Science Journal of Circuits, Systems and Signal Processing
Volume 3, Issue 3, June 2014, Pages: 14-25
Received: Oct. 12, 2014;
Accepted: Oct. 22, 2014;
Published: Oct. 30, 2014
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Walid Ahmed, Verizon Wireless, New Jersey, USA
Ajit Reddy, Alcatel-Lucent, New Jersey, USA
In this paper, a novel real-time closed-loop device linearization technique has presented. In this paper the focus application is on AM/AM and AM/PM linearization of power amplifiers (PA) and/or radio transmitters. In such an application, the novel approach performs on-the-fly measurement-and-prediction of the nonlinear characteristics of the PA, stores such non-linear characteristics and calculates their inverse functions in order to pre-distort the base-band amplitude and phase signals modulating the PA such that the combination, or the resultant, of the pre-distorter and the PA leads to a linear behavior at the output of the PA. The predictive nature of the presented approach overcomes the inevitable delay between the time a measurement is collected through the feedback loop and the time it has taken place at the output of the forward loop, which is a must-have delay encountered in any natural causal system. Such a delay results in imperfect on-the-fly pre-distortion of the output signal due to the mismatch between the applied pre-distortion, which has been based on a past measurement, and the actual effect of the non-linearity at the time the signal is produced . In addition, this novel approach promises the advantage of operating a highly non-linear (compressed) PA - hence, highly efficient - with minimal factory pre-calibration. We present a model of our predictor based approach and evaluate its performance for a GSM/EDGE/UMTS a cellular transmitter scenario, where the performance requirements on the transmitted signal are stringent and distortion due to non-linearity must be minimized. The key performance metrics we evaluate have mostly been based on the GSM/EDGE/UMTS requirements, such as the Error Vector Magnitude (EVM), Switching Transients (ST), and Adjacent Channel Power Ratio (ACPR), transmit time mask, modulation spectrum and power-added efficiency (PAE). Although the focus of the numerical results in this paper is on the GSM/EDGE/UMTS application, the novel approach discussed is applicable to any TDMA or TDD system where switching transients and spectral performance is tightly controlled.
Novel Real-Time Closed-Loop Device Linearization via Predictive Pre-Distortion, Science Journal of Circuits, Systems and Signal Processing.
Vol. 3, No. 3,
2014, pp. 14-25.
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