# Compact Bistatic Iterative Passive Radar Based on Terrestrial Digital Video Broadcasting Signals
## 1 Introduction
The ambiguity function for radar has been analyzed in numerous references in the literature, and there are recent proposals to enhance it, especially in spaceborne synthetic aperture radars [19] using multiple antennas to leverage the diversity in the echoes. Furthermore, the ambiguity function of DVB-T signals was evaluated in [15; 20] in which some methods of reducing the peaks in this function were introduced, focusing in particular on the 8k-mode of operation. In order to reduce ambiguity, three main alternatives have been proposed: the use of several transmitters or receivers [21]; the use of several antennas [22]; or estimation of/compensation for the different effects, i.e., Doppler or interference with the received signals [23]. Recently, machine learning approaches have been used to tackle this problem [24]. The use of multiple-input-multiple-output (MIMO) at the receiver allows the estimation of the angle of arrival (AoA) [25; 26]. Although these techniques improve accuracy and solve the problem of having several transmitters and/or receivers distributed within the space, their accuracy is flawed when there is not a line of sight (LoS) between the receiver and the target. In these cases, such schemes will probably place the target at the position of the main reflection instead of its real position because most of the energy will come from this direction. Moreover, a large number of antennas are often required for good performance. Much of the aforementioned proposals and approaches demonstrate good performance when antennas are a long distance apart (distributed), but they fail when this distance is small. This paper proposes and analyzes a novel approach to a compact single receiver using a small number of close and co-located antennas (only four are needed) based on channel estimates obtained from pilot signals of the DVB-T/2 standard [27; 28]. Then, an iterative algorithm is employed to refine the estimation of position. On this basis, our compact yet portable passive radar demonstrates good performance. Thus, our proposal can be used to improve other techniques and extend other systems' coverage or surveillance capabilities at a low cost. We evaluated our technique and algorithm through simulations. Although they proposed a different technique, the authors of [29] presented some intriguing ideas, wherein a trilateration-based proposal was used to estimate the position of a DVB-T transmitter with several synchronous receivers. Channel estimates in orthogonal frequency division multiplexing (OFDM) for passive radar were also recently analyzed in [30]. The main advantages and contribution of the paper are the following:  
* A compact and accurate passive radar based on DVB-T signals is designed, allowing implementation in a reduced space. Moreover, its portability expands its range of applications. * The design of an iterative algorithm with reduced complexity that improves accuracy and that can be applied to enhance other implementations. * The use of a novel function for positioning, the channel estimates, that guarantees more robustness against interference and non-line-of-sight scenarios. This paper is organized as follows. After this introduction, in Section 2, the system and its numerical derivation are described. Then, in Section 3, the proposed iterative algorithm and the whole scheme are presented. Some simulations and the main results are depicted and discussed in Section 4, and then our conclusions are drawn. In this paper, the following notation is used. Lowercase bold text is used to denote vectors, while uppercase bold text indicates matrices. The symbol (*) denotes complex convolution, and (T) and (*) superscripts denote transposition and Hermitian, respectively.