Digital signal transmission

We know that digital signals are discrete in the time domain and have only two states, 1 and 0. In short-distance transmission (under 100 meters), baseband transmission can be used, and when long-distance transmission is to be used, carrier waves are used Transmission method. The carrier transmission system modulates the digital signal onto the carrier and sends it to the transmission channel. It is the same as the baseband transmission. It only adds a modulator to the output of the digital signal, and adds a demodulator before the digital input port. the same.
1. Baseband transmission system
In a digital communication system, the code output by the channel encoder also needs to undergo code conversion to become a code suitable for transmission. Commonly used baseband transmission codes mainly include the following: 1. Bipolar non-return to zero code; 2. Unipolar non-return to zero code; 3. Bipolar non-return to zero code; 4. Unipolar non-return to zero code; 5. , Manchester tear special code. The so-called bipolar refers to the use of positive pulses and negative pulses to represent digital signals 1 and 0; the so-called unipolar refers to the use of positive pulses and zero to represent digital signals 1 and 0; The pulse of the symbol is followed by a pulse representing the second symbol, and there is no time interval between the two, so-called zeroing. In the Mantra code, a half-symbol positive first and then negative (1, 0) pulse represents the digital signal 1, and a half-symbol negative first and positive pulse (0, 1) represents the digital signal 0. As shown in Figure D-1. In the bipolar non-return-to-zero code, if the probability of 0 and 1 is the same, the positive and negative voltages just cancel out the DC component, which is beneficial to transmission and has strong anti-interference ability.
In the baseband transmission system, multiplexing technology is usually adopted. Multiplexing is to combine various channels of information from different information sources into one channel in a certain way and transmit them to the receiving end through the same channel. Separate the various signals to different users. The multiplexing methods are: 1. Frequency division multiplexing; 2. Time division multiplexing; 3. Code division multiplexing; 4. Wavelength division multiplexing; 5. Time compression multiplexing. In digital communication, more time-division multiplexing technology is used. The so-called time-division multiplexing refers to the use of different time slots of the same channel for the communication of various signals. It is easy to produce intermodulation and intermodulation distortion, so the nonlinear distortion index of time division multiplexing system is not high.
Two main devices are used in the time division multiplexing system: One is the multiplexer, its function is to synthesize several signals into a combined digital signal according to the principle of time division multiplexing. The other is the tap, which is opposite to the function of the multiplexer, which is to restore the combined signal into digital signals of several branches. Putting multiplexer and tap together is called digital multiplexing device. Digital multiplexing must solve two problems: one is synchronization, and the other is multiplexing. Synchronization consists of a timing system and a code rate adjustment unit. The internal clock of the timing system provides a time reference signal to the multiplexer. The code rate adjustment unit is to adjust each branch with a different code rate to a number that is synchronized with the multiplexer timing signal. Signal, multiplexing is to complete the multiplexing task, and the signals of each branch are merged into one signal.
The digital baseband signals are all rectangular waves. Since rectangular wave pulses contain rich harmonic components, they will inevitably suffer distortion during transmission in a limited channel bandwidth, which will cause a large bit error rate. In addition, because the harmonics generated by each symbol overlap each other in the time domain, intersymbol interference occurs. Fortunately, the symbol waveform is sent at a certain interval, as long as the waveform amplitude at a specific time is not Distortion, even if other parts are greatly distorted, has no effect on the symbol regeneration decision.
Second, the carrier transmission of digital signals When digital signals are to be transmitted over a longer distance, the carrier transmission method is used. The main difference between digital signal carrier transmission and baseband transmission is the addition of modulation and demodulation. A modulator is added after the multiplexer, and a demodulator is added before the tap.
Digital signals have only a few discrete values, which is like using digital signals to control switches to select oscillations with different parameters. For this reason, the modulation method of digital signals is called keying. Modulation methods include amplitude keying (ASK); frequency shift keying (FSK); and phase shift keying (PSK).
1. Amplitude keying (ASK)
Amplitude keying can be achieved by multipliers and switching circuits. The carrier is turned on or off under the control of digital signal 1 or 0. When the signal is 1, the carrier is turned on, and there is a carrier on the transmission channel; when the signal is 0, the carrier is turned off, and the transmission channel is now No carrier transmission. Then at the receiving end, we can restore the digital signal 1 and 0 according to the presence or absence of the carrier. The bandwidth of the binary amplitude keying signal is twice the width of the binary baseband signal. The waveform is shown in Figure D-2.
2. Frequency shift keying (FSK)
Frequency shift keying is to use two oscillation sources with different frequencies F1 and F2 to represent signals 1 and 0. Its waveform is shown in Figure D-3. The digital signals 1 and 0 are used to control the two independent oscillation sources to output alternately. . For binary frequency shift keying modulation, the effective bandwidth is B = 2xF + 2Fb, where xF is the bandwidth of the binary baseband signal and the maximum frequency deviation of the FSK signal. Because the bandwidth of the digital signal, that is, Fb value is large, the binary frequency shift key The controlled signal bandwidth B is large, and the frequency band utilization rate is small.
3. Phase shift keying (PSK)
In phase shift keying, the wave phase is controlled by the digital baseband signal. For example, when the binary baseband signal is 0, the carrier phase is 0, and when the carrier phase is 1, the carrier phase is π. The carrier phase corresponds to the baseband signal. Relationship, its waveform is shown in Figure D-4.
Three, multi-ary digital modulation The above discussion is in the case of binary digital baseband signals. In practical applications, we often use a method called multi-ary (such as 4-ary, 8-ary, 16-ary, etc. ) Of the baseband signal. Multi-ary digital modulation carrier parameters have M different values. Multi-ary digital modulation has two prominent advantages over binary digital modulation: First, the multi-ary digital signal contains more information to make the frequency band utilization more efficient High; the second is long duration at the same information rate, which can increase the energy of the symbol, thereby reducing the inter-symbol interference caused by the channel characteristics. Due to the relationship of amplitude, only one of the most used modulation methods is discussed here: multi-ary phase shift keying (MPSK).
Multi-ary phase shift keying is also called polyphase system, because the baseband signal has M different states, so its carrier phase has M different values, these values ​​are generally at equal intervals. There are two types of absolute phase shifting and relative phase shifting in multi-phase system shift keying. In practice, four-phase absolute phase shift keying (4PSK, also known as QPSK) is used. The phases of the four-phase system are 0, π / 2, π , 3π / 2, corresponding to the four states 11, 01, 00, and 10, respectively. The QPSK signal can be expressed as I (t) COS2πft-Q (t) SIN2πfct, where the first term is the in-phase component and the second term is the quadrature component, so QPSK is also called quadrature phase shift keying modulation.
It can be seen from the above that the frequency band utilization rate of QPSK is twice that of the corresponding binary digital modulation, but this is at the expense of power utilization rate. Because the distance between the symbols decreases as the base number increases, it is not conducive to the recovery of the signal, especially when the bit error rate is affected by noise and interference. To solve this problem, we have to increase the signal power (that is, increase the signal-to-noise ratio of the signal to avoid the increase of the bit error rate), which reduces the power utilization. For this reason, is there a way to increase the frequency band utilization and the distance between the symbols is not too small? This introduces a type called QAM (quadrature amplitude modulation). The characteristic of QAM is that not only the amplitude but also the phase are different between the symbols. It belongs to the combination of amplitude and phase modulation. In QPSK, the amplitude of each symbol is the same but the phase is different, so the average power is higher. The amplitude of the symbols is different, so the average power is small. Therefore, when the average power is the same, the value of each symbol of QAM can be higher than that of each symbol of QPSK, so that the signal-to-noise ratio is improved.