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טכניקות בתקשורת מרחיבת סרט (Spread Spectrum) Chapter 2b

מצגת זו תכלול כנראה דיון של הקהל, אשר יביא ליצירת פריטי פעולה. השתמש ב- PowerPoint כדי לעקוב אחר פריטי פעולה אלה במהלך המצגת. בהצגת שקופיות, לחץ באמצעות לחצן העכבר הימני. בחר באפשרות “מפקח הישיבות”. בחר בכרטיסיה “פריטי פעולה”. הקלד את פריטי הפעולה כאשר הם מופיעים.

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טכניקות בתקשורת מרחיבת סרט (Spread Spectrum) Chapter 2b

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  1. מצגת זו תכלול כנראה דיון של הקהל, אשר יביא ליצירת פריטי פעולה. השתמש ב- PowerPointכדי לעקוב אחר פריטי פעולה אלה במהלך המצגת. • בהצגת שקופיות, לחץ באמצעות לחצן העכבר הימני. • בחר באפשרות “מפקח הישיבות”. • בחר בכרטיסיה “פריטי פעולה”. • הקלד את פריטי הפעולה כאשר הם מופיעים. • לחץ על אישור כדי להסיר תיבה זו. פעולה זו תיצור אוטומטית שקופיות לפריטי פעולה בסוף המצגת, והנקודות שהעלית יוזנו בתוכה. טכניקות בתקשורת מרחיבת סרט(Spread Spectrum) Chapter 2b ד"ר משה רן כל הזכויות שמורות לחברת MostlyTek Ltd. אין לצלם, לשכפל או להעתיק בכל צורה שהיא ללא קבלת אישור בכתב מד"ר משה רן Dr. Moshe Ran- Spread Spectrum

  2. נושאי לימוד 8 שו"ת 8 שו"ת 8 שו"ת 8 שו"ת 8 שו"ת 8 שו"ת 4 שו"ת Dr. Moshe Ran / Spread Spectrum

  3. Chapter 2b – Frequency- Hop Spread Spectrum Dr. Moshe Ran / Spread Spectrum

  4. 1. Basics of Frequency Hopping • Method to change the carrier frequency periodically • Typically, modulation code is k-bits that select 1-out-2kfrequencies (or frequency bands) which are spaced approximately • Rx removes the FH by mixing (down-converting) with LO signal which is hopping synchronously with the received signal. • Usually coherent FH is not implemented – since carrier phase estimation is required per each hop. Dr. Moshe Ran / Spread Spectrum

  5. 1.1 Classification of Frequency Hopping Systems Academic Classification Slow Frequency Hopping In every hop there are more than one symbol Fast Frequency Hopping In every hop there is one symbol or less Industrial Classification Slow Frequency Hopping: Less then 50hps Medium Frequency Hopping: 50hps to 500hps Fast Frequency Hopping: More then 500hps Dr. Moshe Ran / Spread Spectrum

  6. Data modulated carrier NRZ data Data Modulator Bandpass filter Frequency synthesizer Code GENERATOR FH code clock 2.1 Coherent slow FH Spread Spectrum Tx implementation for Coherent slow FH Spread Spectrum Dr. Moshe Ran / Spread Spectrum

  7. Image reject filter Data demod Bandpass filter Frequency synthesizer Code GENERATOR FH code clock Coherent slow FH Spread Spectrum Receiver Dr. Moshe Ran / Spread Spectrum

  8. Analysis of Coherent slow FH Spread Spectrum Output of freq. synthesizer at Tx is the “hop carrier” hT(t): bandpass signal with random sequence of tones fn, of duration Th (1) Coherent assumption: the same phase is used each time returns to the same frequency Dr. Moshe Ran / Spread Spectrum

  9. The transmitted signal is the data-modulated carrier up-converted to a new frequency f0+fn for each FH hop Analysis of Coherent slow FH Spread Spectrum – cont. • PSD of transmitted slow FH (2) is the convolution of the two signals (3) (4) Dr. Moshe Ran / Spread Spectrum

  10. Analysis of Coherent slow FH Spread Spectrum – cont. (5) • Notations: (6) (7) PSD Sh(f) can be simplified by assuming This assumption is valid whenever 1/Th is small compared to the minimum frequency spacing i.e. Slow FH Dr. Moshe Ran / Spread Spectrum

  11. Analysis of Coherent slow FH Spread Spectrum – cont. • Simplified PSD – further assume (8) Note: PSD has discrete components, due to the “coherent FH assumption”. The same phase is used each time hT(t) returns to frequency fm . If Then - the discrete components are negligible. Dr. Moshe Ran / Spread Spectrum

  12. Example: • Calculate the PSD of a transmitted coherent slow FH signal with the following parameters.Modulation: BPSK, Rb=1Mbps, Rh=100Khops/sec, 4 frequencies are employed, minimum spacing equals data rate. • Solution: Since minimum spacing >> hop rate we can use simplified PSD formula. Dr. Moshe Ran / Spread Spectrum

  13. Receiver for slow FH • The received signal (9) This signal is down converted using hR(t) (10) Assuming No tracking errors Dr. Moshe Ran / Spread Spectrum

  14. Receiver for slow FH (11) Recovered data-modulated carrier Usually- there are tracking errors and Thus – the recovered carrier is phase modulated by terms with the form: (12) Dr. Moshe Ran / Spread Spectrum

  15. Dealing with tracking errors in slow FH: • Need means for coherent carrier tracking, independent of the FH tracking loop. • Possible way: estimating Rx phase every hop • “feed forward carrier phase estimation” using known sync. Word • Need to protect sync. word to avoid hostile jammer (I.e., by changing periodically the sync word) Dr. Moshe Ran / Spread Spectrum

  16. Non coherent slow FH • Coherent FH are complex to implement • Non-coherent • Differentially coherent • Whenfreq. synthesizer phase is random for each successive time interval, PSD of hT(t) (13) Dr. Moshe Ran / Spread Spectrum

  17. Typical Example of Slow non coherent FH: • Data modulation: 2L-ary FSK • Each Ts=LTb sec. the modulator outputs 1-out-2Ltones • Spacing between the tones 1/LTb • Bd, Bandwidth of modulated signal: Bd = 2L/LTb • Each Th the data modulated signal is translated to a new frequency by a FH modulator, 2k frequency bands of wide Bd • Total BW of system: Bss=2kBd • Th > LTb slow FH condition Plot this for k=3 and L=2. That is 4-FSK and 8 bands for the FH signals Dr. Moshe Ran / Spread Spectrum

  18. Non Coherent fast FH • Here • Hop-bands can change many times per symbol • Data modulator can operate in different modes with different complexities. Can use the K hops per symbol based on majority vote or Maximum Likelihood sequence estimation. • Benefits: frequency diversity gain on each transmitted symbol • Partial-band jammer • improving performance in “fast fading” multipath channel Dr. Moshe Ran / Spread Spectrum

  19. Typical Example of Fast non coherent FH: • Data modulation: 2L-ary FSK • Each Th=(1/K) * Ts sec. the modulator outputs 1-out-2Ltones. That is – the tones are subdivided into K “chips” • Spacing between the tones 1/Th = K / LTb =K / Ts • Bd, Bandwidth of modulated signal: Bd = 2L / Th= K2L / LTb • Each Th the data modulated signal is translated to a new frequency by a FH modulator, 2k frequency bands of wide Bd • Total BW of system: Bss=2kBd • Th < LTb fast FH condition Plot this for L=K=k = 2. That is 4-FSK and 4 bands for the FH signals Dr. Moshe Ran / Spread Spectrum

  20. Combined DS-FH spread spectrum transmitter + DPSK data modulator Data modulated carrier NRZ data Differential encoder Bandpass filter Frequency synthesizer Code GENERATOR FH code clock Dr. Moshe Ran / Spread Spectrum

  21. DPSK demod Combined DS-FH spread spectrum receiver + DPSK data demodulator Bandpass filter BP filter Bandpass filter Frequency synthesizer Code GENERATOR FH code clock Dr. Moshe Ran / Spread Spectrum

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