The Comparison of Digital Communication Systems

Two methods are proposed for comparing the performances of digital communication systems differing in the number of available signals, in bandwidth, in rate of transmission, and in complexity. The systems are ranked on the basis of the probability of correct reception of transmitted messages. The signal durations are adjusted so that the systems have either equal bandwidths or equal rates of transmission. The concept of ´strictly comparable systems´ is introduced; for these the two methods lead to the same ranking. From one group of strictly comparable systems to another, the reliability of communication tends to decrease as the relative rate of transmission increases or as the bandwidth decreases. A number of systems are graphically compared by these methods, including some to which parity-check coding has been applied. The advatage over binary systems of systems of higher order, in which more than one kind of signal can be transmitted, is illustrated.