MindMap Gallery Digital bandpass transmission system
The principles of binary and multi-ary digital modulation, the anti-noise performance and performance comparison of digital modulation systems, the anti-noise performance of multi-ary digital modulation systems, etc. are introduced.
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Digital bandpass transmission system
Binary digital modulation principle
Binary Amplitude Keying (2ASK)
principle
s(t)→carrier amplitude
waveform
expression
produce
analog modulation method
keying method
demodulation
envelope detection method
coherent demodulation method
Binary Frequency Shift Keying (2FSK)
principle
s(t)→carrier frequency
waveform
expression
produce
Analog FM
keying method
Features: fast conversion speed, simple circuit, good waveform generated, and high frequency stability.
demodulation
envelope detection method
modulation rules
Judgment rules
coherent demodulation method
zero crossing detection method
Binary Phase Shift Keying (2PSK)
principle
s(t)→carrier phase
waveform
expression
produce
analog modulation method
keying method
demodulation
Binary Differential Phase Shift Keying (2DPSK)
principle
Use the carrier relative phase of adjacent symbols to represent information
Vector illustration
waveform
Differential coding rules
⊕ is modulo 2 plus bn-1 is the previous code element of bn. Initially bn-1 can be set arbitrarily.
produce
Conceive
Model
demodulation
2DPSK coherent demodulation code inverse transform method
2DPSK differential coherent demodulation (phase comparison) method
Power spectrum (PSD) of a binary digital modulated signal
2ASK signal power spectral density
P2ASK (f) is the linear shift of Ps (f) (belongs to linear modulation)
Power spectral density of 2PSK/2DPSK signal
2FSK signal power spectral density
Anti-noise performance of binary digital modulation system
2ASK system anti-noise performance
2ASK---Coherent demodulation
2ASK---envelope detection
2FSK system anti-noise performance
coherent demodulation
envelope detection
Anti-noise performance of 2PSK/2DPSK system
2PSK coherent demodulation system
2DPSK coherent demodulation system (polarity comparison)
2DPSK differential coherent demodulation (phase comparison)
Performance comparison of binary digital modulation systems
Bit error rate - reliability
r is certain, the same demodulation method, the order of anti-Gaussian white noise performance is: 2PSK, 2DPSK, 2FSK, 2ASK Pe is certain, the required signal-to-noise ratio:
Frequency Bandwidth - Effectiveness
Sensitivity to changes in channel characteristics
Device complexity
Generally, the non-coherent method is simpler than the coherent method. This is because coherent demodulation needs to extract coherent carriers, so the equipment is relatively complex and the cost is slightly higher.
Principle of multi-digit digital modulation
Multiple Amplitude Keying (MASK)
MASK modulation:
The generation method is similar to that of 2ASK, but the difference is that the binary digital baseband signal input by the transmitter needs to be converted into an M-level baseband pulse by a level converter first, and then modulated.
MASK demodulation:
Similar to 2ASK signal demodulation, there are two types of coherent and non-coherent demodulation.
Multiple Frequency Shift Keying (MFSK)
MFSK can be regarded as a promotion of the 2FSK method.
Principle block diagram of MFSK modulation and demodulation:
Polyphase shift keying (MPSK)
basic concept
M different phases of the carrier are used to represent digital information.
Signal vector diagram (constellation diagram)
Fundamental
4PSK modulation
Relationship between double bits and carrier phase
Vector illustration
waveform
QPSK modulation
quadrature phase modulation method
phase selection method
principle
According to the two-bit ab at that time, the phase selection circuit selects the carrier output of the corresponding phase from the four candidate phases.
QPSK demodulation
principle
Coherent demodulation decomposed into two 2PSK signals
Anti-noise performance of multi-ary digital modulation system
Suppressed bit error rate of carrier MASK-coherent demodulation system:
MFSK – bit error rate of non-coherent demodulation system:
Bit error rate of MPSK (M≥4) coherent demodulation system:
Bit error rate of MDPSK (M≥4) coherent demodulation system: