Removal of interference in passive radar reference channel based on digital TV broadcasting signal by terrestrial transmitters with reproduction approach

Master thesis in the field of electrical engineering-communication system

Removing interference in passive radar reference channel based on digital TV broadcast signal by terrestrial transmitters with a reproduction approach

In this thesis, a digital receiver for signal processing in a passive radar reference receiver based on orthogonal frequency division modulation (OFDM) of digital terrestrial TV broadcast (DVB-T) is presented. This receiver includes synchronization blocks, frequency offset estimation and channel estimator. After synchronization, we estimate and compensate the frequency offset, both of which are done by detecting the position of the pilot subsymbols. Then using two methods of linear interpolation and least square error (LS) channel estimation is done. After the channel is estimated, we will compare the channel and finally a reproduction version of the sent signal will be made. In order to check the efficiency of the intended receiver, we have drawn the curve of probability of symbol detection error according to the ratio of signal power to noise power for different methods of channel estimation. Also, to check the effectiveness of the proposed algorithms, we have drawn the curve of the attenuation loss of clutter in a passive radar surveillance receiver based on DVB-T signal to determine that if we weaken the clutter in a surveillance receiver by using the signal reproduced in the proposed receiver, this amount of attenuation will be lost compared to the situation when we try to remove the clutter in a surveillance receiver using an ideal version of the transmitted signal.

Key words: simultaneous Creation, synchronization, reproduction

Introductions to passive radar

The space around us is full of radio waves that are spreading in all directions. Radio waves are magnetic waves that are usually emitted by antennas. The word radar (Radar) [1] is made from the first letters of several English words meaning detection and ranging using radio waves. This word, which is used all over the world today, has become an international term like radio and television. With radar, you can see inside the environment that is impenetrable to the eye, such as darkness, rain, fog, snow, dust, etc. . Radio waves have a long range, they cannot be sensed by humans and it is easy to detect and receive them even when they are weakened. Therefore, radar is a device that can detect the existence of an object and determine its distance by means of radio waves. Conventional radar systems consist of a transmitter and a receiver, which often use an antenna for sending and receiving. The first experiment on the reflection of radio waves was obtained by the German Hertz in 1886. In the years 1920 to 1930, progress was made in the direction of building radar with the capabilities of determining the distance of targets. In 1960, the use of air and space radars was developed and, in addition to military use, for geographic mapping and scientific discoveries. were used. Based on the location of the transmitter and receiver, radars are divided into single-base[2], double-base[3] or multi-base radars. The early radars were all bipods. With the advancement of technology, antennas were made, which were able to switch from transmitting to receiving. In 1936, bipod radars were replaced by monopod radars. The constituent parts of the radar system are transmitter, receiver, antenna and electrical systems for recording and processing information.

One of the types of radars are passive radars. Passive radar is known as PCL [4] and PBR [5] [1]. Passive radar is a two-stage radar that can detect targets without being detected by using a variety of magnetic transmitters and measures the time difference between the signal received directly from the transmitter and the signal received as a result of radiation. This allows to determine the target's status and mobility. There are many analog and digital VHF radio and UHF TV transmitters available that passive radar can use as magnetic transmitters.

The advantages of PBR radars include the following:

Low maintenance cost due to the lack of a transmitter, low manufacturing cost, radar stealth due to the lack of transmitted waves, smaller size than active radars, the possibility of tracking and dealing with stealthy fighters.

Passive radars that use magnetic transmitters have a two-stage structure according to Figure 1-1. In this case, the signal that is exchanged between the magnetic transmitter and the bi-base radar receiver is called the direct path signal, and the signal that is exchanged between the target and the bi-base radar receiver is called the target signal [2-3]. 

Figure 1-1: Geometry of passive radar

The basic idea of ??passive radar is that multipath signals including reference signal, clutter signals and targets in the surveillance channel are taken and separated. For proper separation of these signals, we need to have a pure version of the reference channel signal, usually this pure version is not available and by performing pre-processing on the received signal, this pure signal is obtained. One of the methods of obtaining the original version of the reference channel signal is reproduction[6].

In normal radars, the time of sending and receiving the pulse is completely known, and it allows the radar to easily calculate the distance to the target and determine the signal-to-noise ratio by a matching filter. A passive radar does not receive any information directly, so it must use a dedicated channel (called the source channel).

A passive radar uses the following steps to detect targets:

Searching the covered area to receive waves by digital receivers without noise

Generating digital waves to detect the direction of receiving waves and the distance sent and the strength of the transmitter source

Filtering Adaptation to isolate any unwanted direct signal in the surveillance range

Prepare the specified signal for the sender

Cross-correlation for the source channel with the surveillance channels to determine the bistatic and Doppler range of the target

Detection using the false alarm rate[7] (CFAR

Communication and target tracking in the covered Doppler space known as linear tracking[8]

Communication and combination of linear tracking from each sender is displayed in the form of a final assessment of the position, direction and speed of a target [3]. As we enter the 21st century, digital television is an integral part of the information highway of the new millennium. The reason for this is that digital TV can deliver large amounts of information to the largest number of viewers at the lowest cost. Digital TV can now be completely placed next to other digital transmission networks or integrated, and it can even be made into an information package, as it was not possible before.

Compared to traditional analog TV, digital TV can send more programs on any transmission medium because digital information can be processed and manipulated in ways that were never possible in analog TV. Storing digital images on computers and disks and continuously broadcasting them on digital networks without any signal loss is easily possible. Images can be edited and improved data quality, compressed and stored as well as sent and printed. By displaying images as binary numbers (0 and 1), digital TV has a very high flexibility in how to use information. Sending television signals in analog form requires dedicated circuits that can be mixed with telephone conversations and computer data in digital format and then sent over telecommunication networks for broadcasting to remote stations. Also, programs can be stored on computer hard disks and retrieved at the same time as requested by a viewer. Sending multimedia elements (audio, image and data) in digital format allows the customer to store their content using cheap and accessible technology of personal computers. A computer hard drive can store a movie and retrieve and process it in new ways. It is clear that these innovations represent a revolutionary change in traditional analog broadcasting.