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I've read that OFDM normally uses a one-tap equalizer. It seems that a one-tap equalizer could only scale and delay a signal. Why is this useful?

I thought multipath was handled through the cyclic-prefix and by choosing a symbol width that is large compared to the delay spread. Why is an equalizer needed at all?

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The OFDM signal as a whole is affected by frequency selective filtering. It is usually designed such that the subcarrier bandwidth is smaller than the channel coherence bandwidth. This yields you frequency flat fading for each subcarrier which can be described by a single complex multiplication and equalization can equally done with just one tap.

Equalization in OFDM usually happens in the frequency domain. Let $X$ be the vector of subcarriers in the frequency domain and $x=IDFT(X)$ the corresponding time domain signal. Then, if the impulse response of your channel is a vector $h$, the received signal $y$ will be $y=x*h$ where $*$ referes to convolution. Then $Y=DFT(Y)=X \cdot H$ with $H=DFT(h)$ and $\cdot$ refering to element-wise multiplication. However, this is only true if $y$ is the circular convolution of $x$ and $h$ while in reality the convolution is linear. Therefore, the cyclic prefix is prepended (it needs to be at least as long as $h$) so that the linear convolution contains the circular convolution.

Then, $Y=DFT(Y)=X \cdot H$ with $H=DFT(h)$ and $\cdot$ as stated before. Since each element of $Y$ and $X$ refers to one subcarrier it can be seen that each subcarrier is multiplied with just one complex element from $H$ and hence equalization just becomes $Z=Y/H$ where again $/$ referes to element-wise division. This describes zero-forcing equalization which might not be the best choice if additive noise and interferers are present but it transports the idea.

And this is nothing else but one-tap equalization for each individual subcarrier.

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  • $\begingroup$ Hi Jan. Thanks for the thorough answer. I don't understand the physical phenomena which you're calling "frequency selective filtering" -- what does this mean? Is this a different issue than multipath? $\endgroup$ – Dan Sandberg Jul 25 '13 at 18:47
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    $\begingroup$ Frequency selectivity in mobile channels stems from multiplath propagation and just means that some frequencies are attenuated by the channel (in contrast to a "frequency flat" channel) $\endgroup$ – Deve Jul 26 '13 at 6:24
  • $\begingroup$ So if the signal took four separate paths to arrive at the receiver, causing three "echos" of different magnitude, this would be corrected by this one-tap equalizer? I don't see how it could do as good a job as an MMSE equalizer. And if it doesn't, what is it accomplishing vs. a more complicated one? $\endgroup$ – Dan Sandberg Jul 26 '13 at 18:53
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    $\begingroup$ If the guard interval is long enough, the OFDM one tap equalizer can correct this, yes. It's a zero forcing equalizer and it's performance is inferior to that of an MMSE EQ. Its advantage is lower complexity than MMSE, though. $\endgroup$ – Deve Aug 30 '13 at 7:34

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