OWC advantages and challenges
OWC offers a number of advantages compared to RF systems. We summarize them as follows. High data rate: The use of optical spectrum offers ultra-large bandwidth of 400 THz, which potentially enables data transmission with high data-rates.
Unlicensed spectrum: The very large optical spectrum is currently free and unlicensed, unlike the RF spectrum which is expensive and regulated, in addition to being increasingly congested. Inherent security: Due to the confinement properties of light in indoor environment and the narrow beams used in outdoor applications, OWC links benefit from inherent security and make their interception a relatively hard task.
Ease of deployment: OWC transceivers often use off-the-shelf components, which allows their easy deployment in both indoor and outdoor environments with a relatively affordable cost. Immunity to EMIs: The use of very large optical bandwidth and the confinement properties of light, mentioned above, enables easy frequency reuse and thus dealing with electromagnetic interference (EMI).
On the other hand, OWC faces some challenges that need to be addressed for a robust, operational implementation. We mention the most important challenges of OWC in the following.
Signal level fluctuations: OWC links are susceptible to LOS link blockage by obstacles be-tween the Tx and Rx, which can cause transmission interruption or, otherwise, random signal fluctuations due to shadowing. Note that small-scale fading is mostly negligible due to the fact that the PD surface area is much larger than the optical signal wavelength. Nevertheless, signal level fluctuations may occur due to pointing errors and misalignment in point-to-point links or due to atmospheric turbulence in outdoor scenarios.
Eye safety: Laser safety standards impose an upper-limit on optical transmit power, which restricts the achievable performance especially for narrow beam and narrow field-of-view (FOV) links.
OWC applications can be classified based on their transmission range, starting from ultra-short range communications in inter-chip wireless communications to ultra-long range communica-tions as for space communications [47–49]. Figure 1.4 illustrates some of these applications. We distinguish the following applications.
Ultra-short range applications: The standard copper-based electrical interconnects have become a major bandwidth bottleneck for ultra high data rate, low latency, data centers and super computer systems. Also, their lack of reconfigurability does not enable them to adjust to the network topology and traffic patterns. Within this context, free-space optics interconnects (FSOI) can provide ultra-high data rate and EMI-robust alternative . FSOI have a great potential to address issues such as bandwidth, latency, and power consumption. In addition, the development of optical transmission, switching, and control and management have an important impact on the flexibility and scalability of data centers .
Short range applications (tens of centimeters): The growing deployment of LEDs for illumination infrastructures has encouraged the investigation of VLC systems to enable both lighting and communication in indoor environments. These systems are commonly referred to as light-fidelity (LiFi), which can improve the spectral density, compared to their RF counterparts (i.e., wireless fidelity (WiFi)) networks . Nowadays, many short-range indoor applications can be addressed through the use of OWC. For instance, indoor localization using VLC technologies can provide a low-cost, high accuracy system where the classical GPS cannot be used, such as in indoor public spaces (i.e., malls, train stations, airports), tunnels, hospitals, factories, etc. [53, 54]. Another example is optical camera communications (OCC), where cameras (imaging sensors) are used as optical Rx to enable machine to machine (M2M)) communications . Also, OWC was proposed for aircraft cabin communications to replace the wired networks and thus reduce the weight and flying costs [56, 57]. Lastly, optical WBANs were proposed to complement the growing RF-based WBANs as discussed above .
Medium range applications (several meters): Similar to indoor applications, the availability of LED-based lighting systems in outdoor environment has led to the development of optical vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) communications [58, 59]. Here, the front and back lighting of cars can be used to transmit information to the adjacent cars in order to enable applications such as platooning, autonomous driving, etc. Another application of OWC is in underwater communications (UWC) [60–63], where VLC (generally in the green and blue wave-length range) can be used to augment/complement the traditional acoustic communications, enabling much higher data-rates.
Long range applications (hundreds of meters to several kilometers): Free-space optical (FSO) systems can be deployed for high speed point-to-point data transmission. Here, the use of narrow laser beams gives the link an inherent security and an immunity to EMI, as well as a high reuse factor . However, FSO links are vulnerable to fog and snowfalls, which can cause a severe performance degradation and failure. To solve this, hybrid millimeter wave (mmW) RF/FSO links are used to complement each other in cases of fog or rain . FSO networks are, in particular, proposed as first- and last-mile solution for high data rates applications to bridge the gap between the end users and the existing fiber optics infrastructure [66, 67]. Furthermore, FSO can provide a viable solution for high capacity inter-building links in enterprises with decentralized offices or university campuses . Due to their ease of deployment, FSO links can serve in remote areas and disaster situations, where the communication infrastructure could be damaged or non existing . Last, FSO links have been investigated for mobile platforms such as for unmanned-aerial-vehicle (UAV)-to-ground and UAV-to-UAV communications . These applications would require reliable tracking algorithms to enable strong LOS links .
Ultra-long range applications: Aerospace communications could also benefit from the advantages of OWC, mentioned above. For instance, coherent FSO links can be used for space-to-ground and space-to-space communications,thus benefiting from a high Rx sensitivity and a high spectral efficiency [71, 72]. Also, FSO links can be used for heterogeneous satellite and terrestrial networks (HSTN)s, especially for air-to-air, air-to-ground, and space-to-air links between UAVs, low earth orbit (LEO) satellite, high altitude platforms (HAP)s, and terrestrial base stations (TBS) [73–75].
This dissertation focuses on the characterization and modeling of optical-based WBAN chan-nels as well as on the design of suitable signaling schemes to address the requirement of mul-tiple access management (regarding to data transmission from multiple sensors) in the physical (PHY) and MAC transmission layers. This work has been supported by VisIoN, a European project funded by the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie Grant Agreement No. 764461. Under the framework of vision project, I have been involved in the work package 4, on « Manufacturing and Medical ». I have benefited from a secondment in the communications lab of Czech technical university in Prague (CTU) (for 4 months) and a virtual secondment to the technical university of Berlin (TUB) (for 2 months).
In summary, regarding WBAN channel characterization, in order to accurately model the mobil-ity and shadowing effects of the body, we have considered animation based 3D body walk cycles and walk trajectories based on an improved random waypoint (RWP) mobility model. In fact, the movements of the body parts change the link geometry (related to body posture) between the Tx and the Rx, which could occasionally result in LOS blockage or beam shadowing. Moreover, body movement inside a room exhibits a correlation between consecutive sequences of the walk trajectory, which needs to be taken into consideration for a realistic channel model. As a result, ap-propriate non-uniform mobility modeling needs to be considered within this context. Also, given the potentially large number of nodes that could be used for each WBAN, developing a robust MA scheme is quite challenging. Therefore, we have considered efficient MA management through the MAC layer by minimizing packet collisions and their scheduling to ensure acceptable energy consumption and access delay.
The main questions we try to answer in this thesis are: how to evaluate the impact of body shad-owing and mobility on the intra-WBAN and extra-WBAN communication channels, and how to represent this impact within the channel models in order to evaluate the link performance. We also look for appropriate MA schemes for the case of intra-WBAN links that satisfy the low-energy, low-complexity, high-reliability and low-latency requirements of WBANs. To reach these objec-tives, we have proceeded as follows:
• Reviewed the fundamentals of the indoor OWC systems, with a special focus on the modeling of dynamic channels via Monte-Carlo raytracing (MCRT) simulations (presented in Chapters 2 and 3).
• Characterized the WBAN channel using first- and second- order statistics while considering the effects of mobility and body shadowing during a walk cycle and evaluated their perfor-mance considering different ambient noise conditions, SN, CNs and APs configurations, as well as different Rx types (presented in Chapter 4 and 5).
• Evaluated the performance of MA techniques such as MU m-CAP and their combination with the DQ random access (DQRA) and analyzed its performance in terms of energy efficiency and access delay, while comparing it to its RF counterpart (presented in Chapter 6).
Thesis overview and contributions
Chapter 2 describes the main blocks of an OWC system, i.e., the Tx, the channel and the Rx. The LED characteristics and the types of modulation used in our work are also explained. Also, we explain the optical channel properties and the main metrics used to characterize it. As for the Rx, we explain the photo-detection and demodulation processes for PIN and avalanche photodiode (APD)-based Rxs and discuss the main noise sources that impact the reliability of the transmission for each Rx.
In Chapter 3, we discuss in detail the theoretical concepts behind channel impulse response (CIR) computation via the MCRT approach and the link performance analysis methodologies. After pre-senting the analytical model of MCRT, we introduce the numerical method using Zemax (Optic-studio) software. There, the details of ray-tracing, the 3D environment specifications, local and global mobility modeling as well as sensor placement are presented. Next, we discuss the statis-tical characterization of dynamic channels using best-fit and kernel density estimation, as well as and outage probability calculation.
Chapter 4 deals with the channel modeling and performance analysis of intra-WBANs uplink, i.e., form the SNs to the CN. There, we take into consideration both local and global movements of the body, where we model the body parts movements by using a 3D animation of a walk cycle and random trajectories based on a modified RWP mobility model. After specifying the simu-lation parameters, we present simulation results on the temporal evolution of the channel gain, delay spread, and coherence time, as well as on their statistical distribution for the underlying intra-WBAN links. Next, based on best-fit statistical characterization, we evaluate the link perfor-mance for a simple OOK modulation scheme based on the outage probability criterion. We further evaluate the performances of PIN and APD Rxs under different background noise conditions. We show the high impact of the body position and orientation inside the room on the variations of the channel gain: Intra-WBANs links are shown to be highly impacted with shadowing and block-age, where their statistics can be appropriately modeled with a gamma distribution. Meanwhile, we show that the intra-WBAN channels can nevertheless be considered as effectively flat for low-to-moderate data-rates. They also have slower time variations (typical coherence times of a few seconds), compared to the RF counterparts (coherence times of tens of ms). This allows using larger packet sizes for the former, thus a lower pilot overhead and a simplified power control over successive frames. Concerning the link reliability, we show that it is significantly affected by shad-owing resulting from local body mobility, which also depends on the position of the CN. We also show that given the constraints on the system complexity and power consumption, a PIN-based Rx (CN) could be a suitable choice.
Similar to the approach used in Chapter 4, we study the extra-WBAN channel by focusing on the uplink data transmission, i.e., from the CN to the AP, and evaluate its performance in Chapter 5. We consider different configurations for the CN and investigate the benefit of using multiple APs in the room. Numerical results for the main channel metrics presented previously and outage prob-ability analysis for PIN- and APD-based Rxs are presented and the impact of noise is discussed. We propose a kernel density estimate (KDE) based on Gaussian kernels for the extra-WBAN chan-nels and show that using an APD at the Rx (AP) is best suited except under strong background radiations; this allows increased robustness against shadowing when using a more sensitive PD, as well as reduced required IR power level at the Tx (CN). Lastly, we demonstrate the significant improvement in the extra-WBAN link reliability through the use of multiple APs, which is due to a significant reduction of beam shadowing during user movements. Although the use of mul-tiple APs increases the implementation complexity of the network, it can be justified given the achieved performance improvement, in particular, regarding the substantial reduction of the re-quired transmit power.
After studying the intra- and extra-WBANs channels and evaluating their performance, we focus in Chapter 6 on the design of appropriate MA techniques. Given the potentially large number of SNs that could be used for each patient, and, furthermore, the presence of several patients in a hospital, developing a robust MA scheme in the PHY layer is quite challenging. More efficient MA management can be achieved through the MAC layer by minimizing packet collisions and their scheduling to ensure acceptable energy consumption and access delay. Therefore, after provid-ing a general overview of PHY and MAC layer techniques solutions, we evaluate the use of fre-quency multiplexing schemes such as MU m-CAP and investigate their performance considering realistic WBAN channel models. Also, we present the contention-based schemes such as ALOHA and carrier-sense MA with collision avoidance (CSMA/CA) techniques. Next, given the limitations of scheduled- and contention-based MAC schemes, we propose the use of multi-channel DQRA scheme for energy-efficiency, seamless access to the channel, and the flexibility to cope with het-erogeneous data traffic. We present the system model of optical-based DQRA scheme and provide an analytical analysis of the main metrics such as access delay and energy consumption.
• Characterization of the intra- and extra-WBAN channels in terms of first- and second-order statistics based on MCRT simulations via Zemax software and proposed a dynamic model that consider realistic mobility and shadowing effects. Derivation of statistical distributions to fit the channel models.
• Evaluation of the outage probability performance of WBAN links for different configurations of ambient light and Rx types.
• Proposition of a DQ-based MAC scheme to decrease energy consumption and access delay in intra-WBAN links.
Table of contents :
1 General Introduction
1.2 WBAN overview
1.2.1 WBANs forMedical Applications
1.2.2 Architecture of a WBAN
1.2.3 Specific Requirements ofMedical WBANs
1.2.4 ExistingWireless Technologies for WBANs and RelatedWorks
1.3 OWC overview
1.3.2 OWC advantages and challenges
1.3.3 OWC applications
1.4 Thesis objectives
1.5 Thesis overview and contributions
1.5.1 Thesis outline
1.5.2 Author’s contributions
1.5.3 Author’s publications
2 Fundamentals of OWC Communication
2.2.1 Optical source
2.5 Noise sources
2.5.1 Thermal noise
2.5.2 Background noise
2.5.3 Dark noise
2.5.4 Shot noise
2.6 Channel Characterization andModeling
2.7 Chapter summary
3 WBAN ChannelModeling and Link Performance
3.2 Modeling of dynamic channel
3.2.1 Potential Equation
3.2.3 Channel Characterization
3.2.4 Simulation validation
3.2.5 Statistical ChannelModeling
3.3 Numerical Simulation of intra-WBAN links
3.3.2 Sensor Placement
3.3.4 Link Performance Analysis
3.4 Chapter summary
4 ChannelModeling and Performance Analysis of Intra-WBANs
4.2 Environment Description and Parameter Specification
4.3 Channel characterization
4.3.1 Channel DC Gain
4.3.2 Delay Spread
4.3.3 Channel Dynamic Behavior
4.3.4 Statistical Analysis of Channel Gain
4.3.5 Channel Coherence Time
4.4 Performance analysis
4.4.1 Effect of PD Type and Data-Rate
4.4.2 Comparison of Intra-WBAN Links for Low Background Noise Level
4.4.3 Comparison of Intra-WBAN Links for Relatively High Background Noise Level
4.5 Chapter summary
5 ChannelModeling and Performance Analysis of Extra-WBAN links
5.2 Simulation Approach
5.2.1 General Assumptions
5.3 Channel Characterization, Numerical Results
5.3.1 Single AP Configurations
5.3.2 Multiple AP Configurations
5.3.3 Channel RMS Delay Spread
5.3.4 Channel Coherence Time
5.4 Link Performance Analysis
5.4.1 StatisticalModel of Channel Gain
5.4.2 Performance Comparison of Single andMultiple AP Configurations
5.4.3 Performance Comparison of PIN- and APD-based Rxs
5.5 Chapter summary
6 Multiple AccessManagement for Intra-WBAN Links
6.3.1 multi-user multi-band CAP (m-CAP) (MU m-CAP)
6.4 Contention-based schemes
6.5 DQRA Scheme
6.5.1 DQ-MAC Description and modeling
6.5.3 Performance analysis
6.6 Chapter summary
7 Conclusions and Perspectives