Analog representation and digital implementation of OFDM systems

Many existing results on the analysis of OFDM systems are based on an analog representation. The actual implementation of OFDM transmitters typically consists of a discrete DFT matrix and a digital-to-analog (DAC) converter. In this paper, we show that the analog representation admits the implementation of a DFT matrix followed by a DAC converter only in special cases. Necessary and sufficient conditions for such cases will be given. Analyses based on the analog representation, e.g., spectral roll-off of transmitter outputs and carrier frequency offset, may not be appropriate when digital implementation is employed.