How Does an OFDM Receiver Decode Signals Efficiently?

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An OFDM signal, such as DVB-T, features a wide but sparse spectrum with distinct peaks at specific frequencies and no signals in between. The challenge for OFDM receivers lies in efficiently decoding these signals without the impracticality of using 1705 oscillators for each subcarrier. A common method involves sampling the entire baseband signal and applying FFT, but this requires an ADC with a bandwidth of at least 15.22 MHz, which can be resource-intensive. Given that only 1705 frequencies are active and their locations are known, there is a need for more efficient reception techniques. Insights into practical OFDM reception methods are sought to optimize this process.
divB
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Hi,

Suppose you have an OFDM signal, e.g. DVB-T. In this case the spacing between the carriers is 4464 Hz and there are 1705 carriers. So the total bandwidth is 7.61 Mhz in baseband.

First: Am I right that the spectrum is wide but very sparse? So There is a peak at 0Hz, 4464 Hz, 8928 Hz, ..., 7606656 Hz? And nothing between?

How does a real OFDM receiver work in practice?

The first approach would be to demodulate each subcarrier. But this yields a 1705 oscillators ... not really good.

The second approach, as suggested by Wikipedia [1] is to sample the whole baseband signal and then apply the FFT to obtain the carriers. However, the bandwidth is 7.61 Mhz and this would require at least an ADC of 15.22 Mhz!

On the other hand this is a waste of resources because it is known that only 1705 frequencies are active and even the locations are known!

Can anyone give me a hint how OFDM reception is done in practice?

Regards, divB



[1] http://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing#Receiver
 
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