The formation of OFDM signal is a constellation of n subcarriers, and the complex number is the ifft output, which is OFDM. This output is also a complex number. Is OFDM a complex signal?

Generally speaking, digital-to-analog conversion is needed before carrier modulation and after parallel-to-serial conversion, that is, digital OFDM data processed by DSP chip is converted into analog OFDM signal. Since the output of IFFT is complex data, two DACs are needed to convert the real and imaginary data at the same time, and then the up-converted real RF signal is obtained by IQ modulation. It is worth noting that IQ modulation can also be realized in the digital domain, so only 1 DAC is needed for direct up-conversion during digital-to-analog conversion. In addition, another method is to let IFFT output real data. Therefore, we only need to constrain the IFFT input data and satisfy Hermite symmetry. However, this method reduces the spectrum utilization of the system. Because of its simple system structure and low cost, there are many researches on optical fiber communication system at present.