To improve the cellular energy efficiency, without sacrificing
quality-of-service (QoS) at the users, the network topology must be densified
to enable higher spatial reuse. We analyze a combination of two densification
approaches, namely "massive" multiple-input multiple-output (MIMO) base
stations and small-cell access points. If the latter are operator-deployed, a
spatial soft-cell approach can be taken where the multiple transmitters serve
the users by joint non-coherent multiflow beamforming. We minimize the total
power consumption (both dynamic emitted power and static hardware power) while
satisfying QoS constraints. This problem is proved to have a hidden convexity
that enables efficient solution algorithms. Interestingly, the optimal solution
promotes exclusive assignment of users to transmitters. Furthermore, we provide
promising simulation results showing how the total power consumption can be
greatly improved by combining massive MIMO and small cells; this is possible
with both optimal and low-complexity beamforming.Comment: Published at International Conference on Telecommunications (ICT
2013), 6-8 May 2013, Casablanca, Morocco, 5 pages, 4 figures, 2 tables. This
version includes the Matlab code necessary to reproduce the simulations; see
the ancillary files. This version also corrects errors in Table 1 and in the
simulations, which affected Figs. 3-
