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

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

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

capable of offering relatively high-data rates, they exhibit limited mobility and coverage. The drawbacks can be reduced by employing Wi-Fi mesh networks

Telecommunication Systems – Principles and Applications of Wireless-Optical Technologies

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.

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].

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

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

[11, 28].

2.2 Wired fronthaul solution

2.2.1 Dark fiber solution

2.2.2 Ethernet solution

specification into Ethernet packets [11, 29].

2.2.3 WDM-based solution

142

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].

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

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

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,

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

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 low-

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)

supported with the implementation of WDM schemes.

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

coupled with easy of deployment, are really desirable [11].

fronthaul solutions are depicted in Figure 2.

3. Passive optical network (PON)

maintenance cost [11, 31, 32].

145

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.

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

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.
