# A Novel Scheme for MIMO-SAR Systems Using Rotational Orbital Angular Momentum
); [EMAIL_ADDRESS] (H.T. ); [EMAIL_ADDRESS] (H.T. ); [EMAIL_ADDRESS] (Z.Z.) [PERSON]  
1 Science and Technology on Microwave Imaging Laboratory, Institute of Electronics, Chinese Academy of Sciences, Beijing 100190, China; [EMAIL_ADDRESS] (X.B. ); [EMAIL_ADDRESS] (Z.Z. ); [EMAIL_ADDRESS] (L.C. ); [EMAIL_ADDRESS] (H.T. ); [EMAIL_ADDRESS] (Z.Z.) [PERSON]  
1 Science and Technology on Microwave Imaging Laboratory, Institute of Electronics, Chinese Academy of Sciences, Beijing 100190, China; [EMAIL_ADDRESS] (X.B. ); [EMAIL_ADDRESS] (Z.Z. ); [EMAIL_ADDRESS] (L.C. ); [EMAIL_ADDRESS] (H.T. ); [EMAIL_ADDRESS] (H.T. ); [EMAIL_ADDRESS] (Z.Z.) [PERSON]  
1 Science and Technology on Microwave Imaging Laboratory, Institute of Electronics, Chinese Academy of Sciences, Beijing 100190, China; [EMAIL_ADDRESS] (X.B. ); [EMAIL_ADDRESS] (Z.Z. ); [EMAIL_ADDRESS] (L.C. ); [EMAIL_ADDRESS] (H.T. ); [EMAIL_ADDRESS] (H.T. ); [EMAIL_ADDRESS] (Z.Z.) [PERSON]  
1 Science and Technology on Microwave Imaging Laboratory, Institute of Electronics, Chinese Academy of Sciences, Beijing 100190, China; [EMAIL_ADDRESS] (X.B. ); [EMAIL_ADDRESS] (Z.Z. ); [EMAIL_ADDRESS] (L.C. ); [EMAIL_ADDRESS] (H.T. ); [EMAIL_ADDRESS] (Z.Z.) [PERSON]  
1 Science and Technology on Microwave Imaging Laboratory, Institute of Electronics, Chinese Academy of Sciences, Beijing 100190, China; [EMAIL_ADDRESS] (X.B. ); [EMAIL_ADDRESS] (Z.Z. ); [EMAIL_ADDRESS] (L.C. ); [EMAIL_ADDRESS] (H.T. ); [EMAIL_ADDRESS] (H.T. ); [EMAIL_ADDRESS] (Z.Z.) [PERSON]  
1 Science and Technology on Microwave Imaging Laboratory, Institute of Electronics, Chinese Academy of Sciences, Beijing 100190, China; [EMAIL_ADDRESS] (X.B. ); [EMAIL_ADDRESS] (Z.Z. ); [EMAIL_ADDRESS] (L.C. ); [EMAIL_ADDRESS] (H.T. ); [EMAIL_ADDRESS] (H.T. ); [EMAIL_ADDRESS] (Z.Z.) [PERSON]  
1 Science and Technology on Microwave Imaging Laboratory, Institute of Electronics, Chinese Academy of Sciences, Beijing 100190, China; [EMAIL_ADDRESS] (X.B. ); [EMAIL_ADDRESS] (Z.Z. ); [EMAIL_ADDRESS] (L.C. ); [EMAIL_ADDRESS] (H.T. ); [EMAIL_ADDRESS] (Z.Z.) [PERSON]  
1 Science and Technology on Microwave Imaging Laboratory, Institute of Electronics, Chinese Academy of Sciences, Beijing 100190, China; [EMAIL_ADDRESS] (X.B. ); [EMAIL_ADDRESS] (Z.Z. ); [EMAIL_ADDRESS] (L.C. ); [EMAIL_ADDRESS] (H.  
radar echoes from the multiple transmit signals, which is therefore a fundamental challenge for the implementation of MIMO-SAR systems [6]. Considering the ambiguity function introduced by [PERSON] [7], the energy of the imperfect orthogonal waveforms is spread but not removed in the time-frequency plane. For the distributed target with numerous closely spaced scatterers, the ambiguous energy from each scatterer will be accumulated, and thus the background noise level in both range and azimuth directions will be increased. Consequently, the imperfect orthogonal waveforms will deteriorate the MIMO SAR image quality dramatically [8]. Orthogonal waveforms designed in merely one dimension cannot avoid the ambiguous energy effectively. From the viewpoint of multidimensional modulation, orbital angular momentum (OAM) carried by vortex electromagnetic (EM) waves is introduced for MIMO-SAR implementations in this paper. Independent of frequency and polarization, OAM has been widely studied in many realms; for example, the improvement of the spectrum efficiency in wireless communication [9], stealth target detection [10], and the super-resolution of radar imaging [11]. Recently, studies have shown that vortex EM waves of different OAM modes can be utilized in SAR imaging with great performance, and OAM presents a brand new degree of freedom (DOF) in SAR [12]. As a unique merit of vortex EM waves, the Rotational Doppler Effect (RDE) is associated with a frequency shift that is proportional to the product of the relative rotational angular frequency and the OAM mode [13]. The RDE can be used, not only to detect the spinning object [14], but also to distinguish each signal of various OAM modes [15; 16].