# Distributed Triggered Access for BSM Dissemination in 802.11 bd V2V Networks
## 2 Proposed Scheme  
In the proposed Distributed Triggered Access (DTA) scheme, vehicular stations (VSTAs) leverage the Triggered Uplink Access (TUA) scheme in the IEEE 802.11 ax standard [29] in order to trigger simultaneous broadcast packet transmission from nearby multiple VSTAs under distributed V2V communication environment. According to the IEEE 802.11 ax standard, when an Access Point (AP) station obtains a transmission opportunity (TXOP) through channel access, it may transmit a Trigger Frame (TF) to initiate a TUA OFDMA transmission sequence. In TUA, since AP is assumed to have knowledge about stations' (STAs) buffer status through buffer status report procedure, it solicits specific STAs by listing the STAs' addresses in the TF. The proposed DTA scheme is carefully designed to work under distributed vehicular communication environments. In order to minimize the changes of the standard, DTA is designed on top of TUA. In order to explain the proposed scheme, the overview of TUA is presented in the following. In TUA, the main purpose of TF is to share time synchronization information among scheduled stations as well as scheduling information for each scheduled station. The lack of resource reservation and periodic scheduling procedure makes WLAN difficult to implement uplink multi-user simultaneous transmission. With increased processing capability of station, IEEE 802.11 ax implements uplink simultaneous transmission in a manner of immediate responding to the scheduling of triggering station within one TXOP, instead of resource reservation. The remaining challenges to enable the simultaneous uplink transmission as an immediate responding to the scheduling are: (1) making the uplink frame detectable, and (2) the synchronization of the uplink transmission timing. In order to do so, TF contains all necessary physical layer parameters so that all scheduled stations can construct an identical preamble. The Trigger Based (TB) PPDU is transmitted immediately Short Inter-frame Spacing (SIFS) after the reception of the preceding TF, and the triggered STAs calculate the transmission duration using the Modulation and Coding Scheme (MCS) and the PHY layer Service Data Unit (PSDU) length information contained in the TF. The format of TF is depicted in Figure 1.  
The Common Info field contains general information which can be shared by all triggered stations such as the length and the total occupying bandwidth of the triggered PHY layer Protocol Data Unit (PPDU), the properties of the channel coding scheme and the transmit power of the triggering station. The User Info List field may contain multiple User Info fields as many as the number of scheduling Resource Units (RU). Each User Info field indicates the identification of the target triggered station and the scheduled RU as well as user-specific transmission parameters such as MCS, Forward Error Correction (FEC) coding type etc. If a station receives a TF with valid scheduling information, the station responds with a data packet using the TB PPDU format. Upon the transmission of a TB PPDU, the station needs to construct the first part of the preamble as instructed in the Common Info field of the preceding TF. The first part of the  
Figure 1: Trigger frame format. preamble follows the legacy format of IEEE 802.11 PPDU preamble format and occupies the whole 10 MHz channel bandwidth using the conventional 64-FFT OFDM symbols for backward compatibility. Since all scheduled stations transmit an identical preamble signal at the same time, the recipient can detect the start of the PPDU and obtain the length information of the PPDU. For the latter part of the PPDU, each scheduled station transmits individual data packet using the allocated resource with novel 256-FFT OFDM numerology. In this case, all triggered stations need to align the duration of the transmitting PPDU to maintain the synchronization of the transmission sequence.