Xiping Wu

On the performance of non-orthogonal multiple access in visible light communication

In this paper, the performance of non-orthogonal multiple access (NOMA) is characterized in a downlink visible light communication (VLC) system. Analytical expressions of the system performance are derived for two separate scenarios. In the scenario of achieving guaranteed quality of service (QoS), the outage probability of each user is studied and the effect of power allocation coefficients on the system coverage probability is investigated. In the scenario of providing opportunistic best-effort service, system ergodic sum rate is formulated based on a fixed power allocation (FPA) strategy. Both simulation and analytical results demonstrate that, in the first scenario, the maximum coverage probability can be achieved through the exhaustive search (ES) method to find the optimum set of power allocation coefficients. In the second scenario, it is shown that unlike orthogonal multiple access (OMA) techniques, NOMA can achieve a higher system capacity for a larger number of users. Also, the performance of NOMA can be further enhanced by choosing LEDs with a suitable semi-angle. When compared with OMA, NOMA can increase the system capacity by 125% if LEDs with 30° semi-angle are used.

Performance Evaluation of Non-Orthogonal Multiple Access in Visible Light Communication

In this paper, the performance of non-orthogonal multiple access (NOMA) is characterized in a downlink visible light communication system for two separate cases. In the case of guaranteed quality of service (QoS) provisioning, we derive an analytical expression of the system coverage probability and show the existence of optimal power allocation coefficients on two-user paired NOMA. In the case of opportunistic best-effort service provisioning, we formulate a closed-form expression of the ergodic sum rate, which is applicable for arbitrary power allocation strategies. The probability that NOMA achieves higher individual rates than OMA is derived. Also, we give an upper bound of the sum rate gain of NOMA over OMA in the high signal-to-noise ratio regime. Both the theoretical and simulation results prove that the performance gain of NOMA over OMA can be further enlarged by pairing users with distinctive channel conditions. We also find out that the choice of light emitting diodes (LEDs) have a significant impact on the system performance. In the case of guaranteed QoS provisioning, the LEDs with larger semi-angles have better performance; while in the case of opportunistic best-effort service provisioning, the LEDs with 35° semi-angle give nearly optimal performance.

Channel Estimation for Spatial Modulation

In this paper, a novel channel estimation (CE) method is proposed for spatial modulation (SM), a unique single-stream multiple-input-multiple-output transmission technique. In SM, there is only one transmit antenna being active at any time instance. While this property completely avoids inter-channel interference, it results in a challenge to estimate the channel information. In conventional CE (CCE) methods for SM, all transmit antennas have to be sequentially activated for sending pilots. Therefore, the time consumed in CE is proportional to the number of transmit antennas, which significantly compromises the throughput. By exploiting channel correlation, the proposed method, named transmission cross CE (TCCE), has the following characteristics: i) the entire channel is estimated by sending pilots through one transmit antenna; ii) it requires no overhead or feedback; and iii) it achieves a low computational complexity at the receiver. In addition, we propose an analytical framework to compute the distribution of the CE errors over time-varying fading channels. The corresponding average bit error probability (ABEP) bound of SM is also derived for the proposed method. Results show that the proposed ABEP bound matches with the simulations very well. When compared with CCE, the new method obtains a signal-to-noise ratio gain of up to 7.5 dB for medium and high correlations between the transmit antennas. Moreover, an adaptive CE technique can be readily implemented for SM via switching between CCE and TCCE.

Adaptive Selection of Antennas for Optimum Transmission in Spatial Modulation

In this paper, we propose an optimum transmit structure for spatial modulation (SM), a unique single-stream multiple-input multiple-output (MIMO) transmission technique. As a three-dimensional modulation scheme, SM enables a trade-off between the size of the spatial constellation diagram and the size of the signal constellation diagram. Based on this fact, the novel method, named transmission optimized spatial modulation (TOSM), selects the best transmit structure that minimizes the average bit error probability (ABEP). Unlike the traditional antenna selection methods, the proposed method relies on statistical channel state information (CSI) instead of instant CSI, and feedback is only needed for the optimal number of transmit antennas. The overhead for this, however, is negligible. In addition, TOSM has low computational complexity as the optimization problem is solved through a simple closed-form objective function with a single variable. Simulation results show that TOSM significantly improves the performance of SM at various channel correlations. Assuming Rayleigh fading channels, TOSM outperforms the original SM by up to 9 dB. Moreover, we propose a single radio-frequency (RF) chain base station (BS) based on TOSM, which achieves low hardware complexity and high energy efficiency. In comparison with multi-stream MIMO schemes, TOSM offers an energy saving of at least 56% in the continuous transmission mode, and 62% in the discontinuous transmission mode.

Indoor Visible Light Positioning with Angle Diversity Transmitter

A new concept for indoor positioning using visible light communication (VLC) is presented in this paper. The feasibility of uplink localization is verified so that the proposed system can work in conjunction with downlink positioning systems to improve overall localization accuracy. We propose to use an angle diversity transmitter (ADT) associated with accelerometers for uplink three-dimensional localization. Unlike other techniques using VLC for positioning, the proposed system can achieve indoor localization without making assumptions about the height or orientation angle of a mobile user. The received signal strength (RSS) method is used to minimize implementation costs while achieving satisfactory positioning accuracy. Simulation results show that an average localization error of less than 0.15 m can be achieved even when the receiver is tilted by 45°. Also, compared with a single light emitting diode (LED), the ADT is found to be more robust to accelerometer measurement errors.

Structure optimisation of spatial modulation over correlated fading channels

A unique characteristic of spatial modulation (SM) is the three dimensional constellation diagram. This enables to trade-off traditional signal constellation diagrams with spatial constellation diagrams where the latter is defined by the physical antenna array. In this paper we investigate the optimum pairs of signal and spatial constellation sizes with respect to average bit error probability (ABEP) and energy efficiency. The analysis is performed for varying antenna correlations and channel conditions. Both numerical and closed-form results are presented which show that a significant performance gain can be obtained when the optimal constellation pairs are used.

Base station energy consumption for transmission optimised spatial modulation (TOSM) in correlated channels

Radio frequency (RF) chains are responsible for a large percentage of the total BS energy consumption and each RF chain can only support one activated transmit antenna at a time. This fact greatly benefits spatial modulation (SM) since at any given time instance only one antenna in an array is active. The proposed transmission optimised spatial modulation (TOSM) is an adaptive SM scheme which trades off the spatial constellation size with the signal constellation size to minimise the average bit error probability (ABEP) for a given channel correlation. It outperforms fixed-SMscheme for different channel correlation degrees. In this paper, we construct a single RF chain BS based on TOSM which has low complexity and high energy-efficiency. Simulation results show that for a given spectral efficiency of 6 bits/s/Hz and two receive antennas, the proposed scheme outperforms 2×2 V-BLAST by at least 3.8 dB in energy consumption.

Optimization of Load Balancing in Hybrid LiFi/RF Networks

Light fidelity (LiFi) uses light emitting diodes (LEDs) for high-speed wireless communications. Since an LED lamp covers a small area, a LiFi system with multiple access points (APs) can offer a significantly high spatial throughput. However, the spatial distribution of data rates achieved by LiFi fluctuates because users experience inter-cell interference from neighboring LiFi APs. In order to guarantee a quality of service (QoS) for all users in the network, an RF network is considered as an additional wireless networking layer. This hybrid LiFi/RF network enables users with low levels of optical signals to achieve the desired QoS by migrating to the RF network. With regard to moving users, the hybrid LiFi/RF system dynamically allocates either a LiFi AP or an RF AP to users based on their channel state information. In this paper, a dynamic load balancing scheme is proposed, which considers the handover overhead in order to improve the overall system throughput. Joint optimization algorithm (JOA) and separate optimization algorithm (SOA), which jointly and separately optimize the AP assignment and resource allocation, respectively, are proposed. Simulation results show that SOA can offer a better performance/complexity tradeoff than JOA for system load balancing.

Direct Transmit Antenna Selection for Transmit Optimized Spatial Modulation

To improve the performance of spatial modulation (SM) over correlated MIMO channels, transmit optimized spatial modulation (TOSM) has been proposed recently. It trades off traditional signal constellation diagrams with spatial constellation diagrams to minimize the average bit error probability (ABEP). After the optimum number of transmit antennas is determined, the specific antennas need to be carefully chosen from the entire array to provide a minimum ABEP. Like in conventional transmit antenna selection (TAS) schemes, the problem can be solved by an exhaustive search. However, this results in an unaffordable complexity especially when the spectral efficiency is high. In this paper, we propose a creative TAS approach for TOSM. Given a required number of antennas, the novel technique determines the selection solution based on circle packing. Simulation results show that for various channel correlations and spectral efficiencies, the proposed method achieves performance results close to exhaustive search with a gap of less than 0.3 dB. The complexity is reduced to an extremely low level.

Distributed load balancing for Internet of Things by using Li-Fi and RF hybrid network

The Internet of Things (IoT) is a new generation of network that can remotely and intelligently control distributed objects. Due to the large number of objects in the IoT, a high data traffic for the object communications is required, which is mostly routed through wireless links. However, the available spectrum for radio frequency (RF) wireless communications is exhausted so that each user in the IoT can only achieve very low data rate. In order to offer a better service to users, a light fidelity (Li-Fi) and radio frequency (RF) hybrid network is considered, where Li-Fi uses the large spectrum of visible light to achieve a high data rate, and the RF system guarantees a seamless coverage. In this study, a load balancing (LB) algorithm for the Li-Fi/RF hybrid IoT network is proposed based on evolutionary game theory (EGT). A key feature of the proposed algorithm is that users autonomously select the APs and adapt their strategies. Thus, compared with the conventional centralised algorithm, the computation load of the central unit (CU) can be reduced by using the EGT algorithm. Moreover, simulation results show that the proposed algorithm outperforms the conventional centralised algorithms in terms of the user satisfaction.