A Cooperative Transmission Method Based On Beamforming For Wireless Sensor Networks

Wireless sensor network has diverse application and broad prospect. However, there are some limits due to its peculiarity, especially when it comes to longdistance communication. Applying sink node and transmitting through multiple-hop are ways of solving the problem, but it will bring inconvenience or lower communication performance.In this thesis, traditional theory of beam forming is introduced to WSN. Several sensor nodes cooperate to transmit same information to the distant destination. By carrier synchronization, the signals can be aligned and superimposed at the destination. In this way, receiving gain can be got. Traditional beamforming is usually used in antenna array. Nodes of WSN is independent with each other and there is no uniform time reference or accurate location information. Under the situation of limited known information, the focus of the study is how to realize carrier synchronization of both frequency and phase of multi-node signal at the distant destination.A cooperative transmission method based on beamforming is designed for WSN in this thesis. Range of sensor nodes is delimited according to RSSI ranging method. Carrier of receiver is designed to be intermittent to extent carrier cycle to avoid phase ambiguity problem. Under the limit above, the carrier synchronization at the destination is designed. Sensor nodes work in master-slave mode, nodes follow carrier of the destination and adjust their own vibration and send their own carrier to master node. The master node reverses phase of slave nodes’ signal benchmarked against its own vibration and delay each signal from slave nodes according to its arrival time, then send the signal back. In this way path accumulation phase can be offset so the signals of different slave nodes can be of the same frequency and phase when arrive at the destination. At the same time, mathematical derivation and proof of the scheme is given. Important parameters of the scheme are calculated. Performance of cooperative transmission scheme is deduced and calculated.Finally, typical application scenario of cooperative transmission is given reference to 802.15.4 protocol, under which the proposed cooperative transmission scheme is simulated, verified its principle and analyzed its performance on cooperative transmission beam and receiving gain. Also several other classical cooperative transmission schemes are simulated and compared with the proposed method.The study shows that the application of beam forming in WSN is feasible. On the basis of reasonable design, by using the cooperative transmission scheme, communication range of WSN can be extended to establish direct communication link with the high-altitude platform.