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

performance, in this work, we focus on the NG-PON2 system. Its PHY architecture

Telecommunication Systems – Principles and Applications of Wireless-Optical Technologies

Furthermore, in an effort to make considerable profit, different operators have

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

Loss Min. (dB) 5 10 13 15 14 16 18 20

Note: The degree of severity of specific class requirements could vary from one system category to another.

Max. (dB) 20 25 28 30 29 31 33 35

Class A B B+ C N1 N2 E1 E2

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 evolu-

tion and the present GPON system is highly essential [35, 44].

and development are presented in Section 6.

4. PON system coexistence

Table 2.

Figure 6.

152

PON system coexistence.

ODN optical path loss classes [42, 46].

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

capacity of optical systems and the related deployment simplicity of wireless net-

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

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,

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.

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

Conceptually, multiple virtual aperture systems are generated in the relayassisted transmission with the intention of realizing salient MIMO technique

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

works [11, 13, 14].

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

and availability [11].

5.3 Hybrid RF/FSO scheme

beyond networks [11, 13, 14, 52, 53].

5.4 Relay-assisted FSO scheme

[2, 11, 13, 14].

155

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

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

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

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

Telecommunication Systems – Principles and Applications of Wireless-Optical Technologies

It is noteworthy that the RoF scheme employment is contingent on physical

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 bandwidth-

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

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

intensive services transmission via the mobile networks.

with its implementation [47].

[11, 13, 14].

154

5.2 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

access control layer functions like HARQ flow and physical layer function

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

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 NG-PON2, 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

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 solu-

allocation for different service requirements [12, 29].

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

system [55].

tions [39, 55].

157

6.1 Photonic integrated circuit

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

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].
