Baseband transmission: When the transmission distance of a wired channel (twisted pair, cable) with low-pass characteristics is not too far, the signal can be transmitted directly without modulation. Such as: computer LAN, telephone signals, this kind of transmission is called digital baseband transmission system

Frequency band transmission: The transmission characteristics of wireless channels and long-distance wired channels are bandpass.

Baseband signal formation: pattern formation--waveform formation

System composition: channel signal former (code pattern and waveform), channel, receiving filter, sampling decision maker, synchronization extraction

Unipolar non-return to zero NRZ: contains DC component, no synchronization information; not suitable for long-distance transmission, only suitable for internal equipment

Bipolar non-return to zero BNRZ: no DC component, strong anti-interference ability

Unipolar return to zero RZ: including timing information

Bipolar return-to-zero BRZ with timing information

(Absolute code waveform)

Differential waveform (relative code waveform): It can eliminate the influence of the initial state of the equipment, especially used to solve the problem of carrier phase ambiguity in PM modulation systems.

Multi-ary waveform: Under certain conditions, a higher bit rate can be transmitted

Selection principles: contains no DC components and few low-frequency components; the code conversion process is transparent to any source; contains timing information; has inherent error detection capabilities; the transmission efficiency is as high as possible; the encoding and decoding equipment is as simple as possible

AMI: 1-1 alternation does not contain DC components; there are few high and low frequency components; the power spectrum energy is concentrated at half the code rate; B = code rate; it has error detection capabilities; there is no timing component, but the signal passes through the full wave Rectification becomes RZ, which can extract timing. It is one of the transmission codes recommended by CCITT; interfaces below the u-law fourth group use scrambled AMI (scrambling reduces the number of consecutive 0s in AMI)

HDB3: 3rd-order high-density bipolar code; within three consecutive zeros can be used for error detection, which is beneficial to extracting timing information. Interface patterns below A-law PCM quartic group

Digital biphase code 1:10 0:01 No DC component, including timing information, the bandwidth is doubled than the original code; computer Ethernet adopts

CMI: 1:11 00 0:01 Contains timing information. Since the disabled code group "10" can be used for macro error detection, the interface code type used by the PCM high-order group recommended by CCITT

Miller code: clock information; has good anti-interference ability

nBmB: m=n 1 in optical fiber digital transmission system, commonly used 5B6B Advantages: extract synchronization signals; 4B5B 8B10B is used for high-speed Ethernet

4B/3T: Reduce the code rate to improve frequency band utilization; suitable for higher-rate data transmission systems such as high-order group coaxial cable transmission systems

2B1Q: Coding type used by high-speed data subscriber line HDSL

The power spectral density of digital baseband signals may contain a continuous spectrum to obtain a discrete spectrum

The continuum is always present and is used to determine the signal bandwidth

The discrete spectrum does not necessarily exist. The condition exists that pg1 (1-p)g2 ≠0 and at least one of G1 and G2 is not 0.

Discrete spectra can be used to extract synchronization signals

NRZ：B=fs=RB

RZ:B=2fs=2RB

BNRZ:B=fs=RB

BRZ:B=2fs=2RB

AMI and HDB3: no DC component, no other discrete components; few low-frequency components; bit synchronization extraction method: add a waveform converter (can be a comparator, differentiator, rectifier), and its waveform conversion output is RZ

Frequency domain conditions without inter-symbol crosstalk (where RB=1/Ts)

Nyquist bandwidth (minimum bandwidth for baseband transmission): fN=RB/2=1/(2Ts) Nyquist rate (highest rate for transmission without inter-symbol interference) RB maximum frequency band utilization: n=2Baud/Hz

H(ω) satisfies complementary symmetry

Advantages: It reaches the limit value of 2Baud/Hz of the frequency band utilization of the baseband transmission system, that is, the frequency band utilization is high

Disadvantages: slow time domain response attenuation, sensitive to phase jitter of timing signals (sampling signals and bit synchronization signals)

Odd symmetry at fN

α=▲f/fN

B= fN(1 α) RB=2fN n=2/(1 α)

Disadvantages: low frequency band utilization and poor effectiveness

Advantages: short tail, fast attenuation, slight deviation in judgment, no serious bit errors, not strict requirements on the phase jitter of the sampling signal, and physically achievable

best sampling moment

The slope determines how sensitive the system is to sampling timing errors (the larger the value, the more sensitive it is)

Shadow vertical height: distortion range

Decision threshold level

noise margin

zero crossing distortion