## 4 Sensing-Based SPS Scheduling with Lookahead
### SPS with Lookahead Scheme  
We propose that we take a more direct approach to removing uncertainty in the SPS algorithm while not undermining the necessary randomness in standard parameters. Specifically, we let each packet carry the resource location information for the next streak of packets. Moreover, we require each UE determine the location before the current streak ends. Please note that in the original SPS algorithm, the location is determined just before RC reaches 0 [20]. We call this scheme _SPS/LA_ for \"SPS with lookahead\". Unlike [PERSON] et al. [32], we try to realign only the individual single-subframe resources with the same individual resource location determined as the starting location for the next streak. Since such information is available earlier than the actual movement to the planned location, UEs can have chances to change their decision and advertise the newly adjusted location. In this paper, we let UEs determine the next location at \(RC_{LA}=1\). This is because larger \(RC_{LA}\) values face more uncertainty in the observation of resource usage. Namely, the resource reservation information by other nodes through their published lookaheads and the current use pattern are subject to changes until \(RC=0\), especially when the congestion level is already high.  
The advertised location information for the next streak, the lookahead, is composed of three fields that point to the next resource location and size of the first packet of the next streak. Namely, it is  
\[LA=<c,L,n>\]  
where \(c\) is the starting subchannel, \(L\) is the number of consecutive subchannels it uses to transmit the packet (a.k.a. transport block (TB)), and \(n\) is the subframe index, of the resource location. Notice that other UEs can easily infer the current RC value of the advertising UE, because the last packet in the current streak will be in \([n-(T2-T1+1),n-1]\).  
Figure 2 shows an example where the lookahead helps avoid the message collision that would be unavoidable in the original SPS. In Figure 2a, UEs \(A\) and \(B\) collides at a resource in subframe \(n^{\prime}\) since they do not know each other's internal decision as to the location of the next streak. In contrast, in Figure 2b, \(B\)'s decision to use the resource is conveyed in the lookahead, which \(A\) notices. It changes its earlier decision to move to the contended resource, and it moves to a different resource location when \(RC_{A}\) reaches 0.  
As discussed above, in SPS, UEs do not know the \(RC\) values of others, so some resource is unnecessarily marked unusable by its sensing component. For example, in Figure 2a, other UEs will sense the transmissions of \(B\) at subframe \(n_{B}\) and \(A\) and \(n_{A}\). Then they will consider the same single-subframe resources at \(n_{B}+RRI\) and \(n_{A}+RRI\) are not available. For those whose sensing windows contain these resource locations will have to vie for the remaining resource although the locations will be unused by \(A\) and \(B\). When the channel busy ratio (CBR) [39] is high so that the available resources are limited, such inefficiency would be highly undesirable. In contrast, in SPS/LA, such inefficiency is prevented. Not because SPS/LA explicitly transmits the \(RC\) count, but because it broadcasts the planned initial resource location of the next streak. The \(RC\) count can be inferred to reach zero within \(T\) of the planned location. As with explicit RC advertisement [32], when the currently used location will become unused, which other UEs can use it in SPS/LA.  
Finally, we note that the use of the lookahead does not replace the sensing mechanism. As in the original SPS algorithm, the UE that reselects at \(RC=0\) should check the sensing window for the resource use history, in addition to the lookaheads that it has received that fall in the selection window. In essence, the lookahead is a reinforcement of the existing sensing-based SPS algorithm, not a replacement.