Non-Coherent and Backscatter Communications: Enabling Ultra-Massive Connectivity in 6G Wireless Networks

With the commencement of the 5th generation (5G) of wireless networks, researchers around the globe have started paying their attention to the imminent challenges that may emerge in the beyond 5G (B5G) era. Various revolutionary technologies and innovative services are offered in 5G networks, which, along with many principal advantages, are anticipated to bring a boom in the number of connected wireless devices and the types of use-cases that may cause the scarcity of network resources. These challenges, partly emerged with the advent of massive machine-type communications (mMTC) services, require extensive research innovations to sustain the evolution towards enhanced-mMTC with the scalable network cost in 6th generation (6G) wireless networks. Towards delivering the anticipated massive connectivity requirements with optimal energy and spectral efficiency besides low hardware cost, this paper proposes a framework that is fundamentally based on the amalgamation of non-coherent and backscatter communications (BsC). Considering the potentials of these technologies, starting with the review of 5G target services and enabling technologies, a comprehensive review of their state-of-the-art is conducted, and their potential applications and use-cases in B5G era are identified. Along with the main focus on non-coherent and BsC technologies, various emerging enabling technologies for 6G networks including unmanned aerial vehicles -assisted communications, visible light communications, quantum-assisted communications, reconfigurable large intelligent surfaces, non-orthogonal multiple access, and machine learning -aided intelligent networks are identified and thoroughly discussed. Subsequently, the scope of these enabling technologies for different device types (e.g., body implants), service types (e.g., e-mMTC), and optimization parameters (e.g., energy) is analyzed in detail. ...