Characterizing nonlinearity in multiantenna multibeam transmitters
1University of Oulu, Faculty of Information Technology and Electrical Engineering, Communications Engineering
|Online Access:||PDF Full Text (PDF, 15 MB)|
|Persistent link:|| http://urn.fi/URN:NBN:fi:oulu-201712053277
|Publish Date:|| 2017-12-05
|Thesis type:||Master's thesis
In this thesis, effects of power amplifier (PA) distortion in multiantenna transmitter is studied. Input signal of each PA in the array is modelled by two or multiple tones to characterize the nonlinearity in terms of intermodulation distortion (IMD). In intermodulation, the phase of the nonlinearity depends on the phases of the corresponding input tones. Hence, in beamforming, progressive phase of the nonlinear components over the antenna elements creates a steered beam for the nonlinearity. Measurement setup is created to measure the phase and amplitude of the IMD components in the PA output. The theoretical polynomial relation of the IMD phase dependency on the input tones is validated by measurements. For flexible measurements, the setup is automatized by standard commands for programmable instruments.
Second part of the thesis studies the array IMD by simulations in MATLAB. The used PA model is a memoryless polynomial fitted against the measured amplitude-to-amplitude modulation and amplitude-to-phase modulation responses of a real amplifier. The effects of nonlinearity are studied by using two tones to present each independent data stream in the PA inputs. Hence, in multibeam scenario, each data stream is modelled by two tones having individual phase and amplitude depending on the beamforming coefficients of given stream. The simulations are performed in frequency domain by utilizing the concept of spectral convolution to model the intermodulation distortion, and array factor to model the far-field radiation of the linear and nonlinear PA output components. By utilizing the simulator, PA nonlinearity is analyzed in single-beam and multi-beam scenarios by varying the steering angles, allocated stream powers and amplitude distribution over the PAs. It is observed that IMD terms which depend on only one stream are steered to same direction as the linear terms whereas the IMD terms depending on both streams spreads more in space. This has potentially positive impacts on the signal-to-distortion ratio of the streams observed in beamforming directions.
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