Zhiwen Zhu

Blind Self-Calibration Technique for I/Q Imbalances and DC-Offsets

The gain/phase imbalances and DC-offsets in the in-phase and quadrature branches in direct conversion transmitters (DCTs) are main sources of errors, especially at microwave frequencies and above. These impairments distort the spectrum of the transmitted signal and degrade the communications performance. In this paper, a blind technique based on higher order statistics is proposed to estimate the calibration parameters in order to compensate for these impairments. By minimizing a cost function composed of second and fourth moments of the modulator output signal, the DC-offsets, gain imbalance, and phase imbalance can be successively calibrated. The convergence of the proposed technique is also analyzed. This technique is blind in the sense that it does not require any prior knowledge about the DCT input signal, and is applicable to many communications systems including QAM, OFDM, and SC-FDMA. Computer simulations and experiments at 20 GHz demonstrate that the proposed technique can significantly improve the DCT performance in terms of local oscillator leakage and image suppressions.

Ka-Band Multi-port Power Amplifier Calibration Experiment and Results

A multi-port power amplifier (MPA) is a multi-input multi-output system that is capable of amplifying multiple input signals simultaneously by a set of shared power amplifiers without mutual interference. In a practical MPA, the component imperfections reduce the port isolation, which introduces leakage or cross-port interference. A patent-pending type-based calibration technique is studied in this paper. It exploits the uniqueness of the statistics for a given communication signal to estimate a calibration matrix that minimizes the effects of the MPA impairments and suppresses the cross-port interference. Experimentation with a 4-port 20 GHz prototype MPA subsystem is conducted to evaluate the type-based calibration technique. Results show that it can accurately estimate the calibration matrix to improve the MPA performance significantly.

Analytical Performance of M-PSK Communications Systems in the Presence of Gain/Phase Imbalances and DC-Offsets

This letter addresses the performance of M-PSK communications systems in the presence of gain/phase imbalances and DC-offsets in in-phase and quadrature channels. A model is established to represent the gain/phase imbalances and DC-offsets in both the transmitter and the receiver, and two closed-form formulas are derived to calculate the symbol error rate (SER) of the M-PSK communications systems in the presence of these impairments. Computer simulations are conducted to validate the derivation, and the results show that the SER calculated from the formulas closely matches the simulation results. The formulas provide an accurate guideline for communications system architects/engineers to evaluate the impacts of various imbalances and offsets on the system performance and to determine the required component specifications.

Theoretical and Experimental Studies of a Probabilistic-Based Memoryless PA Linearization Technique

This paper studies the performance of a memoryless power amplifier (PA) linearization technique based on a probabilistic approach. This technique employs a nonparametric method to derive a predistorter function, which does not need any parametric modeling and explicit parameter estimation. It only needs to calculate a probabilistic cumulative distribution function (CDF) and a quantile function (an inverse function of the CDF). Histogram and order statistic methods are proposed to perform the calculation. A rigorous analytic formula is derived for the inter-modulation product power (IMPP) of the PA output signal when a finite number of samples as well as a finite number of bins are used to calculate the CDF and the quantile function. The analytic results show that, with the probabilistic-based technique, the IMPP approaches zero as the number of samples approaches infinity and the bin width approaches zero.Computer simulations are utilized to verify the theoretical analysis and to compare the performance of the probabilistic-based linearization technique with those of other memoryless PA linearization techniques, while a prototype experiment is carried out to demonstrate its performance in a practical application. Results show that the technique can accurately determine the predistortion function that effectively compensates for the nonlinearity in the PA, and that it achieves a much better linearization performance compared to other existing methods, especially in the presence of a loop delay in the feedback circuit.

A Blind AM/PM Estimation Method for Power Amplifier Linearization

A higher order statistics (HOS)-based method is proposed in this letter to estimate the nonlinear amplitude-to-phase (AM/PM) conversion function of power amplifiers (PAs) in communications systems. By utilizing the symmetry property of the modulated signal, the proposed method estimates the AM/PM conversion function using only the PA output signal, from which the phase predistorter (PD) function can be derived. Simulation results show that the HOS-based method can accurately determine the phase PD function required to compensate for the AM/PM distortion in the PA, yielding a linearization performance close to that of non-blind methods in terms of adjacent channel power ratio and error vector magnitude.

A 30GHz 155Mbit/s self-calibrating direct transmitter

This paper describes a 155Mbps self-calibrating direct transmitter for Ka-band satellite communications. It uses an 8-PSK modulation to fit the data rate of 155Mbps into a 72MHz channel bandwidth, and employs an adaptive pre-compensation technique to maximize the LO and image suppressions. An FPGA implements the 8-PSK mapper, the pulse-shaping filters and the pre-compensation, while a DSP adaptively estimates the pre-compensation coefficients. A subharmonic vector modulator brings analog inphase and quadrature signals directly from baseband to the 30GHz carrier. Experimental results show that the adaptive pre-compensation technique can effectively suppress the LO leakage and image

Enabling Satellite Communication Technologies

Over the last few years, various enabling technologies and techniques for future Ka-band satellite communication systems were developed. In this paper, we present objectives and results of several projects on direct conversion transceivers and power amplifier linearization, targeting the cost reduction of terminals and satellite subsystems with the use of advanced, enabling techniques. Performances of these techniques are presented based on either hardware prototype tests or computer simulation. It is shown that low complexity digital signal processing can be used to compensate traditional limitations of direct transceivers as well as to linearize power amplifiers. Although the paper concentrates on terminal technologies, the application and benefit of these techniques to satellite subsystems are readily seen.