# Resolving Persistent Packet Collisions through Broadcast Feedback in Cellular V2X Communication +
## 3 Broadcast Feedback on Packet Collisions
### Packet Collision Sensing
#### 3.2.3 Feedback Provider Qualification Based on In-Band Emission  
Even if a received transmission satisfies the conditions of (1) and (3), the transmission still may not be a collision. It is because the conditions can be met under the influence of IBE when a vehicle much closer to the receiving vehicle transmits on an adjacent subchannel. Figure 4 depicts such a situation. The transmission from vehicle \(Z\) is free from a collision and strong enough so that \(\gamma^{\prime}(R_{k}^{n})>\theta_{\gamma}\), satisfying condition (3). However, in the same slot n, vehicle \(X\) also transmits from a much closer distance from \(Y\). Then, a strong IBE from vehicle \(X\) can interfere with \(Z\)'s transmission received at \(Y\) so much so that the interference component in the SINR can become high enough to cause a decoding failure. For example, suppose \(\gamma^{\prime}(R_{k-1}^{n})\) is 30 dB higher than \(\gamma^{\prime}(R_{k}^{n})\). If \(\alpha_{k-1,k}=3.5\%\), the IBE power will be 35 times higher than the received power on \(R_{k}^{n}\), pulling down the SINR drastically to satisfy the condition (1). Therefore, if the IBE is not accounted for, false positives will trigger unnecessary collision feedbacks. In fact, the frequency of false feedbacks is observed to sharply increase without the IBE consideration [14]. Because this phenomenon may occur quite frequently under higher channel utilization levels due to more occupied adjacent subchannels, it is imperative to account for the IBE in the feedback generation logic. To exclude the impact of IBE, one could simply require that those vehicles experiencing the sum of the IBE powers leaked on \(R_{k}^{n}\) larger than a certain threshold be disqualified as a feedback provider. However, care should be taken in configuring the threshold because the SINR that the IBE affects in condition (1) is a ratio. Specifically, the IBE powers below the fixed threshold can still reduce the SINR significantly if \(\gamma^{\prime}(R_{k}^{n})\) is also small. Therefore, the IBE-filtering threshold should be based not on the absolute IBE power but on the _ratio_ of the powers of the signal and the IBE. Specifically, the ratio should meet inequality (5),  
\[\frac{\gamma\prime(R_{k}^{n})}{\sum_{i\
eq k}\alpha_{i,k}\cdot\gamma\prime(R_{ i}^{n})}>\theta_{\sigma}, \tag{5}\]  
Figure 4: In-band emission from a much closer vehicle. where the denominator is the sum of all IBE power reaching the resource \(R_{k}^{n}\). Essentially, the left-hand side represents the SINR on \(R_{k}^{n}\) except for the noise. Therefore, if the IBE caused the SINR condition (1) to be satisfied, condition (5) would contradict condition (1). On the other hand, if a collision caused condition (1) to be met, the sum power of the two colliding packets will overwhelm the IBE power, satisfying condition (5). Therefore, condition (5) filters out the IBE-induced decoding failures in the low SINR, high RSSI region, so as to suppress the feedback from the receiver under the influence of the IBE. Note that our scheme is an opportunistic mechanism that assists SPS. In case the sum of the interference (i.e., the denominator) becomes too large by some environmental factors, the consequence would be that the vehicle in consideration is disqualified as a feedback provider. However, with the lack of the feedback, the system reverts to the vanilla SPS, and it does not adversely affect the operation of SPS. Moreover, if the vehicle traffic density is high, there can be multiple candidate feedback providers, so one (and only one) of them will provide the feedback. On the other hand, if the denominator is small, it will be a desirable condition because it means that the detected collision on the given resource is not being affected by IBE. Algorithm 1 is the pseudo-code for the feedback provider qualification check that comprises conditions (1), (4), and (5) above. For each resource in every slot, a receiver performs the three checks discussed above. If they are met, the collision event is registered in the \"feedback queue\" (line 20).