The performance enhancement of OCDMA System based on New Zero Cross Correlation (ZCC) Code with OFDM Modulation
A .Cherifi ^{1}, B. Yagoubi ^{2}, B. S. Bouazza ^{1} , A.O.Dahman^{3}.
^{1} Technology of Communication Laboratory (LTC)
University Of Tahar Moulay Saida
Algeria
^{2} University of Abdelhamid Ibn Badis
Mostaganem
Algeria
^{3} Laboratory of Microsystems and Telecommunications (LMST)
University Of Quebec
Canada
Cherifi.abdelhamid@gmail.com, sbmsvy@gmail.com, master2005_78@yahoo.fr, dahmane@uqtr.ca
Abstract:  In order to increase the number of users and data rate, and to reduce the impact of multi access interference (MAI), this study propose the optical code division multiple access (OCDMA) systems with orthogonal frequency division multiplexing modulation (OFDM) based on new zero cross correlation (ZCC) code .
The system proposed (OCDMA new ZCCOFDM) is compared to the (OCDMAFCC (Flexible Cross Correlation)OFDM) in term of SNR and BER. The results showed that the proposed system with the new ZCC code displayed improved performance with an increased number of users up to 120%. The system adopted saved around 3 dBm of power at the receiver. In comparison to (OCDMAFCCOFDM) system, this improvement is due to the effect of the good auto/cross correlation properties of new ZCC code.
KeyWords:  Optical CDMA, OFDM, New ZCC code, Flexible Cross Correlation (FCC) code, (OCDMAFCCOFDM) systems, (OCDMAnew ZCCOFDM) systems.
1 Introduction
Currently, the most common way to allow several transmitters to send the information at the same time by single channel is code division multiple access (CDMA). It is a unique code given to each transmitter; a spread spectrum technique by the same physical resources.
The main advantages of OCDMA are to provide multiple simultaneous users with the same bandwidth along with high security, and to improve optical communication applications [1]. It can be considered as a good solution for optical networks. So, to obtain the best spectral efficiency possible at reduced cost, a practical optical system that combines the technical orthogonal frequency division multiplexing (OFDM) and CDMA is used to enhance the data rate transmission and to increase the number of simultaneous users. This combination has received increased attention as a means to overcome various limitations of optical transmission systems, such as the multipath dispersion and multi access interference (MAI).
OFDM has become a popular transmission technique for highdatarate wireless communications in recent years due to its high spectral efficiency and good resistance to multipath fading [2]. Thus, by using this method, we can get better spectrum utilization, increase transmission rate, and also generate higher number of subcarrier.
In this article, we examined and evaluated the advantages of (OCDMAnew ZCC –OFDM) Optical system. The SNR expression was derived by taking into account the nonlinearity of subcarriers using the new ZCC code. OFDM modulation provided many orthogonal subcarriers which are transmitted to a single optical fiber. In reception part, the desired user will be detected from few subcarriers by using the optical filters (FBG narrowband), the code signature, similar to those using in emission part. The use of this filter removes the effect of multi access interference.
This project is a continuation of previous work presented in [3]. And the main goal of previous study is to combine OFDM technique with OCDMA system to improve the performance of optical network. In other side, the results of the current study (OCDMA new ZCCOFDM) are compared to the work (OCDMAFCC (Flexible Cross Correlation)OFDM) reported by A.O. Aldhaibani and al in [4]. This paper is organized as follows. In Section 2 provides (OCDMA –OFDM) system based on new ZCC code. Section 3 introduces our proposed design the (new ZCC code), their proprieties and their performances. In Section 4 Simulation results are presented. And the Conclusions are drawn in Section 5.
2 Principle of OCDMAOFDM system
In the part of OFDM signal, data bits are encoded using a QAM constellation, this electrical signal passes through the IFFT block, the guard interval is inserted between these block. This eliminates interference between successive blocks in the presence of multipath channels and facilitates the more equalization.
Each OFDM symbols are modulated with the optical ZCC code by using the external modulator. This code is implemented by the Wavelength Division Multiplexing (WDM) like a Fiber Bragg grating (FBGs) which is a technique used in optical communications. This allows transmitting several wavelength signals on a single optical fiber. In the receiver part, the received signal is detected by the photodetector (see Fig.1).
Fig.1 Scheme illustrates the (OFDMOCDMA) system.
Finally, the signal can be recovered after using OFDM demodulator, FFT operations, matched filtering and other electrical treatments.
2.1 OCDMAOFDM performances
To analyze and evaluate the performance of the proposed method (OFDM CDMA optical) with the new ZCC code, the Gaussian approximation for computing the signal to noise ratio (SNR) and bit error rate (BER) was used. As shown in fig 1, SNR is expressed by: SNR = , where : represent the current received at the photodiode given by and is the variance of the noise signal. The effect of phase induced intensity noise (PIIN) is neglected due to the zero cross correlation condition with no overlapping of spectra from multiuser [5], [6]. The first stage concerns with studying the design of the new ZCC code. The general form of ZCC code is:
ZCC1= (1)
We use the mapping technique in ZCC code to avoid the overlapping of 1 for both users, in addition to increasing both; the number of users and the code length.
ZCCn= (2)
Example:
ZCC2=
ZCC2=
Regarding the mapping phase, the relationship between the three parameters (the number of user “K”, the code length “C” and the number of mapping process “M= W" where W represents the codeweight) is given as follows:
KM = 2M
CM = 2M ×w
To extend the number of weight and transform the code w = "N1" to w = N, (N ≥2), the matrix form is given by
ZCCN= (3)
[A] comprises a matrix of [2,2 × w], this matrix Contains the w replication of the matrix (Zw1) and as illustrated in the equation (9).
[B] Comprises a matrix of zero of [2,2 × w] .
[C] Comprises a matrix of zero of [(K−2), w×2].
[D] comprises a matrix of zero of [(K−2), w×(K−2)]
Taking an example for k = 3 the transformation code for w = 1 to w= 2 are presented as:
ZCC2=
Where the expression between w, k and C is defined by: C=w×K (4)
For increasing the number of users and the code length without modifying the weight, a mapping technique is used as:
ZCCm= (5)
The expression of the equation which connects the coefficients of mapping :(mapped number of users cm, the code length Lm and the mapping processes m are expressed as:
Cm = 2^{m} × (CB)
Lm=2^{m }× (L) (6)
3 The Proposed Design
The new ZCC code is constructed by a binary matrix of K row; represent the number of users and C= K × W column represent the code length,
= (7)
Where
is the half matrix; consist of ( , )
The construction of half matrix is given by: Z( , )= where1≤i≤ (8)
The code contain an (k×w) chips,
w chips”1” and “(k×w)w" chips ‘0’. So we can write as:
Where 0 ≤ j≤ .
= (9)
The other Half of matrix is the rotation of of the matrix Zi.
So, the new ZCC code is: =
Example
For example Let: K=4 and w = 2; the wave length is limited to 8. The generation of the half matrix is shown in tab 1.Using equation (9) .
J= 0 1 

i=1 
1 
5 
i=2 
2 
6 
The half matrix is:
The new code is given by:


The new ZCC code properties for Direct Detection technique is given as :
= (10)
Let denote the ith element of new Zcc code sequence with weight w. K denotes the number of each user, in fig. 2, All users send the same data bits ( ) equal to "1”. With = = =1 (11)
The power spectral density (PSD) at the receiving end during a single bit period for PIN photodiode detection can be defined as follows [7, 8]:
. (12)
4 Conclusion
Fig.2 Block diagram of a new ZCC code transmitter system.
Where Psr is the power received of a broadband source and u (v) represents a unit step function is equal to:
Then, integrating equation of the PSD can be determined by:
(13)
(14)
From Eq. (4), when all the users are transmitting bit “1,” using the average value as
= = = 1
so: (15)
The photocurrent Idd can be expressed as:
= (16)
Where represents the responsively of the PDs given by: Here, is the quantum efficiency, isthe electron’s charge, h is the Plank’s constant (6,626068 × 1034 m2 kg / s), and is the central frequency of broadband optical pulse [9].
(17)
In demodulate OFDM signal phase. Signal received of photocurrent ( ) can be expressed as
. n=1, 2, k (18)
The orthogonality conditions are put to ensure the absence of interference between the different carriers which is given by the following Expression [10].
The noise power of PIN can be written as:
(19)
Where : Shot noise
And : Thermal noise
= (20)
Noting that the probability of sending bit ‘1’ at any time by each subscriber is ½ [11], than Eq. (18) becomes:
(21)

Fig.3 diagram illustrates the new ZCC code receiver system.
Now using equations (17), (19) and (20), the signal to noise ratio (SNR) of direct detection technique by using the value of proprieties in [11]; the new expression of SNR can be written as:
= (22)
Thus BER can be obtained as:
Table 1, parameters used in the calculation of the proposed systems.
Photodetector quantum efficiency( 
0.6 
Linewidthbroadband source (∆ν) 
3.75 THz 
Operating wavelength (λ0) 
1550 nm 
Electrical band width(B) 
311 MHz 
Data bit rate(Rb) 
622 Mbps 
Receiver noise temperature( 
300 K 
Receiver load resistor 
1030 Ω 
Number of subcarriers(k) 
256 
4. Numerical results
In order to evaluate the performance of new ZCC code using (OFDMOCDMA) systems, the current study used the same parameters previously reported [1,4,12,13,14,15]. Table 1 indicates the chosen parameters for the calculation of new ZCC code to reduce the BER and to increase the number of simultaneous users, and to improve the power received. These results were simulated using matlab. The weight used in optical system (OCDMAOFDM) for this study is w=4 at data rate 622Mbps and power received equal to 20 dBm. We found in a study by A.O. Aldhaibani [4] that the authors used B=0.75R…. (*), R bit rate. But in table 1 it was mentioned that the electrical bandwidth was B=311.so, due to the calculation of the equation… (*) we find that B=0.5R is the right value. In our study, B=311 was used in comparison to the A.O. Aldhaibani results [4].
Fig 4.shows the variations of the BER with the number of users (k) for (OCDMAOFDM) systems using the two codes (new ZCC code and FCC Code) . We observe that the BER of proposed system using new ZCC code has better performance than using FCC code; this enhancement is due to effect of the new ZCC codes length and the good proprieties of auto and Cross correlation. At the acceptable BER (109), we note that the number of simultaneous users (OCDMAOFDM code new ZCC) and (OCDMAOFDM  FCC Code) are respectively 107 and 100; we say that the cardinality has increased up to 10.7% when we use the new ZCC code.
Fig.4, BER against number of users for ODCMAOFDM systems using two codes (New ZCC) code and ( FCC) code With (W= 4).
Fig.5. represents the BER against the received power for the two systems (OFDMOCDMAnew ZCC ) and (OFDMOCDMAFCC ) at 622 Mbps of data rate and at k=50 (k the number of users) . We observe that the power received for (OFDMOCDMAnew ZCC ) and (OFDMOCDMAFCC ) are 24dBms and 27dBms respectively at the acceptable BER(109), Therefore, when we use (OFDMOCDMAnew ZCC ) system, the performances become better compared with (OFDMOCDMAFCC) and we can save around 3 dBm of power.
Fig.5 BER against power receive (Psr) for ODCMAOFDM systems using two codes (New ZCC )code and( Fcc) code at 622 Mbps data rate and (W=4).
Fig.6 represents the BER as a function of number of users at acceptable BER (109) when we fix Psr=20dB.the BER expression is given by the electrical bandwidth of direct detection (b=0.5R), R: bit rate.
We notice that within 2.5 GB for (OFDMOCDMAnew ZCC) we obtain 107 users compared with (OFDMOCDMAFCC) which is 100 users.
Thus, within 10GB for (OFDMOCDMAnew ZCC) we obtain 68 users compared with (OFDMOCDMAFCC) which is 43 users, apparently the proposed system using the new ZCC code has better performance than using the FCC code.
Fig.6 BER against number of users for ODCMAOFDM systems using new ZCC code (w=4)
4 Conclusion
In this paper, we have derived an analytical expression of SNR of OCDMA OFDM systems based on new ZCC code. The performance are evaluated in terms of BER, the proposed system with the new ZCC code provided better performance compared to FCC code. The major advantage of proposed code has short code lengths and the large flexibility proprieties.
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