## 3 Co-Channel Coexistence Mitigation Methods
### M\({}_{a}\)-Time-Split  
_Brief description._ With \(\mathrm{M_{A}}\)-time-split, both types of vehicles \(\mathrm{V_{NR}}\) and \(\mathrm{V_{11p}}\) are supposed to be synchronized to the superframes, with knowledge of \(\mathrm{S_{NR}}\) and \(\mathrm{S_{11p/bd}}\). This means that the vehicles are all synchronized and informed of the adopted settings. The VUEs are allowed to transmit only in their respective slots, as shown in Figure 2.  
\begin{table}
\begin{tabular}{l l l} \hline \hline
**Message** & **Required Methods** & **Characteristics** \\ \hline \multirow{2}{*}{Energy signal} & \multirow{2}{*}{\(\mathrm{M_{B}}\)-E-signals} &  • Energy without information \\  & & • Transmitted in all of the idle intervals in the time domain of \(\mathrm{S_{NR}}\) \\ \hline \multirow{3}{*}{\(11\)p-preamble} & \(\mathrm{M_{C}}\)-preamble; & • Not really a message, but only a portion of signal which is always identical; can be implemented as a fixed sequence of IQ samples \\  & & • Appended just before each NR-V2X packet \\ \hline \multirow{3}{*}{Reservation} & \(\mathrm{M_{C}}\)-reservation; & • In-type message with CSMA/CA channel access protocol \\  & & • Sent before each NR-V2X packet \\ \hline \multirow{3}{*}{CTS-to-Self} & \multirow{3}{*}{\(\mathrm{M_{F}}\)-CTS-to-Self} & • 11p-type message without CSMA/CA channel access protocol \\  & & • Different from transmitter to transmitter \\  & & • Sent at the beginning of each \(\mathrm{S_{NR}}\) by selected \(\mathrm{V_{NR}}\) \\ \hline \hline \end{tabular}
\end{table}
Table 2: Comparison between messages required by different methods.  
Figure 1: Illustration of the superframe used in the mitigation methods. Each superframe consists of two parts: the NR-V2X slot used by \(\mathrm{V_{NR}}\) and the IEEE 802.11p/bd slot used by \(\mathrm{V_{11p}}\).  
Enhanced versionOne issue arising with M\({}_{\rm A}\)-time-split is the so-called _channel rush problem_[10]; all V\({}_{\rm 1 lp}\) who have new messages to transmit during S\({}_{\rm NR}\) would hold on, and they all would start the backoff procedure immediately when the time goes into the S\({}_{\rm 1 lp/bd}\), leading to a higher probability of collisions at the beginning of S\({}_{\rm 1 lp/bd}\). To solve this issue, enhanced M\({}_{\rm A}\)-time-split (hereafter eM\({}_{\rm A}\)-time-split) was defined, where V\({}_{\rm 1 lp}\) always add a certain delay before starting the channel sensing procedure, in a way that uniformly distributes the beginning of transmissions. More details can be found in [10].  
ConfigurationsThis method is easily implemented only with static and controlled slot settings. The other options need further work.  
Modifications requiredThis method is easily implemented in NR-V2X through the definition of resource pools, which can include only the TTIs belonging to the S\({}_{\rm NR}\). A difference is that it requires modifications to the IEEE 802.11p VUEs, which are normally not synchronized; the V\({}_{\rm 1 lp}\) need to align to the superframe and assume as busy the slot reserved by NR just like the sensed power was above the threshold. Given that modifications are required anyway for the IEEE 802.11p specifications and that the performance of the M\({}_{\rm A}\)-time-split is substantially worse, only the eM\({}_{\rm A}\)-time-split is considered further.  
Implications of NR instead of LTEGiven that each technology has exclusive access to its own slot, the use of NR-V2X instead of LTE-V2X does not impact the inter-technology interference received or generated by IEEE 802.11p.