# A Review of DSP-Based Enabling Technologies for Cloud Access Networks
## 3 CAN Network Elements  
### Reconfigurable Optical Transceivers  
#### 3.1.1 Operating Principle and Design Aspects  
The reconfigurable optical transceivers are based on DSP-embedded digital orthogonal filters implemented with finite impulse response (FIR) filters with fully software-reconfigurable filter coefficients. The ROTs can be implemented as single or multi-channel devices. For each ROT channel the transmitter DSP performs signal modulation, up-sampling, filtering by digital orthogonal shaping filters and channel summation in the case of multi-channel devices, whereas the receiver DSP performs filtering by digital orthogonal matching filters, down-sampling and signal demodulation for each of the supported channels, thus the ROT DSP implements the digital orthogonal filtering-based channel multiplexing as described in Section 2.2 and illustrated in Figure 1. As different modulation formats offer trade-offs between signal parameters, such as spectral efficiency, minimum optical signal-to-noise ratio (OSNR) and power consumption, the ROT's DSP can be designed to support multiple modulation formats and/or adaptive modulation formats for enhanced transmission performance flexibility. The ROTs can also be designed to support variable bandwidth channels by incorporating programmable up/down-sampling factors. To select a ROT's required operating channel(s), the FIR filter coefficients are simply reconfigured in the shaping (matching) filters in the transmitter (receiver) DSP, to select the desired sub-band and channel phase (I or Q). A ROT-embedded DSP controller performs filter configuration by which communicating with a centralized SDN controller responsible for managing all connectivity within the CAN.  
The ROTs incorporate a suitable DAC and IM in the transmitter side and a PD and ADC in the receiver side, along with any necessary RF gain stages, IM and PD biasing and temperature control circuitry. As the DAC/ADC will operate at multi-GS/s it is necessary to employ parallel processing in the DSP designs to achieve the required total sample throughput, as the digital integrated circuit (IC) operating speeds are of the order of several 100 s of MHz, thus parallel architecture FIR filters must be adopted.  
As the digital orthogonal filters are implemented as FIR filters the design of the filters impacts on transmission performance. Parameters, such as the selected baseband pulse [9] for the Hilbert-pair filters, tap count and sample/coefficient bit resolutions can all impact the filter characteristics in comparison to the ideal filter, therefore when designing the filters a trade-off is needed between filter complexity and performance. Trade-offs in performance and filter design complexity have been extensively investigated in [8; 9] where the effect of filter length and the \(\alpha\) parameter (which determines the filters excess of bandwidth) were analysed, the analysis allowed the identification of optimum filter design guidelines for various scenarios.  
An important characteristic of the digital orthogonal filters is that their complexity does not increase with increasing CAN channel count (all channels at minimum bandwidth) as the up-sampling and down-sampling result in inherent redundancy within the filter implementations [14].  
Sample timing synchronization is required for correct operation of the ROTs. Not only is correct sample timing critical in the ROT receiver, but it is also critical in the ROT transmitter for the case of multipoint-to-point (MPTP) networks where orthogonal sub-bands are combined in the optical domain [15]. In all of our experimental demonstrations manual sample timing alignment is employed, however investigations into the effect of sample timing offset (STO) in point-to-point and MPTP networks has been investigated [16] for different levels of channel frequency response roll-off.