6. Summary

This chapter presents a summary about massive access for the cellular IoT in 5G and beyond. In particular, we discuss the theories and techniques of massive access and their applications in the cellular IoT according to the characteristics of available CSI at the BS in different scenarios. Firstly, a massive access scheme for a fixed cellular IoT where full CSI is available at the BS is designed. Especially, spatial beam and transmit power are jointly optimized according to instantaneous CSI from the perspectives of maximizing the weighted sum rate and minimizing the total power consumption, respectively. Then, a low-mobility cellular IoT operated in FDD mode that partial CSI is obtained through a quantization codebook is studied, and the corresponding massive access scheme is provided by optimizing the feedback resource. Furthermore, a TDD mode-based cellular IoT is considered, and a fully non-orthogonal massive access scheme is proposed. To exploit the benefits of fully non-orthogonal massive access, the transmit power at both the BS and IoT devices is optimized. Finally, to satisfy the requirement of high mobility, a non-orthogonal beamspace massive access scheme is given, which can achieve a better spectral efficiency over fast-varying fading channels. Moreover, we analyze the challenging issues in the existing massive access schemes, and point out the future research directions for further improving the overall performance of the cellular IoT.