Yuelin Ma

Wideband Digital Predistortion Using Spectral Extrapolation of Band-Limited Feedback Signal

With the ever increasing demands for higher data rate, wider bandwidth is required for improving the throughput. This trend, however, imposes design challenges for the digital predistortion (DPD) in many aspects. In order to sample the broadband power amplifier (PA) output signal, it requires the use of high-speed analog-to-digital converters (ADCs), which tend to be the most expensive components in a transmitter with DPD. The sampling speed of the ADC for conventional DPD has to be several times of the original signal bandwidth in order to cover the out-of-band intermodulation components caused by nonlinear PA. In this paper, a novel technique, which allows the use of low-speed ADCs by introducing spectral extrapolation to the band-limited feedback signal, is proposed. This allows efficient implementation of DPD for very wideband signals. Experimental results demonstrate that the bandwidth of the acquisition path can be even less than the bandwidth of original signal applying the proposed technique. In addition, satisfactory linearization performance has been achieved employing wideband signals up to 160 MHz bandwidth.

FPGA Implementation of Adaptive Digital Predistorter With Fast Convergence Rate and Low Complexity for Multi-Channel Transmitters

This paper reports an adaptive digital predistorter (DPD) with fast convergence rate and low complexity for multi-channel transmitters, which is fully implemented in a field programmable gate array. The design methodology and practical implementation issues are discussed, with concerns about the impact caused by carrier power shutdown and transmission power control. The proposed DPD is composed of multiple adaptive lookup table (LUT) units of uniform structures, allowing configurability for desired memory depth. A simplified multiplier-free normalized least mean square algorithm for fast adapting the LUT is introduced. The proposed DPD is also experimentally exploited to linearize a Doherty amplifier. The adjacent channel leakage ratio reaches -60 dB for both lower and upper bands in the test applying a long-term evolution signal. It is also demonstrated in this paper that the proposed DPD shows high robustness when a multi-channel global system for mobile communications signal with occasional carrier power shutdown is applied.

Adaptive Compensation of Inter-Band Modulation Distortion for Tunable Concurrent Dual-Band Receivers

For concurrent dual-band receiver with shared radio frequency (RF) front-ends, the received signals are prone to be contaminated by the inter-band modulation components even if the signals are located in distant frequency bands. A novel signal processing technique for blind compensation of nonlinear distortion in concurrent dual-band receiver is presented in this paper. To enhance the performance of the compensator, de-embedding is employed to reconstruct the polynomial model. The compensator coefficients updating is accomplished in a block-by-block manner, such that the computing complexity and adaption speed can be well balanced. Finally, the performance of the compensator is validated by the results obtained from actual measurements.

Fast Baseband Polynomial Inverse Algorithm for Nonlinear System Compensation

Digital predistortion based on nonlinear system modeling and its inverse is an important technique for the linearization of radio frequency power amplifiers (PAs). Compensation for the nonlinear systems represented by polynomials involves polynomial inverses. In this paper, we propose a fast algorithm to invert a given baseband memoryless polynomial based on the predefined orthogonal basis. The orthogonal basis is predefined based on the distribution of the signal, which is known a priori. Compared with the well-established pth-order inverse method, the proposed algorithm can find the optimal coefficients of the inverse polynomial. Both numerical simulation and experiment with an actual PA demonstrate that the proposed algorithm can provide very good performance for compensating the nonlinear systems represented by baseband polynomial.

Experimental Results of Digital Predistorter for Very Wideband Mobile Communication System

This paper presents the experimental results utilizing spectral-extrapolation digital predistortion (SE-DPD) for the linearization of very wideband mobile communication signals. With the limited feedback bandwidth, SE-DPD allows the use of analog-to-digital converter (ADC) with lower sampling frequency. The experiments in this paper validate the effectiveness of the SE-DPD employing multi-carrier LTE-Advanced signals with contiguous and intra-band carrier aggregation of up to 320 MHz bandwidth. The achieved bandwidth is the highest- record of digital predistorter for mobile communications. With the use of reasonable hardware, it is concluded that the DPD is a rather promising technique for future wideband systems such as 5G.