Out-of-Band Radiation Measure for MIMO Arrays with Beamformed Transmission

The spatial characteristics of the out-of-band radiation that a multiuser MIMO system emits in the environment, due to its power amplifiers (modeled by a polynomial model) are nonlinear, is studied by deriving an analytical expression for the continuous-time cross-correlation of the transmit signals. At a random spatial point, the same power is received at any frequency on average with a MIMO base station as with a SISO base station when the two radiate the same amount of power. For a specific channel realization however, the received power depends on the channel. We show that the power received out-of-band only deviates little from the average in a MIMO system with multiple users and that the deviation can be significant with only one user. Using an ergodicity argument, we conclude that out-of-band radiation is less of a problem in massive MIMO, where total radiated power is lower compared to SISO systems and that requirements on spectral regrowth can be relaxed in MIMO systems without causing more total out-of-band radiation

On Out-of-Band Emissions of Quantized Precoding in Massive MU-MIMO-OFDM

We analyze out-of-band (OOB) emissions in the massive multi-user (MU) multiple-input multiple-output (MIMO) downlink. We focus on systems in which the base station (BS) is equipped with low-resolution digital-to-analog converters (DACs) and orthogonal frequency-division multiplexing (OFDM) is used to communicate to the user equipments (UEs) over frequency-selective channels. We demonstrate that analog filtering in combination with simple frequency-domain digital predistortion (DPD) at the BS enables a significant reduction of OOB emissions, but degrades the signal-to-interference-noise-and-distortion ratio (SINDR) at the UEs and increases the peak-to-average power ratio (PAR) at the BS. We use Bussgang's theorem to characterize the tradeoffs between OOB emissions, SINDR, and PAR, and to study the impact of analog filters and DPD on the error-rate performance of the massive MU-MIMO-OFDM downlink. Our results show that by carefully tuning the parameters of the analog filters, one can achieve a significant reduction in OOB emissions with only a moderate degradation of error-rate performance and PAR.Comment: Presented at the 2017 Asilomar Conference on Signals, Systems, and Computers, 6 page

Out-of-Band Radiation from Antenna Arrays Clarified

Non-linearities in radio-frequency (RF) transceiver hardware, particularly in power amplifiers, cause distortion in-band and out-of-band. Contrary to claims made in recent literature, in a multiple-antenna system this distortion is correlated across the antennas in the array. A significant implication of this fact is that out-of-band emissions caused by non-linearities are beamformed, in some cases into the same direction as the useful signal.Comment: IEEE Wireless Communications Letters, 2018, to appea

Spatial Characteristics of Distortion Radiated from Antenna Arrays with Transceiver Nonlinearities

The distortion from massive MIMO (multiple-input--multiple-output) base stations with nonlinear amplifiers is studied and its radiation pattern is derived. The distortion is analyzed both in-band and out-of-band. By using an orthogonal Hermite representation of the amplified signal, the spatial cross-correlation matrix of the nonlinear distortion is obtained. It shows that, if the input signal to the amplifiers has a dominant beam, the distortion is beamformed in the same way as that beam. When there are multiple beams without any one being dominant, it is shown that the distortion is practically isotropic. The derived theory is useful to predict how the nonlinear distortion will behave, to analyze the out-of-band radiation, to do reciprocity calibration, and to schedule users in the frequency plane to minimize the effect of in-band distortion

Analytical Approaches to Load Modulation Power Amplifier Design

In future mobile communication networks, there will be a shift toward higher carrier frequencies and highly integrated multiple antenna systems. The system performance will largely depend on the available radio frequency (RF) hardware. As such, RF power amplifiers (PAs) with improved performance, e.g. energy efficiency, are needed. Active load modulation (ALM) is one of the most common PA efficiency enhancement techniques. Unfortunately, different ALM techniques come at the cost of degrading other PA attributes. Through investigation of new ALM design techniques, the overall objective of this thesis is to improve upon different attributes and performance trade-offs in ALM PAs for future wireless systems.\ua0The working principle of ALM PAs is determined by both how the individual transistors are operated and how their outputs are combined. In the first part of the thesis, an analytical approach, where the output combiner is assumed to be an arbitrary black-box, is applied to the Doherty PA. The fundamental interaction between the main and auxiliary transistors is analyzed and generalized. New solutions with improved performance are identified, such as higher gain and an improved efficiency-linearity trade-off. This approach also introduces improved integration possibilities, which are demonstrated by a transmitter where the antenna acts as both the radiator and the Doherty combiner. Additionally, the analytical approach is applied to an isolated two-way power divider. This unlocks many new possibilities, such as improved integration and layout flexibility. \ua0In the second part, one embodiment of the emerging ALM architecture, the load modulated balanced amplifier (LMBA), is proposed: the RF-input Doherty-like LMBA. Design equations are derived and the fundamental operation is studied. This variant presents several advantages over other known architectures, such as higher gain and device periphery scaling of the different transistors.\ua0The third part proposes a new measurement-based ALM PA design procedure, which emulates the full behavior of the transistors in any ALM architecture using active load-pull measurements. This method can predict the intricate behavior in ALM PAs and it gives measurement-based insights into the internal operation of the circuit already at the design stage. This facilitates the design for optimal ALM PA performance. \ua0The thesis contributes with several promising techniques for reducing performance trade-offs and improving the overall performance of ALM PAs. Therefore, the results will contribute to the development of more energy efficient and high capacity wireless services in the future