MIMO radar demystified and where it makes sense to use

Abstract

Contrary to claims made Multiple Input and Multiple Output (MIMO) radars do not provide an order of magnitude better angle resolution, accuracy and identifiability over conventional radars. What is claimed: MIMO array radar system consisting of a full transmit array and thinned receive array (or vice versa; called here a full/thin array) provides an order of magnitude or more of accuracy, resolution and identifiability, the ability to resolve and identify targets than a conventional array. This claim for MIMO results from making the wrong comparison to a full conventional array rather than to a conventional full/thin array. It is shown here that a conventional full/thin array radar can have the same angle accuracy, resolution and identifiability as a MIMO full/thin array. Moreover the conventional full/thin array example given here can have a better search energy efficiency. Where does the MIMO radar provide a better angle accuracy than a conventional radar? A monostatic MIMO array radar does provide a better angle accuracy than its conventional monostatic equivalent, but it is only about a factor of 1/√2 (29 percent) better and its resolution is the same. Alternately, a monostatic MIMO array radar can offer the advantage of the same accuracy as a conventional monostatic array radar with a smaller aperture size, one that is 1/√2 = 0.707 smaller, or equivalently 29 percent smaller. This improved accuracy comes at a heavy computation cost. For a MIMO monostatic linear array of N elements ≥ N2 matched filters (MFs) are needed versus just N for its conventional equivalent. The factor ≥ results from the need for a bank of F ≥ 1 MFs being needed for the MIMO array. A bank of F MFs are need because the MF for an orthogonal waveform is usually Doppler intolerant. In contrast a conventional array would use a linear FM waveform (chirp waveform) which is Doppler tolerant. An alternate approach for achieving this factor of √2 advantage is to simply increase the radiated power of a conventional radar by a factor of 2. This latter approach to getting the √2 advantage has to be traded off against the cost resulting from the throughput increase required when using MIMO. MIMO radar is best for search not for track, unless track-while-scan. For track, conventional array processing should be used for maximum energy efficiency and to reduce the signal processing requirements. When a MIMO array is used to search a large scan angle, it is best for maximum search energy efficiency to use subarrays of the array as the elements of the MIMO array to form what we call a subarray-MIMO (SA-MIMO). When searching a small scan angle, the subarrays should be sized so that the volume of space illuminated by the subarrays of the SA-MIMO array matches, or is smaller than, the volume of space to be searched. Using SA-MIMO reduces the processing throughput requirements. MIMO radar in the near term will be useful for coherent and incoherent combining of existing radars to achieve about a 9 dB better power-aperture-gain (PAG).