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In wireless data systems, Aloha channels are often used to send requests or short messages from a mobile station to the base station receiver. This paper evaluates a slotted Aloha channel with receiver antenna diversity; packets are transmitted in randomly selected time slots. The packet success-rate is studied as it relates to the carrier to noise plus interference ratio observed in each received branch. The ReFLEX Aloha channel is simulated under slow moving fading. A single cell's throughput, packet success-rate and mean number of transmission attempts per packet are presented from the simulation. The results show that as the transmission retry limit increases, the Aloha throughput decreases and the packet reliability increases. Based on these results, the number of retries can be selected to maximize the throughput for a given packet reliability. Other performance metrics such as the mean number of transmissions and mean packet delay are presented. Although the simulation results are specific to ReFLEX systems, the analysis methods and the system optimization approach can he applied to other Aloha systems.