Defense Intelligence Reference Document High-Frequency Gravitational Wave Communications

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3.0 Operational Concerns
3.1 LINK BUDGET
3.1.1 Signal-to-Noise Ratio
Signal-to-noise ratio (SNR) is an important figure of merit in communication systems
because it is an indicator of whether or not a transmitted signal will be useful upon
arrive at its destination, the receiver. Without processing gain an SNR > 1 will be
required to maintain a link budget. On the transmitter's end, the signal to noise is
determined by the useful signal that is produced by the transmitter after it is already in
its transmission mode, such as the GW power at the output of the GW generator
antenna, divided by the RSS (Root Sum Square) of the uncorrelated noise sources
referred to the same spot in the signal chain-that is, output referred noise equivalent
power (NEP). This signal to noise ratio is represented by the left hand column in Figure
16.
The components of the transmitter's noise equivalent power may be sorted by the
source of the noise. First, before the signal is converted to GW it is in the realm of EM
or photon radiation. Photons themselves make noise, and this component goes as the
square root of the total number of photons. Then there is thermal noise-that is, the
photons generated by blackbody radiation of the transmitter components themselves.
Other electronic and semiconductor components providing the source signal generate
their own photon noise due to carrier activity. All these noise sources are carried along
with the original EM signal and may be converted just as faithfully as if they were
signals should they fall within the transmission bandwidth. All of this is just for the EM
noise component.
The generation process itself may also be a source of noise, and will vary widely
depending upon the generator method used. For example, the generation process noise
created in the GASER would be significantly different than that created in a tuned
resonant EM toroid cavity. This of course would be an important consideration in
selecting a generator type.
Finally, it is expected that there are a variety of GW noise sources. Background sources
from space are predicted, in low levels, across the entire frequency spectrum. Also, in a
GW generator situation, parasitic vibrations may also have quadrupole moments, such
as the walls of a generation cavity for instance, or an unwanted vibration within a slab
of SC, and these could also generate GW noise.
Then there is link loss to contend with. While it is expected that the attenuation of GW
due to absorption and scatter will be quite low, geometry alone will dictate that a
spherically uniform radiating source will fall off as 1/R2 . This link loss will affect both the
transmitted signal and the transmitted noise.
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