Showing papers by "Bell Labs published in 2011"
Posted Content•
TL;DR: It is shown that the use of moderately large antenna arrays can improve the spectral and energy efficiency with orders of magnitude compared to a single-antenna system.
Abstract: A multiplicity of autonomous terminals simultaneously transmits data streams to a compact array of antennas. The array uses imperfect channel-state information derived from transmitted pilots to extract the individual data streams. The power radiated by the terminals can be made inversely proportional to the square-root of the number of base station antennas with no reduction in performance. In contrast if perfect channel-state information were available the power could be made inversely proportional to the number of antennas. Lower capacity bounds for maximum-ratio combining (MRC), zero-forcing (ZF) and minimum mean-square error (MMSE) detection are derived. A MRC receiver normally performs worse than ZF and MMSE. However as power levels are reduced, the cross-talk introduced by the inferior maximum-ratio receiver eventually falls below the noise level and this simple receiver becomes a viable option. The tradeoff between the energy efficiency (as measured in bits/J) and spectral efficiency (as measured in bits/channel use/terminal) is quantified. It is shown that the use of moderately large antenna arrays can improve the spectral and energy efficiency with orders of magnitude compared to a single-antenna system.
2,421 citations
TL;DR: The paper shows that the precoding matrix used by the base station in one cell becomes corrupted by the channel between that base station and the users in other cells in an undesirable manner and develops a new multi-cell MMSE-based precoding method that mitigates this problem.
Abstract: This paper considers a multi-cell multiple antenna system with precoding used at the base stations for downlink transmission. Channel state information (CSI) is essential for precoding at the base stations. An effective technique for obtaining this CSI is time-division duplex (TDD) operation where uplink training in conjunction with reciprocity simultaneously provides the base stations with downlink as well as uplink channel estimates. This paper mathematically characterizes the impact that uplink training has on the performance of such multi-cell multiple antenna systems. When non-orthogonal training sequences are used for uplink training, the paper shows that the precoding matrix used by the base station in one cell becomes corrupted by the channel between that base station and the users in other cells in an undesirable manner. This paper analyzes this fundamental problem of pilot contamination in multi-cell systems. Furthermore, it develops a new multi-cell MMSE-based precoding method that mitigates this problem. In addition to being linear, this precoding method has a simple closed-form expression that results from an intuitive optimization. Numerical results show significant performance gains compared to certain popular single-cell precoding methods.
1,306 citations
TL;DR: The principal feasibility of COMP is shown in two field testbeds with multiple sites and different backhaul solutions between the sites, and significant gains can be shown for both the uplink and downlink.
Abstract: Coordinated multipoint or cooperative MIMO is one of the promising concepts to improve cell edge user data rate and spectral efficiency beyond what is possible with MIMOOFDM in the first versions of LTE or WiMAX. Interference can be exploited or mitigated by cooperation between sectors or different sites. Significant gains can be shown for both the uplink and downlink. A range of technical challenges were identified and partially addressed, such as backhaul traffic, synchronization and feedback design. This article also shows the principal feasibility of COMP in two field testbeds with multiple sites and different backhaul solutions between the sites. These activities have been carried out by a powerful consortium consisting of universities, chip manufacturers, equipment vendors, and network operators.
1,093 citations
TL;DR: The existence of excessively large (bloated) buffers in systems, particularly network communication systems, has been identified as a major cause of network latency problems as discussed by the authors, which is referred to as bufferbloat.
Abstract: We have conflated "speed" with "band width." As Stuart Chesire wrote in "It's the Latency, Stupid" (http://rescomp.stanford.edu/~cheshire/rants/Latency.html), "Making more bandwidth is easy. Once you have bad latency, you're stuck with it." Bufferbloat is the existence of excessively large (bloated) buffers in systems, particularly network communication systems. Bufferbloat is now (almost?) everywhere. Today's routers, switches, gateways, broad band gear, and so on have bloated buffer sizes to where we often measure latency in seconds, rather than microseconds or milliseconds.
622 citations
TL;DR: It is shown that 6×6 MIMO processing can be used to almost completely compensate for crosstalk and intersymbol interference due to mode coupling in a system that transmits uncorrelated 28-GBaud QPSK signals on the six spatial and polarization modes supported by a novel few-mode fiber.
Abstract: Mode-division multiplexing over 33-km few-mode fiber is investigated. It is shown that 6×6 MIMO processing can be used to almost completely compensate for crosstalk and intersymbol interference due to mode coupling in a system that transmits uncorrelated 28-GBaud QPSK signals on the six spatial and polarization modes supported by a novel few-mode fiber.
489 citations
10 Apr 2011
TL;DR: This first detailed measurement analysis of network resource usage and subscriber behavior using a large-scale data set collected inside a nationwide 3G cellular data network delivers important insights into network-wide resource usage.
Abstract: We conduct the first detailed measurement analysis of network resource usage and subscriber behavior using a large-scale data set collected inside a nationwide 3G cellular data network. The data set tracks close to a million subscribers over thousands of base stations. We analyze individual subscriber behaviors and observe a significant variation in network usage among subscribers. We characterize subscriber mobility and temporal activity patterns and identify their relation to traffic volume. We then investigate how efficiently radio resources are used by different subscribers as well as by different applications. We also analyze the network traffic from the point of view of the base stations and find significant temporal and spatial variations in different parts of the network, while the aggregated behavior appears predictable. Broadly, our observations deliver important insights into network-wide resource usage. We describe implications in pricing, protocol design and resource and spectrum management.
438 citations
TL;DR: In this paper, the authors point out that bufferbloat, the existence of excessively large and frequently full buffers inside the network, is a major cause of unnecessary latency and poor system performance.
Abstract: Today’s networks are suffering from unnecessary latency and poor system performance. The culprit is bufferbloat, the existence of excessively large and frequently full buffers inside the network. Large buffers have been inserted all over the Internet without sufficient thought or testing. They damage or defeat the fundamental congestion-avoidance algorithms of the Internet’s most common transport protocol. Long delays from bufferbloat are frequently attributed incorrectly to network congestion, and this misinterpretation of the problem leads to the wrong solutions being proposed.
421 citations
TL;DR: In this paper, an optical-fiber-based non-degenerate PSA link consisting of a phase-insensitive parametric copier followed by a PSA that provides broadband amplification, signal modulation format independence, and nearly 6dB link noise-figure (NF) improvement over conventional, erbium-doped fiber amplifier based links.
Abstract: Optical phase-sensitive amplifiers (PSAs) are known to be capable, in principle, of realizing noiseless amplification and improving the signal-to-noise-ratio of optical links by 3 dB compared to conventional, phase-insensitively amplified links. However, current state-of-the-art PSAs are still far from being practical, lacking e.g. significant noise performance improvement, broadband gain and modulation-format transparency. Here we demonstrate experimentally, for the first time, an optical-fiber-based non-degenerate PSA link consisting of a phase-insensitive parametric copier followed by a PSA that provides broadband amplification, signal modulation-format independence, and nearly 6-dB link noise-figure (NF) improvement over conventional, erbium-doped fiber amplifier based links. The PSA has a record-low 1.1-dB NF, and can be extended to work with multiple wavelength channels with modest system complexity. This concept can also be realized in other materials with third-order nonlinearities, and is useful in any attenuation-limited optical link.
419 citations
TL;DR: This paper discusses MIMO capacities in optical SDM systems, including related outage considerations which are an important part in the design of such systems, and extracts scaling rules for mode-average and mode-dependent loss.
Abstract: With wavelength-division multiplexing (WDM) rapidly nearing its scalability limits, space-division multiplexing (SDM) seems the only option to further scale the capacity of optical transport networks. In order for SDM systems to continue the WDM trend of reducing energy and cost per bit with system capacity, integration will be key to SDM. Since integration is likely to introduce non-negligible crosstalk between multiple parallel transmission paths, multiple-input multiple output (MIMO) signal processing techniques will have to be used. In this paper, we discuss MIMO capacities in optical SDM systems, including related outage considerations which are an important part in the design of such systems. In order to achieve the low-outage standards required for optical transport networks, SDM transponders should be capable of individually addressing, and preferably MIMO processing all modes supported by the optical SDM waveguide. We then discuss the effect of distributed optical noise in MIMO SDM systems and focus on the impact of mode-dependent loss (MDL) on system capacity and system outage. Through extensive numerical simulations, we extract scaling rules for mode-average and mode-dependent loss and show that MIMO SDM systems composed of up to 128 segments and supporting up to 128 modes can tolerate up to 1 dB of per-segment MDL at 90% of the system's full capacity at an outage probability of 10(-4).
401 citations
19 Aug 2011
TL;DR: A systematic evaluation of the suitability of existing software and hardware components in today's routers for the support of CCN concludes that a CCN deployment is feasible at a Content Distribution Network and ISP scale, whereas today's technology is not yet ready to support an Internet scale deployment.
Abstract: Content-Centric Networking (CCN) is a novel networking paradigm centered around content distribution rather than host-to-host connectivity. This change from host-centric to content-centric has several attractive advantages, such as network load reduction, low dissemination latency, and energy efficiency. However, it is unclear whether today's technology is ready for the CCN (r)evolution. The major contribution of this paper is a systematic evaluation of the suitability of existing software and hardware components in today's routers for the support of CCN. Our main conclusion is that a CCN deployment is feasible at a Content Distribution Network (CDN) and ISP scale, whereas today's technology is not yet ready to support an Internet scale deployment.
394 citations
06 Mar 2011
TL;DR: In this article, the authors demonstrate the transmission of 6 independent, spatially and polarization multiplexed 28-Gb/s QPSK signals over 10 km of three-mode fiber using mode-selective excitation and full coherent 6 × 6 MIMO processing.
Abstract: We demonstrate the transmission of 6 independent, spatially- and polarization multiplexed 28-Gb/s QPSK signals over 10 km of three-mode fiber using mode-selective excitation and full coherent 6 × 6 MIMO processing.
Posted Content•
TL;DR: How many antennas per UT are needed to achieve η % of the ultimate performance and how much can be gained through more sophisticated minimum-mean-square-error (MMSE) detection and how many more antennas are needed with the matched filter to achieve the same performance are derived.
Abstract: We consider a multicell MIMO uplink channel where each base station (BS) is equipped with a large number of antennas N. The BSs are assumed to estimate their channels based on pilot sequences sent by the user terminals (UTs). Recent work has shown that, as N grows infinitely large, (i) the simplest form of user detection, i.e., the matched filter (MF), becomes optimal, (ii) the transmit power per UT can be made arbitrarily small, (iii) the system performance is limited by pilot contamination. The aim of this paper is to assess to which extent the above conclusions hold true for large, but finite N. In particular, we derive how many antennas per UT are needed to achieve \eta % of the ultimate performance. We then study how much can be gained through more sophisticated minimum-mean-square-error (MMSE) detection and how many more antennas are needed with the MF to achieve the same performance. Our analysis relies on novel results from random matrix theory which allow us to derive tight approximations of achievable rates with a class of linear receivers.
TL;DR: A new multicore fiber having seven single-mode cores arranged in a hexagonal array, exhibiting low crosstalk among the cores and low loss across the C and L bands is described.
Abstract: We describe a new multicore fiber (MCF) having seven single-mode cores arranged in a hexagonal array, exhibiting low crosstalk among the cores and low loss across the C and L bands. We experimentally demonstrate a record transmission capacity of 112 Tb/s over a 76.8-km MCF using space-division multiplexing and dense wavelength-division multiplexing (DWDM). Each core carries 160 107-Gb/s polarization-division multiplexed quadrature phase-shift keying (PDM-QPSK) channels on a 50-GHz grid in the C and L bands, resulting in an aggregate spectral efficiency of 14 b/s/Hz. We further investigate the impact of the inter-core crosstalk on a 107-Gb/s PDM-QPSK signal after transmitting through the center core of the MCF when all the 6 outer cores carry same-wavelength 107-Gb/s signals with equal powers, and discuss the system implications of core-to-core crosstalk on ultra-long-haul transmission.
22 May 2011
TL;DR: The so-called pilot contamination effect discovered in previous work is analyzed, and it is shown that this effect persists under the finite-dimensional channel model that is considered.
Abstract: We consider multicell multiuser MIMO systems with a very large number of antennas at the base station. We assume that the channel is estimated by using uplink training sequences, and we consider a physical channel model where the angular domain is separated into a finite number of directions. We analyze the so-called pilot contamination effect discovered in previous work, and show that this effect persists under the finite-dimensional channel model that we consider. We further derive closed-form bounds on the achievable rate of uplink data transmission with maximum-ratio combining, for a finite and an infinite number of base station antennas.
06 Sep 2011
TL;DR: An analytical model of CCN in-network storage and receiver-driven transport, that more generally applies to a class of content ori ented networks identified by chunk-based communication, is developed.
Abstract: Content-centric networking proposals, as Parc's CCN, have recently emerged to define new network architectures where content, and not its location, becomes the core of the communication model. These new paradigms push data storage and delivery at network layer and are designed to better deal with current Internet usage, mainly centered around content dissemination and retrieval. In this paper, we develop an analytical model of CCN in-network storage and receiver-driven transport, that more generally applies to a class of content ori ented networks identified by chunk-based communication. We derive a closed-form expression for the mean stationary throughput as a function of hit/miss probabilities at the caches along the path, of content popularity and of content/cache size. Our analytical results, supported by chunk level simulations, can be used to analyze fundamental trade-offs in current CCN architecture, and provide an essential building block for the design and evaluation of enhanced CCN protocols.
15 May 2011
TL;DR: The necessary enhancements over existing performance evaluation frameworks are discussed, such that the energy efficiency of the entire network comprising component, node and network level contributions can be quantified.
Abstract: In order to quantify the energy savings in wireless networks, the power consumption of the entire system needs to be captured and an appropriate energy efficiency evaluation framework must be defined. In this paper, the necessary enhancements over existing performance evaluation frameworks are discussed, such that the energy efficiency of the entire network comprising component, node and network level contributions can be quantified. The most important addendums over existing frameworks include a sophisticated power model for various base station (BS) types, which maps the RF output power radiated at the antenna elements to the total supply power of a BS site. We also consider an approach to quantify the energy efficiency of large geographical areas by using the existing small scale deployment models along with long term traffic models. Finally, the proposed evaluation framework is applied to quantify the energy efficiency of the downlink of a 3GPP LTE radio access network.
01 Sep 2011
TL;DR: In this paper, the authors consider a multicell MIMO uplink channel where each base station (BS) is equipped with a large number of antennas N. The BSs are assumed to estimate their channels based on pilot sequences sent by the user terminals (UTs).
Abstract: We consider a multicell MIMO uplink channel where each base station (BS) is equipped with a large number of antennas N. The BSs are assumed to estimate their channels based on pilot sequences sent by the user terminals (UTs). Recent work has shown that, as N → ∞, (i) the simplest form of user detection, i.e., the matched filter (MF), becomes optimal, (ii) the transmit power per UT can be made arbitrarily small, (iii) the system performance is limited by pilot contamination. The aim of this paper is to assess to which extent the above conclusions hold true for large, but finite N. In particular, we derive how many antennas per UT are needed to achieve η % of the ultimate performance. We then study how much can be gained through more sophisticated minimum-mean-square-error (MMSE) detection and how many more antennas are needed with the MF to achieve the same performance. Our analysis relies on novel results from random matrix theory which allow us to derive tight approximations of achievable rates with a class of linear receivers.
18 Sep 2011
TL;DR: In this article, the effect of in-band crosstalk on several advanced optical modulation formats, including QPSK, 16-QAM, and 64 QAM, was quantified through simulations and experiments.
Abstract: We quantify, through simulations and experiments at 21.4 GBaud, the effect of in-band crosstalk on several advanced optical modulation formats, showing a 1-dB penalty at a bit-error ratio of 1×10−3 from a crosstalk of −18 dB, −24 dB, and −32 dB for QPSK, 16-QAM, and 64-QAM, respectively.
TL;DR: An ideal case for aggressive network-efficiency improvement measures is evaluated within this framework and shown to lead to roughly unchanged consumption over the next decade.
Abstract: Power trends in communication networks are analyzed using a transaction-based model. Traffic models are developed for North America and used to evaluate the relative power trends of wireline networks and mobile networks through 2020. An ideal case for aggressive network-efficiency improvement measures is evaluated within this framework and shown to lead to roughly unchanged consumption over the next decade. Implications for future technology requirements are discussed.
TL;DR: This paper provides three methods for CFD discovery, based on techniques for mining closed item sets, and is used to discover constant CFDs, namely, CFDs with constant patterns only.
Abstract: This paper investigates the discovery of conditional functional dependencies (CFDs). CFDs are a recent extension of functional dependencies (FDs) by supporting patterns of semantically related constants, and can be used as rules for cleaning relational data. However, finding quality CFDs is an expensive process that involves intensive manual effort. To effectively identify data cleaning rules, we develop techniques for discovering CFDs from relations. Already hard for traditional FDs, the discovery problem is more difficult for CFDs. Indeed, mining patterns in CFDs introduces new challenges. We provide three methods for CFD discovery. The first, referred to as CFDMiner, is based on techniques for mining closed item sets, and is used to discover constant CFDs, namely, CFDs with constant patterns only. Constant CFDs are particularly important for object identification, which is essential to data cleaning and data integration. The other two algorithms are developed for discovering general CFDs. One algorithm, referred to as CTANE, is a levelwise algorithm that extends TANE, a well-known algorithm for mining FDs. The other, referred to as FastCFD, is based on the depth-first approach used in FastFD, a method for discovering FDs. It leverages closed-item-set mining to reduce the search space. As verified by our experimental study, CFDMiner can be multiple orders of magnitude faster than CTANE and FastCFD for constant CFD discovery. CTANE works well when a given relation is large, but it does not scale well with the arity of the relation. FastCFD is far more efficient than CTANE when the arity of the relation is large; better still, leveraging optimization based on closed-item-set mining, FastCFD also scales well with the size of the relation. These algorithms provide a set of cleaning-rule discovery tools for users to choose for different applications.
19 Aug 2011
TL;DR: This work provides an analytical characterization of statistical bandwidth and storage sharing, under fairly general assumption on total demand, topology, content popularity and limited network resources, and presents some applications of the model.
Abstract: Internet usage has dramatically evolved towards content dissemination and retrieval, whilst the underlying infrastructure remains tied up to hosts interconnection. Information centric networking (ICN) proposals have recently emerged to rethink Internet foundations and design a natively content-centric network environment.Important features of such networks are the availability of built-in network storage and of receiver-driven chunk-level transport, whose interaction significantly impacts overall system and user performance. In the paper, we provide an analytical characterization of statistical bandwidth and storage sharing, under fairly general assumption on total demand, topology, content popularity and limited network resources. A closed-form expression for average content delivery time is derived and its accuracy confirmed by event-driven simulations. Finally, we present some applications of our model, leveraging on explicit formulae for the optimal dimensioning and localization of storage resources.
TL;DR: A novel optical transmission system based on a programmable liquid crystal on silicon panel, a prototype few-mode fiber, and a 4×4 multiple input multiple output algorithm processing the information of two polarization diversity coherent receivers to experimentally demonstrate the possibility of mode division multiplexing.
Abstract: We present a novel optical transmission system to experimentally demonstrate the possibility of mode division multiplexing. Its key components are mode multiplexer and demultiplexer based on a programmable liquid crystal on silicon panel, a prototype few-mode fiber, and a 4x4 multiple input multiple output algorithm processing the information of two polarization diversity coherent receivers. Using this system, we transmit two 100Gb/s PDM-QPSK data streams modulated on two different modes of the prototype few-mode fiber. After 40km, we obtain Q2-factors about 1dB above the limit for forward error correction.
TL;DR: In this paper, a coherent optical orthogonal frequency-division multiplexing (CO-OFDM) scheme with reduced guard interval (RGI) was proposed for high-speed high-spectral-efficiency long-haul optical transmission.
Abstract: We propose a novel coherent optical orthogonal frequency-division multiplexing (CO-OFDM) scheme with reduced guard interval (RGI) for high-speed high-spectral-efficiency long-haul optical transmission. In this scheme, fiber chromatic dispersion is compensated for within the receiver rather than being accommodated by the guard interval (GI) as in conventional CO-OFDM, thereby reducing the needed GI, especially when fiber dispersion is large. We demonstrate the generation of a 448-Gb/s RGI-CO-OFDM signal with 16-QAM subcarrier modulation through orthogonal band multiplexing. This signal occupies an optical bandwidth of 60 GHz, and is transmitted over 2000 km of ultra-large-area fiber (ULAF) with five passes through an 80-GHz-grid wavelength-selective switch. Banded digital coherent detection with two detection bands is used to receive this 448-Gb/s signal. Wavelength-division multiplexed transmission of three 80-GHz spaced 448-Gb/s RGI-CO-OFDM channels is also demonstrated, achieving a net system spectral efficiency of 5.2 b/s/Hz and a transmission distance of 1600 km of ULAF.
05 May 2011
TL;DR: This paper devise various online and offline algorithms to arrive at a good ordering of jobs to minimize the overall job completion times, and proposes approximation algorithms that work within a factor of 3 of the optimal.
Abstract: Large-scale data processing needs of enterprises today are primarily met with distributed and parallel computing in data centers. MapReduce has emerged as an important programming model for these environments. Since today's data centers run many MapReduce jobs in parallel, it is important to find a good scheduling algorithm that can optimize the completion times of these jobs. While several recent papers focused on optimizing the scheduler, there exists very little theoretical understanding of the scheduling problem in the context of MapReduce. In this paper, we seek to address this problem by first presenting a simplified abstraction of the MapReduce scheduling problem, and then formulate the scheduling problem as an optimization problem.We devise various online and offline algorithms to arrive at a good ordering of jobs to minimize the overall job completion times. Since optimal solutions are hard to compute (NP-hard), we propose approximation algorithms that work within a factor of 3 of the optimal. Using simulations, we also compare our online algorithm with standard scheduling strategies such as FIFO, Shortest Job First and show that our algorithm consistently outperforms these across different job distributions.
10 Apr 2011
TL;DR: CloudStream is presented: a cloud-based video proxy that can deliver high-quality streaming videos by transcoding the original video in real time to a scalable codec which allows streaming adaptation to network dynamics.
Abstract: Existing media providers such as YouTube and Hulu deliver videos by turning it into a progressive download. This can result in frequent video freezes under varying network dynamics. In this paper, we present CloudStream: a cloud-based video proxy that can deliver high-quality streaming videos by transcoding the original video in real time to a scalable codec which allows streaming adaptation to network dynamics. The key is a multi-level transcoding parallelization framework with two mapping options (Hallsh-based Mapping and Lateness-first Mapping) that optimize transcoding speed and reduce the transcoding jitters while preserving the encoded video quality. We evaluate the performance of CloudStream on our campus cloud testbed.
TL;DR: It is shown that communications networks at the IP layer possess global negative curvature, and that it has a major impact on core congestion: the load at the core of a finite negatively curved network with N nodes scales as N(2), as compared to N(1.5) for a generic finite flat network.
Abstract: Large-scale data networks form the infrastructure for contemporary global communications. Increasingly, a single network may provide a variety of disparate services, flatter architectures (i.e., fewer controlling hubs) are used to achieve robustness against failure, and networks have to be dynamically and automatically reconfigurable to allow services to be set up quickly. With these trends in mind, it is impractical to perform detailed case-by-case simulations in order to predict and understand the behavior of such large-scale networks. Instead, one has to identify the key structural properties that affect network performance, reliability, and security. These structural properties can then be used to construct models that estimate network behavior in an efficient and scalable manner. A key observation regarding large-scale communications and biological and social networks has been the “small-world” property [1‐3]. More recent network models have focused on power-law degree distributions (PLDD) (for a few examples, see Refs. [4‐6]) as an explanation of or correlated with the small-world property. Evidence for PLDD has been found in data networks at the Internet protocol (IP) layer [7], for the worldwide web [4], and for the virtual network of social connections [8]. Although these features are interesting and important, the impact of intrinsic geometrical and topological features of large-scale networks on performance, reliability, and security is of much greater importance. Intuitively, it is known that traffic between nodes tends to go through a relatively small core of the network, as if the shortest path between them is curved inward. It has been suggested that this property may be due to global curvature or hyperbolicity of the network [9].
06 Mar 2011
TL;DR: In this paper, two 100Gb/s PDMQPSK data streams over two different modes of a 40km-long prototype few-mode fiber were transmitted with an LCOS-based mode multiplexer/demultiplexer and 4×4 MIMO algorithm in a coherent receiver.
Abstract: We transmit two 100Gb/s PDM-QPSK data streams over two different modes of a 40km-long prototype few-mode fiber. Our experiment is performed with an LCOS-based mode multiplexer/demultiplexer and 4×4 MIMO algorithm in a coherent receiver.
TL;DR: In this paper, the role of parallel transport options such as spatial multiplexing or multiband transmission in the capacity scaling of optical transport systems is discussed, and it is shown that a system supporting 20 b/s/Hz over 1500 km can be about 100 times more energy efficient than when it is built using a single waveguide at high capacity.
Abstract: We discuss the role of parallel transport options such as spatial multiplexing or multiband transmission in the capacity scaling of optical transport systems. Taking current experimental spectral efficiency records as a baseline, a system supporting 20 b/s/Hz over 1500 km can be about 100 times more energy efficient when it is built using spatial multiplexing than when it is built using a single waveguide at high capacity.
TL;DR: In this paper, the authors discuss the generation, wavelength-division-multiplexed (WDM) longhaul transmission, and coherent detection of 224-Gb/s PIMO 16-ary quadrature amplitude modulation (16-QAM) at a line rate of 28 Gbaud.
Abstract: We discuss the generation, wavelength-division-multiplexed (WDM) long-haul transmission, and coherent detection of 224-Gb/s polarization-division-multiplexed (PDM) 16-ary quadrature amplitude modulation (16-QAM) at a line rate of 28 Gbaud. We measure a required optical signal-to-noise ratio of 23.4 dB (0.1-nm reference bandwidth; 10-3 bit-error ratio), 3.4-dB off the theoretical limit. Using ultra-large-area fiber, we achieve 2000-km single-channel transmission. We also demonstrate 1200-km WDM transmission on a 50-GHz grid (4-b/s/Hz spectral efficiency), including three passes through a wavelength-selective switch.
TL;DR: A polarization rotator based on adiabatic mode evolution on silicon for polarization-diversified circuits and combined with a compact, broadband polarization beam splitter based on cascaded directional couplers enhances the polarization conversion extinction ratio to over 30 dB with less than 1.5 dB total insertion loss over a 60 nm spectral range.
Abstract: We demonstrate a polarization rotator based on adiabatic mode evolution on silicon for polarization-diversified circuits. The rotator has a device length of 420 μm, a polarization-conversion efficiency of more than 90%, and an insertion loss less than 1 dB for a wavelength range of 80 nm. Combining the rotator with a compact, broadband polarization beam splitter based on cascaded directional couplers enhances the polarization conversion extinction ratio to over 30 dB with less than 1.5 dB total insertion loss over a 60 nm spectral range.