5 Conclusions

The capacity of the Webb+Gaussian channel was derived analytically, and evaluated numerically in four cases. In conjunction with a Jacobian matrix that describes the relationship between four fundamental parameters and ten physical parameters, this allowed us to determine the sensitivity of the capacity with respect to any of the four fundamental parameters and any single physical parameter.

In all cases considered, the capacity was found to be most sensitive to the primary SNR parameter $\rho_0$, with the other three fundamental parameters playing lesser roles.

Perhaps the most surprising result is that, in all cases evaluated, the signal intensity has a substantially larger influence on capacity than the background intensity, usually by a factor of two or more. This is particularly true during night reception, in which a 1% reduction in background intensity would not increase the capacity as much as, for example, a 1% reduction in the equivalent noise temperature or a 1% reduction in the excess noise factor of the detector.

There are a number of expected results, as well. The quantum efficiency was shown to be the critical detector parameter, and was found to be more influential than any other physical parameter except the signal intensity. When the gain was not optimized, the capacity was sensitive to small gain fluctuations; when the gain was optimized, the capacity was not sensitive to small gain fluctuations. The leakage currents affected capacity much less than the other parameters, by multiple orders of magnitude.

The overall relative importance of the physical parameters, with respect to capacity sensitivity, was found to be fairly consistent. The physical parameters, in order of influence, are: the signal intensity, the quantum efficiency, the excess noise factor, the background intensity, the gain, the slot width, the equivalent noise temperature, the load resistance, the surface leakage current, and the bulk leakage current.