Multiple-access strategies typically attempt to provide orthogonal, or noninterfering, communication channels for each active link. The most common way to divide the available dimensions among the multiple users is through the use of frequency, time, or code division multiplexing.
In frequency division multiple access (FDMA), each user receives a unique carrier frequency and bandwidth.
In time division multiple access (TDMA), each user is given a unique time slot, either on demand or in a fixed rotation.
In code division multiple access (CDMA) systems allow each user to share the bandwidth and time slots with many other users and rely on orthogonal binary codes to separate out the users.
OFDMA is essentially a hybrid of FDMA and TDMA:
Users are dynamically assigned subcarriers (FDMA) in different time slots (TDMA).In OFDMA, the subcarrier and the power allocation should be based on the channel conditions in order to maximize the throughput.
Multiuser diversity and Adaptive modulation are the two key principles that enables high performence in OFDMA. Multiuser diversity describes the gains available by selecting a user or sub-set of users having "good" conditions. Adaptive modulation is the means by which good channels can be exploited to achieve higher data rates.
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