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