I'd like to hear your thoughts comparing "center of goodness" vs. "zero crossing adjust" techniques for recovering bit timing and deframing in an oversampled NRZ sample stream (I'm sure there are better names for these algorithms!)
Take an incoming sample stream which represents an 8X oversampled NRZ stream of 0s and 1s. This would mean there are 8 samples to each baud, and you need to pick at which point 0-7 you make the decision. A known sync code is sent first. In gr_simple_correlator, you maintain eight "trial" shift registers, each of which gets shifted into it a different bit in the sequence of eight. Then the shift register is compared to the sync code, and if it's within a certain hamming distance, you note it as the start of goodness. The first shift register to not meet the hamming distance threshold is considered the end of goodness. The "center of goodness" metric is then the modulo average of these two points, and is used as the decision point to convert the oversampled stream into a symbol stream. (Did I get that right?) The above both recovers clock/symbol timing and "start of frame" in the same operation. The alternative, simpler technique I've used in the past is to use zero crossings of the oversampled stream to estimate the center of the baud period, and slice there. This assumes the filtering ahead of the sampling makes the center between the crossings the best place to slice, and that there are sufficient zero crossings to keep clock skew from causing the slice decision to move out too far from the center. Once the bit timing is established the chosen bits are shifted into a single shift register, and when the sync code is seen (within a certain hamming distance), you are at the start of a frame. The former technique appears more general, less reliant on prior filtering, and immune to long strings of 1s or 0s. On the other hand, the latter technique is simpler, requires fewer calculations and less memory. So if the sample stream is known to have sufficient zero crossings and has been properly filtered, do you see any hazards to going with the latter technique? -Johnathan
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