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Digital Communications Principles
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Forward Error Correction (FEC)
A technique used for controlling errors in data transmission over unreliable or noisy communication channels, where the transmitter adds redundant data to its messages.
Automatic Repeat reQuest (ARQ)
A protocol for error control in data transmission where the receiver has the ability to detect errors and to request a retransmission of the data.
Differential Encoding
A technique used in digital communications in which data is represented by relative rather than absolute changes in the signal.
Equalization
The process of adjusting the balance between frequency components within an electronic signal.
Bit rate
The number of bits transmitted per second.
Modulation
The process of varying one or more properties of a periodic waveform, called the carrier signal, with a modulating signal that typically contains information to be transmitted.
Quadrature Amplitude Modulation (QAM)
A method of combining two amplitude-modulated signals into a single channel, thereby doubling the effective bandwidth.
Attenuation
Reduction in the amplitude or power of a signal as it travels through a transmission medium.
Inter-symbol Interference (ISI)
A form of distortion of a signal in which one symbol interferes with subsequent symbols; an important factor limiting the data rate in digital communication systems.
Spread Spectrum
A form of wireless communications in which a signal generated with a particular bandwidth is deliberately spread in the frequency domain resulting in a signal with a wider bandwidth.
Signal-to-Noise Ratio (SNR)
A measure used in science and engineering that compares the level of a desired signal to the level of background noise.
Pulse Code Modulation (PCM)
A method used to digitally represent sampled analog signals, which is the standard form for digital audio in computers and various Blu-ray, Compact Disc and DVD formats.
Shannon Capacity
The theoretical maximum data rate of a noise-free channel with a given bandwidth.
Hamming Distance
The number of positions at which the corresponding symbols are different in two strings of equal length.
Symbol Rate
The number of symbol changes (waveform changes or signaling events) made to the transmission medium per second in a digitally modulated signal or a line code.
Noise
Any unwanted form of energy that disturbs a communicated or received message or signal.
Cyclic Redundancy Check (CRC)
An error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data.
Frequency Shift Keying (FSK)
A form of modulation that represents digital data through discrete frequency changes of the carrier wave.
Nyquist Rate
Twice the highest frequency of a given signal, the minimum rate at which a signal can be sampled without introducing errors.
Code Division Multiple Access (CDMA)
An access method used by various radio communication technologies in which several transmitters can send information simultaneously over a single communication channel.
Nyquist Criterion for Zero ISI
A necessary condition for a communication channel to be free of inter-symbol interference (ISI) is that the overall impulse response of the channel at the sampling times is an impulse.
Orthogonal Frequency-Division Multiplexing (OFDM)
A method of encoding digital data on multiple carrier frequencies that are orthogonal to each other.
Amplitude Shift Keying (ASK)
A form of modulation that represents digital data as variations in the amplitude of a carrier wave.
Phase Shift Keying (PSK)
A modulation technique in which the phase of the carrier wave is varied to represent digital binary data.
Signal Constellation
A representation of a signal modulated by a digital modulation scheme such as quadrature amplitude modulation or phase-shift keying.
Bandwidth
The range of frequencies within a given band, in particular that used for transmitting a signal.
Error Detection and Correction (EDC)
Methods used in digital communications to detect and correct errors in the transmitted data.
Multiplexing
A method to combine multiple signals into one signal over a shared medium.
Carrier Recovery
The process of obtaining a phase-coherent carrier signal from the received signal to enable demodulation and decoding.
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