60 HS.Abbas,M.A.Gregory Journal of Network and Computer Applications 67(2016)53-74 wavelength splitter operate over the power splitter,which improved and the potential eavesdropping issue is eliminated imposes the major challenge in terms of link budget.Addition- (Srivastava,2013;Urata et al.,2012). ally,the possibility of attaining more than 1 Gbps of data rate is Despite these features,a number of restrictions make WDM- not clear as it exceeds the capability of the current system PON an inappropriate technology for NG-PON2.With the limita- (Nesset,2015). tion of the number of wavelengths allowed in the system and with 2)Wavelength re-use WDM-PON (Nesset,2015):This approach the large bandwidth requirement,it leads to inefficient utilization assigns a wavelength to each user for downstream and of the bandwidth (Hernandez et al.,2012).Additionally,the cost is upstream transmission.The re-use of the wavelength is enabled a prominent issue in WDM-PON where it increases due to the by the transmitter based on semiconductor amplifier.This need for extra equipment such as colored ONUs and a transceiver amplifier modulates the downstream signal in inverse Return- for every wavelength at the OLT(Sotiropoulos et al.,2013;Urata to-Zero format and the upstream signal in Return-to-Zero etal,2012). format (Nesset,2015). 3)Tunable WDM-PON (Begovic et al.2011):This approach is 4.3.OCDM-PON based on a low cost tunable transmitter module instead of the conventional module.The reduction of the cost is achieved by Introducing OCDM-PON technology leads to considerable removing thermoelectric coolers and the wave-lockers from the improvements for NG-PON2.The advantages include highly effi- conventional modules.Tuning at the upstream is performed cient use of bandwidth,good correlation performance,asynchro- utilizing the shared OLT based wave-locker.However,tunable nous transmission,flexibility of user allocation,low signal pro- receivers are needed at each ONU to perform colorless function cessing latency as well as improving network security (Yoshima (Nesset,2015). et al.,2013:Kataoka et al.,2010). 4)Ultra-dense Coherent WDM-PON (Begovic et al.2011):This OCDM can be classified into two main categories:coherent approach is based on coherent detection where the channels are system and incoherent system.In coherent system,OCDM is tightly spaced(around 3 GHz )1 Gbps data rate is allocated to implemented through a bipolar approach that requires informa- every user utilizing dedicated Quadrature Phase Shift Keying tion about the phase of the carriers.On the other hand,the inco- (QPSK)modulated wavelength.However,the transmitters and herent system is implemented through a unipolar approach. the receivers are very complex systems and expensive.Thus, Owing to the simplicity of incoherent hardware as well as its non- more improvements in photonic integration are essential to be reliance on phase synchronization detection,incoherent system used in practical implementation(Nesset,2015). has emerged as the preferred detection scheme.Fig.10 shows the 5)Self-seeded WDM-PON (Tanaka et al,2010):In this scheme,the basic structure of the OCDM network,which has four main com- seed light of the ONU is self-generated by a reflector at the ponents including transmitter,encoder,decoder,and the receiver. common port of the wavelength splitter.However,the length of At the transmitter,an information source provides a data bit for a the drop fiber(the fiber between the splitter and the ONU)is laser at every T second.The encoder then multiplies the data bit limited (Nesset,2015). "when it equals 1"by a code-word.The code-word can be formed by one-dimensional encoding using the time or wavelength Several schemes have been proposed to allow migration from domain or by a two-dimensional encoding scheme,which is a TDM-PON to WDM-PON.Hybrid TDM/WDM PON or SUCCESS- combination of both domains.Yet,recent studies have shown HPON (The Stanford University aCCESS)provides a cost effective advantages of three dimensional codes(Yen and Chen,2015:Wang and smooth migration path from TDM to WDM.SUCCESS-HPON is and Chang.2015;Jindal and Gupta,2012;Garg and Kaler,2013; based on the lasers at ONUs and shares tunable WDM components Shum,2015).The pulses generated are referred to as chips and at the OLT.Hence,it achieves bandwidth equivalent to the pure have a duration of Tc=T/n,where I donates the duration of each WDM-PON bandwidth with lower costs (Gutierrez et al..2005). bit and n denotes the code length. In (Chow and Yeh,2013),another migration scheme has been The multiplexed signal is broadcast to all of the users.The proposed.In this scheme,the differential phase-shift keying (DPSK)technique is used for the downstream signal.The signal arrives at the receiver and passes through the decoder.The decoder matches the code and accepts only the intended user's wavelength-shifted amplitude-shift keying (WS-ASK)is used for the upstream signal.At the ONU,an optical filter is implemented signal.Then the output of the decoder passes through photo- to select the intended downstream wavelength and to demodulate detection and integration.Later,the output power is sampled for each bit interval and compared to the threshold value to provide the downstream signal.The upstream signal is generated by signal demodulation that is based on reusing the downstream wave- length.Another benefit of this scheme,beside the smooth OLT migration,is that it does not require any changes to the existing r】 ONU1 fiber infrastructure. In (Shachaf et al.,2007),a multi-PON architecture based on a Modulator Decoder rmation Receive Encoder 1 coarse AWG at the OLT has been introduced to allow smooth migration path from TDM-PON to WDM-PON.The AWG is designed to support several TDM-PON and WDM-PON by employing tunable laser at the OLT.In addition,the splitter in the distribution side is replaced by a multiplexing unit that works to justify parallel processes of TDM-PON and WDM-PON.This pro- ONU N aser N vides the required bandwidth for the ONUs.At the ONU side, RSOAs is required to implement colorless transceivers,hence,no Decoder Recelve N change is needed at the customers'side. N The multiple-wavelength characteristic in WDM-PON offers several unique features.Firstly,each user can upgrade its capacity without the need for pre-designing a new fiber.Furthermore,the upgrade will not impact other users.Secondly.security is Fig.10.OCDM architecturewavelength splitter operate over the power splitter, which imposes the major challenge in terms of link budget. Addition￾ally, the possibility of attaining more than 1 Gbps of data rate is not clear as it exceeds the capability of the current system (Nesset, 2015). 2) Wavelength re-use WDM-PON (Nesset, 2015): This approach assigns a wavelength to each user for downstream and upstream transmission. The re-use of the wavelength is enabled by the transmitter based on semiconductor amplifier. This amplifier modulates the downstream signal in inverse Return￾to-Zero format and the upstream signal in Return-to-Zero format (Nesset, 2015). 3) Tunable WDM-PON (Begovic et al., 2011): This approach is based on a low cost tunable transmitter module instead of the conventional module. The reduction of the cost is achieved by removing thermoelectric coolers and the wave-lockers from the conventional modules. Tuning at the upstream is performed utilizing the shared OLT based wave-locker. However, tunable receivers are needed at each ONU to perform colorless function (Nesset, 2015). 4) Ultra-dense Coherent WDM-PON (Begovic et al., 2011): This approach is based on coherent detection where the channels are tightly spaced (around 3 GHz ). 1 Gbps data rate is allocated to every user utilizing dedicated Quadrature Phase Shift Keying (QPSK) modulated wavelength. However, the transmitters and the receivers are very complex systems and expensive. Thus, more improvements in photonic integration are essential to be used in practical implementation (Nesset, 2015). 5) Self-seeded WDM-PON (Tanaka et al., 2010): In this scheme, the seed light of the ONU is self-generated by a reflector at the common port of the wavelength splitter. However, the length of the drop fiber (the fiber between the splitter and the ONU) is limited (Nesset, 2015). Several schemes have been proposed to allow migration from TDM-PON to WDM-PON. Hybrid TDM/WDM PON or SUCCESS￾HPON (The Stanford University aCCESS) provides a cost effective and smooth migration path from TDM to WDM. SUCCESS-HPON is based on the lasers at ONUs and shares tunable WDM components at the OLT. Hence, it achieves bandwidth equivalent to the pure WDM-PON bandwidth with lower costs (Gutierrez et al., 2005). In (Chow and Yeh, 2013), another migration scheme has been proposed. In this scheme, the differential phase-shift keying (DPSK) technique is used for the downstream signal. The wavelength-shifted amplitude-shift keying (WS-ASK) is used for the upstream signal. At the ONU, an optical filter is implemented to select the intended downstream wavelength and to demodulate the downstream signal. The upstream signal is generated by signal demodulation that is based on reusing the downstream wave￾length. Another benefit of this scheme, beside the smooth migration, is that it does not require any changes to the existing fiber infrastructure. In (Shachaf et al., 2007), a multi-PON architecture based on a coarse AWG at the OLT has been introduced to allow smooth migration path from TDM-PON to WDM-PON. The AWG is designed to support several TDM-PON and WDM-PON by employing tunable laser at the OLT. In addition, the splitter in the distribution side is replaced by a multiplexing unit that works to justify parallel processes of TDM-PON and WDM-PON. This pro￾vides the required bandwidth for the ONUs. At the ONU side, RSOAs is required to implement colorless transceivers, hence, no change is needed at the customers' side. The multiple-wavelength characteristic in WDM-PON offers several unique features. Firstly, each user can upgrade its capacity without the need for pre-designing a new fiber. Furthermore, the upgrade will not impact other users. Secondly, security is improved and the potential eavesdropping issue is eliminated (Srivastava, 2013; Urata et al., 2012). Despite these features, a number of restrictions make WDM￾PON an inappropriate technology for NG-PON2. With the limita￾tion of the number of wavelengths allowed in the system and with the large bandwidth requirement, it leads to inefficient utilization of the bandwidth (Hernandez et al., 2012). Additionally, the cost is a prominent issue in WDM-PON where it increases due to the need for extra equipment such as colored ONUs and a transceiver for every wavelength at the OLT (Sotiropoulos et al., 2013; Urata et al., 2012). 4.3. OCDM-PON Introducing OCDM-PON technology leads to considerable improvements for NG-PON2. The advantages include highly effi- cient use of bandwidth, good correlation performance, asynchro￾nous transmission, flexibility of user allocation, low signal pro￾cessing latency as well as improving network security (Yoshima et al., 2013; Kataoka et al., 2010). OCDM can be classified into two main categories: coherent system and incoherent system. In coherent system, OCDM is implemented through a bipolar approach that requires informa￾tion about the phase of the carriers. On the other hand, the inco￾herent system is implemented through a unipolar approach. Owing to the simplicity of incoherent hardware as well as its non￾reliance on phase synchronization detection, incoherent system has emerged as the preferred detection scheme. Fig. 10 shows the basic structure of the OCDM network, which has four main com￾ponents including transmitter, encoder, decoder, and the receiver. At the transmitter, an information source provides a data bit for a laser at every T second. The encoder then multiplies the data bit “when it equals 1” by a code-word. The code-word can be formed by one-dimensional encoding using the time or wavelength domain or by a two-dimensional encoding scheme, which is a combination of both domains. Yet, recent studies have shown advantages of three dimensional codes (Yen and Chen, 2015; Wang and Chang, 2015; Jindal and Gupta, 2012; Garg and Kaler, 2013; Shum, 2015). The pulses generated are referred to as chips and have a duration of Tc¼T/n, where T donates the duration of each bit and n denotes the code length. The multiplexed signal is broadcast to all of the users. The signal arrives at the receiver and passes through the decoder. The decoder matches the code and accepts only the intended user's signal. Then the output of the decoder passes through photo￾detection and integration. Later, the output power is sampled for each bit interval and compared to the threshold value to provide Fig. 10. OCDM architecture. 60 H.S. Abbas, M.A. Gregory / Journal of Network and Computer Applications 67 (2016) 53–74