We provide nonasymptotic upper and lower bounds on the sum-rate capacity ofRayleigh block-fading multiple-access channels for the setup where a priorichannel state information is not available. The upper bound relies on a dualformula for channel capacity and on the assumption that the users can cooperateperfectly. The lower bound is derived assuming a noncooperative scenario, whereeach user employs unitary space-time modulation (independently from the otherusers). Numerical results show that the gap between the upper and the lowerbound is small already at moderate SNR values. This suggests that the sum-ratecapacity gains obtainable through user cooperation are minimal.
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