Error rate analysis of subcarrier QPSK With receiver I/Q imbalances over Gamma-Gamma fading channels

A free-space optical system using subcarrier intensity modulated quadrature phase shift keying (QPSK) is investigated for Gamma-Gamma fading channels with receiver in-phase and quadrature (I/Q) imbalances. The I/Q imbalance caused by imperfection of analog components is unavoidable using any direct-conversion transceivers. In addition to the receiver I/Q imbalances, the performance of a subcarrier QPSK system is impaired by the fluctuation of irradiance over the Gamma-Gamma fading channels. By taking into consideration of both receiver I/Q imbalances and fading, we derive closed-form symbol error rate (SER) and bit-error rate (BER) expressions. It is discovered that the value of amplitude imbalance is irrelevant to the error rate performance of the QPSK system.