Humphrey Rutagemwa

Cross-Layer Performance Study of Cooperative Diversity System With ARQ

In this paper, a cross-layer design of a wireless communication system is proposed by considering a quadrature signaling (QS)-based cooperative diversity (CD) system employing truncated stop-and-wait automatic repeat request (ARQ) for error control. The proposed CD-ARQ scheme employs selection relaying at the partner, and all the transmission channels are assumed to exhibit Nakagami-m fading. To analyze the cross-layer performance of the proposed scheme, a Markov model is developed by capturing the effects of correlated transmission errors of the Nakagami-m fading cooperative channels. Based on the analysis, the effective cooperative channel of QS-CD-ARQ is uncorrelated when the normalized Doppler shift of at least one of the user-to-destination channels is higher than 0.2. Performance metrics (e.g., channel efficiency, packet loss rate, throughput, average delay, and jitter) are taken into account in the study. Our numerical results show that the QS-CD-ARQ scheme outperforms both an incremental-relaying-based CD system and a non-cooperative system if the average of the received signal is higher than the receiver sensitivity and if the channel between users is better than the direct transmission channel. In addition, QS-CD-ARQ provides the lowest packet loss rate with the lowest delay among all the schemes for any number of maximum retry limits. Moreover, compared with a scheme that employs incremental relaying, the proposed QS-CD-ARQ scheme is less complex to implement.

Differentiated Reliable Routing in Hybrid Vehicular Ad-Hoc Networks

In hybrid vehicular ad hoc networks (VANETs), roadside units (RSUs) are more powerful and robust than onboard units (OBUs) equipped on vehicles, which can exchange information and synchronize with other RSUs quickly. Due to the fast mobility of vehicles, wireless links in VANETs are particularly vulnerable to failure. Therefore, it is necessary to provide redundancy in terms of provision multiple link-disjoint paths between source and destination. In addition, VANETs support multiple applications, such as road safety applications and commercial applications, which may require different reliabilities. Accordingly, it is important to discover different number of link-disjoint paths for different applications. In this paper, we propose two notions, virtual equivalent node and differentiated reliable path, and develop an on-demand differentiated reliable routing (DRR) protocol for hybrid VANETs. Extensive simulations show that DRR is beneficial to reduce blocking probability and to maintain lower control overhead while providing differentiated services.

Robust Cross-Layer Design of Wireless-Profiled TCP Mobile Receiver for Vertical Handover

In this paper, we consider downward and upward vertical handovers in integrated wireless LAN and cellular networks and address wireless-profiled Transmission Control Protocol premature timeouts due to step increase of round-trip time (RTT) and false fast retransmit due to packet reordering. Specifically, we develop a mobile-receiver centric loosely coupled cross-layer design, which is easy to implement and deploy, backward compatible with the Wireless Application Protocol version 2 architecture, and robust in the absence of perfect cross-layer information. We propose two proactive schemes, called RTT Inflation and RTT Equalization. The RTT Inflation scheme suppresses premature timeouts by carefully inflating retransmission timeout time, and the RTT Equalization scheme prevents false fast retransmit by equalizing the round-trip delay experienced by all packets. We conduct extensive simulations to evaluate the performance in downward and upward vertical handovers. It is demonstrated that the proposed schemes significantly improve the performance in a wide range of network conditions.

A Service-Agent-Based Roaming Architecture for WLAN/Cellular Integrated Networks

In this paper, an agent-based integrated service model for wireless local area network (WLAN)/cellular networks and the relevant authentication and event tracking for billing support schemes are proposed. The service model does not require inefficient peer-to-peer roaming agreements to provide seamless user roaming between the WLAN hotspots and the cellular networks, which are operated by independent wireless network service providers. The proposed authentication and event-tracking schemes take the anonymity and intractability of mobile users into consideration and operate independently so that the integrated billing service can be applied to the cellular network, even if it still uses a traditional authentication scheme. Security analysis and overhead evaluation are given to demonstrate that the proposed service model and the supporting schemes are secure and efficient.

Performance Analysis of Mobile Hotspots with Heterogeneous Wireless Links

Mobile hotspot enabling Internet access services in moving vehicles is an important service for ubiquitous computing. In this paper, we propose an analytical framework for studying the packet loss behavior and throughput in a mobile hotspot with heterogeneous wireless links. We first develop a two-state Markov model for the integrated wireless wide area network (WWAN) and wireless local area network (WLAN). We then derive the expressions that describe the experienced packet loss probability, packet loss burst length, and throughput. Finally, we present simulation results to verify the accuracy of our analysis. It is concluded that adaptive and cross-layer approaches should be deployed to improve the performance of mobile hotspots.

Proxy-Based Wireless Data Access Algorithms in Mobile Hotspots

In this paper, we investigate efficient wireless data access algorithms in mobile hotspots. We introduce a proxy cache (PC) and propose PC-based poll-each-read (P-PER) and PC-based callback (P-CB) data access algorithms to reduce the transmission cost over wireless links in mobile hotspots. An analytical model is developed, and extensive simulations are conducted to demonstrate the performance of P-PER and P-CB. It is shown that P-PER and P-CB can improve cache hit performance and significantly reduce transmission cost. A tradeoff between P-PER and P-CB suggests the need to use a hybrid proxy-based approach to attain optimal performance of wireless data access in mobile hotspots.

Modeling and Performance Analysis of Multihop Cooperative Wireless Networks

In this paper, we develop a cross-layer analytical framework for studying the performance of reliable relaying schemes in multihop cooperative diversity systems. A generalized signaling model over independent nonidentical Nakagami-m fading channels is assumed, and mathematical expressions for end-to-end throughput rate, spectral efficiency, and latency distribution are derived. The proposed framework can capture the effect of correlated packet errors caused by cooperative retransmission attempts and, thus, adequately evaluate the network performance. Using the framework, the performance of automatic-repeat-request-based relaying schemes is demonstrated over a range of network conditions and system parameters. It is shown that tradeoffs are necessary among network robustness, throughput rate, spectral efficiency, and latency variability.

Performance Analysis of Hybrid Medium Access Protocol in IEEE 802.15.3 WPAN

In this paper, the performance of a hybrid medium access protocol in IEEE 802.15.3 has been studied. An analytical model has been developed for the coordination between the contention access period (CAP) and the contention-free channel time allocation period (CTAP). Given the traffic characteristic and the number of contending devices, the piconet coordinator (PNC) determines the duration of the CAP. Based on the successfully received requests and the scheduling scheme, the PNC decides the duration of the CTAP. Extensive simulations are performed to validate our analysis.

Efficiency and Goodput Analysis of Dly-ACK in IEEE 802.15.3

An analytical model for studying the performance of the delayed acknowledgment (Dly-ACK) mechanism in the IEEE 802.15.3 standard over wireless Rayleigh-fading channel is developed. A three-state Markov chain is applied to approximate the correlated transmission errors. Explicit mathematical expressions for the goodput and efficiency of Dly-ACK are derived. It is found that the correlation between consecutive transmission errors has a significant impact on the goodput and efficiency of the Dly-ACK mechanism. The goodput tends to increase as the size of the Dly-ACK burst increases; however, the amount of increase depends on the underlying delay. Simulation results are given to validate the analytical results.

Performance of Adaptive Relaying Schemes in Cooperative Diversity Systems with ARQ

In this paper, the performance of adaptive relaying schemes in cooperative diversity (CD) systems equipped with automatic repeat request (ARQ) is studied. Both selection relaying and incremental relaying are considered with a regenerate and forward CD system. The performance metrics of the CD-ARQ system such as efficiency, packet drop rate, average delay and jitter are presented. Numerical results show that selection relaying with a good partner outperforms incremental relaying. In addition, selection relaying performs better than incremental relaying without the ARQ scheme.