Signal, Modulation, Attenuation, and Multiplexing in Communication Systems

SIGNAL

SIGNAL: electromagnetic field which, thanks to its variations, is able to convey information from one place to another. It can be ANALOGUE (continuous) or DIGITAL (sequence).

MODULATION: the process of varying a periodic signal (car: constant amplitude and frequency) to use it for conveying some information. It can be done through AMPLITUDE (amplitude of carrier modified according to changes on modulating signal), FREQUENCY (frequency of carrier modified according to changes on modulating signal), or PHASE (phase of carrier is...). The transmitted modulated signal uses a frequency band around the carrier frequency. Reasons to use modulation: No signal mixing, long communication range, reduced antenna size.

ATTENUATION: As a signal propagates along a transmission medium, its amplitude (energy) decreases.

DISPERSION: Arises when the propagation delay of the transmission medium varies with the frequency. It causes changes/distortion in the shape of the transmitted signal.

NOISE: Unwanted signals not from the transmitter.

DELAY: The required time for a signal to be converted from one place to another. It includes propagation delay and transmission delay.

JITTER: Delay variation that can result in severe distortion of perceived quality in real-time services (e.g. VoIP).

BANDWIDTH: The width of the range of frequencies that a user/signal/channel uses on a transmission medium. It is calculated as the difference between high and low frequencies.

CHANNEL CAPACITY: The limitation on the highest data rate that a channel is able to support. In a noiseless channel, it is determined by Nyquist formula, while in a noisy channel, it is determined by Shannon-Hartley formula.

SPECTRAL EFFICIENCY: The net bit rate divided by the bandwidth in hertz used by a communication system (bps/hz).

MODULATION EFFICIENCY: The gross bit rate divided by the bandwidth in h... Sometimes there is no difference between them, but if we introduce error connection mechanism, we achieve a reliable link but with a penalty on bandwidth.

MULTIPLEX: When there is a shared medium, a mechanism to be used by different users is needed. If all signals are transmitted together at the same point, they can be multiplexed using different frequency bands, time slots, or orthogonal codes, and demultiplexed at the end. If signals are transmitted directly to the same medium from different points, a multiple access mechanism is required. Each user must use a different frequency band, different time slot, or different orthogonal code.

LINK BUDGET: There is a basic parameter to consider in every communication system. At the other end of the channel, the receiver must get enough signal power to provide information. That level is receiver sensitivity. Some attenuation and gain compensate for the losses, and we can get the level of power available at the receiver, which must be greater than the receiver sensitivity. We work with logarithm. || How to maintain maximum capacity and reduce noise? Increase bandwidth and signal power. MULTIPLE ACCESS: Marks which time slot, frequency range, or orthogonal code each transmitter should use.