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Unit 1 Introduction to SpreadSpectrum Systems Department of Communication Engineering, NCTU 1 Spread Spectrum Lecture 1 Introduction Sau-Hsuan Wu 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 Jammer Interferenec Broadband noise Barrage jammer Multiuser interference Narrowband noise Partial band jammer Multi-system / multiuser Interference Pulse noise Pulse jammer Multiuser Interference Multiple sinusoids Multi-tone jammer Harmonic/image Department of Communication Engineering, NCTU 2 Spread Spectrum Lecture 1 Introduction Sau-Hsuan Wu 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 Sj = J/2B Department of Communication Engineering, NCTU 3 Spread Spectrum Lecture 1 Introduction Sau-Hsuan Wu Matched-filter the received signal and threshold The discretized received signal is of the form where S = Es / T and Department of Communication Engineering, NCTU 4 Spread Spectrum Lecture 1 Introduction Sau-Hsuan Wu The bit error rate of detection is where Rs = 1/ Ts 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 Spread Spectrum Lecture 1 Introduction Sau-Hsuan Wu Define an effective Es / N0 as Suppose the target BER is 10-3 ⇒ (Es/N0)eff =6.8dB The J/S satisfies If Rs / B =1 S > 4.8J Now, suppose we use a signaling scheme which expands the bandwidth by a factor of 100 so that Rs / B =1 /100 ⇒ S > J / 21 How to dilute the noise PSD? Department of Communication Engineering, NCTU 6 Spread Spectrum Lecture 1 Introduction Sau-Hsuan Wu Power spectrum densities of the signal and the interference PSD after spreading Department of Communication Engineering, NCTU 7 Spread Spectrum Lecture 1 Introduction Sau-Hsuan Wu 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 Spread Spectrum Lecture 1 Introduction Sau-Hsuan Wu Direct sequence spread spectrum (DS-SS) Date sequence : d(t) = 1 1 -1 1 -1 Code sequence : c(t) = -1 -1 -1 1 1 -1 1 Department of Communication Engineering, NCTU 9 Spread Spectrum Lecture 1 Introduction At the transmitter, we do signal spreading At the receiver, we do de-spreading Department of Communication Engineering, NCTU Sau-Hsuan Wu 10 Spread Spectrum Lecture 1 Introduction Sau-Hsuan Wu Slow frequency hopping (FH) Data sequence: +1 -1 | -1 -1 | +1 +1 | -1 +1 | -1 -1 Code sequence: -1 -1 -1| +1 -1 +1| +1 -1 -1| -1 +1 -1| +1 +1 -1 0 1 2 3 4 5 6 7 Department of Communication Engineering, NCTU 11 Spread Spectrum Lecture 1 Introduction Sau-Hsuan Wu Hybrid time and frequency hopping (Ts > Tc) Department of Communication Engineering, NCTU 12 Spread Spectrum Lecture 1 Introduction Sau-Hsuan Wu Time-hopping (pulse position modulation) TH-PPM code sequence : 0 1 0 0 0 0 0 1 0 0 -1 0 -1 Transmitted waveform 0 1 0 0 frame 0 0 0 1 0 0 Department of Communication Engineering, NCTU -1 0 -1 13 Spread Spectrum Lecture 1 Introduction Sau-Hsuan Wu 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 Spread Spectrum Lecture 1 Introduction Sau-Hsuan Wu 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 Spread Spectrum Lecture 1 Introduction Department of Communication Engineering, NCTU Sau-Hsuan Wu 16 Spread Spectrum Lecture 1 Introduction Sau-Hsuan Wu 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