cdma capacity and quality optimization

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 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Source: CDMA CAPACITY AND QUALITY OPTIMIZATION 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. 2 Key Radio Concepts Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Key Radio Concepts 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. 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 3 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Source: CDMA CAPACITY AND QUALITY OPTIMIZATION 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 4 Key Radio Concepts Electric Field Magnetic Field Wave Motion Figure 1.1 An electromagnetic radio wave. 1 More mathematical texts often measure frequencies in radians per second and usually use the Greek letter omega ␻ for radian frequency values, where ␻ϭ2␲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. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. 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 ϫ 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) ϭ Ύ ∞ tϭϪ∞ v(t)e ist dt (1.1) v(t) ϭ Ύ ∞ sϭϪ∞ V(s)e Ϫits ds (1.2) Radio Engineering Concepts 5 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 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. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Radio Engineering Concepts While our time and frequency intervals in real life do not go from minus infinity (Ϫ∞) to plus infinity (ϩ∞), 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. A sig- nal v(t) consisting of a single continuous unchanging wave v(t) ϭ sin(2␲ft) has only one frequency f, 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 6 Key Radio Concepts t v (t) = sin(2πft) Figure 1.2 A sine wave is just one frequency. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Radio Engineering Concepts 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. Radio Engineering Concepts 7 volts volts time time Figure 1.3 Phase relationships. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Radio Engineering 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 TDMA system with eight time slots aggregated into a sin- gle larger frequency band. FDMA and TDMA have some kind of absolute protection from other broadcasts on the signal channels in the radio medium. A single 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 CDMA happens to be resistant not only to deliberate outside interference but also to other users on the same channel. 8 Key Radio Concepts Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Radio Engineering Concepts 1.4 Bandwidth as Real Estate We define radio territory by land area and frequency range. A transmission 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 bandwidth or spectrum. 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 CDMA discussions. 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. The power of 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 Radio Engineering Concepts 9 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Radio Engineering 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). A typical 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 not our 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, 10 13 , 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 trillion in the United States, but it is called 10 billion in 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 bel is 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 10 Key Radio Concepts 5 A summary of units and notation is in the “Physical Units” section at the end of this book. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Radio Engineering Concepts [...]... signal-to-noise (S/N) demands of the base-station receiver and the air interface While CDMA can demand high peak power levels in areas of poor reception or during busy periods, most of the time a CDMA channel is operating at a very low power level This gives CDMA an advantage over AMPS and GSM in terms of power consumption and battery recharge interval One way to prolong battery life and to reduce radio... In CDMA, we are trying to manage our power levels to within a few tenths of a decibel CDMA systems access S/I ratios many times per second and adjust power up or down according to the S/I ratio measurement This measurement can be wrong, too, and the effect on CDMA power control of these measurement errors is significant.9 The effect of these errors on CDMA capacity is discussed later in Secs 8.5 and. .. This can form local radio “hot spots” near lakes and rivers Because of the multiplicative nature of path gain reductions from terrain, buildings, and shrubbery, the statistical effect is a log-normal signal distribution, as described in Sec 1.6 Typical standard deviations are about 10 dB.9 Understanding radio propagation is essential to CDMA capacity and quality planning Chapter 47 goes into the subject... telephone is powered on Thus GSM and CDMA proscribe a time schedule for paging so that the mobile telephone can conserve more power using discontinuous reception (DRx) The transmitter and receiver share enough technology and components that it makes sense to form them into a single unit, a transceiver AMPS and CDMA require the 1 We have seen the SEND and END keys marked YES and NO on some portable user... Companies All rights reserved Any use is subject to the Terms of Use as given at the website Source: CDMA CAPACITY AND QUALITY OPTIMIZATION Chapter 2 Radio Signal Quality This chapter briefly defines the theoretical issues behind major challenges for cellular engineers We look at noise, interference, and related issues, as well as issues related to radio signal measurement The practical consequences... degrees, then the voltage sum is a power sum, and one plus one is two In Sec 1.10 we mentioned the in-phase I and quadrature Q components of a modulated signal, and these are 90 degrees out of phase with each other Thus, while the total power is their sum, the I and Q components are orthogonal to each other, and a receiver can discriminate between them and one plus one is one A few paragraphs cannot... is subject to the Terms of Use as given at the website Radio Signal Quality Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Source: CDMA CAPACITY AND QUALITY OPTIMIZATION Chapter 3 The User Terminal A user terminal may be defined... 8-ary, and 16QAM constellations 90 degrees vertically up on the Y axis, 180 degrees horizontally to the left, and 270 degrees vertically down Each point on the graph represents an amplitude and phase relation where digital information can be transmitted In radio we do not call the two dimensions x and y or real and imaginary as we do in mathematics class Rather, we call the same two axes in-phase and. .. the remainder of this book 2.1 Radio Impairments Between a modulated and transmitted signal at one end and the received and demodulated version at the other end, a lot of bad things can happen Since code division multiple access (CDMA) demands more of our radio links than most earlier technology, these impairments have to be understood and managed more carefully than before We can divide radio impairments... environment, broader bandwidth always offers more protection from multipath fading Resistance from multipath fading is a tremendous advantage for spread spectrum The 25- or 30-kHz channel of analog cellular is very susceptible, the 200-kHz channel of the Global System for Mobility (GSM) is more resistant, and the 1.25-MHz of cdmaOne is better still The 5-MHz or greater wideband CDMA is almost immune . 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 bandwidth. 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. website. Source: CDMA CAPACITY AND QUALITY OPTIMIZATION 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