  Sharing the spectrum among multiple operators seems promising in millimeter
wave (mmWave) systems. One explanation is the highly directional transmission
in mmWave, which reduces the interference caused by one network on the other
networks sharing the same resources. In this paper, we model a mmWave cellular
system where an operator that primarily owns an exclusive-use license of a
certain band can sell a restricted secondary license of the same band to
another operator. This secondary network has a restriction on the maximum
interference it can cause to the original network. Using stochastic geometry,
we derive expressions for the coverage and rate of both networks, and establish
the feasibility of secondary licensing in licensed mmWave bands. To explain
economic trade-offs, we consider a revenue-pricing model for both operators in
the presence of a central licensing authority. Our results show that the
original operator and central network authority can benefit from secondary
licensing when the maximum interference threshold is properly adjusted. This
means that the original operator and central licensing authority have an
incentive to permit a secondary network to restrictively share the spectrum.
Our results also illustrate that the spectrum sharing gains increase with
narrow beams and when the network densifies.
