We propose an asymmetric 3×2 multi-input multi-output (MIMO) system for polarization division multiplexing (PDM) transmission in visible light communication (VLC). In PDM transmission, independent channels are constructed by polarization orthogonality, which is vulnerable to misalignment of polarization between the transmitter and the receiver. Although PDM-based VLC system may not maintain polarization orthogonality, the proposed asymmetric MIMO system can achieve the polarization diversity required for a multichannel system. We experimentally demonstrate the performance enhancement of PDM VLC transmission using the proposed asymmetric 3×2 MIMO technique.
A modulation format of differential pulse amplitude modulation in visible light communication using a single transmitter module composed of multiple semiconductor light‐emitting diode (LED) chips is proposed and experimentally demonstrated. Precoded different nonreturn‐zero on–off‐keying signals were assigned to each LED chip, and linearly overlapped signals were detected by a single photodiode. In this demonstration, pre‐equalization and optical filtering were applied to compensate the modulation bandwidth limitation and nonlinearity of the light source. Data rate up to 200 Mbit/s with less than 7.8×10−5 bit error rate was verified by experiment using the proposed modulation method.
We experimentally demonstrated a gigabit visible light communication system employing polarization division multiplexing and adaptive modulation optical-discrete multitones by using a phosphor-based white light-emitting diode. An optimized pre-equalization circuit was used to prevent clipping of the baseband signal, which exceeded the linear operating range of the LED, and to control the power distribution of each subcarrier in order to obtain an efficient bit-loading profile. Using this technique, we achieved 1.025 and 1.016 Gbps transmissions for each polarization channel and a total of 2.041 Gbps transmissions were experimentally verified. In each transmission, bit error rate performances were below the forward error correction limit.
Utilizing the modulation capability of LEDs, there have been many studies about convergence technology to combine illumination and communication. The visible light communication (VLC) system has several advantages such as high security, immunity to RF interference and lower additional cost than comparing to LEDs just for illumination. However, modulation bandwidth of LEDs is not enough for various wireless communication systems. Since the commercial LEDs are designed only for lighting systems; we need an effort to enhance the modulation characteristics of LEDs. When the area of LED is increased, internal junction capacitance of LED is also increased depending on the area of LEDs and then the RC delay time of LED is increased. As a result, the modulation bandwidth of LEDs is limited by large RC delay time. In addition, frequency response of commercial white LED is degraded by the slow response time of the used yellow phosphor. Thus, modulation bandwidth of VLC system is limited to several MHz which is not enough to accommodate high data rate transmission. In this paper, we designed equalization circuit using RLC component for compensating the white LEDs frequency response. Also, we used blue filtering to improve frequency response of white LEDs, which is degraded by yellow phosphorescent component. Power loss by optical filtering and distance is compensated by convex lens. Consequently, we extend the modulation bandwidth of VLC system from 3 MHz to more than 180 MHz, and it allows NRZ-OOK data transmission up to 400 Mbps at 50 cm.
White light-emitting diodes (LEDs) are widely used for lighting due to their energy efficiency, eco-friendly, and small size than previously light sources such as incandescent, fluorescent bulbs and so on. Optical wireless visible light communication (OWVLC) based on LED merges lighting and communications in applications such as indoor lighting, traffic signals, vehicles, and underwater communications because LED can be easily modulated. However, physical bandwidth of LED is limited about several MHz by slow time constant of the phosphor and characteristics of device. Therefore, using the simplest modulation format which is non-return-zero on-off-keying (NRZ-OOK), the data rate reaches only to dozens Mbit/s. Thus, to improve the transmission capacity, optical filtering and pre-, post-equalizer are adapted. Also, high-speed wireless connectivity is implemented using spectrally efficient modulation methods: orthogonal frequency division multiplexing (OFDM) or discrete multi-tone (DMT). However, these modulation methods need additional digital signal processing such as FFT and IFFT, thus complexity of transmitter and receiver is increasing. To reduce the complexity of transmitter and receiver, we proposed a novel modulation scheme which is named differential pulse amplitude modulation. The proposed modulation scheme transmits different NRZ-OOK signals with same amplitude and unit time delay using each LED chip, respectively. The ‘N’ parallel signals from LEDs are overlapped and directly detected at optical receiver. Received signal is demodulated by power difference between unit time slots. The proposed scheme can overcome the bandwidth limitation of LEDs and data rate can be improved according to number of LEDs without complex digital signal processing.
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