I. Introduction
With the rapid evolution of technologies, such as 5G, IoT and AI technologies, consumer electronic devices, including mobile devices, smartphones, and wearables, have developed rapidly [1], [2]. The number of devices has been growing exponentially and might reach 27 billion by 2027 [3]. Moreover, a large amount of private user data is generated with the use of electronic devices. Many intelligent services based on these data [4], [5], such as personalized recommendations, have been developed for various fields, including medicine [6], [7], new energy [8], construction [9], and smart cities [10], [11]. These intelligent services increase the use of intelligence in our daily lives, but it is a challenge to provide better intelligent services while ensuring user data privacy [12], [13], [14], [15].