## 4 Network Layer: The Main Building Block of Fixed Networks
### _Ofdm-Pon_  
Orthogonal frequency division multiple access (OFDMA) is a well-established technology in wireless and fixed access systems such as WiMAX, LTE, ADSL and Wi-Fi and can be efficiently utilized in the PON context to provide significant advantages [43, 48, 49, 50]. This technology employs several low bit rate subcarriers to carry different QAM symbols simultaneously and was originally conceived as a modulation method for media to overcome bandwidth limitations (copper, radio). When the scheme is employed to allow for OFDMA, different users are assigned to different OFDM subcarriers. The use of OFDM in optical access networks has been mainly driven by the necessity to increase transmission rates without increasing the bandwidth of optoelectronic transceivers, as opposed to focusing on link optimization. The first advantage of OFDM-PON is the finer granularity compared with TDM-PON, enabled by efficient multiplexing of orthogonal subcarriers, leading to a better spectrum utilization. The second advantage is the elasticity of spectrum allocation for each user. As traffic demands per user may vary with time, an elastic spectrum allocation is very attractive, and the OFDM-PON enables the bandwidth assignment on an as-needed basis. Third, the OFDM is a more spectrally efficient method than, e.g., NRZ. Next, OFDM can be used for both coherent and direct detection schemes. This is important, because when the coherent scheme is adopted, the overall performance can be greatly increased, and on the other hand, when a direct detection (DD) scheme is considered, low-cost components, such as directly modulated lasers (DML), vertical cavity surface emitting lasers (VCSEL) and low-bandwidth electronics along with discrete multi-tone (DMT), can be used, significantly suppressing the overall costs. Finally, OFDMA-PON enables the convergence between wireless and fixed networks, allowing it to manufacture universal components for both uses. Figure 14 illustrates an example of how OFDM-PON enables convergence. The topology consists of the OLT located at the central office (CO) and several ONUs located either at the user (residential or corporate) premises or at wireless base stations (BSs) that are used for providing connectivity to mobile terminals. One single feeder optical fiber from the OLT is used to carry all upstream and downstream signals up to a power splitter, which broadcasts them to multiple network segments. Then, additional power splitters (for FTTH) are used to broadcast the signals to all ONUs of each segment. Alternatively, some of those fibers can directly lead to wireless antennas or switches with DSL interfaces (FTTB case). Therefore, the OLT manages the assignment of traffic to a large number of subcarriers, which travel all along the Optical Distribution Network (ODN), to be demultiplexed only at the user side and vice versa. The subcarriers are grouped to form FDM channels (with several tens or hundreds of subcarriers contained within one channel), each of them used for carrying traffic, possibly by different providers and employing different technologies (e.g., GPON, EPON, but also wireless or copper-based ones). In summary, the CO can generally be thought of as a PON OLT but combining the additional functionalities of a DSLAM or a wireless BS. Figure 13: Network architecture of a Nyquist-WDM-PON [47]. Regarding the physical layer performance of Nyquist-WDM-PON and WDM-OFDM-PON, a comparative analysis can be found in [51]. This analysis shows that the physical layer performance is highly depended on the linewidth, which results in significant phase noise. In this work, two phase noise compensation methods were considered, namely, common phase error (CPE) and orthogonal basis expansion (OBE). Nyquist-WDM-PON showed a greater performance compared with WDM-OFDM-PON. This happened as the inter-carrier-interference (ICI) in WDM-OFDM-PON cannot be mitigated by CPE.