The pros and cons of optical wireless communication

Defining OWC

OWC is a wireless technology that uses light waves and optoelectronic elements to exchange data. Unlike traditional connectivity options, OWC doesn't use physical optical fibers to transmit data. Instead, it uses free space as the transmission medium and operates on the following light spectrums:

• Visible light spectrum -- 380 nanometers to 700 nm.
• Infrared (IR) spectrum -- 750 nm to 1 millimeter.
• Ultraviolet (UV) spectrum -- 10 nm to 400 nm.

To transmit information, OWC uses an encoder, modulator, source and transmitter optics. Semiconductor devices, such as LEDs and laser diodes, function as the light source. These devices are typically mounted on top of a ceiling or embedded in a roof with transmitter optics. The transmission setup must adhere to eye and skin safety standards. The information signal containing the data modulates over a carrier signal via advanced modulation techniques, such as the following:

• Intensity modulation/direct detection.
• Pulse amplitude modulation.
• Pulse position modulation.
• Carrierless amplitude and phase modulation.
• Orthogonal frequency-division multiplexing.

Once the transmitter converts data into optical signals for transmission, the modulated light transmits through the free space channel. OWC uses an assortment of components to receive data, including a photodetector -- such as a photodiode or an avalanche diode, which can detect incoming light signals -- an amplifier to enhance the signal quality, a demodulator to extract the original data and a decoder to convert the signal.

The receiver must be present in a wide field of view to collect the optical signal. To achieve spectral efficiency, the receiver should be flat enough to have an area for detection. When a client device receives the optical signal from the transmitter, the receiver demodulates and decodes it to extract the original data.

Benefits of OWC

Optical wireless communication has many benefits, including the following.

High available bandwidth

Unlike radio frequencies (RF), OWC spectrum is unregulated and unlicensed. Theoretically, optical wireless communication can access bandwidth in the petahertz range. The visible light and IR spectrums typically offer bandwidths of several hundred terahertz, while UV provides similar bandwidth but is less common because of its higher absorption and scattering.

Current technology standards and devices cannot support such high frequencies because of technical limitations and safety concerns. Due to a lack of congestion in the optical spectrum, however, OWC is anticipated to enable higher data rates in future 6G networks.

Standardized technology

OWC primarily uses the visible light range and portions of the IR spectrum. UVC band usage is slowly developing for solid-state device, underwater and wide field-of-view wireless communication. Depending on the light source, different types of OWC are available, such as the following:

• Visible light communication (VLC).
• IR.
• Free-space optical communication (FSO).

These technologies have been standardized by a few regulatory bodies, though the degree of standardization varies. Enterprises typically use VLC and IR indoors and FSO outdoors, where it's impractical to lay cables.

No EMI

Most electromagnetic interference (EMI) use cases fall within the RF range, and this is known as radio frequency interference. As a result, OWC doesn't incur EMI like standard wireless networks do. Light's inability to penetrate solid barriers, such as walls, eliminates EMI, even in adjacent rooms using the same networking frequencies. While other near-field electronic devices in the same room can produce EMI, this interference tends to affect lower frequencies, rather than the high frequencies OWC uses.

Security

OWC generates small cell sizes, which enhance security by confining transmission to specific areas. Eavesdropping is less likely due to restricted access to enterprise premises, and OWC further reduces the risk because of this signal confinement. It's more difficult for bad actors to hack an OWC network, even when the eavesdropper is in an adjacent room, because signals can't penetrate walls. Highly directional beams ensure that data is transmitted only to receivers within LOS.

Spot diffusion

Enterprises can use a smart link design with multiple input, multiple output to improve OWC performance. Across the networking industry, professionals use various methods to design OWC systems. One of the most common designs is using a multispot diffusing transmitter. This technology directs optical signal beams to multiple locations in a room. This design reduces the need for exact alignment between the transmitter and receiver, which makes the system more user-friendly. It also offers mobility and shadow immunity within the enterprise environment.

Low implementation cost

Laying cables throughout an enterprise is expensive, sometimes costing thousands of dollars. The unregulated nature of the optical spectrum OWC operates on makes it free to use without the licensing fees that come with RF spectrum. Enterprises can further lower costs by using LEDs and laser diodes instead of installing traditional networking devices. The use of LED and laser diodes in residential and commercial spaces has rapidly surged since the 2000s, driven by their energy efficiency and versatility. When in operation, LEDs and laser diodes consume less electric power, leading to lower overall installation and operation costs.

Hybrid networking

Integrating optical fibers and OWC networking elements creates a hybrid network. In this type of network, enterprises deploy multiple optical wireless access points throughout the location, often in different rooms, and connect them via optical fibers. Hybrid networks incorporate both LOS and non-LOS communication technologies. Varying levels of directionality between the transmitter and network devices enable on-site mobility, multipoint communication and higher data rates.

Challenges of OWC

Though OWS presents many promising benefits, it also comes with certain challenges, such as the following.

Short range

Due to eye and skin safety regulations, the maximum permissible transmitter power for OWC systems is relatively low. Low-power optical transmitters can operate effectively within a single room, but OWC equipment can't transmit data from one room to another because visible and IR light cannot penetrate solid barriers. This limits the range of OWC to only a few meters. As a result, OWC isn't as scalable in enterprises with outdoor areas or indoor facilities with large rooms and halls.

LOS maintenance

For effective OWC communication, the transmitter and the receiver must maintain direct LOS. Pointing error loss occurs when the transmitter and receiver are misaligned, which can happen in environments where on-site mobility and seating arrangements prevent multiple client devices from maintaining alignment.

To address this challenge, transmitters are often mounted on the ceiling to create a wider radiation pattern and ensure receivers are in the field of view. This setup can lead to multipath dispersion, however, in which reflections from walls and other surfaces degrade signals. This reduces the signal-to-noise ratio and causes intersymbol interference (ISI).

Multipath dispersion

Opaque objects, like walls, ceilings or furniture, obstruct or shadow the client device. This contributes to multipath dispersion, in which multiple beams from the transmitter travel through different paths to reach the receiver. In this case, some signal components take the original LOS path, while others reflect or scatter off surfaces before detection. As a result, signal components arrive at the receiver at different intervals, resulting in propagation delay. This multipath dispersion and delay spread lead to channel distortion and ISI.

Intersymbol interference

As mentioned earlier, EMI isn't present in OWC systems. However, OWC can experience EMI-like noises with ISI, which occurs when one symbol -- or information bit -- overlaps with successive symbols due to multipath dispersion or signal delays. This data overlap degrades the quality of received signals. Smart link design and beam directionality can mitigate ISI in OWC networks.

Atmospheric susceptibility

OWC receivers can detect different light sources, such as natural sunlight and various light bulb types. This is a common effect known as shot noise or light noise. Temperature and pressure conditions can also cause atmospheric turbulence, including the following:

• Signal absorption.
• Scattering.
• Refraction.
• Attenuation.

These fluctuations can affect the amplitude, phase and signal intensity of the OWC signal and potentially induce flickering or increase the error rate. Therefore, outdoor OWC systems, such as FSO communication, are less suitable in areas with frequently changing weather.

Optoelectronic errors

OWC networks are prone to noise and performance issues due to the limitations of optoelectronic devices. LEDs, laser diodes and photodetectors are highly sensitive to temperature variations and have a limited lifetime operation. LEDs used in Li-Fi -- a wireless OWC technology that transfers data using light -- are susceptible to optical feedback and pollution. On the receiver side, photodiodes have large detection areas but have limited spectral range. High dark currents and capacitance in photodiodes can deteriorate signal quality and connectivity.

Regular replacements

Lifetime maintenance costs for OWC network devices can be much higher than those for optical fiber networks. The average life span of LEDs or laser diodes typically ranges from two to five years, while fiber typically lasts up to 40 years. Enterprise OWC networks might need more frequent maintenance and replacement every three to four years. As such, OWC is more suitable for vehicle networks, traffic lights and IoT, where such replacements are common.

OWC vs. traditional fiber optics

In comparison with OWC, traditional fiber optics is a well-established field. Both technologies have some similarities and major differences, as seen in this table.

Conclusion

OWC has recently been implemented in indoor settings via Li-Fi technology. Smart city and vehicle network developments are also based on OWC. Meanwhile, traffic and open-space transmission towers use FSO. Enterprises with indoor spaces can use hybrid networks to improve data rate and mobility.

Venus Kohli is an electronics and telecommunications engineer, having completed her engineering degree from Bharati Vidyapeeth College of Engineering at Mumbai University in 2019. Kohli works as a technical writer for electronics, electrical, networking and various other technological categories.