Abstract

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.

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) [2]. 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

Enabling Optical Wired and Wireless Technologies for 5G and Beyond 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 [9]. 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 [11–14]. 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 [11, 12]. 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 [15]. A number of approaches such as cooperative radio resource allocation and data compression technologies have been adopted to address the challenges; however, the fronthaul

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 [11, 12]. As aforementioned

and as depicted in Figure 1, optical and wireless network convergence is a

be required on the backhaul/fronthaul networks [10].

DOI: http://dx.doi.org/10.5772/intechopen.85858

capacity demand is still considerable high [11, 12].

139

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 [2, 4] 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) [5–7].

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 as discussed in Section 3. 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 [2].

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 [9]. 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

### Enabling Optical Wired and Wireless Technologies for 5G and Beyond Networks DOI: http://dx.doi.org/10.5772/intechopen.85858

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) [2]. 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 [10].

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 [9]. 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 [11–14]. 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 [11, 12]. 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 [15]. 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 [11, 12].

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 [11, 12]. As aforementioned and as depicted in Figure 1, optical and wireless network convergence is a

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

Telecommunication Systems – Principles and Applications of Wireless-Optical Technologies

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 [2, 4] 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

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 as discussed in Section 3. 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

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 [9]. 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

services in close proximity to the numerous subscribers [2].

solutions that can satisfy the network requirements.

access network (RAN) [5–7].

138

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 [19–21]. In [11], 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.

Enabling Optical Wired and Wireless Technologies for 5G and Beyond Networks

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 [11, 22, 23].

employed is feasible due to the availability of various compact and high-

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 [26]. Although Wi-Fi networks are

comparatively shorter transmission range [11, 22, 24, 25].

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

dimensional 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

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 [11, 26]. 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 [27]. Furthermore, it is remarkable that the Wi-Fi unlicensed spectrum is a promising

2.1 Wireless fronthaul solution

DOI: http://dx.doi.org/10.5772/intechopen.85858

141

#### 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 [4, 16–18]. 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 NG-PON2 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 solutions in Section 2. 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 in Section 6. In Section 7, the obtained simulation results with further discussion are presented. Section 8 concludes the chapter.

### 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

Enabling Optical Wired and Wireless Technologies for 5G and Beyond Networks DOI: http://dx.doi.org/10.5772/intechopen.85858

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 [19–21]. In [11], 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

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 [4, 16–18]. 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 NG-PON2 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

Telecommunication Systems – Principles and Applications of Wireless-Optical Technologies

A scenario for optical and wireless access networks convergence (adapted from Alimi et al. [2]).

Furthermore, with concise information on the enabling optical wired and wire-

The fronthaul protocol can be transported by different viable means. Apart from

the usually employed small form pluggable and serial constant bit rate CPRI

less technologies and the need for alleviating the stringent requirements in the network being introduced, we present comprehensive overview of the fronthaul transport solutions in Section 2. 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 in Section 6. In Section 7, the obtained simulation results with

further discussion are presented. Section 8 concludes the chapter.

integration for optical architectures.

Figure 1.

2. Fronthaul transport solutions

140

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 [11, 22, 23].

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 [11, 22, 24, 25].

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 [11, 26]. 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 [27].

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 [26]. 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 [11, 28].

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

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.

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

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 quad-

rature 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

for the system monitoring at the CO demarcation point [11, 22, 23, 29].

Enabling Optical Wired and Wireless Technologies for 5G and Beyond Networks

DOI: http://dx.doi.org/10.5772/intechopen.85858

2.2.3.1 Passive WDM

demarcation points [11, 22, 23, 29].

2.1.3.2 WDM/OTN

143
