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Performance analysis of subcarrier quadrature phase-shift keying systems with I/Q imbalance over Gamma-Gamma fading channels Zhu, Changle


The error rate performance is studied for subcarrier intensity modulated (SIM) quadrature phase-shift keying (QPSK) system with in-phase and quadrature (I/Q) imbalance over Gamma-Gamma fading channels. In free-space optical (FSO) communication system, the transmitted signals are typically affected by the atmospheric turbulence over the transmission links. In order to study the system performance analytically, different statistical distributions have been proposed to describe the random variation in signal irradiance due to the scintillation caused by the inhomogeneities in both temperature and pressure along the transmission path. Besides the atmospheric turbulence, other sources can also introduce performance degradation to an FSO system. Using quadrature conversion, the performance of the practical system is affected by the phase and amplitude offsets in the two branches. This phenomenon is referred to as I/Q imbalance. Using direct-conversion transceivers, the I/Q imbalance is unavoidable due to the considerable mismatches between the circuit components. This imbalance can happen at transmitter end, receiver end, or both ends of the transceiver. First, we study the error rate performance for a SIM QPSK system with transmitter I/Q imbalance over the Gamma-Gamma fading channels. Then, the error rate performance of a SIM QPSK system with receiver I/Q imbalance is investigated. Finally, the error rate performance is analyzed for a SIM QPSK system with I/Q imbalance at both ends of the transceiver over the Gamma-Gamma fading channels. Closed-form symbol error rate (SER) expressions are derived by taking into account of both the I/Q imbalance and the fading. It is discovered that the value of the receiver amplitude imbalance is irrelevant to the SER of a subcarrier QPSK system. Truncation error analyses are carried out to ensure the accuracy of the approximate series solutions and support the asymptotic analyses. We also present the numerical results to show the performance improvement using a calibrated transceiver. We will treat these three cases separately since they have slightly different models.

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