Passive Radar using COFDM (DAB or DVB-T) Broadcasters as Opportunistic Illuminators

Abstract

This chapter is not dedicated to improve DVB-T (Digital Video Broadcasters-Terrestrial) reception in critical broadcasting conditions. Our purpose is to explain and illustrate the potential benefits related to the COFDM (Coded Orthogonal Frequency Division Multiplex) waveform for passive radar application. As we’ll describe, most of the benefits related to COFDM modulation (with guard interval) for communication purpose, could be derived as advantages for passive radar application. The radar situation considered is the following: the receiver is a fixed terrestrial one using COFDM civilian transmitters as illuminators of opportunity for detecting and tracking flying targets. The opportunity COFDM broadcasters could be either DAB as well as DVB-T ones even in SFN (Single Frequency Network) mode for which all the broadcasters are transmitting exactly the same signal. Such application is known in the literature as PCL (Passive Coherent Location) application [Howland et al 2005], [Baker & Griffiths 2005]. This chapter will be divided into three main parts. The first ones have to be considered as simple and short overviews on COFDM modulation and on radar basis. These paragraphs will introduce our notations and should be sufficient in order to fully understand this chapter. If not, it is still possible to consider a „classical“ radar book as well as some articles on COFDM like [Alard et al 1987]. More specifically, the COFDM description will outline the properties that will be used in radar detection processing and the radar basis will schematically illustrate the compulsory rejection of the „zero-Doppler“ paths received directly from the transmitter or after some reflection on the ground. Then the most important part will detail and compare two cancellation filters adapted to COFDM waveform. These two filters could be applied against multipaths (reflection on ground elements) as well as against multiple transmitters in SFN mode. In this document, no difference will be done between SFN transmitters contributions and reflections on fixed obstacles : all these zero-Doppler paths will be considered as clutter or propagation channel. Obviously, these filters will be efficient also in a simple MFN (Multiple Frequency Network) configuration. Most of the results presented below concerns experimental data, nevertheless some simulations will also be used for dealing with some specific parameters.