## 2 Background Technology  
In the IEEE 802.11p standard, different from other IEEE 802.11 standards, authentication and association processes, which cause a large delay before real data transmission, are not required. Each IEEE 802.11p STA can transmit its data to an AP after receiving the beacon frame with the channel access procedure. Figure 4 shows the conventional IEEE 802.11 transmission procedure with large delay link establishment and the IEEE 802.11p procedure with simplified link establishment. Although IEEE 802.11p has a simplified link establishment procedure, the channel access procedure is still time-based random access; hence, if multiple STAs transmit their data simultaneously, all the data frames collide in the air. Because the random-access-based channel access procedure of IEEE 802.11 is distributed and involves a large retransmission delay, it is not suitable for highly dense connected car scenarios like the interchange and urban traffic jam cases. Figure 4: \(Cont.\)  
An uplink delay of IEEE 802.11p frame delivery can be described as  
\[ \begin{array}{l}T_{V2X}=\pi T_{CCH\_duration}+T_{TXOP}\\ +\sum\limits_{i=0}^{M}\Big{\{}T_{back-off}(i)+T_{SCH\_busy\_time}(i)+N(i)T_{CCH\_ duration}\Big{\}}\end{array}  \tag{1}\]  
where \(\alpha\) represents the ratio of the partial control channel (CCH) duration if the frame arrives during the CCH duration. \(M\) is the number of retransmissions until transmission success or failure. \(N(i)\) is the number of CCHs for each retransmission. If \(i\) is 0, the value came from initial transmission. \(T_{CCH\_duration}\) is a value of a CCH duration usually set to 50 ms or 0 ms. \(T_{TXOP}\) is a time component for transmitting a data frame and receiving its immediate response. \(T_{back-off}\) represents a random delay during which an IEEE 802.11 STA must wait before transmitting its frames to avoid wireless channel collision. The back-off value is chosen randomly between 0 and the value of the congestion window (CW) size, and the STAs must reduce the back-off value every time slot interval if the channel is idle during the time slot and the value is positive. Because IEEE 802.11p could switch its logical channel repeatedly, an uplink channel access procedure by connected cars can be performed during the service channel (SCH) duration only. Basically, IEEE 802.11p could switch its logical channel every 50 ms. In IEEE 802.11p, there are two types of logical channel: CCH and SCH; CCH is used for transmitting downlink ITS information by an ITS AP, and SCH can be used for every ITS station if the STAs have received a beacon frame from the ITS AP. If a V2X system deploys a dedicated frequency channel for the logical CCH and SCH, the V2X delay, \(T_{CCH\_duration}\) can be set to 0. In this case, \(T_{V2X}\) can be described as below. \[ \begin{array}{l}T_{V2X}=T_{TXOP}+\sum\limits_{i=0}^{M}\Big{\{}T_{back-off}(i) +T_{SCH\_busy\_time}(i)\Big{\}}\end{array}  \tag{2}\]  
Because recent V2X systems tend to use a dedicated channel structure rather than to use a channel switching structure in a single channel [35; 36], we determined to use this dedicated CCH structure in this study. Figure 5 describes an example of the channel structure considered in this study. Figure 4: IEEE 802.11 procedures for transmitting its data frame to an unassociated AP: (**a**) Conventional IEEE 802.11 procedure; (**b**) IEEE 802.11p procedure. Figure 5: Example of IEEE 802.11p Channel Structure. V2X systems consist of several components including vehicle-to-vehicle (V2V), vehicle-to-infrastructure (V2I), vehicle-to-device (V2D), etc., and V2V and V2I communications shall be operated in identical frequency bands if the V2X system is supported by a single communication protocol rather than using complex multiple vehicular communication systems. Only the channels in the bands can be diversified for each purpose. Some channels could be defined as CCHs, and others as SCHs. Some SCHs might be used for V2I, and others for V2V or a mixed purpose.