Radar-Communications Systems Coexistence and Agile Multicarrier Radars

Abstract

Rigid allocation policies can no longer satisfy the growing demand for spectrum. There is a great need for flexible and efficient use of spectral resources. Spectrum sharing between different wireless systems is one approach to solve this problem. This requires that the systems are fully adaptive and cognitive. Consequently, these can take advantage of their operational environment knowledge such as the state of the radio spectrum. For such demanding task, multicarrier waveforms provide a convenient and flexible way of generating and designing waveforms. Thanks to their multiple degrees of freedom in time, frequency and code domain, multicarrier waveforms allow for performance optimization as well. In this thesis, theories, methods and justifications for coexistence and spectrum sharing among radar and wireless communication systems are developed.

A generalized multicarrier radar (GMR) model is proposed to enable the required flexible use of spectrum and adaptation to different target scenarios and operational environments. It allows for conveniently representing most of the known radar waveforms as well as designing and optimizing new ones. Using the developed model novel multicarrier waveforms are proposed. These waveforms relax the subcarrier orthogonality constraint for an improved ambiguity function and consequently better radar performance. Constrained radar waveform optimization using the GMR model and a mutual information (MI) based objective function is proposed. Several examples of waveforms employing fast frequency hopping, which are special cases of the GMR model, are also provided.

The problems of radar waveform optimization and adaptation are addressed in the coexistence context using multicarrier waveforms, in particular OFDM. The optimization problems are formulated for different radar tasks, i.e. target characterization, target detection and target parameter estimation. The optimization is performed using information theoretic criteria such as mutual information (MI) or criteria stemming from detection and estimation theory, for example, Neyman-Pearson (NP) criterion or Cramer-Rao bound (CRB). The optimization problems proposed in this thesis are formulated by imposing constraints on the total transmitted radar power and an interference mask provided by the communication system. This interference mask ensures that a desired rate for the communication users can be achieved. The solutions to the optimization problems represent power allocations over the available subcarriers. The performance of the proposed optimized waveforms in this thesis is evaluated in simulations using receiver operating characteristic or root mean squared error plots, for the detection and estimation tasks respectively. Recommendations on which optimization objective functions produce the best performing waveforms are given. It is also demonstrated that exploiting the communication signal reflected off the target can improve the radar performance for all considered tasks.