Anfu Zhou

Signpost: Scalable MU-MIMO Signaling with Zero CSI Feedback

Poor scalability is a long standing problem in multi-user MIMO (MU-MIMO) networks: in order to select concurrent uplink users with strong channel orthogonality and thus high total capacity, channel state information (CSI) feedback from users is required. However, when the user population is large, the overhead from CSI feedback can easily overwhelm the actual channel time spent on data transmission. Moreover, due to spontaneous uplink traffic, uplink user selection cannot rely on the access points central assignment and needs a distributed realization instead, which makes the problem even more challenging. In this paper, we propose a fully scalable and distributed uplink MU-MIMO protocol called Signpost. Firstly, Signpost is scalable, i.e., it achieves zero CSI overhead by exploiting a novel orthogonality evaluation mechanism that enables each user to speculate its orthogonality to other users, using its own CSI only. Secondly, Signpost realizes distributed user selection through a two-dimensional prioritized contention mechanism, which can single out the best users efficiently by utilizing both the time and frequency domain resources. The contention mechanism includes a unique collision recovery scheme, which enables Signpost to achieve a collision probability as low as one-tenth compared with traditional 802.11-like mechanism. Software-radio based implementation and testbed experimentation show that Signpost significantly outperforms state-of-the-art user selection methods under various traffic patterns and node mobility.

Cross-Layer Design for Proportional Delay Differentiation and Network Utility Maximization in Multi-Hop Wireless Networks

One major problem of cross-layer control algorithms in multi-hop wireless networks is that they lead to large end-to-end delays. Recently there have been many studies devoted to solving the problem to guarantee order-optimal per-flow delay. However, these approaches also bring the adverse effect of sacrificing a lot of network utility. In this paper, we solve the large-delay problem without sacrificing network utility. We take a fundamentally different approach of delay differentiation, which is based on the observation that flows in a network usually have different requirements for end-to-end delay. We propose a novel joint rate control, routing and scheduling algorithm called CLC_DD, which ensures that the flow delays are proportional to certain pre-specified delay priority parameters. By adjusting delay priority parameters, the end-to-end delays of preferential flows achieved by CLC_DD can be as small as those achieved by delay-order-optimal algorithms. In contrast to high network utility loss in previous approaches, we prove that our approach achieves maximum network utility. Furthermore, we incorporate opportunistic routing into the cross-layer design framework to improve network performance under the environment of dynamic wireless channels.

Acoustic Eavesdropping through Wireless Vibrometry

Loudspeakers are widely used in conferencing and infotainment systems. Private information leakage from loudspeaker sound is often assumed to be preventable using sound-proof isolators like walls. In this paper, we explore a new acoustic eavesdropping attack that can subvert such protectors using radio devices. Our basic idea lies in an acoustic-radio transformation (ART) algorithm, which recovers loudspeaker sound by inspecting the subtle disturbance it causes to the radio signals generated by an adversary or by its co-located WiFi transmitter. ART builds on a modeling framework that distills key factors to determine the recovered audio quality. It incorporates diversity mechanisms and noise suppression algorithms that can boost the eavesdropping quality. We implement the ART eavesdropper on a software-radio platform and conduct experiments to verify its feasibility and threat level. When targeted at vanilla PC or smartphone loudspeakers, the attacker can successfully recover high-quality audio even when blocked by sound-proof walls. On the other hand, we propose several pragmatic countermeasures that can effectively reduce the attackers audio recovery quality by orders of magnitude.

Joint Traffic Splitting, Rate Control, Routing, and Scheduling Algorithm for Maximizing Network Utility in Wireless Mesh Networks

The existence of multiple gateways, as is a common case in wireless mesh networks (WMNs), brings the possibility to improve network performance. However, previous studies, including both heuristic-based works and theory-driven cross-layer design works, cannot guarantee an optimal exploitation of multiple gateways. In this paper, we focus on exploiting multiple gateways optimally to achieve maximum network utility. We first extend the current framework of cross-layer design and formulate a network utility maximization (NUM) problem under WMNs with multiple gateways as a constrained optimization problem. Then, by solving this optimization problem, we propose a novel joint traffic splitting, rate control, routing, and scheduling algorithm called cross-layer control with dynamic gateway selection (CLC_DGS), which splits and distributes network traffic into multiple gateways in an optimal way. We prove that CLC_DGS can achieve maximum network utility. Finally, we run extensive simulations to demonstrate that, compared with the previous methods, CLC_DGS significantly improves the performance of WMNs under various network environments, including gateway heterogeneity, link heterogeneity, and different interference models.

Modeling and Optimization of Medium Access in CSMA Wireless Networks with Topology Asymmetry

Recent studies reveal that the main cause of the well-known unfairness problem in wireless networks is the ineffective coordination of CSMA-based random access due to topology asymmetry. In this paper, we take a modeling-based approach to understand and solve the unfairness problem. Compared to existing works, we advance the state of the art in two important ways. First, we propose an analytical model called the G-Model, which accurately characterizes the ineffective coordination of medium access in asymmetrical topologies. The G-Model can estimate network performance under arbitrary parameter configurations. Second, while previous works decompose a wireless network into embedded basic asymmetric topologies and study each basic topology separately, we go beyond the basic asymmetrical topology and design a model-driven optimization method called Flow Level Adjusting (FLA) to solve the unfairness problem for larger wireless networks. Through extensive simulations, we validate the proposed G-Model and show that FLA can greatly improve the overall fairness of wireless networks in which basic asymmetric topologies are embedded.

Cross-layer design with optimal dynamic gateway selection for wireless mesh networks

The existence of multiple gateways, as is a common case in Wireless Mesh Networks (WMNs), brings the possibility to improve network performance. However, previous studies, including both heuristic-based gradual-optimization work and theory-driven cross-layer design work, cannot guarantee an optimal exploitation of multiple gateways. In this paper, we first extend the current framework of cross-layer design to incorporate a dynamic gateway selection strategy, and propose a novel joint traffic splitting, rate control, routing and scheduling algorithm called CLC_DGS, which distributes traffic of a flow into multiple gateways in an optimal way so as to guarantee maximum network utility. Secondly, based on CLC_DGS, we propose an enhanced CLC_DGS_DD algorithm which in addition takes into account the delay requirements for network flows. CLC_DGS_DD provides a flexible framework for adjusting delays among different flows, and thereby achieves as low as order-optimal delays for preferential flows while simultaneously guaranteeing maximum network utility. Through theoretical analysis and simulation experiments, we demonstrate that compared with previous studies, CLC_DGS and CLC_DGS_DD significantly improve performance of WMNs.

Low latency IP mobility management: protocol and analysis

Mobile IP is one of the dominating protocols that enable a mobile node to remain reachable while moving around in the Internet. However, it suffers from long handoff latency and route inefficiency. In this article, we present a novel distributed mobility management architecture, ADA (Asymmetric Double-Agents), which introduces double mobility agents to serve one end-to-end communication. One mobility agent is located close to the MN and the other close to the CN. ADA can achieve both low handoff latency and low transmission latency, which is crucial for improvement of user perceived QoS. It also provides an easy-to-use mechanism for MNs to manage and control each traffic session with a different policy and provide specific QoS support. We apply ADA to MIPv6 communications and present a detailed protocol design. Subsequently, we propose an analytical framework for systematic and thorough performance evaluation of mobile IP-based mobility management protocols. Equipped with this model, we analyze the handoff latency, single interaction delay and total time cost under the bidirectional tunneling mode and the route optimization mode for MIPv6, HMIPv6, CNLP, and ADA. Through both quantitative analysis and NS2-based simulations, we show that ADA significantly outperforms the existing mobility management protocols.

PCLF: A Practical Cross-Layer Fast Handover Mechanism in IEEE 802.11 WLANs

As is known to us, the handover latency of FMIPv6 in its predictive mode is given little concerns. However our previous work [4] shows that FMIPv6 may suffer long handover latency in its predictive mode, and [4] identifies three key issues raising such problems. In this paper, we propose a practical cross-layer fast handover management mechanism (PCLF) to address these issues and improve success rate of mobility prediction. To solve the problem, PCLF includes a smart link layer trigger, a TBScan algorithm, a TBAPS algorithm, a buffering support Bi-Binding scheme and the smart link event notification policy. Experiment results show that our mechanism can achieve reasonable mobility prediction and seamless handover with no interruptions on upper layer applications (VoIP) in IEEE 802.11 WLANs. The average handover latency is less than 50ms, the success rate of mobility prediction is 97.7% and no packet loss is observed.

FEDCVS: A fair and efficient scheduling scheme for dynamic cooperative video streaming on smartphones

As video applications are increasingly popular over smartphones, many cooperative video streaming mechanisms have been proposed. These mechanisms use cellular link as well device-to-device links simultaneously to provide higher quality video streaming to mobile users. However current works solely focus on throughput enhancement in static scenarios. Consequently these mechanisms result in unfairness since smartphones with higher download rate expend more cellular traffic and monetary costs. Additionally, previous works assume a static scenario that all smartpone users start to watch the same video at the same time. Obviously, the static scenario is unrealistic in actual mobile environments. Based on these insights, in this paper, we focus on a more practical dynamic cooperation scenario and propose a scheduling scheme to achieve efficient cooperative video streaming and guarantee fluent user experience. More importantly, the proposed scheduling scheme achieves a significant improvement in fairness among cooperators. Through extensive simulations across a wide range of scenarios, we show that the proposed scheme significantly outperforms other works by 52%, 24% and 27% respectively in terms of fairness, without sacrificing efficiency.

Exploiting the full potential of multi-AP diversity in centralized WLANs through back-pressure scheduling

Centralized WLANs widely deployed in enterprise environment or university campus often have high density of Access Point (AP). The high density leads to multi-AP diversity, which brings possibility to improve network performance. Previous studies have proposed different schemes to exploit multi-AP diversity, however, these schemes are all based on heuristic and cannot guarantee an optimal exploitation of multi-AP diversity. In this paper, we propose a Theory Based Centralized Scheduling (TBCS) to exploit the full potential of multi-AP diversity. TBCS is based on the well-known back-pressure scheduling. Although back-pressure scheduling is proved to be throughput-optimal, most of previous studies are purely theoretical. To make a practical use of the theoretical back-pressure scheduling, we design new mechanisms in TBCS to handle the problem caused by the wired/wireless mixed scenario of centralized WLANs and to synchronize the scheduling. We evaluate TBCS through NS-2 simulations and show that compared with previous methods, TBCS can support the largest capacity region and greatly improves the throughput of a network.