Fiber for satellite signals
Trading bulky coaxial cable for modern fiber optics yields a solution for issues such as attenuation, slope and electrical isolation of satellite L-band signals. Known for its ability to transport signals over long distances, fiber optics opens up new possibilities, particularly with remotely located satellite antennas, while the promise of excellent signal quality bolsters critical applications. The implementation of fiber-optic systems does require extra care and considerations, but, when done effectively, they can provide excellent performance for years to come.
Why fiber optics for satellite L-band signals?
Simply put, fiber-optic links provide better performance over longer distances than coaxial cable. The quality and capability of coaxial cable varies considerably based on the type, but often its practical boundaries start at about 100m. This length can be extended with the help of amplifiers and slope compensation, but with this can come added noise to the signal. With lower-end fiber-optic equipment, the attainable distance can be as long as 20km. When fiber transmitters are equipped with higher-quality DFB lasers and receivers with high-sensitivity photodiodes, a fiber link can extend to more than 100km without optical amplification. These distances are attainable with little to no degradation in signal C/N for low error rates at the IRD. Also, coax loss increases with respect to frequency, causing the signal to slope or tilt. With fiber, there is no slope imparted to the L-band spectrum regardless of the length of the link.
Physically, fiber is small in diameter, lightweight and flexible. This offers space and routing advantages over stiff, bulky coax, especially when compared with hard-line cables. Fiber also provides electrical isolation, eliminating ground loop and lighting issues.
Applications for satellite fiber links
The most obvious use for fiber links in satellite applications is to connect antennas far from the facility that can't be reached using coax. This may be on the same campus or to meet long-haul requirements, which can be fulfilled as long as there is fiber connectivity available between points. A company requiring new satellite feeds may have within its organization another facility with antennas that can supply those feeds. Fiber-optic links make it possible to transport and use those feeds, avoiding the expense and space requirements of installing antennas.
In high-reliability applications, satellite antenna redundancy is straightforward; effectively using the redundancy is another matter. Rain fade can wipe out the signals on both main and redundant antennas if they're physically too close together. Connecting antennas at a sufficient distance from each other can place them in different local weather conditions. If one antenna's signal is degraded by weather, then the other antenna's signal can be used instead. This also protects against physically damaging weather phenomena, which may pass through an area and damage one set of antennas, while leaving the antennas located at a safe distance intact.
Fiber is superior in situations where signal quality is of the utmost concern. Motivated by the increased bandwidth requirements of HD programming, efforts to increase transponder throughput have spawned DVB-S2. In return for throughput gains at higher-order modulation and coding rates (e.g., 8PSK and 9/10 FEC), this technology demands higher input signal quality and C/N at the IRD, which strains the abilities of coax infrastructures.
Another application is carrier monitoring, in which the signal from the low-noise block converter must be preserved. In circumstances like these, even shorter runs benefit from a fiber link's ability to maintain signal quality.
How L-band fiber links work
L-band fiber-optic links use a straightforward process to transport signals. At the fiber transmitter, the incoming electrical signal is converted to light using amplitude modulation. This light travels down the fiber and is converted back to an electrical L-band signal by the fiber receiver. As with other devices looking for a satellite signal as input, fiber transmitters and receivers operate within a fixed dynamic power range, so the dish size and low-noise block converter gain must provide a suitable signal level. L-band fiber transmitters typically also generate low-noise block converter power, eliminating the need for discrete low-noise block converter power supplies.
Multimode fiber can be used for L-band transport where existing infrastructure dictates, but this comes at the expense of attainable distance. Given the choice, single-mode fiber should always be used for the best performance and longest distance.
Signal loss on the fiber must also be considered because attenuation of the optical signal directly translates into attenuation of the L-band signal. Fiber transmitters are specified with a laser launch power, while fiber receivers are specified with an optical input sensitivity. These two figures dictate the allowable link loss budget on the fiber while still being able to maintain a certain C/N level.
The condition of the fiber itself and fiber connectors also comes into play. Issues such as broken fiber cores, poor fusion splices and dirty or unseated fiber patches cause extra fiber loss and can also cause the light within the fiber to reflect at these points. When reflections occur, the noise floor of the signal at the output of the fiber receiver can be negatively affected in various ways, impacting the recoverability and error rate of the desired signal. Optical time-domain reflectometer reports can show whether there are any significant discontinuities that could cause reflection issues.
Connector types also have a strong influence. In terms of reflection, angle-polished connectors (APC) are preferred because they dissipate reflections generated at the connector interface. Ultra-polished connectors (UPC) are the next best choice because they too dissipate reflections, but not as effectively as APCs. Standard flat connectors (PC) are the least favorable and should only be implemented in cases where there are existing infrastructure limitations. Usable performance can be obtained from PC connectors; however, extra attention must be paid to achieving the best possible seating and cleanliness of every connector interface.
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It's important to choose the right fiber equipment for your specific application to ensure that it does the job as required for years to come. Be mindful of the following:
- Form factorConsider the choice of centrally located rack-mount equipment or small, stand-alone devices that are temperature-hardened and may be mounted on or near the antennas.
- ExpandabilityDense, modular systems require the minimum amount of rack space and allow for easy future expansion as signal requirements increase.
- Monitoring and controlSNMP remote monitoring and control of parameters such as gain levels, RF power and optical levels are useful not only for the fiber link and associated gear, but also other parts of the RF system, such as monitoring the health of connected low-noise block converters.
- Specifications and featuresCheck for good system RF specs, including return loss and frequency response. Look for fiber transmitters and receivers with flexible, manually adjustable and automatic gain control modes to provide optimal laser drive and fiber receiver output levels.
- Lasers and receiversLook for sufficient laser launch power and receiver sensitivity to provide the required C/N performance over the link loss of your fiber. In fiber-limited or leased-fiber applications, wavelength-division multiplexing (WDM) is a technology that allows multiple lasers with different wavelengths, or colors of laser light, to be combined and separated on a single fiber. Coarse wavelength-division multiplexing (CWDM) combines up to 16 signals on a single fiber, while dense wavelength-division multiplexing (DWDM) combines up to 40 signals. Aggregating signals this way saves cost on leased fibers.
Fiber-optic systems enable signals to traverse longer distances than what is possible with traditional coaxial cables. Fiber optics' lack of signal degradation not only improves the reliability of existing applications, but also opens up the possibility of new ones. As satellite technology moves forward, fiber optics will continue to play an important role in providing the signal quality and distance needed to support future innovations.
Darren Ward is RF product manager for Evertz Microsystems.
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