Experimental performance evaluation of error estimation and compensation technique for quadrature modulators and demodulators

In recent mobile communication systems, impairments in analog circuits cannot be disregarded owing to the need to achieve high performance. Errors in analog circuits consist of a distortion of a power amplifier(PA), gain and phase imbalance in 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 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 a loop-back path, errors in a QMOD and a QDEMOD affect the performance of DPD. Previously, we proposed an error estimation technique for a QMOD and a QDEMDO by using a variable phase shifter and evaluated the performance through computer simulation. In this paper, we evaluate the performance through an experiment by image rejection ratio(IRR) and error vector magnitude(EVM) in the case that the output of a QMOD is directly connected to the input of a QDEMOD. The results show that IRR improvement after compensation is 10dB at the output of the QMOD and EVM improvement after compensation is 10dB at the output of the QMOD and 20dB at the output of the QDEMOD.

I. INTRODUCTION

High data rate wireless communication systems are sensitive to signal degradation caused by analog circuits such as a PA, a QMOD and a QDEMOD. DPD is attracting much attention as a technique for compensating nonlinearity of PA [1]. PA nonlinearity causes the distortion, which degrades adjacent channel power ratio(ACPR) and EVM. Errors in a QMOD and a QDEMOD in the loop-back path also degrade the performance of DPD[2] because DPD needs the loopback path so that amplitude and phase relationship between input signals and output signals of a PA can be estimated. QMOD errors prevent DPD output signals from transmitting to PA correctly and QDEMOD errors cause an estimation error of PA characteristics.