On the Stability of Two-User Slotted ALOHA with Channel-Aware and Cooperative Users

The stability region of the two-user slotted ALOHA system is analyzed under two scenarios: the case with channel- aware transmission control and the case with user cooperation. By exploiting these advantages in the physical layer, we show that the system throughput can be significantly increased with cross- layered designs on the medium access control (MAC) protocol that take into consideration the properties of these technologies. Suppose that the users transmit over a wireless fading channel and that the channel state information (CSI) is available to the users at the beginning of each time slot. We first study the case where each user transmits with a transmission probability that depends on their local CSI. In this case, we show that the optimal transmission control takes on the form of a thresholding function where each user transmits with probability 1 if the channel is above a certain quality; and with probability 0, otherwise. The stability region of the slotted ALOHA system is significantly enlarged with the use of channel-aware transmission control functions. Secondly, we study the case with user cooperation, where each user is allowed to receive and relay the information of the other user. We derive inner bounds on the stability region for cooperative system and show the significant improvements over the case without cooperation.