Proceedings of the 17th Annual International Conference on Mobile Computing and Networking, MOBICOM 2011, Las Vegas, Nevada, USA, September 19-23, 2011. ACM 【DBLP Link】
【Paper Link】 【Pages】:1-12
【Authors】: Shravan K. Rayanchu ; Vivek Shrivastava ; Suman Banerjee ; Ranveer Chandra
【Abstract】: This paper introduces models and a system for designing 802.11 wireless LANs (WLANs) using flexible channelization -- the choice of an appropriate channel width and center frequency for each transmission. In contrast to current 802.11 systems that use fixed width channels, the proposed system, FLUID, configures all access points and their clients using flexible channels. We show that a key challenge in designing such a system stems from managing the effects of interference due to multiple transmitters employing variable channel widths, in a network-wide setting. We implemented FLUID in an enterprise-like setup using a 50 node testbed (with off-the shelf wireless cards) and we show that FLUID improves the average throughput by 59% across all PHY rates, compared to existing fixed-width approaches.
【Keywords】: channel width; conflict graph; scheduling; spectrum; wifi
【Paper Link】 【Pages】:13-24
【Authors】: Cheng-Lin Tsao ; Sandeep Kakumanu ; Raghupathy Sivakumar
【Abstract】: While a remote computing solution such as VNC is an effective technology for PC users to access a remote computer, it is not as effective while being used from smartphones. In this paper, we propose techniques to improve remote computing from smartphones that help deliver near-PC level experience to users. We introduce a key building block called smart-macros that have the robustness of application macros but at the same time possess the generality of raw macros. Using smart-macros we design and prototype SmartVNC, a remote computing solution for smartphones. We show using experimental studies and a trace based analysis of real user activity, that SmartVNC can improve user experience considerably.
【Keywords】: VNC; remote computing; remote desktop; smart-macros; smartphones
【Paper Link】 【Pages】:25-36
【Authors】: Mustafa Y. Arslan ; Jongwon Yoon ; Karthikeyan Sundaresan ; Srikanth V. Krishnamurthy ; Suman Banerjee
【Abstract】: The demand for increased spectral efficiencies is driving the next generation broadband access networks towards deploying smaller cells (femtocells) with sophisticated air interface technologies (Orthogonal Frequency Division Multiple Access or OFDMA). The projected dense deployment of femtocells however, makes interference and hence resource management both critical and extremely challenging. In this paper, we design and implement one of the first resource management systems, FERMI, for OFDMA-based femtocell networks. As part of its design, FERMI (i) provides resource isolation in the frequency domain (as opposed to time) to leverage power pooling across cells to improve capacity; (ii) uses measurement-driven triggers to intelligently distinguish clients that require just link adaptation from those that require resource isolation; (iii) incorporates mechanisms that enable the joint scheduling of both types of clients in the same frame; and (iv) employs efficient, scalable algorithms to determine a fair resource allocation across the entire network with high utilization and low overhead. We implement FERMI on a prototype four-cell WiMAX femtocell testbed and show that it yields significant gains over conventional approaches.
【Keywords】: OFDMA; femtocell
【Paper Link】 【Pages】:37-48
【Authors】: Eugenio Magistretti ; Krishna Kant Chintalapudi ; Bozidar Radunovic ; Ramachandran Ramjee
【Abstract】: The increase in WiFi physical layer transmission speeds from 1~Mbps to 1 Gbps has reduced transmission times for a 1500 byte packet from 12 ms to 12 us. However, WiFi MAC overheads such as channel access and acks have not seen similar reductions and cumulatively contribute about 150 us on average per packet. Thus, the efficiency of WiFi has deteriorated from over 80% at 1 Mbps to under 10% at 1 Gbps. In this paper, we propose WiFi-Nano, a system that uses 800 ns slots} to significantly improve WiFi efficiency. Reducing slot time from 9 us to 800 ns makes backoffs efficient, but clear channel assessment can no longer be completed in one slot since preamble detection can now take multiple slots. Instead of waiting for multiple slots for detecting preambles, nodes speculatively transmit preambles as their backoff counters expire, while continuing to detect premables using self-interference cancellation. Upon detection of preambles from other transmitters, nodes simply abort their own preamble transmissions, thereby allowing the earliest transmitter to succeed. Further, receivers speculatively transmit their ack preambles at the end of packet reception, thereby reducing ack overhead. We validate the effectiveness of WiFi-Nano through implementation on an FPGA-based software defined radio platform, and through extensive simulations, demonstrate efficiency gains of up to 100%.
【Keywords】: 802.11; WiFi
【Paper Link】 【Pages】:49-60
【Authors】: Rafael P. Laufer ; Theodoros Salonidis ; Henrik Lundgren ; Pascal Le Guyadec
【Abstract】: Contemporary wireless multi-hop networks operate much below their capacity due to the poor coordination among transmitting nodes. In this paper we present XPRESS, a cross-layer backpressure architecture designed to reach the full capacity of wireless multi-hop networks. Instead of a collection of poorly coordinated wireless routers, XPRESS turns a mesh network into a wireless switch. Transmissions over the network are scheduled using a throughput-optimal backpressure algorithm. Realizing this theoretical concept entails several challenges, which we identify and address with a cross-layer design and implementation on top of our wireless hardware platform. In contrast to previous work, we implement and evaluate backpressure scheduling over a TDMA MAC protocol, as it was originally proposed in theory. Our experiments in an indoor testbed show that XPRESS can yield up to 128% throughput gains over 802.11.
【Keywords】: backpressure scheduling and routing
【Paper Link】 【Pages】:61-72
【Authors】: Tianji Li ; Mi Kyung Han ; Apurv Bhartia ; Lili Qiu ; Eric Rozner ; Yin Zhang ; Brad W. Zarikoff
【Abstract】: Efficiently sharing spectrum among multiple users is critical to wireless network performance. In this paper, we propose a novel spectrum sharing protocol called Collision-Resistant Multiple Access (CRMA) to achieve high efficiency. In CRMA, each transmitter views the OFDM physical layer as multiple orthogonal but sharable channels, and independently selects a few channels for transmission. The transmissions that share the same channel naturally add up in the air. The receiver extracts the received signals from all the channels and efficiently decodes the transmissions by solving a simple linear system. We implement our approach in the Qualnet simulator and show that it yields significant improvement over existing spectrum sharing schemes. We also demonstrate the feasibility of our approach using implementation and experiments on GNU Radios.
【Keywords】: CDMA; CSMA; OFDM; collision decoding; mac protocol
【Paper Link】 【Pages】:73-84
【Authors】: Kiran K. Rachuri ; Cecilia Mascolo ; Mirco Musolesi ; Peter J. Rentfrow
【Abstract】: The interactions and social relations among users in workplaces have been studied by many generations of social psychologists. There is evidence that groups of users that interact more in workplaces are more productive. However, it is still hard for social scientists to capture fine-grained data about phenomena of this kind and to find the right means to facilitate interaction. It is also difficult for users to keep track of their level of sociability with colleagues. While mobile phones offer a fantastic platform for harvesting long term and fine grained data, they also pose challenges: battery power is limited and needs to be traded-off for sensor reading accuracy and data transmission, while energy costs in processing computationally intensive tasks are high. In this paper, we propose SociableSense, a smart phones based platform that captures user behavior in office environments, while providing the users with a quantitative measure of their sociability and that of colleagues. We tackle the technical challenges of building such a tool: the system provides an adaptive sampling mechanism as well as models to decide whether to perform computation of tasks, such as the execution of classification and inference algorithms, locally or remotely. We perform several micro-benchmark tests to fine-tune and evaluate the performance of these mechanisms and we show that the adaptive sampling and computation distribution schemes balance trade-offs among accuracy, energy, latency, and data traffic. Finally, by means of a social psychological study with ten participants for two working weeks, we demonstrate that SociableSense fosters interactions among the participants and helps in enhancing their sociability.
【Keywords】: computation offloading; energy-accuracy trade-offs; mobile phone sensing; social interactions; social psychology.
【Paper Link】 【Pages】:85-96
【Authors】: Nam P. Nguyen ; Thang N. Dinh ; Sindhura Tokala ; My T. Thai
【Abstract】: Many practical problems on Mobile networking, such as routing strategies in MANETs, sensor reprogramming in WSNs and worm containment in online social networks (OSNs) share an ubiquitous, yet interesting feature in their organizations: community structure. Knowledge of this structure provides us not only crucial information about the network principles, but also key insights into designing more effective algorithms for practical problems enabled by Mobile networking. However, understanding this interesting feature is extremely challenging on dynamic networks where changes to their topologies are frequently introduced, and especially when network communities in reality usually overlap with each other. We focus on the following questions (1) Can we effectively detect the overlapping community structure in a dynamic network? (2) Can we quickly and adaptively update the network structure only based on its history without recomputing from scratch? (3) How does the detection of network communities help mobile applications? We propose AFOCS, a two-phase framework for not only detecting quickly but also tracing effectively the evolution of overlapped network communities in dynamic mobile networks. With the great advantages of the overlapping community structure, AFOCS significantly helps in reducing up to 7 times the infection rates in worm containment on OSNs, and up to 11 times overhead while maintaining good delivery time and ratio in forwarding strategies in MANETs.
【Keywords】: communication network; community detection; forwarding routing strategies; mobile network; worm containment
【Paper Link】 【Pages】:97-108
【Authors】: Jie Yang ; Simon Sidhom ; Gayathri Chandrasekaran ; Tam Vu ; Hongbo Liu ; Nicolae Cecan ; Yingying Chen ; Marco Gruteser ; Richard P. Martin
【Abstract】: This work addresses the fundamental problem of distinguishing between a driver and passenger using a mobile phone, which is the critical input to enable numerous safety and interface enhancements. Our detection system leverages the existing car stereo infrastructure, in particular the speakers and Bluetooth network. Our acoustic approach has the phone send a series of customized high frequency beeps via the car stereo. The beeps are spaced in time across the left, right, and if available, front and rear speakers. After sampling the beeps, we use a sequential change-point detection scheme to time their arrival, and then use a differential approach to estimate the phone's distance from the car's center. From these differences a passenger or driver classification can be made. To validate our approach, we experimented with two kinds of phones and in two different cars. We found that our customized beeps were imperceptible to most users, yet still playable and recordable in both cars. Our customized beeps were also robust to background sounds such as music and wind, and we found the signal processing did not require excessive computational resources. In spite of the cars' heavy multi-path environment, our approach had a classification accuracy of over 90%, and around 95% with some calibrations. We also found we have a low false positive rate, on the order of a few percent.
【Keywords】: acoustic ranging; bluetooth; car speakers; driving safety; location classification; smartphone
【Paper Link】 【Pages】:109-120
【Authors】: Zengbin Zhang ; Xia Zhou ; Weile Zhang ; Yuanyang Zhang ; Gang Wang ; Ben Y. Zhao ; Haitao Zheng
【Abstract】: Today's WiFi access points (APs) are ubiquitous, and provide critical connectivity for a wide range of mobile networking devices. Many management tasks, e.g. optimizing AP placement and detecting rogue APs, require a user to efficiently determine the location of wireless APs. Unlike prior localization techniques that require either specialized equipment or extensive outdoor measurements, we propose a way to locate APs in real-time using commodity smartphones. Our insight is that by rotating a wireless receiver (smartphone) around a signal-blocking obstacle (the user's body), we can effectively emulate the sensitivity and functionality of a directional antenna. Our measurements show that we can detect these signal strength artifacts on multiple smartphone platforms for a variety of outdoor environments. We develop a model for detecting signal dips caused by blocking obstacles, and use it to produce a directional analysis technique that accurately predicts the direction of the AP, along with an associated confidence value. The result is Borealis, a system that provides accurate directional guidance and leads users to a desired AP after a few measurements. Detailed measurements show that Borealis is significantly more accurate than other real-time localization systems, and is nearly as accurate as offline approaches using extensive wireless measurements.
【Keywords】: access point location; smartphones; wifi
【Paper Link】 【Pages】:121-132
【Authors】: Chunyi Peng ; Suk-Bok Lee ; Songwu Lu ; Haiyun Luo ; Hewu Li
【Abstract】: Base stations (BSes) in the 3G cellular network are not energy proportional with respect to their carried traffic load. Our mea- surements show that 3G traffic exhibits high fluctuations both in time and over space, thus incurring energy waste. In this paper, we propose a profile-based approach to green cellular infrastruc- ture. We profile BS traffic and approximate network-wide energy proportionality using non-load-adaptive BSes. The instrument is to leverage temporal-spatial traffic diversity and node deployment heterogeneity, and power off under-utilized BSes under light traf- fic. Our evaluation on four regional 3G networks shows that this simple scheme yields up to 53% energy savings in a dense large city and 23% in a sparse, mid-sized city.
【Keywords】: 3g network traffic; cellular networks; energy efficiency
【Paper Link】 【Pages】:133-144
【Authors】: Supratim Deb ; Kanthi Nagaraj ; Vikram Srinivasan
【Abstract】: There are two emerging trends in the mobile data world. First, mobile data is exploding at a rapid rate with analysts predicting 25-50X growth by the year 2015. The second trend is that users are demanding greater degree of flexibility in selecting their operators at fine timescales. Across Asia, dual-SIM phones have become popular, while Apple is rumored to be designing a Universal SIM that will allow iPhone users to toggle between different operators. This latter trend points towards an impending disruption in wireless service models which could also be the need of the hour from the spectrum shortage perspective. This points towards a new service model where users can choose an operator based on application needs. However, if users make this choice greedily without network assistance, it can exacerbate spectrum scarcity and degrade user experience. In this work, we consider user devices with multiple network interfaces (3G, LTE etc.) that can be simultaneously active and each running multiple applications. We propose the MOTA service model to enable users to associate each interface with the operator of choice at fine time scales. Under the MOTA service model, through concise signalling information, operators provide information about their own network, so that each user can (i) choose a suitable operator for each interface, and (ii) choose an interface for each active application. We make the following contributions in this paper. First, we propose concise network signalling that assists users to make informed choices even under mobility. Second, we develop user-choice algorithms that maximize a suitable notion of user satisfaction while using spectrum resources efficiently. Third, we perform extensive evaluation over actual base station deployment in a city coupled with real signal propagation maps. Our results with two operators show that, MOTA service model provides capacity gain in the range 2.5-4X over the current existing service model. Finally, we argue that our solution is practically implementable by combining appropriate IEEE standards and IETF proposals.
【Keywords】: operator agnostic
【Paper Link】 【Pages】:145-156
【Authors】: Hui Zang ; Jean Bolot
【Abstract】: We examine a very large-scale data set of more than 30 billion call records made by 25 million cell phone users across all 50 states of the US and attempt to determine to what extent anonymized location data can reveal private user information. Our approach is to infer, from the call records, the "top N" locations for each user and correlate this information with publicly-available side information such as census data. For example, the measured "top 2" locations likely correspond to home and work locations, the "top 3" to home, work, and shopping/school/commute path locations. We consider the cases where those "top N" locations are measured with different levels of granularity, ranging from a cell sector to whole cell, zip code, city, county and state. We then compute the anonymity set, namely the number of users uniquely identified by a given set of "top N" locations at different granularity levels. We find that the "top 1" location does not typically yield small anonymity sets. However, the top 2 and top 3 locations do, certainly at the sector or cell-level granularity. We consider a variety of different factors that might impact the size of the anonymity set, for example the distance between the "top N" locations or the geographic environment (rural vs urban). We also examine to what extent specific side information, in particular the size of the user's social network, decrease the anonymity set and therefore increase risks to privacy. Our study shows that sharing anonymized location data will likely lead to privacy risks and that, at a minimum, the data needs to be coarse in either the time domain (meaning the data is collected over short periods of time, in which case inferring the top N locations reliably is difficult) or the space domain (meaning the data granularity is strictly higher than the cell level). In both cases, the utility of the anonymized location data will be decreased, potentially by a significant amount.
【Keywords】: cellular data; k-anonymity; location; privacy
【Paper Link】 【Pages】:157-168
【Authors】: Adel Aziz ; Julien Herzen ; Ruben Merz ; Seva Shneer ; Patrick Thiran
【Abstract】: The goal of jointly providing efficiency and fairness in wireless networks can be seen as the problem of maximizing a given utility function. In contrast with wired networks, the capacity of wireless networks is typically time-varying and not known explicitly. Hence, as the capacity region is impossible to know or measure exactly, existing scheduling schemes either under-estimate it and are too conservative, or they over-estimate it and suffer from congestion collapse. We propose a new adaptive algorithm, called Enhance & Explore (E&E). It maximizes the utility of the network without requiring any explicit characterization of the capacity region. E&E works above the MAC layer and it does not demand any modification to the existing networking stack. We first evaluate our algorithm theoretically and we prove that it converges to a state of optimal utility. We then evaluate the performance of the algorithm in a WLAN setting, using both simulations and real measurements on a testbed composed of IEEE 802.11 wireless routers. Finally, we investigate a wireless mesh network setting and we find that, when coupled with an efficientmechanismfor congestioncontrol, the E&E algorithm greatly increases the utility achieved by multi-hop networks as well.
【Keywords】: E&E; WLAN; adaptive algorithm; congestion control; convergence proof; experimental evaluation; fairness; multi-hop networks; utility maximization; wireless networks
【Paper Link】 【Pages】:169-180
【Authors】: Dinan Gunawardena ; Thomas Karagiannis ; Alexandre Proutière ; Elizeu Santos-Neto ; Milan Vojnovic
【Abstract】: We consider the problem of delivering information streams to interested mobile users, leveraging both access to the infrastructure and device-to-device data transfers. The goal is to design practical relaying algorithms that aim at optimizing a global system objective that accounts for two important aspects: first, the user interest in content with respect to its type and delivery time; and, second, resource constraints such as storage and transmission costs. We first examine a set of real-world datasets reporting contacts between users moving in relatively restricted geographic areas (e.g. a city). These datasets provide evidence that significant performance gains can be achieved by extending the information dissemination from one to two hops, and that using longer paths only brings marginal benefits. We also show that correlation of delays through different paths is typically significant, thus asking for system design that would allow for general user mobility. We then propose a class of relaying strategies (referred to as SCOOP) that aim at optimizing a global system objective, are fully decentralized, require only locally observable states by individual devices, and allow for general user mobility. These properties characterize a practical scheme whose efficiency is evaluated using real-world mobility traces.
【Keywords】: DTNs; decentralized opportunistic relaying; two-hop relaying
【Paper Link】 【Pages】:181-192
【Authors】: Xiaozheng Tie ; Arun Venkataramani ; Aruna Balasubramanian
【Abstract】: Our work is motivated by a simple question: can we design a simple routing protocol that ensures robust performance across networks with diverse connectivity characteristics such as meshes, MANETs, and DTNs? We identify packet replication as a key structural difference between protocols designed for opposite ends of the connectivity spectrum---DTNs and meshes. We develop a model to quantify under what conditions and by how much replication improves packet delays, and use these insights to drive the design of R3, a routing protocol that self-adapts replication to the extent of uncertainty in network path delays. We implement and deploy R3 on a mesh testbed and a DTN testbed. To the best of our knowledge, R3 is the first routing protocol to be deployed and evaluated on both a DTN testbed and a mesh testbed. We evaluate its performance through deployment, trace-driven simulations, and emulation experiments. Our results show that R3 achieves significantly better delay and goodput over existing protocols in a variety of network connectivity and load conditions.
【Keywords】: forwarding; replication; wireless routing
【Paper Link】 【Pages】:193-204
【Authors】: Hsu-Chun Hsiao ; Ahren Studer ; Chen Chen ; Adrian Perrig ; Fan Bai ; Bhargav Bellur ; Aravind Iyer
【Abstract】: Digital signatures are one of the fundamental security primitives in Vehicular Ad-Hoc Networks (VANETs) because they provide authenticity and non-repudiation in broadcast communication. However, the current broadcast authentication standard in VANETs is vulnerable to signature flooding: excessive signature verification requests that exhaust the computational resources of victims. In this paper, we propose two efficient broadcast authentication schemes, Fast Authentication (FastAuth) and Selective Authentication (SelAuth), as two countermeasures to signature flooding. FastAuth secures periodic single-hop beacon messages. By exploiting the sender's ability to predict its own future beacons, FastAuth enables 50 times faster verification than previous mechanisms using the Elliptic Curve Digital Signature Algorithm. SelAuth secures multi-hop applications in which a bogus signature may spread out quickly and impact a significant number of vehicles. SelAuth pro- vides fast isolation of malicious senders, even under a dynamic topology, while consuming only 15%--30% of the computational resources compared to other schemes. We provide both analytical and experimental evaluations based on real traffic traces and NS-2 simulations. With the near-term deployment plans of VANET on all vehicles, our approaches can make VANETs practical.
【Keywords】: VANETs; broadcast authentication; flooding resilience; signatures
【Paper Link】 【Pages】:205-216
【Authors】: Xinyu Zhang ; Kang G. Shin
【Abstract】: WiFi interface is known to be a primary energy consumer in mobile devices, and idle listening (IL) is the dominant source of energy consumption in WiFi. Most existing protocols, such as the 802.11 power-saving mode (PSM), attempt to reduce the time spent in IL by sleep scheduling. However, through an extensive analysis of real-world traffic, we found more than 60% of energy is consumed in IL, even with PSM enabled. To remedy this problem, we propose E-MiLi (Energy-Minimizing idle Listening) that reduces the power consumption in IL, given that the time spent in IL has already been optimized by sleep scheduling. Observing that radio power consumption decreases proportionally to its clock-rate, E-MiLi adaptively downclocks the radio during IL, and reverts to full clock-rate when an incoming packet is detected or a packet has to be transmitted. E-MiLi incorporates sampling rate invariant detection, ensuring accurate packet detection and address filtering even when the receiver's sampling clock-rate is much lower than the signal bandwidth. Further, it employs an opportunistic downclocking mechanism to optimize the efficiency of switching clock-rate, based on a simple interface to existing MAC-layer scheduling protocols. We have implemented E-MiLi on the USRP software radio platform. Our experimental evaluation shows that E-MiLi can detect packets with close to 100% accuracy even with downclocking by a factor of 16. When integrated with 802.11, E-MiLi can reduce energy consumption by around 44% for 92% of users in real-world wireless networks.
【Keywords】: CSMA wireless networks; adapting clock-rate; dynamic frequency scaling; energy efficiency; idle listening; packet detection
【Paper Link】 【Pages】:217-228
【Authors】: Shan-Hsiang Shen ; Aaron Gember ; Ashok Anand ; Aditya Akella
【Abstract】: Many systems have leveraged the broadcast nature of wireless radios to improve wireless capacity and performance. While conventional approaches have focused on overhearing entire packets, recent designs have argued that focusing on overheard content may be more effective. Unfortunately, key design choices in these approaches limit them from fully leveraging the benefits of overhearing content. We propose a cleaner refactoring of functionality where-in overhearing is realized at the sub-packet payload level through the use of IP-layer redundancy elimination. We show that this dramatically improves the effectiveness of prior overhearing based approaches and enables new designs, e.g., enhanced network coding, where content overhearing can be more effectively integrated to improve performance. Realizing the benefits of IP-layer content overhearing requires us to overcome challenges arising from the probabilistic nature of wireless reception (which could lead to inconsistent state) and the limited resources on wireless devices. We overcome these challenges through careful data structure and wireless redundancy elimination designs. We evaluate the effectiveness of our system using experimentation on real traces. We find that our design is highly effective: e.g., it can improve goodput by nearly 25% and air time utilization by nearly 20%.
【Keywords】: redundancy elimination; throughput; wireless networks
【Paper Link】 【Pages】:229-240
【Authors】: Zhangyu Guan ; Tommaso Melodia ; Dongfeng Yuan ; Dimitrios A. Pados
【Abstract】: It is well known that the data transport capacity of a wireless network can be increased by leveraging the spatial and frequency diversity of the wireless transmission medium. This has motivated the recent surge of research in cooperative and dynamic-spectrum-access networks. Still, as of today, a key open research challenge is to design distributed control strategies to dynamically jointly assign (i) portions of the spectrum and (ii) cooperative relays to different traffic sessions to maximize the resulting network-wide data rate. In this article, we make a significant contribution in this direction. First, we mathematically formulate the problem of joint spectrum management and relay selection for a set of sessions concurrently utilizing an interference-limited infrastructure-less wireless network. We then study distributed solutions to this (nonlinear and nonconvex) problem. The overall problem is separated into two subproblems, (i) spectrum management through power allocation with given relay selection strategy, and (ii) relay selection for a given spectral profile. Distributed solutions for each of the two subproblems are proposed, which are then analyzed based on notions from variational inequality (VI) theory. The distributed algorithms can be proven to converge, under certain conditions, to VI solutions, which are also Nash equilibrium (NE) solutions of the equivalent NE problems. A distributed algorithm based on iterative solution of the two subproblems is then designed. Performance and price of anarchy of the distributed algorithm are then studied by comparing it to the globally optimal solution obtained with a centralized algorithm. Simulation results show that the proposed distributed algorithm achieves performance that is within a few percentage points of the optimal solution.
【Keywords】: cooperative networks; nash equilibrium; relay selection; spectrum management
【Paper Link】 【Pages】:241-252
【Authors】: Souvik Sen ; Romit Roy Choudhury ; Srihari Nelakuditi
【Abstract】: Conventional WiFi networks perform channel contention in time domain. This is known to be wasteful because the channel is forced to remain idle while all contending nodes are backing off for multiple time slots. This paper proposes to break away from convention and recreate the backing off operation in the frequency domain. Our basic idea leverages the observation that OFDM subcarriers can be treated as integer numbers. Thus, instead of picking a random backoff duration in time, a contending node can signal on a randomly chosen subcarrier. By employing a second antenna to listen to all the subcarriers, each node can determine whether its chosen integer (or subcarrier) is the smallest among all others. In fact, each node can even determine the rank of its chosen subcarrier, enabling the feasibility of scheduled transmissions after every round of contention. We develop these ideas into a Back2F protocol that migrates WiFi backoff to the frequency domain. Experiments on a prototype of 10 USRPs confirm feasibility, along with consistent throughput gains over 802.11. at high bit rates. Trace based simulations affirm scalability to larger, real-world network topologies.
【Keywords】: backoff; contention resolution; cross-layer; wireless
【Paper Link】 【Pages】:253-264
【Authors】: Apurv Bhartia ; Yi-Chao Chen ; Swati Rallapalli ; Lili Qiu
【Abstract】: Wireless multicarrier communication systems transmit data by spreading it over multiple subcarriers and are widely used today owing to their robustness to multipath fading, high spectrum efficiency, and ease of implementation. In this paper, we use real measurements to show there is significant frequency diversity in Wi-Fi channels, and propose a series of techniques to explicitly harness such frequency diversity. In particular, we leverage the Channel State Information (CSI), which captures the SNR on each subcarrier to (i) map symbols to subcarriers according to their importance, (ii) effectively recover partially corrupted FEC groups and facilitate FEC decoding, and (iii) develop MAC-layer FEC to offer different degrees of protection to the symbols according to their error rates at the PHY layer. We further develop a rate adaptation approach that works together with these optimization schemes. Our trace-driven simulation and testbed experiments based on USRP clearly demonstrate the effectiveness of our approaches.
【Keywords】: IEEE 802.11; cross layer design; forward error correction (FEC); orthorgonal frequency division multiplexing (OFDM); rate adaptation
【Paper Link】 【Pages】:265-276
【Authors】: Hang Yu ; Lin Zhong ; Ashutosh Sabharwal ; David Kao
【Abstract】: In this work, we report the first study of an important realization of directional communication, beamforming, on mobile devices. We first demonstrate that beamforming is already feasible on mobile devices in terms of form factor, device mobility and power efficiency. Surprisingly, we show that by making an increasingly profitable tradeoff between transmit and circuit power, beamforming with state-of-the-art integrated CMOS implementations can be more power-efficient than its single antenna counterpart. We then investigate the optimal way of using beamforming in terms of device power efficiency, by allowing a dynamic number of active antennas. We propose a simple yet effective solution, BeamAdapt, which allows each mobile client in a network to individually identify the optimal number of active antennas with guaranteed convergence and close-to-optimal performance. We finally report a WARP-based prototype of BeamAdapt and experimentally demonstrate its effectiveness in realistic environments, and then complement the prototype-based experiments with Qualnet-based simulation of a large-scale network. Our results show that BeamAdapt with four antennas can reduce the power consumption of mobile clients by more than half compared to a single antenna, while maintaining a required network throughput.
【Keywords】: beamadapt; beamforming; mobile devices; power efficiency
【Paper Link】 【Pages】:277-288
【Authors】: Siripuram Aditya ; Sachin Katti
【Abstract】: Video streaming performance on wireless networks is choppy. The culprit is the unpredictable wireless medium, whose fluctuations results in fluctuating throughput and bit errors. Current video codecs are not equipped to handle such variations since they exhibit an all or nothing behavior. If the channel is strong and above a threshold, the video stream gets decoded perfectly. If not, typically nothing gets decoded. Thus, there is no graceful degradation with wireless conditions. In this paper, we present a new technique FlexCast, that delivers a video reconstruction whose quality automatically varies with the channel conditions. The key idea is a novel joint-source channel code, that allows the sender to continuously transmit video bits, and the receiver to decode a video quality corresponding to the number of bits it receives and the instantaneous wireless channel quality. We show via experimental evaluation that name performs almost as well as the optimal scheme, and outperforms the state of the art graceful video delivery systems by nearly 6dB PSNR.
【Keywords】: coding; video; wireless
【Paper Link】 【Pages】:289-300
【Authors】: Szymon Jakubczak ; Dina Katabi
【Abstract】: Today's mobile video suffers from two limitations: 1) it cannot reduce bandwidth consumption by leveraging wireless broadcast to multicast popular content to interested receivers, and 2) it lacks robustness to wireless interference and errors. This paper presents SoftCast, a cross-layer design for mobile video that addresses both limitations. To do so, SoftCast changes the network stack to act like a linear transform. As a result, the transmitted video signal becomes linearly related to the pixels' luminance. Thus, when noise perturbs the transmitted signal samples, the perturbation naturally translates into approximation in the original video pixels. This enables a video source to multicast a single stream that each receiver decodes to a video quality commensurate with its channel quality. It also increases robustness to interference and errors which now reduce the sharpness of the received pixels but do not cause the video to glitch or stall. We have implemented SoftCast and evaluated it in a testbed of software radios. Our results show that it improves the average video quality for multicast users by 5.5dB, eliminates video glitches caused by mobility, and increases robustness to packet loss by an order of magnitude.
【Keywords】: joint source-channel coding; scalable video communications; wireless networks
【Paper Link】 【Pages】:301-312
【Authors】: Mayank Jain ; Jung Il Choi ; Taemin Kim ; Dinesh Bharadia ; Siddharth Seth ; Kannan Srinivasan ; Philip Levis ; Sachin Katti ; Prasun Sinha
【Abstract】: This paper presents a full duplex radio design using signal inversion and adaptive cancellation. Signal inversion uses a simple design based on a balanced/unbalanced (Balun) transformer. This new design, unlike prior work, supports wideband and high power systems. In theory, this new design has no limitation on bandwidth or power. In practice, we find that the signal inversion technique alone can cancel at least 45dB across a 40MHz bandwidth. Further, combining signal inversion cancellation with cancellation in the digital domain can reduce self-interference by up to 73dB for a 10MHz OFDM signal. This paper also presents a full duplex medium access control (MAC) design and evaluates it using a testbed of 5 prototype full duplex nodes. Full duplex reduces packet losses due to hidden terminals by up to 88%. Full duplex also mitigates unfair channel allocation in AP-based networks, increasing fairness from 0.85 to 0.98 while improving downlink throughput by 110% and uplink throughput by 15%. These experimental results show that a re- design of the wireless network stack to exploit full duplex capability can result in significant improvements in network performance.
【Keywords】: full-duplex wireless
【Paper Link】 【Pages】:313-324
【Authors】: Victoria Manfredi ; Mark Crovella ; Jim Kurose
【Abstract】: Structural change and uncertainty are fundamental properties of an ad hoc network, making it difficult to develop communication strategies, i.e., network-level approaches to transport data from sender to receiver. At a basic level, change and uncertainty affect how long any state maintained by a communication strategy remains useful, and so influence the trade-offs made to collect that state. In this paper, we introduce a framework for organizing the decision space for deciding when a communication strategy should maintain state, and what type of state should be maintained, in an ad hoc network. The framework is based on our observation that three network properties (connectivity, unpredictability, and resource contention) determine when state is useful. Using the framework, we make three contributions. First, we illustrate the framework by showing an instantiation in terms of specific measures that can be used to describe a network setting. Second, we validate the framework by showing it correctly and consistently organizes the decision space for different communication strategies. Finally, we demonstrate the analytic power of the framework by using it to (1) uncover surprising aspects of well-known traces, and (2) identify the need for, and value of, a new strategy for network communication.
【Keywords】: ad hoc networks; communication strategies; delay tolerant networks; entropy; state maintenance
【Paper Link】 【Pages】:325-336
【Authors】: Yang Song ; Starsky H. Y. Wong ; Kang-Won Lee
【Abstract】: We consider a coalition network where multiple groups are interconnected via wireless links. Gateway nodes are designated by each domain to achieve a network-wide interoperability. Due to the inter-domain communication cost, the optimal gateway selection for one single domain depends on the gateway selections of other domains and vice versa. In this paper, we investigate the interactions of gateway selections by multiple domains from a potential game perspective. The equilibrium inefficiency in terms of price of stability is characterized under various conditions. In addition, we examine the well-established equilibrium selective learning algorithm B-logit and show that B-logit is a special case of a general family of algorithms, denoted by Γ collectively. A novel learning algorithm named MAX-logit is proposed, which retains the favorable equilibrium selection property with the provably fastest convergence speed than any other algorithms in Γ, and can be applied to many other applications of potential games. Simulation results show that MAX-logit can improve the convergence speed of B-logit by up to 33.85%.
【Keywords】: gateway selection; mixing time analysis; potential games
【Paper Link】 【Pages】:337-348
【Authors】: Shizhen Zhao ; Luoyi Fu ; Xinbing Wang ; Qian Zhang
【Abstract】: We investigate the fundamental relationship between node density and transmission delay in large-scale wireless ad hoc networks with unreliable links from percolation perspective. Previous works[11][2][10] have already showed the relationship between transmission delay and distance from source to destination. However, it still remains as an open question how transmission delay varies in accordance with node density. Answering this question can provide guidance for determining the number of nodes to meet the delay requirement when designing ad hoc networks. In this paper, we study the impact of node density λ on the ratio of delay and distance, denoted by γ(λ). We analytically characterize the properties of γ(λ) as a function of λ. And then we present upper and lower bounds to γ(λ). Next, we take propagation delay into consideration and obtain further results on the upper and lower bounds of γ(λ). Finally, we make simulations to verify our theoretical analysis.
【Keywords】: connectivity; delay; density