Design and evaluation of Downlink broadband in-flight LiFi
Item statusRestricted Access
Embargo end date31/07/2023
There are still only a limited number of applications for radio frequency (RF) based wireless communications systems inside aircraft cabins. Extreme user density and potential interference in small, confined areas such as an aeronautical cabin, pose significant challenges for broadband data access. Furthermore, safety concerns and regulations in the aviation industry introduce additional challenges for system designs as each scenario has their own specific requirements and constraints. Both optical wireless communications (OWC) and light-fidelity (LiFi), can safely be employed in these exceptional environments to meet the mobile traffic demand. By transforming each reading lamp into a wireless access point (AP), LiFi could provide seamless on-board connectivity alongside high spectral and energy efficiency. Moreover, a significant deployment ease and cost reduction could also be gained by the utilization of the available reading lights. In this thesis, analytical, numerical and measurement methods based evaluation of downlink in-flight LiFi systems is investigated. The achievable system performance and limitations are evaluated via signal-to-interference ratio (SIR) maps and throughput analysis, which are obtained by measurements in a mock aircraft cabin. Depending on the manufacturer and cabin model, the reading lamps may be adjustable. The effect of light emitting diode (LED) adjustment and half power angles are investigated by three passenger service unit (PSU) designs. Furthermore, three measurement points per seat, one in the cell centre and two at the edges, are chosen. The obtained SIR measurement results are also compared to both line-of-sight (LoS)-only analytical expression and realistic Monte Carlo ray-tracing (MCRT) simulations with high order reflections. In order to enhance the system performance, two cellular formation techniques, single point (SP) and coordinated multi-point (CoMP) transmission, are also proposed for onboard connectivity. The results show that both the half power and adjustment angles play a significant role in the achievable SIR and throughput values, where CoMP transmission yields the best overall system performance. Once the reading lights based LiFi is shown to be promising, further characterization on the optical channels is required to ensure a robust in-flight connectivity. Accordingly, MCRT simulations for complete downlink LiFi channel modelling methodology is proposed. Two narrow body cabin models, with realistic surfaces/curvature and geometrical simplification of it, are adopted. In order to assess the importance of wavelength dependency, two source-receiver pairs for both visible light (VL) and infra-red (IR) bands are also utilized. The measurements based relative reflectivity characteristics of the interior surfaces, including the seating elements, are chosen and inputted to the proposed simulation environment. The MCRT simulations are conducted to obtain the direct-current (DC) channel gain, root-mean-squared (RMS) delay spread and flatness factor along with the channel impulse response (CIR) for each measurement point. It has been shown that the proposed methodology is able to model the in-flight OWC (IFOWC) channels more accurately compared to both the analytical LoS expression and simplified cabin model based assumptions. To investigate practical downlink in-flight LiFi systems and their performance, a multi-carrier multiple-input-multiple-output (MIMO)-optical spatial modulation (OSM) technique, namely enhanced time domain spatial modulation (eTD-SM), is proposed. Since it has been shown that the in-flight OWC channels show time dispersive characteristics, the adverse channel effects are mitigated by the utilization of orthogonal frequency division multiplexing (OFDM). Unlike the conventional transmission method, the proposed technique achieves better spectral and energy efficiency by using the time domain samples for additional information encoding in LED indexes. The proposed idea is evaluated under both the analytical LoS and realistic MCRT channels. Simulation results show that the proposed technique outperforms the conventional frequency domain spatial modulation (FD-SM) system by more than 20 and 2 dB under highly correlated LoS and multipath dispersive MIMO channel models, respectively. Therefore, the results have indicated that a combination of both optical OFDM and spatial modulation (SM) could potentially be a good candidate for future generation in-flight LiFi applications. The contributions of this thesis are summarized as follows. Firstly, a comprehensive SIR and throughput investigation for downlink cellular LiFi system is presented. Moreover, both SP and CoMP transmission methods are proposed and evaluated for such on board optical wireless networks. Secondly, the complete downlink CIR characterization methodology is proposed for time dispersive single-input-single-output (SISO) IFOWC links. The obtained CIR results showed that the LoS analytical model is not enough to capture the practical characteristics of IFOWC channel models. Finally, a novel OFDM based OSM technique is proposed for time dispersive optical MIMO channels. Accordingly, practical CIRs for MIMO-IFOWC are obtained. Then, the numerical simulation results have shown that the proposed technique performs significantly better than the conventional one, which is vastly adopted in the literature.