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1. A Comparison of Hybrid Beamforming and Digital Beamforming with Low-Resolution ADCs for Multiple Users and Imperfect CSI

   https://doi.org/10.1109/JSTSP.2018.2813973  

   Roth, Kilian ; Pirzadeh, Hessam ; Swindlehurst, A. Lee ; Nossek, Josef A. ( March 2018 , IEEE Journal of Selected Topics in Signal Processing)

   For 5G it will be important to leverage the available millimeter wave spectrum. To achieve an approximately omni- directional coverage with a similar effective antenna aperture compared to state-of-the-art cellular systems, an antenna array is required at both the mobile and basestation. Due to the large bandwidth and inefficient amplifiers available in CMOS for mmWave, the analog front-end of the receiver with a large number of antennas becomes especially power hungry. Two main solutions exist to reduce the power consumption: hybrid beam forming and digital beam forming with low resolution Analog to Digital Converters (ADCs). In this work we compare the spectral and energy efficiency of both systems under practical system constraints. We consider the effects of channel estimation, transmitter impairments and multiple simultaneous users for a wideband multipath model. Our power consumption model considers components reported in literature at 60 GHz. In contrast to many other works we also consider the correlation of the quantization error, and generalize the modeling of it to non- uniform quantizers and different quantizers at each antenna. The result shows that as the Signal to Noise Ratio (SNR) gets larger the ADC resolution achieving the optimal energy efficiency gets also larger. The energy efficiency peaks for 5 bit resolution at high SNR, since due to other limiting factors the achievable rate almost saturates at this resolution. We also show that in the multi- user scenario digital beamforming is in any case more energy efficient than hybrid beamforming. In addition we show that if mixed ADC resolutions are used we can achieve any desired trade-off between power consumption and rate close to those achieved with only one ADC resolution. 

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2. Virtual DPD Neural Network Predistortion for OFDM-based MU-Massive MIMO

   https://doi.org/10.1109/IEEECONF53345.2021.9723343  

   Tarver, Chance ; Balalsoukas-Slimining, Alexios ; Studer, Christoph ; Cavallaro, Joseph R. ( October 2021 , 2021 55th Asilomar Conference on Signals, Systems, and Computers)

   The nonlinearities of power amplifiers in massive MIMO arrays introduce unwanted spectral regrowth, which is typically avoided via digital predistortion at each amplifier. However, as the number of base station antennas scales up, so does the computational burden of per-antenna linearization. This work introduces a neural-network virtual digital predistortion (vDPD) scheme that operates before the linear precoder for OFDM-based massive MU-MIMO systems. By applying predistortion before the precoder, complexity scales primarily with the number of users. We can achieve comparable linearization along the user beams by training our neural network based on the memory polynomial, predistortion-per-antenna approach. We verify our algorithm through an exhaustive simulator that includes high-order amplifier nonlinearities, memory effects, and variance across the amplifier models. 

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3. Synthetic Diversity To Mitigate Out-of-Band Interference in Widely Tunable Wireless Receivers

   https://doi.org/10.1109/IEEECONF44664.2019.9048662  

   Molnar, Alyosha ; Boynton, Zachariah ; Soni, Sweta ; Sadeghi, Sanaz ( November 2019 , Synthetic Diversity To Mitigate Out-of-Band Interference in Widely Tunable Wireless Receivers)

   null (Ed.)

   Here we present a combined RF hardware/DSP technique to synthesize effective channel diversity in single-antenna wireless systems. This allows digital suppression of out-of-band interference artifacts in widely tunable wireless receivers with one or more antennas, including artifacts from LO phase noise. A passive inductor-capacitor (LC) network provides gain and phase diversity between channels and across frequency. Since amplitude and phase of in-band artifacts are set by the amplitude and phase of the out-of-band interference that generates them, they can be suppressed in DSP without knowledge about the interferer itself. The feasibility of this approach is demonstrated mathematically, with numerical system simulations, and full circuit simulation. 

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4. Energy efficient downlink massive MIMO: Is 1-bit quantization a solution ?

   https://doi.org/10.1109/ISWCS.2019.8877256  

   Marcastel, Alexandre ; Fijalkow, Inbar ; Swindlehurst, Lee ( August 2019 , Proc. 16th International Symposium on Wireless Communication Systems (ISWCS))

   Massive MIMO aims to build wireless base stations with hundreds of coherently operating antennas serving tens of single antenna users in order to improve both the transmission capacity by a factor 10-50 and the energy-efficiency trade-off by up to a thousand times. Pre-coding at the base station has been proposed to efficiently implement digital beamforming. It implies a high signal dynamic range and therefore a power backoff resulting in less energy-efficiency. One-bit quantized Zero-Forcing precoding has been proposed to efficiently handle the RF front-end when the array is implemented with so many antennas. In this paper, we analyze the energy-efficiency of the quantized Zero-Forcing precoded systems for a large number of users and a massive MIMO base station. 

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5. A Compact Wireless Headstage based Optogenetic Neuromodulation and 32-channel Electrophysiological Recording System

   https://doi.org/10.1109/DCAS57389.2023.10130233  

   Saha, Nabanita ; Alvarez, Erik Pineda ; Becker, April ; Mahbub, Ifana ( April 2023 , 2023 IEEE 16th Dallas Circuits and Systems Conference (DCAS))

   This paper demonstrates a commercial off-the-shelf components (COTS)-based miniaturized wireless optogenetic headstage with simultaneous optical stimulation and electro-physiological recording capability for freely moving rodents. The proposed headstage contains 32 recording channels. The optical stimulation system is a battery-powered neural stimulator, comprised of a low dropout regulator (LDO), an oscillator, and a µLED. The electrophysiological signal recording system includes an intracortical neural probe made of a GaN-on-silicon substrate, an array of neural amplifiers with an integrated analog-to-digital converter (ADC), a transceiver integrated circuit, and a ceramic antenna. The integrated MUX with the ADC allows sampling of the amplified voltage at a sampling rate of 4000 kSamples/s. By placing the headstage on the head of a rodent and recording the neural signals from the Ventral Tegmental area of the brain, the system is experimentally validated in in-vivo. Experimental result shows that the proposed headstage can trigger neuron activity while collecting and detecting single-cell microvolt amplitude activity from multiple channels. 

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