This introduction illustrates an OFDM data transmission using example values. Initially a pilot sequence initializes the equalizer according to the frequency-selective channel. Then a data sequence forming an OFDM symbol is sent and can be detected ISI-free.

1. OFDM transmission step-by-step
This introduction illustrates an OFDM data transmission
using example values.

2. 1. Channel estimation

3. Initially a pilot sequence is sent for channel estimation and equalizer initialization.

4. d(i): The pilot sequence is a series of real constant symbols.
In the frequency domain (before IFFT) it contains all frequencies with amplitude 1 V.

5. u(i): It IFFT-transforms to an OFDM symbol, in this case an impulse symbol.

6. s(i): The cyclic prefix (CP) is added.
The OFDM symbol is extended by a prefix containing the last single symbols.

7. r(i): The received symbol is degenerated by the channel.
The echo of the example channel has a relative power of 0.9.

8. v(i): The OFDM symbol is obtained by removing the guard interval,
i.e. the first two single symbols.

9. w(i): It FFT-transforms to the spectral representation of the OFDM symbol w(i).

10. w(i) is the frequency response to the pilot sequence containing all frequencies
with amplitude 1 V. Hence, w(i) is the channel transfer function HC,i.

11. To invert the channel transfer function, the equalizer coefficients HE are set to
the reciprocal of HC (zero-forcing).
reciprocal

12. 2. End-to-end data transmission

13. d(i): A block containing 8 data symbols (representing 16 bits) is generated
and serial-to-parallel converted.

14. u(i): It IFFT-transforms to an OFDM symbol.

15. s(i): The guard interval (GI) is added, i.e. the OFDM symbol is extended
by a cyclic prefix containing the last single symbols.

16. r(i): The received symbol is degenerated by the channel.

17. v(i): The OFDM symbol is obtained by removing the guard interval,
i.e. the first two single symbols.

18. w(i): It FFT-transforms to the spectral representation of the OFDM symbol.

19. d’(i): The equalized signal represents the 8 data symbols.

20. The data symbols are detected error free: d’(i)=d(i). In this scenario no noise is present.
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OFDM transmission implements ISI free detection.