Kong Meiwei, Chen Yifei, Sarwar Rohail, Sun Bin, Xu Zhiwei, Han Jun, Chen Jiawang, Qin Huawei, Xu Jing
Opt Express. 2018 Feb 5;26(3):3087-3097. doi: 10.1364/OE.26.003087.
In this work, we propose an underwater wireless optical communication (UWOC) system using an arrayed transmitter/receiver and optical superimposition-based pulse amplitude modulation with 4 levels (PAM-4). At the transmitter side, we design a spatial summing scheme using a light emitting diode (LED) array, which is divided into two groups in a uniformly interleaved manner. With on-off keying (OOK) modulation for each group, optical superimposition-based PAM-4 can be realized. It has enhanced tolerance to the modulation nonlinearities of LEDs. We numerically investigate the feasibility of the proposed spatial summing scheme in various underwater channels via Monte Carlo simulation. With the increase of divergence angle of LEDs and link distance, the optical power distribution tends to be more uniform at the reception plane. It can significantly relax the requirement on the link alignment. Furthermore, we conduct a proof-of-concept experiment employing two blue LEDs. A multi-pixel photon counter (MPPC), containing an array of single-photon avalanche diodes (SPADs), is used as the detector. It has a much higher sensitivity and can further relax the requirement for pointing. Over a 2-m tap water channel, data rates of 6.144 Mb/s, 8.192 Mb/s, and 12.288 Mb/s were achieved by using the PAM-4 signal generated by optical superimposition, within a 2.5-MHz system bandwidth. With 0.570-mg/L Mg(OH), the measured optical power is just 12.890 µW after a 2-m underwater channel. The corresponding bit error rate (BER) of the 12.288-Mbs PAM-4 signal is 2.9 × 10, which is still below the forward error correction (FEC) limit of 3.8 × 10. It implies that the UWOC system based on the high-sensitivity MPPC with array structure has superior power efficiency and robustness.
在这项工作中，我们提出了一种水下无线光通信（UWOC）系统，该系统使用阵列式发射机/接收机以及基于光叠加的四电平脉冲幅度调制（PAM-4）。在发射端，我们设计了一种使用发光二极管（LED）阵列的空间求和方案，该阵列以均匀交错的方式分为两组。通过对每组进行开关键控（OOK）调制，可以实现基于光叠加的PAM-4。它增强了对LED调制非线性的容忍度。我们通过蒙特卡罗模拟数值研究了所提出的空间求和方案在各种水下信道中的可行性。随着LED发散角和链路距离的增加，接收平面上的光功率分布趋于更加均匀。这可以显著放宽对链路对准的要求。此外，我们进行了一个概念验证实验，使用了两个蓝色LED。一个包含单光子雪崩二极管（SPAD）阵列的多像素光子计数器（MPPC）被用作探测器。它具有更高的灵敏度，并且可以进一步放宽对指向的要求。在2米长的自来水信道上，在2.5兆赫兹的系统带宽内，通过使用光叠加产生的PAM-4信号，实现了6.144兆比特每秒、8.192兆比特每秒和12.288兆比特每秒的数据速率。在含有0.570毫克/升氢氧化镁（Mg(OH)₂）的情况下，经过2米水下信道后，测得的光功率仅为12.890微瓦。12.288兆比特每秒的PAM-4信号对应的误码率（BER）为2.9×10⁻³，仍低于3.8×10⁻³的前向纠错（FEC）极限。这意味着基于具有阵列结构的高灵敏度MPPC的UWOC系统具有卓越的功率效率和鲁棒性。
