Optical Attocell Research Articles

Interference mitigation using optimised angle diversity receiver in LiFi cellular network

Light-fidelity (LiFi) is an emerging technology for high-speed short-range mobile communications. Inter-cell interference (ICI) is an important issue that limits the system performance in an optical attocell network. Angle diversity receivers (ADRs) have been proposed to mitigate ICI. In this paper, the structure of pyramid receivers (PRs) and truncated pyramid receivers (TPRs) are studied. The coverage problems of PRs and TPRs are defined and investigated, and the lower bound of field of view (FOV) for each PD is given analytically. It is shown that the lower bound of FOV for TPR and PR are 20° and 30°, respectively. The impact of random device orientation and diffuse link signal propagation are taken into consideration. The performances of PRs and TPRs are compared, and optimised ADR structures are proposed by jointly considering the impact of tilt angle, FOV, and the number of PDs. For a transmitter-bandwidth limited system, the optimal PD values are 6 for PR and 9 for TPR, whereas, for a receiver-bandwidth limited system, the optimal PD value is 15. In addition, the double source (DS) cell system, where each LiFi AP consists of two sources transmitting the same information signals but with opposite polarity, is proved to outperform the single source cell (SS) system in interference limited or noise-plus-interference limited scenario. However, the SS cell system outperforms the DS cell system in a noise-limited scenario.

Self-Optimizing Data Offloading in Mobile Heterogeneous Radio-Optical Networks: A Deep Reinforcement Learning Approach

In addition to the exploration of more spectrum at high-frequency bands, next-generation wireless networks will witness an intelligent convergence of radio frequency (RF) and non-RF links such as optical and visible light communication. Optical attocell (OAC) networks provide an additional layer to RF-based wireless networks with gigabit-per-second data transmission rate and centimeter-level location accuracy. However, the directionality, line-of-sight constraints, as well as strong sensitivity to the location and orientation of user terminals challenge the stringent requirements for throughput and latency. In this article, we consider mobile heterogeneous networks (HetNets) incorporating indoor OAC with femtocells and macrocells to provide a low-cost and energy-efficient solution. The HetNets solution satisfies diverse service requirements in terms of user-experienced data rate, mobility, latency, accuracy, and security in the Internet of Things. To support seamless connectivity and optimal resource allocation in the proposed HetNets with mobility awareness, handover in dynamic environments needs to be addressed efficiently. Incorporating rich environmental parameters into such a decision making problem facilitates the self-optimization process, but extensively expands the state space. To achieve a fast convergence speed, a deep reinforcement learning approach is proposed to optimize the handover parameters (e.g., time-to-trigger and hysteresis margin). This is a model-free and off-policy reinforcement setting that trains and employs a deep neural network to predict future rewards for successions of states and actions. Thus, the optimal parameters are obtained by selecting the best actions to take. Through numerical simulation and performance analysis, we discover the gain from enriching the state space and the adaptability of the system to dynamic environments.

Analyzing optical TDMA to mitigate interference in downlink LiFi optical attocell networks

Abstract Co-channel interference in the downlink of LiFi attocell networks significantly decreases the network performance in terms of rate. Analysis of multiple access schemes is essential to mitigate interference and improve rate. The light-emitting diodes (LEDs) being centrally monitored, the time division multiple access (TDMA) scheme over the LEDs will be suitable to analyze. This work considers the interference characterization in Ref. (Surampudi A, Ganti RK. Interference characterization in downlink Li-Fi optical attocell networks. J Lightwave Technol 2018;36:3211–28) over M-PAM modulated signals to derive an exact expression for the goodput G of the time scheduled attocell network, which is arranged as a deterministic square lattice in two dimensions. Given this TDMA over the LEDs, numerical simulations show that the LEDs can be optimally time scheduled to maximize the goodput, which implies that the TDMA mitigates interference in an attocell network compared to the case when the LEDs are unscheduled.

Analysis of interference on mirror-aided non-LOS backhaul data transmission in VLC attocell networks

Visible light communication (VLC) is a promising technology that addresses the bandwidth bottleneck of radio-frequency based indoor wireless networks. Optical attocell networks using VLC backhaul links have been proposed. Line-of-sight (LOS) and mirror-aided non-LOS links can be used for backhaul transmission. As the visible light spectrum is used for both access downlink and bidirectional backhaul transmission, co-channel interference is a major issue. In this paper, we analyze the performance of optical attocell networks using VLC backhaul links taking into account illumination and eye safety requirements. Based on the signal-to-interference-plus-noise ratio of different backhaul links, the bandwidth allocation ratio when using an in-band frequency reused scheme is discussed. Finally, aggregate data rates of networks using different VLC backhaul configurations are compared. Results show that LOS backhaul configuration can achieve nearly optimal network performance while mirror-aided non-LOS backhaul configuration is more robust against misalignment.

Performance Analysis of Optimization Algorithms for a Cross-Layer Shadowing Recover Scheme

Optical attocell networks based on visible light communication (VLC) are cost-efficient solutions which complement the spectrum shortage of radio-frequency(RF)-based indoor wireless networks. However, communication using visible light is vulnerable to shadowing effect caused by random pedestrians as visible light cannot penetrate opaque objects. A cross-layer resilience scheme RASP (routing, load assignment, subcarrier selection, power allocation) is proposed to address the shadowing effect. Network performance using different combinations of cross-layer control policies, i.e., algorithms and allocation methods, is evaluated and compared by RASP. An optimization algorithm is used to find the optimal/near-optimal combination of RASP control policies. In order to find the proper optimization algorithm for RASP, we investigate and compare the performance of different algorithms, including partial enumeration and neighbor-searching algorithms like hill-climbing and Tabu-search. An improved Tabu-search algorithm is proposed to reduce the time-complexity.

Joint Synchronization and Attocell Identification With Received Power Estimation in VLC Networks

Optical attocell networks building upon visible light communication (VLC) offer several advanced features including load balancing, mobility management etc. as in radio-frequency networks. Attocell identification is crucial to perform the features' functionalities. Attocells are allocated with identities and their own synchronization signals (SSs) are generated accordingly. The signal is periodically broadcasted and used for cell identification and synchronization in receiver side. In this letter, we revise SS design for VLC domain and propose a joint synchronization and cell identification including SS received power estimation mechanism. The performance of proposed mechanisms is then evaluated though Monte-Carlo simulations.

Throughput Enhancement of the Edge User Equipments Based on the PowerBandwidth Tradeoff in the Optical Attocell Networks

Throughput Enhancement of the Edge User Equipments Based on the PowerBandwidth Tradeoff in the Optical Attocell Networks

Integration of Visible Light Communication and Positioning within 5G Networks for Internet of Things

With the widespread deployment of Internet of Things (IoT), more and more devices are involved in wireless networks, and the fifth generation (5G) network requires to support the massive connectivity and diverse services for the huge number of IoT devices. Visible light communications (VLC) and visible light positioning (VLP) are two promising supplementary technologies to assist 5G networks to support the massive connectivity, high reliability, high data rate, high positioning accuracy, low latency, low power consumption and improved security of IoT. Hence, this article presents a multi-layer network architecture by integrating VLC and VLP within 5G networks, in order to support the above mentioned diverse requirements of IoT devices. In the multi-layer network, the macrocell and picocell layers support better coverage and reliability via the radio frequency (RF) spectrum, while the optical attocell layer provides the high-speed transmission and high-accuracy positioning services operating at the visible light spectrum. We then briefly describe some key technologies for the performance improvement of the multi-layer network, including energy harvesting, modulation and multiple access schemes. An exemplary case study and simulation analysis are provided to demonstrate the advantage and significance of the presented multi-layer network for IoT. Finally, we point out some future research directions.

Multi-Hop Wireless Optical Backhauling for LiFi Attocell Networks: Bandwidth Scheduling and Power Control

The backhaul of hundreds of light fidelity (LiFi) base stations (BSs) constitutes a major challenge. Building on an indoor wireless optical backhauling approach, this paper presents the top-down design of a multi-hop wireless backhaul configuration for multi-tier optical attocell networks by proposing super cells. Such cells incorporate multiple clusters of attocells that are connected to the core network via a single gateway. Consequently, new challenges arise for managing the bandwidth and power of the bottleneck backhaul. By putting forward user-based bandwidth scheduling (UBS) and cell-based bandwidth scheduling (CBS) policies, the system-level modeling and analysis of the end-to-end multi-user sum rate is elaborated. In addition, optimal bandwidth scheduling under both UBS and CBS policies are formulated as constrained convex optimization problems, and solved by using the projected subgradient method. Furthermore, the transmission power of the backhaul system is opportunistically reduced using a fixed power control (FPC) strategy. The notion of backhaul bottleneck occurrence (BBO) is introduced. An accurate approximate expression of the probability of BBO is derived, and verified using Monte Carlo simulations. Several insights are provided by studying different aspects of the performance of super cells including the average sum rate, the BBO probability and the backhaul power efficiency (PE).

Pre-Distorted Enhanced ADO-OFDM for Hybrid VLC Networks: A Mutual-Interference-Free Approach

Mutual interference is a crucial issue for the visible light communication (VLC) systems to be addressed, including both co-channel interference (CCI) and inter-carrier interference (ICI). In this paper, to solve the CCI problem, a novel optical attocell network is introduced, which can be regarded as a hybrid network based on both single frequency network and multi-frequency network. Meanwhile, to eliminate the ICI, a pre-distorted enhanced asymmetrically clipped DC biased optical orthogonal frequency division multiplexing (PEADO-OFDM) approach is designed. In addition, the corresponding properties of PEADO-OFDM are analyzed, and different modulation schemes are employed for downlink and uplink in the same ADO-OFDM modulation framework to reduce the complexity of user processing. Simulation results indicate that the proposed PEADO-OFDM scheme outperforms the conventional ADO-OFDM approach in terms of bit error rate, while the processing latency and computational complexity are greatly reduced at the receiver.

Hybrid femtocell–attocell optical links for indoor free-space optical communication

The demand for wireless bandwidth is increasing rapidly; however, the throughput of short-range radio frequency (RF) solutions, such as WiFi, are quickly reaching limits due to limited bandwidth in the RF spectrum. This problem can be overcome by utilizing unregulated infrared wavelengths in the optical spectrum. On the other hand, free-space optical (FSO) communication presents a challenge for mobility due to the line-of-sight nature of optics. We present a dual-channel optical link for short-range FSO communication, which consists of a 100 Mb / s, large diameter (60 cm at 3 m distance) optical femtocell with a 1.5 Gb / s, small diameter (1 cm at 3 m distance) optical attocell embedded within the femtocell. This hybrid femtocell–attocell optical link addresses the needs of various users by providing a high mobility femtocell link combined with an enhanced bandwidth attocell hotspot. A prototype hybrid system was assembled and evaluated in terms of bandwidth, range, and bit error rate (BER). In addition, the relationship between the incident angle of the transmitted beam relative to the receiver and the BER of the received data was evaluated. At a communication distance of 3 m, the BER is

A Wireless Optical Backhaul Solution for Optical Attocell Networks

The problem of backhauling for optical attocell networks has been approached by a number of wired solutions such as in-building power line communication (PLC), Ethernet, and optical fiber. In this paper, an alternative solution is proposed based on the wireless optical communication in visible light and infrared (IR) bands. A thorough analysis of signal-to-noise-plus-interference ratio (SINR) is elaborated for a multi-user optical attocell network based on the direct current biased optical orthogonal frequency division multiplexing (DCO-OFDM) and decode-and-forward (DF) relaying, taking into account the effects of inter-backhaul and backhaul-to-access interferences. Inspired by concepts developed for radio frequency (RF) cellular networks, full-reuse visible light (FR-VL) and in-band visible light (IB-VL) bandwidth allocation policies are proposed to realize backhauling in the visible light band. The transmission power is opportunistically minimized to enhance the backhaul power efficiency. For a two-tier FR-VL network, there is a technological challenge due to the limited capacity of the bottleneck backhaul link. The IR band is employed to add an extra degree of freedom for the backhaul capacity. For the IR backhaul system, a power-bandwidth trade-off formulation is presented. Closed form analytical expressions are derived for the corresponding power control coefficients. Finally, the network sum rate performance is studied using extensive Monte Carlo simulations.

Interference Mitigation for Indoor Optical Attocell Networks Using an Angle Diversity Receiver

In this paper, interference mitigation techniques based on angle diversity receivers (ADRs) are studied for optical attocell networks. Consisting of multiple photodiodes (PDs), an ADR requires appropriate signal combining schemes in order to mitigate intercell interference (ICI) in optical attocell networks. Four signal combining schemes, namely select best combining, equal gain combining, maximum ratio combining, and optimum combining are investigated. To further mitigate ICI, a novel double-source cell configuration with two transmission modes is also proposed. Results show that the systems with ADRs significantly outperform those with single-PD receivers in terms of signal-to-interference-plus-noise ratio (SINR). Also, compared to the conventional single-source cell configuration, the double-source cell configuration can provide an SINR improvement of over $\text{20}$ dB. Furthermore, an analytical framework is proposed to analyze the performance of optical attocell networks with ADRs, where different propagation scenarios such as line-of-sight and nonline-of-sight are considered. The accuracy of the proposed analytical model is validated by Monte Carlo simulations.

Interference graph-based dynamic frequency reuse in optical attocell networks

Indoor optical attocell network may achieve higher capacity than radio frequency (RF) or Infrared (IR)-based wireless systems. It is proposed as a special type of visible light communication (VLC) system using Light Emitting Diodes (LEDs). However, the system spectral efficiency may be severely degraded owing to the inter-cell interference (ICI), particularly for dense deployment scenarios. To address these issues, we construct the spectral interference graph for indoor optical attocell network, and propose the Dynamic Frequency Reuse (DFR) and Weighted Dynamic Frequency Reuse (W-DFR) algorithms to decrease ICI and improve the spectral efficiency performance. The interference graph makes LEDs can transmit data without interference and select the minimum sub-bands needed for frequency reuse. Then, DFR algorithm reuses the system frequency equally across service-providing cells to mitigate spectrum interference. While W-DFR algorithm can reuse the system frequency by using the bandwidth weight (BW), which is defined based on the number of service users. Numerical results show that both of the proposed schemes can effectively improve the average spectral efficiency (ASE) of the system. Additionally, improvement of the user data rate is also obtained by analyzing its cumulative distribution function (CDF).

Space Division Multiple Access for Optical Attocell Network Using Angle Diversity Transmitters

In this paper, an optical space division multiple access (SDMA) scheme is proposed for optical attocell networks. In the system, the conventional single-element transmitter in each optical cell is substituted by an angle diversity transmitter which can simultaneously serve multiple active users at different positions. The type of configuration can increase the available bandwidth resource and also mitigate intercell interference in optical attocell networks. The results show that an optical SDMA scheme significantly outperforms the conventional optical time division multiple access scheme. The SDMA scheme improves the average spectral efficiency of the system by a factor of 26 for a 37-element light emitting diode angle diversity transmitter. In addition, the upper and lower bound of the optical SDMA performance are derived analytically. These bounds can precisely estimate the performance of SDMA systems. Also, the study is extended to take account of user position errors, and Monte-Carlo simulations show that the system is very robust to user position errors.

Ultra-Thin Optical Sheets for Parallel Data Transmission of Visible Light Communications

Visible light communications (VLCs) have become an important and promising supplement to the Wi-Fi network for the coming 5G communications. Photodetectors with narrow field-of-view are essential for parallel data transmission, such as optical attocell networks and spatial multiple-input and multiple-output arrays. Here, we propose a design of sub-millimeter-scale flexible optical sheets with strong angular selectivity. They can be mounted to any photodetectors even with irregular surfaces and significantly narrow the field-of-view (down to 4ř or less) with a plug-and-play feature. These optical sheets pave the way toward unprecedented progress in cell densification and channel capacity for VLC-based 5G networks.

Applying VLC in 5G Networks: Architectures and Key Technologies

Visible light communication has gained tremendous attention recently and has become a favorable complementary technology to millimeter- wave communication in short-range communication scenarios for future 5G networks. VLC possesses a number of prominent features to address the highly demanding 5G system requirements for high capacity, high data rate, high spectral efficiency, high energy efficiency, low battery consumption, and low latency. These prominent features include but are not limited to abundant license-free spectrum, the ability to provide multiple gigabit-per-second data rates, low energy consumption, and low implementation costs. In this article a heterogeneous multi-layer 5G cellular architecture with a control plane and user plane separation scheme is presented. Three layers — the macrocell layer operating below 3 GHz, the picocell layer operating in the mmWave spectrum, and the optical attocell layer operating at the visible spectrum — are considered in this architecture, with each layer targeting a different coverage and communication need. We further address potential solutions for uplink communications, backhaul connections, and downlink communications in an optical attocell. Several key technologies employed in VLC downlink communications, including modulation, multiple access, and error control schemes, are also discussed.

Downlink Performance of Optical Attocell Networks

An optical attocell network is proposed as an indoor small-cell cellular network based on visible light communication. In this paper, the downlink performance of optical attocell networks is comprehensively analyzed. In particular, signal-to-interference-plus-noise ratio, outage probability, and the resulting achievable cell data rates of optical attocell networks with optical orthogonal frequency division multiplexing are analyzed. With different lighting network designs, the cell deployments of optical attocell networks may vary considerably. Hence, attocell networks with different cell deployments are considered and compared. The results show that the hexagonal and Poisson point process random cell deployments represents the best- and the worst-case performance of practical optical attocell deployments, respectively. In addition, the performance of optical attocell networks is compared with that achieved by other radio frequency small-cell networks. The results show that a well-designed optical attocell network can perform better than the state-of-the-art femtocell network or millimeter-wave system in terms of indoor area data rate (data rate per unit area).

Fractional Frequency Reuse in DCO-OFDM-Based Optical Attocell Networks

In this paper a fractional frequency reuse (FFR) technique is considered in a direct-current optical orthogonal frequency-division multiplexing-based optical attocell network. An optical attocell network is proposed as a special type of visible light communication system that has the complete function of a cellular network. The cellular network is composed of many cells of extremely small size—the optical attocells. Two FFR schemes, strict fractional frequency reuse and soft frequency reuse, are considered. The signal-to-interference-plus-noise ratio (SINR) statistics and the spectral efficiency of the optical cellular system with FFR are analyzed. The performance of the systems with full frequency reuse and FFR is evaluated and compared. The results show that the FFR scheme can effectively achieve interference mitigation in an optical attocell network. The cell edge user SINR and spectral efficiency are significantly improved. Additionally, FFR provides improvements in average spectral efficiency. The effects of important parameters such as cell radius are also studied.