## 5 Performance Evaluation
### Channel Impacts on the Proposed Algorithm  
In this paper, we focused on the algorithmic aspects of the sensing-based SPS algorithm, and abstract away the physical-layer issues such as coverage, path loss, fading, and hidden terminals. We intended to demonstrate the point that even under perfect physical-layer conditions, the current standard SPS algorithm exhibits serious shortcomings in resolving resource conflicts and providing high-reliability communication. However, to evaluate the performance impact of the proposed lookahead scheme more precisely, we would include the physical-layer properties in the simulation. We defer it to a future work, but here we provide a brief discussion on the expected impacts of the physical-layer features.  
In case there are hidden terminals attempting to reserve an overlapping set of resources using the lookahead, the UEs that are in the hearing distance of both the hidden terminals will not be able to hear from either. However, this problem is not specific to our proposed scheme. It is also a problem for the sensing-based SPS as well. In fact, it is a problem for any resource reservation scheme based on the physical sensing. Even the virtual carrier sensing feature in 802.11 is not applicable in DSRC because the beacon messages are broadcast. Therefore, we believe that this problem is beyond the scope of this paper and should be separately addressed.  
In case the lookahead is not received correctly due to various physical-layer degenerations such as path loss and fading, the receiving UEs can consider the reserved resource as still available. The lack of lookahead information, however, reverts the system back to the original sensing-based SPS algorithm because in our algorithm all UEs will check the validity of their reservations when their RC reaches zero, regardless of their published lookahead (line 26, Algorithm 2). Therefore, the addition of lookahead does not produce a side-effect when combined with its loss due to the adverse channel conditions.  
Finally, let us consider the case where two UEs, A and B, have the asymmetric channel condition. Without loss of generality, let us suppose [PERSON] can hear the reservation made by [PERSON] through the lookahead but not vice versa. In our algorithm, the node that sees a resource block being reserved by other UE should change its target (Section 4.3). Thus, if only [PERSON] hears [PERSON] making the reservation first, [PERSON] will choose another resource as the algorithm intends. In case [PERSON] makes the first reservation through the lookahead but [PERSON] cannot hear it, [PERSON] will go ahead and make the reservation for the overlapping resource through the lookahead. If [PERSON] hears it before it commits at \(RC=0\), [PERSON] recognizes the asymmetric channel condition and should change its reservation. If [PERSON] does not make the reservation before [PERSON] finally commits at \(RC=0\), the case becomes similar to Figure 4, except that [PERSON] and B do not have to use the same subframe to experience the problem. However, again, this problem does not make the proposed algorithm worse than the original SPS algorithm. In the SPS, [PERSON] and [PERSON] do not even have a chance to realize the conflict at all. The lookahead scheme at least has a chance to resolve one case where [PERSON] can hear [PERSON].