Abstract :
One of the major interference problems in the field of vehicular communications is brought about by the concentration of a number of base station installations in a relatively small area, or even at the same antenna site. Most cases of interference resulting from such multiple installations are difficult to analyze after the fact. A method is described, herein, for predicting such interference before it occurs. Futhermore, as a method for alleviating radio interference problems due to frequency and geographic congestion, a technique is suggested for processing new applications for frequency assignment. There have been so many papers written and presented on the subject of interference due to spectrum crowding, that it has become trite to state that this is the most serious problem facing vehicular communications today. Nevertheless, the problem becomes more accute with each passing day. Every new installation is regarded with apprehension by existing licensees as another possible source of interference. In a great many cases, equipment is installed only after considerable thought and calculation has gone into the selection of a frequency and a location. Unfortunately, there are also quite a few licensees that romp merrily into a new installation, blissfully unaware of the complexity of the situation. All too frequently, these latter cases result in the laborious task of tracking down a source of interference either to the new licensee or one of his radio neighbors. Determining the source of interference, after the fact, is a time consuming and complex task. Only the most careful evaluation of the interference can determine whether a receiver or a transmitter is at fault. The fault is difficult to establish, since similar interference symptoms will occur due to the limiting performance of the receiver and extra band radiation of an interfering transmitter. A somewhat more straight-forward job is that of predicting interference before it occurs. This is the ounce of prevention. However, I do not mean to imply that the job is a simple one. When analyzing a proposed new installation, or a proposed new system, possible interference must be considered in two general categories (Chart 1). In the first category are those causes of interference caused by the limiting - performance of the new receiver, or of existing receivers at the same site. The first case is intermodulation produced in the front end of the receiver, due to two or more transmitters operating nearby on closely spaced frequencies. The second is modulation splatter from an adjacent channel transmitter thatmight be heard in the newly installed receiver, due to the limitations of the receiver selectivity. A third possibility is desensitization of the desired signal due to a strong nearby undesired signal, and a fourth is interference due to spurious and image responses of the newly installed receiver.
