Abstract
For higher data throughput, the Massive Multi-input Multi-output (MIMO) system has been widely used in modern communication systems. Due to the circuit size and power consumption, it is difficult to integrate one Radio Frequency (RF) chain for each power amplifier (PA). As a result, MIMO transmitters with hybrid beam-forming don't have enough RF chains, and this demands the digital predistortion (DPD) block to deal with several power amplifiers (PA) simultaneously, which causes a shared DPD problem. This article presents an Analog Predistorter Averaged DPD (A 2 -DPD) for hybrid beam-forming massive MIMO transmitters. To linearize more than one PAs with one shared digital predistortion unit, continuously tunable Analog Predistortion (APD) modules are employed to uniform the nonlinear behavior of PAs in each channel. Based on iterative learning control (ILC) technique, crossed normalized mean square error (CNMSE) is derived to judge the uniformity of PAs. By comparing the CNMSE of each PAs, the best similarity state of PAs can be found. The nonlinear behavior of PAs can be adjusted to a similar state by tuning the APD control voltage. The adjustment makes PAs easier to be linearized by a shared DPD. Furthermore, an averaged DPD algorithm is proposed to improve the total linearity performance for each PA. Simulations are performed to study the relationship between CNMSE and PA uniformity and further simulations show the ability of A 2 -DPD to deal with a multi-PA scenario. In the end, to verify the feasibility of the proposed A 2 -DPD, two class-AB PAs with operating frequency at 3.5 GHz with the same type transistor and similar design are tested. Experimental results show the proposed A 2 -DPD can significantly improve the uniformity of PAs and the Adjacent Channel Power Ratio (ACPR) can be improved by 8 dB compared with using the DPD parameter of the other PA.
Highlights
Radio Frequency (RF)Power Amplifiers, as the critical components in communication base stations, have been extensively studied for many years
It is not easy to conduct numerous experiments for shared digital predistortion (DPD) with different power amplifier (PA), which is why simulations are carried out to verify the relationship between crossed normalized mean square error (CNMSE) of particular pairs of PAs and the performance of shared DPD with the corresponding PAs
Based on iterative learning control (ILC) technology, the CNMSE is derived to judge the uniformity of PA for linearization and help to find the best fitting status of Analog Predistortion (APD)
Summary
Power Amplifiers, as the critical components in communication base stations, have been extensively studied for many years. Due to the high power consumption of PAs, the efficiency of PAs directly affect the over-all maintenance cost of the whole base station [1]. Of the inherent contradiction of efficiency and linearity of PAs [2], linearization technologies are crucial subsystems in transmitters to guarantee the linearity of PAs when the PAs are working in the high-efficiency region. Digital predistortion (DPD) is one of the most widely employed linearization techniques in commercial base stations as a result of the good linearity performance and low cost [3].
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