A Near-Field Iterative Adaptive Approach (NF-IAA) was proposed for the joint estimation of Direction Of Arrival (DOA) and range of near-field sources. Firstly, all possible source locations in the neaar field region were represented by dividing two-dimensional grids. Each location was considered to have a potential incident source mapping to an array, indicating the output data model of the array. Then, through the loop iteration, the signal covariance matrix was constructed by using the previous spectral estimation results, and the inverse of the covariance matrix was used as the weighting matrix to estimate the energy of the potential sources corresponding to each location. Finally, the three-dimensional energy spectrum was figured. Since only the energy of real existing source is not 0, the angles and distances corresponding to the peaks are the two-dimensional location parameters of real existing sources. Simulation experimental results show that the DOA resolution probability of the proposed NF-IAA reaches 90% with 10 snapshots, while the DOA resolution probablity of Two-Dimension Multiple Signal Classification (2D-MUSIC) algorithm is only 40%. When the number of snapshots is reduced to 2, 2D-MUSIC algorithm has failed, but NF-IAA can still distinguish 3 incident sources and accurately estimate the two-dimension location parameters. As the number of snapshots and Signal-to-Noise Ratio (SNR) increase, NF-IAA always performs better than 2D-MUSIC. The experimental results show that NF-IAA has the ability to estimate the two-dimensional location parameters of near-field sources with high precision and high resolution when the number of snapshots is low.

Concerning the ranging inaccuracy problem based on radio signal phase information under multi-path environments, a two-step ranging approach based on double tags was proposed. Each target was attached with double tags. Through single frequency subcarrier amplitude modulation, firstly, the wrapped phase information of carrier signal was extracted, the distance between reader and tag within half wavelength of carrier signal was calculated and fine ranging estimation value was achieved. Secondly, the unwrapped phase information of subcarrier signal was extracted, and the integral multiple of half wavelength within the distance of reader and tag was calculated. Thirdly, the average multiple was calculated between double tags, the distance of average multiple of half wavelength was used as coarse ranging value. Finally, the final ranging result was estimated by the sum of the fine ranging value and coarse ranging value. Additionally, single reader and double-tag based geometric localization method was introduced to reduce the cost of hardware facilities. The simulation results show that, under multi-path environments, compared with the directly ranging with subcarrier phase, the average ranging error of double tags based two-step ranging approach is reduced by 35%, and the final average localization error is about 0.43 m, and the maximum error is about 1 m. The proposed approach can effectively improve the accuracy of phase based localization technology and also reduce the hardware cost.

Conventional Direction-Of-Arrival (DOA) estimation approaches suffer from low angular resolution or relying on a large number of snapshots. The sparsity-based SPICE can work with few snapshots and has high resolution and low sidelobe level, but it only applies to narrowband signals. To solve the above problems, a new FrFT-SPICE method was proposed to estimate the DOA of wideband chirp signals with high resolution based on a few snapshots. First, the wideband chirp signal was taken on the Fractional Fourier Transform (FrFT) under a specific order so that the chirp wave in time domain could be converted into sine wave with single frequency in FrFT domain. Then, the steering vector of the received signal was obtained in FrFT domain. Finally, SPICE algorithm was utilized with the obtained steering vector to estimate the DOA of the wideband chirp. In the simulation with the same scanning grid and same snapshots, the DOA resolution level of the proposed FrFT-SPICE method was better than that of the FrFT-MUSIC method which combines MUltiple SIgnal Classification (MUSIC) algorithm and FrFT algorithm; and compared to the SR-IAA which utilizes Spatial Resampling (SR) and IAA (Iterative Adaptive Approach), the proposed method had a better accuracy. The simulation results show that the proposed method can estimate the DOA of wideband chirp signals with high accuracy and resolution based on only a few snapshots.