Self-Calibration of Joint RF Impairments in a Loopback Wideband Transceiver

Abstract

This research studied transmitter/receiver (TX/RX) wideband (WB) RF impairments, namely frequency-dependent, frequency-independent I/Q imbalances and DC I/Q offsets. A real-based parallel structure is proposed to estimate and calibrate TX/RX WB RF impairment factors. RX impairment estimation and compensation are performed first using a frequency offset BPSK training signal. Then, the TX impairments are calibrated using a QPSK training signal. The proposed methods exhibited the following successes. First, a commercial off-the-shelf (COTS) AD9371 RF module with impairments was calibrated. After TX/RX calibration, the improvement of error vector measurement (EVM) of an OFDM 64QAM test signal was approximately 9.42 dB. Second, for the Taiwan Industrial Technology Research Institute's WB RF module with impairments, the EVM of an OFDM 16QAM test signal was calibrated and increased by about 20 dB. In summary, the proposed techniques can overcome WB RF impairments and enable high-quality WB communication. INDEX TERMS Calibration procedures, DC I/Q offsets, frequency-dependent I/Q imbalance, frequency-independent I/Q imbalance, RF impairment. I. INTRODUCTION Direct up/down RF chipsets are commonly used in wideband (WB) communication systems such as the 5G mobile communication system and digital video broadcasting systems. These chipsets have advantages such as low cost, simple design, and small area. However, for direct up/down conversion, multiple RF chips and oscillation circuits induce the different impairments including I/Q imbalance, DC offset, transmitter and receiver nonlinearities, phase noise, and coupling effects. In these impairments, many studies have extensively discussed I/Q imbalance. WB communication systems incur two I/Q imbalance effects: frequency-independent and frequency-dependent I/Q imbalances. Specifically, RF local oscillators generate I and Q carriers with amplitude and phase imbalance effects, resulting in so-called frequency-independent I/Q imbalance. In WB systems, the I and Q channels have the different impulse response effects and exhibit frequency-selective distortion, which is called frequency-dependent I/Q imbalance. The associate editor coordinating the review of this manuscript and approving it for publication was Nagendra Prasad Pathak.

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

“…As in [11] and [13], the frequency-domain image rejection ratio (IRR) is defined as the ratio of the spectral energy of the useful signal to the spectral energy of the image signal, and is used to describe the non-ideality of the signal. It can be written as…”

“…Assuming f s is the sampling rate of the baseband signal, the number of calibration points is N -2, and they are evenly distributed over the entire frequency band. The frequency calculation equation for each calibration point is (11). When the signal frequency is 0 or f s /2, it is difficult to separate the overlapping image from the signal; therefore, we do not calculate the IQI parameters at k = 0 and k = N /2, and use the estimated parameters of adjacent calibration points instead.…”

“…References [11], [12], [13], [14], [15], and [16] conducted relevant research on IQI calibration based on the training sequence method. A loopback path is required to send a designed training sequence before the useful signal, and then estimate the IQI parameters of the receiver.…”

“…The algorithm proposed in this paper belongs to the training sequence method, so the relevant research on the training sequence method is introduced in detail. References [11], [12], [13], [14], [15], [16], [17], and [18] compensated the IQI with a CVC filter. In [11], Deng et al studied transmitter and receiver wideband RF impairments based on the least squares (LS) algorithm.…”

“…References [11], [12], [13], [14], [15], [16], [17], and [18] compensated the IQI with a CVC filter. In [11], Deng et al studied transmitter and receiver wideband RF impairments based on the least squares (LS) algorithm. The CVC filter is equalized by four paralleled real-based filters.…”