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In this paper, we analyze the throughput performance of two co-existing downlink multiuser underlay secondary networks that use fixed-rate transmissions (FRT). We assume that the interference temperature limit (ITL) is apportioned to accommodate two concurrent transmissions using an apportioning parameter 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 transmitting concurrently is higher than the throughput achieved by a single secondary network. We derive an expression for this critical fixed-rate. Below this critical rate, we show that careful …