Channel in communication system and its classification

Channel

A channel is the physical medium (and associated equipment) that carries the signal from the transmitter to the receiver. It may be a pair of wires, a coaxial cable, an optical fibre, a waveguide, or free space (radio). Every channel has a limited bandwidth, causes attenuation and distortion, and adds noise and interference; these characteristics set the maximum data rate (Shannon capacity $C = B\log_2(1+S/N)$) and decide which modulation is suitable.

Classification of channels

1. Based on physical medium

• Wired (guided) channels: the signal is guided along a physical path.
  • Twisted pair: telephone local loop, LAN (Cat-5/6); low cost, limited bandwidth.
  • Coaxial cable: cable TV, older Ethernet; larger bandwidth, better shielding.
  • Optical fibre: backbone networks; very wide bandwidth (THz), very low loss, immune to EMI.
  • Waveguide: microwave links inside equipment.
• Wireless (unguided) channels: signal propagates through free space by electromagnetic waves.
  • Ground wave (LF/MF, AM broadcasting), sky wave (HF, ionospheric reflection), line-of-sight / space wave (VHF and above: FM, TV, mobile, microwave links), satellite channels.
  • Underwater acoustic channels and optical free-space links are also examples.

2. Based on the nature of input/output

• Continuous (analog) channel: input and output are continuous waveforms, e.g. a telephone voice channel.
• Discrete (digital) channel: input and output are symbols, e.g. the binary symmetric channel (BSC), a channel of error probability $p$.

3. Based on time behaviour

• Time-invariant channel: characteristics do not change with time, e.g. coaxial cable, fibre.
• Time-varying (fading) channel: characteristics change with time, e.g. mobile radio with multipath fading, ionospheric HF links.

4. Based on linearity

• Linear channel: obeys superposition, e.g. cables.
• Non-linear channel: e.g. satellite transponder with TWT amplifier near saturation.

5. Based on noise model

• AWGN channel: only additive white Gaussian noise; a basic model for satellite and deep-space links.
• Band-limited channel: limited bandwidth causing ISI, e.g. telephone line.
• Multipath fading channel: e.g. cellular radio.

6. Based on direction of transmission

• Simplex: one direction only, e.g. radio/TV broadcasting.
• Half-duplex: both directions but one at a time, e.g. walkie-talkie.
• Full-duplex: both directions simultaneously, e.g. telephone, mobile phone.