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May, 1967

The advent of lasers raises the possibility of extending coherent techniques into the optical region, particularly for deep-space communications. This article compares microwave and laser links for spacecraft-to-earth communications. Fundamental and practical limitations are discussed.

The performance of a telecommunication link can be measured by such criteria as amount of data, trustworthiness, and economy in transmission of data. In this analysis, primary emphasis is placed on increasing the amount of data while maintaining its quality. Economy of data transmission remains important but secondary; realistic cost evaluations must be related to specific candidate systems as they develop from basic systems studies and research.

The product of transmitted data quality and quantity (the information rate parameter Ro) is then taken as a performance criterion, and the one-way transmission equation is written in terms of this product:

Here the signal-to-noise ratio, S/N, represents signal quality; the bandwidth, B, represents quantity; the expression 

represents the ideal noise limit where T is the temperature at an ideal receiver input. In the microwave region, where kT ≫ hf, this expression converges to the familiar quantity kT; in the optical region, where kT ≪ hf, to hf. Insertion of the functional frequency dependencies of the transmitter and receiver antenna gain (G = 4π A / λ2 ) in Eq. 1 gives:

Lo represents the fixed system losses and Lf the frequency-dependent losses (greater than unity). M may be considered a measure of potential performance in the absence of practical restrictions and is given by

Eqs. 2 and 3 document the explicit functional frequency dependencies and separate them from the physical parameters of the transmission link equation. However, in practical systems, some of these physical parameters indirectly depend on frequency. Consider for example, the frequency dependence placed on a spaceborne transmitting antenna by achievable pointing accuracy. Such a limit sets a minimum allowable bandwidth (or maximum gain), thereby requiring that the effective diameter of the transmitting antenna decrease inversely with frequency. This and other such restrictions must be included when applying the transmission equation to select an optimum frequency.

The explicit functional frequency given by M is discussed next as are the implicit frequency dependences due to various practical limitations imposed on the transmitting antenna area, AT; receiving antenna area, AR; transmitted power, PT; and losses in atmospheric transmission Lf.