Nonlinear Behavioral Modeling of Quadrature Modulators and Analysis of Impacts on Wireless Communication Systems
Direct-conversion quadrature modulators have been widely used in modern communication systems because of their capability to conserve bandwidth with low cost, low complexity, and small form factor. In this dissertation, three different quadrature modulator applications: 1) generic quadrature modulation system; 2) zero-IF OFDM system; and 3) low-IF OFDM system are reviewed. The different usages and implementation costs of the modulators are compared. Impacts of carrier leakage, gain/phase imbalances, and nonlinear distortion on system performance degradation in terms of spectrum regrowth and waveform quality are analyzed for three applications of the quadrature modulators. The adverse impacts of these physical imperfections demand system-level techniques to accurately characterize them for system design and verification.
A new nonlinear behavioral model is developed for characterization of both correlated/uncorrelated nonlinear distortion and the linear static errors including DC offset and gain/phase imbalances for direct-conversion quadrature modulators. This enables fast and accurate prediction of spectrum regrowth and waveform quality degradation due to these physical imperfections. A low-pass equivalent model structure is developed based on the assumptions that I and Q channels are isolated and the dominant nonlinearities of quadrature modulators are the baseband transconductors. The responses of I and Q inputs are modeled by two independent complex power series. These capture both nonlinear distortion and linear static errors. Amplitude-to-amplitude (AM-AM) and amplitude-to-phase (AM-PM) measurements and 4-point vector network analyzer (VNA) measurements are used to extract the model parameters for characterization of the nonlinear distortion and the linear static errors, respectively. An existing orthogonalization technique for power amplifiers is implemented in the quadrature modulator model to decompose the correlated and uncorrelated nonlinear distortion. The modeling technique is applied to both passive and active RF quadrature modulators and the models are verified by adjacent channel power ratio (ACPR) and error vector magnitude (EVM) measurements of systems excited by digitally modulated signals.
One fundamental quadrature modulator model assumption, which is validated by the modeling results, is that the baseband transconductors dominate the nonlinear characteristics of integrated quadrature modulators. This motivates design and modeling work for characterization of the nonlinear distortion of a special category of transconductors: bipolar multi-tanh transconductors. Three baseband and bandpass bipolar transconductors: the bipolar differential pair; the multi-tanh doublet; and the multi-tanh triplet; are designed and their nonlinear characteristics are modeled using the same modeling technique as used with the quadrature modulator modeling work: the AM-AM and AM-PM based complex power series model. The bandwidth limitations of the AM-AM and AM-PM based model for characterizing baseband transconductors are studied and measured and an augmented model structure is proposed to overcome the limitations for broadband quadrature modulator applications.
Advisor:Kevin G. Gard; Mark Johnson; Michael B. Steer; Huaiyu Dai
School:North Carolina State University
School Location:USA - North Carolina
Source Type:Master's Thesis
Date of Publication:08/29/2007