“…In other words, this is the symmetric key that is used both in encryption and decryption process. For example, if the original OFDM symbol is [a 0 , a 1 , a 2 , a 3 , a 4 , a 5 ], then the resulted randomized sequence is [a 5 , a 3 , a 1 , a 2 , a 4 ], where the key is [5,3,1,2,4]. The cyclic prefix (CP) is added after that to be able to extract frequency response of the channel at the receiver, which is detailed in § IV-B.…”