Asymmetric Traffic Patterns Research Articles

A kinematic wave approach to traffic statics and dynamics in a double-ring network

Recently there has been much interest in understanding macroscopic fundamental diagrams of stationary road networks. However, there lacks a systematic method to define and solve stationary states in a road network with complex junctions. In this study we propose a kinematic wave approach to defining, analyzing, and simulating static and dynamic traffic characteristics in a network of two ring roads connected by a 2×2 junction, which can be either an uninterrupted interchange or a signalized intersection. This study is enabled by recently developed macroscopic junction models of general junctions. With a junction model based on fair merging and first-in-first-out diverging rules, we first define and solve stationary states and then derive the macroscopic fundamental diagram (MFD) of a stationary uninterrupted network. We conclude that the flow-density relationship of the uninterrupted double-ring network is not unique for high average network densities (i.e., when one ring becomes congested) and unveil the existence of infinitely many stationary states that can arise with a zero-speed shockwave. From simulation results with a corresponding Cell Transmission Model, we verify that all stationary states in the MFD are stable and can be reached, but show that randomness in the retaining ratio of each ring drives the network to more symmetric traffic patterns and higher flow-rates. Furthermore we model a signalized intersection as two alternate diverge junctions and demonstrate that the signalized double-ring network can reach asymptotically periodic traffic patterns, which are therefore defined as “stationary” states in signalized networks. With simulations we show that the flow-density relation is well defined in such “stationary” states, and asymptotic traffic patterns can be impacted by signal cycle lengths and retaining ratios. But compared with uninterrupted interchanges, signalized intersections lead to more asymmetric traffic patterns, lower flow-rates, and even gridlocks when the average density is higher than half of the jam density. The results are consistent between this study and existing studies, but the network kinematic wave model, with appropriate junction models, is mathematically tractable and physically meaningful. It has offered a more complete picture regarding the number and type of stationary states, their stability, and MFD in freeway and signalized networks.

Impact of Channel Estimation Error on Bidirectional MABC-AF Relaying With Asymmetric Traffic Requirements

Channel estimation error results in severe performance deterioration in wireless networks. In this paper, we study the impact of imperfect channel estimation (ICE) on the outage performance of bidirectional relaying where the two sources have asymmetric traffic requirements (ATRs). In particular, we focus on the amplify-and-forward (AF) relay-assisted multiple-access broadcast (MABC) protocol, i.e., MABC-AF, which has received much attention due to its high spectrum efficiency and low complexity. In the single-relay scenario, we derive exact and generalized closed-form expressions for system outage probability, which indicate that the system outage is determined by the unidirectional outage in the case of highly asymmetric traffic patterns. For more insights into our approach, the closed-form asymptotic expressions are also evaluated, manifesting the interesting error floor (EF) phenomenon due to ICE. Using these analytical results, we further develop a robust and practical optimum power-allocation (OPA) algorithm that minimizes the system outage probability under aggregate and individual node power constraints. In the multirelay scenario, by taking into account the traffic knowledge, a novel relay selection criterion is proposed for asymmetric MABC-AF, followed by its impact on the system outage probability in the presence of ICE. Numerical results validate the accuracy of our analytical results and highlight the effect of the proposed OPA algorithm under various traffic requirements and channel estimation errors. Furthermore, the results also show that the proposed relay selection criterion is superior to the classical max-min criterion with ATRs, and the performance improvement becomes remarkable as channel estimation error increases.

Performance analysis of an adaptive, energy-efficient MAC protocol for wireless sensor networks

We propose an adaptive and energy-efficient TDMA-based MAC protocol that significantly reduces energy consumption in the network, while efficiently handling network traffic load variations and optimizing channel utilization through a timeslot stealing mechanism and a timeslot reassignment procedure. We have analytically derived the average delay performance of our MAC protocol, with and without the timeslot stealing mechanism. Our delay model, validated via simulations, shows that the timeslot stealing mechanism can substantially improve the protocol throughput in scenarios with varying and asymmetric traffic patterns. Evaluation results show that the timeslot reassignment procedure is efficient in handling the longer timescale changes in the traffic load, while the timeslot stealing mechanism is better in handling the shorter timescale changes in the traffic patterns.

Strictly nonblocking grooming of dynamic traffic in unidirectional SONET/WDM rings using genetic algorithms

We propose a genetic algorithm (GA) to solve the strictly nonblocking grooming problems with dynamic traffic in unidirectional synchronous optical network/wavelength division multiplexing rings with arbitrary asymmetric traffic patterns. We give out the theoretical lower and upper bounds on the numbers of both add/drop multiplexers (ADMs) and wavelengths required and define the blocking properties in grooming dynamic traffic in detail. In applying GAs to this problem, we propose a first fit approach incorporated with a greedy local improvement algorithm to decode the chromosome. Four different local improvement scenarios are designed and compared in this paper. We also introduce the concepts of Link Load Imparity, Weighted Traffic Imparity and Average Link Load of each traffic pattern and use the latter two to estimate the first parameter before grooming. Comparison between the genetic and the random search results is also made. Computer simulation results show that it is important to combine the GA with an appropriate local improvement scenario in order to achieve near optimal results and to reduce the running time and that the algorithm proposed in this paper is able to achieve good results in reducing the numbers of ADMs and wavelengths.

Simple and reliable bidirectional optical amplifier suitable for variable traffic pattern networks

We proposed a simple and flexible bidirectional optical amplifier (BOA) that consists of one erbium-doped fiber amplifier (EDFA) and an arrayed waveguide grating (AWG). This BOA is especially suitable for networks having asymmetric traffic patterns since the circuit configuration of BOA can be modified easily by manipulation of AWG ports connection. We constructed the BOA and demonstrated its performance for 10-Gb/s symmetric traffic over 80-km single-mode fiber. The proposed BOA worked well with negligible relative intensity noise by virtue of intrinsic tolerance of backscattering in an AWG.