For UWB, preamble symbols have nothing to do with bytes
refer to section 16.2.5.1 (SYNC field of UWB PHY) of IEEE 802.15.4-2015
or alternatively to the DW1000 user manual section 10.3 Synchronisation header modulation scheme
Although less detailed, the paper you mentionned properly describes UWB preamble symbols as a sequence of coded symbols from ternary alphabet
Fabrice