# A Review of DSP-Based Enabling Technologies for Cloud Access Networks
## 3 CAN Network Elements
### Soft-ROADMs
#### 3.2.2 Soft-ROADM Experimental Demonstrations  
The experimental system setup for the soft-ROADM add/drop operation is depicted in Figure 8, and the associated experimental system parameters are detailed in Table 2. The implemented system contains four channels, thus a single optical wavelength supports two Sub-\(\lambda\)s at different centre frequencies (baseband BB and passband PB) each containing an orthogonal I and Q channels. These four channels are defined as BB-I, BB-Q, PB-I and PB-Q. Two 2-channel transmitters (Txs) are employed, each based on a software-configurable FPGA for DSP execution. Each FPGA generates and digitally adds two of the four channels. A 2 GS/s DAC supports a total signal bandwidth of 1 GHz, therefore Sub-\(\lambda\)s are positioned at 0-0.5 GHz (BB) and at 0.5-1.0 GHz (PB). In the demonstration OFDM modulation is used on all channels, however, in theory, any signal modulation formats can be adopted. Sample timing synchronisation is achieved by delaying each RF signal appropriately via RF delay lines (DLs). To eliminate the OBI effect [23] associated with direct-detection (DD), the two employed laser sources have a 0.3 nm wavelength spacing. In Tx 1 the IM is an EML and in Tx 2 the IM is a tuneable laser source (TLS) and a Mach-Zehnder modulator (MZM). Erbium doped fibre amplifiers (EDFAs) followed by 0.8 nm OBPFs are used to equalize the optical launch powers. As shown in Figure 8, to configure the soft-ROADM add operation, the optical signals from Tx 1 and Tx 2 are passively combined in a 3 dB optical coupler (OC)-based soft-ROADM add element. The receiver (Rx) connects to the OC output port, where a variable optical attenuator (VOA) is used to adjust the ROP before O/E conversion with an integrated PIN and TIA, the generated RF signal amplitude is optimized by an RF amplifier and VEA before digitization by an ADC. The Rx FPGA then performs the required DSP functions for signal recovery. The Rx-DSP first performs MF-enabled Sub-B demultiplexing followed by OFDM demodulation to recover the PRBS data for subsequent BER analysis. To measure BERs before the add operation (BAD) both Txs are connected to the OC with their lasers on, but the FPGA's digital signal output is only enabled in the transmitter to be measured, thus the associated laser will be unmodulated. This avoids optical power fluctuations induced by variations in optical fibre alignment in the connectors. BER measurements after the add operation (AAD) are made with both digital signals enabled simultaneously. To examine the soft-ROADM add performance for different channel add combinations, two different add cases are defined; (i) Sub-\(\lambda\) Add is BB-I + BB-Q optically added to PB-I + PB-Q, and (ii) Sub-B Add is BB-Q + PB-I optically added to BB-I + PB-Q. Figure 8: Soft-ROADM add and drop operations experimental system setup. (**a**) Transmitter system for add operations; (**b**) transmitter system for drop operation; (**c**) add element; (**d**) drop element; and (**e**) receiver system. To configure the soft-ROADM to perform the after drop (ADR) operation, the transmitter shown in Figure 8b, generates the PB Sub-\(\lambda\) comprising both I and Q Sub-Bs. Here an EDFA and OBPF are not necessary as the available optical power is sufficient and power balancing is not applicable. A drop element based on an MZM is employed in combination with a manually tuned LO to generate the required drop RF signal. To drop either PB-I or PB-Q, the RF drop signal's frequency is set to 0.75 GHz (PB Sub-\(\lambda\) CF), the phase is adjusted to match that of the desired drop channel and the amplitude is optimised in order to minimize the drop channel's BER. To recover the data from the baseband shifted Sub-B, firstly a digital LPF eliminates the unwanted residual sub-bands, then OFDM decoding is applied with subcarrier conjugation and order reversal included to compensate for the drop operation-induced spectral reversal, finally, BER analysis is performed.