## 1 Introduction  
Ultrasound imaging has been widely used in clinical disease diagnosis and treatment evaluation due to its non-invasive, real-time, and cost-effective characteristics. Tele-ultrasound imaging is also an important part of telemedicine imaging. Telemedicine has evolved in the last few years to improve healthcare system not only for patient outcomes, but also for education purpose [1]. The obstetric tele-ultrasound has been proved feasible with a bandwidth of 2 Mbit/s video link [2]. Transmission of real-time ultrasound video to a remote iPhone with a WiFi connection and a 3G connection were both accomplished. However, the frame rate was reduced to 1.1 frames/s from 11.9 frames/s of the original transmitted signal [3]. In order to obtain better image quality and higher frame rate in telemedicine imaging, the amount of data can be reduced, in addition to increasing the network bandwidth. B-mode ultrasound imaging is currently the most widely used clinical ultrasound diagnostic methods. Traditional B-mode imaging has a frame rate of 30-40 frames/s, which can meet the requirements of observing the static physiological structure of tissues, such as imaging of the liver, kidney, and abdomen. With the development of more medical ultrasound technologies, such as shear wave elastography [4], cardiac imaging, ultrasound localization microscopy (ULM) [5], etc., higher requirements are placed on the frame rate of ultrasound imaging. Methods to augment the frame rate of ultrasound imaging include multi-line-transmit/receive method, synthetic aperture emission imaging and wide beam emission imaging [6]. Wide beam imaging is a novel transmission method of ultrasound, which includes plane waves and divergent waves. A single plane wave transmission of ultrasound can cover the entire imaging area, and then the full aperture receives and records the echo signal in order to produce an ultrasound image [7]. At present, plane wave imaging can obtain the entire image through a single full-aperture transmit-receive, which can increase the frame rate to more than 10,000 frames/s [8]. It has been applied to shear wave elastography [9], brain function imaging [10], and ultrasound localization microscopy [5]. Compared with the focused wave scanning transmission method, the plane wave transmission method greatly increases the frame rate. However, the signal-to-noise ratio, the image resolution and the contrast of the echo signal are deteriorated because the transmitting beam is not focused. The imaging results of a plane wave emitted once have the problems of poor resolution and low signal-to-noise ratio. In practical applications, multi-angle compound imaging methods are often used [9]. The multi-angle compound imaging mode improves imaging quality with the increase of the angle, but the frame rate will linearly decrease in this way as well. In order to retain the advantages of plane wave imaging's high frame rate as much as possible, different beamforming algorithms are applied to plane wave imaging to improve image quality without reducing the frame rate. Currently, the commonly used beamforming methods are delay-and-sum (DAS) and adaptive beamforming algorithm based on minimum variance (MV) [11]. The DAS algorithm is the most basic beamforming algorithm, which applies the fixed weights to the echo signal after delay processing to reduce the signal side lobes. However, the weight of the window function used in the traditional amplitude apodization process is usually a set of fixed parameters preset according to the depth. Therefore, the sidelobe attenuation is at a certain degree, and the width of the main lobe will also increase, that is, the resolution will not improve [12]. Compared with DAS, the adaptive beamforming uses the signal received by the transducer to adaptively calculate the weighting value added to each element. Because this weighting value is dynamic, it has better resolution and anti-interference ability [13].