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

Interference mitigation in LiFi networks

Due to the increasing demand for wireless data, the radio frequency (RF) spectrum has become a very limited resource. Alternative approaches are under investigation to support the future growth in data traffic and next-generation high-speed wireless communication systems. Techniques such as massive multiple-input multiple-output (MIMO), millimeter wave (mmWave) communications and light-fidelity (LiFi) are being explored. Among these technologies, LiFi is a novel bi-directional, high-speed and fully networked wireless communication technology. However, inter-cell interference (ICI) can significantly restrict the system performance of LiFi attocell networks. This thesis focuses on interference mitigation in LiFi attocell networks. The angle diversity receiver (ADR) is one solution to address the issue of ICI as well as frequency reuse in LiFi attocell networks. With the property of high concentration gain and narrow field of view (FOV), the ADR is very beneficial for interference mitigation. However, the optimum structure of the ADR has not been investigated. This motivates us to propose the optimum structures for the ADRs in order to fully exploit the performance gain. The impact of random device orientation and diffuse link signal propagation are taken into consideration. The performance comparison between the select best combining (SBC) and maximum ratio combining (MRC) is carried out under different noise levels. In addition, the double source (DS) system, where each LiFi access point (AP) consists of two sources transmitting the same information signals but with opposite polarity, is proven to outperform the single source (SS) system under certain conditions. Then, to overcome issues around ICI, random device orientation and link blockage, hybrid LiFi/WiFi networks (HLWNs) are considered. In this thesis, dynamic load balancing (LB) considering handover in HLWNs is studied. The orientation-based random waypoint (ORWP) mobility model is considered to provide a more realistic framework to evaluate the performance of HLWNs. Based on the low-pass filtering effect of the LiFi channel, we firstly propose an orthogonal frequency division multiple access (OFDMA)-based resource allocation (RA) method in LiFi systems. Also, an enhanced evolutionary game theory (EGT)-based LB scheme with handover in HLWNs is proposed. Finally, due to the characteristic of high directivity and narrow beams, a vertical-cavity surface-emitting laser (VCSEL) array transmission system has been proposed to mitigate ICI. In order to support mobile users, two beam activation methods are proposed. The beam activation based on the corner-cube retroreflector (CCR) can achieve low power consumption and almost-zero delay, allowing real-time beam activation for high-speed users. The mechanism based on the omnidirectional transmitter (ODTx) is suitable for low-speed users and very robust to random orientation

Measurements-Based Channel Models for Indoor LiFi Systems

Light-fidelity (LiFi) is a fully-networked bidirectional optical wireless communication (OWC) that is considered a promising solution for high-speed indoor connectivity. Unlike in conventional radio frequency wireless systems, the OWC channel is not isotropic, meaning that the device orientation affects the channel gain significantly. However, due to the lack of proper channel models for LiFi systems, many studies have assumed that the receiver is vertically upward and randomly located within the coverage area, which is not a realistic assumption from a practical point of view. In this paper, novel realistic and measurement-based channel models for indoor LiFi systems are proposed. Precisely, the statistics of the channel gain are derived for the case of randomly oriented stationary and mobile LiFi receivers. For stationary users, two channel models are proposed, namely, the modified truncated Laplace (MTL) model and the modified Beta (MB) model. For LiFi users, two channel models are proposed, namely, the sum of modified truncated Gaussian (SMTG) model and the sum of modified Beta (SMB) model. Based on the derived models, the impact of random orientation and spatial distribution of LiFi users is investigated, where we show that the aforementioned factors can strongly affect the channel gain and system performance

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

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