**2.3 Remote PHY**

*Fiber Optics - From Fundamentals to Industrial Applications*

their analog waveforms via air interface.

**2.1 Analog fiber**

**2.2 BDF/BDR**

bi-annual calibration of fiber nodes.

*(c) remote PHY; (d) delta-sigma digitization.*

baseband unit (BBU) is defined as mobile backhaul (MBH), which transmits the control and payload bits of LTE signals in digital baseband. The digital bits are received by BBUs, which synthesizes OFDM modulation and generates analog waveform of LTE signals. The network segment from BBU to remote radio heads (RRH) is defined as mobile fronthaul (MFH), where the LTE signals are transmitted over fiber in either analog waveform using RoF technology or digital waveform using CPRI digitization interface. The last segment of C-RAN involves the wireless transmission from the RRHs to mobile users, where LTE signals are transmitted in

Similarly, HFC networks in **Figure 1(a)** can also be divided into three segments, i.e., the core network segment from headend to hub, the fiber distribution network from hub to fiber nodes, and the coaxial cable plant from fiber nodes to cable

modems (CMs). Similar to MBH, the segment from headend to hub transmits net bit information; similar to MFH, the fiber distribution segment from hub to fiber node is supported by either analog or digital fiber technologies, e.g., C-RAN uses RoF technology to deliver analog mobile signals; HFC uses analog fiber links to deliver analog DOCSIS/video signals; C-RAN uses CPRI as a Nyquist digitization interface; HFC has a similar interface called baseband digital forward or return (BDF/BDR). The last segment from fiber node to CMs is also similar to the wireless segment of C-RAN, where both DOCSIS and LTE signals are transmitted in their analog waveform over coaxial cable or air interface, respectively. Different analog or digital

**Figure 2(a)** shows the architecture of an analog fiber link. DOCSIS and video signals are aggregated in the hub and delivered to the fiber node in analog waveforms. Then at the fiber node, the received analog signals are delivered to CMs via cable distribution networks. An analog fiber link features simple, low-cost implementation, and high spectral efficiency, but imposes high linearity requirements on channel response. Since it does not perform any data reformation, an analog fiber link is a waveform/service agnostic pipe, and can be used for various services, e.g., DOCSIS, MPEG, and analog TV. On the other hand, it suffers from noise and nonlinear impairments, limited signal-to-noise ratio (SNR), short fiber distance, and small number of WDM wavelengths. It also requires complex RF amplifiers and

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

*Different analog/digital technologies for fiber distribution network: (a) analog fiber; (b) BDF/BDR;* 

implementations of the fiber distribution segment are shown in **Figure 2**.

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**Figure 2.**

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).

## **2.4 Delta-sigma digitization**

**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

paper, we designed a delta-sigma ADC to digitize five DOCSIS 3.1 channels with channel bandwidth of 192 MHz and total frequency range from 258 to 1218 MHz (5 × 192 = 960 MHz). Due to the limited quantization bits, Nyquist sampling rate will lead to significant quantization noise (**Figure 3b**). Oversampling is utilized to extend Nyquist zone and spread the quantization noise over a wide frequency range, so the in-band noise is reduced (**Figure 3c**).

Furthermore, noise shaping technique is used to push the quantization noise out of the signal band, which acts as a high-pass filter (HPF) to the quantization noise and separates the signal and noise in the frequency domain, as shown in **Figure 3(d)**. It should be noted that during the delta-sigma digitization, the signal spectrum is kept intact; it is the out-of-band quantization noise added by the deltasigma ADC that converts the signal waveform from analog to digital. Therefore, in **Figure 3(e)** at the receiver side, when the out-of-band quantization noise is eliminated, the signal waveform will automatically be converted back from digital to analog, i.e., a passive filter can not only filter out the desired signal channel, but also realize the digital-to-analog conversion. In HFC networks, a high-speed delta-sigma ADC is centralized in the hub and shared by many fiber nodes, and each fiber node only needs a low-cost passive filter to select the desired DOCSIS channels and at the same time convert them to the analog waveform. Given the fact that there are many more fiber nodes than hubs/headends, this design significantly reduces the cost and complexity of fiber nodes. It should be note that Nyquist ADC has evenly distributed quantization noise, whereas delta-sigma digitization has a shaped distribution of quantization noise, so the retrieved analog signal has an uneven noise floor.

In time domain, if we use an analog sinusoidal signal as an example, a Nyquist ADC samples the analog input with Nyquist rate and each sample is quantized individually (**Figure 3a**); whereas delta-sigma ADC samples the analog input at a much higher rate and the samples are digitized consecutively (**Figure 3c** and **d**), i.e., the current digitization bits not only depend on the current analog input, but also depend on previous input. One-bit delta-sigma digitization outputs a high data rate on-off keying (OOK) signal with the density of "1" bits being proportional to the amplitude of analog input. For maximum input, it outputs continuous "1"s; for minimum input, it outputs continuous "0"s. For intermediate inputs, the densities of "0"s and "1"s are almost equal. Two-bit digitization outputs a 4-level pulseamplitude-modulation (PAM4) signal. Both one-bit and two-bit digitization are implemented in our experiment.

**Figure 3.**

*Operation principle of Nyquist ADC and delta-sigma ADC: (a) Nyquist ADC and (b-e) delta-sigma ADC.*

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**Analog** Linear optics

Interface Operation principles

AD/DA

Pros Cons

Short distance

Small capacity

Limited by noise/nonlinearity

Few WDM wavelengths

**Table 1.**

*Comparison of analog/digital fiber technologies in HFC networks.*

Simple High spectral efficiency

fiber deep migration

No modification of bits

Waveform agnostic

Simple, low-cost AD/DA

Low spectral efficiency

No Ethernet encapsulation

Always run at full data rate

Vendor proprietary

Waveform agnostic

N/A

2.5 oversampling ratio

12 quantization bits

N/A High capacity, Large SNR, High order modulation, Long distance, Scalability, Many WDM wavelengths, facilitate node split and

Ethernet packet encapsulation

No modification of bits

Waveform agnostic

Low-cost DA based on passive filters

High cost delta-sigma ADC

Statistical multiplexing

Reduced traffic load

Modification of bits

Not service transparent

Increased complexity/cost of fiber nodes

Analog RF over fiber

**Digital** BDF/BDR Nyquist AD/DA in hub and fiber node

Move PHY circuits of OFDM/QAM to fiber node

Transmit net bit information over fiber

High sampling rate

1–2 quantization bits

Remote PHY

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

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

Delta-sigma digitization

Delta-sigma ADC in hub

Passive filter in fiber node as DAC


*Comparison of analog/digital fiber technologies in HFC networks.*

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

*Fiber Optics - From Fundamentals to Industrial Applications*

range, so the in-band noise is reduced (**Figure 3c**).

implemented in our experiment.

paper, we designed a delta-sigma ADC to digitize five DOCSIS 3.1 channels with channel bandwidth of 192 MHz and total frequency range from 258 to 1218 MHz (5 × 192 = 960 MHz). Due to the limited quantization bits, Nyquist sampling rate will lead to significant quantization noise (**Figure 3b**). Oversampling is utilized to extend Nyquist zone and spread the quantization noise over a wide frequency

Furthermore, noise shaping technique is used to push the quantization noise out of the signal band, which acts as a high-pass filter (HPF) to the quantization noise and separates the signal and noise in the frequency domain, as shown in **Figure 3(d)**. It should be noted that during the delta-sigma digitization, the signal spectrum is kept intact; it is the out-of-band quantization noise added by the deltasigma ADC that converts the signal waveform from analog to digital. Therefore, in **Figure 3(e)** at the receiver side, when the out-of-band quantization noise is eliminated, the signal waveform will automatically be converted back from digital to analog, i.e., a passive filter can not only filter out the desired signal channel, but also realize the digital-to-analog conversion. In HFC networks, a high-speed delta-sigma ADC is centralized in the hub and shared by many fiber nodes, and each fiber node only needs a low-cost passive filter to select the desired DOCSIS channels and at the same time convert them to the analog waveform. Given the fact that there are many more fiber nodes than hubs/headends, this design significantly reduces the cost and complexity of fiber nodes. It should be note that Nyquist ADC has evenly distributed quantization noise, whereas delta-sigma digitization has a shaped distribution of quantization noise, so the retrieved analog signal has an uneven noise floor. In time domain, if we use an analog sinusoidal signal as an example, a Nyquist ADC samples the analog input with Nyquist rate and each sample is quantized individually (**Figure 3a**); whereas delta-sigma ADC samples the analog input at a much higher rate and the samples are digitized consecutively (**Figure 3c** and **d**), i.e., the current digitization bits not only depend on the current analog input, but also depend on previous input. One-bit delta-sigma digitization outputs a high data rate on-off keying (OOK) signal with the density of "1" bits being proportional to the amplitude of analog input. For maximum input, it outputs continuous "1"s; for minimum input, it outputs continuous "0"s. For intermediate inputs, the densities of "0"s and "1"s are almost equal. Two-bit digitization outputs a 4-level pulseamplitude-modulation (PAM4) signal. Both one-bit and two-bit digitization are

*Operation principle of Nyquist ADC and delta-sigma ADC: (a) Nyquist ADC and (b-e) delta-sigma ADC.*

**74**

**Figure 3.**

**Table 1** compares different analog/digital fiber technologies in HFC networks. As a waveform-agnostic interface, delta-sigma ADC works with not only OFDM signals but also 5G multicarrier waveforms, such as filter-band multicarrier signals, as we reported in [50]. Since analog fiber, BDF/BDR, and delta-sigma digitization do not modify the bit information, they are service-agnostic and can carry various and a combination of services, even though these services evolve in the future. Remote PHY, on the other hand, is not service-transparent. Its RPD in the fiber node is only designed for one specific service.
