In Orthogonal Frequency Division Multiple Access (OFDMA) systems, all users transmit and receive data at the same time. Each subchannel is allocated to at most one user, although more recent standards have allowed fractional reuse of spectrum at the cell edge. For the downlink direction, i.e. from basestation to mobile user, the mobile user could be allocated subchannels from across the downlink transmission band. In the uplink direction, a mobile user could receive a cluster of adjacent subchannels. A significant research problem has been, and continues to be, the optimal allocation of power and subchannels (and hence rates) to mobile users, which is known as the multiuser resource allocation problem.
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