Active Pre-Equalizer for Broadband over Visible Light

This paper introduces a new technology called Broadband over Visible Light (BVL) which combines two technology solutions like Visible Light Communication(VLC) and Broadband over Power Line (BPL). This new technology is suitable for converting modern LED lighting systems into communication systems. However, there are some deficiencies in BVL technology such as the low bandwidth of LED optical transmitters. Pre-equalization may be solution of this problem. This paper proposes higher bandwidth using the pre-equalization circuit. Also, it shows real experimental results demonstrating an improvement of bandwidth and transmission rate.


Introduction
The recent development in the area of white LEDs caused their common use as a highly efficient alternative to the conventional sources of optical radiation in the visible range.This development brought progressive changes in the lighting technology.The physical principle of white LEDs allows their use for communication purposes.The physical principles, including changes in trends of the lighting technology, caused the emergence of a new research direction generally called Visible Light Communication (VLC), which is a deriva-tive of original research direction generally known as indoor Optical Wireless Communication (indoor OWC), operating exclusively in the infra-red spectrum of optical radiation.The objective of this research direction is merging lighting and communications [1], [2] and [?].
The Broadband over Visible Light (BVL) is a new research direction based essentially on VLC technology.Again, it is intended to utilize the visible spectrum of optical radiation as a communication direction to the end user (downlink) and to utilize the infra-red spectrum of optical radiation (940 nm) in the reverse communication direction (uplink).Moreover, compared to the VLC concept, in the case of BVL, it is intended to use the chipset of the Broadband over Power Line (BPL) technology, which, inter alia, allows the use of the OFDM MQAM modulation format at the number of 1155 sub-carriers in the frequency range from 2 MHz to 32 MHz (for example HomePlug AV).The BVL technology should, by its nature, enable transmission speed of 100 Mb•s −1 .Additionally, the BVL technology provides connectivity over power conductors in an efficient way.It gives us the opportunity to transmit the modulated signal to the optical transmitter by its power lines, and use visible light as a wireless data transmitter.
White light LEDs as transmitters for communication link have the big disadvantage as low bandwidth.Low bandwidth is caused by optoelectronic response of the LED and due to physical principles of fluorescence in a thin layer of phosphor which is responsible for creating white light from blue light.Fluorescence inserts some delay to the optical signal and thus influences the maximum bit rate.White power LEDs achieve several MHz of bandwidth [8].Some researchers achieve 10 MHz of bandwidth due to optical band pass filter on detection side, where only blue part of wavelength processes pass on photodetector without delayed yellow part from fluorescence.Equalization techniques are used for elimination of LEDs optoelectronic response to achieve higher bandwidth [6] and [7].This article deals with this actual issue and brings new unpublished results, which can help to develop BVL technology.

2.
Bandwidth of LEDs  eliminate roughness caused by noise and other signal distortion.The smoother curve of frequency response provides ideal conditions to design the pre-equalization circuit.−3 dBm bandwidth was achieved at 2 MHz with a high power white light LED as shown in Fig. 6, and at 7 MHz with blue part of radiation without phosphor effect.

2.1.
Design of Pre-Equalizer The purpose of equalization is to compensate signal distortion in a communication channel.The main distortion of the signal was produced by the optical transmitter at VLC channel due to an optoelectronic response of LED and delay of the phosphor [6].We used well-known equalization techniques and techniques for designing filters to reach suitable bandwidth.Appropriate bandwidth was reached with a really simple electronic circuit, because the simplicity of designed circuit was the important requirement.
Pre-equalization circuit was designed according to the reversal of measured frequency response, which can be seen in Fig. 6.The circuit is composed of an active and mainly passive part, which determines shape of frequency response.The passive part of the circuit is high pass filter, which causes 35 dBm attenuation, as it can be seen in Fig. 3.The active part of the circuit contains operational amplifier OPA847 eliminating attenuation of the passive part.The complete circuit provides frequency response closed to the reversal of measured frequency response.Circuit diagram of preequalizer can be seen in Fig. 2. The transfer function of the pre-equalization circuit was expressed as: where ω is defined as 2πf .

Passive Active
Fig. 2: Circuit diagram of active pre-equalizer with OPA847.

Simulations
Passive part of the circuit shapes the curve of frequency response and contains parallel connection of resistor R 2 and capacitor C and resistor R 3 , parallelly connected to them. Figure 3 shows the curves from simulations, where the effect of components values on the shape of frequency response can be noticed.The best result of reversal frequency response was reached by the components R 2 = 2 kΩ, C = 22 pF and R 3 = 200 Ω.
Operational amplifier OPA847 was used in active part of the pre-equalization circuit.The OPA847 provides a unique combination of a very low input voltage noise, along with a very low distortion output stage to give one of the highest dynamic range of op amps available.Voltage-feedback of op amps, unlike currentfeedback designs, can use a wide range of resistor's values to set up their gain.R 4 was set to 39.2 Ω and R 5 was optimized according to desired gain.Using this guideline ensures that the noise added at the output due to the Johnson noise of the resistors does not significantly increase the total noise over the 0.85 V/ √ Hz input voltage noise for the op amp itself.This R 4 value is suggested as a good starting point for the design of the circuit.
Curves gained by simulations of different adjusted values of feedback resistor R 5 are shown in Fig. 4, where open-source simulation software Qucs was used.R 4 value was 39.2 Ω whereas we were trying to achieve ideal amplification for the pre-equalization circuit by adjusting value R 5 .R 5 = 750 Ω provided the best result of reversal frequency response and achieved gain +20 V/V of operation amplifier.Values of R 4 and R 5 affected complete amplification of active part of the circuit.On the other hand, they also affected shape of the frequency response of overall pre-equalization circuit as shown in Fig. 4.

Measurement
The frequency response of constructed pre-equalizer was measured and the results were compared with simulations.Network analyzer Rhode-Schwarz ZVB 4 (3 kHz to 4 GHz) was used.Results from simulations, measurements and reverse are shown in the Fig.

Testing Effect of Pre-Equalizer on Bandwith and Modulation
To verify designed and constructed pre-equalizer effect, a new measurement was done on Philips Fortimo LED DLM 3000 44 W/830 by aforementioned vector network analyzer.The frequency response of LED light source without designed circuit was measured at first, then with pre-equalizer.To allow determining influence of fluorescence in the phosphor, measurement was repeated without diffuser with the phosphor layer on the mentioned white light LED source.Uniform distance 40 cm between the optical transmitter and photodetector was set.
The pre-equalizer measurement results are shown in Fig. 6.It is evident how designed circuit of preequalizer influences frequency response of phosphor based white light and also blue part of radiation without influencing the delay due to fluorescence at phosphor layer.A bandwidth of −3 dBm was achieved at 2 MHz with white light LED without pre-equalization, whilst, a bandwidth of −10 dBm was achieved at 6 MHz.In the case of white light LED without pre-equalization, −3 dBm bandwidth was achieved at 2 MHz and −10 dBm bandwidth at 6 MHz.When with the pre-equalization circuit connected between the network analyzer and Bias-T, −3 dBm bandwidth was obtained at 3 MHz and −10 dBm bandwidth at 40 MHz.It proves how pre-equalization mitigates natural inclinations to low bandwidth of semiconductor phosphor based LED light transmitters.The bandwidth of blue part of radiation without the effect of luminescence achieved amplification by 5 dBm at 20 MHz frequency due to pre-equalization.Pre-eq response White part without pre-eq White part with pre-eq Blue part without pre-eq Blue part with pre-eq -39.07 dBm -35.64 dBm Fig. 6: Effect pre-equalizer on frequency response of white light LED and blue light LED.
The power level of the signal has higher inclination to drop toward increasing frequency, due to influence of phosphor layer.Diffuser with phosphor layer decrease power level.BVL technology with respect to HomePlug AV technology solution operates from 2 MHz to 32 MHz, hence BVL solution needs to achieve this bandwidth.An attenuation of 3.43 dBm was achieved with mentioned bandwidth due to designed pre-equalization circuit.

Experimental Setup
The block diagram for the experimental setup is shown in Fig. 7. RF VSG NI PXI-5670 (Vector signal generator) [11] was used to generate the digitally modulated signal.MQAM digital modulation was tested [10] and [14], 4QAM was used specifically.
Vector signal analyzer RF VSA NI PXI-5661 [11] was used on the receiver side.The signal modulated by a digital modulation scheme was monitored by constellation diagram and simultaneously an Error Vector Magnitude (EVM) was measured.The EVM provides a comprehensive measure of the quality of the digitally modulated signal.We used it to verify pre-equalization circuit effect on the transmitted digital signal, depending on the symbol rate ( used bandwidth ).SI PIN photodetector ThorLabs PDA10A-EC is operating in the wavelength range from 200 nm to 1100 nm.Photodetector PDA10A-EC has an effective area A ef f = 0.8 mm 2 only, therefore N-BK7 Plano-Convex Lens with a focal length of 25.4 mm was used.Thanks to the lens, an adequate signal output was obtained to verify the functionality at a realistic distance of 3 m between transmitter and receiver.Center frequencies 5 MHz, 10 MHz, 15 MHz and 20 MHz were used in digital modulation scheme.
The detected signal can be represented by: where g(n) and η(n) represent the multiplicative and additive impairments to the detected signal.The multiplicative impairments can be a result of channel estimation errors or IQ imbalances, for example.The additive impairments are usually caused by thermal noise and are modeled as an i.i.d.(Independent and Identically Distributed random variables) complex AWGN samples with Power Spectral Density (PSD) of N 0 /2.
EVM can be designed as the root-square (RMS) value of the difference between an array of measured symbols and ideal symbols.The EVM can be represented as: where N is the number of symbols over which the value of EVM is measured.S r (n) is the normalized received n th symbol which is disrupted by Gaussian noise.S t (n) is the ideal transmitted value of the n th symbol x(n), and P 0 is either the maximum normalized ideal symbol power or the average power of all symbols for the RF+DC Fig. 7: Block diagram of experimental measurement.
chosen modulation.P 0 can be represented by: The EVM value is normalized with average symbol energy to remove the dependency of EVM on the modulation order.Consider the detected signal in Eq. ( 2), where g(n) ≈ 1.For non-data-aided receivers, the EVM is: where x(n) are transmitted symbols, which are estimated and used to measure the EVM value.
According to [13], the EVM for QAM signals is: where The EVM of a QAM signal in Eq. ( 6) can be divided into two parts.The first part is 1/SNR, which represents the ideal EVM when no errors are introduced to the symbol detection.The second part is QAM signal, which is the sum of the exponential and error function, representing the reduction in measured EVM due to the error detection.

Results and Discussions
The results were measured by the block diagram shown in Fig. 7. 4QAM digital modulation scheme was transmitted via white part of radiation with effect of the luminescence and results are shown in Fig. 8. Small differences were measured between EVM values of VLC system without the pre-equalizaion circuit and with pre-equalization for center frequency 5 MHz.The preequalization circuit had significant influence at higher frequencies.
Significant improvement of EVM values was verified with VLC system with pre-equalization.Most significant difference of EVM values was for center frequency of 20 MHz due to frequency response of preequalization circuit seen in Fig. 6.EVM value increased when symbol rate was increased, due to nonlinearity of frequency spectrum.
EVM values were increased due to low signal power level and natural inclinations of the frequency response as shown in Fig. 6.Higher bandwidth was used with higher symbol rate and it increased EVM and decreased communication possibility because of inadequate frequency response.The VLC system was not suitable for use in higher central frequencies and higher bandwidth for digital modulations without pre-equalization.The VLC system with the pre-equalization circuit provided compliant conditions, thus higher central frequencies and bandwidth could be used.

Conclusion
In this paper pre-equalization of VLC transmitter has been presented.In order to get suitable frequency response of VLC transmitter based on phosphor white LED light source, we have been proposed equalization circuit used in our VLC system.The aforementioned HomePlug AV technology bandwidth from 2 MHz to 32 MHz was achieved with 3.43 dBm attenuation by commercial phosphorescent white light LED and proposed equalizer circuit.The objectives of this paper were to achieve suitable frequency response for the mentioned BVL technology solution.The proposed system demonstrably improves operational bandwidth in VLC system and could be considered as suitable system improvement for future Broadband over Visible Light (BVL) technology deploy.

FrequencyFig. 3 :
Fig. 3: Simulation results of pasive part of pre-equalizer and effect components values on frequency response.

Fig. 4 :
Fig. 4: Simulation results of active part of pre-equaliser and different feedback resistor values effect on frequency response.

FrequencyFig. 5 :
Fig. 5: Frequency response of designed circuit given by reverse, simulation and measurement.

Fig. 8 :
Fig. 8: Resulst of measurement EVM depending on symbol rate and carry frequency transmit 4QAM modulation via white light LED.
5. Frequency response from simulations and measurements of the pre-equalization circuit are almost same to the reverse frequency response, which is desirable.Operational amplifier OPA847 operates up to 40 MHz as seen in Fig.5.This is due to high amplification of OPA847, however 40 MHz is sufficient for BVL technology solution.The designed pre-equalizer circuit attenuates the input signal by about 1 dBm in the frequency range 0.2 to 2 MHz.Designed and constructed circuit achieves power level 15 dBm at 40 MHz.Bandwidth from 2 MHz to 40 MHz of designed circuit is compliant according to HomePlug AV technology solution of Broadband Power Line communication.