12 Sensitivity and Selection of Sync Word

The sensitivity can be impacted by the selection of the sync word and the sync threshold.

First, the sync word should have good auto correlation properties. The auto correlation could be visualized in Matlab or similar using plot(xcorr(2*vector-1)) where vector is the sync word in the following format: Vector = [1,0,0,1,1,0,.....]. Ideally the plot should have only one peak. If the plot has more than one peak this shows that it is a risk for getting a false sync. The probability for finding a false sync increases with the amplitude of the peak. Preamble is a good example of a sync word with poor auto correlation since the plot has many peaks where some are fairly high. A false sync is when the radio gives a sync found when the wanted sync word is not fully received which cause the received bits to be shifted which will give a CRC error. For short sync words, it is often recommended to include the preamble as part of the sync word.

Typically, the sync search consists of two parts. First, a correlator compares the incoming bit stream with the programmed sync. The sync threshold sets how equal the incoming steam has to be with the wanted sync word. Too strict threshold and the sensitivity will be limited by the threshold. Too loose and false sync will occur. In the second part the incoming stream is compared bit by bit to the programmed word. The second part is not done for all phys.

The sync threshold can be set as

CC13x0: HW_REG_OVERRIDE(0x5104,0xYYZZ)

CC13x2: HW_REG_OVERRIDE(0x5114,0xYYZZ)

where YY and ZZ is the threshold for the two correlators. For simplicity the two thresholds can be set equal. Higher value is stricter. The default sync threshold is given by: