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Where am I?

Frame Synchronization. Where am I?. BASIC ISDEA. For a receiver to demultiplex a TDM signal, it must know where the beginning of a frame is To do this, transmitters embed a special N -bit sync word called marker Initially, several repetitions of the sync word precedes the data. message.

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Where am I?

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  1. Frame Synchronization Where am I?

  2. BASIC ISDEA • For a receiver to demultiplex a TDM signal, it must know where the beginning of a frame is • To do this, transmitters embed a special N-bit sync word called marker • Initially, several repetitions of the sync word precedes the data message start of message N-bit sync

  3. SLIDING CORRELATOR • Knowing the synchronization codeword, the receiver constantly “looks” for it in the data • This task is accomplished by a correlation or matched filter detector. Let’s look at a 4-bit sync: {c1,c2,c3,c4} Ak data bits slide in and out Ak-1 Ak-2 Ak-3 Ak-4 c1 c2 c3 c4 frame indicator Vk + V -

  4. WORKINGS OF A SLIDING CORRELATOR Ak Ak-1 Ak-2 Ak-3 Ak-4 c1 c2 c3 c4 frame indicator Vk + V - {C1, C2, C3,C4} define the frame marker. Then

  5. HOW FRAME BEGINNING IS SENSED? • As data slides by the N-bit synch word, Vk takes on different values • There comes a time when the N-bit sync word fills the N-stage shift register. In this case, Vk achieves its maximum=N • If the register word differsfrom the sync word in just one bit, then Vk=N-2(assuming 1 pulses) • To sense the frame beginning, therefore, set the threshold voltage to slightly below N-2

  6. CASE OF FALSE FRAME • False synchronization is sensed when N or N-1 successive message bits match the sync-word bit • Can this happen?. Yes, question is with what probability? • Assuming equally likely +1’s and -1’s

  7. T1 SYNCHRONIZATION • T1 line uses a single bit per frame but framing bit sequence is known, i.e. if many individual framing bits are strung along, they form a known pattern • To acquire sync, several frames must be buffered, framing bits extracted and matched against the known pattern 1 0 1 0 frame 1 0 1 0 ...

  8. PROPERTIES OF GOOD SYNC WORDS • Since large false correlations result in wrong synchronization, synch word pattern must have several important properties • One such property is that they should have very small partial correlation with their shifted versions • The reason is that at certain times the shift register contains a part of the sync word as data is passing through buffer sync word

  9. AN EXAMPLE buffer MATCH sync word buffer Shifted Vs. sync partial cross correlation should be small sync word

  10. DESIRABLE CROSS CORRELATIONS N 1/N lag 1

  11. LENGTH-5 SYNC WORD +1 -1 C0=5 C1=0

  12. BARKER SYNC CODES • Barkers sequences have the property that their largest sidelobes have a magnitude of 1. N(length of code) Barker sequence(+/- are pulse ampl.) 1 + 2 ++ or + - 3 + + - 4 + + + - or + + - + 5 + + + - + 7 + + + - - + - 11 + + + - - - + - - + - 13 + + + + + - - + + - + - +

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