Assume that a multi-user multiple-input multiple-output (MIMO) system is
designed from scratch to uniformly cover a given area with maximal energy
efficiency (EE). What are the optimal number of antennas, active users, and
transmit power? The aim of this paper is to answer this fundamental question.
We consider jointly the uplink and downlink with different processing schemes
at the base station and propose a new realistic power consumption model that
reveals how the above parameters affect the EE. Closed-form expressions for the
EE-optimal value of each parameter, when the other two are fixed, are provided
for zero-forcing (ZF) processing in single-cell scenarios. These expressions
prove how the parameters interact. For example, in sharp contrast to common
belief, the transmit power is found to increase (not to decrease) with the
number of antennas. This implies that energy-efficient systems can operate in
high signal-to-noise ratio regimes in which interference-suppressing signal
processing is mandatory. Numerical and analytical results show that the maximal
EE is achieved by a massive MIMO setup wherein hundreds of antennas are
deployed to serve a relatively large number of users using ZF processing. The
numerical results show the same behavior under imperfect channel state
information and in symmetric multi-cell scenarios.

本研究探讨了在单元格场景下，使用零强制处理时，当其他两个参数固定时，为实现最大能量效率，所需的天线数，有效用户数和发射功率的最佳数量，并提出了新的功耗模型以揭示这些参数如何影响能量效率。研究结果表明，在高信噪比区域操作的能源效率系统中，必须进行抑制干扰信号处理，从而发射功率是随着天线数量增加而增加，而不是减少。