The Experts below are selected from a list of 4941 Experts worldwide ranked by ideXlab platform

Robert W. Heath - One of the best experts on this subject based on the ideXlab platform.

  • GLOBECOM Workshops - Geometric Tracking of Vehicular mmWave Channels to Enable Machine Learning of Onboard Sensors
    2018 IEEE Globecom Workshops (GC Wkshps), 2018
    Co-Authors: Erich Zochmann, Markus Rupp, Robert W. Heath
    Abstract:

    Estimating Time-selective millimeter wave wireless Channels with directive antennas poses a challenging task. A feasible way of relaxing this Channel estimation problem is to focus on the tracking of a few multipath components (MPCs). Aligning antenna beams to the tracked MPCs increases the Channel Coherence Time by several orders of magnitude. We propose to track the MPCs geometrically. Our geometric trackers are based on algorithms known as Doppler-bearing tracking. We reformulate recent work on geometric-polar tracking into an efficient recursive version. If the relative position of the MPCs are known, other sensors on board of a vehicle, for example, lidar, radar, camera, will be capable of performing supervised learning based on their own observed data. Learning the relationship between sensor data and MPCs allows onboard sensors to participate in the Channel tracking. Joint tracking from many onboard sensors possibly increases the reliability of the MPC tracking.

  • The Impact of Beamwidth on Temporal Channel Variation in Vehicular Channels and Its Implications
    IEEE Transactions on Vehicular Technology, 2017
    Co-Authors: Junil Choi, Robert W. Heath
    Abstract:

    Millimeter wave (mmWave) has great potential in realizing high data rates, thanks to the large spectral Channels. It is considered as a key technology for fifth-generation (5G) wireless networks and is already used in wireless LAN (e.g., IEEE 802.11ad). Using mmWave for vehicular communications, however, is often viewed with some skepticism due to a misconception that the Doppler spread would become too large at these high frequencies. This is not necessarily true when directional beams are employed. In this paper, closed-form expressions relating the Channel Coherence Time and beamwidth are derived. Unlike prior work that assumed perfect beam pointing, the pointing error due to the receiver motion is incorporated to show that there exists a nonzero optimal beamwidth that maximizes the Coherence Time. We define a novel concept of beam Coherence Time, which is an effective measure of beam alignment frequency. Using the derived correlation function, the Channel Coherence Time, and the beam Coherence Time, an overall performance metric considering both the Channel Time variation and the beam alignment overhead is derived. Using this metric, it is shown that beam realignment in every beam Coherence Time performs better than beam realignment in every Channel Coherence Time.

  • a lower bound on the optimum feedback rate for downlink multi antenna cellular networks
    International Symposium on Information Theory, 2016
    Co-Authors: Jeonghun Park, Robert W. Heath, Jeffrey G. Andrews, Namyoon Lee
    Abstract:

    We consider a multi-antenna downlink cellular network using either single-user maximal ratio transmission (MRT) or multi-user zero-forcing (ZF) transmission. The locations of the base stations are modeled by a Poisson point process to allow the inter-cell interference to be tractably analyzed. A tight lower bound on the optimum number of feedback bits maximizing the net spectral efficiency is derived, whereby the cost of feedback sent via uplink is subtracted from the corresponding gain in downlink spectral efficiency. When using MRT, the optimum number of feedback bits is shown to scale linearly with the number of antennas, and logarithmically with the Channel Coherence Time. With ZF, the optimum amount of feedback scales the same as with MRT, but additionally also increases linearly with the pathloss exponent.

  • Multiple-Antenna Transmission With Limited Feedback in Device-to-Device Networks
    IEEE Wireless Communications Letters, 2016
    Co-Authors: Jeonghun Park, Robert W. Heath
    Abstract:

    We consider an overlay device-to-device network, where each multiple-antenna transmitter sends data to a designated single-antenna receiver by using maximum ratio transmission based on limited feedback, and their locations are modeled by a homogeneous Poisson point process. We obtain a lower bound on the optimum number of feedback bits that maximizes net spectral efficiency, capturing the overall system gain of using limited feedback. Our main finding is that the optimum number of feedback bits scales linearly with the number of transmit antennas and logarithmically with the Channel Coherence Time and the inverse of the node density.

  • ISIT - A lower bound on the optimum feedback rate for downlink multi-antenna cellular networks
    2016 IEEE International Symposium on Information Theory (ISIT), 2016
    Co-Authors: Jeonghun Park, Robert W. Heath, Jeffrey G. Andrews, Namyoon Lee
    Abstract:

    We consider a multi-antenna downlink cellular network using either single-user maximal ratio transmission (MRT) or multi-user zero-forcing (ZF) transmission. The locations of the base stations are modeled by a Poisson point process to allow the inter-cell interference to be tractably analyzed. A tight lower bound on the optimum number of feedback bits maximizing the net spectral efficiency is derived, whereby the cost of feedback sent via uplink is subtracted from the corresponding gain in downlink spectral efficiency. When using MRT, the optimum number of feedback bits is shown to scale linearly with the number of antennas, and logarithmically with the Channel Coherence Time. With ZF, the optimum amount of feedback scales the same as with MRT, but additionally also increases linearly with the pathloss exponent.

Jawad A. Salehi - One of the best experts on this subject based on the ideXlab platform.

  • Statistical Studies of Fading in Underwater Wireless Optical Channels in the Presence of Air Bubble, Temperature, and Salinity Random Variations (Long Version)
    arXiv: Information Theory, 2018
    Co-Authors: Mohammad Vahid Jamali, Ali Mirani, Alireza Parsay, Bahman Abolhassani, Pooya Nabavi, Ata Chizari, Pirazh Khorramshahi, Sajjad Abdollahramezani, Jawad A. Salehi
    Abstract:

    Optical signal propagation through underwater Channels is affected by three main degrading phenomena, namely absorption, scattering, and fading. In this paper, we experimentally study the statistical distribution of intensity fluctuations in underwater wireless optical Channels with random temperature and salinity variations as well as the presence of air bubbles. In particular, we define different scenarios to produce random fluctuations on the water refractive index across the propagation path, and then examine the accuracy of various statistical distributions in terms of their goodness of fit to the experimental data. We also obtain the Channel Coherence Time to address the average period of fading temporal variations. The scenarios under consideration cover a wide range of scintillation index from weak to strong turbulence. Moreover, the effects of beam-collimator at the transmitter side and aperture averaging lens at the receiver side are experimentally investigated. We show that the use of a transmitter beam-collimator and/or a receiver aperture averaging lens suits single-lobe distributions such that the generalized Gamma and exponential Weibull distributions can excellently match the histograms of the acquired data. Our experimental results further reveal that the Channel Coherence Time is on the order of $10^{-3}$ seconds and larger which implies to the slow fading turbulent Channels.

  • Statistical Studies of Fading in Underwater Wireless Optical Channels in the Presence of Air Bubble, Temperature, and Salinity Random Variations
    IEEE Transactions on Communications, 2018
    Co-Authors: Mohammad Vahid Jamali, Ali Mirani, Alireza Parsay, Bahman Abolhassani, Pooya Nabavi, Ata Chizari, Pirazh Khorramshahi, Sajjad Abdollahramezani, Jawad A. Salehi
    Abstract:

    Optical signal propagation through underwater Channels is affected by three main degrading phenomena, namely, absorption, scattering, and fading. In this paper, we experimentally study the statistical distribution of intensity fluctuations in underwater wireless optical Channels with random temperature and salinity variations, as well as the presence of air bubbles. In particular, we define different scenarios to produce random fluctuations on the water refractive index across the propagation path and, then, examine the accuracy of various statistical distributions in terms of their goodness of fit to the experimental data. We also obtain the Channel Coherence Time to address the average period of fading temporal variations. The scenarios under consideration cover a wide range of scintillation index from weak to strong turbulence. Moreover, the effects of beam-expander-and-collimator (BEC) at the transmitter side and aperture averaging lens (AAL) at the receiver side are experimentally investigated. We show that the use of a transmitter BEC and/or a receiver AAL suits single-lobe distributions, such that the generalized Gamma and exponentiated Weibull distributions can excellently match the histograms of the acquired data. Our experimental results further reveal that the Channel Coherence Time is on the order of 10-3 s and larger which implies to the slow fading turbulent Channels.

Xinchen Zhang - One of the best experts on this subject based on the ideXlab platform.

  • beam switching for millimeter wave communication to support high speed trains
    Vehicular Technology Conference, 2015
    Co-Authors: Vutha Va, Xinchen Zhang, Robert W. Heath
    Abstract:

    This paper considers a millimeter wave (mmWave) system that enables multi-Gbps wireless service for high speed trains. MmWave systems require proper beam alignment to achieve good performance making it difficult to apply to high speed trains due to the need for frequent realignment. In this paper, using the IEEE 802.11ad system parameters, we first show that the Channel Coherence Time will not be enough for communication with wide beams, and thus conventional approaches based on beam sweeping become inefficient if not impossible. Then we consider a beam switching approach that leverages the position information from the train control system for efficient beam alignment. Using this network architecture, we investigate the optimal choice of beamwidth and show that a properly optimized system can achieve multi-Gbps throughput.

  • VTC Fall - Beam Switching for Millimeter Wave Communication to Support High Speed Trains
    2015 IEEE 82nd Vehicular Technology Conference (VTC2015-Fall), 2015
    Co-Authors: Xinchen Zhang, Robert W. Heath
    Abstract:

    This paper considers a millimeter wave (mmWave) system that enables multi-Gbps wireless service for high speed trains. MmWave systems require proper beam alignment to achieve good performance making it difficult to apply to high speed trains due to the need for frequent realignment. In this paper, using the IEEE 802.11ad system parameters, we first show that the Channel Coherence Time will not be enough for communication with wide beams, and thus conventional approaches based on beam sweeping become inefficient if not impossible. Then we consider a beam switching approach that leverages the position information from the train control system for efficient beam alignment. Using this network architecture, we investigate the optimal choice of beamwidth and show that a properly optimized system can achieve multi-Gbps throughput.

Christian Bettstetter - One of the best experts on this subject based on the ideXlab platform.

  • Autocorrelation and Coherence Time of Interference in Poisson Networks
    IEEE Transactions on Mobile Computing, 2020
    Co-Authors: Udo Schilcher, Jorge F. Schmidt, Mahin K. Atiq, Christian Bettstetter
    Abstract:

    This article takes an analytical approach to investigate the temporal dynamics of interference in wireless networks. We propose a framework to calculate the autocorrelation of interference in Poisson networks and derive closed-form expressions for the case of Nakagami fading. The framework takes three correlation sources into account: the location of interferers, the wireless Channel, and the data traffic. We introduce the interference Coherence Time—in analogy to the well-established Channel Coherence Time—and show how its basic qualitative behavior depends on the source of correlation. The insights gained can be useful in the design of medium access control and retransmission protocols.

  • Medium Access with Adaptive Relay Selection in Cooperative Wireless Networks
    IEEE Transactions on Mobile Computing, 2014
    Co-Authors: Helmut Adam, Evsen Yanmaz, Christian Bettstetter
    Abstract:

    We specify and evaluate a protocol for cooperative relay communications in wireless networks targeted for low-budget and energy-constrained off-the-shelf hardware. The protocol located at the Medium Access Control (MAC) layer integrates radio resource reservation, relay selection, and packet flow. Performance is evaluated with different parameters, such as node density, Channel Coherence Time, and data packet size. Higher network-wide reliability and throughput compared to noncooperative protocols can be achieved in dense networks and unreliable Channels. At the same Time, throughput does not degrade in sparse networks or good Channel conditions.

Namyoon Lee - One of the best experts on this subject based on the ideXlab platform.

  • a lower bound on the optimum feedback rate for downlink multi antenna cellular networks
    International Symposium on Information Theory, 2016
    Co-Authors: Jeonghun Park, Robert W. Heath, Jeffrey G. Andrews, Namyoon Lee
    Abstract:

    We consider a multi-antenna downlink cellular network using either single-user maximal ratio transmission (MRT) or multi-user zero-forcing (ZF) transmission. The locations of the base stations are modeled by a Poisson point process to allow the inter-cell interference to be tractably analyzed. A tight lower bound on the optimum number of feedback bits maximizing the net spectral efficiency is derived, whereby the cost of feedback sent via uplink is subtracted from the corresponding gain in downlink spectral efficiency. When using MRT, the optimum number of feedback bits is shown to scale linearly with the number of antennas, and logarithmically with the Channel Coherence Time. With ZF, the optimum amount of feedback scales the same as with MRT, but additionally also increases linearly with the pathloss exponent.

  • ISIT - A lower bound on the optimum feedback rate for downlink multi-antenna cellular networks
    2016 IEEE International Symposium on Information Theory (ISIT), 2016
    Co-Authors: Jeonghun Park, Robert W. Heath, Jeffrey G. Andrews, Namyoon Lee
    Abstract:

    We consider a multi-antenna downlink cellular network using either single-user maximal ratio transmission (MRT) or multi-user zero-forcing (ZF) transmission. The locations of the base stations are modeled by a Poisson point process to allow the inter-cell interference to be tractably analyzed. A tight lower bound on the optimum number of feedback bits maximizing the net spectral efficiency is derived, whereby the cost of feedback sent via uplink is subtracted from the corresponding gain in downlink spectral efficiency. When using MRT, the optimum number of feedback bits is shown to scale linearly with the number of antennas, and logarithmically with the Channel Coherence Time. With ZF, the optimum amount of feedback scales the same as with MRT, but additionally also increases linearly with the pathloss exponent.

  • space Time interference alignment and degree of freedom regions for the miso broadcast Channel with periodic csi feedback
    IEEE Transactions on Information Theory, 2014
    Co-Authors: Namyoon Lee, Robert W. Heath
    Abstract:

    This paper characterizes the degree-of-freedom (DoF) regions for the multiuser vector broadcast Channel with periodic Channel state information (CSI) feedback. As a part of the characterization, a new transmission method called space-Time interference alignment is proposed, which exploits both the current and past CSI jointly. Using the proposed alignment technique, an inner bound of the sum-DoF region is characterized as a function of a normalized CSI feedback frequency, which measures CSI feedback speed compared to the speed of user's Channel variations. One consequence of the result is that the achievable sum-DoF gain is improved significantly when a user sends back both current and outdated CSI compared to the case where the user sends back current CSI only. Then, a tradeoff between CSI feedback delay and the sum-DoF gain is characterized for the multiuser vector broadcast Channel in terms of a normalized CSI feedback delay that measures CSI obsoleteness compared to Channel Coherence Time. A crucial insight is that it is possible to achieve the optimal DoF gain if the feedback delay is less than a derived fraction of the Channel Coherence Time. This precisely characterizes the intuition that a small delay should be negligible.

  • Space-Time Interference Alignment and Degrees of Freedom Regions for the MISO Broadcast Channel with Periodic CSI Feedback
    arXiv: Information Theory, 2013
    Co-Authors: Namyoon Lee, Robert W. Heath
    Abstract:

    This paper characterizes the degrees of freedom (DoF) regions for the multi-user vector broadcast Channel with periodic Channel state information (CSI) feedback. As a part of the characterization, a new transmission method called space-Time interference alignment is proposed, which exploits both the current and past CSI jointly. Using the proposed alignment technique, an inner bound of the sum-DoF region is characterized as a function of a normalized CSI feedback frequency, which measures CSI feedback speed compared to the speed of user's Channel variations. One consequence of the result is that the achievable sum-DoF gain is improved significantly when a user sends back both current and outdated CSI compared to the case where the user sends back current CSI only. Then, a trade-off between CSI feedback delay and the sum-DoF gain is characterized for the multi-user vector broadcast Channel in terms of a normalized CSI feedback delay that measures CSI obsoleteness compared to Channel Coherence Time. A crucial insight is that it is possible to achieve the optimal DoF gain if the feedback delay is less than a derived fraction of the Channel Coherence Time. This precisely characterizes the intuition that a small delay should be negligible.

  • Not Too Delayed CSIT Achieves the Optimal Degrees of Freedom
    arXiv: Information Theory, 2012
    Co-Authors: Namyoon Lee, Robert W. Heath
    Abstract:

    Channel state information at the transmitter (CSIT) aids interference management in many communication systems. Due to Channel state information (CSI) feedback delay and Time-variation in the wireless Channel, perfect CSIT is not realistic. In this paper, the CSI feedback delay-DoF gain trade-off is characterized for the multi-user vector broadcast Channel. A major insight is that it is possible to achieve the optimal degrees of freedom (DoF) gain if the delay is less than a certain fraction of the Channel Coherence Time. This precisely characterizes the intuition that a small delay should be negligeable. To show this, a new transmission method called space-Time interference alignment is proposed, which actively exploits both the current and past CSI.