Modeling of Power Amplifier Distortion in MIMO Transmitters
Examensarbete för masterexamen
Wireless, photonics and space engineering (MPWPS), MSc
Facing an always growing need for higher data rate, telecommunications operators cannot rely on a bandwidth increase any more as the frequency spectrum in the UHF band is nowadays crowded. Therefore new technologies need to be developed to boost the data rate. Among those, multiple inputs multiple outputs systems (MIMO) provide both a significant rise in bit rate and a better reliability for the link. Nevertheless it is based on many antennas transmitting at the same frequency. The effect of crosstalk between antennas is prevented by ferrite isolators which introduce losses and do not fit in typical transmitter design (mostly MMIC). This thesis focuses on modeling and analysing the signal distortion due to crosstalk at the output of the power amplifier and of the MIMO transmitter in absence of isolators. First, a static behavioural model and a dynamic model -both with dual inputs- have been developed to model the PA under conditions of crosstalk, and have been extracted for a 6W PA from CREE thanks to an active load pull measurement system. The latter model guarantees a NMSE of -32.6 dB and a ACEPR of -43dB for a WCDMA signal at 2.14 GHz. Secondly, antenna arrays have been designed based on 2 microstrip patch antennas, providing different coupling between the antennas: high (-14dB), medium (-20.9dB) and low S12 (-28.4dB); the goal being to use the arrays for simulations and measurements. Third, simulations of a complete MIMO transmitter were performed for a WCDMA signal at 2.12GHz and using the S-parameters of the aforementioned antenna arrays. Separating the distortion due to crosstalk from the classic non linearities of the PA, the adjacent channel leakage ratio (ACLR) is -47.3dB in case of high coupling (S12 = -14dB) and -54.3dB in case of medium coupling (S12 = -20.9dB). These values can be seen as the best achievable ACLR for a given coupling. Finally, measurements have been performed with the same antenna arrays using a vector switched generalised memory polynomial as digital pre-distorter. The results confirm the simulations observations: the measured ACLR for the strongest coupling is -46.4dB for the WCDMA signal at 2.12GHz. Therefore the ACLR tends to increase linearly in dB with the coupling and can thus reach the -45dB limit set by 3GPP standards for a coupling around -12.4dB. The influence of crosstalk on the PA distortion is thus significant and hardly negligible in the design of MIMO transmitters for wireless communication applications.
Informations- och kommunikationsteknik , Kommunikationssystem , Telekommunikation , Elektronik , Information & Communication Technology , Communication Systems , Telecommunications , Electronics