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In recent mobile communication systems, impairments in analog circuits cannot be disregarded due to achieving high performance. Errors in analog circuits consist of a distortion of a power amplifier(PA), gain and phase imbalance iii a quadrature modulator(QMOD) and a quadrature demodulator(QDEMOD), dc offset, frequency offset, and so on. Because of high accuracy of compensation, many techniques of error compensation by using a digital signal processing have been studied recently. Digital predistorter(DPD), which can eliminate odd-order distortion caused by PA nonlinearity, is one of the compensation techniques, but in the case that direct counversion architecture is used in a loop-back path, errors in QMOD and QDEMOD affect the performance of DPD. In this paper, we propose a novel technique for error separation for QMOD and QDEMOD error estimation by adding a phase shifter to the conventional DPD architecture.

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In the case of recent wireless communication systems, the need to achieve high performance means that impairments in analog circuits cannot be disregarded. In view of the need for highly accurate compensation, many error compensation techniques using digital signal processing have been studied recently. Digital predistorter (DPD), which can eliminate odd-order distortion caused by PA nonlinearity, is one of the compensation techniques. However, in the case that direct conversion architecture is used in both a forward path and a loop-back path, errors in a quadrature modulator (QMOD) and a quadrature demodulator (QDEMOD) affect the performance of DPD. Previously, we proposed an error estimation technique for a QMOD and a QDEMOD that uses a variable phase shifter and evaluated its performance by an experiment using a narrow-band signal. In this paper, we evaluate the performance of the error estimation technique using the signal based on the 3GPP Long Term Evolution (LTE), whose bandwidth is wider than that in the previous experiment. The results show that image rejection ratio (IRR) after compensation is -59dBc and error vector magnitude (EVM) is -52dBc at the output of the QMOD.