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Abstract
We demonstrate a non-Hermitian symmetry orthogonal frequency division multiplexing (NHS OFDM), known as serial-complex-valued (SCV)-OFDM for the white-light phosphor-based light-emitting-diode (LED) visible light communication (VLC) system. The white-light LED system can provide both VLC and lighting simultaneously. The proposed scheme is based on the fast-Fourier-transform (FFT)/invert-FFT (IFFT) size efficient technique proposed before. The real (Re) and imaginary (Im) parts of the complex-valued OFDM signals are extracted and time-serially multiplexed to achieve NHS. The proposed scheme is simple and only needs a single LED transmitter (Tx) and one receiver (Rx). The encoding and decoding of the proposed SCV-OFDM are also discussed.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Visible light communication (VLC) has attracted much attention recently because of its transmission advantages, such as simple and low installation cost, high security, license-free and electro-magnetic interference (EMI) free [1–7]. VLC using phosphor based white-light light emitting diode (LED) can offer both lighting and communication simultaneously. Intensity-modulation/direct-detection (IM/DD) is usually used in VLC detection since LED emits incoherent and real-valued optical signal. One challenges of the LED based VLC is the small modulation bandwidth [1]. Different techniques were proposed to increase the VLC data rates, such as using pre- or post-equalization circuits [4,5], multiple-input and multiple-output (MIMO) [6,7], or spectral efficient modulation format [8]. Due to the high spectral efficiency and resilient to inter symbol interference (ISI), orthogonal frequency division multiplexing (OFDM) is a popular modulation format for the VLC. In OFDM based VLC, Hermitian symmetry (HS) is adopted before the inverse fast Fourier transform (IFFT) to produce real-valued signals suitable for the LED modulation. However the need of HS doubles the sizes of IFFT and FFT during the encoding and decoding of OFDM; hence increasing the transceiver (TRx) complexity. Several schemes known as non-Hermitian symmetry (NHS) are proposed to mitigate the HS requirement, such as using fast Hartley transform (FHT) [9]. However, only real constellation signals, such as M-ary pulse amplitude modulation (M-PAM) or binary phase shift keying (BPSK) formats can be deployed. By separately transmitting the real (Re) and imaginary (Im) parts of the complex-valued OFDM using red/green/blue (RGB) LEDs could achieve NHS [10]; however this scheme requires multiple LEDs and is not suitable for white-light illumination. NHS can be achieved by sending the Re and Im parts of the complex-valued OFDM signal via a pair of white-light LEDs [11]; however, this scheme needs multiple LEDs and MIMO for demodulation. Barrami et al. proposes a FFT/IFFT size efficient technique to generate real-valued OFDM signals for IM/DD systems [12] using simulations. The FFT/IFFT size efficient OFDM technique has been applied to several laser-diode based systems, such as in a 1550 nm distributed feedback laser (DFB) based passive optical network (PON) [13], a 1550 nm direct modulated laser (DML) short reach optical transmission system [14], a 1560 nm DML optical system [15], and a 450 nm blue laser diode visible laser light communication (VLLC) system [16]. Besides, the FFT/IFFT size efficient OFDM has also been applied in a blue-LED VLC system together with orthogonal circulant matrix transform (OCT) pre-coding and pre-emphasis [17].
In this work, we demonstrate and apply the NHS OFDM scheme, known as serial-complex-valued (SCV)-OFDM for the white-light phosphor-based LED VLC system. By using phosphor-based white-light LED, the proposed scheme can provide both VLC and lighting simultaneously. The proposed scheme is based on the FFT/IFFT size efficient technique proposed in [12]. The Re and Im parts of the complex-valued OFDM signals are extracted and time-serially multiplexed to achieve NHS. The proposed scheme is simple and only needs a single LED transmitter (Tx) and one receiver (Rx). As reported in [12,13], the proposed scheme is theoretically and experimentally compared with the conventional HS based direct-current optical (DCO)-OFDM, and the results showed that they have similar BER performance. Here, experimental SCV-OFDM VLC is demonstrated illustrating that only a single white-light LED can achieve a free-space transmission distance of 140 cm with bit-error-ratio (BER) satisfying the 7% forward-error-correction (FEC) requirement (BER = 3.8 × 10−3).
2. Algorithms
The proposed SCV-OFDM is based on the FFT/IFFT size efficient technique proposed in [12]. By extracting the Re and Im parts of the complex-valued OFDM signals and time-serial multiplexing them, NHS can be achieved. Figure 1 shows the encoding and decoding block diagrams of the proposed SCV-OFDM. The data is first changed into a complex symbol by symbol mapping into quadrature amplitude modulation (QAM). These symbols are then launched into the IFFT to produce OFDM signal. The IFFT input (N) is used for the data constellation symbol. In the generation of the SCV-OFDM signal, the FFT size is 256. The DC and Nyquist subcarriers are set zero, and the effective subcarriers of the SCV-OFDM is 254. A complex-valued OFDM signal is produced at the IFFT output. This complex signal is time-serially separated as Re and Im sequences. In the real-time implementation, the Tx needs more buffers to construct the time-serial data. Then, a proper DC base is added to the SCV-OFDM signal via external bias-tee to modulate the LED. The symbol length of the proposed SCV-OFDM is 2N. At the Rx side, the SCV-OFDM will be converted back to its original complex-valued OFDM signal by combining the Re and Im parts for the input of the FFT as defined in Eq. (1). Then, the FFT module will convert them back to its constellation symbol. Finally, the QAM signal will be decoded, and the BER performance will be evaluated.
3. Evaluation
The experimental setup of the proposed SCV-OFDM white-light LED-based VLC system is shown in Fig. 2(a). The SCV-OFDM signal is created offline by using the MATLAB program. In the experiment, no cyclic prefix (CP) is used to provide high bandwidth utilization, and a training sequences (preamble) with length 128 is used for the channel estimation procedure. The VLC system only needs to be trained once if the distance between the Tx and Rx is fixed. An arbitrary waveform generator (AWG; Tektronix AFG3252C) is used to drive the phosphor-based white-light LED via a bias-tee. The AWG is set with a sampling rate of 12.5 MSample/s. After different free-space transmission distances from 60 to 160 cm, the optical signals are received by a PIN based photo-detector (PD) connected to a real-time oscilloscope (RTO; Tektronix MDO3024). The RTO is set with a sampling rate of 50 MSample/s, which is oversampled by 4 times of the transmitted signal. The data rate of SCV-OFDM signal is 12 Mbit/s; and it is limited by the bandwidth of the phosphor-based white-light LED which has a 3-dB modulation bandwidth of about 1.2 MHz [1]. Figure 2(b) shows the photograph during the experiment, and it is observed that the proposed system can provide practical transmission distance for both lighting and communication.
Fig. 2. (a) Experiment of the white-light LED based VLC system using SCV-OFDM. (b) Photograph during the experiment. AWG: arbitrary waveform generator, RTO: real-time oscilloscope.
4. Results and discussions
The SCV-OFDM modulation depth is adjusted by applying different peak-to-peak driving voltages to the LED. In all cases, the driving currents to the LED is limited to < 0.45 A, which is the damage threshold of the bias-tee circuit. Using smaller modulation depth will increase the BER since the PD will be saturated by the high level of un-modulated optical power; while using higher modulation depth will create OFDM signal clipping. As shown in Fig. 3(a), by applying 5 Vpp electrical SCV-OFDM signal to the LED, BER performance satisfying the FEC threshold at free-space transmission distance of 140 cm can be achieved. Figure 3(b) shows the BER performance against different transmission distances by adjusting the powers of the white-light LEDs. At the operation power of 1.2 W, the BER increases from 2 × 10−5 at transmission distance of 100 cm to 1.3 × 10−4 at transmission distance of 130 cm due to the decrease of received optical signal. At transmission distance of 140 cm, BER = 3.7 × 10−3 is achieved, satisfying the FEC threshold.
Fig. 3. BER performance of the SCV-OFDM at (a) different applied voltages to the white-light LED; and (b) different output powers of the white-light LED.
We also study different FFT/IFFT size effects of the SCV-OFDM signal by using numerical simulations. Figure 4 shows the simulated BER performance against different SNRs of the 128- and 256-FFT/IFFT SCV-OFDM signals respectively. Figures 5(a)–5(f) illustrate the corresponding simulated constellation diagrams of the SCV-OFDM signals at different FFT/IFFT sizes and SNRs. We can observe that the effect of different FFT/IFFT sizes are negligible. According to [12], longer symbol duration will have a higher peak-to-average power ratio (PAPR). However, based on our analysis shown in Fig. 4, the BER performance is similar by using SCV-OFDM with FFT sizes of 128 and 256.
Fig. 4. Simulated BER performance against different SNRs of the 128- and 256-FFT/IFFT SCV-OFDM signals.
Fig. 5. (a)-(f) Simulated constellation diagrams of the SCV-OFDM formats at different FFT/IFFT sizes and SNRs.
5. Conclusion
Phosphor-based white-light LED VLC can offer both lighting and communication simultaneously; and has attracted much attention recently. IM/DD is usually used in VLC detection since LED emits incoherent and real-valued optical signal. Due to the high spectral efficiency and resilient to ISI, OFDM is a popular modulation format for the VLC. In OFDM based VLC, HS is adopted before the IFFT to produce real-valued signals suitable for the LED modulation. However HS doubles the sizes of IFFT and FFT. Here, we experimentally demonstrated a NHS OFDM scheme, known as SCV-OFDM. Experimental SCV-OFDM VLC was demonstrated illustrating that only a 1.2 W white-light LED can achieve a free-space transmission distance of 140 cm satisfying the FEC threshold. The encoding and decoding of the proposed SCV-OFDM were discussed.
Funding
Ministry of Science and Technology, Taiwan (MOST-107-2221-E-009-118-MY3, MOST-108-2218-E-009 -031).
Disclosures
The authors declare no conflicts of interest.
References
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