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Radio-over-Fiber and Microwave Photonics

Princeton University_070820A
(Photo: Princeton University, Office of Communications)


 - Microwave Links and Microwave Radio Transmissions

Microwave is a line-of-sight wireless communication technology that uses high-frequency radio beams to provide high-speed wireless connections that can send and receive voice, video and data information.

Microwave links are widely used for point-to-point communications because their small wavelengths allow appropriately sized antennas to direct them into narrow beams that can be pointed directly at the receiving antenna. This allows nearby microwave devices to use the same frequency without interfering with each other like low-frequency radio waves do. Another advantage is that the high frequency of microwaves gives the microwave band a very large information-carrying capacity; the bandwidth of the microwave band is 30 times wider than that of all other radio spectrum below it.

Microwave radio transmissions are commonly used in point-to-point communication systems on the Earth's surface, satellite communications, and deep space radiocommunications. Other parts of the microwave radio band are used for radar, radio navigation systems, sensor systems and radio astronomy.

The higher part of the radio electromagnetic spectrum, with frequencies above 30 GHz and below 100 GHz, is called "millimeter waves" because their wavelengths are usually measured in millimeters, and their wavelengths range from 10 mm to 3.0 mm. Radio waves in this band are generally strongly attenuated by the Earth's atmosphere and the particles it contains, especially during wet weather. Furthermore, in a wide frequency band around 60 GHz, radio waves are strongly attenuated by molecular oxygen in the atmosphere. The electronics required at mmWave frequencies are also much more complex and more difficult to manufacture than those at microwave frequencies, so mmWave radios are generally more expensive.


- Radio and Microwave over Fiber

Radio over fiber (RoF) or RF over fiber (RFoF) refers to a technology whereby light is modulated by a radio frequency signal and transmitted over an optical fiber link. Main technical advantages of using fiber optical links are lower transmission losses and reduced sensitivity to noise and electromagnetic interference compared to all-electrical signal transmission. 

Applications range from the transmission of mobile radio signals (3G, 4G, 5G and Wi-Fi) and the transmission of cable television signals (CATV) to the transmission of RF L-Band signals in ground stations for satellite communications.

In the area of Wireless Communications one main application is to facilitate wireless access, such as 5G and Wi-Fi simultaneous from the same antenna. In other words, radio signals are carried over fiber-optic cable. Thus, a single antenna can receive any and all radio signals (5G, Wi-fi, cell, etc..) carried over a single-fiber cable to a central location where equipment then converts the signals; this is opposed to the traditional way where each protocol type (5G, Wi-Fi, cell) requires separate equipment at the location of the antenna.


- Radio over Fiber or Microwave over Fiber

In principle, electrical radio frequency (RF) and microwave signals – for example, carrying audio, video or general internet data – can be directly transmitted through suitable electrical cables, for example coaxial cables. However, such cables exhibit substantial attenuation losses, which rapidly increase with increasing frequency. The frequency dependence of those losses can also introduce signal distortions. For such reasons, it normally becomes difficult to reach distances substantially more than a few tens of meters, and even then one may require additional RF amplifiers and signal regenerators to maintain the signal power at a sufficiently high level and preserve the signal quality.

The explained problem can be solved with the method radio over fiber or microwave over fiber, the former in short often called RF over fiber and sometimes radio frequency over fiber. Essentially, the idea is to modulate a light wave with the radio or microwave frequency signal, transmit that light in an optical fiber and finally detect the light in order to recover the radio frequency signal. So one employs an electrical-to-optical (E/O) conversion and an optical-to-electrical (O/E) conversion at the two ends of an optical transmission medium. Effectively, one realizes an integration of wireless and fiber-optic networks and gets into area of RF or microwave photonics. 

Although the transmitted signal frequencies can be quite different, ranging from some megahertz to dozens of gigahertz or even more, but the basic technology is always the same; only various technical details may be optimized differently in different frequency regions. The technology can to some extent be adapted even to terahertz signals, although detection is difficult in that area.


- Advantages over RF Cables

The main advantages of radio or microwave over fiber are the following:

  • As the propagation losses in the fiber are quite small – for example of the order of only 0.2 dB/km in single-mode telecom fibers around 1550 nm –, and these are nearly frequency-independent in the whole optical frequency range of interest (flat frequency response), one can easily use fibers which are many kilometers long. When using fiber amplifiers in addition, even much longer transmission distances can be easily realized. Over such long lengths, propagation losses of RF or even microwave cables would be prohibitive. (For moderate propagation distances, one may also use other optical wavelength regions, where the transmission losses are higher, but transmitters and receivers are cheaper – for example, the 0.8-μm region.)
  • Because the RF or microwave bandwidth is far lower than the width of the optical frequency range which can be transmitted through the fiber with low propagation losses, the cable is future-proof: if a larger RF bandwidth or mean frequency are required later on, only the transmitter and receiver may have to be adapted, but not the optical cable. In contrast to that, many coaxial cable installations in buildings became useless when moving to a satellite-based TV signals, where higher frequencies are required and the cable losses became too high.
  • Compared with RF coaxial cables, optical fiber cables can be thinner and possibly more flexible, making it easier to lay them down in buildings, for example. Furthermore, the cost per meter of cable can be substantially lower. Substantial cost savings are possible in some scenarios. 
  • Because the receiver end can be made relatively simple and at the same time open to further changes of transmission formats (see below), the technology can provide substantial cost savings particularly in cases where many remote stations need to be operated.
  • Signal transmission through optical fibers is immune to electromagnetic interference.
  • For applications with antennas, for example, it can be an advantage that the cable is non-conducting, because the electronics at the lower end can be better protected against lightning strikes.

- Typical Applications of Microwave

 Some typical applications are briefly described in the following sections.


- Radio and TV Broadcast Networks

With radio over fiber technology, it is easily possible to create an infrastructure with a central station and multiple radio antenna units as base stations, which are connected to the center via radio over fiber. For each base station, one requires only comparably simple and cheap technology, which is also open for future changes of our example of transmission performance (format transparency). Changes to the sent-out signals can easily be implemented at the central location without affecting the base stations. 


- Satellite Communications

Satellites in earth orbits typically communicate with ground stations through microwaves. It can be preferable to use microwave over fiber between the transmitter/receiver station and the building containing the control center, particularly if the distance between those is larger than e.g. 50 m. One may also use multiple antenna sites, all connected with microwave over fiber to the operation center, even over distances of many kilometers. That way, one may for example divert the traffic to another station if local weather conditions are bad for one of the stations, or a station has a technical defect.


- Closing Dead Zones

Some remote areas such as road tunnels are difficult to provide with internet access using broadband wireless technology only. RF over fiber from the outside world to local wireless stations can then be a solution. Alternatively, one may use a digital communications cable connecting the zone with the outside world, but this requires more sophisticated technology at the remote location. Particularly when a large number of antenna units is required, radio over fiber tends to be substantially more economical.


- Antennas for Cell Phone Communications

For a base station of a cell phone communications network, the antenna needs to be placed at a high altitude, while it is preferable to keep most of the electronics near the ground for easy access e.g. during maintenance operations. However, if antenna and electronics are simply connected with a long RF cable, the substantial propagation losses are problematic, particularly because of the substantial transmitter power required: the RF power amplifier at the bottom then needs to generate substantially more power in order to have sufficient power at the antenna despite the cable losses. With radio or microwave over fiber technology, one can place a simple optical receiver and microwave power amplifier next to the antenna, while having all of the other electronics at the bottom. That can result in substantial savings of electrical energy.


- Road to Vehicle Communications

There are various preliminary concepts for future road to vehicle communications, for which traditional cell phone systems may not be suitable. One will then probably use a large number of antenna units along the roads, which would probably be connected with microwave over fiber.


- Broadband Wireless Indoor and Outdoor Communications

Indoor communications like WLAN as well as cell phone systems work with high radio or microwave frequencies of several gigahertz, and in the future (e.g. for 5G) even at tens of gigahertz, which can hardly transmitted through electrical cables. One might then use microwave over fiber between a transmitter/receiver antenna and the corresponding electronics, if one does not want to have them close together, for example for reasons of easier maintenance. That technology may become quite important for future systems, operating at substantially increased frequencies, where the data rates are quite high, while the usable range (cell size) is small, so that many base stations are required. One may then connect many such base stations via fibers with a remote access node, which itself is connected through a broad bandwidth optical core network, e.g. based on dense wavelength division multiplexing (DWDM). 


- Non-telecom Applications

RF and microwave over fiber can be applied not only to communication signals, but also to other RF or microwave signals, e.g. carrying GPS data or sensor data, or signals used for certain technologies such as particle accelerators and radio frequency astronomy.



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