## 11 Further Research Directions and Future Considerations
### Software-Defined Modulation  
It is noteworthy that the net data rate that can be achieved will only change in coarse discrete steps when a fixed symbol rate and a fixed-rate code scheme are employed. To address the coarse discrete steps' change and realize a continuous tradeoff between the SE and reach while keeping the symbol rate fixed, a variable-rate FEC code and advanced modulation techniques can be employed [187]. Furthermore, the modulation format can be software-defined when the transmitter-site DSP and the collaborative receiver-side DSP are exploited. In this regard, the system performance can be optimized based on the link conditions. Besides the conventional square QAM, advanced modulation formats such as the 4D modulation formats and iterative polar modulation in which the four orthogonal dimensions are jointly exploited can be employed. In addition, the tolerance of the signal to nonlinear fiber transmission impairment and linear noise can be enhanced by means of an optimized 4D constellation shaping [162].  
In addition, software-defined transceivers are evolving and have been attracting applications in long-haul optical networks. Furthermore, a number of access-network-based transceiver implementations that are capable of flexible exploitation of multiplexing schemes and modulation formats have been emerging [188]. In this regard, a number of software-defined transceivers have been experimentally demonstrated for access networks. For instance, a DSP-based DMT modulation for a 100 Gb/s/\(\lambda\) IM-DD transmission over 10 km SMF by employing high-speed CMOS DAC technology has been demonstrated [189]. The demonstration demands fiber chromatic dispersion mitigation to extend the reach to 40 km. Furthermore, the benefits of software-defined OpenFlow1.0-based flex-grid \(\lambda\)-flow configuration for dynamic 150 Mb/s per-cell 4G OFDM transceivers in relation to the CPRI-based method for a high-speed, low-latency mobile backhaul were established over 20 km SSMF in [179]. The scheme can offer interoperable on-demand spectrum allocation by means of software-defined control.  
In general, different transceiver and network architectures have been developed for short-reach applications. Owing to different hardware being demanded by each architectural implementation, there may be relatively different limitations that will be imposed on the level at which each configuration can exploit the software-based advantages. For instance, since different transceiver architectures' modulation and detection techniques, as well as the offered DoF are diverse, some hardware implementations should be more promising than the others for short-reach-based software-defined transceiver applications [178]. In addition, the full integration of programmability and softwarization into the optical transceiver systems to exploit their intrinsic benefits demand sufficient information about the peculiarities of the related devices and elements to be programmed. Moreover, the potentials and the limitations of the associated photonic technologies have to be well known. In this context, the programmable and accessible parameters have to be understood. This will facilitate the definition of an information model to be implemented by the SDN controller for the effective configuration of the network elements and underlying devices [190]. Apart from the fact that more research attention has to be paid to reconfigurable transceivers, concerted efforts are still required to make the transceivers cost-effective enough for short-reach applications. Further information regarding the related design challenges and tradeoffs for practical optical transceiver implementations is given in [35; 161] and the references therein.