Orthogonal Frequency Division Multiple Access (OFDMA)

OFDM is the modulation of choice for the LTE downlink, the key advantages being the low complexity of the receiver, high spectral efficiency and avoidance of Inter-Symbol Interference (ISI). This chapter presents an overview of the theoretical background of OFDM together with its parameterization for LTE. Firstly, the theory of OFDM is explained by means of a mathematical formulation of its ability to combat Inter-Carrier Interference (ICI) and to avoid ISI by means of the Cyclic Prefix (CP). Secondly, the main limitations of OFDM are highlighted together with their effects on the system performance. These include the Peak-to-Average Power Ratio and sensitivity to frequency offsets. This analysis leads to an explanation of how the parameters of an OFDM system can be traded off to maximize system spectral efficiency, resulting in the particular parameter choices for LTE.