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RF EngineeringContinuing Education & Training

Introduction toCDMA

Prepared by:

SAFCO Technologies, Inc.

600 Atlantis Rd.Melbourne, FL 32904 USA

Phone: (407) 952-8300Fax: (407) 725-5062

www.safco.com

Revision 3

Copyright1997 by SAFCO Technologies, Inc.All rights reserved. No part of this book shall be

reproduced, stored in a retrieval system, or transmittedby any means, electronic, mechanical, photocopying,

recording, or otherwise, without written permissionfrom SAFCO Technologies, Inc.


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Approximate Unit Length: 8 hr.

The purpose of this unit is to expose personnel unfamiliar with CDMA Technology to the basicproperties of CDMA. This unit assumes that the attendees have a basic understanding of wirelessdigital and analog communications systems. The target audience for this unit includes Associatelevel and above RF engineers as well as engineering managers.

Upon successful completion of this unit, the student should be able to describe:

The definition of CDMA and its theoretical advantages The direct sequence modulation technique

The concept of physical and logical channels The concept of call quality, how it is measured, and how it affects system capacity The CDMA advantage as provided by the utilization of the RAKE receiver The factors affecting the capacity of CDMA systems The various handoffs associated with CDMA The basic reverse link and forward link processes of a CDMA system

Some basic concerns associated with engineering a CDMA system


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RF Engineering Continuing Education & TrainingIntroduction to CDMA

Table of Contents

1 DEFINITION OF CDMA...............................................................................................................................................9

1.1 CDMA BASICS...................................................................................................................................................91.2 CDMA POWER SPECTRAL DENSITY & NOISE...........................................................................................................9

1.3 ADVANTAGESOF CDMA.....................................................................................................................................111.3.1 Frequency Reuse.............................................................................................................................................111.3.2 Coherent Signal Combination.........................................................................................................................111.3.3 User Privacy....................................................................................................................................................12

1.4 COVERAGEAND CAPACITY LIMITATIONS..................................................................................................................121.5 COMPARISONOF MULTIPLE ACCESS TECHNIQUES......................................................................................................12

1.5.1 FDMA..............................................................................................................................................................121.5.2 TDMA..............................................................................................................................................................121.5.3 Multiple access: division by code...................................................................................................................13

2 CDMA SPREAD SPECTRUM TERMINOLOGY...................................................................................................14

2.1 IS-95 AND IS-95-A CDMA:..............................................................................................................................142.2 FORWARDAND REVERSE LINKS.............................................................................................................................142.3 CORRELATIONAND ORTHOGONALITY.......................................................................................................................142.4 PN SEQUENCE ...................................................................................................................................................152.5 CHIPSAND CHIP RATE.........................................................................................................................................162.6 BIT RATE..........................................................................................................................................................162.7 TRAFFIC FRAME..................................................................................................................................................162.8 PROCESSING GAIN...............................................................................................................................................162.9 EB/NT, BER, ANDOTHERFIGURESOF MERIT.........................................................................................................172.10 SUMMARYOF CODES.......................................................................................................................................17

2.10.1 PN Long Code.............................................................................................................................................172.10.2 PN Short Codes...........................................................................................................................................182.10.3 Walsh Codes................................................................................................................................................18

2.11 CDMA CALL QUALITY (EB/NT)........................................................................................................................192.12 COHERENTVS. NON-COHERENT DETECTION...........................................................................................................20

3 CDMA PHYSICAL AND LOGICAL CHANNELS..................................................................................................21

3.1 PHYSICAL CHANNEL............................................................................................................................................213.2 LOGICAL CHANNEL..............................................................................................................................................21

3.2.1 Forward Link (Downlink)...............................................................................................................................213.2.1.1 Pilot..........................................................................................................................................................................223.2.1.2 Sync Channel...........................................................................................................................................................223.2.1.3 Paging Channel........................................................................................................................................................223.2.1.4 Traffic Channel........................................................................................................................................................223.2.1.5 Power Control Sub-Channel....................................................................................................................................23

3.2.2 Reverse Link (Uplink).....................................................................................................................................233.2.2.1 Access Channel........................................................................................................................................................243.2.2.2 Traffic Channel........................................................................................................................................................24

4 CDMA MODULATION & DEMODULATION.......................................................................................................25

4.1 TYPESOF SPREAD SPECTRUM MODULATION.............................................................................................................254.1.1 Frequency Hopping.........................................................................................................................................254.1.2 Direct Sequence..............................................................................................................................................25

4.2 SPREAD SPECTRUM (CDMA) MODULATION EXAMPLE: ENCODINGAND DECODINGOF INFORMATION................................264.2.1 Spread Spectrum Transmit Process................................................................................................................264.2.2 Spread Spectrum Receive Process..................................................................................................................274.2.3 Multiple Signal Case.......................................................................................................................................27

5 THE CDMA ADVANTAGE - THE RAKE RECEIVER AND THE MULTIPATH ENVIRONMENT............30

5.1 A BRIEFREVIEWOF MULTIPATHANDITSEFFECTON ANALOGAND DIGITAL TRANSMISSIONS.............................................30

Copyright 1997 by SAFCO Technologies, Inc. 3 Version 3.0


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5.2 THE RAKE RECEIVER........................................................................................................................................325.3 COMPARISONOFTHEEFFECTSOF MULTIPATHON FDMA, TDMA, AND CDMA..........................................................36

5.3.1 FDMA..............................................................................................................................................................365.3.2 TDMA..............................................................................................................................................................365.3.3 CDMA..............................................................................................................................................................385.3.4 Summary of Multipath Effects.........................................................................................................................38

5.4 RAKE RECEIVEREXAMPLE: IMPROVEMENTIN CALL QUALITY (EB/NT) ....................................................................38

6 DYNAMIC POWER CONTROL................................................................................................................................39

6.1 THE NEAR-FAR PROBLEM.................................................................................................................................396.2 REVERSE LINK...................................................................................................................................................40

6.2.1 Open-Loop.......................................................................................................................................................406.2.2 Closed-Loop....................................................................................................................................................40

6.3 FORWARD LINK..................................................................................................................................................41

7 CDMA IMPLEMENTATION AND DIGITAL RADIO LINK PROCESSES.......................................................42

7.1 FORWARD LINK..................................................................................................................................................427.1.1 Variable Rate Speech Coding.........................................................................................................................437.1.2 Channel Coding..............................................................................................................................................447.1.3 Bit Interleaving...............................................................................................................................................45

7.1.4 Encryption: Long Code Scrambling..............................................................................................................457.1.4.1 Paging Channel Encryption ............................................................................................................................. .....467.1.4.2 Access Channel Encryption .................................................................................................................................477.1.4.3 Traffic Channel Encryption ............................................................................................................................... ...47

7.1.5 Walsh Function Modulation...........................................................................................................................477.1.5.1 Power Control Signaling Subchannel Modulation..................................................................................................477.1.5.2 Forward Link Base Station Transmit Power Control..............................................................................................48

7.1.6 Quadrature Spreading & Carrier Modulation...............................................................................................497.2 REVERSE LINK...................................................................................................................................................50

7.2.1 Variable Low Bit Rate Speech Coding...........................................................................................................517.2.2 Channel Coding..............................................................................................................................................527.2.3 Bit Interleaving...............................................................................................................................................537.2.4 64-ary Orthogonal Walsh Symbol Modulation..............................................................................................537.2.5 Encryption: Long Code Spreading................................................................................................................547.2.6 Quadrature Spreading & Carrier Modulation...............................................................................................55

7.3 SYSTEM BLOCKDIAGRAM....................................................................................................................................56

8 CDMA CAPACITY.......................................................................................................................................................57

8.1 THE GENERAL CASE............................................................................................................................................578.2 ADJUSTMENTSTOTHE GENERAL CASE....................................................................................................................58

8.2.1 Sectorization Gain...........................................................................................................................................588.2.2 Voice Activity Factor......................................................................................................................................588.2.3 Frequency Reuse Efficiency (IADJ.)..............................................................................................................59

8.3 DEFINITIONOF POLE POINT...................................................................................................................................598.4 THE POLE POINT EQUATION..................................................................................................................................59

9 CDMA HANDOFF........................................................................................................................................................61

9.1 HANDOFF TERMINOLOGY......................................................................................................................................61

9.1.1 Introduction to TADD, TDROP & TCOMP...................................................................................................619.1.2 Handoff Candidate Classification..................................................................................................................629.2 TYPESOF HANDOFFS...........................................................................................................................................62

9.2.1 Soft Handoff.....................................................................................................................................................629.2.1.1 Forward Link................................................................................................................................................ ........ .639.2.1.2 Reverse Link..........................................................................................................................................................639.2.1.3 Joint Power Control......................................................................................................................................... ......63

9.2.2 Soft - Soft Handoff...........................................................................................................................................639.2.3 Softer Handoff.................................................................................................................................................639.2.4 Soft - Softer Handoff.......................................................................................................................................63



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9.2.5 Hard Handoff..................................................................................................................................................649.2.6 CDMA to Analog Handoff..............................................................................................................................64

9.3 HANDOFF CRITERIA.............................................................................................................................................649.4 HANDOFF PROCESS..............................................................................................................................................64

9.4.1 Example 1........................................................................................................................................................649.4.2 Example 2........................................................................................................................................................659.4.3 Example 3........................................................................................................................................................67

10 CDMA CALL EXAMPLE..................... ............... .............. .............. .............. .............. ............... ........... ..... ..... ....68

10.1 INITIAL SYSTEM ACCESS.....................................................................................................................................6810.2 CALL INITIATIONAND SETUP ..............................................................................................................................6810.3 SOFT HANDOFF.................................................................................................................................................6810.4 CALL TERMINATION...........................................................................................................................................69

11 BASIC SYSTEM ENGINEERING ISSUES................... .............. .............. .............. .............. ........ ..... ..... ...... .....69

11.1 PROPAGATION MODELINGOFTHE WIDEBAND CDMA RF SIGNAL.............................................................................7011.2 LINKBUDGET...................................................................................................................................................7011.3 NOMINAL CELL CONFIGURATIONS & NOMINAL CELL RADII CALCULATIONS.................................................................7211.4 NOMINAL SYSTEM PARAMETERS..........................................................................................................................7511.5 COVERAGE & CAPACITY RELATIONSHIP................................................................................................................75

11.5.1 Sensitivity Analysis: Effects of Loading on the System..............................................................................7511.5.2 Sensitivity Analysis Example........................................................................................................................7611.6 PN OFFSET PLANNING.......................................................................................................................................7611.7 PN INTERFERENCE............................................................................................................................................7811.8 NOMINAL ASSIGNMENTOF PN (RAKE) SEARCH WINDOW......................................................................................78



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List of FiguresFIGURE 1-1: COMPARISON OF INFORMATION AND TRANSMISSION BANDWIDTH.......... ............ ..... ..10

FIGURE 1-2: NOISE IN NARROW BAND AND SPREAD SPECTRUM COMMUNICATION SYSTEMS.. ..10

FIGURE 1-3: COMPARISON OF MULTIPLE ACCESS TECHNIQUES....... .............. .............. .............. ..... ..... ..13

FIGURE 2-4: AUTOCORRELATION OF PSEUDO-NOISE BIT SEQUENCE......... .............. .............. ......... ..... .15

FIGURE 2-5 FOUR-STAGE SHIFT REGISTER: GENERATION OF PN SEQUENCE......... .............. ........... ..16

FIGURE 2-6: SUMMARY OF SEQUENCES USED IN CDMA SPREAD SPECTRUM........ .............. ........ ..... ...18

FIGURE 2-7: EXAMPLE OF FER TO EB/NT RELATION: DIFFERENT FOR FORWARD ANDREVERSE LINK..............................................................................................................................................................20

FIGURE 3-8: FORWARD LINK CHANNEL ASSIGNMENTS............. ............... .............. .............. ............. ..... .....21

FIGURE 3-9: REVERSE LINK CHANNEL ASSIGNMENTS.......... ............... .............. .............. ............ ...... .....23

FIGURE 4-10: SPREAD SPECTURM TRANSMIT PROCESS........... .............. .............. ............... .............. ..... ......26

FIGURE 4-11: SPREAD SPECTRUM RECEIVE PROCESS........... .............. .............. .............. ........... ...... ...... ......27

FIGURE 5-12: DESTRUCTIVE INTERFERENCE DUE TO MULTIPATH................... ............... .............. ........ .31

FIGURE 5-13: SINGLE TRANSMITTER WITH MULTIPATH.................. ............... .............. .......... ..... ..... ..... ....32

FIGURE 5-14: TYPICAL SINGLE TRANSMITTER BAND-LIMITED CHANNEL IMPULSE RESPONSEWITH FIVE DISCRETE MULTIPATH COMPONENTS.........................................................................................33

FIGURE 5-15: COHERENT COMBINATION OF THREE STRONGEST MULTIPATH COMPONENTS

FROM A SINGLE TRANSMITTER.............................................................................................................................34

FIGURE 5-16: MULTIPLE TRANSMITTERS WITH MULTIPATH................... .............. ............... ............. ..... ..35

FIGURE 5-17: TYPICAL MULTIPLE TRANSMITTER BAND-LIMITED CHANNEL IMPULSERESPONSE WITH DISCRETE MULTIPATH COMPONENTS.............................................................................35

FIGURE 5-18: COHERENT COMBINATION OF THREE STRONGEST COMPONENTS OF A TYPICALMULTIPLE TRANSMITTER BAND-LIMITED CHANNEL IMPULSE RESPONSE WITH DISCRETEMULTIPATH COMPONENTS......................................................................................................................................36

FIGURE 5-19: TIME DISPERSION.............. .............. .............. .............. .............. ............... .............. .............. ............37

FIGURE 7-20: CDMA DIGITAL RADIO FORWARD LINK PROCESS......... .............. ............... ....... ..... ...... ......43

FIGURE 7-21: FORWARD LINK SPEECH PROCESSING AT THE NETWORK SIDE....................................44

FIGURE 7-22: CHANNEL CODING PROCESS.......... ............... .............. .............. .............. .............. .............. .........45

FIGURE 7-23: BIT INTERLEAVING.......... .............. .............. .............. .............. .............. ............... ...... ..... ..... ...... ....45

FIGURE 7-24: FORWARD LINK SCRAMBLING FOR TRAFFIC AND PAGING CHANNELS.............. ..... ..46



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FIGURE 7-25: POWER CONTROL SIGNALING SUBCHANNEL.......................................................................48

FIGURE 7-26: FORWARD LINK BASE STATION TRANSMIT POWER CONTROL................ ...... ..... ...... ....48

FIGURE 7-27: FORWARD LINK QUADRATURE SPREADING AND CARRIER MODULATION............ ...50

FIGURE 7-28: CDMA REVERSE LINK RADIO PROCESS........... .............. .............. .............. ............... ..... ...... ....51

FIGURE 7-29: SPEECH PROCESSING AT MOBILE SIDE................ ............... .............. .............. .............. ...... ....52

FIGURE 7-30: REVERSE LINK CHANNEL CODING PROCESS.......... ............... .............. ............. ..... ..... ...... ....53

FIGURE 7-31: REVERSE LINK BIT INTERLEAVING........... .............. ............... .............. .............. ......... ..... ..... ...53

FIGURE 7-32: REVERSE LINK TRAFFIC CHANNEL SPREADING, POWER CONTROL GROUPGATING, AND ENCRYPTION.....................................................................................................................................55

FIGURE 7-33: REVERSE LINK QUADRATURE SPREADING AND CARRIER MODULATION........ ..... ....56

FIGURE 7-34: CDMA FORWARD LINK (BASE TO MOBILE) PHYSICAL LAYER.............. ............ ..... ..... ...56

FIGURE 7-35: CDMA REVERSE LINK (MOBILE TO BASE) PHYSICAL LAYER................... ..... ...... ...... .....57

FIGURE 9-36: MOBILE UNIT TRANSITIONS INTO A REGION DEFINED BY TWO PILOT CHANNELSGREATER THAN T_ADD (SOFT HAND-OFF).........................................................................................................65

FIGURE 9-37: MOBILE UNIT TRANSITIONS INTO A REGION DEFINED BY FOUR OR MORE PILOTCHANNELS GREATER THAN T_ADD......................................................................................................................66

FIGURE 9-38: MOBILE UNIT TRANSITIONS THROUGH A REGION DEFINED BY TWO PREVAILINGPILOTS GREATER THAN T_ADD..............................................................................................................................67

FIGURE 11-39: TYPICAL CDMA SYSTEM PARAMETERS................................................................................75

FIGURE 11-40: COMPARISON OF COVERAGE DUE TO CHANGE IN TRAFFIC (5% TO 80% OFTHEORETICAL CAPACITY).......................................................................................................................................76



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LIST OF TABLES

TABLE 4-1: SUMMARY OF FREQUENCY HOPPING QUALITIES........... .............. .............. .............. ........ ......25

TABLE 4-2: SUMMARY OF DIRECT SEQUENCE SPREAD SPECTRUM QUALITIES................... ....... ..... ..25

TABLE 5-3: CALL QUALITY DB TO LINEAR CONVERSION TABLE...................... .............. .............. ...... .....39

TABLE 6-4: FORWARD LINK TCE ATTENUATION LEVEL VS. VOICE CODING RATE ........... ..... ..... ...41

TABLE 6-5: BASE STATION NOMINAL CHANNEL POWER ALLOCATIONS....... .............. .......... ..... ..... .....41

TABLE 7-6: BASE STATION TRANSMIT POWER VS. DATA RATE............. .............. .............. .............. ..........49

TABLE 7-7: I AND Q BITS AND CORRESPONDING PHASE MODULATION STATE..................................50

TABLE 7-8: I AND Q BITS AND CORRESPONDING PHASE MODULATION STATE..................................55

TABLE 9-9: PILOT SEARCH PARAMETERS....... .............. ............... .............. .............. .............. ......... ...... ...... ......62

TABLE 11-10: RECEIVER SENSITIVITY FOR DIFFERENT CDMA CHANNEL TYPES...............................71

TABLE 11-11: SIMPLIFIED EXAMPLE OF IS-95 CDMA LINK BUDGET FOR IN-VEHICLE COVERAGE............................................................................................................................................................................................72

TABLE 11-12: SUMMARY OF PARAMETERS USED TO CALCULATE NOMINAL CELL RADIUS, ANDCALCULATED CELL RADIUS FOR EACH AREA TYPE AND ANTENNA CONFIGURATION OF ATYPICAL SYSTEM AT 50% LOADING.....................................................................................................................74

TABLE 11-13: TYPICAL DELAY SPREAD VALUES FOR DIFFERENT ENVIRONMENT TYPES........ .... .77

LIST OF EQUATIONS

EQUATION 2-1: DEFINITION OF CORRELATION.................... .............. .............. ............... .............. ...... ...... .....15

EQUATION 2-2: PROCESS GAIN................................................................................................................................16

EQUATION 2-3: FRAME ERROR RATE........... .............. ............... .............. .............. .............. .............. ....... ...... .....19

EQUATION 5-4: PATH LENGTH..........................................................................................................................33EQUATION 5-5: CALL QUALITY DB TO LINEAR CONVERSION........ .............. .............. ............ ..... ..... ..... ....39

EQUATION 8-6: CAPACITY EQUATION (GENERAL FORM) ...........................................................................58

EQUATION 8-7: POLE POINT EQUATION........ .............. .............. ............... .............. .............. .............. ........... ...60

EQUATION 11-8: CALCULATION OF NOMINAL CELL RADII.........................................................................74



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T

t

t

FrequencyDomain

n

n

f

f

Figure 1-1: Comparison of Information and Transmission Bandwidth

The thermal noise encountered in a narrow band communication system is typically considered tobe constant (for a given temperature) over frequency. This level of background noise power

contained in a given bandwidth is called the noise floor. In the case of narrow bandcommunications, concentrating the transmitting energy in a narrow frequency band provides areceived RF signal that is above the noise floor. Having the signal sufficiently above the noisefloor is critical to being able to detect and receive (demodulate) the narrow band signal. This ismeasured as ratio of the desired signal energy per bit (Eb) to total system noise (Nt). For spreadspectrum systems, the transmitted energy is spread over such a wide bandwidth that the receivedsignal density may be below the noise floor yet it is still recoverable knowing the correctspreading sequence (code). This is illustrated in Figure 1-2.

Narrow Band & Wide Band Signal/Noise

-140

-120

-100

-80

-60

-40

1 3 5 7 9 11 13 15 17 19 2 1 23 25 2 7 29

Distance

Pwr

(dBm)

RSL

Narrow Band

Noise Floor

(1.23 MHz)

(30 kHz)

Wide Band

Noise Floor

Figure 1-2: Noise in Narrow Band and Spread Spectrum Communication Systems



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Unique Features

The following is a list of features that differentiate CDMA from analog cellular telephone (AMPS).These features will be explained in later sections.

Spread Spectrum Modulation Narrow band information is transmitted

over a wide band RF channel. N=1 Frequency Reuse Multiple users (in adjacent cells) operate on thesame frequency. Code Division Access Each user and base station is associated with aunique code rather than a frequency or time slot. Coherent Multiple Transmission (CMT) Multiple base stationssimultaneously transmit to a given mobile user. Coherent Multiple Reception (CMR) mobile units coherently combinemultipath components and signals from multiple base stations. Dynamic Power Control Forward and reverse link transmit power iscontrolled to the minimum required to achieve the link.

Variable Rate Speech Encoding Voice is encoded at a slower ratewhen the user is not speaking in order to minimize transmitted power andsystem interference.

1.3 Advantages of CDMA

The use of CDMA technology offers several advantages including:

Increased capacity due to adjacent cell frequency reuse (N=1), Coherent combination of signals, and User privacy.

These features are described in the following sections.

1.3.1 Frequency Reuse

Capacity gain is achieved with CDMAs inherent N=1 frequency reuse pattern. This is distinctlydifferent from the typical AMPS N=7 frequency reuse pattern in which only one-seventh of theavailable frequencies are used in a given cell. N=1 indicates that the same (wide band) frequenciesare used in each cell. When sectored cells are used, the same frequencies can be used in eachsector. Adjacent cell frequency reuse is possible because each signal in the system is associatedwith a unique code not a frequency.

1.3.2 Coherent Signal Combination

CDMA has the ability to coherently combine signals from multiple sources. This multiplecorrelation system employs a RAKE receiver. The RAKE receiver combines signals arriving at agiven location, with different time delays, thus mitigating fading due to multipath. In addition, thisfeature allows the mobile receiver to use signals from multiple base station transmitters, thusimproving cell-boundary performance and minimizing dropped calls.



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1.3.3 User Privacy

CDMAs spread spectrum modulation technique distributes the user information over an RF bandwidth that is much larger than the information bandwidth. The resulting power spectraldensity (PSD) of the transmitted wide band signal resembles thermal noise making the signal verydifficult to detect. In addition, a unique address code is required to recover user information.

1.4 Coverage and Capacity Limitations

The capacity of a CDMA cell site is effectively limited by the amount of interference in theenvironment. Interference is generated by several sources including:

Users of the given cell sight interfering with each other, Users of adjacent cell sites interfering with users of the given cell site, Adjacent base stations interfering with users of the given cell site, as well as Thermal and spurious noise.

It will be shown that system interference is a function of the number of users and their transmit

power. Dynamic power control is used to minimize forward and reverse link transmit power tomitigate interference. The dynamic nature of interference due to system load must be carefullyconsidered during system design.

1.5 Comparison of Multiple Access Techniques

In addressing CDMA, it is useful to understand other commonly used multiple access techniquessuch as FDMA and TDMA. CDMA can be considered a combination of these techniques as itpossesses elements of frequency and time diversity.

1.5.1 FDMA

Frequency Division Multiple Access (FDMA) is used in conventional analog cellular systems (e.g.AMPS, NMT). The FDMA process assigns discrete frequencies (i.e. channels) to individual users.It is considered multiple access in that a number of users can simultaneously use the systemproviding there is sufficient spectrum to accommodate each user. Accordingly, the capacity of thissystem is limited by the amount of available spectrum.

1.5.2 TDMA

Time Division Multiple Access (TDMA) is employed in digital communication systems. TDMA isused in cellular systems such as Digital-AMPS and GSM. It is considered multiple access in that anumber of encoded messages can be transmitted over time on a common carrier frequency. TDMA

assigns discrete time slots on a common carrier frequency to each user. During the time slotdesignated for a specific user, digital information is burst out using the entire allocated RF channel.Information is recovered by the receiver which decodes information only in its designated time slot.As the number of users increases, the transmission bit rate and associated bandwidth increases.Hence, TDMA is also limited by the amount of available spectrum.

Note that TDMA may be coupled with FDMA to further increase system capacity. Each channel inan FDMA system may be time-division multiplexed between several users.



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1.5.3 Multiple access: division by code

In the CDMA scheme, the digital information from each user is allowed to access the systemsimultaneously (as each user requests) using the same frequency spectrum. Frequency division isstill used, but a large bandwidth is used for each carrier. A user channel in CDMA is defined bya specific code and an associated carrier frequency. The user code is correlated against the receive

signal to recover only the information specific to that user. The capacity of a CDMA system isgoverned by the amount of interference in the environment that the receiver can tolerate before it isunable to recover the desired user information.

User3

User1

Frequency

User4Us

er3

Frequency

AllocatedBandwidth

F

Code

User2User

1

User1

User2

TDMA

Time

Code

User1

CDMA

User2...

Time

Code

User3

User4

User2

.

.

.

Frequency

Time

Figure 1-3: Comparison of Multiple Access Techniques



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2 CDMA Spread Spectrum Terminology

There are several key words and tricky phrases that are used in discussing CDMA processing andSpread Spectrum modulation. The following sections define some of the common terms that willbe used in the following sections.

2.1 IS-95 and IS-95-A CDMA:

CDMA as described in this document is based on an document known as IS (Interim Standard) -95.IS-95 is the Mobile Station - Base Station Compatibility Standard for Dual-Mode WidebandSpread Spectrum Cellular. IS-95 is also known as the CDMA Air Interface specification1.

CDMA air interface for PCS applications is described in Interim Standard 95-A (IS-95-A). The basic CDMA process is the same in both standards. Note however that IS-95-A specifies amaximum data rate of 14.4 kbps where as IS-95 specifies a maximum rate of 9.6 kbps.

Some standards that encompasses are:

IS-96-A Voice Encoder SpecIS-97 Base Station Performance SpecIS-98 Mobile Station Performance SpecJ - STD 8 Defines RF requirements at 1900 MHzJ - STD 18 Recommends minimum performance for 1900 MHz personal stations

2.2 Forward and Reverse Links

The definitions of the forward and reverse links are the same in CDMA as in other cellular systems.The Forward link (also known as the Downlink) refers to transmissions from the base station

(cell/sector) to the mobile user. The Reverse link (also known as the Uplink) refers totransmission from the mobile user to the serving base station (cell/sector).

2.3 Correlation and Orthogonality

In discussing spread spectrum CDMA modulation, we often refer to the correlation properties ofdifferent signals or sequences. In conceptual terms, two binary sequences that are being receivedare correlated if their patterns of 1s and 0s are alike as they are received over time. If theirreceived bit patterns are different or random with respect to each other, the sequences or signalsare said to be uncorrelated. Correlation can be thought of as the degree of similarity of signals asthey are received over time.

The correlation of two sequences can be determined by multiplying the received signals andsumming them over time. Correlation of two bit sequences is defined by

1 Term cdmaOne has been adopted by CDG as a designator for CDMA technology based on IS-95 and accompanyingstandards.



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=

+=L

k

AB nkBkAL

nR1

)()(1

)(

Equation 2-1: Definition of Correlation

Where:

n is a relative shift (offset) of the two sequences)(kA and )(kB are bit sequences of the lengthL

For some offset n, two bit sequences are totally correlated if )(nRAB is 1. If the correlation

)(nRAB is zero, sequences are orthogonal.

2.4 PN Sequence

The Pseudo-Noise (PN) Sequence (periodic and noise like) is fundamental to all direct sequencespread spectrum systems. The PN sequence is a finite length binary sequence (code) that exhibitsproperties similar to those of an infinite length random sequence. A good PN sequence is such thatthe number of 1's versus the number of 0's (or -1's) are equal. The correlation of a PN sequencewith itself results in only 1 peak. It is illustrated in Figure 2-4, for any offset other than zero PNsequence is totally uncorrelated with itself. This property is the foundation for finding the desiredcode among all other PN codes.

time [Tc]

RAA(n)

0

1 2 3 L

1

-1/L

4-1-2-3-4-L

Figure 2-4: Autocorrelation of pseudo-noise bit sequence

An example of PN sequence generator (four-stage shift register):



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For CDMA as defined in IS-95:

Rc = 1.2288 Mcps,Rb = 9.6 kbps (max), resulting inProcess Gain = 128 or 21.07 dB.

2.9 Eb/Nt, BER, and other Figures of Merit

There are several figures of merit that are bantered about when discussing CDMA as well as digitalcommunication systems in general.

Eb/No = Ratio of Transmitted energy per bit (Eb) to Thermal Noise (No)usually expressed in dB. Eb/Nt = Ratio of Transmitted energy per bit (Eb) to Total Noise (Nt) includingthermal, spurious, and interference from other CDMA users usually expressed in dB. Ec/Nt = Ratio of Transmitted energy per chip (Ec) to Total Noise (Nt) usuallyexpressed in dB.

Ec/Io = Ratio of Transmitted energy per chip (Ec) to Total Noise including self-interference (Io) usually expressed in dB. BER (Bit Error Rate) = Probability that a transmitted bit will be receivedincorrectly (i.e. 1 received as a 0 or a 0 received as a 1) FER (Frame Error Rate) = Probability that a transmitted frame will be receivedincorrectly.

2.10 Summary of Codes

In discussing CDMA modulation, several different PN sequences or codes are bantered aboutincessantly. In attempting to make sense out of CDMA modulation, it is helpful to know therelative length (time period) of these codes as well as what they are used for.

2.10.1 PN Long Code

The Long Code is a PN sequence that is 242 - 1 bits (chips) long. It is generated at a rate of 1.2288Mbps (or Mcps) giving it a period (time before the sequence repeats) of approximately 41.4 days.The long code is used to encrypt user information. Both the base station and the mobile unit haveknowledge of this sequence at any given instant in time based on a specified private long codemask that is exchanged.

The generation of a Long Code is governed by Long Code Mask. A long code mask is a 42 bitcode which define the initial values used by the long code generator. Knowledge of this long codemask allows the base station or mobile user to generate the same PN Long Code. Generating thesame long code (synchronized in time) at both end of the link allows information to be encryptedand decrypted.

A unique and private, long code mask (thus, PN long code) is assigned to each CDMA user. Thiscode is referred to as a user mask. The user mask is exchanged between the mobile and theserving cell(s)/sector(s), which allows user traffic data to be encrypted on both the forward andreverse links.



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A different long code mask is used to generate the long code for encryption and decryption ofAccess and Paging information more on this later.

2.10.2 PN Short Codes

The Short Code is a PN sequence that is 215 bits (chips) in length. This code is generated at1.2288 Mbps (or Mcps) giving a period of26.67 ms. This code is used for final spreading of thesignal and is transmitted as a reference known as the Pilot Sequence by the base station. All base stations use the same short code. Base stations are differentiated from one another bytransmitting the PN short code at different offsets in absolute. This time offset is known as a PNOffset. All base stations and mobiles have knowledge of this code, however, mobile units do nothave initial knowledge of absolute time. Mobile units initially search (in time) until theysynchronize with a pilot code transmitted by a base station. The base station then conveys timinginformation to the mobile more on this stuff later.

2.10.3 Walsh Codes

CDMA defines a group of 64 orthogonal sequences, each 64 bits long, known as Walsh Codes.These sequences are also referred to as Wash Functions. These codes are generated at 1.2288 Mbps(Mcps) giving them a period of approximately 52 s. These are used to identify users on theforward link. For this reason they are loosely referred to as CDMA channels. All base stations andmobile users have knowledge of all Walsh codes.

W a l s h C o d e s

6 4 b i t s

6 4 b i t W a l s h C o d e su s e d t o i d e n t i f y u s e r s

o n d o w n l i n k

2 4 2 - 1 b i t s

4 2 b i t u s e r m a s k

i d e n t i f ie s u s e r o n u p l i n k

6 4 c h i p o f f s e t su s e d t o i d e n t i f y

b a s e s t a t io n / s e c t o r

t o t h e m o b i l e

P N L o n g

C o d e s

2 1 5 b i t s

P N s h o r t c o d e s : P N - i( t ) = P N - 0 ( t - i x 6 4 Tc

)

Figure 2-6: Summary of Sequences used in CDMA Spread Spectrum



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2.11 CDMA Call Quality (Eb/Nt)

With CDMA the raw channel bits have no inherent information and are not available outside of thespread spectrum receiver. For this reason the fundamental performance measure is theframe errorrate (FER) rather than the bit error rate. Note that a frame includes signaling information and errordetection bits as well as user voice/data. This metric includes the error detection/correction codinginherent in the system. Frame error rate is defined as:

Xrateatdtransmitteframesofnumber

XrateatcorrectlyreceivedframesofnumberFER x

=1

Equation 2-3: Frame Error Rate

The rate X term refers to the specific rate at which voice information is being encoded by thevariable rate vocoder.

System performance is typically characterized by plotting Frame Error Rate vs. Received signalEb/Nt. These plots are known as waterfall curves due to their shape. These are similar to Bit

Error Rate (BER) curves for other digital communication systems. An example plot of this type isshown in Figure 2-7 for different modulation types. Specific CDMA performance curves are notshown as they are specific to vendor hardware. CDMA systems require a Frame Error Rate of lessthan 1% for acceptable call quality. This roughly corresponds to a Bit Error Rate (BER) of 10-3 .



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10-2

10-1

10-3

10-4

10-5

10-6

10-0

5 7 9 1131-1-3

Avera e Des read Eb/Nt

Reverse Link FER

Performance

Forward Link FER Performance

Good Call Quality

Figure 2-7: Example of FER to Eb/Nt relation: different for Forward and Reverse Link

2.12 Coherent vs. Non-Coherent Detection

The typical values Eb/Nt required to maintain a 1% FER have a more-or-less Log Normaldistribution with a standard deviation of 2.5. A 1% FER corresponds to a mean Eb/Nt of 5 dB for

the forward link and 7 dB on the reverse link. The difference in the required signal strength is dueto the use of coherent reception on the forward link and non-coherent reception on the reverse link.Coherent reception implies knowledge of the received signals phase (or timing). In the case of theforward link, this is provided by the Pilot sequence which is transmitted by each cell/sector.

Non-coherent reception implies detection of only the magnitude of received signals. The phase ofthe incoming signals is not known. As there is no pilot sequence transmitted on the reverse link,this type of receiver must be used. CDMA systems are therefore considered to be reverse linklimited with regards to call quality.



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3 CDMA Physical and Logical Channels

3.1 Physical Channel

Physical channels are described in terms of a wideband RF channel and code sequence. As definedin IS-95, each RF channel is 1.2288 MHz wide. For each RF channel, there are 64 Walshsequences (W0 through W63) available for use on the forward link. These Walsh sequences arecommonly referred to as CDMA channels (though this is not correct for the uplink).

3.2 Logical Channel

Divisions on the physical channel that carry specific types of information are known as logicalchannels. Logical channels in CDMA are divided into two categories: Traffic Channels andControl Channels. For the forward link there are three types of Control/Signaling channels andone Traffic Channel (per user). For the Reverse Link there is one type Signaling Channel and oneTraffic Channel per user.

It is important to note that signals on the forward link are identified by Walsh codes, however, signalson the reverse link are identified by Long Codes.

3.2.1 Forward Link (Downlink)

The logical channels for the Forward Link must provide identification of the Base station, timingand synchronizing of the transmissions between the base station and mobile station, hailing ofmobile units in the area, and the voice/data transmission from the base station to the mobile unit.The forward link is comprised of:

The Pilot Channel,

Up to one Sync Channel, Up to seven Paging Channels, and Up to 55 Traffic Channels.

Forward CDMA Channel1.23 Mhz radio channel

transmitted by base station)

PilotChan

PagingCh 1

SyncChan

PagingCh 7...

up toW0 W32 W1 W7

TrafCh 1

TrafCh n

TrafCh 24

W8

... ...up to

W31

TrafCh 25

W33

TrafCh 55

W63

...up to

Traffic

Data

Overhead

Control Bits

Figure 3-8: Forward Link Channel Assignments



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3.2.1.1 Pilot

The Pilot Channel allows a mobile station to acquire the timing of the Forward Traffic Channel -user information. It provides a phase reference for coherent demodulation and provides a meansfor signal strength comparisons between base stations, which is used to determine when to handoff.It consists of the unmodulated final spreading sequences (PN short codes). The Pilot signal is

transmitted continuously on Walsh 0 by each CDMA base station at the transmitter (cell/sector)level.

3.2.1.2 Sync Channel

The Synchronization Channel is an encoded, interleaved and modulated spread spectrum signalthat is used with the Pilot Channel to acquire initial system time and synchronization. The syncchannel is always transmitted on Walsh 32.

3.2.1.3 Paging Channel

The Paging Channel is used for transmission of control information to the mobile. When a mobileis to receive a call it will receive a page from the base station. Up to seven (7) channels may beconfigured for paging depending on the expected demand.

Page channel messaging to each user takes place in an 80 ms slot. The 80 ms slots are groupedinto cycles of 2048 slots (cycle duration 163.84 s) referred to as maximum slot cycles. The basestation can limit the maximum slot cycle used by the mobile. The mobile randomly picks a slotcycle index and informs the base station of its choice when it registers. The mobile now onlymonitors the Page channel during its assigned 80 ms slot defined by:

Slot Cycle = 1.28 x 2 SLOT_CYCLE_INDEX (in seconds)

where: SLOT_CYCLE_INDEX is {0 7}

That is to say for a slot cycle index of 5, the mobile powers up and monitors the Page channelfor 80 ms once every 1.28 x 2 5 = 40.96 seconds. This process of periodic monitoring allowsconsiderable power savings by the mobile unit.

3.2.1.4 Traffic Channel

The Traffic Channel orTraffic Channel Element (TCE) carries all the phone calls (voice or datasignal) from a given base station to all the mobile units active in the coverage area. Each user has adedicated TCE, and corresponding Walsh code, on the down link. The forward traffic channel

message consists of user voice (or data), power control data, and error correction bits. The messageis transmitted as a series of traffic frames. The traffic channel may also carry signaling informationwith or in place of user voice (or data). A Walsh code is assigned by the base station for eachTraffic Channel in use.



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3.2.1.5 Power Control Sub-Channel

A Power Control Sub-Channel is continuously transmitted on the forward traffic channel as part ofthe traffic frame. Information on this channel commands the mobile unit to adjust its transmittedpower + 1 dB every 1/16 of a speech frame (800 times per second).

3.2.2 Reverse Link (Uplink)

The logical channel requirements of the reverse link must provide for the identification and accessrequest by the mobile unit to the base stations in the area and the voice/data transmission from themobile unit to the base station. The reverse link is composed of:

Access Channels and Traffic Channels.

These channels share the same CDMA center frequency on the reverse link (a different frequency isused for forward link transmissions). The total number of channels is determined by base stationactivity. The example in Figure 3-9 shows 55 Traffic Channels available for all reverse links at a

given base station in accordance with the previous forward link channelization discussion. Inactuality, an individual subscriber unit is limited to one access channel and one traffic channel. Thereverse link capability of a given base station is limited by the number of traffic channels assigned(up to 55) and up to seven (7) access channels (correlating to a maximum of 7 paging channels).Note that a mobile does not tie up an access channel, it only borrows it for a short amount oftime.

Reverse CDMA Channels

(1.23 Mhz radio channel

received by base station)

Access

Ch 1

Access

Ch n...up to

Traf

Ch 1

Traf

Ch 55.................

User Dataand/or

Control

Addressed by Long Codes

Figure 3-9: Reverse Link Channel Assignments



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3.2.2.1 Access Channel

The Access Channel is used for the transmission of control information to the base station. Whena mobile is to place a call it uses the access channel to inform the base station. This channel isalso used when responding to a page. Each Access Channel is identified by a distinct AccessChannel Long PN Code . An Access Channel is selected randomly by the mobile unit from the

total number of access channels available from the serving cell/sector.

3.2.2.2 Traffic Channel

The Traffic Channel for the reverse link is identical to the forward link Traffic Channel Elementin function and structure. Each traffic channel is identified by a User Long PN Code which isunique to each CDMA user.



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4 CDMA Modulation & Demodulation

4.1 Types of Spread Spectrum Modulation

CDMA is a spread spectrum modulation scheme. This implies that the transmission bandwidth ismuch larger than the information bandwidth. The types of spread spectrum modulation commonlyused in communication systems are classified as:

Frequency Hopping Direct Sequence

GSM and PCS-1900 are TDMA systems with the ability to frequency hop. CDMA is a directsequence technique. These modulation schemes are described further below.

4.1.1 Frequency Hopping

The carrier frequency is varied and the bandwidth of the transmitted signal is comparable to the

bandwidth of the information signal. Information is modulated on top of a rapidly changing carrierfrequency. Some advantages and disadvantages of frequency hopping systems are listed inTable4-1.

Table 4-1: Summary of Frequency Hopping Qualities

Advantages Disadvantages

Carrier can be hopped over largeportions of the spectrum

Complex Frequency Synthesizer

Can be programmed to avoidportions of the spectrum Not useful for location andvelocity measurements

Shorter Acquisition Time thandirect sequence

Error correction required

Less affected by near-far problemthan direct sequence

4.1.2 Direct Sequence

In direct sequence modulation the carrier frequency is fixed and the bandwidth of the transmitted

signal is larger and independent of the bandwidth of the information signal. Some properties ofdirect sequence spread spectrum systems are listed in Table 4-2.

Table 4-2: Summary of Direct Sequence Spread Spectrum Qualities

Advantages Disadvantages

Better noise & anti-jam Requires wide band channel with



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performance than frequency hoppingfor a fixed transmission bandwidth.

little phase distortion

More difficult to detect thanFrequency hopping or narrow band

transmissions.

Longer acquisition time thanfrequency hopping systems

Best discrimination againstmultipath due to inherent frequencydiversity

Fast code generator needed

4.2 Spread Spectrum (CDMA) Modulation Example: Encoding andDecoding of Information

This section provides a simple example of CDMA spread spectrum modulation. The exampleillustrates how information bits are encoded by a PN sequence an the recovered in presence of

another spread spectrum signal.

4.2.1 Spread Spectrum Transmit Process

Transmitting a spread spectrum signal involves

*0 Modulating the information signal with the spreading PN sequence,*1 Modulating the resulting signal with the desired carrier wave,*2 Band Pass Filtering the output, and*3 Transmitting the resulting RF signal.

This is illustrated below in Figure 4-10

B P F

C o s c

tC1

( t )

S ( t )

S ( t ) C1

( t ) C o s (c

t )

I n f o r m a t i o n

S i g n a lS p r e a d i n g

C a r r i e r

M o d u l a t io nB a n d P a s s

F i l t e r

R F S i g n a l

Figure 4-10: Spread Specturm Transmit Process

Where:S(t) = Desired information signal as a function of time (digital signal).C1(t) = CDMA PN code as a function of time (comprised of a known binary

pattern).Cos( ct) = Desired RF carrier frequency.



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S(t)*C1(t)*Cos(ct) = Transmitted RF signal.

4.2.2 Spread Spectrum Receive Process

Receiving a spread spectrum signal involves

*4 Demodulating the signal with the RF carrier,*5 Low Pass Filtering the resulting wide band signal,*6 Demodulating with the signal with the known spreading sequence, and*7 Integrating the de-spread signal over a bit time to recover the information signal

This process is illustrated below in Figure 4-11.

L P F

C o s ( c

t ) C1

( t )

S ( t )

S ( t ) C1

( t ) C o s (c

t )

I n f o r m a t i o n

S i g n a l

D e - S p r e a d i n g

C a r r i e r D e -

M o d u l a t i o n

L o w P a s s

F i l t e r

R e c e i v e d

R F S i g n a l

t

t+

I n t e g r a t e o v e r

B i t T i m e &

D u m p

C o r r e l a t i o n

w i t h t h e P N

s e q u e n c e

Figure 4-11: Spread Spectrum Receive Process

[S(t)*C1(t)*Cos(ct)]*Cos(ct) = [S(t)*C1(t)*Cos(ct)]*Cos(ct)

= 1/2*[S(t)*C1(t)] + 1/2[S(t)*C1(t)*Cos(2ct)] LPF

= 1/2*[S(t)*C1(t)]1/2*[S(t)*C1(t)] *C1(t) = 1/2*[S(t)] after integration over the information period

Where:[S(t)*C1(t)*Cos(ct)] = Received RF signal

LPF = Low Pass Filter with bandwidth equal to the spread bandwidth (W)S(t) = Signal as a function of time (Digital)C1(t) = PN code as a function of time (comprised of pseudo random binary

sequence)Cos(ct) = Desired RF carrier frequency.

C1(t)*C1(t) = 1 when the codes are aligned in time because of correlation properties of

the PN codes.

4.2.3 Multiple Signal Case

What if the Code 1 signal was also received by a Code 2 receiver ?



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C1(t)*C2(t) = C3(t) because of correlation properties of the PN codes. Knowledge of (and time

synchronization to) the PN code associated with a specific information signal allows us to recoverthat signal from among other spread spectrum transmissions.

A simple example illustrates how the CDMA signal is transmitted and then recovered in thepresence of another CDMA signal. In the example shown, two (2) information bits are encoded

onto a repeating 7 chip CDMA like code sequence. Note that the effects of noise and interferenceare not considered.

Question: What is the processing gain of the spread spectrum signal in this example?

Hint: Rc/Rb

Information bits from two different transmitters

S1 S2

Encoding PN Sequences from those two transmitters

C1 C2

Encoded Information

S1*C1 S2*C2

Receipt of Multiple Encoded Signals

S1*C1 + S2*C2 S1*C1 + S2*C2



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Receiver Decoding Sequences (Same as TX Sequences)

C1 C2

Decoding (Correlation) of Received Signals

(S1*C1 + S2*C2)*C1 (S1*C1 + S2*C2)*C2



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Integration of the Correlated Received Signals

Integrator Output Integrator Output

Output of Translated Information Bits (at T + 1 Bit)

Comparitor Output Comparitor Output

5 The CDMA Advantage - The RAKE Receiver and theMultipath Environment

The land based wireless telephone environment is a multipath environment. Multipath is generallya destructive force in TDMA and FDMA systems and has been factored as a loss in the engineeringof those networks. In CDMA systems, as proposed by the interim standards and proponents of thetechnology, multipath is converted to a positive force through the application of the RAKEreceiver. In order to clearly illustrate the benefits associated with the RAKE receivers uniqueability to demodulate signals in a multipath environments, it is prudent to briefly review theadditive properties of waves and the multipath phenomena.

5.1 A Brief review of Multipath and its effect on Analog and DigitalTransmissions.

Multipath, as it is referred to in RF engineering, is the result of reflections and scattering of radiowaves off of buildings, water towers, mountains, etc. Multipath will exist anywhere the incidentwave and one or more reflected and/or defracted waves can reach the receiver as shown in Figure5-13

Multipath, in effect, creates multiple versions of the transmitted signal which arrive at thereceiver at different times. These multiple versions of the transmitted signal are known asmultipath components. The arrival of multipath components results in destructive interference dueto the superposition of the various waves. The received signal for a given frequency will be thesum of all the multipath components. When the components arrive perfectly in phase, the overall



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Received Signal Level (RSL) will be stronger than any of the individual components. When theyarrive out of phase, as a result of the reflective/defractive process, the overall RSL is less than thestrongest individual component.

Lets consider a single transmitted wave that is scattered such that the receiver detects thetransmitted wave and three multipath components of differing magnitudes and relative phase anglesfrom the incident wave. Mathematically these waves are given as:

f(t)Incident (Direct) Wave = 2.0 sin ( t)f(t)Multipath 1 = 1.5 sin (t+ 90

o)f(t)Multipath 2 = 1.0 sin (t+ 180

o)f(t)Multipath 3 = 0.5 sin (t+ 270

o)

The figure below provides a graphic representation of the incident waveform, multipath waveformsand the resultant waveform. Notice that magnitude of the resultant waveform is less than theincident waveform as a result of the superpositioning of the multipaths on the incident wave.

Destructive Interference Due to Multipath

-2.50

-2.00-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

2.00

2.50

0 2 4 6 810

12

14

16

18

20

22

24

Time

RelativeAmplitude Incident Wave

Multipath 1

Multipath 2

Multipath 3

Resultant Wave

Figure 5-12: Destructive Interference due to Multipath

Destructive (and constructive) interference due to the arrival of equal amplitude and random phasemultipath components is referred to as Rayleigh Fading. The significance or degree that RayleighFading affects system operation is determined by the surrounding environment. If we assume four

(4) different land classifications based on the concentration and size of structures in a given areaand designate them in decreasing concentration as Dense Urban, Urban, Suburban, Rural. Ingeneral we would expect to see the greatest effects of Rayleigh fading in the Dense Urbanenvironment and the least in a Rural Environment. This is due to the greater concentration ofscattering structures in a Dense Urban Environment than in rural areas



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5.2 The RAKE Receiver

The RAKE receiver is the optimum demodulator structure for multipath propagation paths in aland mobile telephone environment. It was first implemented in static form in the late 1950s.Essentially this device has the capability of looking at a given window in time, picking outmultipath components of a given signal and lining them up so that they are in phase again. This process is referred to as coherent addition and results in a greater probability of making ormaintaining the forward link in areas where it would otherwise be prohibited. The RAKE receiveris also applied to reverse link, however, because of a lack of a coherent reference (pilot signal) thereverse link uses a non-coherent RAKE demodulator.

To explain the conceptual processes of the RAKE receiver, consider the forward link scenario inFigure 5-13below in which a mobile unit (in the car), is being served by the nearby base stationsdesignated BSA.

Figure 5-13: Single Transmitter with Multipath

For a single pulse transmitted from BSA, the mobile receives many copies of the pulse, delayed intime, with amplitude which depend on the interaction with buildings, terrain, and the antenna. Theplot of the received signal vs. Time for a pulse is called an impulse response. In theory, a pulse ofzero time duration requires an infinite bandwidth. In practice this is not possible, therefor, thetransmitted pulse has a finite time duration resulting in a finite bandwidth. For this reason the plotof receive signal vs. time for a pulse is referred to as a Band Limited Impulse Response.

A typical band-limited channel impulse response for the above scenario would be composed ofmultipath components from BSA arriving at MU1 at different points in time as shown below inFigure 5-14. The spikes indicate discernible multipath signals. The surrounding envelope is caused

by smaller multipath components, scattering, and background noise.


2

Direct wave

Wave

1

1=

2


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A

A

|h(t)

|

2A

1

3

A

A4

5

t

Figure 5-14: Typical Single Transmitter Band-Limited Channel Impulse Response with FiveDiscrete Multipath Components

The time delay between the received components is related to the different distances traveled by thevarious components as they propagate from BSA to MU1. The difference in path length betweentwo signals can be found by multiplying the time difference between the received signals by thespeed of light.

Path Length Between A1 and A2 = (T2 - T1) 3 x 108 m/sEquation 5-4: Path Length

In the time domain, these multipath components differ in amplitude and time shift. In the frequencydomain, these differences correspond to differences in amplitude and phase. In IS-95 CDMA, thefunction of the RAKE receiver is to align up to a maximum of three multipath components in timeby selectively adjusting the phase of the multipath components so that they are all equal. Whencorrectly adjusted and put in a summing device the result is the coherent addition of the multipathsignals as shown in Figure 5-15. This figure shows the magnitude of the received and combinedsignals, however the phase information of the signals is also maintained.



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ReceivedSignals

A1

|h(t)

|

2A

A3

A5

A4

A4

t

MagnitudeofCoherentlyCombinedMultipath

A2

A1

Figure 5-15: Coherent Combination of Three Strongest Multipath Components from a SingleTransmitter

It is important to note that the only means of adjusting these components is by having a referencethat is also transmitted by BSA along with the traffic information. All IS-95 CDMA base stationswithin a given system continuously transmit a pseudorandom (PN) binary (short) code for thepurpose of synchronization and timing (Pilot Channel). Synchronization to the pilot signal allows

the RAKE receiver to operate in an efficient manner.

Each base station starts the PN short code at a unique time which is offset from the systemreference (which is maintained by GPS time). The PN offset makes it appear to a mobile that eachbase station is transmitting a unique code because of the correlation properties of the PN sequence.Note that the PN Code has properties such that when the received PN short code and the PN shortcode generated by the mobile unit are aligned in time, a correlation peak occurs. When they are notaligned, the correlation between the codes is noise.

The RAKE receiver provides for the coherent combination of multipath components from a singlebase station andmultiple cells/sectors jointly in a CDMA Handoff scenario (see Section 9). In IS-

95 CDMA, the RAKE receiver is limited to resolving and combining a maximum of threemultipath components from either a single transmitter, multiple transmitters, or a combination ofboth. The limit of resolution in time of the received signals may be as small as of a chip. Themaximum number of signals considered is defined in the system specification and results from thefact that there is very little added benefit from using more than three components. Typically theRAKE receiver processes the three strongest three signal components, however, the precisedetermination of which signals will be process depends on the handoff type, desired traffic flow,and relevant thresholds seat at each serving cell/sector.



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Consider the forward link scenario given below in which a mobile unit, MU1, is being served bythree base stations designated BSA, BSB, BSC. The lines from the base stations indicate multipaththat could exist for the geometry indicated.

BSC

BS B

BS AMU 1

Figure 5-16: Multiple Transmitters with Multipath

A typical band-limited channel impulse response for the above scenario could be composed ofmultipath components from serving base stations BSA, BSB, BSC and arriving at MU1 at differentpoints in time as shown below.

t i m e

| h ( t ) |

A 2

A 3

A 1B 1

B 2

C 1

C 2

C 3

C 4

Figure 5-17: Typical Multiple Transmitter Band-Limited Channel Impulse Response withDiscrete Multipath Components

Given that the RAKE receiver MU1 has knowledge that BSA, BSB, and BSC are all serving basestations (See Section 9 for details on joint handoffs), the receiver performs the following functions:



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Identifies the components which are the strongest (maximum of three), Performs time alignment of the select components, and Sums the components.

When correctly time aligned and put into a summing device, the result is the coherent combinationof the multipath signals as shown in Figure 5-18.

C 4A 3

B 2

C 2

A 2

A 3

A 1B 1

B 2

C 1

C 2

C 3

t i m e

R e l a t i v e

P o w e r

R e c e i v e d M u l t i p a t h s

M a g n i t u d e o f C o h e r e n t l y C o m b i n e d

M u l t i p a t h s

Figure 5-18: Coherent Combination of Three Strongest Components of a Typical MultipleTransmitter Band-Limited Channel Impulse Response with Discrete Multipath Components

5.3 Comparison of the effects of Multipath on FDMA, TDMA, andCDMA.

5.3.1 FDMA

The quality of service provided by a Frequency-Division Multiple-Access System is a function ofthe received signal level and proper frequency planning. Assuming no frequency reuse or theassignment of adjacent channels within the system, the problem becomes one dimensional as afunction of signal strength. In FDMA, the carrier wave is subjected to the multipath fading(Rayleigh fading) as discussed above. The human ear is an excellent discriminator of echoes,noise, fading. Multipath may greatly impact voice quality.

5.3.2 TDMA

Multipath in a digital system adversely effects the performance in two ways that must becompensated for in the design and implementation of the hardware. First, multipath fading of thecarrier wave results in reduced signal strength. The reduction in signal strength results in increasedbit error rate as Eb/Nt falls below what is required for acceptable call quality.

The second effect of multipath, the time delay in arrival over which multipath components arrive(delay spread), can be large enough to create Inter Symbol Interference (ISI). This effect is known



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as time dispersion. ISI may result in false 1s when a zero is sent or visa versa. This effect isillustrated in Figure 5-19.

B u i l d i n g s

0

B u i l d i n g s

1

1

B u i l d i n g s

0

1

0 1

1

1 . 2 . 3 .

Figure 5-19: Time Dispersion

Time Delay Compensation

TDMA systems use a variety of techniques to compensate for the effects of multipath fading andtime dispersion including

*8 Reduced transmitted bit rate to increase Eb/Nt,

*9 Encoding the digital signal to allow detection / correction of bit errors,*10 Use of an equalizer to compensate for time delays, and*11 Frequency hopping to combat frequency selective Rayleigh fading.

The following sections expand on the application of channel coding and equalization to combat theeffects of multipath fading.

Channel Coding

Channel coding (encoding of the binary signal) is the process of modifying the bit structure of theoriginal information so that there is redundancy and increased predictability of the transmitteddigital signal. The receiver knows the encoding process so that it can be reversed. Several coding

schemes exist and may be used alone or in some combination. Some common channel codingschemes are block coding, convolution coding, and interleaving. A detailed discussion of theseschemes is beyond the scope of this document.

5.3.2.1.1 Equalization

The equalizer creates a model of the transmission channel and calculates the most probabletransmitted sequence. This is accomplished by transmitting a known bit pattern with good



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correlation properties, called a training sequence. The equalizer compares the received trainingsequence with the standard and makes the appropriate adjustments in its bit recognition algorithm.This process repeats itself with every frame of data transmitted (on the order of tens ofmilliseconds).

5.3.3 CDMA

CDMA is a digital technology that is subject to all of the multipath phenomena that TDMA issubjected to but, because of the properties of spread spectrum and the use of the RAKE receiver, itis less susceptible to the adverse effects of multipath. Specifically, degradation of systemperformance due to frequency selective Rayleigh fading is reduced because the signal energy isspread over 1.25 MHz as opposed to 200 kHz for GSM TDMA or 30 kHz channels for AMPSTDMA. In effect spread spectrum is a form of frequency diversity. Also, by coherently combiningmultipath components, the effects of time dispersion and destructive interference are reduced byaligning the three strongest multipath components in time and combining for a net positive sumgreater than any individual component. Note that the time alignment occurs in a maximal sensewhen there is some prior knowledge of phase.

5.3.4 Summary of Multipath Effects

TDMA systems may use equalizers to compensate for multipath effects, however, the equalizationsettings are typically static while the multipath environment is changing in time. FDMA systemsrely on the total received power from all multipath components, however, the random phase of themultipath components results in constructive and destructive interference (fading). CDMA is ableto pool available signal resources (received multipath components) for a combined effort. Thenet result is the ability to close a link in an area that is not adequately supported by a singlemultipath component, but by combining up to three components or incident waves, the link can beclosed. As stated earlier, the incident waves or multipath components can come from the same

transmitter or multiple transmitters.

To illustrate this point, lets walk through a numerical example.

5.4 RAKE Receiver Example: Improvement in Call Quality (Eb/Nt)

Lets suppose the minimum acceptable Eb/Nt for a given system is 7 dB and the three strongestmultipath components have a corresponding Eb/Nt of 5, 3, and 2 dB. It is clear that the individualcomponents will not provide for service. However, the RAKE receiver will coherently combinethese components such that the resultant Eb/Nt is sufficient to provide the quality of service wedesire. This can be accomplished as follows:

Linearize the respective Eb/Nt measurements so as to allow for coherent combination by usingEquation 5-5.



39/79


= dBtb

N

E

eLinearValu

1.0

10

Equation 5-5: Call Quality dB to Linear Conversion

The linearized values for each of the multipath components are 3.16, 2.00, 1.58 respectively.Assuming perfect phase alignment and zero processing losses, the combined value for all of thecomponents is 6.74 which corresponds to a calculated Eb/Nt of 8.29 dB which provides the desiredlevel of call quality.

Additional examples can be made up and solved using Equation 5-5 or Table 5-3 for thelinearization of Eb/Nt.

Table 5-3: Call Quality dB to Linear Conversion Table

Eb/Nt(dB)

LinearizedValue

Eb/Nt(dB)

LinearizedValue

Eb/Nt(dB)

LinearizedValue

Eb/Nt(dB)

LinearizedValue

0.1 1.02 2.1 1.62 4.1 2.57 6.1 4.070.2 1.05 2.2 1.66 4.2 2.63 6.2 4.170.3 1.07 2.3 1.70 4.3 2.69 6.3 4.270.4 1.10 2.4 1.74 4.4 2.75 6.4 4.37

0.5 1.12 2.5 1.78 4.5 2.82 6.5 4.470.6 1.15 2.6 1.82 4.6 2.88 6.6 4.57

0.7 1.17 2.7 1.86 4.7 2.95 6.7 4.680.8 1.20 2.8 1.91 4.8 3.02 6.8 4.79

0.9 1.23 2.9 1.95 4.9 3.09 6.9 4.901.0 1.26 3.0 2.00 5.0 3.16 7.0 5.01

1.1 1.29 3.1 2.04 5.1 3.24 7.1 5.131.2 1.32 3.2 2.09 5.2 3.31 7.2 5.25

1.3 1.35 3.3 2.14 5.3 3.39 7.3 5.371.4 1.38 3.4 2.19 5.4 3.47 7.4 5.50

1.5 1.41 3.5 2.24 5.5 3.55 7.5 5.621.6 1.45 3.6 2.29 5.6 3.63 7.6 5.751.7 1.48 3.7 2.34 5.7 3.72 7.7 5.89

1.8 1.51 3.8 2.40 5.8 3.80 7.8 6.031.9 1.55 3.9 2.45 5.9 3.89 7.9 6.172.0 1.58 4.0 2.51 6.0 3.98 8.0 6.31

6 Dynamic Power Control

One of the fundamental requirements for successful IS-95 CDMA operation is the implementationof Dynamic Power Control (DPC) on the forward and reverse links. Using DPC the power of allmobile units is controlled so their transmitted signals arrive at the base station at an equal andminimum received power level. In addition, the traffic channel power on the forward link is variedas a function of voice coding rate. In this way, the interference generated from one mobile unit toanother is kept to a minimum resulting in increased system capacity.

6.1 The Near-Far Problem

The near-far problem in spread-spectrum systems relates to the problem of very strong signals ata receiver swamping out the effects of weaker signals located on the edge of the coverage area in aCDMA system resul

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