Introduction

Orthogonal Frequency Division Multiplexing (OFDM) is used in several wireless communication standards, including digital audio broadcast, digital video broadcast, IEEE 802.11a/g wireless local area networks, and IEEE 802.16a metropolitan area networks. OFDM divides a broadband channel into many narrowband subchannels and modulates encoded signals onto the narrowband subchannels by using the fast Fourier transform (FFT). In many OFDM standards, the subchannels on the edges of the transmission band are not used. Each subchannel would generally carry a QAM signal. In each time slot, only one user transmits. In the receiver, frequency-domain equalization is commonly employed, and channel shortening (a.k.a. time domain equalization) is sometimes used as well.

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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