ALOHA Random Access that Operates as a Rateless Code

Abstract

Various applications of wireless Machine-to-Machine (M2M) communications have rekindled the research interest in random access protocols, suitable to support a large number of connected devices. Slotted ALOHA and its derivatives represent a simple solution for distributed random access in wireless networks. Recently, a framed version of slotted ALOHA gained renewed interest due to the incorporation of successive interference cancellation (SIC) in the scheme, which resulted in substantially higher throughputs. Based on similar principles and inspired by the rateless coding paradigm, a frameless approach for distributed random access in slotted ALOHA framework is described in this paper. The proposed approach shares an operational analogy with rateless coding, expressed both through the user access strategy and the adaptive length of the contention period, with the objective to end the contention when the instantaneous throughput is maximized. The paper presents the related analysis, providing heuristic criteria for terminating the contention period and showing that very high throughputs can be achieved, even for a low number for contending users. The demonstrated results potentially have more direct practical implications compared to the approaches for coded random access that lead to high throughputs only asymptotically.Index Terms-random access protocols, slotted ALOHA, distributed rateless coding, successive interference cancellation, M2M communications 1 The idea of users transmitting multiple replicas of the same data in the frame, but without SIC, was originally proposed in [4].

An energy-efficient adaptive frameless ALOHA protocol

“…This comes from the inherent nature of SIC where suddenly a large number of users' transmissions are resolved by adding a single equation 4 . Further, the authors in [17] have shown that the avalanche point happens at the same time slot for both T I (m) and F R (m) most of the time. In addition, after the avalanche point, resolving the users' packets happens very slowly causing only marginal increment in T I (m) and F R (m).…”

“…Andor-tree evaluations [16] of the frameless slotted ALOHA in [17] show that T I (m) and F R (m) both have an avalanche behavior versus the number of elapsed time slots. That is, a sudden jump from a low value to a much higher value is observed at a specific time slot.…”

“…In this case, T I (m) reaches its maximum value, while F R (m) may not be at its peak. That said, the stopping criteria to terminate a contention round is when T I (m) = 1 or F R (m) reaches a threshold F. To ensure the maximum achievable throughput, in the original frameless ALOHA, F is found through a numerical optimization for different values of N as discussed in [17]. The optimized value for F is named F opt .…”

“…Both of these schemes aim at improving the energy efficiency of the original frameless ALOHA protocol [17] without compromising the throughput. In the following, we first give the general intuition behind our schemes and then provide the details of these schemes later in this section.…”

“…Note that in the original frameless ALOHA protocol [9,17], p i (m) = p init for any i and m, where p init is a fixed probability assigned to the users by the BS. The value of p init is determined based on BS's estimation from the number of contending users N in a contention round [17].…”