Editor's Note:
Here arePart I, Part II, Part III of the article, Part IV and Part V.
Adjacent-Channel Interference
Adjacent-channel interference is interference to a receiver listening on a given channel from a transmission occurring on an adjacent channel. A cellular receiver must be designed to receive on all channels in the cellular system band, as a particular telephone connection may be assigned to any of the possible channels. The receiver separates one channel from another by using a highly selective filter. The passband bandwidth of the filter is equal to the channel bandwidth.
The filter must cut off sharply at the passband edges, so that signals in the adjacent channels will not be passed to the demodulator. Now a "brick wall" filter, which cuts off abruptly and completely at the passband edges, is a physical impossibility. It turns out, moreover, that sharp cutoff filters may be far too expensive for mass consumer markets. Analog filters require a large number of components to achieve high selectivity (sharp cutoff). Also, for both analog and digital implementations, the performance of a sharp cutoff filter is very sensitive to small errors in the component or coefficient values.
Figure 4.15 shows the frequency response of a tenth-order Chebyshev filter with a 30 kHz passband. The center frequency has been arbitrarily selected as 870.03 MHz. This frequency response is not intended to represent any specific manufacturer's equipment but to suggest what a selective bandpass frequency response might look like. Notice from the figure that this filter produces an attenuation of about 50 dB at the center of the adjacent channels, 30 kHz above or below the filter's center frequency. One might imagine that 50 dB of attenuation in the adjacent channel might be enough to render interference from these channels negligible. The following example illustrates that the situation might be otherwise.
We can see from the example that adjacent-channel interference can be a problem, even with highly selective channel filtering. Several strategies are available for dealing with this problem. A common strategy in the broadcast services is to avoid using adjacent channels in the same market area. This strategy is used in both AM and FM broadcasting and in television. In cellular systems, however, the number of channels available translates directly into the number of customers who can be supported, which, in turn, translates directly into revenue. Channels are too valuable to be set aside for interference avoidance.
The power level of a mobile unit transmitter can be controlled dynamically, so that it transmits less power when it is nearer the base station than it does when it is at a cell edge. Power control has been in use in some form since the earliest cellular systems. In modern cellular systems a mobile unit's transmitted power is adjusted in 1 dB increments every few milliseconds, to keep the power level received at a base station constant as the mobile unit moves over the cell's coverage area.
Finally, channels can be partitioned so that adjacent channels are not assigned to the same cell or to cells that are immediate neighbors. This will guarantee that an interference source cannot get physically close to a base station receiver. When cluster sizes are small, however, the available channels will be divided up among a relatively small number of cells. In this case it may be difficult to avoid assigning adjacent channels to the same or nearby cells, and adjacent-channel interference may significantly limit how small the clusters can be made.
Next: Cellular System Planning and Engineering
About the Authors
Bruce A. Black completed his B.S. at Columbia University, his S.M. at Massachusetts Institute of Technology, and his Ph.D. at the University of California at Berkeley, all in electrical engineering. Since 1983 he has been on the faculty of the Department of Electrical and Computer Engineering at Rose-Hulman Institute of Technology in Terre Haute, Indiana, where he has been advisor to Tau Beta Pi and is advisor to the Amateur Radio club (W9NAA). In 2004 he was named Wireless Educator of the Year by the Global Wireless Education Consortium. He is a member of Tau Beta Pi, Eta Kappa Nu, and Sigma Xi.
Philip S. DiPiazza received a B.E.E from Manhattan College in 1964, an M.E. in electrical engineering from New York University in 1965, and a Ph.D. (electrical engineering) from the Polytechnic Institute of New York in 1976. Dr. DiPiazza was responsible for the system integration and test of the first North American deployment of AMPS.. He is currently an Adjunct Professor at the Rose-Hulman Institute of Technology and a Senior Consultant with Award Solutions, Inc. Dr. DiPiazza is an advisor and member of the Global Wireless Educational Consortium and a member of the IEEE.
Bruce A. Ferguson received the B.S., M.S., and the Ph.D. degree in electrical engineering from Purdue University, West Lafayette, Indiana in 1987, 1988, and 1992 respectively. He is currently a Communication System Engineer with Northrop Grumman Space Technology. He has worked with space and ground communication systems and photonics at TRW Space and Electronics (now NGST), and taught at Rose-Hulman Institute of Technology and The University of Portland in Oregon. Dr. Ferguson is a member Eta Kappa Nu and IEEE.
David R. Voltmer received degrees from Iowa State University (B.S.), University of Southern California (M.S.), and The Ohio State University (Ph.D.), all in electrical engineering. During nearly four decades of teaching, Dr. Voltmer has maintained a technical focus in electromagnetics, microwaves, and antennas. His more recent efforts are directed toward the design process and project courses. He has served in many offices of the ERM division of ASEE and in FIE. Dr. Voltmer is an ASEE Fellow and a Life Senior member of IEEE.
Frederick C. Berry received the B.S., M.S., and D.E. degrees from Louisiana Tech University in 1981, 1983, and 1988 respectively. He taught in the Electrical Engineering Department at Louisiana Tech University from 1982 to 1995. Currently Dr. Berry is Professor and Head of the Electrical and Computer Engineering Department at Rose-Hulman Institute of Technology. In 2007 he became Executive Director of the Global Wireless Education Consortium. He is a member of Tau Beta Pi, Eta Kappa Nu, and Sigma Xi.
Title: Introduction to Wireless Systems ISBN: 0132447894 Chapter 4: Radio Frequency Coverage: Systems Engineering and Design
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