In this paper, the ergodic sum-rate of a fading cognitive multiple accesschannel (C-MAC) is studied, where a secondary network (SN) with multiplesecondary users (SUs) transmitting to a secondary base station (SBS) shares thespectrum band with a primary user (PU). An interference power constraint (IPC)is imposed on the SN to protect the PU. Under such a constraint and theindividual transmit power constraint (TPC) imposed on each SU, we investigatethe power allocation strategies to maximize the ergodic sum-rate of a fadingC-MAC without successive interference cancellation (SIC). In particular, thispaper considers two types of constraints: (1) average TPC and average IPC, (2)peak TPC and peak IPC. For the first case, it is proved that the optimal powerallocation is dynamic time-division multiple-access (D-TDMA), which is exactlythe same as the optimal power allocation to maximize the ergodic sum-rate ofthe fading C-MAC with SIC under the same constraints. For the second case, itis proved that the optimal solution must be at the extreme points of thefeasible region. It is shown that D-TDMA is optimal with high probability whenthe number of SUs is large. Besides, we show that, when the SUs can be sortedin a certain order, an algorithm with linear complexity can be used to find theoptimal power allocation.
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