A Digital Predistortion Scheme Exploiting Degrees-of-Freedom for Massive MIMO Systems

The primary source of nonlinear distortion in wireless transmitters is the power amplifier (PA). Conventional digital predistortion (DPD) schemes use high-order polynomials to accurately approximate and compensate for the nonlinearity of the PA. This is not practical for scaling to tens or hundreds of PAs in massive multiple-input multiple-output (MIMO) systems. There is more than one candidate precoding matrix in a massive MIMO system because of the excess degrees-of-freedom (DoFs), and each precoding matrix requires a different DPD polynomial order to compensate for the PA nonlinearity. This paper proposes a low-order DPD method achieved by exploiting massive DoFs of next-generation front ends. We propose a novel indirect learning structure which adapts the channel and PA distortion iteratively by cascading adaptive zero forcing precoding and DPD. Our solution uses a 3rd order polynomial to achieve the same performance as the conventional DPD using an 11th order polynomial for a 100x10 massive MIMO configuration. Experimental results show a 70% reduction in computational complexity, enabling ultra-low latency communications.Comment: IEEE International Conference on Communications 201

Cartesian Pre-distortion using a Sigma Delta Modulator for Multi-Standard RF Power Amplifiers

The system implements a pre-distortion algorithm for a non-linear PA by placing a pseudo static forward model of a PA in the feedback path of a Sigma Delta Modulator. Performing the predistortion in this way reduces the computation overhead incurred in calculating pre-distorter weights for a given power amplifier. In this paper a Cartesian LUT is used to represent the PA model, allowing the system to compensate for PA non-linearity caused by multiple signal standards. The system was tested using 3 modulation standards and showed improvement of up to 28dB while the PA model remained constant

When Can a Narrowband Power Amplifier Be Considered to Be Memoryless and when Not?

This paper tries to get a response to the following question: When can a narrowband power amplifier (PA) be considered to be memoryless and when can it not be considered memoryless? To this end, a thorough and consistent analysis of the notions and definitions related with the above topic is carried out. In the considerations presented, two models of the narrowband PA are exploited interchangeably: the black box model widely used in the literature and a model developed here, which is based on the Volterra series. These two models complement each other. In this paper, the conditions for a linear or nonlinear narrowband PA to be memoryless or approximately memoryless or possessing memory are derived and illustrated. They are formulated in terms of the signal delay as well as in terms of the  amplitude-to-phase (AM/PM) conversion of the amplifier. Furthermore, the two possible interpretations of the amplitude-to-amplitude (AM/AM) and AM/PM conversions are given a mathematical framework. That is these conversions are presented through some operations. One set of these operations allows to treat the AM/AM and AM/PM conversions as distortions of the modulating signals. Or equivalently as distortions of a given signal constellation when it passes through the PA. Finally, it is proved that the Saleh’s and Ghorbani’s models of the AM/AM and AM/PM conversions occurring in the PAs, which were published in the literature, are not memoryless ones

On Correct Understanding and Classification of Saleh’s and Related Models of AM/AM and AM/PM Conversions

In this paper, some of the existing classifications of the Saleh’s and related models of the AM/AM and AM/PM conversions occurring in communication power amplifiers (PA) are reviewed. It is shown that these classifications are inconsistent and must be refined. Obviously, carrying out such a refinement properly needs a good knowledge and correct understanding of the mechanisms leading to the AM/AM and AM/PM conversions. This was achieved in this paper by performing a thorough analysis of the PA behavior using an analytical tool, the Volterra series. The main points of this analysis are presented here in great detail. Among others, it is shown that the influence of the PA memory on the AM/PM conversion is two-fold: direct and indirect. The former can be however fully neglected. On the other hand, the indirect influence caused by “the interaction of the carrier with the PA memory” cannot be neglected when the PA has not enough wideband frequency characteristics. The latter effect mentioned causes changes in the carrier phase that are received as the phase changes of the baseband modulating signal

Nonlinearities Influence to RF Satellite Downlink Model with QAM and Raised Square Cosine Filter

Reliability of communications is of vital importance in military applications. Constellations are connecting coded words at different ends of the communication channel that indicate the correctness of the transmitted message. In this paper, we compare the influence of the selected nonlinearity in the transmit amplifier on the constellation diagrams in radio frequency (RF) geostationary satellite downlink and bit-error-rate (BER). Two cases were analyzed: negligible and severe noise in the communication channel. Considering the cubic, hyperbolic tangent, Saleh, Ghorbani, and Raap models, it is shown that the Raap and Saleh models can be used for the lowest BERs when the noise is negligible. In case of severe noise, it is best to use the Raap model from the set of nonlinearities considered. The ANOVA-test showed that there is a dependence between the Raap and Saleh models in the presence of negligible noise, but not in the presence of severe noise