Optical transmission of OFDM m-QAM RF signals: Comparison of direct and external modulations

For interconnections between the stations of mobile networks optical links are frequently used. The mobile networks require new modulation methods to achieve higher transmission capacity. For that purpose the m-QAM modulation method is a proper approach. On the other hand, the signal transmission in the mobile network is strongly affected by multipath wave propagation. The OFDM modulation can counterbalance that problem. Therefore an OFDM m-QAM RF signal is transposed into the optical domain by an optical modulator or by direct laser modulation. However, both m-QAM and OFDM are sensitive to the modulation nonlinearity. In this paper the nonlinear effect of direct and external optical modulations is investigated by simulations and experiments obtaining this way conditions for optimum operation.