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Abstract
Next-generation visible light communication (VLC) is envisioned to evolve into a high-speed and multi-user system. In this work, a 75-µm single layer quantum dot (QD) micro-LED was fabricated, packaged and used to experimentally demonstrate a 3-meter QAM-OFDMA VLC system affording multiple users with a 1.06-GHz modulation bandwidth. The OFDMA system realized data rates of 1.2 Gbps and 750 Mbps with a BER of 0 and 3.6×10−3 for two independent users with a 1:1 bandwidth ratio, respectively. Additional sub-carrier allocation strategies and scenarios of 2∼6 users have been further evaluated, and all proposed strategies reach the sum-rate of beyond 1.41 Gbps while satisfying the forward error correction (FEC) criteria.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Visible light communication (VLC) is emerging as a promising wireless technology to realize communication and illumination simultaneously [1–4], with the advantages of high-security, electromagnetic interference (EMI) immunity and compatibility to existing lighting facilities [5,6]. The electrical-to-optical (E-O) bandwidths of commercial large area light emitting diodes (LEDs) are limited owing to the lifetime of the yellow phosphor [7]. After removing the phosphor luminescence by a filter [8,9], the bandwidth can be increased to 10∼20 MHz. Thus, the E-O modulation characteristics make it difficult to achieve a high-speed multi-user VLC system. To further increase the system bandwidth, gallium nitride (GaN) based micro-size LEDs (micro-LEDs), with superior characteristics such as small size, short carrier lifetime and low RC constant, are proposed to break the current bandwidth bottleneck. The typical quantum well (QW) structured micro-LEDs can reach an E-O bandwidth up to hundreds of MHz [10]. Due to the advantages of high E-O bandwidth and parallel driving, micro-LED and its arrays have great potential for high-speed VLC applications [11]. Despite the efforts have been made to increase the E-O bandwidth, the operation current densities of micro-LEDs are still at an order of kA/cm2. Thus, a practical high E-O bandwidth LED with lower injecting current density has become the keen motivation of the quantum dot (QD) micro-LED fabrication.
Many previous efforts have been mainly focused on improving the data rate of single user in micro-LED based VLC systems [12–18]. In the typical indoor environment, the LEDs as illumination devices are always installed on the ceiling with store-and-forward information, the emitted light carrying the data for different users is broadcasted to every single terminal. The system serving multiple users has earned much attention due to the increasing demands for downlink in a crowded indoor communication environment. However, the implementation of multi-user techniques in VLC system is still challenging if considering the limited bandwidth, and hence accounts for the reason for us to investigate a QD micro-LED based high-bandwidth VLC system. Orthogonal frequency division multiple access (OFDMA) is a multiple access technology which allows the data transmission of multiple users with a limited bandwidth. It has been proved effective in the implementation of passive optical networks (PONs) [19–21]. A LED-based OFDMA VLC system has been reported in [22], which only achieved a data rate of 13.6 Mbps. Enabling multiple users to occupy independent sub-carriers in frequency domain, OFDMA benefits from low interference which corresponds to a better bit-error performance than frequency division multiple access (FDMA) and time division multiple access (TDMA). Recently, non-orthogonal multiple access (NOMA) in VLC has also attracted significant attentions due to its high spectral efficiency [23–25]. With properly designed sub-carrier and power allocation strategy, OFDMA VLC system has the potential to fully exploit the limited bandwidth and can afford multiple terminals with high-speed VLC in various scenarios.
In this paper, a 75-µm single layer QD blue micro-LED is fabricated and packaged for VLC implementation. Its optical and electrical characteristics are both experimentally measured. The modulation bandwidth of the VLC system can reach up to 1.06 GHz with an ultra-low current density of 178.8 A/cm2. The E-O bandwidth of QD micro-LED is better than other high-speed LEDs grown on standard c-plane sapphire and this result is comparable to semi-polar LEDs fabricated by Dinh et al. [26]. For the first time, expanding from our previous works in [27], we demonstrate a multi-user QD micro-LED based QAM-OFDMA VLC system over 3 m link which realizes 1.2 Gbps and 750 Mbps data transmission for two users with BERs of 0 and 3.6×10−3 respectively. Also, two-user sub-carrier allocation strategies with various frequency-band ratios are evaluated to obtain a sum-rate of 1.5 Gbps. Subsequently, a sub-carrier allocation strategy for three to six users is presented to provide users with equal frequency-band occupation, and all demonstrated scenarios achieve a high sum-rate of 1.41 Gbps. The aforementioned transmission links are below the forward error correction (FEC) criterion. To achieve higher data rate, the strategy with greater ratio is suggested to mitigate the channel loss of the high-frequency user.
2. Experimental setup
2.1 Structure and packaging of QD micro-LED
An InGaN-based QD micro-LED epitaxial structure is grown on conventional c-plane sapphire substrate using metal organic chemical vapor deposition (MOCVD, AIXTRON 2000HT) system. This epitaxial structure sample contains a series of components including: a 1-µm undoped GaN layer, a 3.5-µm Si-doped GaN layer, a 10-pair of In0.03Ga0.97N (3 nm)/GaN (3 nm) n-type superlattice (SL) insertion layer, a 1.5-nm InGaN QD layer, a 20-nm undoped GaN barrier, a 20-nm Mg-doped Al0.2Ga0.8N electron blocking layer (EBL), and finally a 150-nm Mg-doped GaN contact layer [28]. The core InGaN QD layer is obtained by a two-step growth interruption method which has been reported in our pervious articles [29,30]. Figure 1(a) shows the pattern of QD micro-LED which is grown after the material epitaxy and the active area with 75 µm is formed by wet etching. Then the indium tin oxide (ITO) is placed on the surface of the QD micro-LED to form an ohmic contact layer. Subsequently, the mesa is passivated by SiO2 layer to lower leakage current by dry etching. Finally, a ring-shaped electrode is designed to link to the ITO layer. After forming the QD micro-LED array, we need to cut them off individually and encapsulate them to enable the implementation for VLC applications. Figure 1(b) is the micrograph of the packaged QD micro-LED with a surface pattern same as that shown in Fig. 1(a). Two metal pins are formed as the anode and cathode. Figure 1(c) is the photograph of the packaged QD micro-LED and its size is very compact compared to a coin. Figure 1(d) is the transmitter of our proposed QD micro-LED based VLC system which includes a driving printed circuit board (PCB), a 3-dimension (3D) tunable stage, a Tx. lens, a bias-tee and a pre-amplifier.
Fig. 1. (a) Schematic diagram and (b) the top view micrograph of the QD micro-LED. (c) Its packaged diagram compared with a coin and (d) the lighting image installed on the 3D tunable stage as a transmitter.
2.2 VLC system and QAM-OFDMA signal processing
We have set up a 3-meter experimental QAM-OFDMA VLC system utilizing the packaged QD micro-LED in a typical indoor environment. Figure 2 shows the schematic diagram of the multi-user QD micro-LED based QAM-OFDMA VLC system with a transmission distance of 3 meters. The experimental setup consists of two main parts including the real-time light propagation link based on optical devices and the off-line data processing module using MATLAB. In the proposed schematic, the QAM-OFDMA VLC system can support multi-user input and multi-user output. Here, in order to simplify the demonstration process, we only adopt two users as a typical scenario. To simulate the information transmitted for the two different users, two pseudo-random-binary-sequences (PRBSs) are generated independently first, and then mapped into QAM format. Then the serial-to-parallel (S/P) conversion is performed before the mapped QAM signal is further modulated into orthogonal frequency division multiplexing (OFDM) format. Considering of both the limited system bandwidth and the performance of two users, different sub-carrier allocation strategies, which would be discussed in the next session, are realized and evaluated for better performance. The 256 points inverse fast Fourier transform (IFFT) is performed to operate OFDM modulation, where Hermitian symmetry is indispensable for generating the real signal. Then by discarding the direct current (DC) components, the QAM data for both of user are totally loaded on 120 effective sub-carriers. Subsequently, all 120 independent subcarriers will be distributed to two different users at a given rate which will be described in detail in the following section. To decrease inter-symbol interference (ISI), cyclic prefixes (CP) are inserted and the obtained OFDM signal is transformed into serial format after parallel-to-serial (P/S) conversion. At the final step for data generation in MATLAB, synchronizing sequence is added to the QAM-OFDMA serial signal.
The complete discrete QAM-OFDMA signal is up-sampled and sent to the arbitrary waveform generator (AWG, AWG7000A, Tektronix) to generate continuous output waveform. Amplified by an amplifier (ZX60-43+, Mini-circuit) first, then the output signal is superposed with DC component provided by a bias-tee (ZFBT-6GW+, Mini-circuit) under 5.5 V bias voltage. Then the corresponding real positive signal is sent to drive the packaged QD micro-LED. To improve the directivity of the emitted beam, convex lenses are utilized at both transmitter and receiver sides. The receiving lens focuses the beam right on the center of active area of a high-sensitivity silicon avalanche photodiode (APD) module (APD210, Menlo Systems) with 1 GHz modulation bandwidth located at 3 m away from the light source, which converts the optical signal into electrical signal. The received electrical signal flows through an amplifier (ZX60-43+, Mini-circuit) for loss compensation before being captured by a real-time oscilloscope (OSC, DPO75902SX, Tektronix). Afterwards, digital signal processing (DSP) for the collected data from oscilloscope is performed to recover the transmitted binary data for the two users. Down-sampling and synchronization operation extract the effective received data first, followed by S/P conversion. Subsequently after removing the CP, the fast Fourier transform (FFT) realizes the OFDM demodulation and obtains frequency domain signal. The received signals for the two users are then extracted from corresponding sub-carriers after channel estimation and equalization. Eventually, the binary sequences are recovered from QAM de-mapping and compared with the original transmitted bits streams to evaluate corresponding BER of the two users.
2.3 OFDMA sub-carrier and bandwidth allocation schemes
OFDMA method enables multiple users to occupy independent orthogonal sub-carriers within the limited system bandwidth. In our proposed OFDMA VLC system, a user is exclusively represented via a sequence of orthogonal sub-carriers in the frequency domain. Properly designed sub-carrier allocation strategy is significant to take a full advantage of whole effective bandwidth and make a compromise between users. Sub-carrier gain follows an uneven distribution, where high-frequency sub-carriers always suffer from more channel loss and interference, owing to the limit of bandwidth of devices. In our experiment, the whole bandwidth with total 120 data sub-carriers are divided into serval parts according to the following strategies. Five different sub-carrier allocation strategies for two users are proposed as shown in the Fig. 3(a), of which the BER performance is evaluated under different sampling rates. The ratios of sub-carrier numbers assigned to user 1 and users 2 are set as 1:9, 1:4, 3:7, 2:3 and 1:1, respectively, to evaluate the effects of bandwidth allocation on the BER performance. Then, we increase the number of users while equally assigning sub-carriers in the limited bandwidth resource. The same method is adopted to evaluate the communication performance of each individual user as shown in the Fig. 3(b). This simple allocation method of dividing different users according to the order of subcarriers can generate independent channels of various data rates between the transmitter and the receiver of different users. It is very beneficial for Internet of Thing (IoT) applications of different data rate with a lot of various sensors.
Fig. 3. Diagrams of the bandwidth allocation strategies for (a) 2 users with different frequency-band ratios and (b) more users with a same ratio.
As an intensity modulation and direct detection (IM/DD) VLC system with incoherent light source, the OFDMA signals are real and unipolar obtained by the mentioned Hermitian symmetry. In our proposed system, a total ${N_{FFT}} = 256$ sub-carries are equally distributed in the modulation bandwidth of $[{{{ - 1} \mathord{\left/ {\vphantom {{ - 1} {2{T_s},{1 \mathord{\left/ {\vphantom {1 {2{T_s}}}} \right.} {2{T_s}}}}}} \right.} {2{T_s},{1 \mathord{\left/ {\vphantom {1 {2{T_s}}}} \right.} {2{T_s}}}}}} ]$ where ${T_S}$ is the sample period in a specific time domain. However, considering frequency domain gain, we clip a part of the sub-carriers, which actually will decrease the total data rate. The spectral efficiency ${\eta _{use{r_i}}}$ of each useri can be given by the following formula:
3. Results and discussions
The L-J-V characteristics and emission spectra of the packaged QD micro-LED are shown in Figs. 4(a) and (b), and the results in Fig. 4(a) attest a linear relationship between the optical power and the current. The peak emission wavelengths in Fig. 4(b) are centralized at 463 nm under different driving currents from 10 mA to 50 mA, which illustrates the wavelength stability of the QD micro-LED. The EL emission spectrum of the QD micro-LED actually includes the shoulder peak of the QDs wetting layer near 420 nm and the main peak near 460 nm. Compared with typical single QW micro-LEDs, our QD micro-LED operates at a higher voltage which enables a large modulation depth of more than 4 V in this work. As aforementioned, a QD micro-LED was fabricated and packaged first, then utilized in our experimental QAM-OFDMA based VLC system with a transmission distance of 3 meters. A vector network analyzer (VNA, N5227A, Agilent) is connected across the transceiver to measure the modulation bandwidth of the designed VLC system with 0 dBm input signal power. Correspondingly, the obtained normalized responses of the system under different driving currents are plotted in Fig. 4(c). From the figure, we can observe the value of -3 dB bandwidth exceeds 1.06 GHz with some particular driving currents, which indicates an excellent modulation performance of our implemented QD micro-LED. Furthermore, the extracted bandwidths from Fig. 4 (c) versus driving current are further drawn in Fig. 4(d). Under normal circumstances, the modulation bandwidth of the QD micro-LED will rise as the drive current increases. However, the VLC system remains unchanged or even drops as shown in the Fig. 4(d) which is limited by the 1 GHz modulation bandwidth of the APD module. With the increase of the driving current, the value of modulation bandwidth ascends sharply first and then stabilizes at around 1 GHz, where a maximum bandwidth of 1.06 GHz is achieved at a voltage of 5 V.
Fig. 4. (a) The L-J-V characteristics of the QD micro-LEDs. Inset: The L-V characteristic. (b) The spectra at driving currents from 10 mA to 50 mA. (c) Normalized frequency responses of the QD micro-LED system at driving currents from 2.16 mA to 44.01 mA. (d) Summarized -3 dB modulation bandwidth values versus driving current.
In our experiment, the static point of the QD micro-LED is set at a voltage of 5.5 V, corresponding to a current density of 178.82 A/cm2 and an emitted optical power of 0.39 mW. The QAM-OFDMA signal containing the information intended for different users are transmitted under various data rates. The SNR of the independent 120 sub-carriers with different baud rates are illustrated as shown in the Fig. 5(a). It is obvious that the sub-carriers in the low-frequency band obtain higher gain than that in the high-frequency band at a same baud rate. As the baud rate increases, the SNR decreases faster with the ascending of the sub-carrier index. The SNR and different sub-carrier allocation strategies of the OFDMA VLC system have a decisive influence on communication performance of specific users. The BER curves versus data rate for the two-user scenarios with five different sub-carrier allocation ratios, i.e. 1:9, 1:4, 3:7, 2:3 and 1:1, are plotted from Fig. 5(b) to Fig. 5(f), where user 1 and user 2 are distributed at low-frequency and high-frequency segment of the existing bandwidth, respectively. With different spectrum efficiencies, users actually obtain inconsistent data rates after calculation and sub-carrier clipping even at the same baud rate. User 1 outperforms user 2 because of the good channel characteristic at low-frequency band in Fig. 5(a). Consequently, user 1 is always decoded successfully with 0 errors, i.e. BERs stabilize at 0, for all measured baud rates while the BERs of user 2 change remarkably. Furthermore, the constellation diagrams of user 2 at different data rates are embedded in Fig. 5 correspondingly, which are gradually getting blurred as the data rate increases. For user 2 under different scenarios, the highest rate and corresponding BER are provided in the Table 1. With the decrease of occupancy ratio of the user 2 from 9/10 to 1/2, the highest data rate decline from 1.35 Gbps to 0.75 Gbps with the corresponding BERs of 2.1×10−3 and 3.6×10−3 satisfying the FEC criterion of 3.8×10−3, respectively. Hence, the sum data rate for our two-user can reach 1.50 Gbps.
Fig. 5. (a) Measured SNR of 120 sub-carriers index with different baud rates. The BER performance of 2 users (User 1: low-frequency user, User 2: high-frequency user) measured under different frequency-band ratios as (b) 1:9; (c) 1:4; (d) 3:7; (e) 2:3; (f) 1:1. (Inset: the constellation diagrams of user 2 at various data rates.)
Table 1. Communication performance of user 2 with different frequency-band allocation schemes.
In order to further observe the communication performance of the QD micro-LED based OFDMA VLC system while supporting more users, we increase the number of users from 3 to 6, and each user occupies a same ratio of bandwidth as shown in the Fig. 3(b). Limited by an approximate 1 GHz modulation bandwidth of the VLC system, the BERs of four allocation strategies under a certain baud rate show a similar tendency. Obviously, the users at high-frequency band gradually degrade with the data rate increases as shown in Fig. 6. The corresponding constellation diagrams of high-frequency user are inserted at certain data rates. The achievable highest data rates of users under different allocation strategies are given in Table 2, in which the BER values all satisfy the FEC limitation. For the fairness strategy of three users, user 1 and 2 can achieve a data rate of 781 Mbps with the BER of 0 and 7.5×10−4 while user 3 can obtain a data rate of 468 Mbps with the BER of 6.2×10−4. The increase of users directly reduces the number of subcarriers obtained by each user, which has been reduced from 60 to 20 with the number of equally supported users rising from 2 to 6. For the case of 6 users, user 1, 2, 3 and 4 can achieve a data rate of 400 Mbps with the BER gradually increasing from 0 to 3.8×10−3, while users 5 and 6 can only obtain a data rate of 250 Mbps with the BER of 2.6×10−3 and 2.9×10−3. Obviously, the sum-rates for all designed multiple-user system reach 1.41 Gbps. The number of subcarriers allocated to each independent user, the highest data rate and its corresponding BER are shown in Table 2. The obtained results show the potential of QD micro-LED based OFDMA VLC system to realize a high-speed communication system for multiple users, which will especially benefit future IoT applications.
Fig. 6. The BER performance of multi-user with equal band ratio: (a) 3 users; (b) 4 users; (c) 5 users; (d) 6 users. (Inset: the constellation diagrams of the user distributed at high-frequency band at various data rates.)
Table 2. Communication performance of multiple users with equal frequency-band allocation scheme
4. Conclusions
Supporting multiple users simultaneously in a bandwidth-limited VLC system is a problem urgently needed to be solved. In this work, based on a 75-µm single layer quantum dot blue micro-LED, a multi-user high-speed OFDMA VLC system with QAM modulation scheme has been firstly proposed and experimentally demonstrated. The packaged single layer QD micro-LED based VLC system has ultra-high modulation bandwidth of 1 GHz at a fairly low current density 178.8 A/cm2. Then we accomplished high-speed data transmission of multiple users over 3-m distance with various sub-carrier allocation strategies. Five schemes with different allocation ratios have been evaluated under varying baud rates for two-user scenario, and a sum-rate of 1.50 Gbps was achieved. For the case of equal bandwidth, 1.2 Gbps and 750 Mbps for two users are achieved corresponding to BER of 0 and 3.6×10−3, respectively. In addition, we have extended the allocation strategy with more users while keeping a fair frequency-band assignment among users. All strategies have achieved the sum-rate of 1.41 Gbps for multi-user scenarios with the BERs below the FEC criteria. The user occupied high-frequency band always suffers more from channel loss than the others due to the limited bandwidth. To make a tradeoff between users, more sub-carriers should be allocated to the high-frequency user, which corresponds to the strategy with greater allocation ratio. The experimental results show the potential of QD micro-LED based OFDMA VLC system to afford multiple users with a limited bandwidth, and further certify the feasibility of providing high-speed communication in a crowded indoor environment.
Funding
National Key Research and Development Program of China (2016YFB0401803); Science, Technology and Innovation Commission of Shenzhen Municipality (JCYJ20170818094001391, JCYJ20180507183815699, KQJSCX20170727163424873); Tsinghua-Berkeley Shenzhen Institute (TBSI) Faculty Start-up Fund; Shenzhen Data Science and Information Technology Engineering Laboratory; Shenzhen Fundamental Research and Discipline Layout project (JCYJ20170817161720819); Overseas Research Cooperation Fund of Tsinghua Shenzhen International Graduate School (HW2018003).
Disclosures
The authors declare no conflicts of interest.
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