# Performance Analysis of Existing ITS Technologies: Evaluation and Coexistence
## 3 Performance Evaluation and Comparision
### Theoretical Evaluation of Channel Occupancy  
The channel occupancy for a data packet with ITS-G5 and LTE-V2X is theoretically compared. However, as mentioned above, the ITS-G5 is an asynchronous network that allows channel allocation to one user at a time (no users-multiplexing). Therefore, the occupancy time depends on the message size as well as the transfer rate. For example, a data packet of 350 bytes (with 40 \(\upmu\)s for overhead) requires 0.973 ms with MCS 0, 0.506 ms with MCS 2 and 0.273 with MCS 4 to be transmitted.  
In contrast, LTE-V2X is a synchronous network that allows multiple users to transmit (users-multiplexing possible in frequency domain) but with message duration fixed to 1 ms. In Table 3, different transport block (TB) sizes and coding rates are tabulated for 10 MHz frequency channels according to [25]. As the table shows, the number of RBs needed to transmit a message varies according to the message size and the coding schemes. The coding rates suggested by standards recommendation [26] are approximately around 0.5 for BPSK and 0.5 or 0.7 for QPSK modulation. For example, a data packet of 350 bytes with MCS 6 requires 28 RBs for TB and 2 RBs for control information SCI. Depending on the subchannel configuration, only one message can be transmitted during a TTI (1 ms) since a 10 MHz channel provides only 50 RBs. Using the same packet length, we can improve the transmission for two messages if we consider MCS 9 and three messages with MCS 13 in the same TTI.  
As we can see, the relevant number of transmitted messages is based on the packet size and the configured MCS. First, if we consider the ideal configuration, the performance of both ITS technologies can be similar in terms of transmissions number in the same period of channel occupation. However, suppose the number of RBs of a data packet exceeds 26. Only one message is sent during the entire TTI transmission, and the performance decreases, considering that the remaining resources are unused. Then, the access time to the channel depends on the access technique itself, where LTE-V2X requires additional time for resource allocation according to the selection window before transmitting. For ITS-G5, access is managed by the back-off algorithm if the channel is busy. In this case, the ITS-G5 depends on the efficiency of the back-off procedure. Finally, the resource re-selection counter and the probability of re-using the resource allow keeping the same resource for several transmissions, introducing more performance in specific scenarios (dense situations). However, the ITS-G5 requests channel access at each communication.  
The effect of these different examinations, such as channel access delay, message size, access techniques performances and optimal coding scheme, is this paper's primary focus through evaluating and comparing ITS technologies under the same conditions.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**MCS Index**} & \multirow{2}{*}{**TBS Index**} & \multicolumn{2}{c}{**Nb RBs TB =**} & \multirow{2}{*}{**Coding Rate**} & \multicolumn{2}{c}{**Nb RBs TB =**} & \multirow{2}{*}{**Coding Rate**} & \multicolumn{2}{c}{**Nb RBs TB =**} \\  & & **350 Bytes** & & **800 Bytes** & & **1200 Bytes** & \\ \hline  
\hline \end{tabular}
\end{table}
Table 3: Coding rate and RBs required for different Transport Block (TB) sizes.