Analysis and compensation of nonlinear power amplifier effects in multi-antenna OFDM systems

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Doctoral thesis (article-based)
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Department of Electrical and Communications Engineering
Sähkö- ja tietoliikennetekniikan osasto
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117, [46]
Report / Helsinki University of Technology, Signal Processing Laboratory, 61
The high peak to average power ratio (PAPR) levels associated with multicarrier systems require the use of linear power amplifiers (PA) with large dynamic range. Unfortunately, linearity and power efficiency are two conflicting specifications for practical PAs. However, systems with high power efficiency are required, for example, in case of battery operated mobile terminals. Thus, the PA needs to operate in a high power efficiency region which compromises the linearity constraints. The evaluation of the effect of nonlinear amplification on the system performance is an important issue that must be considered in a realistic system design. This thesis first addresses the performance analysis of multiuser OFDM systems (space division multiple access (SDMA)-OFDM) employing nonlinear PAs. Bit error probability (BEP) and capacity expressions are derived for an OFDM system with diversity, assuming a memoryless PA. Moreover, BEP expressions are derived for an orthogonal space-time coded (OSTBC)-OFDM system that include the effect of a broadband nonlinear PA and imperfect memory compensation. Thereafter, techniques to reduce the effects of power amplifier distortion are proposed. We consider techniques that can be implemented either in the receiver, the transmitter, or as a combination of both, all with the same goal of relieving the linearity constaints and improving the power efficiency. A low PAPR OFDM implementation is addressed where subcarriers are allocated to different users in such a way that the generated intermodulation distortion is kept at minimum. Then, we propose the combination of multiuser detection and nonlinear distortion mitigation techniques. Initially, knowledge of the PA model (modelled as memoryless) is required for the implementation. The extension to broadband PAs and to PA model estimation in the receiver are also made. Efficient channel estimation strategies are developed that work in the presence of nonlinear distortion. Finally, a split predistorter structure to remove nonlinear distortion caused by a broadband PA is proposed. The idea is to reduce the transmitter complexity by distributing the predistorter tasks such that the transmitter compensates for the static nonlinearity, and the receiver equalizes the PA memory.

Lähetystehon suuri vaihtelu monikantoaaltojärjestelmissä asettaa tiukkoja vaatimuksia tehovahvistimille. Vahvistimien lineaarisen alueen pitää olla suuri, mikä johtaa huonoon hyötysuhteeseen ja suureen tehonkulutukseen vahvistimen operoidessa lineaarisella alueella. Jos vahvistimen hyötysuhde halutaan pitää korkeana, kuten esimerkiksi akuilla toimivissa kannettavissa päätelaitteissa, vahvistettu signaali väistämättä vääristyy. Näiden epälineaaristen vääristymien kompensointi on tärkeä osa langattomien tietoliikennejärjestelmien suunnittelua. Tässä työssä tarkastellaan epälineaaristen tehovahvistimien vaikutusta monen käyttäjän OFDM-järjestelmiin ja näiden vaikutusten kompensointia lähettimessä ja/tai vastaanottimessa, kun päätelaitteissa voi olla useampia antenneja. Lähettimessä vääristymien epäedullista vaikutusta vähennetään lomittelemalla lähetettyjä kantoaaltoja eri lähetysantennien kesken. Vastaanottimessa monen käyttäjän ilmaisu yhdistetään vääristymien kompensointiin ja kanavaestimointiin. Lopuksi työssä kehitetään algoritmi, jossa vääristymien kompensointi jaetaan lähettimen ja vastaanottimen kesken. Työssä myös johdetaan vastaanottimen virhetodennäköisyyslausekkeet ja kanavan kapasiteettilausekkeet erilaisille systeemimalleille, kun lähetetty signaali on vääristynyt epälineaarisesti.
nonlinear power amplifiers, OFDM, PAPR, nonlinear distortion, tietoliikennetekniikka, tehovahvistin, OFDM, moniantennitekniikka
Other note
  • F. Gregorio, T.I. Laakso and R. Wichman. Comparison of clustered and antenna interleaving OFDM in multiple-antenna systems with amplifier nonlinearities, in Proceedings of the 2004 IEEE International Conference on Signal Processing and Communications, SPCOM 2004, pp. 111-115, December 2004, Bangalore, India. [article1.pdf] © 2004 IEEE. By permission.
  • F. Gregorio and T.I. Laakso. A novel carrier allocation method for multiuser OFDM system with amplifier nonlinearities, in Proceedings of the 13th European Signal Processing Conference, EUSIPCO 2005, September 2005, Antalya, Turkey. [article2.pdf] © 2005 by authors.
  • F. Gregorio, T.I. Laakso and J. Cousseau. Receiver cancellation of nonlinear power amplifier distortion in SDMA-OFDM systems, in Proceedings of the 2006 IEEE International Conference on Acoustics, Speech and Signal Processing, ICASSP 2006, pp. IV 325-328, May 2006, Toulouse, France. [article3.pdf] © 2006 IEEE. By permission.
  • F. Gregorio, S. Werner, J. Cousseau and T.I. Laakso. Iterative channel estimation for multiuser OFDM systems in the presence of power amplifier nonlinearities, in Proceedings of the 17th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, PIMRC 2006, September 2006, Helsinki, Finland. [article4.pdf] © 2006 IEEE. By permission.
  • F. Gregorio, S. Werner, T.I. Laakso and J. Cousseau. Receiver cancellation technique for nonlinear power amplifier distortion in SDMA-OFDM systems, IEEE Transactions on Vehicular Technology, vol. 56, no. 5, pp. 2499-2516, September 2007. [article5.pdf] © 2007 IEEE. By permission.
  • F. Gregorio, S. Werner, R. Wichman and J. Cousseau. Split predistortion approach for reduced complexity terminal in OFDM systems, in Proceedings of the 65th IEEE Vehicular Technology Conference, VTC 2007, pp. 2697-2701, April 2007, Dublin, Ireland. [article6.pdf] © 2007 IEEE. By permission.
  • F. Gregorio, S. Werner, J. Hämäläinen, R. Wichman and J. Cousseau. BEP analysis of OSTBC-OFDM systems with broadband PA and imperfect memory compensation, IEEE Communications Letters, 2007, to appear. [article7.pdf] © 2007 IEEE. By permission.
  • F. Gregorio, S. Werner, J. Cousseau and R. Wichman. Broadband power amplifier distortion cancellation with model estimation in the receiver, in Proceedings of the 8th IEEE Workshop on Signal Processing Advances in Wireless Communications, SPAWC 2007, June 2007, Helsinki, Finland. [article8.pdf] © 2007 IEEE. By permission.
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