Slotted ALOHA System Research Articles

Performance Analysis of Connection Establishment Procedure Under Beamforming in 5G NR Networks

High demand for reliability, low latency, seamless connectivity lead to the 5 G era. 5 G cellular networks employ a random access based preamble transmission procedure for the initial locking of User Equipment (UEs) to a base station (gNB) and uplink resource scheduling. It is of utmost importance that we evaluate the factors that aid and those that limit the random access procedure in achieving its envisaged goal. In this paper, we develop an analytical model for random access procedure for connection establishment in 5 G when beamforming is employed. The random access medium is modelled as a multi-channel slotted Aloha system. We use an equilibrium point analysis framework to derive the performance metrics namely, the rate at which UEs can lock on to a gNB and the average and variance of the time taken by a new UE to establish connection with the base station. We analyse the impact of the retransmission limit and the number of available preambles for random access on the performance of connection establishment of a UE. Our analysis brings out the importance of the choice of base station configurations with respect to the user demographics of a 5 G NR cell.

Two-Tier Multi-Rate Slotted ALOHA for OWC/RF-Based IoT Networks

We consider a massive Internet of Things (IoT) scenario where indoor IoT devices access the network via optical wireless communication (OWC) IoT systems that relay data via a backhaul radio frequency (RF) low-power wide-area network (LP WAN). We propose a novel two-tier multi-rate Slotted ALOHA (SA) system model to design and analyse such hybrid OWC/RF networks. For a particular hybrid OWC/RF setup, we present an in-depth numerical investigation that provides insights into the proposed two-tier multi-rate hybrid OWC/RF SA system design.

An Uplink Random Access Scheme Based on ALOHA System Assisted by Gain Division Multiple Access

Gain division multiple access (GDMA) is a non-orthogonal multiple access (NOMA) technique for uplink communications. In GDMA, the receiver employs both the magnitudes and phases of channel gains associated to the users to separate users. In the power-domain NOMA, the receiver employs the power levels associated to users to separate users.We apply the GDMA concept together with orthogonal frequency division multiple access (OFDM) to combat the collision problem in the ALOHA system. Our goal is to minimize the complexity of the devices (users) required for coordination and to obtain high throughput simultaneously in the random access protocol. For the proposed unslotted OFDM-GDMA-ALOHA, each user randomly chooses a Zadoff-Chu (ZC) sequence as its preamble for the transmitted packet so that the receiver can simultaneously obtain the information for timing and the information for coarse channel estimation. Moreover, fine channel estimation is obtained by using a cluster-based method. Simulation for a coded unslotted OFDM-GDMA-ALOHA system with BPSK modulation demonstrates that the obtained throughput performance can be better than that obtained by the slotted ALOHA system without using GDMA.We also provide simulation for slotted OFDM-GDMA-ALOHA, which shows that very high throughput can be achieved if the synchronization of slots can be obtained.

Time-Offset ALOHA With SIC

Internet-of-Things (IoT) applications for real-time control gradually increase and become computationally demanding. To provide better quality-of-service (QoS) in random access (RA) system based on slotted ALOHA (S-ALOHA), this work proposes a novel S-ALOHA system with cross-slot successive interference cancellation (SIC). To facilitate SIC, we design each slot with several time offsets (TOs) and one packet transmission time, where the length of overall TOs is a fraction of a packet transmission time. Users (re)transmit at the boundary of a TO randomly selected. This enables the base station (BS) to distinguish who makes the first and last transmissions in a collision slot and ask immediate retransmissions from them in the subsequent one or two slots. With these retransmitted packets, the BS performs SIC for the previously collided packets. We analyze the system throughput and the distribution of RA delay. The results show that the proposed system can achieve throughput from 0.5 (packets per packet transmission time) at minimum to 0.856 at maximum, depending on the number of TOs and the length of TO. In addition, to run this system stably, we propose a Bayesian-optimized backoff algorithm that enables users to use throughput-optimal (re)transmission probability. It is demonstrated that the proposed backoff algorithms can achieve the throughput close to genie-aided (GA) system.

Short-Packet Transmission in Irregular Repetition Slotted ALOHA System Over the Rayleigh Fading Channel

Random access systems are potential for Internet of Things in the future wireless communication network for its operational simplicity. Irregular repetition slotted ALOHA (IRSA) system is one of the high-efficiency random access systems. In this paper, performance analysis of the irregular repetition slotted ALOHA systems with short-packet, i.e. finite-blocklength, transmission for the quasi-static Rayleigh fading channel is given. A cumulative distribution function of signal-to-interference power ratio (SIR) is derived and thus a closed-form expression of an average packet error probability (PEP) at the SIR with short packets for the Rayleigh fading channel is given. The closed-form expression makes it possible to optimize the degree distributions at a specific blocklength in the sense that the systems give the maximum system load.

S-ALOHA Systems With Successive Transmission: Emulating CSMA System

In slotted-ALOHA (S-ALOHA) system, users should contend for the channel to transmit a packet in their queue every single time, irrespective of how many packets they have in their queue. The maximum throughput of S-ALOHA system is known as $e^{-1}\approx 0.3679$ (packets/slot) for a large number of users. This work proposes a novel S-ALOHA system, where the user with a successful (re)transmission of the packet at the head of queue is allowed to keep on transmitting his packet with some reservation probability if his queue is not empty. We call this system S-ALOHA system with successive transmission (ST) and show that the throughput of the proposed system becomes one packet per slot as the reservation probability is raised. We analyze the stability region of S-ALOHA systems with ST for two heterogeneous users and also investigate the stability condition of the system with $N$ symmetric users. Finally, we develop a Bayesian-learning backoff algorithm, with which users can control their (re)transmission with real-time estimation on the backlogged users. It is shown that the system with the proposed backoff algorithm can achieve the theoretical limit of the throughput.

Direct-to-Satellite IoT Slotted Aloha Systems with Multiple Satellites and Unequal Erasure Probabilities.

Direct-to-satellite Internet of Things (IoT) solutions have attracted a lot of attention from industry and academia recently, as promising alternatives for large scale coverage of a massive number of IoT devices. In this work, we considered that a cluster of IoT devices was under the coverage of a constellation of low-Earth orbit (LEO) satellites, while slotted Aloha was used as a medium access control technique. Then, we analyzed the throughput and packet loss rate while considering potentially different erasure probabilities at each of the visible satellites within the constellation. We show that different combinations of erasure probabilities at the LEO satellites and the IoT traffic load can lead to considerable differences in the system’s performance. Next, we introduce an intelligent traffic load distribution (ITLD) strategy, which, by choosing between a non-uniform allocation and the uniform traffic load distribution, guarantees a high overall system throughput, by allocating more appropriate amounts of traffic load at different positions (i.e., different sets of erasure probabilities) of the LEO constellation with respect to the IoT cluster. Finally, the results show that ITLD, a mechanism with low implementation complexity, allows the system to be much more scalable, intelligently exploiting the potential of the different positions of the satellite constellation.

Poisson Receivers: A Probabilistic Framework for Analyzing Coded Random Access

In this paper, we develop a probabilistic framework for analyzing coded random access. Our framework is based on a new abstract receiver (decoder), called a Poisson receiver, that is characterized by a success probability function of a tagged packet subject to a Poisson offered load. We show that various coded slotted ALOHA (CSA) systems are Poisson receivers. Moreover, Poisson receivers have two elegant closure properties: (i) Poisson receivers with packet routing are still Poisson receivers, and (ii) Poisson receivers with packet coding are still Poisson receivers. These two closure properties enable us to use smaller Poisson receivers as building blocks for analyzing a larger Poisson receiver. As such, we can analyze complicated systems that are not possible by the classical tree evaluation method. In particular, for CSA systems with both spatial diversity and temporal diversity, we can use the framework of Poisson receivers to compute the exact (asymptotic) throughput. We demonstrate that our framework can be used to provide differentiated services between ultra-reliable low-latency communication (URLLC) traffic and enhanced mobile broadband (eMBB) traffic. By conducting extensive simulations, we also verify that our theoretical results match extremely well with the simulation results.

Covert Communications in Multi-Channel Slotted ALOHA Systems

The fundamental limits of covert communication, where a message is sent from transmitter Alice to intended recipient Bob without detection by an attentive adversary warden Willie, has been considered extensively in recent years at the physical layer. The covert throughput depends critically on the warden's understanding of the characteristics of the radio environment and the type of receiver that he employs, and, as expected, the throughput increases when the warden has some uncertainty about the environment or some non-idealities in his receiver. In this paper, we consider the covert throughput when the adversary is only able to observe the medium access control (MAC) layer in a wireless communication system. In particular, given that the system has a rate of <inline-formula><tex-math notation="LaTeX">$\lambda$</tex-math></inline-formula> packets per slot transmitted over <inline-formula><tex-math notation="LaTeX">$n$</tex-math></inline-formula> channels by allowable system users, we study the allowable rate <inline-formula><tex-math notation="LaTeX">$\lambda _a$</tex-math></inline-formula> by covert users while maintaining covertness from an attentive warden observing the channel status in a slotted ALOHA system. We characterize performance for wardens with different abilities to discern the number of packets on a given channel, ranging from simple receivers that detect only whether there was a packet present to complicated receivers that can determine the number of packets involved in any collision, and also consider intended recipients Bob with varying abilities to perform multi-packet reception. In contrast to prior work in covert communications, the application considered motivates the consideration of results for finite (often small) observation vector lengths <inline-formula><tex-math notation="LaTeX">$n$</tex-math></inline-formula> at the adversary. Numerical results are provided both to illustrate the tightness of our achievability regions for the packet transmission rate of the covert transmitters and to demonstrate the covert throughput of the system as a function of <inline-formula><tex-math notation="LaTeX">$\lambda$</tex-math></inline-formula> and <inline-formula><tex-math notation="LaTeX">$n$</tex-math></inline-formula> .

Offered load estimation in random access multipacket perception systems using the expectation-maximization algorithm

This paper considers offered load estimation in probabilistic dynamic frame slotted ALOHA systems where the receiver can perceive multiple concurrent packets in a slot. This type of random access protocol finds current application in the areas of machine-to-machine communications and tag processing in RFID communications. The perception of multiple concurrent packets happens when a receiver is able to detect the number of received packets or to decode them. In the first case, we talk about multiple packet detection (MPD) whereas in the second we refer to multiple packet reception (MPR). The finite detection capability of MPD or separation capacity of MPR implies that, ideally, whenever the number of simultaneously received packets exceeds a given value M, a collision results and no information about the packets or their number can be extracted. Hence, we are confronted with an incomplete data estimation problem. We reformulate the problem so as to apply the expectation-maximization (EM) algorithm and derive the required iterative equations to obtain a maximum likelihood estimator for the offered load. A numerical example of the algorithm's execution is included and application scenarios are indicated. Simulation studies are presented to show that the proposed estimation method compares favorably with alternative techniques.

On receiver diversity for grant-free based machine type communications

Grant-free access schemes are steadily gaining momentum for emerging machine type communications and internet of things applications. Relying on variations of the classical ALOHA protocol to allow a vast population of users share a common channel in an uncoordinated fashion, these solutions have recently been embedded in international standards as well as proprietary commercial products. In this context, the use of multiple low-complexity receivers that act as relays, collecting packets from users and forwarding them towards a central unit represents an interesting approach to increase the efficiency of the system, which finds natural application in both terrestrial and non-terrestrial settings. To gain insights on the potential of such a configuration, this paper investigates a two-tier slotted ALOHA system under an erasure channel model. Closed form expressions for both end-to-end throughput and packet loss rate are derived for an arbitrary number of receivers, and the role of correlation among wireless channels connecting users to relays is thoroughly discussed. Non-trivial tradeoffs and systems design hints are highlighted, pinpointing some fundamental benefits and limitations of receiver diversity for grant-free based machine type communications.

Impact of Segmentation and Capture on Slotted Aloha Systems Exploiting Interference Cancellation

The performance of the widely adopted slotted Aloha (SA) scheme has been recently improved thanks to the introduction of novel mechanisms, including interference cancellation (IC), packet segmentation, and slot slicing. The combined effect of these mechanisms in the presence of capture has however not yet deeply investigated, even if the resulting impact on the network behavior is determinant for properly quantifying the achievable throughput. To deal with this issue, this paper analyzes the influence of capture on a framed SA (FSA) system adopting IC, segmentation, and slicing, by considering a reliable decoding criterion that accounts for the actually experienced signal to interference-plus-noise ratio. A theoretical model is developed to evaluate the capture probability in fast and slow Rayleigh fading conditions, deriving closed-form expressions for the interference-limited case. The IC-based FSA throughput is then estimated adopting a Markov chain approach validated by Monte Carlo simulations. Finally, the performance of an actual system using a quadrature phase-shift keying modulation in conjunction with a turbo encoder is compared with that estimated by adopting the considered decoding criterion, in order to check its applicability to practical communication networks.

Impact of Backoff Algorithm on IoT over Multichannel Slotted Aloha System

The increasing trend of Internet of Things brings new challenges to efficiently manage the resource in the access network. As a prominent solution to serve IoT services, LTE incorporates a small data transmission scheme for this reason. In this scheme, the overall system performance is greatly influenced by its random access procedure. Hence, studying the behavior of backoff during random access procedure is important. This work implements an iterative contending-user estimation model to analyze the performance of the contention-based random access procedure supporting finite-population. For the sake of generality, the system is modeled as a multichannel slotted Aloha. This allows our model to be used in wider specifications, including LTE and WiMAX systems. The behavior and performance of the system supporting multiple load scenarios is studied under different network loads. The simulation result demonstrates the accuracy of our proposed method to predict the normalized throughput, the packet-dropping probability, and the average access delay of each access-class. The performance of different backoff algorithm to resolve the collision in the system is compared and evaluated.

Revisiting Two-User S-ALOHA Games

This letter considers non-cooperative slotted ALOHA (S-ALOHA) games, in which two users contend for the channel by taking one of two actions, i.e., transmit or wait. We obtain an achievable (expected) payoff region of the games with mixed strategy and its Nash equilibria (NE) as a closed form, in which two users want to fulfill their own payoff demand. We further examine the correlated equilibria (CE) of this game and the corresponding payoff region. Finally, by visualizing the achievable payoff region with NE and CE, we discuss how to improve the performance of S-ALOHA systems.

Modeling and Analysis of Slotted ALOHA Systems With Energy Harvesting Nodes and Retry Limit

We analyze the performance of slotted ALOHA systems with energy harvesting nodes and the retry limit by developing a node-centric two-dimensional discrete-time Markov chain (DTMC) model. We consider not only the number of energy packets consumed when a data packet is transmitted but also a general probability distribution of an energy packet arrival process. We assume that the capacities of data and energy buffer at a node are one packet and $E$ packets, respectively. According to the concept of the equilibrium point analysis, we derive a fixed point equation with respect to the ratio of nodes transmitting a data packet. The accuracy of theoretical results obtained from the fixed point equation is verified by computer simulation. Numerical results under Poisson arriving process of energy packets indicate that throughput, the offered traffic, and the discard probability demonstrate the weak symmetric relationship between the data packet generation probability and the average number of arriving energy packets. However, the average transmission delay rapidly increases when the average number of arriving energy packets becomes small.

Distributed Transmission Control in Multichannel S-ALOHA for Ad-Hoc Networks

Wireless ad-hoc networks are marked without a basic infrastructure and global information for the current status of the network. For multichannel slotted ALOHA systems of wireless ad-hoc networks, this letter proposes a (re)transmission control algorithm for the transmitters to employ. The algorithm aims at minimizing Lyapunov (negative) drift on the sum of all transmitters’ queue, while it is implemented in a distributed way such that each transmitter only needs local information, such as the number of idle channels. To control (re)transmission probability optimally, with the local information, each transmitter solves a subproblem of estimating the number of backlogged nodes in each time slot. Simulation results show that our proposed algorithm yields near-optimal queuing performance.

On Frame Asynchronous Coded Slotted ALOHA: Asymptotic, Finite Length, and Delay Analysis

We consider a frame asynchronous coded slotted ALOHA (FA-CSA) system for uncoordinated multiple access, where users join the system on a slot-by-slot basis according to a Poisson random process, and in contrast to standard frame synchronous CSA (FS-CSA), users are not frame-synchronized. We analyze the performance of FA-CSA in terms of packet loss rate and delay. In particular, we derive the (approximate) density evolution that characterizes the asymptotic performance of FA-CSA when the frame length goes to infinity. We show that, if the receiver can monitor the system before anyone starts transmitting, a boundary effect similar to that of spatially coupled codes occurs, which greatly improves the iterative decoding threshold. Furthermore, we derive tight approximations of the error floor (EF) for the finite frame length regime, based on the probability of occurrence of the most frequent stopping sets. We show that, in general, FA-CSA provides better performance in both the EF and waterfall regions as compared to FS-CSA. Moreover, FA-CSA exhibits better delay properties than FS-CSA.

Unequal Error Protection in Coded Slotted ALOHA

We analyze the performance of coded slotted ALOHA systems for a scenario where users have different error protection requirements and correspondingly can be divided into user classes. The main goal is to design the system so that the requirements for each class are satisfied. To that end, we derive analytical error floor approximations of the packet loss rate for each class in the finite frame length regime, as well as the density evolution in the asymptotic case. Based on this analysis, we propose a heuristic approach for the optimization of the degree distributions to provide the required unequal error protection. In addition, we analyze the decoding delay for users in different classes and show that better protected users experience a smaller average decoding delay.

On Bistable Region of Slotted-ALOHA Systems with Delay Constraints

Bistable behavior of slotted ALOHA systems has been extensively investigated. It is proved that bistable region can be eliminated by limiting the number of allowable transmission attempts per packet to eight or less. In this paper, we investigate bistable behavior of slotted ALOHA systems with delay constraints in terms of the catastrophe theory. A lifetime of a packet is confined within D slots, that is, a packet will be discarded after timer of D time-slots expired, unless packet transmission succeeds. Letting r be the packet transmission probability in a slot, we indicate that there exists bistable region, if the product Dr is greater than approximately 8.30, and that the system can operate with the unique stable equilibrium point, if Dr is less than approximately 8.30.

Game-Theoretic Queue-Stabilizing Backoff Algorithm in S-ALOHA and CSMA Systems

This letter investigates a queue-stabilizing backoff algorithm for slotted ALOHA and carrier-sense multiple access systems from game-theoretic perspective. Since queue stability of each user could be achieved when user’s throughput is greater than its mean packet arrival rate, every user tries to achieve its throughput up to a certain target level, by playing a non-cooperative game of determining a (re)transmission probability. We characterize Nash equilibrium points (NEPs) of this game and the convergence of a distributed backoff algorithm for users to employ to reach the NEPs.