The sub-THz frequency spectrum ranging from 90 GHz to 300 GHz offers huge untapped frequency band, up to several tens of GHz, which is highly promising forachieving extremely high data rate transmission. Therefore, sub-THz technology is considered as one of the key technology candidates for beyond 5G communication systems. However, sub-THz communication relies on dominant propagation path in the environments. It is essential to identify and track thedominant paths in the channel to maintain the communication link. Understanding the spatial profiles of the channel is therefore essential. In this work, we present a long range sub-THz channel sounder we developed at Aalborg University Denmark, which is capable of capturing the spatial profile of the channel with high spatial resolution based on the virtual array concept. The channel sounder is experimentally validated in field measurements

Fan, Wei

Lyu, Yejian

Olesen, Kim

English

    Aalborg UniversitetMassive MIMO Channel Sounder for Sub-THz Based on Virtual Array ConceptFan, Wei; Lyu, Yejian; Olesen, KimPublished in:IEEE SAP 2023Publication date:2024Link to publication from Aalborg UniversityCitation for published version (APA):Fan, W., Lyu, Y., & Olesen, K. (2024). Massive MIMO Channel Sounder for Sub-THz Based on Virtual ArrayConcept. In IEEE SAP 2023 IEEE.General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright ownersand it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.            - Users may download and print one copy of any publication from the public portal for the purpose of private study or research.            - You may not further distribute the material or use it for any profit-making activity or commercial gain            - You may freely distribute the URL identifying the publication in the public portal -Take down policyIf you believe that this document breaches copyright please contact us at vbn@aub.aau.dk providing details, and we will remove access tothe work immediately and investigate your claim.Downloaded from vbn.aau.dk on: January 23, 2024Massive MIMO Channel Sounder for Sub-THzBased on Virtual Array ConceptWei FanSchool of Information Science and Engineering, Southeast University,  China weifan@seu.edu.cnYejian LyuDepartment of electronic systemsAalborg UniversityDenmarkyely@es.aau.dkKim OlesenDepartment of electronic systemsAalborg UniversityDenmarkko@es.aau.dkAbstract—The sub-THz frequency spectrum ranging from90 GHz to 300 GHz offers huge untapped frequency band,up to several tens of GHz, which is highly promising forachieving extremely high data rate transmission. Therefore, sub-THz technology is considered as one of the key technologycandidates for beyond 5G communication systems. However,sub-THz communication relies on dominant propagation pathin the environments. It is essential to identify and track thedominant paths in the channel to maintain the communicationlink. Understanding the spatial profiles of the channel is thereforeessential. In this work, we present a long range sub-THz channelsounder we developed at Aalborg University Denmark, which iscapable of capturing the spatial profile of the channel with highspatial resolution based on the virtual array concept. The channelsounder is experimentally validated in field measurements.Index Terms—sub-THz communication, channel sounder, mul-tipath profile, virtual array, radio-over-fiber conceptI. INTRODUCTIONSub-THz technology is one of the candidate solutions for 6Gcommunication, since it can offer huge data-rate transmission.However, there are also lots of challenges to achieve high date-rate reliable communication for mobile users in deploymentscenario scenarios for this frequency bands. Moreover, thepropagation channel will be sparse and specular, composedof a few dominant multipaths originated from line of sight(LOS) or reflections, whereas diffraction and penetration losswill be more much significant. To ensure good SNR for thesub-THz communication, we can in principle boost powerthe transmitted power at the transmitter (Tx) or improve thereceiver sensitivity at the receiver (Rx). However, those arelimited due to current technology bottleneck. Therefore, itis required to employ high-gain and beam-steerable antennasboth at the Tx and Rx to track the dominant propagation paths.Therefore, from propagation aspects, it is important to knowthe spatial profile of the propagation channel.There are a few known strategies to obtain the channelspatial profiles in the state-of-art, namely, mechanical steering(also called directional scanning scheme, DSS), virtual antennaarray (VAA), switched antenna array, electrical beam-steering(based on analog beamformer structure) and real antenna array(based on digital beamformer structure). The basic idea ofthe DSS scheme is that we can mount a highly directiveantennas in the center of a mechanical turntable, which canbe automated and rotated to directly record the channel spatialprofile. It is the most popular one for the mmWave andsub-THz frequency bands, due to its low cost, simplicityand high SNR offered by the employed high gain antenna.However, it suffers from the long measurement time, andthe limited spatial resolution. For the VAA scheme, a singleantenna is mechanically moved to different spatial locationsby a positioner, which can be automated and controlled toform arbitrary virtual array configurations. Compared to theDSS scheme, we can achieve much better spatial resolution,without increasing the cost and measurement time. However, itrequired phase coherent measurements among different spatiallocations.VAA has been widely adopted by the industry and differentresearch groups, covering different frequency bands and mea-surement ranges [1-5]. There are several challenges to achievelong-range VAA channel sounding at sub-THz:• Frequency limitation. We need to generate and analyzesub-THz radio signals in the channel sounder. This canbe done via employing external frequency extenders.• Measurement range limitation. Vector network analyzer(VNA) is widely used for channel sounding purpose atsub-THz bands. However, the Tx and Rx are collocatedand housed in the VNA. For measurements in typicallydeployment scenario, we need to use cables to remote theantennas. Radio frequency (RF) cables are very lossy atsub-THz and expensive. RoF scheme can be employed toaddress this challenge, which however are not commer-cially available at sub-THz bands.• Phase coherent measurements. It was highlighted in [6]that “a decent phase accuracy of virtual arrays is mechan-ically and RF technically non-achievable.”In [7], a long-range VAA channel sounder was reported,where several schemes are introduced to address the above-discussed challenges, i.e. frequency extension scheme, RoFscheme and phase compensation scheme.II. LONG RANGE VAA SUB-THZ CHANNEL SOUNDERA diagram of the developed channel sounder for the Wband (i.e. 75 GHz to 110 GHz) is illustrated in Fig. 1.External Tx and Rx frequency extenders are employed forthe sub-THz signal generation and analysis. The RoF scheme979-8-3503-4114-0/23/$31.00 ©2023 IEEEFig. 1. system diagram of the long range VAA sub-THz channel sounder(a) (b)Fig. 2. system diagram of the long range VAA sub-THz channel sounderwas implemented in the local oscillator (LO) chain beforethe frequency multiplier. The RoF units, i.e. laser and photodetector, are commercially available in this frequency range.By doing so, we effectively avoid using RoF units at sub-THzfrequency bands directly. A phase compensation scheme wasimplemented in the LO path to remove the random phase inthe optical cable.The developed channel sounder is very powerful, coveringthe frequency range offered by the VNA and the frequencyextender and extremely good system dynamic range. Themeasurement range is mainly limited by the over-the-air signaltransmission loss in the propagation channel. Arbitrary VAAcan be implemented as well to record channel spatial profiles.The main limitation is the measurement speed, which limitsits application for static scenarios only.III. MEASUREMENT VALIDATIONSThe developed channel sounder was utilized for channelmeasurement in an indoor scenario. The frequency range is setto 99 to 101 GHz, with a bandwidth of 2 GHz. The number offrequency points is set to 1001. The VNA IF bandwidth andtransmitted power are set to 500 Hz and 10 dBm, respectively.The Tx-Rx heights are set to the same height. An omni-directional antenna is used at the Tx side, while a directionalantenna with half power bandwidth of 45o is utilized at theRx side. Both the DSS and VAA are implemented at the Rxside with the help of a turntable with 1o rotation step. Themeasurement system is used for the DSS and VAA, with adifference that the directive antenna is centered at the turntablefor the DSS and offset with 50cm radius for the VAA.The measured power-angle-delay profiles (PADP) for the DSS and VAA schemes are shown in figure 2 (a) and (b), respectively. The measured channels are highly sparse and specular, with a few multipath components identified. Same multipaths are obtained in the DSS and VAA scheme. How-ever, it is shown that the VAA scheme offers much better spatial resolution and SNR, thanks to the array factor and array gain offered by the VAA scheme.IV. CONCLUSIONA channel sounder based on VNA is presented, which covers up to sub-THz frequency range enabled by the frequency extender scheme, long measurement range offered by the RoF scheme, and virtual large scale array capability, enabled by the phase compensation scheme. The channel sounder capability is illustrated by the channel mea-surements in the field, where the VAA scheme is demonstrated to offer a much better SNR and spatial resolution compared the DSS scheme.Acknowledgment The research is supported by the start-up Research Fund of Southeast University, China (RF1028623309) and European Partnership on Metrology Project MEWS (21NRM03). REFERENCES[1] J. Medbo, H. Asplund, J. -E. Berg and N. Jalden, ”Directional channelcharacteristics in elevation and azimuth at an urban macrocell basestation,” 2012 6th European Conference on Antennas and Propagation(EUCAP), 2012.[2] C. Gentile, S. M. Lopez and A. Kik, ”A Comprehensive Spatial-Temporal Channel Propagation Model for the Ultrawideband Spectrum2–8 GHz,” in IEEE Transactions on Antennas and Propagation, vol. 58,no. 6, pp. 2069-2077, June 2010.[3] C. Gustafson, K. Haneda, S. Wyne and F. Tufvesson, ”On mm-WaveMultipath Clustering and Channel Modeling,” in IEEE Transactions onAntennas and Propagation, vol. 62, no. 3, pp. 1445-1455, March 2014.[4] J. Medbo, H. Asplund and J. . -E. Berg, ”60 GHz channel directionalcharacterization using extreme size virtual antenna array,” 2015 IEEE26th Annual International Symposium on Personal, Indoor, and MobileRadio Communications (PIMRC), 2015, pp. 176-180.[5] M. Li, F. Zhang, Y. Ji and W. Fan, ”Virtual Antenna Array withDirectional Antennas for Millimeter-Wave Channel Characterization,”in IEEE Transactions on Antennas and Propagation, 2022.[6] A. Pärssinen, M.-S. Alouini, M. Berg, T. Kürner, P. Kyösti, M.E. Leinonen, M. Matinmikko-Blue, E. McCune, U. Pfeiffer, and P.Wambacq, “White paper on RF enabling 6G opportunities and chal-lenges from technology to spectrum,” 6G Flagship Ecosystem, Apr.2021. [Online]. Available: https://www.6gchannel.com/items/6g-white-paper-rf-spectrum/[7] Lyu, Yejian ; Yuan, Zhiqiang ; Li, Mengting et al. Enabling Long-RangeLarge-Scale Channel Sounding at Sub-THz Bands: Virtual Array andRadio-Over-Fiber Concepts. In: I E E E Communications Magazine.2023.

Massive MIMO Channel Sounder for Sub-THz Based on Virtual Array Concept

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Massive MIMO Channel Sounder for Sub-THz Based on Virtual Array Concept

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