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The concept of signal regeneration
Processing a damaged signal , The process of removing noise interference and restoring the damaged part of the signal is called signal regeneration.
Common transmission signals are divided into electrical signals and optical signals, so signal regeneration is also divided into electrical signal regeneration and optical signal regeneration.
The information transmitted in the digital signal is a digital code string composed of "1" and "0". The typical signal is a PCM pulse code modulation signal. In the long-distance transmission process of PCM signal, regenerative relay technology must be adopted.
Regenerative relay requires that under the condition that the signal-to-noise ratio of the baseband signal is not too large, the system can identify and judge the distorted waveform in time, identify the "1" code and the "0" code, and go through the regenerative relay The subsequent output pulses will be completely restored to the original digital signal sequence.
The regenerative relay system of baseband transmission is shown in Figure 1.
Features of regenerative relay system
(1) No noise accumulation. The signal amplitude distortion caused by the digital signal in the transmission process can be removed by equalizing amplification and regeneration decision in the regenerative relay system, so the ideal regenerative relay system does not have noise accumulation;
(2) There is an accumulation of errors. In the process of regeneration judgment, due to the influence of inter-code crosstalk and noise interference, it will lead to the wrong judgment of the judgment circuit, that is, the "1" code is misjudged as the "0" code, and the "0" code is misjudged as the "1" code. Errors occur. Once the error occurs, it cannot be eliminated, resulting in error accumulation.
The structure of the regenerative repeater
shown in Figure 2.
The function of equalization and amplification is to equalize and amplify the received distorted signal into a waveform suitable for sampling decision. This waveform is called an equalization waveform and is represented by r(t).
Timing clock extraction is to extract the timing pulse synchronized with the timing clock of the transmitter from the received signal, so as to identify the "1" code and "0" code of the equalized wave at the best time.
What the sampling decision and code formation circuit completes is the decision regeneration function, also called identification regeneration. Recognition refers to identifying the "1" code or the "0" code from the equalized waveform that has been equalized; regeneration is to reshape and transform the judged symbols to form a half-duty bipolar code, that is, code formation.
Basic principles of signal regeneration
Significance of signal regeneration
The channel equivalent model of the baseband transmission system is shown in Figure 3.
Assuming that the channel input signal is ei(t), the characteristic of the channel is h(t), and the additive interference noise introduced by the channel is n(t), then the channel output signal eo(t) is:< /p>
eo(t)= ei(t)*h(t)+n(t)
If the channel characteristics h(t) and noise characteristics n(t) are known , Given a certain transmission signal ei(t), the received signal eo(t) after transmission through the channel can be determined.
According to the basic theory of transmission lines, the attenuation characteristics of transmission lines are proportional to the square root of the transmission signal frequency. The higher the frequency, the greater the attenuation. After a rectangular pulse signal is transmitted through the channel, the waveform will be distorted, which is mainly reflected in the following aspects: (1) The amplitude of the received signal waveform becomes smaller. It indicates that the energy of the signal is attenuated after transmission through the channel. The longer the transmission distance, the greater the attenuation.
(2) The wave crest is delayed. Reflect the delay characteristics of the transmission system.
(3) The pulse width is widened. This is caused by the frequency characteristics of the transmission system and is the main reason for the distortion of the waveform.
The complete signal regeneration system is shown in Figure 4.
Optical signal regeneration
When the optical signal is transmitted in the optical fiber channel, large optical fiber loss and serious dispersion will cause the distortion of the optical signal.
Loss causes the amplitude of the optical signal to attenuate exponentially with the transmission distance. The attenuation of the optical signal can be solved by amplifying the power of the optical signal with an all-optical amplifier.
Dispersion will cause the light pulse to broaden and generate inter-symbol interference, which will increase the bit error rate and seriously affect the quality of communication.
The regeneration of the optical signal is realized by the photoelectric repeater.
The photoelectric repeater first converts the optical signal into an electrical signal through the optical signal, and then re-drives a light source to regenerate the optical signal after the circuit is reshaped and amplified.
All-optical information regeneration technology, that is, connect optical modulators and filters on the optical fiber link, extract the synchronous clock signal from the optical signal transmitted by the link, and input it into the optical modulator. Perform periodic synchronous modulation to narrow the optical pulse, broaden the spectrum, reduce frequency drift and system noise, and calibrate and retiming the position of the optical pulse.
All-optical information regeneration technology can not only fundamentally eliminate the influence of unfavorable factors such as chromatic dispersion, but also overcome the shortcomings of photoelectric repeaters and become one of the basic technologies of all-optical information processing.