Yanfeng Zhu

A phase-type based Markov chain model for IEEE 802.16e sleep mode and its performance analysis

To support battery powered mobile broadband wireless access devices efficiently, IEEE 802.16e defines a sleep mode operation for conserving the power of mobile terminals. In this paper we propose a theoretical Phase-type (PH) based Markov chain model to analyze the performance of IEEE 802.16e sleep mode operation. The model describes the behavior of the mobile stations working in sleep mode. In particular, the service process is designed as a discrete PH model. By means of the mathematic tools of PH theory we then derive the closed-form expressions of the mean sojourn time of packets and power consumption. Comparison with simulation results shows that the model provides an accurate prediction of the system performance. Furthermore, we propose a simple utility function to quantify the efficiency of sleep mode operation which takes the joint effect of sojourn time and power saving into account. This function allows mobile stations to decide when to enable sleep mode operation for power saving.

On Optimal Physical Carrier Sensing: Theoretical Analysis and Protocol Design

Traditional Physical Carrier Sensing (PCS), which aims at eliminating hidden terminals in wireless networks, causes too many exposed terminals and deteriorates the throughput per user seriously. Some existing work has proven that an aggressive PCS can improve the throughput by balancing the tradeoff between hidden terminals and exposed terminals. However, little work has been conducted to optimize the PCS according to the network conditions. In this paper, we develop an analytical model to study the behaviors of a user in the aggressive PCS scenario, with which the optimal PCS threshold can be derived. Then, to avoid the complicated computation, we present a heuristic algorithm, in which the parameters required for PCS tuning are estimated by the carrier sensing mechanism employed in IEEE 802.11. Simulation results show that the proposed heuristic algorithm can make the PCS threshold approach to the theoretical optimal one. Moreover, the proposed heuristic algorithm can obtain significant throughput gain compared to the traditional PCS threshold setting solutions.

Integration of SNR, load and time in handoff initiation for wireless LAN

The performance of handoff initiation algorithm is one of the key issues for providing roaming capability in wireless LAN (WLAN). In this paper, we propose a new handoff initiation algorithm for data transmitting in WLAN. Our proposal combines association time, system load and signal/noise ratio (SNR) together to decide whether mobile node should handoff or not. The goal is to reduce the number of unnecessary handoff, decrease packet loss and balance the system load. In this way, we can provide quality of service (QoS) guarantee. Simulation results show that the proposed handoff initiation algorithm is efficient in WLAN.

A Multi-AP Architecture for High-Density WLANs: Protocol Design and Experimental Evaluation

Fast proliferation of IEEE 802.11 wireless devices has led to the emergence of High-Density (HD) Wireless Local Area Networks (WLANs), where it is challenging to improve the throughput because each device has to share channel with all the other devices within its carrier sensing range. Although the existing adaptive Physical Carrier Sensing (PCS) techniques can improve the throughput, they result in high frame loss rate. In this paper, we investigate a Multi-AP (MAP) architecture, in which each user can associate with multiple APs according to the network topology and traffic distribution, for adaptive PCS based HD-WLANs. One of important features of the MAP architecture is that it can obtain Multi-AP diversity in both uplink and downlink. In uplink (from users to APs) the frame loss rate can be decreased by combining the reception of all associated APs, and in downlink the throughput can be improved significantly by dynamically selecting one of associated APs for transmissions according to the channel fading and traffic distribution. We first study the uplink and downlink performance of the MAP theoretically, and then propose an AP association algorithm for deciding which APs to associate with, an AP selection algorithm for dynamically selecting an AP for downlink transmissions, and an ACK management solution for avoiding ACK collisions. We build a testbed based on Intel StarEast platform to make real experiments for performance evaluation. In a typical experiment scenario, compared to the scheme with the adaptive PCS only, up to 30% throughput gain can be observed in uplink, and nearly 100% throughput gain can be found in downlink.

User-aware rate adaptive control for IEEE 802.11-based ad hoc networks

The existing rate adaptive control schemes for ad hoc networks just adapt to the time variation of the fading channels but not to the variation of the network congestion level. In this paper, we propose a user-aware rate adaptive control (UARAC) scheme for IEEE 802.11-based ad hoc networks, which exploits not only time diversity but also multi-user diversity. The UARAC is RTS/CTS based, where a receiver measures the SNR of a RTS frame, and then selectively returns the CTS frame with a piggyback information showing the maximum feasible rate. The key feature of the UARAC lies in that the probability that the receiver returns the CTS frame depends on the SNR of the RTS frame and the number of active stations in the network, so that the throughput can be further improved by multi-user diversity besides time-diversity. Furthermore, to optimize the throughput on-line, the UARAC tunes the probability that the receiver returns the CTS frame in heuristic mode. Extensive numerical results and simulation results show that the proposed scheme significantly outperforms the existing adaptive schemes

On Optimal QoS-aware Physical Carrier Sensing for IEEE 802.11 Based WLANs: Theoretical Analysis and Protocol D esign

In Wireless Local Area Networks (WLANs), traditional Physical Carrier Sensing (PCS), which aims at eliminating hidden terminals completely, brings too many exposed terminals and degrades the throughput seriously. Some existing work has proven that an aggressive PCS, which turns up the PCS threshold to allow the existence of hidden terminals, can improve the throughput by balancing the tradeoff between hidden terminals and exposed terminals. However, little work has been conducted to compute the optimal PCS threshold. To address this issue, in this paper we develop an analytical model, which can be used to compute the optimal PCS threshold and investigate the impact of the aggressive PCS on the Quality-of-Service (QoS) in terms of the packet loss rate. Then, we propose a QoS-aware aggressive PCS tuning algorithm, with which users can adapt the PCS threshold to the varying network conditions and the QoS requirement. Extensive simulation results show that the proposed algorithm obtains significant throughput gain compared to the traditional PCS and bounds the packet loss rate below the QoS requirement. The unfairness issue brought by the aggressive PCS is also discussed and evaluated with experimental results at the end of the paper.

Joint tuning of physical carrier sensing, power and rate in high-density WLAN

Fast proliferation of IEEE 802.11 wireless devices has led to the emergence of High-Density WLAN, where a large number(10s-100s) of Access Points(AP) are deployed to service a larger number(100s-1000s) of users. In such network it is a challenge to improve the throughput because each device has to share channel with all the other devices within its carrier sensing range. So some traditional settings of 802.11 devices need to be self-adaptively tuned. In this paper, we investigate the joint tuning of physical carrier sensing, transmission power and data rate in High-Density WLAN. With introducing an analytical model, we propose two simple yet effective tuning rules to maximize spatial reuse and keep the fairness among users. We also develop a distributed joint tuning algorithm based on these rules. Extensive simulation results show that the proposed joint tuning algorithm achieves significant improvements in both aggregate network throughput and fairness.

Improve transmission reliability with multi-AP diversity in wireless networks: architecture and performance analysis

With the increasing development of IEEE 802.11 based Wireless Local Area Network (WLAN) devices, large-scale WLANs with high dense deployment of user terminals and access points (APs) have emerged widely in various hotspots. Enhancing transmission reliability has been a primary challenge for scaling the WLANs because high dense deployment of user terminals and APs results in too many collisions. In this paper, we investigate the defects of single association mechanism defined in IEEE 802.11 on transmission reliability from network perspective. Then, we propose a multi-AP architecture, with which an AP Controller (AC) is employed to enable each user terminal to associate and cooperate with multiple APs. In this way, the user terminals can benefit from the diversity effect of multi-paths with independent collisions and transmission errors. This paper concentrates on the performance comparison between the proposed multi-AP architecture and that in IEEE 802.11 standard. Extensive simulation results show that the proposed mechanism can obtain much better performance in terms of the throughput per user and the total throughput, and the performance gain is position dependent. Moreover, the unfairness issue in traditional WLANs due to capture effect can be alleviated properly in the multi-AP framework.

A channel-aware adaptive control to the MAC protocol in rate adaptive wireless LANs

Due to the low efficiency of legacy IEEE 802.11 MAC protocol, it has been the main bottleneck of wireless local area networks (WLANs). In this paper, we focus on multi-rate WLANs and propose an adaptive transmission control scheme, which adapts the transmission probability according to the number of active stations, transmission rates, and channel conditions. At first, an analytical model is built in terms of the proposed scheme to explore the throughput of the network. Then, by means of the characteristics of practical system, a heuristic algorithm is developed to approach the maximum throughput on-line. Extensive numerical calculations and simulations based on NS-2 are implemented to evaluate the performance of the proposed algorithm and the impact of inaccurate estimation of parameters. The results show that our algorithm outperforms the existing adaptive algorithms in imperfect channel, and insensitive to inaccurate estimation of parameters. Copyright

QoS-aware Adaptive Physical Carrier Sensing for Wireless Networks

Traditional physical carrier sensing (PCS) threshold setting, which aims at eliminating hidden terminals completely in wireless networks, brings too many exposed terminals and degrades the throughput per user seriously. Some existing work has proven that an aggressive PCS threshold can improve the throughput by balancing the tradeoff between the existence of hidden terminals and exposed terminals. However, the aggressive PCS results in high packet loss, which degrades the quality-of-service (QoS) seriously. To address this issue, in this paper we develop a QoS aware aggressive PCS threshold tuning algorithm, in which the PCS threshold is tuned dynamically according to the varying network conditions. One important feature of the proposed algorithm is that it can bound the packet loss in the region defined by QoS requirement. Extensive simulation results show that the proposed algorithm can bound the packet loss in the QoS requirement and at the same time obtain remarkable throughput gain compared to the traditional PCS threshold setting solutions.