In the past decade, RF-over-Fiber technology has been rising in capacity and decreasing in cost. Coupling these variables with the increasing price of
Coaxial-cable material for distributed antenna systems (DASs) and small-cell applications has led many designers to select RF-over-Fiber optic systems. Fiber optics offer additional advantages, including low loss, interference immunity, high reliability, and safety, compared to conventional RF-transmission-line technologies. Despite these benefits, in some instances, their cost and the need for specialized equipment and components beyond a conventional telecommunications toolbox have slowed acceptance.
1. Compared to a coaxial or waveguide transmission cable, the transmission loss experienced by fiber-optic links is significantly lower over distance.
RF-over-Fiber refers to the use of fiber-optic communication cabling and RF-to-optical signal-conversion equipment used, usually over long distances, to transmit RF signals. The RF signal is sent straight to the electronics that control a laser diode with several of the new RF-to-optical converters. That diode is used along a single-mode fiber cable to transmit the optical signal. The optical signal with RF signals can be intensity-modulated, a mechanism similar to amplitude modulation. A distributed feedback (DFB) semiconductor laser can be used for high-dynamic-range and low-noise applications. A Fairy-Perot (FP) laser is also introduced for lower-performance requirements.
Using a time-intensive and quite costly process involving high-quality, specialized glass, the optical fibers are made. This glass is molded into fibers with an emphasis on an outer diameter that is extremely consistent and special jacketing that prevents optical leakage. To link two fiber-optic lines together and minimize reflective losses, specialized connectors and fiber-processing tools are required. Because of its normal return loss of 60 dB and low 0.25-dB connector-to-connector loss, an angle-physical-contact (FC/APC) connection is used for the single-mode fiber that serves RF applications. At the RF-to-optical (RF/O) conversion unit, a low return loss helps to avoid any damage to the laser emitter.
A high-speed PIN diode usually converts signals to a rate of 0.9 amps per watt for the optical-to-RF (O/RF) conversion. Luckily, the PIN diode has a relatively linear response curve that reduces the nonlinearities and distortions that the conversion introduces into the signal chain. If transformed and amplified into RF energy, the RF signal can be transmitted over short runs to remote radio heads (RRUs) and then to dispersed antennas with conventional RF transmission lines.
The main advantage of using fiber-optic cables for long-distance transmission of RF signals is that fiber-optic transmission shows a signal loss of less than 0.5 dB per transmission kilometer. Tens to hundreds of times worse than fiber optics, coaxial cables tend to experience losses. The frequency of the RF signals depends upon this loss.
Although there are identical bending radius limits for fiber-optic and RF cables, fiber optics experience additional bending loss. Thus, under direct-run installations, they typically work better. These straighter runs do not always pose a major obstacle, as fiber-optic cabling is non-metallic and can be placed with fewer protection and interference issues near high-voltage and electrical cabling than those faced with coaxial cable.
2. As optical fibers are not conductive and not prone to RF interference, the use of an RF-to-Fiber conversion will translate into much lower interference in the transmission line. (Courtesy of the Jack Daniel Business)
In the RF-over-Fiber method, the non-metallic and optical transfer often prevents electromagnetic interference (EMI) and RF interference (RFI) from the transmission line (Fig. 2). Resistive components also create noise naturally, such as non-perfect conductors. Fiber-optic networks, as a result, tend to incorporate less noise than coaxial cabling into the signal chain. Nonetheless, any interference experienced by the electronics converting RF-to-optical or optical-to-RF can translate into noise and interference in the converted signal.
Without direct access to the optical fiber, signal interception is almost impossible, due to the optical existence of fiber cables. The fact that many fiber-optic cables contain several single-mode fibers is another aspect to remember. With the inclusion of additional fibers, since there is little extra cost per cable meter, big, multi-fiber optical cables are also available. These “dark fibers” are easily integrated into future systems, making it easy without substantial costs to update an optical device.
While coaxial cables typically suffer from corrosion, moisture intake, connector loosening, over-voltage sparking, and dielectric breakdown, fiber-optic lines are generally not concerned with such factors. Inside a fiber-optic connector, it is possible for particulates to build up, which would increase connector losses. In addition, in high-heat scenarios, fiber-optic cables may be resilient, which may cause coaxial cable dielectric breakdown. Fire-resistant fiber-optic cables are also available.
A distinct distinction between fiber-optic and coaxial cables is that the conversion electronics “compress” the RF transmission power envelope (Fig. 3). As a consequence, any electrical spikes or noise beyond the RF-to-optical transmitter transmission band will not be carried by optical conversion. Yet the maximum transmission frequency through an optical line is also reduced by this bandpass-like feature, a restriction determined by the RF-to-optical conversion electronics. Equipment widely available varies from up to 4 GHz. It should be remembered that the fiber-optic system acts as a wideband system that can transmit a maximum conversion frequency bandwidth (including many RF and some microwave bands).
3. For transmission and reception from remote radio systems, an RF-over-Fiber system includes RF-to-optical and optical-to-RF conversions. (Courtesy of Fiber-Span; to expand, click the image)
As is the case for an RF system, there is a maximum transmitting capacity that can be maintained and controlled with linearity by an optical transmitter, optical fiber, and optical receiver. The total power is a combination of all the power of the signal over the entire conversion process bandwidth. Band pass filters are mostly used to allow the bands of interest to be passed while reducing power at non-useful frequencies. The channel-power headroom is improved by this strategy. The optical transmission system is somewhat similar to the RF system, because if the same light frequency is present on the fiber, it can create interference. Multiple fibers can be used for multiple uplink and downlink lanes to allow duplex service. There are several pairs of fiber-optic cables, often more than 50.
Duplexing can be achieved on a single fiber using different frequencies of transmission under conditions where restricted fibers are available. This technique is called multiplexing of wavelength-division (WDM). As the various frequencies within a waveguide propagate with different transmission coefficients, an optical fiber has a minimum and maximum frequency of operation. However, unlike an RF system, optical fiber lines are unidirectional and may have many frequencies that transmit through the fiber in the same or different directions.
The popularity of RF-over-Fiber has started to rival conventional RF transmission lines, given its many advantages. There is a strong demand for a neutral host system that can accommodate multiple carriers and public-safety bands with more buildings integrating DASs. Fiber-optic technology is capable of meeting such demands, but in some applications, fiber optics have historically been cost prohibitive. But the maintenance and return-on-investment considerations for RF-over-Fiber will definitely surpass those of coaxial cable as demand for multiple services and digital marketing strategist extremely wide-bandwidth operations have increased. A rapid rate of adoption may also play a role in reducing the cost of optical technology, improving the RF-over-Fiber value proposition.We are publishing this article on the behalf of Servant which is one of the Top manufacturer of microwave components such as TRIHEDRAL CORNER REFLECTOR, Low Noise Amplifiers, DC Blocks, Conical Horn Antennas, Power Dividers, Bias Tees, Frequency Multipliers, and Coaxial Fixed Attenuators & Coaxial High-Pass Filters.
