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Is it possible for multiple users to simultaneously transmit on the same subcarrier in a MIMO-OFDMA system?

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For a given OFDM symbol, no subcarrier can be assigned to two users simultaneously. Each subcarrier is uniquely assigned to a unique user. Else you have to implement an Interference cancellation at receiver (which would require having channel knowledge of other users as well, meaning global CSI) or at the transmitter and extremely complex interference alignment/precoding has to be done.

In practical systems like LTE or 5G, thus one subcarrier in a given OFDM symbol is allocated to only one user for the reasons listed above.

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  • $\begingroup$ But isn't OFDMA, a multi-user version of OFDM? I have encountered a paper where each sub-carrier serves a unique set of users simultaneously. $\endgroup$ – MaxFrost Apr 19 at 19:18
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    $\begingroup$ @MaxFrost exactly, and the multiuser separation happens through one subcarrier in one OFDM symbol belonging to exactly one user. In the next OFDM symbol, it might be a different user, but at any time in OFDMA, one point in the OFDM symbol / OFDM subcarrier 2D plane belongs to at most one user. $\endgroup$ – Marcus Müller Apr 19 at 19:47
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    $\begingroup$ @MaxFrost you can of course staple on even more layers of multi-user systems on that (do CDMA within OFDMA, for example), but that would be super special-case, and not something you'd generally call OFDMA. $\endgroup$ – Marcus Müller Apr 19 at 19:47
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    $\begingroup$ Just as @Marcus said, it's multi user in the sense that multiple users are simultaneously "scheduled" in one OFDM symbol, but their data is on uniquely assigned sub carriers, in the next symbol the sub carrier allocation can change technically. $\endgroup$ – Dsp guy sam Apr 19 at 19:51
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    $\begingroup$ @MaxFrost In short users are assigned orthogonal radiol resources $\endgroup$ – Dsp guy sam Apr 19 at 19:54
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It is possible if the system supports MU-MIMO.

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