## 1 Introduction  
Scholars estimate that the number of visually impaired people worldwide will increase from 38.5 million in 2020 to more than 115 million by 2050 [1]. So increased societal and governmental attention will be needed in the future. Specifically, the visually impaired need assistive tools when they walk outdoors. White canes and guide dogs are currently the most well-known assistive tools for the visually impaired [2]. Although white canes are cheap and easy to use, they cannot provide important visual information, such as obstacle location, type, and proximity. Visual information is indispensable for environmental perception and movement safety during outdoor navigation [3]. Guide dogs can assist the visually impaired to avoid obstacles; however, their life span is about 8 to 12 years [4], and the cost of breeding and training a guide dog is very high. Many researchers have developed various assistive devices for the visually impaired [5, 6, 7]. To safely guide the visually impaired while walking, the assistive tools should have the ability to recognize the surrounding environment quickly and accurately. A wearable device for visually impaired people was developed by [5] in which the device used different types of sounds to inform users whether there were obstacles in front of them or not. It also used different frequencies and decibels to indicate the location of the obstacles. In the paper [6], the authors combined wearable glasses and augmented reality technology and then integrated the traversable direction visual enhancement function to help partially sighted people to walk safely. In addition, they developed three-voiceprompts to guide severely or profoundly visually impaired people in a safe direction. A wearable navigation device [7] with an RGB-D camera, a gyroscope, and a smartphone were proposed to guide the user in bypassing obstacles when the visually impaired travel through indoor and outdoor environments. When visually impaired people walk outdoors, obstacle avoidance, safe intersection transition, and accurate route navigation are of great importance. Many assistive frameworks have been developed to achieve these goals, which rely on smartphones, sensors, or computer vision. The authors of the paper [8] reviewed several related articles and analyzed the advantages and disadvantages of the different techniques. In the paper [9], the authors used the images captured by RGBD camera to train a semantic segmentation network and a classifier such that the visually impaired can recognize the road environment and walk toward their correct direction. However, there is no guidance if it is a dead-end in front of the user. [PERSON] et al. [10] used smart glasses and a laptop with a real-time semantic segmentation network to lead the user. The system proposed by [11], composed of a pair of smart glasses and a portable processor, can recognize guide bricks and crosswalks for visually impaired users in outdoor environments. Paper [12] provided a segmentation network to recognize crossroads and used an image processing method of color space to identify traffic lights and help the visually impaired use the crosswalk safely. The study of paper [13] used a smartphone to help the visually impaired avoid aerial obstacles. For the visually impaired, a set of glasses with integrated infrared (IR) transceiver sensors and a white cane with accelerometer and gyroscope sensors [14] are used to avoid obstacles and detect whether the user falls. In the paper [15], the authors proposed a sensor-based wearable device for the walking assistance of visually impaired people. An algorithm based on image depth information and fuzzy logic was proposed in [16] to detect multiple objects in real-time, and it achieved high accuracy on obstacle avoidance. The inertial measurement unit (IMU) and the smartphone camera were utilized in [17] to design a navigation system for assisting the visually impaired during indoor and outdoor activities. [PERSON] et al.