With the thriving of Internet of Things (IoT) industry, an emerging scenario named massive machine-type communication (mMTC) is considered in the future 5G networks. One main challenge in mMTC is how to support massive connections with limited radio resources and low energy consumption. Therein, novel multiple access scheme is the key to overcome the challenge. This SpringerBrief targets researchers and engineers, who are engaged in 5G and future IoT related communications system. It can also be a used as a reference book or a secondary text for graduate students, who major in communications.