# An Effective Wide-Bandwidth Channel Access in Next-Generation WLAN-Based V2X Communications
## 2 Background  
### Conventional V2X Communication Scheme in 5.9 GHz Band  
The most widely deployed V2X communication scheme is DSRC, which employs a set of WAVE standards including the IEEE 1609 series and IEEE 802.11p, as described in [11]. The most widely used V2X frequency band used for V2X communication is the 5.9 GHz band, which is a licensed band. As described in reference [12], the Federal Communications Commission (FCC) in the U.S. initially allocated seven channels in the 5.9 GHz band in 1999 and defined each channel's usage and rules in 2003 and 2006, as in Figure 1.  
As shown in the figure, Channel 178 (5.885 GHz-5.895 GHz band) is called the Control Channel (CCH), which is used only for safety messages and control messages. Channel 172 (5.855 GHz-5.865 GHz band) is dedicated to safety messages between vehicles, and Channel 184 (5.915 GHz-5.925 GHz band) is used to transmit frames to vehicles in a long communication range. The remaining channels can be used to send data frames regardless of message or data type. Two sets of 20-MHz channels (Channel 174 and Channel 176, and Channel 180 and Channel 182) can be used for 20-MHz frame transmissions. IEEE 802.11p defines the Medium Access Control (MAC) and Physical (PHY) layers of V2X communication schemes in the 5.9 GHz band. The channel access schemes in IEEE 802.11p employ the Enhanced Distributed Channel Access (EDCA) mechanism of IEEE 802.11e with modified parameter values. EDCA is a contention-based channel access scheme with four different Access Categories (ACs) depending on traffic type: Video, Voice, Best Effort, and Background. Except for EDCA parameters for differentiated channel access per traffic type, EDCA is basically a Distributed Coordination Function (DCF) scheme. In a DCF scheme, when a station has data to transmit, it first senses for a DCF Interframe Space (DIFS) period to check if the medium is idle before it transmits the data frame. There are two kinds of channel sensing methods--Preamble Detection (PD) and Energy Detection (ED). PD senses the channel status by decoding the signal duration included in the IEEE 802.11 preamble and the frame duration value indicated in the Duration field of frame. ED senses the channel status by detecting if there is any received signal power over a threshold value that is regarded as decoding a failure or an error. When the channel has a busy status, there are different procedures depending on the used channel sensing methods (PD or ED). If the channel is sensed as busy by PD during a DIFS period, then a back-off procedure is triggered after the DIFS period from the end of the sensed busy period. The back-off procedure waits for the back-off duration, which is determined by a randomly selected back-off counter. Figure 1: The 5.9 GHz channel allocation in the US. On the other hand, if the channel is sensed as busy by ED during a DIFS period, then the stations must wait for an Extended Interframe Space (EIFS) period, which is a longer time period than DIFS after the end of the channel's busy status sensed by ED. An EIFS period is sufficiently long to protect the ongoing frame transaction where the transmitter of the frame that caused the channel to be busy needs to receive an acknowledge (ACK) frame to successfully complete the ongoing frame transaction. Therefore, an EIFS period includes the time for ACK frame transmission, i.e., EIFS = Transmission time of Ack frame at lowest PHY mandatory rate + SIFS + DIFS. By providing enough time in the EIFS period for a transmitter to receive an ACK frame, the hidden node problem caused by the ACK frame transmitter can be avoided. In EDCA, in order to provide differentiated channel-access priorities depending on the traffic type, as shown in Figure 2, an Arbitration Interframe Space (AIFS) is used instead of the DIFS of the previously described normal DCF operation. AIFS values are different for the AC. The EIFS value is also adjusted with the AIFS period, i.e., EIFS = Transmission time of Ack frame at lowest PHY mandatory rate + SIFS + AIFS.