What actual protocols exist out there that employ Offset Quadrature Phase-Shift Keying (OQPSK)?
What do they use for the frame synchronization marker (or "preamble" or "sync word" or "flag")?
Do they use a unique word, a unique pattern of bits? If yes, do they do some kinda bogus bit insertion (similar to the zero-insertion what the old SDLC protocol did)? If not, how can they guarantee that the data never mimics the frame sync marker and a spurious frame sync occurs?
What do they do for Offset QPSK to synchronize the I/Q phase of the intermediate frequency (IF) signal? Does the frame sync marker give the receiver an unambiguous signal from which to derive the correct I/Q reference phase?
Lotsa questions. I've been thinking of OQPSK a bit and have rambled out loud about it at the DSP Stack Exchange. OQPSK naturally pairs the bits from a single serial stream into "dibits". The even and odd bits naturally are routed to the I and Q signals without any contrived pairing of bits. Then it seems to me that the 16 bits
0011001100110011 would be the natural marker for the lowest instantaneous frequency and
0110011001100110 would be the highest frequency. One pattern could be the frame start marker and the other pattern a channel idle marker.
But then the 15 bits
011001100110011 should never occur in the data and a bogus
0 should be inserted after the 14 bits
01100110011001 so that neither marker would ever appear in the data. In the style of SDLC, the receiver would always remove that bogus
0 that occurs after
01100110011001. If, before the zero-removal, a
1 appears after
01100110011001, then I think either a channel idle or frame start marker is occurring (one more bit is needed on one side or the other of the 15 bits
011001100110011 to know for sure).
But this paper suggests other bit patterns (Barker sequences) for markers.
What are the protocols in use? What is the wisdom of those here that have actually worked on OQPSK at a low level?