{'fraction_non_alphanumeric': 0.027911997532641102, 'fraction_numerical': 0.013467667317775265, 'mean_word_length': 4.596950517836594, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 3, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Resilient Architectures for Free Space Optical Wireless Interconnection Systems

In this paper, we propose the use of two Passive Optical Network (PON)-based network architectures to connect free-space Optical Wireless Communication (OWC) Access Points (APs) within a room with multiple users. We optimize through a Mixed Linear Integer Programming (MILP) model the assignment of mobile OWC users to more than one AP to improve the resilience of the fronthaul network, i.e the OWC system and the wired network linked to APs, and study the impact of users distribution and channel characteristics.


INTRODUCTION
Optical wireless communication (OWC) systems can offer a solution to the spectrum crunch in access networks 1,2

. The
Radio Frequency (RF) spectrum is becoming increasingly exhausted as the traffic is currently continuing to increase at 30% to 40% per year leading to expectations of traffic growth by 30x in 10 years and by 1000x in 20 years 3 . It was reported that about 80% of all Internet traffic starts and terminates indoors 4 . Thus, efficient and high capacity designs for wired and wireless access networks inside building are required to meet these indoor demands. Optical wireless free space systems offer numerous advantages including abundant unregulated optical spectrum, freedom from fading, added physical layer security as light does not penetrate opaque objects, and the ability to re-use the same wavelengths in adjacent rooms. Various receiver and transmitter design considerations were studied to improve the performance of Infrared OWC systems by reducing the impact of noise, interference, and multipath dispersion while accounting for eye safety [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] . These include the use of different modulation formats and coding schemes [5][6][7][8] , fly eye receivers with multiple detectors [6][7][8][9][10][11] , spot diffusing transmitters with angle diversity receivers [12][13][14][15][16] , imaging receivers 17 , with delay adaptation methods 18 and with relay nodes 19 , and the use of adaptive holograms in transmitters with imaging receivers 20 or angle diversity receivers 21 . Those solutions can then provide up to 10 Gbps OWC systems [22][23][24][25] . Visible Light Communication (VLC) was also considered to achieve multi Gigabit OWC systems with multiple users [26][27][28][29][30][31] . For downlinks, VLC systems can use Red Yellow Green Blue (RYGB) Laser Diodes (LDs) in a number of distributed Access Points (APs) for lighting and communication. The assignment of wavelengths and access points to users can be optimized while accounting for the users' distribution and indoor channel characteristics 2,32 .
For demands that originate and terminate indoor such as user-to-user communication and for user-to-Internet traffic, improving the backhaul network (i.e. the connections between the APs within the room and connections between the APs and gateways) can reduce data transmission delay and networking power consumption, which is beneficial for users and service providers, and can increase the resilience of the system against AP failures. Links between APs located within a room can be wired or wireless while linking APs within a building requires a wired network. Wang 34 . In this work, we propose the use of two Passive Optical Network (PON)-based network designs, a WDM-based architecture and a point-to-point architecture,  to connect the APs within a building and to provide connections for the APs with a central office. For the wireless part of  this access network design, we consider a WDM OWC downlink system with multiple APs and Angle Diversity Receivers  (ADR) 2 . To provide resilient services, we propose assigning users to more than one AP and we use a Mixed Integer Linear Programming (MILP) model to optimize users assignments. We use the model to evaluate the performance and resilience of the OWC system against AP failure and consider the impact of user distribution and optical wireless channel characteristics. We also provide a preliminary evaluation of the resilience of the two proposed backhaul networks compared to switch-based data network architectures.
The remainder of this paper is organized as follows: Section 2 describes the resilient access network system model, the parameters considered, technologies used in the OWC system and the backhaul network and provides the optimization model. Section 3 provides the results while Section 4 presents the conclusions and future work.

RESILIENT ACCESS NETWORK MODEL
This section describes the WDM OWC system with multiple APs and users, provides a brief overview of the resource allocation optimization MILP model and introduces the two proposed PON-based data networks for connecting the access points within a room. The room setup we used to model the OWC system 2 is shown in Figure 1. The room is 3m heigh with 8 access points in the ceiling and the communication floor is 1m above the room floor. Each access point provides WDM downlinks by utilizing LDs with four wavelength, i.e. RYGB LDs, which are in the visible light range, hence, can be used for communication and illumination. The parameters of the room, users, transmitters in the APs and the receivers are summarized in Table 1. An ADR with 4 branches is used as the receiver. These parameters were used to ray trace 2,27,35 and simulate all possible transmitted beams (i.e. 8 × 4 × 4) with up to second order reflections from the APs to 32 predefined and equally distributed points in the communication floor 2 . Also, similar evaluation for the background noise was performed. Up to 7 users are considered with uniform random allocation in one of the predefined 32 locations. Prior knowledge for the number of users and their locations is assumed to be provided by a controller connected to all APs 32 . The APs are connected through a data network that allows direct connections between the APs and also provides connections with central office equipment such as an optical line terminal (OLT) to provide connections to the Internet. Selecting a high bandwidth data network with multiple paths between the APs can enable direct user-to-user data transmission without the need for indirect transmission through the Internet. Hence, better performance in terms of delay and power consumption can be achieved. We propose the use of two PON-based network architectures for connecting the APs. A brief review of these networks is presented in the following subsection. In this work, we propose resilient downlink connections, where each user can connect to more than one AP. We assume the use of On-Off Keying (OOK) and Forward Error Correction (FEC) and allow connections with Signal-to-Noise-and-Interference (SINR) more than or equal to 13.8 dB which provides Bit Error Rate (BER) of 1×10 -4 . With an FEC code such as RS (255 239), and with an input BER of 1×10 -4 , an output BER of 1×10 -14 is produced with a small overhead of 6%. We utilize a MILP model 2 to optimize the assignment of users to one or more WDM APs. For detailed derivations and linearization of non-linear terms in the MILP used to calculate the SINR, the reader is referred to our previous work 2 .

Room and ray tracing parameters
It is worth noting that the objective function in our previous work was to maximize the sum of user SINR values and that each user is assigned to only one AP. In this work, we optimize the assignment of users to one or more APs and consider an objective function to increase the users assignments. The following provides a brief summary of the model used including key sets, parameters and variables, the objective function, and key constraints.
Sets: Set of users. Set of APs. Set of wavelengths. Set of receiver branches.

Parameters:
&' () The squared electrical current due to received optical power of user at receiver branch from AP using wavelength ; ∈ , ∈ , ∈ , ∈ . &' () The background light noise mean square current received for user at receiver branch from AP using wavelength ; ∈ , ∈ , ∈ , ∈ . Mean square receiver noise current = Receiver noise power spectral density in A 2 /Hz times the receiver bandwidth.

&' ()
Binary variable that is equal to one if user at receiver branch is receiving from AP using wavelength , and is equal to zero otherwise; ∈ , ∈ , ∈ , ∈ .
The SINR of user at receiver branch when receiving from AP using wavelength can be expressed as 2 : The term in the numerator represents the received signal while the first term in the denominator represent the interference, which is the sum of all received signals from other APs at the same wavelength when used for communication. The second term in the denominator represents the background light noise, which is the sum of all received signals from other APs at the same wavelength when they are only used for illumination (i.e. not assigned to any user) 2 . The objective is to maximize the SINR and the total number of assignments for all users to increase the system resilience against APs failures: Subject to the following allocation constraints: Equation (3) ensures that each user can be assigned only one wavelength in an AP. Equation (4) ensures that each wavelength within an AP is only allocated once. Equation (5) ensures that a user is allocated only if the SINR will guarantee a BER of 1×10 -4 .
This model takes the number of users and their locations information and the channel characteristics, and provides the optimum allocation results of users to branches in the receiver, APs, and wavelengths and the corresponding SINR values for the users where a user can be allocated to more than one AP.

Proposed Access Points Data Networks
Traditional Time Division Multiplexing (TDM) Passive Optical Networks (PONs) and traditional WDM PONs have been used to provide reliable, high performance and low maintenance and low power consumption connections with end users at the last mile of information and communication networks 37 . Using these passive networks in addition to three improved architectures was proposed for connecting end points in data centre networks with arbitrary and high bandwidth requirements 38 . The improved designs increase the network bandwidth and were proposed as the traditional TDM and WDM PONs were more suitable for lower end user to end user demands and asymmetric upload and download bandwidth requirements.
In this work and to improve the resilience of connecting the APs of the OWC system with the remaining access network (i.e. the OLT and higher network levels), we propose using two of the improved PON-based data networks to connect the APs. These designs are namely an Arrayed Waveguide Grating Router (AWGR)-based design that uses TDM/WDM 39-41 , and a TDM or WDM point-to-point-based design that uses end points (i.e. the APs in this work) to forward traffic 42 . Both designs can provide multipath connections between any end point pairs. Improving the connectivity between the APs in terms of the capacity and multipath availability reduces the delay for any AP to AP or user to user communication and also improves the resilience of the whole access network for users within a room or a building.
Figure (2) shows an example of an AP data network that uses the AWGR-based PON design. In this example, we connect the 8 APs in a room together and with the OLT in the CO by using only passive components. Two AWGRs are proposed to provide non-blocking all-to-all connections 39 between all sets of APs and with the OLT. In the example provided in Figure (2), the APs are clustered into four sets. A proposed connections can use two 4×4 AWGRs and will require four wavelengths to provide the connections of each set with the OLT and the connections between different AP sets 38 . Each 2 APs in a set connect to a coupler/splitter that connects to a unique input/output port of one of the two AWGRs. The OLT is connected to both AWGRs to provide higher capacity for the user to OLT traffic (i.e. user to Internet traffic and vice versa). The remaining ports in the AWGRS provide multi hop connections between elements connected to different AWGRs. The additional splitters and Arrayed Waveguide Grating (AWG) ensure proper multiplexing of wavelengths. Each set of APs also has a passive and direct all-to-all network 38 to connect all the APs.
The routing and wavelength assignment to realize this design was detailed in previous work 38,39 . Each AP is equipped with a tuneable transceiver and can tune the wavelength to communicate with other APs in other sets or with the OLT. With 10 Gbps tuneable transceivers, a bisection bandwidth of 5×4×10=200 Gbps is achievable.   42 that can be used to connect the APs within a room or within a building. In this design, APs are first clustered into groups, and each group is clustered into subgroups. APs within each subgroup connect to the remaining network through a splitter/coupler. Each subgroup connects to another subgroup in each group. Each group has a dedicated subgroup to connect with the OLT. The APs within a group are to be passively interconnected to provide all-to-all communication 38 . Each AP has a forwarding mechanism (e.g. by a Network Interface Card (NIC)) to enable transmitting other APs demands. Single wavelength, hence, only TDM, or multiple wavelengths can be used in the design. Due to the large number of paths between any AP pairs, higher resilience can be achieved against APs failures.

RESULTS
This section provides the MILP-based optimized resource allocation results for the downlink OWC system when each user is able to connect to multiple APs. It also provides the optimum allocation results for the cases when one or two APs are not available.

Optimized resources allocation with multiple AP links
We first show the average SINR results of all the users when each user is allowed to connect with one AP only. The choice of AP is optimized 2 . Then, we show the allocation optimization results when multiple APs can be assigned to each user. These results are generated for 20 different and uniform random user locations (called allocations of the users) in each considered case for up to 7 users. Figure (4) and Figure (5) show the optimum allocation results when one and two users are present in the system, respectively. High SINR values between 18 dB and 19 dB are achievable in both cases when one AP is assigned to each user. This is due to the cases where the users locations allow assigning the user or the two users in interference-free allocations. Figure (4) shows that when assigning a single user to more than one AP per user, the average SINR value for all users is reduced by about 2 dB. This is because allocating more links in the system introduces additional interference between the links that use the same wavelength. For the case of two users in the room as in Figure (5), the average SINR can drop by about 5 dB to a value around 13.8 dB, which is the lowest allowed SINR value, as the impact of the interference and background noise is higher due to the additional users compared to the results in Figure (4). In both cases, there is at least one assigned link for each user that provides a very high SINR value. Due to the limited FOV of each AP, the results in Figure 4(a) and Figure 5(b) show that a user cannot be allocated to more than 3 APs at the same time. The average SINR results when allocating resources for up to 7 users in the room are provided in Figure (6). The results show that even for a high number of users, the OWC system is able to provide multiple APs connections to users while ensuring a BER value of 1×10 -4 for assigned users. Some of the allocation cases, for example for 3 users, examined some worst case scenarios, where according to the used objective function and constraints, it is more optimum to allocate some users to many APs while some users are not assigned to any AP. This has resulted an average SINR value below 13 dB.
For the examined allocations with up to a maximum of 5 users in the room, the mode for the number of assigned APs is 2. For 6 and 7 users, the mode is 1. Figure 6. SINR results for the optimized resource allocation for up to 7 users.

Optimized resources allocation with AP failures
We evaluated the average SINR values based on the optimized resources allocation with three cases of AP failures. An AP can be inaccessible to a user either under hardware or software failures or if its optical beams are blocked by an object. The impact of optical beams blockage due to a disc, with several dimension parameters, between a user and APs was studies in our previous work 43 . The results show that when assigning two APs to a user, the maximum separation between the assigned APs results in reduced blockage.
In this work, we consider two cases of single AP failure and a case of two failures, which are randomly selected to be in access point 1, access point 5, and in both simultaneously. We use the same 20 uniform random allocations (ie user locations) for the users and evaluate the optimum resource allocation and average SINR results for 1, 2 and 3 users allocations. Figure (7  Using the proposed AWGR-based PON or the point-to-point data networks, provides passive connections between each AP and the OLT, which improves the resilience of the access network. Using a switch-based data network 44 to connect the APs and improve user-to-user communication can worsen the resilience of the system. In these networks, a number of APs will be connected through an electronic switch to provide connections with other switches or the OLT. A failure in a switch is then equivalent to a failure in multiple APs.

CONCLUSIONS
In this paper, we proposed a resilient free space optical wireless system for multiple users that integrates the use of a visible light OWC system and PON based data networks. The OWC system utilizes RYGB LDs in multiple APs in the ceiling to provide downlink connections for indoor users with ADRs. This system allows each user to be assigned to more than one AP to improve the resilience against AP failures. We optimized the system through a MILP model considering the assignment of users to APs resources in cases of no AP failure and in three cases of AP failures and evaluated the impact of the optimized assignments on the average SINR values. Random allocations for the indoor users are considered to account for the indoor channel characteristics.
To improve the resilience of the access network and its performance for user-to-user traffic, we proposed the use of an AWGR-based PON or a point-to-point PON to passively connect the APs together and with the OLT. We showed that using a switch-based data network to connect the APs can worsen the resilience of the system as switch failures cause multiple APs disconnection whereas the proposed PONs improve the APs connections without reducing the resilience of the access network.
Future work includes considering angle diversity transmitters and imaging receivers in the WDM OWC system to improve the channel characteristics, evaluating and optimizing the average sum rate to improve the fairness between users in allocating AP resources and in resilience, considering Infrared downlinks, and evaluating the performance of the proposed PON data networks with realistic user-to-user indoor demands. Average SINR

Number of users
No failure Failure in AP1 Failure in AP5 Failures in AP1 and AP5