# Resolving Persistent Packet Collisions through Broadcast Feedback in Cellular V2X Communication +
## 3 Broadcast Feedback on Packet Collisions
### Packet Collision Sensing
#### 3.2.2 Feedback Provider Qualification Based on Received Signal Strength Indicator  
As to the question of which vehicles can report the witnessed collision event, the proposed feedback scheme limits them to those within a certain distance from the vehicles that transmitted the collided packets. This is because the closer the vehicle is, the more accurate the power-based assessment will be. Moreover, the main concern is in the proximity of the transmitter where the safety beacons play a more critical role (i.e., ROI) to prevent the crashes with the transmitting vehicle. The collisions in the closer range must be quickly resolved and have a higher priority over those in the fringe of the communication range where the hidden terminal problem is unavoidable. It should be avoided that the hidden terminal problem causes confusion so that a perfectly clean transmission is reported as a collision by a remote vehicle in the fringe. For this reason, the proposed scheme requires a broadcast receiver to sense a high RSSI on the given resource R (denoted \(\gamma\) (R)) to qualify for a feedback provider for the resource. Namely, if the receiving vehicle senses less power than a certain threshold \(\theta_{\gamma}\) in R, it is disqualified as a feedback provider. One caveat in evaluating the RSSI on a particular resource is that \(\gamma\) (R) is the sum of two entities: received signal power from the transmitters and in-band emission (IBE) leaked from other subchannels in the same time slot (see Figure 3). Let \(\gamma^{\prime}\) (R) be the sum of the powers received purely from potentially colliding packets in R. Additionally, let \(\alpha_{i,j}\) be the ratio of the power leaked to subchannel \(j\) due to IBE from \(\gamma^{\prime}\) loaded on subchannel \(i\
eq j\). The \(\alpha_{i,j}\) is a function of the distance between \(i\) and \(j\) on the frequency axis and of the modulation and channel coding (MCS) level among others. It can be empirically measured by actual transmission or determined based on a model [18]. Knowing the RSSI carried in each subchannel \(k\) in a time slot \(n\) and the coefficients \(\alpha\) for nearby subchannels, a broadcast receiver can estimate the IBE-excluded received power of each resource \(R_{k}^{n}\) as Equation (3):  
\[\gamma^{\prime}(R_{k}^{n})=\gamma(R_{k}^{n})-\sum_{i\
eq k}\alpha_{i,k}\gamma^ {\prime}(R_{i}^{n}) \tag{3}\]  
where \(\alpha_{i,k}\) is the IBE leaked from subchannel \(i\) to \(k\) and \(\gamma^{\prime}(R_{i}^{n})\) is the power on the same subframe resource \(R_{i}^{n}\). Then, the second necessary condition the proposed scheme requires for a vehicle to provide a collision feedback on resource \(R_{k}^{n}\) is given in inequality (4),  
\[\gamma^{\prime}(R_{k}^{n})>\theta_{\gamma}, \tag{4}\]  
where \(\theta_{\gamma}\) is the RSSI threshold. This condition disqualifies the receivers in the low RSSI region. In this paper, a threshold value \(\theta_{\gamma}=-85.2\) dBm is used, which is approximately twice the power a receiver senses from the transmitter located just over 100 m away in the experimented channel model [18]. The double power requirement is to account for at  
Figure 3: In-band emission (IBE)-induced interference from other subchannels. least two colliding packets. In practice, the distance and the corresponding \(\theta_{\gamma}\) should be determined by the application requirements, such as in terms of the minimum required communication range for the given safety application [19]. Note that a too low and too high \(\theta_{\gamma}\) would increase false positives and false negatives, respectively. Among the two false classification errors, the false negative is less of a problem because multiple feedback providers will be available in most packet collision cases. This is because packet collisions increase with the channel utilization, which in turn is driven by increased vehicular population. Even if there is no qualified feedback provider, the proposed scheme can always fall back to the original SPS.