I. Introduction
High-resolution wide-swath SAR has attracted extensive attentions due to its powerful military and civilian applications [1]. There is a contradiction between high resolution and low pulse repetition frequency. To address this problem, multiple-input and multiple-output synthetic aperture radar (MIMO-SAR) is used by employing multiantenna arrays transmitting multiple orthogonal coding waveforms [2]. Most of the available orthogonal coding waveform systems utilize the same carrier frequency coding waveforms, such as azimuth phase coding. However, one of the disadvantages of this method is that its cross correlation noise cannot be ignored in a real system. To address this drawback, MIMO-SAR using stepped-frequency (SF) chirps is presented in [3], which transmits multiple SF waveforms simultaneously without overlapping bandwidth; the echoes can be separated by a bandpass filter without any aliasing. In addition, SF chirps are utilized to synthesize a wide bandwidth to attain high range resolution [4].