The Constraints of HAPS

Two types of communication services are required for HAPS: "service link" and "feeder link". The service link is a one-to-multiple communication between HAPS and the user's smartphone, while the feeder link is a one-to-one communication between HAPS and the ground gateway.

Of course, it is impossible to provide a service without a feeder link. This is because the data communicated with the users through the service link must be connected to the 5G core system or the Internet. While the HAPS platform itself can fly freely through the sky, this feeder link is akin to a “chain” that binds HAPS, becoming a major factor limiting its freedom. Even if HAPS is deployed in isolated deserts, oceans, or a disaster-stricken area where the ground infrastructure has been destroyed, it would be necessary to lay optical fibers and build a ground gateway there. If this is the case, the effectiveness and advantages of HAPS would be significantly reduced.
To address this issue, we are considering two approaches: “Inter-HAPS”, which is to make a connection between HAPS with HAPS, and "Satellite Feeder Link", which is to use constellation satellites. If a mesh network could be built with multiple HAPS, only a few terrestrial gateways would need to be installed close to cities with existing terrestrial infrastructure. In addition, when only one HAPS needs to be deployed immediately, for example in the event of a disaster, the feeder link can be secured via satellite to provide flexibility.

What is Laser Communication?

To date, we have demonstrated and experimented with 4G services using HAPS. During this process, we realized that feeder links using millimeter waves would eventually run out of frequency bands.
The 4G/5G communications people use every day are made possible through the allocation of frequency bands by the competent authorities in each country, and in addition to 4G/5G communications, most RF wireless communications are restricted as to who can use them, where, and how.
Therefore, the feeder link frequency band used in the stratosphere must also be allocated by each country. It may already be in use for other purposes, and even if it is available, not all of it can be allocated to a single company. Since the frequency band is a finite resource shared by all humankind, its use is limited. If it is used without permission, it will cause interference and no one will be able to communicate, and even if it is allocated, it is a valuable resource that needs to be used as effectively as possible.
One technology that could replace the scarcity of radio resources is laser communications, also known as optical wireless communications. Laser communications are highly directional, so they rarely interfere with other communications and are not limited by frequency bandwidth allocations. Furthermore, like terrestrial fiber-optic communications, it is capable of transmitting overwhelmingly high-bandwidth communications of several hundred Gbps or Tbps. For this reason, this technology is gaining a great deal of attention in NTN, or Non-Terrestrial Network applications, such as HAPS and LEO constellation satellites.

However, laser communication demands an extraordinarily high level of technology. It is necessary to aim a laser at a transmitter hundreds or thousands of kilometers away and send the laser into an optical fiber with a diameter of 9 μm (0.009 mm) at the end of the receiving lens. The transmitter is constantly moving, due to the maneuvering and vibrations of HAPS, and even though the stratospheric atmosphere is only slightly thicker, which is 2 to 5% of the atmospheric pressure on the ground, the fluctuations constantly refract the laser. This is like aiming at a needle hole in a plane flying over Nagoya from a train running in downtown Tokyo, all amidst a summer mirage.
Another problem is that lasers are greatly attenuated by weather conditions. This problem is virtually non-existent in the stratosphere or in space, where there is little or no atmosphere, but it is often an issue for communications to the ground. When there are clouds over the ground gateway or when it rains, radio waves may be able to communicate, but laser communications will be interrupted. Laser communications cannot be used for feeder links if they are easily interrupted.

Benefits of Extensible SDN

These problems are believed to be solved by an extensible SDN, or Software Defined Network, that incorporates weather prediction models. Extensible SDN understands the deployment status of HAPS, predicts weather and traffic volume, and optimizes communication paths in advance. This technology was used in the aforementioned millimeter-wave communications, but laser communications can benefit even more from it because of its higher bandwidth, allowing more traffic to be rerouted with more time to spare.
In order to realize a stratospheric optical mesh network, the HAPS must have sufficient on-board weight and power. This is one of the reasons why ”Sunglider”, the stratospheric unmanned aircraft we are developing, is a large-scale HAPS. We believe that a large HAPS, which at first glance may take time to implement, is optimal in terms of cost and wireless communication efficiency.
Japan has an excellent environment in which to lead the world in the stack of technologies necessary for laser communications, including not only laser and optical technologies, but also optical communication technologies, control technologies, and manufacturing of aircraft components. We will continue our research on stratospheric laser and optical wireless communication technologies in cooperation with several research institutes and companies.