Principles of Modern Radar - Volume 2 1891121537

142 CHAPTER 4 MIMO RadarSINR Loss (dB)0−5−10−15−20−25−30−35−40Orthogonal WaveformsPhased Array−45−1000 −800 −600 −400 −200 0 200 400 600 800 1000Doppler Frequency (Hz)FIGURE 4-10 Notional SINR loss for GMTI. If SINR loss is near 0 dB, then a target with thevelocity corresponding to that Doppler frequency is easily detected in the presence of clutter.The SINR loss for orthogonal waveforms is significantly lower than that for the conventionalphased array for many frequencies, so an improved minimum detectable velocity is expected.4.6.4 Beampattern SynthesisPhased array radars that employ digital beamforming on receive are able to spoil theirbeam on transmit to cover a larger area. In doing so, the search rate may be increased atthe cost of signal-to-noise ratio. While this is typically accomplished by applying a phasetaper across the array, this can also be considered in a MIMO context. As described in [26],beam spoiling by transmitting orthogonal waveforms relaxes some of the requirements onthe radar hardware when clutter is limiting detection performance.As described before, the transmit beampatterns that may be synthesized are a functionof the MIMO signal correlation matrix, R φ . To synthesize a desired pattern, the appropriatecorrelation matrix must be identified. Further, a suite of signals must be found thatpossesses these correlations. Some examples of such techniques are presented in [27,28].4.7 SUMMARYAs is hopefully evident from this chapter, analysis of the utility of MIMO techniquesfor a particular radar application may not be straightforward. It should also be clear thattransmitting orthogonal waveforms is not advantageous in every situation. The primarygoal of this chapter has been to provide a framework for evaluating the appropriatenessof a particular suite of MIMO waveforms for a specific radar mission. This is necessaryto decide if performance will be enhanced by using a MIMO radar instead of a traditionalphased array configuration.By synthesizing virtual phase centers, we saw how a MIMO radar can provide enhancedangular resolution. This may be of utility in the case of GMTI and air-to-air radar.A similar analysis revealed the ability of a MIMO radar to trade peak gain for a moreflexible transmit pattern. An application to SAR was presented.