Christian Fager

Distributed multiple-input-multiple-output (D- MIMO) is a promising technology for future wireless communication systems, but it still has many implementation challenges to overcome. In this paper, we investigate a D-MIMO setup based on sigma-delta-over-fiber (SDoF) and optical up-conversion, which allows low-complexity remote radio heads (RRHs) while simultaneously exploiting the high bandwidth available in the millimeter-wave (mm-wave) spectrum. The optical up-conversion is implemented by mixing two optical frequencies separated by $26.5 \,{\mathrm{GHz}}$ with a band-pass sigma-delta modulated signal at a lower intermediate-frequency (IF) at $1.5 \,{\mathrm{GHz}}$ in a photodetector, to create a mm-wave carrier at $28 \,{\mathrm{GHz}}$. The same optical frequencies can be used for many RRHs by splitting the power and inserting optical amplification. The fact that the frequency up-conversion is done at the RRH makes it possible to lower the requirement on the central-unit (CU) sample rate, while the phase coherency among the RRHs is preserved as the optical frequencies originate from the same signal source in the CU. We demonstrate through measurements both single- and multi-user MIMO with two phase-synchronized RRHs, showing that SDoF with optical up-conversion is a promising solution for implementing D-MIMO at mm-wave bands.

A digital predistortion (DPD) scheme for concurrent multi-beam transmission in fully digital multiple-input, multiple-output (MIMO) systems, using Fast Fourier Transform (FFT) beamforming and so-called virtual-array processing, is proposed. In a MIMO array with nonlinear power amplifiers (PAs), transmitting multiple beams concurrently yields intermodulation products that end up in both user and non-user directions. In the setting with few users in a large array, the array dimension will typically be much larger than the number of generated intermodulation products. At the same time, linearization per PA is excessively costly for large arrays. This work shows that it is instead possible to linearize the system by producing predistorted user beams, and non-user intermodulation products, through DPD processing in a virtual array of a much smaller dimension than the physical array. Theoretical derivations and simulation examples show how this approach can lead to manyfold reductions in DPD complexity.

This letter addresses measurement-based coupling emulation of power amplifier (PA) arrays with more than four elements, enabling scalable characterization of multiantenna transmitters (TXs). A refined estimation procedure for driver gain is proposed, improving the convergence stability and accuracy over prior methods, while also addressing the estimation of the involved load reflection coefficients. Measurements with up to 16-PA arrays demonstrate reliable error convergence and accurate S-parameter recovery, while we also confirm the anti-symmetric variation of the load reflection coefficient with beamforming angle. The results show a reduced branch-to-branch injection-error variance compared to prior art methods, further supporting the scalability of the proposed estimator. Furthermore, combined far-field adjacent-channel power ratio (ACPR) and error vector magnitude (EVM) metrics for a phased-array TX exhibit the expected symmetry versus beam angle and reveal direction-dependent nonlinearity, validating realistic array-level radiated performance after spatial combining. Notably, this work also integrates a 5G-NR compliant waveform for the first time in a coupling-emulation framework, paving the way for efficient modeling and linearization of next-generation wireless TXs under load modulation.

A digital pre-distortion (DPD) scheme for concurrent multi-beam transmission in fully digital multiple-input, multiple-output (MIMO) systems, using fast Fourier transform (FFT) beamforming and so-called virtual array processing, is proposed. In a MIMO array with nonlinear power amplifiers (PAs), transmitting multiple beams concurrently yields intermodulation products that end up in both user and non-user directions. In the setting with few users in a large array, the array dimension will typically be much larger than the number of generated intermodulation products. At the same time, linearization per-PA is excessively costly for large arrays. This work shows that is instead possible to linearize the system by producing predistorted user beams, and non-user intermodulation products, through DPD processing in a virtual array, of a much smaller dimension than the physical array. Theoretical derivations and simulation examples show how this approach can lead to manyfold reductions in DPD complexity.

We consider a distributed multiple-input multiple-output (D-MIMO) testbed in which, to enable coherent-phase transmission without over-the-air synchronization, the remote radio heads (RRHs) are connected to a central unit (CU) via a 1-bit radio-over-fiber fronthaul. Specifically, 1-bit samples of the radio frequency (RF) signal are exchanged over the fronthaul. We investigate via both measurements and simulations based on an accurate model of the testbed hardware, the capability of the proposed architecture to provide uniform quality of services over the coverage area—one of the promises of D-MIMO. Our results are encouraging: for the case in which two user equipments (UEs) communicate over the same 75-MHz signal bandwidth, the measured error-vector magnitude (EVM) meets the 3GPP New Radio specification of 12.5% for 16 QAM across all tested D-MIMO scenarios. We also determine that uplink transmission is a potential bottleneck, due to the limited dynamic range of the automatic gain controller, which prevents the 1-bit quantizer to benefit from dithering. We show that this issue can be mitigated via UE power control.

We consider a distributed multiple-input multiple-output (D-MIMO) testbed in which, to enable coherent-phase transmission without over-the-air synchronization, the remote radio heads (RRHs) are connected to a central unit via a 1-bit radio-over-fiber fronthaul. Specifically, 1-bit samples of the radio-frequency signal are exchanged over the fronthaul. We investigate via both measurements and simulations based on an accurate model of the testbed hardware, the capability of the proposed architecture to provide uniform quality of services over the coverage area--one of the promises of D-MIMO. Our results are encouraging: for the case in which two user equipments (UEs) communicate over the same 75MHz signal bandwidth, the measured error-vector magnitude meets the 3GPP New Radio specification of 12.5\% for 16QAM across all tested DMIMO scenarios. We also determine that uplink transmission is a potential bottleneck, due to the limited dynamic range of the automatic gain controller, which prevents the 1-bit quantizer to benefit from dithering. We show that this issue can be mitigated via UE power control.

The vast bandwidth available at sub-THz frequencies holds great promise for high-speed wireless access, precise localization, and advanced sensing applications. However, fundamental physical constraints and technological limitations make the deployment of reliable sub-THz networks challenging. We propose a new paradigm for sub-THz coverage by transmitting the RF signals over polymer microwave fibers (PMFs) that interconnect low-complexity radio units (RUs) in a daisy-chain configuration. The distributed architecture ensures that user equipments (UEs) connect to RUs in their proximity, reducing path loss and mitigating blocking. The RUs leverage low-complexity, compact integrated antenna modules. Additionally, dual-frequency tandem operation is proposed, integrating the sub-THz system with a sub-10 GHz system that provides control signalling and a robust fallback solution for the sub- THz system. This proposed tandem architecture can open up the full potential of sub-THz technology and paves the way to cost- and energy-efficient, high-performance, real-time connectivity in dynamic environments.

This article investigates an analog phase correction method for a K-band 4-W drain-modulated power amplifier (PA) in 150-nm GaN-on-SiC technology. A tuneable phase modulator monolithic microwave integrated circuit (MMIC) is integrated directly in the PA RF input path to counteract the output phase shift of the PA under supply modulation and reduce output phase distortion. The phase modulator does not require an additional control signal as it directly takes the supply-modulated drain voltage of the PA as its control input and has negligible power consumption with an insertion loss below 3.4dB. Modulated measurements with a 33.75-MHz 16-QAM RF input signal with 6-dB peak-to-average power ratio (PAPR) at 20.5 GHz show a power-added efficiency (PAE) improvement of 6.5–9.5 percentage points (pp) for a variety of tracking functions. Moreover, with the phase modulator enabled, the PA output phase variation is reduced from 30° to 14°. Simultaneously, with phase compensation, the adjacent channel power ratio (ACPR) is improved by up to 5.2 dB to a value of −40.2 dB, while the rms error vector magnitude (EVM) is improved by up to 4.4 pp to a value of 6.9%, showing better linearity compared to a static 20-V bias case, while the average PAE is significantly improved.