Analysis and Compensation of Nonlinear Power Amplifier Effects in Multi-antenna Ofdm Systems

OF DOCTORAL DISSERTATION HELSINKI UNIVERSITY OF TECHNOLOGY P.O. BOX 1000, FI-02015 TKK http://www.tkk.fi Author Fernando Hugo Gregorio Name of the dissertation Analysis and compensation of nonlinear power amplifiers effects in multi-antenna OFDM systems Manuscript submitted 08.06.2007 Manuscript revised 27.09.2007 Date of the defence 19.11.2007 Monograph Article dissertation (summary + original articles) Department Electrical and Communications Engineering Laboratory Signal Processing Laboratory Field of research Digital Signal processing in wireless communication systems Opponent(s) Prof. Markus Rupp (Vienna University of Technology) and Docent Mikko Valkama (Tampere University of Technology) Supervisor Risto Wichman Instructor Stefan Werner Abstract 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.