Efficient DPD Coefficient Extraction For Compensating Antenna Crosstalk And Mismatch Effects In Advanced Antenna System

University essay from Lunds universitet/Institutionen för elektro- och informationsteknik

Abstract: The demand for high data rates in a wireless communication system has experienced a remarkable growth over the last years. This demand is continually increasing with the growing number of wirelessly connected devices. Thus, there is a continuing hunt for increasing the data transmission rate in wireless communication systems. To increase the data rate, the modern wireless communication system cannot only rely on bandwidth, as usable frequency spectrum is getting crowded. However, advanced antenna system, i.e Multiple input multiple outputs (MIMO), Multi-user MIMO, massive MIMO, lead to significant advancement in the modern wireless system by increasing data rate, spectrum efficiency, channel capacity, etc. These advanced antenna systems rely on the multi-antenna transmitter, using radio frequency (RF) power amplifiers (PAs) to excite an antenna array. These RF PAs are one of most power consuming devices in a wireless transmitter. Thus, the Energy efficiency of PA is a major concern. Higher efficiency can be achieved by increasing the input power to the PA. However, signals, using modern modulation schemes, like Orthogonal Frequency Division Multiplexing (OFDM) and Wideband Code Division Multiple Access (W-CDMA), have non-constant envelope and high Peak to average power ratio (PAPR). These signals create the conflict between power efficiency and frequency spectrum efficiency (linearity). Linearization techniques such as Digital pre-distortion (DPD) is most widely used to linearize the PA so that it can be driven in high power mode for maximizing energy efficiency. However, the coupling between array elements modulates the load on the output of each PA, creating post-PA crosstalk. Thus, Power Amplifier behavior changes which affect the DPD linearization of the PAs since DPD technique is performed without considering these coupling effects. Thus, some degradation of the system performance in terms of linearity is expected. To address this problem, PA linearization techniques have been explored in this thesis. This thesis work is related to the field of research towards a study of post-PA crosstalk effects and exploring a solution to compensate for both nonlinearity and crosstalk at the output of the power amplifier. To emulate multi-antenna transmitter system, a uniform linear array with four transmit antennas and four PAs is built in this thesis. Digital beamforming technique is applied to the antenna array to mimic advanced antenna system. The coupling between array elements is extracted using S-parameters of the array elements. There is no significant impact found on PA output signal Adjacent channel power while post-PA crosstalk is present but without DPD. However, the impact of post-PA crosstalk found on DPD linearization performance. A simulation model is developed using static behavioral PA models in crosstalk environment and extracted S-parameter from the array antenna. The simulation using conventional DPD in crosstalk environment shows around 30dB degradation of linearization performance in the high coupling scenario. An algorithm called Crosstalk Digital Pre-distortion (CDPD) is developed and proposed in this thesis to compensate crosstalk effects on DPD. The proposed solution is tested in a simulation environment. The simulation results show that the proposed method improves the DPD linearization performance about 25dB under the very high coupling scenario. The new algorithm also improves the performance in extracting the model parameters for digital pre-distortion in a multi-antenna system.

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