## 2 Preliminaries and Related Works
### Protocols Exploiting Link Correlation  
We summarize recent works aiming at improving the protocol performance of opportunistic routing and flooding. Opportunistic routing: The key idea of opportunistic routing is to exploit spatial diversity to improve the OR efficiency, _i.e._, a forwarder broadcasts a packet, and any relay nodes that overhear the packet can become the next forwarders. Compared to traditional unicast routing, it can reduce the number of retransmissions. [PERSON] _et al._[14] identify that link correlation indeed has a large impact on the spatial diversity. From the same sender, if the outbound links are weakly correlated, each receiver has different received packets, and there is much spatial diversity. The key insight is that negative correlated links are beneficial to opportunistic routing. The reason is that with negative link correlation, if a receiver cannot receive the packet, it is more likely that another receiver can receive that packet. Then, we can infer that with negative correlated links, when a packet is broadcast, at least one receiver can receive the packet, and retransmissions are avoided. By selecting forwarders with negative correlated links, the protocol can effectively exploit the spatial diversity in the network and reduce transmission and delay overhead. Compared to negative correlated links, independent links (with weak or no link correlation) are less beneficial. [PERSON] _et al._[15] extend the work in [14] by formally designing a link correlation-aware routing metric (expected number of transmissions, ETX), which considers both link quality and link correlation. Moreover, it designs an efficient prioritization approach to prioritize the transmissions from high metric value forwarders. By efficiently selecting the weakly correlated links with high link qualities, the routing efficiency is improved. Flooding: Traditional flooding is simple. The sink node broadcasts the flooding packets, and nodes that received the packets reply ACKs and then re-broadcast the packets. This process goes recursively until all network nodes receive the packets. [PERSON] _et al._[16] employ link correlation for efficient flooding. The proposed protocol, CF (collective flooding), improves the flooding performance compared to previous work. CF uses link correlation to infer the probability of a packet's reception at different receivers. Based on this knowledge, CF calculates the impacts of different forwarders and selects the best forwarder with the largest impact. [PERSON] _et al._[17] exploit link correlations for efficient flooding in low duty cycle sensor networks. Nodes with high correlated links are grouped together in order to reduce the number of ACKs. The grouping of nodes, however, requires each node to perform a time-consuming k-means algorithm, which incurs a large computational overhead on resource-constrained sensor nodes. [PERSON] _et al._[23] extend [PERSON] _et al. 's_ design [16] for multiple packet scenarios by employing rateless codes, which improve the performance in terms of transmissions. [PERSON] _et al._[21] consider link correlation in the clustering of network nodes (CorLayer). CorLayer models the relationship between link correlation and the expected number of transmissions (ETX) and then selects nodes with small ETXs to form clusters. Then, by connecting all clusters, the underlying structure for efficient flooding is constructed. [PERSON] _et al._[20] employ link correlation for dissemination (multi-message reliable flooding). They use link correlation, as well as link quality to model the ETX of each potential sender during the dissemination process, such that strong link correlation neighborhoods in the network are selected for data forwarding. After that, many optimizations on the protocols are done to further reduce the transmission overhead and energy consumption. The above-mentioned protocols rely on the beacon-based link correlation measurement (BCM). The inaccuracy of BCM may balance out the benefits of considering link correlation.