Electromagnetic Signals: Reflection, Focusing, Distortion, and Their Practical Applications
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According to the standards, illuminance in an office environment should take a value between and lx. A common recommendation is that desktop illuminance distribution should be made as even as possible.
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Someone in the literature defined a uniform illuminance rate UIR as the ratio of the minimum illumination and the average illumination. According to the lighting engineering guidelines, UIR should be made greater than 0. Currently, VLC systems are implemented mainly in an indoor environment, whose channel models is affected by many factors, including the design of light source, the placement of receivers, room shape, and the impact of different reflectors.
We will discuss the room model, LED array design, and reflection characteristics of indoor environment in this section. The length and width were usually made equal in most studies As VLCs and infrared communications are very similar, in the reflectance model for indoor infrared channels first proposed by Gfeller and Bapst 37 , communication quality was related closely to the room size. It will have a big impact to VLC signal detection without signal processing techniques, and the maximum transmission rate of the system depends ultimately on the room size.
In 37 , three typical environments were examined as examples. From the experiment results, under the same ambient light conditions, the larger the ratio between S and H is, the greater optical power will be received VLC is typically used in rooms, and thus, the room models may directly affect the quantity and distribution of a LED array.
In some test environments, the room model was specially designed for experiments, such as 7. In terms of LED designs for VLC, the main research topic is the number and arrangement of LED arrays on a room ceiling, to meet both office lighting and communication functions.
Electromagnetic Signals: Reflection, Focusing, Distortion, and Their Practical Applications
Nakagawa et al. LED layout was designed for the ceiling in the literature, and it was a square design. This room model has been widely used in subsequent literature. The LED arrays on the ceiling can be divided into four groups, each of which uses different numbers of LEDs and spacings. Zeng et al. Yang et al. The room model of 59 is the same as In 59 , two LED design methods were compared. Two types of designs could meet the illuminance requirements minimum lx , and the experiments showed that the illuminance distribution of Type B design is more gentle and cover a larger area.
Dominc O'Brien et al. When the total luminous flux was set at about 27klm, the luminance of the square design is greater than the hollow design. If the requirement of the standard deviation is more stringent, then the use of hollow design lighting will be better. In terms of communication performance, signal noise ratio SNR of the receiver in the square design was 5dB higher than that with only LOS, and 3dB higher in the hollow design. SNR range of the hollow design of the receiving surface is 77—78dB, and the square design is 75—80dB.
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Visible light communications is a short range wireless technology, which is more suitable for its applications in an indoor environment. This particular operational scenario determines multipath dispersion characteristics of VLCs.
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The broad band nature of visible light between and nm makes the channel models for traditional narrowband infrared wireless technology unsuitable for their direct applications in VLC. It is worth mentioning that the current commercial infrared LED of full width at half maximum is about 40—50nm. In an indoor environment, the received optical signals will experience time dispersion because of interior walls and other objects, which will reflect optical signals.
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Although the optical signal may generate specular reflections when it hits a mirror or other smooth objects, the vast majority of the reflections is typical diffused reflections, and it can be well modeled by a lambert model. To simplify the analysis, we assume that room interior surface materials have lambert diffusion characteristics.
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Reflection characteristics of a given indoor environment for visible light and other wavelengths are different somehow. Generally, reflectance in infrared bands is higher than that in visible light bands. At present, researchers have measured the reflectance of indoor building materials changing with the bands with the help of spectrophotometers. Reflectivity of plastic walls is lower than plaster walls, floor, and ceiling in the main bands of visible light range. In general, the reflectance of these materials is somewhere between 0.
Reflectivity of plastic walls has a trough near nm wavelength. From the beginning of nm, as wavelength increases, reflectance of plaster walls, floor, and ceiling may grow slowly. Average reflectivity of three reflecting materials are approximately 0. Indoor VLC systems are considered as optical communications within a limited space. The works reported in the existing literature have performed a lot of theoretical analysis on channel characteristics, using channel impulse response function h t to describe the properties of VLC channels.
There are many similarities between VLC channels and indoor infrared communication channels, except that they use different media. The earliest studies on the indoor infrared communication channel models appeared in , when Gfeller and Bapst first proposed a reflectance model for indoor infrared channels In , Barry presented a recursive channel simulation model using Lambert reflection, including multiple reflected light in an empty room on the basis of the first reflection model In , Tanaka et al.
In , Rajbhandari et al. In , Higgins et al. Jupeng Ding et al.
In , Lee et al. Pohl in , based on the observation that the characteristics of scattering signal gain around a room are the same 59 , In , Zhou Zhou et al. In this paper, based on the works in the literature, we would like to classify all aforementioned models into five different types of channel models in VLC systems. They include the VLC channel model proposed by Lee et al.
Pohl et al. After classifying the five different types of channel models in VLC systems, we provide a comparison on their advantages, disadvantages, application scenarios, propagation paths, and basis or theories among those models, as shown in Table 1. The model is in particular suitable for empty rooms. Taking into account the fact that an indoor environment is stable, we assume that noise in Lee model is Gaussian white noise.
They considered the direct and reflected propagations and derived mathematical expression for the channel impulse response function to describe channel characteristics. Multiple reflections cause multipath transmissions and result in relative time delays of received signals. Thus, the impulse response h t is employed to analyze the effect of multipath dispersion between different transceivers.
The model includes path loss L , delayed, and reflected powers. The first two are related to the distances between the transceivers, and the reflected power is related to the reflectance of the materials. In the modeling process of VLC channels, let us consider k reflections. We can obtain the impulse response as 28 where N L E D is the total number of LEDs, each of which is assumed using the same transmitted power. In LOS, the impulse response is given by 33 , and the path loss is 33 For the evaluation of the models, simulations were used in the literature, where received powers of VLC and infrared communications were compared specifically.
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The results showed that the bandwidth of visible light is much larger than that of infrared light because of the fact that the reflectivity of visible light is lower than that of infrared. Calculation methods of VLC channel impulse response were proposed in the literature, which are extremely important.
However, in most of the aforementioned modeling methods, only one path was considered, and actual VLC systems may experience more than one path.
Therefore, it is necessary to consider multiple paths and multiple reflections in VLC channel models. Rajbhandari et al. The channel model they used was based on an infrared communication channel model established by Carruthers and Kahn et al. In a real channel, a has two values.
The model is simple and easy to use, and it can be employed for evaluating the performance of complex VLC systems, which serves as a useful reference in the literature. However, if we want to discuss the impact of the model parameters, we need to take into account each parameter of channel models. If there are multiple light sources in a VLC system assume that the number of light sources is N , the channel impulse response is expressed as The channel impulse response of the model is divided into LOS and scattering paths, and this is very useful to understand the VLC channel modeling approaches.
In this sense, it is a useful reference indeed. Zhou Zhou et al.
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Based on the aforementioned findings, they proposed a method to reduce the modeling calibration errors caused by RMS delay spread estimation errors and improve the computational efficiency. The transmitter is located in the center of a room and produces multiple scattering hot spots on the ceiling, and the receivers are located in the room to receive data from the hot spots.
Indoor wireless channel is represented by its impulse response. The receivers are placed in different positions in the room with different channel impulse responses. Suppose that there are M hotspots and N receivers. Assume that equal gain combining is used.