IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—II: EXPRESS BRIEFS, VOL. 60, NO. 2, FEBRUARY 2013 61 A Novel Adaptive Calibration Scheme for Frequency-Selective I/Q Imbalance in Broadband Direct-Conversion Transmitters Jian Luo, Andreas Kortke, Wilhelm Keusgen, and Mikko Valkama Abstract—This brief proposes a novel adaptive calibration scheme for frequency-selective I/Q imbalance in broadband direct-conversion transmitters (Tx) with multiband multi- standard capability. The calibration consists of pilot-based ini- tial calibration, followed by pilot-free parameter tracking against time-variant characteristics. Compared to the state-of-the-art (SoA) schemes, the proposed scheme has much lower complexity. Furthermore, the pilot-free tracking can provide good perfor- mance independently from the communication signal character- istic and avoids the ill-conditioned matrix problem in the SoA schemes. Its performance is verified by both computer simulations and hardware-in-the-loop experiments. Index Terms—Digital precompensation, direct-conversion ar- chitecture, dirty RF, frequency-selective I/Q imbalance, multi- band (MB), multistandard, parameter tracking, time variant. I. I NTRODUCTION A T PRESENT, the application of direct (up/down) con- version architecture (DCA) is a common trend in wire- less transceiver development. Aside from low cost, low power, and highly integrated implementation, DCA also allows flexible RF signal generation and demodulation in different frequency bands [1], favoring the implementation of software- defined-type flexible radio. Combined with broadband ana- log front ends, DCA even allows simultaneous multiband (MB) transmission/reception [2] and facilitates MB and multi- standard transceivers. However, the performance of DCA can be severely limited by RF impairments like I/Q imbalance, which causes mirror-frequency interference (MFI) [2] and re- stricts the achievable spectral efficiency. Furthermore, I/Q imbalance degrades the baseband predistortion for the power amplifier [1]. Generally, I/Q imbalance exists in both mod- ulator (MOD)/demodulator and the baseband low-pass filters (LPFs). While the former is frequency independent, the latter is Manuscript received July 24, 2012; revised October 2, 2012; accepted December 9, 2012. Date of current version March 13, 2013. This work was sup- ported in part by the German Federal Ministry of Economics and Technology (BMWi) under Grant 01MT07007 and in part by the Academy of Finland under Grant 251138 and the Austrian Competence Center in Mechatronics (ACCM). This brief was recommended by Associate Editor W.-P. Zhu. J. Luo is with European Research Center, Huawei Technologies Duesseldorf GmbH, 80992 Munich, Germany (e-mail: jianluo@huawei.com). A. Kortke is with Technische Universität Berlin, 10623 Berlin, Germany (e-mail: andreas.kortke@mk.tu-berlin.de). W. Keusgen is with Fraunhofer Heinrich Hertz Institute, 10587 Berlin, Germany (e-mail: wilhelm.keusgen@hhi.fraunhofer.de). M. Valkama is with Tampere University of Technology, 33720 Tampere, Finland (e-mail: mikko.e.valkama@tut.fi). Color versions of one or more of the figures in this brief are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TCSII.2012.2235735 frequency selective [1]–[3]. For simplicity, we summarize the overall effect as “frequency-selective I/Q imbalance.” This brief investigates the calibration of frequency-selective I/Q imbalance in broadband DCA transmitters, where MOD dc offset is also taken into account. The calibration consists of I/Q imbalance and dc offset parameter estimation and digital precompensation of the Tx signals [3]. For parameter estimation, an I/Q-imbalance-free feedback path from the MOD output to the baseband is used [1], [2], [4]. Generally, the calibration is carried out in two phases: initial calibration, followed by parameter tracking. The initial calibration is carried out at device start-up or after operation mode switching, where dedicated pilot signals can be transmitted for parameter estima- tion. After the initial calibration, the parameters can vary slowly in time, mainly due to temperature change. Such variation has to be tracked periodically, as described at the preliminary level in [3] to keep the precompensation effective. Unlike in the initial calibration, the device is now in normal operation and transmits communication signals. When considering MB and multistandard applications, no dedicated pilot signals can be used for the parameter estimation during tracking. Instead, the transmitted communication signal has to be used, which is known at the transmitter but can have unfavorable characteristic for parameter estimation. In other words, the estimation method should be pilot-free and standard independent, giving challenge to the calibration. There are a number of works on I/Q imbalance calibration in DCA transmitter, e.g., [1]–[10] and the references therein. The works [4], [5], and [9] consider only frequency-independent I/Q imbalance and are not sufficient for broadband applica- tions. In contrast, [6] considers only the LPF I/Q imbalance and requires redundant hardware for calibration. Furthermore, the schemes in [7], [9], and [10] rely on dedicated pilot signals and are not standard independent. The most advanced state-of- the-art (SoA) schemes are represented by [1]–[3] and [8], which can cope with frequency-selective I/Q imbalance and work without dedicated pilot signals. The schemes in [1]–[3] are based on least squares estimation (LSE) or widely linear LSE. The common drawback is that, if the transmit signal has spectral gaps or even transition bands, the LSE can suffer from an ill- conditioned matrix problem and thus has a large estimation error [8]. The reason is that the corresponding spectral compo- nents of the desired parameters cannot be correctly estimated. One conventional solution is to add low-power superimposed pilot signals (SIPSs) to refill the spectral gaps. This solution is unfavorable due to interference (to both own Tx signal and the neighboring channels) or ineffectiveness. An alternative solu- tion is the iterative frequency-domain estimation (IFDE) in [8], which has the disadvantage of high computational complexity and SIPS usage when encountering asymmetric spectral gaps. 1549-7747/$31.00 © 2013 IEEE