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实现深紫外光通信的一个关键器件是深紫外光源。早期深紫外光源利用高压汞灯实现,但汞灯的调制带宽非常小,这严重影响了深紫光通信的传输速率。最近几年,随着深紫外LED技术的不断发展[16-19],深紫外LED的高调制带宽、低功耗及设计灵活的特性使其在深紫外光通信领域受到越来越多的关注。尤其是随着高效率商用化的深紫外LED的出现,并且实现寿命大于1000 h。采用深紫外LED作为日盲紫外光通信的光源技术已经开始具备实际应用价值并快速成为当前研究热点。2002年,SET (Sensor Electronic Technology) 公司生产出了可以商用化的波长在247~365 nm之间的深紫外LED,而美国麻省理工大学林肯实验室首先在2004年进行了利用深紫外LED尝试完成紫外光通信的设计[20]。根据香农定理,通信中的数据传输速率由带宽和信噪比决定。为了增加传输速率,必须尽量增加光源带宽和提高信噪比。而光源的调制带宽直接受RC时间常数的制约[21-23],对器件电容(C)有很强的依赖性。C随着器件尺寸的减小而减小[24-25],因此实现快速调制的关键技术是缩小LED器件尺寸,特别是小于100 μm×100 μm,即微型发光二极管(μLED)。
目前,基于DUV μLED下进行的无线光通信的研究中,紫外通信的带宽和数据传输速率已经达到了前所未有的水平。2019年,He等将直径小于100 μm的深紫外μLED应用到深紫外通信系统里面,其用尺寸为566 μm2的芯片,系统带宽在71 A/cm2条件下达到438 MHZ[26]。Zhu等[27]阐述在基于276.8 nm的短波紫外线μLED的紫外通信系统中,电流密度为400 A/cm2的条件下,获得452.53 MHz的−3 dB带宽和0.854 mW的光输出功率。采用16位正交调幅正交频分复用和预均衡技术,实现长度在0.5 m以上,数据速率为2 Gb/s的紫外通信链路。此外,Maclure等[28]采用了一个由8个梯形像素组成的深紫外μLED阵列,每个像素的面积相当于直径约40 µm的圆形器件,调制带宽最大可达915 MHZ,并实现了在10 m处大于6 Gb/s和在17 m处大于4 Gb/s的数据速率,其进一步采用紫外μLED,且通过优化系统参数来提高光通信系统的性能[29],分别在10 m和60 m处实现6.5 Gb/s和4 Gb/s的数据速率,其调制带宽最大可达960 MHz。表1总结了DUV LED作为光源时,光通信中带宽和数据传输速率的发展情况。
表 1 LED作为光源时紫外通信的发展
Table 1. Development of UV communication with LED as a light source
Year Bandwidth/MHz Modulation scheme Distance/m Data rate Ref. 2022
2022
2021960
915
452OFDM
OFDM
OFDM10
17
0.56.50 Gb/s
>4 Gb/s
2 Gb/s[29]
[28]
[27]2020 170 PAM-16 1 2.4 Gb/s [30] 2020 170 PAM-16 5 1.09 Gb/s [31] 2019 438 OFDM 0.3 1.1 Gb/s [26] 2019 153 - 1.5 1.18 Gb/s [32] 2018 153 PAM-4 1.6 1.6 Gb/s [2] 2018 1.9 - 150 921.6 Kb/s [33] 2018 20 OOK - 1.92 Mb/s [34] 2017 29 OFDM - 71 Mb/s [35] 从通信系统的带宽和数据传输速率可以看出,DUV μLED的运用提高了带宽。同时,在同一调制方式下,带宽越大,能达到的最大数据传输速率就越高。此外,由于紫外光功率在大气中的快速衰减,当数据传输范围较远时,传输速率会下降。因此保证大的调制带宽的同时提高深紫外μLED的光输出功率,是DUV μLED光通信系统获得高速传播的关键。当深紫外LED尺寸变小时,会使调制带宽变大,但是其有源区面积的减少也导致相同电流密度下光功率急剧下降。而深紫外光在空气中传播很容易被吸收,这严重影响了特定距离的光信号信噪比。而为了解决这一矛盾点,可以通过对单颗芯片加大电流实现。但是深紫外LED较低的发光效率和严重的侧壁缺陷辅助的非辐射复合,导致大电流下存在严重的自发热效应,功率无法随着电流持续增加。因此,要实现高传播速度的深紫外μLED光通信,关键点是使深紫外μLED具有高调制带宽、发光效率好、散热能力好等特性。
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首先,尺寸会严重影响μLED的调制带宽。由于μLED比大尺寸LED的尺寸减少,且整体表面积比增加,因此μLED具有较低的电容、较好的散热效应和良好的电流扩展,所以μLED工作电流密度可以比大尺寸LED更高。表征LED调制带宽的−3 dB响应频率f−3 dB的关系式可表示为[36]:
$$ {f}_{-3\;{\rm{dB}}}=\frac{\sqrt{3}}{2\pi \tau }=\frac{\sqrt{3}}{2\pi }\left(\frac{1}{{\tau }_{r}}+\frac{1}{{\tau }_{nr}}+\frac{1}{{\tau }_{RC}}\right) $$ (1) 式中:τ为少数载流子的寿命;τr为辐射载流子的寿命;τnr为非辐射载流子的寿命;τRC为RC时间常数。对于尺寸为100 μm×100 μm及以下的LED,载流子辐射寿命是限制调制带宽的主要因素。在这种情况下,载流子辐射寿命取决于注入的电流密度,τr= 1/B×N,其中B和N分别表示辐射系数和载流子密度。因此,假设存在双分子重组机制时,f−3 dB符合如下公式[37]:
$$ {f}_{-3\;{\rm{dB}}}=\frac{\sqrt{3}}{2\pi }\times \left(\sqrt{\frac{BJ}{qd}}+\frac{1}{{\tau }_{nr}}+\frac{1}{{\tau }_{RC}}\right) $$ (2) 式中:J为注入电流密度;q为基本电荷;d为有源区厚度。因此,公式(2)表明,通过增加工作电流密度或减小芯片尺寸,可以实现更高的−3 dB带宽。
随着μLED尺寸减小,其芯片电容随之减小,同时其可工作在较高的电流密度下,从而缩短内部差分载流子寿命,因而小尺寸μLED获得更高的调制带宽[38]。Maclure等[39]对DUV μLED的尺寸依赖研究表明,器件的带宽有明显的尺寸依赖性,器件越小,带宽越高,20 μm器件的带宽达到570 MHz。Qian等[40]研究了基于整个通信系统的UVC LED的尺寸效应。他们发现随着电流的增大,μLED的−3 dB带宽不断增大。此外,小尺寸LED具有更高的调制带宽,并且随着电流密度的增加,LED调制带宽迅速增加。然后,随着电流的进一步增加,调制带宽趋于饱和甚至略有下降。带宽下降可能是热效应造成的,如载流子溢出,导致辐射复合效率下降,有效载流子寿命增加,从而降低调制带宽[28]。另外,提高深紫外LED的带宽还可以利用等离子激元[41-42]与微腔[43]等技术。Zhang等[42]通过将高密度的Al纳米三角阵列转移到μLED的指定p-AlGaN区域,实现了局域表面等离子体共振耦合的效果,以提高−3 dB的调制带宽。但由于μLED较小的有源区面积,等离子激元与微腔技术在小尺寸深紫外LED中应用很少,还需科研工作者的努力探索。
其次,μLED尺寸严重影响芯片的I-V特性曲线。随着芯片整体直径(D)减小从而导致串联电阻增加,串联电阻近似与D−2成正比[44]。即台面尺寸越小,串联电阻越大,即相同工作电流下,小尺寸的芯片工作电压急剧升高,如图2(a)所示。但是如果考虑电流密度的话,在相同工作电压下,小尺寸的器件电流密度更大,如图2(b)所示,这是由于器件整体面积减小的原因。另外 ,还需要注意到,随着器件尺寸减少,其电流扩展效应是更好的,这能使电流分布更均匀,从而使器件可以承受更大的电流密度,这将非常有利于提高调制带宽特性。
图 2 (a) I–V特性曲线的尺寸依赖性;(b)相同电压下电流密度的尺寸依赖性[44] ;(c) 有侧壁损伤的LED A、B、C的示意结构(LED A、B和C的可用面积比分别为85%、75%和36%[45])
Figure 2. (a) Size-dependent I–V characteristics curves; (b) Size dependence of current density under the same voltage[44]; (c) LEDs A, B, C with sidewall damages (The usable area ratios for LEDs A, B and C are 85%, 75% and 36%[45])
除此之外,LED器件的漏电流也表现出尺寸依赖性。由于较高的表面积体积比,台面尺寸较小的LED的漏电流往往较大,因为台面边缘的侧壁缺陷会作为漏电流通道[44]。随着芯片尺寸的减少,由于刻蚀台面而引起的侧壁缺陷所占有的整个芯片的面积将急剧增加,如图2(c)所示。因此,与大尺寸LED器件不同,由侧壁缺陷引起的表面非辐射复合是µLED的不可忽略的影响因素[45]。
第三,随着μLED的芯片尺寸减小,其光提取效率能得到有效提升。这是由于更小的芯片尺寸能使光更快逃离,从而减少被芯片内部吸收材料的吸收概率。Floyd等通过阴极发光线扫描实验测量器件的深紫外发射的横向吸收长度[46]。将阴极发光线扫描强度映射到扫描电子显微镜(SEM)测量的台面直径,可得到台面结构内的横向吸收长度约为15 μm。在尺寸越小的LED中,光子横向传输距离越短,从而光子抵达侧壁并逃逸之前被吸收的比例越小,从而改善了器件的LEE。另外,通过FDTD模拟也表明,对于有倾斜侧壁的深紫外LED,其尺寸越小,光提取效率越高,如图3所示。具有倾斜侧壁结构的LED尺寸的减小使更多横向传输的光子首先抵达到倾斜侧壁而不是AlGaN与蓝宝石的界面,从而使更多的光子尽快被倾斜侧壁反射到逃离锥里面,有效提高了光提取效率。
图 3 FDTD中模拟的TM偏振光的光提取效率与尺寸的关系
Figure 3. Relationship between light extraction efficiency and size of TM polarized light simulated in FDTD
第四,芯片尺寸对于光功率有非常重要的影响。如图4(a)所示,随着芯片尺寸面积的减小,在相同工作电流下,小尺寸器件整体的光功率比大尺寸器件要小。从前面分析可知,尽管尺寸减小,其光提取效率得到提升,但是由于小尺寸芯片好的电流扩展和高的侧壁缺陷面积比,导致侧壁缺陷引起的非辐射复合增加更大,从而使整体的光功率下降。但由于小尺寸器件面积更小,电流扩展效应更好的原因,它能承受更高的电流密度,有较高的光功率密度,如图4(b)所示。值得注意的是,最小的器件(D=20 μm)的最大功率密度可达86 W/cm2,比最大的器件(D=300 μm)的最大功率密度(4.14 W/cm2)高出20倍以上。图2(b)可以佐证较小台面尺寸的LED可以承受更高的电流密度,最小的LED可以承受的正向电流密度达到4500 A/cm2以上。而光提取效率的增加也会促使小尺寸的LED具有更高的光功率密度。2022年,中国科学院半导体研究所Guo等[47]研究了并联的深紫外μLED阵列结构的尺寸效应,其也发现了类似的现象。随着尺寸减少,在相同电流密度下,由于小尺寸μLED面积小,总电流更小并且非辐射复合更严重,导致小尺寸μLED阵列的光功率更小。但是小尺寸μLED的光功率密度是更大的,这是因为减小芯片尺寸可以减少光子抵达高吸收的电极,从而减少光逃离路径的损耗,增加了光提取效率,获得更高的光功率密度。
另外,从图4(a)和(b)可以看到,随着芯片尺寸减少,器件功率饱和电流减少,但是功率饱和电流密度却是增加的。功率饱和电流的减少,主要是由于芯片尺寸减少,串联电阻增加,在相同电流下其发热量增加。而功率饱和电流密度增加一方面是由于体表面积随着芯片尺寸增加,从而小尺寸芯片表面散热能力更强;另一方面是由于小尺寸芯片具有更好的光提取效率,导致更多光被提取,而不是被吸收转化成热,从而进一步减少了热的产生。因此可以看到,μLED尽管可以承受大的电流密度,但是却无法承受和常规LED一样大的电流,主要是由于严重的热效应使得器件的功率过早趋于饱和甚至下降[48-50],这限制了μLED利用大电流去进一步的提升光功率。而严重的热效应也会导致μLED发射波长红移和器件寿命减少[51]。
图 4 (a)不同台面尺寸的DUV LED的光输出功率作为注入电流的函数;(b)光功率密度的尺寸依赖性[44] ;(c)三种研究器件在不同电流下的光输出功率和DUV LED的LEE增强因子(d)[52]
Figure 4. (a) Light output power as a function of injection current for the DUV LEDs with different mesa sizes; (b) Size dependence of optical power density[44] ; (c) Light output power and (d) the LEE enhancement factor of the DUV LEDs at different currents for the three investigated devices[52]
综上所述,器件尺寸变小时可以有效改善电流拥挤增加带宽,提高器件的光提取效率,提升器件工作的最大电流密度和功率密度,但有源区面积的减小使得侧壁非辐射复合增加,串联电阻变大,自热效应严重,输出功率减小。当设计出合适的阵列形式的μLED时,相比较单个μLED的情况下可以弥补较低的输出光功率[52]。中国科学院半导体研究所利用4×4的阵列使光输出功率达到mW级,并实现了380 MHz的调制带宽[47]。而Yu等[53]设计并制造了一个由10×10个μLED组成的深紫外发光阵列,并对发光阵列的电学和光学行为进行了系统的研究。与具有相同发射面积的传统大型LED芯片相比,该阵列在100 mA的注入电流下,总光输出功率显著提高,DUV μLEDs阵列中单个LED尺寸变小时电流扩展效应更好,因此功率明显表现出随LED尺寸变小而变大的特性,如图4(c)所示。此外,图4(d)显示了通过μLED阵列与平面LED的EQE之比获得的LEE增强因子。μLED阵列(50 μm)和μLED (20 μm)的LEE分别是平面LED的1.3倍和1.5倍。这是由于尺寸的减小,使侧壁处的光反射增强以及光子的横向传播距离缩短。此外,μLED阵列的LEE增强因子在较高的注入电流水平下更大。这些结果表明,μLED阵列可以有效地提高DUV LED的LEE。
但是并联的深紫外μLED阵列结构明显是又增加了整体器件的电容,并不能完全发挥μLED高的调整带宽的特性。考虑到带宽的损失,以串联的形式制造微型DUV μLED,这可以在不降低带宽的情况下增加输出功率。Jin等[54]提出了一种具有高输出光功率的3×3紫色串联偏置μLED阵列。因此其器件光功率得到极大提高的同时保持着高的带宽,从而提高长距离可见光通信中的数据速率,其首次实现了使用μLED在10 m处超过9.5 Gbps的通信演示。但串联的DUV μLED阵列结构会急剧增加整个器件的电阻,从而增加芯片发热,使器件结温更高,不能最大化的输出功率。因此,最好的解决方法还是需要提高单个μLED芯片的效率,减少自热效应,增加散热能力,提高光功率。下面从增加光提取效率和改善热学特性这两个方面综述提高LED效率的方法途径。
Research progress of AlGaN-based DUV μLED (invited)
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摘要: 随着AlGaN基深紫外发光二极管(DUV LED)的发展,其不仅在杀菌消毒领域得到广泛应用,在日盲紫外光通信领域的应用也受到越来越多的关注。这主要是由于相比其他的紫外光源(如汞灯、激光),其具有功耗低、设计灵活且调制带宽高的优势。而DUV LED的带宽严重依赖于器件尺寸,器件尺寸越小,其带宽越高。但是,随着深紫外微型发光二极管(μLED)的尺寸减少,尽管其带宽得到提高,但是其光功率却急剧下降,这严重限制了深紫外μLED在光通信中的应用。文中主要总结了深紫外μLED作为日盲紫外光通信光源的研究现状和综合分析尺寸效应引起器件性能的变化及其机理;并分析出低的光提取效率和严重的自热效应是影响深紫外μLED光功率的两个主要因素。进而综述了各种提高深紫外μLED光提取效率和改善热学特性的方法。文中将为从事深紫外μLED研究的工作者提供一定的研究方向指导。
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关键词:
- AlGaN /
- 深紫外微型发光二极管 /
- 调制带宽 /
- 光提取效率
Abstract:Significance Recently, deep-ultraviolet (DUV) communications based on DUV micro-LED technology have drawn a significant interest. This is because deep-ultraviolet (DUV) communications possess a number of advantages such as the low back-ground noise, a non-line-of-sight (NLOS) link and high security. However, its development is constrained by the lack of light sources with high power and high modulation bandwidth. In recent years, the rapid advancement of low-cost, high-output power AlGaN-based DUV LEDs has greatly accelerated the development of UVC communication and its application in various fields. Moreover, the DUV μLEDs with small chip size have the advantages of high modulation speed and low power consumption, making them attractive for implementing high-speed UVC systems. However, the low luminous efficiency of AlGaN-based μLED seriously affects the data transmission rate in deep ultraviolet communication. Therefore, we provide a review and comprehensive analysis of the size effect on the optical, electrical, thermal and modulation properties for AlGaN-based μLED, including its underlying physics mechanism. In addition, we also review various approaches to improve the light extraction efficiency and thermal characteristics of DUV μLED, which is of great significance for the study of DUV μLED. Progress Firstly, the current research status of DUV μLED as a solar blind UV communication source is introduced. The performance for UV communication system utilizing LED as a light source is summarized (Tab.1). It can be seen that under the same modulation mode, larger bandwidth and higher data transmission rate can be achieved with DUV μLED. In addition, due to the rapid attenuation of ultraviolet light power in the atmosphere, the transmission rate decreases for long distance communication. Therefore, ensuring both a large modulation bandwidth and a high optical output power for DUV μLED are very crucial for the high-speed propagation of DUV μLED optical communication system. The optical and electrical properties of DUV μLED are significantly affected by its size. The smaller size of the μLED enable them to withstand a higher current density, while the capacitance decreases as the size decreases. Consequently, the μLED with smaller size exhibits a higher modulation bandwidth. However, the reduction of the active area results in a decrease in output power as the size decreases. Additionally, the severe self-heating effect induces a thermal droop in EQE, making it challenging to achieve high power with high work currents. The low light extraction efficiency (LEE) and increased series resistor further deteriorate the self-heating effect. Therefore, to break the bottle of the light output power of μLED, it is necessary to improve the LEE, the series resistor and heat dissipation. Various micro-nano structures for nAlGaN, pAlGaN and sapphire can be used as scatter centers to improve the LEE. The patterned pAlGaN exhibits the most significant effect in improving LEE due to its proximity to the active region. However, it generally brings in a higher work voltage. Increasing the ohmic contact area and only patterning the area around the p-electrode can avoid the disadvantage. In addition, the inclined sidewall technology shows a significant potential for enhancing the LEE of DUV LED. And the shape and the sidewall reflector for the inclined sidewall have a substantial influence on the LEE of DUV μLED. Furthermore, to mitigate the self-heating effects of the device, the ohmic contact resistivity of DUV μLED device should be decreased, and the reflectivity of electrode should be increased. Therefore, the designed electrode needs to possess excellent ohmic contact and high reflectivity. Meanwhile, the device heat dissipation can be improved by increasing the electrode contact area and the device side wall area. Various technologies, such as a rectangle chip shape and a metal radiator can be utilized to enhance the device heat dissipation of DUV μLED. Conclusions and Prospects This paper presents a systematically review of the research status of DUV μLED in the field of wireless optical communication. And the size effect on the modulation characteristics, light extraction efficiency, current and voltage characteristics, optical power characteristics and side wall defect ratio are comprehensive analyzed and its underlying physical mechanism is also shown. Various technologies for improving the efficiency of light extraction and heat dissipation are summarized and discussed in detail. Although a great progress have been made in the development of DUV μLED, further research should be dedicated to enhancing the LEE and heating dissipation of DUV μLED. Especially, the electrode and the chip shape need to be designed to ensure high reflectivity and excellent ohmic contact, high efficiency scatter and good heating dissipation. -
Key words:
- AlGaN /
- DUV μLED /
- modulation bandwidth /
- light extraction efficiency
-
图 2 (a) I–V特性曲线的尺寸依赖性;(b)相同电压下电流密度的尺寸依赖性[44] ;(c) 有侧壁损伤的LED A、B、C的示意结构(LED A、B和C的可用面积比分别为85%、75%和36%[45])
Figure 2. (a) Size-dependent I–V characteristics curves; (b) Size dependence of current density under the same voltage[44]; (c) LEDs A, B, C with sidewall damages (The usable area ratios for LEDs A, B and C are 85%, 75% and 36%[45])
图 4 (a)不同台面尺寸的DUV LED的光输出功率作为注入电流的函数;(b)光功率密度的尺寸依赖性[44] ;(c)三种研究器件在不同电流下的光输出功率和DUV LED的LEE增强因子(d)[52]
Figure 4. (a) Light output power as a function of injection current for the DUV LEDs with different mesa sizes; (b) Size dependence of optical power density[44] ; (c) Light output power and (d) the LEE enhancement factor of the DUV LEDs at different currents for the three investigated devices[52]
图 5 (a) 具有网状p-GaN的深紫外LED结构图;(b)具有网状p-GaN接触的DUV LED的LEE作为纳米棒间距的函数;(c) 具有网状p-GaN与截锥p-AlGaN接触的DUV LED的LEE与纳米棒间距的关系[59];(d)网状p-GaN/ITO DUV LED的示意图;(e) 两种DUV LED的L–I特性和I–V特性[60]
Figure 5. (a) Structure diagram of deep ultraviolet LED with meshed p-GaN; (b) LEEs for DUV LED with the meshed p-GaN contacts as a function of the spacing of nanorods; (c) Relationship between LEE and nanorod spacing of DUV LEDs with meshed p-GaN and truncated p-AlGaN contacts[59]; (d) Schematics of the mesh p-GaN/ITO DUV LED; (e) Measured L–I characteristics and I–V characteristics of the two kinds of DUV LEDs [60]
图 7 (a) 具有Ga面n-AlGaN纳米结构阵列的DUV LED结构图 ;(b) 传统器件(左)和表面纹理Ga面n-AlGaN器件(右)的TE偏振的XY截面电场分布[67]
Figure 7. (a) Structure diagram of DUV LED with n-AlGaN nanostructure array on Ga surface; (b) TE polarized XY cross section electric field distribution of traditional device (left) and surface finish Ga plane n-AlGaN device (right)[67]
图 8 (a) 参考DUV LED的示意图;(b) CSG DUV LED的示意图[74];(c) 具有倾斜侧壁和垂直侧壁的器件结构示意图和SEM图像;(d) 垂直和倾斜侧壁的20 µm直径的LED(实线)和40 µm直径LED (虚线)在不同电流密度下的输出功率密度[75]
Figure 8. Schematic of (a) the Ref. DUV LEDs and (b) CSG DUV LEDs [74]; (c) Schematic diagram and SEM image of device structure with inclined and vertical sidewalls; (d) Output power density under different current densities for 20 µm-diameter LEDs (solid lines) and 40 µm-diameter LEDs (dashed-dotted lines) with vertical and inclined sidewalls[75]
图 11 (a) MIS结构的DUV LED的示意图,即器件 A;(b) 器件 R的n-AlGaN层和n-电极金属的计算能带图;(c)器件 A的n-AlGaN层、绝缘体和n-电极金属的计算能带图[84]
Figure 11. (a) Schematic diagrams for MIS-structured DUV LED, i.e., Device A; Calculated energy band diagrams of (b) n-AlGaN layer and n-electrode metal for Device R; (c) n-AlGaN layer, insulator, and n-electrode metal for Device A[84]
表 1 LED作为光源时紫外通信的发展
Table 1. Development of UV communication with LED as a light source
Year Bandwidth/MHz Modulation scheme Distance/m Data rate Ref. 2022
2022
2021960
915
452OFDM
OFDM
OFDM10
17
0.56.50 Gb/s
>4 Gb/s
2 Gb/s[29]
[28]
[27]2020 170 PAM-16 1 2.4 Gb/s [30] 2020 170 PAM-16 5 1.09 Gb/s [31] 2019 438 OFDM 0.3 1.1 Gb/s [26] 2019 153 - 1.5 1.18 Gb/s [32] 2018 153 PAM-4 1.6 1.6 Gb/s [2] 2018 1.9 - 150 921.6 Kb/s [33] 2018 20 OOK - 1.92 Mb/s [34] 2017 29 OFDM - 71 Mb/s [35] -
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