## 1 Introduction  
Recent advances in wireless communication, computer technology and micro-electronics technology have enabled the rapid development of tiny, low-cost, and multi-functional sensor nodes. These sensor nodes can be randomly deployed in a certain area to sense, process and transmit their monitored data to some remote base station. In this way, wireless sensor networks (WSNs) are formed with large number of tiny sensor nodes which usually work in a local and collaboratively way. WSNs have broad application fields such as military surveillance, environment monitoring and forecast, structural monitoring and target tracking, home network and healthcare _etc._[1; 2; 3; 4]. Energy efficiency is a key research concern and challenging issue during the design of routing algorithms for WSNs. The autonomous sensor nodes are usually powered with very limited energy supply. Besides, the number of sensor nodes is usually large, and it is impractical to recharge or replace the battery power of sensor nodes. Moreover, long distance communication between sensor nodes and base station (or sink nodes) may also increase overall energy consumption. A well-known problem is that the sensors close to sink node will bear more traffic burden to forward to the sink nodes and they will deplete their energy much faster than those sensor nodes far away from sink nodes, leading to network disconnection and much shortened network lifetime. Those sensor nodes become bottleneck nodes and they are called hot spots, and this phenomenon is referred to as the energy holes problem. In the meantime, a large quantity of energy may not be used efficiently for the transmission of the rest of the data among many other residual sensors, which is not very desirable. Exploiting sink mobility based technology is gaining more and more popularity in recent years to achieve better energy efficiency and lifetime performance. Many studies have shown that sink mobility technology can significantly balance traffic load and improve network performance [3; 4]. Mobile sink nodes moving at a certain speed throughout a sensing field can collect monitored data from the static sensors in a single-hop or multi-hop transmission manner. In this way, one can effectively reduce energy overhead at sensor nodes near sink nodes, and enable the sensor network to last longer. As the mobile sink nodes change position, their neighbor nodes will also change. Intuitively, the introduction of sink mobility technology into WSNs can effectively alleviate the hot spot problem. Another advantage of utilizing sink mobility technology is that, since the energy consumption will grow exponentially with the transmission range (typically with an exponent range between 2 and 4), energy consumption with mobile sink nodes can be greatly reduced by using a relatively short transmission distance on average, as sink nodes will choose nodes located at a short distance to communicate. The main contribution of this paper lies in the following three aspects: First, we perform a detailed review of the literature about data collection mechanisms with multiple mobile sinks for wireless sensor networks, and contrast the main features of this research work. Second, we propose an energy efficient distance-aware routing algorithm with multiple mobile sinks for WSNs, which consists of route setup phase, steady phase and maintenance phase. Both distance and residual energy factors are considered during the routing phase, and sink nodes will move along the network boundary to collect monitored data. Finally, we perform an extensive simulation and analysis to validate the performance of our proposed algorithm in terms of energy consumption, network lifetime, _etc._  
The rest of the paper is organized as follows: Section 2 presents the literature survey, and Section 3 gives the system model which includes energy and network model. In Section 4, our proposed routing algorithm is explained in detail.