## labAlive - Virtual Communications Lab

 Tutorial Experiment App

Introduction

Oscilloscope

Spectrum analyzer

Experiments

OFDM step-by-step

OFDM Guard interval

QPSK BER

QAM BER

Equivalent baseband

Wi-Fi IEEE 802.11ac

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OFDM step-by-step

QPSK BER

QAM BER

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Wireless Communications

Telecommunications

Digital Modulation

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# OFDM transmission step-by-step

In this experiment an OFDM data transmission via a frequency-selective channel is established. Check the tutorial.

## Channel estimation

Initially a pilot sequence $d\left(i\right)$ 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 spectral representation of the pilot OFDM symbol $w\left(i\right)$ represents the channel transfer function.

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

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

## End-to-end data transmission

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

 $d\left(i\right)$ A block containing 8 data symbols (representing 16 bits) is generated and serial-to-parallel converted. $u\left(i\right)$ It IFFT-transforms to an OFDM symbol. $s\left(i\right)$ The guard interval (GI) is added, i.e. the OFDM symbol is extended by a cyclic prefix containing the last single symbols. $r\left(i\right)$ The received symbol is degenerated by the channel. $v\left(i\right)$ The OFDM symbol is obtained by removing the guard interval, i.e. the first two single symbols. $w\left(i\right)$ (After further 8 steps.) It FFT-transforms to the spectral representation of the OFDM symbol. ${d}^{\prime }\left(i\right)$ The equalized signal represents the 8 data symbols.

The data symbols are detected error free: ${d}^{\prime }\left(i\right)=d\left(i\right)$. OFDM transmission implements ISI free detection.

Note that in this case no noise is present.

## Next steps

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 $\mu s$ ).