| With rapid development of wireless communication technology, technology regarding to wireless sensor network is more and more widely used in test and measurement activities. For some special measurement environment, the wireless sensor networks shows much more advantages for realize regional signal acquisition and monitoring than using cables. However, the performance of wireless communication system is greatly constrained by characteristic of the wireless channel. In order to realize the physical signals of layout environment, wireless sensor nodes has to be deployed on the earth's surface or on objects with low altitude, signal transmission will be greatly impacted by the outside environment even if not in a complicated terrain. Lots of practical tests show that when the antenna is deployed at a low altitude, it is impossible to predict the connection state or link quality between nodes accurately by using existing transmission model due to uncertainty of surface spread and complexity of the shadow fading distribution.This paper using the theoretical analysis and the method of field measurement to analyze the link loss and coverage scope of the wireless transmission under the circumstances that the wireless sensor nodes are deployed on the earth surface with the outdoor environment is flat and open. In order to acquire transmission losses of the wireless link, the wireless signal strength test system is designed. The system has much more characteristics, such as low cost,small volume,low power consumption,high precision,portability and signal strength real-time display function. The strength value of the receiving signal can be measured at any point through the system. The loss characteristic of wireless wave transmission in the current wireless channel can be acquired from the fitting result based on data processing of multi-point measurements,and the coverage of wireless nodes can be predicted by using the models.A practical test is conducted for near-earth transmission of 2.4GHz wireless signal by adopting different scenarios and different height of antenna. The test and data processing result show great differences existing between the surface wireless channel and traditional micro-cellular wireless channel, however, still can be characterized by the logarithm distance path loss model. Through the comparative analysis method, the loss of wireless link can be described intuitively under different environments and antenna heights. Meanwhile, through the contrast measured data and the model prediction results, the importance and the accurate degree of prediction models were pointed out intuitively. These results will conduce to studying the transmission characteristic of near-earth wireless channel, and will provide basis for design and planning of the surface wireless sensor network. |
