Visible light communications is an energy efficient and cost-effective solution for indoor wireless access. In this paper, we propose a multiple input multiple output system using centralized or ...decentralized transmitted power allocation algorithms with multiple LEDs and photodetectors. The proposed system uses an optical code division multiple-access technique to support multiple users. Time-space minimum mean squared error filters at the receivers are designed to diminish the effect of multiple-access interference. In the centralized power allocation algorithm, all the LED lamps in the room are coordinated and controlled by a central controller; each LED lamp supports all the users within the indoor area. The decentralized power allocation algorithms we propose have similar bit error rate performance yet less computational burden compared to the centralized algorithm. In our decentralized algorithms, users are supported by a subset of the LEDs, and so the optimization problem size can be reduced by as much as 93%. For each receiver, multiple photodetectors with different orientations are employed to improve the signal to interference plus noise ratio. In addition, some practical considerations such as shadowing effects, illumination requirements, dimming control, and transmitted power quantization are taken into account.
Visible light communications (VLC), a new optical wireless communication technology that uses illumination light-emitting diodes as transmitters, requires a modulation scheme that is well suited to ...these devices' nonlinear response. Optical orthogonal frequency division multiplexing (OFDM) is a promising technique to provide high-speed data transmission for VLC. However, the peak transmitted power limitation and nonnegative transmitted signal constraint of the lighting sources can result in nonlinear signal distortion from clipping. In this paper, we propose a novel optical OFDM scheme for VLC systems called clipping-enhanced optical OFDM (CEO-OFDM) that transmits via extra time slots the information clipped by the peak power constraint. CEO-OFDM sacrifices bandwidth to allow a higher modulation index to improve the signal to noise ratio and reduce the clipping distortion caused by the peak power limitation. From analytical and numerical results, the proposed CEO-OFDM provides better bit error rate performance and higher data rate than DC-biased optical OFDM, unipolar OFDM, and asymmetrically clipped optical OFDM. Furthermore, CEO-OFDM can provide a better illumination performance that supports light dimming.
Light-emitting diode (LED)-based communications, such as visible light communications and infrared communications, are candidate techniques to provide short-range and high-speed data transmission. In ...this letter, <inline-formula> <tex-math notation="LaTeX">M </tex-math></inline-formula>-ary pulse amplitude modulation (M-PAM), used as a high bandwidth efficiency scheme, is compared with three well-known optical orthogonal frequency-division multiplexing (OFDM) schemes. Considering the bandwidth limit and constrained peak transmitted power characteristics of LEDs, a bit loading algorithm and a single-tap equalizer with an optimized modulation index are used for the optical OFDM schemes tested. To reduce the inter-symbol interference caused by the bandlimited channel, an optimized pulse shape and a minimum mean squared error equalizer are applied to the M-PAM system. From numerical results, M-PAM can provide a substantially higher data rate than OFDM for bandlimited channels. When the channel bandwidth is ample compared with the symbol rate, optical OFDM outperforms M-PAM.
This paper studies the performance of multi-hop and mesh networks composed of millimeter wave-based radio frequency (RF) and free-space optical (FSO) links. The results are obtained in cases with and ...without hybrid automatic repeat request (HARQ). Using the central limit theorem as well as other state-of-the-art approximation schemes, we derive closed-form expressions for the networks' outage probability and ergodic achievable rates. We also evaluate the effect of various parameters such as power amplifiers efficiency, number of antennas as well as different coherence times of the RF and the FSO links on the system performance. Finally, we determine the minimum number of the transmit antennas in the RF link such that the same rate is supported in the RF- and the FSO-based hops. The results show the efficiency of the RF-FSO setups in different conditions. Moreover, HARQ can effectively improve the outage probability/energy efficiency, and compensate for the effect of hardware impairments in RF-FSO networks. For common parameter settings of the RF-FSO dual-hop networks, outage probability of 10 -4 and code rate of 3 nats-per-channel-use, the implementation of HARQ with a maximum of 2 and 3 retransmissions reduces the required power, compared to cases with open-loop communication, by 13 and 17 dB, respectively.
Dense wavelength-division multiplexing (DWDM) is a promising approach to design ultrahigh-capacity fiber-optic communication systems ( >; 50 Tb/s). However, DWDM gives rise to severe physical ...impairments that adversely affect system performance. To mitigate various physical impairments in DWDM systems and exploit their system capacity, there is a need to develop a 2-D (time and wavelength) discrete-time input-output model of physical impairments that can become the foundation of signal processing for optical communications. This paper develops such a model based on the Volterra series transfer function (VSTF) method. We overcome the well-known triple integral problem associated with the VSTF method and reduce it to a simple integral. This model takes into account multiple channel effects, fiber losses, frequency chirp, optical filtering, and photo detection, which are ignored in the current literature. The model is in excellent agreement with results obtained by split-step Fourier simulation. Furthermore, with this model, we define coefficients that capture intersymbol interference, interchannel interference, self-phase modulation, intrachannel cross-phase modulation (XPM), intrachannel four-wave mixing (FWM), XPM, and FWM to characterize the impact of these effects individually on the system performance. We also apply this model to analyze the effects of varying system parameters and pulse shapes on the individual physical impairments.
This paper presents a routing, modulation, spectrum, and core allocation (RMSCA) algorithm for space-division multiplexing-based elastic optical networks (SDM-EONs). A network state-dependent route ...and core selection method is proposed using a multi-attribute decision-making method based on the analytic hierarchy process (AHP) and preference ranking organization method for enrichment evaluations (PROMETHEE) methods. This systematic resource allocation allows the network designer to choose which resources are most valuable. It is followed by a spectrum allocation algorithm using a weighted score function to rate and select the best spectrum blocks. Physical layer impairments, including inter-core cross talk, amplified spontaneous emission, and Kerr fiber nonlinearities, are considered alongside fragmentation and energy consumption. The proposed RMSCA approach is compared with published benchmarks incorporating quality of transmission constraints and evaluated on two network topologies, NSFNET (7- and 12-core multicore fiber links) and COST. It is shown to be superior in terms of blocking probability, bandwidth blocking probability, network fragmentation, and energy consumption compared to standard and published benchmarks.
Visible light communications (VLC) is a short-range optical wireless communication technology that uses light-emitting diodes (LEDs) as lighting devices and data transmitters. This paper describes a ...multiuser VLC system using Hadamard-coded modulation (HCM) for indoor data transmission. Considering the peak transmitted power limit of the LEDs, a DC-reduced HCM (DCR-HCM) is used to reduce the nonlinear clipping distortion. Since the Hadamard codewords have different bandwidth requirements for a given symbol rate, they can be assigned to users with varying hardware capabilities. Optimally assigning codewords to users is found to significantly improve the average throughput, up to twice higher than a random assignment for a typical scenario. When the number of active users is less than the size of the Hadamard matrix used, more than one codeword can be assigned per user, which further improves the throughput. This paper also examines a scenario where multiple lamps in an indoor space transmit the same data. Since the time of arrival for the received signals emitted from different lamps is different, the Hadamard codes received are no longer orthogonal, resulting in multiple access interference and inter-chip interference. The number of acceptable codewords is computed based on the specific interference experienced in different parts of the indoor space. The spatial distribution of the maximum throughput is also simulated, showing that the ratio of the maximum to the minimum data rate can be as high as 10 when considering the entire area of a typical indoor room. This article is part of the theme issue 'Optical wireless communication'.
Visible light communications (VLC) in indoor environments suffer from the limited bandwidth of LEDs as well as from the inter-symbol interference (ISI) imposed by multipath. In this work, ...transmission schemes to improve the performance of indoor optical wireless communication (OWC) systems are introduced. Expurgated pulse-position modulation (EPPM) is proposed for this application since it can provide a wide range of peak to average power ratios (PAPR) needed for dimming of the indoor illumination. A correlation decoder used at the receiver is shown to be optimal for indoor VLC systems, which are shot noise and background-light limited. Interleaving applied on EPPM in order to decrease the ISI effect in dispersive VLC channels can significantly decrease the error probability. The proposed interleaving technique makes EPPM a better modulation option compared to PPM for VLC systems or any other dispersive OWC system. An overlapped EPPM pulse technique is proposed to increase the transmission rate when bandwidth-limited white LEDs are used as sources.
In long-haul dense wavelength-division multiplexing (DWDM) systems with periodic dispersion compensation and amplification, system performance is adversely affected by severe physical impairments due ...to fiber losses, dispersion and nonlinearity. Fiber modeling is a prerequisite for the development of physical impairment mitigation techniques to improve system performance. The distance between two interacting symbols in time and wavelength, i.e., the range of influence (RoI) of each physical impairment, plays an important role in the development of these mitigation techniques. In this paper, we use the Volterra series transfer function (VSTF) method to define impairment characteristic coefficients that capture intersymbol interference (ISI), self phase modulation (SPM), intrachannel cross phase modulation (IXPM), intrachannel four wave mixing (IFWM), cross phase modulation (XPM) and four wave mixing (FWM), to characterize the impact of these impairments individually on the system output. We then investigate the impact of system parameters, namely, duty cycle, spectral efficiency, frequency chirp, and span length, on the RoI for long-haul DWDM systems.
Roadmap of optical communications Agrell, Erik; Karlsson, Magnus; Chraplyvy, A R ...
Journal of optics (2010),
06/2016, Letnik:
18, Številka:
6
Journal Article
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Lightwave communications is a necessity for the information age. Optical links provide enormous bandwidth, and the optical fiber is the only medium that can meet the modern society's needs for ...transporting massive amounts of data over long distances. Applications range from global high-capacity networks, which constitute the backbone of the internet, to the massively parallel interconnects that provide data connectivity inside datacenters and supercomputers. Optical communications is a diverse and rapidly changing field, where experts in photonics, communications, electronics, and signal processing work side by side to meet the ever-increasing demands for higher capacity, lower cost, and lower energy consumption, while adapting the system design to novel services and technologies. Due to the interdisciplinary nature of this rich research field, Journal of Optics has invited 16 researchers, each a world-leading expert in their respective subfields, to contribute a section to this invited review article, summarizing their views on state-of-the-art and future developments in optical communications.
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