CDMA One. International summer students courses: "Plugged In: Modern Networks and Services in Telecommunication"

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1 INTRAROM S.A. ROMANIAN TELECOMMUNICATIONS AND ELECTRONICS INDUSTRY ROMANIA, Buchrest, 17 Fbric de Glucoz St., Sector 2 Cod 72322, Tel:(+40 1) Fx:(+40 1) Interntionl summer students courses: "Plugged In: Modern Networks nd Services in Telecommuniction" CDMA One Presented by: Author: Web pge: Emil: Ph.D. Eng. Liviu CHIRCA (RF / Trnsmission Engineer) chircl@yhoo.com (privte) liviuc@intrrom.ro (office) t Politechnicl University of Buchrest, on 9 th August, 2001 room AN034, between 9.30 m 1.30pm. Orgnized by BEST Buchrest - (Bord of Europen Students of Technology - ) 2001 Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 1

2 CDMA One - Summer Students Course CONTENTS: 1 INTRODUCTION 1 2 RADIO PROPAGATION FUNDAMENTALS OF DIGITAL RF COMMUNICATION PRINCIPLES OF CODE DIVISION MULTIPLE ACCESS 21 5 LINK STRUCTURE CALL PROCESSING CDMA DESIGN ENGINEERING CDMA PERFORMANCE ENGINEERING SYSTEM NOISE MANAGEMENT CDMA TRAFFIC ENGINEERING TDMA OR CDMA - COMPARISON OF ADVANTAGES AND DISADVANTAGES Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 2

3 1. INTRODUCTION Architecture of GSM system Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA Pge 3

4 1 INTRODUCTION CDMA One - Summer Students Course History of CDMA (Code Division Multiple Access) Qulcomm Compny, throught Jcobs nd Viterbi, suddenly nnounced to the world in 1991 tht it hd invented new cellulr system bsed on CDMA nd tht the cpcity of this system ws 20 or so times greter thn ny other cellulr system in existence. GSM mnufcturers becme concerned tht they would strt to lose mrket shre to this new system. CDMAone is the new nme for rdio system tht ws previously known s IS-95, or Qulcomm CDMA. Much of CDMAone is the sme s GSM: The use of pging, loction updting, rndom ccess, nd security, is ll the sme s it is for GSM. The only rel difference lies in the detiled prmeters of the rdio signl sent between cell nd mobile. The biggest difference is tht CDMAone uses the CDMA multiple ccess method, wheres GSM uses the TDMA multiple ccess method. This presenttion primrily dels with type of spred spectrum tht is employed in the IS-95 stndrd clled Direct-Sequence Spred Spectrum (DS-SS). Another form of spred spectrum is clled frequency-hopping spred spectrum (FH-SS) where the crrier frequency of the signl is moved (hopped) round in the bnd in pseudorndom fshion. The result is n increse in effective bndwidth over time Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 4

5 Principle of DS SS multiple ccess scheme: The properties of the set of orthogonl codes: 1. The cross-correltion should be zero or very smll Ech sequence in the set hs n equl number of 1s nd 1s, or the number of 1s differs from the number of 1s by t most The scled dot product of ech code should be equl to 1. There re two problems: 1) the ner-fr problem required power control, 2) the prtil correltion problem required synchronize codes. Applictions of DS-SS in Mobile Communiction 1) CDMA system cn redily tke dvntge of the voice ctivity of norml humn speech - reduction in interference - cpcity gin for the system. 2) Frequency reuse fctor of close to 1 - there is no need to frequency pln in CDMA. 3) The third dvntge is CDMA's bility to mitigte multipth distortion Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 5

6 Link Anlysis 2 RADIO PROPAGATION Crrier-to-noise rtio t the receiver (C/N) s n indictor of link qulity: Propgtion Loss We will present here only three models: 1) Free Spce: 2) Lee model: - t the cellulr frequency we hve 3) Ht model: - is bsed on extensive empiricl mesurements tken in urbn environments 2001 Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 6

7 2 RADIO PROPAGATION Shdowing A receiver trverses wy, obstcles tht prtilly block the signl pth (such s trees, building, nd moving trucks) cuse occsionl drops in received power. This decrese in power occurs over mny wvelengths of the crrier nd is thus clled slow fding. Slow fding is usully modeled by log-norml distribution with men power nd stndrd devition. Multipth Ryleigh Fding When mobile receiver is completely out of sight of the bse sttion trnsmitter the received signls re mde up of group of reflections from objects, nd none of the reflected pths is ny more dominnt thn the other ones. The envelope of received crrier signl for moving mobile is Ryleigh distributed nd this type of fding is clled Ryleigh fding or fst fding. Multipth Dely Spred Multipth occurs when signls rrive t the receiver directly from the trnsmitter nd, indirectly, due to trnsmission through objects or reflection. Intersymbol interference (ISI) cn occur Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 7

8 Introduction 3 FUNDAMENTALS OF DIGITAL RF COMMUNICATION There re mny dvntges for migrting from nlog AM or FM to digitl such s TDMA nd CDMA, but there re t lest four resons for the recent trend in the industry, s follows: 1) Qulity of service - bility to regenerte the signl 2) Incresed cpcity - convert A/D nd speech compression (Vocoder in CDMA) 3) Privcy - encyption tehniques 4) Enbling technology nd economics - new twhnologies such s digitl signl processing (DSP) techniques nd their implementtion on ppliction-specific integrted circuits (ASICs) System Components - Functionl block digrm of typicl digitl communiction system Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 8

9 3 FUNDAMENTALS OF DIGITAL RF COMMUNICATION Source Coding Pulse Code Modultion (PCM), where the nlog voice is converted into 64-Kbps bit strem. Other wireline techniques, such s dptive pulse code modultion (ADPCM) nd delt modultion (DM), re lso used. In wireless pplictions n ttrctive solution is offered by the vocoders (see Figure 3.3). Vocoders IS-95 vocoder produces frmes tht re 20 ms. Liner-predictive coding (LPC) is widely used to estimte filter coefficients for T(Z) 1/T(z) - filter modelling the vocl trct E(z) - excittion signl In IS-95 CDMA system bove prmeters nd coefficients re the informtion tht is communicted between the trnsmitter nd receiver. S(z) - speech signl 2001 Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 9

10 Chnnel Coding 3 FUNDAMENTALS OF DIGITAL RF COMMUNICATION The gol of chnnel coding is, given desired probbility of error, to reduce the required E b /N 0, or lterntively, given n chievble E b /N 0, to reduce the probbility of error. The cost of this gol is more bndwidth, or more redundnt bits tht the system hs to trnsmit. There re two mjor clsses of error-correcting codes: 1) block codes - code n informtion sequence one block t time, 2) convolutionl codes - hve memory property. Liner Block Codes A liner block code cn be chrcterized by the (n, k) nottion, nd for given code, the encoder trnsforms block of k informtion bits into longer block of n code bits. IS-95 CDMA uses block coding to indicte the qulity of ech trnsmitted frme (which contins block of informtion bits). The IS-95 system uses cyclic redundncy check (CRC), which is one of the most common block codes. Convolutionl Codes The convolutionl codes do hve memory nd in ddition to using CRC, IS-95 CDMA uses convolutionl coding to further improve the error performnce. For convolutionl codes, the encoded bits re functions of informtion bits nd functions of the constrint length. Specificlly, every encoded bit (t the output of the convolutionl encoder) is liner combintion of some previous informtion bits. Interleving Most error-correcting codes perform well in correcting rndom errors. Interleving is technique for rndomizing the bits in messge strem so tht burst errors introduced by the chnnel cn be converted to rndom errors Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 10

11 3 FUNDAMENTALS OF DIGITAL RF COMMUNICATION Multiple Access Multiple ccess refers to the shring of common resource in order to llow simultneous communictions by multiple users, nd this common resource is the RF spectrum. The IS-95 CDMA system hs symmetric links: the forwrd link uses Wlsh codes, the reverse link uses pseudorndom noise (PN) codes (for chnneliztion) Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA Pge 11

12 3 FUNDAMENTALS OF DIGITAL RF COMMUNICATION Wlsh Codes Wlsh codes s used in IS-95 re set of 64 binry orthogonl sequences nd they re generted by using the Hdmrd mtrix. H N (complex) contins the sme but inverted elements of H N. The IS-95 forwrd link uses set of 64 orthogonl Wlsh sequences. PN Codes The incoherent nture of the reverse link clls for the use of nother clss of codes, PN codes, for chnneliztion hving length of chipsnd is generted using 42-stge register. The forwrd link uses lso the short PN code (hving length of chips) tht is superimposed on top of the Wlsh code to provide isoltion mong the different bse sttions (or sectors). The discrette time Autocorreltion help the mobile to quire unmodulted pilot chnnel! 2001 Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 12

13 3 FUNDAMENTALS OF DIGITAL RF COMMUNICATION Modultion Why modultion? - If one wishes to trnsmit bsebnd signl t 9.6 khz, the ntenn size would be km! Binry Phse-Shift Keying (BPSK) Modultor Whenever the trnsmitter wnts to send +1, it will trnsmit positive cosinusoid; whenever the trnsmitter wnts to send -1, it will trnsmit negtive cosinusoid. Demodultor Probbility of bit error versus E b /N Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 13

14 3 FUNDAMENTALS OF DIGITAL RF COMMUNICATION Qudrture Phse-Shift Keying (QPSK) QPSK mkes use of the qudrture component in ddition to the in-phse component trnsmit two bits of informtion per symbol period. Modultor In order to send two bits of informtion, the QPSK system needs to use four symbols: symbols 0, 1, 2, nd 3. The even bit strem is multiplied by the in-phse crrier, nd the odd bit strem is multiplied by the qudrture crrier 2001 Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA Pge 14

15 3 FUNDAMENTALS OF DIGITAL RF COMMUNICATION Qudrture Phse-Shift Keying (QPSK) Demodultor Note tht BPSK nd QPSK hve the sme probbility of bit error s function of E b /N 0 (!) The IS-95 CDMA system uses offset qudrture phse-shift keying (OQPSK) for reverse link to void 180-degree phse trnsition tht occurs in conventionl QPSK systems Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 15

16 Introduction 4 PRINCIPLES OF CODE DIVISION MULTIPLE ACCESS Cpcity is defined s the totl number of simultneous users the system cn support Qulity is defined s the perceived condition of rdio link ssigned to prticulr user ~ bit error rte (BER). CDMA system tht need to be optimized in order to reduce interference nd increse qulity. Cpcity CDMA system cpcity is presented using the mount of user interference in the bnd. We re primrily interested in link metric clled E b /N 0, or energy per bit per noise power density. S - verge modulting signl power R- bit rte W bndwidth of signl N totl power noise The cpcity M is: Note tht the rtio W/R is lso known s the processing gin of the system Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 16

17 4 PRINCIPLES OF CODE DIVISION MULTIPLE ACCESS Effects of Loding Cell A is sid to be loded by users from other cells. Where η is the loding fctor (between 0% nd 100%) Frequency reuse fctor F is defined s: (idelly F = 1) 2001 Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 17

18 Effects of Sectoriztion A sectorized ntenn rejects interference from users tht re not within its ntenn pttern. 4 PRINCIPLES OF CODE DIVISION MULTIPLE ACCESS where λ is the sectoristion gin: where: G(θ) is the horizontl ntenn pttern of the sector ntenn; G(0) is the pek ntenn gin, I(θ) is the received interference power from users of other cells s function of θ. Typiclly: λ = 2.5 for three-sector configured systems, λ = 5 for six-sector configured systems Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 18

19 4 PRINCIPLES OF CODE DIVISION MULTIPLE ACCESS Effects of Voice Activity Speech sttistics shows tht user in converstion typiclly speks between 40% nd 50% of the time. By employing vrible-rte vocoding, the system reduces the totl interference power by this voice ctivity fctor v. where v is voice ctivity fctor. The cpcity M is: Severl conclusions regrding CDMA cpcity: 1. Cpcity, or number of simultneous users M, is directly proportionl to the processing gin of the system. 2. The link requires prticulr E b/ N 0 to ttin n cceptble BER nd ultimtely n cceptble frme error rte (FER). Cpcity is inversely proportionl to the required E b /N 0 of the link. The lower the required threshold E b /N 0, the higher the system cpcity. 3. Cpcity cn be incresed if one cn decrese the mount of loding from users in djcent cells. 4. Sptil filtering, such s sectoriztion, increses system cpcity. For exmple, six-sector cell would hve more cpcity thn three-sector cell Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 19

20 4 PRINCIPLES OF CODE DIVISION MULTIPLE ACCESS Power Control Why Power Control? Power control is implemented to overcome the ner-fr problem nd to mximize cpcity. Ner-fr problem 2001 Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA Pge 20

21 Reverse link Open Loop - only mobile controlled opertion 4 PRINCIPLES OF CODE DIVISION MULTIPLE ACCESS Closed Loop - to compenste for power fluctutions due to fst Ryleigh fding nd involved both mobile nd bse sttion To mintin n cceptble FER mens evlute continuously E b /N Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 21

22 4 PRINCIPLES OF CODE DIVISION MULTIPLE ACCESS Power Control - Forwrd link The mobile continuously monitors the FER of the forwrd link, nd it reports this FER bck to the bse sttion in messge clled the power mesurement report messge (PMRM). The bse sttion, knowing the qulity of the forwrd link, my then djust its trnsmit power to tht prticulr mobile. Open Loop -nd Closed Loop Implementtion 2001 Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 22

23 Hndoff 4 PRINCIPLES OF CODE DIVISION MULTIPLE ACCESS 1) Soft Hndoff - CDMA uses soft hndoff where, during hndoff, mobile simultneously mintins connection with two or three bse sttions. As the mobile moves from its current cell (source cell) to the next cell (trget cell), trffic chnnel connection is simultneously mintined with both cells (for ex. see figures below). 2) Softer Hndoff - This type occurs when mobile trnsitions between two different sectors of the sme cell. 3) Hrd Hndoff - two types: ) CDMA-to-CDMA hndoff (D-to-D hndoff ) occurs when the mobile is trnsitioning between two CDMA crriers (i.e., two spred-spectrum chnnels tht re centered t different frequencies). b) CDMA-to-nlog hndoff (D-to-A hndoff) occurs when CDMA cll is hnded down to n nlog network Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 23

24 4 PRINCIPLES OF CODE DIVISION MULTIPLE ACCESS Hndoff Process In mnging the hndoff process, the mobile mintins in its memory four exclusive lists of bse sttion sectors. The sectors re stored in the form of pilot PN offsets of those sectors. 1) Active (A) set contins the pilots of those sectors tht re ctively communicting with the mobile on trffic chnnels. 2) Cndidte set set contins those pilots whose E c /I 0 re sufficient to mke them hndoff cndidtes. 3) Neighbor (N) set contins those pilots tht re in the neighbor list of the mobile s current serving sector. 4) Remining (R) set contins ll possible pilots in the system for this CDMA crrier frequency, excluding pilots tht re in ctive, cndidte, nd neighbor sets. Note: To void limittion cusing by multipths components, serch windows re provided to serch for pilots tht re in the ctive, cndidte, neighbor, nd remining windows (prmeters SRCH_WIN_A, SRCH_WIN_N nd SRCH_WIN_R tht defines the serch-window width used to serch for pilots in the corresponding sets) Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 24

25 Asymmetric Links 5 LINK STRUCTURE The forwrd link consists of four types of logicl chnnels: pilot, sync, pging, nd trffic chnnels. The reverse link consists of two types of logicl chnnels: ccess nd trffic chnnels Forwrd link Pilot Chnnel The pilot chnnel is effectively the PN sequence itself. The PN sequence with specified offset uniquely identifies the prticulr sector tht is trnsmitting the pilot signl. The pilot chnnel is trnsmitted continuously by the bse sttion sector. The pilot chnnel provides the mobile with timing nd phse reference Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 25

26 Forwrd Link 5 LINK STRUCTURE Sync Chnnel - crries bsebnd informtion. The informtion is contined in the sync chnnel messge tht notifies the mobile of importnt informtion bout system synchroniztion nd prmeters (offset PILOT_PN, + informtion tht help mobile to syncronize with long PN sequence) 2001 Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 26

27 Forwrd Link Pging Chnnel - crries bsebnd informtion. Although there cn be up to seven pging chnnels per sector, ech mobile only monitors one pging chnnel Messges over pging chnnel: 1) overhed - notify to mobile the system configurtion 2) pging - my contin pge to one or group of mobiles 5 LINK STRUCTURE 2001 Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 27

28 Forwrd Link - Trffic Chnnel 5 LINK STRUCTURE Is used to trnsmit user nd dt voice; signling messges re lso sent over the trffic chnnel. There re four dt rtes output of the vocoder: 1.2, 2.4, 4.8 nd 9.6Kbps The reson for repeting symbols is to reduce overll interference power t given time when lower rte dt re trnsmitted The msk used to produce long PN is function of the mobile s ESN, 2001 Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 28

29 Reverse Link 5 LINK STRUCTURE Acsess Chnnel Is used by the mobile to communicte with the bse sttion when the mobile doesn t hve trffic chnnel ssigned. The mobile uses this chnnel to mke cll origintions nd respond to pges nd orders. The set of 64 Wlsh functions is used, to modulte, or represent, groups of six symbols ( group of six binry symbols corresponds to deciml vlue between 0 nd 63 nd the corresponding Wlsh is trnsmitted) Informtion is trnsmitted on the ccess chnnel in ccess chnnel slots nd ccess chnnel frmes. Messges over ccess chnnel: 1) response messge - in response to bse sttion messge 2) request messge - sent by the mobile sttion 2001 Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 29

30 Reverse Link 5 LINK STRUCTURE Trffic Chnnel Is used to trnsmit user dt nd voice; signling messges re lso sent over the trffic chnnel. The structure of the reverse trffic chnnel is similr to tht of the ccess chnnel. The mjor difference is tht the reverse trffic chnnel contins dt burst rndomizer The function of the dt burst rndomizer is to tke dvntge of the voice ctivity fctor on the reverse link Remember! 1) The reverse link use OQPSK where the dt in the Q pth is delyed by one hlf PN chip (to void QPSK signl envelope to colpse to zero. 2) The IS-95 CDMA system uses Multiplex Option 1 to trnsmit primry (i.e., voice) nd secondry (i.e., dt) trffic - but only t full rte Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 30

31 Cll Processing Sttes 6 CALL PROCESSING 1. Initiliztion Stte After power-up, the mobile enters the initiliztion stte. This stte contins four substtes, which the mobile sequentilly goes through:. System determintion substte; b. Pilot chnnel cquisition substte; c. Sync chnnel cquisition substte; d. Timing chnge substte. 2. Idle Stte. Pging Chnnel Monitoring In the idle stte, the mobile monitors the pging chnnel on the forwrd link. b. Idle Hndoff If the mobile detects tht the pilot strength from nother bse sttion is sufficiently stronger thn tht of the current bse sttion, then the mobile proceeds to perform n idle hndoff. c. Pging Chnnel Messges There is totl of six overhed messges tht re sent to the mobile on the pging chnnel Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 31

32 3. Access Stte 6 CALL PROCESSING The mobile trnsmits messges to the bse sttion using the ccess chnnel nd in ddition, the mobile lso receives messges from the bse sttion on the pging chnnel. Access Procedures - see figures below There re two types of messges sent on the ccess chnnel: response messge nd request messge. 4. Trffic Chnnel Stte The mobile communictes with the bse sttion using the forwrd nd reverse trffic chnnels 2001 Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 32

33 7 CDMA DESIGN ENGINEERING Are ll the rdio prmeters dequte to mintin high-qulity rdio link between the bse sttion nd the mobile? Forwrd Link Anlysis: Pilot Chnnel + Trffic Chnnel The E c /I 0 is the energy per chip per interference density mesured on the pilot chnnel; it is effectively the signl strength of the pilot chnnel. The E b /N 0 trnsltes directly into BER, which hs implictions on forwrd-link voice qulity P 0 (θ 0 ) = home bse sttion (sector 0) overhed ERP including pilot, pging, nd sync powers in the direction θ 0 to the probe mobile. T 0 (θ 0 ) = home bse sttion (sector 0) trffic chnnel ERP in the direction θ 0 to the probe mobile. α 0 = frction of home bse sttion overhed ERP llocted to pilot power. L 0 (θ 0,d 0 ) = pth loss from home bse sttion in the direction θ 0 to the probe mobile distnce d 0 wy. G = receive ntenn gin of probe mobile. I h = power received t the probe mobile from overhed power emitted by home bse sttion. I n = power received t the probe mobile from other interference of non-cdma origins. N = therml noise power. I 0 = is the sum of overhed powers from other bse sttions. I m = power received t the probe mobile from totl trfic chnnel power (from the home bse sttion); I m = power received from totl trfic chnnel interference. I t = power received t the probe mobile from totl trfic chnnel power (from ll other bse sttions). (W/R) = processing gin Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 33

34 Reverse Link - Trffic Chnnel 7 CDMA DESIGN ENGINEERING The link E b /N 0 trnsltes directly into BER, which hs implictions on reverse-link voice qulity. T' = reverse trffic chnnel ERP of the probe mobile; the trnsmit pttern is ssumed to be omnidirectionl. L' 0 (θ 0,d 0 ) = reverse pth loss from the probe mobile in the direction θ 0 to the home bse sttion distnce d 0 wy. G 0 (θ 0 ) = receive ntenn gin of home bse sttion in the direction θ 0 to the probe mobile. I' m = power received t the home bse sttion from totl interference introduced by the reverse trffic chnnel trnsmissions of ll the mobiles tht re served by home bse sttion, I' t = power received t the home bse sttion from totl interference introduced by the reverse trffic chnnel trnsmissions of ll the mobiles tht re not served by home bse sttion, I' n = power received t the home bse sttion from other interference of non- CDMA origins. N = therml noise power, nd (W/R) = processing gin. Importnt! R is the rise of the interference level bove the therml noise level Reverse - Link Rise Frequency Reuse Fctor Loding fctor R is very good indictor of whether or not the bse sttion is hevily loded on the reverse link Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 34

35 7 CDMA DESIGN ENGINEERING PN Offset Plnning To relize seprtion between the logicl chnnels from different bse sttions, ech bse sttion (or sector) is ssigned different short PN sequence Short PN Sequences Ech short PN sequence is generted using shift register with 15 dely elements nd the length of such PN sequence is bout 2 15,or 32,768 chips tht mens theoreticlly bout 32,768 different PN sequences to ssign to different bse sttions! Given tht the trnsmission rte is Mcps, the durtion of ech chip is: A time durtion of μsec corresponds to propgtion distnce of m; tht is, Is required mimimum seprtiom (chips) between usble PN sequences! 2001 Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 35

36 PN Offset Plnning Short PN Sequences 7 CDMA DESIGN ENGINEERING To provide more isoltion mong PN sequences tht cn be ssigned (used), the IS-95 stndrd specifies tht usble PN sequences need to hve minimum seprtion of 64 chips between ech other. Ech usble PN sequence is defined by its PN offset. The seprtion cn be incresed further by using the PILOT_INC prmeter. The totl number of usble PN sequences is: For PILOT_INC=1 then result 512 PN sequences! If PILOT_INC=4 then there re only 128 usble PN sequences vilble for ssignment nd PN sequence plnning now becomes nlogous to AMPS or GSM frequency plnning Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 36

37 7 CDMA DESIGN ENGINEERING Co-PN Offset If two bse sttions re using the sme PN sequence wht is the minimum required distnce between the two bse sttions? (in chips) where d is in meters, r is the coverge rdius of bse sttion 2 (home bse sttion) in meters, nd W is the size of SRCH_WIN_A in chips 2001 Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 37

38 7 CDMA DESIGN ENGINEERING Adjcent PN Offset If two bse sttions re using djcent PN sequences (i.e., PN sequences tht re seprted by (I=PILOT_INC x 64) chips), wht is the necessry condition for no PN offset lising? (in chips) where d is in meters, r is the coverge rdius of bse sttion 2 (home bse sttion) in meters, nd W is the size of SRCH_WIN_A in chips 2001 Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 38

39 7 CDMA DESIGN ENGINEERING 9.6-Kbps nd 14.4-Kbps Systems Voice Qulity For the two vocoders, is illustrted n pproximte reltionship between men opinion score (MOS) nd FER. Tking into ccount the level of E b /N 0 versus FER result tht higher voice qulity demnds tht the system hve less coverge nd/or less cpcity. Power Control - Forwrd Link In the 14.4-Kbps system, this fst power control enbles the mobile to report the forwrd-link condition more frequently (50 times second ginst 4 times second s it is t 9.6-Kbps system) nd the benefit of this enhnced trcking performnce is reducing the required E b /N 0 on the forwrd link. Coverge: Forwrd Link - 1) Differences in processing gin of 1.76dB: 2) The 14.4-Kbps system uses convolutionl code of R = 3/4, while the 9.6- Kbps system uses more powerful convolutionl code of R = 1/2 tht mens for the 14.4-Kbps system less error protection nd thus, higher E b /N 0 is required on the forwrd link to chieve specified FER. Reverse Link - 1) Differences in processing gin of 1.76dB; 2) The Kbps system uses convolutionl code of R = 1/2, while the 9.6-Kbps system uses more powerful convolutionl code of R = 1/3. Cpcity: 2001 Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 39

40 8 CDMA PERFORMANCE ENGINEERING Performnce engineering mens process of optimizing the RF performnce of cell, cluster of cells, or system. Chnnel Supervision - On Forwrd nd Reverse link CRC is monitoring representing frme qulity indictor. Power-Control Prmeters - The mobile reports in two wys to the bse sttion the condition of the forwrd link (periodic nd threshold). Serch-Window Sizes - SRCH_WIN_A The size of this window needs to be set ccording to the nticipted propgtion environment. Direct Pth: Multipth The difference in distnce trveled between the two pths is: A smller SRCH_WIN_A cn be used to limit the number of multipths llowed! 2001 Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 40

41 Serch-Window Sizes - SRCH_WIN_A 8 CDMA PERFORMANCE ENGINEERING SRCH_WIN_A cn be used to reduce the re where the mobile cn conduct soft hndoff. If the RF engineer wishes to contin the soft hndoff re to smller re between points nd b, he or she should set SRCH_WIN_A to t lest twice the mximum of the pth differences (i.e., t lest (2 x 16.4) chips, or 32.8 chips wide) Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 41

42 8 CDMA PERFORMANCE ENGINEERING Serch-Window Sizes - SRCH_WIN_N nd SRCH_WIN_R SRCH_WIN_N is the serch window tht the mobile uses to trck the neighbor set pilots. The size of this window is typiclly lrger thn tht of SRCH_WIN_A. The mximum size of this serch window is limited by the distnce between two neighboring bse sttions. SRCH_WIN_R is the serch window tht the mobile uses to trck the remining set pilots Field Optimiztion It is importnt to note tht field conditions chnge due to loding. Pilot Strength The mobile requires sufficient E c /I 0 to lock on or to remin on the system. FER An re with high FER indictes tht E c /I 0 hs decresed below certin threshold. Some Concluding Remrks 1) CDMA performnce problems re divided into four cuses, which re: poor coverge nd high interference on forwrd nd reverse links. 2) It is lso importnt to keep in mind tht mnging CDMA performnce is effectively mnging interference nd noise Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 42

43 Introduction 9 SYSTEM NOISE MANAGEMENT Contrry to the conventionl FDMA nd TDMA systems where noise rejection dels primrily with out-of-bnd noise, CDMA system concerns mostly with in-bnd noise. The gol of n RF system engineer is to design network tht minimizes the mount of unwnted noise introduced into the receiver on both forwrd nd reverse links. Therml Noise - remind tht N = k T W (K = Boltzmnn s constnt, T = temperture in kelvin; W = bndwidth in hertz) Low-Noise Amplifier A technique to reduce the system noise figure on the reverse link is ccomplished by instlling low-noise mplifiers (LNAs) t CDMA bse sttion System Without LNAs - Figure 9.2 (left) σ is the noise enhncement given by: (R is the reverse link rise) 2001 Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 43

44 9 SYSTEM NOISE MANAGEMENT System With LNA The low-noise mplifier is so clled becuse its noise figure F' 2 is typiclly less thn the noise figure F' 4 of the rdio. σ is the noise enhncement given by: If G 2 is lrge then: Signl-to-Noise Rtio Improvement The SNR improvement quntifies how much the SNR hs improved by dding n LNA between the ntenn nd the rdio. The SNR improvement is defined s Q: If G 2 is lrge then: Cpcity Improvement Since the required E b /N 0 hs not chnged, the cpcity of bse sttion with LNAs hs not improved. However, the cpcity of neighboring bse sttion (with no LNAs) my improve becuse the neighboring bse sttion is lower loded by the mobile which trnsmit now with lower power Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 44

45 Intermodultion 9 SYSTEM NOISE MANAGEMENT Intermodultion (IM) is nonliner process tht genertes n output signl contining frequency components not present in the input signl. If we model the input x s two sinusoids t two different frequencies f 1 nd f 2 tht is: By exmining the output due to the third order term, we cn see tht there re six new frequency components tht were not present in the originl input: (3f 1 ), (3f 2 ), (2f 1 +f 2 ), (2f 2 +f 1 ), (2f 1 -f 2 ) nd (2f 2 -f 1 ). However, the frequency components (2f 1 -f 2 ) nd (2f 2 -f 1 ) cn cuse severe problem becuse these two components fll inside the operting bnd (see right). Interference Due to Other Mobiles - (t the reverse link expression E b /N 0 contin the interference term I t ) Downtilting n bse sttion ntenn is n effective wy of reducing the receive gin to those mobiles tht re fr wy while mintining n dequte gin to those mobiles tht re close by. In essence, downtilting reduces I' t Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 45

46 Introduction 10 CDMA TRAFFIC ENGINEERING For cellulr nd PCS systems, trffic engineering is the process of provisioning communiction circuits for given service re Fundmentl Concepts - Trffic Intensity Trffic is mesured in terms of trffic intensity. Trffic intensity is commonly mesured in the unit of Erlng. An Erlng is defined s the verge number of simultneous clls or is the totl usge during time intervl divided by the time intervl. Lods; Grde of Service Wht we cn directly mesure is the crried lod of bse sttion. The trffic demnd, or offered lod, during the sme time intervl cn be estimted by the following: The blocking rte is defined s mesured quntity for prticulr bse sttion. The blocking probbility is the probbility tht cll is blocked due to no chnnel vilble; The term blocking probbility is often used interchngebly with grde of service. The blocking probbility is typiclly evluted for the offered lod during the busy hour (see right fig. for bse sttion) Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 46

47 Grde of Service 10 CDMA TRAFFIC ENGINEERING Erlng-B nd Erlng-C re two widely used mthemticl models tht describe the reltionship mong the blocking probbility (grde of service), offered lod (demnd), nd number of chnnels. Erlng-B Erlng-B ssumes tht blocked clls re clered nd tht the cller tries gin lter. Erlng-C Erlng-C ssumes tht blocked clls re retried until the cll is estblished CDMA Applictions (Soft Blocking + Hrd Blocking) Soft Blocking - There my be plenty of chnnels vilble t bse sttion, but since there re mny users in the sme cell lredy, the interference level is such tht dding n dditionl user would increse the interference bove predetermined threshold. The cll is thus denied. This is the soft-blocking scenrio. Hrd Blocking - A cll my hve n excellent qulity, but if there is no chnnel vilble t the bse sttion, the cll is still blocked. This is the hrd-blocking scenrio Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 47

48 11 TDMA or CDMA - COMPARISON OF ADVANTAGES AND DISADVANTAGES Cpcity The min debte between CDMA nd TDMA concerned which provides the gretest cpcity. It turns out tht it is lmost impossible to clculte the cpcity of CDMA systems; the mthemtics is just too complicted. As experience hs subsequently shown, those who did try to clculte it generlly got it wrong. There re ctully TDMA systems nd CDMA systems working nd the esiest pproch is simply to compre the cpcity of ech in rel life. It turns out tht CDMA systems currently provide cpcity probbly round 30% greter thn TDMA systems (s gross simplifiction). This my chnge in the future s new CDMA nd TDMA techniques re introduced, but it seems likely tht CDMA systems will mintin smll but not insignificnt cpcity increse into the foreseeble future. In ddition to cpcity, number of other dvntges re climed for CDMA systems. Ech of these is exmined in this section to ssess whether the clims cn be supported. Greter rnge It is climed tht CDMA systems hve greter rnge thn equivlent TDMA systems. Rnge is relted to the minimum signl level tht the receiver cn relibly decode, so the clim is bsiclly one tht CDMA cn work with lower received signl strength thn TDMA. Sectoriztion In summry, the CDMAcpcity cn be incresed by fctor of 2 to 3 by sectoriztion with only smll increse in cost. This option is not vilble to TDMA nd, hence, represents n dvntge to CDMA Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 48

49 11 TDMA or CDMA - COMPARISON OF ADVANTAGES AND DISADVANTAGES Discontinuous trnsmission GSM use circuit in the mobile known s voice ctivity detector tht detects when you re not speking. Tht mens: Clipping - tht the first syllble tht you sy is often lost, Less interference with neighbors, The sme cpcity. CDMA use grdully reduction of informtion trnsmitted t ech speech stop, tht mens: Less interference, Incresed cpcity. No frequency plnning In generl CDMA does not require frequency plnning lthough it my require PN code ssignment plnning. However, this is not mjor dvntge. Frequency plnning cn be redily ccomplished with tody's plnning tools nd esily djusted if problems occur. Mcrocell/microcell The concept of using smll cells in high-density res is well known nd there re situtions where smller cells re deployed within the coverge re of lrger cells. Subscriber units configured for the lrger cell will be operting with much higher powers thn those configured for the smller cells. This is problem for CDMA systems nd in the worst cse this is equivlent with reducing the CDMA cpcity by fctor of two. This reduction does not occur in TDMA Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 49

50 11 TDMA or CDMA - COMPARISON OF ADVANTAGES AND DISADVANTAGES Power control Another fctor tht is used to improve the cpcity of both TDMA nd CDMA systems is power control (used to compenste ttenution cusing by distnce/obstruction/propgtion between bse sttion-mobile). So in GSM, ccurte power control cn increse cpcity, wheres in CDMA, lck of ccurte power control cn reduce cpcity. For both systems, power control hs the dded dvntge of reducing bttery drin incresing the possible tlk time. Risk TDMA systems hve been widely deployed round the globe. CDMA systems re only just strting to emerge, so there is much higher risk with CDMA tht equipment will be delyed, will not provide the promised cpcity, or will prove difficult to frequency pln, for exmple. This risk is continully reducing s experience with CDMA systems grows very rpidly. Cost Everything eventully comes down to cost. At the moment, CDMA system components cost more thn TDMA system components. However, becuse of the higher cpcity of CDMA systems, fewer bse sttions re required, resulting in lower equipment bills nd lower site nd line rentl costs. How the two fcts blnce depends on the ctul difference in equipment costs nd the extent to which the network is cpcity limited. Certinly, in highly cpcity-limited sitution, CDMA systems should prove less expensive. Other situtions re less cler. Bndwidth flexibility CDMA systems cn increse the user bndwidth simply by reducing the number of bits in the spreding sequence. TDMA systems cn lso be bndwidth flexible by ssigning more thn one TDMA slot per frme to user. Hence, both ccess methods cn be mde to be pproximtely eqully flexible Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 50

51 11 TDMA or CDMA - COMPARISON OF ADVANTAGES AND DISADVANTAGES Frequency Hopping versus Direct Sequence CDMA systems come in two vrints DS-CDMA nd FH-CDMA. The type of CDMA tht hs been discussed so fr is known s direct sequence CDMA (DS-CDMA) becuse the input dt is multiplied or spred by sequence or codeword. There is n lterntive known s frequency-hopped CDMA (FH-CDMA) where the bndwidth of the signl is not directly incresed but the trnsmitter jumps from frequency to frequency. Becuse more thn one frequency is used, the effect ppers to be to spred the bndwidth. In prctice, only one frequency is being used t one time, so the trnsmitted bndwidth is not incresed, just the spectrum required. The FH is defined s "fst" when the jumps occur more thn once in bit period nd "slow" otherwise. At the moment, fst FH is restricted to militry pplictions. Reders will remember tht FH ws lredy introduced in the context of TDMA systems tht move from chnnel to chnnel to void interference. By some quirk of history or definition, these re exctly the sme thing, tht is, FH-TDMA nd FH-CDMA re identicl Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 51

52 11 TDMA or CDMA - COMPARISON OF ADVANTAGES AND DISADVANTAGES Summry Remrks If the CDMA nd TDMA debte ws simple it would hve been resolved long go. When trying to understnd the issues, the key point to remember is tht both re simply wys of dividing up the spectrum. The only rel issue is which one cn divide up the spectrum with the lowest inefficiency nd hence relize the gretest cpcity. Initil results seem to show tht CDMA is better t this thn TDMA nd so cn provide greter cpcity. Some of the other importnt points to ber in mind re: It is unlikely tht there will be ny significnt rnge difference between the two systems, The dvntge tht CDMA hs ssocited with not hving to frequency pln the system is insignificnt, CDMA systems perform less well when microcells re deployed, CDMA my be less costly but is more risky Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 52

53 INTRAROM S.A. ROMANIAN TELECOMMUNICATIONS AND ELECTRONICS INDUSTRY ROMANIA, Buchrest, 17 Fbric de Glucoz St., Sector 2 Cod 72322, Tel:(+40 1) Fx:(+40 1) Interntionl summer students courses: "Plugged In: Modern Networks nd Services in Telecommuniction" CDMA One - THE END! Thnk you for your ttention! (the detil mteril is hosted on the uthor s web pge t the below link ddress) Presented by: Author: Web pge: Emil: Ph.D. Eng. Liviu CHIRCA (RF / Trnsmission Engineer) chircl@yhoo.com (privte) liviuc@intrrom.ro (office) Orgnized by BEST Buchrest - (Bord of Europen Students of Technology - ) 2001 Buchrest Summer Courses CDMA One Presented by Liviu CHIRCA (chircl@yhoo.com) Pge 53