Orthogonal Chirp-Division Multiplexing for
Future Converged Optical / Millimeter-Wave Radio Access Networks
Selection of constellation signals
received over the full optical and wireless transmission link with a
ROP of 0 dBm; (a) wideband 16-QAM OCDM, (b) wideband 16-QAM OFDM
with FDE, (c) mobile 256-QAM OCDM and (d) mobile 128-QAM OCDM.
Envisaged network scaling in the beyond 5G and 6G era makes the
optical transport of high bandwidth radio signals a critical aspect
for future radio access networks (RANs), while the move toward
wireless transmission in millimeter-wave (mm-wave) and terahertz
(THz) environments is pushing a departure from the currently
deployed orthogonal frequency division multiplexing (OFDM)
modulation scheme. In this work, the orthogonal chirp-division
multiplexing (OCDM) waveform is experimentally deployed in a
converged optical/mm-wave transmission system comprising 10 km
analog radio-over-fiber (A-RoF) transmission, remote mm-wave
generation and 2 m wireless transmission at 60 GHz. System
performance is evaluated in terms of both bit error ratio (BER) and
error vector magnitude (EVM) for a wideband 4 GHz 16 Gb/s signal and
128/256-Quadrature Amplitude Modulation (QAM) mobile signals
compatible with 5G new radio numerology. OCDM is shown to outperform
OFDM by offering enhanced robustness to channel frequency
selectivity, enabling performances below the forward error
correction (FEC) limit in all cases and exhibiting an EVM as low as
3.4% in the case of the mobile signal transmission.
The delivery of 5G
mobile communications is well underway and is driving network
evolution today. From the current standpoint it is difficult to say
exactly what will constitute the 6G era of mobile communications but
it is clear that, as well as the key drivers of increased bandwidth,
lower latency and enhanced connectivity, future wireless
communications must facilitate scaling in a way that supports a vast
array of user types (human and otherwise) in potentially challenging
environments. This points to the ‘super-convergence’ of emerging
flexible radio and optical technologies and network types which are
capable of providing a variety of extremely high-throughput services
in diverse environments. 5G’s frequency range 2 (FR2) has
standardized the use of mm-wave frequencies up to 48 GHz in order to
deliver enhanced mobile channel bandwidths of up to 400 MHz. This
trend looks set to continue with higher frequency mm-wave ranges
(around 60 GHz and 90 GHz) and THz frequencies (0.1 - 1 THz) being
identified as key platforms for developing high bandwidth and multi-Gb/s
wireless technologies for ‘beyond’ 5G and 6G. Indeed, outside of
current mobile standards, WiGig protocols, including the recently
released 802.11ay standard, specify multi-GHz mm-wave channel
bandwidths for the delivery of 10’s Gb/s per channel in the 60 GHz
unlicensed band.
The key optical infrastructures charged with the delivery of
broadband and mobile services are the passive optical network (PON)
and the RAN, respectively. A central unit (CU), distributed unit
(DU) and remote unit (RU) constitute the three main functional
elements of a RAN. Various RAN topologies can be implemented through
the relative distribution of these elements along the path from the
RAN’s central office (CO) to the antenna site, and these are well
summarized in. A more centralized RAN (C-RAN) design, allowing the
CU and DU functionalities to be co-located at the CO, is preferable
in cases where the footprint and cost of the antenna site are of
primary concern. Considering the vast proliferation of antenna sites
envisaged beyond 5G, and the antenna site complexity synonymous with
the transmission of very high frequency carriers, it is likely that
a highly centralized RAN architecture will be required to facilitate
scaling for wireless networks incorporating THz and mm-wave
functionality. Increased centralization of RAN resources at the CO
emphasises the role of RoF transport of mobile data between the CO
and the RU (i.e. fronthaul) which can span 10–20 km’s. As the number
of users and antenna sites grow, the associated increase in
fronthaul capacity demands has shifted focus away from simplistic
point-to-point fronthaul links, to those harnessing optical access
networking technologies such as wavelength division multiplexing
(WDM). Unsurprisingly, these trends have led to proposals for the
co-operation of PON and C-RAN infrastructures whereby RAN traffic
can be provisioned over a PON topology — and indeed such operation
is included in the ITU G-series recommendations.
The recent deployment of enhanced-common public radio interface (eCPRI)
technology has provided some improvement in spectral efficiency for
current fronthaul transmission, but this is ultimately limited by
the binary modulation scheme currently employed. An alternative
approach is to use Analog (i.e. multi-carrier) RoF (A-RoF), whereby
the mobile signal is transmitted over the fronthaul link in its
native multi-carrier format. At the cost of decreased robustness to
system nonlinearity, this technique provides excellent spectral
efficiency and results in the simplification of the RU site compared
to a D-RoF approach — ultimately facilitating network antenna
‘densification’.
The authors’ prior works have examined how remote mm-wave generation
through optical means can be incorporated into a C-RAN architecture.
This type of optical-wireless converged system design makes use of
the optical heterodyne operation whereby two optical carriers,
centrally distributed over a fronthaul link, undergo heterodyne
detection at a photo-diode (PD) stage at the antenna site; producing
a mm-wave signal for wireless propagation. This approach avoids the
difficulty/expense of mm-wave carrier generation in the electronic
domain, is capable of providing a wide mm-wave carrier tuning range
and offers relative ease of convergence with the hardware
centralized implementation of C-RAN. Optical heterodyning in this
way can introduce significant amounts of optical phase noise and
frequency offset which are performance limiting factors for A-RoF
systems. Our prior work has proposed several hardware and software
based techniques to mitigate these effects, thereby alleviating
strict optical source linewidth and stability requirements and
making remote optical heterodyne operation a viable option for
future mm-wave A-RoF fronthaul.
Nevertheless, other challenges associated with operation in the
mm-wave and THz regime, such as frequency selective fading, severe
multi-path interference and enhanced amplitude noise effects remain
highly problematic. To tackle these, a departure from the 4G/5G
waveform of choice — OFDM — is expected. Recently, we demonstrated
the use of the novel waveform orthogonal chirp division multiplexing
(OCDM) in a converged optical / mm-wave system; OCDM was shown to
outperform equivalent OFDM signals in the system owing to its
robustness to channel fading (assigned by its chirp spread-spectrum
form and its capability of providing enhanced channel estimation.
The authors of have previously demonstrated the application of OCDM
in a RAN implementation incorporating optical heterodyne operation
and mm-wave wireless transmission — with results aimed at
highlighting OCDM’s tolerance to interference from adjacent wireless
services. In this work, we augment our previous OCDM converged
system analysis by demonstrating an end-to-end C-RAN transmission
path including 10 km A-RoF fronthaul and 2 m wireless transmission
at 60 GHz. In addition to performance evaluation using equivalent 5G
new radio (NR) OCDM/OFDM compatible signals with up to 256-QAM, a 4
GHz bandwidth OCDM signal delivering 16 Gb/s is successfully
transmitted over the link, exhibiting near-uniform performance
across the frequency range; highlighting OCDM’s potential to provide
extremely high throughput future mm-wave wireless communications
enabled through converged optical networking.
Orthogonal Chirp-Division Multiplexing for Future Converged
Optical/Millimeter-Wave Radio Access Networks | IEEE Journals &
Magazine | IEEE Xplore
Orthogonal Chirp-Division Multiplexing for Future Converged Optical
/ Millimeter-Wave Radio Access Networks (pdf)
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TACS is a leading top consultancy in the field of information, communication
and energy technologies (ICET).
The heart of our consulting spectrum comprises strategic,
organizational, and technology-intensive tasks that arise from the use of new
information, communication and energy technologies. The major emphasis in our work is found in innovative consulting and
implementation solutions which result from the use of modern information,
communication and energy technologies.
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