Positioning technology of UWB-based wireless sensor networks

In many wireless sensor network (WSN) applications, monitoring without node location information is often meaningless. When an event is detected, an important issue is the location of the event, such as forest fire monitoring, natural gas pipeline leakage monitoring, etc. When these events occur, the first thing to know is the geographic location information. In addition to reporting the location of the event, positioning information can also be used for target tracking, target trajectory prediction, routing assistance, and network topology management. Therefore, the node positioning problem has become a primary problem to be solved in wireless sensor networks.

1 Problem Description

Ultra Wide Band (UWB) communication technology is a wireless communication technology that uses nanosecond impulse pulses to transmit data at high speed over a short distance. This communication technology has the characteristics of good concealment, strong penetration, high positioning accuracy and low power consumption. It plays a very important role in the ranging and positioning applications of wireless sensor networks, and has been applied to the actual material supply tracking and positioning.

UWB signals have a very wide bandwidth, and applying them to TOA positioning methods helps achieve higher ranging accuracy. However, in actual application environments, multipath interference often occurs, and it is difficult to accurately detect UWB direct signals. Therefore, this paper proposes a method to obtain the arrival time of the direct signal by weighting the first arrival signal time and the strongest signal time through fuzzy logic technology, making it possible to apply UWB in wireless sensor network node positioning.

2 TOA distance estimation method

For a single-path additive white noise (AwGN) channel condition, the root mean square error of the distance estimation of TOA ranging can be used:

Where S(f) is the Fourier transform of the transmitted signal. Obviously, the estimated root mean square error is related to the RSNR and effective bandwidth of the signal. The larger the RSNR and effective bandwidth, the smaller the estimated root mean square error. Since the UWB signal bandwidth is very wide, the UWB wireless signal can achieve relatively accurate positioning by using time-based technology.

In general, the TOA positioning method measures the distance between the communication transceiver nodes by detecting the arrival time of the direct path in the received signal. Therefore, it is crucial to accurately estimate the arrival time of the direct path signal. The node positioning method discussed in this article is aimed at a typical wireless sensor network. Generally speaking, the arrival time of the direct signal is determined by detecting whether the amplitude of the received signal is the largest, but this method is difficult to achieve high measurement accuracy under multipath conditions. The UWB received signal in a typical multipath environment is shown in Figure 1. The direct signal (direct path) is not the first arrival signal (first path, related to the threshold) or the strongest amplitude signal (strongest path). Therefore, in this case, the arrival time of the first arrival signal or the strongest amplitude signal cannot accurately estimate the distance between the transmitting node and the receiving node. The maximum likelihood estimation method can be used to detect the arrival time of the direct path signal to calculate the distance between the sensor nodes, but it is easy to cause distortion of the signal waveform in a complex multipath environment, so it is difficult to implement. In view of the fact that UWB direct signals are difficult to detect accurately, this paper proposes to obtain the arrival time of the direct signal by weighting the first arrival signal time and the strongest signal time, and the weighting coefficient is obtained by fuzzy logic technology.

3 Weight selection based on fuzzy logic

Assume that the UWB signal is sent at time T0, the arrival times of the first arrival signal and the signal with the strongest amplitude received at the receiving node are Tf and Ts respectively, and the arrival time of the direct signal is calculated by the following formula:

Where c=3×108m/s is the propagation speed of radio signals in free space.

Here, fuzzy logic technology is used to select the weight a. Let the amplitudes of the first arrival signal and the signal with the strongest amplitude be Ef and Es respectively, and define E=|Ef|／|Es|, Tr=(Tf- T0)／(Ts-T0). Er and Tr are the inputs of the fuzzy logic function, and a is the output. Er, Tr and a define three values: low, medium and high respectively. The rules for selecting the value of a are listed in Table 1.

4 Simulation Analysis

The ranging accuracy of the UWB receiving signal method in the measured multipath environment is verified, and the node is located by combining the node positioning technology. The membership functions of Er, Tr and a are shown in Figure 2.

Take the time of transmitting signal T0=0, and record the time of first arrival signal Tf, the time of arrival of the signal with the strongest amplitude Ts, the amplitude of first arrival signal Ef and the amplitude of the signal with the strongest amplitude Es at the receiving end. Calculate the input values ​​Er and Ts of fuzzy logic respectively, and get a according to the membership function. Substitute Tf, Ts and a into formula (3) to get the arrival time T of direct signal, and then calculate the distance between two points by combining formula (4).

Assuming there are three reference nodes (0, 0), (10, 0), and (10, 10), the actual distances from the blind node to the reference nodes are 6.20, 2.88, and 9.46, respectively. The positions of the nodes can be obtained using the trilateral measurement method, as shown in Table 2.

Here, the node positioning error is defined as the Euclidean distance between the true position of the node and the estimated position. As can be seen from the table, in node positioning, the application of UWB-based ranging technology can greatly improve the node positioning accuracy.