Cell-Free Massive MIMO for Massive Low-Power Internet of Things Networks
Byung Moo Lee
Abstract
In this article, we consider the application of a cell-free (CF) massive multiple-input–multiple-output (MIMO) system to low-power Internet of Things (IoT) networks. CF Massive MIMO distributes many access points (APs) in a given area and supports many IoT devices simultaneously using the same time and frequency resources. To support many IoT devices that have more than the number of service antennas within limited coherence time/frequency, the reference signal (RS) should be reused. Generally, CF Massive MIMO with massive connectivity requires a lot of power and, thus, it is difficult to use in low-power IoT networks. In this regard, we propose an energy-efficient (EE) power control scheme that can significantly reduce the radiation power of IoT devices. We note that with many IoT devices, there is a quite large amount of interference, and if we choose the radiation power based on this information, the signal-to-interference-plus-noise ratio (SINR) becomes approximately independent of the radiation power. We show that our scheme can reduce the radiation power more than 90% compared to the normal operation. We also show that with many IoT devices, there is a spectral efficiency hardening effect, and maximum ratio (MR) processing and minimum mean square error (MMSE) processing present similar performance and, thus, MR processing could be sufficient for the use in CF Massive MIMO with massive IoT connectivity.