What Is Radio Over Fiber?

Wireless fiber optics is a wireless transmission technology based on optical transmission methods that uses infrared lasers to carry high-speed signals. The laser is used as the carrier and the air is used as the medium. Or laser diode is the light source, so it is also called "virtual fiber".

Wireless fiber

Introduction to wireless fiber

In fact, FSO has made great breakthroughs in technology in recent years. For atmospheric attenuation, stability and other issues, the industry can currently solve it in the following ways:

Select the appropriate wavelength for wireless fiber

For FSO-based systems, the most commonly used optical wavelength is 850 nanometers (nm) in the near-infrared spectrum. At this frequency, there is enough energy to efficiently convert a current signal into an optical signal. In addition to its fast modulation capability, high energy density is also helpful for obtaining high directivity. The receiver is responsible for efficiently converting a high-capacity modulated optical signal into an electronic signal.

In addition, there are some FSO-based systems that use a wavelength of 1500 nanometers and can support greater system power, but only when the communication distance exceeds 1000 meters, can they show their advantages.

Wavelength ^[1]

Recently, people tend to develop FSO systems that can work outside the near-infrared spectrum, such as the mid-infrared spectrum around 2.2 microns, and even quantum cascade lasers (about 10 microns). However, no matter what wavelength is used, in weather such as rain and snow (with larger suspended matter particles), all FSO-based systems have similar performance. In foggy weather, the long-wave system is more advantageous because it is less affected by atmospheric attenuation. Figure 1 shows the attenuation curves of FSO at different wavelengths and visibility.

Improve stability with automatic power control and positioning tracking technology

In order to develop a future-oriented FSO-based system with higher availability and online time, a new approach is needed. For example, wireless optical fiber systems are equipped with automatic power control (APC) and automatic tracking features. APC allows the transmission power of the product to be adjusted automatically, so that the receiver will never be over-modulated. The advantage is that users can use higher power transmitters to ensure that the wireless fiber optic system can be used in foggy days. In the example shown in the attached table, two FSO-based systems are deployed 500 meters apart. Among them, the transmission power of System A is 1 mW and does not have automatic power control characteristics; System B has integrated automatic power control characteristics. In this case, the advantages of System B are obvious. Because System A can only work at 1 mW, overmodulation may occur in fine weather. On the contrary, System B can control the power. Once the visibility is low due to the weather, it has more than 15dB of additional power available.

In wireless fiber-optic systems, the lens design of the transmitter and receiver must be balanced in the following aspects: large aperture (the large aperture of the receiver helps to obtain a larger light incidence), short focal length (depending on the size of the device) (Decision) and easier adjustment functions. Correspondingly, in business applications, people adopt various ways to achieve this balance.

On the one hand, some FSO-based systems use Fresnel (Fresnel) lenses, which are characterized by a large receiving angle and mirror surface, but the disadvantage is that the high incidence of direct light will significantly affect the sensitivity of the receiver; On the other hand, some FSO-based systems use a multi-lens design, and their cumulative optical signals can significantly enhance the transmission quality. However, the low transmit and receive angles of this system (1 to 5 mrad) have proposed adjustment and system stability Higher requirements, especially for long-distance communications beyond 2000 meters. With this in mind, some leading manufacturers of FSO products (such as LightPointe) have implemented a far-reaching technological innovation, namely "automatic tracking", using a gimbal that can move in the x and y axes, and can actively adjust the receiver , In any case can be in an ideal position. The software that controls the FSO system can compensate for atmospheric fluctuations within 1ms to 10ms, and can complete the compensation within an extremely short period of 1 to 10s.

Wireless fiber wireless fiber industry application

In general, FSO technology can be used in situations where high-bandwidth connections are needed and fiber optic cables / cables cannot be laid. Its advantages include:

Low cost-easy installation, no need to estimate the line cost in advance;

Industry applications ^[2]

No construction permit required;

No interference;

High bandwidthfrom 100Mbps to 2.5Gbps;

Flexible (easy to scale);

Applicable to any environment (not dependent on a certain protocol).

The exciting possibilities of the information industry will only become reality when the network connection is fully expanded to reach a sufficiently wide customer base. By combining wireless fiber and fiber technology, networks can be quickly established and provide affordable, easily expandable connections for end users who desire high bandwidth.

Industry applications ^[3]