B.A. Johnson, Y.I. Abramovich, and G.J. Frazer (Australia)
MIMO radar, orthogonal waveforms, over-the-horizon radar, transmit beamforming.
Transmission of spatially diverse orthogonal waveforms in radar to support enhanced illumination control and surveil lance timeline management has been recently discussed in the radar literature. Such use of orthogonal waveforms in radar (sometimes referred to as MIMO radar) is poten tially attractive in modern HF over-the-horizon (skywave) radar systems. However, HF radar implementation of or thogonal waveforms present some special challenges to waveform design. HF radars operate over several octaves of RF frequency, have array inter-element spacing which deviates significantly from one half-wavelength, use non linear power amplifiers,and have stringent spectral occu pancy requirements. Implementation of specific orthogonal waveform types present their own particular design require ments, but some issues are generic and span a wide range of waveform classes. In particular, operation with over sampled arrays can give rise to excessive reactive power. This can be mitigated by the use of a unitary transform into a separate orthonormal beamspace, but this also trans forms constant modulus waveforms favored in HF skywave radar into waveforms with significant peak to mean varia tion. Such amplitude variation results in loss in high-power amplifier efficiency. The impact of operation with those waveforms with HF skywave radars is examined.
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