Direct transmission of baseband signals without modem

Channel: Provides a transmission channel for baseband signals. Transmit filter, i.e. channel signal former: Function: Original baseband signal → Baseband signal suitable for channel transmission Purpose: Match the channel, reduce the code string, and facilitate synchronization extraction. Receive filter: Function: Filter out-of-band noise and equalize channel characteristics. Purpose: Make the output baseband waveform conducive to sampling decisions. Sampling decision maker: Function: Make sampling decisions on the output waveform of the receiving filter. Purpose: Determine the transmitted signal code sequence and regenerate the baseband signal. Synchronous extraction: Extract bit timing pulses for sampling.

(a) Unipolar waveform

(b) Bipolar waveform

(c) Single return-to-zero code

(d) Double return-to-zero code

(e) Differential waveform (relative code waveform)

(f)Multi-level waveform

If the waveforms of each symbol are the same but have different values, it can be expressed as:

Generally:

Purpose

method

Ideas

The statistical average component of the random sequence s(t), the statistical average waveform of each symbol is the same:

Power Spectral Density

Power Spectral Density

bilateral spectrum

One-sided spectrum

①No DC component and small low-frequency component;

② Rich timing information;

③The main lobe width of the power spectrum is narrow;

④ Not affected by the statistical characteristics of the source;

⑤Have self-checking ability

⑥Easy to compile and decode.

AMI code

HDB3 code

Biphase code (Manchester code)

CMI code

nBmB code (m>n)

nBmT code (m≤n)

Baseband transmission system model

Baseband transmission system composition

Waveforms at each point of the baseband transmission system

Baseband transmission overall characteristics

The baseband signal corresponding to {an}

Receive filter output signal

The current symbol only generates inter-symbol crosstalk for the next symbol

If g(t) is the waveform of the transmitted signal, and the interval between transmitted symbols is TB

Then the sampled value of this symbol is only subject to crosstalk from the same amplitude sample of the previous symbol.

Highest band utilization without ISI

The current symbol only generates inter-symbol crosstalk for the next symbol

Notice

shortcoming

It refers to a pattern observed at the receiving end with an oscilloscope; when transmitting a binary signal waveform, the pattern displayed on the oscilloscope is very similar to the human eye - hence the name "eye diagram". The size of ISI and the strength of n(t) can be observed from it; In order to intuitively evaluate the performance of the system; Adjustments to the receive filter may also be indicated to reduce ISI.

Without ISI - big "eyes", the stitches are thin and clear; with ISI - small "eyes", and the stitches are messy. When there is noise, the eye diagram traces become blurred strip lines; the greater the noise, the wider and blurrier the lines are, and the smaller the "eyes" open, or even close.

Analytical model

nR(t)Characteristics

x(t)Characteristics

For a bipolar baseband signal, its sampling value is (A,-A), then the value of the synthetic wave x(t)=s(t) nR(t) at the sampling time is:

Bit error rate Pe

Total bit error rate of bipolar baseband system: (the value of Pe depends on P(1), P(0), A, and Vd)

optimal threshold level

Meaning: This code element has a value at the sampling time; other code elements are 0 at the sampling time.

"Cut, translate/fold, superimpose" When the ideal LPF transmits at a rate of Re=1/Te, there is no inter-code crosstalk.

Ideal low pass characteristics

Cosine roll-off characteristic