Unit 1 Introduction to Spread- Spectrum Systems. Department of Communication Engineering, NCTU 1

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1 Unit 1 Introduction to Spread- Spectrum Systems Department of Communication Engineering, NCTU 1

2 What does it mean by spread spectrum communications Spread the energy of an information bit over a bandwidth which is much larger than the information bit rate Why spread the energy of information bit? Originally to provide a system which is robust to jamming in a military environment Intentional jammer v.s. unintentional interference Broadband noise Narrowband noise Pulse noise Multiple sinusoids Jammer Barrage jammer Partial band jammer Pulse jammer Multi-tone jammer Interferenec Multiuser interference Multi-system / multiuser Interference Multiuser Interference Harmonic/image Department of Communication Engineering, NCTU 2

3 The effect of a typical narrow band interference Suppose BPSK is used, the transmitted signal of the form In the presence of interference plus noise, we have r(t) = s(t) + j(t) + n(t) Suppose the power spectral density of j is equal to S j = J/2B Department of Communication Engineering, NCTU 3

4 Matched-filter the received signal and threshold The discretized received signal is of the form where S = E s / T and Department of Communication Engineering, NCTU 4

5 The bit error rate of detection is where R s = 1/ T s The loss is this system relative to a system with no interference is How to cut this loss? Increase the signal energy Design a system to operate at a lower signal to noise ratio better channel CODEC Increase B Spread the interference energy (?) Department of Communication Engineering, NCTU 5

6 Define an effective E s / N 0 as Suppose the target BER is 10-3 (E s /N 0 ) eff =6.8dB The J/S satisfies If R s / B =1 S > 4.8J Now, suppose we use a signaling scheme which expands the bandwidth by a factor of 100 so that R s / B =1 /100 S > J / 21 How to dilute the noise PSD? Department of Communication Engineering, NCTU 6

7 Power spectrum densities of the signal and the interference PSD after spreading Department of Communication Engineering, NCTU 7

8 Goal is to have the PSD of the desired signal after despreading looks like below How to do energy spreading The most simpler way is direct-sequence spread spectrum Department of Communication Engineering, NCTU 8

9 Direct sequence spread spectrum (DS-SS) Date sequence : d(t) = Code sequence : c(t) = Department of Communication Engineering, NCTU 9

10 At the transmitter, we do signal spreading At the receiver, we do de-spreading Department of Communication Engineering, NCTU 10

11 Slow frequency hopping (FH) Data sequence: Code sequence: Department of Communication Engineering, NCTU 11

12 Hybrid time and frequency hopping (Ts > Tc) Department of Communication Engineering, NCTU 12

13 Time-hopping (pulse position modulation) TH-PPM code sequence : Transmitted waveform frame Department of Communication Engineering, NCTU 13

14 Important issues related to spread spectrum techniques How to get receiver s spreading sequence in sync with the transmitter s spreading sequence? How to generate spreading sequence which is difficult to predict or at least exhibit certain properties? How many chips per bit should be used? How to build a frequency synthesizer which is programmable and capable of rapid transitions from one frequency to the next (inexpensive would be nice too)? Department of Communication Engineering, NCTU 14

15 Are there other reason for spread spectrum communications Provide multiple access mechanism Traditional approaches are frequency division multiple access and time division multiple access Spread spectrum enables another multiple access scheme called spread spectrum multiple access, also know as code division multiple access (CDMA) DS-CDMA Each user is assigned a different spreading sequence and all users occupy the entire frequency band at all times FH-CDMA Each user follows a different hopping pattern, TH-CDMA Department of Communication Engineering, NCTU 15

16 Department of Communication Engineering, NCTU 16

17 Advantages of CDMA over FDMA or TDMA Provides a means to support multiple users and simultaneously to be robust to interference from sources other than the system users Problems with CDMA Multiple access interference Effective interference PSD Department of Communication Engineering, NCTU 17

Spread Spectrum (SS) is a means of transmission in which the signal occupies a

SPREAD-SPECTRUM SPECTRUM TECHNIQUES: A BRIEF OVERVIEW SS: AN OVERVIEW Spread Spectrum (SS) is a means of transmission in which the signal occupies a bandwidth in excess of the minimum necessary to send