Glossary of Key Terms
Glossary of Key Terms
| Term | Definition |
| Aliasing | The unwanted phenomenon where a high-frequency component in the spectrum of a signal takes on the identity of a lower-frequency component in the spectrum of its sampled version, caused by sampling below the Nyquist rate. |
| Amplitude Modulation (AM) | A technique where the amplitude of the high frequency carrier wave is varied in accordance with the instantaneous amplitude of the modulating signal. |
| Amplitude Shift Keying (ASK) | A digital modulation technique which represents binary data in the form of variations in the amplitude of a signal. |
| Analog Signal | A continuous time varying signal, which represents a time varying quantity. |
| Angle Modulation | A modulation process in which the frequency or the phase of the carrier varies according to the message signal. It is divided into Frequency Modulation (FM) and Phase Modulation (PM). |
| Aperiodic Signal | Any analog or digital signal that doesn’t repeat its pattern over a period of time. |
| Carrier Signal | A high frequency signal which has a certain phase, frequency, and amplitude but contains no information. It is used to carry the message signal to the receiver after modulation. |
| Channel | The medium through which message signals travel to reach the destination. |
| Communication | The process of exchange of information through means such as words, actions, signs, etc., between two or more individuals. |
| Companding | A non-linear technique used in PCM which compresses the data at the transmitter and expands the same data at the receiver to reduce the effects of noise and crosstalk. The word is a combination of Compressing and Expanding. |
| Delta Modulation (DM) | A modulation technique where the sampling rate is much higher than the Nyquist rate and the step-size after quantization is of a smaller value, Δ. It is a simplified form of DPCM. |
| Demultiplexing | The reverse process of multiplexing; extracting multiple channels from a single combined signal at the receiver end. |
| Digital Signal | A signal which is discrete in nature or which is non-continuous in form. |
| Direct Sequence Spread Spectrum (DSSS) | A spread spectrum technique where each bit of user data is multiplied by a secret “chipping code” before transmission. It is also known as Code Division Multiple Access (CDMA). |
| Double Sideband Suppressed Carrier (DSB-SC) | A modulation process where the carrier signal is suppressed and the power saved is distributed to the two sidebands. |
| Entropy | A measure of the average information content per source symbol. |
| Envelope | An imaginary line interconnecting the positive and negative peaks of a carrier wave in Amplitude Modulation, which recreates the shape of the modulating signal. |
| Figure of Merit (F) | The ratio of the output Signal-to-Noise Ratio (SNR) to the input SNR, used to describe the performance of a device. |
| Frequency Division Multiplexing (FDM) | An analog multiplexing technique that uses various frequencies to combine streams of data for sending them on a communication medium as a single signal. |
| Frequency Hopped Spread Spectrum (FHSS) | A spread spectrum technique where users are made to change their frequencies of usage from one to another in a specified time interval. |
| Frequency Modulation (FM) | The process of varying the frequency of the carrier signal linearly with the message signal. |
| Frequency Shift Keying (FSK) | A digital modulation technique in which the frequency of the carrier signal varies according to discrete digital changes (e.g., a high frequency for a binary 1 and a low frequency for a binary 0). |
| Geosynchronous Earth Orbit (GEO) | An orbit at an altitude of 22,300 miles above the Earth, synchronized with a sidereal day (23 hours, 56 minutes). A geostationary orbit is a specific type of circular GEO in the plane of the equator. |
| Information Theory | A mathematical approach to the study of coding of information along with the quantification, storage, and communication of information. |
| Low Earth Orbit (LEO) | An orbit at a distance of 500 to 1000 miles above the Earth’s surface, characterized by very low transmission delays. |
| M-ary Encoding | A digital modulation technique where two or more bits are transmitted at a time as a single signal, reducing channel bandwidth. ‘M’ represents the number of possible conditions or levels. |
| Medium Earth Orbit (MEO) | An orbit at a distance of about 8000 miles from the Earth’s surface, offering a balance between the characteristics of LEO and GEO. |
| Message Signal (Modulating Signal) | The signal which contains a message to be transmitted. It is a baseband signal that undergoes modulation. |
| Modulated Signal | The resultant signal after the process of modulation, which is a combination of the modulating signal and the carrier signal. |
| Modulation | The process of changing the parameters of the carrier signal in accordance with the instantaneous values of the modulating signal. |
| Modulation Index | A measure of the level of modulation that a carrier wave undergoes in Amplitude Modulation. |
| Multiplexing | The process of combining multiple signals into one signal over a shared medium. |
| Noise | An unwanted signal which interferes with the original message signal and corrupts its parameters. |
| Nyquist Rate | The minimum rate of sampling required to perfectly reproduce a signal, which is equal to twice the maximum frequency of the signal (fs = 2W). |
| Periodic Signal | Any analog or digital signal that repeats its pattern over a period of time. |
| Phase Modulation (PM) | The process of varying the phase of the carrier signal linearly with the message signal. |
| Phase Shift Keying (PSK) | A digital modulation technique in which the phase of the carrier signal is changed by varying the sine and cosine inputs at a particular time. |
| Pulse Amplitude Modulation (PAM) | An analog modulating scheme in which the amplitude of the pulse carrier varies proportional to the instantaneous amplitude of the message signal. |
| Pulse Code Modulation (PCM) | A digital modulation process where an analog signal is converted into a binary sequence (1s and 0s) by representing it in discrete form in both time and amplitude. |
| Pulse Position Modulation (PPM) | An analog modulating scheme in which the amplitude and width of the pulses are kept constant, while the position of each pulse varies according to the instantaneous sampled value of the message signal. |
| Pulse Width Modulation (PWM) | An analog modulating scheme in which the duration or width of the pulse carrier varies proportional to the instantaneous amplitude of the message signal. Also known as Pulse Duration Modulation (PDM). |
| Quantization | The process of representing sampled values of an analog signal’s amplitude by a finite set of levels, converting a continuous-amplitude sample into a discrete-time signal. |
| Receiver | The person or station that receives a message. |
| Sampling | The process of converting continuous time signals into equivalent discrete time signals by collecting data at certain instants. |
| Sampling Theorem | A theorem stating that a signal can be exactly reproduced if it is sampled at a rate (fs) which is greater than or equal to twice the maximum frequency (W) of the signal. |
| Sender | The person or station that sends a message. |
| Sideband | A band of frequencies containing power, which are the lower and higher frequencies of the carrier frequency, created during modulation. |
| Signal | A source of energy which transmits some information, such as an electrical impulse or an electromagnetic wave. |
| Signal-to-Noise Ratio (SNR) | The ratio of the signal power to the noise power. A higher SNR value indicates better quality of the received output. |
| Single Sideband Suppressed Carrier (SSB-SC) | A modulation process of suppressing one of the sidebands along with the carrier and transmitting only a single sideband to save power and bandwidth. |
| Spread Spectrum Modulation | A collective class of signaling techniques employed before transmitting a signal to provide secure communication that is resistant to interference and jamming. |
| Vestigial Sideband (VSB) Modulation | A modulation process where a part of the signal, called a vestige, is modulated along with one full sideband. It is a compromise between DSB-SC and SSB, primarily used in television transmissions. |