©2000 BG Mobasseri Where am I?

©2000 BG Mobasseri 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

©2000 BG Mobasseri 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} AkAk A k-1 A k-2 A k-3 A k-4 frame indicator V Vk + - c1c2c3c4 data bits slide in and out

©2000 BG Mobasseri WORKINGS OF A SLIDING CORRELATOR AkAk A k-1 A k-2 A k-3 A k-4 frame indicator V Vk + - c1c2c3c4 {C1, C2, C3,C4} define the frame marker. Then

©2000 BG Mobasseri HOW FRAME BEGINNING IS SENSED? As data slides by the N-bit synch word, V k takes on different values There comes a time when the N-bit sync word fills the N-stage shift register. In this case, V k achieves its maximum=N If the register word differs from the sync word in just one bit, then V k =N-2(assuming  1 pulses) To sense the frame beginning, therefore, set the threshold voltage to slightly below N-2

©2000 BG Mobasseri 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

©2000 BG Mobasseri 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 1010 frame

©2000 BG Mobasseri 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

©2000 BG Mobasseri AN EXAMPLE buffer sync word buffer sync word MATCH partial cross correlation should be small Shifted Vs. sync

©2000 BG Mobasseri DESIRABLE CROSS CORRELATIONS 1 lag N 1/N

©2000 BG Mobasseri LENGTH-5 SYNC WORD C0=5 C1=0 +1

©2000 BG Mobasseri 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.) or or