## 1 Introduction  
Rapid advances in sensor technology, wireless communications, and digital electronics have made it possible to develop low-cost, low-power and multi-functional small sensor nodes, which are capable of not only gathering and processing but of transmitting data as well [1; 2; 3; 4]. A wireless sensor network (WSN) comprises a great number of small sensor nodes generally deployed over a large harsh field. We further web them together via wireless communication to gather data. WSNs can be employed in many applications such as environmental and habitat monitoring, target tracking, object detection, and intelligent transport monitoring [5; 6; 7].  
This paper mainly addresses the problem of data transmission from all sensor nodes to a remote base station (BS), where the end-user can access the data. In a WSN, sensor nodes are generally deployed over a severe environment. In addition, these sensor nodes are typically powered by batteries so their energy is restricted. It is usually inconvenient or impossible to recharge or replace the batteries of the senor nodes in such a severe environment. Thus the network should be designed to endure. The energy consumption of sensor nodes is chiefly used in data transmission, especially over long distances. Since the location of the BS is remote, the energy consumed by each node to directly transmit its data to the BS is considerable and the node will die very soon. Therefore, it is important to use as few transmissions as possible to the BS and reduce the amount of data that needs to be transmitted to the BS [8].  
Lots of improved approaches have been proposed. Most of these efforts focus on energy efficiency [8; 9; 10; 11; 12; 13]. In order to diminish the data transmissions, only a few nodes are responsible for disseminating the data to the BS instead. On the other hand, the dissemination nodes aggregate their own data with the data sensed by others and then transmit it out to reduce the amount of data transmitted to the BS. Aggregated data moves from node to node, and finally a designated node transmits to the BS. It is our design goal to operate sensor nodes in an energy efficient manner to maximize the nodes' lifetimes, so as to further extend the lifetime of the whole network.  
In this paper, we present a Cycle-Based Data Aggregation Scheme (CBDAS) for grid-based WSNs. In order to extend the lifetime of a WSN, we initially construct a grid-based infrastructure by partitioning the entire sensor field into a 2-D grid of cells. Each cell has a head (henceforth called a cell head) which is the node with the most residual energy, and the ordinary nodes which are the other nodes of the cell. In this paper, the cell head is equivalent to a cluster head, as in some cluster-based approaches. Each cell head is further linked together to form a cyclic chain. In each round, one cell head with the most residual energy on the chain takes a turn as cycle leader, responsible for directly transmitting data to the BS. In CBDAS, all ordinary nodes periodically transmit the sensed data to its cell head. After receiving the data from the ordinary nodes of the cell, the cell head aggregates it with its own data and then sends it out. Only cell heads need to disseminate data to the cycle leader. The ordinary sensor nodes just go into sleep mode based on the GAF protocol [14].  
The rest of this paper is organized as follows: Section 2 briefly reviews some related works. In Section 3, our proposed scheme is described. The simulation results are discussed in Section 4. Finally, Section 5 draws the conclusions.