aps审核---数字通信原理Principles of Digital Communication数字通信原理
Topics covered include: digital communications at the block diagram level, data compression, Lempel-Ziv algorithm, scalar and vector quantization, sampling and aliasing, the Nyquist criterion, PAM and QAM modulation, signal constellations, finite-energ...
数字通信原理
Topics covered include: digital communications at the block diagram level, data compression, Lempel-Ziv algorithm, scalar and vector quantization, sampling and aliasing, the Nyquist criterion, PAM and QAM modulation, signal constellations, finite-energy waveform spaces, detection, and modeling and system design for wireless communication.
Digital communication systems, by definition, are communication systems that use such a digital sequence as an interface between the source and the channel input.
1. Modulation
The reasons for modulation
● Antennas can be smaller
● Better transmission/propagation properties
● Fractional bandwidth much smaller
● Antennas and other components easier to design
1.1 Analog Modulation(to impress an information-bearing analog waveform onto a carrier for transmission)
(1) Double-sideband, suppressed carrier AM
(2) Conventional double-sideband AM
(3) single-sideband AM
1.2 Digital Modulation (to convert an information-bearing discrete-time symbol sequence into a continuous-time Waveform)
● Modulation waveforms
● Amplitude Shift Keying, ASK
In the case of ASK, a finite number of amplitudes are used.
In binary ASK, the two binary values are represented by two different amplitudes of the carrier frequency.
● Frequency Shift Keying, FSK
In the case of FSK, a finite number of frequencies are used.
In BFSK, the two binary values are represented by two different frequencies near the carrier frequency.
● Phase Shift Keying, PSK
In the case of PSK, a finite number of phases are used.
PSK modulation encodes data on a sine wave by shifting the phase of the carrier signal to represent the binary data stream.
● Quadrature Amplitude Modulation (QAM)
In the case of QAM, a finite number of phases (at least two) and amplitudes (at least two) are used.
A combination of amplitude and phase modulation
2. Channel coding
2.1 Block codes (redundancy added to blocks of data )
● Code rate= input (original data) /output (transmitted)
● Hamming distance -- [11110000] , [11010001] =2
● Minimum distance—minmum Hamming distance out of all possible codewords
Assume K=4: number information bits
Choose the number of check bits r=3 and the length of codeword n=k+r =7.
7-bit Codeword: [a6 a5 a4 a3 a2 a1 a0]
2.2 Convolutional codes (redundancy added to data continuously)
? A convolutional code is a type of error-correcting code
? Continuous sequence of information bits mapped to continuous sequence of output bits
? Pass information sequence through finite state shift register
Trellis diagram
● Show transition between states over time
● Specify all states on vertical axis and repeat vertical axis along horizontal time axis
● Each transition from a state to another state denoted by a line connecting the two states on two adjacent vertical axes corresponding to two time instances
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