Agapi Mesodiakaki

Pavlos Maniotis
email: ppmaniot@csd.auth.gr

Georgios Kalfas
email: gkalfas@csd.auth.gr

Christos Vagionas
email: chvagion@csd.auth.gr

John Vardakas
email: jvardakas@iquadrat.com

Elli Kartsakli
email: ellik@iquadrat.com

Angelos Antonopoulos
email: aantonopoulos@cttc.es

Eftychia Datsika
email: edatsika@iquadrat.com

Christos Verikoukis

Nikos Pleros
email: npleros@csd.auth.gr

A Gated Service MAC Protocol for 5G Fiber-Wireless Cloud-Radio Access Networks

Keywords: Fifth Generation (5G) networks, mean packet delay model, fiber-wireless (FiWi), millimeter wave (mmWave), cloud-radio access networks

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Next generation, i.e., Fifth Generation (5G), networks increase peak data rate and cell edge data rate needs to 20 Gbps and 1 Gbps, respectively, compared to Forth Generation (4G) networks. In parallel, latency-critical 5G applications, also known as Ultra-Reliable Low Latency Communication (URLLC) applications, require lower than 1 ms end-to-end delay [START_REF] Chen | Ultra-Reliable Low Latency Cellular Networks: Use Cases, Challenges and Approaches[END_REF]. In order to meet these everincreasing capacity and latency demands, the exploitation of higher spectrum bands, e.g., Millimeter Wave (mmWave), is expected to play a key role due to the huge bandwidth availability they offer. This trend is also reflected in Rel. 15 of 5G New Radio (NR) standard by 3GPP, which refers to the exploitation of spectrum bands up to 52.6 GHz, with Rel. 16 including even higher frequencies [START_REF] Parkvall | NR: The New 5G Radio Access Technology[END_REF]. Nevertheless, mmWave bands experience high propagation losses, and therefore, enable shorter link ranges than traditional sub-6GHz networks. As a result, when used in the Radio Access Network (RAN) part, they stress the need for antenna densification.

Hence, centralized architectures are favored, e.g., Cloud-RANs (C-RANs) [START_REF] Agiwal | Next Generation 5G Wireless Networks: A Comprehensive Survey[END_REF], which separate BaseBand Units (BBUs) from Remote Radio Heads (RRHs), placing the former at centralized locations. Thereby, efficient network management as well as energy supply and climate control are achieved. Centralization, however, sets challenging capacity requirements for the BBU-RRH connection, also known as fronthaul, which should be able to support a massive number of broad mmWave channels. In parallel, the protocol specification used for the BBU-RRH communication, also known as Common Public Radio Interface (CPRI) [START_REF]CPRI Specification: Common Public Radio Interface (CPRI); Interface Specification[END_REF], is highly inefficient in this case as it imposes up to two orders of magnitude bandwidth penalty compared to the IP rate. In this context, the need to design new efficient protocol solutions is imperative.

To that end, Analog-Radio-over-Fiber (A-RoF) technology has received increased attention from the research community mainly by virtue of its capability to meet both the centralization and high fronthaul capacity needs [START_REF] Liu | Efficient Mobile Fronthaul via DSP-Based Channel Aggregation[END_REF][START_REF] Ishimura | 1.032-Tb/s CPRI-Equivalent Rate IF-Over-Fiber Transmission Using a Parallel IM/PM Transmitter for High-Capacity Mobile Fronthaul Links[END_REF][START_REF] Giannoulis | Analog Radio-over-Fiber Solutions for 5G Communications in the Beyond-CPRI Era[END_REF][START_REF] Delmade | Performance analysis of analog IF over fiber fronthaul link with 4G and 5G coexistence[END_REF]. This is achieved by placing all main hardware, i.e., sampling and digital-to-analog conversion components, within the BBU, hence significantly simplifying the RRHs compared to Digital-Radio-over-Fiber (D-RoF) technology [START_REF] Novak | Radio-Over-Fiber Technologies for Emerging Wireless Systems[END_REF]. As a result, no bandwidth penalty is imposed on the fronthaul link, since no radio waveform digitization takes place and hence the full fiber bandwidth can be exploited to carry the mmWave channels [START_REF] Miyamoto | Split-PHY processing architecture to realize base station coordination and transmission bandwidth reduction in mobile fronthaul[END_REF]. On the other hand, by removing the inter-mediate digitization at the RRH, the optical and the wireless domains get completely separated one from the other. Thus, controlling both the optical and wireless resources from the centralized BBU pool, while meeting the strict 5G C-RAN delay requirements becomes a challenging problem to face.

A Medium Transparent-MAC (MT-MAC) scheme has been proposed to efficiently manage both the optical and wireless resources in Fiber-Wireless (FiWi) networks that employ A-RoF [START_REF] Kalfas | An Agile and Medium-Transparent MAC Protocol for 60 GHz Radio-Over-Fiber Local Access Networks[END_REF][START_REF] Kalfas | Non-Saturation Delay Analysis of Medium Transparent MAC Protocol for 60 GHz Fiber-Wireless Towards 5G mmWave Networks[END_REF]. In MT-MAC, the users have a direct communication with the BBU, while the RRHs perform solely the Radio Frequency (RF)/ optical signal conversion. An enhanced MT-MAC version has been also proposed aiming at optimizing the size of the transmission window allocated to each RRH. In particular, contrary to the fixed allocation included in the traditional MT-MAC version, a more sophisticated allocation based on the number of active users, i.e., users that have traffic to be sent, located in the range of each RRH is performed. In other words, an RRH with a higher number of active users will be allocated a longer transmission window. Due to its client-weighted data wavelength allocation, this protocol version is also known as Client-Weighted MT-MAC [START_REF] Kalfas | Client-weighted medium-transparent MAC protocol for user-centric fairness in 60 GHz radio-over-fiber WLANs[END_REF][START_REF] Mesodiakaki | Medium-transparent Dynamic Bandwidth Allocation for 5G Fiber Wireless Dense Fronthaul Networks[END_REF]. Client-Weighted MT-MAC was shown to achieve improved performance in terms of throughput and mean packet delay fairness compared to the fixed allocation of the traditional MT-MAC. Specifically, the fact that the size of the transmission window allocated to each user was the same regardless of the number of users that were sharing the same RRH resources, results in improved service equalization.

Although the aforementioned schemes achieve fairness either among RRHs (traditional MT-MAC, where the same transmission window size is allocated to each RRH), or among users (client-weighted MT-MAC, where the transmission window size allocated to each RRH is proportional to its number of active users), more sophisticated schemes are needed that will offer service equalization on a packet basis. To this end, the gated service scheme addresses this challenge by allocating transmission windows proportional to the number of packets located at the buffer of each user during the reporting phase. This scheme has received great research attention and has been also studied in the context of the Interleaved Polling with Adaptive Cycle Time (IPACT) protocol for Ethernet Passive Optical Networks (EPONs) [START_REF] Ngo | A mean value analysis approach for evaluating the performance of EPON with Gated IPACT[END_REF][START_REF] Miyata | Exact mean packet delay for delayed report messages multipoint control protocol in EPON[END_REF]. As shown in [START_REF] Lannoo | Analytical model for the IPACT dynamic bandwidth allocation algorithm for EPONs[END_REF], gated service IPACT achieves considerable lower delay compared to fixed-service IPACT. Nonetheless, despite its unquestionable benefits, to the best of our knowledge, the gated service has never been exploited to MT-MAC protocol design for A-RoF FiWi C-RANs.

To that end, in this paper, we propose a gated service MT-MAC protocol (gMT-MAC), i.e., a protocol that grants a transmission window to each user equal to the amount of time needed to successfully send its reported traffic. An analytical model for the mean packet delay is also proposed and verified by means of simulation. The protocol performance is also evaluated for different network load conditions, number of RRHs and optical wavelength availability levels. Part of this work is based on the analysis of gated IPACT scheme for EPONs, presented in [START_REF] Lannoo | Analytical model for the IPACT dynamic bandwidth allocation algorithm for EPONs[END_REF]. It has been appropriately adapted though for A-RoF FiWi C-RANs to account for the additional delay induced by the wavelength allocation and contention periods during the joint optical and wireless gMT-MAC time cycle. Thereby, the proposed work constitutes the first gated service model that addresses the joint wireless and optical resource allocation problem for FiWi C-RANs. On the contrary, the state-of-the-art either consists of RoF models addressing only the wireless resource allocation problem [START_REF] Fan | Performance analysis for IEEE 802.11 distributed coordination function in radio-over-fiber-based distributed antenna systems[END_REF][START_REF] Pal | Performance analysis of IEEE 802.11 distributed coordination function in presence of hidden stations under non-saturated conditions with infinite buffer in radio-over-fiber wireless LANs[END_REF][START_REF] Mjeku | Analysis of the Request to Send/Clear to Send Exchange in WLAN Over Fiber Networks[END_REF] or operates with fixed transmission window allocation [START_REF] Kalfas | Non-Saturation Delay Analysis of Medium Transparent MAC Protocol for 60 GHz Fiber-Wireless Towards 5G mmWave Networks[END_REF]. The proposed model is shown to be in a very good agreement with the simulation results, thus proving the validity of both. Our results also prove the suitability of gMT-MAC for 5G C-RANs, since it is shown to be able to meet the sub-ms mean packet delay requirements of 5G latency-critical applications.

The rest of the paper is organized as follows. In Sections 2 and 3, the system model and the gMT-MAC operation are described, respectively. In Section 4, the mean packet delay analytical model for is presented. Section 5 refers to the model validation by means of simulation. Specifically, the simulation scenario as well as the evaluation results are given, while useful insights are gained for the performance of gMT-MAC. Finally, Section 6 concludes the paper.

System Model

As shown in Fig. 1, the system model under study consists of a C-RAN network, in which a BBU is placed at a centralized location l f meters away from R RRHs. A Passive Optical Network (PON) topology is also employed for the RRH interconnection. The parameter U r refers to the number of users served by the RRH r. Poisson packet arrivals are assumed with bit rate equal to λ U bps and a fixed packet length of l p . The buffer size of each user is B packets.

In order to tackle the increased complexity, a symmetric network in terms of number of users is assumed, i.e., U 1 =U 2 =...=U r =U R =U . Hence, all RRHs have the same average packet arrival rate (bps), i.e., λ=U λ U . Nevertheless, it is worth pointing out that the number of packets located at the buffer of each user at a specific instant is a random parameter, although it follows the same distribution for all users and RRHs. Consequently, the same holds for the number of active users per RRH. These instantaneous changes are exploited by gated service schemes, which grant transmission windows proportional to the number of packets that each user requests at a specific time period.

We also assume W data wavelengths which can be transmitted through the same fiber link exploiting Wavelength Division Multiplexing (WDM). Another wavelength is also assumed, which is dedicated for control information exchange. The BBU-user communication over the FiWi link follows the gMT-MAC proto-col, which will be detailed in Section 3. Finally, uplink and downlink operate at different frequencies, i.e., a node can receive and transmit concurrently.

Gated Service MT-MAC Protocol (gMT-MAC)

The proposed protocol comprises two parallel procedures, the allocation of data wavelength to the RRHs with active users and the allocation of resources to their users. For the data wavelength assignment, the BBU should initially be informed for the RRHs with users that have traffic to send. Therefore, it transmits a short pulse which is broadcasted to all users through their RRHs [START_REF] Kalfas | Non-Saturation Delay Analysis of Medium Transparent MAC Protocol for 60 GHz Fiber-Wireless Towards 5G mmWave Networks[END_REF]. The users with pending traffic reply with the same pulse to their respective RRHs and from there the pulses are transmitted back to the BBU. For the identification of the RRHs that contain users with pending traffic, the difference in the distance between the BBU and each RRH is exploited, as it eventually leads to time difference between the reception of the pulses originated by different RRHs. Once the RRHs with pending traffic have been identified, a round robin allocation of the data wavelengths takes place among them. Specifically, a list is created including the RRHs with active users that have not been served yet. The RRH located first in the list has the highest priority in case a new data wavelength becomes available. On the other hand, in case a new RRH becomes active, it will be placed last.

Regarding the resource allocation to the active users of an RRH that has been assigned a data wavelength, it includes Request Resource Frames (RRFs) and data exchange frames. The first type of frames refers to the identification of the active users, while the second to the exchange of data information.

During an RRF, each active user picks up a random number between [0, s RRF -1], which represents the number of slots it has to wait until it transmits its ID packet to the BBU. The parameter s RRF denotes the number of RRF slots. Hence, in each slot, the BBU transmits a POLL and the user with the respective chosen number replies with its ID, including its buffer (BF) status. Upon successful reception of the ID packet by the BBU, an ACK packet is transmitted to the user. In case more than one users choose the same number, a collision occurs and the BBU does not send an ACK. In this case, the collided users will participate to another RRF by choosing a new number from the set [0, s RRF -1]. The RRF procedure will be repeated until either the maximum number of RRFs has been reached or all users have been successfully identified.

As for the data frame exchange process, each one of the identified users is being sequentially polled by the BBU to send its data. The polling sequence follows the user identification order. Due to the gated service employed by gMT-MAC, the transmission window allocated to each user is proportional to the number of reported packets located at its buffer during the identification process. Upon successful reception of the data packets of a polled user, the BBU sends an ACK packet following a procedure similar to the RRF process.

Operational Example of gMT-MAC

For a better understanding of gMT-MAC operation, an operational example referring i) to the RRF process and ii) to the data exchange process is given.

Request Resource Frame (RRF) process

As shown in Fig. 2, we consider a simple example with 3 active users and s RRF =4 RRF slots. At the beginning of the RRF, each user selects a random number from the set [0,3]. In the considered example, U1 selected number 3, U2 number 0 and U3 number 1. As a result, U1 will send its ID packet after waiting 3 slots, i.e., at slot 4, U2 without waiting at all, i.e., at slot 1 and U3 at slot 2. In each slot, the BBU transmits a POLL packet. Upon receiving the POLL, i.e., after T P OLL + δ f + δ a , the user that selected the respective slot replies with its ID packet including its buffer status, BF. The parameters T P OLL , δ f and δ a stand for the time duration of a POLL packet and the propagation delay in the fiber and in the wireless medium, respectively. The next POLL, which corresponds to slot 2, is scheduled T P OLL + δ f +δ a before the end of slot 1. As a result, U3 that is scheduled to send its ID packet at slot 2, transmits it at the beginning of slot 2. The same procedure is repeated for the rest of the slots. Notice that slot 3 corresponds to an empty slot, since no user has selected it. Regarding the acknowledgement of the ID packet reception, ACK packets are sent together with the next POLL frames for higher bandwidth efficiency. In particular, an ACK is sent together with the next scheduled POLL frame as long as i) the ID packet has been successfully received by the BBU, and ii) there is another POLL packet to be sent. Hence, in the considered example, the ACK for the ID packet of U2 is sent together with the third POLL, while the ACK for the ID packet of U3 is sent together with the forth POLL. For the ID packet of U1, however, the ACK is sent immediately after the ID packet reception by the BBU, given that there is no other scheduled POLL frame left.

Thus, the RRF process duration can be given by

T RRF = T P OLL + s RRF T ID + T ACK + 3δ f + 3δ a , (1)
where T ID , T ACK is the time duration of an ID and an ACK packet, respectively. Data frame exchange process Regarding the data frame exchange process, a simple example is given in Fig. 3, assuming the same users that have been identified in the previous RRF example of Fig. 2. During the data frame exchange, a POLL packet is transmitted by the BBU for each active user sequentially in the order that they have been previously identified. Hence, initially U2 will be polled, then U3 and finally U1. For each user that is being polled, a transmission window is being granted based on the buffer status it reported during the identification process. Thus, a transmission window equal to the duration of 4 data packets is being granted to U2, of 2 data packets to U3 and of 6 data packets to U1. After each polled user has successfully sent its reported data to the BBU, an ACK is sent to the user. For higher bandwidth efficiency, similar to the RRF process, the ACK is sent together with the next POLL packet, as long as there is one. Hence, the ACK for the data packets of U2 is transmitted together with the third POLL. In the case there is no other POLL packet left, i.e., after all identified users have been polled, a single ACK is sent in the end of the process, which contains acknowledgement information for all the remaining data packets of the users. Moreover, given that there is no need to wait for the ACK packet to be received by the users, the duration of the data exchange process equals

T DE = T P OLL + Uact i=1 T DAT Ai + T ACK + 2δ f + 2δ a , (2)
with U act denoting the number of active users that have been successfully identified in the previous RRF process.

Mean Packet Delay Analysis

We focus our analysis on the time period between two successive transmission windows of a specific RRH, denoted by T cyc . This parameter increases with the traffic load, since in gMT-MAC all users send as many data packets as they requested during the identification process. To calculate the average T cyc , we first derive its minimum value that equals a round-trip time in the fiber. Hence,

T min cyc = 2δ f , (3)
where δ f = l f /c f is the propagation delay in the fiber, with l f denoting the average fiber length and c f being the speed of light in the fiber. Thereafter, based on T min cyc calculation, we derive the minimum cycle time under traffic load as the sum of T min cyc and the amount of time needed to send the packets that have been generated by all RRHs in a minimum cycle time, i.e.,

T min cyc = T min cyc + (Λ/l p )T min cyc T p + T P OLL + T ACK + 2δ f + 2δ a , (4)
where Λ = (R/W )λ denotes the total packet arrival rate (bps) of all RRHs. The parameters T p , T P OLL , T ACK refer to the duration of a data packet, a POLL and an ACK packet, respectively. In general, the duration of a packet x of l x bits is given by T x = l x /D u , with D u being the uplink channel capacity (bps).

The properties of the Poisson traffic enable us to derive an approximate distribution of cycle times. Given that the number of packets is an integer number, the duration of a cycle m can take solely discrete values, i.e.,

T m cyc = R W (T P OLL + T ACK + 2δ f + 2δ a ) + T RRF + mT p , m ≥ 0, (5)
with T RRF being the RRF duration, given by (1). By solving [START_REF] Liu | Efficient Mobile Fronthaul via DSP-Based Channel Aggregation[END_REF] in terms of m and setting T m cyc =T min cyc , we derive the minimum value of m, denoted by m min . We then model the evolution of cycle times as a discrete Markov chain and calculate the matrix of transition probabilities from a cycle to another, denoted by P. The longest cycle corresponds to the case where all user buffers are full. In this case, the total number of packets is B all max =B•U •(R/W ). Hence, the transition probability to cycle times that are longer than the minimum cycle time under traffic load can be calculated as

p i,j = P r[T mmin+j cyc |T mmin+i cyc ] = e -Λ lp T m min +i cyc Λ lp T mmin+i cyc j j! , i ≥ 0, j > 0. (6)
The transition probability to the minimum cycle time under traffic load equals

p i,0 = P r[T mmin cyc |T mmin+i cyc ] = n∈[0,mmin] e -Λ lp T m min +i cyc Λ lp T mmin+i cyc n n! , i ≥ 0. (7)
Thus, we derive the steady state probabilities by solving the following system. where π is the eigenvector reflecting the steady state transition probabilities to cycle time i, with i ranging from 0 to B all max . The average cycle time then equals

πP = π (8) B all max i=0 π i = 1, (9)
T cyc = B all max i=0 π i T mmin+i cyc . ( 10
)
Considering that a packet is more likely to arrive at a longer cycle time, we can conclude that the probability that a packet arrives at a specific cycle is proportional to its length. Hence, the steady state probabilities are rewritten as

π i = π i T mmin+i cyc B all max n=0 π n T mmin+n cyc . ( 11
)
It is also worth pointing out that a packet that arrives on a specific cycle will not be sent during the first transmission window, given that the user has first to notify the BBU for its buffer status and successfully receive the POLL packet before initiating the data transmission. Moreover, packet arrivals take place on average half-way between two transmission windows due to non-bursty nature of Poisson traffic. Hence, a packet stays in the queue on average for one-and-a-half cycles, and consequently the mean packet delay can be approximated by

X = 3 2 B all max i=0 π i T mmin+i cyc . ( 12
)
5 Model Validation

Simulation Scenario

The proposed mean packet delay model was developed in MATLAB R , whereas for the simulations the Java discrete-event simulator, presented in [START_REF] Kalfas | Non-Saturation Delay Analysis of Medium Transparent MAC Protocol for 60 GHz Fiber-Wireless Towards 5G mmWave Networks[END_REF], was modified accordingly so as to consider the gated service related features of gMT-MAC.

In our results, we study different R/W ratio values due to the high impact of this parameter on the system performance. Per RRH, U =5 users are considered, while the fiber length has a mean value of l f =1 km and variance 5 m. The rest of the simulation parameters are summarized in Table 1.

Fig. 1 .

 1 Fig. 1. System model under study composed of a BaseBand Unit (BBU) located at a distant location and R Remote Radio Heads (RRHs) forming a Passive Optical Network (PON). Wavelength Division Multiplexing (WDM) is also employed in case more than one data wavelengths are available.

Fig. 2 .

 2 Fig. 2. Example of Request Resource Frame (RRF) process with 3 active users and sRRF =4 RRF slots.

Fig. 3 .

 3 Fig. 3. Example of data exchange process between the BBU and the 3 active users that have been identified based on the RRF process example of Fig. 2.

Table 1 .

 1 Simulation Parameters.

	Parameter	Value	Parameter	Value
	Speed of light in the fiber (cf ) 2•10 8 m/s	ID packet size (lID)	72 bytes
	Air propagation delay (δa)	0.2 μs	POLL packet size (lPOLL)	72 bytes
	Number of RRF slots (sRRF )	10	Bit rate per data wavelength (Du) 1 Gbps
	ACK packet size (lACK )	16 bytes Maximum consecutive RRF frames	1
	Data packet size (lp)	2000 bytes	User buffer size (B)	40 packets

Supported by H2020-5G PPP 5G-PHOS (grant agreement 761989), MSCA ITN 5G STEP-FWD (grant agreement 722429), SPOT5G (TEC2017-87456-P) and AGAUR (2017 SGR 891).

Results

In Fig. 4, the mean packet delay versus the normalized offered load is shown for different R/W ratio values for both the analytical model and the simulations. Please note that the system operates under stable conditions for normalized offered load up to 0.8. In other words, the network throughput is equal to the offered load and consequently there are no dropped packets. However, above this value, the network becomes unstable, i.e., with non-zero packet drops.

The analytical model is shown to be in a very good agreement with the simulation results for all R/W ratio values, which proves the validity of both. Nevertheless, for 0.9 normalized offered load, the network becomes unstable, making the queues susceptible to small variations caused by the probabilistic Poisson traffic, which accounts for the small model-simulations gap.

We can also observe that the mean packet delay increases with the offered load and R/W ratio value increase. It is worth pointing out, however, that in all cases, when the system operates under stable conditions, the mean packet delay remains below 1 ms. Thereby, the suitability of gMT-MAC is proved to satisfy the challenging 5G delay requirements e.g., of URLLC services.

Conclusions

In this paper, we proposed gMT-MAC, an MT-MAC protocol for mmWave A-RoF C-RANs, which employs gated service for maximum bandwidth efficiency. We also proposed a mean packet delay model, which was verified by means of simulation for different R/W ratio values and network load conditions. Our results showed: i) that the proposed model closely matches the simulations, which proves the validity of both, and ii) that gMT-MAC can be considered a promising 5G protocol able to address the challenging requirements of 5G C-RANs by offering sub-ms delay performance.


Hidenori Takahashi
email: takahashi@kddi-research.jp

State-of-the-Art and Future of Submarine Cable System Technology

Keywords: Submarine cable, optical communication, digital coherent transmission

The FASTER cable system has been developed as the first trans-pacific optical submarine cable system designed for digital-coherent transmission at the initial state. With this significant change for submarine cables, the design capacity is continuously being upgraded following the improvement of the stateof-the-art modulation format to maximize spectral efficiency even at the limited optical signal-to-noise ratio (OSNR). For the next generation, the novel technologies are expected to increase the capacity per cable under the conditions of limited feeding power and space in a cable. This report reviews some technologies from the current to the promising future submarine cable systems such as the introduction of space-division-multiplexing (SDM) technologies.

INTRODUCTION

The demand for increasing the capacity of optical submarine cable systems for global communication is steadily growing. The traffic increase rate is maintained at around 40% per year for transpacific segments. To support the traffic growth, over 10 submarine cable systems have been built and operated following the technology evolutions. Nowadays, digital-coherent transmission technology is considered as the default for a transponder at the initial design stage of optical submarine cables. For example, in the FASTER cable system whose ready-for-service (RFS) launch was June 2016, the initial design capacity is 60 Tbit/s consisting of 6 fiber pairs (FP) x 100 ch x 100 Gbit/s utilizing wavelength-division-multiplexing (WDM) technology [START_REF]FASTER Cable System is Ready for Service, Boosts Trans-Pacific Capacity and Connectivity[END_REF][START_REF] Kamalov | FASTER Open Submarine Cable[END_REF][START_REF] Kamalov | Evolution from 8QAM Live Traffic to PS 64-QAM with Neural-Network Based Nonlinearity Compensation on 11000 km Open Subsea Cable[END_REF]. This trend never seems to change, so the higher capacity optical submarine cable system will be indispensable for future global communication. This paper reviews the evolution of telecommunication technology for submarine cable systems, explains the currently available state-of-the-art technologies, and finally describes the promising technologies that are expected to overcome some significant issues and be applicable for future optical submarine cable systems.

Table 1 shows the evolution of the once in-service transpacific submarine cable system for communication by RFS year, including those already retired. Typical cable length of transpacific systems is around 10,000 km. The "TPC-1" was the first telephone cable between Japan and the US, consisting of a coaxial cable, whose initial design capacity was just 128 phone lines. Assuming the bit rate of a phone line as 64 kbit/s, the initial design capacity has increased ~560 million times that of the original 50 years ago, leading to the introduction of FASTER. The optical fiber communication has been introduced with TPC-3 as the first transpacific optical submarine cable system. A regeneration repeater was used so the signal quality was independent of the total transmission distance; however, the capacity had never changed from the initial design stage. TPC-5 was the first transpacific cable to introduce an erbium-doped fiber amplifier (EDFA) for the re-amplifying scheme with the initial design capacity of 10 Gbit/s (5 Gbit/s x 2FP) in 1995. With its larger capacity, the mainstream of international video broadcasting was shifted from satellites to submarine cables after the Atlanta Olympic games in 1996. Thanks to the physical characteristics of EDFA, it is independent of the modulation format of optical signals. Even though it has the drawback of noise accumulation being generated in EDF, it is possible to increase the design capacity by changing the signal modulation format, known as upgrading. This means that the evolution of the transponder increases the value of the deployed cable. In other words, the limitation is determined by the usable bandwidth of a repeater. For example, already deployed cables seem obsolete when a new generation cable is constructed with a wider bandwidth. This is because the operation and maintenance cost for submarine cables is not so different between the old and the newly deployed version. Therefore, the narrower bandwidth cable has relatively higher running costs. Even if the design lifespan of submarine cables is typically 25 years, the actual "business" lifespan tends to be shortened with traffic migration to a higher capacity cable because of the previously described reason. For example, the China-US cable as shown in Table 1 was retired in Dec. 2016 even16 years after RFS [4].

Technology Trend of Optical Submarine Cable

The repeatered system with EDFA is widely adopted currently. Figure 1 shows the historical evolution of the initial design capacity per one FP for transpacific optical submarine cable systems. The TCP-5 used a dispersion shifted fiber (DSF) cable designed for single channel transmission. After that, the non-zero DSF (NZ-DSF) cable was introduced for WDM transmission such as the Japan-US cable network, etc. Furthermore, the dispersion managed fiber (DMF) cable was introduced for dense WDM (DWDM) systems such as Unity. For these cable types, the direct-decision (DD) receiver was considered as the transmission method. On the other hand, the digital-coherent transmission technology was considered at the initial design stage for FASTER. This enables the cable to only consist of positive dispersion fibers (D+ fiber).

STATE-OF-THE-ART TECHNOLOGIES

This section explains the cutting-edge submarine cable technologies that are currently available. There are two types of application: that for new cable deployment, and that for the upgrading of already deployed cables by simply replacing the submarine line terminal equipment (SLTE).

+D fiber Transmission Line

The newly deployed cable can support digital-coherent transmission since the introduction of FASTER for transpacific cables. This change has led to the introduction of the dual-polarization (DP), phase and amplitude modulation to increase the spectral efficiency (SE). Compared to DSF, NZ-DSF and DMF cables, the +D fiber cable gives increments of accumulated dispersion monotonously, which contributes to the reduction of the signal quality degradation caused by the nonlinear effect such as self-phase modulation (SPM), cross-phase modulation (XPM) and four-wave mixing (FWM), etc. However, the quantity of accumulated dispersion becomes huge, e.g., it hits 200,000 ps/nm when the signal transmits over 10,000-km fiber with +20 ps/nm/km dispersion.

For the digital coherent transmission, the dispersion is compensated by digital signal processing (DSP). Thanks to the development of the high-performance DSP, which had an ability to compensate such a huge dispersion, it has become possible to introduce +D fiber for transoceanic submarine cables. Moreover, the evolution of +D fiber is rapidly proceeded in terms of the larger effective area (Aeff) and lower attenuation. Compared to the standard single mode fiber (SSMF) whose Aeff is 80 µm 2 , the ultra-lowloss 130 µm 2 fiber is used in FASTER [START_REF] Kamalov | FASTER Open Submarine Cable[END_REF]. Furthermore, the fiber with 0.152-dB/km attenuation has been commercialized recently [START_REF] Yamaguchi | Ultra-Low Loss and Large Aeff Pure-Silica Core Fiber Advances[END_REF]. The large Aeff fiber contributes to the reduction of the nonlinear effect, and low loss fiber is effective for increasing the signalto-noise ratio (SNR) or extension of the repeater span, which gives a higher capacity or lower cost with fewer required numbers of repeaters. At R&D bases, as a loss of 0.1419 dB/km is reported with an Aeff of 147 µm 2 , further improvement can be expected [START_REF] Tamura | Lowest-ever 0.1419-dB/km Loss Optical Fiber[END_REF].

Additionally, this evolution leads to significant changes not only for the transmission capacity, but for the operation and maintenance works. In the case of cable repairment, it is required to insert additional cables whose length must be twice as long as the water depth. For example, if a cable fault happens at a water depth of 6,000 m, a 12-km cable at least must be inserted. This means that the accumulated dispersion is changed according to the length of the inserted cable. In the cable of NZ-DSF and/or DMF types, the transmission line is composed of at least 2 types of fiber, so the repair procedure depends on the type and the length of fiber at the faulty point with precise calculation of the accumulated dispersion. However, for the +D fiber cable, basically only one type of fiber is required to prepare the spare cable; additionally, the operator no longer has to pay heed to the change of the accumulated dispersion utilizing the DSP function in the transponder. This contributes to simplifying the operation and maintenance procedures.

Advanced Modulation Formats

As shown in Fig. 1, the initial design capacity has increased 10-fold from Unity to FASTER with the introduction of digital coherent transmission technology. Furthermore, it can achieve higher SE with the advanced modulation formats. This progress is applicable both to new cable systems and all of the already deployed cables such as DSF, NZ-DSF, DMF and +D fiber types. By upgrading the design capacity of the existing cable, the "business" lifespan of the cable can be extended. Especially, it is more effective to apply the format for +D fiber cables than the other fiber types. As +D fiber cables give smaller nonlinearity, they are suitable for the typical higher SE modulation format that is weak for nonlinear effects. In the case of FASTER, while the initially designed modulation format is dual-polarization (DP)-quadrature phase shift keying (QPSK) with SE of 2.5 bit/s/Hz between the Japan and US segment, the DP-8QAM (quadrature amplitude modulation) signals have been introduced in 37.5-GHz spacing with SE of 4.0 bit/s/Hz [START_REF] Kamalov | FASTER Open Submarine Cable[END_REF]. Additionally, at the MAREA for transatlantic cables with a length of 6,644 km, it has been reported that the SE of 6.21 bit/s/Hz is achievable with DP-16QAM, which indicates that it will contribute to the upgrading of the design capacity from the initial one of 160 Tbit/s/cable to 200 Tbit/s/cable [START_REF]Infinera Breaks Real-time Subsea Spectral Efficiency Records with 6.21 b/s/Hz on 6,644 km MAREA Trans[END_REF]. Furthermore, recently the commercialization of the constellation shaping modulation format has proceeded which can approach the theoretical limit, the so-called Shannon limit [START_REF] Kamalov | Evolution from 8QAM Live Traffic to PS 64-QAM with Neural-Network Based Nonlinearity Compensation on 11000 km Open Subsea Cable[END_REF][START_REF] Cho | Trans-Atlantic Field Trial Using High Spectral Efficiency Probabilistically Shaped 64-QAM and Single-Carrier Real-Time 250-Gb/s 16-QAM[END_REF]. Even these are offline processing, SE of 7.46 bit/s/Hz is confirmed at AEC-1 of transatlantic cables (5,523 km) with probabilistic constellation shaping (PS)-64QAM modulation [START_REF] Cho | Trans-Atlantic Field Trial Using High Spectral Efficiency Probabilistically Shaped 64-QAM and Single-Carrier Real-Time 250-Gb/s 16-QAM[END_REF]. Additionally, it has been confirmed that 5.5 bit/s/Hz is achievable with a Q-margin of 0.45 dB without the application of the nonlinear compensation function at the Taiwan to US segment of FASTER with 11,000 km [START_REF] Kamalov | Evolution from 8QAM Live Traffic to PS 64-QAM with Neural-Network Based Nonlinearity Compensation on 11000 km Open Subsea Cable[END_REF]. Still, the decision about the availability of these SE should be defined by operators according to their policies in light of the communication sustainability and reliability.

C+L Band Transmission Technologies and Raman Amplification

One of the restrictions for submarine cables is the limited number of fibers in a cable, therefore the technologies to increase the transmission bandwidth is attractive. A typical submarine cable system is designed for C-band transmission, but recently, C+L band transmission has been introduced. For example, PLCN, which is under construction at this moment, will be the first transpacific system with C+L band transmission between Hong Kong and the US of over 12,917 km [9]. Roughly, L-band can support almost the same capacity as C-band, so the C+L band can be treated as virtually double the number of FPs compared to the C-band only system. However, as shown in the schematic view of Fig. 2, the C-and L-band must be separated in a repeater, so twice the number of EDFAs is required, and therefore it does not contribute to the reduction of power consumption per signal basically. Another case of L-band application is the hybrid type with Raman amplification. As shown in the actual case of ARBR (2,700 km with 4 FPs) between Argentina and Brazil, even though the initial design capacity is 12 Tbit/s per FP, the potential capacity can be extended to 44 Tbit/s per FP utilizing the 70 nm of C+L band transmission bandwidth [10,11].

RESTRICTIONS FOR OPTICAL SUBMARINE CABLES

Submarine cable systems have particular restrictions compared to terrestrial ones. In this section, the two main limitations are explained.

Power Feeding Limitation for Submarine Cable

First, there is only one type of power supply for optical submarine cables. In the case of a typical point-to-point configuration, the electrical power is supplied from both ends of the cable by the power feeding equipment (PFE) in the cable landing station (CLS). For the transpacific cables, it supplies electrical power to 100 repeaters. The electric potential difference is mainly determined by the power consumption of repeaters and power loss at cables. The voltage of each side of the cable is set to be positive and negative half that of the required electric potential difference. For example, when the required potential difference is 15 kV, the voltage of each side is set to be +7.5 kV and -7.5 kV, respectively. So, the voltage in the middle point becomes virtual earth of 0 V, the same as ground (earth) shown in Fig. 3, even if the conductor is not electrically connected to the ground. This configuration is a remedy for cable fault types called shunt faults. When the conductor of a cable is partly exposed in the water caused by scratch, the faulty point is forced to be grounded (earthed) as 0 V. It means that it becomes impossible to supply power from the initially assigned PFE to the repeaters that are located on the far side of the faulty point. To repair this situation, the PFE on the other side of the cable increases the output power to provide sufficient power to the repeater. If a shunt fault happens near the shore, all of the required power must be supplied from one side. It seems like a single-end feeding that needs twice the power as that of the normal setting. Therefore, typically the maximum voltage for PFE is specified as twice the voltage for the normal setting. Note that the maximum voltage of current commercialized PFE is 15 kV, so a limited number of EDFAs can be activated. Especially, the transatlantic and/or transpacific cables must be equipped with a large number of repeaters, so the usable power is relatively small for each repeater, and accordingly, the number of implementable FPs is limited. The other design is to reduce the power of the pump laser or to use fewer numbers of repeaters to increase the supportable number of FPs; however, the achievable OSNR will be decreased, and accordingly, the ultimate capacity per FP becomes smaller.

Fig. 3. Schematic view of double-end feeding for a submarine cable.

The cable capacity with the utilization of dual-polarization (DP) transmission is restricted by the Shannon limit as shown in Eq. ( 1):

� = � � � � 2��� � �1 � ���� ( 1
)
where C is the maximum capacity in theory, N, B and SNR are the number of FP, bandwidth and signal-to-noise ratio (SNR), respectively. The SNR is related to OSNR in the optical fiber communication field, so usually:

���� = � ��� /� ��� (2)
Psig and PASE are the power per signal within the signal bandwidth of BWsig and amplified spontaneous emission (ASE) noise power density, respectively. The noise power density is defined with the bandwidth BWnoise. In the case of the bandwidth of Psig and PASE are equalized, the OSNR can be treated as SNR. The Psig can be assigned by the output power setting of the repeater; on the other hand, the PASE depends on span loss, which also means gain, noise figure (NF) of the repeater, and the number of repeaters. Therefore, the OSNR is decreased as the number of repeaters increases, so the theoretical maximum capacity becomes smaller for longer distances.

While the ultimate performance of submarine cables is determined by Eq. ( 1) and ( 2), the existence of nonlinear effect cannot be disregarded with the introduction of digital coherent transmission. So, the characteristics of optical submarine cables should be defined by the generalized OSNR (GOSNR) that includes the nonlinear effect as follows [START_REF] Mateo | Capacity Limits of Submarine Cables[END_REF]: where PNL is the noise caused by the nonlinear effect. Additionally, Generalized SNR (GSNR) is also proposed which is independent of signal bandwidth [START_REF] Mateo | Linear Vs Nonlinear Submarine Systems[END_REF]:

����� = � ��� /�� �� � � ��� � (
���� = ����� -10 log �� ��� ��� /�� ����� � (4)
As described previously, the +D fiber type cable gives smaller PNL than the other types such as DSF, NZ-DSF and DMF, and this is one of the main reasons why +D fiber can have higher potentials than the other types in terms of the capacity. Conventionally, the optical submarine cable is procured for both wet and dry plants including SLTE at the same time. Therefore, it is common sense to use the Q-factor to show signal quality as the criteria for acceptance of the system. In addition to this condition, FASTER has adopted another criterion defined by OSNR for the first time in the world [START_REF] Kamalov | FASTER Open Submarine Cable[END_REF]. Furthermore, there is a new trend of using the GOSNR (or GSNR) as the criteria instead of the Q-factor [START_REF] Mateo | Nonlinear Characterization of Fiber Optic Submarine Cables[END_REF]. The background to this trend is that recently the style of procurement has changed to the so-called "Open cable", which is contracted for wet plants only. It is advantageous because operators can choose SLTE freely. However, it also means no SLTE introduction at the initial stage, so it is impossible to define the criteria with Q-factor for acceptance. Therefore, there are some difficulties to be resolved, for example, how to confirm the reliability of estimated GOSNR (or GSNR) before making a contract, and how to measure GOSNR (or GSNR) after construction, etc. Currently, the GN-model is used for the estimation of GOSNR (or GSNR) at the design stage [START_REF] Poggiolini | The GN-Model of Fiber Non-Linear Propagation and its Applications[END_REF], while the measured nonlinear coefficient can be correlated with the value of the fiber specification that is obtained by the transmission performance of an actual real-time transponder [START_REF] Mateo | Nonlinear Characterization of Fiber Optic Submarine Cables[END_REF]. The result contributes to the strategy of the cable system acceptance test.

Limitation of Fiber Counts for Submarine Cables

Another constraint for optical submarine cables is the implementable number of fibers, which is significantly less compared to terrestrial cables and/or unrepeatered systems. As shown in Table 1 and the case of MAREA, the number of FPs is equal to or less than 8. This restriction is related to the limitation of the power supply described in the previous section. Even if it is difficult to define the limitation of implementable fiber counts per cable, the cable capacity can reach several 100 Tbit/s per cable considering these limitations such as GOSNR, power supply and fiber counts [START_REF] Mateo | Capacity Limits of Submarine Cables[END_REF].

POLICY AND TECHNOLOGIES FOR NEXT GENERATION

Maximum Power Operation for Double-End Feeding

The normal setting for PFE is typically half that of the required electric potential difference on both ends to repair shunt faults at double-end feeding configuration. In other words, these PFEs have potentials to support double the capacity utilizing maximum power even in normal situations, if accepted by operators. However, as this configuration is quite weak for shunt faults, it means that communication can fail suddenly. To make matters worse, the cable cannot be used until the completion of the cable repairment. Therefore, this operational policy will be one of choices if the operator has robustness with diversity using the other cables. Note that the repair period for a submarine cables will take a few weeks or more [START_REF]The International Cable Protection Committee[END_REF]. So, it is important for operators to keep in mind the estimated duration for recovery.

Shared Pumping for Multiple EDFA with Multicore Fiber (MCF) of Space-Division-Multiplexing (SDM) Technologies

Related to the issues described previously, it is important to reduce the power consumption per FP or channel. To provide the same gain for all of the WDM channels, each EDFA has a gain flattening filter (GFF) at the output side of EDF. However, The GFF reduces the total output power because the transmission profile of GFF is designed to highly attenuate around the higher gain band to equalize the power or OSNR among all WDM channels. Recently, however, it is considered to be one of the causes of wasted energy. So, a novel configuration is proposed to increase capacity that utilizes a higher number of fiber cores with narrow bandwidth EDFA [START_REF] Turukhin | 105.1 Tb/s Power-Efficient Transmission over 14,350 km using a 12-Core Fiber[END_REF][START_REF] Turukhin | Power-Efficient Transmission Using Optimized C+L EDFAs with 6.46 THz Bandwidth and Optimal Spectral Efficiency[END_REF]. The attractive point of Ref. [START_REF] Turukhin | 105.1 Tb/s Power-Efficient Transmission over 14,350 km using a 12-Core Fiber[END_REF] is the introduction of multicore fiber (MCF) as a method of space-division multiplexing (SDM) technologies. In that report, WDM signals are transmitted via a 12-core fiber with 12 EDFAs in parallel whose pump light power is provided and shared from a pump laser output of 800 mW. As a result, it is confirmed that 105.1 Tbit/s signals successfully transmit over 14,350 km of a 12-core fiber transmission line. The drawback of this proposal is that the required number of cores becomes 12 times, so it seems difficult to install in a submarine cable just by utilizing the multiple conventional single core single-mode fibers (SC-SMF). Therefore, the introduction of MCF is feasible with the reduction of the fiber counts. Moreover, the multiple parallel EDFA can be replaced by multicore-EDFA (MC-EDFA) as explained in the following section.

Uncoupled-core MCF and Coupled-core MCF

The MCF is regarded as a possible candidate for an applicable technology for optical submarine cable systems in terms of its energy and space-saving efficiencies. The MCFs are categorized into two groups: uncoupled MCF (UC-MCF) and coupled-core MCF (CC-MCF), depending on the quantity of crosstalk (XT) between cores. The first advantage of UC-MCF is the easier upgradability from the current conventional SC-SMF based system. With the minimized XT, it can be treated as multiple SC-SMFs virtually. To reduce the XT, each core is surrounded by a lower refractive index layer [START_REF] Hayashi | Uncoupled multi-core fiber enhancing signal-to-noise ratio[END_REF][START_REF] Gonda | 125 μm 5-core fibre with heterogeneous design suitable for migration from single-core system to multi-core system[END_REF]. It contributes to the application of the conventional transponder without any modification, so it seems easier to realize. one of the topical merits of CC-MCF is lower attenuation. For example, the attenuation of 0.158 dB/km is achieved even if its cladding diameter is the standard size of 125 µm for a 4-core fiber [START_REF] Hayashi | 125-μm-cladding Coupled Multi-core Fiber with Ultra-low Loss of 0.158 dB/km and Record-low Spatial Mode Dispersion of 6.1 ps/km 1/2[END_REF]. The value of attenuation is approaching the value of SC-SMF with a pure-silica core that is commonly used for submarine cable systems, so it is effective for system design in terms of maintaining the repeater span, etc. Furthermore, it is indicated that CC-MCF gives lower nonlinearity compared to SC-SMF that has equivalent Aeff, as confirmed by the transmission experiment [START_REF] Ryf | Long-Haul Transmission over Multi-Core Fibers with Coupled Cores[END_REF]. This means that CC-MCF may contribute to the increase in GOSNR or reduction of the number of repeaters.

Multicore EDFA

Regardless of the multi-fiber or MCF cases, the required number of EDFAs in a repeater is increased as a function of the number of cores. Because of the space limitation in a repeater, it is required to minimize the size of an EDFA unit. From this perspective, multicore EDFA (MC-EDFA) is attractive to decrease the number of EDFs and then amplify multiple cores with one MC-EDF. The low XT characteristics have been confirmed and the applicability for transoceanic distance transmission is demonstrated experimentally [START_REF] Takahashi | First Demonstration of MC-EDFA-Repeatered SDM Transmission of 40 x 128-Gbit/s PDM-QPSK Signals per Core over 6,160-km 7-core MCF[END_REF][START_REF] Igarashi | 1.03-Exabit/s.km Super-Nyquist-WDM Transmission over 7,326-km Seven-Core Fiber[END_REF][START_REF] Takeshima | 51.1-Tbit/s MCF Transmission over 2,520 km Using Cladding Pumped 7-core EDFAs[END_REF][START_REF] Tsuchida | Cladding Pumped Seven-Core EDFA Using an Absorption-Enhanced Erbium Doped Fibre[END_REF]. Moreover, the cladding pump MC-EDFA (CP-MC-EDFA) method is proposed to excite multiple cores at once [START_REF] Takeshima | 51.1-Tbit/s MCF Transmission over 2,520 km Using Cladding Pumped 7-core EDFAs[END_REF][START_REF] Tsuchida | Cladding Pumped Seven-Core EDFA Using an Absorption-Enhanced Erbium Doped Fibre[END_REF][START_REF] Kawaguchi | Ultra-Long-Haul Multicore Fiber Transmission Over 5,000 Km Using Cladding Pumped Seven-Core EDFA[END_REF][START_REF] Takasaka | Cladding Pump Recycling in 7-core EDFA[END_REF]. First, it has a further space saving efficiency utilizing some optical components that can handle multiple cores at once. Additionally, it contributes to reduction of the power consumption per core with the introduction of double cladding MC-EDF and a multimode laser diode (LD) for a pump laser instead of multiple single mode pump LDs. The drawback is the difficulty in increasing the excitation efficiency because of the huge difference of the mode field diameters between MM-LD and each core of MC-EDFA. Basically, the excitation efficiency is increasing as a function of the length of CP-MC-EDFA. Typically, the length of EDF is longer for L-band than C-band, therefore, in the case of cladding pumping, L-band CP-MC-EDFA is intrinsically energy efficient [START_REF] Tsuchida | Cladding Pumped Seven-Core EDFA Using an Absorption-Enhanced Erbium Doped Fibre[END_REF]. It is confirmed that 50 x 256 Gbit/s can be transmitted over 5,040 km of a 7-core fiber with CP-7C-EDFA [START_REF] Kawaguchi | Ultra-Long-Haul Multicore Fiber Transmission Over 5,000 Km Using Cladding Pumped Seven-Core EDFA[END_REF].

To increase the excitation efficiency, it is proposed to recycle the remaining pump lightwave from output to input of CP-MC-EDFA [START_REF] Takasaka | Cladding Pump Recycling in 7-core EDFA[END_REF][START_REF] Takeshita | Transmission of 200Gbps PM-16QAM Signal Through 7-Core MCF and MC-EDFA Using Novel Turbo Cladding Pumping Scheme for Improved Efficiency of the Optical Amplification[END_REF]. It is reported that the pump power can be decreased by 14% compared to the no-recycling case at the same output power condition [START_REF] Takasaka | Cladding Pump Recycling in 7-core EDFA[END_REF], and the other report shows that a 32% reduction in power consumption is observed in the same gain condition [START_REF] Takeshita | Transmission of 200Gbps PM-16QAM Signal Through 7-Core MCF and MC-EDFA Using Novel Turbo Cladding Pumping Scheme for Improved Efficiency of the Optical Amplification[END_REF]. Regarding these results, CP-MC-EDFA is expected to be a possible means of reducing the power consumption of a repeater.

SUMMARY

This paper reviewed the evolution of transpacific communication cables, current cutting-edge technologies and promising technologies for future optical submarine cable systems. The typical design lifespan of submarine cable systems is as long as 25 years, so technical evolution is progressed for new cables and for already deployed cables with new type fibers and advanced modulation format in parallel. The main issues are limitation of power feeding for submarine cables and space for a cable and a repeater. So, MCF and MC-EDFA of SDM technologies are expected to overcome these limitations. It seems that the traffic demands will never abate for global networking; therefore, the technical evolution will continue into the future for optical submarine cable systems.

Fig. 1 .

 1 Fig. 1. Historical evolution of the initial design capacity per one FP for transpacific optical submarine cable systems.

Fig. 2 .

 2 Fig. 2. Schematic view of C-band only and C+L band repeaters.

Table 1 .

 1 In-service transpacific submarine cable systems

	System	RFS	Tech.	Ini.	Ini. des. signals/FP	FP	Initial design
		[Year]		des.	[bit/s]		capacity
				Rx
	TPC-1	1964	Coax.	-	-	-	128 lines *1
	TPC-2	1976	Coax.	-	-	-	845 lines *2
	TPC-3	1989	Regen.	DD	1 x 280M	2	560 Mbit/s
	TPC-4	1992	Regen.	DD	1 x 560M	2	1.1 Gbit/s
	TPC-5	1995	EDFA	DD	1 x 5G	2	10 Gbit/s
	China-US	2000	EDFA	DD	8 x 2.5G	4	80 Gbit/s
	PC-1	2001	EDFA	DD	16 x 10G	4	640 Gbit/s
	Japan-US	2001	EDFA	DD	16 x 10G	4	640 Gbit/s
	TGN-P	2002	EDFA	DD	64 x 10G	8	5.12 Tbit/s
	TPE	2008	EDFA	DD	64 x 10G	4	2.56 Tbit/s
	Unity	2010	EDFA	DD	96 x 10G	5	4.8 Tbit/s
	FASTER	2016	EDFA	DC	100 x100G	6	60 Tbit/s
	NCP	2018	EDFA	DC	N x 100G	7	80 Tbit/s
	RFS: Ready for Service
	Coax: Copper cable
	Regen.: Regenerator as the repeater for the optical fiber cables
	EDFA: Optical amplifier as the repeater for the optical fiber cables
	Ini. des.: Initial design
	Rx: Receiver
	DD: Direct detection
	DC: Digital coherent Displayed equations are centered and set on a separate line.

*1 Assuming the bitrate for a telephone line as 64 kbit/s, it will be 8.2 Mbit/s. *2 Assuming the bitrate for a telephone line as 64 kbit/s, it will be 54 Mbit/s.

Acknowledgment

This work is partly supported by the national project of the Ministry of Internal Affairs and Communications of Japan (#0155-0041).


Charoula Mitsolidou

Christos Vagionas

Agapi Mesodiakaki

Pavlos Maniotis

George Kalfas

Chris G H Roeloffzen

Paulus Dijk

Ruud M Oldenbeuving

Amalia Miliou

Nikos Pleros

Pav- Los Maniotis

Paulus W L Van Dijk

Analog IFoF/mmWave 5G Optical Fronthaul Architecture for Hot-Spots Using Multi-channel OFDM-Based WDM Signals

Keywords: 5G networks, analog fronthaul, Intermediate Frequency over Fiber, OFDM, Photonic Integrated Circuit, ROADM

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The insatiable mobile data traffic growth in combination with the exponential proliferation of smart mobile devices [START_REF]Ericsson Mobility Report[END_REF], have created a new set of bandwidth and latency requirements beyond the capabilities of the legacy 4G mobile networks. To this end, Fifth Generation (5G) networks have already stepped in as the next generation technology for broadband mobile connectivity [START_REF] Andrews | What will 5G be?[END_REF]. Towards satisfying user's experience, 5G broadband access has been defined by expert alliances through Key Performance Indicators (KPIs) that foresee user rates of 1 Gb/s and peak rates up to 10 Gb/s at latencies less than 10 ms, resulting in high area connection densities up to 10 6 devices per km 2 [START_REF] Ngmn | NGMN 5G White paper[END_REF]. To address the 5G goals, intense research efforts have directed on the development of New Radio (NR) systems [START_REF] Lien | 5G New Radio: Waveform, Frame Structure, Multiple Access, and Initial Access[END_REF] and the introduction of millimeter Wave (mm-Wave) spectral bands of 28 GHz and 60 GHz in the access networks [START_REF] Rappaport | Millimeter wave mobile communications for 5G cellular: It will work![END_REF].

The initial 5G trials are expected to be deployed at densely populated areas and overcrowded hotspots, such as stadiums, airports etc., where the density of the users may result on financially viable installations [START_REF]G Infrastructur Association, 5G Trials Roadmap[END_REF]. However, moving to the New Radio systems comes with the need for densification of the Access Points (APs), in order to cope with the high propagation losses of the mm-Wave band [START_REF] Ge | 5G Ultra-Dense Cellular Networks[END_REF], bringing at the same time increased deployment costs. To alleviate the densification costs, operators gradually focus on the Centralized RAN (C-RAN) technology [START_REF] Checko | Cloud RAN for mobile networks-A technology overview[END_REF] that shifts the demanding operations at the Central Office (CO), while simplifying the hardware of the APs.

However, the cost-reduction due centralization does not alleviate on its own the upcoming fronthaul explosion [START_REF] Kani | Solutions for Future Mobile Fronthaul and Access-Network Convergence[END_REF], which necessitates the transmission of ultrabroadband mm-Wave channels at a small latency and energy envelope. Towards developing 5G NR systems capable to follow this explosion, research and industry efforts have been directed on the adoption and optimization of specific key technologies.

One of the directions is to employ the technology that can seamlessly combine the wireless access networks with the optical transport. Since optical fiber has chosen as the means to transport data to the centralized Base-Band Units (BBUs), analog Intermediate Frequency over Fiber (IFoF) technology [START_REF] Sung | Demonstration of IFoF based 5G mobile fronthaul in 28 GHz millimeter wave testbed supporting giga-bit mobile services[END_REF] is currently investigated as an efficient fronthaul technique for distributing data from the BBU to a large number of Remote Radio Heads (RRHs) in a bandwidth-and cost-efficient manner. The advantage of IFoF is that it utilizes aggregation of mobile signals in intermediate frequencies and modulation with cost-efficient and low bandwidth devices [START_REF] Vagionas | A 6-band 12Gb/s IFoF/V-band Fiber-Wireless Fronthaul link using an InP Externally Modulated Laser[END_REF], without adding any extra delay due to digital procedures as in the equivalent Common Public Radio Interface (CPRI)-based schemes [START_REF]Common Public Radio Interface[END_REF]. Up to now, most of the analog IFoFbased demonstrations have been mainly evaluated for single carrier data signals with bandwidth up to 1Gb/s [START_REF] Vagionas | A 6-band 12Gb/s IFoF/V-band Fiber-Wireless Fronthaul link using an InP Externally Modulated Laser[END_REF], or for OFDM signals with channel bandwidth less than 200 MHz [START_REF] Sung | Demonstration of IFoF based 5G mobile fronthaul in 28 GHz millimeter wave testbed supporting giga-bit mobile services[END_REF][START_REF] Martin | 28 GHz 5G radio over fibre using UF-OFDM with optical heterodyning[END_REF][START_REF] Shibata | 256-QAM 8 Wireless Signal Transmission with DSP-Assisted Analog RoF for Mobile Front-Haul in LTE-B[END_REF].

Another physical-layer key technology for cost efficient fronthauling, is the Photonic Integrated Circuit (PIC) technology that has already proved beneficial for optical communications [START_REF] Winzer | Fiber-optic transmission and networking: the previous 20 and the next 20 years[END_REF]. Based on already developed PIC technologies which mainly include linear analog transceivers with high peak data rates beyond the 10 Gb/s [START_REF] Debregeas | Record 6dBm electroabsorption modulated laser for 10Gb/s and 25Gb/s high power budget access networks[END_REF] and low-loss integrated Reconfigurable Add/Drop Optical Multiplexer (ROADMs), 5G networks can be evolved into larger-scale and denser topologies utilizing the cost, energy and size benefits of PICs. So far, the advantages of using PIC -based transceivers and wavelength multiplexing devices have been investigated only for metrorings and PON networks [START_REF] Marom | Survey of Photonic Switching Architectures and Technologies in Support of Spatially and Spectrally Flexible Optical Networking[END_REF].

Finally, another key direction for achieving efficient fronthauling is the development of Medium Access Control (MAC) protocols capable to allocate all available optical, wireless and time-domain resources of a converged Fiber-Wireless (FiWi) network, while simultaneously satisfying the low latency KPI constraints [START_REF] Shokri-Ghadikolaei | Millimeter Wave Cellular Networks: A MAC Layer Perspective[END_REF]. Recently, we investigated the employment of a Medium-Transparent resource allocation scheme in a hotspot FiWi 5G network, achieving adequate throughput and latency values [START_REF] Kalfas | Medium-transparent Packet-based Fronthauling for 5G Hot-spot Networks[END_REF]. However, this analysis was based on certain physical -layer assumptions since the hardware specifications were not available at that time.

Although significant research advances have been reported for each of the above key technologies individually, yet it remains a research gap on defining the physicallayer specifications of a fronthaul architecture that will incorporate all distinct technological aspects in the same scheme, while simultaneously meet the 5G KPI requirements. Towards addressing this research gap, we extend our previous MAC-layer related work [START_REF] Kalfas | Medium-transparent Packet-based Fronthauling for 5G Hot-spot Networks[END_REF], by providing a detailed physical -layer study of the proposed analog IFoF 5G fronthaul architecture for hot-spot areas. The architecture employs photonic integrated Indium Phosphide (InP) transmitters at the BBU and optical busses of WDM-ROADMs at the hot-spot area. Our analysis relies on experimentally verified simulation models of the optical transceivers as well as the signal degradation induced by the fiber-propagation, while WDM functionalities are leveraged by a low-loss photonic integrated Si 3 N 4 /SiO 2 ROADM design. The transmission of 6-IF band OFDM 16-QAM signals was successfully confirmed by Error Vector Magnitude (EVM) measurements, revealing performance above the commonly acceptable limits.

This paper is organized as follows: Section II presents the concept of the proposed 5G hot-spot architecture, while Section III shows the simulation modeling of the employed building blocks. The physical -layer evaluation of optical fronthaul link is presented in Section IV. Section V provides the conclusion of the work.

5G Fronthaul architecture for stadiums

The concept of the proposed 5G fronthaul architecture, serving a typical sportstadium is depicted in Fig. 1. In this architecture, the BBU is located in a centralized location outside the hot-spot area of the stadium and is interconnected with the RRH units which are dispersed around the stadium, through fiber links. WDM optical transmitters based on photonic integrated Externally Modulated Laser (EML) modulators are employed at the BBU side, in order to generate and transmit the IFoF signals to the hot-spot area where the RRHs are placed at different regions of the stadium in an optical bus topology. Each RRH consists of an optical ROADM that either drops any of the wavelengths at the connected RRH or sends it to the next RRH of the bus. Moreover, each RRH incorporates a number of antennas that are responsible for optical -to -IF and subsequently IF -to -mmWave conversion before the wireless transmission to the clients. It should be noted that multi -IF bands transmission per wavelength is supported, allowing in this way each wavelength to carry different user channels on different IF bands.

Based on this concept, our goal is to design a fronthaul architecture according to the requirements of the 5G vision by identifying what is the optimum set of resources in terms of fiber-lengths, optical losses, number of wavelengths, modulation etc,, while simultaneously complying with the physical layer performance metrics of the available hardware and providing a low cost and power consumption solution.

Simulation Modeling

Ιn order to ensure that the simulation results used for the physical layer performance evaluation of the proposed fronthaul are reliable, we created experimentally-verified simulation models of the main building blocks employed in the simulation setup.

Particularly, the response of the employed optical modulator was matched with the respective experimental characterized behavior of a high -power linear InP -based EML [START_REF] Debregeas | Record 6dBm electroabsorption modulated laser for 10Gb/s and 25Gb/s high power budget access networks[END_REF], while the signal impairments (losses, chromatic dispersion) caused by the fiber transmission, were matched with the respective response of a Standard Single Mode Fiber (SMF) [START_REF] Corning | LEAF Optical Fiber[END_REF]. The simulation model of the ROADM was also matched with the estimated spectral response of a MZI -based ROADM design being currently fabricated on the TriPleX platform [START_REF] Roeloffzen | Low-Loss Si3N4 TriPleX Optical Waveguides: Technology and Applications Overview[END_REF]. Fig. 2 depicts the simulation model results of the EML, perfectly matched with the experimentally-verified response of a fabricated InP -based linear EML operating in the C -band and comprising a laser source cointegrated with an Electro Absorption Modulator (EAM) [START_REF] Debregeas | Record 6dBm electroabsorption modulated laser for 10Gb/s and 25Gb/s high power budget access networks[END_REF]. The arm difference dL1 for the cascade of MZIs with FSR equal to 200 GHz, was chosen to be 872.1 um, while the arm difference dL2 for the cascaded MZIs of 400 GHz FSR, was chosen to be equal to 436 um. The total losses of the ROADM were calculated to be up to 3.1 dB, taking into account the fiber in/out interfaces and the propagation losses of the waveguides.

Fig. 4 (a) depicts the spectral response for the output ports of the cascaded MZIs with the FSR of 200 GHz, which is responsible for splitting λ 1 (1536.9 nm), λ 3 (158.46 nm) from λ 2 (1537.67 nm), λ 4 (1539.25 nm) at its output ports 1 and 2, respectively. Fig. 4 (b) illustrates the spectral response for both outputs of the cascade of MZIs that has a FSR equal to 400 GHz and is depicted with the brown color in Fig. 3 (a). The filtering response of the cascade shown in Fig. 4 (b) allows for separating λ 1 from λ 3 and λ 2 from λ 4 . It is worth mentioning that both spectral responses, shown in Fig. 4, exhibit flat-top filter shape by properly setting the values of the coupling coefficients TC1, TC2, TC3 equal to 0.9248, 0.28 and 0.5, respectively.

Optical Fronthaul Performance Evaluation

In this section, we present the physical -layer simulations performed with the aid of VPI photonics software [START_REF]VPIphotonics[END_REF], in order to evaluate the performance of the proposed analog optical fronthaul link when a multi-IF band WDM signal with OFDM -based format and SC modulation of 16 -QAM is transmitted. The employed setup of the WDM hot-spot link as well as the physical layer results will be presented. It should be mentioned that the matched simulation models, described in the previous section were employed in order to provide a reliable performance evaluation study.

Simulated Setup

Fig. 5 illustrates the setup employed for the performance evaluation of the optical WDM downlink transmission from the BBU to the RRH side comprising four ROADMs interleaved in an optical bus for serving half of hot-spot area (e.g. sports stadium). The other half of the stadium is served by another independent and identical transmission link, thus without loss of generality we focus only on one of the two transmission links. As shown in Fig. 5, the BBU consists of a WDM transmitter that The voltage swing of the electrical signal entering the EAM section of the respective EML is adjusted to the value of 0.7 Vpp and the bias voltage is set to -0.4 V achieving in this way operation in the linear region of the EAM. The laser section of each EML was set to its maximum optical power, resulting on an optical signal with an average modulated power of 2.8 dB [START_REF] Debregeas | Record 6dBm electroabsorption modulated laser for 10Gb/s and 25Gb/s high power budget access networks[END_REF] for each of the WDM channels exiting the respective EML. The outputs of the four EMLs are multiplexed by a 4:1 optical Multiplexer (MUX) and the resultant WDM stream is transmitted through a SMF to the ROADM bus located in the stadium hot-spot area (half part of the stadium). The data rate of the transmitted WDM signal is equal to 48 Gb/s (12 Gb/s per λ), resulting in a total data rate of 96 Gb/s when two identical optical links are used, as it is proposed in our fronthaul network. At the stadium side, the employed four ROADMs are capable to add or drop any of the four WDM channels originated by the BBU. The four ROADMs are placed around the stadium's periphery in a bus topology with the neighboring ROADMs equally spaced at 80 m. The distance between the ROADMs was chosen based on the actual diameter of a typical football -stadium [START_REF]Toumba stadium[END_REF]. In this setup, we focused on the worst case scenario, where all the WDM channels crossed through the first three ROADMs of the bus and dropped by the longest reach forth ROADM. The dropped demultiplexed optical signals are fed into an array of four photo-receivers for optical-to-electrical conversion by the Photo-Diode (PD) and amplification by the Transimpedance Amplifier (TIA). The outputs of the TIAs are down-converted and captured by the EVM tester. VOAs were used for controlling the received optical power entering the PD.

Results

In this sub-section, we present the physical-layer performance evaluation study of the optical fronthaul link, considering that the WDM multi-IF data signal crosses through the three first ROADMs and is received by the last 4 th ROADM of the optical bus.

The length of the SMF connecting the BBU with the 1 st ROADM was set equal to 500 m and the minimum Received Optical Power (ROP) corresponding to the first IF band was equal to -13 dBm, calculated by taking into account the maximum losses of all building blocks of the evaluated link. of PDs connected with the drop ports of the fourth ROADM. The WDM signal was transmitted from the BBU to the hot-spot stadium area through a 500 m SMF, crossed the first three ROADMs and dropped by the 4 th ROADM. As it is shown, all wavelengths (λ 1 , λ 2 , λ 3 , λ 4 ) exhibit similar performance, achieving an EVM below the 12.5 % threshold for a ROP value of -15 dBm. This measurement indicates that the dropped data signals reaching the PDs of the longest-reach ROADM should have an optical power higher than -15 dB for exhibiting adequate performance.

Thus, in order to investigate the feasibility of the proposed fronthaul link with the employed hardware, we performed a power budget study for calculating the actual optical power entering the PDs of the fourth ROADM, based on the losses of the individual devices. Table 2 presents the power budget parameters of all building blocks incorporated in the link, with the EML exhibiting a modulated average output optical power (P OUT ) of 2.8 dBm [START_REF] Debregeas | Record 6dBm electroabsorption modulated laser for 10Gb/s and 25Gb/s high power budget access networks[END_REF], the MUX (L MUX ) and the ROADM (L ROADM ) adding 1.5 dB and 3.1 dB losses, while the SMF (L SMF ) inserting propagation losses of 0.25 dB/km [START_REF] Corning | LEAF Optical Fiber[END_REF]. The actual ROP is calculated as by subtracting the sum of the losses from the output power of the signal exiting the EML, as shown in (1):

actual * * = - - - OUT MUX ROADM SMF ROP P L N L length L (1)
, where Ν is the number of the ROADMs in the optical bus and length the total SMF length including both the fiber connecting the BBU with the first ROADM and the fiber links connecting the neighboring ROADMs. By considering a number of 4 ROADMs, a fiber link of 500 m between the BBUfirst ROADM and fiber links of 80 m between the adjacent ROADMs and utilizing the parameter values shown in Table 2, the actual ROP was calculated equal to -11.3 dBm. Since this value is higher than the required ROP of -15 dBm derived by the EVM measurements in Fig. 7, the applicability of the proposed 5G fronthaul link was successfully confirmed for WDM signals with OFDM waveform and 16-QAM SC modulation.

Towards investigating the longest distance between the BBU and the hot-spot area, we performed a fiber length study with the main criteria being the actual ROP calculated from the power budget analysis of the system (Eq. 1) to be higher than the ROP required in order to achieve an EVM value less than 12.5 %. In this study we employed the setup shown in Fig. 5, using different values for the length of the SMF interconnecting the BBU with the first ROADM.

Fig. 8 illustrates the ROP versus the SMF length connecting the BBU and the first ROADM, where the red line corresponds to the actual ROP in dBm reaching the PDs of the 4 th ROADM, calculated by Eq. 1. By substituting the values of Table 2 and varying the length parameter from 0 km to 12 km, the actual ROP was linearly decreased from -11.1 dB at 0 km to -14.1 dB at 12 km. The other four curves show the ROP required at the input of the PDs connecting with the fourth ROADM, in order to achieve an EVM value bellow the acceptable limit of 12.5 %. Each of these curves corresponds to the worst performing SC of a different WDM channel. As it can be observed, the EVM degrades for all channels as the fiber length increases due to the signal impairing effect of the chromatic dispersion. By comparing the actual and the required ROP, it can be seen that these curves cross at a length of 9.5 km. Before this crossing point the actual ROP stemming from Eq. 1 is higher than the required ROP, revealing that the longest reach to place the BBU from the hot-spot area is equal to 9.5 km. Considering that the proposed fronthaul architecture targets on hot-spot networks where all the clients are gathered to a specific geographical area, the BBU box is expected to be placed not more than a few km from the hot-spot, with the fiber length of 9.5 km satisfying the above requirement.

Conclusion

We designed and evaluated an analog optical 5G fronthaul architecture for serving the bandwidth -demanding needs of hot-spot areas, such as sport-stadiums. The proposed cost-effective fronthaul scheme comprises a centralized Baseband Unit (BBU) with low cost WDM PIC-based optical transmitters and Remote Radio Heads (RRHs) on bus topology equipped with wavelength selective PIC-based ROADMs. By exploiting the WDM technology and using 6-band OFDM with 0.5 Gbaud 16-QAM modulation format, an aggregate capacity up to 96 Gb/s was achieved when two WDM links were employed. We also carried out a power budget study, showing the feasibility to locate the BBU equipment in a distance up to 9.5 km from the hot-spot area.

Fig. 1 .

 1 Fig. 1. Conceptual representation of the 5G IFoF/mmWave fronthaul architecture for hot-spot stadiums.

Fig. 2 .

 2 Fig. 2. EML modeling: a) Power versus Volts experimental and simulated transfer functions, b) experimental and simulated S21 curves.

Fig. 3 .

 3 Fig. 3. a) ROADM design based on cascaded MZIs, b) building block of the cascaded MZIs.

Fig. 4 .

 4 Fig. 4. ROADM modeling: Simulated spectral response of the cascaded MZIs' filters with: a) FSR = 200 GHz (green MZIs), b) FSR = 400 GHz (brown MZIs).

Fig. 5 .

 5 Fig. 5. Simulated setup of the analog multi -IFoF optical fronthaul, comprising the WDM EML-based transmitter at the BBU side and the ROADM bus at the stadium side.employs four EML modules, modulating the respective Continues Waves (CWs) at the peak wavelengths of 1536.9 nm, 1537.67 nm, 1538.46 nm and 1539.25 nm. Each EML is driven by a different multi -IF band electrical signal consisting of six OFDM waveforms that are generated and up-converted to six different IF frequencies.Table1summarizes the waveform parameters of the aggregated multi -IF band OFDM signal. Particularly, six IF bands with peak frequencies at 3.8 GHz, 4.4 GHz, 5 GHz, 5.6 GHz, 6.2 GHz and 6.8 GHz were employed, with each of them occupying a bandwidth of 0.5 GHz and consisting of 256 SCs. Each SC has a baud rate of 1.953 Mbd and a modulation format of 16 -QAM, resulting on a data rate of 2 Gb/s per IF channel. The six IF bands are separated by a guard band of 100 MHz. Thus, the aggregated data rate of all IF bands fed into the EML is equal to 12 Gb/s for a total occupied bandwidth of 3.5 GHz.

Fig. 6 (

 6 a) presents the EVM value per SC index of all IF bands for the first WDM channel at the λ1 = 1536.9 nm. As it can be observed, the first IF band at 3.8 GHz exhibited the worst performance with the EVM values of the SCs ranging from 7.3 % to 9.4 %, while the last IF of 6.8 GHz showed the best performance with EVMs between 4.6 % -5.9 %. Fig.6(b) and (c) depicts the constellation diagrams for the worst performing SCs of the outermost IF bands of 3.8 GHz and 6.8 GHz, exhibiting EVMs equal to 9.4 % and 5.9 %, respectively. This non-equal performance of the 6 IF channels, is attributed to the filtering effect that the signal experiences as it crosses through the hot-spot link, resulting to partially uneven losses and not ideally flat channel response. Similar performance was obtained for all the other WDM channels with EVM values below the 3GPP threshold of 12.5 % [26]. Fig.7depicts the EVM measurements for the worst performing 16-QAM modulated SC of each WDM channel versus the ROP of the data signal inserted into the array

Fig. 6 .

 6 Fig. 6. a) EVM per sub-carrier for all IF bands for λ1 = 1536.9 nm at a minimum received optical power of -13 dBm, b) constellation diagram for the first IF band at 3.8 GHz and c) the last IF band at 6.8 GHz.

Fig. 7 .

 7 Fig. 7. EVM versus ROP (dBm) for the worst performing SC of each WDM channel.

Fig. 8 .

 8 Fig. 8. ROP versus the length of the SMF fiber connecting the BBU with the first ROADM. Curve of the actual ROP deriving from the power budget of the hot-spot link and curves of the ROP required to achieve an EVM = 12.5 % (3GPP threshold) for all WDM channels.

Table 1 .

 1 Waveform Parameters

	Parameter	Value	Parameter	Value
	Modulation	OFDM 16-QAM SC baud-rate	1.953 MBaud
	IF bands	6	Data rate per IF	2 Gb/s
	IF center frequencies	3.8, 4.4, 5, 5.6, 6.2, 6.8 (GHz)	Channel guard band	100 MHz
	BW per IF band	0.5 GHz	BW per λ	3.5 GHz
	SC per IF band	256	Data rate per λ	12 Gb/s

Table 2 .

 2 Power Budget Parameters

	Building Block	Value
	EML modulated average output power (P OUT )	2.8 dBm
	MUX losses (L MUX )	1.5 dB
	SMF propagation losses (L SMF )	0.25 dB/km
	ROADM losses (L ROADM )	3.1 dB (max.)

Acknowledgment: This work is supported by H2020-5G PPP 5G-PHOS (grant agreement 761989) and MSCA ITN 5G STEP-FWD (grant agreement 722429).