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37
Design and Experimental Evaluation of Multi-User Beamforming in Wireless LANs
"... Multi-User MIMO promises to increase the spectral efficiency of next generation wireless systems and is currently being incorporated in future industry standards. Although a significant amount of research has focused on theoretical capacity analysis, little is known about the performance of such sys ..."
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Cited by 49 (6 self)
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Multi-User MIMO promises to increase the spectral efficiency of next generation wireless systems and is currently being incorporated in future industry standards. Although a significant amount of research has focused on theoretical capacity analysis, little is known about the performance of such systems in practice. In this paper, we present the design and implementation of the first multiuser beamforming system and experimental framework for wireless LANs. Using extensive measurements in an indoor environment, we evaluate the impact of receiver separation distance, outdated channel information due to mobility and environmental variation, and the potential for increasing spatial reuse. For the measured indoor environment, our results reveal that two receivers achieve close to maximum performance with a minimum separation distance of a quarter of a wavelength. We also show that the required channel information update rate is dependent on environmental variation and user mobility as well as a per-link SNR requirement. Assuming that a link can tolerate an SNR decrease of 3 dB, the required channel update rate is equal to 100 and 10 ms for non-mobile receivers and mobile receivers with a pedestrian speed of 3 mph respectively. Our results also show that spatial reuse can be increased by efficiently eliminating interference at any desired location; however, this may come at the expense of a significant drop in the quality of the served users.
Mirror mirror on the ceiling: flexible wireless links for data centers
- In SIGCOMM
, 2012
"... Modern data centers are massive, and support a range of dis-tributed applications across potentially hundreds of server racks. As their utilization and bandwidth needs continue to grow, traditional methods of augmenting bandwidth have proven complex and costly in time and resources. Recent measureme ..."
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Cited by 21 (5 self)
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Modern data centers are massive, and support a range of dis-tributed applications across potentially hundreds of server racks. As their utilization and bandwidth needs continue to grow, traditional methods of augmenting bandwidth have proven complex and costly in time and resources. Recent measurements show that data center traffic is often limited by congestion loss caused by short traffic bursts. Thus an at-tractive alternative to adding physical bandwidth is to aug-ment wired links with wireless links in the 60 GHz band. We address two limitations with current 60 GHz wire-less proposals. First, 60 GHz wireless links are limited by line-of-sight, and can be blocked by even small obstacles. Second, even beamforming links leak power, and potential interference will severely limit concurrent transmissions in dense data centers. We propose and evaluate a new wireless primitive for data centers, 3D beamforming, where 60 GHz signals bounce off data center ceilings, thus establishing in-direct line-of-sight between any two racks in a data center. We build a small 3D beamforming testbed to demonstrate its ability to address both link blockage and link interfer-ence, thus improving link range and number of concurrent transmissions in the data center. In addition, we propose a simple link scheduler and use traffic simulations to show that these 3D links significantly expand wireless capacity compared to their 2D counterparts.
Efficient error estimating coding: Feasibility and applications
- In Proc. ACM SIGCOMM
, 2010
"... Motivated by recent emerging systems that can leverage partially correct packets in wireless networks, this paper proposes the novel concept of error estimating coding (EEC). Without correcting the errors in the packet, EEC enables the receiver of the packet to estimate the packet’s bit error rate, ..."
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Cited by 19 (1 self)
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Motivated by recent emerging systems that can leverage partially correct packets in wireless networks, this paper proposes the novel concept of error estimating coding (EEC). Without correcting the errors in the packet, EEC enables the receiver of the packet to estimate the packet’s bit error rate, which is perhaps the most important meta-information of a partially correct packet. Our EEC design provides provable estimation quality, with rather low redundancy and computational overhead. To demonstrate the utility of EEC, we exploit and implement EEC in two wireless network applications, Wi-Fi rate adaptation and real-time video streaming. Our real-world experiments show that these applications can significantly benefit from EEC. 1
XPRESS: A Cross-Layer Backpressure Architecture for Wireless Multi-Hop Networks
"... 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 po ..."
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Cited by 17 (1 self)
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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.
FLUID: Improving Throughputs in Enterprise Wireless LANs through Flexible Channelization
, 2011
"... 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, ..."
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Cited by 17 (0 self)
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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.
Directional antenna diversity for mobile devices: characterizations and solutions
- in Proc. MobiCom: ACM
, 2010
"... We report a first-of-its-kind realization of directional transmission for smartphone-like mobile devices using multiple passive directional antennas, supported by only one RF chain. The key is a multi-antenna system (MiDAS) and its antenna selection methods that judiciously select the right antenna ..."
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Cited by 16 (7 self)
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We report a first-of-its-kind realization of directional transmission for smartphone-like mobile devices using multiple passive directional antennas, supported by only one RF chain. The key is a multi-antenna system (MiDAS) and its antenna selection methods that judiciously select the right antenna for transmission. It is grounded by two measurement-driven studies regarding 1) how smartphones rotate during wireless usage in the field and 2) how orientation and rotation impact the performance of directional antennas under various propagation environments. We implement MiDAS and its antenna selection methods using the WARP platform. We evaluate the implementation using a computerized motor to rotate the prototype according to traces collected from smartphone users in the field. Our evaluation shows that MiDAS achieves a median of 3dB increase in link gain. We demonstrate that rate adaptation and power control can be combined with MiDAS to further improve goodput and power saving. Real-time experiments with the prototype show that the link gain translates to 85 % goodput improvement for a low SNR scenario. The same gain translates to 51 % transmit power reduction for a high SNR scenario. Compared to other methods in realizing directional communication, MiDAS does not require any changes to the network infrastructure, and is therefore suitable for immediate or near-future deployment.
Practical Conflict Graphs for Dynamic Spectrum Distribution
"... Most spectrum distribution proposals today develop their allocation algorithms that use conflict graphs to capture interference relationships. The use of conflict graphs, however, is often questioned by the wireless community because of two issues. First, building conflict graphs requires significan ..."
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Cited by 15 (1 self)
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Most spectrum distribution proposals today develop their allocation algorithms that use conflict graphs to capture interference relationships. The use of conflict graphs, however, is often questioned by the wireless community because of two issues. First, building conflict graphs requires significant overhead and hence generally does not scale to outdoor networks, and second, the resulting conflict graphs do not capture accumulative interference. In this paper, we use large-scale measurement data as ground truth to understand just how severe these issues are in practice, and whether they can be overcome. We build “practical”conflict graphs using measurement-calibrated propagation models, whichremovetheneedforexhaustivesignal measurements by interpolating signal strengths using calibrated models. These propagation models are imperfect, and we study the impact of their errors by tracing the impact on multiple steps in the process, from calibrating propagation models to predicting signal strength and building conflict graphs. At each step, we analyze the introduction, propagation and final impact of errors, by comparing each intermediate result to its ground truth counterpart generated from measurements. Our work produces several findings. Calibrated propagation models generate location-dependent prediction errors, ultimately producing conservative conflict graphs. While these “estimated conflict graphs ” lose some spectrum utilization, their conservative nature improves reliability by reducing the impact of accumulative interference. Finally, we propose a graph augmentation technique that addresses any remaining accumulative interference, the last missing piece in a practical spectrum distribution system using measurement-calibrated conflict graphs.
Beamforming on Mobile Devices: A First Study
"... 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 m ..."
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Cited by 12 (2 self)
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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.
3D Beamforming for Wireless Data Centers
"... Contrary to prior assumptions, recent measurements show that data center traffic is not constrained by network bisection bandwidth, but is instead prone to congestion loss caused by short traffic bursts. Compared to the cost and complexity of modifying data center architectures, a much more attracti ..."
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Cited by 6 (1 self)
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Contrary to prior assumptions, recent measurements show that data center traffic is not constrained by network bisection bandwidth, but is instead prone to congestion loss caused by short traffic bursts. Compared to the cost and complexity of modifying data center architectures, a much more attractive option is to augment wired links with flexible wireless links in the 60 GHz band. Current proposals, however, are severely constrained by two factors. First, 60 GHz wireless links are limited by line-of-sight, and can be blocked by even small obstacles between the endpoints. Second, even beamforming links leak power, and potential interference will severely limit concurrent transmissions in dense data centers. In this paper, we explore the feasibility of a new wireless primitive for data centers, 3D beamforming. We explore the design space, and show how bouncing 60 GHz wireless links off reflective ceilings can address both link blockage and link interference, thus improving link range and number of current transmissions in the data center.
SIR Based Interference Modeling For Wireless Mesh Networks: A Detailed Measurement
- Study”, COMSNETS 2012, 4 th International Conference on Communication Systems & Networks, January 3-7, 2012 at
"... Abstract—Spatial reuse is a central aspect in the efficiency of wireless mesh networks. Accurate inter-link interference measurement and estimation is necessary for such spatial reuse. In this work, we explore in depth Signal to Interference Ratio (SIR) based interference modeling. We take a measure ..."
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Abstract—Spatial reuse is a central aspect in the efficiency of wireless mesh networks. Accurate inter-link interference measurement and estimation is necessary for such spatial reuse. In this work, we explore in depth Signal to Interference Ratio (SIR) based interference modeling. We take a measurement centric approach, characterizing the SIR versus PDR (Packet Delivery Ratio) relationship in outdoor mesh network settings. Our significant findings are the following. (1) In outdoor environment there is a range of SIR values (intermediate SIR region), wherein it is difficult to predict the PDR accurately. (2) Fortunately, the width of this intermediate SIR region is small: about 4-5dB for most data rates. (3) The SIR vs PDR relationship depends significantly on the modulations used by transmitter as well as the interferer: we characterize this dependence. The use of an outdoor 802.11g-based testbed is a significant aspect of our measurements. The above findings have important implications for the design of interference measurement and prediction schemes. We quantify the accuracy of our SIR-based technique by applying an offline prediction model in our outdoor mesh testbed and observe that of the 44 pairs of links the SIR technique predicts performance with less than 10 % error for over 85 % of the links for which the SIR values lie outside the intermediate SIR region. An evaluation of existing interference modeling techniques reveals that these make certain incorrect assumptions which make them perform significantly worse than our proposed technique. I.
