# Programmable Adaptive BVT for Future Optical Metro Networks Adopting SOA-Based Switching Nodes
## 2 S-BVT Architectures Tailored for Optical Metro Networks  
The S-BVT is a key element in elastic optical networks since it supports programmable functions and multi-adaptive, software-defined optical transmission. In fact, a multi-rate, multi-format, multi-reach, and multi-flow transmission is enabled on demand with sub- and super-wavelength granularity thanks to the SDN programmability and S-BVT functionalities/capabilities [6]. The slice-ability concept confers more flexibility to the BVT since it implies the capability of aggregating and distributing different flows. Furthermore, the adoption of OFDM at the DSP offers the possibility to load the subcarriers individually, according to the channel profile [3,5,6]. Hence, the spectrum can be optimized according to the traffic demand. Figure 1 shows a modular S-BVT architecture composed by N BVT modules. The multiple flows are aggregated/distributed at the output of the array of the N BVTs by an additional element using, for example, a bandwidth variable WSS, which can be implemented in Liquid Crystal on Silicon (LCoS) technology or can be a simple and cost-effective passive element, implemented for example in Planar Ligthwave Circuit (PLC) [6,20]. To design the optoelectronic front-ends for an S-BVT, we can consider simple architectures using Direct Detection (DD) receivers. Several possibilities considering the DMT modulation can be used including Single Side Band (SSB) or Vestigial Side Band (VSB) modulations [5,21] On the other hand, we can adopt more complex S-BVT using coherent detection, also combined with simple DMT transmission, as it has been demonstrated in [7]. For the experiments, we consider a single BVT. For a cost-effective solution, the BVT proposed is based on DMT, as the simplest version of the OFDM, and simple DD. Despite the simplicity of DMT, the Chromatic Dispersion (CD) can limit the system performance due to the transmission over the fiber [5]. At the increase of the fiber length, the impact of the CD is higher. As a result, various subcarriers, corresponding with certain frequencies, are highly attenuated. The frequencies were the attenuation peak appears, depends on the speed of light, the center wavelength, the fiber length, or the dispersion parameter as it is explained in Section 4. To mitigate this effect loading algorithms can be implemented to adapt the modulation format to the channel profile [5]. In DMT system, Hermitian Symmetry (HS) is forced on the input of the Inverse Fast Fourier Transform (IFFT) obtaining a real signal [5]. Thus, the complexity of the DSP module is reduced. The main building blocks of a DMT-based BVT are illustrated in Figure 1. At the Transmitter (Tx), the input data is parallelized. After that, the signal is mapped adapting loading using Binary Phase-Shift Keying (BPSK) and M-ary Quadrature Amplitude Modulation (M-QAM) or with uniform loading, using 4-QAM. According to the Signal-to-Noise Ratio (SNR) profile, estimated at the Receiver (Rx), with a uniform loaded probe signal, the DMT subcarriers are modulated at the transmitter side using BL and PL algorithms. The algorithms can be implemented under two criteria. The MarginAdaptive (MA) criterion considers fixed bit rate and energy minimization and the Rate Adaptive (RA) criterion considers a fixed energy for bit rate maximization [5; 22]. Training Symbols (TS) are added for zero-forcing equalization at the receiver side. Then, the IFFT is performed forcing the HS, the Cyclic Prefix (CP) is included, the signal is serialized (P/S) and finally it is symmetrically clipped (CLIP) [5]. After that, the digital signal is converted from digital to analog by means of a Digital to Analog Converter (DAC). The Tx optoelectronic front-end consists of an external modulator and a Tunable Laser Source (TLS), for arbitrary wavelength selection. At the Rx side, the signal is photo-detected by a DD optoelectronic front-end and converted from analog to digital by an Analog to Digital Converter (ADC). Finally, the signal is processed by the receiver DSP module.