Research On Target Direction Finding Based On Synthetic Wi-Fi Antenna Array

Direction finding with radio is a common way to use in target positioning.To achieve accurate direction finding,complex dedicated hardware based on antenna array is often needed to analyze the characteristic parameters of RF signals,which makes it difficult to be used in ordinary civil applications.In recent years,people can obtain Channel State Information(CSI)of radio from commercial WLAN NICs by modifying the firmware of it,and calculate the target orientation based on CSI.WLAN NIC,however,as a kind of non-special radio frequency measurement hardware,in the process of obtaining CSI will inevitably introduce a variety of hardware measurement error(including the residual carrier frequency offset and sampling frequency offset,packet detection delay and initial phase shift,etc.),and only has limited radio frequency chains,thus brings challenges to the precise direction finding.In this thesis,we will analyze the measurement error sources and correct them accordingly.In addition,the method of how the radio frequency channels of a WLAN NIC can be flexibly extended to a multi-dimensional antenna array will be introduced,so as to achieve more accurate target direction finding.The main contributions of this thesis are as follows:(1)The characteristics of hardware measurement errors in CSI output by WLAN NIC and their influence on the CSI phase are analyzed.In particular,the phase offset between RF chains of the WLAN NIC is measured,and it is found to be statistically stable.According to the measurement results,the initial phase of the array antenna can be compensated.(2)Enlightened by the idea of synthetic antenna array put forward by some researchers,with the help of low-cost RF switching circuit,the uniform nine-antenna circular array and nine-antenna cross array are built on the regular three-chain WLAN NIC,and an effective correction method is presented for the phase difference among the antenna combinations due to RF switchs.(3)It is discussed that the cross shaped measuring array has a high measuring efficiency(the number of measurements required for single target)in the synthetic antenna array measurement mode proposed by this thesis.A switching algorithm based on multi-target direction finding is proposed to solve the coherent time constraint in the synthetic antenna array measurement mode.(4)Hardware experiments show that the measurement accuracy of nine-antenna circular array and nine-antenna cross array based on synthetic Wi-Fi antenna array can reach 4.3° and 5.9° respectively.Simulation results show that the RF switching frequency of nine-antenna cross array is obviously lower than that of nine-antenna circular array.