This article appeared inMicrowaves & RF并已在此处出版。
What you’ll learn:
- A glimpse of the potential that 6G offers—such as data rates up to 1 Tb/s.
- 毫米和Terahertz波段可能不是某些人预见的RF的黑洞。
- 一些重大的技术障碍阻碍了6G的成果，但解决方案正在进行中。
Even though 5G is far from being fully realized, researchers throughout the world are already concentrating on the next generation. There’s obviously considerable speculation about precisely what 6G will be, and while it’s far too early to make precise predictions, it’s not too early to see how researchers are approaching the challenges.
当发布5G的细节和期望时，有很多怀疑论者（许多人仍然如此）将运行频率推向高于4G的数量级。这些频率中最高的仅用于Wi-Fi和其他一些应用程序的非常短的范围变体。也就是说，基础设施已经在24和28 GHz迅速部署，在未来几年中，频率更高。
到6G到来时，要克服的挑战将出现更多的挑战，在某些情况下，它们将比其前任更困难。然而，还有更多的机会，超越5G提供的机会。
More Revolutionary than Evolutionary
如果将“细胞”一代的历史用作预测指标，则每个标准中定义的内容部分是通过其后代来实现的。也就是说，最终在3G中实现了2G计划的一些计划，而4G则可以更全面地衡量3G打算实现但没有实现的目标。每一代的目标主要是实现更高的数据速率和更广泛的覆盖范围。
That changed with 5G, which might be considered a wholesale reinvention of cellular technology, expanding for the first time beyond smartphones, tablets, and laptops to the far broader landscape under the IoT umbrella. It ushered in the era of software-defined architectures for user equipment, infrastructure, and the entire network, and extended the frequencies used by wireless systems well into the millimeter-wave region.
Any one of those advances would arguably be a major achievement—5G achieves them all. When launched around 2030, 6G will attempt to accomplish what 5G did not, whether it’s achieving extremely low latency or multi-gigabit speeds in a broad swath of the country.
6G will likely be of the same magnitude as 5G as it reaches frequencies near the light-wave spectrum, drives downlink speeds to 1 Tb/s, and employs technologies that either don’t yet exist today or have been in development for years without a market to drive them harder. It will also fully enable applications that require latency much lower than what’s ultimately achieved by 5G, and allow for instantaneous communications between consumers, devices, vehicles, and the surrounding environment（表格1）。
但是，6G将不仅仅提供AI和机器学习的进步，并启用诸如自动驾驶汽车，机器人控制，高清全息游戏等应用的优势。所有这些应用程序都符合无线认知，传感，成像，无线通信以及位置和导航的类别。
6G promises data rates of as much as 1 Tb/s (1 million Mb/s), which is 1,000 times faster than 5G. That will require immense channel bandwidths well north of 1 GHz. The only available spectrum that provides the required bandwidth is between 100 GHz and 3 THz. Not surprisingly, this region has been used only for scientific research, including radio astronomy, the Earth exploration-satellite service (EESS), the space research service (SRS), and to a limited degree, amateur radio.
FCC认识到使6G实现6G所需的技术巨大和时间，并在2019年启动了一个名为New Horizons的计划。它为在95 GHz和3 THz之间的无载频率上进行实验提供了几乎没有规则的途径。委员会在其第5部分实验无线电服务（ERS）规则中多次使用了实验许可。这次，它创建了一种称为Spectrum Horizons实验无线电许可证（或Spectrum Horizons许可证）的新类型的许可证。
The goal of the program is to lower the barrier to entry by making experimental licenses easy to obtain, with few of the typical hurdles required in conventional licensing. It provides wide flexibility in specifications such as frequency range, power, and emissions, with the only caveat being that the experimenter must refrain from creating interference to existing services.
将有超过21 GHz的频谱可用，为116至123 GHz，174.8至182 GHz，185至190 GHz，244至246 GHz(Table 2)。即使在较低的毫米波频率下，可用的光谱也很大。例如，仅在60 GHz的无执照的频段就可以提供可用的带宽，等同于当今几乎所有许可和许可服务从DC上方到7 GHz使用的带宽。
The FCC notes that in addition to traditional communications, these frequencies might have use for data links that enable transmission of wideband, uncompressed high-definition video signals and other high-speed data for other types of applications. As an example, the commission specified that Japan’s NTT used 120-GHz wireless links to provide live TV coverage of the 2008 Beijing Olympics.
Terahertz的恐惧
The view among many scientists and engineers is that the higher the frequency, the higher the loss through space. Therefore, operation in the terahertz region is a fantasy. This position, along with a lack of suitable semiconductor technologies, is arguably one of the major reasons why millimeter wavelengths haven’t been used. However, this is far from a complete picture for several reasons.
首先，在当前用于无线通信的UHF和微波频率下，路径损失主要来自分子吸收，与较高的频率相比，这非常低。但是，在频谱的更高范围内，其他因素也发挥了作用，例如从降水和叶子（以及几乎所有其他物体）散射，它们会极大地阻碍范围和可靠性。因此，可以预料的是，在1 GHz的波长达到0.3毫米的情况下，传统无线服务的通信似乎至少是不切实际的。
其次，亚terahertz波长固有地跨越了较短的距离，但是从较低频率到较高的损失不是线性的(Fig. 1)。This is because the resonant frequencies of oxygen, hydrogen, and other gases in the atmosphere absorb more electromagnetic energy than others. This is why the few applications operating at these wavelengths operate at only specific frequencies. It’s also why the FCC chose the unlicensed bands it did for Spectrum Horizons.
However, Dr. Ted Rappaport, David Lee/Ernst Weber Professor of Electrical Engineering at the NYU Tandon School of Engineering, founding director of NYU Wireless, and a long-time pioneer in wireless communications, makes an important point: For a given antenna’s effective isotropic radiated power (EIRP), path loss through space decreases quadratically as frequency increases if the antenna aperture is the same size (i.e., number of elements) at each end of the transmission path. It’s been demonstrated that for the same RF output power when this condition is met, the signal strength at 140 GHz in free space is actually 5.7 dB greater than at 73 GHz and 14 dB greater than at 28 GHz.
事实上,根据Rappaport,许多millimeter-wave and terahertz bands have remarkably little loss when compared with sub-6-GHz bands, adding only 10 dB/km of loss at frequencies as high as 300 GHz. These frequency bands could easily be used for high-speed 6G mobile wireless networks with up to km-size coverage range, and perhaps even up to 10 km or beyond in fixed applications. In fact, although much of the spectrum between 600 and 800 GHz suffers from 100 to 200 dB/km attenuation, this is only 10 to 20 dB over a 100-m distance, which is the typical radius of a small cell.
已经表明，诸如雨滴，雪和冰雹之类的颗粒在10 GHz以上的频率下大大衰减。在73 GHz时，信号以10 mm/h的降雨速率以10 dB/km的速度减弱。降雨衰减从100 GHz变平至500 GHz，这意味着在100 GHz以上的工作频率下，雨水不会引起任何其他衰减。
尽管所有这些似乎都在面对传统思维时飞行，但它认为，在如此高的频率下，使用带有许多元素的微型电子转导天线可以实现真正的大量前向增益。这将抵消大气衰减的效果，同时保持与较低频率相同的信噪比。简而言之，这些天线将使移动系统能够很好地进入Terahertz地区。
挑战和解决方案
Immense challenges must be overcome before 6G can become a reality. They span from the development of semiconductor technologies able to produce measurable power at sub-terahertz frequencies, to the electronically steered, phased-array antennas required to deliver enough gain to overcome the various factors that make communication extremely difficult in this spectral region.
为了达到约100 GB/s的数据速率，调制方案将需要大于14 b/s/hz的光谱效率，远远超出了当今可用的效率。即将到来的RF功率放大器将需要使用硅锗（Sige），硅启用器（SOI）和BICMOS半导体技术，也可能是磷化磷脂。随着频率的增加和天线元件的缩小，基本站所需的空间可以大大降低，并且即使在Terahertz状态的较低范围内，阵列元素之间的阵列面上的距离也只有几百微米。
Not surprisingly, DARPA is involved in the development of semiconductor technologies for terahertz applications. The agency’s Technologies for Mixed-mode Ultra Scaled Integrated Circuits (T-MUSIC) program is investigating SiGe HBT, CMOS, SOI, and BiCMOS circuit integration in hopes of achieving power amplifiers operating at up to 1 THz. T-MUSIC’s goal is to develop terahertz mixed-mode devices that integrate digital processing and intelligence on the same chip(Fig. 2)。
The program focuses on advanced materials, device processing, and mixed-mode circuit designs based on an advanced CMOS fabrication platform that it hopes will vastly improve the speed and accuracy of integrated mixed-mode electronics. The T-MUSIC program’s participants (BAE Systems, Raytheon, University of California Los Angeles, University of California San Diego, and the University of Utah) will develop advanced mixed-mode foundry technologies with transistors operating to at least 1 THz, as well as broadband precision mixed-mode integrated circuits.
由1,000个元素组成的天线阵列可适应小于4 cm的区域2at 250 GHz. For user equipment, this means that mobile devices could host several tens of thousands of antennas, and base stations will become tiny cells with a range of 10 m. The pencil-wide beams produced by these antennas can also reduce interference, jamming, and detection.
Coverage extension technology will be necessary to provide services for drones, ships, and spacecraft as their service areas aren’t fully covered by conventional cellular networks. To remedy this, geostationary, low-earth-orbit satellites (LEOs), and high-altitude pseudo-satellites (HAPS) may be employed to cover mountainous and remote areas, sea, and space, and to provide communication services to new areas. HAPS are garnering more attention because they can be stationed at a fixed location at an altitude of about 20 km, producing wide coverage with a cell radius greater than 50 km on land. In addition, HAPS offer a solution for providing backhaul service to portable base stations during disasters and possibly for industrial IoT scenarios.
结论
的第一个确凿证据表明可以达到6克d is still years away, but with industry, academia, and other entities already fully engaged in development, it will be fascinating to watch how the various elements come together. There’s much to be gained when 6G is deployed, well beyond downloading a feature film in two seconds, from extremely precise location and other sensing applications to anti-jam communications where short range is actually desirable, among dozens more.
到2035年，在软件定义的硬件，AI，计算能力和其他技术方面，可能会取得巨大进展。网络将从纯粹的无线无线发展到包含填充地球结合平台无法达到的空白的卫星，并且还将实现许多其他进步。时间会证明6G的表现方式，但就像5G一样，现在看起来不太可能正在部署。
用于进一步阅读
“A Vision of 6G Wireless Systems: Applications, Trends, Technologies, and Open Research Problems,” Walid Saad, Virginia Tech; Mehdi Bennis, Centre for Wireless Communications, University of Oulu, Finland; and Mingzhe Chen, Future Network of Intelligence Institute, The Chinese University of Hong Kong, Shenzhen, China and Department of Electrical Engineering, Princeton University.
Theodore S. Rappaport，Nyu Wireless等人，有关6G及以后的机会和挑战，有关毫米波和Terahertz繁殖，渠道建模和应用程序的特别部分，IEEE Access，2019年6月。
“Key Drivers and Research Challenges for 6G Ubiquitous Wireless Intelligence,” University of Oulo (Finland), September 2019.
“5G Evolution and 6G,” NTT DoCoMo, January 2020.
“光谱范围，规则制定的请愿书，允许在95-1,000 GHz乐队中执行无执照的操作，”联邦通讯委员会，报告和命令，2019年3月21日。
