# A Review of DSP-Based Enabling Technologies for Cloud Access Networks
## 2 Cloud Access Network Concept
### Digital Orthogonal Filtering-Based Channel Multiplexing  
The CAN concept is built upon the principle of DSP-based channel multiplexing using digital orthogonal filters. As channel multiplexing and demultiplexing are performed in the digital domain [9], this allows great flexibility in network connectivity due to the intrinsic versatility of DSP circuits. Furthermore, low-cost implementation is achieved by avoiding expensive RF and optical components and leveraging the economies of scale associated with mass produced digital integrated circuits (ICs). Figure 1 illustrates the basic principle of channel multiplexing/demultiplexing by digital orthogonal filtering, where reconfigurable digital orthogonal filters embedded in digital ICs within the optical transceivers are used to multiplex/demultiplex N bandwidth-variable channels. For each channel, an independently modulated signal is first up-sampled via the insertion of M-1 zeros between two consecutive samples, thus the up-sampling factor is M, denoted as (M\(\uparrow\)). The up-sampling generates M/2 double-sideband spectral images within the Nyquist frequency band, the up-sampling factor, therefore, allows control of the generated channel bandwidth (bandwidth = sampling frequency/M). Subsequently, a digital shaping filter (SF) is employed to select the appropriate spectral image to locate the signal at the desired spectral location, that is, in the desired sub-wavelength channel. By using Hilbert-pair filters, two signals can be co-located in the same sub-wavelength channel. The Hilbert-pair filters generate orthogonal channels as the In-phase (I) filter applies a \(0^{\circ}\) phase shift to all frequency components, whereas the Quadrature-phase (Q) filter applies a \(-90^{\circ}\) phase shift to all frequency components, thus two orthogonal sub-band channels occupy the same sub-wavelength channel. The two orthogonal channels are thus denoted as the I (In-phase) and Q (Quadrature-phase) channels. The generated channels are all summed together in the digital domain and fed to a single DAC. The resulting analogue electrical signal then drives a suitably biased optical intensity modulator (IM) and the optical signal is launched into an intensity modulation and direct-detection (IMDD) network employing standard single mode fibre (SSMF). After transmission through the network, in the receiver a photo detector (PD) converts the optical signal to an electrical for digitisation by an ADC, the digital signal is then fed to one or more DSP-based receivers, each dedicated to recovering a specific channel. In the DSP receivers the digital signal is first filtered by the appropriate digital matching filter (MF) to demultiplex the desired channel from the aggregated signal, subsequent down-sampling by a factor of M, denoted as (M\(\downarrow\)), is performed by selecting every \(M\)-th sample, which returns the signal to baseband at the original sample rate. Sample timing alignment is necessary for the receiver (not shown in Figure 1) to ensure correct filter operation. Finally, the received signal from each channel is demodulated according to the adopted signal modulation to recover the encoded binary data. Full details of the theory underpinning the digital orthogonal filer-based multiplexing can be found in [9; 10]. Figure 2a,b show example spectra of different combinations of digital orthogonal filter multiplexed channels for the cases of equal and varying channel bandwidths respectively.