**2. Operation principles**

*Fiber Optics - From Fundamentals to Industrial Applications*

more than one DOCSIS channels to a single user.

coherent optical transmission, can be exploited.

population.

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

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

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

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

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

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complexity of fiber nodes.

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

**Figure 1.** *Architecture of HFC network and C-RAN: (a) HFC network and (b) C-RAN.*

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 their analog waveforms via air interface.

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