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Cardinality improvement of Zero Cross Correlation (ZCC) code for OCDMA visible light communication system utilizing catenated-OFDM modulation scheme
Optik - International Journal for Light and Electron Optics 170 (2018) 220–225
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Cardinality improvement of Zero Cross Correlation (ZCC) code for OCDMA visible light communication system utilizing catenatedOFDM modulation scheme
T
⁎
N.M. Nawawi , M.S. Anuar, M.N. Junita School of Computer & Communication Engineering, Universiti Malaysia Perlis, Malaysia
A R T IC LE I N F O
ABS TRA CT
Keywords: Visible light communication (VLC) Optical Code Division Multiple Access (OCDMA) Zero Cross Correlation (ZCC) code OCDMA-VLC Catenated-OFDM
In this paper, we proposed a new OCDMA-VLC system design utilizing new modulation scheme known as catenated-OFDM (orthogonal frequency division multiplexing) technique. The catenated-OFDM-OCDMA-VLC system is demonstrated by implementing Zero Cross Correlation (ZCC) code. ZCC code is the preferred code generation in OCDMA system as the effect of multiple access interference (MAI) can be cancelled out due to reduction in Phase Intensity Induced Noise (PIIN). In particular, the use of catenated-OFDM and its application in intensity modulation and direct detection fiber optic visible light communication link is theoretically investigated by taking into account all the noises contribute in the visible light spectrum. The theoretical results demonstrated that cardinality enhancement with doublefold, triplefold and fourfold increase in number of user for (Band = 2), (Band = 5) and (Band = 8) respectively compared to previous ZCC-VLC system designs. The results also indicates a good performance with high data rate up 10Gbps using proposed system which is predicted to be new considerable for upcoming OCDMAVLC mobile communication system design.
1. Introduction Optical wireless especially visible light communication (VLC) can be a potential candidate solution for 5G networks. The VLC system has been investigated for about one and half decade and received a lot of interest [1]. VLC systems have more flexibility and integrity compare to other communication forms in many regards. Since the transmission medium in VLC systems is through visible light spectrum and not radio waves that can penetrate walls, the security issue is essentially elucidated because the light only travel in the same room, which means the data and information transmitting and receiving process happened in one location. Therefore, the information cannot be retrieve and access by outsider. Only a user that is in a direct path of the light being used to transmit the data can received the data. VLC technology encounters several challenges in the implementation level. One of the major challenge in VLC systems has been in improving transmission speed with high spectral efficiency. Similar to other broadband wireless access network, VLC is aimed to allow high data rate communication between users. Unfortunately, the VLC with OOK data formatting approaches still leads to poor efficiency of optical wireless communication for future broadband access network [2]. Nonlinear frequency response of VLC only occurred in single-carrier modulation because in this scheme the inter-symbol interference (ISI) did not handle properly [3]. Hence, a suitable and good modulation technique may lead to increase the speed (> 1Gbps) of the overall system. Compared with NRZ-OOK
⁎
Corresponding author. E-mail address: [email protected] (N.M. Nawawi).
https://doi.org/10.1016/j.ijleo.2018.05.125 Received 12 April 2018; Accepted 26 May 2018 0030-4026/ © 2018 Published by Elsevier GmbH.
Optik - International Journal for Light and Electron Optics 170 (2018) 220–225
N.M. Nawawi et al.
Fig. 1. Catenated OFDM ZCC-VLC architecture based on IM-DD.
modulation, orthogonal frequency division multiplexing (OFDM) modulation attracts much attention due to its advantage of high spectral efficiency, reduced the complexity in equalizers [4] and might be a promising alternative for high-speed VLC systems [5]. Hence, there are still rooms of improvement in modulation scheme that could improve bandwidth efficiencies of the existing modulation scheme while maintaining the high data rates system. Catenated-OFDM scheme is a multiband modulation technique which exclusively utilized the available electrical bandwidth and offers higher spectral efficiency. Conventional OFDM only transmit one band at time while catenated-OFDM tranmits more than one band simultaneously. Another issue to enable VLC system suited for future 5G networks is an access control in this multiuser environment [6]. Therefore, merging OCDMA with VLC system is useful to envision the most promising candidate for future mobile communication which offers multiple access control based on optical encoding technique. The maximum channel capacity can be improved using the proposed combination OCDMA-VLC with catenated-OFDM modulation by increasing the OCDMA codewords and/or by transmittting multiple OFDM bands. Zero Cross Correlation (ZCC) code generation is preferred in OCDMA system because limitation due to Phase Intensity Induced Noise (PIIN) can be reduced and consequently suppressed the MAI effect [7]. In this paper, we assumed PIIN is totally suppressed due to no correlation between users. Hence, the total noise variance only affected by shot noise and thermal noise. 2. Catenated-OFDM ZCC-VLC system architecture The architecture of catenated-OFDM ZCC-VLC system illustrates in Fig. 1. Catenated-OFDM is a method for multiband modulation generated by concatenation of several basic OFDMs modulation so that they can be transmitted to the receiver at the same time. The most advantage of this scheme is it is fully utilized the available electrical bandwidth so more data can be sent through the medium compared to traditional OFDM scheme. As a ZCC code encoder has been adapted into this proposed design, n-band of catenatedOFDM are spread by ZCC codes simultaneously over visible light medium based on several sub-channels. Each user has specific spreading code refer to different wavelength which means the generated catenated-OFDM signal band is modulated and carried out through the communication medium in code domain. At the receiver, the catenated-OFDM data is demultiplexed to individual user by applying specific code. Photodiode is used to detect the optical signal of each channel and pass through to the splitter to separate and extract back each catenated-OFDM signal band by filtering at their respective frequency band. Each OFDM band is then demodulated through OFDM demodulator to recover the original transmitted data. 3. Theoretical analysis of catenated-OFDM ZCC-VLC system The theoretical analysis was carried out based on some assumptions. There are n bands of catenated-OFDM signal for each optical channel. It means that for each optical channel has similar n bands and the number of users is equal to number of optical channels multiplies by n bands. The number of optical channels are labelled with K transmitters and K receivers. The spectrum of light source is Δv Δv flat for a given bandwidth ⎡v0 − 2 , v0 + 2 ⎤ where vo and Δv is the center frequency and bandwidth (in hertz) of optical source. Each ⎣ ⎦ power spectral component has identical spectral width, thus each user has equal power at the receiver. Each bit stream from each user is synchronized. 221
Optik - International Journal for Light and Electron Optics 170 (2018) 220–225
N.M. Nawawi et al.
The optical signal of k-th transmitter can be written as
bk (t ) = P0 sk (t ) z (t ) 0 ≤ t ≤ T
(1)
where P0 is the input pulse power, sk (t) is the catenated electrical signal, Z(t) is the ZCC code waveform assigned to k user with code length L. The catenated electrical signal can be written as Nsc − 1
sk (t ) =
∑
Nsb
ck exp(2πfk t )
k=0
∑ mn exp(2πfn t )
(2)
n=1
where ck is the information symbol, mn is the modulation index, n is number of band and fm is the subband/center frequency. The fn is calculated using fn = nfm . The modulation index is assumed to be identical between sub-bands [8], hence it is defined as 1 0 < mn ≤ N . sb After combiner in Fig. 1, the transmitted optical signal can be written as K
b (t ) =
∑ bk (t )
(3)
k=1
From methodology in [9], the ZCC code is the code set that satisfied the correlation functions in Eq. (4). N
w K = l auto correlation K ≠ l cross correlation
∑ zK (i) z l (i) = ⎧⎨ 0 ⎩
i=1
(4)
where ZK(i) and Zl(i) denote the ith elements of Kth and lth ZCC code sequences respectively, w denotes the code weight which equal to number of ‘1’ in the code. For K = l, the process is called autocorrelation which compared the same code. If K≠l, the correlator is going through cross correlation process. In other word, autocorrelation is the cross-correlation of a signal with itself and crosscorrelation is a measure of similarity of two waveforms. The power spectral density of received optical signals at the photodiode can be written as
r (v ) =
Pr Δv
Nsc
N
∑ sk ∑ zK (i) z l (i) Π (i) k=1
(5)
i=1
where Pr is the effective power of the broadband source at the photodiode and П(i) denotes the unit step function as given below:
Δv Δv Π (i) = u ⎡v − vo − (−L + 2i)⎤ − u ⎡v − vo − (−L + 2i + 2)⎤ 2L 2L ⎦ ⎣ ⎦ ⎣
(6)
Δv Δv = u ⎡− (−L + 2i) + (−L + 2i + 2)⎤ 2L ⎣ 2L ⎦
(7)
Δv = u⎡ ⎤ ⎣ L⎦
(8)
Now, let G(v) is the integrating function of r(v) can be written as
∫0 =
∞
G (v ) dv =
Pr w L
Pr Δv
Nsc
∑ sk ⎡w.1. k=1
⎣
Δv P ⎤+ r L⎦ Δv
Nsc
∑ sk ⎡0.1. k=1
⎣
Δv ⎤ L⎦
(9)
Nsc
∑ sk
(10)
k=1
The intensity of optical signal is encoded by ZCC code varies the photocurrent by the catenated OFDM data Sk and properties of w and L. Thus, the photocurrent incident at the receiver can be computed as
Ir = R
Pr w L
Nsc
∑ sk
(11)
k=1
Substituting Eq. (2) into Eq. (11), the photocurrent can be written as
Ir = R
Pr w L
Nsc
Nsb
∑ ck exp(2πfk t ) ∑ mn exp(2πfn t ) k=1
(12)
n=1
In an indoor VLC system, the first noise source is ambient light noise either it is radiation from background solar through windows or radiation from incandescent and fluorescent lamps. The second noise sources are from signal and ambient light induced the shot noise in the photodetector and the third noise source is from thermal noise or electrical preamplifier noise. However, the ambient light noise induced by background solar radiation and incandescent lamps represents essentially a DC interference that could be easily eliminated using an electrical highpass filter. The noise induced by fluorescent lamps needs to be determined in different 222
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N.M. Nawawi et al.
application scenarios based on what kind of driving circuit is used [10–12]. Mathematically, the signal and ambient light induced quantum shot noise in visible spectrum can be expressed as 2 σshot = 2eIr B + 2e (Ir ambient I2) B
(13)
1.6x10−19C ,
Ir and Ir_ambient are the generated signal and ambient currents in the where e is the electron charge which equal to photodetector, and I2 is the noise bandwidth factor. Usually the induced shot noise can be preserved as additive white Gaussiian noise (AWGN) if the ambient currents is much greater than generated signal current [13]. However, in this paper the ambient current is defined as 0.124 mA and noise bandwidth factor is defined as 0.562 [11]. While the thermal noise is defined as a function of Botzman constant (k), the absolute temperature (T), photodetector electrical bandwidth (B) and load resistor at the receiver (RL). Note that, the thermal noise can be reduced by using large value or load resistor but still consistence with the receiver bandwidth requirement. Therefore, the mean square of the random current or thermal noise can be expressed as 2 σthermal =
4kTB RL
(14)
Thus, the total noise can be expressed as the sum of the variance of the shot noise and the thermal noise which can be written as 2 σTotal = 2eIr B + 2e (Ir ambient I2) B +
4kTB RL
(15)
The average signal to noise ratio (SNR) of combination OCDMA based on ZCC code with catenated OFDM technique can therefore be written as
SNR =
Ir2 2 σTotal 2 2
SNR =
P w sc − 1 | c |2 cos ω t ∑Nsb | m |2 cos ω t R2 r 2 ∑kN= n n k k n=1 0 L
2eIr B + 2e (Ir ambient I2) B +
4kTB RL
(16)
The BER can be obtained from SNR and can be written as
BER =
3 log 2M M −1 SNR erfc 2(M − 1) M log 2 M
(17)
where M is the value of M-ary QAM in each subchannel. Table 1 shows the parameters used in the theoretical analysis. 4. Results and discussions Fig. 2 shows the cardinality performance comparison of cat-ZCC-VLC with conventional ZCC-VLC and OZCZ-VLC systems. A comparison is made with OZCZ-VLC based on similar code n platform. Therefore, in this result the parameters setting are based on OZCZ-VLC system design in [14]. Here, the bit rate is fixed to 622Mbps to ensure a fair comparison with OZCZ-VLC [14] and conventional ZCC-VLC. In the Fig. 2, it is seen that for BER less than 1.0E-9, the cat-ZCC-VLC system with 8, 5 and 2 bands achieved approximately 206, 152 and 92 number of active users, respectively. While for conventional ZCC-VLC only 42 active users are allowed and 45 active users for OZCZ-VLC at Pr =-9 dBm and number of weight of 4. According to the graph, there is doublefold increase in number of user for Band = 2, triplefold increase for Band = 5 and more than fourfold increase for Band = 8 compared to OZCZ-VLC system. Each optical channel brings more than one of OFDM band concurrently, therefore its increase the capacity of the system, hence improved the number of permissible users. On the other hand, the performance of signal to noise ratio is better because power of the signal increases due to many subcarriers used for each band. It is also observed from this result, the new combination method is not only an effective approach to expand the capacity of optical channel but also improved the electrical spectral efficiency by transmitting more bands at an available electrical bandwidth. Table 1 Typical parameters used in the analysis. Parameter
Value
Responsivity, R Operating wavelength, λ 0 Electrical bandwidth, B Data rate, Receiver noise temperature, T Receiver load resistor, RL Electron charge, e
0.6 480 nm 311 MHz/2.5 GHz/5 GHz 622Mbps/5Gbps/10Gbps 300K 1030Ω
1.6 × 10−19 C 1.38 × 10 −23 J/K
Boltzmann’s constant, k
223
Optik - International Journal for Light and Electron Optics 170 (2018) 220–225
N.M. Nawawi et al.
Fig. 2. Comparison of cat-ZCC-VLC with conventional ZCC-VLC and OZCZ-VLC system.
Fig. 3 illustrates the effect when varies the data rate at 622Mbps, 5Gbps and 10Gbps to the number of active users and BER performance. It is shown that the performance of cat-ZCC-VLC at higher data rate is degraded compared to cat-ZCC-VLC at lower bit rate. The maximum acceptable BER is achieved for 622Mbps with 92 active users compared to 61 active users for 10Gbps and 42 active users for 5 Gbps. However, the number of active user is almost similar for cat-ZCC-VLC at high data rate 10Gbps with OZCZVLC performance at lower data rate 622Mbps. This graph proves the proposed system can achieved higher data rate at similar cardinality performance compared OZCZ-VLC. Noted that in this analysis, number of band is set to 2. Fig. 4 indicates the effects of effective power level on the number of users for cat-ZCC-VLC system for 622Mbps data rate. As expected, the lower the effective power the worse BER performance. Hence, it is worth noting that, for proposed combination scheme at large number of bands (5 bands), the number of users at Pr = −9 dBm has high cardinality than the system with Pr=−15 dB m. The maximum accommodated number of users for Pr=−15 dB m is 30, while for high effective power −12 dBm system could accommodate up to 78 number of users and −9 dBm system could accommodate up to 153 number of users for BER = 1E-9. It is also observed that there is trade off between number of users and BER performance as BER performance decreases when there are many users in the system. In the following analysis the data rate is increased to 10Gbps to scrutinize the relationship between effective power and the BER at higher data rate. Also, the impact of cat-OFDM bands that used to increase the electrical channel capacity are investigated in OCDMA ZCC-VLC system as shown in Fig. 5. In this case, the optical codes are set to be four optical channels. At BER value of 1E-9, the effective power for Band = 3, Band = 5 and Band = 8 are −16.5 dB m, −15.5 dB m and −14.5 dB m respectively. As expected, an increasing the number of bands need higher effective power for similar BER level. 5. Conclusion In this paper, the performance of OCDMA based on ZCC code has been investigated in VLC environment using catenated-OFDM modulation scheme. Together this work provides important insights into designing process of OCDMA-VLC system. The number of users can be improved by transmitting more catenated bands simultaneously with similar number of optical channel and ZCCOCDMA code length. One unanticipated finding was that, our work can be operated at higher data rate compared to previous study in this ZCC-VLC field for a similar number of user. It is because the data rate in catenated-OFDM-OCDMA has been divided among the subbands, thus each band has lower date rate which is not affected by other lossed and impairments. As ZCC code has been used in this system, with zero cross correlation between users, the proposed system totally eliminated the PIIN noise thus significanlt improved the system performance. Thus, with a larger capacity, the catenated-OFDM-ZCC OCDMA visible light spectrum is the next best thing for the purpose of high speed data transmission. Acknowledgements This work has been funded and supported by the Department of Higher Education, Ministry of Higher Education Malaysia under Fundamenal Research Grant Scheme, FRGS (9003-00609) leads by Prof. Madya Ir. Dr. Anuar Mat Safar, Universiti Malaysia Perlis.
Fig. 3. Performance comparison at various data rate. 224
Optik - International Journal for Light and Electron Optics 170 (2018) 220–225
N.M. Nawawi et al.
Fig. 4. Effect of effective power received on number of users.
Fig. 5. BER performance of cat-ZCC-VLC versus effective power for W = 3 and K = 12.
References [1] D.C. O’Brien, Visible light communications: challenges and potential, IEEE Photonic Soc. 24th Annu. Meet. PHO 3 (2011) 365–366. [2] Y.C. Chi, D.H. Hsieh, C.T. Tsai, H.Y. Chen, H.C. Kuo, G.R. Lin, 450-nm GaN laser diode enables high-speed visible light communication with 9-Gbps QAM-OFDM, Opt. Express 23 (10) (2015) 13051–13059. [3] A. Khalid, H.M. Asif, OCDMA and OSTBC based VLC transceiver design using NI cDAQ, Photonic Netw. Commun. (2017) 1–12. [4] F. Jiang, H. Deng, W. Xiao, S. Tao, K. Zhu, An ICA based MIMO-OFDM VLC scheme, Opt. Commun. 347 (2015) 37–43. [5] Y.-C. Chi, D.-H. Hsieh, C.-Y. Lin, H.-Y. Chen, C.-Y. Huang, J.-H. He, B. Ooi, S.P. DenBaars, S. Nakamura, H.-C. Kuo, G.-R. Lin, Phosphorous diffuser diverged blue laser diode for indoor lighting and communication, Sci. Rep. 5 (1) (2016) p. 18690. [6] W. Abdallah, N. Boudriga, Enabling 5G Wireless Access Using Li-Fi Technology: An OFDM Based Approach, pp. 1–6 (2016). [7] M.S. Anuar, S.A. Aljunid, A.R. Arief, N.M. Saad, LED spectrum slicing for ZCC SAC-OCDMA coding system, 7th Int. Symp. High-Capacity Opt. Networks Enabling Technol. HONET 2010, (2010) pp. 128–132. [8] J.M. Nordin, S.A. Aljunid, A.M. Safar, A. Razif, A. Jamil, R.A. Rahim, Performance of Hybrid Subcarrier Multiplexing Over Optical CDMA Network Based on Zero Cross Correlation Code vol. 2, (2012) pp. 37–46. [9] M.S. Anuar, S.A. Aljunid, N.M. Saad, S.M. Hamzah, New design of spectral amplitude coding in OCDMA with zero cross-correlation, Opt. Commun. 282 (14) (2009) 2659–2664. [10] J. Duan, A. Shi, Y. Liu, A practical indoor visible light communication system, 9th Int. Symp. Commun. Syst. Networks Digit. Signal Process. CSNDSP 2014, (2014) pp. 1170–1175. [11] T. Komine, S. Member, M. Nakagawa, Fundamental analysis for visible-light communication system using LED lights, IEEE Trans. Consum. Electron. 50 (1) (2004) 100–107. [12] T. Komine, J.H. Lee, S. Haruyama, Adaptive equalization system for visible light wireless communication utilizing multiple White LED lighting equipment, IEEE Trans. Wirel. Commun. 8 (6) (2009) 2892–2900. [13] K. Cui, G. Chen, Z. Xu, R.D. Roberts, Line-of-sight visible light communication system design and demonstration, 2010 7th Int. Symp. Commun. Syst. Networks Digit. Signal Process, (2010) pp. 621–625. [14] L. Feng, J. Wang, R.Q. Hu, L. Liu, New design of optical zero correlation zone codes in quasi-synchronous VLC CDMA systems, EURASIP J. Wirel. Commun. Netw. 2015 (1) (2015) p. 120.
225
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.com/locate/ijleo
Original research article
Cardinality improvement of Zero Cross Correlation (ZCC) code for OCDMA visible light communication system utilizing catenatedOFDM modulation scheme
T
⁎
N.M. Nawawi , M.S. Anuar, M.N. Junita School of Computer & Communication Engineering, Universiti Malaysia Perlis, Malaysia
A R T IC LE I N F O
ABS TRA CT
Keywords: Visible light communication (VLC) Optical Code Division Multiple Access (OCDMA) Zero Cross Correlation (ZCC) code OCDMA-VLC Catenated-OFDM
In this paper, we proposed a new OCDMA-VLC system design utilizing new modulation scheme known as catenated-OFDM (orthogonal frequency division multiplexing) technique. The catenated-OFDM-OCDMA-VLC system is demonstrated by implementing Zero Cross Correlation (ZCC) code. ZCC code is the preferred code generation in OCDMA system as the effect of multiple access interference (MAI) can be cancelled out due to reduction in Phase Intensity Induced Noise (PIIN). In particular, the use of catenated-OFDM and its application in intensity modulation and direct detection fiber optic visible light communication link is theoretically investigated by taking into account all the noises contribute in the visible light spectrum. The theoretical results demonstrated that cardinality enhancement with doublefold, triplefold and fourfold increase in number of user for (Band = 2), (Band = 5) and (Band = 8) respectively compared to previous ZCC-VLC system designs. The results also indicates a good performance with high data rate up 10Gbps using proposed system which is predicted to be new considerable for upcoming OCDMAVLC mobile communication system design.
1. Introduction Optical wireless especially visible light communication (VLC) can be a potential candidate solution for 5G networks. The VLC system has been investigated for about one and half decade and received a lot of interest [1]. VLC systems have more flexibility and integrity compare to other communication forms in many regards. Since the transmission medium in VLC systems is through visible light spectrum and not radio waves that can penetrate walls, the security issue is essentially elucidated because the light only travel in the same room, which means the data and information transmitting and receiving process happened in one location. Therefore, the information cannot be retrieve and access by outsider. Only a user that is in a direct path of the light being used to transmit the data can received the data. VLC technology encounters several challenges in the implementation level. One of the major challenge in VLC systems has been in improving transmission speed with high spectral efficiency. Similar to other broadband wireless access network, VLC is aimed to allow high data rate communication between users. Unfortunately, the VLC with OOK data formatting approaches still leads to poor efficiency of optical wireless communication for future broadband access network [2]. Nonlinear frequency response of VLC only occurred in single-carrier modulation because in this scheme the inter-symbol interference (ISI) did not handle properly [3]. Hence, a suitable and good modulation technique may lead to increase the speed (> 1Gbps) of the overall system. Compared with NRZ-OOK
⁎
Corresponding author. E-mail address: [email protected] (N.M. Nawawi).
https://doi.org/10.1016/j.ijleo.2018.05.125 Received 12 April 2018; Accepted 26 May 2018 0030-4026/ © 2018 Published by Elsevier GmbH.
Optik - International Journal for Light and Electron Optics 170 (2018) 220–225
N.M. Nawawi et al.
Fig. 1. Catenated OFDM ZCC-VLC architecture based on IM-DD.
modulation, orthogonal frequency division multiplexing (OFDM) modulation attracts much attention due to its advantage of high spectral efficiency, reduced the complexity in equalizers [4] and might be a promising alternative for high-speed VLC systems [5]. Hence, there are still rooms of improvement in modulation scheme that could improve bandwidth efficiencies of the existing modulation scheme while maintaining the high data rates system. Catenated-OFDM scheme is a multiband modulation technique which exclusively utilized the available electrical bandwidth and offers higher spectral efficiency. Conventional OFDM only transmit one band at time while catenated-OFDM tranmits more than one band simultaneously. Another issue to enable VLC system suited for future 5G networks is an access control in this multiuser environment [6]. Therefore, merging OCDMA with VLC system is useful to envision the most promising candidate for future mobile communication which offers multiple access control based on optical encoding technique. The maximum channel capacity can be improved using the proposed combination OCDMA-VLC with catenated-OFDM modulation by increasing the OCDMA codewords and/or by transmittting multiple OFDM bands. Zero Cross Correlation (ZCC) code generation is preferred in OCDMA system because limitation due to Phase Intensity Induced Noise (PIIN) can be reduced and consequently suppressed the MAI effect [7]. In this paper, we assumed PIIN is totally suppressed due to no correlation between users. Hence, the total noise variance only affected by shot noise and thermal noise. 2. Catenated-OFDM ZCC-VLC system architecture The architecture of catenated-OFDM ZCC-VLC system illustrates in Fig. 1. Catenated-OFDM is a method for multiband modulation generated by concatenation of several basic OFDMs modulation so that they can be transmitted to the receiver at the same time. The most advantage of this scheme is it is fully utilized the available electrical bandwidth so more data can be sent through the medium compared to traditional OFDM scheme. As a ZCC code encoder has been adapted into this proposed design, n-band of catenatedOFDM are spread by ZCC codes simultaneously over visible light medium based on several sub-channels. Each user has specific spreading code refer to different wavelength which means the generated catenated-OFDM signal band is modulated and carried out through the communication medium in code domain. At the receiver, the catenated-OFDM data is demultiplexed to individual user by applying specific code. Photodiode is used to detect the optical signal of each channel and pass through to the splitter to separate and extract back each catenated-OFDM signal band by filtering at their respective frequency band. Each OFDM band is then demodulated through OFDM demodulator to recover the original transmitted data. 3. Theoretical analysis of catenated-OFDM ZCC-VLC system The theoretical analysis was carried out based on some assumptions. There are n bands of catenated-OFDM signal for each optical channel. It means that for each optical channel has similar n bands and the number of users is equal to number of optical channels multiplies by n bands. The number of optical channels are labelled with K transmitters and K receivers. The spectrum of light source is Δv Δv flat for a given bandwidth ⎡v0 − 2 , v0 + 2 ⎤ where vo and Δv is the center frequency and bandwidth (in hertz) of optical source. Each ⎣ ⎦ power spectral component has identical spectral width, thus each user has equal power at the receiver. Each bit stream from each user is synchronized. 221
Optik - International Journal for Light and Electron Optics 170 (2018) 220–225
N.M. Nawawi et al.
The optical signal of k-th transmitter can be written as
bk (t ) = P0 sk (t ) z (t ) 0 ≤ t ≤ T
(1)
where P0 is the input pulse power, sk (t) is the catenated electrical signal, Z(t) is the ZCC code waveform assigned to k user with code length L. The catenated electrical signal can be written as Nsc − 1
sk (t ) =
∑
Nsb
ck exp(2πfk t )
k=0
∑ mn exp(2πfn t )
(2)
n=1
where ck is the information symbol, mn is the modulation index, n is number of band and fm is the subband/center frequency. The fn is calculated using fn = nfm . The modulation index is assumed to be identical between sub-bands [8], hence it is defined as 1 0 < mn ≤ N . sb After combiner in Fig. 1, the transmitted optical signal can be written as K
b (t ) =
∑ bk (t )
(3)
k=1
From methodology in [9], the ZCC code is the code set that satisfied the correlation functions in Eq. (4). N
w K = l auto correlation K ≠ l cross correlation
∑ zK (i) z l (i) = ⎧⎨ 0 ⎩
i=1
(4)
where ZK(i) and Zl(i) denote the ith elements of Kth and lth ZCC code sequences respectively, w denotes the code weight which equal to number of ‘1’ in the code. For K = l, the process is called autocorrelation which compared the same code. If K≠l, the correlator is going through cross correlation process. In other word, autocorrelation is the cross-correlation of a signal with itself and crosscorrelation is a measure of similarity of two waveforms. The power spectral density of received optical signals at the photodiode can be written as
r (v ) =
Pr Δv
Nsc
N
∑ sk ∑ zK (i) z l (i) Π (i) k=1
(5)
i=1
where Pr is the effective power of the broadband source at the photodiode and П(i) denotes the unit step function as given below:
Δv Δv Π (i) = u ⎡v − vo − (−L + 2i)⎤ − u ⎡v − vo − (−L + 2i + 2)⎤ 2L 2L ⎦ ⎣ ⎦ ⎣
(6)
Δv Δv = u ⎡− (−L + 2i) + (−L + 2i + 2)⎤ 2L ⎣ 2L ⎦
(7)
Δv = u⎡ ⎤ ⎣ L⎦
(8)
Now, let G(v) is the integrating function of r(v) can be written as
∫0 =
∞
G (v ) dv =
Pr w L
Pr Δv
Nsc
∑ sk ⎡w.1. k=1
⎣
Δv P ⎤+ r L⎦ Δv
Nsc
∑ sk ⎡0.1. k=1
⎣
Δv ⎤ L⎦
(9)
Nsc
∑ sk
(10)
k=1
The intensity of optical signal is encoded by ZCC code varies the photocurrent by the catenated OFDM data Sk and properties of w and L. Thus, the photocurrent incident at the receiver can be computed as
Ir = R
Pr w L
Nsc
∑ sk
(11)
k=1
Substituting Eq. (2) into Eq. (11), the photocurrent can be written as
Ir = R
Pr w L
Nsc
Nsb
∑ ck exp(2πfk t ) ∑ mn exp(2πfn t ) k=1
(12)
n=1
In an indoor VLC system, the first noise source is ambient light noise either it is radiation from background solar through windows or radiation from incandescent and fluorescent lamps. The second noise sources are from signal and ambient light induced the shot noise in the photodetector and the third noise source is from thermal noise or electrical preamplifier noise. However, the ambient light noise induced by background solar radiation and incandescent lamps represents essentially a DC interference that could be easily eliminated using an electrical highpass filter. The noise induced by fluorescent lamps needs to be determined in different 222
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application scenarios based on what kind of driving circuit is used [10–12]. Mathematically, the signal and ambient light induced quantum shot noise in visible spectrum can be expressed as 2 σshot = 2eIr B + 2e (Ir ambient I2) B
(13)
1.6x10−19C ,
Ir and Ir_ambient are the generated signal and ambient currents in the where e is the electron charge which equal to photodetector, and I2 is the noise bandwidth factor. Usually the induced shot noise can be preserved as additive white Gaussiian noise (AWGN) if the ambient currents is much greater than generated signal current [13]. However, in this paper the ambient current is defined as 0.124 mA and noise bandwidth factor is defined as 0.562 [11]. While the thermal noise is defined as a function of Botzman constant (k), the absolute temperature (T), photodetector electrical bandwidth (B) and load resistor at the receiver (RL). Note that, the thermal noise can be reduced by using large value or load resistor but still consistence with the receiver bandwidth requirement. Therefore, the mean square of the random current or thermal noise can be expressed as 2 σthermal =
4kTB RL
(14)
Thus, the total noise can be expressed as the sum of the variance of the shot noise and the thermal noise which can be written as 2 σTotal = 2eIr B + 2e (Ir ambient I2) B +
4kTB RL
(15)
The average signal to noise ratio (SNR) of combination OCDMA based on ZCC code with catenated OFDM technique can therefore be written as
SNR =
Ir2 2 σTotal 2 2
SNR =
P w sc − 1 | c |2 cos ω t ∑Nsb | m |2 cos ω t R2 r 2 ∑kN= n n k k n=1 0 L
2eIr B + 2e (Ir ambient I2) B +
4kTB RL
(16)
The BER can be obtained from SNR and can be written as
BER =
3 log 2M M −1 SNR erfc 2(M − 1) M log 2 M
(17)
where M is the value of M-ary QAM in each subchannel. Table 1 shows the parameters used in the theoretical analysis. 4. Results and discussions Fig. 2 shows the cardinality performance comparison of cat-ZCC-VLC with conventional ZCC-VLC and OZCZ-VLC systems. A comparison is made with OZCZ-VLC based on similar code n platform. Therefore, in this result the parameters setting are based on OZCZ-VLC system design in [14]. Here, the bit rate is fixed to 622Mbps to ensure a fair comparison with OZCZ-VLC [14] and conventional ZCC-VLC. In the Fig. 2, it is seen that for BER less than 1.0E-9, the cat-ZCC-VLC system with 8, 5 and 2 bands achieved approximately 206, 152 and 92 number of active users, respectively. While for conventional ZCC-VLC only 42 active users are allowed and 45 active users for OZCZ-VLC at Pr =-9 dBm and number of weight of 4. According to the graph, there is doublefold increase in number of user for Band = 2, triplefold increase for Band = 5 and more than fourfold increase for Band = 8 compared to OZCZ-VLC system. Each optical channel brings more than one of OFDM band concurrently, therefore its increase the capacity of the system, hence improved the number of permissible users. On the other hand, the performance of signal to noise ratio is better because power of the signal increases due to many subcarriers used for each band. It is also observed from this result, the new combination method is not only an effective approach to expand the capacity of optical channel but also improved the electrical spectral efficiency by transmitting more bands at an available electrical bandwidth. Table 1 Typical parameters used in the analysis. Parameter
Value
Responsivity, R Operating wavelength, λ 0 Electrical bandwidth, B Data rate, Receiver noise temperature, T Receiver load resistor, RL Electron charge, e
0.6 480 nm 311 MHz/2.5 GHz/5 GHz 622Mbps/5Gbps/10Gbps 300K 1030Ω
1.6 × 10−19 C 1.38 × 10 −23 J/K
Boltzmann’s constant, k
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Fig. 2. Comparison of cat-ZCC-VLC with conventional ZCC-VLC and OZCZ-VLC system.
Fig. 3 illustrates the effect when varies the data rate at 622Mbps, 5Gbps and 10Gbps to the number of active users and BER performance. It is shown that the performance of cat-ZCC-VLC at higher data rate is degraded compared to cat-ZCC-VLC at lower bit rate. The maximum acceptable BER is achieved for 622Mbps with 92 active users compared to 61 active users for 10Gbps and 42 active users for 5 Gbps. However, the number of active user is almost similar for cat-ZCC-VLC at high data rate 10Gbps with OZCZVLC performance at lower data rate 622Mbps. This graph proves the proposed system can achieved higher data rate at similar cardinality performance compared OZCZ-VLC. Noted that in this analysis, number of band is set to 2. Fig. 4 indicates the effects of effective power level on the number of users for cat-ZCC-VLC system for 622Mbps data rate. As expected, the lower the effective power the worse BER performance. Hence, it is worth noting that, for proposed combination scheme at large number of bands (5 bands), the number of users at Pr = −9 dBm has high cardinality than the system with Pr=−15 dB m. The maximum accommodated number of users for Pr=−15 dB m is 30, while for high effective power −12 dBm system could accommodate up to 78 number of users and −9 dBm system could accommodate up to 153 number of users for BER = 1E-9. It is also observed that there is trade off between number of users and BER performance as BER performance decreases when there are many users in the system. In the following analysis the data rate is increased to 10Gbps to scrutinize the relationship between effective power and the BER at higher data rate. Also, the impact of cat-OFDM bands that used to increase the electrical channel capacity are investigated in OCDMA ZCC-VLC system as shown in Fig. 5. In this case, the optical codes are set to be four optical channels. At BER value of 1E-9, the effective power for Band = 3, Band = 5 and Band = 8 are −16.5 dB m, −15.5 dB m and −14.5 dB m respectively. As expected, an increasing the number of bands need higher effective power for similar BER level. 5. Conclusion In this paper, the performance of OCDMA based on ZCC code has been investigated in VLC environment using catenated-OFDM modulation scheme. Together this work provides important insights into designing process of OCDMA-VLC system. The number of users can be improved by transmitting more catenated bands simultaneously with similar number of optical channel and ZCCOCDMA code length. One unanticipated finding was that, our work can be operated at higher data rate compared to previous study in this ZCC-VLC field for a similar number of user. It is because the data rate in catenated-OFDM-OCDMA has been divided among the subbands, thus each band has lower date rate which is not affected by other lossed and impairments. As ZCC code has been used in this system, with zero cross correlation between users, the proposed system totally eliminated the PIIN noise thus significanlt improved the system performance. Thus, with a larger capacity, the catenated-OFDM-ZCC OCDMA visible light spectrum is the next best thing for the purpose of high speed data transmission. Acknowledgements This work has been funded and supported by the Department of Higher Education, Ministry of Higher Education Malaysia under Fundamenal Research Grant Scheme, FRGS (9003-00609) leads by Prof. Madya Ir. Dr. Anuar Mat Safar, Universiti Malaysia Perlis.
Fig. 3. Performance comparison at various data rate. 224
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Fig. 4. Effect of effective power received on number of users.
Fig. 5. BER performance of cat-ZCC-VLC versus effective power for W = 3 and K = 12.
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