As optical signals move deeper and deeper into access networks, achieving the ambitious performance goals of 5G architectures requires more optics than ever between small cell sites.
Extending fiber optics deeper into remote communities “is a critical economic driver, promoting competition, increasing connectivity for the rural and underserved, and supporting densification for wireless,” according to Deloitte, a financial and tech consulting firm.1
But there are cases in which fiber isn’t cost-effective to deploy. For example, a network carrier might need to quickly increase its access network capacity for a big festival, and there is no point in deploying extra fiber. In many remote areas, the customer base is so small, the deployment of fiber won’t produce a return on investment. These situations can be addressed via some kind of wireless access solution.
Carriers have used fixed microwave links for years to handle these situations. But radio microwave frequencies might not be enough as the world demands greater internet speeds.
Simply changing over to higher carrier frequencies will limit the reach of microwave links. The radio spectrum is also quite crowded, and carriers might not have the available licensed spectrum to deploy this wireless link. And microwave point-to-point links produce plenty of heat while struggling to deliver capacity beyond a few gigabits per second.
This is where free-space optics (FSO) comes into play. With FSO, a high-power laser source converts data into laser pulses and sends them through a lens system and into the atmosphere. The laser travels to the other side of the link and goes through a receiver lens system. A high-sensitivity photodetector then converts these laser pulses back into electronic data that can be processed (see Fig. 1). So instead of using an optical fiber as a medium to transmit the laser pulses, FSO uses air as a medium. The laser typically operates at an infrared wavelength of 1550 nm, which is safer on the eye.
FSO is often referred to as a futuristic technology for space applications, but can also be used for ground-to-ground links in access networks. FSO can deliver a wireless-access solution for quick deployment and with more bandwidth capacity, security features, and less power consumption than traditional point-to-point microwave links. And since it does not use the RF spectrum, there is no need to secure spectrum licenses.
Alignment and atmospheric turbulence
FSO has struggled to break through into practical applications despite these benefits because of certain technical challenges. Communications infrastructure, therefore, focused on more stable transmission alternatives such as optical fiber and RF signals. But research and innovation during the last few decades is removing these technical barriers.
One obstacle to achieving longer distances with FSO is the quality of the laser signal. Over time, FSO developers have found a solution to this issue in adaptive optics systems. These systems compensate for distortions in the beam by using an active optical element—such as a deformable mirror or liquid crystal—that dynamically changes its structure depending on the shape of the laser beam. Dutch startup Aircision uses this kind of technology in its FSO systems to increase their tolerance to atmospheric disruptions.
Another drawback of FSO is aligning the transmitter and receiver units. Laser beams are extremely narrow and if the beam doesn’t hit the receiver lens at just the right angle, the information may be lost. The system requires almost perfect alignment (see Fig. 2), which it must maintain even when there are small changes in the beam trajectory due to wind or atmospheric disturbances.
FSO systems can handle these alignment issues with fast steering mirror (FSM) technology. These mirrors are driven with electrical signals and are fast, compact, and accurate enough to compensate the disturbances in the beam trajectory. But even if the system can maintain its beam trajectory and shape, atmospheric turbulence can still degrade the message and cause interference in the data.
Fortunately, FSO developers also use sophisticated digital-signal processing techniques (DSP) to compensate for these impairments. These DSP methods allow reliable, high-capacity, quick deployments even through thick clouds and fog. FSO links can now handle gigabit-per-second capacity over several kilometers, thanks to all these advances in technologies.
A collaboration between Aircision and TNO demonstrated in 2021, for example, their FSO systems could reliably transmit 10 Gbit/s over 2.5 km. “It’s an important milestone to show we can outperform millimeter E-band antennas and provide a realistic solution for the upcoming 5G system,” says Aircision’s scientific director John Reid.
Safe, private networks
One understated benefit of FSO is, from a physics perspective, they’re arguably the most secure form of wireless communication available today.
Point-to-point microwave links transmit a far more directional beam than mobile antennas or WiFi systems, which reduces the potential for security breaches. But even these narrower microwave beams are spread out enough to cover a wide footprint vulnerable to eavesdropping and jamming (see Fig. 3). At a 1 km distance, the beam can expand enough to cover roughly the length of a building, and at 5 km, it could cover an entire city block. Microwave systems have side and back lobes radiating away from the intended direction of transmission that can be intercepted, as well. If an attacker is close enough to the source, even the reflected energy from buildings can be used to intercept signals.
Laser beams in FSO are so narrow and focused that these issues don’t exist. At 1 km, a typical laser beam only spreads out about 2 m, and at 5 km, only about 5 m. There are no side and back lobes to worry about and no near-zone reflections. The beam is so narrow that intercepting the transmission becomes an enormous challenge. An intruder would need to get within inches of a terminal or the line of sight, making it easier to be discovered. To complicate things further, the intruder’s terminal would also need to be very well aligned to pick up enough of a signal.
Highly integrated transceivers in FSO
While fiber-optic communications drove the push for smaller and more efficient optical transceivers, this progress also has a beneficial impact on FSO.
As the industry has seen during the last several years, optical transmission systems are being miniaturized from big, expensive line cards to small, affordable pluggables the size of a large USB stick. These compact transceivers with highly integrated optics and electronics have shorter interconnections, fewer losses, and more elements per chip area. These features all led to reduced power consumption over the last decade. Even greater efficiency gains are made possible by an optical system-on-chip (SoC) that integrates all photonic functions on a single chip—including lasers and amplifiers.
FSO systems can now take advantage of affordable, low-power transceivers to transmit and receive laser signals in the air. For example, a transceiver based on an optical SoC can output a higher power into the FSO system. By using this higher laser power, the FSO does not need to amplify the signal as much before transmitting it, improving its noise profile. This benefit happens with both direct detect and coherent transceivers, and is a key reason why Aircision has partnered with EFFECT Photonics to create both direct-detect and coherent free-space optical systems. Aircision ultimately aims to reach transmission speeds of 100 Gbit/s over the air, and an example of such a system is shown in Figure 4.
FSO can deliver a wireless access solution to be deployed quickly and with more bandwidth capacity, security features, and less power consumption than traditional point-to-point microwave links. And since it does not use the RF spectrum, it is unnecessary to secure spectrum licenses. Affordable direct-detect and coherent transceivers based on SoCs can further improve the quality and affordability of FSO transmission.
REFERENCE
1. See https://bit.ly/3J4SQpq.