One-Tap Wideband I/Q Compensation for Zero-IF Filters

Abstract

Abstract-The I/Q imbalance is one of the performance bot tlenecks in transceivers with stringent requirements imposed by applications such as 802.11a. The mismatch between the frequency responses of two analog low-pass filters, used, e.g., for channel selection in zero-IF receivers, makes this I/Q imbalance frequency dependent. Usually, frequency-dependent I/Q mismatch is esti mated and corrected by adaptive techniques, which are complex to implement and may converge slowly due to noise. In this work, a simple, delay-based I/Q compensation scheme is proposed based on an extensive statistical analysis. Its digital implementation uses only two coefficients, which are tuned by a one-step two-tone error estimation. Simulations show that this hardware-efficient scheme significantly reduces the I/Q imbalance. Fig. 1. Usually, the RX chain in cludes an antenna (A), a low-noise amplifier (LNA), a pair of mixers driven by quadrature local-oscillator (LO) signals I ( ) and Q ( ), a pair of real low-pass filters (LPFs), a pair of analog-to-digital converters (ADCs) sampled at , and a digital signal processor (DSP) [3]. An orthogonal frequency-division multiplexing (OFDM) DSP, used in 802.11a [4], inherently contains a fast-Fourier transform (FFT) block. A zero-IF transmitter (TX) chain starts with a DSP and is followed by a pair of digital-to-analog converters (DAC), a pair of LPFs, a mixer/LO block, a power amplifier (PA), and an antenna [3]. Index Terms-ComplexThe main contributors of the RX or TX chain's I/Q imbal ance 1 are the gain error ( ) of the mixers, the phase error ( ) in the LO signals, the gain ( ) and phase ( ) mismatch between the LPF's transfer functions, and, finally, the gain error ( ) between the data converters (ADCs in RX, DACs in TX). The I/Q imbalance contribution of gain and phase errors can be modeled as a two-input two-output linear network with some inter-coupling coefficients [5]. These simple models can be individually applied to each block of mixers/LO, LPFs,

I/Q Compensation of Broadband Direct-Conversion Transmitters

“…An ideal complex mixer takes the filtered I/Q baseband data, two quadrature shifted version of the LO reference signal at the generic carrier frequency f c , and generates the output signalx(t) = y I (t) cos(2π f c t) − y Q (t) sin(2π f c t). The description adopted by almost all authors, such as [4], [6], [7], [19], is to describe the impaired signal as x(t) = y I (t) + j g M e jφM y Q (t) where the constants g M and φ M represent the gain and phase error terms of the I/Q mixer, respectively. In this case, the I/Q imbalance effects are frequency-independent.…”

“…The I/Q imbalance problem has been extensively studied in the literature: some algorithms compensate for narrowband signals [3]- [6] while others implement wideband signal compensation techniques [7]- [17]. As far as transmitter devices are concerned, some I/Q compensation algorithms rely on blind methods [18], decorrelation and adaptive filtering techniques [8], [14], [19], and a priori knowledge about the signal waveform statistical behavior such as WSS Gaussian assumption [4], [20], [21], signal circularity [9], [11], [22] or signal properness [10] features.…”

“…As far as transmitter devices are concerned, some I/Q compensation algorithms rely on blind methods [18], decorrelation and adaptive filtering techniques [8], [14], [19], and a priori knowledge about the signal waveform statistical behavior such as WSS Gaussian assumption [4], [20], [21], signal circularity [9], [11], [22] or signal properness [10] features. Other TX compensation techniques depend on availability of known simple baseband signals (e.g., sine/cosine test signals) such as those shown in [3]- [5], [7], [12], multiple test signals such as OFDM pilot tones [13] or demodulated signal samples [4], [5], [15], [16]. Finally, some other methods [5], [6], [11], [15], [23], [24] compensate I/Q and non-linearity distortion effects by merging both compensation architectures in a single joint processing scheme.…”

“…Estimation and compensation architectures strictly depend 1536-1276/14$31.00 c 2014 IEEE on the RF synchronization method adopted for the auxiliary receiver: for instance, power measurements only are performed in [3], [7] according to specific baseband test signals while adaptive filtering requires the use of the demodulated samples [13], [15]. These samples may be recovered by a synchronous I/Q imbalance-free receiving chain as indicated in [13] while RF signal measurements may be obtained with an auxiliary synchronous [4], [10], [15] or asynchronous [16], [20] I/Q imbalance-free auxiliary receiver.…”

New estimation and compensation method for quadrature modulator and demodulator errors in digital predistortion system

“…Models of the QDM and QDM compensator are shown in Figure 1b, and similarly the QDM also uses the asymmetric form model [16]. If we define a 2 and b 2 as the gain of the in-phase and quadrature branches, respectively, h 2 and f d as the phase error and DC offset vector, the quadrature demodulated signal can be represented as…”