Wireless Cable - A Competing Technology:  Technical Note 106

By Glyn Bostick, President
And Elizabeth Buck, Marketing Research and Publicity
Microwave Filter Co. Inc.

Figure 1. Typical wireless cable system. 

The picture still remains rose-colored for the growth of wireless cable, despite the gray landscape painted by the economic climate this year. Over the last three years, Wireless Cable systems have been materializing in all regions of the country where few if any previously existed. The Wireless Cable Report (May 29, 1991) cited that Wireless Cable systems currently exist in the top 10 TV markets. While traditionally urban areas have been ideal settings for Wireless Cable, the medium also is becoming more popular out in the country.

"There are few rural systems at this point. However, some system developers are looking at rural areas and see great potential. We’re working on close to 100 rural systems right now," said Lawrence Behr of Lawrence Behr Associates, a consulting engineering firm specializing in Wireless Cable and other telecommunications areas. "With the proper business plan you can do business anywhere. The emergence of Wireless Cable systems closely resembles the development of FM radio in rural areas—an audience has to be developed," he said.

Two schools of thought have traditionally existed with respect to Wireless Cable. The first is the medium should locate in areas where Cable TV would never venture because low population densities would make laying cable cost-prohibitive. The other is to locate in an urban area and take away a percentage of Cable TV’s customers.

"At the present time competition is at best marginal,” said Behr. "However, it’s possible to examine the demographics of an urban area with several million people and cherry pick away 10 percent of Cable TV’s subscribers by offering a new service to unhappy Cable TV customers," he added.

Congress wants competition

Anti-cable sentiment in Congress has facilitated the growth of Wireless Cable. There is a commonly held belief among some members on Capitol Hill that there should be some competition to Cable TV. Wireless Cable is perceived as one of those alternatives. Over the last few years Wireless Cable has been struggling to acquire programming at the same rate card as Cable TV systems. Some progress has been made. There is current legislation in Congress concerning access to programming. The Wireless Cable industry also now has access to all cable programming except for TNT and a few operator-owned regional sports services. However, this programming is not always available at "fair and equitable prices," according to information published in the previously mentioned issue of Wireless Cable Report.

The positive political sentiment and growth of systems throughout the United States also has inspired confidence within a previously reticent financial community. This is despite the fact that MetroTEN, a Wireless Cable company in Cleveland, filed for Chapter 11 recently and last year Microband also faced financial problems.

"Both companies were burdened by programming contracts. It’s possible to cut deals at better prices now. MetroTEN also set up a system with the anticipation of selling the same service as Cable TV when it would have been better to market itself as a lower cost alternative," said Behr.

Some changes in Federal Communications Commission filing rules also have offered a boost to the growth of the Wireless Cable industry. Cable TV operators are precluded from applying for multichannel multipoint distribution service (MMDS) frequencies in their franchise area. One applicant may now file for both MMDS Groups E and F, whereas previously these frequency groups could not be held by one party. A one-day filing rule also has substituted the public notice waiting period. As well, Wireless Cable is allowed to transmit up to 100 watts with maximum allowable output now defined in terms of effective isotropic radiated power.

Cable TV and Wireless Cable are odd bedfellows in that they compete and complement each other. Both have weaknesses technologically and neither one is designed for installation in all areas of this country. In one instance where a Cable TV and Wireless Cable system compete, the wireless system has 31 channels and a large MSO-owned system has 35. The Wireless Cable system charges $2 less per month for basic rate. Wireless Cable is a less expensive medium to install. "At significant penetration the cost per sub could be one-third to one-half of that for cable. It may cost Cable TV $1,000 per sub vs. $350 for Wireless Cable," said Behr.

However, Wireless Cable is limited to only 32 channels in the majority of areas while Cable TV can offer around 60. Cable TV is impeded by the high cost of coax while Wireless Cable, as a line-of-site transmission medium, is easily hindered by obstacles in its broadcast path. Cable TV is subject to occasional outages provoked by weather conditions and Wireless Cable experiences occasional signal fade as a result of atmospheric conditions. It is easier to some extent to repair problems on Wireless Cable systems because the entire system is above ground, however expertise in maintenance must be standardized .

"Cable TV has a trade association that concerns itself with system standards. This is an area Wireless Cable needs to develop because individual installations are more intense. You risk your business if you use untrained installers who mount rooftop antennas incorrectly or have no knowledge of antenna grounding," said Behr.

The emergence of wireless cable

The origin of wireless cable dates back to the mid-1970s when the FCC allocated two channels in the 2,150 to 2,162 MHz band for multipoint distribution service (MDS). MDS 1 is 6 MHz wide and capable of video carriage. MDS 2 is 6 MHz only in the top 50 markets of the country. In remaining markets, channel width is 4 MHz and it is used only for data or voice delivery. In the late-1970s, MDS was a video delivery system used to provide single-channel programming (such as HBO) to subscribers. The service rapidly lost appeal when cable came into town and began offering multiple channels of programming. Since MDS was broadcast over-the-air, it also was prone to pirates who intercepted the then unscrambled signal. However, an ability to offer more channels became the primary concern of the MDS industry.

In the mid-1980s two groups, E and F, consisting of four channels each (2,596-2,644 MHz) were reallocated for entertainment video delivery. Originally these channels were part of the instructional television fixed services (ITFS). ITFS, which consists of five groups of four channels (A, B, C, D and G), is a frequency allocation for educational programming available to educational institutions, hospitals and religious organizations. In the late-1980s three channels located in the H group (2,650-2,680 MHz) were allocated for operational fixed services (OFS). The intended use for this band was for data communications, but it was not restricted from video delivery. Hence, the OFS became another source for video.

With eight reallocated channels, three OFS channels, special leasing arrangements with ITFS license holders for excess channel space and two MDS frequencies, the new Wireless Cable market formed. The service became capable of offering 33 channels in the top 50 markets, which have two 6 MHz MDS channels, and can offer 32 channels in all other markets.

A large obstacle faced by prospective Wireless Cable operators is assembling channels for a system. This requires applying for frequency licenses and/or forming lease agreements with other frequency license holders. In the case of leasing channels from an educational institution, the operator must make arrangements to channel map - provide continued channel service when those frequencies are used for educational purposes. Educational institutions holding ITFS licenses are obligated to use the channels a minimum of 12 hours a week for the first two years, and then 20 hours a week thereafter. Wireless Cable operators do not have to acquire a franchise although this has been challenged in at least one area.

How wireless cable operates

Wireless Cable receives most of its programs via satellite and rebroadcasts them over-the-air on microwave frequencies to its subscribers (see Figure 1). A typical transmitter emits one of the channels in the MMDS, MDS, ITFS or OFS groups. Transmitters are made with various wattage outputs, depending on planned coverage, with a maximum of 100 watts visual power.

Effective isotropic radiated power (EIRP) is defined in the direction of strongest antenna radiation in-line with the beam axis. If the antenna radiates power equally in all directions and the total power radiated was 100 watts, then the EIRP, seen at any receiver site, would be 100 watts. However, Wireless Cable antennas have shaped beams that "bunch" the power in favored directions and increase the power received in the strongest direction by the "bunching" or gain of the antenna. For example, an antenna rated at 20 dBi gain at its peak, increases normal power density at a receiving site in that direction by a factor of 100. Therefore, the EIRP in the strongest direction is 100 watts x 100 =10,000 watts EIRP. The receiving site, in-line with the beam, "thinks" the antenna is throwing its power equally in all directions and that the total power radiated is 10,000 watts.

The radius by which a receive site may be separated from the transmitting station is proportional to the EIRP of the transmitter/antenna combination. Specifically, it is proportional to the square root of the EIRP. Thus, if the EIRP is suddenly reduced by 50 percent, the maximum receiving distance reduces to 70 percent of the original radius. For the Wireless Cable operator, maximizing EIRP means reaching the greatest number of potential subscribers. If the system experiences a 50 percent reduction in EIRP, its coverage circle (with a radius reduced to 70 percent) is only half the former area - and hence, has lost 50 percent of its original potential subscribers. Therefore, it is important that the installation’s transmission line system, which connects transmitters to the antenna, dissipates as little of the microwave power as possible. The transmission line system consists essentially, of the special filter networks that combine transmitters to the tower transmission line and the transmission line itself.

The transmission line is a critical system component whose selection affects coverage and capital equipment cost. Available antennas have relatively high efficiencies with no prospect soon for substantial improvement. Coverage can be optimized only by selecting the type or model that best fits the potential subscriber area. The key parameter of the transmission line is its power dissipation, which reduces the power available to the antenna. Given a mandated coverage, transmission line attenuation is a factor in determining the required transmitter output power. And an increased transmitter power output requirement is a significant cost factor.

Waveguide transmission line is most frequently used because of its lower attenuation in tower runs, which may be 500 feet or more. The most frequently used type is a flexible, copper, elliptical waveguide EW-20, which has an attenuation of approximately 0.49 dB/100 feet. A 600-foot run of this waveguide would have an attenuation of approximately 3 dB and reduce the transmitter power to only 50 percent of the value leaving the transmitter. To minimize line losses, the actual transmitter room is often located on the top floor of a tall building. Coaxial lines have much more loss than waveguide, but for very short runs it is often used because of its lower cost. Rigid 7/8-inch coaxial line is frequently used for very short runs. This line consists of a copper tube (outer conductor), inner copper rod (center conductor) and air or low-loss foam dielectric.

Figure 2.
Wireless Cable broadcast antenna.

For an installation broadcasting more than one channel, and hence using multiple channel transmitters (see accompanying photograph), special filters called channel combiners are used to combine the outputs of the channel transmitters to the tower transmission line for transport to the common broadcast antenna (Figure 2). The various brands and schemes of combining filters can be arranged to combine any number of channels from two to 16. The limitation to 16 channels is due to the fact that most current filter techniques cannot combine channels adjacent to one another at low-loss and there are only 32 channels available to Wireless Cable.

The important characteristics of wireless broadcast antennas are radiation patterns (to fit the shape of the potential subscriber area), bandwidth (to receive multiple or random channels with high efficiency), power capacity (to withstand the combined power of several channels), and polarization (vertical or horizontal). Antennas with a variety of radiation patterns are available to fit almost any coverage shape.

A broadcast station located in the center of its coverage area would have a horizontally omnidirectional radiation pattern (Figure 3), radiating equally in all azimuth directions. But it would have a very sharp elevation pattern, reducing its radiation sharply above and below the horizon so as not to waste power radiating into the sky and into the ground. The deflection of the beam tip (below horizontal) can be controlled to further concentrate power on subscribers within the exact target area.

Sometimes a suitable tower site is available only on the edge of the desired coverage area. In this case the antenna can be designed with a “lopsided” azimuth distribution to concentrate the power only within the desired area and to minimize radiation in other directions (see Figures 4 and 5).

Many Wireless Cable broadcast antennas are broadband and can accommodate any channel in the 2,500-2,686 MHz band. They are available with either vertical or horizontal polarization. Omnidirectional antennas (equal radiation in all directions) must be mounted as the highest element on the tower (except for safety lights) to prevent blockage, while antennas of less than 360x coverage may be mounted to the side of the tower or building.

The transmitting antenna can handle up to five full-power (100 watt) channels or a greater number of lower wattage channels. A popular design is tubular and its surface populated with discreet radiating elements, forming antenna arrays. Elements can be phased to give the various azimuth distributions and beam tilts below horizontal. Increasing gain is realized by stacking an increasing number of “bays” or shorter tubes. Antennas are available with gains of about 8 to 22 dBi, and with downward tilt angles (for the main beam) to optimize targeting to the coverage area. Antennas with a number of horizontal plane radiation patterns are available. These include those with uniform 360x coverage for centrally located broadcast facilities as well as special power azimuthal distributions to fit situations where the broadcast facility is at the edge of its coverage area or where subscribers are concentrated in two separate areas.

A transmitter malfunction removes a single channel from the broadcast menu. If the antenna malfunctions, the entire installation is out. Therefore, design and construction must concentrate on high reliability and long life. To achieve the necessary ruggedness, each radiating element must be stable during wide temperature swings and sealed against moisture. The all-metal cylinder containing the radiating elements must have a mounting provision to ensure mechanical rigidity against high winds and the entire antenna is usually encased in a tough plastic radome to prevent the accumulation of foreign matter near the radiating elements.

At the subscribers' end

The receiving site antenna (Figure 6) is equipped with a block downconverter, which converts the microwave channels to regular VHF TV channels. The output of the block downconverter is fed into the internal cable system of the building, which may be a private home, an apartment house, hospital, school or campus building. Most subscriber or institutional TV sets are equipped with a channel converter to facilitate adding local off-air TV programming to the receiving network. The converters are similar or identical to those used by Cable TV systems for their subscribers. In the case of Wireless Cable, the converter also incorporates a device for decoding or descrambling the signal. The receive antenna is used in large quantities by Wireless Cable operators and must be relatively inexpensive. Nevertheless, it must have high mechanical reliability to minimize service calls. Since its function is critical to reception quality and in establishing the maximum operating radius from the system (and therefore maximizing potential audience), it must have good electrical performance as well. Most receiving antennas are parabolic dishes with the surface of the dish formed by a series of parallel metal rods. This construction minimizes wind loading and ice buildup. The feed is usually a dipole with reflector connected directly to the block downconverter to minimize cable loss at the microwave receive frequency.

Antennas are available with gains from about 12 to 27 dBi. The greater the gain, the farther the subscriber can be located from the broadcast tower and still receive clear pictures. Gain is proportional to the capture area of the antenna, which is determined by the physical area. The 27 dBi antenna captures about 32 times as much signal as does the 12 dBi antenna and can therefore be about 5.5 times (square root of 32) the distance from the broadcast station and experience the same reception quality. The antenna is mounted to a mounting mast through a swivel that allows it to be rotated for either polarization - horizontal or vertical - to match the transmitted polarization. Where the broadcast facility radiates some channels on vertical and some on horizontal polarization, the receiving antenna may be rotated 45x to receive all channels. Where the broadcast facility maintains two separate towers, some distance apart, two receive antennas are required (one pointed at each tower).

The block downconverter amplifies and changes the microwave signals to VHF channels, in the midband, superband or hyperband so as not to conflict with off- air channels (2-13), which the subscriber may be receiving on a regular TV antenna. Models are available for targeting Wireless Cable channels to any of these three bands, as well as special dual models where more than one Wireless Cable receiving antenna is used. The operation of the block downconverter is critical to receiving quality and system range. Its gain, noise figure and other characteristics, together with other system characteristics, establish the noise floor, which determines the minimum useful received signal.  Since they must be mounted as close to the antenna as possible to minimize microwave cable loss, they are weatherized. The set-top converter is similar or identical to those used by Cable TV subscribers. It allows subscribers to continue to receive any off-air TV channels they have been receiving plus the additional wireless downconverted channels.

Figure 6. Subscriber receive antenna.