony has moved into the microwave band, above 1 GHz. It has been a constant
v (t) = sin(2πft)
t
Figure 1.2 Asine wave is just one frequency.
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Radio Engineering Concepts
Radio Engineering Concepts 7
volts
time
volts
time
Figure 1.3 Phase relationships.
challenge to design cost-effective radio transmitters and receivers in the microwave
band, and it becomes more difficult as frequencies get higher.
1.3 Multiple Access
In the very first days of radio, it sufficed to get a signal from here to there over the ra-
dio airwaves. We can imagine the listeners’excitement the first time they heard a live
voice carried across the ocean on a radio wave and received for their ears. We also can
imagine the desire to carry more than one radio signal. While radio link users can wait
their turns in an ordered sequence, radio is only really useful when many users can use
it simultaneously. The ability to send more than one signal at the same time is called
multiple access.
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Radio Engineering Concepts
8 Key Radio Concepts
Radio has been used for both broadcast and two-way communication. In broadcast, a
single signal is meant for a large community of receivers, whereas we typically picture
two-way radio as having two individual stations communicating with each other.
The usual picture of broadcast is a commercial radio station, but there are private
broadcast channels of distribution. Pagers are a form of broadcast radio; a single source
sends data over the airwaves for a large community of receivers.
The two-way walkie-talkie has evolved into sophisticated communications systems
used in aviation, trucking, railroads, police, and the wireless telephone systems of to-
day. Some systems often have one broadcast direction, a dispatcher talking to all the
taxicabs or an air traffic controller talking to all the airplanes, with individuals reply-
ing on a common frequency with no privacy.
Wireless telephone systems require another level of sophistication because they
manage separate two-way communication links in the same system. Unlike airline pi-
lots, wireless callers do not want to be bothered by other telephone conversations on
the same system. Wireless telephone users take their privacy seriously, and maintain-
ing separate and confidential calls is an important component of system design.
In the earliest days of radio, we used frequency to discriminate among radio signals,
and we called the system frequency division multiple access(FDMA). Each radio user
gets a frequency range, although there may be other users on other frequencies. Com-
mercial radio stations (both AM and FM) are assigned frequencies in their large geo-
graphic areas, and our receiver sets easily discriminate among the stations and allow
us to exclude all but the one to which we are listening. Like broadcast radio, the first
mobile telephone systems and, later, the first cellular systems were FDMA-based.
When the leap was made from analog to digital modulation described in Chap. 7, it
was more efficient to use a larger frequency band and to divide it up among several sig-
nals using time slots. The system is synchronized so that each receiver knows which
transmitted time slots belong to that receiver’s signal. This time division multiple ac-
cess(TDMA) is more complicated than FDMA, but it uses the radio frequency more ef-
ficiently. The Global System for Mobility (GSM), which started in Europe and became
a worldwide standard, is a TDMAsystem with eight time slots aggregated into a sin-
gle larger frequency band.
FDMAand TDMAhave some kind of absolute protection from other broadcasts on the
signal channels in the radio medium. Asingle frequency band or time slot gets minimal
interference from other frequency bands or time slots because at the exact moment of re-
ception, no other transmitter is broadcasting on the specified frequency. However, in the
world of spread spectrum,a single stream of radio is shared simultaneously. Code divi-
sion multiple access (CDMA), the subject of this book, is a spread-spectrum system that
transmits many signals in the same radio band at the same time.
Allow us to use our favorite analogy for explaining CDMA. Consider a few dozen peo-
ple in a small room all talking in pairs. Each listener knows his or her speaker’s voice
and can tune out the other voices that interfere with his or her own conversation. This
tuning-out ability has limitations and the ability of these listeners to understand their
speakers runs out if we have too many people talking at the same time. In CDMA, we
assign each digital stream its own distinct voice in the form of a digital code, and all of
these streams coexist on the same radio channel all at the same time.
Spread spectrum came from military research where resistance to enemy jamming
was the major design issue. There are several spread-spectrum technologies, and
CDMAhappens to be resistant not only to deliberate outside interference but also to
other users on the same channel.
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Radio Engineering Concepts
Radio Engineering Concepts 9
1.4 Bandwidth as Real Estate
We define radio territory by land area and frequency range. Atransmission license au-
thorizes its owner to transmit only within a specified geographic region and between
lower and upper frequency bounds set in the license. These licenses are regulated by
governments in just about every country in the world today. Here in the United States,
the Federal Communications Commission (FCC) gives out broadcast licenses based on
the service being offered and the technology being used. In the United States, it would
be fair to say that regulation of the radio spectrum is tighter and more restrictive than
mineral rights but looser and freer than airspace, which is controlled minute by minute
by the Federal Aviation Administration (FAA).
The allocation of radio frequency carries with it the obligation not to transmit on any
other frequency bands. More frequency bandwidth means more capacity, in the form
of more channels for FDMA and TDMA systems and more bits when using CDMA
technology.
We refer to radio frequency ranges as bandwidthorspectrum.The usual discussion
is about the frequency range, with the geographic coverage region assumed. There are
hot debates over where one region ends and a neighboring region begins, but we will
concentrate on the bandwidth issues in our CDMAdiscussions. Readers should keep in
mind, however, that negotiating with geographic neighbors for compatible service is
every bit as important as making sure radio transmission is contained within allocated
spectrum. We will discuss the technical issue of calls being served by different wireless
systems in Sec. 12.5.5.
As in any other acquisition, some frequencies are more desirable than others. Just as
the downtown real estate commands a premium in most big cities, so also, in radio,
lower frequencies are less demanding to operate. Lower-frequency amplifiers and an-
tennas are simpler, and radio coverage is broader at a given power level. The lower fre-
quencies are already firmly claimed by radio and television stations, police communi-
cations, and other long-established users.
In the early days of cellular, we were lucky enough to get radio spectrum allocations
in the 900-MHz band, the upper end of “the UHF wasteland.” We called it that because
cable television was clearly alleviating the need for 70 UHF TV stations. Even before
cable, most of us remember there being only a few U.S. TV stations numbered 14
through 83.
More recently, wireless has been pushed up into the microwave band. Much of this
push is a consequence of our own success. As the demand for wireless telephone service
has increased, we have become hungry for more bandwidth to satisfy that demand, and
that bandwidth is out there in the microwave band.
1.5 Amplitude and Power
The amplitude of a radio wave is the electromagnetic voltage level as it propagates
through space. Similarly, the amplitude of a sound wave is the pressure variation as it
propagates through the air or other medium. Alas, amplitude and power are different,
and this creates more than a little confusion.
Thepowerof an electromagnetic (or audio) event is the energy per unit time. We are
all familiar with power in units of watts or horsepower. In a car engine, for example, it
is the driving force or thrust being applied multiplied by the speed at which it is being
applied. In an electromagnetic event, power is the voltage multiplied by the magnetism
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Radio Engineering Concepts
10 Key Radio Concepts
or current. In an audio event, power is the air pressure multiplied by the air velocity.
In both radio and sound, this means that the power level is proportional to the square
of the amplitude.
To expand this idea a little more fully: In an electromagnetic wave, the amplitude is
the voltage, and the current is proportional to that voltage. Power is voltage times cur-
rent, so the power is proportional to the square of amplitude. In an audio event, am-
plitude affects both the air pressure and air velocity, so power is proportional to the
square of amplitude there as well.
Amplitude is measured in volts, which we almost never use in discussions of radio
systems, and power is measured in watts, which we use constantly.5
A typical base-station radio transmitter in our mobile telephone world has about
100 W of effective radiated power(ERP). Atypical broadcast radio or television (TV) sta-
tion might have 1 million W ERP, so our mobile radio stations are in the lightweight
division. The telephone itself is usually limited to 1 W, the bantamweight division.
However, this comparatively puny signal reliably gets hundreds of millions of calls
through every day.
An effective medium for sending information, radio is not an efficient medium
for sending energy. The mobile telephone signal transmitted at 1 W is typically
0.0000000000001 W, or 100 fW, at the base-station receiver. Losing 99.99999999999
percent of the energy sounds wasteful, but our receivers are able to demodulate this
tiny signal to recreate the signal modulated at the transmitting end. We can demodu-
late and understand such a weak signal by engineering our wireless systems so that
the other signals competing with it are even weaker.
The crucial issue in getting a signal through is not the amount of power; it is the
ratio between the power of the signal and the power of the noise or interference, the
signal-to-noise (S/N) ratio or the signal-to-interference(S/I) ratio.As long as this ratio
is high enough at the receiver, the amount of power received is irrelevant.
Since every electrical amplifier has its own noise, the receiver has some internal
noise level, and we want our signal to be stronger than the noise. Maximizing the S/N
ratio is the key to good radio design. Electrical engineers are designing superb low-
noise receivers, and it is the job of the wireless telephone system planners to get as
much signal and as little that is notour own signal to the receiver as possible.
1.6 Decibel Notation
The power ratios in radio are often huge. We broadcast 100 W of power only to have
10 fW get into the receiver. Throwing numbers like 10,000,000,000,000, 1013, around
gets tiresome and confusing. The Americans and British do not even agree on what to
call such a large number; it is called 10 trillionin the United States, but it is called 10
billionin England.
There is a more fundamental point, however. Ratios are often the essential matter,
and we need some notation for describing ratios as ratios and articulating the differ-
ences between ratios. Fortunately, the decibel scale does the job nicely.
Named after Alexander Graham Bell, the belis defined as a factor of 10, so 2 bels is
a factor of 100 and 3 bels is a factor of 1000. As shown in Table 1.1, bels add where the
5Asummary of units and notation is in the “Physical Units” section at the end of this book.
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