Optical MIMO communication systems under illumination constraints

Technology for wireless information access has enabled innovation of 'smart' portable consumer devices. These have been widely adopted and have become an integral part of our daily lives. They need ubiquitous connectivity to the internet to provide value added services, maximize their functionality and create a smarter world to live in. Cisco's visual networking index currently predicts wireless data consumption to increase by 61% per year. This will put additional stress on the already stressed wireless access network infrastructure creating a phenomenon called 'spectrum crunch'. At the same time, the solid state devices industry has made remarkable advances in energy efficient light-emitting-diodes (LED). The lighting industry is rapidly adopting LEDs to provide illumination in indoor spaces. Lighting fixtures are positioned to support human activities and thus are well located to act as wireless access points. The visible spectrum (380 nm - 780 nm) is yet unregulated and untapped for wireless access. This provides unique opportunity to upgrade existing lighting infrastructure and create a dense grid of small cells by using this additional 'optical' wireless bandwidth. Under the above model, lighting fixtures will service dual missions of illumination and access points for optical wireless communication (OWC). This dissertation investigates multiple-input multiple-output (MIMO) optical wireless broadcast system under unique constraints imposed by the optical channel and illumination requirements. Sample indexed spatial orthogonal frequency division multiplexing (SIS-OFDM) and metameric modulation (MM) are proposed to achieve higher spectral efficiency by exploiting dimensions of space and color respectively in addition to time and frequency. SIS-OFDM can provide significant additional spectral efficiency of up to (Nsc/2 - 1) x k bits/sym where Nsc is total number of subcarriers and k is number of bits per underlying spatial modulation symbol. MM always generates the true requested illumination color and has the potential to provide better color rendering by incorporating multiple LEDs. A normalization framework is then developed to analyze performance of optical MIMO imaging systems. Performance improvements of up to 45 dB for optical systems have been achieved by decorrelating spatially separate links by incorporating an imaging receiver. The dissertation also studies the impact of visual perception on performance of color shift keying as specified in IEEE 802.15.7 standard. It shows that non-linearity for a practical system can have a performance penalty of up to 15 dB when compared to the simplified linear system abstraction as proposed in the standard. Luminous-signal-to-noise ratio, a novel metric is introduced to compare performance of optical modulation techniques operating at same illumination intensity. The dissertation then introduces singular value decomposition based OWC system architecture to incorporate illumination constraints independent of communication constraints in a MIMO system. It then studies design paradigm for a multi-colored wavelength division multiplexed indoor OWC system

Downlink Performance of Optical OFDM in Outdoor Visible Light Communication

Visible light communication (VLC) is a promising ubiquitous design alternative for supporting high data rates. Its application has been primarily oriented to indoor scenarios, but the proliferation of light-emitting diodes in the streets warrants its investigation in outdoor scenarios as well. This paper studies the feasibility of VLC in a conventional outdoor scenario, when optical orthogonal frequency division multiplexing techniques are employed. The presence of sunlight reduces the system's performance, hence sophisticated adaptive techniques must be applied. Closed-form expressions of the signal-to-noise ratio and of the mean cell data rate are derived and our simulations demonstrate their accuracy. Besides, the outage probability when adaptive modulation and coding schemes are employed is analytically expressed. It is shown that, when modulation bandwidth adaptation is carried out depending on the time of day and the illuminance from ambient light, the mean cell data rate is increased and the outage probability is reduced.This work was supported in part by the Spanish National ELISA Project under Grant TEC2014-59255-C3-3-R, the TERESA-ADA Project under MINECO/AEI/FEDER, UE Grant TEC2017-90093-C3-2-R and the 5RANVIR Project under MINECO/AEI/FEDER, UE Grant TEC2016-80090-C2-1-R. The work of B. Genovés Guzmán was supported by the Spanish MECD FPU Fellowship Program. The work of M. C. Aguayo-Torres was supported by the Universidad de Málaga. The work of H. Haas was supported in part by EPSRC through the Established Career Fellowship Extension under Grant EP/R007101/1 and in part by the Wolfson Foundation and the Royal Society. The work of L. Hanzo was supported in part by EPSRC under Project EP/Noo4558/1 and Project EP/PO34284/1, in part by the Royal Society's GRFC Grant, and in part by the European Research Council's Advanced Fellow Grant QuantCom

Transceiver Design for MIMO DCO-OFDM in Visible Light Communication

Direct current-biased optical-orthogonal frequency-division multiplexing (DCO-OFDM) is a simple yet spectrally efficient multicarrier modulation scheme for visible light communication (VLC). But in multiple-input multiple-output (MIMO) scenario, which is more practical for VLC due to the LED deployment, the research on DCO-OFDM is still limited and calls for in-depth investigation. In this chapter, we first study the basic modulation scheme of DCO-OFDM, including the design of conventional receiver without considering the clipping noise. Secondly, we present a novel receiver for combating clipping distortion in the DCO-OFDM system, which can reconstruct the clipping noise and subtract it from the received signal. Thirdly, we generalize the results to MIMO scenario and investigate the preliminary transceiver design, which is based on the minimum mean-square error (MMSE) criteria. Based on this, we propose a precoding algorithm to further enhance the performance. Finally, the symbol error rate performance is compared through computer simulations to give the reader a whole picture of the performance of MIMO VLC system

Mitigation of Side-Effect Modulation in Optical OFDM VLC Systems

Side-effect modulation (SEM) has the potential to be a significant source of interference in future visible light communication (VLC) systems. SEM is a variation in the intensity of the light emitted by a luminaire and is usually a side-effect caused by the power supply used to drive the luminaires. For LED luminaires powered by a switched mode power supply, the SEM can be at much higher frequencies than that emitted by conventional incandescent or fluorescent lighting. It has been shown that the SEM caused by commercially available LED luminaires is often periodic and of low power. In this paper, we investigate the impact of typical forms of SEM on the performance of optical OFDM VLC systems; both ACO-OFDM and DCO-OFDM are considered. Our results show that even low levels of SEM power can significantly degrade the bit-error-rate performance. To solve this problem, an SEM mitigation scheme is described. The mitigation scheme is decision-directed and is based on estimating and subtracting the fundamental component of the SEM from the received signal. We describe two forms of the algorithm; one uses blind estimation while the other uses pilot-assisted estimation based on a training sequence. Decision errors, resulting in decision noise, limit the performance of the blind estimator even when estimation is based on very long signals. However, the pilot system can achieve more accurate estimations, thus better performance. Results are first presented for typical SEM waveforms for the case where the fundamental frequency of the SEM is known. The algorithms are then extended to include a frequency estimation step and the mitigation algorithm is shown also to be effective in this case

Study And Analysis Of An Optical OFDM Based On The Discrete Hartley Transform For IM/DD Systems

Projecte fet en col.lboració amb CTTC. Centre Tecnològic de Telecomunicacions de CatalunyaOrthogonal frequency division multiplexing (OFDM) is used extensively in broadband wired and wireless communication systems because it is an effective solution to inter channel. While many details of OFDM systems are very complex, the basic concept of OFDM is quite simple: data is transmi number of different frequencies, and as a result the symbol period is much longer than for a serial system with the same total data rate. Because the symbol period is longer, ISI affects at most one symbol, and equalization is simplified. In most OFDM implementations any residual ISI is removed by using a form of guard interval called a cyclic prefix

PAPR reduction in multicarrier modulation techniques based visible light communication systems

Visible light communication (VLC) is an optical wireless communication (OWC) technology that has the potential to provide high data rate transmission for indoor applications. VLC is a promising alternative technology with a large and unlicensed spectrum to complement the congested radio frequency (RF) based communication in order to meet the exponential growth and popularity of smart devices, data intensive services and applications. The use of low-cost commercially available front-end devices further highlights the attraction of VLC system. However, nonlinear dynamic range of front-end devices and optical channel impairments limit full exploitation of VLC available modulation bandwidth. To fully benefit from the inherent resources and mitigate these limitations, multicarrier modulation (MCM) techniques are adopted. However, these techniques are affected by high peak-to-average power ratio (PAPR) which imposes constraints on the limited dynamic range of the front-end devices and the average radiated optical power. The main focus throughout this thesis is to reduce the high PAPR of MCM modulation techniques-based VLC system by implementing pilot-assisted (PA) technique. Additionally, performance of PAPR reduced modulation techniques is investigated through analytical, simulation, and experimentally. This thesis first presents background of VLC system principles including the front-end devices, VLC channel, system impairments and challenges, and employed solutions. The principles, limitations, and performance of MCM modulation variants that are implemented in this work are presented. Moreover, principles of PAPR challenge in MCM based VLC, PAPR evaluation, impact on the transmitted signal as well as the existing PAPR reduction techniques are discussed. Looking at the gap, a PA is implemented as PAPR reduction technique which is presented in this work including its implementation and performance. Following that, multiple experimental studies on PAPR reduction of PA technique are presented. Two experimental demonstrations on the efficacy of PA PAPR reduction for PAM-DMT and DCO-OFDM based VLC using a single blue LED are presented. These studies are comparing the bit-error-rate (BER) performance of the proposed systems with conventional counterparts over a range of sampling rate. This shows that, the proposed systems perform better than conventional systems without PAPR reduction. The results are validated through simulation. Other two experimental studies on the previous systems with parameters optimisation and available modulation bandwidth utilisation are presented, which show that the proposed systems outperform the conventional systems in terms of BER. This is followed by investigating the PA PAPR reduction effect on the achievable data rate of a wavelength division multiplexing (WDM) based VLC system using three different LEDs for PAPR reduced DCO-OFDM and PAM-DMT systems. The proposed systems have achieved more than 8% data rate higher than that of conventional systems without BER performance degradation. Finally, analytical investigation of clipping noise that leads to distortion in a VLC system due to front-end devices limitations is presented. To mitigate the clipping noise, PAPR of the system is reduced by the PA technique. The analytical BER performance of the system with PAPR reduction is verified through simulation and then compared to that of the conventional system without PAPR reduction at similar clipping levels. The PA proposed system shows better BER performance at all clipping levels

High speed energy efficient incoherent optical wireless communications

The growing demand for wireless communication capacity and the overutilisation of the conventional radio frequency (RF) spectrum have inspired research into using alternative spectrum regions for communication. Using optical wireless communications (OWC), for example, offers significant advantages over RF communication in terms of higher bandwidth, lower implementation costs and energy savings. In OWC systems, the information signal has to be real and non-negative. Therefore, modifications to the conventional communication algorithms are required. Multicarrier modulation schemes like orthogonal frequency division multiplexing (OFDM) promise to deliver a more efficient use of the communication capacity through adaptive bit and energy loading techniques. Three OFDM-based schemes – direct-current-biased OFDM (DCO-OFDM), asymmetrically clipped optical OFDM(ACO-OFDM), and pulse-amplitude modulated discrete multitone (PAM-DMT) – have been introduced in the literature. The current work investigates the recently introduced scheme subcarrier-index modulation OFDM as a potential energy-efficient modulation technique with reduced peak-to-average power ratio (PAPR) suitable for applications in OWC. A theoretical model for the analysis of SIM-OFDMin a linear additive white Gaussian noise (AWGN) channel is provided. A closed-form solution for the PAPR in SIM-OFDM is also proposed. Following the work on SIM-OFDM, a novel inherently unipolar modulation scheme, unipolar orthogonal frequency division multiplexing (U-OFDM), is proposed as an alternative to the existing similar schemes: ACO-OFDMand PAM-DMT. Furthermore, an enhanced U-OFDMsignal generation algorithm is introduced which allows the spectral efficiency gap between the inherently unipolar modulation schemes – U-OFDM, ACO-OFDM, PAM-DMT – and the conventionally used DCO-OFDM to be closed. This results in an OFDM-based modulation approach which is electrically and optically more efficient than any other OFDM-based technique proposed so far for intensity modulation and direct detection (IM/DD) communication systems. Non-linear distortion in the optical front-end elements is one of the major limitations for high-speed communication in OWC. This work presents a generalised approach for analysing nonlinear distortion in OFDM-based modulation schemes. The presented technique leads to a closed-form analytical solution for an arbitrary memoryless distortion of the information signal and has been proven to work for the majority of the known unipolar OFDM-based modulation techniques - DCO-OFDM, ACO-OFDM, PAM-DMT and U-OFDM. The high-speed communication capabilities of novel Gallium Nitride based μm-sized light emitting diodes (μLEDs) are investigated, and a record-setting result of 3.5Gb/s using a single 50-μm device is demonstrated. The capabilities of using such devices at practical transmission distances are also investigated, and a 1 Gb/s link using a single device is demonstrated at a distance of up to 10m. Furthermore, a proof-of-concept experiment is realised where a 50-μm LED is successfully modulated using U-OFDM and enhanced U-OFDM to achieve notable energy savings in comparison to DCO-OFDM

Resource Allocation for Cooperative Transmission in Optical Wireless Cellular Networks With Illumination Requirements

This work has been partially funded by the Spanish MECD FPU fellowship program granted to the author Borja Genovés Guzmán, the Catalan Government under Grant 2017-SGR-1479, and the Spanish Government under the national project ’TERESA-ADA’ with ID no. TEC2017-90093-C3-2-R and TEC2017-90093-C3-1-R (MINECO/AEI/FEDER, UE)