  In this paper, we propose adaptive techniques for multi-user multiple input
and multiple output~(MU-MIMO) cellular communication systems, to solve the
problem of energy efficient communications with heterogeneous delay-aware
traffic. In order to minimize the total transmission power of the MU-MIMO, we
investigate the relationship between the transmission power and the M-ary
quadrature amplitude modulation~(MQAM) constellation size and get the energy
efficient modulation for each transmission stream based on the minimum mean
square error~(MMSE) receiver.Since the total power consumption is different for
MU-MIMO and multi-user single input and multiple output~(MU-SIMO), by
exploiting the intrinsic relationship among the total power consumption model,
and heterogeneous delay-aware services, we propose an adaptive transmission
strategy, which is a switching between MU-MIMO and MU-SIMO. Simulations show
that in order to maximize the energy efficiency and consider different Quality
of Service (QoS) of delay for the users simultaneously, the users should
adaptively choose the constellation size for each stream as well as the
transmission mode.
