1. Introduction

The applications of portable communication devices (PCDs) i.e. smartphones for consumer use have been extremely rapid in the last decade and it is widely expected that in 2017, there will be approximately 2 billion active tablets and 4~5 billion smartphones to access Internet service [1]. Although these PCDs are getting great, one of their limitations is the size of the screen, which of necessity has to be small. To solve this problem, the idea of building a projector into a PCD so that images can be displayed on any nearby flat surface was raised by makers. However such PCDs proved were unfortunately too bulky for users to accept until Samsung firstly announced its new smartphone, Galaxy Beam [2], which is a new, small, thin and lightweight phone that has a projector built into it that allows users to project whatever is on the screen onto any nearby surface. As a precursor to the adoption of the next generation wireless access (5G) which is projected to be in place by 2020, digital services are being upgraded allowing for more data bandwidth transmission since the aim of 5G is to provide connectivity for any kind of device and any kind of applications that may benefit from being connected, including mobile connectivity for people and various objects in user’s environment [3]. However this raises new challenges for the design of a new generation PCD to meet the aim of 5G since those already available short-range communication technologies on current PCDs such as Near Field Communication (NFC) and Bluetooth which have a limited data transmission rate less than 5 Mb/s seems not competitive.

With the recent development of optical wireless communication technologies, a white light emitting diode (LED) based visible light communication (VLC) technology [4,5] offers simultaneously illumination and high-speed data transmission, making it a good candidate as an alternative short-range communication technology for 5G PCD application since LEDs are widely used on PCD either for camera or for micro-projector application. Compared with the conventional white light LED which is composed of blue LED and phosphor limiting the available modulation bandwidth, Red-, Green-, and Blue-based white light LEDs have the advantages of providing a higher modulation bandwidth and they can potentially be used to perform a multi-channel transmission simultaneously through the use of multiple wavelengths. In order to increase the functionality and simultaneously reduce the physical size of a PCD, it is widely expected that VLC technology using the light sources of micro-projector system i.e. RGB-based LEDs will be integrated into the micro-projector module on PCD [6] to provide a new communication approach. This will dramatically provide not only a new efficient but also a faster and a secure communication approach for PCD applications.

In this paper, an Liquid Crystal on Silicon (LCoS) based polarization modulated image (PMI) system architecture which offers simultaneously micro-projection and high-speed data transmission nearly Gbit/s, serving as an alternative short-range communication (SRC) approach for PCD application in 5G that has experimentally implemented is reported and demonstrated. In order to make the proposed system architecture transparent to the future possible wireless data modulation format, baseband modulation schemes such as Multilevel Pulse Amplitude Modulation (M-PAM), M-ary Phase Shift Keying modulation (M-PSK) and M-ary Quadrature Amplitude Modulation (M-QAM) [7] which represent the amplitude, phase and quadrature amplitude were used in this research. Although, it is well known from communication theory that for the same bit error rate (BER), QAM requires a lower signal to noise ratio (SNR) than PAM (also than PSK) assuming the same order of modulation M, the N sub-carriers transmission will require $5{\log }_{10}N$ dB more optical power than the corresponding single sub-carrier scheme [7]. In this research, baseband modulation schemes based on the criteria of single sub-carrier were therefore experimentally implemented to investigate the highest possible data transmission rate of the proposed system architecture. However these baseband modulation schemes can be further employed by more advanced multicarrier modulation schemes (such as DMT, OFDM and CAP).

In our experiments, the performance of the proposed system architecture was evaluated when data transmission were performed with and without micro-projection simultaneously. The results demonstrated that the highest aggregative data transmission rate of 892 Mb/s and 900 Mb/s can be achieved by using 16-QAM modulation scheme when data transmission were performed with and without micro-projection simultaneously. The estimated BERs of these results were all less than ${10}^{-3}$ which satisfies the limitation of Forward Error Correction (FEC) standard ($\text{BER}\le 3.8\times {10}^{-3}$) since it is sufficient to achieve a packet BER below ${10}^{-14}$ using a standard Reed-Solomon (255, 239) FEC code [8]. Although, performing micro-projection simultaneously would result a slightly degradation of data transmission performance (less than $1\text{\%}$), the proposed system architecture still offers an alternative and faster communication approach for the application of a new PCD in 5G since to the best of our knowledge, this is the first time that a high-speed data transmission nearly Gigabit using RGB-based LEDs without any bandwidth improvement techniques such as equalization design [9] can be performed simultaneously with micro-projection in the same system architecture.

2. Experimental setup

In our experimental setup as illustrated in Fig. 1, a low cost commercially available component, Philip Luxeon Z series RGB-based LEDs [10] was used as the light source. It has three LED chips which had the peak wavelengths of 650nm (Red), 550nm (Green) and 450nm (Blue) respectively. The measured 3-dB modulation bandwidths of these LED chips within the proposed system architecture are approximately 8 MHz, 16MHz and 12 MHz for Red, Green and Blue wavelengths respectively. These chips were fixed on a 20 mm x 20 mm metal base printed circuit board (metal core PCB, MCPCB) serving as a RGB-based LEDs transmitter (TX) for SRC link and mico-projection in our experimental architecture. The signals for TX transmission of SRC link were randomly generated and modulated offline by a Matlab program on a personal computer (PC) and uploaded to the memory of an arbitrary waveform generator (AWG, Keysight 33621A, 1GSa/s sampling rate). Since intensity modulations were utilized to encode data on the LEDs for VLC links, the emitted light of each LED chip on TX were then individually driven by modulated signals from AWG and separately biased by DC currents through low-frequency Bias Tees. A DC current of 350 mA which has been observed to have the best performance from our experiments was used.

 

Fig. 1 Experimental implementation of micro-projection enabled SRC system.

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In order to experimentally demonstrate the feasibility of establishing SRC links while performing micro-projection simultaneously, a reflective LCoS based polarization modulated image (PMI) system was proposed and implemented in this research. The proposed system architecture was composed of a collimated lens, L1 (Thorlabs, 50 mm diameter), a polarization beam splitter (PBS, Thorlabs, 50:50), an LCoS device, and an image lens, L2 (Thorlabs, 50 mm diameter). The incident light from TX was firstly collimated by a L1 and was then selected by a PBS. Although the LCoS device is a polarization sensitive device, in which only one polarization state of the incident light selected by the PBS can be used in our experiments, this disadvantage can be further compensated through a conventional polarization conversion system (PCS) [11,12] in order to reduce the system light loss in the practical applications. The LCoS device used was JD9554 manufactured from Jasper Display Corp. (JDC), Taiwan [13]. It is a phase only microdisplay with full HD resolution (1920 x 1080 pixel) and 6.4 µm pixel pitch leading to an active area diagonal of 0.7” with aspect ratio of 16:9. Each pixel on the LCoS device was controlled by a PC.

At the receiver side (RX), transmitted lights from LCoS-based PMI system was firstly collimated by a focus lens, L3 (Thorlabs, 50 mm diameter) before focusing on a Si transimpedance amplified photodetectors (Thorlabs, PDA10A). The photodetector used have responsivities of 0.385 A/W, 0.25A/W and 0.175 A/W for Red, Green and Blue wavelength respectively and has an active area of 0.8 mm² [14]. The received signals were amplified again by means of a power amplifier (Minicircuits, 11 dB gain), before recording by a real-time oscilloscope (Keysight DSOX 4104A, 5 GSa/s sampling rate). The signals from the oscilloscope were online analyzed directly through a vector signal analysis software (Keysight VSA89600). Although a transmission distance of 0.65m between TX and RX was initially used in this proof of concept research, it could be further extended by using an array of identical LED emitters. The experimental works as illustrated in Fig. 1 were carried out via two stages which are data transmission with and without performing micro-projection simultaneously.

2.1 Multilevel pulse amplitude modulation

Multilevel pulse amplitude modulation (M-PAM) is the generalization of non return-to-zero (NRZ) on-off keying (OOK) from a set of two symbols to a set of M symbols. In our experimental works, the measurement results of Q-factors from eye diagrams observed from Oscilloscope were used to determine the BER of M-PAM (i.e. 4-PAM) modulation scheme [15]. The BER of M-PAM can be calculated as [7]:

2.2 M-ary phase shift keying modulation

Phase shift keying (PSK) is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal (the carrier wave). The motivation behind M-ary PSK (M-PSK) is to increase the bandwidth efficiency of the PSK modulation scheme. In M-PSK, n, ${\log }_{2}M$data bits are represented by a symbol, thus the bandwidth efficiency is increased to n times. The BER of M-PSK can be calculated as [7]:

In our experimental works, the measurement results of Error Vector Magnitude (EVM) from vector signal analyzer (Keysight VSA89600) were used to determine the BERs through the calculation of SNRs in light of the equation derived from [16]:

2.3 M-ary quadrature amplitude modulation

Quadrature amplitude modulation (QAM) is a class of nonconstant envelope schemes that can achieve higher bandwidth efficiency than M-PSK with the same average signal power. The theoretical relation of BER and SNR for M-QAM can be expressed as [7]:

From the measurement results of the EVM, we can calculate the SNR by (3) and use it to estimate the BER of M-QAM through (4).

3. Results and discussions

The performance of the proposed PMI system architecture which offers simultaneously micro-projection and high-speed data transmission was evaluated in two different experimental works. In the first stage of the experimental works, data transmission tests were performed without performing micro-projection function simultaneously. The data transmission performance of baseband M-PAM, M-PSK and M-QAM modulation schemes was evaluated through the estimated and calculated BERs from the measured eye diagrams and constellation diagrams. The results shown that the highest possible aggregative data transmission rates of 4-PAM, 8-PSK and 16-QAM modulation schemes at a BER of 10^-3 are 114 Mb/s, 576 Mb/s and 900 Mb/s respectively.

In the second stage of the experimental works, data transmission and micro-projection were performed simultaneously based on the proposed system architecture. In this experimental work, a computer generated hologram (CGH) composed of sub-holograms was uploaded to the LCoS device. For the proof of the concept research, one of the sub-holograms (800 x 600 pixels) corresponding to a super video graphics array (SVGA) resolution was designed to perform the micro-projection function. The rest of the other sub-holograms on the LCoS device were designed to perform the polarization modulation and reflect the light into the RX to establish a SRC link. The data transmission performance of 4-PAM, 8-PSK and 16-QAM modulation schemes was compared and the results as illustrated in Fig. 2-4 shown that the highest possible aggregative data transmission rates of 4-PAM, 8-PSK and 16-QAM modulation schemes at a BER of ${10}^{-3}$ are 108 Mb/s, 573 Mb/s and 892 Mb/s respectively.

 

Fig. 2 The highest data rate of Red LED (BER = 10⁻³).

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Fig. 3 The highest data rate of Green LED (BER = 10⁻³).

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Fig. 4 The highest data rate of Blue LED (BER = 10⁻³).

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4. Conclusion

In this paper, we propose and experimentally demonstrate a micro-projection enabled short range communication system (SRC) for the application of PCD in 5G. RGB-based LEDs which have widely employed as the light source of micro-projector system was used as the light source in our proposed system. In order to make the proposed system architecture transparent to the future possible wireless data modulation format, M-PAM, M-PSK and M-QAM modulation schemes which can be further employed by more advanced multicarrier modulation schemes (such as DMT, OFDM and CAP) were used to investigate the highest possible data rate of our proposed system architecture when data transmission was performed with and without performing micro-projection simultaneously. Although performing micro-projection with data transmission simultaneously will slightly degrade the SNR which will further reduce the highest possible data rate (less than 1%), an aggregative data transmission rate nearly Gigabit at a BER of ${10}^{-3}$ can be achieved by using 16-QAM modulation scheme without any bandwidth improvement techniques. Our works presented in this paper will potentially offer an alternative and faster communication approach for the application of a new PCD in 5G.