## 2 Related Work  
Many WSN protocols have been developed in recent years to extend the system lifetime. Among of those, we briefly review some relevant designs: chain-based, cluster-based, grid-based, and diffusion-based. In the chain-based protocol, PEGASIS [8] links all sensor nodes with a greedy algorithm to form a chain. Each node receives from one neighbor data fused with its own and then transmits to the other on the chain. Gathered data moves from node to node along the chain and finally a designated node called leader transmits to the BS. To reduce the average energy spent by each node per round, nodes take turns transmitting to the BS. EECB [11] improves PEGASIS by applying a distance threshold to avoid the formation of Long Links (LLs) on the chain to further distribute the energy load evenly among the nodes. Besides, EECB selects the node with minimum cost as leader to finally transmit gathered data to the BS. The cost is a function of the remaining energy of the node over the distance between the node and the BS. ECBSN [15] adopts 2-layer hierarchical chains. In the low layer, ECBSN based on PEGASIS adopts multiple chains instead. Each chain has a leader, the node with the most remaining energy. ECBSN further links the leader of each low layer chain to form a single high layer chain. In the high layer chain, the node with the shortest distance to the BS is selected as the high layer leader, responsible for transmitting the aggregated data finally to the BS. In the data transmission phase, the token passing mechanism of each low layer chain is the same as that of PEGASIS. After receiving the gathered data, the leader of each low layer chain transmits to the high layer leader which eventually transmits to the BS. In the cluster-based protocol, LEACH [16] combines the ideas of cluster-based routing and media access together with application-specific data aggregation for WSNs. In LEACH, distributed clusters are self-organized by the nodes into a local cluster. Each cluster has a head responsible for aggregating data and transmitting it to the remote BS. LEACH incorporates randomized rotation of cluster head position to evenly distribute the energy load among all the nodes so as to prolong the system lifetime. In GROUP [17], one of sinks proactively, dynamically and randomly constructs a cluster grid structure to relay queries and data packets. Only a small part of the sensor nodes will participate in the election of cluster heads. Cluster heads can expediently aggregate data to reduce the volume of data packets. GROUP can also distribute the energy load among the sensor nodes in the network. [PERSON]'s model [4] modifies the LEACH protocol and allows cluster heads to form a multi-hop backbone by incorporating a cooperative MIMO scheme [9] through the selection of cooperative sending and receiving nodes. In the grid-based protocol, TTDD [18] builds the grid on a per-source basis. A source transmits data by building a grid structure. Each grid point has a dissemination node responsible for storing and forwarding data. A sink sends the immediate dissemination node a query which is in turn forwarded towards the source. The query forwarding process lays the information of the path to the sink to enable the requested data from the source to the sink. Unlike TTDD's per source-based grid structure, CODE [19] divides the whole sensor field into grids. Each grid has one coordinator which acts as an intermediate node to cache and relay data. In data announcement and query transfer phases, CODE establishes a data dissemination path first so that the source can send data to the mobile sink along the path. EEDD [10] also divides the whole sensor field into small virtual grids so that each grid has a headto forward data. Each grid is further divided into four sub-grids if an event is unceasing. Working nodes in the sub-grids are scheduled to stay active according to their corresponding sub-grid time slot.