DIGITAL SPREAD SPECTRUM SYSTEMS

1. DIGITAL SPREAD SPECTRUM SYSTEMS ENG-737 Lecture 2 Wright State University James P. Stephens

2. DIGITAL MODULATION TECHNIQUES • Information signals (baseband signals) must be processed before transmission to be compatible with the channel • A baseband signal modulatesa high frequency carrier by varying the carrier’s phase, amplitude, or frequency • In digital systems there are 3 basic modulation schemes: • Phase shift keying (PSK) • Amplitude shift keying (ASK) • Frequency shift keying (FSK) • In general modulated signal is of the form: S(t) = A(t) cos [2pfot + f(t)]

3. BINARY PHASE SHIFT KEYING (BPSK)

4. BPSK CIRCUIT IMPLEMENTATION

5. BINARY AMPLITUDE-SHIFT KEYING (BASK)

6. SPECTRUM OF BASK • ASK differs from PSK in that ASK has carrier component due to DC value of m(t)

7. BINARY FREQUENCY-SHIFT KEYING (BFSK)

8. BFSK POWER SPECTRUM

9. SPREAD SPECTRUM DEFINITION: Spread spectrum (SS) is a means of signal transmission in which: • The transmitted signal occupies a bandwidth which is much greater than the minimum necessary to send the information. • Spreading is accomplished by means of a spreading signal called a ‘code’ signal, which is independent of the data. • At the receiver, despreading is done by correlating the received SS signal with a synchronized replica of the spreading signal.

10. WHY USE SPREAD SPECTRUM ? • Interference Suppression • Antijam capability • Natural interference rejection • Self-interference (multipath protection) • Energy Density Reduction • Low probability of intercept (LPI) • Low probability of exploitation (LPE) • National allocation regulations • High-Resolution Ranging • Multiple Access • Communications resource sharing • Communications privacy

11. SPREAD SPECTRUM TECHNIQUES • Direct Sequence (DS) - A carrier is modulated by a digital code sequence in which bit rate is much higher than the information signal bandwidth. • Frequency Hopping (FH) - A carrier frequency is shifted in discrete increments in a pattern dictated by a code sequence. • Time Hopping (TH) - Bursts of the carrier signal are initiated at times dictated by a code sequence. • Hybrid Systems - Use of combination of the above. • Others - Carrier-less based, transform domain systems

12. Interference Signal Spread Signal Recovered Data Data Signal Spreading Code Spreading Code Synchronized BASIC SPREAD SPECTRUM TECHNIQUE The essence of interference rejection capability in SS systems can be summarized as: Demod • Multiplication once by the spreading code spreads the signal bandwidth • Multiplication twice by the spreading code followed by filtering, recovers the original data signal • The desired signal gets multiplied twice, but the interference gets multiplied only once

13. SIGNAL DIMENSIONALITY • An arbitrary M-ary signal set: • Can be completely specified by a linear combination of orthonormal basis functions: • The signal set is said to be D-dimensional when D is the minimum number of orthonormal basis functions necessary to span the signal set • It can be shown that : D  2 BD T Where, T = signaling interval BD = bandwidth of the D-dimensional signal set • AWGN has infinite power and constant energy in all dimensions, therefore increasing D yields no performance against AWGN • A jammer or interference source has a fixed finite power, increasing the dimensionality of our signal space in a manner unknown to the jammer will provide increased performance • The jammer is forced to spread his finite power over all the coordinates that we might use

14. Jammer J(t) Xmitter Receiver s(t) DIMENSIONALITY EXAMPLE • r(t) = s(t) + J(t), neglecting noise • r(t) = m(t) b(t) + J(t) Where, m(t) is the message data (1) b(t) is the spreading code (1) • Note that b(t) b(t) = b2(t) = 1 , for all t • At the receiver r(t) b(t) = m(t) b(t) b(t) + J(t) b(t) Where, b(t) is an embedded reference code at the receiver r(t) b(t) = m(t) + J(t) b(t) • The jammer has been spread and the message m(t) has been despread

15. CONCEPT OF DIMENSIONALITY Low SNR ½ PT Jammer = ½ J c - f f c c At the receiver’s antenna Low SNR ½ PT Signal = ½ PT c c Jammer - f f c c After despreading by receiver

16. CONCEPT OF DIMENSIONALITY (Cont) High SNR Signal = ½ PT c Jammer - f f c c Output of filter

17. PROCESSING GAIN = PG Processing gain is the improvement seen by a spread spectrum system in SNR, within the system’s information bandwidth, over the SNR in the transmission channel. PG = BS / Ri PG(dB) = 10 log (Bs / Ri) Typical PG = 20 to 60 dB

18. JAMMING MARGIN = MG Jamming margin takes into account the requirement for a useful system output SNR and allow for internal losses. MG = GP – [Lsys + (S/N)out ] , dB Where, Lsys = system implementation losses (S/N)out = SNR at information, despread, output

19. BASIC SPREAD SPECTRUM SYSTEM CONFIGURATION Transmitter Receiver DBM DBM FM Receiver FM Exciter RF Preamp X Amp/Filter X Audio Audio PN Code Generator Synchronous Oscillator PN Code Generator Embedded Reference

20. AFIT BUILT DSSS THEORY OF OPERATION TRANSMITTER So(t) FM Exciter Audio DBM FM Osc BPF X Amp X4 X3 X3 ST1(t) ST2(t) 12.388 MHz 111.5 MHz PN Code Gen Divide by 40 bT(t) 2.7875 MHz Detailed DSSS System Block Diagram

21. AFIT BUILT DSSS THEORY OF OPERATION RECEIVER SI(t) DBM SR1(t) SR2(t) X FM Receiver Audio Preamp 446.0 MHz bR(t) PN Code Gen Divide by 40 Synchronous Oscillator 2.7875 MHz 111.5 MHz Detailed DSSS System Block Diagram

22. DIRECT SQUENCE BPSK SPECTRAL CHARACTERISTICS Rc = 2.7875 MHz Tc = 0.3587 s N = 255 BW = 5.575 MHz 1/NTc = 10.93 kHz • PSD = {Sin (x)/x}2 • Spectral line spacing = 1/NTc • Null-to-Null BW = 2 * 1/Tc • Number of spectral lines = N 0 to 1st Null 40

23. DIRECT SEQUENCE BPSK Direct sequence, suppressed carrier, biphase, code modulated Direct sequence, unsuppressed carrier, biphase, code modulated

24. POWER DISTRIBUTION OF SINC2 SPECTRUM

25. EFFECT OF HELICAL BANDPASS FILTER

26. SPECTRAL CHARACTERISTICS Rc = 2.7875 MHz Tc = 0.3587 s N = 255 BW = 5.575 MHz 1/NTc = 10.93 kHz • {Sin (x)/x}2 • Spectral lines – 1/NTc • Null-to-Null BW – 2 * 1/Tc • Number of spectral lines = N 40

27. EFFECT OF NARROWBAND FM MODULATION ON THE PSD OF DSSS

28. NRZ data d(t) Sd(t) x High Pass Filter Data Modulator St(t) ht(t) √2P cos(ω0t) Frequency Synthesizer 1 2 3 . . . . k Code Generator FH code clock TRANSMITTER BASIC FREQUENCY HOPPER ARCHITECTURE Typically FSK

29. Reference Oscillator Phase Detector Filter VCO ÷ n Digital word from frequency map BASIC INDIRECT FREQUENCY SYNTHESIZER Phase-Locked Loop

30. 2 1 Frequency Synthesizer PN Generator f3 + IF f4 + IF f1 f2 f3 f4 f2 + IF f1 + IF BASIC FREQUENCY HOPPER SYSTEM WITH WAVEFORMS Mixer IF BPF To demodulator 4 3 PN Generator Frequency Synthesizer 1 – PN Code 3 – Code Reference 4 – FSK Reference 2 – FH Carrier

31. FREQUENCY HOPPING CORDLESS TELEPHONE

32. Hop time Dwell time t f1 f2 DWELL TIME AND HOP RATE • Hop time is the period of the hop cycle • Hop rate = 1 / Hop time • Dwell time is the time when radio is transmitting • Duty cycle is the % time the radio is transmitting versus the hop time: • % Duty Cycle = (Dwell time / Hop time) x 100

33. CORDLESS PHONE WAVEFORMS Base and Handset Unit Handset Unit

34. CORDLESS PHONE ACQUISITION FREQUENCIES Spectrum Plot Time-Frequency Plot

35. CORDLESS PHONE COMMUNICATION FREQUENCIES f1 f1 f2 f2 Spectrum Plot Time-Frequency Plot

36. TIME-FREQUENCY DISTRIBUTION OF FH RADIO (a) Dwell 4-ary (b)

37. Legend Tone # Tone Data Symbol f0 + 175 Hz f0 + 125 Hz f0 + 75 Hz f0 + 25 Hz f0 f0 - 25 Hz f0 - 75 Hz f0 - 125 Hz f0 - 175 Hz 000 001 010 011 100 101 110 111 0 1 2 3 4 5 6 7 FREQUENCY HOPPING EXAMPLE USING 8-ARY FSK Data Tone 1 Tone 6 Tone 3 0 0 1 1 1 0 0 1 1 Frequency Hopping Band Time Symbol Interval (20 ms)

38. Legend Tone # Tone Data Symbol f0 + 700 Hz f0 + 500 Hz f0 + 300 Hz f0 + 100 Hz f0 f0 - 100 Hz f0 - 300 Hz f0 - 500 Hz f0 - 700 Hz 000 001 010 011 100 101 110 111 0 1 2 3 4 5 6 7 Symbol Interval (20 ms) FREQUENCY HOPPING EXAMPLE WITH DIVERSITY (N = 4) Data Tone 1 Tone 6 Tone 3 0 0 1 1 1 0 0 1 1 Frequency Hopping Band 5 ms / chip Each Symbol Repeated 4 times

39. 1 0 Bits Frequency Time Chip Duration Bits 101 100 101 110 011 000 011 110 Frequency Time Chip Duration FAST HOPPING VS. SLOW HOPPING (a) Fast Hopping Example: 4 hops / bit (b) Slow Hopping Example: 3 bits / hop