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OFDM

Data rate is same as single carrier modulation. But when the coherence bandwidth is less than the bandwidth of the signal (in case of wide-band channels), single carrier modulation suffers from inter symbol interference (ISI).  Whereas in multi-carrier modulation the bandwidth is divided into sub-carriers with bandwidth less than the coherence bandwidth; it’s more like converting wide-band into narrow band channels.

Thus OFDM converts frequency selective fading channel into flat fading channel.

Since multi-carrier modulation requires many modulators/demodulators for the transmission of the data, OFDM comes with IDFT (done by IFFT) which maps the data onto the respective sub-carrier, and can be transmitted.

A ZP–OFDM system has lower transmission power and a simpler transmitter structure. Unfortunately, the ZP–OFDM scheme introduces ICI, as the orthogonality among subcarriers is destroyed when multiple
copies of the time-shifted ZP–OFDM waveform are received. To remove ICI, cyclic
prefixing (CP) transmission is preferred.

Cyclic prefix converts the linear convolution into circular convolution, thus DFT can be invoked on the received signal.

In MIMO-OFDM each transmit antenna implements OFDM and transmit.  Say, we have 4 antennas and 256*4 symbols to transmit, divide the 256 into 4 sets; and implement OFDM using the 256 sub-carriers at each antenna and transmit.

OFDM has high PAPR, which results in the non-linear region of the amplifier (in IFFT block).  Thus high PAPR makes OFDM loses its properties such as orthogonality of the carriers.

Equalization at the receiver is done to mitigate the ISI, equalizers such as ZF equalizer, MMSE, decision feedback equalizer (check Goldsmith’s book) are used.

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