Chai-Hien Gan

Design and Performance Study for a Mobility Management Mechanism (WMM) Using Location Cache for Wireless Mesh Networks

Wireless mesh networks (WMNs) have emerged as one of the major technologies for 4G high-speed mobile networks. In a WMN, a mesh backhaul connects the WMN with the Internet, and mesh access points (MAPs) provide wireless network access service to mobile stations (MSs). The MAPs are stationary and connected through the wireless mesh links. Due to MS mobility in WMNs, mobility management (MM) is required to efficiently and correctly route the packets to MSs. We propose an MM mechanism named Wireless mesh Mobility Management (WMM). The WMM adopts the location cache approach, where mesh backhaul and MAPs (referred to as mesh nodes (MNs)) cache the MSs location information while routing the data for the MS. The MM is exercised when MNs route the packets. We implement the WMM and conduct an analytical model and simulation experiments to investigate the performance of WMM. We compare the signaling and routing cost between WMM and other existing MM protocols. Our study shows that WMM has light signaling overhead and low implementation cost.

Charge scheduling of electric vehicles in highways

Abstract Today, charging stations (CSs) for electric vehicles (EVs) are much less popular than gas stations. Therefore, searching and selecting CSs is an important issue for the drivers of EVs. This paper investigates the EV charging problem. We propose two types of CS-selection algorithms. The first type only utilizes local information of an EV. The second type utilizes the global information obtained through interactions between the EVs and a Global CS-selection (GCS) server through the mobile telecommunications network. Our study indicates that by using the global information (specifically the workload status of each CS), the EVs can be effectively charged with short waiting times at the CSs.

Modeling opportunity driven multiple access in UMTS

The Third Generation Partnership Project (3GPP) specification TR 25.924 proposed the opportunity driven multiple access (ODMA) mechanism for the universal mobile telecommunication system (UMTS). According to the maximum transmission bit rate that can be achieved between a UMTS Node-B and user equipment (UE), the coverage area of a Node-B is divided into two regions: high-bit-rate region (where a high bit rate can be used to deliver the packets for the UEs) and low-bit-rate region (where, due to hostile losses, the packets for the UEs can only be transmitted with low bit rate). The ODMA mechanism increases the transmission bit rate for the UEs in Region L by relaying the transmissions between a Node-B and UEs in Region L over a number of hops. However, more radio resource is consumed to serve a transmission with ODMA than that without ODMA, which may affect the quality of service (QoS) for other packet transmissions. In this paper, we propose an analytical model and simulation experiments to investigate the performance of the ODMA and non-ODMA networks in terms of the average transmission rate, packet blocking probability, and code channel utilization. Our study indicates that when the network traffic is not heavy, the ODMA mechanism significantly improves the average transmission rate and code channel utilization for the user packets by slightly reducing the QoS for other packet transmissions (i.e., packet blocking probability).

Operating electric taxi fleets: A new dispatching strategy with charging plans

In this paper, we propose a dispatching strategy with charging plans upon the client requests for a commercial fleet of pure electric taxis. To boost the green industry, promoting the development of electrical vehicles is one of the most important policies of many governments. In a new scenario that the commercial taxi fleets run electrical vehicles as the main transportation, traditional dispatching policies for general gasoline taxis are no longer effective. It is because those policies do not need to consider the newly generated issues for the electrical vehicles such as the endurance and the related charging problems when dispatching them. In addition, taxi drivers may doubt whether their working hours would be occupied by the possibly long waiting time of power recharging and thus decrease the chances to carry clients. To overcome the above issues, this paper proposes a new dispatching policy in consideration of the taxi demand, the remaining power of electrical taxis, and the availability of battery charging/switching stations in order to lower the waiting time of power recharging and thus increase the workable hours for taxi drivers. Simulation results show that our dispatching strategy can effectively reduce the waiting time for charging and increase the chances of taking clients compared to some random dispatching strategy.

Credit allocation for UMTS prepaid service

Prepaid phone service requires a user to make payment before calling. In 2.5G or the 3G networks, a user may be engaged in multiple voice and/or data prepaid sessions at the same time. For such services, it is important to distribute appropriate amounts of prepaid credit units to simultaneously executed sessions of a user. By considering the universal mobile telecommunication system (UMTS) as an example, this paper studies the prepaid credit allocation for the prepaid sessions. To simultaneously accommodate more prepaid sessions for a user, we propose a credit reclaim mechanism called prepaid credit reclaim (PCR). Analysis and simulation experiments are conducted to investigate the performance of our mechanism. Our study indicates that PCR can significantly improve the performance of the prepaid mechanism.

Push-to-Talk Service for Intelligent Transportation Systems

Push to talk (PTT) is a walkie-talkie like service. In this service, several predefined group members participate in one PTT session. At any time, only one group member is allowed to speak. Therefore, a mechanism is required to determine the member that is permitted to speak. In Open Mobile Alliance (OMA), a central arbitrator coordinates the permission to speak among the group members. In the Intelligent Transportation System (ITS) environment, the centralized OMA approach is not appropriate. This paper proposes a distributed PTT mechanism for the ITS environment, which does not require any central arbitrator. The group member permitted to speak is automatically determined through distributed learning interaction. We also explore the properties and model the performance of the proposed PTT mechanism.

An Effective Power Conservation Scheme for IEEE 802.11 Wireless Networks

In IEEE 802.11 wireless networks, a mobile station (MS) can power down the transceiver to save energy. To achieve power conservation, an access point (AP) periodically broadcasts beacons to notify the associated MSs about their traffic indications, and the MSs periodically wake up to listen to the beacons. The MS awake time scheduling is an important issue, which affects the number of simultaneously awake MSs to compete for access. We propose a power conservation scheme to optimally schedule the awake times among the MSs, such that the number of MSs awaking at the same time is minimal. Our study indicates that the proposed scheme demonstrates good performance in terms of frame loss and delay time.

Performance measurements of TD-LTE, WiMax and 3G systems

Long Term Evolution (LTE) and Worldwide Interoperability for Microwave Access (WiMAX) are two major technologies toward next-generation mobile broadband standards, which are both expected to provide higher throughput and lower transmission latency for mobile users. This article measures and compares the latency and the throughput of Time Division - Long Term Evolution (TD-LTE), WiMAX, and third-generation (3G) systems based on several technical trials. Our quantitative measures and comparisons provide guidelines for the operators to deploy their future networks.

Reducing Call Routing Cost for Femtocells

Femtocell is an effective solution to improve indoor coverage for cellular networks, where short-range and low-power Base Stations (BSs) called Femto BSs are deployed in small areas (e.g., buildings). We propose the Femto Private Branch Exchange (FPBX) to concentrate the traffic of the Femto BSs in a specific area, such as a campus or an enterprise with central or distributed locations. Through the FPBX, a normal cellular call between two Femto users can be replaced by a low-cost extension call. Therefore, the call routing cost can be significantly reduced. In this approach, the existing mobile network nodes are not modified. We analytically analyze the call routing performance for the FPBX approach, and develop simulation experiments to validate against the analytic model. Our study indicates that the FPBX approach can effectively reduces the call setup costs and the voice trunk costs among Femto users by slightly increasing the normal cellular call setup costs.

Effective VoIP call routing in WLAN and cellular integration

In cellular-WLAN integration, a dual-mode mobile station (MS) typically disables the WLAN module for power saving. A major problem is that for an incoming VoIP call (or data session), the MS will not be able to receive this call from the WLAN. It turns out that the call is directed to the cellular network. This letter proposes a simple push solution where an MS can accurately detect a VoIP call from paging signaling of the cellular network. Then the WLAN module of the MS is turned on and the VoIP call is connected to the MS through the relatively inexpensive WLAN.