Open wire, cable, fiber optic

free space or atmosphere

Study modulation and demodulation issues

Study encoding/decoding problems

Frequency: <2MHz Characteristics: Diffraction capability Distance: hundreds or thousands of meters Used for: AM broadcasting

Frequency: 2~30MHz Characteristics: Reflected by the ionosphere Distance: <4000km (one hop) Used for: long-range, short-wave communication

Frequency: >30MHz Characteristics: Straight line propagation, penetrating ionosphere Usage: Satellite and outer space communications, ultra-short wave and microwave communications Distance: Depends on antenna height

Each pair is twisted to reduce mutual interference between pairs

Large transmission attenuation, long distance, and crosstalk interference between adjacent channels

Transmission media in telephone lines, local area networks and integrated wiring projects

Composed of two coaxial conductors. Inner core: metal wire. Outer conductor: metal braid.

Strong anti-electromagnetic interference capability, wider bandwidth and higher speed

The cost is higher; solution: use optical cable instead

Wide transmission bandwidth, large communication capacity; small transmission attenuation, long transmission distance without relay; anti-electromagnetic interference, good transmission quality, anti-eavesdropping, corrosion resistance; small size, light weight, saving non-ferrous metals, environmental protection

Fragile, expensive interfaces, installation and maintenance require specialized skills

In the main lines of long-distance telephone networks, cable TV networks, etc.

Superposition of noisy linear time-varying/time-invariant networks

There is a pair (or multiple pairs) of input terminals and output terminals; most channels satisfy the linear superposition principle; there are fixed or time-varying delays and losses for signals; when there is no signal input, there may still be output (noise)

Additive noise is always present

constant parameter channel

accompanying channel

It can be described by transition probability.

The average amount of information transmitted by the channel per second

r ‒The number of symbols transmitted by the channel per second is (symbol rate) [H(x) – H(x/y)] ‒It is the average amount of information obtained by the receiving end

Maximum information transmission rate: probability distribution of all possible information sources

Meaning: The maximum average amount of information that each symbol can transmit

Equivalence

When the average power of the signal and channel noise is given, the theoretical limit value of the amount of information that can be transmitted per unit time on a channel with a certain frequency bandwidth.

If R, ≤ C, then a channel coding method can always be found to achieve error-free transmission; if the transmission rate is greater than the channel capacity, error-free transmission is impossible.

Channel capacity C depends on B, S and n0

Increasing S can increase C. If S →∞, then C→∞

Decreasing n0 can increase C. If n0 → 0, then C→∞

When C is constant, the channel bandwidth B and signal-to-noise ratio S/N can be converted to each other.

Increasing B can lead to a decrease in S/N; vice versa

man-made noise

natural noise

Internal noise (thermal noise)

impulse noise

Narrowband/Single Frequency Noise

Ripple noise

Impulse response of ideal constant parameter channel

Amplitude frequency distortion

phase frequency distortion

Attenuation changes with time Delay changes with time Multipath propagation

Multipath propagation causes Rayleigh fading of signals

Frequency dispersion caused by multipath propagation

Diversity reception

spread spectrum technology

OFDM etc.