  In this paper we address the problem of fully distributed assignment of users
to sub-bands such that the sum-rate of the system is maximized. We introduce a
modified auction algorithm that can be applied in a fully distributed way using
an opportunistic CSMA assignment scheme and is $\epsilon$ optimal. We analyze
the expected time complexity of the algorithm and suggest a variant to the
algorithm that has lower expected complexity. We then show that in the case of
i.i.d Rayleigh channels a simple greedy scheme is asymptotically optimal as
$\SNR$ increases or as the number of users is increased to infinity. We
conclude by providing simulated results of the suggested algorithms.
