5 G
Enabling Optical Wired and Wireless Technologies for 5G and Beyond Networks
The emerging fifth-generation mobile communications are envisaged to support massive number of deployment scenarios based on the respective use case requirements. The requirements can be efficiently attended with ultradense small-cell cloud radio access network (C-RAN) approach. However, the C-RAN architecture imposes stringent requirements on the transport networks. This book chapter presents high-capacity and low-latency optical wired and wireless networking solutions that are capable of attending to the network demands. Meanwhile, with optical communication evolutions, there has been advent of enhanced photonic integrated circuits (PICs). The PICs are capable of offering advantages such as lowpower consumption, high-mechanical stability, low footprint, small dimension, enhanced functionalities, and ease of complex system architectures. Consequently, we exploit the PICs capabilities in designing and developing the physical layer architecture of the second standard of the next-generation passive optical network (NG-PON2) system. Apart from being capable of alleviating the associated losses of the transceiver, the proposed architectures aid in increasing the system power budget. Moreover, its implementation can significantly help in reducing the opticalelectrical-optical conversions issue and the required number of optical connections, which are part of the main problems being faced in the miniaturization of network elements. Additionally, we present simulation results for the model validation.
Keywords:
5G, backhaul, centralized unit (CU), common public radio interface
(CPRI), distributed unit (DU), fiber to the X (FTTX), fronthaul, functional
split, optical wireless communication (OWC), passive optical network (PON),
photonic integrated circuits (PICs), radio access network (RAN), radio over
fiber (RoF)
1. Introduction
There have been growing concerns regarding the
increasing number of unprecedented bandwidth-intensive mobile applications and
services being experienced by the Internet. A notable cause of the increase in
the traffic and the subsequent pressure on the network is the Internet of
things (IoT) technologies. For instance, massive IoT (mIoT) schemes have caused
remarkable revolutions in the amount of mobile devices and applications in the
networks. This is in an effort to enhance the user experience in delivering
enhanced mobile broadband (eMBB) services and providing ultra-reliable
low-latency communication (uRLLC) for critical communication and control
services. In theory, IoT comprises universal existence of a collection of
things like mobile PCs, tablets, smartphones, actuators, sensors, wireless
routers, as well as radio-frequency identification (RFID) tags. It is
remarkable that these devices are capable of cooperating not only with each
other but also with their neighbors. By this approach, they are able to achieve
common network goals by means of unique addressing scheme [1, 2]. Furthermore,
it has been predicted that massive number of mobile devices on which various
bandwidth-intensive applications and services will be operating and will be
Internet connected [3]. In actual fact, there is a tremendous demand for
effective systems that are capable of delivering various services in a
cost-effective manner while meeting the essential network demands.
Consequently, in an effort to accomplish the next-generation mobile network
technical demands, there have been intensive researches on viable solutions
that can satisfy the network requirements.
Additionally, to support the anticipated
massive devices, there has been general consensus that the fifth-generation
(5G) wireless communication system is the viable and promising solution.
Meanwhile, massive multiple-input multiple-output (M-MIMO) antenna and
millimeter-wave (mm-wave) technologies are anticipated to be integrated into
the 5G networks, so as to enhance the wireless system bandwidth. This is due to
the fact that radio-frequency (RF)-based wireless system transmission speeds
are highly constrained by the regulated RF spectrum. This limitation can be
attributed to numerous advanced wireless systems and standards such as UWB
(IEEE 802.15), iBurst (IEEE 802.20), WiMAX (IEEE 802.16), Wi-Fi (IEEE 802.11),
as well as the cellular-based 3G and 4G. On the other hand, there is a vast
amount of unexploited and underutilized frequency at high bands as
expatiated in Section 2. Nevertheless, the radio propagation at higher
frequency bands is comparatively demanding. Consequently, advanced scheme like
beamforming (BF) technique is essential for radio operation at the bands. The
technique will help in compensating mm-wave band inherent path loss in the
radio access network (RAN).
In addition, owing to several innovative
technologies that have been implemented in the optical communications,
significant improvements have been noted in the network performance [8]. Among
the remarkable improvements are the increase in the network reach, optical
system capacity, and the number of users that can be effectively supported.
This is as a result of cutting-edge optical fiberbased technologies. The
optical schemes have been increasingly advancing deeper into different access
networks, in order to provide various services such as mobile
backhaul/fronthaul and multitenant fiber to the X (FTTX) with some variants of
fiber-based broadband network architectures. For
instance, the optical broadband network architectures, such as fiber to the
curb or cabinet (FTTC), fiber to the node (FTTN), fiber to the building (FTTB),
fiber to the premise (FTTP), and fiber to the home (FTTH), proffer commercial
solutions to the communication network performance bottleneck, by progressively
delivering services in close proximity to the numerous subscribers.
It is noteworthy that various 5G use cases
like uRLLC and eMBB can be effectively achieved by radio elements and BSs that
are not far-off the end users or wireless devices. This is due to the fact that
close proximity helps in facilitating better signal quality, with lower latency
and higher data rates in the system. This can be effectively realized by
means of passive optical network (PON) technologies such as gigabit PON (GPON),
10Gbps PON (XG-PON), as well as Ethernet PON (EPON). It is noteworthy that one
of the key issues is the process of supporting different service demands with
the intention of realizing ubiquitous and elastic connections. As a result,
optical and wireless networks convergence is very indispensable. This is not
only a cost-effective approach but also enables high-network penetration, in
order to achieve the envisaged ubiquitous feature of the nextgeneration network
(NGN). Based on this, there is a growing consensus of opinion that
high-capacity optical fronthaul scheme is one of potential solutions for
addressing the network demands. For instance, if the CPRI standard is to be
directly employed for the transportation of a considerable number of long-term
evolutionadvanced (LTE-A) and/or 5G radio signals, an enormous aggregate
bandwidth will be required on the backhaul/fronthaul networks.
Furthermore, it has been observed that the
reference system architectures for the 5G standardizations are based on the notion
of heterogeneous networks where mm-wave small cells are overlaid on the larger
macrocells. This will enable the RAN to handle the growing traffic demands.
In addition, to contain the massive deployment of small-cell BSs, cloud RAN
(C-RAN) has been adopted as a promising architecture to ensure effective
scalability regarding deployment cost as well as energy consumption.
The C-RAN offers an innovative architecture that is really different from the
traditional distributed RAN (DRAN). In the C-RAN architecture, the baseband
unit (BBU) is shifted away from the cell sites where it is normally located in
the DRAN. Consequently, BBU collections that are usually referred to as BBU
pools are centralized at the central office (CO). With this configuration, the
remote radio heads (RRHs) are left at the cell sites.
As a result, C-RAN implementation offers
significant benefits such as improved system spectral efficiency and better
flexibility for further RRH deployments than the DRAN. Likewise, with the centralized
BBUs, C-RAN supports greener infrastructure, enhanced interference
mitigation/coordination, better resource pooling, improved BS virtualization,
as well as simplified management and operation. Besides, multiple technologies
can be supported with smooth and scalable evolution. Furthermore, in the C-RAN
architecture, the BBU pools are connected via the fronthaul network to the
RRHs. It is remarkable that the de facto air interface standard that is usually
employed for connecting the BBU pools to the RRHs is the common public radio
interface (CPRI) protocol. This is an interface that helps in the digital
baseband sample distribution on the C-RAN fronthaul. However, stringent
requirements concerning jitter, latency, and the bandwidth are imposed on the
fronthaul network for seamless connectivity. This makes the CPRI-based
fronthaul links to be prone to flexibility and bandwidth limitations, which may
prevent them from being visible solutions for the next-generation networks. Meanwhile, it has been noted that the 5G systems will impose higher
requirements on the transport network regarding latency, bandwidth,
reliability, connectivity, and software-defined networking (SDN) capability
openness. A number of approaches such as cooperative radio resource
allocation and data compression technologies have been adopted to address the
challenges; however, the fronthaul capacity demand is still considerable high.
The viable means of addressing the capacity
requirement is through the implementation of passive optical network (PON)
solutions such as wavelength division multiplexed PON (WDM-PON) and ultradense
WDM-PON (UDWDM-PON). The PON architectures are compatible with the 5G networks
and are capable of supporting both wired and wireless services. Based on the
PON architecture, individual RRH has the chance to communicate with the BBU
pools using a dedicated wavelength. Besides, in the upstream direction, the
aggregate wavelengths can be further multiplexed into a single shared fiber
infrastructure at the remote node (RN). They can eventually be de-multiplexed
at the CO. As aforementioned and as depicted in Figure
1, optical and wireless network convergence is a
Figure 1.
A
scenario for optical and wireless access networks convergence (adapted from
Alimi et al. [2]).
promising scheme for exploiting the optical system inherent
bandwidth and the mobility advantage of wireless connectivity, which can help
in realizing the 5G network envisaged capacity and energy efficiency. In
addition, optical wireless communication (OWC) is another feasible and
attractive optical broadband access solution that is capable of supporting
high-capacity, high-density, and low-latency networks. Therefore, it can
effectively address the network requirements for different applications and
services at a comparatively lower cost. So, it has been seen as an alternative
and/or complementary solution for the existing wireless RF solutions. This chapter presents optical wired and wireless networking solutions
for high-capacity, high-density, and low-latency networks. Furthermore, because
of its potential for intense revolution and salient advantages, we focused on
the second standard of the next-generation PON (NG-PON2) system. In addition,
with the exploitation of notable features of photonic integration, we design
and develop the physical (PHY) layer architecture of the NG-PON2 system. The
proposed NGPON2 architectures offer an enabling platform for active device
integration into the chip to ensure a significantly low propagation loss. We
also present simulation results for model validation. This helps in
demonstrating the potential of photonic integration for optical architectures.
Furthermore, with concise information on the enabling optical wired and
wireless technologies and the need for alleviating the stringent requirements
in the network being introduced, we present comprehensive overview of the
fronthaul transport solution. The salient needs for PON in the
envisaged ultradense network deployments are considered in Section 3. In
Section 4, a practical method for network investment optimization by the
operators based on PON system coexistence is discussed. In Section 5, we
present a number of viable schemes for alleviating the imposed stringent
requirements in the system. The NG-PON2 PHY architecture design and development
based on photonic integration are demonstrated. The
obtained simulation results with further discussion are presented.
2. Fronthaul transport solutions
The fronthaul protocol can be transported
by different viable means. Apart from the usually employed small form pluggable
and serial constant bit rate CPRI specification that is based on digital radio
over fiber (D-RoF) implementation, there are other innovative and standard
fronthaul interfaces such as Open Base Station Architecture Initiative (OBSAI),
next-generation fronthaul interface (NGFI), open radio interface (ORI), and
enhanced CPRI (eCPRI) that can be used. We give an overview of
various prospective and standard fronthaul interfaces. In this chapter, for
reference purposes, we focus on the extensively employed CPRI protocol.
However, it should be noted that the transport methods to be discussed in this
section are applicable to other fronthaul interfaces. The transport methods
discussed in this section are grouped into wired and wireless fronthaul
solutions.
2.1 Wireless fronthaul solution
Wireless transport schemes are very viable
fronthaul solutions that have resulted into tremendous evolutions in the
communication systems. This is due partly to the inherent advantages such as
operational simplicity, ease of deployment, scalability, roaming support,
effective collaboration, and cost-effectiveness. Furthermore, it is an
appropriate scheme for complementing fiber-based fronthaul solutions. However,
their susceptibility to transmission channel conditions makes their implementation
effective for short range. Besides, the current solution can only support few
CPRI interface options. This brings about bandwidth limitation for this
solution. Moreover, to alleviate this, promising wireless technologies like
mm-wave and wireless fidelity (Wi-Fi) can be employed in the fronthaul.
As aforementioned in Section 1, there is a
huge amount of unexploited and underutilized frequency at high bands. The
fronthaul in which mm-wave is being employed is feasible due to the
availability of various compact and highdimensional antenna arrays for
commercial use in the band. Besides, as a result of 60 GHz standards like
802.11ad, 802.15.3c, and WirelessHD that have been issued, considerable
attention has been given to mm-wave communications. Nonetheless, the inherent
high propagation losses of the mm-wave communications give rise to
comparatively shorter transmission range.
In addition, as stated in Section 1, RF-based
system transmission speeds are substantially limited due to a number of
advanced wireless systems being deployed in the network. Consequently, to meet
the demands of the current and future wireless networks, many chipset suppliers
and wireless operators have been paying significant attention to the unlicensed
spectrum. The major focus is in the 2.4 GHz and 5 GHz frequency bands that are
under implementation by the Wi-Fi. This is being used for the 5G LTE-Unlicensed
communication systems. With this implementation, the unlicensed
spectrum resources could be effectively allotted to the LTE system, in order to
have more capacity for supporting the Wi-Fi users.
Furthermore, it is remarkable that the Wi-Fi unlicensed spectrum is a promising solution for the fronthaul network. A notable advantage of exploiting the unlicensed spectrum for the fronthaul network is due to the fact that separate frequency procurement for the fronthaul might not be necessary for the network providers. Besides, the same spectrum could be effectively reused in the access and fronthaul links. This can be accomplished by means of time-division multiplexing (TDM) and frequency-division multiplexing (FDM) schemes. Another way of achieving this is through opportunistic fronthauling, in which unlicensed spectrum can be sensed. For instance, the RRH can sense unlicensed spectrum that is available (unused unlicensed spectrum) and then employ it for fronthauling. Besides, in a situation where the active user signal is considerably lower than the predefined threshold, the RRH can also make use of the spectrum. In addition, the fronthaul link constraints could be effectively eased via the Wi-Fi. This is majorly due to the fact that it can be employed for offloading. Although Wi-Fi networks are capable of offering relatively high-data rates, they exhibit limited mobility and coverage. The drawbacks can be reduced by employing Wi-Fi mesh networks.
2.2 Wired fronthaul solution
The wired network offers a number of
advantages such as low interference, enhanced coverage, low latency, and high
reliability and security. Due to these advantages, they have been able to stand
the test of time and continue to be relevant despite the advent of wireless
systems. Some of the fronthaul solutions that are based on wired links are dark
fiber, passive WDM, WDM-PON, WDM/optical transport network (OTN), and Ethernet.
In this subsection, we present potential wired-based fronthaul solutions that
can support the network requirements.
2.2.1 Dark fiber solution
Dark fiber offers an attractive
fronthaul solution. With this implementation, transmission equipment is not
required between the BBU pools and the radio remote units (RRUs), consequently
resulting in easiest deployment solution with least possible latency.
Nevertheless, since dark fiber solution is based on point-topoint (P2P) direct
connections, it lacks the required network protection, making it not a good candidate
to support 5G use cases such as uRLLC services in which high reliability is
required. Besides, its implementation demands huge amount of fiber resources.
In the 5G systems in which ultradense networks are envisaged, the required
amount of fiber is even more challenging. So, the fiber resources may be
inadequate to support mIoT devices and other envisaged multimedia devices.
Therefore, availability of fiber and the associated deployment cost may be the
limiting factors for the dark fiber solution employment. This inefficiency can
be addressed with the aids of different WDM and Ethernet solutions [11, 22, 23,
29].
2.2.2 Ethernet solution
In Ethernet-based fronthaul solution, packet technologies that
encourage statistical multiplexing feature are employed. This helps in
achieving traffic convergence and in enhancing the line bandwidth usage.
Besides, considerable fiber resources can be saved due to its support for
point-to-multipoint (P2M) transmission. Nevertheless, a number of issues such
as identification as well as fast forwarding of low latency services deserve
considerable research attention in this approach. Also, further efforts are
required for backward compatibility with CPRI transmission and high-precision
synchronization. Based on these, the Institute of Electrical and Electronics
Engineers (IEEE) has established a task group known as time-sensitive
networking (TSN) which is a part of the IEEE 802.1 working group, to study the
latency-sensitive Ethernet forwarding technology. Reasoning along the same
lines, the IEEE 1914 next-generation fronthaul interface (NGFI) working group
has been established not only for the development of the NGFI transport
architectures and the associated requirements but also for the definition of
radio signal encapsulation specification into Ethernet packets [11, 29].
2.2.3 WDM-based solution
The requirement for low-latency transmission in the range
of 10-Gb/s makes
WDM-based network the usually adopted option for the fronthaul
links. At large,
WDM-based fronthaul methods can be grouped
into two solutions which are active and passive. In active solution, other
protocols are used for the CPRI traffic encapsulation, before being multiplexed
on the fronthaul network. Also, the solution offers robust network topologies
with considerable flexibility. Moreover, with optical amplifiers, the network
reach can be significantly extended. Another important distinguishing feature
of an active solution is that the cell site demarcation point requires power
supply for operation. On the other hand, a passive solution mainly depends on
CPRI link passive multiplexing (MUX)/demultiplexing (DEMUX). Besides, this
solution’s demarcation point can function effectively without any battery
backup and power supply. Nonetheless, active equipment can be employed for the
system monitoring at the CO demarcation point [11, 22, 23, 29].
In general, the main dissimilarities between the passive and active
solutions can be recognized in the nature of their routing table and switching
granularity. For instance, unlike the active solution, routing table can be
statically and dynamically configured as well as associated with the interface;
that of passive solution is fixed and lacks configuration capability. Likewise,
the passive solution switching granularity is based on spectrum or time slot as
being implemented in the TWDM-PON, while an active solution presents finer
switching granularity which can be based on packet or frame switching.
Consequently, the active solution offers better configuration flexibility;
however, it is power-consuming and relatively complicated [12]. In the
following, we expatiate on different WDM-based fronthaul solutions.
2.2.3.1 Passive WDM
In this approach, a passive optical
MUX/DEMUX is employed for multiplexing a number of wavelengths on a shared
optical fiber infrastructure for onward transmission. Therefore, the
implementation can save considerable fiber resources via the support for
multiple channels per fiber. Also, the employed optical components introduce
negligible latency, so, the stipulated jitter and latency requirements for CPRI
transport can be effectively met. Moreover, due to the passive nature, power
supply is not required for the associated equipment operation. This brings
about high power efficiency in the network. Besides, this approach is not only
a costeffective solution but also offers simple maintenance. Nevertheless, the
cost implication of the wireless equipment deserves significant attention. This
is due to the required colored optical interfaces at the BBU and RRU. Also,
factors that need consideration are the limited transmission range and
inadequate optical power budget of a relatively complex topology such as chain
or ring network. This can be attributed to the accumulated insertion loss owing
to multiple passive WDM components. Besides, the approach offers no robust
operations, administration, and maintenance (OAM) potentials, and usually, line
protection is not provided. Passive WDM implementation can also be limited by
the need for well-defined network demarcation points [11, 22, 23, 29].
2.1.3.2 WDM/OTN
When WDM/OTN scheme is employed, multiplexed
and transparent signal transmissions can be achieved over the fronthaul link to
multiple sites. Thus, the fiber capacity is increased by enabling multiple
channels on a shared fiber infrastructure [11, 23, 29]. This can be realized by
encapsulating the inphase and quadrature component (I/Q) data by means of OTN
frame; this is subsequently multiplexed to the WDM wavelength. Consequently,
any wavelength can be employed for routing the resulting frame to the
destination port [12]. Apart from being able to save fiber resources, other
notable advantages of this solution are provision for OAM capabilities, network
protection, service reliability, as well as service level agreement (SLA)
management and network demarcation. Furthermore, this solution presents
attractive features regarding low latency and high bandwidth. It is also a good
approach for attending to the required colored optical interface at BBU and RRU
by the passive WDM. Since colored optical interface is not demanded, wireless
equipment deployment challenges are alleviated drastically by the WDM/OTN
solution. Another significant advantage of the approach is the offered easy
scalability. This is due to the fact that there is no need for replacing the
wireless equipment optical interfaces while upgrading from non-C-RAN to the
C-RAN architecture. Notwithstanding, the major drawback of the solution is the
relatively higher cost of the equipment. Although power supply is not required
for WDM transport in the approach, it is essential for wavelength translation
and active management [11, 23, 29].
In addition, the WDM-based systems such as coarse WDM (CWDM) and dense
WDM (DWDM) exhibit promising features for the fronthaul transport applications.
For instance, apart from the offered high throughput and low latency, CWDM is
very cost-effective regarding fiber resource usage and equipment expenses.
Also, DWDM is widely known for the higher channel counts that can be
efficiently supported. This can help further in increasing the number of small
cells and the associated RRHs that can be deployed effectively. Furthermore, it
helps in improving the fiber resource efficiency.
Figure 2.
Potential 5G
fronthaul solutions: (a) microwave, (b) point-to-point, (c) WDM-PON, (d) OTN,
and (e) Ethernet.
It is
remarkable that WDM-based schemes can be used in conjunction with PON
technology in order to further enhance the system performance. This scheme is
highly appropriate for the anticipated massive RRHs and ultradense small cell
deployment as explicated in Section 3. It should be noted that, for RAN to be
well deployed, especially in the urban environments, the radio elements should
be, as much as possible, in close proximity to the subscribers. So, the remote
elements could be mounted on different places such as buildings and street lamp
poles. Therefore, the arbitrary nature of the remote element placement can be
efficiently supported with the implementation of WDM schemes.
Furthermore, as discussed, there are a number
of ways by which the C-RAN fronthaul can be realized; nonetheless, the imposed
stringent requirements make fiber-based method the widely adopted in the C-RAN.
However, optical fiber implementation for ultradense networks, besides being
time-consuming, may render the C-RAN schemes uneconomical and less flexible. It
is remarkable that wireless fronthaul offers attractive and flexible solutions
for information exchange between the centralized unit (CU) and distributed unit
(DU). This is owing majorly to the offered advantages such as higher
flexibility, lower cost, and undemanding deployment when than the fixed wired
fronthaul counterparts. Therefore, innovative optical wireless solutions with
good scalability and operational simplicity, coupled with easy of deployment,
are really desirable [11].
In addition, apart from physical fiber-based methods being discussed, OWC
system, also known as a free-space optical (FSO) communication system, is
another attractive and feasible optical wireless fronthaul. The FSO provides a
range of benefits such as low latency and high capacity that make it viable for
addressing network requirements in a cost-effective manner [4, 16–18]. The
potentials for the FSO implementation in the fronthaul network and different
innovative concepts that are appropriate for improving the FSO system
performance, while easing the stringent system requirements, are discussed in
Section 5. Different potential 5G fronthaul solutions are depicted in Figure
2.
3. Passive optical network (PON)
The existing fiber-based methods as well
as active P2P Ethernet might unable to meet the envisaged bandwidth-intensive
traffic requirements by the 5G and beyond networks. For instance, ultradense
network deployments with the associated huge network resources are envisaged in
the 5G network. As illustrated in Figure 3, PON system can make
better use of the current fiber infrastructures than the existing P2P system
such as CPRI. This helps considerably in reducing the required number of
interfaces in the network. As a result, it aids not only in reducing the site
space, but also substantial amount of system power can be saved [30]. As
explained in Section 2, PON technology has been deemed as an attractive access
network solution owing to the presented advantages such as low-operation cost,
high bandwidth, and lowmaintenance cost [11, 31, 32].
It should be noted that the PON
architectures have been experiencing continuous and gradual evolution, so as to
considerably enhance the service availability and the related data rates. The
offered technological options and the intrinsic benefits have been attracting
the operators in deploying a number of PON systems. It has been observed that
the most widely deployed one is the gigabit PON (GPON) system. Moreover, the
first standard 10 Gbps PON technology, the next-generation PON (NG-PON) system,
known as 10-gigabit PON (XG-PON1) has also been gaining considerable attention.
With continuous demand for further capacity, there are innovative PON
generations such as 10-gigabit symmetric PON (XGS-PON)
Figure 3.
Potential
fronthaul solutions (a) CPRI-based and (b) PON-based schemes.
and the second standard of NG-PON (NG-PON2) that are now
becoming the target of various providers [33]. In PON system, WDM and TDM
techniques are normally employed to further enhance the capacity and fiber
efficiency. Based on these techniques, the PON system can be broadly grouped
into WDM-PON and TDM-PON.
Moreover, it is noteworthy that the
TDM-PON is capable of giving considerable greater bandwidth for various data
applications; however, availability of the resources that can be delivered to
the end users is limited. In contrast, the issue can be effectively addressed
with the WDM-PON scheme. This can be done by assigning a peculiar wavelength
per subscriber. As a result of this, a distinct, highdata rate, as well as
secure P2P channel, can be delivered over a high-capacity and longer optical
reach, between each of the subscriber and the CU. Consequently, a WDM-PON
scheme is suitable for partitioning the ONUs into a number of distinctive
virtual P2P links over the shared physical optical infrastructure by multiple
operators. This attribute facilitates fiber efficiency compared to P2P Ethernet.
Similarly, in relation to TDM-based systems, it gives lower latency. These
features make WDM-PON a disruptive solution that is very appropriate for FTTX
as well as mobile backhaul and fronthaul applications. This will eventually aid
the operators not only in developing converged networks but also in enhancing
the current access networks. As a consequence of this, some redundant COs can
be eliminated in an attempt to enhance the network performance in
cost-effective ways [11, 31, 32].
Moreover,
it is remarkable that advantages of both WDM-PON and TDM-PON can be effectively
exploited though joint application of the schemes. This results in the TWDM-PON
architecture. The potential PON architectures and their applications in
telecommunication systems are presented in the subsequent subsections.
3.1 TDM-PON application
The TDM-PON can be grouped into broadband PON (BPON), asynchronous
transfer mode (ATM) PON (APON), Ethernet PON (EPON), and GPON. In the existing
telecommunication networks, GPON and EPON are the widely adopted schemes.
Therefore, in the following, we focus on both schemes.
3.1.1 EPON application
The data traffic being encapsulated in the
Ethernet frames as defined by the IEEE 802.3 standard is transported by the
EPON solution. Different network elements such as optical network unit (ONU),
optical line terminal (OLT), and optical distribution network (ODN) are the
building blocks of a standard EPON system and other PON architectures. In the
EPON solution, PON topology is exploited for getting the Ethernet access. Based
on the joint schemes, EPON solution is capable of offering high bandwidth and
good network scalability. Besides, due to the fact that it is highly compatible
with Ethernet, network management can be supported in cost-effective manners.
Likewise, as illustrated in Figure 4, FTTB, FTTC, and
FTTH network architectures can be supported depending on the
ONU deployments and demarcation point between the copper cable and optical
fiber termination [32].
Typically, ONUs can be deployed beside the
telegraph pole junction boxes, or else, at roadside when FTTC system is
employed. Also, different types of twisted pair cables can be utilized for
connecting the ONUs and the respective customer. It has been observed that FTTC
technology offers a cost-effective and practical solution for delivering
narrowband services. However, FTTC solution is not an ideal scheme, when
broadband and narrowband services are to be incorporated [32].
Moreover, the ONU deployment can be made
closer to the users in the FTTB solution. So, it can be located inside the
buildings through further optical fiber penetration into customer homes. This
can be achieved by means of cables, local
Figure 4.
FTTX
architectures.
area networks (LANs), or asymmetric digital subscriber line
(ADSL) broadband communication technologies. Relatively, FTTB employs more
optical fiber in the connection than FTTC solution. This makes it more
appropriate for broadband/ narrowband service integration [32].
Furthermore, ONU deployment can take place
right inside the subscribers’ homes or offices in the FTTH solution. This
facilitates a fully transparent network in which the ONUs are independent of
the wavelength, bandwidth, as well as transmission mode and technology. These
benefits enable FTTH scheme to be very ideal for access network implementations
[32].
In addition, the discussed IEEE 802.3 Ethernet is a 1-Gbit/sec EPON
standard. It is remarkable that there is a 10G EPON standard that is capable of
supporting 10G/ 10G symmetric DS and US transmission. In another effort to
attend to the system requirement, the IEEE 802.3ca task force has been working
relentlessly on the development of 25G/50G/100G EPON standards. A notable
feature of the entire EPON standards is that they are designed to be both
backward and forward compatible. This is to ensure that legacy service, as well
as innovative higher-speed service, can be effectively supported using the same
ODN [34].
3.1.2 GPON application
Furthermore, to address the growing
traffic demands, XG-PON1 has been presented. The XG-PON1 is capable of
delivering higher data transmission than the legacy GPON system. Moreover, in
an effort to keep the existing investments, it is backward compatible with the
GPON. Also, the GPON ODN, as well as framing and management, is inherited by
the XG-PON1. This encourages the reuse of the existing network elements [35].
3.2 WDM-PON application
The WDM-PON enables multiple-wavelength
transmission through the multiple operators’shared optical fiber infrastructure
rather than one wavelength in the PON system. This helps in ensuring that
WDM-PON meets the huge subscribers’ bandwidth demands. Furthermore, it presents
various merits such as high wavelength efficiency and relatively simpler
network management. This encourages support for various services than the
TDM-PON. Besides, all anticipated services can be delivered over a shared
communication network infrastructure.
In addition, it can effectively support different access networks such as
FTTB, FTTH, and FTTC. Also, both small-scale and large-scale subscribers can be
concurrently supported as well. Based on the inherent huge bandwidth, different
types of BS bandwidth requirements can be appropriately met. Its implementation
can also help in the network reach extension and in the current EPON network
transition. This will help in keeping the current network investment while
enhancing the network scalability [32]. In addition, UDWDM-PON offers a
wavelength grid that is relatively denser for the WDM scheme. This helps not
only in supporting a huge amount of aggregated wavelengths per fiber but also
in accommodating higher number of RRHs per feeder fiber. Nonetheless, with the
envisaged NGN stringent transport network requirements, UDWDM will be unable to
maintain the high perwavelength bit rates resourcefully. For instance,
subcarriers’ aggregation for highspeed services usually bring about
considerable latency. Therefore, UDWDM implementation is preferred in
situations where there are ultradense RRH deployments and inadequate feeder
fiber accessibility. Besides, it also finds application in antenna sites which
demand a low-peak but high sustainable rate [6]. As discussed in subsection
3.3, WDM-PON can be employed along with TDM-PON to achieve a hybrid WDM-TDM-PON
solution known as time and wavelength division multiplexed (TWDM-PON) scheme.
Apart from being efficient for both small-scale and large-scale subscribers,
the hybrid scheme offers a promising solution for applications in
telecommunication environment.
3.3 TWDM-PON application
It is notable that TDM-PON implementation in
the 4G networks offers a very cost-efficient solution for a wavelength channel
sharing between the cell sites, by means of diverse time slot allocations for
different cell sites. However, with the evolution of mobile networks, the major
ITU-defined application scenarios such as eMBB, uRLLC, and massive machine-type
communications (mMTC) could make TDM-PON solution unsuitable for the fronthaul
transport network in the 5G and beyond networks. As aforementioned, a hybrid
TWDM-PON scheme is a feasible solution with abundant bandwidth capable of
supporting the fronthaul demands.
With the scheme, time slots, as well as
wavelength resources, can be allocated dynamically between the RRHs. The
offered centralized and virtualized PON BS can considerably help in the system
energy savings. Likewise, the virtualized scheme presents a number of
advantages such as low handover delay, excessive handover reduction, and better
network reliability. This results in cost saving, celledge user throughput
improvement, and enhanced mobility management [32, 36, 37]. The associated
multiple wavelengths, as well as potential for wavelength tenability, give
TWDM-PON unprecedented means of improving the network functionalities compared
with the basic TDM-PONs [36, 37]. Likewise, orthogonal frequency-division
multiplexed PON (OFDM-PON) is another promising PON solution. With OFDM, there
is a comparable high potential for flexible bandwidth resource sharing as
experienced in the TWDM. On the other hand, regarding the reach, the OFDM
variants in which direct detection is employed usually present poor performance.
Similarly, variants in which coherent detection is implemented are
comparatively too expensive [6]. Furthermore, it is noteworthy that among its
counterparts such as standard WDM-PON, optical code division multiplexed PON
(OCDM-PON), and OFDM-PON that are capable of offering 40 Gb/s or higher (80
Gb/s) aggregated bandwidth, the full service access network (FSAN) community
has chosen TWDM-PON as a major broadband solution. Apart from the inherent huge
capacity with 1:64 splitting ratio, it has a long reach of 40 km. The salient
features enable TWDM-PON system to meet the future broadband service
requirements [37–39].
A typical TWDM-PON system architecture is
depicted in Figure 5. In a conventional TWDM-PON solution,
multiple wavelengths can effectively coexist in a shared ODN by means of WDM.
Moreover, each of the wavelengths is capable of serving multiple ONUs through
TDM access. With reference to the ITU-T recommendation, 4–8 wavelengths in L
band (1590–1610 nm) and C band (1520– 1540 nm) can be employed for the
downstream (DS) and upstream (US) transmissions, respectively. Also, each of
the DS wavelengths can operate at 10 Gb/s, while the US can function each at
2.5 or 10 Gb/s data rate [32, 37].
In addition, the TWDM-PON ONUs employ
colorless tunable transceivers for selective transmission/reception of any
US/DS wavelengths (data) via a pair of US/ DS wavelengths. With this approach,
the ONU inventory issue can be prevented. In essence, the transceiver features
help in easing network deployment as well as inventory management. Furthermore,
load balancing can be supported effectively in the TWDM-PON system. Besides,
with dynamic wavelength and bandwidth allocation (DWBA) implementation, large
bandwidth can be flexibly exploited. It is remarkable that TWDM-PON is a stack
of four 10-gigabit PONs (XG-PONs) with
Figure 5.
Typical
TWDM-PON architecture.
four pairs of wavelengths. In the stack, each XG-PON is
operating on different wavelengths. Also, as stated earlier, the GPON and
XG-PON GEM frames are compatible with and can be employed in the TWDM-PON
solution. Based on this and the ability for coexistence with existing PON
solutions, it is a viable scheme for optical access network swift evolution
[11, 32, 37]. Consequently, TWDM-PON has been adopted for the NG-PON2. In
NG-PON2, TWDM-PON can be employed with optional P2P WDM overlay extension. It
is remarkable that DWDM scheme will enable NG-PON2 to deliver multiple unshared
P2P connections, while TDM scheme simultaneously offers multiple P2M
connections. This will enable the operators to efficiently support both
fronthaul/backhaul and business services with the P2P WDM overlay technology,
by using dedicated wavelengths [11, 40, 41].
In addition, based on the inherent colorless
tunable transceivers of the TWDMPON ONUs, three classes of wavelength channel
tuning time have been specified for the NG-PON2 by the physical media dependent
layer recommendation (ITU-T G.989.2). Table 1 illustrates the
specified tuning time classes by the G.989.2 recommendation. It should be noted
that different innovative technologies can be exploited by the wavelength
tunable devices in order to have the capability for supporting various classes.
This will enable a number of potentials for the NGPON2 system at relatively
different costs. Out of the defined three classes, Class 3 is based on the
slowest tunable devices. Consequently, it is applicable in scenarios with
occasional tuning operations or in applications that can tolerate short service
disruption. On the other hand, Class 1 wavelength tunable devices present the
shortest tuning time. This feature makes them attractive for offering DWBA
feature in the network. Besides, with this class implementation, the ONU
transmission wavelengths can be dynamically controlled by the OLT for
wavelength hopping between the transmission periods [42].
|
|
|
Class |
|
|
|
1 |
2 |
3 |
|
Tuning time |
<10 μs |
10 μs to 25 ms |
25 ms to 1
s |
Table 1.
Tuning time
classes [42].
Although a TWDM-PON offers effective
bandwidth resource allocation among multiple clients, meeting the low latency
and jitter requirements of certain services may be challenging. Consequently,
its implementation for the NGN RAN transport network depends mainly on the RAN
use cases and deployment scenario requirements [6]. In Section 5, we present a
number of viable means for alleviating the growing stringent requirements in
the system. Furthermore, as aforementioned, the NG-PON2 system employs multiple
wavelengths that demand for tunable transceivers at the ONUs. However, this
requirement might hinder its implementation as the existing optical tunable
transceivers are uneconomical. Based on this, a number of operators have been
looking for ways around this by envisaging provisional scheme adoption before
the full NG-PON2 migration. This will enable them to have a seamless transition
with least possible or no disruption in the offered services. One of viable
solution is the XGS-PON. It offers an improved commercial solution as a result
of the less costly elements being employed.
3.4 XGS-PON application
The XGS-PON presents a novel technology that
offers a generic solution for the NG-PON system. It can be viewed as an
uncomplicated variant of TWDM-PON in which the wavelength tunability and
mobility are eliminated for a more costeffective reason. In addition, there can
be an efficient coexistence between the XGS-PON and TWDM-PON using the same
fiber infrastructure, since the employed wavelengths by each technology are
different. Consequently, the operators can exploit the lower-cost XGS-PON for
quick delivery of 10 Gbps services. This will also enable them to seize 10 Gbps
services opportunities for immediate deployments. With XGS-PON, there can be
cost-efficient, gradual upgrade, and well-controlled transition to a full
TWDM-PON system, with minimum or no disruption to the offered services. It can
also facilitate TWDM-PON system by enabling its deployment using the wavelength
by wavelength approach. This will really help in pay-as-you-grow scheme for
effective system upgrade and migration [33, 43].
Besides its capability for delivering 10
Gbps in both US and DS directions, XGSPON has high potential for the dual rate
transmission support as well [44]. Based on this, the 10/2.5G XG-PON ONUs and
10/10G XGS-PON ONUs can be coupled to the same OLT port via a native dual US
rate TDMA scheme. It is remarkable that XGS-PON dual rate presents a comparable
cost to XG-PON; nonetheless, it is capable of providing 4 times of the XG-PON
US bandwidth. In addition, XGS-PON has been seen as a transitional scheme to
NG-PON2 due to its ability for offering the associated NG-PON2 high-data rates
in conjunction with the XG-PON1 CAPEX efficiency [33, 43]. Furthermore, it
should be noted that the GPON employs 1490 and 1310 nm in the DS and UP,
respectively. Likewise, XGS-PON utilizes 1578 and 1270 nm in the DS and UP,
respectively. This implies that the XGS-PON service can be effectively overlaid
on the same infrastructure as that of GPON. Similarly, the G.989 standard is
employed in NG-PON2. The G.989 supports TWDM technologies and it is a
multiwavelength access standard [44].
In addition, NG-PON2 is not only a state-of-the-art PON technology with
the potential for intense revolution in the operational models of providers but
also offers them flexible platform that is capable of enhancing their agility
to the market demands as never before. Besides, it has the ability for
cost-effective support for both the scale and capacity of the existing gigabit
services while at the same time having more than enough room for the
multi-gigabit bandwidth requirements of the future networks [38]. Consequently,
based on the aforementioned advantages and its proficiency for multiple
networks converging with outstanding performance, in this work, we focus on the
NG-PON2 system. Its PHY architecture and development are presented in Section
6.
4. PON system coexistence
Furthermore, in an effort to make considerable
profit, different operators have been developing high-bandwidth demanding
applications and services. Good examples of such notable ultra-broadband
systems are high-definition television (TV) and mIoT. It has been envisaged
that there will be a further increase in the bandwidth demand due to the
innovative services such as online gaming, home video editing, interactive
e-learning, next-generation 3D TV, and remote medical services. However, it
should be noted that NG-PON system deployment entails huge initial investments.
For instance, in the greenfield FTTH systems, out of the total network
investments, the ODN deployment takes between 70 and 76%. Therefore, network
investment optimization can be achieved by the operators with the existing ODN
exploitation. Besides, compatibility between the NG-PON evolution and the
present GPON system is highly essential [35, 44].
Moreover, efficient support for
bandwidth-intensive applications and services depends on coexistence of
different PON technologies. The coexistence will help in the network investment
optimization when the existing ODNs are shared. For instance, a network in
which service delivery is being offered by GPON and needs upgrade in order to
support new FTTH access technologies can coexist with the PON technologies such
as XGS-PON and NG-PON2. This can be realized with the aids of a coexistence
element. Based on the desired scenario, various ONT and OLT
|
A B B+ |
|
Class |
|
|
|
|
C |
N1 |
N2 |
E1 |
E2 |
|
|
Loss Min.
(dB) 5 10 13 |
15 |
14 |
16 |
18 |
20 |
|
Max. (dB) 20 25 28 |
30 |
29 |
31 |
33 |
35 |
Note: The degree of severity
of specific class requirements could vary from one system category to another.
Table 2.
ODN optical
path loss classes [42, 46].
Figure 6.
PON system
coexistence.
generations can effectively
coexist over a shared ODN fiber infrastructure. Besides, optical time-domain
reflectometer (OTDR) and RF signals can also coexist with the PON systems. This
is mainly due to the fact that there is no wavelength overlap between each of
the technologies. So, this permits in-band measurement without any service
interruption [34, 45]. Different ODN optical path loss classes are presented in
Table
2.
It is remarkable that, apart from the fact
that the existing GPON subscribers can be kept together with higher-bandwidth
services, the coexistence will also give the operators the profound chance to
take advantage of different approaches such as asymmetrical and symmetrical
data rates. They also have deployment flexibility by operating on fixed or
tunable wavelengths in order to offer appropriate operations and services at
suitable costs. It will also assist the operators in the NG-PON evolution path
not only by allowing them to upgrade their networks accordingly but also for
gradual migration to the evolving PON technologies that are capable of offering
the full optical potential. Thus, they have the liberty of adopting the cost
and deployment pace that best fit their precise business requirements [43].
Moreover, this will enable the operators in making further revenue by
exploiting flexible bandwidth and wavelength plans in order to support any
service type as well as any business need. Figure 6 depicts a PON system
coexistence for a gradual and payas-you-grow expansion [33].
5. System requirement alleviation schemes
As explained in Section 1, C-RAN is
envisioned to be a promising candidate for efficient management of the access
network and the associated emergent complexity. This is due in part to its
cost-effectiveness and remarkable flexibility for the network element
deployments. Normally, the inphase and quadrature (I/Q) component stream
transmission in this architecture is via the D-RoF-based CPRI. It is remarkable
that CPRI-based fronthaul demands huge bandwidth which could be a limiting
factor in the 5G and beyond networks in which mm-wave and massive MIMO are
anticipated to be implemented. Consequently, an advanced optical transmission
technology such as analog RoF (ARoF) has to be employed for an efficient
fronthaul solution realization [11, 13, 14].
5.1 RoF schemes
The RoF schemes offer efficient and
economical methods for modulated RF signal transmission. For instance, it can
be used for transmission from the CO, to a number of distributed RRHs, through
low-loss optical fiber networks, by employing an optical carrier. In addition,
as aforementioned in Section 1, optical and wireless network convergence is
highly imperative for scalable and cost-effective broadband wireless networks.
The envisaged convergence for the next-generation mobile communication networks
can be efficiently achieved with the implementation of RoF. This is due to its
simplicity and efficiency in conveying wireless signal via an optical carrier.
Furthermore, the inherent low attenuation and huge bandwidth of optical link
can effectively support multiple wireless services on a shared optical
fronthaul network. Moreover, with RoF implementation, the CUs and DUs can be
well-supported. This offers effective centralized network control that
subsequently presents advantages such as easy upgrade, simple maintenance, and
efficient resource sharing [11, 47, 48].
It should be noted that there are various
RoF options that can be employed in the network. Furthermore, each of the
viable options presents related distinct merits and demerits. Out of the
variants, the highly spectrally efficient scheme is the ARoF. Besides, its
implementation results in a most power-efficient and least complex RRH design.
Nevertheless, it is susceptible to intermodulation distortion which is as a result
of optical and microwave component nonlinearity. This results in relatively
shorter operating distance. Moreover, the transmitter components such as
oscillators, digital to analog converters (DACs), and mixers consume a
considerable amount of power. On the other hand, with D-RoF implementation, the
ARoFassociated nonlinearity issue can be effectively mitigated. However, in a
scenario where high baud rates and high carrier frequencies are required, the
DAC power consumption and expenditure are excessively high. Also, if
upconversion is required or implemented at the RRHs, it turns out to be
substantially high. Consequently, having a fixed phase relation among various
RRHs is really challenging. Besides, digitized sample transmission, rather than
the analog signal, brings about a significantly low spectral efficiency. The
aforementioned drawbacks can be more challenging when densely distributed RRHs
are to be supported [11, 47, 48]. Therefore, to address the challenges, a
hybrid scheme that is capable of exploiting the ARoF and D-RoF schemes can be
employed. One of notable techniques for a hybrid scheme is based on the
implementation of sigma-delta-over-fiber (SDoF). This scheme helps in ensuring
digital transmission that can support simple and powerefficient RRHs. Besides,
there is no need for high-resolution and high-speed DACs with its
implementation [47].
It is noteworthy that the RoF scheme employment is contingent on physical
optical fiber availability. On the other hand, for the envisaged ultradense small-cell
deployment, fiber deployment is not only time-consuming but also capital
intensive. Likewise, there could be inappropriate system deployment due to the
associated right-of-way acquisition. For these reasons, as well as limited
number of the deployed fiber, the FSO system practicability has been considered
[11, 13, 14].
5.2 FSO scheme
FSO communication presents an alternative
technology for optical fiber systems. It is based on RF signal transmission
between the CU and the DU apertures via the free space. Therefore, being an
optical wireless technology, the fiber media are not required, and,
consequently, trenches are unnecessary for its implementation. Moreover, like a
well-developed, viable, and widely employed RoF technology, FSO scheme is capable
of supporting multiple RF signal transmission. Apart from having inherent
optical fiber features like RoF, FSO scheme offers additional merits regarding
time-saving and cost-effectiveness, since there is no need for physical fiber
deployment. This makes it to be very applicable in scenarios where physical
network connectivity through optical fiber media is challenging and/or
unrealistic. Besides, it is capable of delivering broadband services in rural
area where there is an inadequate fiber infrastructure [11, 13, 14]. It is
noteworthy that, when employed as a complementary solution for fronthauling,
FSO can be a promising mobile traffic offloading scheme for alleviating the
stringent requirements of bandwidthintensive services transmission via the mobile
networks.
In addition, the FSO scheme offers a number
of benefits such as high bit rates, ease of deployment, full duplex
transmission, license-free operation, improved protocol transparency, and
high-transmission security. These salient merits enable the FSO scheme to be
considered as a viable broadband access technology. It is capable of addressing
various services and applications’ bandwidth requirements at low cost for the
NGNs. Based on these, the RF signals over FSO (RoFSO) idea have been presented.
This is in an effort to exploit the inherent massive transport capacity of
optical systems and the related deployment simplicity of wireless networks [11,
13, 14].
Furthermore, a DWDM RoFSO scheme implementation has the capability of
supporting concurrent multiple wireless signal transmission [49]. Nevertheless,
the FSO systems have some drawbacks due to their susceptibility to the
atmospheric turbulence and local weather conditions. The effects of these can
cause beam wandering, as well as scintillation, which in due course results in
the received optical intensity fluctuation. Consequently, the system
reliability and availability can be determined by the extent of the effects. As
a result, FSO technology is relatively unreliable like the normal optical fiber
technology. Therefore, apart from the fact that these can limit the RoFSO
system performance, its employment for uRLLC applications might also be limited
as well. Consequently, the drawbacks hinder the FSO scheme as an effective
standalone solution. Therefore, for the FSO scheme to be effective, the
associated turbulence-induced fading has to be alleviated [2, 17, 18, 50].
Based on this, several PHY layer ideas like maximum likelihood sequence
detection, diversity schemes, adaptive optics, and error control coding with
interleaving have been presented to address the issue [11, 50, 51]. Besides,
innovative schemes such as relay-assisted transmission and hybrid RF/FSO
technologies can be implemented to enhance the system performance regarding
capacity, reliability, and availability [11].
5.3 Hybrid RF/FSO scheme
A hybrid RF/FSO scheme exploits the
inherent high-transmission bandwidth of the optical wireless system and the
related deployment simplicity of wireless links [2]. In addition, the hybrid
RF/FSO system idea does not only base on concurrent means of attending to the
hybrid scheme related limitations, but it also entails ways of exploiting both
approaches for a reliable heterogeneous wireless service delivery. The hybrid
scheme is able to achieve this by incorporating the RF solutions’scalability
and cost-effectiveness with the FSO solutions’ high data rate and low latency.
Consequently, the technology is able to address the high throughput,
costeffectiveness, and low-latency requirements of the system. Besides, it
presents a heterogeneous platform for wireless service provisioning for the
envisaged 5G and beyond networks [11, 13, 14, 52, 53].
5.4 Relay-assisted FSO scheme
One of feasible methods of turbulence-induced
fading mitigation is the spatial diversity scheme. In this technique, there are
multiple deployed apertures at the receiver and/or transmitter sides. This is
in an effort to realize extra degrees of freedom in the spatial domain. It is
remarkable that spatial diversity is an appealing fading mitigation scheme,
owing to the presented redundancy feature. On the other hand, multiple-aperture
deployment in the system causes a number of challenges like an increase in the
cost and system complexity. Moreover, in order to prevent the spatial correlation
detrimental effects, the aperture separation should be sufficiently large.
Furthermore, a notable approach for simplified spatial diversity implementation
is a dual-hop relaying scheme. It is noteworthy that there has been extensive
implementation of the scheme in the RF and wireless communication systems.
Application of the scheme in these fields not only aids in improving the
receive signal quality but also helps considerably in the network range
extension [2, 11, 13, 14].
Conceptually, multiple virtual aperture systems are generated in the
relayassisted transmission with the intention of realizing salient MIMO
technique features. The architecture takes advantage of the RF and FSO features
for an efficient and reliable service delivery. In addition, a relay-assisted
transmission system is an innovative communication technique known as a mixed
RF/FSO dualhop communication system. The dual-hop scheme meaning can be easily
understood from its architecture. In the architecture, the transport networks from
the source to the relay system are RF links; however, the transport networks
between the relay system and the associated destination node(s) are FSO links.
Hence, in a dual-hop system, RF is used for signal transmission at one hop,
while FSO transmission is implemented at the other. The FSO link mainly
functions to facilitate the RF users’ communication with the backbone network.
This is purposely for filling the connectivity gap between the backbone and the
last-mile access networks. Accordingly, the offered architecture can
efficiently address the system-related last-mile transmission bottleneck. This
can be effectively achieved by supporting multiplexed users with RF capacities.
The users can also be aggregated onto a shared high-capacity FSO link. This
will help in harnessing the inherent huge bandwidth of an optical communication
system. Another outstanding advantage of this scheme is that any kind of
interference can be easily inhibited via its implementation. This is due mainly
to the fact that the RF and FSO operating frequency bands are completely
different. Consequently, it offers better performance than the traditional
RF/RF transmission schemes [2, 11, 13, 14].
5.5 RAN functional split
The RAN functional split is another
innovative and practical scheme for alleviating the imposed fronthaul
requirements by the C-RAN architecture [11, 54]. For instance, to address the
drawbacks of CPRI-based fronthaul solutions, an eCPRI specification presents
additional physical layer functional split options and a packet-based solution.
Consequently, unlike the conventional constant data rate CPRI in which the
stream significantly depends on the carrier bandwidth, as well as the number of
antennas, the eCPRI stream does not depend on either of the factors but on the
actual traffic load. In essence, apart from being able to alleviate the
stringent bandwidth demands, multiple eCPRI stream can also be multiplexed onto
a wavelength for onward transmission over the fronthaul network [12].
In addition, with recent network architecture
development, the traditional BBU and RRU have been reformed into different
functional entities which are the CU, DU, and RRU/active antenna unit (AAU).
With the configuration, the CU majorly focuses on non-real time and part of the
traditional Evolved Packet Core functionalities. This involves high-level
protocol processing like dual connectivity and radio resource management. In
addition, the DU is responsible for the real-time media
Figure 7.
Functional
split options between CU and DU with emphasized PHY layer.
access
control layer functions like HARQ flow and physical layer function processing.
Also, when massive MIMO antennas are to be employed, certain parts of the
physical layer functions can also be shifted to the RRU/AAU. The implementation
will not only aid in lessening the associated transmission bandwidth between
the RRU/AAU and DUs but will also help in reducing the transmission cost
considerably. Therefore, a number of functional split options have been
presented in order to reduce the processing and network resource cost
considerably. As shown in Figure 7, each of the option
is categorized according to the demarcation point between the CU and the DU.
Therefore, depending on the deployment scenarios and use cases, each option offers
different degrees of flexibility regarding resource allocation for different
service requirements [12, 29].
6.
NG-PON2 physical layer architecture design and development
The NG-PON2 physical layer requirements are very challenging. Besides,
the requirements are even more strict than the legacy PON technologies. For
instance, when compared with the GPON taken into consideration the related
spectrum, GPON employs only one channel for the transmission and one for the
reception, with a very wide wavelength allocation (up to 100 nm). On the other
hand, in NGPON2, there are <4 nm to accommodate four channels. Consequently, this
means that the thermal control must be very precise in order to keep each
channel inside the specified channel space (which is +/20 GHz). As
aforementioned, there are multiple channels in NG-PON2 transmission; therefore,
the receiver must be tunable so as to work for any one of them at a particular
time while others are rejected. This requirement implies that there is a need
for a very tight band-pass filter too for efficient operation. Also, the tuning
time classes, already presented in Table 1 in Section 3, are
likewise strict and difficult to achieve on the hardware side. Besides, one of
the major related issues is the amount of the required
optical-electricaloptical (OEO) conversions, which can bring about an unviable
and unsustainable system [55].
6.1 Photonic integrated circuit
The optical communications evolution has
initiated enhanced photonic integrated circuits (PICs) that present a
cost-effective alternative to data transmission. With PIC technology
implementation, a number of optical components such as modulators, lasers,
amplifiers, detectors, etc. can be merged/integrated on a single chip.
Consequently, it helps in optical system design simplification, system
reliability enhancement, as well as significant power consumption and space
reduction. In addition, there can be considerable reduction in the amount of
OEO converters required for the system implementation. This subsequently
results in the total network cost reduction [55]. Thus, it is anticipated to be
an enabling and viable technology with immense flexibility and
reconfigurability in a number of fields [56]. A PIC has numerous advantages
over the traditional optical sub-assemblies
(OSAs). For instance, considering the occupied volume, the
PICs allow a very dense architecture in a small area, passing also by the
optical losses; however, the losses in the OSAs are higher because of the
internal free-space alignment between each optical component. Also, other
notable advantages of the PICs compared with the OSAs are lower power
consumption, lower footprint, and cost-effectiveness. Therefore, PICs have the
capability of permitting flexible and high data rate solutions [39, 55].
In the following, for the system realization, we propose three
different architectures: the ONU architecture, the OLT architecture, and the
architecture that can perform both functions just by hardware selection. It
should be noted that all of these architectures have the transmit and the
receive parts.
6.1.1
NG-PON2 ONU transceiver architecture
The ONU transceiver architecture is
represented in Figure 8. This is a very simple structure regarding
the optical setup, but the electrical control is very tough, mostly because of
the tunability (both on the transmitter and on the receiver). In this example,
there is one tunable laser. The laser can be tuned by temperature and can be
directly or externally modulated (the latter would also need a modulator after
the laser). On the receiver part, there is an optical band-pass filter which
has to be tunable to allow one of the downstream channels and cut the rest of
the spectrum. The tunable band-pass filter is followed by an optical receiver.
6.1.2
NG-PON2 OLT transceiver architecture
As explained before, the OLT is not tunable; both transmitter and
receiver should work on the same fixed wavelength pair, as depicted in Figure
9. Consequently, four pairs of optical devices will be needed. Since it
is very difficult to encapsulate everything on the same transceiver, the
solution that is being followed commercially is having four different
transceivers, one for each wavelength pair, and the wavelength multiplexer (WM)
device is external. This WM should, in each port, allow one wavelength pair,
meaning that in each port, it should pass only one downstream and the
respective upstream channel.
6.1.3
NG-PON2 OLT/ONU transmission architecture
The architectures presented in Figures
8 and 9 are the basic ones to have functional devices for
NG-PON2. But taking advantage of photonic integration, it is possible to
develop a much more complex circuit with more functionalities, which is being
presented next. Figure 10 illustrates the block diagram of an
architecture that can be used both as ONU and OLT. This helps in exploiting the
advantage of both functionalities on a single chip. The purpose (OLT or ONU) to
be served can be achieved just by hardware selection. This proposed
architecture fits inside a 4 4.6 mm indium phosphide (InP) PIC. In the
following subsection, we present the final design and some obtained simulation
results.
Figure 8.
ONU
transceiver architecture.
Figure 9.
OLT
transceiver architecture.
Figure 10.
Block
diagram of OLT/ONU transmission architecture.
6.2 PIC implementation of OLT/ONU
and receiver circuits
The architecture comprises four lasers,
four Mach-Zehnder modulators (MZM), and a number of filters. Two of the filters
are for changing the operational frequency band (C band for upstream
transmission and L band for downstream). Also, one filter is employed for
tuning the four lasers to the correct wavelength. Besides, at the output, there
is one filter working as a combiner of the four lasers. The band selection is
made using the two semiconductor optical amplifiers (SOAs) that are placed
after the band filters. It is noteworthy that the two SOAs are working as
switches and determine the chip’s operating mode (i.e., OLT or ONU). Therefore,
one of the SOAs is amplifying the light (active SOA), while the other is
absorbing (passive SOA). Consequently, by this configuration, only one band
filter is contributing to the setup. The employed lasers are built using laser
cavities which contain
SOAs that are being used for gain
purposes, filters, and reflectors on both sides. The C þ L band filter
helps in the selection of the downstream or upstream channel [39].
Moreover, the architecture includes also a
multimode interferometer reflector (MMIR) before the band selection and another
one after each gain SOA. These reflectors define the laser cavity limits. The
second MMIR, after the gain SOAs, only reflects 50% of the light, and the
remaining 50% is the laser cavity output and is sent to the MZM for modulation.
After the modulation on the MZMs, all four channels are combined in just one,
and the resulting light signal is sent to the output
Figure 11.
Receiver
block diagram.
Figure 12.
OLT/ONU
integrated transceiver design masks.
of the PIC, where a fiber will be aligned to collect the
light, and subsequently, it will be sent to the network [39].
6.2.1 PIC implementation of receiver circuit
This PIC has also a receiver circuit, but
it is a simple one, with just a wavelength division multiplexer (WDM) filter
which receives the light from the network and routes each NG-PON2 channel for a
different PIN. The receiver circuit schematic is depicted in Figure
11.
6.2.2 PIC implementation of OLT/ONU circuit
Using the photonic design kit (PDK) from the foundry Smart Photonics and a
software for PIC design (Phoenix Software at the time, meantime bought by
synopsis) for the implementation, the final circuit masks of the chip are shown
in Figure 12.
7. Results and discussion
In this section, we present the obtained
simulation results with further discussion on NG-PON2 physical layer
architecture design and development based on PICs. Figure 13 shows
the spectral simulation results obtained using advanced simulator for photonic
integrated circuits (Aspic) software from filarete. On the left figure, there
is the downstream operation (L band selected), and on the right there is the
upstream (C band selected). In the figure, the spectra in blue, pink, orange,
and green are the four channels. In both cases, it is possible to conclude that
there is about 30 dB of suppression of replicas. The suppression facilitates
smooth operation of the system by preventing intra-channel interference.
The reason for using laser cavities is due
to the limitations on the foundry. During the chip’s design period, the Smart
Photonics did not offer lasers on their process design kit (PDK). Consequently,
improvements in the architecture can be undertaken to potentiate the results.
For instance, the laser cavities could be replaced by distributed feedback
(DFB) or distributed Bragg reflector (DBR) lasers that have narrow linewidth
and a stable single mode operation. In this case, the cavity would disappear,
and the filtering should be done after the lasing. In addition, the
architectures can be simplified using only one modulator; nevertheless, it
Figure 13.
Optical
spectra at the transmitter output (a) downstream and (b) upstream.
would not be possible to transmit the four
channels simultaneously; this implies that only one channel can be transmitted
at a time. The proposed and developed architectures demonstrate the potential
of photonic integration for optical architectures. Consequently, the
architectures not only have the ability of supporting high data rates, high
density, and flexible solutions but also offer advantages such as low power
consumption, improved functionality, low footprint, and cost-effectiveness.
8. Conclusion
The 5G based system is a promising
solution for attending to the growing concerns about the traffic pressure on
the network. Also, the envisaged massive number of deployment scenarios and use
cases to be supported brings about highbandwidth and low-latency requirements
for the 5G networks. The small-cell-based C-RAN approach can efficiently attend
to the associated ultradense deployment. However, the C-RAN-based approach
imposes stringent requirements regarding jitter, bandwidth, and latency for the
mobile transport networks. In this book chapter, we have presented wired and
wireless transport solutions that are capable of addressing the C-RAN-based
stringent requirements and, consequently, the 5G mobile transport network
demands. Furthermore, owing to its significant and inherent advantages for the
5G and beyond networks, we have focused on the NGPON2 system. We have exploited
the salient advantages and the low footprint platform offered by the PICs in the
NG-PON2 system design and implementation. Based on these technologies, the
proposed architectures are capable of alleviating the associated losses in the
system while also helping in increasing the system power budget. In addition,
employment of the proposed architectures can help the device makers,
service/network providers, and infrastructure and chip vendors, in lowering the
footprint of network elements.
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