# Distributed Triggered Access for BSM Dissemination in 802.11 bd V2V Networks
## 2 Proposed Scheme
In the proposed scheme, we assume that the PPDU structure of TB PPDU and the PPDU for TF transmission for IEEE 802.11 bd VSTAs follow the format of HE TB and HE SU PPDU defined in the IEEE 802.11 ax standard, except that the subcarrier spacing is halved with increased OFDM symbol duration. Following the assumption, a triggering station can schedule up to 9 stations per single 10 MHz bandwidth using the smallest RU size with 26 OFDM tones. In this paper, multi-channel transmission in [34] is not considered. The general format of the proposed TB PPDU is depicted in Figure 2.  
In IEEE 802.11 ax, TUA is also applied for a wide-range transmission protection scheme, called MU-RTS. In WLAN, in order to prevent collision, the transmitter and the receiver can perform RTS/CTS (Clear-To-Send) procedure, so that the nearby stations can set a Network Allocation Vector (NAV) and suspend any transmission until the time indicated in the NAV. However, if a transmission is performed by multiple transmitters simultaneously or a transmission is destined to multiple receivers, the protection that covers all communication range cannot be done unless all corresponding transmitters and receivers exchange RTS/CTS one by one sequentially. Thanks to the synchronization feature of TF, a triggering station can solicit simultaneous CTS transmission by transmitting an MU-RTS frame which is a variant of TF. Following the information included in the MU-RTS frame, such as the transmitting bandwidth, the scrambling seed and NAV information, all solicited stations can transmit an identical CTS frame simultaneously. Since the CTS frame follows the legacy OFDM PPDU format, all other stations within the range of simultaneous CTS frame can set a NAV properly. If a transmitted MU-RTS frame had collided with other frame, the transmitter may not receive any CTS frames from nearby stations. In this case, the transmitter release the TXOP and re-initiate an EDCA backoff procedure. The proposed distributed multi-vehicle channel access scheme, DTA, fully exploits all the beneficial features of TUA and MU-RTS. Although the proposed scheme can accommodate all kinds of Outside the Context of Basic Service Set (OCB) traffics, for the sake of simplicity, Basic Safety Message (BSM), which is a broadcast traffic and the most common traffic in V2X communications, is used in the description of the proposed scheme. In the proposed scheme, two types of channel access methods coexist. One is conventional EDCA, the other is Triggered Access (TA) in which VSTA accesses the channel triggered by another VSTA, independent from its own EDCA backoff procedure. Whenever a VSTA acquires a TXOP after finishing its  
Figure 2: The general format of the proposed TB PPDU. backoff procedure, it plays a role of triggering VSTA. In order to solicit multiple VSTAs' simultaneous BSM transmission, the triggering VSTA transmits a TF, which contains the scheduling information for multiple VSTAs, at the beginning of the TXOP. Since broadcast BSM is destined to all nearby VSTAs, all VSTAs within the transmission range of each transmitting VSTAs needs to be protected. Thus, in the proposed scheme, a protection procedure using MU-RTS is proceeded ahead of multi-vehicle BSM transmission. Since the MU-RTS protection in DTA is always intended to all neighboring VSTAs, the Common Info field can contain all the necessary information for MU-RTS procedure without User Info List field. Furthermore, as the Common Info field information of the MU-RTS frame can be reused for the TF soliciting TB PPDU transmission of BSM, the TF for MU-RTS procedure and that for multi-vehicle BSM triggering can be merged into one unified TF. Thus, once a triggering VSTA transmits a TF, all VSTAs that receive the TF transmit simultaneous CTS immediately after the TF reception, following that a simultaneous TB PPDU transmission is made by the scheduled VSTAs.