## 3 Co-Channel Coexistence Mitigation Methods
### M\({}_{\rm F}\)-Cts-to-Self  
_Brief description._ In M\({}_{\rm F}\)-CTS-to-Self, as illustrated in Figure 5, the superframe structure is again only known by V\({}_{\rm NR}\). At the beginning of each S\({}_{\rm NR}\), a Clear-To-Send-To-Self (CTS-To-Self) message is sent by some of the V\({}_{\rm NR}\) to inform the V\({}_{\rm 11p}\) that the channel will be reserved. The CTS-To-Self message is specified in the MAC layer of IEEE 802.11 for channel resource reservation to avoid the hidden terminal problem. The channel could be reserved for up to 32 ms by the CTS-To-Self message.  
When the V\({}_{\rm 11p}\) receive the CTS-To-Self message, they set a parameter named network allocation vector (NAV) to mark that the channel is busy during the indicated time interval, regardless of the actual sensed status of the channel.  
The reservation information of the CTS-To-Self message is included in the DATA field, thus the scrambling operation in the PHY layer would lead to different signals varying with the transmitter. This implies that multiple messages sent simultaneously from multiple sources collide. To reduce this issue, only selected nodes transmit the CTS-To-Self message. In general, there may be multiple possible rules to determine which V\({}_{\rm NR}\) are selected. Consistently with what is proposed in [4], in this paper the vehicles that have resources in the current superframe and do not sense anyone reserving resources before it or during the same slot with lower frequency will send the CTS-To-Self message. An example of this approach is given in Figure 6.  
Figure 5: Illustration of M\({}_{\rm F}\)-CTS-to-Self. Only V\({}_{\rm NR}\) know the superframe structure. At the beginning of each S\({}_{\rm NR}\), a chosen V\({}_{\rm NR}\) would broadcast a CTS-To-Self message to reserve the resources from V\({}_{\rm 11p}\). The subfigure (**a**) is from the V\({}_{\rm NR}\) perspective, and the subfigure (**b**) is from the V\({}_{\rm 11p}\) perspective.  
Figure 6: Procedure of choosing the CTS-To-Self sender of M\({}_{\rm F}\)-CTS-to-Self. V\({}_{2}\) has a resource segment in the SF(\(i\)), and no resources are reserved by other vehicles before it, hence the CTS-To-Self in SF(\(i\)) would be transmitted by V\({}_{2}\) at the beginning of SF(\(i\)). Following the same procedure, V\({}_{7}\) is chosen to send the CTS-To-Self message at SF(\(i\)+1). The red number in the figure is the resource index ordered in the frequency and time domains.  
ConfigurationsGiven that only part of the stations transmit the CTS-To-Self, how to apply it in a dynamic configuration is not clear. Thus, only the static and autonomous superframe setting is investigated in this paper.  
Modifications requiredVUE with NR-V2X should be changed to be able to send the CTS-To-Self message (reported in Table 2) and make decisions about which vehicle needs to send the CTS-To-Self message, while VUE with IEEE 802.11p should be able to recognize the CTS-To-Self message and set the NAV accordingly.  
Implications of NR instead of LTEThe use of NR-V2X instead of LTE-V2X with numerology above 0 implies shorter messages. This is, however, not expected to impact relevantly on the inter-technology interference received or generated by IEEE 802.11p stations.