Network Availability of Hybrid FSO/mmW 5G Fronthaul Network in C-RAN Architecture

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Introduction
5th generation of mobile networks (5G) is the next evolution in the communications society where access to information and sharing of data is available to anyone at anyplace and anytime [1].According to this vision, 5G aims to have [2]: (i) 1000 times higher mobile data volumes per geographical area (i.e., up to Tbps per square kilometer in some ultra-____________________________________________________________ 314 dense scenarios), (ii) 1000 times more connected devices (i.e., with densities that can be over a million terminals per square kilometer), and (iii) requirements for 5 times lower end-to-end service latency (i.e., with values as low as a few milliseconds).To meet these future challenges a tight integration between the 5G radio and the transport network segments is needed [3].The wireless communication systems are increasingly calling for smart wireless network architectures to decrease networking cost and encounter the need of low power consumption.Therefore, Cloud Radio Access Network (C-RAN) is considered to be the most promising and innovated architecture for the next generation mobile and wireless communication.In C-RAN architecture, the traditional base station (BS) is divided into a centralized signal processing unit (shared) called a Baseband Processing Unit (BBU), multiples Remote Radio Heads (RRHs) deployed on site and a high speed fronthaul transport network that connects RRH to the central processing office (BBU Pool) as shown in Fig. 1.
The evolution of the radio access network architecture needs to be complimented by an evolution of the fronthaul transport network.There is a huge need for high capacity, high available, scalable and cost and energy effective fronthaul network that is able to forward the massive traffic to/from the large number of RRHs from/to the BBU [2].
Clearly, there are many wireless communication technologies that can be key enablers for 5G fronthaul transport networks.The first one is using Free Space Optics (FSO) as it can overcome most challenges in C-RAN fronthaul network.The advantages of FSO are it uses a licensed-free

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wavelength, supports full-duplex, high-speed transmission, and is immune from electro-magnetic interference.However, the network availability is decreased at the foggy weather that is considered to be drawback in the FSO systems [4].The second one is using Millimeter Waves (mmW) which is actually the key technology within the evolution of 5G cellular networks.Unlike microwave links, mmW links cast very narrow beams that allow for deployment of multiple independent links in close proximity, Moreover, mmW also has the potential to offer bandwidth delivery comparable to that of optical fiber, but with reasonable cost.However, rainy weather could cause large attenuations in mmW links which could decrease the network availability [5].
Recently, hybrid FSO/mmW systems are considered to be a promising solution to provide wireless fronthauling services for the traffic of RRHs on C-RAN architecture.The benefits of these two emerging technologies can be used together to provide high data rates, scalable and cost efficient fronthaul system that will be suitable for 5G C-RAN requirements.The idea behind a combination between FSO and mmW is the complementary behavior of each technology during various weather conditions.Since fog and rain are the dominant causes for attenuation in the FSO and mmW links, respectively, FSO and mmW can replace or complement each other by providing nearly the same throughput requirements in data transmission when one of them fails.The switching between the two links is based on variations in the channel conditions.Obviously, the most efficient solution for high-throughput wireless connectivity is to use both FSO and mmW links simultaneously as it can increase the capacity and the availability of the fronthaul system [6].
Hybrid FSO/RF networks are used in different applications for the development of the network performance.Several studies have been proposed on hybrid FSO/RF communications systems.FSO/mmW Link switching has been proposed extensively which can categorize in two main schemes i) hard switching proposed in [7][8] that offers one link selected which can support the required QoS.This technique offers a trade-off between link availability and average data rate ii) soft switching proposed in [9] that offers the two links which are always on and rates adapted by channel coding.According to channel conditions this technique offers high availability and high-power consumption.In [10], the authors present Common Public Radio Interface (CPRI) specification, its concept, design and interfaces, and further provide a guideline to fronthaul dimensioning ____________________________________________________________ 316 in realistic LTE scenarios.The characteristics of FSO and mmW are presented in [11] and [12], respectively.
In this paper, hybrid Free Space Optics and Millimeter Wave (FSO/mmW) network is proposed to be used as a fronthaul network to enhance the performance of C-RAN architecture at various weather conditions.Also, a proposed scheme for the selection / switching between FSO and mmWfronthaul links is investigated based on CPRI requirements.This proposed technique requires instantaneous knowledge of bit error rate (BER) in the FSO and mmW transmitters.In other words, full channel state information (CSI) is required.Furthermore, the performance of hybrid FSO/mmWfronthaul network is evaluated in terms of network availability at different foggy and rainy weather conditions and compared to that in all FSO and all mmW systems.
The reminder of this paper is organized as follows: In the next section, the challenges and requirements of fronthaul network in C-RAN architecture is studied.Section 3 presents channel modelling and characterization for both FSO and mmW systems.Hybrid FSO/mmWfronthaul switching criteria is investigated in section 4. Simulation results are provided in section 5. Finally, the paper is concluded in section 6.

Challenges of fronthaul network in C-RAN
In C-RAN architecture, Remote Radio Heads (RRHs) include the radio, the associated amplification/filtering, and the antenna.On the other side, Baseband processing Unit (BBU) is implemented separately and performs the centralized signal processing functionality of the RAN.The RRHs and BBU are connected together using fronthaul transport network.The decentralized BBU enables agility, faster service delivery, cost savings, and improved coordination of radio capabilities across a set of RRHs.The CPRI data-rate results from the following calculation [10]: Where:  is the number of antennas per sector,   is the sampling rate used for digitization (sample/s/carrier),  is the sample width (bits/sample), 2(I/Q) is a multiplicationfactor for in-phase (I) and quadrature-phase (Q) data,   represents the factor of CPRI control word (16/15) and  is a coding factor (either 10/8 for 8B/10B coding or 66/64 for 64B/66B coding).Furthermore, the main requirements for CPRI transmission are delay and bit error rate (BER).Therefore, fronthaullink should operate with at most 5 us delay excluding propagation delay, and a maximum acceptable BER of 10 −12 [10].Actually, there are many challenges in the design of fronthaul network in C-RAN.The first challenge is the requirement of cost-effective and scalable networks that connect a large number of RRHs to/from the BBU, while ensuring sufficient capacity and quality of service (QoS).The second one is the availability of fronthaul transport networks to cope with different weather conditions to guarantee a better user experience with acceptable BER ≤ 10 −12 .Finally, reducing power consumption is also an important issue.
To overcome these challenges, the benefits of FSO and mmWtechnologies can be used together in hybrid system to provide high data rates, scalable and cost efficient fronthaulnetwork suitable for C-RAN architecture.

Hybrid FSO/mmW link Model
The performance of hybrid FSO/mmW link relies on channel models of both FSO and mmW which experience different channel gains at the same weather conditions.FSO link is mainly affected by fog attenuation, while rain attenuation is the dominant one in mmW link.Naturally, fog and rain rarely occur concurrently and their influences can be studied separately.

FSO communication channel
Both link losses and noise can degrade the performance of FSO link.Also, the FSO link performance depends on the selected wavelength, modulation formant and transmitted power level [13].FSO channel losses have both atmospheric and geometric losses.Atmospheric loss includes fog, rain and scintillation [14].In FSO link modeling, the collecting aperture diameter of an optical receiver could be larger than the spatial scale of the optical scintillation that is caused by atmospheric turbulence.In this condition, the FSO receiver will average fluctuations of the received waveform over the aperture area, leading to reduced signal fluctuations, especially if the network experiences weak turbulence [15].In this paper, the receiver diameter is assumed to be large, so that the effect of atmospheric turbulence is relatively very small compared to that caused by fog and rain attenuations [16].Therefore, the total FSO link loss is given by: Among all atmospheric losses, fog is the dominant cause of attenuation in FSO link.Using Kim model, for accurate prediction at low visibility values, the fog attenuation is given as [11]: Where, is total extinction coefficient, and is defined by: Where, is the visibility in kilometers,  is the wavelength in nanometers,  is 550 nm, is the distance in kilometers and  is the distribution of scattering particle size and is described by: From equation ( 5), at V < 0.5 km, fog particle sizes are much larger than the FSO wavelength.As a result, the scattering particles are large enough that the angular distribution of scattered radiation can be described by geometric optics.Therefore, based on Mie scattering theory calculations, the proposed Kim model considered fog attenuation for V < 0.5 km as wavelength independent [17].
Generally, rain attenuation in FSO links is smaller than that caused by fog.This occurs because the rain droplet size is relatively larger than the used ____________________________________________________________ 319 wavelength of the optical signal ( = 1500 nm).The rain attenuation is calculated using Japanese empirical model as [17]: Where,R is the rainfall rate in mm/hr.Even at clear weather, geometric loss occurs due to the divergence of the optical beam over its propagation distance.The geometric Loss is calculated by [18]: Where,  is the receiver diameter,   is the transmitter diameter (both in millimeters), and  is the divergence angle in mm.rad/km.
Therefore, the received optical power at photo-detector is given by [18]: Where,  is the transmitted power,  is the transmitted power,   is the transmitter optical efficiency, and  is the receiver optical efficiency Also, there are two kinds of noise sources presented in FSO links which are external noise (ambient noise) and internal noise (includes dark current and thermal noises).Link noise degrades the received electrical SNR of FSO link which can be calculated as [19]: Where,  is the photo-detector's responsibility in A/W,  is the electron charge,   is the dark current,   is average background power,  Boltzmann constant,  is absolute temperature,   is the load resistor,   is noise figure and  is electrical bandwidth.The first, second and third terms in the denominator represent shot, background and thermal noises, respectively.Furthermore, -PPM modulation format is used in simulated FSO link to enhance receiver sensitivity, especially at high ambient noise.

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Furthermore, the desired BER of the FSO link can be obtained by adapting the modulation order  of the scheme as [21]:

mmW communication channel
The major impairments of mmW propagation in free space are path loss and rain attenuation.Also, like FSO, mmW link performance depends on the selected carrier frequency, modulation formant and transmitted power level.Path loss occurs due to the divergence in transmitted beam through its travel in free space.It is proportional to the square of distance between transmitter and receiver as [21]: Where, is the distance between transmit and receive antennas,   is the operating wavelength.Also, rain attenuation occurs as a result of the absorption and scattering of electromagnetic waves by rain particles.It depends on carrier frequency, temperature, rain droplet size, and rainfall rate.In general, there are two types of rain attenuation models: empirical and theoretical models.The most utilized empirical model is ITU-R which gives the calculations of rain attenuation by: Where, and  are the frequency dependent coefficients, and  is the rainfall rate (mm/hr).The received electrical SNR of mmW link can be defined as [21]: Where, denote the transmitted power,  is the effective gain which can be modeled as [21]:

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Where,  and   denote the transmitting and receiving antenna gains, respectively, and   . is the attenuations caused by oxygen absorption (dB/km). 2 is the RF noise variance and is given by [21]: Where,   , 0 and   denote RF bandwidth, noise power spectral density (in dBm/MHz) and noise figure of the receiver, respectively.Finally, the desired BER can be obtained by adapting the modulation order of M-QAM as [21]: Moreover, a homogeneous foggy and rainy weather is assumed over the entire network with equal joint visibility-rainfall rate value.

Hybrid FSO/mmWfronthaul network in C-RAN
Hybrid FSO/mmW systems presents a promising solution to provide wireless fronthauling services for the traffic of RRHs on C-RAN architecture.The idea behind a combination between FSO and mmW is the complementary behavior of each technology during different weather conditions.As mentioned before, rain is the dominant cause for attenuation in mmW link, whereas fog is the most important cause for attenuation in FSO link.Recently, mmW became a feasible solution in the hybrid FSO/RF systems; due to the mmW technology can provide high data rates similar to FSO (multi Gbps).In addition, the mmW (70-80 GHz) bands are particularly attractive for long range communications because these bands have a very low  2 and  2 atmospheric absorption.Therefore, when the FSO link fails the mmW can provide nearly the same throughput requirements in data transmission.The switching between the two links is depending on variations in the channel conditions as shown in Fig. 2. The flowchart for real time switching operation in hybrid FSO/mmWfronthaul network is indicated in Fig. 3. Initially, depending on the geographical service area, FSO link is selected to be the primary link in coastal cities whose weather is rainy.When the bit error rate on the FSO link exceeds the threshold value (  > 10 −12 )which is CPRI constraint, the hybrid system switches to the mmW link that is considered ____________________________________________________________ 322 to be the secondary one to maintain the availability of the fronthaul network.In contrast, mmW link is considered to be the primary one in Agricultural lands whose weather is mainly foggy.When the bit error rate on the mmW link exceeds the threshold value (  > 10 −12 ), thehybrid system switches to the FSO link that is considered to be the secondary one to maintain the availability of the fronthaul network.

Simulation and numerical results
In this section, FSO, mmW and hybrid FSO/mmWfronthaul networks are numerically evaluated to indicate the superior performance of each system at different foggy and rainy weather conditions based on network availability.Since fog and rain are the dominant causes for attenuation in FSO and mmW links, respectively, each system is evaluated at different visibility and rainfall values.

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Fig. 3.Flow chart showing the real time switching operation in hybrid FSO/mmWfronthaul network.
As a study case, hybrid FSO/mmW system is proposed to be used as a fronthaul network in Egypt.Generally, the Egyptian weather in Giza has visibility values usually greater than 2 km, except it equals zero during 90 minutes only, but it becomes 1 km during 56.5 hr.[23].Therefore, according to Kim model [11], fog attenuation on 1 km FSO link in the Egyptian weather varies between zero dB/km at visibility values  ≥ 2 km and 20 dB/km at visibility value  = 1 km and reaches up to 350 dB/km at visibility values  < 0.2 km.Therefore, the effect of fog attenuation on FSO link on hybrid FSO/mmW system appears at visibility values less than or equal 1 km.Also, Egypt (especially in Alexandria) is awarded an average of 200 mm of rainfall per year, or 16.67 mm per month.The maximum rainfall rate occurs on January with around 75 mm/hr that may cause rain attenuation of 85 dB/km on 1 km mmW link [24].

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The simulated network topology consists of 1RRH connected to BBU pool over distance ( = 1 km) point to point as shown in Fig. 3.The simulation parameters are given in table 1, where each FSO link uses 4-PPM modulation format with maximum bit rate 1.25Gbps, maximum transmitted power 19 dBm and maximum bit error rate 10 −12 (CPRI constraint).From the table, the FSO receiver diameter is assumed to be   = 0. ] −7/5 = 0.000513.Clearly, using this value of aperture averaging, the attenuation variance caused by weak turbulences has small effect and can be neglected [15][16].Also, each mmW link uses BPSK modulation format with maximum bit rate 1.25Gbps, maximum transmitted power 14 dBm and maximum bit error rate 10 −12 .These values are selected to be in the practical range [11][12][13][14][15][16][17][18][19][20][21][22].   Figure 5 shows the network availability of FSO, mmW and hybrid FSO/mmWfronthaul systems at different foggy weather conditions.Clearly, at haze and moderate foggy weather, especially at  ≥ 0.55 km, network availability of FSO, mmW and hybrid FSO/mmWsystemsreach to 100%.However, at  < 0.55 km, the network availability of FSO system decreases gradually until it reaches to 0% at  < 0.24 km which is considered as heavy foggy weather.In contrast, mmW and hybrid FSO/mmW systems maintain their 100% network availability as fog has no effect on mmW links presented in mmW and hybrid FSO/mmW systems.
Figure 6 shows the network availability of FSO, mmW and hybrid FSO/mmWfronthaul systems at different rainy weather conditions.Clearly, at light rain, especially at  ≤ 2 mm/hr, network availability mmW system reaches to 100%.However, at  > 2 mm/hr, the network availability of mmW system decreases gradually until it reaches to 0% at  > 68 mm/hr which is considered as heavy rain.In contrast, FSO and hybrid FSO/mmW systems maintain their 100% network availability until  > 86mm/hr, and then network availability decreases gradually until it ____________________________________________________________ 326 reaches to 19% at  > 150mm/hr due to rain has slightly effect on the FSO links presented in FSO and hybrid FSO/mmWfronthaul systems.

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As a summary, at clear weather, FSO, mmW and hybrid FSO/mmW systems can work with network availability reaches to 100%.At heavy foggy weather ( < 0.24), mmW link in the hybrid system will be the preferred one.In contrast, at heavy rainy weather ( > 68), the hybrid system will select the FSO link.

Conclusion
Hybrid FSO/mmW network is considered to be apromising solution to support 5G fronthauling services in C-RAN architecture.The performance of hybrid FSO/mmW link relies on channel models of both FSO and mmW which experience different attenuations atthe same weather conditions.Generally, FSO link ismainly affected by fog attenuation, while rain attenuation is the dominant in mmW link.By exploiting thecomplementary behavior of FSO and mmW links at various weather conditions, hybrid FSO/mmW fronthaul network achieves better performance than that FSO and mmW ones.

Acknowledgement
This work is supported by National Telecom Regularity Authority (NTRA) in Egypt.
2 m and the maximum computed distance for FSO link in the simulated network topology is  = 1 km.The aperture averaging is then calculated as, = [1

Figures 5 -
Figures 5-6indicates the performance of FSO, mmW and hybrid FSO/mmWfronthaul systems at different foggy and rainy weather conditions in terms of network availability.Obviously, at clear weather, FSO, mmW and hybrid FSO/mmWsystemscan work with network availability reaches to 100%.