Principles of Modern Radar - Volume 2 1891121537

140 CHAPTER 4 MIMO RadarTABLE 4-2 Properties of the virtual arrayscorresponding to physical arrays with M transmittersand N receivers.Configuration Length SpacingVernier array N × D/2 D/2Dense MIMO N × D/2 1/M × D/2Sparse MIMO MN × D/2 D/2can use a PRF that is decreased by a factor of N without impacting image quality and canrealize a potential increase in area coverage rate.The properties of the virtual arrays for these designs are given in Table 4-2. Note thatthe Vernier array is a special case of either a dense MIMO array or a sparse MIMO array.The results in the table for the MIMO configurations coincide with the Vernier array whenM = 1.The Dense MIMO Array Consider a receive array that consists of N receive elementsthat are of length D and spaced at an interval of D so that the receive array is contiguous.The SAR system uses M transmit elements that are spaced by D/M. Thecorresponding virtual array is sampled at an interval of 1/M×D/2. Note that theeffective length of the dense MIMO array is the same as the corresponding Vernier array.However, a higher along-track sampling rate can be achieved by the dense MIMOarray if it uses the same PRF as the Vernier array.The Sparse MIMO Array Once again, begin with a Vernier array of N elements. Now,distribute the M transmitters such that there is a separation of NDbetween transmitsubarrays. In this case, the spacing between virtual phase centers is D/2, as in theVernier array case, but the resulting virtual array is M times as long as that of theVernier array. This allows the sparse MIMO array to use a PRF that is M times lowerthan the Vernier array case, which provides a commensurate increase in area coveragerate.Note that the sparse MIMO array is presented in the bottom of Figure 4-2. The denseMIMO array is similar to the the filled MIMO array, but the spacing between the transmitphase centers is such that they must fit between the first two receive phase centers.An example of MIMO SAR is presented in [16]. An analysis of MIMO SAR collectionapproaches is found in [22]. These concepts for MIMO SAR can also be extended tosynthetic aperture sonar (SAS) systems [23], which face tremendous challenges in maintainingsufficient along-track sampling rates with reasonable area coverage rates to therelatively slow speed at which sound propagates.4.6.2 MIMO GMTIBy studying the angular PSF and the steered response, we saw that a MIMO radar hasimproved angular resolution and has the ability to resteer its transmit beam digitally toenable a higher area coverage rate compared with the phased array. We will see thatthis provides enhanced detection of slow moving targets in the presence of strong clutterreturns for ground moving target indication (GMTI) radar systems.Recall that the enhanced capability afforded by transmitting orthogonal waveformscomes at the cost of SNR. However, in the case of GMTI, for example, this may be