The effect of Multipacket Reception (MPR) on stability and delay of slotted ALOHA based random access systems is considered. A general asymmetric MPR model is introduced and the MAC capacity region is specified. An explicit characterization of the ALOHA stability region for the two user system is given. It is shown that the stability region undergoes a phase transition from a concave region to a convex region bounded by lines as the MPR capability improves. It is also shown that after this phase transition, slotted ALOHA is optimal i.e., the ALOHA stability region coincides with the MAC capacity region. Further, it is observed that there is no need for transmission control when ALOHA is optimal i.e., ALOHA with transmission probability one is optimal. These results are extended to a symmetric N > 2 user ALOHA system, where it is shown that for a large class of symmetric MPR channels no transmission control is optimal from a stability viewpoint. This finding suggests that if the physical layer is even reasonably good, there is no need for sophisticated Medium Access Control protocols.Next, sufficient conditions for stability of slotted ALOHA for the asymmetric N > 2 case are provided.Finally, a complete characterization of average delay in capture channels for the two user system is given. In cases with non-zero capture probability, no transmission control is found to minimize delay for a subset of stable arrival rates. It is shown that in certain capture scenarios, no transmission control is delay-optimal for all stable arrival rates. Further, it is also shown that no transmission control is optimal for stability and delay simultaneously in the two user capture channel.
Abstract-An information theoretic queueing model is proposed in a wireless multiple access communication setup. The proposed symmetric N user model captures physical layer parameters such as the encoding rate, transmit power and Medium Access Control (MAC) layer metrics such as queue stability. Two alternative medium access strategies are considered: centralized scheduling and ALOHA. Next, a cross-layer approach is taken wherein the maximum stable throughput of the system is achieved by a joint optimization over the MAC parameters (viz., scheduling set size with scheduling and transmission probability with ALOHA) and the encoding rate. Performance comparisons with traditional layered designs are given. It is shown that in the low and high SNR regimes, layered designs are close to optimal whereas in the moderate SNR range, cross-layer designs outperform layered schemes. Exact characterizations of the "low" and "high" SNR regimes are given quantitatively. It is also shown that ALOHA with transmission probability one is optimal in the low SNR regime.
The problem of communicating sensor readings over a multinrcess channel for detecting a target is considered. A natural way of communication in target detection is to let sensors simultaneously transmit one of two predetermined frequency tones indicating whether the target is detected or not. Recently, this scheme has been generalized to consider non-hinary sensor observations by letting sensors simultaneously transmit orthogonal wavefarms depending on the value of their observations-Type-Based Multiple Access (TEZMA). TBMA was shown to be asymptotically oprimal in terms of detection-error probability under the idealistic assumptions that the sensor channel gains are identical, and the sensor data are conditionally independent and identically distributed (i.i.d.). In this paper, TBMA is analyzed in a more general framework by considering non-i.i.d. data and non-identical channel gains. An asymptotically optimal detector is proposed and its error-exponents for detection probabilities are characterized using tools from Large deviatiuns theoiy. Numerical simulations are used to demonstrate that the error exponents provide reasonably accurate estimates ofthe performance of TBMA.
Abstract-We consider the problem of stability of slotted ALOHA for a system consisting of N users communicating with a common receiver, which employs spatial diversity to receive multiple transmissions simultaneuosly. We introduce a general packet reception model to incorporate multiple packet receptions since the collision channel model is no longer valid in such a scenario. We characterize the stability region of slotted ALOHA for the two user case explicitly. We also provide a sufficient condition for stability of slotted ALOHA for the N > 2 case. Finally, we apply our results to a simple system in which two users communicate with a base station equipped with a linear antenna array. Using stability region as a performance measure, we compare four different receivers/beamformers viz. Matched Filter, Zero Forcing, pseudo-MMSE and true MMSE employed by the base station to receive information from the users.
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