A Low Complexity I/Q Imbalance Calibration Method for Quadrature Modulator
Abstract : This brief presents a low-complexity I/Q (in-phase and quadrature components) imbalance calibration method for the transmitter using quadrature modulation. Impairments in analog quadrature modulator have a deleterious effect on the signal fidelity. Among the critical impairments, I/Q imbalance (gain and phase mismatches) deteriorates the residual sideband performance of the analog quadrature modulator degrading the error vector magnitude. Based on the theoretical mismatch analysis of the quadrature modulator, we propose a low-complexity I/Q imbalance extraction algorithm. After the parameter extraction, the transmitter is calibrated by imposing the counter imbalanced mismatch of the transmitter through the digital baseband. In comparison with existing I/Q imbalance calibration methods, the novelty of the proposed method lies in that: 1) only three spectrum measurements of the device-under-test are needed for extraction and calibration of gain and phase mismatches; 2) due to the blind nature of the calibration algorithm, the proposed approach can be readily applicable to an existing I/Q transmitter; 3) no extra hardware that degrades the calibration accuracy is required; and 4) due to the noniterative nature, the proposed method is faster and computationally more efficient than previously published methods.
? Based on the existing frequency-domain blind calibration algorithm, we propose a calibration point classification rule based on the distribution characteristics of imbalance parameters, which successfully separates frequency-dependent phase imbalance and frequency-independent phase imbalance, eliminates the interference of useless information, and reduces the filter length.
? However the existence of IQ imbalance brings cross coupling between every tone and its mirrored equivalent.
? The tradeoff caused by analog domain such as area for precision, power and speed does not exist in digital domain with the same potency.
? The IC compensation technique is simple and easy to be implemented, but the output SIR degrades as the input SIR increases due to the signal leakage problem.
? The interference cancellation method (single-tap IQI compensation method) using the LMS algorithm was reviewed briefly mentioning that this technique suffers from the signal leakage problem.
? The single-tap IQI compensation method proposed is suitable for the low range of the input SIR, at moderate and high input SIR the output SIR degrades due to the signal leakage problem and the effect of the signal leakage as the values of the IQI increases.
• In order to accurately estimate the imbalance parameters with low cost, a classification rule is proposed according to the frequency-domain statistical characteristics of the received signal.
• This paper proposes a blind calibration model for I/Q imbalance based on the statistical characteristics of the received signal in the frequency-domain.
• We propose a frequency point classification rule by analyzing the distribution characteristics of frequency-independent imbalance and frequency-dependent imbalance, and classify the frequency points on the baseband into two types (noise points and signal points).
? One of the main performance limitations in the low-IF & Zero-IF down-converters is the components mismatch between the in-phase path and the quadrature-path named the IQ Imbalance (IQI) which limits the achievable image rejection ratio (IRR) of the down converters.
? However, imperfections from gain and phase mismatches as well as dc offset deteriorate the out-of-band emission performance.
? However, extraction of transmitter I/Q imbalance with the loop-back method is disturbed by the limited residual sideband (RSB) performance of the receiver.
? Extraction and calibration performances do not have to be sacrificed with our approach since no additional circuits are required in DUT nor external environment.
|