  In this paper, we consider a two-user multiple access fading channel under
quality-of-service (QoS) constraints. We initially formulate the transmission
rates for both transmitters, where the transmitters have arbitrarily
distributed input signals. We assume that the receiver performs successive
decoding with a certain order. Then, we establish the effective capacity region
that provides the maximum allowable sustainable arrival rate region at the
transmitters' buffers under QoS guarantees. Assuming limited transmission power
budgets at the transmitters, we attain the power allocation policies that
maximize the effective capacity region. As for the decoding order at the
receiver, we characterize the optimal decoding order regions in the plane of
channel fading parameters for given power allocation policies. In order to
accomplish the aforementioned objectives, we make use of the relationship
between the minimum mean square error and the first derivative of the mutual
information with respect to the power allocation policies. Through numerical
results, we display the impact of input signal distributions on the effective
capacity region performance of this two-user multiple access fading channel.
