Pilot Assignment and Power Control for Cell-Free Massive MIMO With HTC/MTC Coexistence

Cell-free massive MIMO (CF-mMIMO) is regarded as one of the most promising wireless technologies for future applications. It is expected to tackle the intercell interference problem with traditional cellular networks in order to achieve high Quality of Service (QoS). The coexistence of human-type communication (HTC) and machine-type communication (MTC) poses a challenge to today's and future communication technologies, including CF-mMIMO. However, there have been few studies on the support of CF-mMIMO for HTC/MTC coexistence. In this article, we present a comprehensive resource allocation mechanism for CF-mMIMO with HTC/MTC coexistence, which consists of two components. First, to handle the device activity detection issue caused by the absence of orthogonal pilot sequence, we propose a pilot assignment scheme based on the continuous method and a device activity detection scheme based on the spatial separation characteristics of CF-mMIMO, aiming at reducing the activity detection error probability. Second, we formulate an multiobjective optimization problem (MOOP) and devise a power control method based on MOOP to deal with the conflict caused by the different service requirements of HTC and MTC. Numerical results show that the proposed pilot assignment scheme outperforms the comparable pilot assignment schemes in terms of activity detection error probability. In addition, combined with the proposed pilot assignment scheme, the devised MOOP power control method leads to higher HTC normalized sum rate and MTC energy efficiency than the comparable power control methods.