Underwater Visible Light Communications Based on Spatial Diversity Yunfeng Wei(1, 2), Bangjiang Lin(1, 2), Xuan Tang(2), Yiwei Li(2), Min Zhang(2), ZabihGhassemlooy(3), Yi Wu(1)and Hui Li(1) (1)
Key Laboratory of OptoElectronic Science and Technology for Medicine of Ministry of Education, Fujian Normal University, China,
[email protected] (2) Quanzhou Institute of Equipment Manufacturing, Haixi Institutes, Chinese Academy of Sciences, Quanzhou, China,
[email protected] (3) Optical Communications Research Group, Faculty of Engineering and Environment, Northumbria University, Newcastle, U.K. ABSTRACT This paper investigates the performance of underwater visible light communications (UVLC) systems employing multiple transmitters and/or multiple receivers to exploit the advantage of spatial diversity. Experimental results are further provided to evaluate the error performance of single input multiple outputs (SIMO), multiple inputs single output (MISO) and multiple inputs multiple outputs (MIMO) UVLC systems when orthogonal frequency division multiplexing (OFDM) modulation is utilized. Our results reveal that the proposed system configuration can indeed offer significant system performance enhancements in terms of the attainable bit error rate. Keywords: Visible Light Communications, Spatial Diversity, Orthogonal Frequency Division Multiplexing 1. INTRODUCTION Underwater applications such as undersea exploration and autonomous underwater vehicle (AUV) remote control have drawn global attention due to the dependence of human life on the ocean. Many efforts have been made in recent decades in finding a reliable, efficient and cost effective communication method [1, 2]. Compared with acoustic communication, visible light communications (VLC) provides a much higher bandwidth and lower cost, which is more suitable to realize high speed transmission. So far, several research groups have investigated and demonstrated data transmission using Light Emitting Diode (LED) source for underwater environments [3-5]. Unlike indoor or atmospheric scenarios, underwater visible light communications (UVLC) faces several challenges, two of which are the exponential water attenuation coefficient and the bandwidth limitation of the used LED sources. Researchers have pushed the bandwidth from several to hundreds of megahertz by signal equalizing circuits and high-order modulation format [6-8]. Moreover, the VLC data rate can be boosted to over Gbit/s by advanced modulation format such as
orthogonal frequency division multiplexing (OFDM) and OFDM/offset quadrature amplitude modulation (OQAM) [9, 10]. It is well known that spatial diversity, i.e. the employments of multiple apertures at the transmitter (Tx) and/or multiple receivers (Rxs), can provide significant performance enhancements for optical wireless communication systems [11-14]. The possibility for temporal blockage of the visible light beams by obstructions is further reduced and longer distances can be covered through heavier weather conditions. In this paper, the spatial diversity schemes including single input multiple outputs (SIMO), multiple inputs single output (MISO) and multiple inputs multiple outputs (MIMO) are proposed to improve the transmission performance of UVLC systems. The experimental results reveal that the proposed system configuration can indeed offer significant system performance enhancements in terms of the attainable bit error rate (BER). Compared with single input single output (SISO), the increased transmission bit rates at a BER of 10-3 are about 3 times and 5 times for the SIMO and MIMO schemes, respectively. Compared with the SISO UVLC system at a bit rate of 9 Mb/s, the increased transmission spans are 54 cm and 44 cm for SIMO and MISO, respectively. 2. EXPERIMENT SETUP AND RESULTS In the underwater channel, underwater optical turbulence (UOT) and multiple scattering will cause deep fading of received signals and communication outage. To mitigate deep fading, we propose the SIMO, MISO and MIMO schemes for UVLC systems. The intensity modulation and direct detection (IM/DD) based UVLC transmission systems equipped with multiple inputs and/or multiple outputs are shown in Figs.1-3. The experimental setup for SIMO UVLC system with OFDM modulation is shown in Fig.1. At the Tx, the OFDM signal with quadrature phase shift keying (QPSK) mapping is generated in the MATLAB domain, which is then uploaded onto an arbitrary waveform generator (AWG). The output of AWG (i.e.,
electrical OFDM signal) is converted into an analog stream and then DC-level shifted prior to IM of a commercially available blue LED (Epileds). As shown in Fig.4, the 3 dB modulation bandwidth of the LED is about 4.3 MHz. At the Rx, the optical signals are detected by two commercial photo detectors (THORLABS PDA36A), respectively. The distance of the two photo detectors is 20 cm. The outputs of the two detectors are converted into two digital streams by two analog digital converters (ADCs), respectively. Then a real-time digital oscilloscope is used to capture the two waveforms for offline signal processing. The two digital OFDM signals are combined directly and then decoded to recover the transmitted signal. The MISO UVLC system is shown in Fig.2, which includes one Rx and two Txs with a distance of 15 cm. The structure of the Txs is similar with that in Fig.1. The transmitted data is the same. At the Rx, the two optical signals are captured by a photo detector and decoded offline.
The BER performance against the transmission bitrate for SISO, SIMO and MIMO UVLC systems is shown in Fig.5, in which a lens is placed before the detector to concentrate the optical signal. The BER is calculated based on more than 1×106 bits. As shown in Fig.5, SIMO and MIMO systems offer improved BER performance compared with SISO system, due to its utilization of spatial diversity. In our experiments, the transmission span is 5 m, and the transmission bit rates are 10.42 Mb/s, 27.78 Mb/s and 55.56 Mb/s at a BER of 10-3 for SISO, SIMO and MIMO UVLC systems, respectively. As the number of Tx or Rx increases, the transmission bit rate at a BER of 10-3 is significantly enhanced. Therefore, for a given transmission span, it improves BER performance compared with the SISO UVLC system for both SIMO and MIMO UVLC systems. Compared with SISO UVLC, the increased transmission bit rates at a BER of 10-3 are about 3 times and 5 times for SIMO and MIMO schemes, respectively.
Fig.1. The scheme of SIMO UVLC system.
Fig.2. The scheme of MISO UVLC system.
Fig.4. The bandwidth of the blue LED used in our experiment.
TABLE I. SYSTEM PARAMETERS
Fig.3. The scheme of MIMO UVLC system.
The experimental setup for MIMO UVLC system with OFDM modulation is shown in Fig.3. At the Tx, the two OFDM signals with QPSK mapping are generated in the MATLAB domain then converted into two analog streams by an AWG simultaneously. The outputs of AWG (i.e., electrical OFDM signals) combined with direct currents (DCs) are used for IM of two commercially available blue LEDs, respectively. At the Rx, the two optical signals are detected by two commercial photo detectors (THORLABS PDA36A), respectively. The outputs of the two detectors are converted into two digital streams by two analog digital converters (ADCs), respectively. Then a real-time digital oscilloscope is used to capture the two waveforms for offline signal processing. The interference of multiple Txs can be removed using MIMO algorithm. All the key systems parameters are provided in TABLE I.
LED Bandwidth Semi-angle of half power Transmit power PIN photodetector Active area Ar Responsivity R Bandwidth OFDM modulation No. of SC CP length Field of view of Rx DC bias Distance of two Txs Distance of two Rxs
