Tham khảo tài liệu 'advances in vehicular networking technologies part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 202 Advances in Vehicular Networking Technologies There are two different approaches of secondary use of spectrum in cognitive radio context. One is in the form of overlay opportunistic usage of idle bands in the primary user s PU spectrum by cognitive radios and another in the form of underlay using Ultra Wide Band UWB technology cabric06 . The rules in secondary use of frequency spectrum specify that licensed users known as Primary Users PUs have the rights for interference-free communication in certain bands. When these bands are not used by the primary users they can be used by Secondary Users SUs . As soon as a primary user starts activity in its channel the SU has to vacate the channel to avoid interference timmers07 . However a cognitive radio using a half duplex transceiver cannot scan the spectrum and transmit simultaneously in the same frequency band. Then for the protection of primary users a maximum detection or sensing time must be established. This detection time represents the maximum time of interference from secondary users that a primary user can tolerate jia07 . Rendezvous in multi-channel protocols In multi-channel MAC protocols Mobile Stations MSs exchange control information to concur on the channel for data transmission in the user plane. Proposed protocols vary in how MSs negotiate the channel to be used for data transmission and the way to solve medium contention these protocols can be divided according to their principle of operation. In single rendezvous protocols the rendezvous between a sender and its receiver can take place on at most one channel at any time while in Multiple Rendezvous protocols several rendezvous can take place in different channels simultaneously thereby mitigating the control channel congestion mo07 . In single rendezvous three different classes of protocols can be distinguished based on the mechanism of channel negotiation sheung07 . The Dedicated Control Channel approach which uses two transceivers TRx .
