## 4 Network Layer: The Main Building Block of Fixed Networks
### _Ofdm-Pon_
Next, when OBE is used, WDM-OFDM-PON attains a greater performance than Nyquist-WDM-PON under the two cases, especially when the laser-linewidth is small. WDM-OFDM-PON is vulnerable to phase noise due to the long OFDM symbol period, but with the aid of effective phase noise suppression method, it can attain a lower BER compared with Nyquist-WDM-PON, being a cost-effective solution for access networks. _CDMA-PON_  
Optical Code Division Multiple Access (OCDMA) exploits the generation, combination and recognition of codes which can be set in the time or in the frequency domain of the optical signal. In the CDMA-PON approach, different codes are used between each pair of ONU and OLT. After the code allocation for the individual users, the data are coded and transmitted into the pulse sequences. As a result, a significantly greater level of security can be achieved using this technology. Another potential advantage of CDM is the combination with TDM and WDM in order to enhance the overall number of supported users [52]. However, its features pose significant constraints on its wide adoption, such as the non-existence of a bandwidth allocation mechanism, the lack of maturity and lack of optical encoders and decoders for this PON technology [48]. Finally, another disadvantage of CDMA-PON is that it is currently constrained by unsolved practical issues such as high insertion losses and phase errors of the optical CDM elements [53]. The principle of operation for the CDMA-PON variant is shown in Figure 15. In this architecture, users connect via an ONU and a 1\(\times\)N optical combiner/splitter to the OLT. Encoders generate the encoded signals in the OLT in the optical domain, which are transmitted on the same medium to the optical splitter. Encoding and decoding take place in the optical domain, so OCDMA systems can support asynchronous transmission modes without the need for complex electronic devices. _Coherent-PON_  
Coherent transmission has been widely commercialized for long-haul communications, but in the access domain, it is considered a cost-ineffective solution [54, 55]. However, as the traffic demands scale over time, coherent transmission with low-cost modules seems  
Figure 14: An architectural example of OFDM-PON along with its proposed meta-MAC scheme for dynamic resource provisioning [49]. appealing for the near future, as it can greatly increase the number of supported users as their signals can be transparently multiplexed in the optical domain. Coherent-PON provides a number of well-known advantages. First, the coherent transceivers can attain an increased power budget compared with their DD counterparts, due to the improved receiver sensitivity, directly leading to an increase in both the number of users and the transmission reach. In practice, the increase in power budget using coherent transmission can be higher than the one attained with an APD or with an optical amplifier. Second, the spectral efficiency can scale flexibly due to the use of quadrature amplitude modulation (QAM) shaped signals. The use of a higher-order constellation instead of an increased number of components, directly reduces the overall costs and power consumption. Third, the channel selectivity can be finely tuned by simply adjusting the local oscillator (LO) at the receiver and the electrical filter. Fourth, if digital signal processing (DSP) is employed, the impact of various performance constraints, such as chromatic dispersion, polarization mode dispersion and various component imperfections, can be mitigated concurrently, but with an additional cost, due to the expensive ad hoc application-specific integrated circuits (ASICs) for DSP and coherent optics [52]. These advantages make coherent-PON seem highly attractive for access networks in the near future as it is fully compatible with deployed TDM-PON standards.