1. Dr. Moshe Ran- Spread Spectrum 1 טכניקות בתקשורת מרחיבת סרט (Spread Spectrum) Chapter 2b ד"ר משה רן 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. לחץ על אישור כדי להסיר תיבה זו. פעולה זו תיצור אוטומטית שקופיות לפריטי פעולה בסוף המצגת, והנקודות שהעלית יוזנו בתוכה. כל הזכויות שמורות לחברת MostlyTek Ltd. אין לצלם, לשכפל או להעתיק בכל צורה שהיא ללא קבלת אישור בכתב מד"ר משה רן

2. Dr. Moshe Ran / Spread Spectrum 2 נושאי לימוד פרק 1 מבוא הסטורי לטכניקות Spread Spectrum הרחבת ספקטרום – לשם מה? חזרה- מושגי יסוד ועקרונות של מערכות תקשורת ספרתיות; רעשים והפרעות במערכות תקשורת, דרישות מערכתיות על התקשורת, השוואת שיטות אפנון ספרתיות, יעילות ספקטרלית. פרק 2 מבוא למערכות מרחיבות סרט (Spread Spectrum) - : קונספט ומודלים למערכות מרחיבות סרט; שיטות הרחבת סרט המבוססות על הרחבה ישירה DS)) דילוגים בזמן TH) ) דילוגים בתדר (FH) פרק 3 סדרות קוד למערכות מרחיבות סרט - LFSR, Gold Sequence, Walsh פרק 4 ביצועים של מערכות עם הרחבת סרט ישירה (DS); ביצועים של מערכות עם דילוגי תדר (FH) ; שיטות גילוי, עקיבה וסנכרון של אותות Spread Spectrum פרק 5 קודים לתיקון שגיאות, ביצועים של מערכות Spread Spectrum עם קודים לתיקון שגיאות, אלגוריתם Viterbi פרק 6 עקרונות CDMA בתקשורת תאית פרק 7 שימושים ואפליקציות של מערכות Spread Spectrum 8 שו"ת 4 שו"ת

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

4. 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-2 k frequencies (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.

5. 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

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

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

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

9. Dr. Moshe Ran / Spread Spectrum 9 PSD of transmitted slow FH Analysis of Coherent slow FH Spread Spectrum – cont. is the convolution of the two signals The transmitted signal is the data-modulated carrier up- converted to a new frequency f 0 +f n for each FH hop (2) (3) (4)

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

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

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

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

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

15. 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)

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

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

18. Dr. Moshe Ran / Spread Spectrum 18 Non Coherent fast FH Here o Hop-bands can change many times per symbol o 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 o Partial-band jammer o improving performance in “ fast fading ” multipath channel

19. Dr. Moshe Ran / Spread Spectrum 19 Typical Example of Fast non coherent FH: Data modulation: 2 L -ary FSK Each T h =(1/K) * T s sec. the modulator outputs 1-out-2 L tones. That is – the tones are subdivided into K “ chips ” Spacing between the tones 1/T h = K / LT b =K / T s B d, Bandwidth of modulated signal: B d = 2 L / T h = K2 L / LT b Each T h the data modulated signal is translated to a new frequency by a FH modulator, 2 k frequency bands of wide B d Total BW of system: B ss =2 k B d T h < LT b fast FH condition Plot this for L=K=k = 2. That is 4-FSK and 4 bands for the FH signals

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

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