Communication Systems

Vocabulary flashcards about Communication Systems, based on lecture notes.

Signal

A function of one or more independent variables which contain some information.

Electric Signal

Electric Voltage or current, such as radio signal, TV signal, telephone signal.

Non-Electric Signal

Pressure signal, sound signal

System

A set of elements or functional blocks that are connected together and produces an output in response to an input signal.

Example of system

Audio amplifier, attenuator machine or engine.

Continuous Time Signal (CT)

Defined continuously with respect to time.

Discrete Time Signal (DT)

Defined only at specific, regular time instants.

Periodic Signal

Repeats at regular intervals.

Non-Periodic Signal

Do not repeat at regular intervals.

Even Signals

Signal inversion of the time axis does not change the amplitude.

Odd Signals

Signal inversion of the time axis also inverts amplitude of the signal.

Energy Signal

Normalized energy is non-zero and finite.

Power Signal

Normalized power is non-zero and finite.

Deterministic Signal

Signal can be completely represented by a mathematical equation at any time

Random Signal

Signal that cannot be represented by any mathematical equation.

Unit Step Function

Has an amplitude of 1 for positive values of the independent variable and 0 for negative.

Unit Impulse or Delta Function

Area under unit impulse approaches 1 as amplitude becomes infinite and width approaches zero.

Unit Ramp Function

Linearly growing function for positive values of the independent variable.

Rectangular Pulse

Pulse centered at t=0, represents the double-sided frequency response of a low pass filter.

Sine Pulse

Very important mathematical model used extensively in mathematical analysis of communication systems.

Static System

The continuous time system output depends upon the present input only.

Time-Invariant System

Continuous time system is time invariant if a time shift in the input signal results in a corresponding time shift in the output.

Causal System

A system is said to be causal if its output at any time depends upon present and past inputs only.

Stable System

Every bounded input produces a bounded output

Linear Time Invariant (LTI) System

The system which is linear and time-invariant.

Impulse Response

Input to the system is an impulse function, its output is called the impulse response.

Fourier Series

Gives time domain representation of signal; represents a periodic signal in terms of complex exponentials.

Fourier Transform

Used to represent any non-periodic signal, as well as periodic, in terms of complex exponential functions.

Amplitude Modulation (AM)

The amplitude of a carrier signal is varied according to the amplitude of a modulating signal.

Modulation Index

Ratio of the maximum amplitude of the modulating signal to the maximum amplitude of the carrier signal.

Transmission Efficiency

Defined as the ratio of power in both sidebands to total transmitted power.

Double Sideband Full Carrier System (DSB-FC)

Carrier and two sidebands are transmitted. The carrier does not contain any information

Double Side Band Suppressed Carrier system (DSB-SC)

Carrier is suppressed; only one (or sometimes both) sidebands are transmitted.

Single Sideband Suppressed Carrier (SSBSC)

Carrier and one sideband are suppressed; only one sideband remains

Frequency Translation

The message spectrum is shifted by an amount equal to the carrier frequency fc.

Vestigial Sideband Transmission (VSB)

One of the sidebands is partially suppressed and a vestige of the other sideband is transmitted.

Pre-Emphasis and De-Emphasis

Used to improve the threshold effect

Low Pass Sampling Theorem

A continuous-time signal can be completely represented by its samples and recovered back if the sampling frequency is twice the highest frequency content of the signal.

Superheterodyne Reciever:

The system problems of TRF receiver are overcome in this receiver. The superheterodyne receiver converts all incoming RF frequencies to a fixed lower frequency.

Output Signal-to-Noise Ratio (SNR)

Is the ratio of the average power of the message signal at the receiver output to the average power of the noise at the receiver output

Channel Signal to Noise Ratio (SNR)

Is the ratio of average power-of-message signal at the receiver input to the average power of noise in message bandwidth at the receiver input.

Practical Filters

If the bandwidth of the signal to be transmitted is infinite, practically it is not realizable. Practical filters will have some finite transition bandwidth.

Sampling

The process of assigning amplitudes to discrete time signals is termed as sampling.

Quantization

The process of converting continuous amplitude signal into discrete amplitude signal is termed as Quantization.

Encoding

The process which converts discrete amplitude and discrete time signal into digital signal is termed as Encoding.

Pre-emphasis

High frequency components are artificially emphasized by the pre-emphasis filter before modulation.

De-emphasis

Employed to restore the signal to its original form by de-emphasizing high frequency components.

Sampling theorem

States that a continuous time random signal can be represented in its samples and recovered back when sampling frequency is greater than or equal to twice the maximum frequency component present in the band limited signal.