## 4 Sensing-Based SPS Scheduling with Lookahead
### Implementation Considerations  
In this paper, our focus is on exploring the performance impact of the lookahead scheme. Implementing it is not an immediate issue, but here we briefly mention possible approaches. First, we could include it in the BSM in Part II [2] where an optional information can be carried. However, it would pose difficulty because the SPS scheduling is far below the application layer in the protocol stack. We would need a cross-layer communication between the application layer that schedules BSM transmissions and the physical layer that controls the resource scheduling. Second, a more desirable way would be including the lookahead in the SCI Format 1 [38], which currently has the following information (numbers in parentheses are number of bits for each field).  
* Priority (3)
* Resource reservation interval (RRI) (4)
* Frequency resource location of initial transmission and retransmission (\(x=\lceil\log_{2}(N_{subCH}^{SL}(N_{subCH}^{SL}+1)/2)\rceil\))
* Time gap between initial transmission and retransmission (4)
* Modulation and coding scheme (5)
* Retransmission index (1)
* Transmission format (1)
* Reserved information (14\(-\)_x_)  
The additional data size to carry the lookahead is the time-frequency location of the transmission and the size of the resource allocation. One issue with this approach is that unless we increase the size of the SCI, there are not many remaining bits in the reserved information field to carry all four fields of a lookahead. Assuming that the lookahead information is added to the existing information above, we need \(x(=\lceil\log_{2}(N_{subCH}^{SL}(N_{subCH}^{SL}+1)/2)\rceil)\) bits to specify the frequency resource location  
Figure 4: Collision is not detected by the UEs using the same subframe.  
Figure 3: SPS sensing prevents the collision of the planned moves if RCs expire in different subframes.  
and the length \(L\) in the selected subframe. For example, if \(N_{subCH}^{SL}=25\), we need 9 bits to specify the two values. In this case, we could use the remaining bits in the reserved information field. Since the subframe specified in the lookahead is not the current subframe, we additionally need bits to encode the subframe number \(n\) in the lookahead. As each vehicle transmits a beacon at least once a second, the offset to the subframe is within 1000 subframes of the current subframe at which the lookahead is being determined. To encode this offset, we need 10 bits. Again, we could use the remaining 6 bits from the added bytes to cope with larger \(N_{subCH}^{SL}\). Figure 5 shows the additional number of bits that we would require by including the lookahead in the SCI, as a function of \(N_{subCH}^{SL}\) and the offset of the subframe designated for the lookahead. The figure assumes that we do not borrow the bits from the reserved information field. If the overhead is excessive to offer in the SCI, we could even consider carrying it in the BSM as discussed in the first alternative or as in [PERSON] et al. [31] at the cost of increased cross-layer signaling. However, we defer this implementation issue to a future work, and in this paper, we focus on the potential impacts of using such lookahead on the message collision mitigation performance.