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Multi-user IFoF uplink transmission over a 32-element 60GHz phased array antenna enabling both Frequency

and Spatial Division Multiplexing Eugenio Ruggeri1, Christos Vagionas1, Nikolaos Karagiorgos2, Apostolos Tsakiridis1, Yigal

Leiba3, George Kalfas1, Agapi Mesodiakaki1, Kostas Siozios2, Amalia Miliou1, Nikos Pleros1

1 Aristotle University of Thessaloniki and Center for Interdiplinary Research and Development Dep. of Informatics, 54124, Thessaloniki, Greece

2Sector of Electronics and Computers, Physics Department, Aristotle University of Thessaloniki, Thessaloniki, Greece

3Siklu Communication Ltd., Petach Tikva 49517, Israel eugenior@csd.auth.gr

Keywords: 5G, Mobile Fronthaul, Analog Radio-over-Fiber, Beamforming, Multi-User.

Abstract

We experimentally present the A-RoF/IFoF uplink transmission of three 60GHz terminals with 200Mb/s QPSK through a 32-element antenna with beamsteering up to a 60o-angle and 10km fiber, comparing for the first time the frequency and spatial division multiplexing for multi-user uplink for 5G mmWave networks.

1 Introduction

The demand for ubiquitous broadband connectivity has been driving an immense growth of the mobile network towards the 5G era [1]-[3]. Centralized Radio Access Network (C-RAN) architecture enables cost-effective deployment through converged Fiber-Wireless (FiWi) Point-to-Multipoint (PtMp) topologies between a single powerful Base-Band Unit and various simple Remote Radio Heads [3]. Meanwhile, first trials and studies focus on crowded hotspots with enhanced mobile broadband (eMBB) services to multiple users, e.g. at malls, stadiums and railway [4]-[6] or dense urban areas with Fixed Wireless Access (FWA) [7][8], as the most probable 5G roll-outs [2], where expert alliances foresee uplink rates of 50Mb/s per user regardless of the connection densities [1]. To enable this, 5G are promoting the development of several technologies, with the most prominent being massive MIMO and large scale phased array antennas with up to 256 elements, performing beamforming and steering for spatial frequency reuse [9] and mmWave with large available spectrum for broad channels [10], while for the optical MFH, analog Radio over Fiber (A-RoF) schemes aim to overcome the bandwidth penalty of CPRI, being capable to carry several user channels with 1Tb/s CPRI equivalent rate [11].

Combining some of the discrete technology developments so far, various recent research efforts achieved optimized unified FiWi links, loading various mmWave user bands on Intermediate Frequency over Fiber (IFoF) with Frequency Division Multiplexing (FDM) for aggregate capacities up to 4.56Gb/s in the 28GHz band [12], 24Gb/s in the 60GHz band [13][14] and 45 Gb/s in the 98 GHz band [15], yet relying on single Point-to-Point (PtP) link between two fixed directional antennas, e.g. horns, with static gains and beam directions, favouring PtP backhaul links without any flexible steering

capabilities. The latter was only recently achieved by a recent prototype of a single downlink of five 125MHz channels by a 28GHz antenna [16], extended later in a 2x2 MIMO system [17], while a 4-channel beamforming antenna was shown to support up to 60o degree steering in A-RoF transmission [8]. Scaling from single user to multi-user environments, the reduced out-of-band leakage of four windowed OFDM waveforms [18], the non-orthogonal multiple access scheme between two users [19] or a neural network equalizer of the impairments of four FDM signals [20] showed promising results for the 60GHz band, yet still running on fixed horn antennas. Finally, a FiWi multi-beam downlink demo this year achieved simultaneous transmission of 10 FDM signals through a Leaky Wave Antenna [21], which still however is limited to certain fixed directions only due to static passive frequency selective steering, without any spatial steering, thus failing to achieve frequency reuse across the FiWi link.

In this work, we experimentally demonstrate the FiWi A-RoF IFoF uplink transmission of three 60GHz terminals deploying an analog phased array V-band antenna with 32 radiating elements, a MZI modulator and 10km fiber spool. The FiWi link is initially statically characterized in terms of supported spectrum featuring a 5dB bandwidths of 1.5GHz. This is followed by single user transmission of 200 Mbd QPSK from a client terminal placed at three different angles of 15o, 30o and 60o, achieving up to 400Mb/s per user with negligible EVM distortion for 120o angle coverage, for application in three sectoral 5G antennas. The FiWi link is then tested in FDM scenario where three users transmit simultaneously 100Mbd QPSK at different frequencies to the 32-element antenna, as shown in Fig. 1(i), and is compared to the Spatial Division Multiplexed (SDM) case of Fig.1(ii), where the three users simultaneously transmit at the same frequency but the 32-element antenna is beam-formed and

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steered to select only one only stream, showcasing frequency reuse. To the authors’ knowledge, this is the first multi-user IFoF/A-RoF uplink demonstration over a 32-element antenna enabling both FDM and SDM for 5G mmWave networks.

2 Experimental Setup and 60GHz antenna

The experimental setup used is shown in Fig. 2(a), including three horn antennas Tx stages, Tx1,2,3 that emulate three end-point terminals at 60.33GHz, 60.64GHz and 59.43GHz, and the 32-element beamsteering Rx antenna, followed by a 10-km fiber-optic link. Initially the data traffic is generated by two Arbitrary Waveform Generators (AWGs), a Keysight M8195A feeding each of the Tx2 and 3 wireless terminals, and a Keysight M8190A for Tx1, while all three antennas utilize a differential data input (𝐼/𝐼 ̅ and 𝑄/𝑄�) followed by a built-in I/Q modulation stage. Tx2 and Tx3 comprise a gotMIC’s gTSC0020 V-Band up-conversion stage assembled on a driver board with a 400MHz bandwidth, that is also fed with a 9.5GHz local oscillator from a PmT PLL. The driver-upconversion stage is powered by two DC sources at -5V and 5V and the PLLs by a 13V supply. The output of the gotMIC upconversion stage was connected to a Quinstar horn antenna with 22.5dBi gain, while the third terminal comprised a Siklu antenna with similar upconversion stage and integrated local oscillator, featuring a 36dBi antenna, and 500MHz baseband input. The three antennas were fed with a QPSK signal at 100Mbd symbol rate, resulting in 200Mb/s uplink per user, and an aggregate uplink data rate of 0.6 Gb/s.

The signals transmitted by the three users at 80cm distance were received by the Rx array antenna with broadband operation in the 57-64 GHz range with 32 radiating elements, comprising a Tile PCB and a Tile Feed Board PCB. The Tile PCB has been integrated on a low-temperature ceramic with each of the 32 dipoles featuring a 6dBi gain with almost isotropic radiation across a 120o angle when being the only one actively radiating. When all 32 elements are active, the antenna can be configured to receive from one direction with a 2o beamwidth and steered from -60o and 60o. In order to achieve this, the output of each dipole is feeding a line with a Low Noise Amplifier (LNA) followed by a φ-phase shifting element. The 32 lines are then combined by a 32:1 combiner, followed by an integrated downconversion stage, that generates a 5GHz IF output of -2dBm. The antenna is also

fed by a 15dBm local oscillator (LO) signal at 10GHz. A photo of the experimental setup is shown in Fig. 2(b), while the insets at Fig. 2(c) and (d) show a closer look at the board-assembly of the antenna and the 32-element Tile.

The simultaneous wireless transmission from the three terminals was tested in two configurations: i) an FDM scenario with one of the 32 elements active to receive isotropically from all three antennas at different frequencies across a 120° sector and ii) an SDM scenario where the three Tx antennas were simultaneously transmitting at the same frequency with the beamforming antenna steered either at 50o, 0o or -50o to receive exclusively one of the three streams. Supplied with 5.3V, the Rx assembly was consuming 1.2A in isotropic mode, but when configured in beamsteering mode with all 32 elements on, it consumed a current of 1.6A.

The wireless links was then extended with an fiber-optical transmission of up to 10km fiber spool. Specifically, the received signal of the Rx antenna was down-converted to 5GHz at its output and driver to an optical transmission stage. Specifically, the -2dBm output (0.5Vp-p) of the Rx antenna was amplified to 5V by a driver amplifier for driving a zero chirp LiNbO3 Mach-Zehnder Modulator (MZM) biased at the quadrature point, modulating the CW output of a 1550nm optical carrier of a DFB Laser Source (LS). The optical IFoF signals was propagated either though a short fiber-patchcord or a 10km SMF spool with 0.2 dB/km losses, emulating the optical MFH distances. At the end of the fiber link, the signal is o-e converted by 10G InGaAs Avalanche Photo-Receiver (PD). Finally, the PD output is captured by Signal Analyser (SA) for monitoring purposes. For the SDM, the setup is the same and the three antennas transmit at the same frequency.

Fig. 3 (a) Fi-Wi link output power vs IF output frequency. (b), (c), (d): Constellation diagram and EVM values of single user uplink at 15°, 30° and 60° incident angle.

Fig.2 (a) Experimental setup of the three user FiWi uplink, (b) Photo of the setup showing the three client terminals and the 32-element Rx (c) the Rx assembly with the Tile PCB and the Tile Feed PCB and (d) the 32-element Tile.

Fig. 1 Multi-user FiWi uplink: i) FDM with isotropic reception at different IFs and ii) SDM with beamforming.

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3 Results

The FiWi link was firstly statically characterized in terms of supported bandwidth and available frequency spectrum, transmitting a single tone with the Tx1 antenna only, and measuring the received power after the PD. The tone frequency was swept from 3.5GHz to 6GHz, which was translated in sweeping of the wirelessly transmitted mmWave carrier from 58.6 to 61.3 GHz. The received power at the SA is plotted, as function of frequency, in Fig. 3(a), featuring a gain-peak at 4.6GHz, i.e. the 59.7GHz. Based on this plot, the end-to-end FiWi link exhibits a 3dB bandwidth of 0.95GHz and a 5dB bandwidth of 1.5GHz, allowing for enough bandwidth to allocate to more than one wireless user. The link was then evaluated in a single user data transmission from various angles of incidence, aiming to evaluate the impact of the different antenna configurations with all 32 elements active. As such, the beam was steered at different angles of 15°, 30° and 60° degrees and the Tx client terminal was transmitting a QPSK signal at 200 Mbd. The obtained constellation diagrams are shown in Fig. 3(b)-(d), featuring EVMs of 12.84%, 13.21% and 13.71%, i.e. a very small deviation with less than 1% EVM penalty, and being well within the acceptable limit of 17.5% suggested by 3GPP for NR system [22], even for the challenging 60o degree angle.

The FiWi link was then tested in a three user FDM scenario using 100 Mbd QPSK signals. The corresponding mmWave frequencies of the Tx1,2, 3 were 60.33GHz, 60.64GHz and 59.43GHz placed at an angle of 30°, 0° and -30°, while the Rx was set in isotropic mode. The constellation diagrams of the received signals after the 10km fiber propagation and the PD are shown in Fig. 5, featuring EVM values of 19.50%, 20.68% and 20.25%. It is worth noting that these results have been obtained for IQ data inputs at Tx1,2,3 of 190mV, 500mV and 350mV, resulting in average power levels for each channel with less than 5dB power variation after the FiWi link, as shown in the RF spectrum in Fig. 4(d) while the EVM values were recorded as RMS values at the SA without any DSP for channel estimation, predistortion or equalization.

Finally, the link is evaluated also in a three user SDM scenario, where all three Tx channels were set at the frequency of 60.33GHz. The received constellation diagrams

are shown in Fig. 5, revealing that all three data streams were successfully received by deploying SDM beamsteering and clearly demodulated despite the use of the same frequency channel, while the EVM values recorded were 16.80%, 14.26% and 18.91%. The received RF spectrum is shown in Fig, 5(d) with all three Tx channels superimposed and overlapping at the same frequency. The results reveal that performing SDM through beamsteering seems to provide lower EVM values due to the increased antenna gain with the lobe directed towards the transmission of one only user, rejecting the other two but FDM with subcarrier multiplexing allows accommodating multiple simultaneous users. All the FiWi links have been tested also without the use of the 10km spool showing similar results with 1.06 EVM improvements. 4 Conclusion

The first multi-user IFoF/V-band FiWi uplink was presented for 5G mmWave networks, employing three horn antennas to transmit 100Mbd QPSK each to a 32-element beamforming antenna, followed by a 10km fiber spool, enabling the use of Frequency or Spatial Division Multiplexing. Although both FDM and SDM exhibited clear constellation diagrams, the SDM achieved lower EVM through beamsteering to one user, while FDM favors co-existence of three simultaneous users.

5 Acknowledgements

The work is supported by H2020 5GPPP Phase II project 5G-PHOS (Contract No 761989) and 5G STEP-FWD (722429).

Fig. 4 Three user FiWi with FDM: (a)-(c) Constellation diagrams and EVM of Tx1,2 and 3 and (d) RF spectrum

Fig.5 Three user FiWi with SDM: (a)-(c): Constellation diagrams and EVM of Tx1,2 and 3 and (d) RF spectrum.

Fig. 3 (a) Fi-Wi link output power vs IF output frequency. (b), (c), (d): Constellation diagram and EVM values of single user uplink at 15°, 30° and 60° incident angle.

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