Fiber-optic cable simplifies in-building video distribution

Mor Allon Foxcom

Used in conjunction with coaxial cable, fiber`s flexibility and low attenuation ensure efficient, economical transmission of video in multi-dwelling units.

Distribution of video within buildings is one of the technical challenges facing service providers today. In itself, video distribution, with its bandwidth requirements, is difficult enough. But the challenge doesn`t end there. Convergence is one of the most popular buzzwords being used in the telecommunications industry today. One service provider may supply customers with video, telephony, high-speed Internet, direct-broadcast, cable television, and a host of other services. Regional Bell operating companies, competitive local-exchange carriers, and cable-TV operators all want to supply the services their customers want. So besides the various financial and legal hurdles that need to be overcome, a basic technical problem must be solved: How do you get these multiple services from the antenna/server/satellite dish/microwave transmitter to the end user?

In a private home, solving this problem is fairly easy because of the short distances involved. But in multi-dwelling units or "garden-style" apartments, the large distances and high number of end users make in-building distribution a complex, time-consuming, and expensive problem. Coaxial cable, the current technology of choice, has several physical limitations that make it an inappropriate vehicle for services with high bandwidth requirements. An alternative technology, fiber-optic cable, can be used as the means to transport bundled services.

Coaxial vs. fiber-optic cable

Before anyone invests in a new technology such as fiber-optic cable, a very basic question must be answered: Why is it needed? What`s wrong with coaxial cable, a known technology that is currently installed in most buildings?

Carriers look for some basic characteristics in their cable technology. The first is low attenuation. Supplying the needed signal strength to the end user is the sine qua non of any distribution system. However, the higher the carrier`s attenuation rate, the more amplifiers and other in-line actives that must be used to supply the needed signal.

The second characteristic is bandwidth. To transmit new services as they become available, higher and higher frequencies are being used. Higher frequencies mean greater bandwidth requirements. While cable TV is transmitted at 500 MHz, transmitting both cable TV and video requires almost 2 GHz of bandwidth.

The third factor is reliability. The more components in a system, the greater the chance the system will fail. That is especially true when the system`s components come from various suppliers. While component X from any one company may be fully reliable, if it is used with component Y from another company, the reliability may be compromised. Therefore, a minimal number of system components is more desirable.

"Future-proofing" represents the fourth factor. As more and more systems, such as LMDS, local wireless loop, and microwave radio, are brought into use, the demands placed on a carrier are compounded. Even if a carrier can handle its current load, will it be able to handle the new technologies as they are brought on-line?

The fifth variable is design. Designing a system for any given building should not amount to reinventing the wheel. System design should not be site-specific, nor should it require special training.

Finally, the sixth factor is expandability. Can the carrier be easily expanded to include another building or floor? If, for example, 200 more units are added to the site, can the property owner easily expand the system to include these new units, or will this be a whole new project requiring a new design?

Several challenges

How well do coaxial cable-based systems face up to all these criteria? Using a coaxial cable-based distribution system poses several major challenges. Designing a distribution system in a multi-dwelling unit with a large number of subscribers, a limited link budget, different building architectures, and serious distances between sites requires a highly trained engineer. And even then the task is not simple.

Coaxial cable`s most important physical characteristic is its high attenuation rate. This rate only increases, and increases rapidly, with the higher frequencies (2 GHz) being brought into use (see Figure). When transmitting a signal at 400 MHz over RG6, a standard coaxial cable loses about 5 dB after 100 ft. However, when transmitting a 2-GHz signal over 1000 ft on the same cable, there is a 100-dB loss. Compensating for this loss means amplifiers, couplers, equalizers, and tabs. All of this equipment means extra complexity to the network. Installing and maintaining such a network becomes much harder. And what is going to happen in the future when services that require 3 or 4 GHz of bandwidth come into use? Are there amplifiers that can handle that load? What else will be required to transmit that signal? More cable?

An additional consideration is the variation in the quality of the components used. Product quality ranges from low (and inexpensive) to high (but expensive). Figuring out where to place all of the needed components is not something an amateur can do. Furthermore, even if someone does know how to design a well-made coaxial cable-based network, a unique configuration must be designed for each building. Any variation in the building architecture requires a corresponding change in the network design.

Lastly, expanding a coaxial cable-based carrier is not so simple. In fact, going from a network that serves 100 subscribers to one that serves 250 subscribers basically entails building a new network.

When the same criteria are applied to a fiber-based system, a very different picture emerges. Fiber-optic cable has two physical characteristics that make it a much more suitable carrier than coaxial cable: an extremely low attenuation rate at any bandwidth (0.6 dB/mile) and unlimited bandwidth. The current bandwidth limit of 2.5 GHz is based on electronic components, not the cable itself. As technology improves, fiber-optic cables will be able to carry even higher bandwidths.

Once the attenuation rate becomes so low, the need for inline amplification equipment is eliminated. No amplification equipment means that the network design becomes extremely easy. Simply lay the line. If for some reason, extra line is needed, the installer simply lays more line. Adding extra amplifiers to make up the lost signal strength is unnecessary. Reliability increases because the number of components has been dramatically reduced. Conversely, maintaining a system that has a small number of components becomes that much easier.

Fiber-optic bandwidth capacities make such technology ideal for planning for future needs. The cable can handle 3 GHz as well as it handles 500 MHz. There is no need to add any other components or even more cable. Any new services the provider offers can be put onto the same cable, without needing to upgrade a host of inline components.

Combining both technologies

With this background in mind, a fiber-optic/coaxial-cable hybrid approach to video distribution benefits from the advantages of both technologies. Such an approach uses a fiber-optic backbone to distribute the video signals to distribution points throughout the building. At the distribution points, all other VHF and UHF signals are added and then transmitted to each end user via a short coaxial cable. The end result is that the customers receive all services over one cable. With the use of a fiber-optic backbone, all of the advantages of fiber optics come into play: a small number of components and virtually no actives or no engineering. Further, the use of a return-path capability allows the user to transmit data, thereby allowing super high-speed Internet services.

The proprietary SDTV system exemplifies such an approach (see photo on page 72). A stacked LNBF unit upconverts one polarity (either the horizontal polarity or the right-hand circular polarity) and stacks it, along with the vertical polarity or the left-hand circular polarity, onto a single output. The signal is sent via coaxial cable to the transmitter, which may have 4, 8, or 12 ports. The entire signal is then transmitted over a single fiber-optic cable to one or several receivers, which may be located throughout the building. In the receiver, the optical signal is reconverted to RF, combined with off-air channels, and transmitted to each apartment via a single coaxial cable.

Each transmitter port can connect to a receiver. Each receiver has 16 ports that connect to individual apartments via the coaxial cable (home run) of the building`s existing infrastructure. Inside the apartment, the cable connects to a subscriber unit (downconverter) for each integrated receiver/decoder (IRD). The subscriber unit downconverts the L-Band signal back into V/H or right-hand circular/left-hand circular polarizations, switched by the 13 or 18 VDC from the IRD. These signals are passed to user-supplied set-top IRDs that descramble the signal. This one-cable approach enables the user to connect multiple TV sets using the apartment`s existing coaxial cable.

Using a fiber-optic backbone allows service providers to supply their customers with video and other services easily and affordably. The simplicity and ease of use of hybrid fiber-optic/coaxial-cable systems is directly related to the physical properties of fiber-optic cable. Since fiber-optic cable has such a low attenuation rate, no in-line devices are needed to deliver a proper signal. Once these devices have been eliminated, system design, installation, and maintenance become correspondingly easier. Expanding a system becomes a simple task.

Fiber-optic cable`s bandwidth allows a multitude of services to be transmitted over a single cable. As technology improves, more and more services can be put on the same cable without any additional inline devices. u

Mor Allon is vice president of business development at Foxcom (Jerusalem, Israel).