Preamble generator for a multiband frequency division multplexing (OFDM) tranceiver of a wireless personal area network (WPAN) being switchable between a time frequency interleaving (TFI)-mode wherein data packets are transmitted by said transceiver in different frequency bands according to a predetermined frequency hopping pattern and a fixed frequency interleaving (FFI) mode wherein data packets are transmitted by the transceiver in at least one fixed frequency band, wherein said preamble generator scrambles in the fixed frequency interleaving (FFI) mode a predetermined preamble of the data packets by multiplying said preamble with a pseudo random data sequence to flatten a power spectrum of said preamble.