Basic Encoding Techniques

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Bai Enoding Tehnique Signal Enoding, Spread Spetrum Chapter 7 Digital data to analog ignal Amplitude-hift keying (ASK) Amplitude differene of arrier frequeny Frequeny-hift keying (FSK) Frequeny

1 Bai Enoding Tehnique Signal Enoding, Spread Spetrum Chapter 7 Digital data to analog ignal Amplitude-hift keying (ASK) Amplitude differene of arrier frequeny Frequeny-hift keying (FSK) Frequeny differene near arrier frequeny Phae-hift keying (PSK) Phae of arrier ignal hifted Bai Enoding Tehnique Amplitude-Shift Keying One binary digit repreented by preene of arrier, at ontant amplitude Other binary digit repreented by abene of arrier Ao () t = 0 ( 2πf t) binary 1 binary 0 where the arrier ignal i Ao(2πf t) Amplitude-Shift Keying Sueptible to udden gain hange Ineffiient modulation tehnique On voie-grade line, ued up to 1200 bp Ued to tranmit digital data over optial fiber Binary Frequeny-Shift Keying (BFSK) Two binary digit repreented by two different frequenie near the arrier frequeny () t = ( 2πf ( 2πf t) Ao 1 Ao 2 1 binary binary 0 where f 1 and f 2 are offet from arrier frequeny f by equal but oppoite amount 1

2 Binary Frequeny-Shift Keying (BFSK) Le ueptible to error than ASK On voie-grade line, ued up to 1200bp Ued for high-frequeny (3 to 30 MHz) radio tranmiion Can be ued at higher frequenie on LAN that ue oaxial able Multiple Frequeny-Shift Keying (MFSK) More than two frequenie are ued More bandwidth effiient but more ueptible to error i ( t ) A o 2π f t = 1 i M f i = f + (2i 1 M)f d f = the arrier frequeny f d = the differene frequeny M = number of different ignal element = 2 L L = number of bit per ignal element i Multiple Frequeny-Shift Keying (MFSK) To math data rate of input bit tream, eah output ignal element i held for: T =LT eond where T i the bit period (data rate = 1/T) So, one ignal element enode L bit Multiple Frequeny-Shift Keying (MFSK) Total bandwidth required 2Mf d Minimum frequeny eparation required 2f d =1/T Therefore, modulator require a bandwidth of W d =2 L /LT=M/T Multiple Frequeny-Shift Keying (MFSK) Phae-Shift Keying (PSK) Two-level PSK (BPSK) Ue two phae to repreent binary digit () t ( ) Ao 2πf t = A o ( 2 π f t +π ) ( ) ( ) Ao 2πf t = Ao 2πf t binary 1 binary 0 1 binary binary 0 2

3 Phae-Shift Keying (PSK) Differential PSK (DPSK) Phae hift with referene to previou bit Binary 0 ignal burt of ame phae a previou ignal burt Binary 1 ignal burt of oppoite phae to previou ignal burt Phae-Shift Keying (PSK) Four-level PSK (QPSK) Eah element repreent more than one bit () t = π Ao 2πf t π Ao 2πf t π Ao 2πf t 00 4 π A o 2πf t 10 4 Phae-Shift Keying (PSK) Multilevel PSK Uing multiple phae angle with eah angle having more than one amplitude, multiple ignal element an be ahieved D = R L = R log 2 M Quadrature Amplitude Modulation QAM i a ombination of ASK and PSK Two different ignal ent imultaneouly on the ame arrier frequeny ( t) = d ( t) 2πf t d ( t) in 2πf t 1 o + 2 D = modulation rate, baud R = data rate, bp M = number of different ignal element = 2 L L = number of bit per ignal element Quadrature Amplitude Modulation Spread Spetrum Input i fed into a hannel enoder Produe analog ignal with narrow bandwidth Signal i further modulated uing equene of digit Spreading ode or preading equene Generated by peudonoie, or peudo-random number generator Effet of modulation i to inreae bandwidth of ignal to be tranmitted 3

Spread Spetrum Spread Spetrum On reeiving end, digit equene i ued to demodulate the pread petrum ignal Signal i fed into a hannel deoder to reover data Spread Spetrum What an be gained from apparent

4 Spread Spetrum Spread Spetrum On reeiving end, digit equene i ued to demodulate the pread petrum ignal Signal i fed into a hannel deoder to reover data Spread Spetrum What an be gained from apparent wate of petrum? Immunity from variou kind of noie and multipath ditortion Can be ued for hiding and enrypting ignal Several uer an independently ue the ame higher bandwidth with very little interferene Frequeny Hoping Spread Spetrum (FHSS) Signal i broadat over eemingly random erie of radio frequenie A number of hannel alloated for the FH ignal Width of eah hannel orrepond to bandwidth of input ignal Signal hop from frequeny to frequeny at fixed interval Tranmitter operate in one hannel at a time Bit are tranmitted uing ome enoding heme At eah ueive interval, a new arrier frequeny i eleted Frequeny Hoping Spread Spetrum Frequeny Hoping Spread Spetrum Channel equene ditated by preading ode Reeiver, hopping between frequenie in ynhronization with tranmitter, pik up meage Advantage Eavedropper hear only unintelligible blip Attempt to jam ignal on one frequeny ueed only at knoking out a few bit 4

FHSS Uing MFSK MFSK ignal i tranlated to a new frequeny every T eond by modulating the MFSK ignal with the FHSS arrier ignal For data rate of R: duration of a bit: T = 1/R eond duration of ignal

5 FHSS Uing MFSK MFSK ignal i tranlated to a new frequeny every T eond by modulating the MFSK ignal with the FHSS arrier ignal For data rate of R: duration of a bit: T = 1/R eond duration of ignal element: T = LT eond T T - low-frequeny-hop pread petrum T < T - fat-frequeny-hop pread petrum FHSS Performane Conideration Large number of frequenie ued Reult in a ytem that i quite reitant to jamming Jammer mut jam all frequenie With fixed power, thi redue the jamming power in any one frequeny band Diret Sequene Spread Spetrum (DSSS) Diret Sequene Spread Spetrum (DSSS) Eah bit in original ignal i repreented by multiple bit in the tranmitted ignal Spreading ode pread ignal aro a wider frequeny band Spread i in diret proportion to number of bit ued One tehnique ombine digital information tream with the preading ode bit tream uing exluive-or DSSS Uing BPSK DSSS Uing BPSK Multiply BPSK ignal, d (t) = A d(t) o(2π f t) by (t) [take value +1, -1] to get (t) = A d(t)(t) o(2π f t) A = amplitude of ignal f = arrier frequeny d(t) = direte funtion [+1, -1] At reeiver, inoming ignal multiplied by (t) Sine, (t) x (t) = 1, inoming ignal i reovered 5

6 Code-Diviion Multiple Ae (CDMA) Bai Priniple of CDMA D = rate of data ignal Break eah bit into k hip Chip are a uer-peifi fixed pattern Chip data rate of new hannel = kd CDMA Example If k=6 and ode i a equene of 1 and -1 For a 1 bit, A end ode a hip pattern <1, 2, 3, 4, 5, 6> For a 0 bit, A end omplement of ode <-1, -2, -3, -4, -5, -6> Reeiver know ender ode and perform eletroni deode funtion ( ) 6 S u d = d1 1 + d2 2+ d3 3+ d4 4+ d5 5+ d6 <d1, d2, d3, d4, d5, d6> = reeived hip pattern <1, 2, 3, 4, 5, 6> = ender ode CDMA Example Uer A ode = <1, 1, 1, 1, 1, 1> To end a 1 bit = <1, 1, 1, 1, 1, 1> To end a 0 bit = < 1, 1, 1, 1, 1, 1> Uer B ode = <1, 1, 1, 1, 1, 1> To end a 1 bit = <1, 1, 1, 1, 1, 1> Reeiver reeiving with A ode (A ode) x (reeived hip pattern) Uer A 1 bit: 6 -> 1 Uer A 0 bit: -6 -> 0 Uer B 1 bit: 0 -> unwanted ignal ignored CDMA for Diret Sequene Spread Spetrum Categorie of Spreading Sequene Spreading Sequene Categorie PN equene Orthogonal ode For FHSS ytem PN equene mot ommon For DSSS ytem not employing CDMA PN equene mot ommon For DSSS CDMA ytem PN equene Orthogonal ode PN Sequene PN generator produe periodi equene that appear to be random PN Sequene Generated by an algorithm uing initial eed Sequene in t tatitially random but will pa many tet of randomne Sequene referred to a peudorandom number or peudonoie equene Unle algorithm and eed are known, the equene i impratial to predit 6

7 Important PN Propertie Linear Feedbak Shift Regiter Implementation Randomne Uniform ditribution Balane property Run property Independene Correlation property Unpreditability Propertie of M-Sequene Property 1: Ha 2 n-1 one and 2 n-1-1 zero Property 2: For a window of length n lid along output for N (=2 n-1 ) hift, eah n-tuple appear one, exept for the all zero equene Property 3: Sequene ontain one run of one, length n One run of zero, length n-1 One run of one and one run of zero, length n-2 Two run of one and two run of zero, length n-3 2 n-3 run of one and 2 n-3 run of zero, length 1 Propertie of M-Sequene Property 4: The periodi autoorrelation of a ±1 m- equene i 1 = 0, N, 2N,... τ R( τ ) = 1 otherwie N Definition Correlation The onept of determining how muh imilarity one et of data ha with another Range between 1 and 1 1 The eond equene mathe the firt equene 0 There i no relation at all between the two equene -1 The two equene are mirror image Cro orrelation The omparion between two equene from different oure rather than a hifted opy of a equene with itelf Advantage of Cro Correlation The ro orrelation between an m-equene and noie i low Thi property i ueful to the reeiver in filtering out noie The ro orrelation between two different m- equene i low Thi property i ueful for CDMA appliation Enable a reeiver to diriminate among pread petrum ignal generated by different m-equene 7

8 Gold Sequene Gold Sequene Gold equene ontruted by the XOR of two m-equene with the ame loking Code have well-defined ro orrelation propertie Only imple iruitry needed to generate large number of unique ode In following example two hift regiter generate the two m-equene and thee are then bitwie XORed Orthogonal Code Orthogonal ode All pairwie ro orrelation are zero Fixed- and variable-length ode ued in CDMA ytem For CDMA appliation, eah mobile uer ue one equene in the et a a preading ode Provide zero ro orrelation among all uer Type Welh ode Variable-Length Orthogonal ode Walh Code Set of Walh ode of length n onit of the n row of an n n Walh matrix: Wn Wn W 1 = (0) W2 n = Wn Wn n = dimenion of the matrix Every row i orthogonal to every other row and to the logial not of every other row Require tight ynhronization Cro orrelation between different hift of Walh equene i not zero 8

Reasons for Choosing Encoding Techniques. Signal Encoding Techniques. Reasons for Choosing Encoding Techniques. Signal Encoding Criteria

Reasons for Choosing Encoding Techniques. Signal Encoding Techniques. Reasons for Choosing Encoding Techniques. Signal Encoding Criteria Reaon for Chooing Enoding Tehnique Signal Enoding Tehnique Chapter 6 Digital data, digital ignal Equipment le omplex and expenive than digital-to-analog modulation equipment Analog data, digital ignal