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YakB

Transmission Channel

  • BER: Bit Error Rate, number of bit errors
  • SER: Symbol Error Rate, number of symbol errors

1. Source Coding

Source coding is used to compress the data to be transmitted in order to reduce the number of bits that need to be sent.

2. Channel Coding

Channel coding adds redundancy to the data to be transmitted. This is used to correct potential errors (noise, interference, etc.) during transmission.

3. Digital Modulator

The modulator transmits bits in groups called symbols. Symbols form an alphabet.

Example of a 4-symbol alphabet:
m₁ = 00, m₂ = 01, m₃ = 10, m₄ = 11

The digital modulator is composed of two parts:

  • the encoder converts the symbol mₖ into a set of parameters (aₖ, φₖ)
  • the modulator constructs a signal Sₖ(t) of duration Tₛ (symbol duration) using the parameters aₖ and φₖ

4. The Channel

The channel acts like a band-pass filter. It also adds noise and causes amplitude and phase distortion to the signal.

The received signal is therefore:

x(t)=Sk′(t)+w(t)x(t) = S'ₖ(t) + w(t)

Caption:

  • x(t): received signal
  • S'ₖ(t): the signal distorted by the band-pass filter
  • w(t): the noise

5. Digital Demodulator

The demodulator transforms the signal into a sequence of bits.

The digital demodulator consists of two parts:

  • the detector retrieves the parameters (aₖ, φₖ)
  • the decoder reconstructs a symbol from the signal x(t) and the parameters (aₖ, φₖ). This value may be incorrect if the signal is degraded.

6. Channel Decoding

Channel decoding uses the added redundancy to correct errors.

7. Source Decoding

Source decoding decompresses the bit sequence.

The final signal may differ from the original signal due to residual errors.