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1. A Direct-Conversion Digital Beamforming Array Receiver with 800 MHz Channel Bandwidth at 28 GHz using Xilinx RF SoC

   Sravan Pulipati, Viduneth Ariyarathna (November 2019, 2019 IEEE International Conference on Microwaves, Antennas, Communications and Electronic Systems (COMCAS))

   null (Ed.)

   This paper discusses early results associated with a fully-digital direct-conversion array receiver at 28 GHz. The proposed receiver makes use of commercial off-the-shelf (COTS) electronics, including the receiver chain. The design consists of a custom 28 GHz patch antenna sub-array providing gain in the elevation plane, with azimuthal plane beamforming provided by real-time digital signal processing (DSP) algorithms running on a Xilinx Radio Frequency System on Chip (RF SoC). The proposed array receiver employs element-wise fully-digital array processing that supports ADC sample rates up to 2 GS/second and up to 1 GHz of operating bandwidth per antenna. The RF mixed-signal data conversion circuits and DSP algorithms operate on a single-chip RFSoC solution installed on the Xilinx ZCU1275 prototyping platform. 

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2. 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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3. Sub-Terahertz Spatial Statistical MIMO Channel Model for Urban Microcells at 142 GHz

   Ju, Shihao; Rappaport, Theodore S. (December 2021, 2021 IEEE Global Communications Conference)

   Abstract—Sixth generation (6G) cellular systems are expected to extend the operational range to sub-Terahertz (THz) frequencies between 100 and 300 GHz due to the broad unexploited spectrum therein. A proper channel model is needed to accurately describe spatial and temporal channel characteristics and faithfully create channel impulse responses at sub-THz frequencies. This paper studies the channel spatial statistics such as the number of spatial clusters and cluster power distribution based on recent radio propagation measurements conducted at 142 GHz in an urban microcell (UMi) scenario. For the 28 measured locations, we observe one to four spatial clusters at most locations. A detailed spatial statistical multiple input multiple output (MIMO) channel generation procedure is introduced based on the derived empirical channel statistics. We find that beamforming provides better spectral efficiency than spatial multiplexing in the LOS scenario due to the boresight path, and two spatial streams usually offer the highest spectral efficiency at most NLOS locations due to the limited number of spatial clusters. Index Terms—Channel Measurement; Channel Modeling; Spatial Statistics; Beamforming; Spatial Multiplexing; MIMO; Sub- Terahertz; 140 GHz; 5G; 6G 

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4. Millimeter Wave and Sub-Terahertz Spatial Statistical Channel Model for an Indoor Office Building

   Shihao Ju, Yunchou Xing (December 2021, IEEE journal on selected areas in communications)

   Abstract—Millimeter-wave (mmWave) and sub-Terahertz (THz) frequencies are expected to play a vital role in 6G wireless systems and beyond due to the vast available bandwidth of many tens of GHz. This paper presents an indoor 3-D spatial statistical channel model for mmWave and sub-THz frequencies based on extensive radio propagation measurements at 28 and 140 GHz conducted in an indoor office environment from 2014 to 2020. Omnidirectional and directional path loss models and channel statistics such as the number of time clusters, cluster delays, and cluster powers were derived from over 15,000 measured power delay profiles. The resulting channel statistics show that the number of time clusters follows a Poisson distribution and the number of subpaths within each cluster follows a composite exponential distribution for both LOS and NLOS environments at 28 and 140 GHz. This paper proposes a unified indoor statistical channel model for mmWave and sub-Terahertz frequencies following the mathematical framework of the previous outdoor NYUSIM channel models. A corresponding indoor channel simulator is developed, which can recreate 3-D omnidirectional, directional, and multiple input multiple output (MIMO) channels for arbitrary mmWave and sub-THz carrier frequency up to 150 GHz, signal bandwidth, and antenna beamwidth. The presented statistical channel model and simulator will guide future air-interface, beamforming, and transceiver designs for 6G and beyond. Index Terms—Millimeter-wave, terahertz, radio propagation, indoor office scenario, channel measurement, channel modeling, channel simulation, NYUSIM, 28 GHz, 140 GHz, 142 GHz, 5G, 6G. 

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5. Dual-Band, Two-Layer Millimeter-Wave Transceiver for Hybrid MIMO Systems

   https://doi.org/10.1109/JSSC.2021.3110520  

   Mondal, Susnata; Carley, Larry Richard; Paramesh, Jeyanandh (October 2021, IEEE Journal of Solid-State Circuits)

   null (Ed.)

   This paper presents a two-layer RF/analog weighting MIMO transceiver that comprises fully-connected (FC) multi-stream beamforming tiles in the RF-domain first layer, followed by a fully connected analog- or digital-domain baseband layer. The architecture mitigates the complexity versus spectral-efficiency tradeoffs of existing hybrid MIMO architectures and enables MIMO stream/user scalability, superior energy-efficiency, and spatial-processing flexibility. Moreover, multi-layer architectures with FC tiles inherently enable the co-existence of MIMO with carrier-aggregation and full-duplex beamforming. A compact, reconfigurable bidirectional circuit architecture is introduced, including a new Cartesian-combining/splitting beamforming receiver/transmitter, dual-band bidirectional beamforming network, dual-band frequency translation chains, and baseband Cartesian beamforming with an improved programmable gain amplifier design. A 28/37 GHz band, two-layer, eight-element, four-stream (with two FC-tiles) hybrid MIMO transceiver prototype is designed in 65-nm CMOS to demonstrate the above features. The prototype achieves accurate beam/null-steering capability, excellent area/power efficiency, and state-of-the-art TX/RX mode performance in two simultaneous bands while demonstrating multi-antenna (up to eight) multi-stream (up to four) over-the-air spatial multiplexing operation using proposed energy-efficient two-layer hybrid beamforming scheme. 

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