In digital communication system, the frequency spectrum occupied by unmodulated digital signal starts from zero frequency or very low frequency, which is called digital baseband signal.[1]
Digital baseband signal is the radio waveform representation of digital information. It can use different levels or pulses to represent the corresponding message code.There are many types of digital baseband signals (hereinafter referred to as baseband signals).Taking rectangular pulse as an example, several basic baseband signal waveforms are shown in Figures 1-6.
1. Unipolar non return to zero waveform
Figure 1 Unipolar Waveform
As shown in Figure 1, the unipolar non return to zero waveform is a simple baseband signal waveform.It uses positive level and zero level to correspond to binary code "1" and "0" respectively;In other words, it represents "1" and "0" with or without pulse in a symbol time.The waveform is characterized by no separation between electrical pulses, single polarity, and easy to generate with TTL and CMOS circuits;The disadvantage is that there is DC component (the average level is not zero), and the transmission line is required to have DC transmission capability, so it is not suitable for long-distance transmission with AC coupling, and it is only suitable for transmission inside the computer or very close (such as in the core box of the printed circuit board).
2. Bipolar non return to zero waveform
Figure 2 Bipolar Waveform
As shown in Figure 2, the bipolar non return to zero waveform uses positive and negative level pulses to represent binary code "1" and "0" respectively.Because the amplitude of positive and negative levels is equal and the polarity is opposite, there is no DC component when the probability of "1" and "0" occurs, which is conducive to transmission in the channel, and the decision level of the recovered signal at the receiving end is zero, so it is not affected by changes in channel characteristics.The anti-interference ability is also strong.The bipolar waveform is used in the V.24 interface standard formulated by ITU-T and the RS-232C interface standard formulated by the American Electrotechnical Association (EIA).
3. Unipolar zero waveform
Figure 3 Waveform of Unipolar Zeroing
The so-called return to zero waveform means that its electrical pulse width tao is less than the symbol width T, that is, the signal voltage always returns to the zero level before the termination of a symbol.Generally, the return to zero waveform uses a half duty code.As shown in Figure 3, the unipolar zeroing waveform can directly extract timing information from the unipolar zeroing waveform, which is a transitional waveform often used for other code type bit taking synchronization information.
4. Bipolar zero waveform
Figure 4 Waveform of Bipolar Return to Zero
As shown in Figure 4, the bipolar return to zero waveform has the characteristics of both bipolar and return to zero waveforms.Because there is a zero potential interval between adjacent pulses, it is easy for the receiver to identify the start and end time of each symbol, so that both the sender and receiver can maintain correct bit synchronization.
5. Differential waveform
Figure 5 Differential Waveform
As shown in Figure 5, the differential waveform represents the message code with the level jump and invariance of adjacent symbols, regardless of the potential or polarity of the symbol itself.In the figure, "1" is represented by level jump, and "0" is represented by level unchanged. The above provisions can also be reversed.Because the differential waveform represents the code by the relative change of the adjacent pulse level, it is also called the relative code waveform, and correspondingly, the unipolar or bipolar waveform is called the absolute code waveform.Using differential waveform to transmit code can eliminate the influence of initial state of equipment, especially in phase modulation system, it can be used to solve the problem of carrier phase ambiguity.
6. Multilevel waveform
Figure 6 Multilevel Waveform
There are only two levels of the above waveforms, namely, one binary code corresponds to one pulse.In order to improve the frequency band utilization, multi-level waveform or multi value waveform can be used.As shown in Figure 6, a four level waveform 2B1Q (two bits are represented by the first level in the four level level) is given, where 11 corresponds to+3E, 10 corresponds to+E, 00 corresponds to - E, and 01 corresponds to - 3E.Because one pulse of the multi-level waveform corresponds to multiple binary codes, the bit rate is increased under the same baud rate condition, so the multi-level waveform is widely used in high-speed data transmission systems with limited frequency bands.
It should be noted that the waveform of a single pulse representing an information symbol is not necessarily rectangular.According to the actual needs and channel conditions, it can also be Gaussian pulse, rising cosine pulse and other forms.
Brief analysis of characteristics
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1. Non return to zero and return to zero
The unipolar non return to zero waveform has a DC component and requires a DC transmission system mechanism.Zeroing code improves this situation.
The return to zero code can extract bit synchronization information.
