Spatial difference smoothing for coherent sources location in MIMO radar

• Four kinds of spatial difference smoothing (SDS) techniques in MIMO radar are developed to estimate the DOD and DOA of coherent sources under unknown spatially colored noise.
• The failure of conventional SDS is analyzed, and modified techniques are proposed.
• The diversity smoothing in MIMO radar is incorporated in SDS techniques.
• The forward, backward, and combined forward–backward forms of these SDS techniques are introduced.
• The aforementioned techniques are compared and extended from bistatic to monostastic MIMO radar.

In this paper, we develop a group of spatial difference smoothing (SDS) techniques in multiple-input multiple-output (MIMO) radar to facilitate the directions-of-departure (DODs) and directions-of-arrival (DOAs) estimation of coherent signals under unknown correlated noise. The conventional SDS technique, which exploits the difference between the forward spatially smoothed covariance matrix and the backward one, has been used to reduce noise for coherent sources location. However, it fails when the number of sources is odd. To solve the problem, we refine the SDS technique into asymmetric SDS (A-SDS) technique. Meanwhile, two smoothing methods exist in MIMO radar, which are transmit–receive spatial smoothing relying on the rotational invariance property of array and transmit–receive diversity smoothing brought by waveform diversity. Then the general SDS and A-SDS techniques using the spatial smoothing are specialized into transmit–receive diversity SDS (TRD-SDS) and asymmetric TRD-SDS (A-TRD-SDS) techniques using the diversity smoothing. Further, the forward, backward, and combined forward–backward forms of these four kinds of smoothing techniques are discussed. Based on them, eigenstructure algorithms are applied for DOD and DOA estimation. The proposed techniques can be used in bistatic or monostatic MIMO radar under spatially colored or white noise, and their effectiveness is confirmed by simulations.