Abstract
The design of predistortion techniques for broadband multiple input multiple output-OFDM (MIMO-OFDM) systems raises several implementation challenges. First, the large bandwidth of the OFDM signal requires the introduction of memory effects in the PD model. In addition, it is usual to consider an imbalanced in-phase and quadrature (IQ) modulator to translate the predistorted baseband signal to RF. Furthermore, the coupling effects, which occur when the MIMO paths are implemented in the same reduced size chipset, cannot be avoided in MIMO transceivers structures. This study proposes a MIMO-PD system that linearizes the power amplifier response and compensates nonlinear crosstalk and IQ imbalance effects for each branch of the multiantenna system. Efficient recursive algorithms are presented to estimate the complete MIMO-PD coefficients. The algorithms avoid the high computational complexity in previous solutions based on least squares estimation. The performance of the proposed MIMO-PD structure is validated by simulations using a two-transmitter antenna MIMO system. Error vector magnitude and adjacent channel power ratio are evaluated showing significant improvement compared with conventional MIMO-PD systems.
Highlights
Emerging broadband communication systems require high spectral efficiency and robustness against multipath channels
In this study we propose a multiple input multiple output (MIMO)-PD based on a parallel memory polynomial (MP) model that renders the following properties: (a) Compensation of nonlinear crosstalk and in-phase and quadrature (IQ) imbalance effects: We propose a modified MP [14] structure to linearize each broadband power amplifier (PA) response and to compensate the crosstalk and IQ imbalance effects
To include all the impairments, i.e., the nonlinear distortion and memory effects due to the PA, crosstalk coupling due to the MIMO structure, and the IQ imbalance distortion, we propose for each PA
Summary
Emerging broadband communication systems require high spectral efficiency and robustness against multipath channels. Phase and amplitude imbalances of the modulator affect the estimation of the PD coefficients This problem motivates the joint compensation of PA nonlinear response and the distortion introduced in the up-conversion process. The use of MIMO transceiver structures implemented on reduced size leads to the coupling case (b) between the branches at the front-end that cannot be avoided This kind of crosstalk is amplified by a nonlinear device (amplifier) and is denoted as nonlinear crosstalk. The proposed PD coefficient estimation technique has high computational complexity, and IQ imbalance effects, and tracking the changes of PA characteristics in time due to temperature or bias point variations have not been considered in [13].
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