It is well known that multipath effect remains a dominant error source that affects the positioning accuracy of Global Navigation Satellite System (GNSS) receivers. Significant efforts have been made by researchers and receiver manufacturers to mitigate multipath error in the past decades. Recently, a multipath mitigation technique using dual-polarization antennas has become a research hotspot for it provides another degree of freedom to distinguish the line-of-sight (LOS) signal from the LOS and multipath composite signal without extensively increasing the complexity of the receiver.
Numbers of multipath mitigation techniques using dual-polarization antennas have been proposed and all of them report performance improvement over the single-polarization methods. However, due to the unpredictability of multipath, multipath mitigation techniques based on dual-polarization are not always effective while few studies discuss the condition under which the multipath mitigation using a dual-polarization antenna can outperform that using a single-polarization antenna, which is a fundamental question for dual-polarization multipath mitigation (DPMM) and the design of multipath mitigation algorithms. In this paper we analyze the characteristics of the signal received by a dual-polarization antenna and use the maximum likelihood estimation (MLE) to assess the theoretical performance of DPMM in different received signal cases.
Based on the assessment we answer this fundamental question and find the dual-polarization antenna’s capability in mitigating short delay multipath—the most challenging one among all types of multipath for the majority of the multipath mitigation techniques. Considering these effective conditions, we propose a dual-polarization sequential iterative maximum likelihood estimation (DP-SIMLE) algorithm for DPMM. The simulation results verify our theory and show superior performance of the proposed DP-SIMLE algorithm over the traditional one using only an RHCP antenna.
RECEIVED SIGNAL MODEL OF A DUAL-POLARIZATION ANTENNA IN THE ONE LOS AND ONE REFLECTED PATH ENVIRONMENT
A dual-polarization antenna is a single antenna whose internal elements are combined in two different ways to produce RHCP-sensitive and LHCP-sensitive outputs. As Figure 1 shows, when the NLOS signal is reflected, it turns to an elliptical polarized (EP) wave; the dual-polarization antenna receives the RHCP LOS signal and both the RHCP and LHCP components of the reflected EP signal.
THEORETICAL PERFORMANCE OF DPMM USING MLE UNDER ONE LOS AND ONE REFLECTED PATH ENVIRONMENT
Figure 3 shows the root mean square error (RMSE) of the TOA of the LOS signal t0 of each ML estimator as a function of the relative delay of the MP signal to the LOS signal in the two-path environments defined by Weak LHCP Signal Cases 1 and 2 in Table 2. The performance of DP-R2L2-MLE (in magenta diamonds lines) under these environments is almost the same as that of RHCP-R2-MLE (in blue circles line), which is based on a single-polarization antenna.
Figure 6 is the performance of the iiDP-R2L1-MLE in different received signal cases defined in Table 2. In Weak LHCP Signal Cases 1 and 2 (DPG = 1.25) when there is not enough power in the LHCP channel, the performance of DP-R2L1-MLE (in green diamonds and green circles) degrades to that of RHCP-R2-MLE (in blue circles). In Strong LHCP Signal Case 2 (DPG = 2), it performs better than DP-R2L2-MLE and RHCP-R2-MLE within the short delay range from 6 m to 20 m.
DUAL-POLARIZATION MULTIPATH MITIGATION ALGORITHM
The implementation of DP-SIMLE algorithm requires the calculation of path parameters in LHCP channel which results in double the computational load of that of the iterative MEDLL algorithm. The RMSEs of t0 estimated by the DP-SIMLE algorithm in different received signal cases defined by Table 2 are depicted in Figure 7 for theoretical performance evaluation when one constructive (lines above zero) or destructive (lines below zero) multipath is present.
The platform consists of three parts, as Figures 8 and 9 illustrate: an intermediate-frequency (IF) signal generator, a software receiver, and a post-processing unit for multipath mitigation. The generator in Figure 8 generates two IF signals to simulate the received signals in RHCP and LHCP channels, respectively. At most four paths can be simulated, with one LOS signal, two static multipath signals and one programmable dynamic multipath signal.
In this paper we demonstrate the capability of a dual-polarization antenna in multipath mitigation for GNSS receivers. In order to find the answers to the fundamental question about the effective conditions under which the DPMM can outperform the single-polarization one, we firstly model the received signal from a dual-polarization antenna concerning the polarization states of both the antennas and the reflected signals. Based on the typical parameters of the GNSS dual-polarization antenna and the electrical properties of the reflective materials, we analyze the characteristics of the received signals and classify them into different received signal cases.
After that we evaluate the theoretical performance of DPMM within the scope of MLE and compare its performance with that of other ML estimators designed for the RHCP antenna in different received signal cases to find that: (1) provided sufficient received power of MP signals for the LHCP antenna (higher than−12 dB relative to the LOS signal in the RHCP channel), the dual-polarization antenna can outperform the single-polarization one in mitigating short delay multipath whose relative delay to the LOS signal is less than 25 m.
(2) the greater the power of the MP signal received by the LHCP antenna, the better the performance of DPMM; and (3) when (1) is satisfied, the iiDP-R2L1-MLE algorithm based on the R2L1 model that ignores the LOS signal in LHCP antenna is more appropriate for mitigating the short delay multipath than the DP-R2L2-MLE algorithm based on the R2L2 model. Inspired by the effective conditions and the R2L1 model, we propose the DP-SIMLE algorithm, which takes advantages of the dual-polarization antenna to mitigate short delay multipath. The simulations of the DP-SIMLE algorithm not only verify our theory but also show its superior performance in mitigating short delay multipath over the conventional RHCP-MEDLL algorithm using a RHCP antenna.
Source: Tsinghua University
Authors: Lin Xie | Xiaowei Cui | Sihao Zhao | Mingquan Lu