Spectral-efficient model for multiuser massive MIMO: Exploiting user velocity

The employment of a massive number of antennas in multiple-input multiple-output systems, known as massive MIMO, has drawn a new horizon for future communications systems to support a very large number of users. However, the actual number of active users in massive MIMO are limited by pilots training via the coherence time of the communication channel which is inversely proportional to the user velocity. The current model applies this coherence time for every user to design multiuser massive MIMO, which might result in a suboptimal solution since the users usually move at different speeds in practice. In this paper, we investigate multiuser massive MIMO by taking into consideration the differences in user velocities. In particular, two multiuser models are proposed to maximize the per-user spectral efficiency and the number of served users, respectively. System capacity of the proposed models is provided in analytical expression. Finally, numerical results demonstrate the advantages of our proposed models compared with the reference model.