Tactical Wireless Communications and Networks: Design Concepts and Challenges

2.5 Power Spectrum

The power spectral density (power spectrum) of the modulated signal is important to understand and is related to the signal energy efficiency studied so far. Note that the power spectrum is measured as an averaged property of the signal and is in the order of seconds (which can hide certain properties of the signal). Thus power spectrum should not be the only factor in choosing a signal for a given channel. However, selecting the signal format and studying the power spectrum leads to good design of communication media. A channel can have a certain frequency response that will affect the signal power spectrum. The channel frequency response can differ, based on the modulation technique. For example, if we have a channel that has poor frequency response at low frequencies, then we should select a signal that has low power density at the low frequency range.

Regulation also plays a major role in selecting a signal with specific power spectrum characteristics. For example, microwave links must have a certain spectrum pattern to satisfy regulations. At 10 MHz away from the microwave center (carrier) frequency, the power spectrum measured over 1 kHz bandwidth should be 60 dB below the signal total power. In the commercial world, telecommunications authorities such as the Federal Communications Commission (FCC) in the USA regulate spectrum patterns to ensure best use of the available spectrum. Even with military communications, there are spectrum regulations, and frequency ...