**1. Introduction**

Video-intensive services, such as virtual reality and immersive applications are driving the growth of data traffic at user premises in an explosive way, making access networks become a bottleneck of user quality of experience. Various optical and wireless access technologies have been investigated, including passive optical networks (PON) [1–3], cloud-radio access networks (C-RAN) [4–7], and hybrid fiber coax (HFC) networks [8, 9]. In the United States, there are more than 50 million subscribers using cable services for broadband access, which is 40% more than digital subscriber line (DSL) and fiber-to-the-home (FTTH) users [10]. Given the emergence of data over cable service interface specification (DOCSIS)

3.1, it is expected that HFC networks will continue to dominate the broadband access market in the United States, delivering fastest access speed to the broadest population.

As a fifth-generation broadband access technology, DOCSIS 3.1 specifications are being commercialized at a historically rapid pace to support ultra-high-resolution videos (4 K/8 K), mobile backhaul/fronthaul (MBH/MFH), and other emerging applications enabled by virtual reality and internet of things [10–14]. DOCSIS 3.1 specifications involve enhancement in both physical and MAC layers, which transform the physical layer signal from single-carrier QAM (SC-QAM) to orthogonal frequency division multiplexing (OFDM), for increased data rate, improved spectral efficiency, and flexible resource allocation. It provides up to 10 Gb/s downstream and 1.8 Gb/s upstream capacities to each subscriber [15, 16]. With subcarrier spacing of 25 or 50 kHz, DOCSIS 3.1 specifications support downstream channel bandwidths 24–192 MHz, and upstream channel bandwidths 6.4–96 MHz [17–19]. Moreover, higher order modulations up to 4096QAM were adopted with optional support of 8192 and 16384QAM [10, 15]. Similar to the LTE carrier aggregation in MFH networks [20, 21], DOCSIS 3.1 specifications support channel bonding to designate more than one DOCSIS channels to a single user.

The continuous envelope and high peak-to-average power ratio (PAPR) of OFDM signals, on the other hand, make them vulnerable to noise and nonlinear impairments in analog HFC networks [22–24]. Combined with demanding carrierto-noise ratio (CNR) requirements of high order modulations (>4096QAM), it is difficult to support DOCSIS 3.1 signals by legacy analog fiber links [15]. In this paper, we for the first time demonstrate the digitization of DOCSIS 3.1 signals to enable the upgrade of fiber distribution networks from analog to digital, so mature digital fiber technologies, e.g., intensity modulation/direct detection (IM/DD) and coherent optical transmission, can be exploited.

To enable digital transmission of DOCSIS 3.1 signals, a digitization interface, i.e., analog-to-digital converter (ADC), is needed in the hub to digitize the analog signals into bits, and a digital-to-analog converter (DAC) is needed in the fiber node to retrieve the analog waveforms from digital bits for the following transmission over coaxial cable plant. Different from conventional Nyquist AD/DA that uses Nyquist sampling rates, such as common public radio interface (CPRI) in MFH networks [25], which has quantization noise evenly distributed in the frequency domain and needs many quantization bits, delta-sigma ADC features high sampling rate but only a few (one or two) quantization bits, and most importantly, it utilizes a noise shaping technique to push the quantization noise out of the signal band, so that signal and noise are separated in the frequency domain, and the in-band CNR of digitized signals can be optimized [26–29]. Moreover, a simplified DAC design based on low-cost passive filters can be used in the fiber node, which filters out the desired signals, eliminates the out-of-band noise, and at the same time, retrieves the analog waveforms. In the hub, a high-speed delta-sigma ADC is shared by multiple fiber nodes; whereas in each fiber node, only a low-cost passive filter is needed to filter out the desired signal and convert it to the analog waveform. Since there are more fiber nodes than hubs, especially given the fact that fiber node number is continuing to grow due to node segmentation and fiber deep migration, replacing Nyquist DAC with a low-cost passive filter can significantly reduce the cost and complexity of fiber nodes.

Delta-sigma digitization has found wide applications in power amplifiers [30–32], RF transmitters [33–37] and receivers [38–42], visible light communications [43, 44], radio-over-fiber (RoF) [45–47], and MFH networks [48–50]. In Ref. [48-50], we first demonstrated delta-sigma digitization as a new digitization

**71**

**Figure 1.**

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

interface in MHF networks to replace CPRI, and 32 LTE carrier aggregation was demonstrated within a single-λ 10 Gb/s PON system to support 3GPP release 13. We then extended delta-sigma digitization to DOCSIS signals for HFC networks [51]. This paper is an extended version of our previous work [51] with substantial details

In this paper, we for the first time demonstrate the delta-sigma digitization of twenty 192-MHz DOCSIS 3.1 channels with 16384QAM modulation, based on a low-pass cascade resonator feedback (CRFB) delta-sigma ADC. We transmit the digitized bits over 80 km fiber by a low-cost single-λ 128-Gb/s dual-polarization (DP)-QPSK or 256-Gb/s DP-16QAM coherent fiber link. Both one-bit and two-bit delta-sigma digitization are realized and supported by the coherent QPSK/16QAM links, respectively. To facilitate its application in access networks, the coherent fiber link is built using low-cost narrowband RF amplifiers and optical modulator. More than 50 dB modulation error ratio (MER) is achieved for all 20 DOCSIS 3.1 channels, and high order modulation up to 16384QAM is demonstrated and delivered over fiber for the first time in HFC networks. The raw DOCSIS data rate is 54 Gb/s with net user information ~45 Gb/s. The bit error ratio (BER) tolerance of deltasigma digitization is also evaluated and negligible MER performance degradation

and 1.7 × 10<sup>−</sup><sup>4</sup>

This chapter is organized as follows. Section 2 discusses the operation principles

The architecture of a HFC network is shown in **Figure 1**. Due to their similarity, a C-RAN architecture is also presented for comparison. In **Figure 1(b)**, the network segment from service gateway (S-GW) or mobile management entity (MME) to

of delta-sigma digitization. Section 3 presents the experimental setup. Section 4 shows the design of delta-sigma ADC. The experimental results are shown in

, for one-bit and two-bit

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

is observed for BER values up to 1.5 × 10<sup>−</sup><sup>6</sup>

Section 5. Section 5 concludes the paper.

*Architecture of HFC network and C-RAN: (a) HFC network and (b) C-RAN.*

digitization, respectively.

**2. Operation principles**

and new results.

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

interface in MHF networks to replace CPRI, and 32 LTE carrier aggregation was demonstrated within a single-λ 10 Gb/s PON system to support 3GPP release 13. We then extended delta-sigma digitization to DOCSIS signals for HFC networks [51]. This paper is an extended version of our previous work [51] with substantial details and new results.

In this paper, we for the first time demonstrate the delta-sigma digitization of twenty 192-MHz DOCSIS 3.1 channels with 16384QAM modulation, based on a low-pass cascade resonator feedback (CRFB) delta-sigma ADC. We transmit the digitized bits over 80 km fiber by a low-cost single-λ 128-Gb/s dual-polarization (DP)-QPSK or 256-Gb/s DP-16QAM coherent fiber link. Both one-bit and two-bit delta-sigma digitization are realized and supported by the coherent QPSK/16QAM links, respectively. To facilitate its application in access networks, the coherent fiber link is built using low-cost narrowband RF amplifiers and optical modulator. More than 50 dB modulation error ratio (MER) is achieved for all 20 DOCSIS 3.1 channels, and high order modulation up to 16384QAM is demonstrated and delivered over fiber for the first time in HFC networks. The raw DOCSIS data rate is 54 Gb/s with net user information ~45 Gb/s. The bit error ratio (BER) tolerance of deltasigma digitization is also evaluated and negligible MER performance degradation is observed for BER values up to 1.5 × 10<sup>−</sup><sup>6</sup> and 1.7 × 10<sup>−</sup><sup>4</sup> , for one-bit and two-bit digitization, respectively.

This chapter is organized as follows. Section 2 discusses the operation principles of delta-sigma digitization. Section 3 presents the experimental setup. Section 4 shows the design of delta-sigma ADC. The experimental results are shown in Section 5. Section 5 concludes the paper.
