Preambles in OFDM USRP2

Hi All,

I understand that the preambles in OFDM are obtained from the PN
sequence. But what does reapeated preambles, Preamble#1 and Preamble#3
mean in the gr-ofdm.pdf? I havent seen preambles after the first one.
Please let me know…

Thanks
T Matt