A 5G Fiber Wireless 4Gb/s WDM Fronthaul for Flexible 360 Coverage in V-Band massive MIMO Small Cells

Abstract

Towards relieving the bandwidth limitations in 5G mobile fronthaul networks, a Fiber Wireless(FiWi) small cell architecture is presented, relying on spectrally efficient analog transport on Intermediate Frequency over Fiber(IFoF) and a millimeter wave(mmWave) Phased Array Antenna(PAA) interfaced with a low-loss Optical Add/Drop Multiplexer(OADM) on Si3N4/SiO2 TriPleX platform with 5 dB fiber-to-fiber losses. Specifically, four 1 Gb/s FiWi links, each carrying a 250 MBd 16-QAM signal on 5.8 GHz IFoF, are Wavelength Division Multiplexed(WDM) and transported across a 10 km Single Mode Fiber(SMF), before being demultiplexed into the constituent channels in the OADM device and subsequently wirelessly transmitted by the PAA over a 1 m-long V-band link with 90 degree beamsteering capability. Featuring 4x 1Gb/s user rate with beamsteering to meet the respective 5G Key Performance Indicator for the peak user rate and an EVM within the acceptable 3GPP limit of 12.5%, the current work forms the first centralized FiWi Point-to-Multipoint small cell architecture with efficient transport scheme on IFoF and ubiquitous 360-degree coverage for emerging 5G mmWave networks.

Full text

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Abstract— Towards relieving the bandwidth limitations
in 5G mobile fronthaul networks, a Fiber Wireless (FiWi)
small cell architecture is presented, relying on spectrally
efficient analog transport on Intermediate Frequency over
Fiber (IFoF) and a millimeter wave (mmWave) Phased
Array Antenna (PAA) interfaced with a low-loss Optical
Add/Drop Multiplexer (OADM) on Si3N4/SiO2 TriPleX
platform with 5 dB fiber-to-fiber losses. Specifically, four 1
Gb/s FiWi links, each carrying a 250 MBd 16-QAM signal
on 5.8 GHz IFoF, are Wavelength Division Multiplexed
(WDM) and transported across a 10 km Single Mode Fiber
(SMF), before being demultiplexed into the constituent
channels in the OADM device and subsequently wirelessly
transmitted by the PAA over a 1 m-long V-band link with
90° degree beamsteering capability. Featuring 4x 1Gb/s
user rate with beamsteering to meet the respective 5G Key
Performance Indicator for the peak user rate and an EVM
within the acceptable 3GPP limit of 12.5%, the current
work forms the first centralized FiWi Point-to-Multipoint
small cell architecture with efficient transport scheme on
IFoF and ubiquitous 360°-degree coverage for emerging
5G mmWave networks.
Index Terms—5G, Mobile Fronthaul, Analog Radio-over-Fiber,
Beamforming, Multiplexing, Point-to-Multipoint.
I. INTRODUCTION
ITH the advent of the 5th Generation era (5G) for
mobile networks, there has been an increasing demand
for ubiquitous high bandwidth mobile connectivity and
a wealth of emerging Use Cases [1], such as enhanced Mobile
Broadband (eMBB) services, augmented reality or Fixed
This work is supported by H2020 5GPPP Phase II project 5G-PHOS
(Contract No. 761989) and 5G STEP-FWD (Contract No. 722429). The
authors would like to acknowledge Keysight for supporting the experiments
with measurement equipment
E. Ruggeri, A. Tsakyridis, C. Vagionas, G. Kalfas, N. Pleros, A. Miliou are
with the Department of Informatics, Center for Interdisciplinary Research and
Innovation, Aristotle University of Thessaloniki, 54636 (e-mail:
eugenior@csd.auth.gr).
Ruud M. Oldenbeuving, Paul W. L. van Dijk, Chris G.H. Roeloffzen are
with LIONIX International B.V., Enschede, The Netherlands.
Y. Leiba is with Siklu Communications Ltd. Petach Tikva, Israel (e-mail:
yigal@siklu.com).
Wireless Access (FWA), driving a burgeoning growth of
mobile traffic [2][3]. Expert alliances have already defined a
set of 5G Key Performance Indicators (KPIs) [4][5], that
dictate user rates up to 1 Gb/s, channel bandwidths of up to
400 MHz, latencies down to 1ms, high connection and traffic
densities etc., stressing significant challenges to the underlying
infrastructure, further exacerbated in hotspots and
overcrowded micro-cellular scenarios, e.g. malls, airports,
squares, etc. Aiming to enhance the Radio Access Network
(RAN) capabilities, millimeter wave (mmWave) frequencies
are being promoted [6][7], migrating from the sub-6 GHz
bands towards larger available spectral bands that will boost
user channel bandwidths up to 400 MHz as currently foreseen
in 3GPP Rel. 15 for 5G networks [5]. MmWave have verified
their high bandwidth credentials, with V-band featuring 7 GHz
continuous spectrum between 57 and 64 GHz operating in an
unlicensed regime [8], while the path loss is compensated by
Phased Array Antennas (PAA) that are integrating MIMO
configurations with up to 256 radiating elements, facilitating
directional beams with beamforming and beamsteering for
intra-cell spatial frequency reuse and reduced radio
interference [9][10].
However, boosting the wireless channels is only one side of
the story, as it raises serious concerns for the efficacy of the
Common Public Radio Interface (CPRI) and Digital Radio
over Fiber solutions [11]. Fronthaul capacity scales
proportionally with the radio-bandwidth, the number of
antennas elements and the sectors of the RRH [12] , requiring
concomitant advances in the optical segment [13] that initially
spurred a speed-up of optical fronthaul interfaces to 25 Gb/s,
e.g. CPRI option 10 line rate [14] and compliant transceivers
[15][16], achieving also uncooled coarse WDM operation at
4x 25 Gb/s. On the other hand, the enhanced CPRI (eCPRI) is
promoting higher layer functional splits to relax the insatiable
data rate demand over the optical interface [17], yet it requires
more complex RRH. Moreover, novel 5G Frequency Range 2
(FR2) above 24 GHz [18] is still forecasted to exploit
extensive capacities, massive MIMO and beamforming [9],
placing unsustainable bitrate scaling for existing commercial
solutions [19][20]. Indicatively a mmWave small-cell with a
128 antenna array radiating an 100 MHz carrier at one sector
would require 78 Gb/s fronthaul rate using split option 2 [21].
To this end, Analog Radio over Fiber (A-RoF) is gaining
increasing momentum as the most spectrally efficient transport
A 5G Fiber Wireless 4Gb/s WDM Fronthaul
for Flexible 360° Coverage in
V-Band massive MIMO Small Cells
Eugenio Ruggeri, Apostolos Tsakyridis, Christos Vagionas, George Kalfas, Ruud M. Oldenbeuving,
Paul W. L. van Dijk, Chris G.H. Roeloffzen, Yigal Leiba, Nikos Pleros, Amalia Miliou
(Top-Scored Paper)
W

2
solution [13], inherently supporting lower latency, simplified
RRH hardware complexity and subcarrier-multiplexing of 20
MHz LTE [22] or even broader channels with CPRI
equivalent rate of 1 Tb/s [23]. For sub-6 GHz bands RoF
transport has been combined with omnidirectional antennas
for 2.4 GHz [24] WLAN applications or transportation of 5
GHz [25] intra-building optical distribution, however
omnidirectional or low gain antennas cannot be used in
mmWave frequencies [26] due to the large path loss, e.g. 15.5
dB/km for the V-Band [27]. For the mmWave frequency
range, Fiber-Wireless A-RoF fronthaul links have achieved
high capacity Point-to-Point transmission through highly
directive static horn antennas [28]-[30] or Point-to-MultiPoint
(PtMP) links using phased array antennas featuring
beamforming and beam-combining, with PAAs [31]-[34] at 28
GHz [32] and 60 GHz bands [33][34], in order to mitigate the
path loss and link distance [26], which comes at the cost of
reduced Field of View (FoV), generally in the order of up to
90° [35][36], requiring multiple beams to provide ubiquitous
360°-degrees coverage.
Scaling from unitary link to multi-user environments, two 3-
user 60GHz uplink transmissions through Frequency (FDM)
or Spatial Division Multiplexing (SDM) were presented in
[35] and a 28 GHz 4x4 MIMO Distributed Antenna System in
[37], transporting up to four mmWave user-beams within a
single RRH sector. Yet, on the path to dense 5G, RRHs are
still incorporating multi-panel and multi-sector PAAs [10],
where each user-radio beam can carry different data stream in
terms of modulation format or frequency, being optically
modulated and transported by different optical transceivers, as
widely performed in field deployments [32][38][39],
underlying the need to further extend the number of fiber-
transported beams or the sector-coverage.
To this end, introducing some form of optical multiplexing
to the antenna site is considered to add one extra dimension in
traffic parallelization [40]. Being already deployed in Metro
networks [41] and Passive Optical Networks (PONs) [42],
WDM has been shown to support 8λ-60GHz access [43], 100
GHz spaced VCSELs or DML transmission [44][45], down to
25 GHz spacing [46], relying exclusively on static mmWave
directional links of horn antennas [43] or even wired setups
only [44]-[46]. Alternative forms of optical multiplexing in
FiWi fronthaul links include Space Division Multiplexing
(SDM) in multicore fibers [47] of 4- [48] or 7-cores [49],
Mode Division Multiplexing (MDM) [50] of 4 modes in few
mode fibers [51] and Polarization Division Multiplexing
(PDM) of 4-polarization modes [52]. While any form of
optical multiplexing can relax the transport limitations, WDM
is the only technique that can operate over field deployed
single mode fiber by inserting simple optical multiplexing
devices at the antenna site [20], avoiding costly brownfield
fiber installation of new specialty fibers and maintaining
compatibility with Fixed Mobile network convergence
[39][53]. Meanwhile, with the growing maturity of Silicon
Photonics (SiPho) achieving to develop low-loss, cost-
effective Si3N4-based waveguide platforms and cascaded
lattice filers [54], low-loss RoF multiplexers [55] and optical
add drop multiplexing (OADM) devices [39][56][57], SiPho-
assisted RRH optical interfaces are already bearing promises
for up to 24 carriers aggregation [51] and reconfigurable
optical fronthaul networks [57]-[62], opening a path towards
efficient multi-user 5G small cell networks with flexible
allocation of various mmWave user radio beams to different
optical carriers and fronthaul transport streams.
In this communication, a 5G FiWi mmWave small cell
architecture with spectrally efficient optical transportation of
four different 90° degree RRH sectors on four different
wavelengths of a 4λ WDM optical fronthaul stream is
presented, achieving for the first time ubiquitous 360° FiWi
small cell coverage, as conceptually depicted in Fig. 1 [56].
Specifically, a 4λ WDM FiWi downlink transmission is
presented, featuring four 250 Mbd 16-QAM waveforms
resulting in about 330 MHz bandwidth and satisfying the
targeted user-data rate of 1 Gb/s in compliance 5G KPIs [5],
each loaded on a 5.8 Intermediate Frequency over Fiber
(IFoF) of a different wavelength, before being transported
through a 10 km single mode fiber (SMF) spool,
demultiplexed by a low-loss 100 GHz 4λ-WDM Si3N4
TriPleX OADM with 5dB fiber-to-fiber losses and radiated
across 1 m V-band beamsteering links by a PAA prototype.
Featuring a 1 Gb/s user rate per FiWi mmWave/IFoF stream
to meet the 5G KPI requirement and an almost-flat EVM
11.03% performance that satisfies the 3GPP requirement [63]
across any wavelength and any angular position of the four
sectors, the current work paves the way towards spectrally
efficient multi-user 5G mmWave C-RAN networks.
II. EXPERIMENTAL SETUP AND DEVICES
The experimental setup of the proposed 5G small cell
architecture, depicted in Fig. 2, comprising four FiWi
downlink transmissions of PAA-based steerable V-band
Fig.
2
. Conceptual schematic of the proposed 5G FiWi small cell architecture,
with
4λ-WDM optical fronthaul and 90°-range beamsteered OTA radiation.
Fig.
1. Experimental setup including a 4λ-
WDM IFoF modulation stage
(light yellow) transmitting through a 10km single mode fiber spool towards
the
OADM/PAA stage in light blue background and 1 m V-band link.

3
wireless beams with 90°-degree steering each, optically
transported on 4λ-WDM IFoF traffic through a 10 km SMF
spool. Initially, the data traffic is generated at the optical
modulation stage of the setup, comprising four Continuous
Wavelengths (CWs), namely λ1-4 at 1545.6nm, 1546.4nm,
1547.1nm and 1548nm respectively, generated by a 4-channel
Tunable Laser Source. The four λ1-4 CW wavelengths are then
multiplexed by an Arrayed Waveguide Grating (AWG),
providing lower optical insertion losses compared to a power
coupler, before being connected to a single LiNbO3 Mach
Zehnder Modulator (MZM) for optical modulation, due to
limited optical components availability, as highlighted with a
light-yellow marker in Fig. 2. The MZM modulator is then
biased at the quadrature point, while its modulation operation
is driven by a Keysight M8195A Arbitrary Waveform
Generator, that digitally synthesizes a 250 MBd 16-QAM data
waveform loaded on an IF carrier of 5.8 GHz using a roll-off
factor of 0.35, which was found to provide linear gain
operational regime for 1 m wireless distance without
saturating the RF components [64]. The 16-QAM waveform is
then amplified to around 5V, before being fed to the electrical
input of the MZM, to generate four Double Sideband
Modulated signals at an optical power of +3 dBm each [65].
After the optical transmission stage, the four λ1-4 WDM IFoF
data are launched into a 10 km long SMF spool, featuring a
typical propagation loss of α=0.25 dB/km and dispersion of
D=17 ps/nm/km, emulating an equidistant fiber propagation
for typical 5G mobile fronthaul networks, inherently providing
propagation losses of around 2.5 dB. Although in field-
deployments [32][38], each IFoF data streams would carry
different traffic and be individually modulated at a different
optical transceiver at the centralized BBU, the 10 km fiber
propagation used here allows decorrelating the four
wavelength 16QAM data signals in–phase, through different
fiber propagation delays, emulating different fronthaul traffic.
Polarization Controllers (PC) were used to tune the
polarization state of the signal previous to both the MZM and
the OADM device stages.
After the fiber propagation, the λ1-4 WDM IFoF data reach
the OADM-assisted PAA antenna site, that is highlighted with
a light blue background in Fig. 2. The data traffic is first
connected to the input port of the Si3N4 OADM, where it gets
demultiplexed to the four individual optical carriers spaced by
100 GHz, in order for each one to emerge at one of its Drop
output ports. Each Drop channel is in turn filtering the
respective singe-wavelength optical carrier plus the two side
bands of the DSB IFoF stream, without significant power
variation across the whole channel bandwidth. Each
demultiplexed wavelength, exhibiting a power of -4.5 dBm in
average, was then sequentially fed to a 10 GHz Avalanche
Photo Diode (APD) photo-receiver with 0.7 A/W responsivity
for opto-electrical conversion, before being supplied as data
input to the V-band beamsteerable PAA subsystem.
At the PAA system, the incoming o/e converted 5.8 GHz
IFoF signal, gets electrically upconverted at a V-band carrier
frequency of 61.3 GHz and subsequently transmitted Over the
Air (OTA) across 1 m distance. After wireless propagation,
each radiated signal was then captured by a portable V-band
horn Rx antenna assembly that featured 22.5 dBi gain
pyramidal horn passive element and an integrated down-
conversion stage, fed by an external 10 GHz Keysight
E8257D Local Oscillator (LO) signal. Finally, each
wirelessly-received and down-converted signal was fed to a
Signal Analyzer for evaluation in terms of EVM figures. A
more detailed description of the OADM and the PAA is
provided in the following subsections.
A. Si3N4 TriPleX Optical Add/Drop Multiplexer (OADM)
The employed low-loss Si3N4 OADM device was designed,
fabricated and packaged by LIONIX International [66], as
shown in the top view photo of the packaged device in Fig.
3(a) and the respective zoom in of the Photonic Integrated
Chip (PIC) in Fig. 3(b). The OADM features four add/drop
wavelengths in the C-band with a channel spacing of 100
GHz, relying on a novel MZI-based interleaver lattice design
layout, as well as a through port that allows connecting
additional channels to another cascaded antenna site.
Specifically, the circuit-layer design is shown in Fig. 3(c)
relying on six stages of cascaded dual-MZI based building
blocks, which are in turn represented in the schematics of Fig.
3(d). The four λ1-4 wavelengths are fed to the input port of the
first dual asymmetric MZI cascade, having a differential arm
length of 872.1 μm that corresponds to a Free Spectral Range
(FSR) of 200 GHz and separating the data traffic into pairs of
odd [λ1, λ3] and even [λ2, λ4] carriers, that respectively emerge
at the upper and lower branch of the structure. In turn, each
pair of wavelengths is propagated towards the second stage of
the design with four dual asymmetric MZI interleaver blocks,
each one featuring half the differential arm length of 436 μm
to correspond to an FSR of 400 GHz, in order to drop one of
the 100 GHz incoming wavelengths. For instance, the first
block of dual-MZIs at the upper arm of the layout of Fig. 3(c)
drops λ1, while the second drops λ3 and equivalently the lower
Fig.
3. a) Packaged OADM top-view photo. b) OADM PIC Zoom-
in. c)
OADM circuit
-
layer design, based on a cascaded configuration of d) MZI-
based interleavers
building blocks on a low-loss Si3N4
TriPleX waveguide
platform
. e) OADM
spectral response including the four Drop channel output
transfer functions superimposed with different coloring

4
branch drops λ2 and λ4, achieving the desired demultiplexing
operation. Additionally, in order to ensure a flat-top response
and compensate any non-ideal filter transfer function due to
fabrication processes, the MZI based building blocks relied on
tunable optical couplers, as illustrated in detail in Fig. 3(d),
allowing to obtain the desired transfer function by tuning their
coupling coefficients, i.e. through Phase Shifters (PS), that can
adjust the spectral response positions of the asymmetric MZI
filtering. Moreover, the thermo-optic couplers allow for fine
wavelength granularity and reconfiguration of the PIC
reconfigurable C-RAN fronthaul network architectures [62].
The complete design parameters and layouts are discussed in
more detail in [66]. The current layout design has been
fabricated on the Si3N4/SiO2 TriPleX platform [66], with
waveguide propagation losses as low as 0.1 dB/cm. The
fabricated PIC was fully assembled on a Temperature
Controlled (TEC) Printed Circuit Board (PCB) submount and
electro-optically interfaced with wire-bonds and a fiber array
in a bench-top package, to facilitate easier testing. Similar
integrated photonic packaged OADMs with fiber-to-fiber
losses in the range of 10 dB [59] or polarization insensitive
operation [67] are currently being developed towards
reconfigurable 5G C-RAN architectures.
Initially, the packaged OADM device was passively
characterized in term of transfer function for each channel and
losses. An Amplified Spontaneous Emission (ASE) lase
source was used to generate a wide flat spectral spectrum in
the C-band that was fed to the input port of the PIC OADM
and the output of each of the Drop channel, after being tuned
to properly demultiplex each wavelength, was recorded using
an Optical Spectrum Analyzer (OSA). The four obtained
transfer functions of the channels are depicted in Fig. 3(e),
superimposed and with different coloring. All four channels
featured a similar flat-top response with 32.5 GHz pass-band
and 3dB bandwidth of 0.66 nm, along with a wavelength
spacing of 0.8 nm between the peaks of neighboring channels.
Moreover, the static crosstalk defined at the central peak of the
pass-band compared to the stop band of adjacent channel was
at least 18 dB, while the fiber-to-fiber losses 5 dB. The current
transfer function allows both the wavelength carrier and the
two sidebands of the IFoF DSB signal to pass through each
channel, while staying intact and not distorting the RoF signal.
B. V-band Phased Antenna Array (PAA) prototype
The employed mmWave PAA prototype is operating in the V-
band range of 57-64 GHz and has been developed by SIKLU.
It is composed of a Control PCB and of a Tile Feed PCB
board, as depicted in Fig 4(a), from left to right, respectively.
The Control PCB is interfaced to a PC using a Qualcomm
QCA9008 Modem and drives the Tile Feed PCB operation,
allowing to activate or deactivate any of the antenna array
elements, as well as controlling the phase term of its incoming
signal constituent. On the other hand, the Tile Feed PCB hosts
the 32-element array on its front panel, as depicted in Fig. 4(b)
and referred to as Tile PCB, along with the required analog
upconverting RF IC circuitry, i.e. V-band mixer and x6
multiplication stage. Specifically, the 5.8 GHz incoming IF
input data signal is fed to the mixing stage to be upconverted
to the V-band carrier frequency of 61.3 GHz by means of an
external 10 GHz LO signal, provided by an Anritsu
MG3692B, split in 32 channels through an 1:32 splitter, with
each one employing a Low Noise Amplifier (LNA) and an
electrically tunable φ-phase Phase Shifter (PS), before
reaching each of the 32 radiating elements of the Tile PCB.
Due to the high targeted frequency, the substrate of the 32-
element Tile PCB array is integrated on a low-temperature
ceramic with each radiating element being a 6 dBi dipole and
featuring an almost isotropic radiation across a 120° angle.
The PAA design allows for control of the φ-phase shifters
and to turn ON/OFF each of the 32 elements independently,
enabling its operation duality: i) radiating isotropically within
a 120°-degrees range when only one single array element is
active, or ii) beamforming towards a desired spatial direction
when all elements are active simultaneously, while steering
the beam by properly tuning the 32 φ-phase shifters.
Specifically, the latter is achieved by exploiting constructive
interference OTA between the 32 wavefronts simultaneously
radiated, resulting in a 10°-width highly directional beam,
steerable with negligible penalty among a 90°-degrees angular
range. The operation of the PAA has been experimentally
tested with 32QAM data transmission [64] and has shown to
support two multi-user scenarios, either through isotropic
PAA operation with FDM user beams within a 90° degree
sector or through beamsteering towards the end user with
SDM of various parallel beams, as experimentally presented
for three user QPSK uplink transmission scenarios in [33].
During operation, the PAA system is provided with a 5.3 V
Fig. 5. a) Phased Array Antenna (PAA)
transmitter subsystem photo,
depicting the Control PCB (left side) and the Tile Feed PCB
(right side).
b) 32-elements array Tile zoom-in.
Fig.
4. Single-
wavelength Fiber Wireless (FiWi) fronthaul link in isotropic or
beamsteering mode:
a) Top view of the
angular location of the portable V-
band horn receiver placed at +
45° degrees
. b) Constellation diagrams for the
received signals, after both optical propagation and OTA transmission:
isotropic
(ISO) on the left, 45°-beamsteered (BS) on the right.

5
DC voltage supply, consuming an external current of 1.2 A for
the isotropic transmission, when only one radiating element is
active, while during beamsteering operation, when all 32
radiating elements are active, a current of 1.6 A is drawn.
III. SINGLE FIBER WIRELESS FRONTHAUL LINK
Initially, two single-λ cases are presented in a single
wavelength FiWi setup, providing a comparison between the
isotropic and beamsteered wireless transmission in terms of
EVM figures for the downlink operation at first, then followed
by a characterization of the beamsteering range, as well as a
reference transmission for the next sections. This is
experimentally implemented by activating only one CW λ1 of
the TLS, that is directly connected to the LiNbO3 MZM
modulator, while the IFoF optical downstream after
propagating through the 10 km SMF spool is connected
directly to the APD Rx, i.e. bypassing the OADM.
A. Fiber Wireless link with beamsteering or isotropic
radiation
In this first investigation, the transmission stage was
configured to perform single-λ fronthauling of 250 MBd 16-
QAM modulation format, corresponding to 1 Gb/s user data
rate as required by 5G KPIs [4] and used in similar recent
beamforming experiments [68], the PAA OTA performance
was evaluated for both beamsteered and isotropic
transmissions. Moreover, the performance of the finally
received constellation diagram for FiWi downlink with
isotropic-beam transmission is compared to the received signal
in beamsteering operation directed towards a portable antenna
at +45° degree angular location, as depicted in the photo of the
experimental setup in Fig. 5(a).
The constellation diagrams of the received signal of the 250
MBd 16-QAM FiWi transmission for the isotropic PAA
configuration is presented in Fig. 5(b), featuring an EVM of
13.9 %. On the contrary, the received constellation diagram of
the beamsteering case at +45° degrees is shown in Fig. 5(c),
exhibiting an EVM of 10.04%. It is worth noting that this
3.86% EVM improvement of the beamsteering mode
compared to isotropic transmission can be mainly attributed to
the beamforming operation of the 32 radiating elements,
achieving higher antenna gain and enhanced concentration of
the wireless received power at the receiver side. Using this
single-λ scenario, the proposed FiWi link can deliver 1 Gbps
user rate per beam, meeting the respective 5G KPI for the
maximum user rate, while the EVM performance in the
beamsteering operation also satisfies the 3GPP EVM signal
quality requirement of 12.5% for a 16-QAM signal [63].
B. Angular sector scan of the fiber wireless link
The performance of the single-λ FiWi link was benchmarked
against delivering 1 Gb/s user data rate across all angles of a
90°-degree sector, with the transmitted waveform being again
a 250 MBd 16-QAM. The PAA was configured in
beamsteering mode and its beam was scanned between -45°
and +45° degrees with steps of 15° degrees, performing
transmission at seven different angular positions of the
portable horn receiver, while its distance was kept constant at
1 m of V-band link.
The received signals and their constellation diagrams were
evaluated at all seven angular locations and the EVM; figures
are plotted in Fig. 6, resulting in a spectral efficiency of 3.1
b/s/Hz. Moreover, the measurement revealed an almost flat
EVM value and nearly equal performance at all angles within
a 90°-sector, with an average EVM of 11.07% well below the
12.5% EVM limit defined by 3GPP [63].
IV. WDM FIBER WIRELESS LINKS FOR 360° CELL COVERAGE
Towards demonstrating 4x WDM fronthaul, initially a
thorough experimental study of the crosstalk was investigated
in the optical fronthaul segment for different modulation
formats transported on different wavelengths and modulated
by separate dedicated modulators. Then, the full 4x WDM
traffic is injected into the end-to-end FiWi link with a single
MZI modulator of Fig. 3 due to limited component availability
to demonstrate the 360° coverage 5G small cell architecture.
Fig.
6 Experimental results for single-λ
beamsteered transmission of a 250
MBd 16
-QAM signal at angles from -45° to +45° with
steps of 15° degrees.
The inset shows the constellation diagrams corresponding to the wors
t
case
of
11.78% EVM at +15° degrees.
Fig. 7. a) Superimposed optical spectra of the 4 demultiplexed optical
signals before the APD, n
ormalized to 0dBm peak power. b
) Constellation
diagrams of the re
ceived signal for λ2 channel,
after optical propagation,
demultiplexing and V
-Band beamsteered transmission at -45°, 0° and +45°.

6
A. Crosstalk study with two IFoF wavelength data streams
Prior to full 4x WDM traffic transmission, the authors have
performed a crosstalk evaluation employing two wavelengths
being modulated by as many different optical modulators,
towards investigating the impact of crosstalk in case of a
potential real system implementation, where separate optical
transmission stages are modulating different radio streams as
shown in the experimental setup in Fig. 8(a). Two
wavelengths were separately generated at λ2=1546.4nm and
λ3=1547.1nm, with 0.7nm of separation, as in the 4xWDM
traffic presented in the next sub-section. The two wavelengths
were fed to two different MZMs and independently modulated
by two 250 MBaud waveforms, carrying 16QAM on λ2 and
QPSK on λ3, which are then combined by a 2x1 coupler into
2x wavelength traffic and propagated through the 10 km of
SMF before reaching the APD. The power of wavelength λ2
was fixed at -4.5 dBm when reaching the APD, which was
demodulated and recorded by the S.A., while the power of λ3
was gradually attenuated to generate increasing levels of
crosstalk by completely decorrelated data traffic, aiming to
study the degradation of the λ2 in terms of EVM.
The experimental results of the crosstalk investigation are
plotted in Fig. 8(b) and (c). Fig. 8(b) reveals the EVM values
recorded for λ2 corresponding to crosstalk generated by tuning
and lowering the average input optical power of λ3 with
respect to λ2 with a thorough step of 1 dB. As it can be seen,
the IFoF link is showing negligible EVM penalty lower than
0.23% for low crosstalk values from -26 dB up -15 dB,
indicating that the λ2 16QAM data transmission is not
impacted by the presence of λ3, featuring stable and equal
response as in the case of a single λ2 FiWi stream at an EVM
of around ~3.3%. Moreover, a 1% EVM penalty for the λ
2
16QAM data transmission was recorded for a crosstalk of -12
dB. Indicatively, three constellation diagrams of λ2 are shown
in Fig. 8(c) for crosstalk values of -9, -15 and -26 dB, where it
can be seen that the latter two were successfully demodulated
at the SA, with negligible noise around the ideal symbols. The
current crosstalk measurements reveal negligible EVM
degradation for crosstalk values worse than -15 dB, after
which the EVM curve exhibits flat and stable performance.
Similar theoretical [69] and experimental studies [70][71] also
indicate that FiWi RoF and IFoF WDM streams with crosstalk
of -15 dB can be successfully demodulated with EVM values
within the acceptable EVM limits.
B. 4x WDM IFoF fiber wireless link
Following the crosstalk studies, we used the experimental
setup of Fig. 2 while the proposed FiWi system was then
investigated in 4λ-WDM optical fronthaul operation with the
data traffic being demultiplexed by the OADM and wirelessly
transmitted. For the downlink operation, the OADM is used
here simply as an AWG demultiplexer. After the
demultiplexing operation of the OADM, the four Drop
channel outputs were recorded with an OSA, as depicted in the
optical spectrum of Fig. 7(a), where the four channel spectra
are superimposed and marked with four different colors. The
four received wavelengths featured an almost equal power
level of around -4.5 dBm, while the 0.66 nm wide bandwidth,
flat-top channel response and small crosstalk of the OADM
channels allowed for clear demultiplexing of the IFoF
wavelength streams without significant power imbalance or
impairment of the transported DSB signals.
Channel λ2 exhibited the highest optical crosstalk of around
-15 dB among the four channels. Its optical signal output of
the OADM was opto-electrically converted and fed to the
PAA for V-band beamsteering transmission towards the center
of its sector at 0° angle and the two edges at -45° and +45°.
After the V-band wireless reception and down-conversion by
the horn receiver, the λ2/V-band data signal was demodulated
and recorded. The thee constellation diagrams are depicted in
Fig. 7(b), featuring EVM values of 10.44%, 10.37% and
10.53%, respectively for -45°, 0° and +45° degrees. The
measurements confirm again a negligible penalty due to the
Fig.
9.
Polar representation of the 24 experimentally captured EVM values
for the 360° coverage. Color
-coding each sector with the corresponding λ
and highlighting the 3GPP EVM threshold in red.
Fig. 8. Crosstalk
experimental investigation employing 2-λ and
two
separate optical transmitting stages, including: (a) dedicated experimental
setup
, (b) plotted experimental results of EVM vs Crosstalk along wit
h (c)
constellation diagrams of the captured EVMs corresponding to crosstalk
values of 9, 15 and 26 dB, respectively.

7
variation in steering angle with an nearly-equal behavior,
while still meeting the 5G KPI user data rate requirement and
the 3GPP EVM limit of 12.5% for a 16-QAM [63].
Finally, towards demonstrating the proposed FiWi system
as suitable for a 360° coverage 5G small cell architecture, each
of the channels of the 4λ-WDM optical fronthaul traffic was
transmitted OTA at different 90°-degree sector, performing a
complete 360°-cycle scan of the beamsteering at angular steps
of 15°-degrees. The 15°-degrees step value was
experimentally found to be the minimum value in order to
appreciate significant performance variation, which is also
compliant to the 10°-degree beamwidth of the portable
receiver, resulting in a thorough investigation employing 24
combinations of four wavelength and six angular locations.
The 24 captured values are presented in the 360°-degrees polar
representation of Fig. 9 by the blue points, with each λ,
corresponding to each 90°-degrees sector. The red dashed line
highlights the acceptable EVM limit of 12.5% for 16-QAM set
by 3GPP, while the inner and outer circles of the
representation correspond to 9% and 14% EVM values,
respectively. The resulting plot reveals an average EVM of
11.03% among all the 24 acquired measurements of the 250
MBd 16-QAM data signal transmission, satisfying both 5G
KPIs for user data downlink and 3GPP’s EVM threshold for
every value of the proposed 360°-degrees coverage. By using
the proposed configuration, a single optical IFoF stream can
be mapped to a single beam that is transmitted by the 32-
element antenna tile. Nevertheless, towards additionally
scaling to higher number of users or beams per sector, placing
multiple 32-antenna tiles in a larger massive MIMO antenna
front-panel would be required for each Small Cell sector,
where each of the tile would then take a different data input
and would generate a separate beam, as demonstrated for 2
[72], 4 [73] or 12 beams [74] by large PAA demonstrations
carrying up to 288 elements, at a trade-off of increased cost
and complexity. The proposed FiWi small cell architecture
requires tuning the wavelength of four lasers to align with the
OADM channels and expand the number of downlink
transmitted IFoF beams, while for the uplink case it could
potentially be combined with recently developed uncooled
reflective devices for colorless uplink [75].
V. CONCLUSIONS
An experimental demonstration of a PtMP FiWi
mmWave/IFoF small cell architecture was presented,
employing a V-band PAA interfaced with analog optical
transmission on a IFoF and a low-loss Si3N4/SiO2 TriPleX
OADM device, resulting in four FiWi 90°-steerable fronthaul
links that are covering four different sectors. The 4λ WDM
IFoF fronthaul data traffic, carrying 250 MBd 16-QAM per
wavelength, was successfully demultiplexed by the OADM
and transmitted in the V-band across any angle of the FiWi
small cell architecture, scanning a complete 360° cycle at a
step of 15°-degree and achieving EVM values that satisfy the
3GPP requirements. Delivering 1 Gb/s mmWave beams that
meet the respective 5G KPI for the user rate and ubiquitously
covering a 360° cycle with flexible beamsteering, the current
work allows assigning various spectrally efficient IFoF optical
streams to individual RRH sectors, relaxing the mobile
fronthaul bandwidth limitations and paving the way towards
5G mmWave small cell environments with high user density.
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