Basics of OFDM In Wireless Communications

Basics of OFDM In Wireless Communications

Wireless Communications

Basics of OFDM:

OFDM Transmitter:

OFDM Transmitter is shown as follows:

The bit sequence is first subjected to channel encoding to reduce the probability of error at the receiver due to the channel effects. 
The bits are mapped to symbols of either 16-QAM or QPSK.
The symbol sequence is converted to parallel format IFFT (OFDM modulation) is applied to convert the block of frequency data to a block of time data that modulates the carrier.
The sequence is once again converted to the serial format. 
Guard time is provided between the OFDM symbols and the guard time is filled with the cyclic extension of the OFDM symbol.
Windowing is applied to the OFDM symbols to make the fall-off rate of the spectrum steeper.
The resulting sequence is converted to an analog signal using a DAC and
passed on to the RF modulation stage. 
The resulting sequence is converted to an analog signal using a DAC and passed on to the RF modulation stage.

OFDM receiver:

OFDM receiver is shown as follows:

At the receiver, first RF demodulation is performed. Then the signal is digitized using an
ADC. Timing and frequency synchronization are performed. The guard time is removed from
each OFDM symbol. The sequence is converted to parallel format.FFT (OFDM
demodulation) is applied to get back to the frequency domain. The output is then serialized.
Symbol de-mapping is done to get back the coded bit sequence. Channel decoding is, then,
done to get the user bit sequence.

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