Ultra-Wideband OFDM Synthetic Aperture Radar Imaging Sensor Newtork

Ultra-wideband (UWB) imaging sensors can be instrumental in number of scenarios that involve the need to monitor, track and identify targets and determine sensor's location relative to the targets, to other friendly sensors in the field and to the observed target scene. Presence of several sensors in RF proximity to each other may allow for data exchange between sensors, enabling enhanced surveillance and location operations. In Fig. 1, two sensor platforms complement and enhance each other's functionality. A sensor operating in radar mode will collect image data and perform data processing. Same sensor in communication mode will receive image data from other sensors, enhancing its own reconnaissance and navigation capabilities.

Various types of UWB radar systems have been explored over the past several decades. Often, extremely wide bandwidths were achieved by employing ultra-short pulses, e.g. Gaussian pulses [1],[2]. These radars suffer from low spectral efficiency which makes dual use of them as radar/communication system device problematic. Also, since pulse shape is a constant, these radars could become susceptible to certain types of jamming - e.g. false-target response generator [3]. This also precludes using these radars in the situations where they could interfere with each other. Some of the approaches to mitigate these concerns included using UWB noise as radar signal [4]. These waveforms are inherently irreproducible, making them resistant to certain kinds of jamming. However, lack of influence over the transmitted signal's shape is a disadvantage resulting in restrictions on transmit power and sidelobe control.

A principally different approach to UWB wave-shaping can be implemented at reasonably low cost, thanks to advances in sampling technology. Multi-band orthogonal frequency division multiplexing (MB-OFDM) is a class of signals that has received strong interest from industrial and academic communities researching UWB communication [5]. This signalling scheme is very spectrally-efficient, it allows for pulse diversity and dynamic spectrum allocation. Moreover, there is a clear potential to design a dual use radar/communication device based on OFDM architecture. Recent study by TU-Delft researchers [6] found that OFDM radar does not exhibit undesirable range-Doppler coupling/ambiguity to the extent inherent to, e.g. conventional LFM radar, while preserving performance characteristics. Other multi-frequency radar signals employing several sub- carriers simultaneously in OFDM fashion include e.g., multi- frequency complementary phase coded (MCPC) waveforms, invented and described by N. Levanon in [7]. Levanon notes superb sidelobe reduction potential of these signals, afforded by using multiple frequencies at the same time.

Fig. 1: Illustration of the scenario of interest

Overall, the area of UWB multi-carrier radar systems is a relatively new field. OFDM systems for communication could not be implemented in UWB due to the limitations of sampling technology until recently. Before the proposed system's benefits can be tested in hardware, it is deemed useful to validate it via simulation study. Description of our modelling setup, assumptions and results follow.