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TABLE OF CONTENTS

CHAPTER ONE.....................................................................................................................................6

..................................................................................................................................................................6

INTRODUCTION....................................................................................................................................6

HOW WIRELESS WORKS ......................................................................................................... ........ .7

SPREADING THE SPECTRUM ..........................................................................................................................8SPREAD SPECTRUM : DOWN TO THE BITS .........................................................................................................9

INTRODUCTION TO SPREAD SPECTRUM..................................................................................10

HOW SPREAD SPECTRUM WORKS ...............................................................................................................12DETAILS ON SPREAD SPECTRUM .................................................................................................................12D IRECT SEQUENCE SYSTEMS ......................................................................................................................12FREQUENCY HOPPING SYSTEMS ...................................................................................................................12D IRECT SEQUENCE VS FREQUENCY HOPPING ..............................................................................................13WHAT SPREAD SPECTRUM DOES ................................................................................................................15

CHAPTER TWO ..................................................................................................................... ........ ....17

CODE DIVISION MULTIPLE ACCESS............................................................................... ........ ....17

THE CDMA STANDARDS..................................................................................................................17

CDMA S TANDARD : IS-95A ..................................................................................................................17

WHY CDMA..........................................................................................................................................17

BACKGROUND OF CDMA................................................................................................................18

CHAPTER THREE..............................................................................................................................20

POWER CONTROL IN CDMA..........................................................................................................20

THE NEAR -FAR PROBLEM .........................................................................................................................20POWER CONTROL TECHNIQUES (PCT).......................................................................................................21

Reverse Link Power Control ..........................................................................................................21Open Loop Control...................................................................................................................................21Closed Loop Control ................................................................................................................................21

Forward Link Power Control ........................................................................................................22 Reverse Outer loop Power Control................................................................................................22 Power Control in Soft Handoff.......................................................................................................22

POWER MANAGEMENT TECHNIQUES (PMT)...............................................................................................23 Forward Handoff Boundary...........................................................................................................23 Reverse Handoff Boundary ............................................................................................................23 Breathing........................................................................................................................................24Wilting.............................................................................................................................................24

Blossoming......................................................................................................................................24

CHAPTER FOUR.................................................................................................................................25

HANDOFF..............................................................................................................................................25

HANDOFFS IN CDMA.........................................................................................................................25

STEPS IN HANDOFF ..................................................................................................................................25SOFT HANDOFF ........................................................................................................................................26

Forward CDMA Channel...............................................................................................................27

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Reverse Power Control................................................................................................................. ..27 Reverse CDMA Channel............................................................................................................... ..27

I NITIATION OF SOFT HANDOFF ...................................................................................................................28 Pilot Search.....................................................................................................................................28 Detection Thresholds......................................................................................................................28

CASES WHERE CDMA DOES NOT USE SOFT HANDOFFS ................................................................................28

Inter Frequency Handoffs............................................................................................................. ..28Timing Changes..............................................................................................................................29

Digital-to-Analog............................................................................................................................29 Handoffs between FM and CDMA systems ...................................................................................29

"SOFTER" H ANDOFF .............................................................................................................................29

CHAPTER FIVE ...................................................................................................................... ......... ..30

CHANNELS...........................................................................................................................................30

CDMA CHANNELS............................................................................................................................. .30

CDMA F ORWARD CHANNELS ..................................................................................................................30 Pilot Channel..................................................................................................................................30Sync Channel..................................................................................................................................30

Paging Channel..............................................................................................................................30 Forward Traffic Channel................................................................................................................30

CDMA R EVERSE CHANNELS ....................................................................................................................31 Access Channel...............................................................................................................................31 Reverse Traffic Channel.................................................................................................................31

CHAPTER SIX.....................................................................................................................................32

CODES....................................................................................................................................................32

PN CODES.............................................................................................................................................32

FORWARD CDMA C HANNEL ....................................................................................................................33CHAPTER SEVEN.................................................................................................................................34

CAPACITY............................................................................................................................................34

CDMA CAPACITY...............................................................................................................................34

CDMA C APACITY I NCREASES .................................................................................................................34CDMA AND CELL R EUSE ........................................................................................................................34EB/NO AND I NTERFERENCE THRESHOLD .....................................................................................................35CDMA C APACITY IMPROVEMENTS ...........................................................................................................35BASIC CAPACITY CALCULATIONS - 3 S ECTOR AMPS TO 3 S ECTOR CDMA..................................................35FIRST CDMA C ARRIER ALLOCATION .......................................................................................................36

CAPACITY OF A CDMA NETWORK..............................................................................................36

SINGLE CELL CDMA C APACITY ..............................................................................................................37 Augmented Performance with CDMA............................................................................................38 Reverse Link Power Control in Multiple-Cell Systems..................................................................39

CAPACITY FOR MULTIPLE CELL CDMA.....................................................................................................39Multiple-Cell Forward Link Capacity with Power Allocation.......................................................40

CONCLUSIONS AND COMPARISONS ..............................................................................................................40

FACTORS INFLUENCING CAPACITY.......................................................................................... .40

VOICE ACTIVITY DETECTION .....................................................................................................................40CDMA P OWER CONTROL ........................................................................................................................40

COVERAGE VERSUS CAPACITY....................................................................................................41

CHAPTER EIGHT...............................................................................................................................42


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CDMA STEPS....................................................................................................................................... .42

MODULATION IN CDMA..................................................................................................................42

CORRELATION...................................................................................................................................42

DE-SPREADING AND DETECTION................................................................................................43

VOICE CODING.................................................................................................................................. .43

MULTIPATH AND CDMA................................................................................................................. .44

FADING DUE TO MULTIPATH .....................................................................................................................45EFFECTS OF FADING ................................................................................................................................45

CHAPTER NINE .......................................................................................................................... .......46

TIMING..................................................................................................................................................46

CDMA AND SYSTEM TIMING.........................................................................................................46

THE "C LOCK ".........................................................................................................................................46The system time scale......................................................................................................................46 The reference state of the system clock relative to system time......................................................46

HOW TO SET A CLOCK & FIND OUT THE TIME ...............................................................................................47 If you are a base station: GPS........................................................................................................47 If you are a mobile, listen to a base station....................................................................................47

PROCESS OF SYNCHRONIZATION TO SYSTEM TIME ........................................................................................47

CHAPTER TEN....................................................................................................................................48

PLANNING............................................................................................................................................48

RADIO PLANNING............................................................................................................................. .48OBJECTIVE .............................................................................................................................................49COVERAGE ..............................................................................................................................................49CAPACITY ...............................................................................................................................................49A NTENNA ...............................................................................................................................................49DRIVE TEST ............................................................................................................................................49OPTIMIZATION .........................................................................................................................................50

Network Parameters Optimization.................................................................................................50

CHAPTER ELEVEN............................................................................................................................51

RF FUNDAMENTALS OF PLANNING.............................................................................................51

THEORY.............................................................................................................................................51RADIO.................................................................................................................................................51SIDEBANDS ......................................................................................................................................51SPREAD SPECTRUM........................................................................................................................51DEMODULATION.............................................................................................................................52ANTENNA THEORY.........................................................................................................................520.6 FRESNEL ZONE..........................................................................................................................52SPACE ATTENUATION...................................................................................................................52DECIBELS..........................................................................................................................................52ANTENNA TYPES.............................................................................................................................53COAXIAL CABLE.............................................................................................................................53DATA QUALITY...............................................................................................................................53SYSTEM CALCULATIONS..............................................................................................................53SITE.....................................................................................................................................................53INSTALLATION TIPS.......................................................................................................................54TESTING.............................................................................................................................................54


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Unified Power Control, Error Correction Coding, and Scheduling for a CDMA Downlink System........................................................................................................................................................54

IMPLICATIONS FOR CELL PLANNING ...........................................................................................................55

CHAPTER TWELVE .............................................................................................................. ......... ..56

FUNDAMENTALS OF NETWORK ENGINEERING................................................................. ....56

NETWORK ENGINEERING WITHIN RF PLANNING............................................................... ..56

RF SERVICES ...........................................................................................................................................56SITE SELECTION & S ITE ACQUISITION .......................................................................................................56

Site Acquisition services ........................................................................................................ .......56 I NSTALLATION & N ETWORK OPTIMIZATION .................................................................................................56COMMUNICATIONS S ITE MANAGEMENT .......................................................................................................57SPECTRUM E NGINEERING & F REQUENCY MANAGEMENT ................................................................................57I NTERFERENCE A NALYSIS - IDENTIFYING & RESOLVING CONFLICT ....................................................................57

Propagation modeling....................................................................................................................57 RF C OVERAGE DESIGNS A ND CAPACITY A NALYSES ...................................................................................57RF D ESIGN STANDARDS & G UIDELINES .....................................................................................................58RF S IGNAL COVERAGE DESIGNS ..............................................................................................................58TRAFFIC CAPACITY PLANNING & D ESIGN ..................................................................................................58MOBILE SYSTEM FREQUENCY PLANNING & D ESIGN ....................................................................................58M ICROWAVE R ADIO SYSTEM PLANNING & D ESIGN .....................................................................................58

NETWORK & S WITCHING I NFRASTRUCTURE DESIGN .....................................................................................58 NETWORK I NTERCONNECTION .....................................................................................................................59 NETWORK SERVICES .................................................................................................................................59R ADIO FREQUENCY NETWORK PERFORMANCE ..............................................................................................59COMPREHENSIVE DESIGN SERVICES .............................................................................................................59COMPREHENSIVE RF O PTIMIZATION SERVICES ............................................................................................60

Prior to Drive Tests:.......................................................................................................................60 Initial Drive Tests:..........................................................................................................................60Cluster Drive Tests.........................................................................................................................60

K EY ELEMENTS IN DESIGNING CELLULAR ...................................................................................................60

CHAPTER THIRTEEN....................................................................................................................... .61

QUALITY OF SERVICE......................................................................................................................61

CHAPTER FOURTEEN...................................................................................................................... .63

NETWORK DEPLOYMENT AND PERFORMANCE....................................................................63

THE BASIC NETWORKING CONCEPTS:..................................................................................... .63

RF E NGINEERING CAPABILITIES .................................................................................................................63

A NALYSIS AND NETWORK IMPLEMENTATION ................................................................................................63 NETWORK OPTIMIZATION ..........................................................................................................................63 NETWORK PERFORMANCE AND QUALITY TESTING ........................................................................................63THE PLANNING TOOL ...............................................................................................................................64

PROBLEMS & THEIR SOLUTIONS IN THE WLL NETWORK.................................................65

THE STATIC AND DROPPED CALLS THAT ARE HAUNTING THE CUSTOMERS MAY BE COMING FROM THE CELL SITE . ......65SITE SELECTION . .....................................................................................................................................66SHIELDING . ............................................................................................................................................66OTHER RF P ROTECTION . .........................................................................................................................66STEPS FOR ABATEMENT ...........................................................................................................................66PRELIMINARY A NALYSIS . .........................................................................................................................66TEST MEASUREMENTS . ............................................................................................................................67

CONCLUSION ..........................................................................................................................................67CDMA IN WIRELESS LOCAL LOOP..............................................................................................68


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NETWORK DEPLOYMENT...............................................................................................................68

WLL SUBSCRIBER TERMINALS....................................................................................................69

WLL INTERFACES TO THE PSTN................................................................................................ ..69

MICROWAVE BACKHAUL CONNECTIVITY................................................................... ......... ..70

I NTER C ITY M ICROWAVE BACKHAUL ..........................................................................................................70I NTRA C ITY M ICROWAVE BACKHAUL .........................................................................................................70STEPS FOR TRANSMITTING I NTER C ITY .......................................................................................................70STEPS FOR R ECEIVING I NTER CITY .............................................................................................................71STEPS FOR D ISTRIBUTION OF E1 I N THE I NTERCITY M ICROWAVE BACKBONE .................................................71STEPS FOR TRANSMITTING I NTRA C ITY .......................................................................................................71STEP FOR R ECEIVING I NTRA C ITY :.............................................................................................................71STEPS FOR D ISTRIBUTION OF E1 W ITHIN THE CITY ....................................................................................71

CONCLUSION......................................................................................................................................71

TEN TOP ADVANTAGES USING CDMA...................................................................................... ..72


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CHAPTER ONE

INTRODUCTION

Designers and planners of the communication systems are often concerned with theefficiency with which the systems utilize the signal energy and bandwidth. In mostcommunication systems these are the most important issues. In some cases, it isnecessary for the system to resist external interference, to operate at low spectralenergy, to provide multiple access capability without external control and securechannel not accessible to the outsiders. Thus, it is sometimes unavoidable tosacrifice some of the efficiency in order to enhance these features. Spread spectrumtechniques allow accomplishing such objectives.

The theoretical aspects of using spread spectrum in a strong interferenceenvironment have been known for over forty years. It is only recently that practicalimplementations became feasible. In the beginning, the spread spectrum technologywas developed and used for military purposes and their implementations were tooexpensive for the commercial applications. New technological advancements such asVLSI, and advanced signal processing techniques made it possible to develop lessexpensive spread spectrum equipment for civilian use. Applications of thistechnology include cellular, wireless data transmission and satellite communications.All of the spread-spectrum systems have to satisfy two criteria:

1. The bandwidth of the transmitted signal must be greater than the transmitted signal

2. Transmitted bandwidth must be determined by some function that is independent of the message and is known to the receiver.

Bandwidth expansion in spread spectrum systems is achieved by using afunction that is independent of the message, thus it is more susceptible to whitenoise as opposed to other communication techniques, such as FM and PCM.Spread spectrum techniques have other applications that make it unique anduseful. These applications include:1. Anti-jam capability-particularly for narrow-band jamming.2. Interference rejection.3. Multiple-access capability.

4. Multi-path protection5. Covert operation or low probability of intercept (LPI)6. Secure communications.7. Improved spectral efficiency-in special circumstances8. Ranging

CDMA is a wireless communications technology that uses the principle of spreadspectrum communication. The intent of CDMA technology is to provide increasedbandwidth in a limited frequency system, but has also other advantages includingextended range and more secure communications. In a CDMA system, a narrow-band message signal is multiplied by a spreading signal, which is a pseudo-noisecode sequence that has a rate much greater than the data rate of the message.

CDMA uses these code sequences as a means of distinguishing between individualconversations. All users in the CDMA system use the same carrier frequency and


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may transmit simultaneously. In this document I will be discussing about CDMA indetail.

CDMA is a driving technology behind the rapidly advancing personal communicationsindustry. Because of its greater bandwidth, efficiency, and multiple accesscapabilities, CDMA is becoming a leading technology for relieving the spectrumcongestion caused by the explosion in popularity of cellular mobile phones, fixedwireless telephones, and wireless data terminals. Since becoming an officiallyrecognized digital cellular protocol, CDMA is being rapidly implemented in thewireless communications networks of many large communications corporations.

CDMA stands for "Code Division Multiple Access." It is a form of spread-spectrum,an advanced digital wireless transmission technique. Instead of using frequencies or time slots, as do traditional technologies, it uses mathematical codes to transmit anddistinguish between multiple wireless conversations. Its bandwidth is much wider than that required for simple point-to-point communications at the same data ratebecause it uses noise-like carrier waves to spread the information contained in a

signal of interest over a much greater bandwidth. However, because theconversations taking place are distinguished by digital codes, many users can sharethe same bandwidth simultaneously. The advanced methods used in commercialCDMA technology improve capacity, coverage and voice quality, leading to a newgeneration of wireless networks.

Old-fashioned radio receivers separate stations and channels by filtering in thefrequency domain. CDMA receivers, conversely, separate communication channelsby a pseudo-random modulation that is applied and removed in the digital domain.Multiple users can therefore occupy the same frequency band. This universalfrequency reuse is crucial to CDMA's distinguishing high spectral efficiency. CDMAhas gained international acceptance by cellular radio system operators as anupgrade because of its universal frequency reuse and noise-like characteristics.CDMA systems provide operators and subscribers with significant advantages over analog and conventional TDMA-based systems.

How Wireless Works

Before we start talking about the CDMA we shall be familiar with the wireless. Whena cellular mobile is switched on it scans the group of control channels to determinethe strongest base station signal. Control channels are only involved in setting up acall and moving it to an unused channel. When a telephone call is placed, signal issent to the base station. The mobile switching center (MSC) dispatches the requestto all base stations in the cellular system. The mobile identification number (MIN),which is the subscriber's telephone number, is then broadcast as a paging messageto the forward control channels throughout the cellular system. The mobile receivesthe page, and identifies itself through the reverse control channel. The base station of the mobile informs the MSC of the "handshake", and the MSC instructs the basestation to move the call to an unused channel. All of these events happen within afew seconds that are unnoticeable by the users.

Wireless technology uses individual radio frequencies repeatedly by dividing aservice area into separate geographic zones called cells. Cells can be as small as anindividual building, such as an office, or as large as 20 miles across. Each cell mustbe equipped with its own radio transmitter/receiver antenna. Because the systemoperates at such low power, a frequency being used to carry a telephoneconversation in one cell can be used to carry a conversation in a nearby cell withoutinterference. (This allows much greater capacity than radio systems like Citizens


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Band (CB) in which all users contend to get their messages on the same limitedchannels.)

When a customer using a wireless phone - car phone or portable - approaches theboundary of one cell, the wireless network senses that the signal is becoming weakand automatically hands off the call to the antenna in the next cell into which thecaller is traveling. This process is known as a "handoff". Roaming occurs whensubscribers travel beyond their home geographical area. The wireless carrier in thearea where they are traveling provides service so they can still make calls.

Spreading the Spectrum

Spread spectrum multiple access transmits the entire signal over a bandwidth that ismuch greater than that required for standard narrow band transmissions in order togain signal-to-noise (S/N) performance. In channels with narrow-band noise,increasing the transmitted signal bandwidth results in an increased probability thatthe received information will be correct. Because each signal is an assembly of manysmaller signals at the fundamental frequency and its harmonics, increasing thefrequency results in a more accurate reconstruction of the original signal. Theeffective drawback of narrow-band data communications is the limitation of bandwidth; thus signals must be transmitted with enough power so the corruption bygaussian noise is not as effective and the probability that the data received is correctwill remain low. This means that the effective SNR must be high enough so that thereceiver should have no problem in recovering the transmitted code without error.

From a system viewpoint, the performance increase for very wideband systems isreferred to as "process gain". This term is used to describe the received signal fidelitygained at the cost of bandwidth. Errors introduced by a noisy channel can bereduced to any desired level without sacrificing the rate of information transfer usingClaude Shannon's equation describing channel capacity:

C =W log2 (1+S/N)

Where C = Channel capacity in bits per second, W = Bandwidth, S/N = Energy per bit/Noise power.

The benefits of increasing bandwidth become more clear. The S/N ratio may bedecreased without decreasing the bit error rate. This means that the signal may be

spread over a large bandwidth with smaller spectral power levels and still achieve therequired data rate. If the total signal power is interpreted as the area under thespectral density curve, then signals with equivalent total power may have either alarge signal power concentrated in a small bandwidth or a small signal power spreadover a large bandwidth.

A CDMA spread spectrum signal is created by modulating the radio frequency signalwith a spreading sequence (a code consisting of a series of binary pulses) known asa pseudo-noise (PN) digital signal because they make the signal appear wide bandand "noise like". The PN code runs at a higher rate than the RF signal anddetermines the actual transmission bandwidth. Messages can be encoded to anylevel of secrecy desired with direct sequencing, as the entire transmitted/received

message is purely digital.


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An SS receiver uses a locally generated replica of pseudo noise (PN) code and areceiver correlator to separate only the desired coded information from all thepossible signals. An SS correlator can be thought of as a specially matched filter -- itresponds only to signals that are encoded with a pseudo noise code that matches itsown code. Thus an SS correlator (SS signal demodulator) can be "tuned" to differentcodes simply by changing its local code. This correlator does not respond tomanmade, natural or artificial noise or interference. It responds only to SS signalswith identical matched signal characteristics and encoded with the identical PN code.

Many spread spectrum radios can share the same frequency band, if each systemuses a unique spreading code to reduce interference between the different radios.Because only the receiver with the identical code can de spread the signal to recover the signal, SS radios can tolerate a high level of interference unlike conventionalradios, providing much greater capacity increase in frequency reuse. SSMA is notvery bandwidth efficient when used by a single user. However, since many users canshare the same spread spectrum bandwidth without interfering with one another, SSsystems become bandwidth efficient in multiple user environments. This reason

makes SS communication an ideal choice for metropolitan areas with large blockingrates. Frequency reuse is universal, that is, multiple users utilize each CDMA carrier frequency. The reuse pattern is N=1.

The spread of energy over a wide band, or lower spectral power density, makes SSsignals less likely to interfere with narrow band communications, because thespreaded signal power is near that of gaussian noise levels. Narrow bandcommunications, conversely, cause little to no interference to SS systems becausethe correlation receiver effectively integrates over a very wide bandwidth to recover an SS signal. The correlator then "spreads" out a narrow band interferer over thereceiver's total detection bandwidth.

Spread Spectrum: down to the bits

CDMA technology focuses primarily on the "direct sequence" method of spreadspectrum. Direct sequence is spread spectrum technique in which the bandwidth of asignal is increased by artificially increasing the bit data rate. This is done by breakingeach bit into a number of sub-bits called "chips". Assuming this number is 10, eachbit of the original signal would be divided up into 10 separate bits, or "chips." Thisresults in an increase in the data rate by 10. By increasing the data rate by 10, wealso increase the bandwidth by 10.

The signal is divided up into smaller bits by multiplying it by a Pseudo-Noise code,PN-code. A PN-code is a sequence of high data rate bits ("chips") ranging from -1 to1 (polar) or 0 to 1 (non-polar). When referring to the number of "chips" used, wemean the number of small data bits in the PN-code per single bit of the originalsignal. Simply by multiplying the original modulated signal by this high data rate PN-code will result in dividing the signal into smaller bits, and hence, increase itsbandwidth.

The greater number of "chips" used results in a wider bandwidth proportional to thenumber of "chips".

The basic operation of the transmitter and receiver for spread spectrum will now be

described briefly. Let's assume there are two transmitters with two differentmessages to be transmitted. We should keep in mind that each transmitter can be


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thought of as separate cell phones. The messages M1(t) and M2(t) first go through amodulator to modulate the message at a higher carrier frequency. For spreadspectrum, all messages are modulated on the same carrier frequency. The output for each of the modulators is S1(t) and S2(t). After the modulator, each signal ismultiplied by its own unique Pseudo-Noise code, C1(t) and C2(t). These are the highdata rate bit patterns which spreads the signal's bandwidth. For this example, let'sassume the range values for the PN-code is -1 and 1. After spreading the bandwidth,each signal is transmitted. Because many signals can be transmitted from differenttransmitters at the same time, we represent these transmissions by simply summingtheir spectrums.

At the receiver end, the incoming signal is the spread spectrum signal. In order for areceiver to extract a single message, it must multiply the incoming signal by thecorrect PN-code. Because we chose the PN-code to range from -1 to 1, thistechnique of multiplying by the PN-code works perfectly. Since the original signal atthe transmitter end was multiplied by the PN-code, and again multiplied by the samePN-code at the receiver end, we effectively canceled out the PN-code for that

particular message.By eliminating the PN-code, we eliminate the spread spectrum effects for thatparticular message signal. The receiver circuit that does this is called a correlator,and it collapses the spread signal back down to just the original narrow bandwidthcentered at the modulated carrier frequency. The resulting signal is then passedthrough a band pass filter (BPF) centered at the carrier frequency. This operationselects only the desired signal while rejecting all surrounding frequencies due toother messages in the spread spectrum. This rejection is known as the processinggain of the de spreading correlation process. Lastly, the desired signal isdemodulated to eliminate the carrier frequency.

Processing gain is a direct consequence of the direct sequence radio signalspreading and de spreading process. It refers to the increase in signal-to-noise ratiothat results from this process, and is required for successful data communications.Processing gain increases as the number of chips per data bit increases, and thiscan be manipulated by the system designer to get the desired effect.

Introduction to Spread Spectrum

The basic characteristics of spread spectrum system as follows:1. The carrier is an unpredictable, or pseudorandom, wideband signal.2. The bandwidth of the carrier is much wider than the bandwidth of the data

modulation.3. Reception is accomplished by cross correlation of the received wide-band signal

with a synchronously generated replica of the wide-band carrier.In case of spread spectrum (SS) systems, if a signal is called pseudorandom, itmeans that it appears to be random but in fact the information is contained within it.One of the most important features of the SS signal is that it contains large number of very different signaling formats used for communicating data symbols. It means thatthe receiver which detects one of these formats cannot detect any other format withina single message. The number of formats used in an SS system is called themultiplicity factor of the communication link. Most of the well known communicationsystems, have a multiplicity factor near unity while SS systems have multiplicityfactors in the thousands. Thus, it can be seen that a jammer attempting to interferewith SS communication has to know exactly which signaling factors are being used,which is not very likely considering the size of SS multiplicity factor, or the jammer


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has to reduce significantly his power per each signaling format by jamming all of theformats.Scholtz recognizes at least five important performance attributes of SS systemswhich are due to the nature of their signal characteristics:1. Low probability of intercept (LPI) can be achieved with high processing gain and

unpredictable carrier signals when power is spread thinly and uniformly in thefrequency domain, making detection against noise by the survailance receiver difficult. A low probability of position fix (LPPF) attribute goes one step further inincluding both intercept and direction finding (DFing) in its evaluation. Lowprobability of signal exploitation (LPSE) may include additional effects, e.g.,source identification, in addition to intercept and DFing.

2. Anti jam (AJ) capability can be secured with an unpredictable carrier signal. The jammer cannot use signal observations to improve its performance in this case,and must rely on jamming techniques which are independent of the signal to be

jammed.3. High time resolution is attained by the correlation detection of wide-band signals.

Differences in the time of arrival (TOA) of the wide-band signal, on the order of the

reciprocal of the signal bandwidth, are detectable. This property can be used tosuppress multipath and, by the same token, to render repeater jammersineffective.

4. Transmitter-receiver pairs using independent random carriers can operate in thesame bandwidth with minimal cochannel interference. These systems are calledspread-spectrum code-division multiple-access (CDMA) systems.

5. Cryptographic capabilities result when the data modulation cannot bedistinguished from the carrier modulation, and the carrier modulation is effectivelyrandom to an unwanted observer. In this case the SS carrier modulation takes onthe role of a key in a cipher system. A system using indistinguishable data and SScarrier modulation is a form of privacy system.

There are three basic configurations used for recovery of the SS carrier:1. Transmitted reference (TR) system achieves detection by transmitting two

versions of the carrier, one modulated by data and other unmodulated. These twosignals enter a correlation detector which extracts the message.

2. In a stored reference (SR) system, both receiver and transmitter keep a 'copy' of the same pseudorandom signal. Carrier generator at the receiver is adjustedautomatically in order to synchronize its output with the arriving carrier. Detectionis then similar to TR system.

3. Matched filtering can also be used for reception of SS signals. Filter systemsproduce a wide-band, pseudorandom impulse response. Matched filter with suchresponse is used at the receiver in order to recover transmitted signal.Pseudorandom characteristic of the impulse response ensures security of thetransmitted signal.

Another way to classify the SS system is by the modulation technique used togenerate the SS signals. Some of the techniques are listed below:

1. In early developments of SS techniques, pure noise was used for a signal carrier.This technique gave superior randomness but could be accomplished only by TRsystem. There is a chance though that the jammer can gain access to bothchannels thus reducing the multiplicity factor to 1 (no anti-jamming capability).

"Spread-spectrum radio communications, long a favorite technology of the militarybecause it resists jamming and is hard for an enemy to intercept, is now on the vergeof potentially explosive commercial development. The reason: spread-spectrumsignals, which are distributed over a wide range of frequencies and then collected


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onto their original frequency at the receiver, are so inconspicuous as to be'transparent.' Just as they are unlikely to be intercepted by a military opponent, soare they unlikely to interfere with other signals intended for business and consumer users -- even ones transmitted on the same frequencies. Such an advantage opensup crowded frequency spectra to vastly expanded use.

How Spread Spectrum Works

them more noise-like.

Spread Spectrum signals use fast codes that run many times the informationbandwidth or data rate. These special "Spreading" codes are called "PseudoRandom" or "Pseudo Noise" codes. They are called "Pseudo" because they are notreal gaussian noise.

Spread Spectrum transmitters use similar transmit power levels to narrow band

transmitters. Because Spread Spectrum signals are so wide, they transmit at a muchlower spectral power density, measured in Watts per Hertz, than narrowbandtransmitters. This lower transmitted power density characteristic gives spread signalsa big plus. Spread and narrow band signals can occupy the same band, with little or no interference. This capability is the main reason for all the interest in SpreadSpectrum today.

Details on Spread Spectrum

Over the last 50 years, a class of modulation techniques usually called "SpreadSpectrum," has been developed. This group of modulation techniques ischaracterized by its wide frequency spectra. The modulated output signals occupy amuch greater bandwidth than the signal's baseband information bandwidth. Toqualify as a spread spectrum signal, two criteria should be met:1. The transmitted signal bandwidth is much greater than the information bandwidth.2. Some function other than the information being transmitted is employed to

determine the resultant transmitted bandwidth.

Direct sequence systemsDirect sequence spread spectrum systems are so called because they employ a highspeed code sequence, along with the basic information being sent, to modulate their RF carrier. The high speed code sequence is used directly to modulate the carrier,thereby directly setting the transmitted RF bandwidth. Binary code sequences as

short as 11 bits or as long as [2^(89) - 1] have been employed for this purpose, atcode rates from under a bit per second to several hundred megabits per second.The result of modulating an RF carrier with such a code sequence is to produce asignal centered at the carrier frequency, direct sequence modulated spread spectrumwith a (sin x/x)2 frequency spectrum. The main lobe of this spectrum has abandwidth twice the clock rate of the modulating code, from null to null. Thesidelobes have a null to null bandwidth equal to the code's clock rate. Directsequence spectra vary somewhat in spectral shape depending upon the actualcarrier and data modulation used.A binary phase shift keyed (BPSK) signal is themost common modulation signal type used in direct sequence systems.

Frequency hopping systems


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The wideband frequency spectrum desired is generated in a different manner in afrequency hopping system. It does just what its name implies. That is, it "hops" fromfrequency to frequency over a wide band. The specific order in which frequencies areoccupied is a function of a code sequence, and the rate of hopping from onefrequency to another is a function of the information rate. The transmitted spectrumof a frequency hopping signal is quite different from that of a direct sequence system.Instead of a [(sin x)/x]^2-shaped envelope, the frequency hopper's output is flat over the band of frequencies used. The bandwidth of a frequency hopping signal is simplyw times the number of frequency slots available, where w is the bandwidth of eachhop channel.

Direct Sequence Vs Frequency Hopping

The nature of radio signals used for data transmissions RF portable data collectionsystems rely on radio waves to transmit information to a remote computer or wirednetwork interface. Most of these portable handheld computers rely on radiotransmissions in the 2.4 GHz band the ETSI has established for unlicensed operation

of spread spectrum radios.Radio signals consist of a carrier frequency to which information (audio, video, or digital data) is added in a process called modulation. Modulation creates sidebandson both sides of the carrier frequency, and it is these sidebands that carry theinformation to be transmitted.

Radio receivers use demodulators to get rid of the carrier frequency and extract thedesired information. The person operating the receiver needs to know whatfrequency to tune into (the carrier frequency) before the receiver can do thedemodulation. Also, the receiver must be using the correct method of demodulationthat corresponds to the method used by the transmitter. This avoids interferencebetween adjacent carrier frequencies, and maintains a degree of privacy.

Spread spectrum radio Spread spectrum transmitters maintain user privacy andavoid interference by the way they encode their frequency signals. They purposelyspread their signals across a very large band of frequencies, and rely on the fact thatothers in that band are doing the same. Each receiver must know what spreadingpattern or code the transmitter is using in order to decode the signals being sent.

Because the ETSI allows very wide bandwidths for spread spectrum radiotransmissions, very high data rates are possible. Typically, the bit rates are in therange of 200 Kbps to 2Mbps. One issue associated with spread spectrum radios isreceiver signal-to-noise as compared to a narrow-band transmission. Reducedsignal-to-noise of a spread spectrum signal is regained by the use of a despreadingprocess in the receiver that boosts the level of the despread signal. This is calledprocessing gain. Range and coverage are often increased by using a network of repeaters that receive, amplify, and retransmit the signal.

The two most popular methods of encoding spread spectrum signals are calledDirect Dequence (DS) spread, and Frequency Hopping (FH).

Frequency Hopping (FH) In FH systems, the radio transmitter hops from one carrier frequency to another at a specific hopping rate, in a specific sequence that appearsto be a random pattern. Each carrier frequency and its associated sidebands must

stay within the channel width defined by the ETSI. If only the intended receiver knowsthe transmitter's hopping pattern, then only that receiver can follow the transmission.


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Other FH transmitters will be using different patterns, which usually will be on noninterfering frequencies. There is a communications protocol that transmitters andreceivers employ to cover those instances when two different transmitters attempt touse the same frequency simultaneously. In those cases the data is retransmitted onthe next hopping frequency.

The ETSI allows FH systems to define their own channel spacing up to a maximum 1MHz bandwidth in the 2.4 GHz band. There also are ETSI requirements on theamount of time the transmitter can spend on any one channel, and the number of channels that must be used. This is done to avoid "collisions" between differenttransmitters.

Direct Sequence Spread Rather than hop around the band, DS spread radiosbroaden the bandwidth of their transmissions by artificially increasing the data bitrate. This is done by breaking each bit into 10 or more sub-bits called "chips". For example, if 10 chips are used, the apparent data rate and resulting bandwidth alsoare increased proportionally.

A spread spectrum transmitter with a unique spread code cannot create the exactsame side-bands (spectral lines) as another transmitter using a different code. Areceiver having the same despreading code as the transmitter can extractinformation from a DS spread signal. The receiver circuit that does this is called acorrelator, and it collapses the spread signal back down to just the data side bands.This is done by matching the proper spread code to the received spread signal, andthus removing the effects of chipping. The resulting signal is demodulated to extractthe data.

Removing the chipping also allows the modulated signal to be filtered at a narrowbandwidth. Filtering helps reject interference from other transmitters. This rejection isknown as the processing gain of the despreading correlation process.

Processing gain is a direct consequence of the direct sequence radio signalspreading and despreading process. It refers to the increase in signal-to-noise ratiothat results from this process, and is required for successful data communications.Processing gain increases as the number of chips per data bit increases, and thiscan be manipulated by the system designer to get the desired effect.

Performance Comparison Frequency hopped signals will generally have better adjacent channel selectivity compared to DS spread signals. But FH radios must hopthrough 50 channels. The ETSI requires this to keep spectrum usage uniform andrandom. Selective use of channels is not allowed in frequency hopping. DS radiousers have the freedom of selecting the channels that have the least amount of trafficand interference in their area. This usually results in better overall reliability.

It is sometimes argued that DS spreading results in a weaker signal-to-noise ratiothan the narrower FH signals. The logic is that DS spreading lowers the signal power at any one frequency. However, the processing gain of the despreading correlator regains the apparent loss in power when the correlator signal is collapsed back downto the data bandwidth. In reality, DS spread signals can actually be received andcorrelated even when they are lower than the accompanying noise on the channel.

DS spread radios also offer the opportunity for better power management than FHradios. ADS radio can more easily rely on the wireless network access points todetermine when it can shut down to conserve power. FH systems are forced to stayon more of the time because of the need to constantly synchronize their hopping


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sequence with that of the RF network access points. Therefore, battery life ispotentially longer with DS spread radios than it is with their FH counterparts.

This document provides an in-depth treatment of the important concepts for architecting, analyzing, developing, and implementing efficient, secure CDMAnetworks. CDMA is an attractive technique for wireless access to broadband servicesand has emerged as the leading technology for today's new mobile communicationssystems. For CDMA Cellular Mobile I have tried to provide some guidelines toplanning, designing, and securing the efficient CDMA cellular systems.

The main objective of the document is to learn all the fundamentals to intensivesystem concepts and innovative implementation techniques for CDMA spreadspectrum multiple access communications.

Understand the techniques for encoding, repeating, interleaving, modulation,spreading, filtering and QPSK transmission that make CDMA possible. Then, take alook at each CDMA code channel. Walk through the design issues surrounding the

CDMA forward channel, including its pilot, sync, paging, and forward traffic codechannels. Examines the access and reverse traffic code channels that make up theCDMA reverse channel. Then go through the fundamentals of cellular systemplanning, the aspects and the does and don'ts while planning a network.

What Spread Spectrum Does

The use of these special pseudo noise codes in spread spectrum (SS)communications makes signals appear wide band and noise-like. It is this verycharacteristic that makes SS signals possess the quality of Low Probability of Intercept. SS signals are hard to detect on narrow band equipment because thesignal's energy is spread over a bandwidth of maybe 100 times the informationbandwidth.The spread of energy over a wide band, or lower spectral power density, makes SSsignals less likely to interfere with narrow band communications. Narrow bandcommunications, conversely, cause little to no interference to SS systems becausethe correlation receiver effectively integrates over a very wide bandwidth to recover an SS signal. The correlator then "spreads" out a narrow band interferer over thereceiver's total detection bandwidth. Since the total integrated signal density or SNRat the correlator's input determines whether there will be interference or not. All SSsystems have a threshold or tolerance level of interference beyond which usefulcommunication ceases. This tolerance or threshold is related to the SS processinggain. Processing gain is essentially the ratio of the RF bandwidth to the informationbandwidth.

A typical commercial direct sequence radio, might have a processing gain of from 11to 16 dB, depending on data rate. It can tolerate total jammer power levels of from 0to 5 dB stronger than the desired signal. Yes, the system can work at negative SNRin the RF bandwidth. Because of the processing gain of the receiver's correlator, thesystem functions at positive SNR on the baseband data.Besides being hard to intercept and jam, spread spectrum signals are hard to exploitor spoof. Signal exploitation is the ability of an enemy (or a non-network member) tolisten in to a network and use information from the network without being a validnetwork member or participant. Spoofing is the act of falsely or maliciouslyintroducing misleading or false traffic or messages to a network. SS signals also are

naturally more secure than narrowband radio communications. Thus SS signals canbe made to have any degree of message privacy that is desired. Messages can also,


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be cryptographically encoded to any level of secrecy desired. The very nature of SSallows military or intelligence levels of privacy and security to be had with minimalcomplexity. While these characteristics may not be very important to everydaybusiness and LAN (local area network) needs, these features are important tounderstand.


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CHAPTER TWOCODE DIVISION MULTIPLE ACCESS

The CDMA Standards

With CDMA, unique digital codes, rather than separate RF frequencies or channels,are used to differentiate subscribers. The codes are shared by both the mobilestation (cellular phone) and the base station, and are called "pseudo-Random CodeSequences." All users share the same range of radio spectrum.

For cellular telephony, CDMA is a digital multiple access technique specified by theTelecommunications Industry Association (TIA) as "IS-95."

In March 1992, the TIA established the TR-45.5 subcommittee with the charter of developing a spread-spectrum digital cellular standard. In July of 1993, the TIA gaveits approval of the CDMA IS-95 standard.

IS-95 systems divide the radio spectrum into carriers which are 1,250 kHz (1.25MHz) wide. One of the unique aspects of CDMA is that while there are certainly limitsto the number of phone calls that can be handled by a carrier, this is not a fixednumber. Rather, the capacity of the system will be dependent on a number of different factors. "This will be discussed in later sections."

There are primarily two standards for CDMA as described below. These standardsensure that a mobile station can obtain service in any cellular system manufacturedaccording to this standard. However, neither one of the following two standardsaddress the quality or reliability of the service.

CDMA Standard: IS-95A

IS-95A defines a compatibility standard for wideband spread spectrum cellular mobiletelecommunications (800 MHz band). IS-95 was first published in July, 1993 and IS-95A revision was published in May, 1995.It describes the Generation of channels, Power Control, Call Processing, Handoff,Compatibility (Radio Interface and Call Processing Protocols are specified to ensurethis), and Registration techniques for cellular system operations. The subscriber stations have more compatibility requirements than the base stations.

The other one is for the CDMA PCS J- STD 008(1800 MHz band) and is notdescribed here in this document.

Why CDMA

The CDMA scheme was developed mainly to increase capacity. The development of digital cellular systems for increasing capacity came just as the analog cellular system faced a capacity limitation in 1987. In theory, it does not matter whether thespectrum is divided into frequencies, time slots, or codes; the capacity provided fromthese three multiple schemes are same.However in cellular systems we might that one is better suited in certaincommunication media than another. Code division multiple access (CDMA) is adigital air interface standard, claiming eight to fifteen times the capacity of analog. It


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employs a commercial adaptation of military spread-spectrum single-sidebandtechnology. Based on spread spectrum theory, it is essentially the same as wirelineservice -- the primary difference is that access to the local exchange carrier (LEC) isprovided via wireless phone.Because users are isolated by code, they can share the same carrier frequency,eliminating the frequency reuse problem encountered in AMPS and DAMPS. EveryCDMA cell site can use the same 1.25 MHz band, so with respect to clusters, n = 1.This greatly simplifies frequency planning in a fully CDMA environment.CDMA is an interference limited system. Unlike AMPS/TDMA, CDMA has a softcapacity limit; however, each user is a noise source on the shared channel and thenoise contributed by users accumulates. This creates a practical limit to how manyusers a system will sustain. Mobiles that transmit excessive power increaseinterference to other mobiles. For CDMA, precise power control of mobiles is criticalin maximizing the system's capacity and increasing battery life of the mobiles. Thegoal is to keep each mobile at the absolute minimum power level that is necessary toensure acceptable service quality. Ideally, the power received at the base stationfrom each mobile should be the same (minimum signal to interference).

The main advantages of CDMA are as follows:1. Increased capacity2. Improved voice quality, eliminating the audible effects of multipath fading3. Enhanced privacy and security4. Improved coverage characteristics which reduce the number of cell sites5. Simplified system planning reduces deployment and operating costs6. Reduced average transmitted power, thus increasing talk time for portable

devices7. Reduced interference to other electronic devices8. Reduction in the number of calls dropped due to handoff failures9. Development of a reliable transport mechanism for wireless data communications10. Coexistence with previous technologies, due to CDMA and analog operating intwo spectras with no interference

Background of CDMA

To understand why there is a demand for CDMA, it is necessary to understand thetechnology that existed prior to its introduction and to know the background behindprevious spread-spectrum systems. Spread spectrum communications have beenused for encrypting military communication for many years. Its strengths in themilitary arena lie in its ability to resist enemy jamming and to provide securecommunications. It is difficult to interfere with or intercept a CDMA signal because of its use of a spread signal. The great attraction of CDMA technology from thebeginning was its inherent ability to boost communications capacity and reusefrequencies to a degree unheard of in narrowband multiple access wirelesstechnology. Its civilian mobile radio application was proposed theoretically in the late1940's, but its practical application in the market did not take place until 40 yearslater due to the many technical obstacles that still needed to be overcome. Theviability of CDMA technology was dismissed by TDMA (Time Division MultipleAccess) supporters as a technology that worked fine in theory but would never workin practice. The rapid development of high density digital ICs, however, combinedwith the realization that regulating all transmitter powers to the lowest level requiredfor a link would achieve optimal multiple access communication, allowed CDMA tomaterialize as a working technology. In 1991, the promising results of the first fieldtrials demonstrated that CDMA could work as well in practice as it did in theory.Commercial CDMA was introduced, tested, standardized, and initially deployed inless then seven years, a relatively rapid maturation cycle compared to other


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technologies such as TDMA. The first commercial CDMA service was launched inHong Kong in 1995, followed by a launch in Korea and Pennsylvania. It has rapidlybecome the primary choice of carriers in the U.S. Now 11 of the top 14 cellular carriers, 10 of the top 17 PCS carriers and the 2 largest PCS C block bidders haveselected CDMA for their new digital network.


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CHAPTER THREEPOWER CONTROL IN CDMA

The Near-Far Problem

The CDMA technology had all the better things to its part but why then it took thislong time to come into the commercial market. CDMA has not been previouslyimplemented due to its "Near-Far Problem." Let's assume there are two users, onenear the base and one far from the base.

The propagation path loss difference between these extreme users may be manytens of dB. In general, the strongest received mobile signal will capture thedemodulator at the base station. In CDMA, stronger received signal levels raise thenoise floor at the base station demodulators for the weaker signals, therebydecreasing the probability that weaker signals will be received.

To help eliminate the "Near-Far Problem", CDMA uses power control . The basestation rapidly samples the radio signal strength indicator levels of each mobile andthen sends a power change command over the forward radio link. This sampling isdone 800 times per second and can be adjusted in 84 steps of 1 dB. The purpose of this is so that the received powers from all users are roughly equal. This solves theproblem of a nearby subscriber overpowering the base station receiver and drowningout the signals of far away subscribers. An extra benefit is extended battery life. Thatis, when a mobile unit is close to a base station, its power output is lower. In other words, the mobile unit transmits only at the power necessary to maintain connection.

The key to high capacity of CDMA is the fact that instead of using constant power,the transmitters can be controlled in such a way that the received powers from all theusers are roughly equal and the subscribers occupy the same spectrum. This solvesthe problem of a nearby subscriber overpowering the base station receiver anddrowning out the signals of far- away subscribers. Power Control is implemented atthe base station by rapidly sampling the Radio Signal Strength Indicator (RSSI) levelof each mobile and sending power change command over the forward radio link. Incellular service areas, the typical total dynamic range of path loss is of the order of 80dB which means that the mobile transmitter must vary its power from about 2.5n W to0.25W.

The critical part of DS-CDMA System is Power Control and Management because of

many reasons Let us summarize what Power Control and Management Techniqueshas to achieve:

Reduce the transmitted power of both mobile and base station.

Received Power level of signals from all mobiles should be same at the base station.

Optimize the network resources.Two terms are used one is Power Control Techniques and the other is Power Management Techniques. Power Control Techniques will achieve both a. and b.described above. Power Management Techniques will achieve c. described above.


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It is clear that with out Power Control Techniques, the DS-CDMA System will gohaywire. By using Power Management Techniques, the DS-CDMA System will beoptimally used. So, in practice both the techniques are used.

Power Control Techniques (PCT)

Power Control Techniques are implemented on per user basis. So, PCT isimplemented on base band of a channel and before adding all the Walsh channels inthat frequency. There are 4 Power Control Techniques, which are explained in detailbelow:

1. Reverse Link Open Loop Power Control2. Reverse Link Closed Loop Power Control3. Reverse Outer Loop Power Control4. Forward Link Power Control

Reverse Link Power Control

Open Loop Control

This handles the wide dynamic range mentioned above. The mobile initiallyestimates the required transmit power based on the received power at the mobileand access parameters provided by the base station. This is under the assumptionthat coarse losses in the both directions are same. The mobile estimates the pathloss to the cell by measuring the received signal level in terms of analog AGC(Automatic Gain Control) voltage. The receiver AGC loop holds constant the total

power entering its 1.25 MHz IF pass band (which includes signal, thermal noise, andinterference) . The measured front-end power is adjusted by a closed loop correctionand then used to control the mobile transmit power accordingly.

But, this will not guarantee perfect power control. So, after the connection isestablished, Reverse Link Closed Loop Power Control techniques are used.

Closed Loop Control

Closed loop power control is a correction applied to the open loop power estimate. Inthis case, the cell measures the received Eb/No and compares it to a set point( setby a cell function ). If the measured Eb/No is above the set point, then a "down"command is sent, otherwise an "up" command is sent. The base station directs themobile to increase or decrease transmit power of mobile. The mobile shall change itstransmit power accordingly. Each command results in increasing or lowering themobile power by one dB , depending on the open loop estimate. The commands aresent once every 1.25 ms, or a rate of 800 corrections per second. The dynamic rangeof the closed loop control is +-24 dB relative to the open loop estimate.

At the boundary of a cell site, the mobile receives power control commandsfrom both the base station and the mobile will increase its transmit power only if boththe base station's command the mobile to increase the transmit power otherwise themobile will decrease its transmit power.


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Power Control Power Control Action taken Resulting changeCommand bit Command bit (per IS-95) by in transmit power from BTS#1 from BTS#2 mobile of mobile

0 0 Diversity combine Increase Power from both BTS's

0 1 Select BTS#2 Decrease Power 1 0 Select BTS#1 Decrease Power 1 1 Diversity combine Decrease Power

from both BTS's

Bit Value: 0 Increase Power.1 Decrease Power.

The base station sets Power control command bit by comparing the receivedpower with the Threshold value or Set point. If the received power is more thanThreshold value, and then BTS commands mobile to decrease transmit power,

otherwise BTS commands mobile to increase transmit power of mobile. Set point isso chosen (by BSC) that required transmit power of mobiles is just enough tomaintain reasonable Frame Error Rate (FER) of received signals from mobile.

Forward Link Power Control

In this, case, base station varies power transmitted in forward direction. IS-95specifies only messaging-based forward control. That is, when the mobile stationconcludes, because of excessive frame error rate, its forward signal quality is poor, itsends a report to the base station. This method is however relatively slow due to theprocessing delay in message parsing by the base station. Rate set 2, however incorporates a rapid power control mechanism which permits a faster and tighter power control.

The main aim of this Power Control is to reduce the transmit power of base station toeach mobile (this will not shrink the cell because transmit power of pilot, sync, pagingchannels are not reduced). The base station decides to raise the transmit power tothose mobile under two conditions:

- Periodic Frame Quality Measure sent by mobile is poor.- Mobile requests a specific threshold needed for it.

Reverse Outer loop Power Control

This is to adjust the Set point of Reverse Link Closed Loop Power Control. The BSC adjusts the Set Point based on the Reverse Link FER. If the FERis more, then BSC increases the Set Point to decrease the FER. Otherwise, BSCdecreases the Set Point which in turn indirectly reduces the transmit power of mobiles. Benefits of Reverse Outer loop Power Control:

- Decrease power consumption of mobiles.- Increase capacity of base station.

The capacity of base station is increased because of reduction in transmit power of mobile which reduces interference in that cell as well as other cells. This is not part of IS-95 because it doesn't have commands that are sent into air.

Power Control in Soft Handoff It is crucial to control the mobile transmit power during handoff by the cell that isreceiving the best signal, so that minimum necessary power is transmitted. Thus,


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each cell and sector participating in the soft handoff makes a separate determinationof the power control bit to be sent. The mobile processes them separately, andperforms an "or of the downs" logic operation as mentioned in detail in the handoff section of the document. However, it is also possible that two soft handoff participantsectors make a joint decision and both transmit. This happens when the soft handoff is between the sectors of one cell and one processing engine handles both branchesof the handoff. The mobile is informed that the power control bits are identical so thatit does diversity combining and single bit decision rather than separate decisions anda logical "or".

Power Management Techniques (PMT)

All the above Power Control Techniques should be implemented for both regular cellsites (equal sized & non-overlapping) and irregular cell sites (unequal sized & over-lapping). In practice, the cell sites are of irregular sizes and are overlapping. Power Management Techniques are not needed for regular cell sites. They are needed for irregular cell sites because with out Power Management Techniques the network

capacity is reduced but CDMA system will not go haywire.Before getting into Power Management Techniques, one has to understandtwo terms that are Forward Handoff Boundary and Reverse Handoff Boundary. Here,assume cell sites of irregular size and are overlapping.

Forward Handoff Boundary

Forward Handoff Boundary between two sectors is defined as the surface where anMobile Station's forward link would perform the same, regardless of which of the twosectors were transmitting.

Reverse Handoff Boundary

Reverse Handoff Boundary between two sectors is defined as the surface where anMobile Station's reverse link would perform the same, regardless of which of the twosectors were receiving.

For regular cell sites, the forward handoff boundary and reverse handoff boundaryare aligned and so PMT is not needed. For irregular cell sites, the forward handoff boundary and reverse handoff boundary are not aligned and so PMT is needed toalign the two handoff boundaries. PMT is meant to align two handoff boundaries,

which is also called as Handoff boundary balancing. Using PMT, one can only adjustthe Forward Handoff Boundary to align with Reverse Handoff Boundary becauseReverse Handoff Boundary is a physical boundary, which cannot be adjusted.

As PMT is not needed for regular cell sites, here assume cell sites are of irregular.PMT is applied to all Walsh channels of CDMA frequency.

PMT is implemented by using the following processes:

1. Breathing.2. Wilting3. Blossoming.


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Breathing

The Breathing Module varies the attenuation in the forward link based on the amountof reverse power that is detected. This causes the radius of the forward link of thesector to follow the radius of the reverse link of the sector, therefore keeping the

reverse and forward link handoff boundaries balanced. This is implemented by usinga suitable equation to find the attenuation to be inserted in the transmit path of thebase station. The equation should able to find the difference between the actualReceived Noise Power and the estimated Received Noise Power. And then calculatethe attenuation to be applied to transmit path, which is the above difference minussome constant value.

WiltingThe motive of Wilting is to avoid calls to drop during bringing down of sector. Suddendeactivation of a sector causes calls in the sector to drop. Thus a gradual decreasein transmitted power of base station is used to bring down the sector. Sector Wiltingis used to achieve handoff boundary balancing during the process to bringing downthe sector. The Wilting Module increases in steps the attenuation and Noise figure intransmit and receive paths of base station and there by mobiles get time to handoff toother base stations.

BlossomingSudden activation of a sector causes an increase in the forward link power leading todegradation in the forward link and possible dropped calls near the new sector. Thusa gradual increase in power is used to bring up the sector. Blossoming is used toachieve handoff boundary balancing during the process of bringing up the sector.

The Blossoming Module decreases in steps the attenuation and Noise figure intransmit and receive paths of base station and there by mobiles get time to handoff.

When transmit power of base station exceeds certain threshold set by BSC, the basestation wilts till the transmit power is sufficiently below the threshold and now thebase station begins to Blossom till transmit power exceeds threshold.


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CHAPTER FOURHANDOFF

Handoffs in CDMA

The act of transferring support of a mobile from one base station to another is termedhandoff. In other words, Handoff occurs when a call has to be handed off from onecell to another as the user moves between cells. In a traditional "hard" handoff, theconnection to the current cell is broken, and then the connection to the new cell ismade. This is known as a "break-before-make" handoff. Since all cells in CDMA usethe same frequency, it is possible to make the connection to the new cell beforeleaving the current cell. This is known as a "make-before-break" or "soft" handoff.Soft handoffs require less power, which reduces interference and increases capacity.The implementation of handoff is different between the narrowband standards andthe CDMA standards.

Traditional AMPS handoffs fail frequently, causing dropped calls and poor qualityservice. Moreover each handoff is preceded and followed by poor link quality,resulting in annoying noise and distortion. But, CDMA is specifically designed toreduce handoff failures, maintain good quality at all times : before, during, and after handoff. In fact, the handoffs are totally undetectable, even by skilled listeners.

The number of handoff is seen to be anywhere from perhaps one every 8 - 10 calls toperhaps 3 - 4 per call (roughly) which indicates that handoff is frequent enough for good performance to be considered important. Moreover, CDMA cell radii can beconsiderably larger than AMPS cell radii for any particular load distribution, whichagain reduces handoff rates simply on the basis of geometry.

As mentioned above, good handoff performance in CDMA is very important. A mobilethat is being served by one base station when another is closer in terms of path losswill be transmitting more power than would be necessary were it using the "right" cell.The fact that that mobile, and all others like it radiate excess power raises the overallinterference level. The higher overall interference level increases the effective reusefactor, and thus reduces overall, average reverse link capacity. Sloppy, late, or slowhandoffs thus should be kept to an absolute minimum.

On the other hand, there is a detrimental effect of handoff due to the asymmetry inpower control design between forward and reverse links. While the reverse linkpower control is fast and accurate, the forward link power control is slow and loose.

In IS-95A it is implemented via messaging this permits a faster power controlimplementation.

Soft handoff requires that multiple base stations transmit the same traffic to themobile in question. The multiple active forward channels raise the overall interferencelevel at the mobile - like the reverse link, this increases the effective frequency reusefactor and thus reduces forward link capacity.

Steps in Handoff

The main steps in any handoff can be summarized as follows:

1. Starting in a state where only one cell is supporting the call in question.


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2. Determining that over-the-air link conditions between the mobile and the oldserving cell are deteriorating, and that there is a potentially better link to a new,candidate cell.

3. Informing the candidate cell of the imminent handoff, including parametersneeded to identify the mobile and execute the handoff.

4. Signaling the mobile to begin executing the handoff.5. New cell beginning to service the mobile.6. Mobile beginning to use the new cell.7. Entering the mid-handoff state.8. Mobile discontinuing use of the old cell.9. Old cell stopping service to the mobile.10. Ending in a state where only the new cell, is supporting the call in question.

CDMA handoff offers several advantages over AMPS as follows:

1. It is "soft", meaning that communication is not interrupted by the handoff. This issometimes called "make before break." This means fewer dropped calls for users

and higher customer satisfaction for operators.2. The handoff is not abrupt, but rather it is a prolonged call state during which thereis communication via two or more base stations. The multi-way communicationdiversity improves the link performance during the handoff. The diversity gainpartially compensates for the large path loss at the cell boundary.

3. The signal measurement that triggers the handoff is performed by the mobilestations, not the base stations.

There is no handoff boundary in CDMA but rather a handoff region. The distributedhandoff alleviates most of the shortcomings of the AMPS-style hard handoffs. For example, a decision that handoff shou

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