Communication Theory (5) - a digital band-pass transmission system (ASK / FSK / PSK / DPSK)

Digital band pass - digital carrier, the digital band digital modulation

A digital modulation method - analog modulation method, a digital keying ( amplitude shift keying ASK, FSK FSK, phase shift keying PSK )

2ASK / OOK modulation: control carrier amplitude . Symbol is 1, the sine wave amplitude A, is 0 when the amplitude is zero. Also known as on-off keying.
2FSK Modulation: controlling the carrier frequency . Symbol 1 and 0:00 correspond to different sine wave frequency. 2FSK can be seen as two superimposed 2ASK different carrier frequencies.
2ASK demodulation: envelope detectors, coherent demodulation .
2FSK demodulation: envelope detectors, coherent demodulation, other . It can be decomposed into two 2ASK.

2PSK modulation: a phase instead of 1 Π, representative of the phase 0 0.
2PSK demodulation: only coherent demodulation .
2PSK Disadvantages: Due to carrier phase ambiguities exist down Π phenomenon that work in reverse .

Solution: 2DPSK.

2DPSK Modulation: using longitudinal adjacent carrier relative phase of the symbol to represent information. 0 the phase difference is 0, a phase difference of Π.
demodulation:
- + Coherent demodulation code inverse transform (differential coding) method ;
- Differential coherent demodulation (carrier adjacent symbol information representing the relative phases before and after).

All the demodulators by the band-pass filters the BPF, a demodulator, sampling determiner composition.

Bandpass filter function: signaling noise filtering .
Design: Based on the spectral characteristics of modulated signals - power spectrum (PSD).

Objective: bandwidth B and a carrier (F _C ) component.

Method: by means of a power spectrum of the baseband signal.

F _B =. 1 / T _B = R & lt _B symbol rate
2ASK: B _2ASK =. 2F _B . Containing carrier component.
2FSK: continuous spectrum shape changes toward the size of the carrier frequency difference between the two, and may be unimodal or bimodal.

B _2FSK ≈ | F _{. 1} -f ₂ |. 2F + _B . Containing carrier component F _{. 1} and F ₂ .
2PSK and 2DPSK: B _2PSK = B _2DPSK =. 2F _B . Free carrier component.

Modulated signal, the channel ( Gaussian white noise ), the BPF band-pass filter ( narrow band noise ), a demodulator (usually a low-pass filter ), sampling judgment.

2ASK- coherent demodulation system: bit error rate and unipolar baseband system the same result.
2FSK- coherent demodulation system: the bit error rate BER is 0 and 1 symmetry.
2PSK- coherent demodulation system: bit error rate and bipolar baseband system the same result.
2DPSK- coherent demodulation system: the error rate is twice 2ASK.

2ASK: susceptible channel parameter changes, suitable for changing the reference transport channel.
2PSK: less susceptible to receiving channel parameter variation.
2FSK: no human decision threshold setting, and thus less susceptible to receiving channel parameters. Variable parameters applicable to transmission channel applications.

Good anti-noise performance 2PSK / 2DPSK of;
2DPSK 2PSK solve the problem of reverse work;
2FSK for varying parameter channel, but lower bandwidth efficiency;
2ASK simple but otherwise poor performance;
IOU incoherent way lower than the complexity of a coherent manner.
Coherent data transmission speed 2DPSK--

Incoherent in slower data transmission 2FSK--, especially for the case with the reference channel