Rationale

Interference is an onerous problem that must be dealt with. It is a fact of life and we can do something about it. There are interference issues with all radio systems. In essence, our concern is electromagnetic compatibility (EMC).

We turn to the IEEE for help from their standard dictionary (Ref. 1) to define EMC. We think the IEEE definition number 2 is appropriate:

[EMC is] the ability of a device, equipment or system to function satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic disturbances to anything in that environment.

EMC breaks down into two subsets: (1) emanation and (2) susceptibility. Nearly all electronic devices "emanate." In other words, they produce measurable RF energy, what we sometimes call electromagnetic interference (EMI) or radio-frequency interference. The path of this RF energy can be conducted or radiated. How much RF energy is produced and the harm it can do to that same device or other nearby devices are vital to good system/equipment design. The PC (personal computer) is a notorious em-anator.

Electronic equipment is "susceptible" to RF energy, whether radiated or conducted. Susceptibility is defined in ANSI C63.14-1998 (Ref. 2) as "the inability of a device, piece of equipment, or system to perform without degradation in the presence of an electromagnetic disturbance.'' Of course, emanation at sufficient level can be called an electromagnetic disturbance.

Radio System Design for Telecommunications, Third Edition By Roger L. Freeman Copyright © 2007 John Wiley & Sons, Inc.

Our concern here is much more narrow and defined. Our interests, the subjects of this text, are a wide variety of microwave communications, HF, cellular/PCS, and meteor burst communication systems.

A radio transmitter is a good example of a device where there is "wanted" radiation. There will also be some unwanted radiation. The scope of this unwanted radiation is the signal radiation from the antenna, either out the main lobe or the sidelobes. Of course, to the companion distant-end receiver, the main lobe radiation is "wanted."

We further reduce the scope because we only will consider unwanted radiation entering the receiving antenna, causing harmful interference in that facility. The system design engineer must be aware that unwanted radiation can enter practically anywhere—through power lines, as a result of a wave impinging on the equipment itself, through a poor ground, any or all signal leads, and so forth. This analysis, though, will only treat unwanted RF energy entrance through the antenna subsystem.

As a result, there are several areas where we might reduce interference to an acceptable level: (1) we can reduce the level of the unwanted energy radiated right at the source and (2) we can make a victim device less susceptible. An excellent example of both (1) and (2) is to reduce antenna sidelobes. Another is the use of filters to reduce harmonics and spurious radiation of an emitted wave at both the transmit and receive side of a radiolink. We can also reduce the EIRP to that necessary to just meet performance requirements. The trouble with this latter approach is that by reducing EIRP, we will reduce the link fade margin. However, we could use space diversity or larger antennas to compensate for the lost margin.

Our emphasis in this chapter will be interference issues of LOS microwave, satellite communication terminals, and PCS facilities. We will also deal with the cellular environment where frequency reuse and adjacent cell operations generate rich interference fields.

Before moving to our main theme, we will briefly discuss spurious response interference windows at a receiver.

0 0

Post a comment