# 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
An optimum RF signal voltage level is generated by an RF gain stage consisting of an RF amplifier and a variable electrical attenuator (VEA), \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Value** & **Parameter** & **Value** \\ \hline Total no. of IFFT/FFT & & & \\ points per channel & 32 & Total samples per symbol & 40 samples (20 ns) * \\ \hline Data-carrying subcarriers & & & \\ per channel & 15 maximum & Error count period & 88,500 symbols \\ \hline \(n\)-th subcarrier frequency & \(n\)\(\times\) 31.25 MHz & EML laser wavelength & 1550 nm \\ \hline Adaptive modulation & & & \\ formats & & & \\ \hline DAC and ADC sample & & & \\ rate/resolution & 2 GS/s/8 bits & Laser bias current & 124 mA \\ \hline Symbol rate & 50 MHz & EAM bias voltage/driving & \(-\)0.7 V\({}_{\text{DC}}\)/320 mVpp \\ \hline Samples per symbol (IFFT) & 32 samples (16 ns) * & PIN detector bandwidth & 12.4 GHz \\ \hline Cyclic prefix & 8 samples (4 ns) * & PIN detector sensitivity & \(-\)19 dBm * \\ \hline \hline \end{tabular}  
* Before up-sampling and after down-sampling
* Corresponding to 10 Gb/s non-return-to-zero data at a bit error rate (BER) of 1.0 \(\times\) 10\({}^{-9}\). FFT, fast Fourier transform; IFFT, inverse FFT; QAM, quadrature amplitude modulation.  
\end{table} 
Table 1: Experimental transceiver and system parameters. the RF signal is then combined, via a bias-T, with an optimised DC bias voltage. The generated RF signal subsequently intensity modulates an electro-absorption modulated laser (EML) device which integrates a 1550 nm DFB laser (as the laser source) and a 10 GHz electro-absorption modulator (EAM). The EML produces an optical output at 2.5 dBm, which is subsequently launched into a 25 km SSMF IMDD system. At the receiver, the received optical power (ROP) level is controlled by a variable optical attenuator (VOA), before optical-electrical conversion by a 12.4 GHz PIN + TIA. The generated analogue electrical signal level is carefully optimized via an RF amplifier and VEA to occupy the full-scale range of the 8-bit, 2 GS/s ADC. An RF delay line (not shown in Figure 5) is also utilised to adjust the receiver sample timing of the ADC. The digitized samples are transferred via the receiving FPGA's deserialiser-based ADC interface before processing by the aforementioned receiver DSP, where the techniques in [18; 19] are employed for OFDM symbol synchronization and subcarrier equalization. The FPGA's embedded logic analyser is utilised for performance monitoring of the system frequency response, the individual subcarrier bit error rates (BERs), total channel BERs and subcarrier constellations. Real-time control of DSP parameters, RF signal gain, operating conditions of the EML and optical signal launch power, combined with the real-time transmission performance monitoring, allows the system performance to be rapidly optimised. It should be highlighted that both channels could be recovered simultaneously by implementing channel-dedicated instances of the matching filter and OFDM receiver function. To demonstrate the operation of the ROT, experimental results in terms of bit error rate (BER) variation with ROP are presented in Figure 6. For the BER curves plotted in Figure 6a all parameters presented in Table 1 are adopted, with 16-QAM used on all employed subcarriers and a uniform subcarrier power loading profile is applied. The first five subcarriers of the quadrature channel are dropped, due to excessive BER levels induced by (i) frequency response ripples due to the low tap count, which are more pronounced at low frequencies in the quadrature-phase channel [20] and (ii) undesirable intermixing frequency products produced by square-law photon detection in the receiver which have higher powers at lower frequencies. The resulting raw line rate is 1.875 Gb/s (1.25 Gb/s) for the in-phase (quadrature) channel, thus the aggregated raw signal line rate is over the 25 km SSMF IMDD system is 3.125 Gb/s. This demonstrates that a single sub-wavelength channel is capable of supporting multi-Gb/s capacities.