Fig: Communication classification by signal type, direction, channel and frequency placementFig: General communication system: the transmitter conditions the message for the channel, an additive-noise node injects n(t), and the receiver recovers the message for the destinationFig: Signal classification by amplitude/time, repetition, predictability and energy measureFig: Noise classification into internal (device) and external (environmental) sourcesFig: One-sided noise PSD: flat thermal (white) noise, low-frequency 1/f flicker noise, and a band-limited white spectrum of bandwidth BFig: FDM spectrum: baseband channels placed in adjacent frequency bands separated by guard bandsFig: TDM frame: user time slots TS0\,-\,TSn repeating within each frame