# Future Scenarios for Software-Defined Metro and Access Networks and Software-Defined Photonics
## 3 Solutions for Software Defined Metro and Access Networks
### Physical Layer Re-Configurability
#### 3.2.1 Flexible Transceivers  
In this context, flexible transceivers are reconfigurable transmitters and receivers installed on OLTs and ONUs. Their parameters should be varied in real-time: bitrate, launch power, modulation format and order, coding technique, optical carrier wavelength, forward error correction (FEC) payload ratio, multiplexing scheme, and number of subcarriers [10]. In order to realize flexible transceivers, it is necessary to work on laser sources, external optical modulators, driver electronic circuits, and digital signal processing (DSP) techniques.  
Flexible transceivers, also called software-defined transceivers (SDTs), offer flexibility, increased resilience to channel impairments, and an upgrade path for future transmission systems, as they allow data rate adaptation and channel bandwidths allocation on the fly. Being more mature in recent years, they are now going to be used in long-haul optical communications but they are expected to have great importance also in future re-configurable access networks [53].  
[PERSON] et al. [31] have demonstrated that software-defined coherent transponders, which digitally processes the burst transmissions according to the distance of a user from the OLT, can double the average transmission capacity per user.  
[PERSON] et al. [54] have developed flexible bandwidth variable transceivers (BVTs). The transmitters are composed by a tunable laser diode and a combination of dual-drive Mach-Zehnder modulator (MZM) and dual-parallel MZM with electrical binary drive signals and a polarization-division-multiplexing (PDM) synthesizer. They are capable of dynamically changing the symbol rate and the modulation format among binary phase-shift-keying (BPSK), quadrature phase-shift-keying (QPSK), 8-ary quadrature amplitude modulation (8 QAM), and 16-ary quadrature amplitude modulation (16 QAM). The receivers are based on a coherent detection scheme dual-polarization 90\({}^{\circ}\) optical hybrid followed by four balanced photodetectors (BPDs) and analog-to-digital converters (ADCs). They are capable not only of detecting the optical signal with any modulation format/rate, but also to send the bit error ratio (BER) information to an OpenFlow controller to automatically change the network parameters.  
Several DSP-enabled flexible transceivers for optical access network have been proposed by [PERSON] et al. [32] in synergy with OLT side access network resource virtualization and software-defined programmable network functions and resource scheduling. One of them uses QPSK, 16 QAM, 32 QAM, or 64 QAM modulations for achieving different optical power budgets of 36, 32, 30, or 28 dB, respectively [32].  
[PERSON] et al. [55] have proposed and demonstrated BVTs for PONs. If compared to the previous proposal by [PERSON] et al. and [PERSON] et al., the approach is obviously simpler: the available modulation schemes are only on-off keying (OOK) and quadrature amplitude modulation (QAM), which are simultaneously sent from the OLT and filtered by the ONUs.  
A recent detailed analysis about the feasibility of elastic transceivers in optical metro rings also in terms of spectrum occupation has been carried on by [PERSON] et al. [33]. In particular, the impact of grid flexibility and of the usage of distance-adaptive modulation techniques is analyzed and significant gains in terms of spectral occupation and number of transceivers have been demonstrated.  
Other proposals aim to obtain increased flexibility by replacing single-carrier with multi-carrier multiplexing schemes, e.g., orthogonal frequency division multiplexing (OFDM) and Nyquist frequency division multiplexing (NFDM), where optical sub-carriers can be dynamically allocated and eventually grouped together in super-channels [53; 54; 55; 56].