《信息网络协议基础》课程教学资源（学习资料）The next generation of passive optical networks：A review Huda Saleh Abbas n , Mark A. Gregory（ng-pon）

58 HS.Abbas,M.A.Gregory Journal of Network and Computer Applications 67(2016)53-74 Table 3 (WBF)is required in order to differentiate the PON data traffic 1G-EPON VS XG-EPON (Erzen and Batagelj,2015;Galveias,2012). 2)XG-GPON2 Feature 1G-EPON XG-EPON The major aim of XG-GPON2 is to offer symmetrical transmis- Bit rate Symmetric 1Gbps Symmetric 10Gbps Asymmetric 10/ sion by increasing the upstream transmission up to 10 Gbps. 1Gbps The expectation of spontaneous movement from GPON to the Wavelength (nm)US:1260-1360 US:1260-1280 XG-GPON1/XG-GPON2 has been discussed in the literature. DS:1480-1500 DS:1575-1580 Line code 8B/10B 64B/66B However,a number of drawbacks associated with the coex- FEC Optional Mandatory istence of these technologies have appeared.This approach SR(255,239) RS(255.223) requires different receivers at the OLT in order to receive the upstream data at different transmission speeds.In addition,it is not certain that the fragmentation process will be supported at The XG-GPON is divided into two classes.The first class called a higher transmission rate in the upstream transmission XG-GPON1 provides asymmetrical transmission with 10 Gbps (Kataoka et al.,2011). downstream and 2.5 Gbps upstream.The second class is XG- GPON2 which provides 10 Gbps symmetrical transmission(Erzen 3.3.Mixed scenario and Batagelj.2015:Leng et al..2013).Details about the XG-GPON1 physical layer have been described in ITU-T G.987.2.Whereas,the The mixed scenario is another possible upgrade to NG-PON1.In XG-GPON2 physical layer standard is still to be finalized. this platform,GPON and XG-EPON coexist with each other and operate on the same infrastructure.However,this scenario 1)XG-GPON1 requires suitable wavelength band separation with the help of a According to the G.987.1 recommendation for XG-GPON1,two WDM filter at the OLT in order to eliminate interference (Kataoka scenarios have been proposed to enable migration from GPON etal,2011). to XG-GPON1.The first scenario is a green-field migration which is the replacement of the copper connection into premises with an optical connection.The other option is the PON brown-field 4.ING-PON2 pure technologies migration scenario which is an upgrade of the existing GPON system and this includes replacing or upgrading some of the Studies have been conducted for several NG-PON2 technologies network components such as ONU units or OLT modules if necessary(Erzen and Batagelj.2015). that offer up to 100 Gbps.This includes high speed TDM-PON The downstream wavelength band selected for XG-GPONI is WDM-PON,OCDM-PON,OFDM-PON,and hybrid technologies between 1575 and 1580 nm and the upstream wavelength band (Cvijetic et al.,2010:Luo et al..2012).The pure technologies will be reviewed in this section. is between 1260 and 1280 nm.The wavelength bands were selected to enlarge the guard band between the wavelengths which reduces signal interference (Erzen and Batagelj.2015). 4.1.High speed TDM-PON The coexistence of XG-GPON1 with the deployed GPON is an important criterion when an upgrade is considered.Even TDM-PON allows multiple users to share the same bandwidth though this approach decreases the overall cost,there is an using a single wavelength.A typical TDM-PON structure is shown additional cost associated with wavelength filtering that is in Fig.8.The downstream traffic is broadcast to all users and a required at the ONUs.Fig.7 shows the coexistence scenario, specific time is assigned by the OLT to every ONU to control where the Co consists of two OLTs,one to carry the GPON upstream transmissions.These time slots are allocated in down- connection and the other to carry the XG-GPON1 connection. stream and upstream frames where a complex algorithm is New equipment named as WDMr1 is installed at the CO. required to arrange and assign the bandwidth in order to avoid Multiplexing/demultiplexing the signal of both OLTs and RF is collisions (Esmail and Fathallah,2013:Muciaccia et al.2014). its functionality.On the user's side,a Wavelength Blocking Filter TDM-PON is a simple and cost effective technology,however;it has limited scalability due to the fact that ONUs share bandwidth ONU XG-PON1 ONU1 OLT ONU ONU WDM r1 123 OLT Splitter ONU 1☑3 GPON OLT ONU ◆Downstream 2 Upstream ONU3 ONU Fig.7.Coexistence of GPON and XG-PON1 Fig 8.TDM architecture

The XG-GPON is divided into two classes. The first class called XG-GPON1 provides asymmetrical transmission with 10 Gbps downstream and 2.5 Gbps upstream. The second class is XG￾GPON2 which provides 10 Gbps symmetrical transmission (Eržen and Batagelj, 2015; Leng et al., 2013). Details about the XG-GPON1 physical layer have been described in ITU-T G.987.2. Whereas, the XG-GPON2 physical layer standard is still to be finalized. 1) XG-GPON1 According to the G.987.1 recommendation for XG-GPON1, two scenarios have been proposed to enable migration from GPON to XG-GPON1. The first scenario is a green-field migration which is the replacement of the copper connection into premises with an optical connection. The other option is the PON brown-field migration scenario which is an upgrade of the existing GPON system and this includes replacing or upgrading some of the network components such as ONU units or OLT modules if necessary (Eržen and Batagelj, 2015). The downstream wavelength band selected for XG-GPON1 is between 1575 and 1580 nm and the upstream wavelength band is between 1260 and 1280 nm. The wavelength bands were selected to enlarge the guard band between the wavelengths which reduces signal interference (Eržen and Batagelj, 2015). The coexistence of XG-GPON1 with the deployed GPON is an important criterion when an upgrade is considered. Even though this approach decreases the overall cost, there is an additional cost associated with wavelength filtering that is required at the ONUs. Fig. 7 shows the coexistence scenario, where the CO consists of two OLTs, one to carry the GPON connection and the other to carry the XG-GPON1 connection. New equipment named as WDMr1 is installed at the CO. Multiplexing/demultiplexing the signal of both OLTs and RF is its functionality. On the user’s side, a Wavelength Blocking Filter (WBF) is required in order to differentiate the PON data traffic (Eržen and Batagelj, 2015; Galveias, 2012). 2) XG-GPON2 The major aim of XG-GPON2 is to offer symmetrical transmis￾sion by increasing the upstream transmission up to 10 Gbps. The expectation of spontaneous movement from GPON to the XG-GPON1/XG-GPON2 has been discussed in the literature. However, a number of drawbacks associated with the coex￾istence of these technologies have appeared. This approach requires different receivers at the OLT in order to receive the upstream data at different transmission speeds. In addition, it is not certain that the fragmentation process will be supported at a higher transmission rate in the upstream transmission (Kataoka et al., 2011). 3.3. Mixed scenario The mixed scenario is another possible upgrade to NG-PON1. In this platform, GPON and XG-EPON coexist with each other and operate on the same infrastructure. However, this scenario requires suitable wavelength band separation with the help of a WDM filter at the OLT in order to eliminate interference (Kataoka et al., 2011). 4. ING-PON2 pure technologies Studies have been conducted for several NG-PON2 technologies that offer up to 100 Gbps. This includes high speed TDM-PON, WDM-PON, OCDM-PON, OFDM-PON, and hybrid technologies (Cvijetic et al., 2010; Luo et al., 2012). The pure technologies will be reviewed in this section. 4.1. High speed TDM-PON TDM-PON allows multiple users to share the same bandwidth using a single wavelength. A typical TDM-PON structure is shown in Fig. 8. The downstream traffic is broadcast to all users and a specific time is assigned by the OLT to every ONU to control upstream transmissions. These time slots are allocated in down￾stream and upstream frames where a complex algorithm is required to arrange and assign the bandwidth in order to avoid collisions (Esmail and Fathallah, 2013; Muciaccia et al., 2014). TDM-PON is a simple and cost effective technology, however; it has limited scalability due to the fact that ONUs share bandwidth. Table 3 1G-EPON VS XG-EPON. Feature 1G-EPON XG-EPON Bit rate Symmetric 1Gbps Symmetric 10Gbps Asymmetric 10/ 1Gbps Wavelength (nm) US: 1260–1360 US: 1260–1280 DS: 1480–1500 DS: 1575–1580 Line code 8B/10B 64B/66B FEC Optional Mandatory SR (255,239) RS (255, 223) Fig. 7. Coexistence of GPON and XG-PON1. Fig. 8. TDM architecture. 58 H.S. Abbas, M.A. Gregory / Journal of Network and Computer Applications 67 (2016) 53–74

H.S.Abbas.MA.Gregory Joumal of Network and Computer Applications 67 (2016)53-74 59 Increasing the bit rate for all of the users will be a challenging task ONU1 because every ONU receiver operates at a bit rate that is higher than the bit rate assigned per ONU.Utilizing a high speed digital BS signal processor and field-programmable gate array to increase the OLT bitrate to higher than 10 Gbps increases cost and complexity ONU Z (Muciaccia et al.,2014;Sotiropoulos et al.,2013).In addition,TDM- TX PON is not very secure due to the shared infrastructure which array opens the possibility of eavesdropping and other attacks.More- 1,2,入3，入4 9 over,the variation in the distance between ONUs and the OLT is G ONU 3 another drawback that causes variation in the optical power and RX consequently,the OLT receiver operates in burst mode (Hara et al.. array 2010;Yoshima et al.,2012). In order to upgrade the current TDM-PON to meet the NG- PON2 requirements,a number of approaches have been investi- gated to increase the capacity of TDM-PON,including: d BS TX .Conventional ON OFF Key (OOK)systems:Applying OOK is the easiest way to increase the capacity of TDM-PON.However,this Fig.9.WDM-PON. solution is not favorable for future PONs because it requires a 40 Gbps burst-mode receiver,high cost 40 GHz electronics and transmissions (Duan et al.,2013)and consequently degrades the photonics as well as it requires highly sensitive receivers transmission distance and the receiver sensitivity (Feng et al., (Sotiropoulos et al.,2013). 2014). Due-binary modulation:this scheme is similar to the deployed There are several schemes that can be used to mitigate RB PON system that uses one wavelength for downstream and noise,for example: another one for upstream.Invest such modulation in the downstream grants the ONUs with 20 GHz bandwidth and reduce the disruption(Nesset,2015). Using phase modulation.In (Chow and Yeh,2013)the authors Bit interleaving:This approach employs two wavelengths,one claim that the RB noise can be reduced by using Wavelength- for downstream that supports a 40 Gbps signal and another Shifted amplitude-shift keying (WS-ASK)modulation.In addi- tion,the role of phase modulation non return to zero(PM-NRZ) wavelength for upstream transmission that supports 10 Gbps. modulation format has been investigated in (Talli et al.,2008)to Bit interleaving is introduced in the downstream frame where each ONU is pre-assigned an offset and an interval.This reduce BR noise which can be farther reduced by utilizing an optical filter. technique requires the ONU receiver operating at a rate lower than 40 Gbps.It simplifies the transmission process,reduces Using dual parallel Mach-Zehnder modulator(DP-MZM) power consumption,and reduces the electronic circuitry of the Four-wave mixing(FWM). ONU receiver (Luo et al,2012). Serial 40G NRZ-40G serial Non-Return-To-Zero (NRZ):is A key advantage of WDM-PON is that it allows every ONU to another approach that has been investigated to increase the transmit at the peak speed as the OLT bandwidth is not shared capacity of legacy TDM-PON.However,it has a transmission Thus,it is capable of supporting a higher data rate(Yoshima et al.. distance limitation due to chromatic dispersion and the asso- 2012:Srivastava,2013).Another type of WDM-PON is based on ciated optical power requirement at the receiver (Srivastava splitter and known as WDM-PON wavelength switched in which 2013)】 the power splitter is implemented to distribute incoming signals equally into all ONUs.However,each ONU is required to equip 4.2.WDM-PON with a wavelength filter to select specific wavelength.Although wavelength switched PON considers simple and distributed WDM-PON has been considered as an alternative technology to structure,its signal loss is higher than wavelength routed PON TDM-PON.A typical WDM-PON structure is shown in Fig.9.It (Banerjee et al.,2005).WDM-PON is classified into two classes provides a virtual point-to-point connection between the OLT and based on the number of wavelengths supported and the wave- several ONUs;where,each ONU is assigned a different wavelength length spacing between the individual wavelengths transmitted for transmission. over a single fiber.The first class is Dense WDM(DWDM)and its The major difference between the implementation of WDM- wavelength plan is defined by ITU-T G.694.1 and the second class PON and TDM-PON is that WDM-PON employs a WDM device in is Coarse WDM(CWDM)and its wavelength plan is defined by the ODN such as an Array Wavelength Gratings(AWG)instead of a ITU-T G.694.2.The main objective of DWDM is to increase the power splitter.This leads to dramatic reduction in the power loss network capacity by minimizing the wavelength spacing:CWDM and consequently supports a large number of ONUs(Nesset,2015). aims to reduce the cost where the wavelength spacing is suffi- This type of WDM is called Wavelength routed.Each port of the ciently high to permit the transmitters to be more accurately AWG is assigned to a specific wavelength;each transmitter at the controlled (Muciaccia et al.,2014;Ragheb and Fathallah,2011). ONU transmits a signal on the wavelength that is specified by the In the literature,there are number of approaches that have port.This architecture offers lower insertion loss and a simple been proposed to be implemented in WDM-PON.The approaches ONU receiver structure.However,the OLT is required to install a are discussed below. standard receiver and a wavelength de-multiplexing device. Upstream transmission in a WDM loop back structure is 1)Externally seeded WDM-PON (Sotiropoulos et al.,2013):In a achieved by utilizing a single or two fiber link.In the case of a wavelength-splitter based ODN,a light source is splitted spec- single fiber link,bidirectional transmission of the light and the trally and distributed to reflective ONUs.This approach is modulated signal leads to Rayleigh Backscattering(RB)noise.This mature and available with the commercially existing systems. issue affects the performance of downstream and upstream However,the commercially available systems require that the

Increasing the bit rate for all of the users will be a challenging task because every ONU receiver operates at a bit rate that is higher than the bit rate assigned per ONU. Utilizing a high speed digital signal processor and field-programmable gate array to increase the bitrate to higher than 10 Gbps increases cost and complexity (Muciaccia et al., 2014; Sotiropoulos et al., 2013). In addition, TDM￾PON is not very secure due to the shared infrastructure which opens the possibility of eavesdropping and other attacks. More￾over, the variation in the distance between ONUs and the OLT is another drawback that causes variation in the optical power and consequently, the OLT receiver operates in burst mode (Hara et al., 2010; Yoshima et al., 2012). In order to upgrade the current TDM-PON to meet the NG￾PON2 requirements, a number of approaches have been investi￾gated to increase the capacity of TDM-PON, including:
Conventional ON OFF Key (OOK) systems: Applying OOK is the easiest way to increase the capacity of TDM-PON. However, this solution is not favorable for future PONs because it requires a 40 Gbps burst-mode receiver, high cost 40 GHz electronics and photonics as well as it requires highly sensitive receivers (Sotiropoulos et al., 2013).
Due-binary modulation: this scheme is similar to the deployed PON system that uses one wavelength for downstream and another one for upstream. Invest such modulation in the downstream grants the ONUs with 20 GHz bandwidth and reduce the disruption (Nesset, 2015).
Bit interleaving: This approach employs two wavelengths, one for downstream that supports a 40 Gbps signal and another wavelength for upstream transmission that supports 10 Gbps. Bit interleaving is introduced in the downstream frame where each ONU is pre-assigned an offset and an interval. This technique requires the ONU receiver operating at a rate lower than 40 Gbps. It simplifies the transmission process, reduces power consumption, and reduces the electronic circuitry of the ONU receiver (Luo et al., 2012).
Serial 40G NRZ- 40G serial Non-Return-To-Zero (NRZ): is another approach that has been investigated to increase the capacity of legacy TDM-PON. However, it has a transmission distance limitation due to chromatic dispersion and the asso￾ciated optical power requirement at the receiver (Srivastava, 2013). 4.2. WDM-PON WDM-PON has been considered as an alternative technology to TDM-PON. A typical WDM-PON structure is shown in Fig. 9. It provides a virtual point–to-point connection between the OLT and several ONUs; where, each ONU is assigned a different wavelength for transmission. The major difference between the implementation of WDM￾PON and TDM-PON is that WDM-PON employs a WDM device in the ODN such as an Array Wavelength Gratings (AWG) instead of a power splitter. This leads to dramatic reduction in the power loss and consequently supports a large number of ONUs (Nesset, 2015). This type of WDM is called Wavelength routed. Each port of the AWG is assigned to a specific wavelength; each transmitter at the ONU transmits a signal on the wavelength that is specified by the port. This architecture offers lower insertion loss and a simple ONU receiver structure. However, the OLT is required to install a standard receiver and a wavelength de-multiplexing device. Upstream transmission in a WDM loop back structure is achieved by utilizing a single or two fiber link. In the case of a single fiber link, bidirectional transmission of the light and the modulated signal leads to Rayleigh Backscattering (RB) noise. This issue affects the performance of downstream and upstream transmissions (Duan et al., 2013) and consequently degrades the transmission distance and the receiver sensitivity (Feng et al., 2014). There are several schemes that can be used to mitigate RB noise, for example:
Using phase modulation. In (Chow and Yeh, 2013) the authors claim that the RB noise can be reduced by using Wavelength￾Shifted amplitude-shift keying (WS-ASK) modulation. In addi￾tion, the role of phase modulation non return to zero (PM-NRZ) modulation format has been investigated in (Talli et al., 2008) to reduce BR noise which can be farther reduced by utilizing an optical filter.
Using dual parallel Mach-Zehnder modulator (DP-MZM)
Four-wave mixing (FWM). A key advantage of WDM-PON is that it allows every ONU to transmit at the peak speed as the OLT bandwidth is not shared. Thus, it is capable of supporting a higher data rate (Yoshima et al., 2012; Srivastava, 2013). Another type of WDM-PON is based on splitter and known as WDM-PON wavelength switched in which the power splitter is implemented to distribute incoming signals equally into all ONUs. However, each ONU is required to equip with a wavelength filter to select specific wavelength. Although wavelength switched PON considers simple and distributed structure, its signal loss is higher than wavelength routed PON (Banerjee et al., 2005).WDM-PON is classified into two classes based on the number of wavelengths supported and the wave￾length spacing between the individual wavelengths transmitted over a single fiber. The first class is Dense WDM (DWDM) and its wavelength plan is defined by ITU-T G.694.1 and the second class is Coarse WDM (CWDM) and its wavelength plan is defined by ITU-T G.694.2. The main objective of DWDM is to increase the network capacity by minimizing the wavelength spacing; CWDM aims to reduce the cost where the wavelength spacing is suffi- ciently high to permit the transmitters to be more accurately controlled (Muciaccia et al., 2014; Ragheb and Fathallah, 2011). In the literature, there are number of approaches that have been proposed to be implemented in WDM-PON. The approaches are discussed below. 1) Externally seeded WDM-PON (Sotiropoulos et al., 2013): In a wavelength-splitter based ODN, a light source is splitted spec￾trally and distributed to reflective ONUs. This approach is mature and available with the commercially existing systems. However, the commercially available systems require that the Fig. 9. WDM-PON. H.S. Abbas, M.A. Gregory / Journal of Network and Computer Applications 67 (2016) 53–74 59

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 architecture

wavelength 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