The preamble is a 16-chip sequence transmitted at the start of a packet. Its primary purpose is to allow the receive correlator to establish the appropriate bit centering and synch up on the data stream before the other structures in the packet are transmitted.
The preamble pattern and length are set through the PREAMBLE_ADR file register 0x24. Byte 1 of the file register establishes the number of repetitions of the 16-chip sequence. The preamble may be disabled by writing 0x00 to this byte, but it is highly advised to make use of the preamble.
Disabling the preamble has a small savings in terms of overall packet length in most data modes, but can have a significant effect on the ability to receive a packet. Without the preamble sequence to allow the receive correlator an opportunity to synch on the data stream, it is much more likely for the receiver to fail to correlate on the SOP symbols in the case of framed data, or on the data itself in the case of unframed modes. Failing to correlate on a SOP means that no data packet will be received. For unframed modes, the result can be corrupted data. The second and third bytes of the file register set the least significant eight chips and most significant eight chips of the preamble sequence respectively. Note that when reading or writing this register, all three bytes must be read or written. Failing to do so will shift the contents by the number of reads or writes. A subsequent access will then not start with the first byte.