Unsaturated Traffic Conditions Research Articles

Performance Analysis of IEEE 802.11p Protocol in IoV under Error-Prone Channel Conditions

The complexity of the channel condition in the Internet of vehicles (IoV) may increase the bit error rate (BER) of the intelligent vehicle terminals, resulting in data transmission failures or errors. Therefore, it is necessary to improve the performance of communication protocols under error-prone channel conditions. This article investigates the influence of the access categories’ (ACs) performance with channel errors by using four A C s to cause service differentiation based on the enhanced distributed channel access (EDCA) mechanism of the IEEE 802.11p. To address the error-prone characteristics of the channel and unsaturated traffic conditions, a three-dimensional Markov model is developed first, followed by an analysis of the characteristics and mutual transition probabilities of the seven classes of states in the model, and then, the steady-state equations of the system are developed to derive the steady-state distribution to study system performance. The model considers the backoff phase, the freezing of the backoff counter, the retransmission limit, the probability of collisions occurring, the size of the maximum and minimum contention window, and the number of interframe intervals. These parameters are chosen to meet the requirements for the protocol to operate, while also preventing overestimation of throughput and avoiding having packets being served all the time. We derive expressions for the throughput and delay of A C s under the conditions of error channels and unsaturated traffic. The impact of channel errors on the throughput and delay of A C s is evaluated by numerical simulation. Numerical results show that too many stations in the system will increase the average access delay and decrease throughput. The increase in BER will seriously decrease the performance of high-priority A C s . The throughput of the A C s is also modeled to vary with frame length and BER, and the variation curves of the optimal frame lengths for A C 3 and A C 2 were obtained.

A meta–reinforcement learning algorithm for traffic signal control to automatically switch different reward functions according to the saturation level of traffic flows

AbstractReinforcement learning (RL) algorithms have been widely applied in solving traffic signal control problems. Traffic environments, however, are intrinsically nonstationary, which creates a convergence problem that RL algorithms struggle to overcome. Basically, as a target problem for an RL algorithm, the Markov decision process (MDP) can be solved only when both the transition and reward functions do not vary. Unfortunately, the environment for traffic signal control is not stationary since the goal of traffic signal control varies according to congestion levels. For unsaturated traffic conditions, the objective of traffic signal control should be to minimize vehicle delay. On the other hand, the objective must be to maximize the throughput when traffic flow is saturated. A multiregime analysis is possible for varying conditions, but classifying the traffic regime creates another complex task. The present study provides a meta‐RL algorithm that embeds a latent vector to recognize the different contexts of an environment in order to automatically classify traffic regimes and apply a customized reward for each context. In simulation experiments, the proposed meta‐RL algorithm succeeded in differentiating rewards according to the saturation level of traffic conditions.

A Markov model for IEEE 802.15.4 MAC protocol with energy-efficient GTS utilization under saturated and unsaturated traffic conditions

In the present work, we have proposed an analytical Markov-chain model of the modified slotted Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) IEEE 802.15.4 Medium Access Control (MAC) protocol, which efficiently utilizes the Guaranteed Time Slots (GTSs) under saturated traffic conditions. The five dimensional Markov chain model based analysis is performed for variable Contention Access Period (CAP). Two constraints are introduced i.e. Global Utilization Factor (GUF) and Local Utilization Factor (LUF), to adjust the duration of CAP in each cycle. The work mainly focuses on utilization of GTS under saturated traffic condition for high traffic load applications. To evaluate the performance of our proposed model, we have compared the performance with Misic’s model of standard IEEE 802.15.4 MAC protocol. In addition, the analysis is carried out for practical networks where the data traffic is bursty in nature. The results reflect that our proposed algorithm improves the throughput of the IEEE 802.15.4 MAC protocol in energy-efficient manner at high traffic load applications.

Performance analysis of game based MAC protocol for cognitive radio based wireless network

A Cognitive Radio based Wireless Network (CRWN) is an emerging trend in wireless networks. Cognitive Radio (CR) has opened up many new opportunities to utilize the frequency spectrum to its fullest utilization. Many researchers and engineers are coming up with new protocols for CRWN. In this paper, existing MAC protocols for CRWN are studied and found that all the protocols are designed for optimized node performance but not the network performance. Moreover, individual node’s optimized performance may harm network’s performance. So, there is a gap of performance to be filled at an individual node and network-level. In this paper, the frame work of existing MAC protocol is modified to achieve the global optimized performance of CRWN under saturated and unsaturated traffic conditions. Also, majority of the existing work proposed their solutions assuming full information-based game theoretical models, whereas in this paper more realistic approach based on incomplete information is presented. A rigorous simulation study is carried out to compute various parameters such as channel capacity, throughput and delay to understand the limitations of the non-game theoretic approach. Later, a game theoretical framework to MAC protocol is applied for CRWN to improve the network performance in terms of these parameters. It is shown that the application of game theory to the MAC layer avoids collision and reduces the delay by 54% and energy consumption leading to enhancement of the overall throughput of a network by 57% respectively.

Restricted Access Window-Based Resource Allocation Scheme for Performance Enhancement of IEEE 802.11ah Multi-Rate IoT Networks

IEEE 802.11ah (Wi-Fi HaLow) is introduced to support Internet of things (IoT) applications. In this paper, we consider the IEEE 802.11ah multi-rate IoT network where stations (STAs) contend for channel access using group-based restricted access window (RAW) mechanism with the default uniform grouping scheme. RAW divides the channel time into various slots, STAs into different groups and assigns a dedicated RAW slot to each group of STAs. As uniform grouping scheme performs grouping on random basis, each group will comprise of distinct data rate STAs, which leads to severe degradation of throughput because of performance anomaly. We propose a novel RAW based resource allocation (RAW-RA) scheme which groups the STAs in a multi-rate IoT network based on data rates and allocates the RAW slots to the groups proportional to their data rates. We also develop an accurate analytical model with unsaturated traffic conditions for calculating the throughput and energy efficiency of the IEEE 802.11ah multi-rate IoT network with the RAW mechanism. From the analytical results, it is evident that the proposed RAW-RA scheme can resolve the performance anomaly as well as significantly improves the aggregate throughput and energy efficiency of the IoT network. Finally, all the analytical results are validated with extensive simulation studies.

QoS provisioning in energy-efficient cooperative networks with power assignment and relay deployment planning

The cooperative network is an energy-efficient technology to provide ubiquitous broadband access and quality-of-service (QoS). This paper investigates the tradeoffs among QoS, capacity, and power consumption in the cooperative networks. By adaptively selecting among the direct, relay, and cooperative transmission (TX) modes for users, we can enhance the capacity with lower transmission power. Furthermore, we determine the proper relay location and transmission powers of base station (BS) and relay station, so that more users can exploit the low-power two-hop TX modes to enhance the capacity and reduce the delay. In the real networks, if the packet queue is empty, the BS can be idle without sending (to save energy). Hence, we also properly increase the link capacity and idle probability to further reduce power consumption. We develop an analytical model to evaluate the delay, variance, capacity, idle probability, and actual power consumption in unsaturated traffic conditions. Then, we apply an optimization approach to analytically determine the optimal relay location and transmission powers, aiming to maximize the overall energy efficiency (EE) subject to the delay requirements. Numerical results show that the adaptive TX-mode selection scheme along with the joint power assignment and relay deployment planning can significantly enhance the capacity and reduce the power consumption. Then, the goal of QoS support can be achieved with higher EE.

A comparative analysis of the performance of turbo roundabouts based on geometric characteristics and traffic scenarios

ABSTRACT This study aimed to investigate the effect of geometric characteristics and traffic scenarios on the performance measures, including delay, volume to capacity (v/c) ratio, capacity, and the level of service (LOS) in basic and rotor turbo roundabouts. In addition, it attempted to compare turbo roundabouts using performance measures based on geometric characteristics and traffic scenarios. The results showed that performance measures reduced significantly by increasing the diameter of four-leg turbo roundabouts although they improved in three-leg turbo roundabouts. Further, four- and three-leg basic turbo roundabouts had a better performance by changing the balanced to imbalanced traffic flow scenario compared with four- and three-leg rotor turbo roundabouts. Additionally, substituting four- and three-leg basic turbo roundabouts for rotor four- and three-leg turbo roundabouts increased the performance measures. Overall, the basic turbo roundabouts had a better performance compared to the rotor turbo roundabout under unsaturated and saturated traffic conditions .

An Isolated Traffic Signal Design using a GA-based Optimization Technique

The Genetic Algorithm (GA) approach is an evolutionary optimization technique, which is developed based on the fundamental theories of natural selection and evolution. The present study focuses on the design of an isolated traffic signal for a two-phase intersection using the conventional method and GA-based optimization technique for unsaturated traffic flow conditions. The methodology of the study includes formulation of an objective function and the constraints, formulation of the constraint violation coefficient, formulation of the modified objective function, formulation of the fitness index (fi), and the GA operations to determine the best green signal timings. The intrinsic nature of Genetic Algorithms in performing elitism, which assures to carry forward the best solution identified in each generation to the next generation. An example problem was solved to demonstrate elitism using binary genetic algorithm with a single variable approach. In the GA operation, parent strings/chromosomes are selected and the crossover is performed along with mutation to form the new offspring’s. Mutation helps in avoiding the convergence of the solution to local optima. In this operation Fitness Index (FI) values of each strings/chromosomes are used as a measure to identify the parent strings to perform GA operations for the next generation. The results of the study indicate that the proposed technique can be used to optimize the signal timings of an isolated traffic signal, this will influence on reducing the delays at the junctions.

Signal control method and performance evaluation of an improved displaced left-turn intersection design in unsaturated traffic conditions

ABSTRACT This paper proposes an improved layout for the Displaced Left-turn (DLT) intersection by combining the conventional DLT intersection with protected left-turn phases, based on the safe-crossing requirements of pedestrians and non-motorized vehicles. The delay and stops are weighted to form an integrated performance index (PI) in a real-time vehicle-to-infrastructure communication environment. The PI models, pertaining to all vehicles in non-DLT lanes, DLT lanes and shared through-right lanes are established based on the improved DLT intersection in unsaturated traffic conditions. In addition, a dynamic optimized method of traffic signal timing parameters is constructed based on minimizing average PI per vehicle and considering the traffic signals’ coordination at the main intersection and left-turn crossovers. The operational performance of the optimized method is validated using data collected at an intersection in Harbin, China.

Parallel PPDU Transmission Mechanism for Wideband Wireless LANs

When the preambles of a wideband 802.11n/ac frame transmission are affected by a collision with a 20MHz frame on one of the subchannels, a receiver might fail to decode the whole wideband frame. Experimental evaluations with off-the-shelf 802.11n hardware showed that the vulnerability of wideband frame transmission to a narrowband collision/interference is a real problem. To mitigate the severe impact of narrowband collision/interference on wideband transmission, we propose to transmit several 20MHz frames in parallel instead of a single wideband frame so that a narrowband collision on one of the subchannels affects only the parallel frame transmitted on that subchannel. Performance evaluations demonstrated that the proposed parallel frame transmission scheme significantly improves the throughput and delay performance of wideband transmissions under different traffic and channel conditions. While the throughput performance of the proposed scheme for saturated traffic conditions was analytically validated, extensive simulation-based evaluations showed that the proposed scheme achieves great performance improvement for unsaturated traffic conditions and hidden node environment.

Distributed Triggered Access for BSM Dissemination in 802.11bd V2V Networks

In this paper, a novel channel access scheme, Distributed Triggered Access (DTA), is proposed for distributed V2V Basic Safety Message (BSM) dissemination in future platooning environment. To meet the stringent delay requirements of platooning communications, the proposed scheme is designed to use Wireless Local Area Network (WLAN) multi-user channel access in a distributed manner. The proposed scheme leverages advanced Medium Access Control (MAC) layer features, such as Triggered Uplink Access and Multi-user Request-To-Send (RTS), introduced in the 6th generation mainstream WLAN standard, IEEE 802.11ax, based upon a conceptual Physical (PHY) layer frame structure for IEEE 802.11bd, the successor of IEEE 802.11p. The proposed scheme is analyzed by mathematical model and simulations from transmission delay and successful transmission rate perspective. The mathematical model includes a Markov chain analysis that models the Enhanced Distributed Channel Access (EDCA) backoff procedure of DTA considering the effect of resumed backoff procedure with empty buffer caused by triggered access. Also, a Markov arrival/General service distribution/1 service channel (M/G/1) queuing model is provided to analyze the transmission delay of a BSM under unsaturated traffic condition. The extensive simulation results corroborate that DTA effectively improves the transmission success rate and reduces the average BSM collecting delay in a highly congested environment.

Enabling Green Wireless Sensor Networks: Energy Efficient T-MAC Using Markov Chain Based Optimization

Due to the rapidly growing sensor-enabled connected world around us, with the continuously decreasing size of sensors from smaller to tiny, energy efficiency in wireless sensor networks has drawn ample consideration in both academia as well as in industries’ R&D. The literature of energy efficiency in wireless sensor networks (WSNs) is focused on the three layers of wireless communication, namely the physical, Medium Access Control (MAC) and network layers. Physical layer-centric energy efficiency techniques have limited capabilities due to hardware designs and size considerations. Network layer-centric energy efficiency approaches have been constrained, in view of network dynamics and available network infrastructures. However, energy efficiency at the MAC layer requires a traffic cooperative transmission control. In this context, this paper presents a one-dimensional discrete-time Markov chain analytical model of the Timeout Medium Access Control (T-MAC) protocol. Specifically, an analytical model is derived for T-MAC focusing on an analysis of service delay, throughput, energy consumption and power efficiency under unsaturated traffic conditions. The service delay model calculates the average service delay using the adaptive sleep wakeup schedules. The component models include a queuing theory-based throughput analysis model, a cycle probability-based analytical model for computing the probabilities of a successful transmission, collision, and the idle state of a sensor, as well as an energy consumption model for the sensor’s life cycle. A fair performance assessment of the proposed T-MAC analytical model attests to the energy efficiency of the model when compared to that of state-of-the-art techniques, in terms of better power saving, a higher throughput and a lower energy consumption under various traffic loads.

Performance analysis of AUL FeLAA and WiFi coexistence in the presence of Rayleigh fading channel and capture effect under unsaturated traffic conditions

Performance analysis of AUL FeLAA and WiFi coexistence in the presence of Rayleigh fading channel and capture effect under unsaturated traffic conditions

Improving Performance by a Dynamic Adaptive Success-Collision Backoff Algorithm for Contention-Based Vehicular Network

The distributed coordination function (DCF) is the core of the IEEE 802.11 standard and is applied routinely to contention-based vehicular networks. Many existing backoff algorithms based on DCF are applied to develop the performance of wireless networks and most of them work in saturated traffic load conditions, which are unlikely to be valid in practical wireless network. The study of the backoff algorithm under unsaturated traffic load conditions is still an open issue. In this paper, a dynamic adaptive success-collision (DASC) backoff algorithm is proposed to improve the performance of contention-based vehicular network. As we know that the probability of collision reflects system performance, and the probability of collision is restricted by the variation of contention window ( $CW$ ) size and it depends on the traffic loads. A threshold is defined to judge the low or high traffic loads. In addition, the DASC backoff algorithm takes proactive measures to optimize throughput and delay through decreasing $CW$ after “ $S$ ” times consecutive successful transmissions and increasing $CW$ after “ $C$ ” times consecutive collisions. The simulation results indicate that the proposed DASC backoff algorithm provides better performance in terms of throughput and delay than dynamic control backoff time algorithm, exponential linear backoff algorithm, and binary exponential backoff algorithm.

Reinforcement Learning-Based Variable Speed Limit Control Strategy to Reduce Traffic Congestion at Freeway Recurrent Bottlenecks

The primary objective of this paper was to incorporate the reinforcement learning technique in variable speed limit (VSL) control strategies to reduce system travel time at freeway bottlenecks. A Q-learning (QL)-based VSL control strategy was proposed. The controller included two components: a QL-based offline agent and an online VSL controller. The VSL controller was trained to learn the optimal speed limits for various traffic states to achieve a long-term goal of system optimization. The control effects of the VSL were evaluated using a modified cell transmission model for a freeway recurrent bottleneck. A new parameter was introduced in the cell transmission model to account for the overspeed of drivers in unsaturated traffic conditions. Two scenarios that considered both stable and fluctuating traffic demands were evaluated. The effects of the proposed strategy were compared with those of the feedback-based VSL strategy. The results showed that the proposed QL-based VSL strategy outperformed the feedback-based VSL strategy. More specifically, the proposed VSL control strategy reduced the system travel time by 49.34% in the stable demand scenario and 21.84% in the fluctuating demand scenario.

Optimisation model for network progression coordinated control under the signal design mode of split phasing

An optimisation model for network progression coordinated control under the signal design mode of split phasing in unsaturated traffic conditions was proposed in this study. Considering the characteristics of the cyclical variation relationship between the bidirectional progression coordinated control effect and intersection spacing, a cosine function of the time difference between the actual green centre point and the ideal green centre point was defined as the evaluation index. The constraints of the common signal cycle, phase time, and phase sequence were introduced in the model, and the phase time allocation was discussed in this study. The model enables the phase time and phase sequence of each intersection to be freely valued within the solution space, and realises the comprehensive optimisation of the phase time, as well as common signal cycle, phase sequence, and offset. The case study demonstrates that the scheme optimised by this model can provide significant progression coordinated control effects for all the through movements on each artery. This method is effective when applied to network progression coordinated control.

Delay-based Passenger Car Equivalent at Signalized Intersections in Iran

Due to their different sizes and operational characteristics, vehicles other than passenger cars have a different influence on traffic operations especially at intersections. The passenger car equivalent (PCE) is the parameter that shows how many passenger cars must be substituted for a specific heavy vehicle to represent its influence on traffic operation. PCE is commonly estimated using headway-based methods that consider the excess headway utilized by heavy vehicles. In this research, the PCE was estimated based on the delay parameter at three signalized intersections in Tehran, Iran. The data collected were traffic volume, travel time for each movement, signalization, and geometric design information. These data were analysed and three different models, one for each intersection, were constructed and calibrated using TRAF-NETSIM simulation software for unsaturated traffic conditions. PCE was estimated under different scenarios and the number of approach movements at each intersection. The results showed that for approaches with only one movement, PCE varies from 1.1 to 1.65. Similarly, for approaches with two and three movements, the PCE varies from 1.07 to 1.99 and from 0.76 to 3.6, respectively. In addition, a general model was developed for predicting PCE for intersections with all of the movements considered. The results obtained from this model showed that the average PCE of 1.5 is similar to the value recommended by the HCM (Highway Capacity Manual) 1985. However, the predicted PCE value of 1.9 for saturated threshold is closer to the PCE value of 2 which was recommended by the HCM 2000 and HCM 2010.

Energy efficient medium access scheme for visible light communication system based on IEEE 802.15.7 with unsaturated traffic

Visible light communication (VLC) networks can provide high-rate transmissions with advantages such as high security, low power consumption and so on. Multiple users get access to the VLC network according to the carrier sense multiple access with collision avoidance (CSMA/CA) mechanism defined by IEEE 802.15.7 media access control layer specifications. In this study, the authors propose an energy efficient medium access scheme to improve the performance of IEEE 802.15.7 CSMA/CA mechanism under unsaturated traffic conditions. Aiming at reducing the power consumed on unnecessary waiting and at improving the channel utilisation efficiency, they first apply a successive transmission scheme on the node, which has completed one successful transmission and gets the chance of transmitting the next packet without backoff delay period according to the network condition. Then, a dynamic contention window is exploited to mitigate the collisions, where they derive the optimal minimum contention window based on throughput analysis. The power consumption can be reduced with improved throughput, when both the traffic load and the number of nodes in the network are taken into account. Simulation results demonstrate that the authors’ proposed scheme outperforms the IEEE 802.15.7 CSMA/CA scheme with respect to the average power consumption, system throughput and packet dropping probability.

Guest Editorial

Guest Editorial

Performance Analysis of Cooperative Communication in Decentralized Wireless Networks With Unsaturated Traffic

In this paper, we investigate the performance of cooperative communication in decentralized wireless networks under unsaturated traffic conditions with randomly positioned single-hop source-destination pairs and relays, where interference is the main performance-limiting factor. The traffic unsaturation and concurrent cooperative transmissions introduce a correlation between the interferer density and the packet retransmission probability, and a correlation of interference power in both space and time domains, which complicate the interference characterization. Based on queueing theory and stochastic geometry, the stationary interferer density is derived by solving a fixed-point equation, which is proved to have a unique solution. According to the relay selection scheme, we characterize the correlation of interference power in two consecutive time-slots by identifying the densities of source and relay retransmissions. Based on the interferer density and interference correlation, we derive the outage probability and average packet delay of the cooperative scheme, while taking into account the dynamic traffic arrivals, interference correlation, relay selection scheme, and spatial node distribution. The performance analysis is validated by extensive simulations. The analytical results provide useful insights on cooperative communication in large-scale networks.