The phrase “radio over fiber” has been bothering me. I do not thing that radio waves would be bent by glass in the way light in a fiber optic cable is bent to stay inside the fiber. I wanted to find a calculation to understand this and below are my notes on this so called Radio over Fiber.
Snell’s Law
Both light and radio waves are forms of electromagnetic waves and are subject to the same basic laws and principles of wave behavior, including reflection and refraction. However, there are differences between the two. One of the main differences is that light has a higher frequency and shorter wavelength than radio waves. Radio waves are lower frequency than light, and the lengths of radio waves are longer than the wave lengths of light. Another big difference is that light is visible to the human eye, while radio waves are not. Both light and radio waves can be refracted when passing through a medium with a different refractive index, causing the path of the wave to bend. The basic law for refraction of both light and radio waves is known as Snell’s Law. Gradual changes in refractive index can cause radio waves to be refracted, and this can happen as the radio waves propagate through the atmosphere where small changes in the refractive index occur.
Snell’s Law relates the angle of incidence (θ1) and the angle of refraction (θ2) of a wave as it passes from one medium to another:
n1 * sin(θ1) = n2 * sin(θ2)
where n1 and n2 are the refractive indices of the two media.
In fiber optic cables, the core of the cable has a higher refractive index than the cladding surrounding it. This causes the light waves to undergo total internal reflection, enabling them to propagate along the fiber.
The critical angle (θc) is the angle of incidence at which the angle of refraction becomes 90 degrees. Beyond this angle, total internal reflection occurs.
For light waves in a fiber optic cable, the critical angle is determined by the refractive index of the core and cladding. Let’s denote the refractive index of the core as n1 and the refractive index of the cladding as n2. Therefore,
sin(θc_light) = n2 / n1
For radio waves in a cable, the refractive indices are typically close to 1 for both the core and the surrounding medium (air or materials with similar refractive indices). Therefore,
sin(θc_radio) ≈ 1 / 1 = 1
Since sin(θ) cannot be greater than 1, we can see that the critical angle for radio waves is 90 degrees. This means that radio waves cannot undergo total internal reflection and, consequently, cannot propagate effectively in a fiber optic cable.
Refractive Indices
The refractive index is a dimensionless number that depends on the temperature of the medium and the wavelength of the light. It can be measured using various techniques, such as refractometers, and is commonly reported using a single value for a specific wavelength, typically measured at 633 nm for visible light. The concept of refractive index applies across the full electromagnetic spectrum, from X-rays to radio waves, and can also be applied to wave phenomena such as sound. In the case of RF signals, the refractive index of a material affects the transmission of the signals through the material, and it can vary with the frequency of the RF signal.
Radio in Fiber?
Radio waves, however, would not be able to propagate through a fiber optic cable. Fiber optic cables are designed to transmit data through the use of light signals rather than electromagnetic waves like radio waves. Inside a fiber optic cable, there is a core made of transparent materials, typically glass or plastic, surrounded by a cladding layer with a lower refractive index. This structure forms a waveguide that allows the transmission of light signals by using the principle of total internal reflection.
Radio waves, on the other hand, are a form of electromagnetic waves with a much longer wavelength than visible light. They are typically used for wireless communication and have different propagation characteristics. Radio waves require antenna systems and free space to propagate effectively.
Therefore, if radio waves were introduced into a fiber optic cable, they would not be able to travel through the waveguide structure of the cable. Instead, they would be absorbed and attenuated by the cable’s materials, resulting in a loss of signal strength.
Radio over Fiber (RoF)
The process of Radio over Fiber (RoF) involves converting the RF signal into an optical signal, transmitting it over the fiber optic cable, and then converting it back into an RF signal at the receiving end. So, you can say fiber optic cables are transporting 5G signals, but not as radio waves.
The refractive index of pure silica, which is commonly used in fiber optic cables, is typically around 1.45 and the refactive
Radio over fiber (RoF) or RF over fiber (RFoF) is a technology that uses fiber optic cables to transmit radio signals (but not radio waves!) over long distances with low loss and reduced interference[1][3]. RF signals are converted to modulated light using RFoF converters and then transmitted over optical fibers. The fiber itself causes only relatively low levels of phase noise, and the propagation losses in the fiber are quite small, making it possible to use fibers that are many kilometers long[2]. Since one mile is about 1.6093 kilometers, this allows fiber optic cables to transmit RF signals for miles with little signal loss.
Applications of RF over fiber include wireless communications, cable television, and sensor networks[1]. Therefore, fiber optic cables can be a suitable alternative to traditional RF coaxial cables for transmitting RF signals over long distances with reduced loss and interference.
Radio over Fiber (RoF) provides coverage in areas with weak or no wireless signals by leveraging its unique capabilities, which include the following:
1. Extensive Wireless Coverage: RoF technology enables the transmission of high-frequency signals, such as microwaves and millimeter-waves, over long distances using optical fibers[9]. This capability allows for the distribution of wireless signals to areas with weak or no wireless coverage, such as indoor environments, tunnels, or remote locations[1].
2. Centralized Signal Processing: RoF systems can centralize the processing of radio signals in one location, with remote antennas attached via fiber optics serving all protocols[1]. This centralized approach reduces the need for complex and costly equipment at each remote site, making it an efficient solution for providing wireless coverage in various locations[1].
3. Low Attenuation Loss: Signals transmitted over optical fibers experience low attenuation loss, enabling the transport of radio signals over longer distances without the need for frequent signal regeneration or amplification[9][8].
4. Immunity to Interference: Optical fibers offer immunity to electromagnetic interference, which can be beneficial in environments where traditional wireless signals may be susceptible to interference[8].
In summary, RoF technology is well-suited for providing wireless coverage in areas with weak or no wireless signals due to its ability to transport high-frequency signals over long distances, its centralized signal processing capabilities, low attenuation loss, and immunity to interference. These features make RoF an effective solution for extending wireless coverage to various challenging environments.
Thus, I now wonder if AT&T putting in new fiber optic lines is a precursor for small 5G antenna being installed in rural areas across the country at a later date.
Citations:
[1] https://en.wikipedia.org/wiki/Radio_over_fiber
[2] https://www.rp-photonics.com/radio_and_microwave_over_fiber.html
[3] https://www.5gtechnologyworld.com/rf-over-fiber-overcoming-an-inherent-transmission-line-problem-pt-1/
[4] https://www.opticalzonu.com/2014/02/why-use-fiber-optics-instead-of-coax-to-transmit-rf-over-long-distances/
[5] https://www.linkedin.com/advice/3/what-distinguishes-fiber-optic-cable-from-microwave-g1cec
[6] https://en.wikipedia.org/wiki/Radio_over_fiber#cite_note-hodson-2
[7] https://turcomat.org/index.php/turkbilmat/article/download/12493/9037/22129
[8] https://www.rfwireless-world.com/Terminology/Advantages-and-Disadvantages-of-Radio-over-Fiber.html
[9] https://www.sciencedirect.com/science/article/abs/pii/S0030402621015692
[10] https://www.degruyter.com/document/doi/10.1515/joc-2016-0020/html?lang=en
[11] http://data.conferenceworld.in/SGTB/P866-873.pdf
[12] https://ieeexplore.ieee.org/document/8993233
[13] https://www.allaboutcircuits.com/technical-articles/radio-meets-fiber-optics-rf-over-fiber/
[14] https://www.sciencedirect.com/topics/materials-science/radio-over-fiber-system
