Enhancing confidentiality protection for ZCZ-OCDMA network using line selection and wavelength conversion based on SOA

1 Introduction

Optical Code Division Multiple Access (OCDMA) schemes have recently garnered significant interest because of their inherent capability of multiplexing multiple users on the same asynchronous access network, secure network connection, flexibility and multimedia services. Numerous parameters such as simultaneous number of active users and the type of codes govern transmission performance analysis of the OCDMA systems [1], [2].

In order to improve the capacity of the system, many techniques have been employed such as the combination of the subcarrier multiplexing (SCM) technique and the OCDMA system resulted in the creation of the hybrid SCM SAC-OCDMA system, which enabled more efficient use of the available band by combining the advantages of both techniques [3], [4].

However, the main cause of degradation of optical CDMA performance is multiple access interference, which can be reduced by using a good optical codes family having a large number of users with maximum weight and the good correlation properties. Various coding techniques have been proposed [5], [6]. Recently, an interesting code called zero correlation zone (ZCZ) code is proposed. This class of codes is an innovative coding, which ensures ideal autocorrelation and cross-correlation properties within the correlation zone. Various constructions of ZCZ codes such as binary, ternary, polyphase and optical codes have been constructed [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17].

On the other hand, in recent years, massive growth in the volume of information exchange notably in Big Data applications such as social networks, Internet of Things (IoTs) and cloud computing, has driven remarkable attention. Therefore, in these applications, strong demands of physical-layer privacy and confidentiality potentially supported by OCDMA have become an interesting research topic where the security is one of the challenges associated with the rise of IoTs and Big Data [18].

For enhance security purpose and thus increase individual privacy in OCDMA systems, various techniques have been proposed such as:

• Using spectral-phase-encoded scheme [19], [20]

• Using multicode-keying encryption [21], [22]

• Introducing optical Ex-oring (XOR gate) between coded data with random key sequences [23]

• Designing a security enhanced decoder based on the XOR logic gate detection technique [24]

• Introducing a semiconductor optical amplifier (SOA)based Code word Multiplexer (CMUX) and Code word De-Multiplexer (CDEMUX) [25].

In the present paper, we propose novel security mechanisms for enhancing physical-layer privacy of zero correlation zone-Optical Code Division Multiple Access (ZCZ-OCDMA) systems.

For this goal, the paper is organizes as follows. In Section 2, we introduce the realization of high-speed all-optical logic gates based on SOAs and tunable filters. Section 3 describes the different architectures that can be used to enhance the level of ZCZ-OCDMA network confidentiality. Section 4 presents proposed system configurations that can generally be broken down into two principles. The first method is based on the modification of the transmitted data information, while the second method consists in modifying both the data information and the set of wavelengths, which were used for encoding the data signal. Results of the simulation under OptiSystem are given in Section 5. Finally, conclusion is given in Section 6.

2 SOA-based logic gate

Nowadays, optical logic gates, such as all-optical NOT, AND, OR and XOR gates become key elements for all-optical high-speed signal processing and show promise for numerous potential applications in optical signal processing systems, such as bit-error rate monitoring, optical bit-pattern recognition, all-optical packet address, package synchronism and clock recovery [26], [27], [28], [29].

Fundamentally, all optical gates may be designed using the nonlinearity properties introduced by SOA. These nonlinear effects make the SOA an attractive device for applications in optical networks in terms of low power consumption, small footprint, and higher optical integration. Among these nonlinear effects are the cross-gain modulation (XGM), cross-phase modulation (XPM) and four-wave mixing [28], [29], [30], [31].

The block diagram to realize different SOA-based logic gates is shown in Figure 1.

Figure 1:

Simulation setup of semiconductor optical amplifier (SOA) based optical logic gates [27].

3 Confidentiality techniques

Enhancing the level of ZCZ-OCDMA system confidentiality and security against eavesdropping is an important issue. For this purpose, several architectures can be exist in which two principals methods can be used such as wavelength conversion (WC) technique using SOA [32] and optical line selector (LS) using SOA [33], [34].

Wavelength Conversion (WC) is a key function present in various contexts. WC convert data signal from one wavelength to another. This transformation can be performed using an SOA’s in a Mach-Zehnder Interferometer (MZI) configuration (see Figure 2) taking advantage of the XPM and XGM phenomenon. Wavelength converter needs a simple SOA that can be controlled optically by gating of CW light. Therefore, inverted or in-phase output signals result [32].

Figure 2:

(a) Cross-phase modulation XPM wavelength conversion; (b) cross-gain modulation XGM wavelength conversion.

Line Selector also referred as multiplexer is a combinational logic device that selects only one signal from 2 ^N input data signals depending on the N control signals [34]. Figure 3 shows the block diagram of a 2:1 (2 to 1) LS with two inputs (A, B) and one control input C (see its truth table; Table 1). The expression for the output Y is given by:

Figure 3:

Block diagram of a 2:1 line selector.

A 2 ^N : 1 LS can be built out of basic gates such as AND, OR and NOT gates using SOA-MZI configuration [33].

4 Proposed system configurations

In this paper, we propose two kinds of protection against eavesdropping. The first method is based on the modification of the transmitted data as shown in Figure 6, while the second method consists in modifying both the data and the set of wavelengths, which were used for encoding the data signal as shown in Figures 9 and 12.

In this work, ZCZ spreading sequences are adopted for all proposed SAC-OCDMA designs. These codes, with ZCZ (see Figure 4), have good correlation properties (less periodic auto correlation function (PACF) and periodic cross correlation function (PCCF)) that make it possible to minimize multiuser interference effect in future communications systems.

Figure 4:

Definition of zero-correlation zone [35].

Using on-off keying (OOK) as modulation scheme, the optical ZCZ sequences used in the simulation with OptiSystem are as described in [36] and are shown in Figure 5.

Figure 5:

Zero correlation zone (ZCZ) sequences for: (a) user 1; (b) user 2.

As we can see, these codes have a zero cross-correlation (λ _C  = 0), making it eligible for the direct detection technique.

4.1 Design 1

Figure 6 present the first security concept based on the modification of the transmitted data for ZCZ-OCDMA networks. The proposed model consists of using the optical 2:1 LS component (see Figure 3). The selection line (input control) is based on the ‘DATA1’ signal. Depending upon whether the value of select line ‘DATA1’ is “0” or “1” the input line (C0 or C1) is selected and given at the output. In this way, the output data signal ‘DATA2’ has completely changed from the original.

Figure 6:

First design of zero correlation zone-Optical Code Division Multiple Access (ZCZ-OCDMA) transmitter based on LS components for one user.

In this design, the number of 2:1LS components depend on the weight of the ZCZ code and C 1 = C 0 ‾ . Thus, we can rewrite Equation (1) in the form:

(3) DATA 2 = C 0. DATA 1 ‾ + C 1. DATA 1

At the receiver side (see Figure 7), it is observed that by making XOR (exclusive OR) gate of the received signal (‘DATA2’) and the input line C0, we can get the original user data (‘DATA1’).

(4) DATA 1 = DATA 2   ⊕   C 0

Where ⊕ denotes the Exclusive OR Boolean function operator.

Figure 7:

First design of zero correlation zone-Optical Code Division Multiple Access (ZCZ-OCDMA) receiver based on XOR operators for one user (all wavelengths direct detection).

In this design, the architecture used for all-optical XOR gates is based on the SOA-MZI. For performing this Boolean function, two SOAs are symmetrically located in each branch of the SOA-MZI structure without WC. The obtained signals are then multiplexed and directly detected by the photodiode as shown in Figure 7 (all wavelengths detection) [4].

In this stage, we note that the receiver architecture can be simplified when using one wavelength detection. It consists of adequately reconstructing the information of the desired user from any of the wavelengths constituting the code that do not overlap with other wavelengths from other spreading codes [4].

By using this detection, the decoder architecture become less complex whereby the received codes can be filtered out using a bandpass optical filter allowing to pass only one wavelength as shown in Figure 8.

Figure 8:

First design of zero correlation zone-Optical Code Division Multiple Access (ZCZ-OCDMA) receiver based on XOR operators for one user (one wavelength direct detection).

4.2 Design 2

Figure 9 presents a second security concept based on the modification of both transmitted data and the set of wavelengths constituting the ZCZ code. The proposed model consists of using 2:1 LS components for both functionalities.

Figure 9:

Second design of zero correlation zone-Optical Code Division Multiple Access (ZCZ-OCDMA) transmitter based on LS components for one user.

Let us denote by Δ λ the spectral spacing between the wavelengths. The new wavelengths are chosen in order to preserve the same spectrum band occupied by the original wavelengths as follows:

(5) λ i ′ = λ i + Δ λ ′

Where i denotes the ith wavelength and Δ λ ′ =   Δ λ / 2 (see Figure 10).

Figure 10:

Principle of operation of the proposed wavelength conversion (WC).

For this design, the receiver has the same structure as described in Section 4.1 (see Figure 7). In this case, the XOR function is performed using SOA-MZI structure with WC (see Figure 11).

Figure 11:

Second design of zero correlation zone-Optical Code Division Multiple Access (ZCZ-OCDMA) receiver based on XOR operators for one user: (a) all wavelengths detection; (b) one wavelength detection.

4.3 Design 3

Figure 12 shows another security concept based on the modification of both transmitted data and the set of wavelengths constituting the code word for ZCZ-OCDMA networks. The proposed model consists of using wavelength converter (WC) components. The number of WC components depends on the weight the ZCZ code.

Figure 12:

Zero correlation zone-Optical Code Division Multiple Access (OCDMA-ZCZ) transmitter based on WC for one user.

In this design, a simplified WC model using a single SOA based on XGM (see Figure 2) is used. The method is capable of inverting the original data signal ( DATA 2 = DATA 1 ‾ ) and convert it to other wavelengths with the same manner in design 2.

At the receiver side (see Figure 13), the incoming signal is injected to the wavelength de-converter (W-DC) that will regenerate the original set of wavelengths which were used for encoding at the transmitter side.

Figure 13:

Zero correlation zone-Optical Code Division Multiple Access (ZCZ-OCDMA) receiver based on wavelength de-converter (W-DC) for one user: (a) all wavelengths detection; (b) one wavelength detection.

5 Results and discussion

In this section, we will analyze and compare the performance of the proposed designs based on ZCZ codes for Optical CDMA system.

Unless otherwise stated, the global simulation parameters are shown in Table 2 and the physical SOA parameters used for our proposed model are set as listed in Table 3.

The performance of ZCZ-OCDMA is evaluated in terms of log of bit error rate (BER) as a function of received power (dBm) for all proposed designs taking into account the aforementioned fiber effects such as attenuation and dispersion and insertion losses in the encoder/decoder including multiplexer/demultiplexer and splitter. The analysis presented here treats the performance comparison of the proposed designs (log of BER as function of the signal power measured at the detector) while varying fiber length, into the range of 5 to 45 Km, for one wavelength detection.

Figure 14 reveals that the log of BER of the ZCZ-OCDMA system based on design 3 is more valuable than that system built from design 1 and 2. Design 3 gives less log of BER and more receive power compared to others for the same fiber link distance. Design 1 and 2 are receiving less power achieving higher value of BER compared to design 3.

Figure 14:

Log of BER vs. received power (dBm) for 2 users.

Indeed, we can see from Figure 14 that, for a transmission distance of 20 km, the received power is −17.86, −18.78 and −20.014 dBm for design 3, 1 and 2, respectively. Therefore, for two users the maximum distance reached is greater than 45, about 45 and greater than 35 Km for designs 3, 1 and 2, respectively.

Design 1 offers a better BER than design 2 due to its less complex structure requiring fewer optical components and therefore less insertion losses. Nevertheless, design 3 is the best solution in terms of received power and BER performance, since this allows high extinction ratio at the optical receiver.

In the next simulation, we analyzed the impact of increasing users’ set cardinality, from 2 up to 4, on the performances of proposed designs.

Unfortunately, simulation results in Figure 15 indicate, that as the number of users increase, the performance of the proposed designs degrades which results in reducing received power levels and consequently in reduced transmission distances within tolerable BER limits.

Figure 15:

Log of BER performance comparisons for 2 and 4 users.

6 Conclusion

In this work, three designs are proposed to enhance ZCZ-OCDMA network confidentiality and simulated on OptiSystem. The obtained results demonstrate that the design 3 receives more power along with lower log of BER compared to other designs at a fixed link distance. Therefore, design 3 is the best confidentiality solution in terms of received power, BER performance and architecture complexity. On the other hand, a high number of users will inevitably result in degradation of users’ performance for all proposed designs. Therefore, for a given link distance we can find the required number of users to keep BER levels within tolerable limits. In contrast, for a defined number of users we can measure the maximum allowable link distance.