Throughput of slotted aloha with encoding rate optimization and multipacket reception

Abstract-This paper considers a slotted ALOHA random access system where users send packets to a common receiver with multipacket reception capability. A collection of m users access the shared medium independently of each other with probability p and, upon access, they choose an encoding rate. A collision occurs when the sum of the rates of all the users exceeds the capacity of the channel. We analytically characterize as a function of m and p the encoding rate which maximizes the expected global thoughput of the system. It is shown that for any value of p the throughput converges to one when m tends to infinity, hence there is no loss due to packet collisions. This is in striking contrast with the well known behavior of slotted ALOHA systems in which users cannot adjust the encoding rate. In that case the throughput decreases to zero as the number of users increases. Finally, assuming that users are selfish, we characterize the encoding rate which maximizes the expected individual throughput of each user, and show that the corresponding Nash equilibrium is not globally optimum

Medium Access Control Protocols With Memory

Many existing medium access control (MAC) protocols utilize past information (e.g., the results of transmission attempts) to adjust the transmission parameters of users. This paper provides a general framework to express and evaluate distributed MAC protocols utilizing a finite length of memory for a given form of feedback information. We define protocols with memory in the context of a slotted random access network with saturated arrivals. We introduce two performance metrics, throughput and average delay, and formulate the problem of finding an optimal protocol. We first show that a TDMA outcome, which is the best outcome in the considered scenario, can be obtained after a transient period by a protocol with (N-1)-slot memory, where N is the total number of users. Next, we analyze the performance of protocols with 1-slot memory using a Markov chain and numerical methods. Protocols with 1-slot memory can achieve throughput arbitrarily close to 1 (i.e., 100% channel utilization) at the expense of large average delay, by correlating successful users in two consecutive slots. Finally, we apply our framework to wireless local area networks.Comment: 32 pages, 7 figures, 2 table