Technical Note 115: 
Wireless Evolution, Definition and Current Practice

Definition
There are several over-the-air delivery systems for TV entertainment, including LPTV: Low Power TV, for example. By general agreement, the one broadcasting at microwave frequencies (2500-2700 MHz US 2300-2400 Australia) is known as “Wireless Cable”. This includes the ITFS service, for broadcast of instructional programming within educational and religious institutions, since it uses the same frequency band and identical broadcast equipment: transmitters and antennas. The name “Wireless Cable” also includes an older band (2150-2162) known as MDS. Programming is broadcast from a location central to the served area and is received by roof-top microwave dishes.

History
The frequency band 2500-2686 MHz was initially reserved to educational institutions for over-the-air transmission of instructional TV programs. Transmission was point-to-point, campus-to-campus, for example. Hence the name: Instructional Television, Fixed Service (ITFS). The power authorized (up to 100 watts) allowed transmit/receive separations of up to 30-40 miles.

The coexisting commercial MDS band 2150-2162 MHz was used by commercial operators for over-the-air Pay-TV transmissions to roof-top antennas on apartment buildings and private homes and for business data transmissions. Transmitters were similar to those for ITFS service and the range of these installations was also 30-40 miles. The transmitter site was centered in the coverage area, usually a city, on a preexisting tower or atop a tall building.

The antennas were usually omnidirectional to reach all subscribers in the “coverage circle”. Hence the name: Multi-Point Distribution Services (MDS). This service was the beginning of what we now know as “Wireless Cable”. MDS was conceived as an alternate or supplement to conventional cable (CATV). It was more successful in areas not covered by CATV. In areas where both services were available it was severely challenged: it could offer only two TV channels versus dozens by CATV. Hence, after the novelty of MDS wore off, MDS revenues declined. Clearly one remedy was more channels and the commercial wireless operators examined with interest the lightly used ITFS channels, reserved for educational purposes.

As a result, the FCC reallocated eight of the ITFS channels (Groups E and F) for use by commercial over-the-air pay-TV operations. Since this allowed simultaneous broadcast of many more channels (other than MDS), the practice of using these new channels became MMDS: Multi-Channel Multi-Point Distribution Service. Many MDS operations have acquired MMDS channel licenses and, in some cases, newly licensed MMDS operators have acquired older MDS operations. For purposes of describing operations we should consider these two services technically identical.

Almost simultaneously, the FCC allocated 3 ITFS channels (in Group H) to the relatively new OFS: Operational Fixed Service. This is a point-to-point transmit/receive service used primarily for the transfer of business information. Since the ITFS-MMDS band now provides several channels for both of the original MDS purposes, the latter has virtually ceased to grow in terms of new installations.

Prospects
Four factors appear to promote the survival and growth of Wireless Cable. Perhaps the factor most responsible for its survival has been its concentration on population areas not cost effective for coverage by conventional CATV. In this area it does not have to compete with CATV. The future of this advantage is uncertain as the cable industry completes the wiring of urban areas and re-examines the business possibilities in traditionally underserved areas. However, for the time being, MMDS has little competition in covering rural areas.

The second factor has been the diligence of the MMDS community in acquiring more channels to approach the superior “subscriber perceived value” of multichannel CATV. In this respect, signal events have been winning allocation of some ITFS channels and the right to lease underused channels from educational institutions.

A third factor is technological advances which allow more programs per channel and/or decrease capital and operating cost per channel. For example, one innovation permits transmitting two separate programs on one channel while another requires only one transmitter for eight different channels.

A typical MMDS installation requires that line-of-sight be maintained between the transmit antenna and receive antenna.

The fourth factor is the inherently lower cost of building and operating a wireless system as compared to a CATV system: less than $1,000 per subscriber versus over $2,000 per subscriber for CATV. The major CATV investment is in constructing the cable network throughout a locality so as to connect it directly to each subscriber. In Wireless this is not necessary. Its omission not only decreases cost of building the system, but network maintenance is obviated. This economic advantage of Wireless is enhanced by the recent escalation of CATV subscriber rates since CATV deregulation.

Each of these factors alone is probably not enough to insure the long term viability of Wireless Cable. However, all four factors operating simultaneously should secure the future for Wireless Cable, especially if the industry is careful to concentrate on unserved areas until advancing technology equips it with enough channels to go head-to-head in CATV areas.

Allocated Frequencies
Table I shows the frequency scheme of the original US ITFS band. It has 31 channels, each 6 MHz wide, as for VHF-TV channels. Relative locations (within a channel) of the video, color and aural frequencies are identical. (See Appendix A for Wireless system channel allocations in Australia and the British Isles). Because the channels are contiguous (no guard band between them) group designations are given to clusters of 4 non-adjacent channels. This facilitates combining several transmitters, with feasible filter techniques, for the purpose of using one transmission line leading up to the common broadcast antenna.

Table I - Instructional TV Fixed Service (ITFS)

Many wonder why some MMDS facilities broadcast more than the 8 channels allocated to MMDS. The answer is that they are allowed to lease ITFS channels from educational institutions during inactive hours. It is estimated that the typical ITFS channel broadcasts less than 20 hours per week, usually during “nonprime” time. Hence, this “horsetrade” allows MMDS to expand its prime time entertainment and the leasing fees defray a significant portion of the ITFS facility capital and operating costs.

The Transmission Site
For simplicity, we’ll refer to all over-the-air systems as Wireless, it being understood that this will include all services operating identical equipment and installations, such as MMDS, ITFS, OFS and MDS.

The Location - High Is Beautiful
The Wireless operator locates the transmitting antenna at the highest feasible elevation. Elevations of 500 feet or more are common. Because microwave transmission is limited to line-of-sight, the higher the antenna the greater the effective range. For the educational system, this means distribution of information to a larger number of campuses disbursed over a wide area. For the over-the-air TV provider, it means a larger potential audience of paying subscribers. To minimize expense, it is common practice to lease space on an existing tower or atop a tall building.

The Transmitter Feed (Input)
Programs carried on college TV networks, or originated in classrooms or studios are up-converted from regular VHF channel frequencies to the microwave frequency band (2500 -2700 MHz) and supplied to the Wireless channel transmitters which amplify and broadcast the information on microwave channels. Entertainment operators receive most of their programs via satellite and, in a similar manner, these are broadcast on the Wireless microwave frequencies. Typical fare is CNN, ESPN, CBN, WTBS, etc. Usually regular off air TV programming is incorporated in wireless broadcasting.

The Microwave Transmitter
The typical transmitter emits one of the channels shown in Table I. Transmitters are made with various wattage outputs, depending on planned coverage, with a maximum of 100 watts visual power. Most transmitters have two outputs: separate outputs for visual (picture) and aural (sound) signals. If so, visual and aural are combined to a single output using a filter called a video/audio DIPLEXER. The combined output is fed to a tower transmission line for transport to the broadcast antenna. Where the site has more than one transmitter (is broadcasting several channels) diplexer outputs are combined with another type of filter network called a CHANNEL COMBINER, before connection to the antenna transmission line.

Single channel transmitters are supplied by a small group of manufacturers. Recently COMBAND has perfected a unique system which allows TWO separate entertainment programs to be transmitted on ONE microwave channel. This doubles the number of entertainment channels from a given Wireless station. MDS Systems Ltd. has just announced a system to allow up to eight separate channels to be broadcast by only one transmitter, greatly reducing the capital cost of station equipment per channel offered.

Effective Isotropic Radiated Power - EIRP
The 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 antennas have shaped beams which “bunch” the power in favored directions and increase the power received in the strongest direction by the “bunching” or GAIN of 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.

Maximizing Radiated Power - and Range
The radius by which a receiving site may be separated from the transmitting station is proportional to the Effective Isotropic Radiated Power (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%, the maximum receiving distance reduces to 70% of the original radius. Therefore, maximizing EIRP is important for reaching distant campuses. For the Pay-TV (MMDS) operator maximizing EIRP means reaching the greatest number of potential subscribers. If he experiences a 50% reduction in EIRP, his coverage circle (with a radius reduced to 70%) is only half the former area - and hence he has lost 50% of his original potential subscribers. Therefore, it is important that the installation’s transmission line system, which connects transmitters to the antenna, dissipate as little of the microwave power as possible. The transmission line system consists, essentially, of the special filter networks which combine transmitters to the tower transmission line and the transmission line itself.

The Tower Transmission Line
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 which best fits the potential subscriber area. The key parameter of the transmission line is its power dissipation (attenuation), 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. 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 (Andrew Corp.) which has an attenuation of approximately 0.49 dB/100 ft. A 600 ft. run of this waveguide would have an attenuation of approximately 3 dB and reduce the transmitter power to only 50% 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" 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.

Combining Transmitters
For an installation broadcasting more than one channel, and hence using multiple channel transmitters, 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. The various brands and schemes of combining filters can be arranged to combine any number of channels from 2 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 31 channels in the Wireless band. When adjacent channels are in service at the same installation, two separate CHANNEL COMBINERS, each feeding a separate tower transmission line, are usually used. This necessitates use of two separate tower antennas, since the two transmission lines cannot be fed to the same antenna without mutual interference. The following Figure illustrates single and double transmission line systems.

Also illustrated is the use of a “magic tee” to combine adjacent channels to a single transmission line. While this saves the substantial expense of a second transmission line (about $28.00/foot plus installation) the “magic tee” has an inherent loss which drastically reduces the EIRP.

Using two tower waveguides to handle
adjacent channels

Using the "Magic Tee" to combine all channels
to one tower waveguide (2 dB Power Loss)

Recently a scheme has been proposed by Microwave Filter Company which allows adjacent channels to be mixed on a square waveguide with very little loss. Two antennas are required, as for the double transmission line scheme described above.

A broadcast station located in the center of its coverage area would have a horizontally omnidirectional radiation pattern: radiation equally in all azimuth directions. But it would have a very sharp elevation pattern - reduce its radiation sharply above and below the horizon so as not to waste power by 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.

Many Wireless broadcast antennas are broadband and can accommodate any channel in the 2500-2686 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 360 degrees coverage may be mounted to the side of the tower or building.

Most types are coaxial fed, requiring an adapter to the tower transmission line, if waveguide is used, and are rated at 500 watts input. Therefore, they can handle up to five full power (100 watt) channels or a greater number of lower wattage channels. A popular design type is made by Bogner and is tubular with its surface populated with discreet radiating elements. These form 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 dBi 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 360 degree 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.

The broadcast antenna is the most critical and, perhaps, the most expensive single component of the broadcast installation.

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 mounting provisions to insure 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. A limited number of suppliers have the experience and facilities to produce an antenna having the required mechanical and electrical properties.

The Receiving Site
The receiving site antenna 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 use by CATV systems for their subscribers. In the case of MMDS (Pay-TV), the converter also incorporates a device for decoding or “descrambling” the signal.

The Receiving Antenna
The receiving antenna is used in a large quantity, at least by the Pay-TV wireless operator, and therefore, must be relatively inexpensive. Nevertheless, it must have a 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 (see illustration). This construction minimizes wind loading and ice build-up.

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 dBi 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 which 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 45 degrees 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
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 his regular TV antenna. Models are available for targeting MMDS channels to any of these three bands, as well as special dual models where more than one MMDS 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
The set top converter is similar or identical to those used by CATV subscribers. It allows the subscriber to continue to receive any off air TV channels he has been receiving plus the additional wireless down converted channels.

Signal Security
Wireless cable has a choice of almost as many signal security methods as does CATV. Both audio and video techniques are available and consist of video inversion, synch suppression, bandwidth compression, etc.

Many systems are addressable, providing flexibility for a number of tiering options including pay-per-view. Where used, decoding means are usually incorporated within the set top downconverter. The low cost POSITIVE security system, originated for CATV, has recently been applied to MMDS.

Microwave Interference
While potential interference to reception exists, its actual occurrence is rare. Potential sources of interference are Amateur microwave bands and Government operational bands both above and below the 2500-2700 MHz band and the intercity TV relay band, below the wireless band. Since the 2500-2700 MHz band is reserved exclusively for wireless, no direct frequency interference is likely, except for second harmonic emissions from equipment operating at about half frequency - such as airport radar. Any interference is most likely to occur as overload of the block downconverter due to strong off band carriers mentioned above. The cure is relatively simple: installation of a preselector bandpass filter between reception antenna and block downconverter. Most microwave interference has been experienced at the broadcast facility and affects TVRO receptions used to feed the MMDS broadcast station. This is due to frequencies in the C-band microwave terrestrial network shared by AT&T, MCI and other common carriers of telephone and business data. These frequencies are allocated midway between C-band transponder center frequencies. Mild to strong interference appears as light “sparklies” on the picture, to complete "wipe-out" of the picture, respectively. Other common carrier frequencies are located below and above the TVRO C-band and can cause overload to either the LNA, microwave converter or both, depending on their relative strength. A number of different filters are available for these problems.

Appendix A - 6 Channel Format (US MMDS/ITFS)

Appendix A (Con't) - 7 Channel Format (Australia)

Appendix A (Con't) - 8 Channel Format (Ireland)

An updated Historical and Regulatory Overview may be found in Technical Note 120.