Part 2-Fiber-based systems can extend antennas away from interference, provide fill coverage and service high-traffic areas.

Filling in "dead" zones, extending coverage into high traffic and providing service where existing system equipment does not penetrate have been serious issues for LMR, SMR, PCS and cellular providers for some time. Fiber-optic-based systems are one route available to service providers and site owners to enhance coverage.

Lack of coverage for wireless applications and service can be caused by a wide range of structures or circumstances. The classic problem of coverage within tunnels, multistory parking structures or large sports complexes can be solved with a number of "off-the-shelf" solutions. Relocatable or low-power repeaters located high in the structure or complex will work. Small, microwave-linked microcell units can also provide acceptable signal strength for any wireless application. Somewhat more difficult to engineer is the radiating cable and bi-directional amplifier solution. Many companies, such as 3dbm Inc, of California, make a complete line of these RF products.

Each of these solutions brings special considerations. Microwave links, for example, must take both line of sight and human exposure issues into consideration. The radiating cable solution (MRT, May 1998) requires careful system design and balance of supporting amplifiers to cable antenuation. In all of the solutions mentioned, ac power, some form of backup power and access for periodic maintenance must be part of the design consideration. Another important factor is the weight of the equipment and cable/antenna assembly. A bi-directional amplifier, for example, can weigh in at 35 pounds with a weather-resistant enclosure. Some microwave-linked microcell system components total more than 100 pounds-an indication of solid construction. These requirements and weights, however, may lead to limitations in some circumstances.

A final system consideration is point source EMC/RFI issues. You may have areas with high noise levels, or other facility users may consider your system a noise source. More facility owners, such as hospitals, are becoming aware of RF levels within their buildings. Routing of cable assemblies becomes critical around high-powered equipment, such as elevator motors and controls or large ac mains transformers. Powerful electrical fields from these devices can induce distortion in, or cause interference to, desired signals.

A fiber-optic-based system may be a good answer to many of these issues. A combination of direct RF-to-optic signal conversion for quality reproduction and lightweight fiber-optic cable can allow you to provide service where a more conventional solution would be problematic. Extending an antenna away from interference, providing fill coverage in large structures without the use of a radiating cable or servicing small, high-traffic areas can all be performed by a fiber-based system.

A typical system has a fiber-optic transmitter that provides a linear RF to optical conversion. Direct modulation of a laser diode to produce the optical signal reduces system design considerations in terms of modulation conversion loss or distortion caused by intermodulation distortion products. Use of a low-power laser diode ensures an ample power budget for larger systems. A linear PIN diode-based receiver converts the optic signal to RF, and an integrated amplifier can be used to set final output power levels to the antenna, thus ensuring that the system is easy to balance. System components can include a fiber-optic transceiver, which allows a single box to perform duties as a microcell. Fiber cable is both lightweight and small, making installations in small or tight spaces much easier on the technician.

New Jersey-based Anacom Systems (www.anacomsystems.com or 908-846-2680) makes a complete line of these products. Photo 1 above shows an optical transceiver unit. Using the connectors as a reference, you can see just how small these units are made. Photo 2 above shows an even smaller transmitter and receiver set. Photo 3 above gives you an idea of the uncomplicated makeup of this type of unit. Maintenance is certainly simplified. You can see just how easy it would be to fit these small units into a ceiling space or the roof of a sports complex. Anacom can supply design guides for a specific wireless application.

In addition to its inherent size and weight advantages, fiber-optic cable is immune from EMI/RFI considerations, so following main ac feeds to a remote location is now an option for the installer, saving both time and money on installs. Antenna placement is still a critical factor, but a wider range of placement is permitted by the use of a fiber-optic system. The short length of coax cable between the antenna and optical transceiver component reduces the amount of interference introduced into the overall system.

As mentioned in Part 1 (MRT, March 1999), the technician will require some additional test equipment. An optical power meter and source are the minimum necessities for either installation or troubleshooting of a fiber-optic system. These can be obtained from Wavetek or Fluke, among others. As in a LAN system, most of the problems I have encountered in a fiber-optic communications system can be traced to a cable problem of some sort. Technicians should carefully read manufacturers' manuals or specification sheets for minimum allowed bend radius, free span (unsupported) lengths and frequency. Fiber cables are manufactured for peak efficiency at specific wavelengths, typically 960nm or 1,300nm, so be sure that the cable you install matches the transmitter/receiver frequency. I mention this only because I have had to fix problems caused by "mix and match" of cable types.

All of this may sound both expensive and difficult to obtain; so the technician who might want to tinker on his own or the student who wants to learn more can be scared off by assumed cost. Nothing could be further from the truth. Several companies make kits and student experimental systems that are reasonably priced. You can use the World Wide Web: www.electronickits.com/kit/complete/fibe/ck1500.htm to purchase a low-cost fiber-optic kit from Carl's Electronics, P.O. Box 182, Sterling, MA 01564, for less than $30. A second vendor, Alltronics (www.alltronics.com/kits.htm) has a kit you can use to drive a remote speaker. Both provide valuable hands-on experience with different types of optical communication systems.

Learning more about, and preparing to use, fiber-optic-based systems will open a new dimension to your busines, allowing you to offer a wider range of solutions to RF carrier problems you or your customers may experience. Fiber-optic components are dropping in price, use less power on the whole and allow greater flexibility in system placement. The ability to virtually ignore EMI/RFI point sources adds a potential for cost savings in installations near high-power ac equipment found in factories or large buildings. Fiber optics has a bright future, use it to ensure yours.