Retransmission Control and Fairness Issue in Mobile Slotted ALOHA Networks With Fading and Near-Far Effect

The effects of different retransmission control policies on the performance of a mobile data system employing the slotted ALOHA protocol are investigated, with the emphasis on the unfairness between close-in and distant users due to the near-far effect. An analytical multi-group model is developed to evaluate both the user and the network performance of the mobile slotted ALOHA network under two classes of retransmission control policies, namely the Uniform Policy and the Nonuniform Policy. The Uniform Policy requires that all users adopt the same retransmission probability, whereas the Nonuniform Policy allows more distant users to have larger retransmission probability in order to compensate for the unfairness caused by the near-far effect. The performance of a slotted ALOHA network with a linear topology in a Rician fading channel under the two policies is compared by the multi-group model and simulation. The Nonuniform Policy is found to be more effective in alleviating the unfairness of user throughput over a wider range of the data traffic load than the Uniform Policy, which is effective only when the data traffic load is very light. Thus, a mobile data network can enjoy the network performance improvement derived from the near-far effect while the unfairness between close-in and distant users can be greatly mitigated without resorting to power control.