# A Review of DSP-Based Enabling Technologies for Cloud Access Networks
## 3 CAN Network Elements
### Reconfigurable Optical Transceivers
#### 3.1.2 DSP Architecture and Real-Time Experimental Demonstrations
In comparison with the quadrature channel the in-phase channel achieves both a higher transmission capacity and a lower ROP at the adopted FEC limit of 1E-3, this is attributed to the fact that the in-phase channel has an inherently flat frequency response compared to the quadrature channel [20]. Figure 5: Reconfigurable optical transceiver real-time experimental system setup. FEA, fixed electrical attenuator; PIN + TIA, integrated photodetector and transimpedance amplifier. To explore the effect of the transceiver's reconfigurability on system BER performances, Figure 6a also shows the BER performance for 25 km SSMF transmission of each channel whilst the other channel is disabled in the digital domain whilst preserving all other transceiver and system parameter settings. Figure 6a shows that, at the adopted FEC limit, the channel multiplexing and demultiplexing operation results in an increase of \(\sim\)2 dB (\(\sim\)3 dB) in ROP for the quadrature (in-phase) channel. This ROP penalty is due to cross-channel interference (CCI) effects [21], however it is possible to eliminate this penalty by applying adaptive bit loading, although this incurs \(\sim\)25% reduction in the maximum channel transmission capacities that can be achieved. To significantly reduce the CCI between orthogonal channels and so improve channel transmission performance, a DSP-based CCI cancellation (CCIC) technique has been demonstrated [21], under certain conditions the technique achieved a reduction in individual OFDM subcarrier BERs of more than 1000 times, an increase in transmission capacity by as much as 19 times and an increase in optical power budget by as much as 3.5 dB. To explore the impact of the OFDM modulation's adaptive bit loading feature, the BER versus ROP performance is plotted for each individual channel in Figure 6b with the transceiver parameters and system operating conditions identical to those employed in Figure 6a, except that now the capacity is maximised by applying adaptive bit loading (selecting subcarrier modulation from 16-QAM, 32-QAM or 64-QAM). In carrying out adaptive bit loading, the highest possible signal modulation format is selected first on the highest possible frequency subcarriers (as they exhibit better SNR) to maximise total transmission capacity whilst maintaining the total BER below the FEC limit of 1E-3. Figure 6b shows that the adaptive bit loading cannot only improve channel transmission capacity, but also reduce the associated power penalty. The optimized bit loading achieves a channel capacity of 2.03 Gb/s for the in-phase channel and 1.41 Gb/s for the quadrature channel with an aggregated capacity of 3.44 Gb/s for the entire 25 km SSMF IMDD system. Therefore, the aggregated transmission capacity has been increased by \(\sim\)10%, due to the adaptive bit loading. This demonstrates that it is possible to exploit adaptive modulation to maximize transmission performance in a transmission system incorporating impairments associated with practical digital orthogonal filter implementations. Figure 6: BER vs. received optical power. (**a**) Identical modulation of 16-QAM on all subcarriers of each channel. (**b**) Adaptive subcarrier bit loading applied on each channel. (Channel 1: In-phase, Channel 2: Quadrature).