# Multi-Source Cooperative Data Collection with a Mobile Sink for the Wireless Sensor Network
## 1 Introduction  
For traditional wireless sensor networks (WSNs), many static nodes are randomly deployed and the data of active sensor nodes are usually forwarded to the sink node through multiple hops [1]. However, this transmission strategy brings a series of problems. On the one hand, the energy consumption of different sensor nodes is not uniform and more energy may be consumed for the nodes closer to the sink node compared with those located farther away from the sink due to the converge-cast traffic pattern [2; 3; 4; 5]. In this way, the network lifetime may degrade and thus the connectivity or the coverage of wireless sensor networks can not be guaranteed [2]. On the other hand, the data rate of the multi-hop transmission decreases as the number of hops increases and it is also a challenging topic to maximize the throughput for some WSNs [6; 7]. Mobile sink nodes were introduced to overcome some challenges above for wireless sensor networks [4; 5; 6; 7; 8; 9]. In this way, the energy consumption of different sensor nodes is balanced and thus the network lifetime can be prolonged [5; 8; 9]. The throughput can also be maximized by employing mobile sinks in some wireless sensor networks [7]. For mobile sink networks, the energy consumption can be further reduced by designing efficient wireless communication strategies because the data sending and receiving by communication modules may consume the major energy in WSNs [10]. Therefore, the power-efficient data transmission schemes should be investigated to further prolong the lifetime of wireless sensor networks with mobile sinks. For example, the virtual multiple-input multiple-output was employed to gather mobile data in [11]. Cooperative diversity with networkcoding was used to improve the communication reliability in [12]. In this paper, we devise the data collection schemes with low power consumption by using cooperative communication technologies such as multi-source cooperation and coded cooperation for wireless sensor networks with mobile sinks. The schemes are based on the cooperative effort of sensor nodes, which is recognized as an important feature of wireless sensor networks [1]. Cooperative communications can achieve the diversity gain to decrease the impairment of wireless fading channels and improve the power efficiency by sharing antennas of different nodes in wireless networks [13; 14]. In cooperative networks, different relay protocols such as amplify-and-forward (AF), decode-and-forward (DF) and coded cooperation (CC) have been designed for cooperative nodes to forward the received data to the destination. For coded cooperation, it can achieve the diversity gain and the coding gain simultaneously by integrating traditional coding schemes into cooperative communications [15]. Various coded cooperation schemes have been designed based on different coding schemes such as low-density parity-check (LDPC) codes, turbo codes, generalized low-density codes and network codes [16; 17; 18; 19; 20; 21; 22; 23; 24; 25]. For example, turbo codes were employed in wireless relay networks to design distributed coding schemes [16]. Low-density parity-check codes were investigated for relay channels in [17] and generalized low-density codes were used in multi-relay networks [18]. Generalized adaptive network coded cooperation strategy based on LDPC codes was proposed for cooperative networks [21] and network coding was used to implement efficient multi-source cooperation schemes in [22]. For performance evaluation, we have implemented a distributed testbed with software-defined radios to test coded cooperation schemes in a real indoor wireless environment [23]. The investigations above disclose that coded cooperation can achieve significant performance gain for wireless networks. For multi-node coded cooperation, each node usually needs multiple cooperative partners so as to obtain the good performance.