Research On Distributed Tansmit Beamforming In Wireless Sensor Networks

Wireless sensor networks(WSNs) are widely used in modern industrial applications. However, organized in structure of Sensor-Sink-Base, WSNs would not be adopted in some special applications, since sinks can’t be placed. Our purpose is to extend WSNs’ communication ranges and build the direct links between WSNs and base stations, based on energy-limited sensors. It’s a great way to enhance WSNs commonly applicable.Antenna arrays obtain power gains by antennas’ collaborative beamforming, which leads to better communication performance. Generally, beamforming with N power-fixed antennas form 2N times power gains, 2N times better SNRs and N times wider communication ranges.The topic in this essay is to adopt beamforming to WSNs, with sensors as the elements of antenna arrays. Then the WSNs perform as a large-scale “virtual antenna array”, which will strengthen the signal power in the targeted direction and extend communication range significantly. The author first analyzes the spatial pattern characteristics of the distributed beam with mathematical methods. The basic conditions how distributed transmit beamforming to make sense is derivated. The results of average beam pattern and power gains with different levels of phase error are provided. The conclusions are the basic theoretical reference to design the communication links between WSNs and the targeted base station. Distributed transmit beamforming requires the carrier signals from sensors reach the targeted base station with the same phase, so the signals coherently combine and form certain power gains. The key issue is how to conduct the carrier synchronization. The author investigates the typical carrier synchronization protocols and desgns the scalable synchronization scheme which is practical to distributed beamforming in large-scale WSNs, with a distant base station as the targeted receiver. All the numerical results demonstrate that the scheme proposed in this e ssay is of great scalability and anti-noise properties. The communication link between the WSNs on the planet and the airship floating in the stratosphere is desgned based on distributed transmit beamforming. Without any modification on the sensors working in BPSK type of Zig Bee protocol, with several sensors adopted to distributed beamforming, the performance of the link between WSNs and an airship is of the same performance with that between any two sensors in the WSN. Furthermore, numerical results show that the proposed scalable carrier synchronization is applied in the communication link between the WSNs and the airship.Distributed transmit beamforming is the only way to build the directed communication links between WSNs and distant base stations. WSNs pose much further communication ranges by distributed transmit beamforming, which will improve the energy efficiency and extend WSNs’ available life periods. On the other hand, distributed transmit beamforming makes the structure of WSNs much simpler than that of conventional WSNs, which will enhance the robustness and the survivability of WSNs in poor conditions, and improve the applicability in the environments without any infrastructure maintained.