MIMO Radar for Automobiles

Abstract

A commonly considered MIMO radar architecture for a car radar consists of a receive uniformly weighted full linear array of N elements with $\gamma/2$ spacing combined with a uniformly weighted transmit array of 2 elements which are spaced $\mathrm{N}\lambda/2$ apart. With this arrangement the MIMO virtual array consists of a uniformly weighted array having 2N elements with $\lambda/2$ spacing which has no grating lobes and twice the resolution of a conventional array consisting of N elements for receive and using a single element for transmit. This however is an unfair comparison. For a fairer comparison the conventional array should have the same number of elements as the MIMO radar array. The MIMO radar will have N+1 elements if one of the transmit array elements is shared with the receive array. Hence it sholdIt should consist of a uniform array of N+1 elements. Also with such a conventional arrays the N+1 elements are normally used for both transmit and receive. As a result the MIMO array has a resolution of a 2N element array while the conventional array has a resolution of a $(N+1)2$ array. The $\sqrt{2}$ results from the fact that the conventional array is a 2-way N+l element array. For N=5 the MIMO array has the one-way resolution of a 2N=10 array while the conventional array has a resolution of a one way $(\mathrm{N}+1)\sqrt{2}=8.5$ element array. Thus the MIMO array has only a 15% better resolution, not a factor of 2 better resolution. For N=3 the MIMO only has only a 6% better resolution. What about the antenna sidelobes for the MIMO and conventional array. For the MIMO array the 2-way sidelobe level is that of the 2N element virtual array with uniform weigting which has a first sidelobe of only 13 dB down. For the conventional array we have a 2-way sidelobe of the N+1 element array which has a uniform weighting. Thus its first sidelobe is 2*13 =26 dB down, So for N =5 the conventional array has 15% worse resolution but much lower sidelobes than the MIMO array having same number of elements, Lower sidelobes can be achieved for both if weighting is used. For the conventional array conventional weighting can be used for the receive and/or transmit arrays. For the MIMO array it has be applied to the virtual array rather than the receive array/. For the MIMO array radar we get 30 dB down sidelobes if a 30 dB Chebyshev weighting is applied to the virtual array. For the conventional array with uniform weighting on transmit and 30 dB Chebyshev on receive we get a first sidelobes that is 32 dB down with the rest 45 dB or more down.