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Investigation on wavelength re-modulated bi-directional passive optical network for different modulation formats
Optik 125 (2014) 5378–5382
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Investigation on wavelength re-modulated bi-directional passive optical network for different modulation formats Simranjit Singh a,∗ , Amit Kapoor a , Gurpreet Kaur a , R.S. Kaler b , Rakesh Goyal c a b c
Department of Electronics and Communication Engineering, Punjabi University, Patiala, India Department of Electronics and Communication Engineering, Thapar University, Patiala, India Department of Electronics and Communication Engineering, Rayat and Bahra, Patiala, India
a r t i c l e
i n f o
Article history: Received 28 September 2013 Accepted 26 May 2014 Keywords: Bi-directional PON Modulation formats Remodulation techniques FTTH
a b s t r a c t In this paper, the cost effective bi-directional passive optical network architecture with wavelength remodulated scheme is investigated. To realize the cost-effective PON, remodulation scheme is used, in which the downstream optical signal is reused as a carrier for the upstream transmission as it eliminates the need for an extra laser source at optical network units. The performance of proposed passive optical network is analyzed and compared for various modulation formats such as Non Return to Zero (NRZ), Return to Zero (RZ) and On–Off Keying (OOK) with 64 optical networks units (ONUs) at different traffic speed for downlink and uplink, respectively. It has been observed that the most suitable data format for proposed PON network is NRZ. Further the proposed system performance is compared with the current state-of-the-art PON architectures. © 2014 Elsevier GmbH. All rights reserved.
1. Introduction Passive optical networks (PONs) are emerging as the ultimate solution in the fiber-to-the-home (FTTH) environment. They can offer more bandwidth than copper-based solution. Current Ethernet PON and Gigabit PON have been widely deployed in many places around the world because of the low installation and maintenance cost [1,2]. Wavelength-division-multiplexing (WDM) transport systems that use different optical channels to carry combined signals would be quite useful for a fiber transport network providing multiple services [3]. There has been increasing attention in implementing next generation broadband network by using WDM-PON; it is because WDM-PON can provide large transmission capacity, network security and flexibility [4,5]. The tremendous growth of the high speed internet and data traffic has created an enormous demand for transmission bandwidth of dense wavelength division multiplexed optical communication systems and differential phase-shift keying is the current technology to achieve better performance even in cost-effective high-speed networks [6,7]. In PONs, a continuous light is distributed over a fiber link from the optical line terminal (OLT) to each optical network unit (ONU). At the ONU side, the continuous wave (CW) light is then
∗ Corresponding author. E-mail address: [email protected] (S. Singh). http://dx.doi.org/10.1016/j.ijleo.2014.06.019 0030-4026/© 2014 Elsevier GmbH. All rights reserved.
modulated and transmitted back over the same fiber so in wavelength re-modulated scheme, a downstream optical signal is reused for upstream transmission. These configurations eliminate the need for an extra CW light source at ONU [8]. In the optical system the optical code pattern is the important factor for deciding the spectrum efficiency, transmission quality and dispersive tolerance of the system [9], so signal modulation format is a key issue in order to maximize optical transmission link capacity, system design and optimization have to take into account all the contributing facts, such as channel data rate, transmission distance, signal optical power, amplifier noise figure, channel wavelength spacing, optical amplifier spacing, fiber dispersion and non-linear parameters, dispersion management strategy, receiver bandwidth and so on [10,11]. One of the most important facts in the system, which affects the choices of all other system parameters, is the signal optical modulation format. The BER and Q-factor may be improved by choosing a proper line coding scheme which includes various modulation formats such as Non Return to Zero (NRZ), Return to Zero (RZ), On–Off Keying (OOK) and many more. In literature, various works on the performance of PONs with various modulation formats has been reported. Kaler et al. [12] presented the comparative investigation and suitability of various data formats like NRZ Rectangular, NRZ Raised cosine, RZ Rectangular, RZ Raised cosine and RZ super Gaussian for optical transmission link at transmission distance of 50 km. Li et al. [13] employed separately different modulation modes of DPSK and OOK in the upstream
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Fig. 2. Internal architecture for optical line terminal (OLT).
Fig. 1. System setup for bi-directional passive optical network architecture.
and downstream link, after the comparison with different modulation formats in the downstream including the codes of NRZ, RZ and Inverse Return-to-Zero (IRZ), the symmetric rate of 10 Gbps with 20 km transmission was realized without the dispersion compensation. Goyal et al. [14] analyzed the performance of a hybrid WDM/TDM PON system and compared the proposed hybrid PON for suitability of various modulation formats for different distance. It had been observed that the most suitable data format for hybrid PON network was NRZ Rectangular. Feng et al. [15] proposed a novel architecture for WDM-PON with centralized light-wave and investigated that the power penalty of the downstream was very small, while the power penalty of the upstream was large. They have also shown that with the decrease of launch power, the power penalty of the upstream can be reduced, at the cost of shortening the transmission distance. Kashyap et al. [16] observed the performance evaluation of Ethernet PON in the absence and presence of dispersion compensation techniques (DCT) and concluded that using DCT transmission speed as well as transmission distance of EPON system can be increased. Hwan et al. [17] reported T-ECL based WDM-PON in which 25 wavelength channels were transmitted up to a distance of 20 km. Urban et al. [18] transmitted only two 1.25-Gbps wavelength channels through single-mode fiber up to a distance of 26 km. Till now, the proposed PON were investigated at lesser transmission distance (≤50 km) [12–18] and limited to lesser data rate [16,18]. Moreover most of the proposed networks were investigated in downlink transmission only [12,16]. In this paper, we extend the previous work by investigating the different modulation formats such as NRZ, RZ, and OOK for bi-directional passive optical network at different transmission distance and reduce the complexity of the system by using the re-modulation technique instead of costly RSOA. This paper is organized into five sections. In Section 1, introduction to passive optical networks and different modulation formats pulses are described. In Section 2, the system setup for investigates the effects of modulation formats in re-modulated bidirectional PON. In Section 3, results have been reported for the BER, Q-factor and received power in terms of distance. In Section 4, the comparison of important performance parameters of our proposed PON architecture with current state-of-the-art PON architectures is presented. Finally, the conclusion is made. 2. System setup The system setup of FTTH using bi-direction PON architecture is shown in Fig. 1. It consists of an OLT and number of ONUs. To
implement a point to multipoint architecture a fiber distribution is used between single OLT to multiple ONUs. Eight passive elements 1:8 splitters and combiners with coupling factor of 0.50 are used to establish connection between a central office to different users. Semiconductor optical amplifier with 10 dB gain is used as a booster amplifier. In this paper, the proposed PON architecture has been investigated for different lengths from a central office to the ONUs in terms of BER, Q-factor, and received power under with different modulation formats. The wavelength is tuned at 1490 nm for downlink and 1310 nm for uplink with 10 Gbps and 2.5 Gbps data rate for downstream and upstream respectively. OLT is a device which consists of transmitter to transmit the data to different ONUs and receiver to detect the information from different ONUs. The internal architecture of OLT is shown in Fig. 2. Transmitter section consists of random data source, electrical driver, laser source and modulator as shown in Fig. 2. CW laser source with 100 MHz line width and electrical sine wave generator is used which superimposed and produced modulated signal for downstream signal. Pseudo random data source generate bit pattern and provide necessary bit rate of 10 Gbps then this sequence is converted into electrical pulses using different electrical coders such as NRZ, RZ and OOK. Mach–Zehnder optical modulator is used to provide modulated output and then 2:1 mux with 3 dB insertion loss is used for multiplexing. OLT receiver section consists of PIN photodiode, clock recovery circuit and Bessel filter. The different signals coming from different ONUs are demultiplexed by using 1:2 de-mux with power attenuation of 3 dB for attenuator then passed through the Bessel filter. To detect the information from ONUs a coherent detection techniques is used. PIN photodiode with 1 A/W responsivity, zero ampere dark √ current, and thermal noise of 10−11 A/ Hz is used to convert optical signal into electrical signal. The clock recovery module is used which determines the time delay between the incoming signal and the original signal hence provides synchronization. The performances of various modulation formats used in transmitter section on bi-directional PON can be observed by plotting the graph for BER, Q-factor at OLT side with transmission distance. ONU lies at subscriber end in PON architecture which provides many kinds of broadband services such as VoIP, HDTV, video conferences, etc. It consists of a transmitter to send the data to OLT at 2.5 Gbps and receiver to detect the data from OLT as shown in Fig. 3. ONU transmitter consists of PRBS, NRZ, RZ and OOK electrical drives and optical modulator. PRBS generates the data rate of 2.5 Gbps as upstream data and passes to different driver which convert it into different electrical coded signal for each input bit. The AM optical modulator with 0.9 modulation index modulates this electrical coded signal with carrier signal taken from downlink signal. In ONU receiver, first signal from OLT is demultiplexed by 1:2 demultiplexers with power attenuation of 0 dB for attenuator. One output of de-mux is connected to the AM optical modulator as shown in Fig. 3, so we are reusing the downstream optical signal for the upstream transmission, which is known as the remodulation
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Fig. 3. Internal architecture for optical network unit (ONU).
scheme. While the second output of de-mux is connected to PIN photodiode with 0 A dark current which will convert optical signal into electrical signal then clock recovery circuit is used for synchronization and then signal fed to the signal analyzer for analyzing the performance with different modulation formats.
3. Results and discussion The most common parameters used for measuring the performance are Q-factor and bit error rate (BER). The BER defines as the probability of incorrect identification of a bit by the decision circuit in the receiver and Q-factor measures the quality of the transmission of signal in terms of its signal-to-noise ratio (SNR), higher the value of Q-factor the better the SNR and therefore the lower the
Fig. 5. Q-factor versus transmission distance for (a) downlink, (b) uplink transmission.
probability of bit errors. The BER and Q-factor can be estimated by the following expression (Eqs. (1) and (2)). BER =
1 erfc 2
Q √ 2
≈
exp(−Q 2 /2) √ Q 2
(1)
Parameter Q can be estimated as Q =
Fig. 4. BER versus transmission distance for (a) downlink, (b) uplink transmission.
m1 − m0 1 − 0
(2)
where m1 and m0 are the mean of the received signal at the sampling instant when a bit 1 and 0 is transmitted and 1 , 0 are the standard deviations, respectively [14]. The Q-factor can be converted into Q-value by taking into decibels. In order to observe the bit error rate for NRZ, RZ and OOK modulation formats at ONU and OLT side, respectively, the graph between BER versus transmission distance is shown in Fig. 4. The acceptable BER of 2.27 × 10−11 , 9.42 × 10−11 and 3.18 × 10−9 is achieved with NRZ, RZ and OOK for downlink, respectively after covering 150 km of transmission distance. For uplink the BER of 1.27 × 10−10 , 7.07 × 10−9 and 2.28 × 10−6 is reported with NRZ, RZ and OOK, respectively at the same distance. We observed that NRZ has less bit error rate than RZ and OOK, which indicates the best performance hence with NRZ modulation format proposed system can transmit over 150-km standard bi-directional fiber carrying data to and from the user. The variation in Q-factor with NRZ, RZ and OOK modulation formats at ONU and OLT side is shown in Fig. 5. The acceptable Q-factor of 6.95, 6.48 and 5.91 for downlink transmission and 6.05, 5.5 and 4.4 for uplink transmission with NRZ, RZ and OOK, respectively
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Fig. 6. Received power versus transmission distance for (a) downlink, (b) uplink transmission.
Fig. 7. BER versus received power for (a) downlink, (b) uplink transmission.
observed after 150 km of transmission distance. It is also observed that quality of the signal decreases with an increase in distance. For the RZ and OOK modulation format the quality factor is less as compared to NRZ hence NRZ provides the better performance. To observe the received power for NRZ, RZ and OOK modulation formats, the graph between received power versus transmission distance at ONU and OLT side is shown in Fig. 6. The variation in received power is −18 to −33 dBm, −21.2 to −35.5 dBm and −23 to −39 dBm with NRZ, RZ and OOK, respectively for downlink transmission. For uplink transmission received power varies from −20.3 to −40.1 dBm, −25.1 to −41 dBm and −26.2 to −46 dBm with NRZ,
RZ and OOK, respectively in transmission spam of 25–175 km. We observed that received power is more at lesser transmission distance; as the transmission distance increases the received power degrades. With NRZ modulation format, the received power is higher than RZ and OOK. We have also drawn the graph for bit error rate versus received power at ONU and OLT side for different modulation formats such as NRZ, RZ and OOK as shown in Fig. 7. The BER of 2.27 × 10−11 with NRZ, 9.42 × 10−11 with RZ and 3.18 × 10−9 with OOK is observed at the received power of −32 dBm for downlink transmission. For the uplink transmission the BER of 1.27 × 10−10 with NRZ, 7.07 × 10−9
Table 1 Comparison of our proposed PON architecture with current state-of art PON architecture for different modulation formats. Parameters
Kaler et al. (2011) Ref. [12]
Li et al. (2013) Ref. [13]
Goyal et al. (2012) Ref. [14]
Feng et al. (2010) Ref. [15]
Kashyap et al. (2013) Ref. [16]
Current investigation
Transmission mode
Bi-directional
Single directional (downlink) NRZ, RZ and Manchester
Bi-directional
Single directional (downlink) NRZ and RZ
Bi-directional
Transmission distance Data rate
Single directional (downlink) NRZ/RZ Rectangular, Raised cosine and RZ Super Gaussian 50 km 10 Gbps
Users System complexity & cost
8 High
Modulation formats
NRZ, RZ and IRZ
20 km Symmetrical 10 Gbps
28 km 1.25 Gbps
85 Low (used re-modulation scheme)
128 High
DQPSK, OOK for downlink and uplink, respectively 20 km Symmetrical 10 Gbps
16 Low (used re-modulation scheme)
30 km 1.25 Gbps
16 High
NRZ, RZ and OOK
150 km 2.5 Gbps and 10 Gbps for uplink and downlink, respectively 64 Low (used re-modulation scheme)
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with RZ and 2.28 × 10−6 with OOK is observed at received power of −34.5 dBm. It is observed that at the higher received power the system has less bit error rate and provides good communication. 4. Comparison of proposed PON architecture with current state-of-the-art PON architectures In this section, we compare the important performance parameters of our proposed PON architecture with current state-of-the-art PON architectures, as described in Table 1. This comparison has been done to determine the level of knowledge and the development achieved in a technique. In Table 1, we present currently published 2010–2013 snapshots of the various current PON architectures. From this table, it can be concluded that our proposed PON architecture shows improvement over current state-of-theart architectures because: (1) we achieved better performance of bi-directional PON with wavelength remodulation scheme which eliminates the laser source from ONUs. (2) The reported results are conducted by the performance comparison of PON NRZ, RZ and OOK modulation format. (3) It includes 64 ONUs so the cost of the overall system is even more reduced because the cost can be shared by number of users. (4) The proposed PON can successfully transmit their data at speed of 10 Gbps and 2.5 Gbps up to 150 km for downlink and uplink, respectively. It can be observed that the performance of the current proposed system can be extended by increasing the number of channels with more data speed. 5. Conclusion The cost effective bi-directional passive optical network architecture has been investigated with different modulation formats such as NRZ, RZ and OOK. A remodulation scheme, where downstream optical signal is reused for upstream transmission has been used for realization of cost effective PON. The downstream (10 Gbps) and upstream (2.5 Gbps) traffic data is successfully transmitted up to 150 km at 1490 nm and 1310 nm, respectively. The acceptable Q-factor of 6.95, 6.48 and 5.91 for downlink transmission and 6.05, 5.5 and 4.4 for uplink transmission with NRZ, RZ and OOK, respectively has been observed. It is also observed that NRZ modulation format has an edge over RZ and OOK modulation formats.
References [1] G. Kramer, G. Pesavento, Ethernet passive optical network (EPON): building a next-generation optical access network, IEEE Commun. Magaz. 40 (February (2)) (2002) 66–73. [2] C.H. Lee, W.V. Sorin, B.Y. Kim, Fiber to the home using a PON infrastructure, J. Lightwave Technol. 24 (2006) 4568–4583. [3] S.J. Park, C.H. Lee, Fiber-to-the-home services based on wavelength-divisionmultiplexing passive optical network, J. Lightwave Technol. 22 (November (11)) (2004) 2582–2591. [4] L. Kazovsky, W. Shaw, D. Gutierrez, N. Cheng, S. Wong, Next-generation optical access networks, J. Lightwave Technol. 25 (November (11)) (2007) 3428–3442. [5] A. Banerjee, Y. Park, F. Clarke, H. Song, S. Yang, G. Kramer, K. Kim, B. Mukherjee, Wavelength-division-multiplexed passive optical network (WDMPON) technologies for broadband access, J. Opt. Network. 4 (November (11)) (2005) 737–758. [6] S. Singh, R.S. Kaler, Flat-gain L-band Raman-EDFa hybrid optical amplifier for dense wavelength division multiplexed system, IEEE Photon. Technol. Lett. 25 (3) (2013) 250–252. [7] S. Singh, R.S. Kaler, Performance evaluation of hybrid optical amplifier for highspeed differential phase-shift keying-modulated optical signals, Opt. Eng. 52 (9) (2013), 096102 (1-6). [8] J.H. Yu, N. Kim, B.W. Kim, Remodulation schemes with reflective SOA for colorless DWDM PON, IEEE J. Opt. Network 6 (2007) 1041–1504. [9] R.S. Simranjit Singh, Kaler, Investigation of hybrid optical amplifiers with different modulation formats for DWDM optical communication system, Optik 124 (2013) 2131–2134. [10] P.J. Winzer, R.J. Essiambre, Advanced optical modulation formats, Proc. IEEE 94 (5) (2006) 952–985. [11] W. Jaworski, Optical modulation formats for high-speed DWDM systems, in: Transparent Optical Networks, Proceedings of Fifth International Conference, Vol. 2, 29 June–3 July, 2003, pp. 162–165. [12] R. Kaler, R.S. Kaler, Comparative investigation and suitability of various data formats for 10 Gb/s optical AWG multiplexer and AWG demultiplexer based transmission links, Optik 122 (2011) 610–615. [13] L. Li, H. Li, J. Zhang, X. Zhang, Influence of modulation formats performance of the combined modulation in WDM-PON system, Optik 124 (July) (2013) 5660–5664. [14] R. Goyal, R.S. Kaler, A novel architecture of hybrid (WDM/TDM) passive optical networks with suitable modulation format, Opt. Fiber Technol. 18 (2012) 518–522. [15] Z.W. Feng, X. Xiang, Q. Zhang, Z. Zhang, N. Wei, S. Yan, Centralized light-wave WDM-PON employing DQPSK downstream and OOK remodulated upstream signals 17 (August (4)) (2010) 125–128. [16] A. Kashyap, N. Kumar, P. Kaushik, Enhanced performance of ethernet passive optical networks using dispersion compensation, J. Opt. Commun. 34 (1) (2013) 15–19. [17] S. Hwan, K. Hong, K. Soo, J. Kim, O. Kyun, D. Kon, Y. Ouk, J. Kyu, H. Jong, Tunable external cavity laser by hybrid integration of a super luminescent diode and a polymer Bragg reflector, Select. Topic Quantum Electron. IEEE 17 (6) (2011) 1534–1541. [18] P.J. Urban, E. Pluk, M.M. de Laat, F.M. Huijskens, G.D. Khoe, A.M.J. Koonen, H. de Waardt, 1.25-Gb/s transmission over an access network link with tunable OADM and a reflective SOA, IEEE, Photon. Technol. Lett. 21 (6) (2009) 380–382.
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Investigation on wavelength re-modulated bi-directional passive optical network for different modulation formats Simranjit Singh a,∗ , Amit Kapoor a , Gurpreet Kaur a , R.S. Kaler b , Rakesh Goyal c a b c
Department of Electronics and Communication Engineering, Punjabi University, Patiala, India Department of Electronics and Communication Engineering, Thapar University, Patiala, India Department of Electronics and Communication Engineering, Rayat and Bahra, Patiala, India
a r t i c l e
i n f o
Article history: Received 28 September 2013 Accepted 26 May 2014 Keywords: Bi-directional PON Modulation formats Remodulation techniques FTTH
a b s t r a c t In this paper, the cost effective bi-directional passive optical network architecture with wavelength remodulated scheme is investigated. To realize the cost-effective PON, remodulation scheme is used, in which the downstream optical signal is reused as a carrier for the upstream transmission as it eliminates the need for an extra laser source at optical network units. The performance of proposed passive optical network is analyzed and compared for various modulation formats such as Non Return to Zero (NRZ), Return to Zero (RZ) and On–Off Keying (OOK) with 64 optical networks units (ONUs) at different traffic speed for downlink and uplink, respectively. It has been observed that the most suitable data format for proposed PON network is NRZ. Further the proposed system performance is compared with the current state-of-the-art PON architectures. © 2014 Elsevier GmbH. All rights reserved.
1. Introduction Passive optical networks (PONs) are emerging as the ultimate solution in the fiber-to-the-home (FTTH) environment. They can offer more bandwidth than copper-based solution. Current Ethernet PON and Gigabit PON have been widely deployed in many places around the world because of the low installation and maintenance cost [1,2]. Wavelength-division-multiplexing (WDM) transport systems that use different optical channels to carry combined signals would be quite useful for a fiber transport network providing multiple services [3]. There has been increasing attention in implementing next generation broadband network by using WDM-PON; it is because WDM-PON can provide large transmission capacity, network security and flexibility [4,5]. The tremendous growth of the high speed internet and data traffic has created an enormous demand for transmission bandwidth of dense wavelength division multiplexed optical communication systems and differential phase-shift keying is the current technology to achieve better performance even in cost-effective high-speed networks [6,7]. In PONs, a continuous light is distributed over a fiber link from the optical line terminal (OLT) to each optical network unit (ONU). At the ONU side, the continuous wave (CW) light is then
∗ Corresponding author. E-mail address: [email protected] (S. Singh). http://dx.doi.org/10.1016/j.ijleo.2014.06.019 0030-4026/© 2014 Elsevier GmbH. All rights reserved.
modulated and transmitted back over the same fiber so in wavelength re-modulated scheme, a downstream optical signal is reused for upstream transmission. These configurations eliminate the need for an extra CW light source at ONU [8]. In the optical system the optical code pattern is the important factor for deciding the spectrum efficiency, transmission quality and dispersive tolerance of the system [9], so signal modulation format is a key issue in order to maximize optical transmission link capacity, system design and optimization have to take into account all the contributing facts, such as channel data rate, transmission distance, signal optical power, amplifier noise figure, channel wavelength spacing, optical amplifier spacing, fiber dispersion and non-linear parameters, dispersion management strategy, receiver bandwidth and so on [10,11]. One of the most important facts in the system, which affects the choices of all other system parameters, is the signal optical modulation format. The BER and Q-factor may be improved by choosing a proper line coding scheme which includes various modulation formats such as Non Return to Zero (NRZ), Return to Zero (RZ), On–Off Keying (OOK) and many more. In literature, various works on the performance of PONs with various modulation formats has been reported. Kaler et al. [12] presented the comparative investigation and suitability of various data formats like NRZ Rectangular, NRZ Raised cosine, RZ Rectangular, RZ Raised cosine and RZ super Gaussian for optical transmission link at transmission distance of 50 km. Li et al. [13] employed separately different modulation modes of DPSK and OOK in the upstream
S. Singh et al. / Optik 125 (2014) 5378–5382
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Fig. 2. Internal architecture for optical line terminal (OLT).
Fig. 1. System setup for bi-directional passive optical network architecture.
and downstream link, after the comparison with different modulation formats in the downstream including the codes of NRZ, RZ and Inverse Return-to-Zero (IRZ), the symmetric rate of 10 Gbps with 20 km transmission was realized without the dispersion compensation. Goyal et al. [14] analyzed the performance of a hybrid WDM/TDM PON system and compared the proposed hybrid PON for suitability of various modulation formats for different distance. It had been observed that the most suitable data format for hybrid PON network was NRZ Rectangular. Feng et al. [15] proposed a novel architecture for WDM-PON with centralized light-wave and investigated that the power penalty of the downstream was very small, while the power penalty of the upstream was large. They have also shown that with the decrease of launch power, the power penalty of the upstream can be reduced, at the cost of shortening the transmission distance. Kashyap et al. [16] observed the performance evaluation of Ethernet PON in the absence and presence of dispersion compensation techniques (DCT) and concluded that using DCT transmission speed as well as transmission distance of EPON system can be increased. Hwan et al. [17] reported T-ECL based WDM-PON in which 25 wavelength channels were transmitted up to a distance of 20 km. Urban et al. [18] transmitted only two 1.25-Gbps wavelength channels through single-mode fiber up to a distance of 26 km. Till now, the proposed PON were investigated at lesser transmission distance (≤50 km) [12–18] and limited to lesser data rate [16,18]. Moreover most of the proposed networks were investigated in downlink transmission only [12,16]. In this paper, we extend the previous work by investigating the different modulation formats such as NRZ, RZ, and OOK for bi-directional passive optical network at different transmission distance and reduce the complexity of the system by using the re-modulation technique instead of costly RSOA. This paper is organized into five sections. In Section 1, introduction to passive optical networks and different modulation formats pulses are described. In Section 2, the system setup for investigates the effects of modulation formats in re-modulated bidirectional PON. In Section 3, results have been reported for the BER, Q-factor and received power in terms of distance. In Section 4, the comparison of important performance parameters of our proposed PON architecture with current state-of-the-art PON architectures is presented. Finally, the conclusion is made. 2. System setup The system setup of FTTH using bi-direction PON architecture is shown in Fig. 1. It consists of an OLT and number of ONUs. To
implement a point to multipoint architecture a fiber distribution is used between single OLT to multiple ONUs. Eight passive elements 1:8 splitters and combiners with coupling factor of 0.50 are used to establish connection between a central office to different users. Semiconductor optical amplifier with 10 dB gain is used as a booster amplifier. In this paper, the proposed PON architecture has been investigated for different lengths from a central office to the ONUs in terms of BER, Q-factor, and received power under with different modulation formats. The wavelength is tuned at 1490 nm for downlink and 1310 nm for uplink with 10 Gbps and 2.5 Gbps data rate for downstream and upstream respectively. OLT is a device which consists of transmitter to transmit the data to different ONUs and receiver to detect the information from different ONUs. The internal architecture of OLT is shown in Fig. 2. Transmitter section consists of random data source, electrical driver, laser source and modulator as shown in Fig. 2. CW laser source with 100 MHz line width and electrical sine wave generator is used which superimposed and produced modulated signal for downstream signal. Pseudo random data source generate bit pattern and provide necessary bit rate of 10 Gbps then this sequence is converted into electrical pulses using different electrical coders such as NRZ, RZ and OOK. Mach–Zehnder optical modulator is used to provide modulated output and then 2:1 mux with 3 dB insertion loss is used for multiplexing. OLT receiver section consists of PIN photodiode, clock recovery circuit and Bessel filter. The different signals coming from different ONUs are demultiplexed by using 1:2 de-mux with power attenuation of 3 dB for attenuator then passed through the Bessel filter. To detect the information from ONUs a coherent detection techniques is used. PIN photodiode with 1 A/W responsivity, zero ampere dark √ current, and thermal noise of 10−11 A/ Hz is used to convert optical signal into electrical signal. The clock recovery module is used which determines the time delay between the incoming signal and the original signal hence provides synchronization. The performances of various modulation formats used in transmitter section on bi-directional PON can be observed by plotting the graph for BER, Q-factor at OLT side with transmission distance. ONU lies at subscriber end in PON architecture which provides many kinds of broadband services such as VoIP, HDTV, video conferences, etc. It consists of a transmitter to send the data to OLT at 2.5 Gbps and receiver to detect the data from OLT as shown in Fig. 3. ONU transmitter consists of PRBS, NRZ, RZ and OOK electrical drives and optical modulator. PRBS generates the data rate of 2.5 Gbps as upstream data and passes to different driver which convert it into different electrical coded signal for each input bit. The AM optical modulator with 0.9 modulation index modulates this electrical coded signal with carrier signal taken from downlink signal. In ONU receiver, first signal from OLT is demultiplexed by 1:2 demultiplexers with power attenuation of 0 dB for attenuator. One output of de-mux is connected to the AM optical modulator as shown in Fig. 3, so we are reusing the downstream optical signal for the upstream transmission, which is known as the remodulation
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Fig. 3. Internal architecture for optical network unit (ONU).
scheme. While the second output of de-mux is connected to PIN photodiode with 0 A dark current which will convert optical signal into electrical signal then clock recovery circuit is used for synchronization and then signal fed to the signal analyzer for analyzing the performance with different modulation formats.
3. Results and discussion The most common parameters used for measuring the performance are Q-factor and bit error rate (BER). The BER defines as the probability of incorrect identification of a bit by the decision circuit in the receiver and Q-factor measures the quality of the transmission of signal in terms of its signal-to-noise ratio (SNR), higher the value of Q-factor the better the SNR and therefore the lower the
Fig. 5. Q-factor versus transmission distance for (a) downlink, (b) uplink transmission.
probability of bit errors. The BER and Q-factor can be estimated by the following expression (Eqs. (1) and (2)). BER =
1 erfc 2
Q √ 2
≈
exp(−Q 2 /2) √ Q 2
(1)
Parameter Q can be estimated as Q =
Fig. 4. BER versus transmission distance for (a) downlink, (b) uplink transmission.
m1 − m0 1 − 0
(2)
where m1 and m0 are the mean of the received signal at the sampling instant when a bit 1 and 0 is transmitted and 1 , 0 are the standard deviations, respectively [14]. The Q-factor can be converted into Q-value by taking into decibels. In order to observe the bit error rate for NRZ, RZ and OOK modulation formats at ONU and OLT side, respectively, the graph between BER versus transmission distance is shown in Fig. 4. The acceptable BER of 2.27 × 10−11 , 9.42 × 10−11 and 3.18 × 10−9 is achieved with NRZ, RZ and OOK for downlink, respectively after covering 150 km of transmission distance. For uplink the BER of 1.27 × 10−10 , 7.07 × 10−9 and 2.28 × 10−6 is reported with NRZ, RZ and OOK, respectively at the same distance. We observed that NRZ has less bit error rate than RZ and OOK, which indicates the best performance hence with NRZ modulation format proposed system can transmit over 150-km standard bi-directional fiber carrying data to and from the user. The variation in Q-factor with NRZ, RZ and OOK modulation formats at ONU and OLT side is shown in Fig. 5. The acceptable Q-factor of 6.95, 6.48 and 5.91 for downlink transmission and 6.05, 5.5 and 4.4 for uplink transmission with NRZ, RZ and OOK, respectively
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Fig. 6. Received power versus transmission distance for (a) downlink, (b) uplink transmission.
Fig. 7. BER versus received power for (a) downlink, (b) uplink transmission.
observed after 150 km of transmission distance. It is also observed that quality of the signal decreases with an increase in distance. For the RZ and OOK modulation format the quality factor is less as compared to NRZ hence NRZ provides the better performance. To observe the received power for NRZ, RZ and OOK modulation formats, the graph between received power versus transmission distance at ONU and OLT side is shown in Fig. 6. The variation in received power is −18 to −33 dBm, −21.2 to −35.5 dBm and −23 to −39 dBm with NRZ, RZ and OOK, respectively for downlink transmission. For uplink transmission received power varies from −20.3 to −40.1 dBm, −25.1 to −41 dBm and −26.2 to −46 dBm with NRZ,
RZ and OOK, respectively in transmission spam of 25–175 km. We observed that received power is more at lesser transmission distance; as the transmission distance increases the received power degrades. With NRZ modulation format, the received power is higher than RZ and OOK. We have also drawn the graph for bit error rate versus received power at ONU and OLT side for different modulation formats such as NRZ, RZ and OOK as shown in Fig. 7. The BER of 2.27 × 10−11 with NRZ, 9.42 × 10−11 with RZ and 3.18 × 10−9 with OOK is observed at the received power of −32 dBm for downlink transmission. For the uplink transmission the BER of 1.27 × 10−10 with NRZ, 7.07 × 10−9
Table 1 Comparison of our proposed PON architecture with current state-of art PON architecture for different modulation formats. Parameters
Kaler et al. (2011) Ref. [12]
Li et al. (2013) Ref. [13]
Goyal et al. (2012) Ref. [14]
Feng et al. (2010) Ref. [15]
Kashyap et al. (2013) Ref. [16]
Current investigation
Transmission mode
Bi-directional
Single directional (downlink) NRZ, RZ and Manchester
Bi-directional
Single directional (downlink) NRZ and RZ
Bi-directional
Transmission distance Data rate
Single directional (downlink) NRZ/RZ Rectangular, Raised cosine and RZ Super Gaussian 50 km 10 Gbps
Users System complexity & cost
8 High
Modulation formats
NRZ, RZ and IRZ
20 km Symmetrical 10 Gbps
28 km 1.25 Gbps
85 Low (used re-modulation scheme)
128 High
DQPSK, OOK for downlink and uplink, respectively 20 km Symmetrical 10 Gbps
16 Low (used re-modulation scheme)
30 km 1.25 Gbps
16 High
NRZ, RZ and OOK
150 km 2.5 Gbps and 10 Gbps for uplink and downlink, respectively 64 Low (used re-modulation scheme)
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with RZ and 2.28 × 10−6 with OOK is observed at received power of −34.5 dBm. It is observed that at the higher received power the system has less bit error rate and provides good communication. 4. Comparison of proposed PON architecture with current state-of-the-art PON architectures In this section, we compare the important performance parameters of our proposed PON architecture with current state-of-the-art PON architectures, as described in Table 1. This comparison has been done to determine the level of knowledge and the development achieved in a technique. In Table 1, we present currently published 2010–2013 snapshots of the various current PON architectures. From this table, it can be concluded that our proposed PON architecture shows improvement over current state-of-theart architectures because: (1) we achieved better performance of bi-directional PON with wavelength remodulation scheme which eliminates the laser source from ONUs. (2) The reported results are conducted by the performance comparison of PON NRZ, RZ and OOK modulation format. (3) It includes 64 ONUs so the cost of the overall system is even more reduced because the cost can be shared by number of users. (4) The proposed PON can successfully transmit their data at speed of 10 Gbps and 2.5 Gbps up to 150 km for downlink and uplink, respectively. It can be observed that the performance of the current proposed system can be extended by increasing the number of channels with more data speed. 5. Conclusion The cost effective bi-directional passive optical network architecture has been investigated with different modulation formats such as NRZ, RZ and OOK. A remodulation scheme, where downstream optical signal is reused for upstream transmission has been used for realization of cost effective PON. The downstream (10 Gbps) and upstream (2.5 Gbps) traffic data is successfully transmitted up to 150 km at 1490 nm and 1310 nm, respectively. The acceptable Q-factor of 6.95, 6.48 and 5.91 for downlink transmission and 6.05, 5.5 and 4.4 for uplink transmission with NRZ, RZ and OOK, respectively has been observed. It is also observed that NRZ modulation format has an edge over RZ and OOK modulation formats.
References [1] G. Kramer, G. Pesavento, Ethernet passive optical network (EPON): building a next-generation optical access network, IEEE Commun. Magaz. 40 (February (2)) (2002) 66–73. [2] C.H. Lee, W.V. Sorin, B.Y. Kim, Fiber to the home using a PON infrastructure, J. Lightwave Technol. 24 (2006) 4568–4583. [3] S.J. Park, C.H. Lee, Fiber-to-the-home services based on wavelength-divisionmultiplexing passive optical network, J. Lightwave Technol. 22 (November (11)) (2004) 2582–2591. [4] L. Kazovsky, W. Shaw, D. Gutierrez, N. Cheng, S. Wong, Next-generation optical access networks, J. Lightwave Technol. 25 (November (11)) (2007) 3428–3442. [5] A. Banerjee, Y. Park, F. Clarke, H. Song, S. Yang, G. Kramer, K. Kim, B. Mukherjee, Wavelength-division-multiplexed passive optical network (WDMPON) technologies for broadband access, J. Opt. Network. 4 (November (11)) (2005) 737–758. [6] S. Singh, R.S. Kaler, Flat-gain L-band Raman-EDFa hybrid optical amplifier for dense wavelength division multiplexed system, IEEE Photon. Technol. Lett. 25 (3) (2013) 250–252. [7] S. Singh, R.S. Kaler, Performance evaluation of hybrid optical amplifier for highspeed differential phase-shift keying-modulated optical signals, Opt. Eng. 52 (9) (2013), 096102 (1-6). [8] J.H. Yu, N. Kim, B.W. Kim, Remodulation schemes with reflective SOA for colorless DWDM PON, IEEE J. Opt. Network 6 (2007) 1041–1504. [9] R.S. Simranjit Singh, Kaler, Investigation of hybrid optical amplifiers with different modulation formats for DWDM optical communication system, Optik 124 (2013) 2131–2134. [10] P.J. Winzer, R.J. Essiambre, Advanced optical modulation formats, Proc. IEEE 94 (5) (2006) 952–985. [11] W. Jaworski, Optical modulation formats for high-speed DWDM systems, in: Transparent Optical Networks, Proceedings of Fifth International Conference, Vol. 2, 29 June–3 July, 2003, pp. 162–165. [12] R. Kaler, R.S. Kaler, Comparative investigation and suitability of various data formats for 10 Gb/s optical AWG multiplexer and AWG demultiplexer based transmission links, Optik 122 (2011) 610–615. [13] L. Li, H. Li, J. Zhang, X. Zhang, Influence of modulation formats performance of the combined modulation in WDM-PON system, Optik 124 (July) (2013) 5660–5664. [14] R. Goyal, R.S. Kaler, A novel architecture of hybrid (WDM/TDM) passive optical networks with suitable modulation format, Opt. Fiber Technol. 18 (2012) 518–522. [15] Z.W. Feng, X. Xiang, Q. Zhang, Z. Zhang, N. Wei, S. Yan, Centralized light-wave WDM-PON employing DQPSK downstream and OOK remodulated upstream signals 17 (August (4)) (2010) 125–128. [16] A. Kashyap, N. Kumar, P. Kaushik, Enhanced performance of ethernet passive optical networks using dispersion compensation, J. Opt. Commun. 34 (1) (2013) 15–19. [17] S. Hwan, K. Hong, K. Soo, J. Kim, O. Kyun, D. Kon, Y. Ouk, J. Kyu, H. Jong, Tunable external cavity laser by hybrid integration of a super luminescent diode and a polymer Bragg reflector, Select. Topic Quantum Electron. IEEE 17 (6) (2011) 1534–1541. [18] P.J. Urban, E. Pluk, M.M. de Laat, F.M. Huijskens, G.D. Khoe, A.M.J. Koonen, H. de Waardt, 1.25-Gb/s transmission over an access network link with tunable OADM and a reflective SOA, IEEE, Photon. Technol. Lett. 21 (6) (2009) 380–382.