5.9.10 Preamble Length and Sync Word

The rules for choosing a good sync word are as follows:

1. The sync word should be significantly different from the preamble.
2. A large number of transitions is good for the bit synchronization or clock recovery. Equal bits reduce the number of transitions. The recommended sync word has at the most 3 equal bits in a row.
3. Autocorrelation. The sync word should not repeat itself, as this will increase the likelihood for errors.
4. In general the first bit of sync should be opposite of last bit in preamble, to achieve one more transition.

The recommended sync words for the CC1021 device are 2 bytes (D391), 3 bytes (D391DA) or 4 bytes (D391DA26) and are selected as the best compromise of the above criteria.

Using the register settings provided by the SmartRF™ Studio software, packet error rates (PER) less than 0.5% can be achieved when using 24 bits of preamble and a 16 bit sync word (D391). Using a preamble longer than 24 bits will improve the PER.

When performing the PER measurements described above the packet format consisted of 10 bytes of random data, 2 bytes CRC and 1 dummy byte in addition to the sync word and preamble at the start of each package.

For the test, 1000 packets were sent 10 times. The transmitter was put in power down between each packet. Any bit error in the packet, either in the sync word, in the data or in the CRC caused the packet to be counted as a failed packet.