Irina Stefan

Area spectral efficiency performance comparison between VLC and RF femtocell networks

In this paper the average area spectral efficiency (ASE) of indoor visible light communication (VLC) wireless network is investigated for various room geometries (2.5m×5m×3m, 5m×5m×3m and 10m×10m×3m). A comparison is made against state-of-the-art radio frequency (RF) systems employing femtocells indoors. A modified version of the wireless world initiative new radio (WINNER) indoor deployment scenario (office building) is used as the common setup where femtocell access points (APs) and VLC APs are installed alternatively. For the RF system the minimum number of femtocell APs per floor area is four as specified. The requirement for the VLC system is to fulfill the lighting conditions inside the room, characterized by the illuminance distribution in rooms with several possible dimensions. The ASE gains of the VLC system compared to the femtocell network range between 12 and 924 (depending on the number of femtocell APs and floor geometry).

Study of dimming and LED nonlinearity for ACO-OFDM based VLC systems

In this paper, the performance of an asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) indoor wireless system is investigated. In particular, the illuminance distribution inside a room (5m×5 m×3m) under different brightness conditions, i.e. dimming levels, and the electrical signal-to-noise ratio (SNR) at the receiver are considered. The system performance is also assessed in terms of bit-error-ratio (BER) as a function of average transmitted optical power. Brightness control is achieved using the continuous current reduction (CCR) technique to assure higher luminous efficacy compared to the pulse-width modulation (PWM) dimming technique. In addition, the impact of the induced distortions due to the nonlinear behavior of the light-emitting diode (LED) as a light source is included in the simulation model. For a circular area of about 1m radius located directly below one of the four LED illumination modules utilized in the room, having illuminance values above 300 lx (10% brightness), a 9 dB average electrical SNR is achieved at the receiver located on a desktop.

Analysis of Optimal Placement of LED Arrays for Visible Light Communication

In this paper the optimum placement of light emitting diode (LED) arrays is investigated for indoor visible light communication (VLC) subject to maximization of average area spectral efficiency (ASE). Design parameters of LED array such as the distance between two neighboring LEDs and the precise location on the ceiling of the LED arrays can be obtained by solving an optimization problem. The formulated optimization problem is applied to a room of 2.5m×5m×3m dimension where two square LED arrays provide the required illumination. Due to the complexity of the optimization problem, a numerical optimization procedure is employed and the maximization is carried out by using algorithms from Matlab optimization toolbox. The results show that when the users employ a wide field of view (FOV) detector (e.g. 85°) the optimum solution involves placing the two LED arrays close to the opposite walls in the direction of the highest dimension of the room. It is also shown that when employing receivers with a smaller FOV (45°) the optimum solution allows the two LED arrays to be placed farther away from the opposite walls in the room. The average ASE achieved in this case is twice as high as the average ASE obtained when employing receivers with 85° FOV. Moreover, when the illumination constraint is also fulfilled, the ASE performance degradation can be quite significant (e.g. 33% for detectors with 85° FOV) as compared with the case where the VLC system is used only for communication purposes.

Optical Wireless OFDM System on FPGA: Study of LED Nonlinearity Effects

Nonlinearities can drastically degrade the performance of OFDM (orthogonal frequency division multiplexing) based optical wireless (OW) communication systems using intensity modulation (IM) of the optical carrier. The light emitting diode (LED) transfer function distorts the signal amplitude and forces the lower signal peaks to be clipped at the LED turn-on voltage (TOV). Additionally, the upper signal peaks can result in optical output degradation. The induced distortion can be controlled by optimizing the bias point (BP) of the LED and/or backing-off the signal power modulating the LED. In this paper, the obtained experimental results using a hardware demonstrator for OW OFDM transmission based on field programmable gate array (FPGA) and off-the-shelf analog components are presented. The conducted measurements for the bit-error performance focus on determining the optimum BP and optimizing the OFDM signal amplitude to obtain best performance. In this context, the experimental bit-error ratio (BER) is obtained as a function of the LED BP and the RMS (root mean square) OFDM signal across the LED.

Hybrid Visible Light and Radio Frequency Communication Systems

In this paper we propose a hybrid visible light communications (VLC) and radio frequency (RF) femtocell system and investigate user association issues to achieve effective load balancing. We propose a scheme for cell association based on the minimum distance to the closest access point (AP) which involves a simple and fully uncoordinated decision. The performance of the three tier (macro-, femto- and optical attocells) heterogeneous network is analysed in terms of area spectral efficiency (ASE). We compare the performance of the minimum distance user association, the maximum signal to interference and noise ratio (SINR) based association and a decentralized iterative algorithm. The latter scheme results in a near optimal solution by considering the load of the AP in the cell association process. Simulation results show that the simple min-distance based user association nearly achieves the optimal load-aware performance. Moreover the average ASE of the hybrid VLC/RF system may be increased by at least two orders of magnitude over the stand-alone RF network.

Time and frequency synchronisation in optical wireless OFDM networks

This paper analyses the impact of imperfect synchronisation in optical orthogonal frequency division multiplexing (OFDM) systems utilising intensity modulation (IM) / direct detection (DD). The use of IM/DD technique inherently eliminates the problem of carrier frequency offset. Therefore, time-frequency synchronisation in optical OFDM system reduces to frame synchronisation and sampling clock synchronisation. Sampling clock offset causes irreducible intercarrier interference (ICI) and symbol timing offset. A technique to mitigate the impact of sampling clock offset is proposed in this paper. In the proposed approach, the received signal is oversampled at twice the Nyquist rate and the odd samples are punctured. The symbol timing offset is estimated using pilots and when the symbol timing offset exceeds ±0.5 of sample duration, the puncturing pattern flips between puncturing the odd series and the even series. This in effect, constraints the symbol timing offset within ±1 sample duration. The residual symbol offset can be perfectly corrected using linear phase equaliser. The results show that the proposed method attains a performance comparable to the system that is ideally synchronised provided that the sampling clock offset is lower than 50 ppm.