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 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].

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

Telecommunication Systems – Principles and Applications of Wireless-Optical Technologies

due mainly to the fact that the RF and FSO operating frequency bands are completely different. Consequently, it offers better performance than the tradi-

a wavelength for onward transmission over the fronthaul network [12].

Functional split options between CU and DU with emphasized PHY layer.

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

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

tional RF/RF transmission schemes [2, 11, 13, 14].

5.5 RAN functional split

Figure 7.

156

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.

6.2 PIC implementation of OLT/ONU and receiver circuits

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

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

Block diagram of OLT/ONU transmission architecture.

Figure 9.

Figure 10.

159

OLT transceiver architecture.

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

Figure 8. ONU transceiver architecture.

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

Figure 9. OLT transceiver architecture.

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

Telecommunication Systems – Principles and Applications of Wireless-Optical Technologies

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 spec-

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

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

trum. The tunable band-pass filter is followed by an optical receiver.

these architectures have the transmit and the receive parts.

6.1.1 NG-PON2 ONU transceiver architecture

6.1.2 NG-PON2 OLT transceiver architecture

downstream and the respective upstream channel.

6.1.3 NG-PON2 OLT/ONU transmission architecture

final design and some obtained simulation results.

Figure 8.

158

ONU transceiver architecture.

Figure 10. Block diagram of OLT/ONU transmission architecture.
