Digital baseband transmission system--digital baseband signal

Digital baseband signal

a digital baseband signal is a waveform that represents digital information, which can be expressed in different levels or pulses. There are many types of digital baseband signals.

This paper introduces several basic baseband signal waveforms with rectangular pulses as an example.

Unipolar waveform

This is one of the simplest baseband signal waveforms. It corresponds to the binary digits "1" and "0", respectively, with a positive and 0 level, or "1" and "0" in a code element time with or without pulses. The characteristic of the waveform is that there is no interval between electric pulses, a single polarity, easy to use TTL, CMOS circuit generation; The disadvantage is that there is a DC component, which requires the transmission line to have DC transmission capacity, so it is not suitable for AC-coupled long-distance transmission, only applicable to the internal or very close to the computer (such as printed circuit board

Bipolar Waveform

This is a positive, negative flat pulse to represent the binary number "1" and "0" respectively. Because of its positive and negative levels of equal amplitude, polarity opposite, so when "1" and "0" and other probabilities occur without DC components, is conducive to transmission in the channel, and in the receiver to restore the signal at the decision level of zero value, thus not affected by the change of channel characteristics, anti-jamming ability is also weak. Bipolar waveforms are used in the V.24 interface standard developed by ITU-T and the RS-232C interface standard developed by the American Electrotechnical Society (EIA).

unipolar Zero Waveform

The

so-called zero (RZ) waveform refers to its electrical pulse width tau is less than the code element width T, that is, the signal voltage in a code element at the end of the time before the total to return to 0 levels. In general, the Zero waveform uses a semi-empty code, that is, the duty ratio (τ/τ) is 50%, from the unipolar RZ waveform can be directly extracted timing information, it is the other pattern extraction bit synchronization information is often used in a transition waveform. corresponding to the zero-return waveform, the above unipolar waveform and bipolar waveform belong to the non-zero (NRZ) waveform with a duty-ratio of τ/τ=100%.

Bipolar Zero Waveform

It is the zero form of the bipolar waveform. It has both bipolar and zero waveform characteristics. Since there are 0 potential intervals between the adjacent pulses, it is easy for the receiver to identify the start-start time of each code element, so that the transceiver can maintain the correct bit synchronization. This advantage makes the Bipolar zero waveform get a certain application.

Differential Waveform

This waveform is used to represent the message with the jump and invariant of the level of the adjacent code element, regardless of the point or polarity of the code element itself. The "1" is indicated by the level-hopping, and the "0" is indicated by the level, but the above regulation can be reversed. Because the differential waveform is a relative change in the adjacent pulse level to represent the message, it is also referred to as the relative code waveform, which is referred to as the preceding unipolar or bipolar waveform is absolute code waveform. The transmission of messages with differential waveforms can eliminate the influence of the initial state, especially in the corresponding modulation system, which can be used to solve the problem of carrier phase ambiguity.

Multi-level waveform

There are only two levels for the first 5 waveforms, that is, one binary code element corresponds to one pulse. Multi-level waveforms or multi-value waveforms can be used to improve the frequency band utilization. Because one pulse of the multilevel waveform corresponds to multiple binary codes, the bit rate increases with the same baud rate (the same transmission bandwidth), so the multilevel waveform is widely used in the high speed data transmission system with limited band.

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Digital baseband transmission system--digital baseband signal