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In cognitive radio settings with highly dynamic primary activities and with small opportunities for secondary access, the requirement to fairly distribute the temporarily available spectral ranges among the unlicensed users turns out to be of particular relevance. The current paper addresses this issue by presenting a new design formulation that aims to optimize the performance of an orthogonal-frequency-division-multiple-access (OFDMA) ad-hoc cognitive radio network, by means of joint subcarrier assignment and power allocation. Besides important constraint on the tolerable interference induced to primary network, to efficiently implement spectrum-sharing fairness, the optimization problem considered here strictly enforces upper and lower bounds on the total amount of temporarily available bandwidth to be granted to individual secondary users. Specifically, the system throughput is maximized via the application of Lagrangian duality theory. More importantly, the dual decomposition framework also gives rise to the realization of distributed solution. As the proposed distributed protocol requires very limited cooperation among the participating network elements, it is especially applicable for the ad-hoc networking environment under investigation, to which any central processing or control is certainly inaccessible. While the computational complexity of the devised algorithm is affordable, its performance in practical scenarios also attains the actual global optimum. The potential of the proposed approach is verified through asymptotic complexity analysis and via numerical examples.