Performance enhancement techniques for visible light communication systems

• Autores: Borja Genovés Guzmán
• Directores de la Tesis: Victor Pedro Gil Jiménez (dir. tes.)
• Lectura: En la Universidad Carlos III de Madrid ( España ) en 2019
• Idioma: español
• Tribunal Calificador de la Tesis: Mounir Ghogho (presid.), Máximo Morales Céspedes (secret.), Majid Safari (voc.)
• Programa de doctorado: Programa de Doctorado en Multimedia y Comunicaciones por la Universidad Carlos III de Madrid y la Universidad Rey Juan Carlos
• Materias:
  • Ciencias tecnológicas
    • Tecnología de las telecomunicaciones
• Enlaces
  • Tesis en acceso abierto en: e-Archivo
• Resumen

  • The increasing demand for ubiquitous wireless Internet access is gradually exhausting the existing radio-frequency (RF) resources. Researchers are making much effort to propose RF techniques that are capable of satisfying the increasing wireless data rate demand. For example, massive multiple-input multiple-output (MIMO) was proposed as a novel technique where many transmitting and receiving antennas are utilized to increase the capacity. However, it is expected that the RF band no longer fulfils the future wireless data services. Thus, to prevent the looming spectrum crunch, the research community is considering diverse new technologies operating in higher frequency bands, such as the millimeter-wave (mmWave) and the visible light communication (VLC) band. The former utilizes frequencies between 30 and 300 GHz. Due to the intrinsic characteristics of such frequencies, mmWave needs line-of-sight (LoS) between the transmitter and the receiver to work properly. Thus, a small multi-cell network will be used, where the hardware cost is much higher than in optical wireless networks. This makes mmWave deployment hard. Differently, VLC utilizes frequencies in the range of 430 THz and 770 THz, which is around 11000 times larger than the one dedicated to traditional RF services, and around 1000 times larger than the one estimated to be used by mmWave. Some of the advantages of the VLC technology are: its complementarity, since the signals in VLC do not interfere with RF systems and so, both technologies can perform jointly. In addition, VLC technology can be installed in scenarios where RF services are not allowed such as hospitals and airplanes; its security, due to, unlike RF where simultaneous systems can jam others located in nearby rooms, the control of optical waves propagation in VLC is possible because they do not go through walls and so systems in separate rooms will be independent of each other; its low cost, because the optical spectrum is unlicensed and the components in VLC are low-cost and off-the-shelf. These encouraging facts show the potential of VLC that can be exploited by employing transmission and detection techniques to achieve a huge data rate in the near future. The VLC solutions rely on light-emitting diodes (LEDs) that are capable of supporting high data rates on the order of Gbps. The widespread deployment of LED-based lighting facilitates the VLC installation creating a dense cell deployment and then, provides illumination and high-speed wireless communication simultaneously.

    In this Thesis, VLC point-to-point issues are investigated both in indoor and outdoor scenarios. Different from single-cell scenarios, a cellular system where there are multiple cells can accommodate multiple users connected simultaneously in a more efficient way. Since access points (APs) in a cellular system can reuse resources, the area spectral efficiency is increased. Because of these advantages, cellular systems are used since the beginning of mobile wireless networks, and currently there is a trend of cell-densification that increments the whole system performance as demonstrated in the literature. Indeed, operators are currently offering the installation of femtocells to offload users from the macro-cells and, in that way, to increase the network efficiency. Along these lines, not only single-cell scenarios are considered in this Thesis, but also VLC multi-cell fully networked scenarios where cooperation transmission techniques are proposed to address the line-of-sight (LoS) and the inter-cell interference link blockage problems.

    When using optical orthogonal frequency division multiplexing (O-OFDM) schemes, time-domain (TD) signals with large envelope fluctuations are generated. The typical metric to quantize the fluctuations of a TD signal are the peak-to-average-power ratio (PAPR) or cubic metric (CM), being the latter a more effective indicator because of using higher order statistics, whereas PAPR only considers the maximum signal peak. Thus, these OFDM signals in TD suffer from non-linear distortions when passing through an LED because of its non-linear transfer function. To avoid an overheating at the LED, a clipping of the input signal must be carried out that introduces the so-called clipping noise. Another alternative to avoid the overheating at the LED, in addition to reducing the clipping noise, is to introduce a back-off, and then to make the signal work within the dynamic range, which is the linear region of the LED’s transfer function. The great back-off necessary to avoid the clipping effect in the LED strongly degrades the performance, and then PAPR and CM reduction techniques are preferable and crucial to improve the system performance. In this Thesis, an adaptive network-based fuzzy inference system (ANFIS) is used to obtain efficient direct current optical orthogonal frequency division multiplexing (DCO-OFDM) signals with a low power envelope profile. First, signals specially designed for DCO-OFDM with very low CM, as the ones obtained from the raw cubic metric (RCM)–active constellation extension method, are used to train the fuzzy systems in time and frequency domains. Second, after the off-line training, the ANFIS can generate a real-valued signal in a one-shot way with a considerable RCM reduction from the original real-valued signal, which involves gains in the input power back-off, illumination-to-communication conversion efficiency and considerable improvements in bit error rate. For all the studied parameters and figures of merit, the implementation of this technique in a DCO-OFDM VLC system involves a better efficiency between the communication capacity and the illumination level.

    The application of VLC has been primarily oriented to indoor scenarios, but the proliferation of LEDs in the streets warrants its investigation in outdoor scenarios as well. Nowadays, there is a proliferation of outdoor scenarios capable of transmitting information by means of light: LED boards, street lights, traffic lights, head lights, etc. The number of these scenarios is increasing, which may lead to the vision of a smart city where all these sources provide Internet access. This fact opens the door for research studies of VLC techniques in outdoor scenarios. This Thesis studies the feasibility of VLC in a conventional outdoor scenario when O-OFDM schemes are employed. When working in outdoor single-cell scenarios, the shot noise produced by the sunlight and the skylight degrades the system performance. The illuminance coming from ambient light is multiplied by 100 in comparison to indoor scenarios, which may blind the receiver. Hence sophisticated adaptive techniques must be applied. Closed-form expressions of the signal-to-noise ratio (SNR) and of the mean cell data rate are derived and simulations demonstrate their accuracy. Besides, the outage probability when adaptive modulation and coding schemes are employed is analytically expressed. The background noise increases with the modulation bandwidth, hence the optimum working bandwidths were found for specific environmental conditions. 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. When DCO-OFDM and asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) are compared, it is observed that ACO-OFDM can perform at higher modulation bandwidths than DCO-OFDM for the same outage probability, but DCO-OFDM provides better performance in terms of mean cell data rate thanks to its better spectral efficiency. Generally, DCO-OFDM achieves a better performance than ACO-OFDM in the absence of background noise than during the daytime.

    Coming back to indoor scenarios, VLC networks are being proposed and deployed to provide a seamless coverage. Performance analysis in multi-cell VLC networks is more critical than in single-cell scenarios due to the impact of the inter-cell interference. Multi-cell VLC networks suffer from a high inter-cell interference power level due to the necessity of complying with the stringent lighting regulations (in terms of mean value and variability of illumination) that imply the deployment of multiple light fixtures in the same indoor area. Thus, inter-cell interference mitigation techniques become essential to keep this impairment under control. Besides, when working with wavelengths in the optical spectrum, LoS-link blockage impacts the communication capabilities notably. In this case, when there is a LoS-link blockage condition between the transmitter and the receiver, most VLC system designs are usually in outage (particularly in presence of strong inter-cell interference).

    It is important to highlight that contemporary RF multi-cell systems also suffer from inter-cell interference and LoS-link blockages but to a much lesser extent. Due to the carrier frequency that RF technology uses is much lower than optical wireless technology, the impact of LoS blockages is notably lower. Radio systems are more resilient to LoS blockages because the energy contribution of physical propagation mechanisms in RF bands (i.e., transmission/reflection, diffraction, and refraction, among others) enables the reception of appropriate signal levels even in case of blockage events; in contrast, optical signals cannot go through opaque objects and the energy contribution of physical propagation mechanisms in LoS-link blockage situations is minimal at optical frequencies. In addition, the deployment of RF cells is made by qualified staff that seeks the minimization of the overlapping area among cells for better network coverage. In contrast, an illumination system cannot be easily optimized for data transmission since the placement of the light fixtures is usually pre-defined. Thus, the inter-cell interference and LoS-link blockage phenomena in VLC systems deserve a complete different study.

    This Thesis studies cooperative transmission techniques in multi-cell VLC systems to decrease the detrimental effect of LoS-link blockages and high levels of inter-cell interference produced in these systems. Different from the state-of-the-art solutions where dynamic resource allocation and AP assignment techniques are employed, the use of static resource allocation techniques is motivated by the reliability, stability and low complexity required in VLC systems. Thus, static resource allocation techniques are presented where cooperation among adjacent cells is carried out. An initial study based on a single-carrier scheme is proposed for corridor scenarios. The main idea behind the proposal is a simple cooperative transmission scheme where the receiver terminal obtains the signal from different APs at the same time using a pulse position division multiplexing and on-off keying. This proposal outperforms traditional VLC schemes, especially when there is a LoS-link blockage, where around 3 dB of gain with respect to traditional schemes can be obtained for unoptimized parameters, and larger than 3 dB could be easily achieved.

    Due to the advantages offered by the multi-carrier schemes such as O-OFDM, cooperative transmission techniques based on O-OFDM schemes are also studied, where a hexagonal cell deployment is considered. A preliminary study is presented, which is based on phosphor-coated white LEDs and time division multiple access (TDMA). Each user is always connected to the nearest AP and receives useful signal from other two neighboring APs through the in phase or quadrature part of the quadrature amplitude modulation (QAM) symbol. The system performance is evaluated in terms of signal-to-interference-plus-noise ratio (SINR) and mean cell data rate, for different cell radius and blockage probability configurations. In view of the results, the potential of cooperative transmission schemes in O-OFDM schemes is notable, but the use of TDMA decreases the mean cell data rate considerably and new alternatives must be investigated. Thus, the use of red-green-blue (RGB) or red-green-blue-yellow (RGBY) LEDs, and wavelength division multiplexing (WDM) are proposed to increase the communication dimensions and flexibility of the VLC system, avoiding the use of TDMA. This Thesis studies different resource allocation patterns for multi-cell cooperative transmission schemes when tri- and tetra-chromatic LEDs and O-OFDM are utilized. Firstly, guidelines are derived for maintaining the same spatial distribution of the SINR in every sector of the multi-cell environment in case of single-point (non-cooperative) and multi-point (cooperative) transmission. These guidelines are formulated in the form of six Lemmas that determine: the number of possible cells that can conform a cluster without cooperation, also called cluster size without cooperation; the number of sectors in which a cell can be divided; the orientation of such sectors within a cell; the minimum number of orthogonal resources (number of colors per LED, number of frequency sub-bands per color and number of sectors per cell) necessary to perform cooperation among APs; the maximum number of cooperating APs in the same region and the maximum number of sectors within such a region; and finally, the cluster size when cooperation is performed. Satisfying all these Lemmas, the same SINR statistics are maintained in every sector of the whole coverage region. Thus, thanks to this spatial homogeneity property, the performance characterization in the whole coverage area can be simplified to the study of the SINR in a reference sector and cell. Secondly, all the possible transmission schemes are classified in terms of the LED type and available orthogonal resources, evaluating the achievable data rate of each of them under different working conditions in terms of cell radius length, half-power semi-angle of the LEDs and blockage probability. Furthermore, working conditions were thoroughly studied in order to determine the transmission schemes that comply with the illumination requirements, which are based on: the average illuminance in the whole indoor area that must be higher than a threshold value; and the illuminance uniformity defined as the ratio between the minimum illuminance level to the average illuminance in the area, which must be high enough such that people would not observe notably different lighting levels in the same room. Finally, the performance gain of different cooperation strategies among APs is also studied. It is shown that, in presence of obstacles that may block the LoS from the serving AP to the user, the implementation of multi-point cooperative transmission schemes is essential. In addition, it is demonstrated that cooperative schemes also offer important gains in mean cell data rate even in case of LoS conditions. The results confirm that the proper design of cooperative transmission schemes will be of paramount importance to improve the performance of ultra-dense VLC systems.

    All the interference mitigation and robustness-increasing techniques that are summarized above, in addition to the ones presented in the literature, need a wired backhaul approach that is often assumed to be ideal, i.e. lossless, noiseless and instantaneous, and sometimes it is supposed to involve unlimited capacity, which is unrealistic. A backhaul link that interconnects APs is essential in cellular networks such as the current multi-cell VLC network that performs as a wireless local area network (WLAN). It must be considered and studied in a realistic way to evaluate the communication system performance. Different backhaul wired techniques for VLC have been demonstrated in the literature, such as power line communication (PLC), Ethernet-VLC and optical fiber. However, the use of wired backhauls imposes an extra cost and cabling infrastructure that sometimes means a high complexity when the network becomes larger. Furthermore, every time the topology changes (e.g. the location of the light fixtures), the wired backhaul needs an update too. Thus, an alternative reflection-based wireless cooperative system is proposed, where no wires are required to interconnect APs and reflections in the floor are used as relaying links to transmit the information to neighboring APs that are provided of an additional PD. This proposal makes VLC technology stronger in the presence of blocking elements as it is common in indoor scenarios, because the system performance can still be satisfactory even in case of LoS-link blockage between the transmitter and the receiver. To evaluate the performance of the proposal, a square cell deployment is considered in this case because it provides a proper illumination distribution and it easily adapts to rectangular-shaped rooms. SNR and data rate analyses are developed, showing a proper performance and validating the analytical results with simulations. Finally, an analysis of the time delay is carried out that confirms this proposal is a valid and easy alternative to deploy robust and cooperative transmission schemes in VLC-based 5G systems.