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In this experiment an OFDM data transmission via a frequency-selective channel is established. Using example values we walk through the signal chain, OFDM symbols and equalizer coefficients. Finally a data sequence can be detected ISI-free. Check the tutorial.

Initially a pilot sequence $d(i)$ is sent for channel estimation and equalizer initialization.

- The app stops after the first symbol $s\left(0\right)$ is sent.
- Send the next symbol by pressing F5 (or
*Run - 1 Step*). - Continue till the first OFDM symbol is completely received (further 9 steps).
- Look at $w\left(i\right)$ , the spectral representation of this OFDM symbol.

The equalizer coefficients are the reciprocal of $w(i)$ .

- Regard the equalizer coefficients by clicking on the Equalizer box and then on Coefficients.

Now a data sequence is sent. Continue the simulation (6 steps). Step through the transmit chain:

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

$u(i)$ | It IFFT-transforms to an OFDM symbol. |

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

$r(i)$ | The received symbol is degenerated by the channel. |

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

$w(i)$ | (After further 8 steps.) It FFT-transforms to the spectral representation of the OFDM symbol. |

${d}^{\prime}(i)$ | The equalized signal represents the 8 data symbols. |

Note that in this case no noise is present.

The app recurrently transmits a fixed data sequence.

- Confirm the ISI free detection for random data sequences. Switch the digital source to
*Random*. - Modify the channel and simulate time selective channel variation. Channel parameters (delay and amplitude) can be adjusted in real time. Note that ISI occurs when there's a mismatch of the channel transfer function and the equalizer coefficients.
- Modify the initial echo delay of the channel:
*Simulation - Setup*(F12).Repeat the same flow as outlined above. Note that after clicking on

*OK*the simulation restarts and the pilot sequence is sent via this channel. Therefore the channel estimation and equalizer initialization work properly.Approve that ISI free detection requires a channel maximum delay spread (

*Echo delay*) lower than the guard interval length (i.e. the adjustable discrete*Prefix length*times the sampling time of e.g. 1,6 $\mu s$ ).