H.S.Abbas.MA.Gregory Joumal of Network and Computer Applications 67 (2016)53-74 61 Table 4 the partitioning process and distribute the total bandwidth over Types of OCDM codes. the subcarriers,over the timeslots or on both to different ONUs Name of Bipolar/ Coherent Codes according to their demand.For downstream transmission,each encoding Unipolar incoherent ONU recognizes its own OFDM subcarriers and/or time slots based on information obtained by the OLT's schedule.For upstream 1D Code Pulse amplitude Unipolar Incoherent -00C PC transmission,the OLT works to assemble the sub-frames coming from different ONUs to generate a complete OFDMA frame(Cvi- -HCC jetic,2012.2010). Pulse phase Bipolar Coherent -M-sequence Various benefits can be achieved by applying the OFDM mul- Spectral Unipolar Incoherent Gold code amplitude tiplexing technique.Firstly,the total cost is reduced because of the Spectral Phase Bipolar Coherent -Walsh- cost of the complex optical modulation at the OLT can be shared Hadamard Codes between the users.In addition,the ONU implements a simple and 2D Code Wavelength- Unipolar Incoherent 2-D WH/TS OOC Hopping Time inexpensive optical modulation in order to identify data for that spreading ONU.Moreover,OFDM-PON technology helps to reduce the cost by Space encoding Unipolar Incoherent 2-D Space Codes using cost-effective electronic devices instead of optical devices. 3D Code Three dimension Unipolar Incoherent Space/time/wave- encoding The overlapping characteristic of OFDM produces no interference codes length or Polarization/time/ which results in the effective utilization of the spectral resources. wavelength codes Furthermore,in comparison with other technologies,OFDM-PON provides a two dimensional bandwidth map with finer granularity. offering flexibility for assigning the bandwidth at different levels. Despite the enormous advantages of OFDM,some limitations have been identified.OFDM-PON requires complex receivers that are reliant on high speed DSP and FPGAs.Furthermore,OFDM-PON an estimation of the transmitted bit (Zahedi and Salehi,2000: is disadvantaged by noise and a high Peak Average Power Ratio Anaman and Prince,2012). (PAPR).The PAPR issue appears as a result of sinusoidal signals The performance of the OCDM network is reliant on the per- from multiple OFDM subcarriers that interfere constructively in formance of the address codes that have been designed to be orthogonal in order to reduce Multiple-Access Interference(MAl) the time domain.This generates a higher amplitude value than the and performance of the receiver structure that must successfully average amplitude value of the signal.The noise is generated as a operate in an environment including various noise sources(Sri and result of interference when multiple signals from multiple users Sundararajan,2013;Zahedi and Salehi,2000). are detected on the photodiode at the OLT.Such interference leads Various types of codes have been investigated and the codes to performance degradation (Cvijetic,2012:Cano et al..2013) and corresponding coding devices are shown in Table 4(Yin and D. Frequency offset is also a disadvantage of OFDM technique which 2007).In Table 5 a comparison of different OCDM receiver struc- occurs due to mismatch of carrier frequencies (Bindhaiq et al.. tures is presented(Zahedi and Salehi.2000). 2015) 4.4.OFDM-PON 4.5.UNI-PON OFDM-PON is considered as the most attractive system because High costs,wastage of resources are the main limitations in the of its scalability and ability to provide bit rate up to 40 Gbps per user.OFDM for NG-PON2 is used as multiplexing technique as it is existing multiplexing techniques insist researchers to think about more appropriate and effective methods.Some researchers came- spectrally efficient modulation method.OFDM technique offers flexibility on dynamic bandwidth allocation,enables multiple up with the idea of UNI-PON (Cloud-Radio Access Network services,and attains high spectral efficiency.OFDM utilizes a large Onlinel). number of orthogonal subcarriers that are closely-spaced in order In UNI-PON data manipulation is done at OLT using cloud to carry traffic.These subcarriers are modulated at a low symbol computing.The advantages of UNI-PON include access of all ser- rate utilizing conventional or advanced modulation techniques vices for all users,lower cost,and connectivity of radio remote (Muciaccia et al.,2014). units,multi-rate adjustment,and dynamic bandwidth allocation. OFDM-PON architecture is similar to the conventional PON.It In(Liu et al.2012).a physical layer adaptive algorithm is used to utilizes two different wavelengths for downstream and upstream attain multi-rate and dynamic bandwidth allocation.With the transmissions (Shaddad et al.,2014).The OLT generates multiple rapid advancement in technology the systems should be resilient orthogonal subcarriers that are assigned to different ONUs.Each to adopt future changes.Therefore,UNI-PON can be a suitable subcarrier is divided into different time slots.The OLT performs choice for future networks. Table 5 comparison of several reciever structures. Receiver structure Characteristics Passive correlation receiver Cheap.not suitable for high speed applications.high power loss Active correlation receiver Expensive,supports high speed applications. Optical hard limiter and passive correlation receiver Not suitable for high speed applications,relies on the availability of optical hard limiter Optical hard limiter and active correlation receiver Supports high speed applications,relies on the availability of an optical hard limiter Double optical hard limiter and passive correlation receiver Not suitable for high speed applications,expensive,good performance. Double optical hard limiter and active correlation receiver Supports medium to high speed applications,high power loss High speed chip detector Not suitable for high speed applications.low power lossan estimation of the transmitted bit (Zahedi and Salehi, 2000; Anaman and Prince, 2012). The performance of the OCDM network is reliant on the per￾formance of the address codes that have been designed to be orthogonal in order to reduce Multiple-Access Interference (MAI) and performance of the receiver structure that must successfully operate in an environment including various noise sources (Sri and Sundararajan, 2013; Zahedi and Salehi, 2000). Various types of codes have been investigated and the codes and corresponding coding devices are shown in Table 4 (Yin and D, 2007). In Table 5 a comparison of different OCDM receiver struc￾tures is presented (Zahedi and Salehi, 2000). 4.4. OFDM-PON OFDM-PON is considered as the most attractive system because of its scalability and ability to provide bit rate up to 40 Gbps per user. OFDM for NG-PON2 is used as multiplexing technique as it is spectrally efficient modulation method. OFDM technique offers flexibility on dynamic bandwidth allocation, enables multiple services, and attains high spectral efficiency. OFDM utilizes a large number of orthogonal subcarriers that are closely–spaced in order to carry traffic. These subcarriers are modulated at a low symbol rate utilizing conventional or advanced modulation techniques (Muciaccia et al., 2014). OFDM-PON architecture is similar to the conventional PON. It utilizes two different wavelengths for downstream and upstream transmissions (Shaddad et al., 2014). The OLT generates multiple orthogonal subcarriers that are assigned to different ONUs. Each subcarrier is divided into different time slots. The OLT performs the partitioning process and distribute the total bandwidth over the subcarriers, over the timeslots or on both to different ONUs according to their demand. For downstream transmission, each ONU recognizes its own OFDM subcarriers and/or time slots based on information obtained by the OLT's schedule. For upstream transmission, the OLT works to assemble the sub-frames coming from different ONUs to generate a complete OFDMA frame (Cvi￾jetic, 2012, 2010). Various benefits can be achieved by applying the OFDM mul￾tiplexing technique. Firstly, the total cost is reduced because of the cost of the complex optical modulation at the OLT can be shared between the users. In addition, the ONU implements a simple and inexpensive optical modulation in order to identify data for that ONU. Moreover, OFDM-PON technology helps to reduce the cost by using cost-effective electronic devices instead of optical devices. The overlapping characteristic of OFDM produces no interference which results in the effective utilization of the spectral resources. Furthermore, in comparison with other technologies, OFDM-PON provides a two dimensional bandwidth map with finer granularity, offering flexibility for assigning the bandwidth at different levels. Despite the enormous advantages of OFDM, some limitations have been identified. OFDM-PON requires complex receivers that are reliant on high speed DSP and FPGAs. Furthermore, OFDM-PON is disadvantaged by noise and a high Peak Average Power Ratio (PAPR). The PAPR issue appears as a result of sinusoidal signals from multiple OFDM subcarriers that interfere constructively in the time domain. This generates a higher amplitude value than the average amplitude value of the signal. The noise is generated as a result of interference when multiple signals from multiple users are detected on the photodiode at the OLT. Such interference leads to performance degradation (Cvijetic, 2012; Cano et al., 2013). Frequency offset is also a disadvantage of OFDM technique which occurs due to mismatch of carrier frequencies (Bindhaiq et al., 2015). 4.5. UNI-PON High costs, wastage of resources are the main limitations in the existing multiplexing techniques insist researchers to think about more appropriate and effective methods. Some researchers came￾up with the idea of UNI-PON (Cloud-Radio Access Network [Online]). In UNI-PON data manipulation is done at OLT using cloud computing. The advantages of UNI-PON include access of all ser￾vices for all users, lower cost, and connectivity of radio remote units, multi-rate adjustment, and dynamic bandwidth allocation. In (Liu et al., 2012), a physical layer adaptive algorithm is used to attain multi-rate and dynamic bandwidth allocation. With the rapid advancement in technology the systems should be resilient to adopt future changes. Therefore, UNI-PON can be a suitable choice for future networks. Table 4 Types of OCDM codes. Name of encoding Bipolar/ Unipolar Coherent/ incoherent Codes 1D Code Pulse amplitude Unipolar Incoherent – OOC – PC – QCC – HCC Pulse phase Bipolar Coherent – M-sequence Spectral amplitude Unipolar Incoherent – Gold code Spectral Phase Bipolar Coherent – Walsh￾Hadamard Codes 2D Code Wavelength￾Hopping Time spreading Unipolar Incoherent 2-D WH/TS OOC Space encoding Unipolar Incoherent 2-D Space Codes 3D Code Three dimension encoding Unipolar codes Incoherent Space/time/ wave￾length or Polarization/time/ wavelength codes Table 5 comparison of several reciever structures. Receiver structure Characteristics Passive correlation receiver Cheap, not suitable for high speed applications , high power loss Active correlation receiver Expensive, supports high speed applications. Optical hard limiter and passive correlation receiver Not suitable for high speed applications, relies on the availability of optical hard limiter Optical hard limiter and active correlation receiver Supports high speed applications, relies on the availability of an optical hard limiter Double optical hard limiter and passive correlation receiver Not suitable for high speed applications, expensive, good performance. Double optical hard limiter and active correlation receiver Supports medium to high speed applications, high power loss High speed chip detector Not suitable for high speed applications, low power loss H.S. Abbas, M.A. Gregory / Journal of Network and Computer Applications 67 (2016) 53–74 61