# A Novel Procedure of Enhancing the Throughput of a Mode-2 Sidelink Based on Partial Sensing
## 2 Sensing-Based Mode-2 Sidelink Communication  
In the 5G NR mode-2 sidelink, the base station does not intervene in the resource selection process of the terminal, meaning that each terminal selects the usable resources itself. Each terminal performs the sensing procedure by decoding the PSCCH to determine which resources are not occupied by other terminals. Therefore, sensing is performed prior to data transmission. In this section, we summarize the basic philosophy of the conventional resource allocation procedures based on resource sensing.  
Figure 1 presents a timing diagram of the sensing and selection process when the length of the sensing window is set to 200 ms. This corresponds to the method of full sensing which can be applied to V-UEs, for which considerably less severe restrictions on power consumption are imposed. In Figure 1, T denotes the timing corresponding to the start of the selection window. The sensing window starts at time \(\mathrm{T}-200\) ms, that is, 200 ms before T. During the sensing window period, each terminal undergoes the sensing process of decoding the PSCCH resource blocks used by all terminals. Thus, sensing is used to identify the occupied resources and exclude the occupied resource blocks from the candidate resource set. After the candidate resource set is obtained through this process, the resource blocks to be used for data transmission are selected from the candidate resource set that is used during the selection window. Through this process, even if the base station does not allocate resource blocks, the terminal can identify the resource blocks being used by all other terminals and thus prevent a collision.  
Random selection is a method in which usable resources are randomly selected and data are transmitted without sensing the resource blocks occupied by other terminals. Although no power is consumed for sensing, high collision probability can be achieved because each terminal transmits data without sensing.  
Figure 2 presents a timing diagram for the method of partial sensing considered mainly for P-UEs, which suffer significant restrictions on power consumption, where sensing is performed only for a partial sensing window of 40 ms for the entire observation period of 200 ms. As shown in Figure 1, T in Figure 2 also denotes the timing where the selection window begins. Here, sensing at each terminal is performed only for 40 ms, corresponding to the time interval from \(\mathrm{T}-200\) to \(\mathrm{T}-160\) ms. Because PSCCH decoding is performed only for 40 ms (from \(\mathrm{T}-200\) to \(\mathrm{T}-160\) ms), the selection window is eventually reduced to 40 ms (from T to T + 40 ms). Consequently, usable resources are selected only for the selection window of 40 ms.  
With partial sensing, power consumption can be reduced to approximately \(1/5\) (=40 ms/200 ms) compared to the case of sensing the resources for the entire 200 ms observation period. It can also prevent collisions, unlike in random selection. However, because sensing is performed only for a 40 ms partial window, selectable resources are also limited only for the corresponding time interval. Consequently, the throughput is reduced to nearly \(1/5\) (=40 ms/200 ms). We claim that, if several P-UEs can share their sensing results with each other, the throughput may be substantially improved. In the next section, we present a method for combining all the partial sensing results.  
Figure 1: Sensing window and selection window.