A Frequency-Domain I/Q Imbalance Calibration Algorithm for Wideband Direct Conversion Receivers Using Low-Cost Compensator

Abstract

The amplitude and phase imbalances of the in-phase (I) and quadrature (Q) branches inherent in the direct conversion receiver structure cause the generation of image frequency interference signals.In this paper, a frequency-domain I/Q imbalance calibration algorithm is proposed for wideband direct conversion receivers. The I/Q imbalance model is rebuilt by applying the infinitesimal method and FFT algorithm. The mathematical expressions for the exact computation of the I/Q imbalance parameters are derived based on the frequency-domain statistical properties of the baseband signal. The two phase parameters, frequency-dependent I/Q imbalance (FD-IQI) and frequency-independent I/Q imbalance (FI-IQI), are separated according to the parity properties of the imbalance parameter. To avoid the interference from the transmitter, the receiver impairment estimation is performed using a frequency offset (FO) DC training signal, and a low-cost real-valued compensation (RVC) filter is introduced to correct the impairments of the received signal. The performances of the proposed calibration model are evaluated through simulations and experiments. The simulation results show that the image rejection ratio (IRR) is improved to 80-120 dBc and can also exceed 40 dBc at high noise levels. The experimental results based on the CX9261A evaluation board show that the average IRR of the multi-tone signal is increased by 24.99 dB, and the IRR of the wideband signal is increased by 19.08 dB.INDEX TERMS Direct conversion receiver, frequency-dependent I/Q imbalance, frequency-independent I/Q imbalance, frequency-domain statistical properties, infinitesimal method, real-valued compensation filter.

A Hybrid Scheme for TX I/Q Imbalance Self-Calibration in a Direct-Conversion Transceiver

“…Because the blind method uses only the inherent characteristics of the received signals to estimate the I/Q imbalance, it is widely used for tracking calibration, and the system does not need to stop its normal operation to enter the calibration process. However, in order to extract the inherent characteristics of the signals, the blind method requires high-complexity calculation blocks, thus usually requiring external digital signal processing (DSP) for the calibration of the whole system [14][15][16]. On the other hand, training sequence methods are easier to implement, and are widely used for power-on calibration (self-calibration) since they can implement a high-precision I/Q imbalance estimation by sending a designed training sequence [17][18][19].…”

“…To further reduce the frequency-dependent I/Q imbalance in the proposed system, an FIR is recommended to implement after the I/Q compensation block [16,20]. Table 1 lists many published I/Q imbalance self-calibration works.…”