Energy Efficiency of Massive MIMO: Cell-Free vs. Cellular

Cell-free Massive MIMO (Multiple Input Multiple Output) employs a large number of AP's (Access Points) that are distributed throughout the intended coverage area to simultaneously serve a much smaller number of user AT's (Access Terminals). Conjugate beamforming is the simplest precoding method for the downlink transmission, and allows decentralized precoding processing. Max-min power control maximizes the minimal effective SINR (Signal-to-Interference-plus-Noise Ratio) among all the active users, therefore provides a uniform throughput to all users. Cell- free Massive MIMO with conjugate beamforming precoding and max-min power control is naturally radiated energy efficient due to two facts: a well-known fact that with high probability at least one AP is nearby every user, and a surprising fact that, with max-min power control, a large portion of the AP's does not transmit with full power. We find that a cell-free Massive MIMO can deliver more than 80 Mb/joule in urban, and more than 40 Mb/joule in suburban and rural scenarios. We compare its energy efficiency and spectral efficiency with single cell Massive MIMO systems and demonstrate that in suburban and rural scenarios, cell-free systems can more than double the radiated energy efficiency and at the same time dramatically increase the 95% likely per user throughput, while in an urban scenario, the gain in radiated energy efficiency can be moderate at less than 50% with a comparable 95% likely per user throughput.