ALOHA System Research Articles

Effect of Variable Backoff Algorithms on Age of Information in Slotted ALOHA Networks

Effect of Variable Backoff Algorithms on Age of Information in Slotted ALOHA Networks

Peak Age of Information Optimization in Heterogeneous Aloha Networks

Peak Age of Information Optimization in Heterogeneous Aloha Networks

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.

Peak Age of Information in Slotted ALOHA Networks

Peak Age of Information in Slotted ALOHA Networks

Stability Region and Transmission Control of Multi-Cell Aloha Networks

As one of the most representative random-access schemes, slotted Aloha has been adopted in various wireless communication networks. A multi-cell Aloha network may easily become unstable as the inter-cell interference grows if the traffic input rates and transmission probabilities of nodes are not properly regulated. Yet how to stabilize a multi-cell Aloha network has remained largely unknown. To address the above open issue, an analytical framework is proposed in this paper for multi-cell Aloha networks to characterize the stability region of traffic input rates and operating region of transmission probabilities of nodes for achieving network stability. Specifically, the inter-cell interference level is captured by the overlapping ratio of each cell, and shown to be a key factor that determines the stability performance. For a two-cell Aloha network, the stability region of input rates and complete operating regions of transmission probabilities are obtained as functions of the overlapping ratios of cells. For the general <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> -cell case, a transmission control algorithm is further proposed to stabilize the network only based on the local information exchange between neighboring cells, with effectiveness demonstrated through simulations.

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.

Modeling age of information in a cooperative slotted Aloha network

Modeling age of information in a cooperative slotted Aloha network

Fairness-Constrained Rate Optimization of Multi-Link Slotted Aloha

This letter optimizes the data rate performance of multi-link slotted Aloha networks, in which Multi-Link Devices (MLDs) coexist with multiple groups of Single-Link Devices (SLDs), where different groups reside in distinct channels. MLDs can transmit/receive packets via multiple channels, causing interference to all groups of SLDs and severe rate unfairness issue. To ensure SLDs in different channels can acquire target ratios of data rate, explicit expressions of sum rate and group data rate are derived, based on which the optimal transmission probabilities of MLDs/SLDs for maximizing the sum rate without or with fairness constraints are obtained. The analysis reveals the coupling effect of the behavior of devices in different channels, the tradeoff between the optimal sum rate performance and the fairness, and the effect of channel fading on benefit of using multi-link transmission capability.

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.

Peak Age of Information Optimization of Slotted Aloha: FCFS Versus LCFS

This paper aims at optimizing the Peak Age of Information (PAoI) performance of slotted Aloha networks with two representative data queue service disciplines, i.e., first-come first-served (FCFS) and last-come first-served (LCFS). Specifically, by assuming a unit-size buffer and Bernoulli packet arrivals, the average PAoI are derived and minimized by properly tuning the packet arrival rate and the channel access probability of each sensor. Depending on whether the packet arrival rate can be tuned or not, the individual optimization and joint optimization are both considered. The optimal system parameters and corresponding minimum PAoI are explicitly characterized. The analysis shows that in the joint optimization case, the PAoI linearly increases with the network scale and the minimum PAoI with LCFS is 16.8% lower than that with FCFS in the massive access scenario. Yet, for achieving such performance gain over FCFS, each sensor with LCFS should generate a new sample in each time slot, indicating soaring energy consumption due to the sampling operation. The analysis shows that the energy efficiency with LCFS is always lower than that with FCFS, and the gap keeps increasing with the number of sensors, which reveals a clear age-energy tradeoff in terms of service disciplines.

Reinforcement Learning for NOMA-ALOHA Under Fading

We consider a non-orthogonal multiple access in a random-access ALOHA system, in which each user randomly accesses one out of different time slots and send uplink packets based on power differences. In the context of an asymmetric game, we propose a non-orthogonal multiple access ALOHA system based on multi-agent reinforcement learning tools that can help each user to find its best strategies of improving the rates of successful action choices. While taking into account not only collisions, but also fading, we analyze the mean rewards of actions under general settings and focus on the case that involves two different groups of users. To characterize the behaviors of accessing strategies, we apply multi-agent action value methods that consider either greedy or non-greedy actions, combined with an acceleration gradient descent. Our results show that in the proposed system, users employing the greedy action-based methods can be randomly divided into two groups of users and increase the rates of successful action choices. Interestingly, in relatively limited channels, such greedy methods turn many of users to be with a state of barring-access. In this case, the proposed acceleration, non-greedy action methods are shown to reduce such unfairness, at a loss of successful action rates.

When Aloha and CSMA Coexist: Modeling, Fairness, and Throughput Optimization

With the emerging unlicensed spectrum sharing and Machine-to-Machine communications, the coexistence performance of multiple devices with different access schemes operating at the same unlicensed bands has received significant research interests. As the two most representative random-access schemes, Aloha and Carrier Sense Multiple Access (CSMA) both found wide applications in unlicensed bands. Yet most of the studies have focused on the coexistence of CSMA-based networks, leaving the coexistence of Aloha-based and CSMA-based networks largely unexplored. The challenge originates from the lack of a coexistence model of slotted Aloha and CSMA. In this paper, the throughput performance of coexisting Aloha and CSMA networks is characterized and optimized by extending a unified analytical framework proposed for random-access networks. The analysis shows that different from the single-network case where the maximum throughput of CSMA is much higher thanks to carrier sensing, when Aloha and CSMA coexist, Aloha would significantly outperform CSMA if each network optimizes its own throughput performance without cooperation, leading to poor throughput performance for CSMA and severe unfairness. To achieve fair coexistence, inter-network cooperation is crucial. The optimal transmission probabilities of Aloha and CSMA for maximizing the total network throughput under a given throughput ratio are further derived, and applied to coexisting LTE Unlicensed and WiFi networks to optimize their coexistence performance.

Comprehensive Throughput Analysis of Unslotted ALOHA for Low-Power Wide-Area Networks

Unslotted ALOHA has been often employed by several low-power wide-area networks (LPWANs) for Internet of Things (IoT) as a random access (RA) protocol. This work analyzes the performance of unslotted ALOHA systems in terms of throughput and RA delay, and investigates their optimization. Our analysis consists of: 1) two- <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">heterogeneous</i> user case, whose backoff rate and packet length are different; 2) <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$N$ </tex-math></inline-formula> - <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">homogeneous</i> user case, whose backoff rate and packet length are identical; and 3) homogeneous users of infinite population model. In the two-user case, we investigate the throughput region of unslotted ALOHA by using a multiobjective optimization problem (MOOP) and derive the Laplace Stieltjes transform (LST) of the probability density function (PDF) of RA delay. For <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$N$ </tex-math></inline-formula> -homogeneous user case, we show how the throughput behaves according to the population size, packet length, and backoff rate. Our work may provide a comprehensive analytical framework for unslotted ALOHA systems.

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.

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.

Modeling and Online Adaptation of ALOHA for Low-Power Wide-Area Networks (LPWANs)

Unslotted ALOHA protocol has been adopted as a channel access mechanism in commercial low-power wide-area networks (LPWANs), such as Sigfox and long-range (LoRa) alliance. This work examines the throughput and random access (RA) delay distribution of unslotted ALOHA systems by considering exponential random backoff (ERB) or uniform random backoff (URB) algorithm. We further characterize the operating region of the systems as unsaturated stable, bistable, and saturated regions in terms of the new packet arrival and retransmission rates. To run the system stably with the maximum throughput, we propose a Bayesian online backoff algorithm that estimates the number of backlogged devices. Its performance is compared with other algorithms, such as particle filter (PF)-based algorithm, binary exponential backoff (BEB) algorithm, and the algorithm of exploiting exact backlog size information. Through extensive simulations, it is demonstrated that the performance of the proposed algorithm is very close to the upper bound and robust to time-varying traffic condition.

Delay and Peak-Age-of-Information of ALOHA Networks With Limited Retransmissions

This letter analyzes the impact of packet retransmissions on the performance of wireless random access protocols. In particular, we focus on the effects of two controllable parameters, namely the rate at which retransmissions occur and the number of allowed retransmissions. In a scenario where packets are generated according to a Poisson process and new packets substitute old ones upon arrival, we determine the conditions in which a given throughput or packet success probability constraint is achievable. We also prove, for a general packet success probability function of the traffic, that selecting the smallest feasible value of the number of allowed retransmissions along with an appropriate retransmission rate is optimal in terms of mean delay to transmit packets and mean <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Peak-Age-of-Information</i> . We illustrate this result with numerical examples.

Rate-Constrained Delay Optimization for Slotted Aloha

Slotted Aloha provides a simple way for accommodating the massive access of Machine-to-Machine (M2M) communications. Yet, the delay performance of slotted Aloha has long been observed to significantly deteriorate as the network size grows. It is therefore important to study how to optimize the delay performance of slotted Aloha in a large-scale network. This paper focuses on the optimization of access delay of a buffered slotted Aloha network, where n nodes transmit to a common receiver in fading channels. Specifically, by deriving the closed-form expressions of the network steady-state points in both unsaturated and saturated conditions, the first and second moments of access delay of each packet are obtained as explicit functions of system parameters, and minimized by optimizing the transmission probability of each node. The analysis shows that to achieve the minimum mean access delay, the transmission probability of each node should be reduced as the network size increases, leading to a diminishing node data rate unless the information encoding rate is jointly optimized. The minimum mean access delay for a given data rate requirement is further characterized, and effects of key parameters such as the minimum required data rate for each node, the mean received signal-to-noise ratio of each packet and the number of nodes on the rate-constrained minimum mean access delay are discussed. The practical insights of the analysis are also demonstrated by taking the example of an LTE-M system with smart grid applications.

Throughput analysis of 2-D OCDMA/pure ALOHA networks with access control and two user classes of variable length for multimedia traffic

In a previous research, the throughput of a 2-D optical code-division multiple-access (OCDMA)/unslotted ALOHA (U-ALOHA)/channel load sensing protocol network using an optical hard limiter and channel code was analyzed. This scheme assumed a fixed message length and one user class. However, the current and future themes of networks is multimedia traffic with variable message length and two different user classes of real-time and non-real-time. In this paper, we propose a 2-D OCDMA/U-ALOHA network with access control and two user classes of variable message length. We assume that the number of fixed-length packets in a message is geometrically distributed and perform access control by assigning two user classes with different access permission probabilities. The numerical results show the high priority user class (e.g. real-time data traffic) can maintain maximum system throughput under the heavy load at the expense of low priority user class (e.g. non-real-time data traffic). The proposed network protocol could obtain 100 Gbps and be a promising alternative for local area networks (LANs) and broadband optical access networks for larger capacity in response to fast growing of multimedia data traffic.

Nonorthogonal CSK/SS ALOHA system under MANET environment

Spread ALOHA systems that combine the spread spectrum techniques with the ALOHA system (a simple random access protocol without carrier sensing function) have been investigated as Mobile Ad-hoc Network (MANET) protocols. This paper presents the effect of the nonorthogonal Code Shift Keying Spread Spectrum (CSK/SS) ALOHA system on improving the throughput and delay performances of the spread ALOHA system under the MANET environment. The nonorthogonal CSK/SS system, which uses systematically-constructed nonorthogonal sequences, is one of the multilevel modulation systems for the spread spectrum technique with increasing the number of bits per frame. Optimum parameters can be used to obtain the best system performance; thus, this paper gives analytical expressions of the data transmission efficiency and delay property of the proposed scheme as a function of the system parameters under the assumption of no power control. Numerical results show that the nonorthogonal CSK/SS ALOHA system is useful in the MANET environment, compared with the conventional spread ALOHA systems.