Analysis of random orientation and user mobility in LiFi networks

Mobile data traffic is anticipated to surpass 49 exabyte per month by 2021. Smartphones, as the main factor of generating this huge data traffic (86%), are expected to require average speed connection of 20 Mbps by 2021. Light-fidelity (LiFi) is a novel bidirectional, high-speed and fully networked optical wireless communication and it is a promising solution to undertake this huge data traffic. However, to support seamless connectivity in LiFi networks, real-time knowledge of channel state information (CSI) from each user is required at the LiFi access point (AP). The CSI availability enables us to achieve optimal resource allocation and throughput maximization but it requires feedback transmitted through the uplink channel. Furthermore, the important aspects of the indoor LiFi channel such as the random orientation of user device, user mobility and link blockage need to be carefully analysed and effective solutions should be developed. In contrast to radio frequency (RF) channels, the LiFi channel is relatively less random. This feature of LiFi channel enables a potential reduction in the amount of feedback required to achieve high throughputs in a dynamic LiFi network. Based on this feature, two techniques for reducing the amount of feedback in LiFi cellular networks are proposed: 1) limited-content feedback scheme based on reducing the content of feedback information and 2) limited-frequency feedback scheme based on the update interval. It is shown that these limited-feedback schemes can provide almost the same downlink performance as full feedback scheme. Furthermore, an optimum update interval which provides maximum bidirectional user equipment (UE) throughput, has been derived. Device orientation and its statistics is an important determinant factor that can affect the users throughput remarkably in LiFi networks. However, device orientation has been ignored in many previous performance studies of LiFi networks due to the lack of a proper statistical model. In this thesis, a novel model for the orientation of user device are proposed based on experimental measurements. The statistics of the device orientation for both sitting and walking activities are presented. Moreover, the statistics of the line-of-sight (LOS) channel gain are calculated. The influence of random device orientation on the received signal-to-noise-ratio (SNR) and bit-error ratio (BER) performance of LiFi systems has been also evaluated. To support the seamless connectivity of future LiFi-enabled devices in the presence of random device orientation, mobility and blockage, efficient handover between APs are required. In this thesis, an orientation-based random waypoint (ORWP) mobility model is proposed to analyze the performance of mobile users considering the effect of random device orientation. Based on this model, an analysis of handover due to random orientation and user mobility is presented. Finally, in order to improve seamless connectivity, a multi-directional receiver (MDR) configuration is proposed. The MDR configuration shows a robust performance in the presence of user mobility, random device orientation and blockage

Bidirectional User Throughput Maximization Based on Feedback Reduction in LiFi Networks

Channel adaptive signalling, which is based on feedback, can result in almost any performance metric enhancement. Unlike the radio frequency (RF) channel, the optical wireless communications (OWCs) channel is fairly static. This feature enables a potential improvement of the bidirectional user throughput by reducing the amount of feedback. Light-Fidelity (LiFi) is a subset of OWCs, and it is a bidirectional, high-speed and fully networked wireless communication technology where visible light and infrared are used in downlink and uplink respectively. In this paper, two techniques for reducing the amount of feedback in LiFi cellular networks are proposed, i) Limited-content feedback (LCF) scheme based on reducing the content of feedback information and ii) Limited-frequency feedback (LFF) based on the update interval scheme that lets the receiver to transmit feedback information after some data frames transmission. Furthermore, based on the random waypoint (RWP) mobility model, the optimum update interval which provides maximum bidirectional user equipment (UE) throughput, has been derived. Results show that the proposed schemes can achieve better average overall throughput compared to the benchmark one-bit feedback and full-feedback mechanisms.Comment: 30 pages, 9 figures, submitted to IEEE Transactions on Communication

Terminal Orientation in OFDM-based LiFi Systems

Light-fidelity (LiFi) is a wireless communication technology that employs both infrared and visible light spectra to support multiuser access and user mobility. Considering the small wavelength of light, the optical channel is affected by the random orientation of a user equipment (UE). In this paper, a random process model for changes in the UE orientation is proposed based on data measurements. We show that the coherence time of the random orientation is in the order of hundreds of milliseconds. Therefore, an indoor optical wireless channel can be treated as a slowly-varying channel as its delay spread is typically in the order of nanoseconds. A study of the orientation model on the performance of direct-current-biased orthogonal frequency-division multiplexing (DC-OFDM) is also presented. The performance analysis of the DC-OFDM system incorporates the effect of diffuse link due to reflection and blockage by the user. The results show that the diffuse link and the blockage have significant effects, especially if the UE is located relatively far away from an access point (AP). It is shown that the effect is notable if the horizontal distance between the UE and the AP is greater than $1.5$ m in a typical $5\times3.5\times3$ m$^3$ indoor room.Comment: This work has been submitted to the IEEE for possible publication. Copyright may be transferred without notice, after which this version may no longer be accessibl

Impact of Device Orientation on Error Performance of LiFi Systems

Most studies on optical wireless communications (OWCs) have neglected the effect of random orientation in their performance analysis due to the lack of a proper model for the random orientation. Our recent empirical-based research illustrates that the random orientation follows a Laplace distribution for a static user equipment (UE). In this paper, we analyze the device orientation and assess its importance on system performance. The reliability of an OWC channel highly depends on the availability and alignment of line-of-sight (LOS) links. In this study, the effect of receiver orientation including both polar and azimuth angles on the LOS channel gain are analyzed. The probability of establishing a LOS link is investigated and the probability density function (PDF) of signal-to-noise ratio (SNR) for a randomly-oriented device is derived. By means of the PDF of SNR, the bit-error ratio (BER) of DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) in additive white Gaussian noise (AWGN) channels is evaluated. A closed-form approximation for the BER of UE with random orientation is presented which shows a good match with Monte-Carlo simulation results. Furthermore, the impact of the UE's random motion on the BER performance has been assessed. Finally, the effect of random orientation on the average signal-to-interference-plus-noise ratio (SINR) in a multiple access points (APs) scenario is investigated.Comment: 10 pages, 11 figures, journa

A Study of Sojourn Time for Indoor LiFi Cellular Networks

Sojourn time is an important parameter in the analysis and design of mobile cellular networks. It shows the expected time that a user equipment (UE) remains connected to the serving access point (AP) while moving in the network, i.e. stays within one cell. Therefore, an accurate estimation of the sojourn time is an essential element for mobility management. In this study, an analysis of the sojourn time for indoor light-fidelity (LiFi) cellular networks is presented based on the random waypoint (RWP) mobility model. It is initially assumed that the UE is oriented vertically upward and closed-form analytical expressions are derived. Monte-Carlo simulations are also provided to validate the analytical derivations and to gain more insight into the performance with different orientations. It is shown that the sojourn time mainly varies depending on the speed of the user. However, other factors such as the device orientation and the coverage area of cells also play a role in determining the sojourn time. The results offer several insights and design guidelines in consideration of this parameter for indoor LiFi networks