International Journal of Antennas and Propagation

Volume 2018, Article ID 7306902, 9 pages

https://doi.org/10.1155/2018/7306902

## Fast Estimation Method of Space-Time Two-Dimensional Positioning Parameters Based on Hadamard Product

Correspondence should be addressed to Haiwen Li; moc.361@66whlykcul

Received 18 October 2017; Revised 22 January 2018; Accepted 29 January 2018; Published 26 March 2018

Academic Editor: Luis Landesa

Copyright © 2018 Haiwen Li et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

The estimation speed of positioning parameters determines the effectiveness of the positioning system. The time of arrival (TOA) and direction of arrival (DOA) parameters can be estimated by the space-time two-dimensional multiple signal classification (2D-MUSIC) algorithm for array antenna. However, this algorithm needs much time to complete the two-dimensional pseudo spectral peak search, which makes it difficult to apply in practice. Aiming at solving this problem, a fast estimation method of space-time two-dimensional positioning parameters based on Hadamard product is proposed in orthogonal frequency division multiplexing (OFDM) system, and the Cramer-Rao bound (CRB) is also presented. Firstly, according to the channel frequency domain response vector of each array, the channel frequency domain estimation vector is constructed using the Hadamard product form containing location information. Then, the autocorrelation matrix of the channel response vector for the extended array element in frequency domain and the noise subspace are calculated successively. Finally, by combining the closed-form solution and parameter pairing, the fast joint estimation for time delay and arrival direction is accomplished. The theoretical analysis and simulation results show that the proposed algorithm can significantly reduce the computational complexity and guarantee that the estimation accuracy is not only better than estimating signal parameters via rotational invariance techniques (ESPRIT) algorithm and 2D matrix pencil (MP) algorithm but also close to 2D-MUSIC algorithm. Moreover, the proposed algorithm also has certain adaptability to multipath environment and effectively improves the ability of fast acquisition of location parameters.

#### 1. Introduction

Wireless target localization has found widespread application requirements in military and civilian fields. Parameter estimation is an important prerequisite for positioning algorithm, which determines the overall effectiveness of the positioning system. The location parameters include time of arrival (TOA), direction of arrival (DOA), time difference of arrival (TDOA), and received signal strength. The key technique of location finding based on TOA method is to estimate the propagation delay of the radio signal arriving from the direct line-of-sight (DLOS) propagation path accurately. The existing TOA methods mainly include cross-correlation method [1], multiple signal classification (MUSIC) algorithm [2], delay estimation algorithm based on Markov Monte Carlo [3], propagator algorithm [4], and matrix pencil algorithm [5]. If the array antenna can be used to obtain the signal DOA for joint estimation [6, 7], then it will effectively reduce the number of nodes and system overhead of the localization system. In theory, a reference node can lock the location of the target source.

OFDM technology is a multicarrier modulation and high-speed transmission technology, which has been widely used in underwater acoustic communication system, IEEE 802.11a/g/n/ac [8], OFDM new system radar [9], WiMAX system [10], 3GPP LTE/LTE-Advanced [11], 5G mobile communication systems, and so on. However, the demand for target location information of OFDM technology is also increasing, especially in indoor, underground places, and other environments. Therefore, using OFDM array antenna receiving system to achieve accurate positioning of the target signal has become a hot spot of current research.

Due to the limitation of bandwidth, sampling rate, and the attributes of narrowband signals, OFDM signals cannot achieve joint estimation of TOA and DOA employing the triangle geometry relation as in [12, 13]. Ni et al. [14] introduces a TOA estimation algorithm for OFDM wireless signal, which combines coarse estimation with fine estimation. The algorithm needs to use the technology-guided detection to suppress multipath propagation, but the positioning accuracy is limited. Three methods including peak detection, modified maximum peak leak rate detection, and channel frequency domain response reconstruction are proposed in [15]. In the multipath environment of WINNER A1 LOS channel, the channel frequency response reconstruction method can provide the best performance, but this method is mainly based on the received preamble sequence. In the environment of indoor multipath, the two-dimensional matrix pencil (2D MP) algorithm is presented in [16] to achieve the joint estimation of time delay and arrival angle, which has low computational complexity, but sacrifices the effective array element and bandwidth. Zhou et al. [17] provide a joint estimation algorithm of delay and DOA, which uses the frequency domain data of single antenna for MUSIC time delay estimation. This algorithm uses the array data for MUSIC DOA estimation, but it fails to give a matching method of time delay and arrival angle, and its computational complexity is high. Ba et al. [18] present a joint estimation of delay and angle using 2-MUSIC for two-dimensional searching, which solves the pairing problem, but its computational complexity is very high, which makes it hard to apply in practice.

In this paper, a fast estimation method of space-time two-dimensional positioning parameters based on Hadamard product is presented, which can offer the closed-form solution of parameter estimation directly without pseudo spectral peak search. This method can implement parameter pairing and improves the efficiency significantly. The computational complexity analysis, CRB derivation, and simulation are also presented. The simulation results show that the proposed algorithm has higher precision of parameter estimation close to the 2D-MUSIC algorithm, but the computational complexity is greatly reduced, which effectively improves the ability of fast acquisition of the positioning parameters.

The rest of the paper is organized as follows: Section 2 introduces the data model. Section 3 presents the method of fast estimation and the computational complexity analysis in detail. The CRB for the method is derived in Section 4. Simulation and experimental results are presented in Section 5. Conclusions are drawn in Section 6.

#### 2. Data Model

Considering a node equipped with an equidistant line array composed of antenna elements, the distance between the signal source and the antenna array is far enough, and and represent DOA and propagation delay of the multipath signal, respectively. is the OFDM symbol transmitted with subcarriers. On the account of the wireless multipath propagation, the received signal at the antenna element can be formulated as where is the number of the multipath components and and are the complex attenuation of the path and propagation delay of the path in antenna, respectively. The phase of the complex attenuation is normally assumed random from one snapshot to another with a uniform probability density function [19]. is additive Gauss white noise. The time delay in each antenna element depends not only on the propagation delay but also on the direction of arrival. In particular, for a uniform linear array, the propagation delay associated to be the arriving path in antenna is given by , with being the distance between adjacent array elements in the array and the speed of light. After the Fourier transform of the received signal , the sampled discrete frequency domain channel response of the subcarrier at the antenna element is given by where . In (2), it is clear that the effect of time delay coming

from the antenna array is negligible compared with , so this formula can be further simplified as where denotes additive Gauss white noise with mean zero and variance at the subcarrier and the antenna element, , is the carrier frequency of OFDM signal, is the net data length of OFDM symbol, and is the sampling interval. We can then write this formula (3) in a vector form. where

Moreover, , set , and denotes Hadamard product. Then,

So, the frequency domain response vector of extended array elements can be expressed as where

By eigenvalue decomposition of , we can obtain mutually orthogonal subspace and noise subspace based on Hadamard product-extended signal. Using space-time pseudo spectral search can estimate and , but this method is very time-consuming, which costs large amount of calculation, and is hard to practical application. Here, the fast estimation method of two-dimensional positioning parameters based on Hadamard product is discussed.

#### 3. Fast Estimation Method of Two-Dimensional Parameters

##### 3.1. Algorithm Description

According to (7), the frequency domain estimation vector of the reference antenna element is obtained as where and are the estimation vectors and response vectors of the channel in frequency domain, respectively, is Vandermonde matrix of the frequency points at the reference antenna element, and , , , and are the additive Gauss white noise vector.

For the reference antenna element, the eigendecomposition of the covariance matrix with *P* snapshot measurement data can be expressed as
where and are, respectively, the signal subspace and noise subspace and and are the signal eigenvalue diagonal matrix and noise eigenvalue diagonal matrix. By using the noise eigenvector based on Hadamard product to extract space parameter information, MUSIC-type algorithm function can be constructed as

The zero value of the function gives the estimation of the time delay, which is equivalent to the value obtained by pseudo spectral peak search. Let and be the power term of , and (11) is not the polynomials of . Then, , the polynomials of Root-MUSIC for TOA, can be expressed as

Then, by utilizing the sampled frequency points at the reference element, the corresponding delay estimation can be obtained as where are roots which are the closest to the unit circle in (12).

Similarly, the DOA estimation of the frequency response of the channel at the first frequency point of the *M* antenna elements can be obtained by the above algorithm.

Since the delay and DOA are estimated separately, the corresponding parameter pairing is required.

##### 3.2. Parameter Pairing

By using the frequency domain channel responses at the reference array element and the next element, the matrix is constructed as where

The covariance matrix of can be an eigenvalue decomposed as , where and are, respectively, the signal subspace and the noise subspace; obviously, the column vector and noise subspace are orthogonal, which make . In order to achieve the pairing of delay and arrival angle, the cost function can be obtained by

So the pairing of the estimated parameters is to minimize the cost function

Firstly, select a fixed from , and let traverse , then the corresponding to the minimum value of the cost function is the correct pairing value.

##### 3.3. Computational Complexity Analysis

The computational complexity of the algorithm in this paper consists of the following four aspects: the estimation of the covariance matrix and , the computational complexity is and , respectively; the eigenvalue decomposition of the covariance matrix and , the computational complexity is and ; the complexity of constructed polynomial is and ; the computational complexity of parameter pairing is . So the total complexity of the algorithm is .

The total complexity of using 2D-MUSIC algorithm is , and and are, respectively, the number of grid of delay and angle of arrival search, that is, the range of measurement search is divided by the search step. The computational complexity of ESPRIT algorithm is . Table 1 compares the computational complexity of the three algorithms.