Random Access Protocol Research Articles

A Game Theoretic Approach to Irregular Repetition Slotted Aloha

Many technological enhancements are being developed worldwide to enable the “Internet of Things” (IoT). IoT networks largely rely on distributed access of billions of devices, but are still lagging in terms of combined reliability and low latency. To mend that shortcoming, it is paramount to adapt existing random access methods for the IoT setting. In this article, we shed light on one of the modern candidates for random access protocols fitted for IoT: the “Irregular Repetition Slotted ALOHA” (IRSA). As self-managing solutions are needed to overcome the challenges of IoT, we study the IRSA random access scheme in a distributed setting where groups of users, with fixed traffic loads, are competing for ALOHA-type channel access. To that aim, we adopt a distributed game-theoretic approach where two classes of IoT devices learn autonomously their optimal IRSA protocol parameters to optimize selfishly their own effective throughput. Through extensive simulations, we assess the notable efficiency of the game based distributed approach. We also show that our IRSA game attains the Nash equilibrium (NE) via the “better reply” strategy, and we quantify the price of anarchy in comparison with a centralized approach. Our results imply that user competition does not fundamentally impact the performance of the IRSA protocol.

A statistical model checking approach to analyse the random access protocol

Mobile cellular networks are becoming the most important technology in the telecom industry, and this made them a preferred subject for research and development of new hardware and software systems. In order to check the validity of these systems, one can use either a simulation or formal methods. Recently, new emerging methods have been proposed as alternative solutions, such as Statistical Model Checking (SMC). In this paper, we present a comprehensive framework based on SMC that could be used to analyse the cellular network protocol Random-Access Procedure (RAP), by using UPPAAL. We model the system using a simplified network of timed automata, we check the validity of our model by running some concrete simulations and after that we perform a formal verification of some properties of the protocol. Finally, the statistical approach, SMC, is used to study the performance of the system.

A statistical model checking approach to analyse the random access protocol

A statistical model checking approach to analyse the random access protocol

User Activity Detection for Irregular Repetition Slotted Aloha Based MMTC

Irregular repetition slotted aloha (IRSA) is a grant-free random access protocol for massive machine-type communications, where a large number of users sporadically send their data packets to a base station (BS). IRSA is a completely distributed multiple access protocol: in any given frame, a small subset of the users, i.e., the active users, transmit replicas of their packet in randomly selected resource elements (REs). The first step in the decoding process at the BS is to detect which users are active in each frame. To this end, a novel Bayesian user activity detection (UAD) algorithm is developed, which exploits both the sparsity in user activity as well as the underlying structure of IRSA-based transmissions. Next, the Cramer-Rao bound (CRB) on the mean squared error in channel estimation is derived. It is empirically shown that the channel estimates obtained as a by-product of the proposed UAD algorithm achieves the CRB. Then, the signal to interference plus noise ratio achieved by the users is analyzed, accounting for UAD, channel estimation errors, and pilot contamination. The impact of these non-idealities on the throughput of IRSA is illustrated via Monte Carlo simulations. For example, in a system with 1500 users and 10% of the users being active per frame, a pilot length of as low as 20 symbols is sufficient for accurate user activity detection. In contrast, using classical compressed sensing approaches for UAD would require a pilot length of about 346 symbols. The results also reveal crucial insights into dependence of UAD errors and throughput on parameters such as the length of the pilot sequence, the number of antennas at the BS, the number of users, and the signal to noise ratio.

On the Impact of Channel Estimation on the Design and Analysis of IRSA Based Systems

Irregular repetition slotted aloha (IRSA) is a distributed grant-free random access protocol where users transmit multiple replicas of their packets to a base station (BS). The BS recovers the packets using successive interference cancellation. In this paper, we first derive channel estimates for IRSA, exploiting the sparsity structure of IRSA transmissions, when non-orthogonal pilots are employed across users to facilitate channel estimation at the BS. Allowing for the use of non-orthogonal pilots is important, as the length of orthogonal pilots scales linearly with the total number of devices, leading to prohibitive overhead as the number of devices increases. Next, we present a novel analysis of the throughput of IRSA under practical channel estimation errors, with the use of multiple antennas at the BS. Finally, we theoretically characterize the asymptotic throughput performance of IRSA using a density evolution based analysis. Simulation results underline the importance of accounting for channel estimation errors in analyzing IRSA, which can even lead to 70% loss in performance in severely interference-limited regimes. We also provide novel insights on the effect of parameters such as pilot length, SNR, number of antennas at the BS, etc, on the system throughput.

Enabling Grant-Free URLLC: An Overview of Principle and Enhancements by Massive MIMO

Enabling ultrareliable low-latency communication (URLLC) with stringent requirements for transmitting data packets (e.g., 99.999% reliability and 1-ms latency) presents considerable uplink transmission challenges. For each packet transmission over dynamically allocated network radio resources, the conventional random access protocols are based on a request-grant scheme. This induces excessive latency and necessitates reliable control signaling, resulting in overhead. To address these problems, grant-free (GF) solutions are proposed in the fifth-generation (5G) new radio (NR). In this article, an overview and vision of the state of the art in enabling GF URLLC are presented. In particular, we first provide a comprehensive review of NR specifications and techniques for URLLC, discuss underlying principles, and highlight impeding issues of enabling GF URLLC. Furthermore, we briefly explain two key phenomena of massive multiple-input–multiple-output (mMIMO) (i.e., channel hardening and favorable propagation) and build several deep insights into how celebrated mMIMO features can be exploited to address the issues and enhance the performance of GF URLLC. Moving further ahead, we examine the potential of cell-free (CF) mMIMO and analyze its distinctive features and benefits over mMIMO to resolve GF URLLC issues. Finally, we identify future research directions and challenges in enabling GF URLLC with CF mMIMO.

MSPA: Multislot Pilot Allocation Random Access Protocol for mMTC-Enabled IoT System

To provide massive connectivity in massive machine-type communications (mMTCs) for the Internet of Things (IoT) system, a novel grant free random access protocol, called multislot pilot allocation (MSPA) is proposed in this article, where the user equipments (UEs) are permitted to jointly transmit randomly chosen pilot sequences along with their data packets over multislot to resolve intracell pilot collision. In addition, by utilizing the belief propagation tool for the MSPA protocol, the closed-form expressions to the access failure probability (AFP) and system throughput in a finite length regime are derived, which are highly desired for practical-interest mMTC network. Further, a guideline for certain mMTC scenarios that target urgent serving requirement UEs is also proposed to minimize the access latency and maximize the system throughput under diverse AFP constraints. Finally, the parametrical analysis of the MSPA protocol is given by theoretical proof and simulation verification, which shed light on the advantages of our MSPA protocol over the existing protocols in terms of achieving high throughput and shortening the access latency.

Optimal distributed filtering for nonlinear saturated systems with random access protocol and missing measurements: The uncertain probabilities case

The saturated-constrained distributed filtering problem is discussed for a class of discrete nonlinear systems with random access protocol (RAP) and missing measurements (MMs) under uncertain missing probabilities (UMPs). In order to prevent data congestions, the RAP is adopted to regulate the information transmission, where only one sensor is scheduled to have the communication right and transmit the information at each time step through a shared network. In addition, the phenomenon of MMs is considered, in which the UMPs are characterized by means of the nominal means and error bounds. The main purpose of this paper is to design a distributed filter under saturation constraint such that, for both RAP and MMs under UMPs, there exists an upper bound (UB) matrix of filtering error covariance and the corresponding distributed filter gain is constructed to minimize the trace of the received UB matrix by resorting to the matrix simplifying technique. Besides, the filtering algorithm performances are discussed regarding the boundedness and monotonicity of the presented distributed filtering scheme from the mathematical perspective. Finally, some comparisons are provided to illustrate the validity of the proposed time-varying distributed filtering algorithm.

MODERN RADIO TECHNOLOGIES FOR M2M COMMUNICATION

The aim of the study is to analyze the functioning of the PRACH system in overload conditions, when a large number of M2M devices with different priorities establish their connection to the access network. Fifth-generation (5G) systems provide increased data rates, use of low-latency over-the-counter (URLLC), have high spectral efficiencies, extensive connectivity, and increased energy efficiency. Multiplexing with orthogonal frequency division multiplexing and filter (F-OFDM) is a signal received at the output of the filter. F-OFDM compared to OFDM provides a reduction in out-of-band radiation, which improves system performance. The article is devoted to the comparison of CP-OFDM and F-OFDM signals. Connecting multiple M2M devices to the network in a short period of time can cause a surge in PRACH channel loading due to its limited capacity. It is also interesting to note that the PRACH preambles, randomly selected by M2M devices, can be considered as code "channels" that do not overlap. When two or more terminals choose an identical preamble (channel), a situation resembling a typical collision between data packets occurs. This interpretation allows us to study the process of competition between M2M devices, based on the known results obtained for multi-channel random access protocols. As a result, it is possible to obtain an exact approximation in a given range of parameters of the PRACH system, such as the number of available preambles (M) and competing terminals (U), the size of the delay window, etc.

Opportunities of hybrid random access protocols for M2M communications in LTE/LTE-A networks

Machine-to-machine (M2M) communications on Long-term evolution (LTE) networks form a substantial part for the Internet-of-things (IoT). The random access procedure is the first step for M2M devices to access network resources. Many researchers have attempted to improve the efficiency of the random access procedure. This work revisits the performance of the hybrid random access protocols which combine congestion control techniques with collision resolution techniques. In particular, we investigate two hybrid protocols. The first one combines the pre-backoff (PBO) with tree random access (TRA), and the second one combines dynamic access barring (DAB) with TRA. The probability analysis is presented for both protocols. The performance is evaluated based on the access success rate, the mean throughput, the mean delay, the collision rate and the mean retransmissions. The simulation results show that the hybrid protocols achieve the highest success rate and throughput with moderate delay and low collision rates with a lower mean number of retransmissions compared to three benchmarks that apply either a congestion control or a collision resolution. The opportunities of future developments of hybrid protocols are listed at the end of this paper to highlight the issues that could be investigated to improve the performance of hybrid random access protocols.

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.

Random Multiple Access With Hierarchical Users

Classical random access protocols support prioritized user groups by allowing a high priority user to transmit with a relatively high probability. However, when a high priority user competes for the channel with a large number of low priority users, transmission success probability of the high priority user can still diminish to zero. In this paper, a distributed medium access control (MAC) framework is proposed to support hierarchical user groups in a random multiple access system in the following senses. First, when the number of primary users is small, the MAC framework guarantees that channel availability should stay above a pre-determined threshold no matter how many secondary users are competing for the channel. Second, when the number of primary users is large, the MAC framework drives transmission probabilities of the secondary users to zero but does not reject channel access to the primary users. These properties are achieved in a distributed environment without direct message exchange between users, without knowledge on the number of users, and without knowing whether each transmission should belong to a primary or a secondary user. The MAC framework is also extended to systems where each user can be equipped with multiple transmission options.

Massive Access in Space-Based Internet of Things: Challenges, Opportunities, and Future Directions

The integration of fifth generation (5G) mobile network and satellite communication promises a promotion on the interconnection of everything, and has become one of the key development directions of beyond 5G and even 6G. As an inevitable development trend of mobile communication, the space-based Internet of Things (S-IoT) is expected to be commercially available in building an integrated information infrastructure network of the space, air, ground and oceans, and so on. However, the tremendous growth in the number of connected user equipments poses a fundamental rethinking of conventional multiple access technologies, which is considered one of the open challenges in future S-IoT networks. In this article, we first review the main challenges of massive access in S-IoT. Then, we present and discuss how to develop a scalable massive access S–IoT network by leveraging introduced technologies, including intelligent hybrid non-orthogonal multiple access (NOMA) transmission, grant-free superimposed pilot NOMA random access and multi-slot pilot allocation random access protocol. Finally, some exciting future directions of massive access in S-IoT are revealed.

Man-in-the-middle attack against cyber-physical systems under random access protocol

This paper investigates the man-in-the-middle (MITM) attack against cyber-physical systems (CPSs) under the random access protocol (RAP) scheduling, where an attacker intercepts and modifies the transmitted data and then forwards them on to degrade the system performance. The RAP schedules the sensing devices to avoid data collisions, where only one node is allowed to access the shared communication channel at each time instant. Hence, it makes the existing stealthy attacks invalid. To overcome the protocol-induced effects, a novel attack model utilizing only part of the measurements at each time instant is proposed, based on which the strictly stealthy and ∊-stealthy attacks are designed. For strictly stealthy attack, the Kullback-Leibler divergence (KLD)-based stealthy constraint is converted into a linear matrix inequality and then a semi-definite program problem is constructed to obtain the optimal attack parameters. In such case, the attack performance is optimal but limited. Furthermore, an ∊-stealthy attack is proposed to achieve higher attack performance, where the analytical attack parameters are obtained by solving an off-line convex optimization problem. Finally, simulations are provided to verify the results.

Energy-Constrained Design of Joint NOMA-Diversity Schemes with Imperfect Interference Cancellation.

This study proposes a set of novel random access protocols combining Packet Repetition (PR) schemes, such as Contention Resolution Diversity Slotted Aloha (CRDSA) and Irregular Repetition SA (IRSA), with Non Orthogonal Multiple Access (NOMA). Differently from previous NOMA/CRDSA and NOMA/IRSA proposals, this work analytically derives the energy levels considering two realistic elements: the residual interference due to imperfect Interference Cancellation (IC), and the presence of requirements on the power spent for the transmission. More precisely, the energy-limited scenario is based on the relationship between the average available energy and the selected code modulation pair, thus being of specific interest for the implementation of the Internet of Things (IoT) technology in forthcoming fifth-generation (5G) systems. Moreover, a theoretical model based on the density evolution method is developed and numerically validated by extensive simulations to evaluate the limiting throughput and to explore the actual performance of different NOMA/PR schemes in energy-constrained scenarios.

Noise covariance estimation for networked linear systems under random access protocol scheduling

This paper investigates the noise covariance estimation problem for the networked linear system under communication protocol. To reduce network burden and avoid data collisions, the data transmission from the sensor nodes to the filter is scheduled by the random access protocol (RAP), where only a sensor node is permitted to send data at each transmission instant. Within the framework, the correlations of the innovations data are utilized to construct a least-squares problem to estimate the noise covariances, where a novel estimation form of the auto-covariance from measurement data is given. Moreover, the two cases of the process noise correlated and uncorrelated with the measurement noise are both discussed. It can be proved that the estimated covariances are unbiased and asymptotically converge to the true values as the sample size increasing. Finally, the simulation results are given to demonstrate the effectiveness of the presented method.

Moving Horizon Estimation of Networked Nonlinear Systems With Random Access Protocol

This paper is concerned with the moving horizon (MH) estimation issue for a type of networked nonlinear systems (NNSs) with the so-called random access (RA) protocol scheduling effects. To handle the signal transmissions between sensor nodes and the MH estimator, a constrained communication channel is employed whose channel constraints implies that at each time instant, only one sensor node is permitted to access the communication channel and then send its measurement data. The RA protocol, whose scheduling behavior is characterized by a discrete-time Markov chain (DTMC), is utilized to orchestrate the access sequence of sensor nodes. By extending the robust MH estimation method, a novel nonlinear MH estimation scheme and the corresponding approximate MH estimation scheme are developed to cope with the state estimation task. Subsequently, some sufficient conditions are established to guarantee that the estimation error is exponentially ultimately bounded in mean square. Based on that the main results are further specialized to linear systems with the RA protocol scheduling. Finally, two numerical examples and the corresponding figures are provided to verify the effectiveness/correctness of the developed MH estimation scheme and approximate MH estimation scheme.

On partially homogeneous nearest-neighbour random walks in the quarter plane and their application in the analysis of two-dimensional queues with limited state-dependency

This work deals with the stationary analysis of two-dimensional partially homogeneous nearest-neighbour random walks. Such random walks are characterized by the fact that the one-step transition probabilities are functions of the state-space. We show that stationary behaviour is investigated by solving a finite system of linear equations, two matrix functional equations, and a functional equation with the aid of the theory of Riemann (–Hilbert) boundary value problems. This work is strongly motivated by emerging applications in flow level performance of wireless networks that give rise in queueing models with scalable service capacity, as well as in queue-based random access protocols, where the network’s parameters are functions of the queue lengths. A simple numerical illustration, along with some details on the numerical implementation are also presented.

Collision-aware distributed detection with population-splitting algorithms

We consider the design of distributed detection algorithms for single-hop, single-channel wireless sensor networks in which sensor nodes send their local decisions to a fusion center (FC) by using a random access protocol. There is also limited time to collect local decisions before a final decision must be made. We thus propose and analyze a modified random access protocol in which the FC combines slotted ALOHA with a population-splitting algorithm called population-splitting-based random access (PSRA) and collision-aware distributed detection according to an estimate-then-fuse approach. Under the PSRA, only sensor nodes whose observations fall in a particular range of reliability will send their decisions in a specific frame by using slotted ALOHA. At the end of the collection time, the FC applies the collision-aware distributed detection to make a final decision. Here, the FC will first observe the state of each time slot—idle, successful, collision—in each frame, use this information to estimate the number of sensor nodes participating in each frame, and, then, compute a final decision using a population-based fusion rule. An approximation of the optimal transmission probability of the slotted ALOHA is determined to minimize the probability of error. Numerical results show that, unlike slotted-ALOHA-based data networks, the transmission probability maximizing the number of successful time slots does not optimize the performance of the proposed distributed detection. Instead, the proposed distributed detection performs best with a transmission probability that induces many collisions.

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.