OFDMA

OFDMA (Orthogonal Frequency-Division Multiple Access) is a multi-user version of OFDM enabling multiple users to transmit simultaneously by allocating them separate subsets of subcarriers within a common time-frequency grid. It uses orthogonal subcarriers to minimize inter-carrier interference, and a central scheduler assigns resource blocks to users according to channel conditions and service requirements.

The radio waveform is based on OFDM; the spectrum is divided into many narrowband subcarriers that can be grouped into resource blocks; transmissions occur in frames and slots with a cyclic prefix to mitigate intersymbol interference. OFDMA supports dynamic, fine-grained allocation in the frequency domain, providing scalability for varying user data rates and quality of service. It integrates with MIMO and advanced antenna techniques and can accommodate mobility.

Applications: It is used in several wireless standards, notably downlink of 4G LTE and WiMAX, and in 5G NR downlink; it is common in many broadband wireless access systems.

Because of its multiuser and adaptive allocation capabilities, OFDMA can exploit frequency-selective fading and spatial diversity to improve capacity.

Advantages and challenges: Advantages include high spectral efficiency, robustness to multipath, flexible scheduling, and compatibility with MIMO. Challenges include sensitivity to precise timing and frequency synchronization, high peak-to-average power ratio, and the complexity of the scheduler and receiver design.