  Dense deployment of base stations (BSs) and multi-antenna techniques are
considered key enablers for future mobile networks. Meanwhile, spectrum sharing
techniques and utilization of higher frequency bands make more bandwidth
available. An important question for future system design is which element is
more effective than others. In this paper, we introduce the concept of
technical rate of substitution (TRS) from microeconomics and study the TRS of
spectrum in terms of BS density and antenna number per BS. Numerical results
show that TRS becomes higher with increasing user data rate requirement,
suggesting that spectrum is the most effective means of provisioning extremely
fast mobile broadband.
