Development of digital predistorters for broadband power amplifiers in OFDM systems using the simplicial canonical piecewise linear function

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School of Electrical Engineering | Doctoral thesis (article-based) | Defence date: 2014-04-25
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121 + app. 191
Aalto University publication series DOCTORAL DISSERTATIONS, 38/2014
Power amplifiers (PAs) are inherently nonlinear devices. Linearity of a PA can be achieved by backing off the PA to its linear region at the expense of power efficiency loss. For signals with high envelope fluctuation such OFDM system, large backoff is required, causing significant loss in power efficiency. Thus, backoff is not a favourable solution. Digital predistorters (PDs) are widely employed for linearizing PAs that are driven to the nonlinear regions. In broadband systems where PAs exhibit memory effects, the PDs are also required to compensate the memory effects. This thesis deals with the development of digital PDs for broadband PAs in OFDM systems using the Simplicial Canonical Piecewise Linear (SCPWL) function. The SCPWL function offers a few advantages over polynomial models. It imposes a saturation after the last breakpoint, making it suitable for modelling nonlinearities of PA and PD. The breakpoints of the function can be freely placed to allow optimum fitting of a given nonlinearity. It is suitable for modeling strong nonlinearities. Analysis of the SCPWL spectra property shows that the function models infinite order of intermodulation distortion, even with small number of breakpoints. The accuracy of the model can be improved by increasing the number of breakpoints. The original real-valued SCPWL function is extended to include memory structure and complex-valued coefficients, resulting in the proposed baseband SCPWL model with memory. The model is adopted in the development of the Hammerstein-SCPWL PD and memory-SCPWL PD. Vector projection methods are developed for static SCPWL PDs identification. Adaptive algorithms employing the indirect and direct learning architectures are developed for identifying the Hammerstein-SCPWL PD and memory-SCPWL PD. By exploiting the properties of the SCPWL function, the algorithms are simplified. A modified Wiener model estimator is employed to circumvent the non-convex cost function problem of block models. This further reduces the complexity of the Hammerstein PD algorithms. The thesis also analyses the effects of measurement noise on indirect learning SCPWL filter. Due to its linear basis function, the SCPWL filter coefficients do not suffer the coefficient bias effects which are observed in polynomial models. The performance of the proposed SCPWL PDs are compared with state-of-the-art polynomial-based PDs by simulations and measurements.
Wichman, Risto, Prof., Aalto University, Department of Signal Processing and Acoustics, Finland
Thesis advisor
Werner, Stefan, Dr., Aalto University, Department of Signal Processing and Acoustics, Finland
predistorter, power amplifier, nonlinear distortion, memory effects, adaptive linearization, piecewise linear function, OFDM
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  • [Publication 1]: Mei Yen Cheong, StefanWerner, Juan Cousseau and Timo I. Laakso, “Predistorter identification using the simplicial canonical piecewise linear function,” in Proceedings of the 12th International Conference on Telecommunications (ICT), Cape Town, South Africa, May 3-6, 2005.
  • [Publication 2]: Mei Yen Cheong, Ernst Aschbacher, Peter Brunmayr, Markus Rupp and Timo I. Laakso,“Comparison and experimental verification of two low-complexity digital predistortion methods,” in Proceedings of the 39th Asilomar Conference on Signals, Systems and Computers, Pacific Grove, CA., USA, pp. 432-436, 30 October-2 November, 2005.
  • [Publication 3]: Mei Yen Cheong, Stefan Werner, Timo Laakso, Juan Cousseau, and Jose L. Figueroa, “Predistorter design employing parallel piecewise linear structure and inverse coordinate mapping for broadband communications,” in Proceedings of the 14th European Signal Processing Conference (EUSIPCO), Florence, Italy, September 4-8, 2006.
  • [Publication 4]: Mei Yen Cheong, Helka-Liina Maattanen, Stefan Werner and Sven-Gustav Haggman, “A combined PAPR reduction and predistorter scheme for OFDM systems in nonlinear channels,” in Proceedings of the IEEE Radio and Wireless Symposium (RWS), Long Beach, CA., USA, pp. 309-312, January 9-11, 2007.
  • [Publication 5]: Ernst Aschbacher, Mei Yen Cheong, Peter Brunmayr, Markus Rupp and Timo I. Laakso, “Prototype implementation of two efficient lowcomplexity digital predistortion algorithms,” EURASIP Journal on Advanced Signal Processing, vol. 2008, pp. 15, 2008.
  • [Publication 6]: Mei Yen Cheong, Stefan Werner, Juan Cousseau, Risto Wichman, “Spectral characteristics of a piecewise linear function in modeling power amplifier type nonlinearities,” in Proceedings of the IEEE 21st International Symposium on Personal Indoor and Mobile Radio Communications (PIMRC), Istanbul, Turkey, pp. 639-644, September 26-30, 2010.
  • [Publication 7]: Mei Yen Cheong, Stefan Werner, Marcelo J. Bruno, Jose L. Figueroa, Juan Cousseau and Risto Wichman, “Adaptive piecewise linear predistorters for nonlinear power amplifiers with memory,” IEEE Transaction on Circuits and Systems–I: Regular Papers, vol.59, no.7, pp. 1519-1532, July 2012.