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Multirate FFH-OCDMA Networks Based on Coherent Advanced Modulation Formats Anderson L. Sanches1,2,Student Member, IEEE, Thiago R. Raddo1,3, Student Member, IEEE, Jose V. dos Reis Jr 1, Student Member, IEEE, and Ben-Hur V. Borges1, Member, IEEE 1 Electrical Engineering Department, University of São Paulo, 13560-250 São Carlos, Brazil,
[email protected] 2 Science and Technology for Energy and Sustainability Center, Federal University of the Recôncavo of the Bahia, 44085-132 Feira de Santana, Brazil,
[email protected] 3 DTU Fotonik, Department of Photonics Engineering, Technical University of Denmark, 2800 Denmark ABSTRACT In this paper, we propose and evaluate the performance of a new scheme for multirate optical code-division multiple-access (OCDMA) networks based on coherent advanced modulation formats, namely binary and quadrature phase shift keying (BPSK and QPSK). The proposed scheme employs BPSK and QPSK modulation formats for low and high rate transmission users, respectively. In addition, we develop a general formalism that allows performance evaluation of legacy networks based on on-off keying (OOK) modulation format. We investigate how the coherent demodulation of encoded fast frequency hopping (FFH) signals under multiple access-interference (MAI) can improve the network performance. It is shown for the first time that simultaneous users of different transmission rates can surprisingly achieve BER levels in the error-free region. Keywords: multirate OCDMA networks, coherent advanced modulation formats, FFH encoding. 1. INTRODUCTION Recently, the growing demand for multimedia services has pushed communications companies to invest in access networks that support heterogeneous data traffic transmission [1]. In their view, networks based on optical code-division multiple-access (OCDMA) have emerged as one of the most prominent optical network architectures. This can be attributed to some remarkable aspects of this access network, such as flexibility in code design, capacity on demand, possibility of offering a virtual point-to-point topology, physical layer security, and high scalability [2]. Among all attributes, the flexibility in code design has been of interest since it can provide multirate traffic transmission. Initially, some works have addressed multirate transmission in direct sequence OCDMA (DS-OCDMA) [3] utilizing optical orthogonal code (OOC). However, even though OOC structure allows multirate transmissions, the performance of high rate users are significantly penalized in terms of Quality of Service (QoS). This characteristic can be inadmissible in some cases, especially when the network enables users to switch traffic type to other transmissions with differentiated QoS requirements. In order to reduce the differences between bit error rate (BER) or QoS requirements of different data traffic transmission, many works have been proposed so far [4]-[7]. Among these, [6] and [7] are particularly attractive since the former uses variable power control attenuators and the latter employs code sequences based on fast frequency hopping (FFH)-OCDMA. In this paper, to the best of our knowledge, we provide for the first time a multirate scheme based on advanced coherent modulation formats for FFH-OCDMA networks. This scheme uses binary phase shift keying (BPSK) to modulate low rate users (one bit per symbol), and exploits the multiple bits per symbol (more specifically two bits per symbol) of quadrature phase shift keying (QPSK) for high rate users. Our proposed multirate network employs all users' codes with the same length, which eliminates the major cause of BER differences between low and high rate users. Furthermore, we show that both low and high rate users can surprisingly achieve BER levels in the error-free region. 2. MULTIRATE NETWORK DESCRIPTION AND PERFORMANCE EVALUATION In this section, we briefly present all considerations utilized to model the multirate FFH-OCDMA network based on PSK modulation formats. Firstly, at the transmission side, the information data bits of low and high rate users are modulated via BPSK and QPSK modulation formats, respectively. For the BPSK format (the simplest form of PSK), the information bits are represented by 180° phase shifts of the optical field. On the other hand, for QPSK format (a more complex form of PSK) the optical field is incremented by 90° phase shifts (45 to 135 degrees). Furthermore, each modulated information data (regardless the user's class) is encoded through FFH scheme and then the users’ signals are inserted simultaneously into the channel. The MAI, during the transmission of each individual user, degrades significantly the users’ signals and severely limits the OCDMA network performance. In our analysis, the MAI level is quantified via chip synchronization overlapping, a situation that does not take advantage of the OCDMA feature, which allows completely asynchronous traffic transmission.
978-1-4673-7880-2/15/$31.00 ©2015 IEEE
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Nevertheless, this assumption, normally considered in the literature [2], allows the evaluation of the worst network performance scenario. Besides delivering the sum of modulated and encoded users’ data bit to every user, the desired user signal can be extracted via FFH decoding and the specific PSK demodulation (BPSK or QPSK if the desired user is in low or high rate class, respectively). In addition, we employ chip level receivers and, therefore, only the MAI signal matched to the desired receiver that is with the chip period at which the autocorrelation peak is formed impact on the bit decision. Specifically, the bit decision variable depends on the number of simultaneous users, and auto- and crosscorrelation average powers. Subsequently, since we do not know a priori the coded symbols that are active at any given time, we further average over all possible coded symbols considering also the random-access network delay of each user. Thus, considering that 𝐾𝐾ℎ𝑟𝑟 interfering users are active in a high-rate class, the average interference from these 2 𝐼𝐼,𝑄𝑄 �������������� users is given by 𝜎𝜎 𝑄𝑄𝑄𝑄𝑄𝑄𝑄𝑄𝑀𝑀𝑀𝑀𝑀𝑀,ℎ𝑟𝑟 . Similarly, considering that 𝐾𝐾𝑙𝑙𝑙𝑙 interfering users are active in low-rate classes, the 2 ������������� . Now, it is necessary to express the variance for the average interference from these users is given by 𝜎𝜎 𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝑀𝑀𝑀𝑀𝑀𝑀,𝑙𝑙𝑙𝑙
cases where the interfering users of a given class are demodulated by the receiver of the desired user that is active in another class. Again, following the same steps and assumptions above, the average interferences among and active users of different classes for the desired user in low or in high rate classes are given by ������������������ 𝜎𝜎 2 𝐼𝐼,𝑄𝑄, 𝑄𝑄𝑄𝑄𝑄𝑄𝑄𝑄/𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝑀𝑀𝑀𝑀𝑀𝑀
2 ������������������ 𝜎𝜎𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵/𝑄𝑄𝑄𝑄𝑄𝑄𝑄𝑄 , respectively. 𝑀𝑀𝑀𝑀𝑀𝑀 Hence, the BER of multirate networks can be adapted from [6], [7] based on the specific features of the hereproposed network, where users achieve different rates via QPSK and BPSK modulation formats. Now, considering that the desired user is active in the high-rate class, the BER of this user is given by
where
and
𝑄𝑄 𝐼𝐼 𝐵𝐵𝐵𝐵𝐵𝐵𝑄𝑄𝑄𝑄𝑄𝑄𝑄𝑄 = 1 − �1 − 𝐵𝐵𝐵𝐵𝐵𝐵𝑄𝑄𝑄𝑄𝑄𝑄𝑄𝑄 ��1 − 𝐵𝐵𝐵𝐵𝐵𝐵𝑄𝑄𝑄𝑄𝑄𝑄𝑄𝑄 �, 𝐼𝐼,𝑄𝑄 𝐼𝐼,𝑄𝑄 𝐵𝐵𝐵𝐵𝐵𝐵𝑄𝑄𝑄𝑄𝑄𝑄𝑄𝑄 = 𝑄𝑄 ��𝑆𝑆𝑆𝑆𝑆𝑆𝑄𝑄𝑄𝑄𝑄𝑄𝑄𝑄 �, 𝐼𝐼,𝑄𝑄 𝑆𝑆𝑆𝑆𝑆𝑆𝑄𝑄𝑄𝑄𝑄𝑄𝑄𝑄_𝑀𝑀𝑀𝑀
2
𝐼𝐼,𝑄𝑄 �𝜇𝜇𝑄𝑄𝑄𝑄𝑄𝑄𝑄𝑄 � 𝑎𝑎𝑎𝑎 = , 2 𝐼𝐼,𝑄𝑄 ����������������� 2 ������������������ 𝐾𝐾ℎ𝑟𝑟 𝜎𝜎𝑄𝑄𝑄𝑄𝑄𝑄𝑄𝑄𝑀𝑀𝑀𝑀𝑀𝑀,ℎ𝑟𝑟 + 𝐾𝐾𝑙𝑙𝑙𝑙 𝜎𝜎𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵/𝑄𝑄𝑄𝑄𝑄𝑄𝑄𝑄 𝑀𝑀𝑀𝑀𝑀𝑀
(1)
(2)
(3)
are the BER and SIR measured in each channel (I- and Q-phase channels) of the users' demodulator, 𝐼𝐼,𝑄𝑄 respectively, and 𝜇𝜇𝑄𝑄𝑄𝑄𝑄𝑄𝑄𝑄 is the autocorrelation signal. 𝑎𝑎𝑎𝑎 Finally, when the desired user is active at the low-rate class, the BER of this user class can be expressed as where
𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵_𝑀𝑀𝑀𝑀 = 𝑄𝑄 ��𝑆𝑆𝑆𝑆𝑆𝑆𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵_𝑀𝑀𝑀𝑀 �, 𝑆𝑆𝑆𝑆𝑆𝑆𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵_𝑀𝑀𝑀𝑀
2
�𝜇𝜇𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝑎𝑎𝑎𝑎 � = , 2 𝐼𝐼,𝑄𝑄 2 ������������� 𝐾𝐾𝑙𝑙𝑙𝑙 𝜎𝜎𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝑀𝑀𝑀𝑀𝑀𝑀,𝑙𝑙𝑙𝑙 + 𝐾𝐾ℎ𝑟𝑟 ������������������ 𝜎𝜎𝑄𝑄𝑄𝑄𝑄𝑄𝑄𝑄/𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵 𝑀𝑀𝑀𝑀𝑀𝑀
(4)
(5)
Full details on the BER formalism as well as how advanced modulation techniques reduce the crosscorrelation among users’ signals will be published elsewhere. 3. SIMULATION RESULTS We assess the performance of the multirate network described in the previous section considering OOK and PSK-based modulation formats. For all investigated scenarios, we employ 29 wavelengths in order to generate the FFH codes. This allows the generation of 29 code sequences with good auto- and cross- correlation properties [8] and, thus, a maximum of 29 users can coexist in the network simultaneously. If demanded, the number of users can be increased by modifying the code parameters. Furthermore, the chip pulse waveforms are assumed to be Gaussian shaped, and compressed by a factor of ten when compared with the chip period to avoid interchip interference. Also, the average power is maintained constant independently of the modulation format adopted (in order to make a fair comparison between them). Next, we describe the specific parameters for legacy multirate FFH-OCDMA based on OOK modulation format. For low rate user class, 12 of 29 available wavelengths in the range of 1.205 µm – 1.625 µm (0.015 µm spacing) are employed for each low rate user's code. This code is disposed in 12 chip times (code length) in agreement with the FFH-based code generated. On the other hand, for high rate users class, we truncate the
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remained sequences (not allocated for low rate users) in order to generate the high rate users' codes. The high rate users' codes sequences have length of 6 time chips. In the proposed multirate FFH-OCDMA based on PSK-based advanced modulation formats, the data information bits of low rate users and high rate users are modulated via BPSK (one bit per symbol) and QPSK (two bits per symbol) respectively. The users of both classes are encoded employing 12 of 29 available wavelengths in the range of 1.205 µm – 1.625 µm (0.015 µm spacing). Consequently, the signal users’ are disposed in 12 time chips in agreement with the FFH code generated. It is worth mentioning that our proposed network achieves multirate transmissions employing code sequences of the same length whereas most approaches found in the literature employ different code lengths [1],[3]-[7]. In order to investigate how the modulation formats affect the network performance, we assume both legacy multirate FFH-OCDMA network (based on OOK) and the proposed multirate FFH-OCDMA (based on PSK) as having 23 users distributed in 6 high rate users and 17 low rate users simultaneously in the network. We adopted this number of simultaneous users in order to validate the legacy multirate FFH-OCDMA network results with those previously presented in [6]. The results are shown in Fig. 1a, with the squares and circles related to legacy and proposed FFH-OCDMA, respectively. The solid and hollow symbols are related to desired user’s class, i.e., low and high, respectively. It can be observed a close agreement between the performance of legacy OOK networks evaluated with the proposed formalism and the results presented in [6 (please see Fig. 4a)]. Regarding the BER level we can observe that error free operation (BER~10-15) is possible only when the desired user transmits in low rate and the maximum number of simultaneous users is 17 (6 high rate users plus 11 low rate users). In contrast, the proposed multirate FFH-OCDMA allows the desired user transmit information data bits in both low and high rate even when the number of simultaneous users is 23 (6 high rate users plus 17 low rate users). Notwithstanding, the proposed multirate FFH-OCDMA network still supports more users than legacy FFH-OCDMA networks under any given BER. This is due to the average power per bit maintenance requirement adopted in PSK-based networks, which increases the symbol distance by √2 in the optical field amplitude. This increased symbol distance associated with a corresponding 3 dB benefit in SIR is responsible for the significant improvement in the BER level.
(a)
(b)
Figure. 1: (a) BER versus number of low rate users plus 6 high rate users. (b) BER versus number of low rate users plus 4 high rate users. Finally, we investigate the BER performance of legacy OOK and the proposed multirate PSK-based network with a reduced number of high rate users. The results are shown in Fig. 1b. Again, the idea is to reproduce the legacy FFH-OCDMA network performance also evaluated in [6, (please see Fig. 4b)] and verify the improvement provided by the proposed multirate FFH-OCDMA network. As expected, the results for the legacy FFH-OCDMA network performance show close agreement with those presented in [6]. In this case, the number of users that coexist with error free transmission is also 17 (6 high rate users plus 11 low rate users). However, the desired user can transmit as long as one low rate user at the most is inserted in the network. For the proposed multirate FFH-OCDMA network, the desired user can transmit in both low and high rates even when the number of simultaneous users is 23 (6 high rate users plus 17 low rate users). 4. CONCLUSIONS In this paper, we proposed a multirate FFH-OCDMA based coherent advanced modulation formats. As opposed to legacy multirate FFH-OCDMA based OOK modulation format, the proposed networks use BPSK modulation for low rate users and QPSK modulation for high rate users. Thus, the multiple bits per symbol (two bits per symbol), an inherent characteristic of the QPSK modulation format, eliminates the need of code length reduction during the process. In addition, the greater symbol distance of PSK modulated network provides a better
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performance for both low and high rate users. Consequently, the maximum number of simultaneous users can achieve error free transmissions regardless the desired user transmits in low or high rate. ACKNOWLEDGEMENTS This work was supported by the Brazilian agencies CAPES, CNPq, FAPEPI and FAPESP. REFERENCES [1] L. Sanches, T.R. Raddo, J.V. dos Reis, Jr, and B.-H.V. Borges, “Highly efficient FFH-OCDMA packet network with coherent advanced modulation formats,” in Proc. ICTON, Graz, Austria, 2014. [2] A.L. Sanches, J.V. dos Reis, Jr., and B.-H.V. Borges, “Analysis of high-speed optical wavelength/time CDMA networks using pulse-position modulation and forward error correction techniques,” J. Light. Technol., vol.27, no. 22, pp. 5134-5144, 2009. [3] J.G. Zhang, “Flexible optical CDMA networks using strict optical orthogonal codes for multimedia broadcasting and distribution applications,” IEEE Trans. Broadcast., vol. 45, pp. 106-115, Mar. 1999. [4] H. Yashima and T. Kobayashi, “Optical CDMA with time hopping and power control for multimedia networks,” J. Lightw. Technol., vol. 21, no. 3, pp. 695-702, Mar. 2003. [5] C.C. Yang, J.F. Huang, and T.C. Hsu, “Differentiated service provision in optical CDMA network using power control,” IEEE Photon. Technol. Lett., vol. 20, no. 20, pp. 1664-1666, 2008. [6] E. Intay, H.M.H. Shalaby, P. Fortier, and L.A. Rusch, “Multirate optical fast frequency-hopping CDMA system using power control,” J. Lightw. Technol., vol. 20, no. 2, pp. 166-177, Feb. 2002. [7] T.R. Raddo, A.L. Sanches, J.V. dos Reis, Jr., and B.-H.V. Borges, “A new approach for evaluating the BER of a multirate, multiclass OFFH-CDMA system,” IEEE Commun. Letters, vol.16, no.2, pp.259-261, 2012. [8] H. Fathallah, L.A. Rusch, and S. LaRochelle, "Passive optical fast frequency-hop CDMA communications system," IEEE J. Lightwave Technol., vol. 17, no. 3, pp. 397-405, Mar. 1999.
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