In cooperative systems, users achieve spatial diversity and multihop gains by transmitting packets over multiple independent fading paths provided by their partners. Most previous works on cooperative communications focus on the physical layer aspects such as coding, modulation, and transceiver signal processing techniques. In this work, we study the advantages of user cooperation from a MAC layer perspective and devise queueing strategies to exploit cooperative gains in random access networks. Based on the conventional slotted ALOHA protocol, we propose a simple cooperative transmission mechanism for a two-user cooperative pair. We derive the two-user stability region of the proposed system and show the improvements compared to noncooperative systems. The benefits can be attributed to both physical layer cooperation, where users with good channels may relay for those with bad channels, and MAC layer cooperation, where system parameters can be chosen to enhance cooperation and reduce competition. Then, we extend the proposed strategy to a finite-user system that consists of multiple cooperative pairs. By treating each pair as a single transmission entity, we derive inner bounds for the finite-user stability region and propose a ranking system to characterize the transmission entities' relative tendency of being stable (or unstable).

In this work, we study the finite-user stability region of a slotted ALOHA random access network with cooperative users. The network consists of multiple cooperating pairs where the users in each pair is allowed to cooperate by relaying each others' messages to the access point (AP). Spatial diversity gains are achieved since the packet of a cooperative user can be transmitted through independent fading channels, eg. the direct transmission path and the cooperative relaying path. Most works in the literature on cooperative communications focus on the physical layer aspects such as coding, modulation, transceiver signal processing etc. In this paper, we study the advantages of user cooperation from a MAC layer perspective and devise queueing strategies to exploit the cooperative diversity gains in a random access network. Extending upon our previous results for the two-user case, we study the stability of the finite-user cooperative system that consists of multiple cooperating pairs. By treating each cooperative pair as a transmission entity, we derive inner bounds for the finite-user stability region and propose a ranking system to characterize the transmission entities' relative tendency of being stable (or unstable).

In this work, we study the effect of imperfect channel-state information (CSI) on the stability and delay of a two-user channel-aware slotted ALOHA system. We assume that the channel is quantized into two states and channel estimation error may occur between channel states. By taking into account the estimation error, we derive the optimal transmission control function that adjusts the users' transmission probabilities based on the estimated channel state. To evaluate the performance, we derive the stability region and average delay of the system and show the loss in performance due to channel estimation errors.

In this work, we analyze the stability region of the slotted ALOHA random access network with cooperative users. With cooperation, the packets corresponding to each user can be transmitted to the destination through the multiple relaying paths formed by the other users. The diversity introduced by the multi-path relaying reduces significantly the probability of outage. We propose two queuing strategies for the cooperative system. In the first strategy, we assume that each user is equipped with two buffers: a source buffer, to record its own packets, and the relay buffer, to store the packets received from its partner. When both buffers are non-empty, the user chooses randomly among the two buffers based on a predetermined probability. An inner bound of the stability region is derived and is shown to outperform the non-cooperative system. In the second case, we assume that each user utilizes only one buffer to store all packets. The relaying packets are stored at the front of the queue at the cooperative user upon arrival. The second scheme outperforms the non-cooperative case only for certain channel conditions and arrival rates. However, the scheme has low complexity and can be implemented with little changes to the original system.