If you look under the section "802.15.4 Frame Format" on this page you will see it defines
Preamble Sequence : 4 octets
Start Frame Delimiter (SFD) : 1 octet
In the AT86RF233's data sheet on pg. 76 Section 8.1.1.1 in the SFD is predefined to be
0xA7
But the preamble does not have a predefined value, so what should I define it to be? I don't see any indication in the data sheet explaining this further.
From the AT86RF233 data sheet (p.76):
8.1.1 PHY Protocol Data Unit (PPDU)
8.1.1.1 Synchronization Header (SHR)
The SHR consists of a four-octet preamble field (all zero), followed by a single byte start-of-frame delimiter (SFD) which has the predefined value 0xA7. During transmission, the SHR is automatically generated by the Atmel AT86RF233, thus the Frame Buffer shall contain PHR and PSDU only, see Section 6.3.2. […]
So, to me it looks like the preamble is made of 32 consecutive Zero-Bits. However, I am curious why you need to "define" the preamble, as it is automatically generated by the hardware and cannot be modified by configuration.