Category: VLC

In Previous Post, we discussed How to calculate SNR for Indoor VLC Check Here

Now Using SNR we can calculate BER(Bit Error Rate )for Indoor VLC system. Normally BER rate would be 10-3 is Great in every communication system. Indoor VLC system gives BER in Range of 10-2 to 10-6. Indoor VLC is Give result variation at Angle of Tx (Transmitter) and Rx(Receiver), In another word we can be called it’s LOS(Line of Sight).  Indoor VLC give accurate Result on -45 To 45 Angle. In that Range, We can get average 10-5 BER.

BER depends on the modulation scheme and SNR of the system. Where Q is a function that depends upon the modulation scheme which used.

BER = Q√SNR

We have implemented the VLC channel in MATLAB. Now irradiance angle and incidence angle depend on the receiver’s position in the room. At different incidence angles and irradiance angles, we have measure SNR and BER. It is measured for the transmitter and receiver are in-line, we get maximum SNR and low BER. Performance graphs of SNR and BER are shown in Fig, 1 and Fig, 2 respectively.


As discuss above FOV affect the performance of VLC system. For different transmitter’s FOV we can calculate SNR. Assume that the receiver is in the line of a transmitter. Transmitter’s FOV versus SNR graph is shown in Fig, 5As shown in the graph, to achieve high SNR, the transmitter’s FOV should be set above 50. One another important point is maximum SNR is achieved at FOV equal to 90.

For analyze VLC system performance, we assume some parameter for a room, LED, receiver and noise. That parameter listed in below Table, 1 [8] [2] [1].

CONCLUSION :

In Single Input Single Output (SISO) VLC system we get High SNR and low BER for LOS transmitter and receiver’s positions. So, Irradiance angle and incidence angle must be less than 45 to achieve good signal. Now transmitter FOV should be always above 50 to get maximum SNR. Using MIMO RC technique increase SNR of the system. VLC system support up to 10 Gbps system.

VLC SYSTEM MODEL SISO

• SISO: Single Input Single Output

In this section, we describe the Single Input Single Output VLC system and discuss the parameter of the VLC link. First, we consider the LOS link between the single transmitter and the receiver. The link configuration is shown in (fig1) [1].

Where θ is the angle of incidence with respect to the axis of the receiver surface. Ҩ is receiver’s FOV. At transmitter side viewing angle (Irradiance) with respect to transmitter surface is Ø and transmitter FOV is ɸ. Lambertian emission defined as m [11].

Optical concentrator gain of the receiver [2] defined as.

VLC channel gain for LOS link determined by HLOS is,

Where 𝑅0(Ҩ) is [11],

𝑅0(Ҩ)=(𝑚+1)/(2∗𝜋)*𝑐𝑜𝑠2(∅) [4]

Where 𝐴𝑝𝑑 the collection area of the receiver is, 𝑑2is the distance from the transmitter to the receiver. θ is the angle of incidence on the receiver and Ҩ is the receiver’s FOV. For SISO system we have to consider only one LOS link so at receiver side only one channel gain calculated. A received signal is given by [2]

𝑅𝑥 = R∗𝑃𝑡∗HLOS + √σ2 [5]

Where R is the photodiode responsively. Pt is transmitted optical power and σ2 is the mean square noise current of a receiver. Transmitted power is defined as [11]

𝑃𝑡=𝑃𝐿𝐸𝐷∗𝑅0(Ҩ) [6]

Where PLED is, the power emitted by LED. Received data is recovered using the inverse of HLOS as [2]

𝑅𝑟𝑒𝑐𝑜𝑣𝑒𝑟𝑒𝑑=𝑅𝑥∗𝐻𝑇 [7]

Received power 𝑃𝑟 depends upon the filter transmission 𝑇𝑠(𝜃) and concentrator gain. Ideally, concentrator gain and transmission filter are considered as 1 [2].

𝑃𝑟= 𝑃𝑡∗𝐻𝐿𝑂𝑆∗g(𝜃)∗𝑇𝑠(𝜃), 0≤θ≤Ҩ [8]

SNR depend upon the photodetector responsivity R, received optical power and noise variance. SNR is [2]

SNR = (𝑅𝑃𝑡)2/𝜎𝑠ℎ𝑜𝑡2+𝜎𝑡ℎ𝑒𝑟𝑚𝑎𝑙2 [9]

Shot noise is photon generated noise in the detector by received signal and calculated as [1]

𝜎𝑠ℎ𝑜𝑡2=2𝑞𝑅𝑃𝑟𝐵 [10]

Thermal noise is given by [1],

𝜎𝑡ℎ𝑒𝑟𝑚𝑎𝑙2= 4𝐾𝑇𝐵𝐹𝑅𝑡 [11]

BER depends on the modulation scheme and SNR of the system. Where Q is a function that depends upon the modulation scheme which used.

BER = Q√𝑆𝑁𝑅 [12]

VLC SYSTEM OVERVIEW

VLC system block diagram is shown in Figure 1.

A. VLC Transmitter

Precise dimming control can be done by LED. Driving circuit combines the input data of communication module and they drive a dimming control of LED array. LED arrays are generating multiple light signal.
For transmitting Light signal, the LED luminaire can be used. LED luminaire unit which consists of an LED lamp, ballast housing, and other components, and Driving circuit which controls the current flowing through LEDs to control its brightness.

White light is the most commonly used form of illumination in both indoor as well as outdoor applications. This is because colors of objects as seen under the white light closely resemble the colors of the same objects under the natural light.

White light is produced in two ways [5]. First one is blue LED with the yellow phosphor coating on the outside. When the blue light traverses through the yellow coating, the combination produces a white light. Different variations of the white light are produced by modifying the thickness of the phosphor layer. The second one is RGB combinations in which white light can be produced by proper mixing of red, green and blue lights. In this method, three separate LEDs are used which increase the cost of LED luminaire compared to using the blue LED with phosphor. RGB combination is more preferable for communication because we use color shift keying to modulate the data using three different wavelengths.

B. VLC Receiver

The optical concentrator is used in the receiver, to set the receiver at a particular angle of light beam. An optical filter is also used to filter out data distorted by noise and multipath effect. Two types of receivers can be used to receive the signal transmitted by an LED [5]. First one is Photodetector, also referred to as photodiode or non-imaging receiver which receives the précised light incident on it and then converts into a current. The second one is Imaging Sensor which is also called a camera sensor. Camera sensor consists of many photodetectors arranged in a matrix or array in an integrated circuit [5].

C. VLC Channel

In VLC channel, experimental arrangement or setup is made with LED as transmitter and photodetectors as a receiver in a Line of Sight(LOS) link. For calculating channel gain we have to analyze indoor link. For VLC, indoor link four types are possible [4]. First is Line of Sight (LOS) link, in which receiver and transmitter are in-line. Second is Non-directed LOS link where at receiver side there is LOS path and multipath due to physical obstacles and the wall of a room. A third is a diffuse link with no LOS path but having only multipath signals on the receiver side Fourth is Quasi-diffuse link with multiple narrow beam transmitters and narrow Field of View (FOV) receiver, achieving lower multipath effects and lower path loss.

What is VLC?

Visible Light Communication system (VLC) is dual use of illumination and communication concept. VLC concepts are that the data is propagated through a visible light wave. Visible Lightwave produced by LED or LASER. But LED has many advantages like energy savings and longer lifespans and also other benefits. Usage of LED is increased exponentially. LED has a short range of illumination so that it can be used in indoor environments for both purpose illuminations as well as communication. The LEDs are capable of switching to different light intensity levels at a very fast rate. The switching rate is fast enough to be imperceptible by a human eye. This functionality can be used for communication where the data is encoded in the emitting light in various ways. A photodetector or an image sensor (matrix of photodiodes) can receive the modulated signals and decode the data. Visible Light Communication provides many benefits. First visible light spectrum in Figure 1 includes hundreds of terahertz of license-free bandwidth which is completely untapped for communication. Second due to its high frequency, visible light cannot penetrate through most object and walls. It can also increase the capability of an available wireless channel. Third, VLC facilitates the reuse of existing lighting infrastructure for purpose of communication.

In MIMO VLC system, as the number of transmitters and receivers increases, large adjacent channel interference is introduced. The adjacent channel interference is helpful to achieve the higher strength of a signal at receiver side due to use of MIMO diversity repetition coding technique. Repetition coding means simply transmit same data by multiple transmitters.

In this section, we discuss equalizer, which is used for removing Inter-Symbol Interference (ISI). Performance of MIMO VLC can also be increased by applying different equalizer. The type of simple equalizer is the Zero Forcing (ZF) and Minimum Mean Squared Error (MMSE). We can also improve the performance of the system by adding the Successive Interference Cancellation (SIC) approach to the equalizer. ZF – SIC and MMSE – SIC equalizers give better performance because SIC uses adjacent channel interference to improve the performance of MIMO VLC system. For remove perfect effect of Inter-Symbol Interference (ISI), we use optimal ordering in a different equalizer. All different equalizers applied in MIMO VLC system, and the related simulations have been implemented in MATLAB.