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Frequency Planning Vital in Microwave System.

Even five years ago, microwave path engineering began with land considerations. Frequency assignment stood as the last step in the process, and only occassionally did a frequency proposed after a thorough computer study and office engineering review fail to pass the scrutiny of existing users. Frequency selection wasn't exactly as easy as pressing a button, but an experienced engineer with access to a complete data base could expect to lead a quiet life.

No longer. Increasingly, frequency considerations, not land availability, drive the design of new microwave routes. As a result, the frequency engineer has become involved in the design process from the beginning. Rare is the case where a proposed frequency plan turns into the final assignment without modification. There are several reasons for the change.

The boom in microwave constructions has slowed from a gallop to a fast trot. The emphasis at this point is on fill-ins and cross-connections between trunk routers. The result is that some rural areas are experiencing the crowding conditions formerly only seen in major urban areas.

The interconnection of microwave networks puts additional demands on the frequency coordinator. Unfortunately, a microwave beam doesn't stop when it reaches its intended destination, so a phenomenon called "overshoot" can prevent two links in a microwave route from using the same frequency. The most common solutions is to offset relay stations from a straight line. The result is a "herringbone" route pattern.

Problem with Bucking Stations

A more serious design problem arises with so-called "bucking stations." To keep transmission and reception from interfering with each other, a station in a microwave route transmits on a high frequency in the same band. The next station on the route receives high and transmits low, so on down the route. Bucking arises when interconnecting existing routes. It is very likely that to continue the existing high-low patterns means that a station both transmit and receive high-low plan on one of the existing routes or change frequency band. As the emphasis in design shifts from backbone routes to interconnection, avoidance of bucking and similar network-wide frequency assignment problems has become a major consideration in network design.

While the use of digital transmission and single-sideband techniques are increasing the capacity of microwave, they pose a new challenge to the frequency coordinator. Each modulation technique has its own interference characteristics--what it interferes with and what interferes with it. Right now FM represents slightly over half of all path modulations, digital represents a fourth, video a fifth and single sideband a mere five percent but growing fast. The increase from three to four modulation techniques effectively doubles the amount of calculation required--with a corresponding increase in the potential for problems.

There is no single response to the increasing complexity of frequency assignment. Rather, ther are varieties of responses to the challenge, ranging from improved antenna technology and increased use of computerized analysis to improved communication among frequency coordinators.

Terrain factors have for some time been a part of computerized office analysis. In recent years, however, sophisticated use of terrain analysis plays a key part in winnowing out the truly significant interference cases. Often terrain analysis takes place in two stages. The first uses terrain points spaced 30 seconds apart (about a half-mile in the continental US). This relatively rough analysis will eliminate many potential cases without further consideration. The cases that remain can be subjected to analysis based on three-second data points (spaced at about 200 feet). Since the latter requires about 60 times the processing resources, the first "rough cut" analysis serves to reduce both the cost and the time required for the the cost and the time required a skilled and experienced communication engineer. Good judgment has a significant impact on both the cost of the study and the quality of the outcome.

The close proximity of microwave paths has also increased the likelihood of reflection interference between microwave systems. Until recently, communication engineers considered only direct interference between microwave antennas. There are two reasons: direct interference explained most of the known interference cases and there was no model for predicting other forms of interference. Recent research at Bell Laboratories, however, has produced evidence that otherwise-unexplained interference cases were probably caused by reflection-type interference. In addition, it appears that the level of interference could be reasonably estimated by using an equation borrowed from radar analysis.

Comsearch, a communication engineering firm, has recently developed a computer program called TBEAM that computerizes the reflection equation. The equation depends on a data base that includes site information for more than 300 highly reflective areas centered on built-up urban regions; the equation itself incorporates assumptions about the transition between built-up and rural-area reflectivity.

The development of the TBEAM program has had several immediate benefits: first, it predicts ground scatter before construction takes place. Ground scatter conditions discovered after a system is in operation are rather difficult and expensive to correct. Second, the program provides directionf or field researchers, who now know where to look first for the "smoking gun" when investigating suspected interference situations--for correction or prevention.

More field testing is another consequence of path crowding. But field techniques are improving in order to cut costs and to increase efficiency. Field studies have been automated with the help of portable personal computers. With the help of automated testing, the field team can accomplish more in less time. In addition, there is much greater digitized antenna patterns for the antennas being tested. Field processing of test results even allows a certain amount of "what if" testing by the field crew. In any case, it greatly reduces the data analysis upon return to the office.

Another important consequence of computerizing is the gain in precision of measurement over analog spectrum analyzer output. The final report of the test crew is supported by clear, multicolor plotted graphs rather than Polaroid photos of a spectrum analyzer, simplifying further office analysis while also ensuring its accuracy.

Antenna technology is giving an important assist in solving the problems of spectrum crowding. The most profound change in crowded areas and bands has been the near-complete abondonment of the plain vanilla parabolic-type antennas in favor of horn antennas and high-performance parabolics. The demands brought on by crowding have produced a very healthy competition among antenna manufacturers for ever-improved side-lobe performance.

Paradoxically, new developments in antennas add to the coordinator's job even as they offer new solutions. More alternatives mean more time must be spent in "what if" analysis.

Another response of technology to microwave crowding is the development by AT&T of Automatic Power Control (APC) transmitters. APC takes advantage of the fact that most instances of deep fading of microwave signals are relatively brief. The APC transmitter operates at a relatively low power level over 99 percent of the time. While the transmitter can detect and compensate for deep fading conditions by boosting power substantially, the transmitter also is designed to operate at full power only 0.01 percent of the time (about 52 minutes in the course of a year). By tailoring power output to environmental conditions, AT&T hopes to facilitate more microwave growth without limiting the reliability of microwave communication.

The whole process of path design and frequency coordination is changing to cope with the demands of limited spectrum. Coordinators for new systems have had to develop negotiating skills in order to clear cases with existing users. With increasing frequency, the new user must upgrade the equipment of the existing use to reduce its susceptibility to interference.

The developments in frequency coordination have greatly increased the need for intra-industry communication and education. In response to this need, frequency coordinators representing common carriers, private users, earth-station owners and communication engineering firms have come together in a group called the National Spectrum Managers Association. NSMA has provided a forum removed from the day-to-day demands of frequency coordination, where coordinators can discuss current concerns within the community. In its first year of operation, the NSMA has already become a valuable vehicle for developing new responses to the demands of frequency coordination.

What follows is a discussion of some of the most pressing challenges in the various common carrier frequency bands:

Light loadings on the 2-GHz band have until recently limited its use by common carriers. But the band's limitations have turned into an advantage for interconnection of cellular telephone nodes. Unfortunately, the limited spectrum at 2 GHZ allocated to common carriers means that the second cellular applicant trying to get frequencies has a difficult time of it.

MMDS Could Be a Problem for 2-GHz Carriers

An additional problem looming for 2-GHz carriers is MMDS on adjacent channels. MMDS, a pay-TV distribution service, uses high-power omnidirectional antennas--typically atop high buildings near city centers where 2-GHz common carrier signals tend to converge.

C-band satellite earth stations continue to complicate life at 4 GHz and 6 GHz. The 4-GHz frequency is used for receiving satellite signals. The proliferation of increasing numbers of small satellite dishes leaves a path designer feeling like Gulliver among the Lilliputians.

The 6-GHz band is used for transmission to satellites. Recent developments in spread-spectrum satellite transmission are posing problems here (despite spread-spectrum's reputation for non-interference). In addition, 6 GHz, presently the most popular common carrier band, is feeling the brunt of new modulation systems.

The 11-GHz area is expected to improve with the move of Intelsat to the standard Ku-band by early next year. Due to crowding at 6 GHz, 11 GHz is becoming the frequency band of choice for the "last hop" into major communication centers.

Until recently, the uncertainty surrounding channelization at 18 GHz has held down its development by common carriers. Engineers still find 18 GHz difficult to work with, due to the patchwork of available bandwidths. Equipment is not yet widely available, although much new equipment has been announced. The result is that 18 GHz is becoming crowded "on paper" in some of the high-volume microwave areas, even though very little is yet on the air.

In contrast to private activity, there is very little common carrier activity to date at 23 GHz. In large measure, this is due to the lack of transmitting equipment with the reliability and capacity common carriers require. Unlike a private carrier, a common carrier is looking to network several 23-GHz paths. For the moment, the longer hops available at 18 GHz make it a more attractive choice.
COPYRIGHT 1985 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1985 Gale, Cengage Learning. All rights reserved.

Article Details
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Author:Hardy, T.; Prives, D.
Publication:Communications News
Date:Oct 1, 1985
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