Sok Ian Sou

Enhancing VANET Connectivity Through Roadside Units on Highways

In vehicular ad hoc network (VANET) safety applications, the source vehicle that detects an accident can generate a warning message and propagate it to the following vehicles to notify other drivers before they reach the potential danger zone on the road. Recent studies have shown that sparse vehicle traffic leads to network fragmentation, which poses a crucial research challenge for safety applications. In this paper, we analyze and quantify the improvement in VANET connectivity when a limited number of roadside units (RSUs) are deployed and to investigate the routing performance for broadcast-based safety applications in this enhanced VANET environment. Our results show that, even with a small number of RSUs, the performance in terms of the probability of network connectivity, the rehealing delay, the number of rehealing hops, and the message penetration time can be significantly improved in highway VANET scenarios.

Performance of decode-and-forward cooperative communications with multiple dual-hop relays over nakagami-m fading channels

In this letter, we examine the symbol-error-rate (SER) performance of decode-and-forward (DF) cooperative communications. Our focus is on the scenario in which multiple dual-hop relays are employed and the channel environment is described by Nakagami-m fading. Cooperative diversity is observed from the derived error-rate expressions, and some insight into how channel conditions in the relay nodes affect the SER performance is obtained. In addition, with the knowledge of the partial channel state information (CSI) at the transmitting sides, we derive an optimal power allocation scheme to further enhance the SER performance. Simulation results are finally presented to demonstrate the correctness of our analysis and perception for the considered DF protocol.

A Power-Saving Model for Roadside Unit Deployment in Vehicular Networks

In this letter, we investigate the impact on connectivity metric when a power-saving model is applied to a vehicular network. An analytic model with linear time complexity is developed to compute the optimal number of active roadside units under a connectivity constraint. We provide guidelines for applying a power-saving model in a vehicular network. The results are very useful for the ministry of transportation.

Modeling Credit Reservation Procedure for UMTS Online Charging System

The IP multimedia core network subsystem (IMS) provides real-time multimedia services for Universal Mobile Telecommunications System (UMTS). Through recharge threshold-based credit reservation (RTCR) mechanism, prepaid IMS services can be supported by the online charging system (OCS) in UMTS. In RTCR, when the remaining amount of prepaid credit is below a threshold, the OCS reminds the user to recharge the prepaid account. It is essential to choose an appropriate recharge threshold to reduce the probability that the in-progress service sessions are forced-terminated. An analytic model is developed to investigate the performance of RTCR for the OCS. Based on our study, the network operator can select the appropriate parameter values for various traffic conditions.

Modeling Emergency Messaging for Car Accident over Dichotomized Headway Model in Vehicular Ad-hoc Networks

In a Vehicular Ad Hoc Network (VANET), the wireless Collision Avoidance (CA) system issues warnings to drivers before they reach a potentially dangerous zone on the road. This paper proposes an analytical model for evaluating the performance of emergency messaging via wireless CA systems. First, we utilize the dichotomized headway model, the braking model, and Greenbergs logarithmic model to generate vehicular mobility traces for analysis. Second, we derive the probability of a rear-end collision between two vehicles that travel in the same direction when a sudden event occurs. Third, we quantify the probability of vehicles failing to receive the emergency message. Numerical results from the model show that the number of car crashes per accident is much higher when a wireless CA system is not used. We also find it interesting that the number of car crashes is not directly proportional to the vehicle density when the vehicular mobility traces follow the speed-density relationship offered by Greenbergs logarithmic model. By integrating flow theory into VANET analysis, our model provides useful insights for future intelligent transportation.

Mobile Data Offloading With Policy and Charging Control in 3GPP Core Network

The Third-Generation Partnership Project (3GPP) has recently standardized the policy and charging control (PCC) system to provide dynamic policy enforcement for network resource allocation combined with real-time charging. The upward trend in smartphone use and the subsequent explosion of data traffic on third-generation (3G) networks have pressed carriers to offload the data traffic from the cellular domain. One key challenge is to elaborate on PCC functionality and mobility management for offloading sessions. This paper proposes an enhanced Wi-Fi offloading model to bring mobile Internet Protocol (IP) integration to a core network with PCC. Specifically, we develop a comprehensive analytical model to quantify the performance of data offloading concerning the amount of 3G resources saved by offloading and the deadline assurance for measuring the quality of user experience with PCC support. Numerical results demonstrate that deadline assurance can be satisfied while saving a significant amount of 3G resources in many situations.

Reducing Credit Re-authorization Cost in UMTS Online Charging System

During an online charging general packet radio service (GPRS) session, a number of mid-session events, such as changes of quality of service (QoS), could dynamically affect the rating of the in-progress service. When such events occur, the GPRS support node needs to re-authorize the granted credit units with the online charging system (OCS). This paper proposes a threshold-based scheme that utilizes a threshold parameter delta to reduce the signaling traffic for the credit re-authorization procedure. By selecting an appropriate delta value, the signaling overhead in the OCS can be significantly reduced while the inaccuracy of the credit information insignificantly increases. The mobile operator can choose appropriate parameter values in the threshold-based scheme based on our study.

Modeling mobility database failure restoration using checkpoint schemes

This paper studies checkpointing and failure restoration of mobility database for Universal Mobile Telecommunications System (UMTS). By utilizing per-user checkpointing technique, individual home location register (HLR) records are saved into non-volatile backup storage form time to time. When a failure occurs, the backup record is restored back to the mobility database. We consider three per-user checkpoint schemes for the HLR. An analytic model is developed to investigate these schemes in terms of the probability that a HLR backup record is obsolete and the expected checkpoint interval. This model is validated against simulation experiments. Our study provides guidelines for selecting an appropriate checkpoint scheme and parameters for various traffic conditions

SCB: Store-Carry-Broadcast Scheme for Message Dissemination in Sparse VANET

To take on challenges of communication in sparse VANET, we propose a Store-Carry-Broadcast (SCB) scheme to assist message dissemination by broadcasting over a specific road segment instead of a single vehicle. In the SCB scheme, an opposite vehicle helps to disseminate the safety messages to oncoming vehicles traveling on the reverse lane by broadcasting. Compare with the well-known store-carry-forward broadcasting in VANET, the SCB scheme consumes much less network bandwidth in terms of the number of broadcasting performed. Extensive simulation results are presented to demonstrate that when the traffic density is low or when the DSCR penetration rate is low, our proposed SCB scheme significantly reduced both broadcast overhead as well as delivery delay in sparse VANET.

Signaling overhead of Policy and online Charging Control for bearer sessions in LTE network

The convergence of radio access technologies with all-IP core network has gained significant attention recently. Through the all-IP based Policy and Charging Control (PCC) architecture standardized in 3GPP R8, the charging rules are consistent with application-level and IP-level policy control. It is essential to investigate the signaling overhead incurred by PCC and the OCS management during the mid-session events, such as the handover and QoS changes. To address this issue, this paper investigates PCC and OCS traffic rate over LTE networks.