  As we make progress towards the era of fifth generation (5G) communication
networks, energy efficiency (EE) becomes an important design criterion because
it guarantees sustainable evolution. In this regard, the massive multiple-input
multiple-output (MIMO) technology, where the base stations (BSs) are equipped
with a large number of antennas so as to achieve multiple orders of spectral
and energy efficiency gains, will be a key technology enabler for 5G. In this
article, we present a comprehensive discussion on state-of-the-art techniques
which further enhance the EE gains offered by massive MIMO (MM). We begin with
an overview of MM systems and discuss how realistic power consumption models
can be developed for these systems. Thereby, we discuss and identify few
shortcomings of some of the most prominent EE-maximization techniques present
in the current literature. Then, we discuss "hybrid MM systems" operating in a
5G architecture, where MM operates in conjunction with other potential
technology enablers, such as millimetre wave, heterogenous networks, and energy
harvesting networks. Multiple opportunities and challenges arise in such a 5G
architecture because these technologies benefit mutually from each other and
their coexistence introduces several new constraints on the design of
energy-efficient systems. Despite clear evidence that hybrid MM systems can
achieve significantly higher EE gains than conventional MM systems, several
open research problems continue to roadblock system designers from fully
harnessing the EE gains offered by hybrid MM systems. Our discussions lead to
the conclusion that hybrid MM systems offer a sustainable evolution towards 5G
networks and are therefore an important research topic for future work.
