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Differential Service in a Bidirectional Radio-over-Fiber System over a Spectral-Amplitude-Coding OCDMA Network

Differential Service in a Bidirectional Radio-over-Fiber System over a Spectral-Amplitude-Coding... hv photonics Article Differential Service in a Bidirectional Radio-over-Fiber System over a Spectral-Amplitude-Coding OCDMA Network 1 2 , 2 2 Chao-Chin Yang , Kai-Sheng Chen *, Jen-Fa Huang and Jia-Cyuan Kuo Department of Electro-Optical Engineering, Kun Shan University, Tainan 701, Taiwan; ccyang@ksu.edu.tw Insitute of Computer and Communication Engineering, Department of Electrical Engineering, National Chen Kung University, Tainan 710, Taiwan; huajf@ee.ncku.edu.tw (J.-F.H.); cyuan0912@gmail.com (J.-C.K.) * Correspondence: weiweibjqskwx@hotmail.com; Tel.: +886-926-887-654 Received: 31 August 2016; Accepted: 5 October 2016; Published: 18 October 2016 Abstract: A new scheme of radio-over-fiber (RoF) network based on spectral-amplitude-coding (SAC) optical code division multiple access (OCDMA) is herein proposed. Differential service is provided by a power control scheme that classifies users into several classes and assigns each of them with a specific power level. Additionally, the wavelength reuse technique is adapted to support bidirectional transmission and reduce base station (BS) cost. Both simulation and numerical results show that significantly differential quality-of-service (QoS) in bit-error rate (BER) is achieved in both downlink and uplink transmissions. Keywords: radio-over-fiber (RoF); spectral amplitude coding (SAC); differential service; wavelength reuse 1. Introduction Radio-over-fiber (RoF) supports broadband interconnections between base stations (BS’s) and the central office (CO) to release the heavy burden of wireless throughput [1–4]. For an adequate RoF transmission, multiplexing techniques are applied to both optical and electrical domains. Optical time-division multiple access (TDMA) and wavelength-division multiple access (WDMA) are main schemes to multiplex signals from different users. For TDMA, users are allocated in different time slots. When the user number increases, the queue time between two successive transmissions becomes longer. Additionally, there is a stringent requirement of synchronization between transmitter and receiver. WDMA has a simple structure and fairly increases the transmission rate, but it is not efficient in bandwidth utilization, since only one wavelength can be used by a certain user. On the other hand, optical code-division multiple-access (OCDMA), providing multiple users with asynchronous access without scheduling, is a promising solution to implementing RoF [2–4]. Spectral amplitude coding (SAC) has been studied in many RoF schemes [3,4] since it is free from multiple access interference (MAI), and the data can be encoded on an optical carrier without pre-sampling. In this paper, we propose a new system including two novel characteristics: supporting multi-service transmission and adapting wavelength reuse structure. These two issues have not been studied in previous research on SAC-based RoF. Due to the variant data traffics and diverse service types, scheduling algorithms on a wireless [5] and processing control scheme on an optical signal [6] is proposed to provide the requested quality of service (QoS). The optical control scheme is preferable because of its short processing time and low cost. We propose a power control scheme that classifies the total users into several classes. Different power levels are assigned to each class according to the specific class-of-service. Figure 1 shows the user classification of a 2-class scheme. K and P is the number of users and power in class i, respectively, i i Photonics 2016, 3, 53; doi:10.3390/photonics3040053 www.mdpi.com/journal/photonics Photonics 2016, 3, 53 2 of 7 Photonics 2016, 3, 53 2 of 7 user classification of a 2-class scheme. Ki and Pi is the number of users and power in class i, respectively, where i = 1, 2. The power level P2 is higher than P1, so a higher signal-to-noise ratio wher (SNRe) iis = dete 1, 2.ct The ed power for Class level 2 and P isbet higher ter service than P is , so achieved. a higherF signal-to-noise urthermore, th ratio e desi (SNR) red uis ser detected can be 2 1 for identi Class fied 2 th and rough better MAI service elimis ina achieved. tion even Furthermor if all codede, sithe gnals desir are ed multipl user can exed bein identified the same thr wough avelength MAI elimination band. even if all coded signals are multiplexed in the same wavelength band. Figure 1. User classification and power distribution in the proposed differential service system with Figure 1. User classification and power distribution in the proposed differential service system with spectral amplitude coding (SAC) optical code-division multiple-access (OCDMA). spectral amplitude coding (SAC) optical code-division multiple-access (OCDMA). Our second contribution is the designation of a cost-effective BS. Wavelength reuse is commonly Our second contribution is the designation of a cost-effective BS. Wavelength reuse is commonly used for reducing the cost of a RoF network [7,8], where both downlink and uplink share the same used for reducing the cost of a RoF network [7,8], where both downlink and uplink share the same light source. This has advantages such as the centralization of the light source at the CO and a light source. This has advantages such as the centralization of the light source at the CO and a simplified BS structure. However, the broadband light source (BLS) used for SAC encoding is not simplified BS structure. However, the broadband light source (BLS) used for SAC encoding is not suitable for generating the optical millimeter, which is required by most wavelength reuse schemes. suitable for generating the optical millimeter, which is required by most wavelength reuse schemes. A coherent light source, generally a laser, is used to generate optical millimeters for up- and downlink A coherent light source, generally a laser, is used to generate optical millimeters for up- and downlink transmissions by modulation schemes such as a double side band (DSB) [9] or a single side band transmissions by modulation schemes such as a double side band (DSB) [9] or a single side band (SSB) [10]. Unlike a laser with a near impulse spectrum, the broadband light source (BLS) for SAC (SSB) [10]. Unlike a laser with a near impulse spectrum, the broadband light source (BLS) for SAC encoding occupies a range of wavelength band of 1 to 10 THz. Employing DSB or SSB on BLS would encoding occupies a range of wavelength band of 1 to 10 THz. Employing DSB or SSB on BLS would result in an overlapping between the down- and uplink carriers. Thus, in this paper, we employ the result in an overlapping between the down- and uplink carriers. Thus, in this paper, we employ the concept of wavelength-division multiplexing (WDM). The spectrum of BLS from the CO is divided concept of wavelength-division multiplexing (WDM). The spectrum of BLS from the CO is divided by by a de-multiplexer (De-Mux) into two carriers. Only one of the carriers is modulated by the a de-multiplexer (De-Mux) into two carriers. Only one of the carriers is modulated by the downlink downlink data, while the other is for the uplink transmission. data, while the other is for the uplink transmission. To investigate the proposed system performance, the bit-error rate (BER) is used to quantify QoS To investigate the proposed system performance, the bit-error rate (BER) is used to quantify QoS by considering the effects of thermal noise and phase-induced intensity noise (PIIN). Along with by considering the effects of thermal noise and phase-induced intensity noise (PIIN). Along with simulations, the numerical results show that we achieve significantly differential service between simulations, the numerical results show that we achieve significantly differential service between individual users when employing power control in the proposed RoF architecture. individual users when employing power control in the proposed RoF architecture. 2. The Proposed Bi-Directional RoF System with Power Control Scheme 2. The Proposed Bi-Directional RoF System with Power Control Scheme The architecture of the bi-directional SAC-based RoF along with the power control and The architecture of the bi-directional SAC-based RoF along with the power control and wavelength wavelength reuse is shown in Figure 2. For simplicity, there are only two classes, and each of them reuse is shown in Figure 2. For simplicity, there are only two classes, and each of them has one user has one user in it. As for Class 1, BLS#1 is separated into two optical carriers by a De-Mux. Downlink in it. As for Class 1, BLS#1 is separated into two optical carriers by a De-Mux. Downlink passband passband signal is modulated on the first carrier by intensity modulation (IM). Then, the modulated signal is modulated on the first carrier by intensity modulation (IM). Then, the modulated carrier is carrier is sent to the SAC encoder employed in [11] for encoding. An optical coupler (OC) combines sent to the SAC encoder employed in [11] for encoding. An optical coupler (OC) combines the coded the coded carrier with the un-modulated one to obtain the downlink signal. The downlink signal of carrier with the un-modulated one to obtain the downlink signal. The downlink signal of Class 2 is Class 2 is similar to that of Class 1, except for the code vector assigned to each user and the power similar to that of Class 1, except for the code vector assigned to each user and the power level of the level of the BLS. Before being transmitted to the BS, downlink signals from Class 1 and Class 2 are BLS. Before being transmitted to the BS, downlink signals from Class 1 and Class 2 are combined again combined again by another OC. by another OC. Photonics 2016, 3, 53 3 of 7 Photonics 2016, 3, 53 3 of 7 Photonics 2016, 3, 53 3 of 7 Figure 2. Configuration of the wavelength-reused radio-over-fiber (RoF) network with power control Figure 2. Configuration of the wavelength-reused radio-over-fiber (RoF) network with power Figure 2. Configuration of the wavelength-reused radio-over-fiber (RoF) network with power control technique. control technique. technique. We use a modified stuffed-shift prime code (MSSP code) [12] in the proposed OCDMA-based We use a modified stuffed-shift prime code (MSSP code) [12] in the proposed OCDMA-based RoF We use a modified stuffed-shift prime code (MSSP code) [12] in the proposed OCDMA-based RoF system. A MSSP code has the following properties: M = p + p + 1, ω = p + 1, and λ = 1, where M system. A MSSP code has the following properties: M = p + p + 1, ! = p + 1, and  = 1, where M is the RoF system. A MSSP code has the following properties: M = p + p + 1, ω = p + 1, and λ = 1, where M is the code length, ω is the code weight, λ is the cross-correlation, and p is a prime number. Two MSSP code length, ! is the code weight,  is the cross-correlation, and p is a prime number. Two MSSP codes is the code length, ω is the code weight, λ is the cross-correlation, and p is a prime number. Two MSSP codes of p = 2—(1, 0, 1, 0, 1, 0, 0) and (0, 1, 0, 1, 1, 0, 0)—are assigned to user#1 and user#2, respectively. of p = 2—(1, 0, 1, 0, 1, 0, 0) and (0, 1, 0, 1, 1, 0, 0)—are assigned to user#1 and user#2, respectively. The codes of p = 2—(1, 0, 1, 0, 1, 0, 0) and (0, 1, 0, 1, 1, 0, 0)—are assigned to user#1 and user#2, respectively. The power of BLS#2 is twice than that of BLS#1, the coded spectrum of these two are (0, λ2, 0, λ4, λ5, power of BLS#2 is twice than that of BLS#1, the coded spectrum of these two are (0,  , 0,  ,  , 0, 0) The power of BLS#2 is twice than that of BLS#1, the coded spectrum of these two are (0, λ2, 0, λ4, λ5, 2 4 5 0, 0) and (2λ1, 0, 2λ3, 0, 2λ5, 0, 0). The corresponding power spectral densities (PSDs) of the two users and (2 , 0, 2 , 0, 2 , 0, 0). The corresponding power spectral densities (PSDs) of the two users are 0, 0) and (2λ1, 0, 2λ3, 0, 2λ5, 0, 0). The corresponding power spectral densities (PSDs) of the two users 1 3 5 are shown in Figure 3. shown in Figure 3. are shown in Figure 3. Figure 3. Downlink transmitted spectrum for user in (a) Class 2 and (b) Class 1. Figure 3. Downlink transmitted spectrum for user in (a) Class 2 and (b) Class 1. Figure 3. Downlink transmitted spectrum for user in (a) Class 2 and (b) Class 1. In In BS, BS, a a De-Mux De-Mux divides divides the the downlink downlink signal signal into into two two parts. parts. One One part part of of the the co coded ded carriers carriers is is In BS, a De-Mux divides the downlink signal into two parts. One part of the coded carriers is separated separated again again by by an an optical optical splitter splitter (OS) (OS) and and sent sent to to the the decoders decoders of of both both classes. classes. Th Then, en, the the pa passband ssband separated again by an optical splitter (OS) and sent to the decoders of both classes. Then, the passband signal signal fr from om the the CO CO is is rreco ecover vered ed after after balanced balanced detection detection and and optical-to-electri optical-to-electric cal al (O/E) (O/E) con conversion. version. signal from the CO is recovered after balanced detection and optical-to-electrical (O/E) conversion. The The ot other her par part t with with th the e un un-modulated -modulated carrier carriers s is is spl split it by by anoth another er OS w OSith with the the power power ratio ratio of 2:1. T of 2:1. he The other part with the un-modulated carriers is split by another OS with the power ratio of 2:1. The The carrier carrier withwith larger lar po ger wer power is used is used for th for e upli the n uplink k trans transmission mission for C for lass Class 2. Since 2. Since the op the ticoptical al uplink uplink data carrier with larger power is used for the uplink transmission for Class 2. Since the optical uplink data generated from the two classes have different power levels, multi-service transmission is still data generated from the two classes have different power levels, multi-service transmission is still generated from the two classes have different power levels, multi-service transmission is still supported. supported. H Her eree, , w we e use use on one e of of the the carcarriers riers from from CO CO to achi to eve achieve a wav aelength wavelength scheme. scheme. Since th Since e upli the nk supported. Here, we use one of the carriers from CO to achieve a wavelength scheme. Since the uplink signal is directly modulated on this carrier, it not necessary to equip an extra light source at the BS. uplink signal is directly modulated on this carrier, it not necessary to equip an extra light source at signal is directly modulated on this carrier, it not necessary to equip an extra light source at the BS. the Therefore BS. Ther , th efor e burdens o e, the burf c dens ost and the s of cost and ystem the system require rmen equir t at eme th nt e up at - the link rece up-link ive rr a eceiver re rele ar as e e rd eleased. . After Therefore, the burdens of cost and the system requirement at the up-link receiver are released. After being encoded by two distinct MSSP codes and combined by an OC, the uplink optical signals from After being encoded by two distinct MSSP codes and combined by an OC, the uplink optical signals being encoded by two distinct MSSP codes and combined by an OC, the uplink optical signals from Photonics 2016, 3, 53 4 of 7 from both classes are transmitted to the CO. The passband signal is down-converted from optical to electrical domains by the decoding and photo-detecting procedures similar to the BS. To retrieve the original data from other users, interference cancellation is performed at the decoder. In the SAC scheme, codes with fixed cross-correlation and proper decoder design remove MAI effect significantly. Correlation property between any two code vectors C and C is described as follows: i j p + 1, i = j C C = , (1) i j 1, i 6= j 0, i = j and C C = , (2) i j p, i 6= j where C is the complementary code vectors of C and is the dot-product symbol. From Equations j j (1) and (2), we find that the MSSP code has unit correlation and can be used for designing the MAI elimination process: 1 p + 1, i = j C C C C = . (3) i j i j 0, i 6= j 3. System Performance Analysis In this section, we investigate the QoS of the proposed architecture by deriving SNR expression. Since MAI is negligible in the SAC scheme, system performance is almost decided by the noise terms induced in the photo-detecting process. Thermal noise and PIIN are considered as the main noise sources. For simplicity, a two-class network with the downlink transmission only is taken in the following analysis. The photocurrent obtained at the output of balanced detector is expressed as follows [13]: I(t) = i (t) + i (t) + i (t) + i (t), (4) P P TH where i (t) is the current of BS #k, i (t) and i (t) are PIIN terms of the correlated and the complimentary k P P correlated signal, and i (t) is the thermal noise. We firstly denote the transmitted spectra from all TH users as a time-varying vector: " # K K +K 1 1 P 1 + r (t) 1 + r (t) sr i i S(t) = C + 2 C , (5) i i å å v 2 2 i=1 i=K +1 where r (t) is the normalized radio signal with mean E [r (t)] = 0 and time-average power i i E r (t) = 1/2. P and v are the power and the band-width of BLS, respectively. The signal of i sr the desired BS #k for two classes is derived by correlation subtraction in Equation (2): " !# h i K K 1 2 RP RP sr 1 sr 1 i (t) = C S(t) C S(t) = C C C + 2 C R (t) å å k k k k i i k k M p M p i=1 i=K +1 , (6) wRP sr R (t), k 2 f1, 2, . . . K g k 1 2wRP sr R (t), k 2 K + 1, K + 2, ...K f g k 1 2 2 M Photonics 2016, 3, 53 5 of 7 where R is the responsivity of the photo-diode (PD), and R (t) = (1 + r (t))/2. One can see that the k k signal amplitude of Class 2 is twice than that of Class 1. PIIN-induced variances of the correlated signal of the complementary one in BS #k are expressed as: 2 2 BR P sr Var [i (t)] = fC [S(t) S(t)]g P k Mv ( " ! !#) K K K K 2 2 1 2 1 2 BR P sr 2 = C C + 2 C C + 2 C R (t) å å å å k i i j j Mv , and (7) i=1 i=K +1 j=1 j=K +1 1 1 2 2 wBR P sr 2 (K + 2K + p)(K + 2K )R (t), k 2 f1, 2, . . . K g 2 1 2 1 2 1 M v 2 2 wBR P sr (K + 2K + 2p + 1)(K + 2K )R (t), k 2 fK + 1, K + 2, ...K g 1 2 1 2 1 2 2 M v 2 2 BR P sr Var i (t) = C [S(t) S(t)] 2 k p Mv ( " ! !#) K K K K 2 2 1 2 1 2 BR P sr 2 = C å C + 2 å C å C + 2 å C R (t) 2 k i i j j p Mv i=1 i=K +1 j=1 j=K +1 , (8) 1 1 2 2 wBR P < sr (K + 2K 1)(K + 2K )R (t), k 2 f1, 2, . . . K g 2 1 2 1 2 1 pM v 2 2 wBR P : sr 2 (K + 2K 2)(K + 2K )R (t), k 2 fK + 1, K + 2, ...K g 2 1 2 1 2 1 2 2 pM v where B is the electrical bandwidth, and is the Kronecker product [14]. The variance of thermal noise is Var [i (t)] = S B, (9) TH TH where S is the power spectral density (PDF) of thermal noise. Therefore, the average SNR of the TH SAC-based RoF system is formulated as follows: fE [i (t)]g SNR =    . (10) EfVar [i (t)]g + EfVar [i (t)]g + E Var i (t) TH P Assuming that r (t) is binary phase shift keying (BPSK) signal, the BER expression is: 1 SNR BER = erfc . (11) 2 4 4. Simulation Results and Discussion In this section, numerical analysis according to Equation (10) and simulations done by TM Optisystem are both demonstrated. Received user power P is defined as P = P = P /2, and sr 1 2 user numbers are K = K = 3. Other parameters are set as follows: B = 0.6 GHz, R = 0.9 A/W, and 1 2 = 7 THz. A 20 km single mode fiber (SMF) was used to connect the CO and the BS. A 3-dB-gain erbium-doped fiber amplifier (EDFA) was employed in front of the down- and uplink receiver to compensate the insertion loss from the components of OS‘s and optical combiners. BER curves for users of Class 1 and Class 2 are shown in Figure 4. The legend of S denotes the user in class n, n = 1, 2. As the user power increases, lower BER values are reached. The BERs of users in Class 2 are better than those in Class 1, since the photocurrent after correlation subtraction leads to a higher SNR. For Class 1, the power penalty is 3.5 dB at BER = 10 . Additionally, the simulation results nearly match the numerical ones. The small difference between these two curves are the results of the vibrations of the light source. The effects mentioned above become severe when BLS power gets large, where the light source vibrates in a larger amplitude and more power leakage occurs. Therefore, the discrepancy between software and numerical simulation is more obvious for Class 2. Figure 5 shows the differential BER curves according to the power margin of downlink and uplink transmissions. The performance gap between these two comes from the power losses of the system components. From the above results, one can see that the proposed power control scheme achieves differential service with different qualities of BER. Photonics 2016, 3, 53 6 of 7 occurs. Therefore, the discrepancy between software and numerical simulation is more obvious for Class 2. Photonics 2016, 3, 53 6 of 7 occurs. Therefore, the discrepancy between software and numerical simulation is more obvious for Photonics 2016, 3, 53 6 of 7 Class 2. Figure 4. Bit-error rate (BER) comparison between numerical and simulation results for Classes 1 and 2. Figure 5 shows the differential BER curves according to the power margin of downlink and uplink transmissions. The performance gap between these two comes from the power losses of the system components. From the above results, one can see that the proposed power control scheme achieves differential service with different qualities of BER. Figure 4. Bit-error rate (BER) comparison between numerical and simulation results for Classes 1 and 2. Figure 4. Bit-error rate (BER) comparison between numerical and simulation results for Classes 1 and 2. Figure 5 shows the differential BER curves according to the power margin of downlink and uplink transmissions. The performance gap between these two comes from the power losses of the system components. From the above results, one can see that the proposed power control scheme achieves differential service with different qualities of BER. Figure 5. Bit-error rate (BER) comparison between numerical and simulation results for Classes 1 and 2. Figure 5. Bit-error rate (BER) comparison between numerical and simulation results for Classes 1 and 2. 5. Conclusions 5. Conclusions In this paper, we proposed a differential RoF structure with power control based on the SAC- In this paper, we proposed a differential RoF structure with power control based on the OCDMA system. By adopting the wavelength-reuse technique, the differe ntial service can be SAC-OCDMA system. By adopting the wavelength-reuse technique, the differential service can accomplished without an increase in BS complexity. Users with large signal power obtain relatively Figure 5. Bit-error rate (BER) comparison between numerical and simulation results for Classes 1 and 2. be accomplished without an increase in BS complexity. Users with large signal power obtain relatively small BERs without MAI. The proposed scheme provides optical networks with different small BERs without MAI. The proposed scheme provides optical networks with different performance perform 5. Conclus anc ions e req uirements while retaining system simplicity. requirements while retaining system simplicity. Acknowledgments: This work was supported by Ministry of Science and Technology, Taiwan, project In this paper, we proposed a differential RoF structure with power control based on the SAC- Acknowledgments: This work was supported by Ministry of Science and Technology, Taiwan, project No. 104-2221-E-168-010. OCDMA system. By adopting the wavelength-reuse technique, the differential service can be No. 104-2221-E-168-010. accomplished without an increase in BS complexity. Users with large signal power obtain relatively Author Contributions: C.C. Yang proposed the concept and initialized the research, K.S. Chen carried out the Author Contributions: C.C. Yang proposed the concept and initialized the research, K.S. Chen carried out the small BERs without MAI. The proposed scheme provides optical networks with different system design and wrote the paper, J.F. Huang guided the studies; J.C. Kuo ran the simulation and analyzed the system design and wrote the paper, J.F. Huang guided the studies; J.C. Kuo ran the simulation and analyzed the data. data. performance requirements while retaining system simplicity. Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest. Acknowledgments: This work was supported by Ministry of Science and Technology, Taiwan, project No. 104-2221-E-168-010. References Author Contributions: C.C. Yang proposed the concept and initialized the research, K.S. Chen carried out the 1. Xu, Z.Z.; Wang, H.X.; Ji, Y.F. Multichannel resource allocation mechanism for 60 GHz radio-over-fiber local system design and wrote the paper, J.F. Huang guided the studies; J.C. Kuo ran the simulation and analyzed the access networks. J. Lightw. Technol. 2013, 5, 254–260. [CrossRef] data. 2. Chang, C.Y.; Yang, G.C.; Chang, C.Y.; Kwong, W.C. Study of a diversity O-CDMA scheme for optical wireless. Conflicts of Interest: The authors declare no conflict of interest. J. Lightw. Technol. 2012, 30, 1549–1558. [CrossRef] 3. Yang, C.C.; Huang, J.F.; Chang, H.H.; Chen, K.S. Radio transmissions over residue-stuffed-QC-coded optical CDMA network. IEEE Commun. Lett. 2014, 18, 329–331. [CrossRef] Photonics 2016, 3, 53 7 of 7 4. Yang, C.C. Optical CDMA-based fiber-radio networks with improved power efficiency. IEEE Trans. Commun. 2012, 60, 810–816. [CrossRef] 5. Ahmed, M.; Ahmad, I.; Habibi, D. Service class resource management for green wireless-optical broadband access networks (WOBAN). J. Lightw. Technol. 2015, 33, 7–18. [CrossRef] 6. Zhu, Q.Y.; Pavel, L. Enabling differentiated services using generalized power control model in optical networks. IEEE Trans. Commun. 2009, 57, 2570–2575. 7. Matsuura, M.; Oki, E. Optical carrier regeneration for carrier wavelength reuse in a multicarrier distributed WDM network. IEEE Photon. Technol. Lett. 2010, 22, 808–810. [CrossRef] 8. Cui, W.T.; Shao, T.; Yao, J.P. Wavelength reuse in a UWB over fiber system based on phase-modulation to intensity-modulation conversion and destructive interferencing. J. Lightw. Technol. 2013, 31, 2904–2912. [CrossRef] 9. Kaszubowska, L.H.; Barry, L. Remote down conversion with wavelength reuse for the radio/fiber uplink connection. IEEE Photon. Technol. Lett. 2006, 18, 562–564. [CrossRef] 10. Attygalle, M.; Lim, C.; Pendock, P.J.; Nirmalathas, A.; Edvell, G. Transmission improvement in fiber wireless links using fiber bragg gratings. IEEE Photon. Technol. Lett. 2005, 17, 190–192. [CrossRef] 11. Chen, K.S.; Yang, C.C.; Huang, J.F. Using stuffed quadratic congruence codes for SAC labels in optical packet switching network. IEEE Commun. Lett. 2015, 19, 1093–1096. [CrossRef] 12. Yang, C.C. Optical CDMA passive optical network using prime code with interference elimination. IEEE Photon. Technol. Lett. 2007, 19, 516–518. [CrossRef] 13. Noshad, M.; Jamshidi, K. Bounds for the BER of codes with fixed cross correlation in SAC-OCDMA Systems. J. Lightw. Technol. 2011, 29, 1944–1950. [CrossRef] 14. Horn, R.A.; Johnson, C.R. Matrix Analysis, 2nd ed.; Cambridge University Press: Cambridge, UK, 2012. © 2016 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Photonics Multidisciplinary Digital Publishing Institute

Differential Service in a Bidirectional Radio-over-Fiber System over a Spectral-Amplitude-Coding OCDMA Network

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hv photonics Article Differential Service in a Bidirectional Radio-over-Fiber System over a Spectral-Amplitude-Coding OCDMA Network 1 2 , 2 2 Chao-Chin Yang , Kai-Sheng Chen *, Jen-Fa Huang and Jia-Cyuan Kuo Department of Electro-Optical Engineering, Kun Shan University, Tainan 701, Taiwan; ccyang@ksu.edu.tw Insitute of Computer and Communication Engineering, Department of Electrical Engineering, National Chen Kung University, Tainan 710, Taiwan; huajf@ee.ncku.edu.tw (J.-F.H.); cyuan0912@gmail.com (J.-C.K.) * Correspondence: weiweibjqskwx@hotmail.com; Tel.: +886-926-887-654 Received: 31 August 2016; Accepted: 5 October 2016; Published: 18 October 2016 Abstract: A new scheme of radio-over-fiber (RoF) network based on spectral-amplitude-coding (SAC) optical code division multiple access (OCDMA) is herein proposed. Differential service is provided by a power control scheme that classifies users into several classes and assigns each of them with a specific power level. Additionally, the wavelength reuse technique is adapted to support bidirectional transmission and reduce base station (BS) cost. Both simulation and numerical results show that significantly differential quality-of-service (QoS) in bit-error rate (BER) is achieved in both downlink and uplink transmissions. Keywords: radio-over-fiber (RoF); spectral amplitude coding (SAC); differential service; wavelength reuse 1. Introduction Radio-over-fiber (RoF) supports broadband interconnections between base stations (BS’s) and the central office (CO) to release the heavy burden of wireless throughput [1–4]. For an adequate RoF transmission, multiplexing techniques are applied to both optical and electrical domains. Optical time-division multiple access (TDMA) and wavelength-division multiple access (WDMA) are main schemes to multiplex signals from different users. For TDMA, users are allocated in different time slots. When the user number increases, the queue time between two successive transmissions becomes longer. Additionally, there is a stringent requirement of synchronization between transmitter and receiver. WDMA has a simple structure and fairly increases the transmission rate, but it is not efficient in bandwidth utilization, since only one wavelength can be used by a certain user. On the other hand, optical code-division multiple-access (OCDMA), providing multiple users with asynchronous access without scheduling, is a promising solution to implementing RoF [2–4]. Spectral amplitude coding (SAC) has been studied in many RoF schemes [3,4] since it is free from multiple access interference (MAI), and the data can be encoded on an optical carrier without pre-sampling. In this paper, we propose a new system including two novel characteristics: supporting multi-service transmission and adapting wavelength reuse structure. These two issues have not been studied in previous research on SAC-based RoF. Due to the variant data traffics and diverse service types, scheduling algorithms on a wireless [5] and processing control scheme on an optical signal [6] is proposed to provide the requested quality of service (QoS). The optical control scheme is preferable because of its short processing time and low cost. We propose a power control scheme that classifies the total users into several classes. Different power levels are assigned to each class according to the specific class-of-service. Figure 1 shows the user classification of a 2-class scheme. K and P is the number of users and power in class i, respectively, i i Photonics 2016, 3, 53; doi:10.3390/photonics3040053 www.mdpi.com/journal/photonics Photonics 2016, 3, 53 2 of 7 Photonics 2016, 3, 53 2 of 7 user classification of a 2-class scheme. Ki and Pi is the number of users and power in class i, respectively, where i = 1, 2. The power level P2 is higher than P1, so a higher signal-to-noise ratio wher (SNRe) iis = dete 1, 2.ct The ed power for Class level 2 and P isbet higher ter service than P is , so achieved. a higherF signal-to-noise urthermore, th ratio e desi (SNR) red uis ser detected can be 2 1 for identi Class fied 2 th and rough better MAI service elimis ina achieved. tion even Furthermor if all codede, sithe gnals desir are ed multipl user can exed bein identified the same thr wough avelength MAI elimination band. even if all coded signals are multiplexed in the same wavelength band. Figure 1. User classification and power distribution in the proposed differential service system with Figure 1. User classification and power distribution in the proposed differential service system with spectral amplitude coding (SAC) optical code-division multiple-access (OCDMA). spectral amplitude coding (SAC) optical code-division multiple-access (OCDMA). Our second contribution is the designation of a cost-effective BS. Wavelength reuse is commonly Our second contribution is the designation of a cost-effective BS. Wavelength reuse is commonly used for reducing the cost of a RoF network [7,8], where both downlink and uplink share the same used for reducing the cost of a RoF network [7,8], where both downlink and uplink share the same light source. This has advantages such as the centralization of the light source at the CO and a light source. This has advantages such as the centralization of the light source at the CO and a simplified BS structure. However, the broadband light source (BLS) used for SAC encoding is not simplified BS structure. However, the broadband light source (BLS) used for SAC encoding is not suitable for generating the optical millimeter, which is required by most wavelength reuse schemes. suitable for generating the optical millimeter, which is required by most wavelength reuse schemes. A coherent light source, generally a laser, is used to generate optical millimeters for up- and downlink A coherent light source, generally a laser, is used to generate optical millimeters for up- and downlink transmissions by modulation schemes such as a double side band (DSB) [9] or a single side band transmissions by modulation schemes such as a double side band (DSB) [9] or a single side band (SSB) [10]. Unlike a laser with a near impulse spectrum, the broadband light source (BLS) for SAC (SSB) [10]. Unlike a laser with a near impulse spectrum, the broadband light source (BLS) for SAC encoding occupies a range of wavelength band of 1 to 10 THz. Employing DSB or SSB on BLS would encoding occupies a range of wavelength band of 1 to 10 THz. Employing DSB or SSB on BLS would result in an overlapping between the down- and uplink carriers. Thus, in this paper, we employ the result in an overlapping between the down- and uplink carriers. Thus, in this paper, we employ the concept of wavelength-division multiplexing (WDM). The spectrum of BLS from the CO is divided concept of wavelength-division multiplexing (WDM). The spectrum of BLS from the CO is divided by by a de-multiplexer (De-Mux) into two carriers. Only one of the carriers is modulated by the a de-multiplexer (De-Mux) into two carriers. Only one of the carriers is modulated by the downlink downlink data, while the other is for the uplink transmission. data, while the other is for the uplink transmission. To investigate the proposed system performance, the bit-error rate (BER) is used to quantify QoS To investigate the proposed system performance, the bit-error rate (BER) is used to quantify QoS by considering the effects of thermal noise and phase-induced intensity noise (PIIN). Along with by considering the effects of thermal noise and phase-induced intensity noise (PIIN). Along with simulations, the numerical results show that we achieve significantly differential service between simulations, the numerical results show that we achieve significantly differential service between individual users when employing power control in the proposed RoF architecture. individual users when employing power control in the proposed RoF architecture. 2. The Proposed Bi-Directional RoF System with Power Control Scheme 2. The Proposed Bi-Directional RoF System with Power Control Scheme The architecture of the bi-directional SAC-based RoF along with the power control and The architecture of the bi-directional SAC-based RoF along with the power control and wavelength wavelength reuse is shown in Figure 2. For simplicity, there are only two classes, and each of them reuse is shown in Figure 2. For simplicity, there are only two classes, and each of them has one user has one user in it. As for Class 1, BLS#1 is separated into two optical carriers by a De-Mux. Downlink in it. As for Class 1, BLS#1 is separated into two optical carriers by a De-Mux. Downlink passband passband signal is modulated on the first carrier by intensity modulation (IM). Then, the modulated signal is modulated on the first carrier by intensity modulation (IM). Then, the modulated carrier is carrier is sent to the SAC encoder employed in [11] for encoding. An optical coupler (OC) combines sent to the SAC encoder employed in [11] for encoding. An optical coupler (OC) combines the coded the coded carrier with the un-modulated one to obtain the downlink signal. The downlink signal of carrier with the un-modulated one to obtain the downlink signal. The downlink signal of Class 2 is Class 2 is similar to that of Class 1, except for the code vector assigned to each user and the power similar to that of Class 1, except for the code vector assigned to each user and the power level of the level of the BLS. Before being transmitted to the BS, downlink signals from Class 1 and Class 2 are BLS. Before being transmitted to the BS, downlink signals from Class 1 and Class 2 are combined again combined again by another OC. by another OC. Photonics 2016, 3, 53 3 of 7 Photonics 2016, 3, 53 3 of 7 Photonics 2016, 3, 53 3 of 7 Figure 2. Configuration of the wavelength-reused radio-over-fiber (RoF) network with power control Figure 2. Configuration of the wavelength-reused radio-over-fiber (RoF) network with power Figure 2. Configuration of the wavelength-reused radio-over-fiber (RoF) network with power control technique. control technique. technique. We use a modified stuffed-shift prime code (MSSP code) [12] in the proposed OCDMA-based We use a modified stuffed-shift prime code (MSSP code) [12] in the proposed OCDMA-based RoF We use a modified stuffed-shift prime code (MSSP code) [12] in the proposed OCDMA-based RoF system. A MSSP code has the following properties: M = p + p + 1, ω = p + 1, and λ = 1, where M system. A MSSP code has the following properties: M = p + p + 1, ! = p + 1, and  = 1, where M is the RoF system. A MSSP code has the following properties: M = p + p + 1, ω = p + 1, and λ = 1, where M is the code length, ω is the code weight, λ is the cross-correlation, and p is a prime number. Two MSSP code length, ! is the code weight,  is the cross-correlation, and p is a prime number. Two MSSP codes is the code length, ω is the code weight, λ is the cross-correlation, and p is a prime number. Two MSSP codes of p = 2—(1, 0, 1, 0, 1, 0, 0) and (0, 1, 0, 1, 1, 0, 0)—are assigned to user#1 and user#2, respectively. of p = 2—(1, 0, 1, 0, 1, 0, 0) and (0, 1, 0, 1, 1, 0, 0)—are assigned to user#1 and user#2, respectively. The codes of p = 2—(1, 0, 1, 0, 1, 0, 0) and (0, 1, 0, 1, 1, 0, 0)—are assigned to user#1 and user#2, respectively. The power of BLS#2 is twice than that of BLS#1, the coded spectrum of these two are (0, λ2, 0, λ4, λ5, power of BLS#2 is twice than that of BLS#1, the coded spectrum of these two are (0,  , 0,  ,  , 0, 0) The power of BLS#2 is twice than that of BLS#1, the coded spectrum of these two are (0, λ2, 0, λ4, λ5, 2 4 5 0, 0) and (2λ1, 0, 2λ3, 0, 2λ5, 0, 0). The corresponding power spectral densities (PSDs) of the two users and (2 , 0, 2 , 0, 2 , 0, 0). The corresponding power spectral densities (PSDs) of the two users are 0, 0) and (2λ1, 0, 2λ3, 0, 2λ5, 0, 0). The corresponding power spectral densities (PSDs) of the two users 1 3 5 are shown in Figure 3. shown in Figure 3. are shown in Figure 3. Figure 3. Downlink transmitted spectrum for user in (a) Class 2 and (b) Class 1. Figure 3. Downlink transmitted spectrum for user in (a) Class 2 and (b) Class 1. Figure 3. Downlink transmitted spectrum for user in (a) Class 2 and (b) Class 1. In In BS, BS, a a De-Mux De-Mux divides divides the the downlink downlink signal signal into into two two parts. parts. One One part part of of the the co coded ded carriers carriers is is In BS, a De-Mux divides the downlink signal into two parts. One part of the coded carriers is separated separated again again by by an an optical optical splitter splitter (OS) (OS) and and sent sent to to the the decoders decoders of of both both classes. classes. Th Then, en, the the pa passband ssband separated again by an optical splitter (OS) and sent to the decoders of both classes. Then, the passband signal signal fr from om the the CO CO is is rreco ecover vered ed after after balanced balanced detection detection and and optical-to-electri optical-to-electric cal al (O/E) (O/E) con conversion. version. signal from the CO is recovered after balanced detection and optical-to-electrical (O/E) conversion. The The ot other her par part t with with th the e un un-modulated -modulated carrier carriers s is is spl split it by by anoth another er OS w OSith with the the power power ratio ratio of 2:1. T of 2:1. he The other part with the un-modulated carriers is split by another OS with the power ratio of 2:1. The The carrier carrier withwith larger lar po ger wer power is used is used for th for e upli the n uplink k trans transmission mission for C for lass Class 2. Since 2. Since the op the ticoptical al uplink uplink data carrier with larger power is used for the uplink transmission for Class 2. Since the optical uplink data generated from the two classes have different power levels, multi-service transmission is still data generated from the two classes have different power levels, multi-service transmission is still generated from the two classes have different power levels, multi-service transmission is still supported. supported. H Her eree, , w we e use use on one e of of the the carcarriers riers from from CO CO to achi to eve achieve a wav aelength wavelength scheme. scheme. Since th Since e upli the nk supported. Here, we use one of the carriers from CO to achieve a wavelength scheme. Since the uplink signal is directly modulated on this carrier, it not necessary to equip an extra light source at the BS. uplink signal is directly modulated on this carrier, it not necessary to equip an extra light source at signal is directly modulated on this carrier, it not necessary to equip an extra light source at the BS. the Therefore BS. Ther , th efor e burdens o e, the burf c dens ost and the s of cost and ystem the system require rmen equir t at eme th nt e up at - the link rece up-link ive rr a eceiver re rele ar as e e rd eleased. . After Therefore, the burdens of cost and the system requirement at the up-link receiver are released. After being encoded by two distinct MSSP codes and combined by an OC, the uplink optical signals from After being encoded by two distinct MSSP codes and combined by an OC, the uplink optical signals being encoded by two distinct MSSP codes and combined by an OC, the uplink optical signals from Photonics 2016, 3, 53 4 of 7 from both classes are transmitted to the CO. The passband signal is down-converted from optical to electrical domains by the decoding and photo-detecting procedures similar to the BS. To retrieve the original data from other users, interference cancellation is performed at the decoder. In the SAC scheme, codes with fixed cross-correlation and proper decoder design remove MAI effect significantly. Correlation property between any two code vectors C and C is described as follows: i j p + 1, i = j C C = , (1) i j 1, i 6= j 0, i = j and C C = , (2) i j p, i 6= j where C is the complementary code vectors of C and is the dot-product symbol. From Equations j j (1) and (2), we find that the MSSP code has unit correlation and can be used for designing the MAI elimination process: 1 p + 1, i = j C C C C = . (3) i j i j 0, i 6= j 3. System Performance Analysis In this section, we investigate the QoS of the proposed architecture by deriving SNR expression. Since MAI is negligible in the SAC scheme, system performance is almost decided by the noise terms induced in the photo-detecting process. Thermal noise and PIIN are considered as the main noise sources. For simplicity, a two-class network with the downlink transmission only is taken in the following analysis. The photocurrent obtained at the output of balanced detector is expressed as follows [13]: I(t) = i (t) + i (t) + i (t) + i (t), (4) P P TH where i (t) is the current of BS #k, i (t) and i (t) are PIIN terms of the correlated and the complimentary k P P correlated signal, and i (t) is the thermal noise. We firstly denote the transmitted spectra from all TH users as a time-varying vector: " # K K +K 1 1 P 1 + r (t) 1 + r (t) sr i i S(t) = C + 2 C , (5) i i å å v 2 2 i=1 i=K +1 where r (t) is the normalized radio signal with mean E [r (t)] = 0 and time-average power i i E r (t) = 1/2. P and v are the power and the band-width of BLS, respectively. The signal of i sr the desired BS #k for two classes is derived by correlation subtraction in Equation (2): " !# h i K K 1 2 RP RP sr 1 sr 1 i (t) = C S(t) C S(t) = C C C + 2 C R (t) å å k k k k i i k k M p M p i=1 i=K +1 , (6) wRP sr R (t), k 2 f1, 2, . . . K g k 1 2wRP sr R (t), k 2 K + 1, K + 2, ...K f g k 1 2 2 M Photonics 2016, 3, 53 5 of 7 where R is the responsivity of the photo-diode (PD), and R (t) = (1 + r (t))/2. One can see that the k k signal amplitude of Class 2 is twice than that of Class 1. PIIN-induced variances of the correlated signal of the complementary one in BS #k are expressed as: 2 2 BR P sr Var [i (t)] = fC [S(t) S(t)]g P k Mv ( " ! !#) K K K K 2 2 1 2 1 2 BR P sr 2 = C C + 2 C C + 2 C R (t) å å å å k i i j j Mv , and (7) i=1 i=K +1 j=1 j=K +1 1 1 2 2 wBR P sr 2 (K + 2K + p)(K + 2K )R (t), k 2 f1, 2, . . . K g 2 1 2 1 2 1 M v 2 2 wBR P sr (K + 2K + 2p + 1)(K + 2K )R (t), k 2 fK + 1, K + 2, ...K g 1 2 1 2 1 2 2 M v 2 2 BR P sr Var i (t) = C [S(t) S(t)] 2 k p Mv ( " ! !#) K K K K 2 2 1 2 1 2 BR P sr 2 = C å C + 2 å C å C + 2 å C R (t) 2 k i i j j p Mv i=1 i=K +1 j=1 j=K +1 , (8) 1 1 2 2 wBR P < sr (K + 2K 1)(K + 2K )R (t), k 2 f1, 2, . . . K g 2 1 2 1 2 1 pM v 2 2 wBR P : sr 2 (K + 2K 2)(K + 2K )R (t), k 2 fK + 1, K + 2, ...K g 2 1 2 1 2 1 2 2 pM v where B is the electrical bandwidth, and is the Kronecker product [14]. The variance of thermal noise is Var [i (t)] = S B, (9) TH TH where S is the power spectral density (PDF) of thermal noise. Therefore, the average SNR of the TH SAC-based RoF system is formulated as follows: fE [i (t)]g SNR =    . (10) EfVar [i (t)]g + EfVar [i (t)]g + E Var i (t) TH P Assuming that r (t) is binary phase shift keying (BPSK) signal, the BER expression is: 1 SNR BER = erfc . (11) 2 4 4. Simulation Results and Discussion In this section, numerical analysis according to Equation (10) and simulations done by TM Optisystem are both demonstrated. Received user power P is defined as P = P = P /2, and sr 1 2 user numbers are K = K = 3. Other parameters are set as follows: B = 0.6 GHz, R = 0.9 A/W, and 1 2 = 7 THz. A 20 km single mode fiber (SMF) was used to connect the CO and the BS. A 3-dB-gain erbium-doped fiber amplifier (EDFA) was employed in front of the down- and uplink receiver to compensate the insertion loss from the components of OS‘s and optical combiners. BER curves for users of Class 1 and Class 2 are shown in Figure 4. The legend of S denotes the user in class n, n = 1, 2. As the user power increases, lower BER values are reached. The BERs of users in Class 2 are better than those in Class 1, since the photocurrent after correlation subtraction leads to a higher SNR. For Class 1, the power penalty is 3.5 dB at BER = 10 . Additionally, the simulation results nearly match the numerical ones. The small difference between these two curves are the results of the vibrations of the light source. The effects mentioned above become severe when BLS power gets large, where the light source vibrates in a larger amplitude and more power leakage occurs. Therefore, the discrepancy between software and numerical simulation is more obvious for Class 2. Figure 5 shows the differential BER curves according to the power margin of downlink and uplink transmissions. The performance gap between these two comes from the power losses of the system components. From the above results, one can see that the proposed power control scheme achieves differential service with different qualities of BER. Photonics 2016, 3, 53 6 of 7 occurs. Therefore, the discrepancy between software and numerical simulation is more obvious for Class 2. Photonics 2016, 3, 53 6 of 7 occurs. Therefore, the discrepancy between software and numerical simulation is more obvious for Photonics 2016, 3, 53 6 of 7 Class 2. Figure 4. Bit-error rate (BER) comparison between numerical and simulation results for Classes 1 and 2. Figure 5 shows the differential BER curves according to the power margin of downlink and uplink transmissions. The performance gap between these two comes from the power losses of the system components. From the above results, one can see that the proposed power control scheme achieves differential service with different qualities of BER. Figure 4. Bit-error rate (BER) comparison between numerical and simulation results for Classes 1 and 2. Figure 4. Bit-error rate (BER) comparison between numerical and simulation results for Classes 1 and 2. Figure 5 shows the differential BER curves according to the power margin of downlink and uplink transmissions. The performance gap between these two comes from the power losses of the system components. From the above results, one can see that the proposed power control scheme achieves differential service with different qualities of BER. Figure 5. Bit-error rate (BER) comparison between numerical and simulation results for Classes 1 and 2. Figure 5. Bit-error rate (BER) comparison between numerical and simulation results for Classes 1 and 2. 5. Conclusions 5. Conclusions In this paper, we proposed a differential RoF structure with power control based on the SAC- In this paper, we proposed a differential RoF structure with power control based on the OCDMA system. By adopting the wavelength-reuse technique, the differe ntial service can be SAC-OCDMA system. By adopting the wavelength-reuse technique, the differential service can accomplished without an increase in BS complexity. Users with large signal power obtain relatively Figure 5. Bit-error rate (BER) comparison between numerical and simulation results for Classes 1 and 2. be accomplished without an increase in BS complexity. Users with large signal power obtain relatively small BERs without MAI. The proposed scheme provides optical networks with different small BERs without MAI. The proposed scheme provides optical networks with different performance perform 5. Conclus anc ions e req uirements while retaining system simplicity. requirements while retaining system simplicity. Acknowledgments: This work was supported by Ministry of Science and Technology, Taiwan, project In this paper, we proposed a differential RoF structure with power control based on the SAC- Acknowledgments: This work was supported by Ministry of Science and Technology, Taiwan, project No. 104-2221-E-168-010. OCDMA system. By adopting the wavelength-reuse technique, the differential service can be No. 104-2221-E-168-010. accomplished without an increase in BS complexity. Users with large signal power obtain relatively Author Contributions: C.C. Yang proposed the concept and initialized the research, K.S. Chen carried out the Author Contributions: C.C. Yang proposed the concept and initialized the research, K.S. Chen carried out the small BERs without MAI. The proposed scheme provides optical networks with different system design and wrote the paper, J.F. Huang guided the studies; J.C. Kuo ran the simulation and analyzed the system design and wrote the paper, J.F. Huang guided the studies; J.C. Kuo ran the simulation and analyzed the data. data. performance requirements while retaining system simplicity. Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest. Acknowledgments: This work was supported by Ministry of Science and Technology, Taiwan, project No. 104-2221-E-168-010. References Author Contributions: C.C. Yang proposed the concept and initialized the research, K.S. Chen carried out the 1. Xu, Z.Z.; Wang, H.X.; Ji, Y.F. Multichannel resource allocation mechanism for 60 GHz radio-over-fiber local system design and wrote the paper, J.F. Huang guided the studies; J.C. Kuo ran the simulation and analyzed the access networks. J. Lightw. Technol. 2013, 5, 254–260. [CrossRef] data. 2. Chang, C.Y.; Yang, G.C.; Chang, C.Y.; Kwong, W.C. Study of a diversity O-CDMA scheme for optical wireless. Conflicts of Interest: The authors declare no conflict of interest. J. Lightw. Technol. 2012, 30, 1549–1558. [CrossRef] 3. Yang, C.C.; Huang, J.F.; Chang, H.H.; Chen, K.S. Radio transmissions over residue-stuffed-QC-coded optical CDMA network. IEEE Commun. Lett. 2014, 18, 329–331. [CrossRef] Photonics 2016, 3, 53 7 of 7 4. Yang, C.C. 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PhotonicsMultidisciplinary Digital Publishing Institute

Published: Oct 18, 2016

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