CDMA Spread Spectrum in Communications Systems

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1 in Communications Systems ORIGINAL SIGNAL NBI Lecturer: Assoc. rof. Dr Noor M Khan Department of Electronic Engineering, Muhammad Ali Jinnah University, Islamabad Campus, Islamabad, AKISTAN h: +9 (5) , Ext. 47 Fax: +9 (5) noor@ieee.org, noormkhan@jinnah.edu.pk SECTRUM SREADING MAI AWGN SIGNAL SECTRUM DESREADING SIGNAL AWGN NBI MAI

2 Introduction Types of Spread Direct Spread Frequency Hopping Time Hopping erformance of SS With BSK With QSK

3 Spread Modulation Spread spectrum multiple access (SSM) uses signals which have a transmission bandwidth that is several orders of magnitude greater than the minimum required RF bandwidth. A pseudo-noise (N) sequence converts a narrowband signal to a wideband noise-like signal before transmission. SSM provides immunity to multi-path interference and robust multiple access capability. 3

4 SS Modulation Bandwidth Efficiency SS modulation is not very bandwidth efficient when used by a single user only. When many users can share the same spread spectrum bandwidth without interfering with one another, spread spectrum systems become bandwidth efficient in a multiple user environment in a multiple cell environment. 4

5 Types of Spread Direct Sequence Spread (DSSS) The carrier is modulated by a digital code Digital code chip rate is much larger then the information signal bit rate Frequency Hopping Spread (FHSS) The carrier frequency is shifted in increments in a random pattern generated by code sequence The FHSS could be fast FHSS or slow FHSS Time Hopping Spread (THSS) Transmission time is divided into frames and frames are divided in time slots. Signal transmission takes place in a random time slot controlled by a random code (THSS has limited application, so far ) 5

6 Frequency Hopped Multiple Access(FHMA) In FHMA the carrier frequencies of the individual users are varied in a pseudorandom fashion within a wideband channel. The instantaneous bandwidth of any one transmission burst is much smaller than the total spread bandwidth. At any given point in time, a frequency hopped signal only occupies a single, relatively narrow channel. M( ) = F{ m(t) } ( ) = F{ p(t) } B m B a 6

7 FH Classification Fast frequency hopping system If the rate of change of the carrier frequency is greater than the symbol rate. Slow frequency hopping If the channel changes at a rate less than or equal to the symbol rate.. A frequency hopped system provides a certain level of security. An intercepting receiver not knowing the N sequence of frequency slots must chase rapidly the signal it wishes to intercept. 7

8 DS SS System Model m(t) Transmitter p(t) m(t) Message a(t) Spreading Code a(t) Channel n(t) p(t) Spreaded Signal n(t) AWGN Noise d(t) Receiver r(t) r(t) Received Signal d(t) Despread Signal a(t) r(t) = m(t) a(t) + d(t) = r(t) a(t) n(t) 8

9 DS SS Modulation in Time and Frequency - - Time Frequency 9

10 Spreading Code, a(t) roperties M( ) = F{ m(t) } ) a(t) a(t) = ) B a >> B m B m A( ) = F{ a(t) } d(t) = r(t) a(t) d(t) = m(t) a(t)a(t) + n(t)a(t) d(t) = m(t) + n a (t) B a 0

11 of SS Signal M( ) = F{ m(t) } p(t) = m(t)a(t) B m A( ) = F{ a(t) } ( ) = M( ) A( ) since B a >> Bm ( ) = F{ p(t) } B a B m B a Bp B a B G = B a m

12 Spread System Before Spreading After Spreading After Despreading M( ) B m ( ) M( ) B m - signal power B m B a N 0 =k T a(t) = σ = N0Bm B G = B Spread a m SNR = Gσ SNR o = σ

13 Hiding Signal in Noise ( ) AWGN N 0 = k T D( ) Before despreading AWGN B m After despreading LI Low robability of Interception 3

14 Two Signals in Noise r(t) = m (t)a(t) + m(t)a(t) + n(t) d (t) = r(t)a(t) d (t) = m (t)a(t)a(t) + p(t)a(t) + n(t)a(t) d (t) = m (t) + p(t)a(t) + n(t)a(t) Signal Interference Noise We want to find output SNR o to evaluate the erformance, i.e. the BER 4

15 Two SS Signals M ( ) M ( ) B m ( ) ( ) B m N 0 =k T M ~ ( ) Signal Interference B a AWGN N 0 =k T B m B a B a 5

16 Output Signal to Noise Ratio Signal power - > Noise power - > Effective interference power - > Total noise SNR power o = σ + G - > SNRm = + G σ G m σ + G 6

17 k Signals in Noise r(t) = m (t)a(t) + m(t)a(t) + + m k(t)ak(t) + n(t) d (t) = r(t)a(t) d (t) = m (t)a(t)a(t) + p(t)a(t) + + pk(t)a(t) + n(t)a(t) d (t) = m (t) + p(t)a(t) + + pk(t)a(t) + n(t)a(t) Signal Interference Noise Again we want to find output SNR o ; -> BER 7

18 K SS Signals M ( ) ( M ) M k ( ) B m () Bm () k ( ) Bm N 0 =k T ~ M ( ) Signal Interference B a AWGN N 0 =k T B m B a B a 8

19 Output Signal to Noise Ratio Signal power - > Noise power - > Effective interference power - > Total noise power - > σ (k -) G (k -) σ + G SNR o = σ + (k -) G = + SNRm (k -) SNR G m 9

20 High SNR m SNR o = σ + (k -) G = + SNRm (k -) SNR G m SNR o = k k -= G k - G SNR o 0

21 CDMA Frequency Reuse Reuse Factor =/7 Reuse Factor = FDMA/TDMA CDMA B G C A F D B E G C B A G C F D A E F D E

22 Overview of Adaptive CDMA Multi-user Detection ORIGINAL SIGNAL NBI SECTRUM SREADING MAI AWGN SIGNAL SIGNAL BS SECTRUM DESREADING AWGN NBI MAI

23 Technical challenges Is the mobile Internet possible in the same form as the wire-line Internet? Does the existing and future 3G mobile communication infrastructure support increased capacity demand of the future mobile Internet? If not, how it can be fixed? Does Adaptive Multi-user Detection help to solve any of the mentioned questions 3

24 Convergence of Applications Current Services: Radio, TV, telephone, VCR, CD, WiFi, cable TV; Large but LIMITED choice, Dedicated hardware. Future Services: Internet, Games, Library, TV, CD, ersonal Video Unlimited choice Internet software terminal (user specified) Communication Capacity requirement - order of Mb/s per user 4

25 Last Mile Access (LMA) Capacities Two major Home-Wire-line Access Options: HCF Cable TV (GHz for homes, ~MHz per home) ADSL Telephone Twisted air,.5 Mbit per home Number of users Distances km 5

26 Wireless Cellular Mobile System MSC STN 6

27 Migration ath to 3G Mobile Comm. Systems First Major Migration ath (Europe ) I Gen, 80 s, ETACS (C-450,NMT-450..),(FDMA), Analog II Gen, 90 s, GSM, GRS, EDGE, (TDMA) Digital III Gen, 00 s, W-CDMA, (CDMA), All Digital Second Major Migration ath (USA) I Gen, 80 s, AMS, (FDMA), Analog II Gen, 90 s, IS-54 (TDMA), IS-95 (CDMA), Digital III Gen, 00 s, CDMA000 (CDMA), All Digital 7

28 Mobile Network Capacity Information Bit Rate, Mb/s Wired Terminal WLAN Cordless MBS 60GHz Course of action 3G Cellular G Cellular and CS This is where we want to be Office Building Stationary Walking Vehicle Indoors Outdoors MOBILITY 8

29 Communication Channel Capacity Communication Channel Resources are: T - Time B - Bandwidth σ - Signal to Noise Ratio S I I = T B S T B S + σ = log Amount of information (number of bits) transmitted/received, with arbitrarily small probability of error (Shannon 948) C + σ = B log 9

30 Multiple Access S T S T S T FDMA AMS B TDMA GSM B CDMA IS-95 B S I T B 30

31 CDMA a Case of DMA Multi-user channel with + = with bit rates R satisfies R + R = log = C. y = x + x + and single user capacity C = log + N + ; R = log + N+ N n + + R + R = log + log N+ N 3

32 Users Multi-user Channel + R + R = log + N+ N log a + logb = log(ab) N+ + N+ = log N+ N N+ + = log N = log + = C N + a b b + a = b + = used identities 3

33 N Users Multi-user Channel The same applies for channel y = x + x xm + n With power limitation And data rates R R R m = log = log = N+ + N+ = log m + N ; m m + + m + + m- m- ; C = R + R m. R 33

34 N Users Multi-user Channel C = R + R m R y = x + x xm + n = m C = m log + log... log ;. N m m N m m N C = log N + m + m N+ m + m N + m + m- N +... N + m + m N + m + m N + m N m N+ m + m = log N C = log + N C C = log + N y = x+ n = 34

35 Multi-user Channel The previous result requires involvement of powerful coding techniques and interference cancellation In reality channel coding is part of every current digital mobile communication standards Interference cancellation/suppression is still a hot research topic In order to implement power division multiple access (DMA) concept in current practice we rely on the spread spectrum technique such as code division multiple access (CDMA) 35

36 Why CDMA? Advantages of CDMA (IS-95) Digital System (extensive use of digital algorithms) Voice activity factor (flexible use of channel resources) Frequency reuse factor (All cells on the same frequency) Fading resistant (wideband system, RAKE diversity) Limitations of CDMA (IS-95) Near-Far effect Necessity of strict user power control 36

37 CDMA System Model x Transmitter y=sx+n y Receiver ^ x s s x Transmitter S y Receiver ^ x s + s x K Transmitter K AWGN y Receiver K ^ x K s K s K 37

38 Multi-user Receivers Classification Multi-user Detectors Adaptive Multi-user Detectors Optimality criterion Source of Sufficient Statistics Type of detector Short Name Ref. Source of Sufficient Statistics Type of detector Short Name Ref. ML; BER MF Single user limiter ML; ZF MF Decorelator R - MF MF- DEC ursley77 ursley8 Schneider80 Lupas89 MMSE MF Symbol MMSE (R+σ I) - MF- MMSE ZF; MMSE MF DFE MF- CDEC Xie90 Madhow94 Duel-Hallen A-MMSE Single user limiter A-MMSE DFE MMSE- DFE DFE MMSE Rapajiic 9-94 Miller95 MMSE- CDFE Abdulrahman9-94 ; Woodward000 ; Rapajiic 94 ML MF ML MF-ML Verdu86 A-MMSE ML MMSE- ML Rapajiic 99 Borah00 38

CDMA Spread Spectrum in Communications Systems

CDMA Spread Spectrum in Communications Systems Muhaad Ali Jinnah University, Islaabad Capus, akistan in Counications Systes ORIGIAL SIGAL BI Lecturer: Assoc. rof. Dr oor M Khan Departent of Electronic Engineering, Muhaad Ali Jinnah University, Islaabad