The effects of I/Q imbalance of the performances of QPSK in uncoded and coded schemes : imbalance detection and compensation techniques

Analogue implementations of quadrature modulation circuits are usually imperfect and suffer from several impairments. Among these impairments, the amplitude imbalance and phase imbalance are the major source of signal distortions especially for direct conversion schemes. The imperfection contributes to the distortion of signal constellation and spurious tones at the transmitter, and also affects the steady state lock point at the receiver. The imperfect rendered from such a quadrature modulation scheme inevitably degrades the system performance. In this thesis, two kinds of mathematic models are proposed. One of them is an equivalent baseband matrix model. Another is a modulated signal mathematic model. Base on our research, a least square estimation method is adapted at the transmitter by means of amplitude of modulated signal, and a compensation method is achieved backward in baseband before the quadrature modulator. At the receiver, maximum likelihood (ML) estimation is used to detect the imbalances from the transmitter and receiver. A compensation method reaches the solution of compensation at the receiver. We also prove the efficient algorithms by the simulation result. Channel coding technologies can be a general error correction technology. With Viterbi decoder algorithm and Turbo code, the simulations for the imbalance effect on the performance of the system have been done. These simulation results show that Turbo code as well as Viterbi decoding has powerful correction ability to the I/Q imbalance