Jing Zhu

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.

Leveraging multi-AP diversity for transmission resilience in wireless networks: architecture and performance analysis

With the increasing development of IEEE 802.11 based wireless local area network (WLAN) devices, large-scale multi-cell WLANs with a high density of users and access points (APs) have emerged widely in various hotspots. Providing resilient data transmission has been a primary challenge for scaling the WLANs because the high density of users and APs results in too many collisions. In this paper, we analyze and point out the defect of the single association mechanism defined in IEEE 802.11 on transmission reliability from a network perspective. Then, we propose a multi-AP architecture with which a MAC layer device called an AP controller (AC) is employed to enable each user to associate and cooperate with multiple APs. In this way, the users can benefit from the diversity effect of multipaths with independent collisions and transmission errors. This paper concentrates on the theoretical analysis of performance comparison between the proposed ldquoMulti-APrdquo architecture and that in IEEE 802.11. Extensive simulation results show that the proposed ldquomulti-APrdquo architecture 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 ldquomulti-APrdquo framework.

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.

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 delayed multiple copy retransmission scheme for data communication in wireless networks

In this paper, we propose a delayed multiple copy retransmission (DMCR) scheme for data communication in wireless networks, by which multiple copies of a lost link layer frame are retransmitted one-by-one with a retransmission delay in between. The number of the copies gradually increases with the number of retransmissions. Furthermore, for implementation of the DMCR scheme in practical mobile communication system, we also propose a dynamic retransmission scheme by interleaving and a new round-robin scheduling algorithm. We compare our scheme with the previous non-delayed retransmission schemes on the performance of frame loss probability, channel capacity and total transmission time. Numerical results show that the DMCR scheme can achieve higher performance. The effect of the delay time on end-to-end TCP throughput is investigated as well.

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.

Capacity Planning for Voice/Data Traffic in IEEE 802.11e Based Wireless LANs

In this paper, we concentrate on IEEE 802.11e based WLANs with mixed Voice-over-IP (VoIP) and data services. Although the enhanced distributed coordination access mechanism of IEEE 802.11e can differentiate VoIP and data services, the unsaturated feature of VoIP traffic and the unbalance between downlink and uplink degrade the capacity for VoIP seriously. To address these issue, we propose a queueing system based analytical model to investigate the contention between unsaturated voice traffic and saturated data traffic. Moreover, our queueing analysis indicates that the bottleneck of increasing the capacity for VoIP is the downlink. Based on the capacity analysis, we propose an admission control mechanism to guarantee the Quality-of-Service of admitted VoIP sessions. Extensive simulation results are given to illuminate the efficiency of the proposed scheme.