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This work investigates a random access (RA) game for a time-slotted RA system, where $N$ players choose a set of slots of a frame and each frame consists of $M$ multiple time slots. We obtain the pure strategy Nash equilibria (PNEs) of this RA game, where slots are fully utilized as in the centralized scheduling. As an algorithm to realize a PNE (Pure strategy Nash Equilibrium), we propose an Exponential-weight algorithm for Exploration and Exploitation (EXP3)-based multi-agent (MA) learning algorithm, which has the computational complexity of $O(N N_{\max }^{2} T)$. EXP3 is a bandit algorithm designed to find an optimal strategy in a multi-armed bandit (MAB) problem that users do not know the expected payoff of each strategy. Our simulation results show that the proposed algorithm can achieve PNEs. Moreover, it can adapt to time-varying environments, where the number of players varies over time. [ABSTRACT FROM AUTHOR]

Coded Slotted ALOHA (CSA) is a random access scheme based on the application of packet erasure correcting codes to transmitted packets and the use of successive interference cancellation at the receiver. CSA has been widely studied and a common assumption is that interference cancellation can always be applied perfectly. In this paper, we study the design of CSA protocol, accounting for a non-zero probability of error due to imperfect interference cancellation (IC). A classical method to evaluate the performance of such protocols is density evolution, originating from coding theory, and that we adapt to our assumptions. Analyzing the convergence of density evolution in asymptotic conditions, we derive the optimal parameters of CSA, i.e., the set of code selection probabilities of users that maximizes the channel load. A new parameter is introduced to model the packet loss rate of the system, which is non-zero due to potential IC errors. Multi-packet reception (MPR) and the performance of 2-MPR are also studied. We investigate the trade-off between optimal load and packet loss rate, which sheds light on new optimal distributions that outperform known ones. Finally, we show that our asymptotic analytical results are consistent with simulations obtained on a finite number of slots. [ABSTRACT FROM AUTHOR]

Massive Machine-type Communications (mMTC) is one of the principal features of the 5th Generation and beyond (5G+) mobile network services. Due to sparse but synchronous MTC nature, a large number of devices tend to access a base station simultaneously for transmitting data, leading to congestion. To accommodate a large number of simultaneous arrivals in mMTC, efficient congestion control techniques like access class barring (ACB) are incorporated in LTE-A random access. ACB introduces access delay which may not be acceptable in delay-constrained scenarios, such as, eHealth, self-driven vehicles, and smart grid applications. In such scenarios, MTC devices may be forced to drop packets that exceed their delay budget, leading to a decreased system throughput. To this end, in this paper a novel delay-aware priority access classification (DPAC) based ACB is proposed, where the MTC devices having packets with lesser leftover delay budget are given higher priority in ACB. A reinforcement learning (RL) aided framework, called DPAC-RL, is also proposed for online learning of DPAC model parameters. Simulation studies show that the proposed scheme increases successful preamble transmissions by up to $75 \%$ while ensuring that the access delay is well within the delay budget. [ABSTRACT FROM AUTHOR]

In this paper, variable measurement interval is proposed for channel-adaptive random access with discontinuous channel measurement. After each channel measurement, the next measurement interval is adaptively determined based on previous channel measurement. Results show that receiver power consumption for channel measurement can be reduced with variable measurement interval under the same average transmission delay constraint compared with fixed measurement interval. [ABSTRACT FROM AUTHOR]

As one of the three main application scenarios of 5 G cellular system, massive Machine-Type Communications (mMTC) has been regarded as the key solution to facilitate the IoT paradigm. One major bottleneck for accommodating mMTC is the severe congestion at the cellular random access channel when plenty of Machine-Type Devices (MTDs) send access requests concurrently while the preamble resources are limited. To remedy this issue, limiting the number of retransmissions and dropping access requests after the limit is reached can be an effective approach. Yet, the effect of the preamble retransmission limit $K$ on the optimal access performance of mMTC in cellular networks remains largely unexploited, which motivates the study in this paper. Specifically, in this paper, we start by characterizing the network steady-state points based on the limiting probability of successful transmission of access requests. We then obtain explicit expressions of the access throughput and the mean access delay of successfully-transmitted access requests as functions of $K$ and the number of preambles $M$. The maximum access throughput and the corresponding optimal backoff window size are further derived. It is shown that the maximum access throughput is independent of $K$ , while the mean access delay can be significantly reduced with a small $K$ , yet, at the expense of increased request dropping ratio. In addition, to improve both the throughput and delay performance, the analysis shows that more preambles should be allocated but the performance gain becomes marginal when $M$ is large. Therewith, an algorithm is proposed for determining the least number of preambles $M^\ast$ that maximizes the access throughput and the preamble resource utilization ratio. Numerical results show that a smaller preamble retransmission limit $K$ can further reduce $M^\ast$. [ABSTRACT FROM AUTHOR]

Integrated satellite-terrestrial communications networks aim to exploit both the satellite and the ground mobile communications, thus providing genuine ubiquitous coverage. For 5 G integrated low earth orbit (LEO) satellite communication systems, the timing advance (TA) is required to be estimated in the initial random access procedure in order to facilitate the uplink frame alignment among different users. However, due to the inherent characteristics of LEO satellite communication systems, e.g., wide beam coverage and long propagation delays, the existing 5 G terrestrial uplink TA scheme is not applicable in the satellite networks. In this paper, we investigate location-based TA estimation for 5 G integrated LEO satellite communication systems. We obtain the time difference of arrival (TDOA) and frequency difference of arrival (FDOA) measurements in the downlink timing and frequency synchronization phase, which are made from the satellite at different time instants. We propose to take these measurements for either UE geolocation or ephemeris estimation, thus calculating the TA value. The estimation is then formulated as a quadratic optimization problem whose globally optimal solution can be obtained by a quadratic penalty algorithm. To reduce the computational complexity, we further propose an alternative approximation method based on iteratively performing a linearization procedure on the quadratic equality constraints. Numerical results show that the proposed methods can approach the constrained Cramér-Rao lower bound (CRLB) of the TA estimation and thus assure uplink frame alignment for different users. [ABSTRACT FROM AUTHOR]

In this paper, we investigate stability region of hybrid uplink non-orthogonal multiple access (NOMA) in conjunction with non-cooperative game, where two users independently send packets with transmit power control according to fading channel gain to the same radio resource so that their packets are received at a base station (BS) with one of two target receive powers (TRPs). The BS then tries to decode the packets with successive interference cancellation (SIC) technique in power domain. We consider a symmetric non-cooperative game where the users (or players) send packets with one of two TRPs or no transmission. We examine the convexity of stability region at the mixed-strategy Nash equilibrium (NE) of the game, where no one's queue grows to infinity. Furthermore, we investigate how the stability region changes depending on the parameters of users’ payoff. [ABSTRACT FROM AUTHOR]

Massive Internet of Things (mIoT) is a major use case of the fifth generation (5G) wireless systems. mIoT aims to support a large number of connection requests from IoT devices. However, the conventional Long Term Evolution (LTE) random access procedure hinders the support of mIoT due to the limited number of available preambles. In this paper, we propose to aggregate two Zadoff-Chu preamble sequences from two different roots to obtain a larger set of preambles by considering all possible combinations of preamble sequence pairs. Decoding the aggregate preambles is challenging because the receiver needs to decode two preamble sequences where each one is allocated half of the transmit power. We propose two receiver architectures for preamble decoding. The first one is a threshold-based receiver which only requires minor changes to the LTE preamble receiver architecture. The second proposed preamble decoder architecture exploits a deep neural network. Simulations show that the proposed aggregate preamble design results in a lower service time for backlogged IoT devices compared to existing collision avoidance techniques. Moreover, the proposed receiver architectures can decode the aggregate preambles with low probabilities of misdetection and false alarm (less than 11%), especially in the high signal-to-noise ratio (SNR) regime. [ABSTRACT FROM AUTHOR]

We consider the problem of uplink timing synchronization for Orthogonal Time Frequency Space (OTFS) modulation based systems where information is embedded in the delay-Doppler (DD) domain. For this, we propose a novel Random Access (RA) preamble waveform based on OTFS modulation. We also propose a method to estimate the round-trip propagation delay between a user terminal (UT) and the base station (BS) based on the received RA preambles in the DD domain. This estimate (known as the timing advance estimate) is fed back to the respective UTs so that they can advance their uplink timing in order that the signal from all UTs in a cell is received at the BS in a time-synchronized manner. Through analysis and simulations we study the impact of OTFS modulation parameters of the RA preamble on the probability of timing error, which gives valuable insights on how to choose these parameters. Exhaustive numerical simulations of high mobility scenarios suggests that the timing error probability (TEP) performance of the proposed OTFS based RA is much more robust to channel induced multi-path Doppler shift when compared to the RA method in Fourth Generation (4G) systems. [ABSTRACT FROM AUTHOR]

Code division multiplexing (CDM)-based random access, wherein a set of sequences is reserved for random access and a remote station transmits a random access packet using a sequence randomly selected from this set, is employed in numerous practical wireless systems. If multiple remote stations simultaneously transmit random access packets using the same sequence, the performance degrades owing to sequence collision. In addition, the performance is degraded because of interference if multiple remote stations simultaneously transmit random access packets using different sequences. Therefore, the performance of CDM-based random access is dependent on both sequence collision and interference. There have been no previous studies on analyzing the performance of CDM-based random access considering both sequence collision and interference. In this paper, the throughput of CDM-based random access is investigated considering both sequence collision and interference based on a signal-to-interference-plus-noise ratio (SINR) capture model. Analyses and numerical simulations are performed to compare the throughputs of several random access schemes, including conventional and channel-adaptive random access. The results show that channel-adaptive random access can achieve significantly higher throughput than that achieved by conventional random access. Moreover, the results demonstrate that it is possible to analyze the trade-off between the throughput and implementation complexity with the increased the number of sequences. [ABSTRACT FROM AUTHOR]

In this paper, we propose a spatial group based optimal uplink power control scheme to enhance the performance of random access (RA) in satellite networks. In the proposed scheme, satellite machine-type terminals (SMTs) are divided into multiple groups based on their spatial locations in a distributed fashion, and the target received power levels of SMTs are the same in the same group but different from group to group by means of uplink power control, which enables the received power diversity. The received power levels of SMTs in different groups are designed by taking into account their path losses, which effectively reduces the energy consumption of SMTs. The threshold based capture model at the gateway demodulator is adopted, and the packet loss ratio (PLR) based on this model is derived by using density evolution analysis. An optimization problem is formulated and solved to jointly find the optimal number of power levels and the optimal values of power levels, which maximizes the RA system throughput. Simulation results show that the proposed scheme can obtain considerable performance improvement with respect to the conventional schemes in terms of normalized throughput, PLR, and average energy consumption. [ABSTRACT FROM AUTHOR]

This work proposes a random access system with uplink non-orthogonal multiple access (NOMA), where users transmit their packets by using the space-time line code (STLC) to fully exploit the spatial diversity without channel state information at the receiver. More specifically, users with a single antenna estimate channel gain from the base station (BS) with two antennas, encode the information using the STLC, and then transmit it using transmit power control so that the received power of their packets at the BS can be one of the predetermined values. The BS decodes the received packets via STLC decoding and the successive interference cancellation technique. Owing to spatial diversity, we show that the proposed scheme improves energy efficiency in comparison to the conventional system with a single antenna BS. [ABSTRACT FROM AUTHOR]

In this paper, we propose an enhanced random access (RA) with a distributed pilot orthogonalization (ERA-DPO) for mitigating packet collisions in Step3 during the RA procedure in cellular internet-of-things (IoT) networks. Even though each IoT device experiences a preamble collision in Step1, it can avoid the packet collision in Step3 by differentiating uplink pilot with the proposed technique, which provides an opportunity for the collided packets to be separated in spatial domain by multiple receive antennas. We also mathematically analyze the performance of the proposed scheme in terms of packet collision probability, throughput, and resource efficiency, based on a Markov chain model. Simulation results reveal that the proposed scheme significantly reduces the packet collision probability and increases resource efficiency compared to the conventional scheme. [ABSTRACT FROM AUTHOR]

In this paper, performance is investigated for a random access packet composed of a preamble and message. To minimize the error probability of a packet with channel-adaptive random access, the optimum energy allocation is obtained between the preamble and the message. The performances with the optimum and sub-optimum energy allocations are compared by simulation. [ABSTRACT FROM AUTHOR]

Satellite Communication (SatCom) systems are a promising solution to extend and complement terrestrial networks in un-/under- served areas, as reflected by several recent commercial and standardisation endeavours. Recently, 3GPP initiated a Study Item for 5 G (New Radio, NR) Non-Terrestrial Networks (NTN) to foster the integration of SatCom in future 5 G systems. After the definition of the system architecture and main design parameters, the focus is currently on the feasibility assessment of NR PHY/MAC layer procedures when a satellite channel is involved. In this letter, we propose a flexible methodology to design the beam footprint taking into account any source of differential delay between User Terminals (UTs) as, e.g., Random Access (RA) and Timing Advance (TA) procedures. In the numerical assessment, we compare the obtained beam footprint sizes with those currently being considered within 3GPP NTN studies, showing that larger/smaller dimensions can be assumed when considering the RA and TA procedures. [ABSTRACT FROM AUTHOR]

This paper proposes a distributed energy-harvesting random access protocol called harvest-or-access by applying slotted ALOHA to wireless powered communication networks (WPCNs). From the fact that there are a considerable number of idle slots during which any nodes do not attempt transmission in slotted ALOHA, the idle slots are made available for wireless energy transfer (WET). If any random access (RA) slot is recognized as an idle slot, the hybrid access point (HAP) opportunistically performs the WET in this slot. The wireless devices (WDs) can harvest energy for every idle slot and attempt random access by using this harvested energy. An asymptotic throughput analysis in a high signal-to-noise ratio (SNR) environment with a sufficient number of accessing WDs shows that there is an optimal number of RA slots allocated to maximize throughput and this optimal number of RA slots depends only on the average of the minimum SNRs of the WDs without knowledge of full channel state information. Monte Carlo simulation results further show that the proposed harvest-or-access protocol outperforms the conventional harvest-then-transmit-based random access protocols especially when the number of WDs is large. [ABSTRACT FROM AUTHOR]

In this paper, we propose a compressive random access (CRA) scheme using multiple resource blocks (RBs) to support massive connections for machine type communications. The proposed CRA scheme is scalable. As a result, if the number of devices increases, more RBs can be added to support them. Thanks to multiple RBs, we can employ fast retrial between RBs for re-transmissions of collided packets, which can result in short access delay. For stable CRA with fast retrial, we derive conditions (with a rate control scheme), and analyze the steady-state performance to find the throughput and delay. Through analysis and simulation results, we can see that the proposed scheme can perform better than conventional multichannel ALOHA and enjoy a tradeoff between the performance and complexity in terms of the number of RBs. [ABSTRACT FROM AUTHOR]

Different from a long-term evolution (LTE) system that monopolizes a licensed channel, the standalone LTE system in an unlicensed spectrum (SA LTE-U) is not allowed to occupy the unlicensed channel uninterruptedly, the starting point of its channel occupancy cannot be determined in advance, and its maximum channel occupancy time is restricted. Therefore, user equipments (UEs) may be backlogged during the channel unoccupancy time in their random access (RA) procedure, which causes severe collision during the channel occupancy time and unnecessary retransmissions of preambles in an SA LTE-U system. To address the aforesaid issues, we propose a user grouping and effective window size based RA protocol. In this protocol, the UEs are divided into several groups, and at any time only one group is activated and allowed for its UEs to send RA attempt, which avoids the intergroup UEs’ collision. The backlogged UEs of the activated group are scattered over multiple RA slots for them to send RA attempt, which alleviates the intragroup UEs’ collision. In addition, to avoid the unnecessary retransmissions of preambles caused by the channel uncertainty, the transmission of RA attempt and the reception of RA response are performed on the effective channel that consists of the channel occupancy periods only. The expressions of success and dropping rates, cumulative distribution function of preamble transmission times, utilization of carrying RA slots, and RA delay are derived. An optimization problem is formulated to maximize the utilization of carrying RA slots to obtain the optimal number of carrying RA slots. Simulation results demonstrate that the proposed protocol outperforms the baseline protocol in SA LTE-U systems. [ABSTRACT FROM AUTHOR]

Contention resolution diversity slotted ALOHA (CRDSA) is a promising solution to meet the challenge of designing efficient random access in future wireless networks. In this paper, we consider CRDSA with transmit power diversity where each packet copy from a device is transmitted at a randomly selected power level. This results in interslot received power diversity, which is exploited by employing a signal-to-interference-plus-noise ratio based successive interference cancellation (SIC) receiver. Leveraging edge-weighted bipartite graph representation, we propose a novel graph-based message passing algorithm to model the SIC decoding. We derive an expression to characterize the recovery-error probability of the scheme. We also formulate and solve an optimization problem to determine the optimal transmit power distribution. The results show that by enhancing the capture effect, the optimal transmit power distribution leads to considerable performance improvement. [ABSTRACT FROM AUTHOR]

This correspondence paper proposes two nonorthogonal random access (NORA) techniques for 5G mobile communication networks, where user equipments (UEs) make use of the channel inversion technique such that their received power at the base station (BS) can be one of the two target values. It enables the BS to decode two packets simultaneously with the successive interference cancelation (SIC) technique if a different power level is chosen. We propose two NORA systems; that is, UEs choose one of the two target power levels based on the channel gain or the region where they are. The performance of the proposed systems is analyzed in terms of access delay, throughput, and energy efficiency. Through analysis and extensive computer simulations, we show that the maximum throughput of the proposed NORA techniques can exceed 0.7, which is a significant improvement compared to the maximum throughput of conventional random access $ 0.368$. [ABSTRACT FROM AUTHOR]

Discontinuous channel measurement can reduce the power consumption of a remote station receiver and enhance the link performance of channel-adaptive random access. Previous research has been limited to the special case when all discontinuous channel measurements are independent of each other. In this paper, performance is investigated for channel-adaptive random access with discontinuous and correlated channel measurements. The performance is evaluated on the detection performance, the transmission delay, and the receiver power consumption of channel-adaptive random access. Results show that the analyses provide accurate performance estimations of channel-adaptive random access. As the period for discontinuous channel measurement decreases, the transmission delay of random access decreases, while the link performance degrades. Therefore, it is important to consider the tradeoff between the link performance and the transmission delay to design an efficient channel-adaptive random access scheme. [ABSTRACT FROM AUTHOR]

In this paper, two-stage random-access-based massive Internet-of-things uplink transmission is investigated. In this scheme, aggregator nodes are additionally deployed to relay packets to deal with a massive number of arrivals. A user node determines whether to utilize aggregator nodes according to the received signal strength from base stations and aggregator nodes. If the user node selects and transmits a packet to an aggregator node, the aggregator node stores and forwards the received packet to the base station. The use of aggregator nodes can reduce the per-hop delay as well as the user energy consumption by reducing the transmission distance from user nodes. However, in the two-stage random access protocol, an increase in the number of aggregator nodes incurs collisions in the wireless medium, and hence, increases the queueing delay at the aggregator node. Thus, improved radio resource allocation and network design are required to reduce additional delay in the two-hop uplink system. We, therefore, focus on optimizing the transmission probabilities of transmitting nodes and the aggregator node density to minimize the delay in two-stage random access networks. [ABSTRACT FROM PUBLISHER]

Recently, compressive random access has been considered for machine-type communications (MTC) as it has a potential to support massive connectivity by exploiting the sparsity of device activity. However, its performance limitations are not well studied yet compared with other well-known candidates for MTC, e.g., multichannel ALOHA. In this paper, we investigate the stability of compressive random access with a controlled access probability strategy under ideal conditions, and derive the maximum stable throughput, which shows that the maximum stable throughput of compressive random access is higher than that of multichannel ALOHA by a factor of 2. We also carry out simulations to see the performance of compressive random access under more realistic conditions with low-complexity multiuser detection (MUD) algorithms. Based on analysis and simulation results, we can claim that compressive random access can support more devices than conventional multichannel ALOHA and is well-suited for MTC. [ABSTRACT FROM AUTHOR]

About 60-GHz millimeter wave (mmWave) unlicensed frequency band is considered as a key enabler for future multi-Gbps WLANs. IEEE 802.11ad (WiGig) standard has been ratified for 60-GHz wireless local area networks (WLANs) by only considering the use case of peer to peer (P2P) communication coordinated by a single WiGig access point (AP). However, due to 60-GHz fragile channel, multiple number of WiGig APs should be installed to fully cover a typical target environment. Nevertheless, the exhaustive search beamforming training and the maximum received power-based autonomous users association prevent WiGig APs from establishing optimal WiGig concurrent links using random access. In this paper, we formulate the problem of WiGig concurrent transmissions in random access scenarios as an optimization problem, and then we propose a greedy scheme based on (2.4/5 GHz) Wi-Fi/(60 GHz) WiGig coordination to find out a suboptimal solution for it. In the proposed WLAN, the wide coverage Wi-Fi band is used to provide the control signalling required for launching the high date rate WiGig concurrent links. Besides, statistical learning using Wi-Fi fingerprinting is utilized to estimate the suboptimal candidate AP along with its suboptimal beam direction for establishing the WiGig concurrent link without causing interference to the existing WiGig data links while maximizing the total system throughput. Numerical analysis confirms the high impact of the proposed Wi-Fi/WiGig coordinated WLAN. [ABSTRACT FROM PUBLISHER]

In cognitive radio (CR) networks, cooperative spectrum sensing is utilized to improve the sensing performance to avoid potential interference to primary users (PUs) and increase spectrum access opportunities for secondary users (SUs). A cooperative spectrum sensing process is divided into three phases: individual sensing/detection, reporting/fusion, and data transmission. In the reporting phase, one or more reporting channels are needed to transmit individual sensing results to a fusion center (FC), and global spectrum sensing results are determined at the FC. The number of required reporting channels depends on the number of spectrum sensors or SUs, which relates to reporting channel efficiency and channel scheduling complexity. That is to say, the reporting channel design can be a challenge, especially when fixed assignment scheduling is used. Therefore, in this paper, we design a reporting channel scheme based on random access protocols, including slotted Aloha and reservation-Aloha. Performance evaluations in terms of PU detection probabilities and false alarm probabilities considering the proposed reporting channels are presented. In addition, the impact of soft/unquantized spectrum sensors or detectors (SUs) and malicious SUs is considered in this paper. Analytical and simulation results illustrate the effectiveness of the proposed reporting channel scheduling methods in improving the cooperative spectrum sensing performance. [ABSTRACT FROM AUTHOR]

In this paper, we consider random-access wireless multihop networks, with multipacket reception capabilities, where multiple flows are forwarded to the gateways through node disjoint paths. We explore the issue of allocating flow on multiple paths, exhibiting both intra- and interpath interference, in order to maximize average aggregate flow throughput (AAT) and provide bounded packet delay. A distributed flow allocation scheme is proposed where allocation of flow on paths is formulated as an optimization problem. Through an illustrative topology, it is shown that the corresponding problem is nonconvex. Furthermore, a simple but accurate model is employed for the AAT achieved by all flows, which captures both intra- and interpath interference through the signal-to-interference-plus-noise ratio (SINR) model. The proposed scheme is evaluated through Ns2 simulations of several random wireless scenarios. Simulation results reveal that the model employed accurately captures the AAT observed in the simulated scenarios, even when the assumption of saturated queues is removed. Simulation results also show that the proposed scheme achieves significantly higher AAT for the vast majority of the wireless scenarios explored than for the following flow allocation schemes: one that assigns flows on paths on a round-robin fashion, one that optimally utilizes the best path (BP) only, and another one that assigns the maximum possible flow on each path. Finally, a variant of the proposed scheme is explored, where interference for each link is approximated by considering its dominant interfering nodes only. [ABSTRACT FROM PUBLISHER]

With the potential to generate numerous connection requests, an explosive growth in the volume of data traffic and the number of mobile and machine-to-machine devices has drawn new attention to the radio access network. Surging random-access attempts cause not only severe preamble collisions but downlink resource shortage as well, thus degrading the performance of the random-access procedure. However, the effect of downlink resource shortage on system performance has yet to be comprehensively studied. In addition, most existing random-access contention resolution mechanisms sacrifice random-access channel (RACH) throughput for a high success probability, and thus, the price is that low-throughput mechanisms need a lot of time to deal with access attempts. In this paper, we evaluate the medium-access-control (MAC)-level performance for the four-step random-access procedure in Long-Term Evolution (LTE) systems, both with and without constrained downlink resources. Furthermore, we propose a novel RACH contention resolution scheme, i.e., the dynamic backoff (DB) scheme. DB can achieve high RACH throughput, yielding a high random-access success probability under various RACH overloaded scenarios. [ABSTRACT FROM AUTHOR]