Research On Cross-layer Design Of Cooperative Schemes In Wireless Sensor Networks

The wireless sensor network (WSN) is usually composed of a large number of low-cost, low-power, and energy-constrained sensors which are responsible for monitoring a certain environment and reporting to the access point. However, due to the limitation of application environment or node size and cost, it is usually difficult or infeasible to replace or recharge the small batteries embedded in the tiny sensor nodes. Hence, maximizing network lifetime becomes the primary objective in the WSN design, leaving other performance metrics as secondary objectives.Multiple-Input Multiple-Output (MIMO) technology along with some channel coding techniques such as space time coding has been studied intensively to mitigate multi-path fading effect and thus achieve space diversity in wireless systems. Compared with single-input single-output (SISO) system, MIMO system can save much more energy under the same performance constraint. However, the implementation of multiple antennas on small sensor nodes is quite difficult due to device-size and cost constraints. Recently, cooperative communication has gained much interest as it can achieve space diversity through nodes'cooperation. Due to the broadcast nature of wireless channels, packets transmitted by a source node can be overheard and retransmitted by relay nodes to facilitate better reception at the destination receiver. Through cooperation, a virtual MIMO antenna array can be formed and therefore, cooperative diversity can be achieved to significantly improve the system robustness. Different cooperative schemes have been proposed on the physical layer. Performances such as bit-error-rate (BER), symbol-error-rate (SER), packet-error-rate (PER), outage probability, throughput and channel capacity etc. have been widely studied for both regenerative and non-regenerative cooperative systems over different fading channels in various communication environments. Based on the constraints of QoS (Quality of Service) parameters, various relay selection techniques have been proposed, and transmission power is optimally allocated for a given power budget.Cooperative communication can also be applied to the data link layer, indeed, some research efforts have been made to design cooperative MAC protocols, which should satisfy the following attributes:fairness and efficiency in the sharing of communication resources between sensor nodes; scalability and adaptability to topology changes. Cooperative MAC protocols are usually designed with objectives of improving system throughput, increasing energy efficiency, or maximizing network lifetime, etc., while the last two items are primary requirements in the distributed cooperative MAC protocol design for WSN. Another important issue on the data link layer is the cooperative truncated ARQ scheme. As we all know, ARQ scheme exploiting temporal diversity is an effective way to combat random packet errors in wire-line networks. However, it can't be effectively used in wireless networks, as packet errors appear in burst rather than randomly. When the link between two nodes is experiencing packet errors, there is a high probability that the bad channel condition will continue for a considerably long period. This drawback can be overcome by jointly combining cooperative diversity at the physical layer and truncated ARQ scheme at the data link layer. Space diversity can be achieved by allocating the retransmission task to the relay nodes, as the relay-destination and source-destination channels are independent and the probability that all of them suffer from deep fading simultaneously is very small.Energy efficient protocol can be approached from different communication layers, much work has been done on the lower two layers, however, there has not been sufficient work done corresponding to the network layer, and in fact, it is still an open problem to design distributed energy efficient cooperative routing schemes. With the objective of minimizing average hops, minimizing total energy consumption or maximizing network lifetime, different incomplete cooperative routing algorithms have been studied. However, most of the existing routing algorithms are implemented by finding a shortest path route first and then improving the route using cooperative communication. As such, these routing algorithms do not fully exploit the merits of cooperative diversity, since the optimal cooperative route might not be similar to the shortest path route. In order to thoroughly take advantage of cooperative communication, and efficiently prolong lifetime of WSN, a completely distributed energy efficient cooperative routing scheme should be proposed from the cross-layer perspective.In order to increase energy efficiency and prolong network lifetime, the application of cooperative communication in wireless sensor network is studied from the three lower layers of the network protocol stack. The main contribution and innovation is:(1) Average SNR, SER performance of AF cooperative communication system over Nakagami-m fading channels with M-PSK modulation is analyzed and new schemes of optimal power allocation and best relay selection are proposed.(2) Jointly considering the channel state information and residual energy information, a new cooperative MAC protocol is proposed to maximize the lifetime of a special wireless sensor network.(3) Through jointly combining cooperative communication technique at the physical layer, truncated ARQ scheme at the data link layer, and distributed routing strategy at the network layer using cross-layer method, a novel cooperative routing algorithm is proposed to maximize the network lifetime. This algorithm considers both the channel state information and residual energy information to avoid the overuse of certain nodes. Besides, with the QoS constraint, optimal power is allocated over every link.Finally, problems to be solved in this field and future research topics are summarized.