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Essentially, system noise has no explicit frequency dependence other than that already included in the parameter M. It depends in practice, however, on external noise sources, on achievable receiver noise temperatures in the RF region and on detector quantum and heterodyne efficiencies in the optical region.
Presently achievable receiver performances in the RF and projected performance in both the RF and optical region are indicated in Fig. 3. A receiver noise figure was not explicitly included in the one-way transmission equation, but has the effect of reducing the value of M. Noise figure and external noise can be accounted for in the RF region by taking the noise-equivalent temperature rather than actual input temperature to give the effective value of M. For coherent detection in the optical region, M must be reduced by the quantum efficiency of the detector as well as by the receiver noise figure when considering the performance to be expected of a real system. The effective value of M has been taken, therefore, as 6 dB below the value indicated in Fig. 1, the loss being equally divided between the detector quantum efficiency and the receiver noise figure.
In addition to frequency and weather-dependent transmission losses and detection losses accounted for by the effective value taken for M, there are several practical losses which apply generally to all systems. Limitations on effective transmitting and receiving apertures have been given for conditions to which a 3-dB decrease in effective gain from the ideal is expected. Thus, a 6-dB loss must be allocated to these components. A further 1 dB is taken to cover miscellaneous transmission losses in the equipment, so that a total of 7 dB is assumed for fixed system losses.
System performance vs. frequency now can be calculated from the one-way transmission equation and from various limiting values of the parameters as discussed above. A calculation of performance at a range of one astronomical unit is made taking representative parametric restrictions given in Table 1. In general, the technological restrictions represent the state of the art projected to the 1975-80 period. For convenience, a transmitted power of one W is used, the assumption being that roughly the same transmitter efficiency can be obtained in any region of the spectrum. Practical restrictions imposed by the atmosphere define these three cases of interest:
- No atmospheric losses (corresponding to a satellite receiving station),
- Atmospheric losses due to clear weather conditions and
- Atmospheric losses due to poor weather conditions.
The product (S/N)B, or information-rate parameter R0, is plotted in Fig. 4 for the three cases considered. The sharp peaks and valleys result from the discontinuous nature of the parametric restrictions selected. Undue significance should not be attached to their exact positions, which depend on the particular values given in Table 1 as representative of practical limits.
It should also be noted that the curves in the optical region represent performance limitations on coherent detection. For noncoherent detection, pointing accuracy and atmospheric distortions do not impose restrictions on receiver aperture area. In general, the performance will vary inversely with frequency in the optical region (for no atmosphere or for clear weather) rather than as the inverse cube. On the other hand, receiver noise levels may be very much higher. A comparison is made between coherent and noncoherent performance at two specific wavelengths as in Fig. 4.