CH 4. Principle of Code Division Multiple Access

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1 CH 4. rinciple of Code Division Muliple Access Korea Aerospace Universiy Mobile Counicaions Lab.

2 Conens Wha is CDMA? Basic Concep of Spread Specru Techniques Spreading Codes Used for CDMA Walsh Codes seudo-rando Noise (N) Codes, ec. Classificaion of CDMA Syses CDMA Feaures Suary: CDMA Syse Benefis Korea Aerospace Universiy Mobile Counicaions Lab.

3 Wha is CDMA? CDMA (Code Division Muliple Access) is a ype of uliple access schees in hich each user expands he signal bandidh in excess of iniu necessary o send he inforaion and shares he sae specru a he sae ie. In CDMA, each user can be differeniaed by is on unique spreading code. Spreading a signal in CDMA is, in usual, accoplished by eans of N (pseudo-rando noise) codes, hich are generaed by using a linear feedback shif regiser (LFSR). Korea Aerospace Universiy Mobile Counicaions Lab.

4 Basic Concep of Spread Specru Techniques [4] Definiion Spread-specru is a radio ransission echnology in hich he signal bandidh is expanded in excess of he iniu necessary o send he inforaion. riarily used for iliary purpose due o is inrinsic naures such as lo probabiliy of deecion and robusness o jaing, ec. Since 99s, applied o coercial obile counicaion syses called CDMA. Types of spread-specru syses; direc-sequence spread specru (DS- SS), frequency-hopping spread specru (FH-SS), hybrid ype spreadspecru, and chirp spread-specru syses. Korea Aerospace Universiy 4 Mobile Counicaions Lab.

5 Basic Concep of Spread Specru Techniques (con.) Fig. 4. Transier side of a DS-SS syse. Korea Aerospace Universiy 5 Mobile Counicaions Lab.

6 Basic Concep of Spread Specru Techniques (con.) Noise AB A A W ( ) SF ( ) o o (SNR) baseband = = = N = SNR = SNR RF RF NB o No No B Fig. 4. Receiver side of a DS-SS syse. Korea Aerospace Universiy 6 Mobile Counicaions Lab.

7 Spreading Codes for CDMA [],[] Tradiional ay for uliple access is o separae signals eiher in he ie doain (TDMA) or in he frequency doain (FDMA). Hoever, in CDMA, differen users occupy he sae bandidh a he sae ie bu are separaed fro each oher via soe code sequences assigned o each user. To represenaive code sequences used for CDMA: Orhogonal codes seudo-rando noise (N) codes Orhogonal codes are soeies used in CDMA syses o isolae differen channels in eiher forard or reverse link. Orhogonaliy To rando processes X() and Y() are called orhogonal if hey saisfy RXY [ ] ( τ) = E X( Y ) ( + τ) = for all τ. To periodic funcions x() and y() are called orhogonal if hey saisfy T Rxy () = x ( ) y ( ) d =, T: period T To periodic sequences x and y are called orhogonal if x T y =. Korea Aerospace Universiy 7 Mobile Counicaions Lab.

8 Spreading Codes: Walsh Code [] In he IS-95 CDMA syse, he Walsh code ih a diension of 64 x 64 is used o isolae forard link channels. The 64 x 64 Walsh code is generaed by folloing recursive procedures. H H H = H, = N N N H HN HN = H H [ ] 4 = = H H H H =,, Korea Aerospace Universiy 8 Mobile Counicaions Lab.

9 Spreading Codes: Walsh Code (con.) To illusrae he isolaion of differen channels using Walsh codes, suppose ha here are hree differen users and ha each user ries o send a separae essage, [ + + ], = [ + + ], = [ + ] = + Each of he hree users is assued o be assigned a Walsh code as follos, = = = [ + + ] [ + + ] [ + + ] Korea Aerospace Universiy 9 Mobile Counicaions Lab.

10 Korea Aerospace Universiy Mobile Counicaions Lab. Spreading Codes: Walsh Code (con.) Afer spreading, he coposie spread-specru signal is given by In order o separae ou he original essages (), (), and () fro he coposie signal C(), he receiver uliplies C() by he assigned Walsh code and inegrae over one bi duraion as follo, Since he decision rule is given by ( ) ( ) ( ) ( ) ( ) ( ) ( ) C + + = ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) { } ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) { } ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) { } ( ) = = + + = = + + = = b b b b b b T T T T T T d d C M d d C M d d C M ( ) ( ) ˆ, if ˆ, if i i i i M M = > = <

11 Spreading Codes: Walsh Code (con.) The esiaed essages are as follos, ( ) ( ) ( ) ˆ = ˆ = ˆ = I should be noed ha he orhogonaliy beeen Walsh codes is ainained only in case here here is no shif beeen he codes. Copare and. Therefore he Walsh code is osly used in case here all channels are aligned on ie, as in he forard link of IS-95. Korea Aerospace Universiy Mobile Counicaions Lab.

12 Spreading Codes: N Code [],[] The os represenaive spreading code used for CDMA is he N code ha ay be generaed by a linear feedback shif regiser. Since here are exacly n - non-zero saes in a linear n-sage shif regiser, he period of he sequence generaed by a linear n-sage shif regiser canno exceed n -. f n n ( x) = + c x + c x + + c x + x, c or. n i = Fig. 4. An n-sage linear feedback shif regiser. Korea Aerospace Universiy Mobile Counicaions Lab.

13 Spreading Codes: N Code (con.) If he period of he N sequence generaed by an n-sage linear shif regiser is n -, he N sequence is called he axial-lengh pseudorando noise sequence or -sequence. The noralized auocorrelaion funcion of a axial-lengh pseudo-rando noise sequence is o-valued and periodic, hich is given by ( ) R k n ( ). k = l = k l N n ( ) here l is any ineger ih n denoing he nuber of sage of he shif regiser and N = n -. Korea Aerospace Universiy Mobile Counicaions Lab.

14 Spreading Codes: N Code (con.) x x x ( x) = + x x f + Fig. 4.4 A -sage linear feedback shif regiser and is oupu. Korea Aerospace Universiy 4 Mobile Counicaions Lab.

15 Spreading Codes: N Code (con.) Fig. 4.5 Auocorrelaion funcion of -sage axial-lengh linear feedback shif regiser. Korea Aerospace Universiy 5 Mobile Counicaions Lab.

16 Spreading Codes: N Code (con.) N codes can be used for uliple access. Suppose again he sae hree users ish o send hree separae essages, [ + + ], = [ + + ], = [ + ] = + Also assue ha each of he hree users is assigned a N code, respecively, 6 [ ] [ ] [ ] = = + + +, = Then he coposie signal is given by ( ) = ( ) p ( ) + ( ) p ( ) ( ) p ( ) C o + 6 In order o separae ou he original essages (), (), and () fro he coposie signal C(), he receiver uliplies C() by he assigned N code and inegraes i. Korea Aerospace Universiy 6 Mobile Counicaions Lab.

17 7 Korea Aerospace Universiy Mobile Counicaions Lab. Spreading Codes: N Code (con.) I should be noed ha he cross-correlaion of differen N codes is no zero, hich becoes a source of oher-user inerference in CDMA. The N codes are osly used o isolae differen users in case here orhogonal codes canno be eployed as in he CDMA reverse. ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) { } ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) { } ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) { } ( ) = = + + = = + + = = b b b b b b T T T T T T d d C M d d C M d d C M

18 Classificaion of CDMA Syses CDMA syses can be classified ino he synchronous CDMA syse and he asynchronous CDMA syse. Synchronous CDMA Syse All base saions are synchronized ih each oher by using he ie inforaion obained fro GS saellies. All base saions use he sae N code ih differen phase shifs so ha he iniial synchronizaion a he obile is relaively easy. IS-95 and cda syses Asynchronous CDMA Syse Unlike synchronous syses, GS saellies are no used for synchronizaion hile base saions are roughly synchronized ih each oher by using a neork clock. Each base saion should eploy a differen N code ==> Iniial synchronizaion requires a long ie and a special synchronizaion circui so called he ached filer should be used. WCDMA syse Korea Aerospace Universiy 8 Mobile Counicaions Lab.

19 CDMA Feaures: High Frequency Reuse Efficiency [],[] The frequency reuse facor is defined for AMS and TDMA syses as he acual bandidh assigned o each base saion divided by he oal bandidh: F = /7 In a CDMA syse, he frequency reuse facor is redefined as he self-cell inerference over oal inerference. self-cell inerference F = =.% oal inerference + f.%.%.%.%.% =.7 + 6k + k + 8k +.%.%.%.%.%.% 6 % 6 % 6 %.%.% k =. 6, k =., k =..% %.%.%.% 6 % 6 % 6 %.%.% f = oher-cell inerference : loading facor self-cell inerference.%.%.%.%.%.%.% ==> High frequency reuse efficiency!.%.%.% Fig. 4.6 Typical inerference in hexagonal cellular paerns. Korea Aerospace Universiy 9 Mobile Counicaions Lab.

20 CDMA Feaures: Secoring Gain [],[] The inerference a he base saion can be reduced significanly if he cell is secored. Three secor anenna is ypically used so ha each secor receives he signals coing over only degrees as shon in Fig This arrangeen decreases he inerference by a facor of approxiaely /, eaning ha secoring gain G a is. If he cell is secored o six secors, hen he inerference is decreased by a facor of approxiaely /6. In acual, hoever, he secoring gain becoes abou.5 for hree-secor anenna and 5 for six-secor anenna due o overlapping of anenna beapaern. Korea Aerospace Universiy Mobile Counicaions Lab.

21 CDMA Feaures: Secoring Gain (con.) Fig. 4.7 Secored anenna paern. Korea Aerospace Universiy Mobile Counicaions Lab.

22 CDMA Feaures: Voice Aciviy Gain [],[] The voice aciviy is defined as he acual alking ie noralized by he user s overall holding ie in voice counicaions. CDMA syses can reduce inerference effecively by using he voice aciviy in such a ay ha he syse does no ransi any signal hile a user is no acually alking. The voice aciviy is abou.4 so ha he channel capaciy can be increased up o.5 ies by siply uilizing he voice aciviy. Korea Aerospace Universiy Mobile Counicaions Lab.

23 CDMA Feaures: oer Conrol [],[] There are o probles in cellular environens, near-far proble and fading, hich can be solved by poer conrol. Especially in CDMA, all users share he sae specru so ha each user looks like inerference o oher users. Therefore, he poer of each user us carefully be conrolled so ha no user is unnecessarily inerfering ih ohers. Fig. 4.8 Near-far proble. Korea Aerospace Universiy Mobile Counicaions Lab.

24 CDMA Feaures: oer Conrol (con.) There are hree poer conrol schees in CDMA: closed-loop, open-loop, and ouer-loop poer conrols. We herein invesigae he ainly for he reverse link. Closed-loop poer conrol The closed-loop poer conrol is used o copensae for rapid signal flucuaion due o ulipah fading. As shon in Fig. 4.9, a base saion easures E b /N o or received poer fro a obile saion. If he easured E b /N o (or poer) exceeds a cerain hreshold, hen he base saion coands he obile o decrease is ransi poer. If he easured E b /N o (or poer) is belo a cerain hreshold, hen he base saion coands he obile o increase is ransi poer. In IS-95 reverse link, he ransi poer a obile is adjused every.5 s (poer conrol rae: 8 Hz) ih a sep size of db. The poer conrol coand is ransied by puncuring forard raffic channel ih poer-up or poer-don coand every.5 s, as shon in Fig. 4.. Korea Aerospace Universiy 4 Mobile Counicaions Lab.

25 CDMA Feaures: oer Conrol (con.) Deod Fig. 4.9 Reverse link closed-loop and ouer-loop poer conrol. Korea Aerospace Universiy 5 Mobile Counicaions Lab.

26 CDMA Feaures: oer Conrol (con.) Fig. 4. uncuring he poer conrol bi in IS-95 syses. Rec. signal poer Trans. signal poer Rec. signal poer Disance or ie No poer conrol Disance or ie Closed-loop poer conrol Disance or ie Closed-loop poer conrol Fig. 4. oer conrol effec agains shor-er fading. Korea Aerospace Universiy 6 Mobile Counicaions Lab.

27 CDMA Feaures: oer Conrol (con.) Open-loop poer conrol The open-loop poer conrol is used o copensae for a large flucuaion of he signal due o long-er fading here here is a high correlaion beeen he forard and reverse links. As shon in Fig. 4., a obile saion coninuously oniors he received poer fro he base saion and adjuss ransi poer in response o he change in received poer fro base saion. The er open-loop is used in ha i is purely a obile-conrolled operaion and does no involve he base saion a all. Ouer-loop poer conrol In CDMA syses, he qualiy of link is finally deerined by he FER (frae error rae) or BER (bi error rae). The ouer-loop poer conrol is he process o adjus arge E b /N o (or hreshold) in response o he FER easureens a he base saion, as shon in Fig Tha is, if he easured FER a he base saion is larger han a cerain hreshold ( - FER for voice for exaple) for a given arge E b /N o (or hreshold), hen he base saion increases he arge E b /N o. Oherise, he base saion decreases he arge E b /N o. Korea Aerospace Universiy 7 Mobile Counicaions Lab.

28 CDMA Feaures: oer Conrol (con.) Fig. 4. Reverse link open-loop poer conrol. Korea Aerospace Universiy 8 Mobile Counicaions Lab.

29 CDMA Feaures: Sof Handoff [],[] Handoff (or handover) is he process ha a obile changes he call connecion fro he curren base saion o he arge base saion ih beer channel condiions in order o coninuously counicae ih he oher pary in cellular neorks. There are hree handoff schees used in CDMA syses; hard handoff, sof handoff, and sofer handoff. Cell A Cell B Fig. 4. Handoff in cellular obile syses. Korea Aerospace Universiy 9 Mobile Counicaions Lab.

30 CDMA Feaures: Sof Handoff (con.) Hard handoff: A obile saion sars o counicae ih a arge base saion afer inerruping counicaions ih a curren base saion (break-before-ake). I occurs beeen CDMA syses ih differen frequency assignens in curren DCN (digial cellular neork) and CS syses. I also occurs beeen analog and CDMA syses or beeen analog syses. Sof handoff: A obile saion sars o counicae ih a arge base saion ihou inerruping counicaions ih a curren base saion (ake-before-break). I occurs beeen CDMA syses ih he sae frequency assignens. During sof handoff, he obile saion receives he signals fro boh arge and curren base saions and boh base saions also receive he signal fro he obile saion. By using sof handoff, a significan increase of cell coverage can be obained in addiion o he dissoluion of call-drop probles. Sofer handoff: A obile saion changes he secor inside a base saion by he sae schee as he sof handoff (sof handoff beeen secors). Korea Aerospace Universiy Mobile Counicaions Lab.

31 CDMA Feaures: Sof Handoff (con.) Fig. 4.4 Cobining of received signals a obile saion during sof handoff. Korea Aerospace Universiy Mobile Counicaions Lab.

32 CDMA Feaures: Sof Handoff (con.) Fig. 4.5 Selecion diversiy during sof handoff a base saion conroller (BSC). Korea Aerospace Universiy Mobile Counicaions Lab.

33 CDMA Feaures: Rake Receiver [],[] A ype of receiver consising of parallel deodulaors o uilize various diversiy echniques in CDMA syses is called a rake receiver. A hree-finger rake receiver is used a Qualco s MSM (obile saion ode) chip, hile a four-finger rake receiver is eployed a Qualco s BSM (base saion ode) chip. Fig. 4.6 Rake receiver. Korea Aerospace Universiy Mobile Counicaions Lab.

34 Suary: CDMA Syse Benefis [],[] High Capaciy: Theoreically ies larger han analog syses due o uilizing voice aciviy, various diversiy echniques, and high frequency reuse efficiency, secoring gain, ec. In acual, abou ies larger han analog syses High rivacy: The user privae long code and daa scrabling provide high degree of privacy High Qualiy of Service: Robus o fading by using various diversiy echniques due o ideband naure. Reduced call-drop by sof hand-off Easy Cell lanning and Syse Deployen: frequency assignen unnecessary Increased Baery Life Tie: Reduced ransi poer due o lo required E b /N o ==> Saller handse size Korea Aerospace Universiy 4 Mobile Counicaions Lab.

35 References. Sauel C. Yang, CDMA RF Syse Engineering, Arech House, Qualco, CDMA Syse Training Handbook-vol., 99. Korea Aerospace Universiy 5 Mobile Counicaions Lab.

L A-B-C dei Segnali Spread-Spectrum

L A-B-C dei Segnali Spread-Specrum Marco Luise Universiy of Pisa, Ialy Diparimeno Ingegneria dell Informazione hp://www.ie.unipi.i/m.luise PAM Signal +A -A s() a 0 a 1 a 2 a 3 a 4 {a k }=+1 Binary Symbols