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Overview of Direct Sequence Spread Spectrum Code Division Multiple Access Prof. Dr. Essam Sourour 1 Overview of Presentation • Spread Spectrum techniques • Direct Sequence Spread Spectrum • DS-SS CDMA • CDMA in Cellular Systems • CDMA versus TDMA • Spreading Codes • Multipath fading • Rake receiver 2 Overview of Presentation (cont.) • Voice Activity and variable rate Vocoder • Interference cancellation • Near Far Problem & Power Control • Soft handoff • Cell breathing • Multi-Carrier CDMA • Advantages and disadvantages • Summary 3 Spread Spectrum Systems • Invented for military communications • Two main types: – Direct Sequence: Wide band all the time – Frequency Hopping: Narrow band hopping signal frequency frequency f10 f9 f8 f7 f6 fc f5 f4 f3 f2 f1 time time Ts Direct Ts Frequency Sequence Fopping 4 Spread Spectrum Systems • Frequency Hopping – Narrow band signal hopping pseudo-randomly – Mitigates narrow band jamming – Different users use different hopping patterns • Direct Sequence – Spreading narrow band signal over wide bandwidth using a high rate spreading code – Receiver de-spreads signal and spreads jammer signal – Most jamming signal power filtered out – Different users use different spreading codes having small cross-correlations 5 DS-SS System • At transmitter: low rate data is spread by a high chip rate pseudo-noise (PN) code • Jamming or interference present • At receiver: data de-spread by a synchronized PN code • Processing Gain=Bs/Bd = Tb/Tc = # of code chips per bit data PN Code data data Filter Tb Tc Jammer PN code Carrier Carrier PN code LPF Jammer f f f Bd Bs 6 DS-SS CDMA • Unique code per user (typically, 64 or 128 chips) • Synchronized Walsh codes = Zero cross-correlation • Example: 2 synchronized users Tb T b i j C i t C j t dt 0 0 i j d1(t) Z1(t) d1(t) . dt d1(t) Tb C1(t) C1(t) Carrier Carrier C1(t) d2(t) d2(t) C2(t) C2(t) Carrier Tb Tc 7 Principle of CDMA • Assume codes take values ±1 • Received signal = d1(t) C1(t) + d2(t) C2(t) • Receiver 1 multiplies by own code C1(t) • Receiver 1 gets: Z1(t)= C1(t) ( d1(t) C1(t) + d2(t) C2(t) ) Z1(t)= d1(t) + d2(t) C1(t) C2(t) • After integration over code period, output is: User 1 Output = Tb d1(t) + Zero • Same applies for more users 8 CDMA Quick Exercise • For the previous signal: • Multiply: s1(t) = d1(t) C1(t) • Multiply: s2(t) = d2(t) C2(t) • Add Z(t) = s1(t) + s2(t) , chip by chip • Multiply the sum Z(t) C1(t) • Accumulate the first 8 chips and the second 8 chips 9 CDMA Quick Exercise: d1 1 1 1 1 1 1 1 1 -1 -1 -1 -1 -1 -1 -1 -1 C1 d1 C1 d2 -1 -1 -1 -1 -1 -1 -1 -1 1 1 1 1 1 1 1 1 C2 d2 C2 d1C1+ d2C2 C1(d1C1+ d2C2) Add 8’s 10 Orthogonal Codes • Codes of previous example are orthogonal Walsh codes • Orthogonal codes have cross-correlation=0 • Orthogonality preserved only if the codes are synchronized, i.e., start and end together • If one code is delayed compared to the other, orthogonalty is lost • System still works, with some interference among users Tb T b i j cross correlation C i t C j t dt 0 0 i j 11 Non-Orthogonal Users • Receiver 1 gets: Z1(t)= C1(t) ( d1(t) C1(t) + d2(t-t) C2(t-t) ) Z1(t)= d1(t) + d2(t-t) C1(t) C2(t-t) • After integration over code period, output is: User 1 Output = Tb d1(t) + Interference • More non-orthogonal users = more interference 12 CDMA Quick Exercise 2: d1 1 1 1 1 1 1 1 1 -1 -1 -1 -1 -1 -1 -1 -1 C1 d1 C1 Delayed d2 1 1 1 -1 -1 -1 -1 -1 -1 -1 -1 1 1 1 1 1 Delayed C2 Delayed d2 C2 Get Z1 C1 Z1 Add 8’s 13 Non-orthogonal CDMA • User orthogonal only in a downlink without large multipath spread • With multipath, delayed paths cause interference to each other • In an uplink, users are not synchronized, they cause interference to each other • CDMA capacity is a soft capacity, number of users limited by allowed interference level • You can always squeeze in one more person 14 Orthogonal Versus Non-Orthogonal Orthogonal if orthogonal codes are used Orthogonal Signals Uplink Building 1 Path 1 Path 2 Downlink Non-Orthogonal Signals 15 Spreading Detailed Code Example: 8 chips spreading code I time Filter D/A I Encoded bits Modulator RF Filter D/A Q time Pulse Code Shaping Spreading Tc code time After Spreading time After D/A time 16 De-Spreading Details Code time I A/D Filter Encoded code Demodulat bits RF or A/D Filter code Q After A/D Pulse Tc & Filter Shaping Code time After Sampling time After Sum time 17 Complex Spreading Codes Complex Multiplication Filter D/A I Encoded bits Modulator RF Q Filter D/A CI CQ Pulse Complex Code Shaping Pulse Complex Shaping Multiplication A/D Filter I S Encoded bits De- RF Modulator A/D Filter Q S Tc CI -CQ Complex Conjugate of Code • CI and CQ are two different orthogonal codes • Chips of CI and CQ take values ±1/SQRT(2) • Despreading with complex conjugate of the code 18 Cellular DS-SS CDMA • In a CDMA Cellular system: – Code(s) per user (assigned per call) c(t) – Code per base station, p(t) di(t) di(t) Tb dt carrier p(t) Ci(t) Ci(t) S p(t) carrier dj(t) dt dj(t) Tb Cj(t) carrier p(t) Cj(t) Base Station Cell Phones 19 DS-SS CDMA Frequency Reuse Frequency re-use factor =1 B G C A F D B E G C B A G C F D A E F D E TDMA Systems, frequency re-use, example 7 CDMA Systems, frequency re-use = 1 20 CDMA versus TDMA • TDMA: Users separated by time slots • CDMA: Users separated by codes user 1 user 2 user 3 Time TDMA Time Time Time CDMA 21 CDMA versus TDMA • TDMA: – Users separated by time slot and frequency – Burst transmission & reception – Narrower bandwidth – Frequency reuse > 1 (4 or 7 usually) • CDMA: – Users separated by code and frequency – Continuous transmission & reception – Wider bandwidth – Frequency reuse = 1 22 Spreading Codes • Codes with low or zero cross-correlation required to reduce multi-user interference • WCDMA & CDMA-2000: Walsh code assigned to user/call • Synchronous Walsh codes zero cross-correlation – Synchronous Forward link without multipath fading no intra-cell multi-user interference • Asynchronous Walsh codes non-zero cross- correlation – Forward link inter-cell interference – Multipath in Forward link intra-cell interference – Reverse link Inter/Intra-cell interference 23 Spreading Codes (Cont.) • Several types of SS codes are used in both 3G CDMA Systems: • Walsh codes: assigned per call in CDMA- 2000 & WCDMA • Maximal length codes (m-sequences): used in pilot code for CDMA-2000 • Gold codes: used as scrambling codes in WCDMA • Kasami codes: Optional additional scrambling code in WCDMA 24 Walsh Codes Generation Length N = 2n +1 +1 +1 -1 +1 +1 +1 +1 Hn H H 2n n +1 -1 +1 -1 H n H n +1 +1 -1 -1 +1 -1 -1 +1 +1 +1 +1 +1 +1 +1 +1 +1 H2 1 1 +1 -1 +1 -1 +1 -1 +1 -1 1 1 +1 +1 -1 -1 +1 +1 -1 -1 +1 -1 -1 +1 +1 -1 -1 +1 +1 +1 +1 +1 -1 -1 -1 -1 +1 -1 +1 -1 -1 +1 -1 +1 +1 +1 -1 -1 -1 -1 +1 +1 +1 -1 -1 +1 -1 +1 +1 -1 25 OVSF Codes OVSF=Orthogonal Variable Spreading Factor • Walsh codes of different lengths • When code used, branching codes not used • Shorter code for higher data rate, but lower processing gain 11111111 1111 11 1 1 1 1 -1 -1 -1 -1 1 1 -1 -1 1 1 -1 -1 1 1 -1 -1 1 1 -1 -1 -1 -1 1 1 1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 -1 1 -1 1 1 -1 1 -1 -1 1 1 -1 -1 1 1 -1 -1 1 1 -1 -1 1 -1 1 1 -1 Set of Walsh Set of Walsh codes N=4 codes N=8 26 M-Sequence Generation • Generating polynomials set feedback nodes • Length N = 2n-1 • Longest sequence that can be generated with shift register of length n (maximal-length sequence) • Generating polynomial g(x) g(x) = 1 + g1x + g2x2 +….. + gn-1xn-1 + xn 0/1 0→1 +1/-1 D D D D 1 →-1 g0=1 g1 g2 gn-1 gn=1 27 M-Sequence properties • Code period is N=2n-1 • If the PN code is represented in 0 and 1 – There 2n-1 ones and 2n-1-1 zeros – Shift and add: any modulo 2 sum of the code with a circular shifted version of itself generates the same code but shifted • If the PN code is represented in +1 and -1 – The autocorrelation of the code is given by N t 0 C t 1 otherwise 28 Autocorrelation of m-sequence 35 Auto correlation of M-sequence of length 31 (n=5) 30 25 20 Auto Correlation 15 10 5 0 -5 -30 -20 -10 0 10 20 30 t 29 Gold Codes Generation • Two preferred m-sequence polynomials added • Can generate N+2 codes of length N= 2n-1 n 2 2 n 2 2 • Low cross-correlations= 1, 1 2 , 1 2 • Example: n=5, cross correlation= {-1,-9,7} D D D D 1 g1 g2 1 0/1 0→ 1 -1/+1 1→-1 D D D D 1 g1 g2 1 30 Codes Examples: m-sequence D D D D D D with n=6 g(x)=1+x+x6 D D D D D g(x)=1+x2+x5 D D D D D Gold sequence with n=5 g(x)=1+x+x2+x4+x5 31 Gold codes example (n=7) • Gold code example with n=7: g1 x 1 x 3 x 7 g 2 x 1 x x 2 x 3 x 7 • This generates 129 Gold codes of period 127 32 Multipath Fading • Multipath causes Rayleigh fading • When Delay Spread > Tc resolvable multipath • WCDMA small Tc more resolvable paths • Each path adds Interference & also Diversity • Example WCDMA: Tc= 0.26 ms • Delay spread 2.5 ms 2.5/0.26 9 paths chip 33 Rake Receiver • Multipath diversity = multipath is advantageous • One finger (correlator) per path • Each finger synchronized to one path • Finger outputs combined (MRC) t g2 g1 . dt Tb chip p(t) C1(t) g1 d(t) Carrier . dt Tb p(t-t) C1(t-t) g2 34 Rake Receiver Fingers • One rake finger for each channel path • Codes on each finger delayed to desired path • Gain in each finger is complex conjugate of path gain (Maximal Ratio Combining) • Additional subsystems needed: – Delay tracking: to track delay changes – Channel gain estimation: to find paths gains {gi} – Searcher: to find new stronger paths (and new t base stations) g2 g1 . dt Tb chip p(t) C1(t) g1 d(t) Carrier . dt Tb p(t-t) C1(t-t) g2 35 Multipath Interference on Rake Rx. • Assume 3 CDMA users • User A Channel with 2 paths User C User B • Phone A sets 2 Rake fingers, one for each path • Each path carries the signal of the 3 users • Within a path, user codes are Building 1 orthogonal Path 1 Path 2 • However, path 1 causes interference to finger 2, and vice versa User A 36 Performance of Rake receiver • Assume K CDMA users, with random codes Ck (may be complex) • Code is N chips per bit (or symbols), random ±1, with chip time is Tc • Channel of user k is multipath with impulse response L 1 g l 0 l ,k t l T c • The L gains gl,k are all independent complex Gaussian with zero mean L 1 and variance v l2 v 2 l 0 l 1 • Assume BPSK modulation (bk=±1). We find the BER of user k=0 desiredsignal • Received signal is given by L 1 r t g l ,0 b 0 t lT c C 0 t lT c l 0 K 1 L 1 g l ,k b k t lT c C k t lT c n t k 1 l 0 AWGN multiple access interference 37 Performance of Rake receiver • Rake receiver includes L fingers, each is performing: – Multiples the received signal by delayed versions of the code of user 0 – Integrates over the bit duration (or symbol if not BPSK) – Multiplies the sum by the conjugate of the path gain – Results of all fingers are added (call it z) – The output z is compared to zero (or sent to demapper if not BPSK) L 1 mT c NT c z g m ,0 * r t C 0 t mT c dt * m 0 mT c P1 P2 P3 P4 desired self interference multiple access interference AWGN • Interference terms will be approximated as Gaussian. • To find the BER we need to find the 4 terms above 38 Illustration of Rake receiver • K users, L paths 39 Performance of Rake receiver L 1 L 1 P1 g m ,0 b0 N T c b0 N T c g 2 2 m ,0 m 0 m 0 L 1 L 1 mTc NTc P2 g m ,0 g l ,0 * b0 t lT c C 0 t lT c C 0 t mT c dt * m 0 l 0 mTc l m • Assume the code are random ±1 and random bits ±1 L 1 L 1 P2 T c g g m 0 * m ,0 l 0 l ,0 Al l m • Al is a random variable which is the sum of N random variables ±1 • All random variable are independent. P2 is zero mean with variance: L 1 L 1 L 1 var P2 T c g var g var A var P2 T c N g 2 2 2 2 m ,0 l ,0 l m ,0 m 0 l 0 m 0 l m 40 Performance of Rake receiver L 1 L 1 K 1 mTc NTc P3 g m ,0 g l ,k * bk t lT c C k t lT c C 0 t mT c dt * m 0 l 0 k 1 mTc L 1 L 1 K 1 Tc g g B m 0 * m ,0 l 0 l ,k k 1 l ,k • Bl,k are independent random variables which are sums of N random variables ±1 • P3 is modeled as Gaussian. The mean is zero. Variance is: L 1 L 1 K 1 var P3 T c g var g var B 2 2 m ,0 l ,k l ,k m 0 l 0 k 1 L 1 T c N K 1 g m ,0 2 2 m 0 41 Performance of Rake receiver • Finally P4 is the AWGN L 1 mT c NT c P4 g m ,0 * n t C 0 t mT c dt * m 0 mT c L 1 NT c g m ,0 * n t dt m 0 0 • Noise n(t) is AWGN with two-sided power spectral density No/2 L 1 NTc var P4 var g m ,0 var n t dt * m 0 0 L 1 N T c N o g m ,0 2 m 0 L 1 g m ,0 Gaussian 0, N T c N o T c2 N T c2 N K 1 2 z b0 N T c m 0 L 1 g m ,0 Gaussian 0, N T c N o T c2 N K 2 z b0 N T c m 0 42 Performance of Rake receiver • The complete output z is: L 1 L 1 L 1 L 1 2 g m ,0 T c N K 1 g m ,0 2 2 2 z b0 N T c g m ,0 Gaussian 0, N T c N o g m ,0 T c N 2 2 m 0 m 0 m 0 m 0 L 1 L 1 Gaussian 0, N T c N o T c2 N K g 2 2 z b0 N T c g m ,0 m ,0 m 0 m 0 • With the normalized bits and normalized channel, the combined average energy per bit is Eb=NTc and the Eb/No =b= NTc/No • Overall SNR is given by: 2 L 1 g 2 N Tc m ,0 SNR m 0 1 2 L 1 m 1 g N T c N o T c N K g m ,0 b K N m ,0 2 2 m 0 1 1 L 1 L 1 m 1 vm g b K N 2 total 1 b K N m 0 m ,0 m 0 m 43 Performance of Rake receiver • To get a closed form we assume equal power for each path 1 1 m b1 K N L • Similar to MRC, the pdf of the total SNR is given by L 1 e p total L L 1! • The BER is given by Pb Q 0 2 p total d L 1 m 1 m 1 L L 1 2 0 m 2 m 1 is given above 44 0 BER for BPSK with Rake receiver 10 numPaths=1 numPaths=2 10 CDMA users, Code length 64 numPaths=4 numPaths=8 -1 10 BER -2 10 -3 10 -4 10 0 2 4 6 8 10 12 14 16 18 20 b dB 45 Voice Activity - Variable Vocoder • CDMA capacity is multi-user interference limited • Ideal: No voice stop transmission reduce interference increase capacity • Practical: No voice lower Vocoder rate & Voice active ,higher data rate lower transmission power • Energy per bit is same • CDMA-2000: Vocoder rates: 1.5, 2.7, 4.8 & 9.6 Tb No Voice ,lower power & kbps or 1.5, 2.7, 4.8 &14.4 kbps lower data rate • WCDMA uses Adaptive Multirate (AMR) Tb Vocoder • AMR rates: 12.2, 10.2, 7.95, 7.4, 6.7, 5.9, 5.15 or 4.75 kbps 46 Interference Cancellation • Base Station: – Knows all assigned PN codes of all users – One receiver synchronized to each user – Performs channel estimation for all users – Demodulates all users signals – Has all information to calculate interference from one user on an other • Intra-cell interference cancellation possible • Reduces Near-Far problem • Implementation is complex 47 Interference Cancellation (Cont.) • Parallel IC (for user 1) + + - - received signal Conventional reconstruct reconstruct Demodulation . Interference . Interference . . . . for all users . felt by user 1 . felt by user 1 . Conventional decision First stage IC decision Second stage IC decision varaible for all users varaible for all users varaible for all users • Sequential IC + + - - received Conventional Conventional signal Regenarte Regenarte Demodulation Demodulation Signal of Signal of 2'nd for stongest for 2'nd Strongest user Strongest user user stongest user Decision varaible for Decision varaible for 2'nd stongest user stongest user 48 Near Far Problem • All users transmit on the same frequency • Signal from near users cause high interference to far users • Reverse link power control is crucial • Also saves phone battery 49 Reverse Link Power Control • Goal: Equal received power from all users – Also saves mobile battery • Open loop Power Control: – Slow, based on average power – Fixes Near-Far Problem po we e tim r – Fixes Shadowing r we – Cannot fix Rayleigh fading po tim • Closed loop Power Control: e – Fast, with instantaneous SNR – Fixes Rayleigh fading 50 Open Loop RL Power Control • Mobile measures averaged forward link power – High MS reduces its transmit power – Low MS increases its transmit power • Open Loop Example: Bt = BS transmit power (dB) Mt = MS transmit power (dB) Br = BS received power (dB) Mr = MS received power (dB) L = Path loss in dB C = correction factor (dB) Mr=Bt-L & Br = Mt-L Power Control Rule: Mt = -Mr + C Br = -Bt + C Constant received power at BS 51 Closed Loop RL Power Control • Closed and open loop power control work simultaneously • BS measures SNR for each MS • BS sends PC bits to MS, up or down • MS adjusts power immediately • WCDMA: PC bits at 1500 Hz, step size is 1, 2 or 3 dB • IS-95 & CDMA-2000 1X: PC bits at 800 Hz, step size 0.25, 0.5 or 1 dB • Effect of fading is partially removed 52 Forward Link Power Control • Goal: Reduce forward link interference – Reduce power to MS in favorable channels – Increase power to MS in unfavorable channels • BS scales each user code in baseband • All channels added for RF transmission User 2 User 1 53 FL Power Control • Slow FL Power Control: – Existed in IS-95 standard – MS measures FER and periodically reports to BS – BS adjusts power – Slow process • Fast FL Closed loop Power Control: – In CDMA-2000 and WCDMA – MS measures SNR at a high rate – MS sends PC bits to BS – PC commands rate similar to RL power control – WCDMA step size is 0.5 and 1 dB 54 Soft Handoff • Each BS transmits unmodulated pilot channel • MS always measures all pilots strength • MS requests handoff to BS with strong pilot • Connection with old BS kept • If granted, MS combines the two signals • In the reverse link, MSC selects the best connection • Old connection dropped if pilot strength gets < threshold • IS-95, CDMA-2000 & WCDMA: maximum 6 BS in soft handoff. Typically 2 or 3. 55 Soft Handoff MSC MSC select combine Step 1 Step 2 MSC select MSC combine Step 3 Step 4 Make before brake (or even don’t brake) 56 Cell Breathing • Cell size controlled by MSC select pilot channel power • Cell/sector overloaded? combine reduce pilot channel power Before • Mobile stations at border MSC will handoff to neighbor Base stations and drop connection to loaded cell/sector After 57 Multi-Carrier CDMA • CDMA-2000 allows 3X bandwidth Forward Link • Multi-Carrier CDMA is used: – Bits encoded and interleaved normally – Encoded bits de-multiplexed to 3 branches – Each branch transmitted on a separate carrier – 3 receive RF chains at mobile station – MS receives separate CDMA carriers and de- multiplex bits – Leverage off current CDMA-2000 1X designs 58 Multi-Carrier CDMA Walsh Complex Code Pilot Code After coding and interleaving: S1 RF1 Carrier 1 b 1 , b 2 , b 3 , b4 , b 5 , b 6 , … De-multiplex S2 Carrier 2 source bits Coding and RF2 Interleaving 1.25 MHz S3 RF3 Carrier 3 Complex Multiply frequency IS-95 and CDMA-2000 1X QPSK Symbols: S1=b1+jb2 S2=b3+jb4 frequency Multi-Carrier CDMA-2000 3X S3=b5+jb6 59 Advantages of CDMA • Anti-interference and Jamming • Takes advantage of voice activity • No frequency cell planning • Soft capacity • Easy for variable data rate • Multipath diversity (Rake receiver) • Soft handoff (make before break) • Transmit diversity • Easier for packet data 60 Disadvantages of CDMA • Sensitive to self interference • Sensitive to power control • Difficult handoff to other frequencies 61 Summary • In CDMA user separation is by codes • Codes designed orthogonal • Codes: Walsh, m-sequences, Gold, Kasami • CDMA is cellular with frequency reuse = 1 • Rake receiver benefits from multipath fading • Open & Closed loop Power Control necessary • Soft handoff possible in cellular CDMA 62

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posted: | 10/5/2012 |

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