Preamble and Sync Word
The preamble is the regular bit sequence at the start of the packet and it lets the receiver settle its gain and lock its bit clock. The sync word that follows immediately marks exactly where the data begins. Without these two fields the receiver cannot sample an incoming bit at the right moment, even on the right frequency.
What the receiver sees when it is listening continuously is noise. When the preamble arrives, three jobs are done at the same time:
- Automatic gain control is adjusted to the signal level.
- The bit clock is locked to the incoming transitions; the sampling instant settles in the middle of the bit.
- Frequency offset is measured and corrected.
The sync word is a chosen pattern with a low probability of occurring in noise. The receiver starts reading the remaining fields only when it sees this pattern. A long preamble increases the chance of capture on a weak signal, but it increases air time and therefore the transmission budget; a short preamble does the opposite. On low-consumption receivers using periodic wake-up, the preamble has to be kept longer than the wake-up interval. In the field this means: a link that drops at the range limit most often drops not from lack of power but because synchronisation does not hold.
Context in which this term is used: Wireless remote control installation: pairing, relay, safety