  In this paper, we analyze the throughput performance of two co-existing
downlink multiuser underlay secondary networks that use fixed-rate
transmissions. We assume that the interference temperature limit (ITL) is
apportioned to accommodate two concurrent transmissions using an interference
temperature apportioning parameter so as to ensure that the overall
interference to the primary receiver does not exceed the ITL. Using the derived
analytical expressions for throughput, when there is only one secondary user in
each network, or when the secondary networks do not employ opportunistic user
selection (use round robin scheduling for example), there exists a critical
fixed-rate below which sum throughput with co-existing secondary networks is
higher than the throughput with a single secondary network. We derive an
expression for this critical fixed-rate. Below this critical rate, we show that
careful apportioning of the ITL is critical to maximizing sum throughput of the
co-existing networks. We derive an expression for this apportioning parameter.
Throughput is seen to increase with increase in number of users in each of the
secondary networks. Computer simulations demonstrate accuracy of the derived
expressions.
