Sure, why not? In fact, you can use one modulation standard for the downlink, and a completely different one for the uplink. Or am I missing some subtlety in your question?
To extend Albert's response, you can even use FBMCs for duplex without guard band between uplink and downlink, since it is less sensitive to out-of-band interference, and synchronization mismatch.
Dear Zeynab,OFDM is a multicarrier modulation scheme where the multiple carriers are orthogonal to each other. This is best implemented by synthesizing the qam symbols in frequency domain and converting this symbol to time domain by inverse fast Fourier Transform.
There is also a multicarrier system which is the frequency division multilplixing where a filter bank can be used in the modulation and demodulation.
Frequency gaps are left between the sub channels in the FDM where in the OFDM the channels are overlapped saving bandwidth and increasing the bandwidth efficiency.
wish you ssuccess
Dear Zeynab, you can use filter banks, but OFDM is a much more efficient and also simple way of doing it. To complement the advantages mentioned by Abdelhalim I mention that with OFDM you can also cope with multipath fading in a simple way by using the Cyclic Prefix. In OFDM the bank of filters is substituted by digital modules (IDFT and DFT) avoiding in this way lot of problems, complexity and costs. Also OFDM is very convenient from a point of view of bandwidth efficency due to the orthogonal overlap of subcarriers. In summary I strongly recomend you to use OFDM instead of a bank of filters.
I agree with Dr. Rodriguez. However, to ensure that the subcarrier symbols remain orthogonal, in OFDM, which is essential because you have no filter banks, you need to have a single transmitter using the channel. That's the tradeoff. If you have many transmitters in various locations sharing the subcarriers, you have a problem. That's when the filter banks become necessary.
Always tradeoffs involved.
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