Investigating the methods of removing interfering signals in the monitoring channel in passive radars based on DVB-T signal

Master's Thesis in Electrical Engineering-Communication System

Abstract

Investigating the methods of removing interference signals in the monitoring channel in passive radars based on DVB-T signal

In this thesis, the performance of adaptive methods In the attenuation of interference signals, including the direct path signal and clutter, and the computational complexity of the methods, the appropriate parameters of each method have been extracted in passive radar based on DVB-T signal. In this thesis, the adaptive filters used to remove interference signals are filters based on LMS, NLMS, VSLMS, VSNLMS, RLS and FT-RLS algorithms. The results show that LMS and NLMS algorithms with proper selection of parameters, although they do not have much computational complexity, but they will not have proper behavior in terms of clutter attenuation and detection. VSLMS and VSNLMS algorithms have better performance than LMS and NLMS algorithms, but the performance loss of these two algorithms is more than RLS and FT-RLS algorithms from the point of view of target detection. RLS and FT-RLS algorithms have more suitable and better behavior than LMS family algorithms in terms of removing interference signals and from the point of view of target detection. Of course, the FT-RLS algorithm also has a performance loss compared to the RLS algorithm, but when the clutter range and the required length of the adaptive filter are large, the implementation of the RLS algorithm imposes a high volume of calculations on the system due to its high computational complexity, and in this case, the use of the FT-RLS algorithm, which has a much smaller calculation volume, seems to be a suitable solution despite the performance loss.

Introductions to signal-based passive radar DVB-T

1- 1- Introduction

Radar is an electromagnetic system that in its simplest form consists of a transmitter and a receiver. Different signaling can be used in the radar transmitter, the signaling used is selected according to the mission and type of radar. In the radar receiver, by receiving return echoes from the targets, their parameters are detected and extracted. Based on the location of the receiver and transmitter, radars are divided into single-base, two-base, or multi-base radars. Since it was not possible to separate the sent and received signals in the early radar systems, the transmitter and receiver were placed in different places. With the advent of circulators and duplexers and the possibility of sending signals in pulse form, two-base radars gave way to single-base radars with less complexity, but in the early 1950s, when the interesting property of returning energy of waves in two-base radars was discovered, these radars regained their former position [1-2]. The probability of radar detection by the enemy detection station has been reduced. One of the results of these efforts is the use of passive radar. Passive radar systems are known as PCL[1] and PBR[2]. Passive radar is actually a type of two-base radar that has more complications than a single-base radar. These radars actually use non-radar transmitters, which are known as magnetic transmitters[3], and this is a great help to stay hidden from enemy eavesdropping receivers. The idea of ??using non-radar transmitters goes back to 1935, when the first experiments in the field of PBR radars were carried out in England, and due to the lack of suitable processors, attempts to implement complex algorithms to improve detection performance in PBR radars failed, and this possibility was postponed to the 1980s, which was accompanied by significant improvements in digital processors and the availability of cheap and high-speed analog-to-digital converters [1-3-4].

The advantages of PBR radars include the following:

Not having a special radar transmitter has turned these systems into cheaper, simpler, smaller and undetectable systems..

Due to multi-bases, these systems have the ability to detect low-altitude targets.

In these radars, due to the use of magnetic transmitters, there is no need to discuss frequency band allocation.

The discussion of ambiguity in range and speed in PBR radars is less prominent than in active single-base radars. [3-5].

Figure 1-1 shows a model of the structure of two bases of passive radars [6]. As shown in the figure, the direct path signal is the signal exchanged between the magnetic transmitter and the two-pole radar receiver, and the target signal is the signal exchanged between the target and the two-pole radar receiver. And mobile signal can play the role of magnetic transmitter for passive radar. The suitability of the waveform for use in passive radar depends on factors such as resolution in the range and Doppler and the level of the side lobes in the ambiguity function [7-8-9]. In this thesis, the aim is to investigate digital terrestrial television signal (DVB-T) [6] as a magnetic transmitter for passive radar and to solve the problem of eliminating interference in these radars and how to detect the target. rtl;"> 

Among the reasons for interest and approach to DVB-T signaling in passive radars, the following can be mentioned [9-10-11]:

TV digital signal transmitters send a signal with good power and high quality at the lowest possible cost from the radar point of view.

This signal has enough bandwidth to provide It has the power of resolution in the appropriate range.

This signal is similar to noise in a way, and this helps the compression capability in the range and the appropriate Doppler estimation.

The statistical properties of the signal are stable over time.

This signaling has less ambiguity in range and Doppler than other modulations.

The transmitter in this signaling has an omnidirectional coverage.

ERP[7] This signaling is in the range of several kilowatts.

High bandwidth and as a result of having a high resolution in the range (High Range Resolution) in this signaling is significant, the resolution in the range for several categories of passive radars based on several types of signals is stated in the table below. As it is known, the range resolution in passive radars based on DVB-T signal is much lower than passive radars based on other signaling [8-12-13]. The appropriate parameters of adaptive algorithms such as LMS, NLMS, VSLMS, VSNLMS, RLS and FT-RLS are used. Besides, the performance of different algorithms for clutter cancellation and target detection are analyzed and compared. Results show that LMS and NLMS do not have good performance neither for clutter cancellation nor for target detection. VSLMS and VSNLMS algorithms have better performances than LMS and NLMS. For both clutter cancellation and target detection. LMS algorithms with variable step-size are not as good as RLS or FT-RLS. Among all mentioned algorithms RLS and FT-RLS have good performance both for clutter cancellation and target detection. Although RLS algorithm has a better performance than FT-RLS, using FT-RLS when the clutter range and the adaptive filter length is high, seems to be better because of the lower order of complexity.