Downlink data transmission of multi-user indoor optical wireless communications and its corresponding fairness analysis is studied by the authors. It is assumed that access points (APs) may use infrared beam (IR-beam) laser or light emitting diodes (LEDs) for data transmissions. For the case of APs with IR-beams, since the bandwidth of the transmitted optical signal can be very large than that of the LED's, a typical time-division multiple access (TDMA) scheme is utilised to serve multiple users and the respective achievable rates are derived. Then, for a general fairness-scheduling utility function with the capability of adjusting the degree of fairness known as α-proportional fairness, optimal timesharing coefficients are analytically derived in closed-forms. To compare the performance of the IR-based communications for the LED-based APs, two non-cooperative and cooperative data transmission schemes are also studied and the corresponding achievable rates are presented based on the classical superposition coding. Finally, to compare the performance of the two types of APs, per user data rate of the proposed systems are compared through numerical results for different scenarios to highlight advantages and drawbacks of the two types of APs. Also, by considering the Jain's index as a fairness metric, the impact of the system parameters on the fairness is discussed through numerical results. K E Y W O R D S 5G mobile communication, laser beam applications, optical communication | INTRODUCTIONThanks to the large bandwidth of optical signals, it is possible to provide users with high-data rate access via wireless optical signaling instead of the radio frequency (RF) . Also, in contrast to RF waves that can be easily eavesdropped, due to the directive and limited free space propagation range of the optical signals, optical wireless communication (OWC) has gained research and practical interests to provide safer, that is, non-electromagnetic radiations, high-rate, reliable and secure communications.To provide high-rate indoor access to the users, one of the promising solutions is to utilise the low-cost and illuminating LEDs as access points (APs) for downlink data transmissions [1,2]. This technique is known as visible light communications (VLC) and it is thoroughly studied in the literature from different perspectives [3][4][5][6][7][8][9][10][11][12][13][14]. The weak signal power received from reflected paths and susceptibility of the OWC to blockages, however, makes it hard to provide almost even quality of service in different locations in a room using OWC. Therefore, it is essential to optimise the design parameters such that all users are evenly served no matter where in the room they are located. Hence, in this paper, we aim to analyse ways to fairly serve users in different locations of a room. Now we provide a literature review of the existing works for VLC. In ref [3], the maximisation of the achievable downlink sum-rate of a VLC-based distributed multiple-input multipleoutput (D-MIMO) communications under p...
<p>Light-fidelity (Li-Fi) is a promising solution to provide high-rate, secure, and green communications to be used in the next generation of wireless networks. Since visible or infrared (IR) light-emitting diodes (LEDs) are used as the optical source and have a nonlinear transfer function, the transmitted modulated signal can be distorted if the signal has a high peak-to-average power ratio (PAPR). Recently, a new modulation scheme called Hadamard-coded modulation (HCM) is proposed which has a low PAPR and produces symbols with discrete levels. In the HCM technique, the symbol levels can be created using multiple LEDs. Therefore, each LED operates in its linear region and just switches on or off. In this paper, a low-complexity transceiver architecture for the HCM-based communication links is proposed and a complete synchronization procedure based on the spread spectrum techniques is presented. Finally, the bit error rate of the system is evaluated by Monte Carlo simulations, and effects of system parameters such as preamble length, fall and rise time of optical devices, and timing jitter on the bit error rate of the link are discussed.<br> </p>
<p>Light-fidelity (Li-Fi) is a promising solution to provide high-rate, secure, and green communications to be used in the next generation of wireless networks. Since visible or infrared (IR) light-emitting diodes (LEDs) are used as the optical source and have a nonlinear transfer function, the transmitted modulated signal can be distorted if the signal has a high peak-to-average power ratio (PAPR). Recently, a new modulation scheme called Hadamard-coded modulation (HCM) is proposed which has a low PAPR and produces symbols with discrete levels. In the HCM technique, the symbol levels can be created using multiple LEDs. Therefore, each LED operates in its linear region and just switches on or off. In this paper, a low-complexity transceiver architecture for the HCM-based communication links is proposed and a complete synchronization procedure based on the spread spectrum techniques is presented. Finally, the bit error rate of the system is evaluated by Monte Carlo simulations, and effects of system parameters such as preamble length, fall and rise time of optical devices, and timing jitter on the bit error rate of the link are discussed.<br> </p>
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