Showing papers by "Bell Labs published in 2010"

Noncooperative Cellular Wireless with Unlimited Numbers of Base Station Antennas

Abstract: A cellular base station serves a multiplicity of single-antenna terminals over the same time-frequency interval. Time-division duplex operation combined with reverse-link pilots enables the base station to estimate the reciprocal forward- and reverse-link channels. The conjugate-transpose of the channel estimates are used as a linear precoder and combiner respectively on the forward and reverse links. Propagation, unknown to both terminals and base station, comprises fast fading, log-normal shadow fading, and geometric attenuation. In the limit of an infinite number of antennas a complete multi-cellular analysis, which accounts for inter-cellular interference and the overhead and errors associated with channel-state information, yields a number of mathematically exact conclusions and points to a desirable direction towards which cellular wireless could evolve. In particular the effects of uncorrelated noise and fast fading vanish, throughput and the number of terminals are independent of the size of the cells, spectral efficiency is independent of bandwidth, and the required transmitted energy per bit vanishes. The only remaining impairment is inter-cellular interference caused by re-use of the pilot sequences in other cells (pilot contamination) which does not vanish with unlimited number of antennas.

Capacity Limits of Optical Fiber Networks

Abstract: We describe a method to estimate the capacity limit of fiber-optic communication systems (or ?fiber channels?) based on information theory. This paper is divided into two parts. Part 1 reviews fundamental concepts of digital communications and information theory. We treat digitization and modulation followed by information theory for channels both without and with memory. We provide explicit relationships between the commonly used signal-to-noise ratio and the optical signal-to-noise ratio. We further evaluate the performance of modulation constellations such as quadrature-amplitude modulation, combinations of amplitude-shift keying and phase-shift keying, exotic constellations, and concentric rings for an additive white Gaussian noise channel using coherent detection. Part 2 is devoted specifically to the "fiber channel.'' We review the physical phenomena present in transmission over optical fiber networks, including sources of noise, the need for optical filtering in optically-routed networks, and, most critically, the presence of fiber Kerr nonlinearity. We describe various transmission scenarios and impairment mitigation techniques, and define a fiber channel deemed to be the most relevant for communication over optically-routed networks. We proceed to evaluate a capacity limit estimate for this fiber channel using ring constellations. Several scenarios are considered, including uniform and optimized ring constellations, different fiber dispersion maps, and varying transmission distances. We further present evidences that point to the physical origin of the fiber capacity limitations and provide a comparison of recent record experiments with our capacity limit estimation.

Multi-Cell MIMO Cooperative Networks: A New Look at Interference

Abstract: This paper presents an overview of the theory and currently known techniques for multi-cell MIMO (multiple input multiple output) cooperation in wireless networks. In dense networks where interference emerges as the key capacity-limiting factor, multi-cell cooperation can dramatically improve the system performance. Remarkably, such techniques literally exploit inter-cell interference by allowing the user data to be jointly processed by several interfering base stations, thus mimicking the benefits of a large virtual MIMO array. Multi-cell MIMO cooperation concepts are examined from different perspectives, including an examination of the fundamental information-theoretic limits, a review of the coding and signal processing algorithmic developments, and, going beyond that, consideration of very practical issues related to scalability and system-level integration. A few promising and quite fundamental research avenues are also suggested.

Distributed Caching Algorithms for Content Distribution Networks

Abstract: The delivery of video content is expected to gain huge momentum, fueled by the popularity of user-generated clips, growth of VoD libraries, and wide-spread deployment of IPTV services with features such as CatchUp/PauseLive TV and NPVR capabilities. The `time-shifted' nature of these personalized applications defies the broadcast paradigm underlying conventional TV networks, and increases the overall bandwidth demands by orders of magnitude. Caching strategies provide an effective mechanism for mitigating these massive bandwidth requirements by replicating the most popular content closer to the network edge, rather than storing it in a central site. The reduction in the traffic load lessens the required transport capacity and capital expense, and alleviates performance bottlenecks. In the present paper, we develop light-weight cooperative cache management algorithms aimed at maximizing the traffic volume served from cache and minimizing the bandwidth cost. As a canonical scenario, we focus on a cluster of distributed caches, either connected directly or via a parent node, and formulate the content placement problem as a linear program in order to benchmark the globally optimal performance. Under certain symmetry assumptions, the optimal solution of the linear program is shown to have a rather simple structure. Besides interesting in its own right, the optimal structure offers valuable guidance for the design of low-complexity cache management and replacement algorithms. We establish that the performance of the proposed algorithms is guaranteed to be within a constant factor from the globally optimal performance, with far more benign worst-case ratios than in prior work, even in asymmetric scenarios. Numerical experiments for typical popularity distributions reveal that the actual performance is far better than the worst-case conditions indicate.

Power consumption modeling of different base station types in heterogeneous cellular networks

Abstract: In wireless communications micro cells are potentially more energy efficient than conventional macro cells due to the high path loss exponent. Also, heterogeneous deployments of both cell types can be used to optimize the energy efficiency. Energy efficiency of any deployment is impacted by the power consumption of each individual network element and the dependency of transmit power and load. In this paper we developed such power models for macro and micro base stations relying on data sheets of several GSM and UMTS base stations with focus on component level, e.g., power amplifier and cooling equipment. In a first application of the model a traditional macro cell deployment and a heterogeneous deployment are compared.

Challenges and enabling technologies for energy aware mobile radio networks

Abstract: Mobile communications are increasingly contributing to global energy consumption. In this article, a holistic approach for energy efficient mobile radio networks is presented. The matter of having appropriate metrics and evaluation methods that allow assessing the energy efficiency of the entire system is discussed. The mutual supplementary saving concepts comprise component, link and network levels. At the component level the power amplifier complemented by a transceiver and a digital platform supporting advanced power management are key to efficient radio implementations. Discontinuous transmission by base stations, where hardware components are switched off, facilitate energy efficient operation at the link level. At the network level, the potential for reducing energy consumption is in the layout of networks and their management, that take into account slowly changing daily load patterns, as well as highly dynamic traffic fluctuations. Moreover, research has to analyze new disruptive architectural approaches, including multi-hop transmission, ad-hoc meshed networks, terminal-to-terminal communications, and cooperative multipoint architectures.

Integral Boundary Points of Convex Polyhedra

Abstract: Here is the story of how this paper was written. (a) Independently, Alan and Joe discovered this easy theorem: if the “right hand side” consists of integers, and if the matrix is “totally unimodular”, then the vertices of the polyhedron defined by the linear inequalities will all be integral. This is easy to prove and useful. As far as we know, this is the only part of our theorem that anyone has ever used.

Spectrally Efficient Long-Haul Optical Networking Using 112-Gb/s Polarization-Multiplexed 16-QAM

Abstract: We discuss the generation, wavelength-division-multiplexed (WDM) long-haul transmission, and coherent detection of 112-Gb/s polarization-division-multiplexed (PDM) 16-ary quadrature amplitude modulation (16-QAM) at a line rate of 14 Gbaud and spectral efficiencies beyond 4 b/s/Hz. We describe the (off-line) digital signal processing and blind filter adaptation algorithms used in our intradyne receiver and characterize its performance using both simulated and measured 16-QAM waveforms. We measure a required optical signal-to-noise ratio of 20.2 dB (0.1-nm reference bandwidth; 10-3 bit-error ratio), 3.2-dB off the theoretical limit. We study the effects of finite analog-to-digital converter resolution, laser frequency offset, laser phase noise, and narrowband optical filtering. Our experiments on a 25-GHz WDM grid (4.1-b/s/Hz spectral efficiency) reveal a 1-dB penalty after 7 passes though reconfigurable optical add/drop multiplexers (ROADMs) and an achievable transmission reach of 1022 km of uncompensated standard single-mode fiber. At a spectral efficiency of 6.2 b/s/Hz (16.67-GHz WDM channel spacing) a transmission reach of 630 km is attained.

Training and testing of recommender systems on data missing not at random

Abstract: Users typically rate only a small fraction of all available items. We show that the absence of ratings carries useful information for improving the top-k hit rate concerning all items, a natural accuracy measure for recommendations. As to test recommender systems, we present two performance measures that can be estimated, under mild assumptions, without bias from data even when ratings are missing not at random (MNAR). As to achieve optimal test results, we present appropriate surrogate objective functions for efficient training on MNAR data. Their main property is to account for all ratings - whether observed or missing in the data. Concerning the top-k hit rate on test data, our experiments indicate dramatic improvements over even sophisticated methods that are optimized on observed ratings only.

Beyond 100G Ethernet

Abstract: With 100G technologies standardized, in the context of both Ethernet and the optical transport network (OTN), 100G router ports and 100G optical transport interfaces are commercially available. At the same time, heavily data-centric users are starting to ask for higher-rate interfaces. First speculations include 400G as well as 1T as the next possible Ethernet standards. In this article we discuss interface technology options for Ethernet and OTN beyond 100G in light of the current 100G standards, taking into account likely evolution paths of interface technologies over the next 10 years.

Scaling optical communications for the next decade and beyond

Abstract: Since the collapse of the market for optical communications equipment at the beginning of the millennium, there has been diminished interest and investment in technologies for long-distance optical transmission. This is largely a result of the enormous capacities afforded by wavelength-division multiplexed systems; the systems deployed early in the decade had capacities comparable to the total network traffic. This situation is about to change. Those systems are becoming full now, and growth in traffic indicates that capacity of systems will become a crucial factor in the years ahead. This paper examines the technical challenges inherent in scaling network capacity to accommodate traffic growth in the coming decades.

Pressure Routing for Underwater Sensor Networks

Abstract: A SEA Swarm (Sensor Equipped Aquatic Swarm) is a sensor "cloud" that drifts with water currents and enables 4D (space and time) monitoring of local underwater events such as contaminants, marine life and intruders. The swarm is escorted at the surface by drifting sonobuoys that collect the data from underwater sensors via acoustic modems and report it in real-time via radio to a monitoring center. The goal of this study is to design an efficient anycast routing algorithm for reliable underwater sensor event reporting to any one of the surface sonobuoys. Major challenges are the ocean current and the limited resources (bandwidth and energy). In this paper, we address these challenges and propose HydroCast, a hydraulic pressure based anycast routing protocol that exploits the measured pressure levels to route data to surface buoys. The paper makes the following contributions: a novel opportunistic routing mechanism to select the subset of forwarders that maximizes greedy progress yet limiting co-channel interference; and an efficient underwater "dead end" recovery method that outperforms recently proposed approaches. The proposed routing protocols are validated via extensive simulations.

Quantum frequency translation of single-photon states in a photonic crystal fiber.

Abstract: We experimentally demonstrate frequency translation of a nonclassical optical field via four-wave mixing (Bragg-scattering process) in a photonic crystal fiber (PCF). The high nonlinearity and the ability to control dispersion in PCF enable efficient translation between nearby photon channels within the visible to-near-infrared spectral range, useful in quantum networks. Heralded single photons at 683 nm were translated to 659 nm with an efficiency of 28.6±2.2 percent. Second-order correlation measurements on the 683- and 659-nm fields yielded g(683)(2) (0)=0.21±0.02 and g(659)(2) (0)=0.19±0.05, respectively, showing the nonclassical nature of both fields.

Linear Precoding in Cooperative MIMO Cellular Networks with Limited Coordination Clusters

Abstract: In a cooperative multiple-antenna downlink cellular network, maximization of a concave function of user rates is considered. A new linear precoding technique called soft interference nulling (SIN) is proposed, which performs at least as well as zero-forcing (ZF) beamforming. All base stations share channel state information, but each user's message is only routed to those that participate in the user's coordination cluster. SIN precoding is particularly useful when clusters of limited sizes overlap in the network, in which case traditional techniques such as dirty paper coding or ZF do not directly apply. The SIN precoder is computed by solving a sequence of convex optimization problems. SIN under partial network coordination can outperform ZF under full network coordination at moderate SNRs. Under overlapping coordination clusters, SIN precoding achieves considerably higher throughput compared to myopic ZF, especially when the clusters are large.

On femto deployment architectures and macrocell offloading benefits in joint macro-femto deployments

Abstract: This article provides an overview of opportunities and challenges related to joint deployments of macro and femtocells. It provides insights on possible deployment architectures for femtocells along with an analysis framework for quantifying macro offloading benefits in realistic network deployment scenarios by means of advanced performance analysis techniques. Such benefits include potential enhancement in quality of radio signals for users served by the macro network in joint macro-femto deployments. This in turn translates into potentially better data rates (throughput) for macrocell users and may offer the possibility of adding more users to the macro network while preserving the network configuration -- resulting in direct capital expenditure savings. The approach taken in this article consists of creating a framework for quantifying macro offloading benefits in joint macro-femto deployments. A baseline configuration where all users (indoor and outdoor) are served by a traditional macro network (state-of-the-art macro only network) is considered first. The analysis is followed through joint macro-femto deployments, where femtocells serve indoor users. Through comparison of the baseline configuration and the joint macro-femto analysis, we quantify the benefits of the joint macro-femto deployment.

Improving Energy Efficiency of Femtocell Base Stations Via User Activity Detection

Abstract: Femtocells, which operate in licensed spectrum and provide improved cellular coverage inside homes and offices, have attracted significant interest in the wireless industry. As a result, an extensive deployment of femtocells is expected in the near future. One prime concern with a large-scale femtocell deployment is the resultant substantial energy consumption. In this paper, we address this issue by proposing a novel energy saving procedure which allows the femtocell base station (BS) to completely switch off its radio transmissions and associated processing when not involved in an active call. The results indicate that based on a certain voice traffic model, the proposed procedure introduces an average reduction of approximately 37.5% in the femtocell's power consumption. Moreover, for a high femtocell traffic scenario, a fivefold reduction in the occurrence of mobility events can also be achieved, compared to a fixed pilot transmit power strategy.

Observation of a one-dimensional spin–orbit gap in a quantum wire

Abstract: The ability to produce spin-polarized currents in a quantum wire is crucial for spin-based electronics. Fortunately, the spin–orbit interaction can be exploited to deliver pure spin currents, without charge currents, that travel in one direction only.

Pilot Contamination Reduction in Multi-User TDD Systems

Abstract: This paper considers the problem of interference mitigation in multi-cell multi-antenna time division duplex (TDD) wireless systems for downlink transmission. An efficient way to obtain channel state information (CSI) at the base station is by using uplink pilots and reciprocity of the downlink channel. At the same time, it has been shown that pilots from different cells contaminate each other, resulting in corruption of precoding matrices used by base stations, and high inter-cell interference. This paper studies the effects of shifting the location of pilots in time frames used in neighboring cells, and its effectiveness in obtaining better channel estimates, and, thereby, inter-cell interference reduction.

Wide Bandwidth Silicon Nitride Grating Coupler

Abstract: We demonstrate a one-dimensional grating coupler in silicon nitride with a 67-nm 1-dB bandwidth, the largest reported for a grating coupler butt-coupled to standard single-mode fiber. The peak coupling efficiency is -4.2dB. It requires no partial etch and requires only 0.6-μm fabrication resolution.

Monolithic Polarization and Phase Diversity Coherent Receiver in Silicon

Abstract: In this paper, we realized a monolithic silicon photonic integrated circuit (PIC) for polarization and phase diversity coherent detection. The PIC includes two polarization beam splitters, two 90° optical hybrids, and four pairs of balanced photodiodes implemented as integrated germanium detectors. We tested the PIC using polarization-division multiplexed quadrature phase-shift keyed signals at 43 and at 112 Gb/s.

Low-Loss and Broadband Cantilever Couplers Between Standard Cleaved Fibers and High-Index-Contrast Si $_{3}$ N $_{4}$ or Si Waveguides

Abstract: We demonstrate a novel facet coupler for coupling between 10.4-m mode-diameter cleaved fibers and high-index-contrast SiN or Si waveguides. By creating a cantilevered glass waveguide surrounding an inverse taper and injecting a low-index cladding around the cantilever, we demonstrate coupling loss as low as 0.7 dB/facet to 500 nm 400 nm Si N waveguides, with only 0.2-dB polarization-dependent loss and 0.4-dB wavelength-dependent loss from 1480 to 1580 nm. Coupling loss of 1.5-2.0 dB/facet to 500 nm 220 nm Si waveguides at 1550 nm is also demonstrated.

Lower Bounds on the Q of Electrically Small Dipole Antennas

Abstract: General expressions are obtained for the lower bounds on the quality factor (Q) of electrically small electric- and magnetic-dipole antennas confined to an arbitrarily shaped volume V and excited by general sources or by global electric-current sources alone. The lower-bound expressions depend only on the direction of the dipole moment with respect to V , the electrical size of V , and the static electric and magnetic polarizabilities per unit volume of hypothetical perfectly electrically conducting and perfectly magnetically conducting volumes V . The lower bounds are obtained directly from the electromagnetic field expressions for Q with the help of current equivalence principles and the uncoupling of Maxwell's equations for electrically small volumes into quasi-electrostatic and quasi-magnetostatic fields.

Generation and 1,200-km transmission of 448-Gb/s ETDM 56-Gbaud PDM 16-QAM using a single I/Q modulator

Abstract: We generate 56-Gbaud PDM 16-QAM using electronic time-division multiplexing (ETDM) and a four-level-driven I/Q modulator. The 448-Gb/s line-rate signal is transmitted over 1,200 km of ultra-large-area fiber and coherently received by two 32.5-GHz oscilloscopes with >5.5 effective bits.

Greening the internet with content-centric networking

Abstract: Our energy efficiency analysis of various content dissemination strategies reveals that a change in network architecture from host-oriented to content-centric networking (CCN) can open new possibilities for energy-efficient content dissemination. In this paper, we consider energy-efficient CCN architecture and validate its energy efficiency via trace-based simulations. The results confirm that CCN is more energy efficient than conventional CDNs and P2P networks, even under incremental deployment of CCN-enabled routers.

FleaNet: A Virtual Market Place on Vehicular Networks

Abstract: In this study, we introduce the concept of a virtual ?flea market? over a vehicular ad hoc network (VANET) called FleaNet. FleaNet customers express their demands/offers to buy/sell items via radio queries. These queries are opportunistically disseminated, exploiting the mobility of other customers, until a matching customer/vendor is found. We identify the key performance metrics, namely, query resolution latency, scalability, mobility, and churning, and evaluate their impact on performance using analytic and simulation models. The results show that FleaNet can function as an effective virtual marketplace.

Optimal Resource Allocation in Clouds

Abstract: Cloud platforms enable enterprises to lease computing power in the form of virtual machines. An important problem for such enterprise users is to understand how many and what kinds of virtual machines will be needed from clouds. We formulate demand for computing power and other resources as a resource allocation problem with multiplicity, where computations that have to be performed concurrently are represented as tasks and a later task can reuse resources released by an earlier task. We show that finding a minimized allocation is NP-complete. This paper presents an approximation algorithm with a proof of its approximation bound that can yield close to optimum solutions in polynomial time. Enterprise users can exploit the solution to reduce the leasing cost and amortize the administration overhead (e.g., setting up VPNs or configuring a cluster). Cloud providers may utilize the solution to share their resources among a larger number of users.

Gaussian Model-Based Multichannel Speech Presence Probability

Abstract: The knowledge of the target speech presence probability in a mixture of signals captured by a speech communication system is of paramount importance in several applications including reliable noise reduction algorithms. In this correspondence, we establish a new expression for speech presence probability when an array of microphones with an arbitrary geometry is used. Our study is based on the assumption of the Gaussian statistical model for all signals and involves the noise and noisy data statistics only. In comparison with the single-channel case, the new proposed multichannel approach can significantly increase the detection accuracy. In particular, when the additive noise is spatially coherent, perfect speech presence detection is theoretically possible, while when the noise is spatially white, a coherent summation of speech components is performed to allow for enhanced speech presence probability estimation.

Capacity Regions and Sum-Rate Capacities of Vector Gaussian Interference Channels

Abstract: The capacity regions of vector, or multiple-input multiple-output, Gaussian interference channels are established for very strong interference and aligned strong interference. Furthermore, the sum-rate capacities are established for Z interference, noisy interference, and mixed (aligned weak/intermediate and aligned strong) interference. These results generalize known results for scalar Gaussian interference channels.

Identifying suspicious activities through DNS failure graph analysis

Abstract: As a key approach to securing large networks, existing anomaly detection techniques focus primarily on network traffic data. However, the sheer volume of such data often renders detailed analysis very expensive and reduces the effectiveness of these tools. In this paper, we propose a light-weight anomaly detection approach based on unproductive DNS traffic, namely, the failed DNS queries, with a novel tool - DNS failure graphs. A DNS failure graph captures the interactions between hosts and failed domain names. We apply a graph decomposition algorithm based on the tri-nonnegative matrix factorization technique to iteratively extract coherent co-clusters (dense subgraphs) from DNS failure graphs. By analyzing the co-clusters in the daily DNS failure graphs from a 3-month DNS trace captured at a large campus network, we find these co-clusters represent a variety of anomalous activities, e.g., spamming, trojans, bots, etc.. In addition, these activities often exhibit distinguishable subgraph structures. By exploring the temporal properties of the co-clusters, we show our method can identify new anomalies that likely correspond to unreported domain-flux bots.