In fifth-generation wireless communication networks, Non-Orthogonal Multiple Access (NOMA) has attracted much attention in both academic and industrial fields because of its higher spectral efficiency in comparison with orthogonal multiple access. Recently, numerous uplink NOMA techniques have been proposed, some of which are based on Successive Interference Cancellation (SIC) and others on Joint Decoding (JD, or simultaneous decoding). In this study, we analyze the outage capacities of SIC and JD in the case of single-block transmission over a two-user Gaussian multiple-access channel with partial channel state information at transmitter from the perspective of information theory. Results of the analysis and numerals show that compared to SIC, JD can achieve a sum-rate gain of up to 10% or sum-power gain of 0.8 dB.