  For energy efficient operation of the massive multiple-input multiple-output
(MIMO) networks, various aspects of energy efficiency maximization have been
addressed, where a careful selection of number of active antennas has shown
significant gains. Moreover, switching-off physical resource blocks (PRBs) and
carrier shutdown saves energy in low load scenarios. However, the joint
optimization of spectral PRB allocation and spatial layering in a heterogeneous
network has not been completely solved yet. Therefore, we study a power
consumption model for multi-cell multi-user massive MIMO 5G network, capturing
the joint effects of both dimensions. We characterize the optimal resource
allocation under practical constraints, i.e., limited number of available
antennas, PRBs, base stations (BSs), and frequency bands. We observe a single
spatial layer achieving lowest energy consumption in very low load scenarios,
whereas, spatial layering is required in high load scenarios. Finally, we
derive novel algorithms for energy efficient user to BS assignment and propose
an adaptive algorithm for PRB assignment and power control. All results are
illustrated by numerical system-level simulations, describing a realistic
metropolis scenario. The results show that a higher frequency band should be
used to support users with large rate requirements via spatial multiplexing and
assigning each user maximum available PRBs.
