spatial optical code division multiple access passive optical networks (OCDMA-PON)

Optical Fiber Technology 54 (2020) 102072

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Optical Fiber Technology journal homepage: www.elsevier.com/locate/yofte

An optimized architecture to reduce the impact of fiber strands in spectral/ spatial optical code division multiple access passive optical networks (OCDMA-PON)

T

⁎

Waqas A. Imtiaza, , Hassan Yousif Ahmedb, Medien Zeghidb,c, Yahia Shariefb a

Department of Electrical Engineering, Jalozai Campus, University of Engineering and Technology, Peshawar, Pakistan Electrical Engineering Department, College of Engineering at Wadi Aldwaseer, Prince Sattam Bin Abdulaziz University, Saudi Arabia c Electronics and Micro-Electronics Laboratory (E. μ. E. L), Faculty of Sciences, University of Monastir, Tunisia b

A R T I C LE I N FO

A B S T R A C T

Keywords: SAC-OCDMA CAPEX 2D-EMD BER PON

Exploitation of the spectral/spatial two dimensional optical code division multiple access (2D-OCDMA) systems in passive optical networks (PONs) is not explored extensively due to the fact spatial encoding requires deployment of multiple star couplers in the topology where each coupler is connected to different ends of either transmitter or receiver nodes. As a result, this scheme is not suitable for PON applications because it requires a significant increase in the amount of fiber deployment within the network along with the complexity of implementation to deploy the required star couplers. To deploy spectral/spatial OCDMA systems at PON and reduce the impact of fiber strands, this work proposes and investigates an optimized architecture of 2D-OCDMA for PONs based on enhanced multi-diagonal (EMD) code. Spectral/spatial encoding technique is used to develop 2D-EMD coding algorithm from the existing 1D-EMD scheme. This operation elevates orthogonality between the adjacent codes and supports high capacity through superior noise mitigation at the receiving end. A complete system architecture is developed and implemented in an excessively used software for optical networks implementation and analysis called Optisystem, for analysis under different performance parameters of PON. It is observed that the proposed system provides better performance in terms of transmission capacity, and cardinality, in comparison with existing 1D-EMD code and other 2D-OCDMA counterparts. Analysis shows that for an agreeable BER of 10−9 the proposed system can support 192 subscribers each communicating at 2 Gbps of data over 25 km single mode fiber. In terms of capital expenditure (CAPEX), four times cost saving per user is achieved for proposed architecture (PA) over conventional architecture (CA) for nearly similar performance.

1. Introduction Two dimensional (2D) optical code division multiple access (OCDMA) systems using spectral/spatial encoding have attracted substantial attention to address the fundamental limitations in optical transmission capacity at the access domain. Spectral encoding uses signature code sequences with specific cross-correlation (CC) values to convert the optical spectrum, by allowing of rejecting the specific wavelength, from a broadband source in accord with the binary 1′s or 0′s of the OCDMA code sequence. Efficient performance parameters of the spectral coding scheme significantly help in the mitigation of multiple access interference (MAI) and connected phase induced intensity noise (PIIN), which results in high transmission capacity [1–4]. Whereas spatial encoding employs communication while using several parallel fiber strands, while each strand has a distinct cladding comprising of ⁎

one or more cores. Spatial encoding further improves orthogonality among adjacent codes and alleviates the limitation of spectral encoding to develop systems with high capacity and cardinality [5–10]. However, exploitation of the spectral/spatial 2D-OCDMA systems is not explored extensively in passive optical networks (PONs) till date up to our best knowledge. This is due to one of the important challenges: spatial encoding requires deployment of multiple star couplers in the topology where each coupler is connected to different ends of either transmitter or receiver nodes as shown in Fig. 1 [5–15]. These connections should be made by using separate fiber strands to allow orthogonality required by the proposed 2D-OCDMA codes. As a result, this scheme is not suitable for PON applications because it requires a significant increase in the amount of fiber deployment within the network along with the complexity of implementation to deploy the required star couplers. In an attempt to deploy spectral/spatial OCDMA

Corresponding author. E-mail addresses: [email protected], [email protected] (W.A. Imtiaz).

https://doi.org/10.1016/j.yofte.2019.102072 Received 18 July 2019; Received in revised form 7 October 2019; Accepted 8 November 2019 1068-5200/ © 2019 Elsevier Inc. All rights reserved.

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SC 1

E1,1

E1,1

User(1,1) SC 2 E1,2

E1,2

User(1,2)

User(K2), (K1)

E(K2),(K1)

SC N2

E(K2),(K1)

Fig. 1. Basic 2D spectral/spatial OCDMA system.

systems at PON and reduce the impact of fiber strands, this paper proposes an optimized architecture for 2D-OCDMA based on enhanced multi-diagonal (EMD) code. Impact from the extensive use of optical fibers among the transmitter (Tx ) and receiver (Rx ) modules is limited by duplicating the stars couplers at optical line terminal (OLT) and remote node (RN) racks. This significantly limits the length of fiber strands that employ spatial encoding and decoding between the Tx and Rx modules. Moreover, number of the parallel paths in spatial encoding is also reduced through efficient design of the newly given 2D-EMD algorithm [15]. Consequently, the proposed optimized 2D-EMD based spectral/ spatial OCDMA system can deliver increased capacity and cardinality at limited cost, which is suitable for deployment at PONs. Working of the proposed 2D OCDMA-PON is evaluated by implementation in an excessively used software for optical networks called Optisystem. Analysis is made by referring to bit-error-rate (BER) and eyes diagrams for diverse system performance parameters including number of subscribers, data rates, etc. Feasibility of the proposed setup is also determined by providing a detailed cost analysis, in terms of capital expenditure (CAPEX), and comparison with the conventional 2D-OCDMA systems.

corresponding element along the main diagonal of the Dn via step 2.2 for X . 2. Fill locations of wi − 1 with ones for X (step 2.3) and wi for Y by using step 3.2, where i = 1 for X and 2 forY . Algorithm: Proposed two dimensional spectral/spatial code for OCDMA-PON system: Input: w1, w 2,k1, k2 Output: Ag, h = YhT Xg 1 N1' = k1 + k1 (w1 − 2); N2' = k2 (w 2 − 1); N3 =k1 (w1 − 2) 2 for (g = 1; g≤ k1; g++) {

2.1 Xg ((N1' − 1)downto0) = (others = > '0')

2.2 Xg (N1' − g + 1) = '1' 2.3 Xg (N3 − (g − 1)(w1 − 2)downtoN3 − g(w1 − 2)) = (others = > '1') } 3 for (h = 1; h≤ k2; h++) { 3.1 Yh ((N2' − 1)downto0) = (others = > '0')

3.2 Yh (N2' − (h − 1)(w 2 − 1)downtoN2' − h(w 2 − 1)) = (others = > '1') } 4 for (h = 1; h≤k2; h++) { 4.1 for (g = 1; g≤ k1 ; g++) {

2. 2D-EMD code The proposed 2D-EMD code is derived from two OCDMA codes extracted from 1D-EMD code namely X and Y (where X represents the spectral code sequence and Y represents the spatial code sequence), with X = [x1, x2 , x3 , ⋯⋯,x N1] and Y = [y1 , y2 , y3 , ⋯⋯,yN2 ] respectively [2,15]. X is the 1D-EMD code sequence with both data matrix (Dn ) and code matrix (Cn ) . Whereas Y coding sequence contains the Cn only to reduce the number of parallel fibers for spatial encoding. N1 = K1 + [K1 (w1 − 2) + 1] and N2 = [K2 (w2 − 1) + 1] represent the spectral (X th ) and spatial (Y th) code lengths with code weights of w1 and w2 respectively. Moreover, K1 represents the X th code sequences and K2 represents the Y th code sequences for the proposed 2D-EMD code. An example of two X th and Y th 1D-EMD code sets with K1 = 2 , K2 = 2 and w1 = 3, w2 = 2 is shown in Table 1. Now, both X th and Y th code sequences are used to build 2D-EMD code matrix using Algorithm 1 [15] through the following two steps:

4.2 Ag, h = YhT Xg

} 5} 6 Return Ag, h

Now using Algorithm 1 and the above-mentioned steps, spectral/spatial code matrix for users (Ug, h) can be obtained from Xg and Yh generating sequences as Egh = YhT Xg , where Xg and YhT are the g th and hth code sequences of X and Y respectively with g = (1, 2, 3, ⋯⋯,K1) and h = (1, 2, 3, ⋯⋯,K2) . Table 2 shows an example of the proposed code sequences for Xg (K1 = 2, w1 = 3) and Yh (K2 = 2, w2 = 2) respectively. Table 2 Proposed 2D-EMD spectral/spatial code.

1. Calculate the first non-zero elements of the matrix that will give a Table 1 Spectral (X th ) and spatial (Y th ) code sequences extracted from 1DEMD code. X th code sequences

Y th code sequences

X1 = {10011} X2 = {01110}

Y1 = {011} Y2 = {110}

2

YhT Xg

X1 = [10011]

X2 = [01110]

⎡0⎤ Y1T ⎢ 0 ⎥ ⎢1⎥ ⎢ ⎣1⎥ ⎦ ⎡0⎤ Y2T ⎢ 1 ⎥ ⎢1⎥ ⎢ ⎣0⎥ ⎦ ⎡1⎤ 1 Y3T ⎢ ⎥ ⎢0⎥ ⎢ ⎣0⎥ ⎦

⎡ 00000 ⎤ ⎢ 00000 ⎥ ⎢ 10011 ⎥ ⎢ ⎣ 10011 ⎥ ⎦ ⎡ 00000 ⎤ ⎢ 10011 ⎥ ⎢ 10011 ⎥ ⎢ ⎣ 00000 ⎥ ⎦ ⎡ 10011 ⎤ ⎢ 10011 ⎥ ⎢ 00000 ⎥ ⎢ ⎣ 00000 ⎥ ⎦

⎡ 00000 ⎤ ⎢ 00000 ⎥ ⎢ 01110 ⎥ ⎢ ⎣ 01110 ⎥ ⎦ ⎡ 00000 ⎤ ⎢ 01110 ⎥ ⎢ 01110 ⎥ ⎢ ⎣ 00000 ⎥ ⎦ ⎡ 01110 ⎤ ⎢ 01110 ⎥ ⎢ 00000 ⎥ ⎢ ⎣ 00000 ⎥ ⎦

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Now substituting Eqs. (3)–(5) into Eq. (6), the new 2D-EMD correlation function becomes:

Moreover, the proposed 2D-EMD code can support a total of K1 × K2 subscriber with code length ofN1 × N2 .

R(3) (g , h) R(4) (g , h) ⎤ R(1) (g , h) − R(2) (g , h) − ⎡ − ⎢ w1 − 1 w1 − 1 ⎥ ⎦ ⎣ w w g h 0 ∀ = = 1 2 =⎧ ⎨ 0otherwise ⎩

2.1. 2D-EMD code properties Performance of the proposed system primarily depends on encoding and decoding of the proposed 2D-EMD code at the Tx and Rx modules respectively. This section evaluates both the auto and CC properties of the proposed code that will facilitate the development of an efficient encoder and decoder combination for the proposed 2D-EMD based OCDMA-PON. Four characteristic matrices are introduced [10–15], to determine the required correlation properties, by using E (d) matrix, where d ∈ (1, 2, 3, 4)

It can be observed from Eq. (7) that the receiving combination yields λa = w1 w2 andλ c = 0 , which is essential for the implementation of high cardinality OCDMA-PON. 3. Proposed 2D-EMD based OCDMA-PON

T Eg(1) , h = Yh Xg

Eg(2) ,h

=

Eg(3) ,h

=

Eg(4) ,h

=

− YhT Xg − YhT Xg − − YhT Xg

The proposed architecture is divided into three parts namely OLT (located at the central office), optical distribution network (ODN), and optical network terminal (ONT) at the subscriber’s premises to mimic the conventional OCDMA-PON architecture [16–17]. Additions are made at OLT and RN racks to reduce the extensive use of fiber strands and place the couplers for spatial division multiplexing at optimal locations. However, before discussing the structures for transmitter and receiver modules in detail, it is imperative to discuss the encoder and decoder design for better understanding of the proposed architecture. Design of the encoder and decoder arrangement of the 2D-EMD based OCDMA-PON is discussed with reference to Table 5, which elaborates the allocation of wavelengths and couplers (CP) for 6 users namely X1 = 10011, X2 = 01110 , U1,1, U2,1, U1,2, U2,2, U1,3, U2,3 using and Y1 = 0011, Y2 = 0110 , Y3 = 1100 respectively.

(1)

− Xg and YhT −

represents complement of the Xg and YhT code sequences here respectively. Furthermore, the 2D-EMD code Eg, h is represented by a set of elements ei, j where i = (1, 2, 3, ⋯⋯,N1) and j = (1, 2, 3, ⋯⋯,N2) . Hence, CC for 2D-EMD coding scheme while using Eg, h and E (d) can be expressed as N2

R(d) (g , h) =

N1

∑ ∑ e((id,j))e(i,j) (g, h)

(2)

i=1 j=1

e((id, j))

E (d)

j )th

3.1. Encoder

j )th

is the (i, entry of and e(i, j) (g , h) is the (i, entry where ofE(g, h) . Table 3 further elaborates the correlation characteristics of 2DEMD code, which is developed by using R(d) (g , h) with w2 = 2. Table 4 further demonstrates the process of attaining the correlation values from (i, j )th entries of E (d) and (i, j )th entries of E(g, h) . Random entries of i, j are considered for elaboration purpose of Table 3. Now a new CC function R(4) (g , h) is defined to remove the effect of CC from R(1) (g , h), R(2) (g , h), R(3) (g , h) that is produced by Eg, h when g ≠ 1 ∩ h ≠ 1. Consequently, the new expression of CC functions by using Table 3 are given as follows.

RI(1) (g , h) = R(1) (g , h) − RI(2) (g , h) = R(2) (g , h) −

R(4) (g , h) w1 − 1

The encoder arrangement is used inside the OLT rack to convert the binary 1′s into spectral and spatial representation for each subscriber based on the X th and Y th code sequences of 2D-EMD code. The encoder module essentially e7ncompasses of two basic operations (i) spectral encoding and (ii) spatial encoding [15]. 3.1.1. Spectral encoding Spectral encoding operation is used to convert the binary 1′s in X th code sequence into spectral representation. Encoder arrangement for the necessary conversion consists of 1: w1 optical splitter (OS) and a w1: 1 multiplexer (MUX) arrangement as shown in Fig. 2(a). The 1: w1 OS is used to split the relatively flat spectrum from a broadband source (BBS) into three equal portions, based on w1 of the X th 2D-EMD code sequence. The split portions are then forwarded towards input ports of three band-pass filters in the MUX arrangement. Each filter in MUX is configured at 0.4 nm bandwidth and is used to allow a specific wavelength [2,15]. This arrangement of 1: w1 OS and w1: 1 MUX is used to perform the spectral encoding operation as per the X th code sequence. For example, the X2 code sequence [0 1 1 1 0] in Table 5 is spectrally encoded using the encoder arrangement given in Fig. 2(a), where the input spectrum is first split into three equal portion in order to represent each chips in X2 code sequence. Each leg of the 1:3 OS is connected to a 3:1 MUX arrangement with bandpass filters centered along 1490 nm, 1490.4 nm and 1490.8 nm. Each filter in the MUX arrangement is used to allow the spectrum in accordance with the chip-wavelength placement as shown in Fig. 2(b). Consequently, the input spectrum is spectrally encoded as each chip is now represented with carefully selected dins from the input spectrum.

(3)

R(4)

(g , h) w1 − 1

(4)

RI(3) (g , h) = R(3) (g , h) − R(4) (g , h)

(5)

RI(2)

RI(3)

(g , h) and (g , h) have nonzero It is evident from Table 3 that values at g = 1 ∩ h ≠ 1 and g ≠ 1 ∩ h = 1. This result can be effectively utilized to remove the negative effects caused byEg, h . Thus, the decoder arrangement that primarily operates using Eq. (6) discards the contribution of MAI due to adjacent interfering users and recovers the intended spectrum with maximum suppression of PIIN figures. Simple manipulation of the above equations yield:

RI(1) (g , h) − RI(2) (g , h) −

RI(3) (g , h) w w ∀g=h=0 =⎧ 1 2 ⎨ w1 − 1 0otherwise ⎩

(6)

Table 3 Correlation properties for 2D-EMD code with w1 = 3, w2 = 2 . Eg, h g g g g

= = ≠ ≠

1, 1, 1, 1,

h h h h

=1 ≠1 =1 ≠1

(7)

R(1) (g , h)

R(2) (g , h)

R(3) (g , h)

R(4) (g , h)

w1 w 2 w1 w2 1

0 w1 0 1

0 0 w 2 (w 2 − 1) w1 − 1

0 0 0 w1 − 1

3.1.2. Spatial encoding End-face of the w1: 1 MUX based spectral encoder is connected with spatial encoder, which consists of a 1: w2 OS that is used to split the encoded spectrum based on w2 of the Y th 2D-EMD code sequence. Split portions of the spectrally encoded spectrum are forwarded towards 3

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Table 4 Demonstration of attaining correlation properties.

Table 5 A graphic representation of 2D-EMD code sequences.

3:1 MUX

1:3 OS

BBS

(a)

(b)

Fig. 2. (a) Spectral encoder (b) Output of the spectral encoder for 0011100 code sequence.

Coupler 3 Spectral Encoder

1:2 OS

input ports of the respective coupler (CP). Tree couplers are employed to facilitate implementation of the proposed setup. Selection of the CP is based on location of binary 1′s in the Y th code sequence [10–15]. For example, Fig. 3 represents the spatial encoding operation for the Y1T code sequence [0 0 1 1], where the placement of chips in the code represents the number of optical CP that will be used to pass the spectrally encoded spectrum. As shown in Fig. 3, CP 3 and 4 are selected as per the chip placement specified in Table 5, which spatially encoded in the spectrally encoded spectrum by allowing the signal to flow in multiple paths as per the Y1T code sequence. Moreover, N2 CPs are used to perform the required operation with K input ports and 1 output port each (K represents the number of subscribers in the system). Small strands of the optical fiber medium are used to connect output ports of the w2: 1 OS with input ports of the

Y1T Coupler 4

Fig. 3. Spatial encoder inside the OLT rack.

4

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Coupler 3

2:1 OC

Y1T

__

DB1

X1

X1

BD1

Y1T

Coupler 4

OA

+

+

PD2

PD0

Coupler 1

2:1 OC

__ Y1T

__

BD2

BD2

X1 PD1 +

X1

Coupler 2 __

Y1T

Fig. 4. Spatial decoder.

OA

+

PD3

respective CP as shown in Fig. 3. Furthermore, CPs are placed inside the OLT rack to avoid the extensive use of long fiber strands between the Tx and Rx module for the accomplishment of spatial encoding. The 2DEMD spectral/spatial encoded signal is forwarded towards the RN rack at ODN using N2 SMFs.

PIN photodiode PD0, which will generate a corresponding current of R(1) (g , h) . Similarly, the productions of PIN PDs 1–3 are proportionate

3.2. Decoder

to the values of R(2) (g , h), w − 1 , and w − 1 . Thus the total output current produced by the given arrangement is proportional to

Fig. 5. Spectral decoder.

R(3) (g , h)

(3)

R(4) (g , h)

(4)

⎡R(1) (g , h) − R (g, h) ⎤ and ⎡R(2) (g , h) − R (g, h) ⎤ respectively. Now both w−1 w−1 ⎣ ⎦ ⎣ ⎦ the currents are combined through an optical adder to generate a resulting current that is proportional to w1 w2 for g = h = 0 and 0 otherwise. Thus, receiver can recover the intended spectrum and completely remove the contribution MAI and associated PIIN, which helps the proposed setup to support large cardinality at high data rates and extended reach.

Decoder module is used to recuperate the desired encoded spectrum with determined auto- and CC values. Decoder arrangement for the proposed 2D-EMD based OCDMA-PON encompasses two operation namely (i) spatial decoding and (ii) spectral decoding. 3.2.1. Spatial decoding The process of spatial decoding is achieved inside the RN rack to minimize the span of fiber strand between RN and ONT modules. Spatial decoder essentially consists of CPs and OSs in reverse order as that of the spatial encoder. N2 CPs are employed at the RN rack, where each CP contains 1 input port and 2 × K output ports. Each port of the CP relates to the respective spectral decoder through two w2: 1 optical combiners (OCs) and balanced detector (BD) BD1 and BD2 for each subscriber as shown in Fig. 4. Optical combiner at top of the decoder (OCT ) is primarily utilized to retrieve and associate the signals from the respective optical CPs, in correspondence with Y1T code sequence. Whereas, the bottom optical combiner (OCB ) is used to retrieve and associate the signals from CPs

3.3. Optical line terminal (OLT) rack Fig. 6 shows the proposed 2D-EMD based OCDMA-PON with OLT rack, ODN and ONTs for three users U1,1, U1,2, U1,3 as an example. OLT rack houses the transmitter and receiver modules for each subscriber. The transmitter module, for userU1,1, consists of a light emitting diode (LED) with 30 nm spectral width to provide a relatively flat spectrum from spectral encoding. End-face of the LED relates to Mach-Zehnder Modulator (MZM), which is used to modulate the user’s data with LED’s spectrum using on–off keying (OOK) operation. Output from MZM is fed into the spectral encoder that is used to perform the necessary binary-to-spectral conversion as per Fig. 2 in Section 3.1.1, where X1 = 10011 and Y1 = 0011 are the X th and Y th code sequences for userU1,1. The spectral encoded signal for user U1,1 is forwarded towards the bidirectionalOST , which is used to split the spectrum in two equal portions for down-link traffic. The split portions are fed into CP3 and CP4 using short span fibers as per location of the binary 1′s in Y1T code sequence. Table 5 can be referred as an example to fully understand the allocation of wavelength and CPs for each subscriber. Thus, the process of spectral/spatial encoding is performed inside the OLT rack to avoid the extensive deployment of optical fiber medium between the Tx and Rx modules. Consequently, a relatively simple architecture with minimum span of optical fiber is achieved for deployment of the proposed 2DEMD built OCDMA system as PON at the access domain. CPs inside the OLT rack are directly connected to CPs at RN racks using N2 SMFs as shown in Fig. 6. For up-link traffic, the flow of information runs in opposite direction as shown in Fig. 6. Receiver module for user U1,1 employs a spectral

−

corresponding to the spatial code sequence Y1T . Furthermore, small strands of optical fiber medium are used to connect 2K output ports of the couplers with OCT and OCB in order to achieve spatial decoding operation inside the RN rack and avoid the extensive use of long fiber strand between CPs and ONTs [10–15]. 3.2.2. Spectral decoding Spectral decoder essentially consists of two BDs in order to recover the intended spectrum with desired correlation properties. Fig. 5 shows architecture for both BDs containing a combination of Uniform FiberBragg Grating (UFBG) filters, optical adders, optical attenuators (OA), PD, and an optical subtractor respectively. BD1 is configured to recover − the spectrum in accordance with X1 Y1T and X1Y1T through special arrangement of UFBG filters and optical adders as shown in Fig. 5. While −

BD2 is configured to recover the spectrum in accordance with X1 Y1T and − − X1Y1T

for user U1,1 respectively. End-face of the FBG filters are fed into corresponding PDs using OA. Signals that are in proximity with the X th and Y th codes can reach 5

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Fig. 6. Proposed 2D-EMD based OCDMA-PON architecture.

down-link traffic through a single fiber as shown in Fig. 6. This further eliminates the need for numerous optical fiber strands between the CPs and ONTs. Consequently, the proposed architecture mimics the conventional PON model and avoids the needs for extensive deployment of optical fiber medium and answers the quest for couplers placement.

decoder arrangement as per Section 3.2.2. The signal to BD1 is forwarded from CP3 and CP4 through OST to recover the spectrum in ac− cordance with X1 Y1T and X1Y1T . Whereas BD2 is connected to CP1 and CP2 −

− −

through OSB to recover the spectrum in accordance with X1 Y1T and X1Y1T for userU1,1.

4. Coupler implementation 3.4. Optical distribution network (ODN) Placement of CPs inside the OLT and RN rack avoids the need for star couplers that are employed in the conventional 2D-OCDMA system. Consequently, tree couplers can be used for physical implementation of the proposed setup. However, due to unavailability of the tree coupler module in Optisystem software, optical combiners and splitters are used to mimic the operation of couplers at the OLT and RN rack respectively. Now, in order reach such a high number of subscribers, O stages of 1 × P couplers are be deployed in the following simulation models. Fig. 7 shows an example of the concept with O = 3 and P = 4 to implement 64 × 1 CP inside the OLT rack. At the first stage of the implementation, one 4 × 1 OC is employed leading to 4 input ports. At second stage of the implementation, one 4 × 1 OC is connected to each input port of the first stage combiner. So, the number of inputs at second stage of the implementation becomes 42 = 16 respectively. Finally, at the third stage, one 4 × 1 OC is employed for each 16 inputs of the second stage. As a result, at the end of the third stage, the CP can support 4 × 16 = 64 connections respectively.

ODN is used to carry the spectral/spatial encoded signal from OLT towards the subscriber premises. In order to mimic the conventional PON, ODN is divided into three parts namely (i) feeder fiber (FF), (ii) RN rack, (iii) distribution fiber (DF). FF entails N2 long span SMFs that are used to connect the OLT rack CPs with those inside the RN rack. FFs are used to carry the spectral/spatial encoded signals in both up- and down-link direction across the ODN. Application of FFs significantly mitigates the extensive use of long span optical fiber medium for spatial encoding and elevates feasibility of the proposed 2D-EMD built OCDMA system for the low cost PON. RN rack is used to receive the N2 SMFs as shown in Fig. 6. RN rack is designed to house a combination of multiple CPs, short span fibers to perform the required spatial decoding for down and encoding for uplink traffic, and OCs. End-face of each FF relates to a specific CP in order to carry the 2D-EMD based encoded signal from OLT towards the RN rack. Each CP is further connected to corresponding bidirectional OC using short span fibers, as discussed in Section 3.2.1, inside the RN rack. For user U1,1 the down-link traffic is divided in two parts for transmission towards the BD1 andBD2 . Encoded signals from CP3 and CP4 are connected to BD1 using OCT inside the RN rack as shown in Fig. 6. This arrangement is used to recover the spectrum in accordance with Y1T

5. Proof of concept As a proof of concept, the proposed optimized architecture for 2DEMD built OCDMA-PON is implemented in a highly recognized software called Optisystem as shown in Fig. 8. The implementation is performed for three users U1,1, U1,2, U1,3 based on Fig. 6 while up- and down-link traffic is observed by referring to eye diagrams at 1 and 1.25 Gbps. System performance parameters used for simulation analysis are given as Table 6 and spatial encoding and decoding is performed using back-to-back approach. Moreover, UFBG filters are used for the analysis due to limitations on availability of suitable components in OPTISYSTEM. Furthermore, different wavelengths are used for both upand down-link traffic, in-order to avoid interference at the system components. Consequently, 1490 nm is used for up-link traffic, while 1550 nm spectrum is adapted for the down-link data. It is also evident from Table 6 that noise sources like dark noise and thermal noise with the values of 10nA and 1 × 10−22 W/Hz are included in the simulation analysis. Furthermore, shot noise having Gaussian distribution is also introduced as a noise source to perform the simulation analysis while

− Y1T

code sequence. While compliment of is forwarded from CP1 and CP2 towards the BD2 using OCB as per the description in Section 3.2.1. For up-link traffic, OCT is connected with CP3 and CP4 to forward the spectral encoded signal for the required spatial encoding inside the RN rack. Consequently, both spatial encoding and decoding operations are performed inside the RN rack to address the issue of fiber span and CPs placement in 2D spectral/spatial OCDMA systems. DFs are placed between the OCs and BDs to avoid extensive use of optical fiber medium between RN and ONT modules. A standard 2 core SMF with primary and secondary fiber is employed to carry the signal to and from the ONTs. For userU1,1, the primary fiber is used to carry the flow of information in both up- and down-link direction from CP3 and CP4 via OCT as shown in Fig. 6. Whereas, secondary fiber is used to carry the down-link traffic from CP1 and CP2 towards the BD2 using OCB . Moreover, different wavelengths are employed to carry both up- and 6

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Fig. 7. Three stage implementation model example for CPs with 4 × 1 OC.

data, reach, and number of subscribers and the flow of traffic in both direction with minuscule addition in system components. Design of OLT and RN rack significantly reduce the span of optical fiber medium employed for the operation of spatial encoding and decoding. However, it is necessary to determine a suitable span for practical implementation of the proposed system. Consequently, Fig. 10 investigates the effect of fiber span used inside the OLT and RN rack on overall performance of the system. Analysis is made by changing the span of optical fiber medium while using system performance parameters as Table 6, while comparison is made by referring to the back-to-

using relatively practical values. Eye diagrams of both up- and down-link traffic for U1,1 are shown in Fig. 9(a)–(b) respectively. Results show that the proposed setup provides the required provision for the flow of traffic in both directions with no requirement for additional components in the system. Moreover, eyes diagrams elaborate that the extensions in OLT and RN racks to mitigate the need for long span optical fibers between the Tx and Rx modules is working efficiently and the proposed setup can support high data rates in both directions. Thus, the proposed optimized architecture for 2D-EMD based OCDMA-PON can provide high capacity in terms of

Fig. 8. Simulation setup for up- and down-link traffic analysis. 7

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based OCDMA-PON for maximum transmission capacity and cardinality. Analysis is made for down-link model only while using the optimization concepts for OLT and RN rack proposed in Section 3. Moreover, BER and eye diagram are observed while varying different performance parameters like number of and data rate while employing system performance parameters in Table 6. The down-link model for investigation purpose is built with X th and Y th code sequences having (K1 = 32, K2 = 6) respectively. Thus, the total number of subscribers that can access the medium simultaneously in the implementation model isK1 × K2 = 192 . Moreover, N2× CPs, CP1 − CP7 are employed at both OLT and RN racks to perform the required spatial encoding and decoding operation as per Section 3. Erbium doped fiber amplifiers (EDFA) with a power gain of 25 dB is also employed at each SMF after the coupler arrangement at OLT rack to mitigate the effect of power losses across different components of the network. The CPs in both OLT and RN racks are connected using 25 km SMFs, distribution fibers are ignored for simplicity of the simulation model, namely SMF1 − SMF7 respectively. Fig. 11 shows performance of the optimized 2D-EMD based OCDMA-PON architecture for BER vs. data rate while 192 subscribers are accessing the medium simultaneously. Furthermore, both back-to-back, and SMF scenarios are considered for the analysis. Graphs in Fig. 11 gives an upward drift in BER as the amount of data increases. This performance can be attributed to the fact that width of the transmitted pulses is in inverse proportion with the amount of data transmitted between the Tx and Rx modules [3]. Consequently, the pulse width reduces when more data is transmitted and that imparts a negative impact of BER at the receiving module. Further investigation of Fig. 11 shows that BER increase gradually

Table 6 System performance parameters. Performance parameters

Values

LED bandwidth Filters bandwidth Sequence length Samples per bit Signal format Received power Responsibility Thermal noise Dark current SMF wavelength SMF dispersion Dispersion slope Attenuation Nonlinear parameters EDFA gain

30 nm 0.4 nm 128 bits 64 NRZ −10 dBm 0.75 A/W 1E-22 W/Hz 10 nA 1550 nm 17 ps/nm/km 0.08 ps/nm2/km 0.25 dB/km Active 25 dB

back results for download traffic at 1 Gbps in Fig. 9. Eye diagrams for different spans of the optical fiber medium shows proximity with eye diagram obtained from the back-to-back setup. Moreover, it is also observed that increase in the span of fiber from 2.5 m, 5 m, 10 m, 50 m, and 100 m has miniscule effect on overall performance of the system.

6. Performance analysis This section investigates performance of the proposed 2D-EMD

(a) Downlink

(b) Uplink

(c) Downlink

(d) Uplink

Fig. 9. Eye diagrams for up-link and down-link traffic at (a) (b) 1 and (c) (d) 1.25 Gbps. 8

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Back-to-back

2.5 m

5m

10 m

50 m

100 m

Fig. 10. Eye diagrams with different lengths of the spatial encoding and decoding fibers inside OLT and RN rack.

B2B SMF

1E-6

increase in the number of subscribers elevates MAI, which results in higher values of PIIN at the receiving PD. Consequently, decreasing the quality of signal in terms of SNR and increase in BER. However, a gradual increase in BER is observed for both 2 and 2.5 Gbps of data. Moreover, it is shown that the proposed 2D-EMD based OCDMAPON is able to support 192 subscribers accessing the medium simultaneously at 2 Gbps of data. It is evident from the fact that performance of the two-matrix EMD code used as X th code sequence with w1 = 3 is further enhanced by added orthogonality of the Y th code sequence with w1 = 2 that results in mitigation of MAI and associated PIIN at the receiving PD. Moreover, efficient arrangement of the BDs at the receiving end further elevates the system’s performance through spectrum recovery with maximum auto and minimum CC. Consequently, the intended spectrum is recovered with maximum power units in comparison with the interfering one, resulting in high values of SNR and low BER. Fig. 13 demonstrates performance of the proposed architecture in comparison with existing 2D-EMD in [17] and 2D-FRS OCDMA system in [16–17]. Analysis is carried out for different values of the subscribers while sing performance parameters in Table 6 and 622 Mbps of data is used for 2D-EMD and FRS codes as per [16–17], while the proposed setup is analyzed at 2 Gbps. BER curves in Fig. 13 shows an upward trend with elevation in the number of users that agrees with the fact that MAI elevates as more subscribers access the medium simultaneously. Further investigation of Fig. 13 shows that the proposed setup performs significantly well in comparison with the existing counter parts. This can be attributed to the addition of low weight spatial code in the proposed algorithm, which significantly mitigates the need for higher powers at the receiving end along with the provision of required correlation and orthogonality between adjacent codes. Furthermore, limiting the number of fiber strands between the transmitter and receiver module also simplifies the proposed architecture and reduces the

1E-8

Bit Error Rate (BER)

1E-10 1E-12 1E-14 1E-16

B2B

1E-18 1E-20

SMF

1E-22

1.5 Gbps 1.00

1.25

1.50

1.75

2 Gbps 2.5 Gbps 2 Gbps

2.00

2.25

2.50

2.75

3.00

Data Rate (Gbps)

Fig. 11. BER vs. data rate for 192 subscribers at −10 dBm received power.

1E-4

0.01

1E-6

1E-4

1E-8

1E-6 1E-8

1E-10

Bit Error Rate (BER)

Bit Error Rate (BER)

between data rates and the proposed 2D-EMD based OCDMA-PON provides desirable performance for an adequate BER value of 10−9 . The unique two-matrix structure of EMD coding algorithm offers desirable CC between adjacent codes that significantly removes the contribution MAI at the receiving PD. Moreover, the process of spatial encoding further elevates the orthogonality between adjacent codes, and hence removal of MAI and associated PIIN. Consequently, the proposed setup can support 192 users simultaneously transmitting at 2 Gbps over 25 km SMF with −10 dBm power at the receiving PD. Fig. 12 shows performance of the proposed system in terms of BER and the number of subscribers accessing the medium simultaneously. It is shown that BER increases as more subscribers access the medium simultaneously. This increase in BER can be attributed to the fact that

1E-12 1E-14 1E-16 1E-18 1E-20

1E-10 1E-12 1E-14 1E-16 1E-18 1E-20

2 Gbps 2.5 Gbps

1E-22

64

96

128

160

2D-EMD @ 622 Mbps 2D-FRS @ 622 Mbps 2D-EMD @ 2 Gbps

1E-22

192

64

Number of Subscribers

96

128

160

192

Number of Subscribers

Fig. 12. BER vs. number of subscribers for 2 and 2.5 Gbps data at −10 dBm received power.

Fig. 13. Comparison of the proposed 2D-EMD architecture with existing solutions. 9

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minimum threshold on efficient recovery of the intended spectrum. Consequently, the proposed architecture is able to provide efficient performance at low power of −10 dBm in comparison with 0 dBm for the considered counterparts.

Table 7 System components with cost unit for CAPEX analysis.

7. Cost analysis This section evaluates the proposed architecture in terms of capital expenditure (CAPEX) for comparison with the conventional 2D-EMD based OCDMA system. Cost analysis is an important tool that highlights the feasibility of network deployment and adaptability by the end user. Moreover, spectral/spatial OCDMA system includes several optical fiber strands and couplers to perform the required spatial encoding, which elevates the need for cost analysis in terms of deployment at the cost sensitive PON [18–19]. CAPEX analysis is carried out by calculating the overall expenses incurred on deployment of the system components including optical fiber infrastructure. Total number of ONTs as given in Eq. (8) obtains total CAPEX per subscribers through dividing the cumulative cost of the system. Where CTS represents the total cost per subscriber, CSi is the cost of each component block inside the system including optical fiber medium, with (i = 1, 2, 3, ⋯⋯,I ) as the total number of components. K in Eq. (8) represents the total number of ONTs, which is computed by K = K1 × K2 respectively. K

(8)

Analysis is performed for two 2D-EMD based OCDMA systems by considering the proposed architecture (PA) and the conventional architecture (CA) as given in Fig. 14 in order to determine the impact of changes in the OLT and RN rack on overall cost of the system. For fair comparison, it is assumed that:

OLTrack = (M × OLTTX ) + (K × OS ) + (2K × .005 × OF1c ) + (N2 × CP ) (10)

RNrack is also modified to provide in-house spatial decoding in order to reduce the span and number of optical fibers employed between the couplers and ONTs. RNrack in the PA collects the 2D encoded spectrum through CPs arrangement from N2 FFs, which is employed using a 12core fiber module. In-house spatial decoding is employed using short span (5 m) single core optical fibers followed by OCs to reduce the overall expenditure on implementation of the spatial decoding process. Consequently, the CAPEX equation for RNrack becomes

RNrack = (N2 × CP ) + (4K × .005 × OF1c ) + (2K × OC )

(11)

Now, total cost for the proposed architecture for 2D-EMD based OCDMA-PON can be written as

90

C(PA)

80

Cost Units (CU)

(9)

OLTrack in the PA essentially consists of OLTTX with modulation and spectral encoding components, followed by in-house spatial encoding arrangement as shown in Fig. 6. Consequently, the CAPEX equation for OLTrack can be written as

PA CA

100

463 0.6 10 15 6 2.6 4 12.5

C(PA) = OLTrack + FF + RNrack + DF + ONT

1. Analysis is made for K = 36 with(K1 = 6, K2 = 6) andN2 = 7 to maintain acceptable cost figures. 2. OLT port in each system can support 18 subscribers; which leads to a total of M = 2 ports in both 2D-EMD based PA andCA . 3. 2D-EMD based spectral encoding and on–off keying modulation is similar in both systems; therefore, spectral encoding and modulation arrangement is combined in a single component block calledOLTTX , for the analysis, with same cost. 4. Spatial encoding is considered separately in the analysis as OLT rack in analysis of the PA .

110

OLTTX OS/OC 1: K Tree Coupler (CP) K: K Star Coupler (SC) ONT 2D Optical fiber 1 Core/Km(F1c ) Optical fiber 2 Core/Km(F2c ) Optical fiber 12 core/Km(F12c )

Now, for 36 subscribers, CAPEX of the proposed 2D-EMD based OCDMA-PON is determined by defining total cost of the system as the sum of all network components inside the OLT rack, ODN including RN rack and ONT, which leads to Eq. (9).

∑i = 1 CSi ∑k = 1 ONTk

Cost (Cost Unit)

5. For all systems, length of the FF between OLT and RN is set to 20 Km, whereas 5 Km DFs are used between RN and ONTs. The optical fiber used for spatial encoding is set to 5 m, based on simulation model, for analysis. 6. Analysis is performed for down-link model only. 7. Digging cost of the fiber is ignored due to high variation between service providers and regions. 8. Local vendors and existing research were adapted for cost of the components used in all systems. For fair analysis generic cost units are adapted as given in Table 7.

I

CTS =

System Components

= (M × OLTTX ) + (K × OS ) + (2K × .005 × OF1c ) + (N2 × CP ) +

70

(20 × FF12c ) + (N2 × CP ) + (4K × .005 × OF1c ) + (2K × OC ) +

60

(5 × K × DF2c ) + (K × ONT )

50

(12)

Similarly, the CPAEX equation for conventional architecture of 2DEMD based OCDMA system based on Fig. 1 can be derived as. It is observed that spatial encoding and decoding arrangement, which is formed by the combination of optical fiber and SC, employed at the ODN. Thus, overall equation of CAPEX for the CA can be written as:

40 30 20 10

C(CS )

0 OLT

OS

SEF

SEC

SDF

OC

= (M × OLTTX ) + (K × OS ) + (2K × 20 × OF1c ) + (N2 × SC ) +

ONT

System Components

(4K × 5 × OF2c ) + (2K × OC ) + (K × ONT )

Fig. 14. Component wise cost comparison for the proposed and conventional 2D-EMD based OCDMA systems.

(13)

Fig. 14 shows the major contributors of cost along with component wise cost comparison in implementation of both architectures. It is 10

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PA CA

220

8. Conclusion

200

This paper proposes an optimized architecture for the deployment of 2D-EMD based OCDMA systems at PON. The proposed architecture is designed with addition in the OLT and RN rack to minimize the length of optical fiber strands for spatial encoding operation. Performance analysis using Optisystem shows that the proposed architecture can support high capacity in terms of data, reach and the number of subscribers at relatively low power at the receiving end. Furthermore, cost analysis of the proposed architecture in comparison with conventional 2D-EMD based OCDMA system shows a significant reduction in cost. Consequently, the proposed 2D-EMD based OCDMA system is feasible for deployment at the low cost PON.

180

Cost Units (CU)

160 140 120 100 80 60 40

Declaration of Competing Interest

20 0 32

64

96

128

160

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

192

Number of Subscribers Fig. 15. CAPEX analysis for the proposed and conventional 2D-EMD based OCDMA systems at different number of subscribers.

References [1] H. Ghafouri-Shiraz, M.M. Karbassian, Optical CDMA Networks: Principles, Analysis and Applications, John Wiley & Sons, 2012. [2] W.A. Imtiaz, M. Ilyas, Y. Khan, Performance optimization of spectral amplitude coding ocdma system using new enhanced multi diagonal code, Infrared Phys. Technol. 79 (2016) 36–44. [3] N.D. Keraf, S. Aljunid, A. Arief, P. Ehkan, The evolution of double weight codes family in spectral amplitude coding ocdma, Advanced Computer and Communication Engineering Technology, Springer, 2015, pp. 129–140. [4] W.A. Imtiaz, Design of high-capacity spectral amplitude coding ocdma system with single photo-diode detection technique, Arabian J. Sci. Eng. 43 (2018) 2769–2777. [5] A.R.A.J. Abdullah, S.A. Aljunid, A.M. Safar, J.M. Nordin, R.B. Ahmad, Mitigation of multiple access interference using two-dimensional modified double weight codes for optical code division multiple access systems, Opt. Eng. 51 (2012) 65–70. [6] B.-C. Yeh, C.-H. Lin, J. Wu, Noncoherent spectral/spatial ocdma system using twodimensional hybrid codes, J. Opt. Commun. Networking 2 (2010) 653–661. [7] B.C. Yeh, C.H. Lin, Non-coherent spectral/spatial optical CDMA system using 2-D diluted perfect difference codes, J. Lightwave Technol. 27 (2009) 2420–2432. [8] C.-H. Lin, J. Wu, C.-L. Yang, Noncoherent spatial/spectral optical CDMA system with two-dimensional perfect difference codes, J. Lightwave Technol. 23 (2005) 3966–3980. [9] R. Kadhim, H.A. Fadhil, S.A. Aljunid, M. Razalli, A new two dimensional spectral/ spatial multi-diagonal code for non-coherent OCDMA systems, Opt. Commun. 329 (2014) 28–33. [10] M. Najjar, N. Jellali, Spectral/spatial optical CDMA code based on Diagonal Eigenvalue Unity”, Opt. Fiber Technol. 38 (2017) 61–69. [11] M. Najjar, N. Jellali, M. Ferchichi, Two-dimensional multi-service code for spectral/ spatial optical CDMA system, Opt. Quantum Electr. 49 (2017) 397–411. [12] C. Kandouci, A. Djebbari, A. Ahmad, A new family of 2D-wavelength-time codes for OCDMA system with direct detection, Optik 135 (2017) 8–15. [13] A. Cherifi, N. Jellali, M. Najjar, S. Aljunid, B. Bouazza, Development of a novel twodimensional-SWZCC–code for spectral/spatial optical CDMA system, Opt. Laser Technol. 109 (2019) 233–240. [14] H.Y. Ahmed, M. Zeghid, W.A. Imtiaz, A. Sghaier, Two-dimensional fixed right shift (frs) code for sac-ocdma systems, Opt. Fiber Technol. 47 (2019) 73–87. [15] W.A. Imtiaz, H.Y. Ahmed, M. Zeghid, Y. Sharief, M. Usman, Design and implementation of two-dimensional enhanced multi-diagonal code for high cardinality OCDMA-PON, Arab. J. Sci. Eng. 3 (2019) 1–18. [16] B. Dai, S. Shimizu, X. Wang, N. Wada, Full-asynchronous gigabit-symmetric DPSK downstream and OOK upstream OCDMA-PON with source-free ONUs employing all-optical self-clocked time gate, Opt. Express 20 (26) (2012) B21–B31. [17] Z. Gao, B. Dai, X. Wang, N. Kataoka, N. Wada, 40 Gb/s, secure optical communication based upon fast reconfigurable time domain spectral phase en/decoding with 40 Gchip/s optical code and symbol overlapping, Opt. Lett. 36 (22) (2011) 4326–4328. [18] Y. Cheng, M. Fiorani, L. Wosinska, J. Chen, Reliable and cost efficient passive optical interconnects for data centers, IEEE Commun. Lett. 19 (2015) 1913–1916. [19] M. Mahloo, J. Chen, L. Wosinska, A. Dixit, B. Lannoo, D. Colle, C.M. Machuca, Toward reliable hybrid WDM/TDM passive optical networks, IEEE Commun. Mag. 52 (2014) 14–23.

observed that spectral encoding arrangement in both architectures requires the same cost as same number of components are employed to modulation and the process of spectral encoding. Moreover, it is observed that the major contributor of cost is the process of spatial encoding and decoding, which is divided into three parts for and in-depth analysis namely spatial encoding fibers (SEF), spatial encoding couplers (SEC), and spatial decoding fibers (SDF). CA shown in Fig. 1 employs long span fiber for carrying spectral encoded signals from OS towards the corresponding SCs, which leads to a total of (2K × 20 × OF1c ) value for SEF in comparison with (2K × .005 × OF1c ) + (N2 × 20 × FF12c ) for the PA . This leads to a significant different in CUs for both architectures. Furthermore, it is observed that PA requires more cost for spatial encoding couplers in comparison with the CA . This can be attributed to the fact that two sets of CPs are employed in PA to reduce the span and number of fibers in the process of spatial encoding. Consequently, cost figures for PA becomes (N2 × CP ) + (N2 × CP ) in comparison with (N2 × SC ) forCA. Thus, more cost of required for deployment of the coupler’s arrangement for spatial encoding in the proposed architecture. Further analysis of Fig. 14 shows the expected conclusion that CA requires more cost for spatial decoding in comparison with the PA. It is evident from the fact that PA employs spatial decoding inside the RNrack , which significantly limits the number and cost of fiber to (4K × .005 × OF1c ) + (5K × DF2c ) in the process. Whereas, (4K × 5 × OF2c ) fibers are employed to perform spatial decoding inCA, which leads to more CUs in comparison with thePA . It is also shown that CUs for ONTs, OCs, and OSs in both arrangements is same, as same number of components are required to perform the required operations. Fig. 15 provides the comparison of CAPEX for both systems at different number of subscribers. It is observed that PA provides better performance in terms of total CAPEX when compared with the CA due to the improvements made at OLTrack and RNrack . Furthermore, it is observed that CUs for CPAEX reduces with increase in the number of subscribers. This can be attributed to the fact that cost per subscriber is obtained by dividing the total cost over number of subscribers. Consequently, the CUs for CAPEX decreases with increases in the number of subscribers. Thus, the proposed architecture for 2D-EMD based OCDMA-PON provides a feasible solution for deployment at the access domain in comparison with conventional 2D and 1D OCDMA systems.

11