  Recent initiatives by regulatory agencies to increase spectrum resources
available for broadband access include rules for sharing spectrum with
high-priority incumbents. We study a model in which wireless Service Providers
(SPs) charge for access to their own exclusive-use (licensed) band along with
access to an additional shared band. The total, or delivered price in each band
is the announced price plus a congestion cost, which depends on the load, or
total users normalized by the bandwidth. The shared band is intermittently
available with some probability, due to incumbent activity, and when
unavailable, any traffic carried on that band must be shifted to licensed
bands. The SPs then compete for quantity of users. We show that the value of
the shared band depends on the relative sizes of the SPs: large SPs with more
bandwidth are better able to absorb the variability caused by intermittency
than smaller SPs. However, as the amount of shared spectrum increases, the
large SPs may not make use of it. In that scenario shared spectrum creates more
value than splitting it among the SPs for exclusive use. We also show that
fixing the average amount of available shared bandwidth, increasing the
reliability of the band is preferable to increasing the bandwidth.
