# An Effective Wide-Bandwidth Channel Access in Next-Generation WLAN-Based V2X Communications
## 5 Results  
In order to investigate the performance of the proposed channel-access schemes in terms of the fairness and efficiency (latency) of the 20-MHz frame defined in IEEE 802.11 bd, rigorous simulations are performed. The time driven simulator implemented using MATLAB is utilized in the performance evaluation. In the simulation, 20-MHz channel is composed of two contiguous 10 MHz-channels and each 10-MHz channel has two states: channel busy state and channel idle state. Multiple STAs independently perform channel access when frames are generated with their interarrival times following an exponential distribution. Two kinds of STAs are implemented: STAs performing 10-MHz channel access and STAs performing 20-MHz channel access following the schemes in this paper. The transmission of each STA changes the state of the corresponding channel(s) to channel busy state. In case of 20-MHz channel access, upon successful channel access, both primary and secondary channels become channel busy state. If 20-MHz frame transmissions are unfair, i.e., greedy with the 10-MHz frame transmissions of the secondary channel, then the 10-MHz frame transmitted in the secondary channel has a longer medium access delay than 10-MHz frames with only channel contention. In order to clearly measure the fairness performance, the traffic of the secondary 10-MHz channel is set to be congested. To implement the different levels of channel load, different number of STAs generating background 10 MHz frames is employed. Under such environments, the mean access delays of frames in the secondary channel for the proposed schemes are compared. Since fairness and efficiency need to be jointly designed, the latency performance of the proposed schemes is also investigated. The best scheme is that which provides the minimum impact on the secondary 10-MHz frame transmissions and short channel access delays of 20-MHz frame transmissions. In order to investigate the efficiency of the proposed schemes, the mean access delays of 20-MHz frames of the proposed schemes are compared. The mean access delay is the main component that causes latency in a broadcast environment, which is the typical communication environment of V2X. In the simulation, stations generate a frame to  
Figure 8: Comparison between analysis and simulation, in terms of the mean access delay of 10 MHz transmissions in the secondary channel. transmit with their interarrival times following an exponential distribution with an average interarrival time of 100 ms. For simplicity, an ideal channel is assumed, and all traffic is assumed for one AC. In addition, for simplicity, only broadcast frames, which do not require ACK, are transmitted in the simulation because the main type of V2X message is a broadcast message among many V2X applications [21]. As the effect of fairness can be clearly measured under a highly congested environment in the secondary channel, the simulation is performed under high traffic in the secondary channel and low traffic in the primary channel. Therefore, in the simulation, 200 stations transmit 10-MHz frames in the secondary channel, and 10 stations transmit 10-MHz frames in the primary channel. The other simulation parameters are listed in Table 1.  
Figure 9 shows mean and standard deviation of the medium access delay of 10-MHz frames transmitted in the secondary channel and mean and standard deviation of the medium access delay of 20-MHz frames as the number of stations transmitting 20-MHz frames increases from 10 to 200. As in Figure 9a,c, the proposed 20-MHz channel access scheme, the all back-off AIFS scheme, provides the minimum impact on the medium access delay of 10-MHz frames transmitted in the secondary channel in terms of mean and standard deviation. The standard deviation of all back-off AIFS scheme provides smaller standard deviation than the conventional method, providing the minimum impact on the number of 10-MHz transmissions with high contention delay caused by 20-MHz transmissions.