Information capacity of fading channels under conditions of intense interference

The newer, more efficient data transmission systems which are designed to combat channel variations or interference, or both, require information concerning the channel variations for proper operation. Although it is seldom recognized, such systems must receive and process two distinct types of information. The first, and quite obvious, type of information concerns the intelligence signal which is provided as an output from the receiving terminal of the system. The second, and much less obvious, type of information concerns medium variations. This information is normally used internally at the receiver terminal in order that proper processing of of the received signal may be obtained. As the channel SNR progressively worsens, the total information capacity of the channel must of necessity decrease. Eventually the total information capacity of the channel becomes so small that the receiver is denied the required information concerning medium variations and proper operation of systems of the above type is no longer possible. This paper is concerned with the limiting case of very small channel SNR's that are below the level required for analysis of the medium at the receiver. The channel capacity of an ideal or nonfading channel is derived for reference purposes and in order that an estimate of the information rate loss due to medium variations at low SNR's may be obtained. The problem of data transmission over a fading channel under conditions of intense interference is approached in an intuitive manner. A system for operation under such conditions is proposed which appears to offer the best performance possible. The performance of this proposed system is then analyzed to yield capacity formulas for the conditions specified. No proof is given that the system proposed is, in fact, ideal but the resulting capacity formulas are believed to be correct in form and sufficiently accurate in a quantitative sense for useful practical application. The mathematical approach used is sufficiently general to permit inclusion of random system variations (such as oscillator phase) as part of the medium variations and hence the results are believed to be applicable to a wide variety of communications situations including deep-space probe telemetry.