Differential Modulation for Grant-free Short-packet Transmission in Flat Fading Channels

In the massive machine-type-communication (mMTC), grant-free transmission allows user devices to access to network randomly and transmit small data packets occasionally. To reduce the signaling overhead, no scheduling is performed and no reference signal is available. Consequently, blind multi-user detection (MUD) is carried out without knowledge on user identity and channel state information. For the conventional demodulation based on the geometric features in the blind MUD, phase ambiguity is inevitable. The elimination of ambiguity requires additional computation complexity. To solve this problem, we propose in this paper the application of differential modulation for the grant-free transmission. Then we propose a new scheme for data recovering free of channel equalization. The proposed structure significantly reduces the complexity and the latency on the receiving side. Meanwhile, numerical results in terms of block error rate show that the newly proposed scheme maintains similar performance to the conventional modulation scheme in the high SNR region.