**2.2 BDF/BDR**

Upgrading fiber distribution networks from analog to digital offers the opportunity to leverage the existing mature digital access technologies. Due to the wide

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**2.4 Delta-sigma digitization**

*Delta-Sigma Digitization and Optical Coherent Transmission of DOCSIS 3.1 Signals in Hybrid…*

**Figure 2(b)** shows a digital fiber link based on BDF/BDR architecture, where a Nyquist ADC is inserted in the hub with 2.5 oversampling ratio and 12 quantization bits. At fiber node, a Nyquist DAC retrieves the analog signals, and feeds them to the coaxial cable plant. Same as CPRI, BDF/BDR provides a simple, low-cost, and servicetransparent digitization interface, but with low spectral efficiency. Similar to CPRI, the digitized bits of BDF/BDR are not encapsulated into Ethernet packets but framed by time-division-multiplexing (TDM), so it always runs at full data rate even without any real payload. This makes traffic engineering based on statistical multiplexing impossible. In today's HFC networks, there is only upstream BDR deployed, but no downstream BDF, and the BDR specifications are vendor proprietary and not interoperable.

Digital fiber link based on remote PHY architecture is shown in **Figure 2(c)**, where the PHY chips for OFDM/QAM modulation and demodulation are moved to fiber node, and an integrated converged cable access platform (CCAP) is separated into the CCAP core in hub and the remote PHY device (RPD) in fiber node [52–56]. In the downstream, payload and control bits are packetized into Ethernet packets and transmitted from hub to fiber node, where the OFDM/QAM modulators synthesize analog DOCSIS/MPEG signals for cable distribution. In the upstream, OFDM/QAM demodulators interpret the received analog signals into baseband bits and transmit back to hub in Ethernet packets. Compared with analog fiber link in **Figure 2(a)**, the RF interface in hub is replaced by an Ethernet interface, and in fiber node, there is an Ethernet interface connecting to the digital fiber and a RF interface connecting to the coaxial cable plant. With the help of Ethernet packetization, remote PHY architecture can exploit Ethernet access technologies, such as Ethernet PON (EPON), gigabit PON (GPON), and Metro Ethernet [52–54], and enable statistical multiplexing for traffic engineering. Compared with other digital solutions, remote PHY features smaller traffic load in the fiber, but with the penalty of increased complexity and cost of fiber nodes. Due to the modulation/demodulation at RPD, the fiber link in remote PHY architecture is no longer a service-transparent pipe, although it maintains the least amount of hardware exported to RPD, and preserves the compatibility with existing hubs in analog fiber links. It should be noted that the concepts of remote PHY/MAC are very similar to the function split of MFH networks [57–59], by moving partial physical and/or MAC layer functions

from the centralized entity (hub/BBU) to a remote node (fiber node/RRH).

**Figure 2(d)** shows the architecture of delta-sigma digitization. Compared with **Figure 2(b)**, Nyquist AD/DA in BDF/BDR are replaced by a delta-sigma ADC in hub and a passive filter in fiber node. Different from the Nyquist ADC with oversampling ratio of 2.5 and 12 quantization bits, delta-sigma ADC trades quantization bits for sampling rate, using high sampling rate but only a few (one or two) quantization bits. Its operation principle is shown in **Figure 3**. For reference, the operation principle of Nyquist ADC is also presented in **Figure 3(a)**. In this

deployment of Ethernet, digital fiber link features low cost, high capacity, long fiber distance, and easy setup/maintenance, as compared with its analog counterpart. Moreover, the contribution of optical noise and nonlinear impairments can be isolated from the received signal quality, so large SNR and high order modulation can be achieved. Since it can support many (>80) WDM wavelengths, digital fiber link facilitates the migration to node split and fiber deep. **Figure 2(b-d)** shows three

*DOI: http://dx.doi.org/10.5772/intechopen.82522*

different digital fiber technologies.

**2.3 Remote PHY**

#### **Figure 2.**

*Different analog/digital technologies for fiber distribution network: (a) analog fiber; (b) BDF/BDR; (c) remote PHY; (d) delta-sigma digitization.*

*Delta-Sigma Digitization and Optical Coherent Transmission of DOCSIS 3.1 Signals in Hybrid… DOI: http://dx.doi.org/10.5772/intechopen.82522*

deployment of Ethernet, digital fiber link features low cost, high capacity, long fiber distance, and easy setup/maintenance, as compared with its analog counterpart. Moreover, the contribution of optical noise and nonlinear impairments can be isolated from the received signal quality, so large SNR and high order modulation can be achieved. Since it can support many (>80) WDM wavelengths, digital fiber link facilitates the migration to node split and fiber deep. **Figure 2(b-d)** shows three different digital fiber technologies.

**Figure 2(b)** shows a digital fiber link based on BDF/BDR architecture, where a Nyquist ADC is inserted in the hub with 2.5 oversampling ratio and 12 quantization bits. At fiber node, a Nyquist DAC retrieves the analog signals, and feeds them to the coaxial cable plant. Same as CPRI, BDF/BDR provides a simple, low-cost, and servicetransparent digitization interface, but with low spectral efficiency. Similar to CPRI, the digitized bits of BDF/BDR are not encapsulated into Ethernet packets but framed by time-division-multiplexing (TDM), so it always runs at full data rate even without any real payload. This makes traffic engineering based on statistical multiplexing impossible. In today's HFC networks, there is only upstream BDR deployed, but no downstream BDF, and the BDR specifications are vendor proprietary and not interoperable.
