© janaka dharmasena dreamstime.com_technical Application Notes | January 20, 2021
Solving the Cable TV Infrastructure Downstream Transmitter Challenge
Driven by demand for faster internet connectivity, the cable TV industry has developed new network architectures for the delivery of multigigabit services to subscribers. This fiber deep approach, using a remote PHY device (RPD), moves critical hardware closer to the users by using digital fiber.
This is comparable with a remote radio head in wireless (cellular) networks and while this saves space and reduces heat dissipation in the headend, it creates new design challenges for remote equipment. Although lower in absolute frequency, cable TV signals have much wider bandwidths than wireless, extending across several octaves from 108 MHz to 1218 MHz, with multiple in-band harmonics. RPDs have created a perfect storm for designers, where the RF and mixed-signal hardware must cover a wider frequency range, with higher RF powers, lower noise floor, and better linearity, while consuming less dc power. The downstream final stage RF amplifiers each typically draw 18 W, and with a 4-port system, this is around 50% of the 140 W to 160 W power budget that can typically be delivered to (and dissipated by) an RPD. ADI’s cable digital predistortion (DPD) efficiency enhancement technology, applied to a DPD optimized power doubler (ADCA3992), combined with advances in high speed data converter technology allows a single DAC (such as the AD9162), and a single ADC (such as the AD9208), complemented by a highly integrated clocking solution (HMC7044) make full-band DPD a reality. This article describes the evolution to remote PHY and how Analog Devices has solved the efficiency and linearity challenge, using a proprietary DPD, with ADI’s algorithms and IP core integrated within the OEM’s existing FPGA implementation. Background Since its introduction as community access television (CATV) more than 60 years ago, cable TV has evolved from a simple unidirectional (downstream only) analog link to a complex multimode, multichannel bidirectional system (including upstream or reverse path) that supports analog TV, IP-based standard definition (SD) and high definition (HD) digital TV, and high speed data for internet download and upload. These services are provided by multiple system operators (MSOs). Cable data and digital TV services are delivered to consumers using the data over cable systems interface specification (DOCSIS), which was developed by CableLabs and contributing companies. There have been multiple evolutions in the configuration of the headend (cable modem termination system or CMTS), including the addition of EdgeQAM modulators either as a separate unit or integrated with the CMTS as part of a converged cable access platform (CCAP). The demand for downstream data capacity is now increasing at a compound annual growth rate (CAGR) of around 50%, meaning that demand doubles roughly every 21 months.1 To meet this demand, since the release of DOCSIS 1.0 in 1997, downstream data rates have increased from 40 Mbps through to 1.2 Gbps (with the widely deployed DOCSIS 3.0 implementation). These downstream speed increases were implemented through the combination of multiple techniques including channel bonding, more complex modulation (moving from 64 QAM to 256 QAM) and higher downstream upper frequency limit (from 550 MHz to 750 MHz to 1002 MHz). In the United States, all of this was implemented while retaining the 6 MHz channel plan from the legacy analog TV service (8 MHz for EuroDOCSIS and C-DOCSIS), but to support downstream rates up to 10 Gbps, it became necessary to make changes that are more fundamental, and in 2013 the DOCSIS 3.1 standard was published. While it maintains support for legacy standards, DOCSIS 3.1 uses the more spectrally efficient orthogonal frequency division multiplexing (OFDM) technique, with channel bandwidths of up to 190 MHz supporting up to 4096 QAM. Additionally, the upper frequency limit of the downstream frequency range was increased by more than 20% to 1218 MHz, with an option for extending to 1794 MHz. One thing that has not changed over time is the use of a coaxial cable with 75 Ω impedance for the physical link to subscribers’ cable modems. Prior to the 1990s, systems used 100% coaxial cable between the headend and the subscriber, but most current deployments are hybrid fiber copper (HFC). In HFC, an analog electrical-to-optical converter is connected to the headend’s coaxial output; the signal is then transferred to a node close to the service area using fiber, and then passed through an optical-to-electrical converter for final distribution to subscribers over coaxial cable. This last-mile connection to the subscriber, with overhead or underground cable, has become a bottleneck in the system, but upgrading to a fiber to the home (FTTH) link is very expensive and disruptive, and cable MSOs are determined to make the most of their existing coaxial cable assets. Compared to twisted pair telephone cable, coaxial cable presents a relatively benign environment, with inherent shielding from interference or crosstalk, and modest levels of signal reflections due to impedance mismatch. However, with a typical distance from the node to the most distant subscriber of up to 1200 feet, the frequency dependent loss characteristics are significant (there is a slope of almost 17 dB between 108 MHz and 1002 MHz), requiring pre-emphasis or tilt, implemented by RF filter inserts with a high-pass response. In a typical HFC deployment, as shown in Figure 1, a single trunk coaxial cable connected from the optical node feeds several hundred subscribers, with multiway RF splitters to distribute the signal to subgroups and taps to connect drop cables to the individual subscribers. In a typical node + n system, wideband booster amplifiers are inserted into the network at regular intervals to increase the signal level to ensure adequate signal-to-noise ratio (SNR) at the cable modem. Figure 1. Cable TV deployment with HFC. Providing Increased Data Capacity to Subscribers The available data bandwidth on a DOCSIS trunk cable is shared between all the connected users, and there are two options for providing more bandwidth to all users:
- Increase the data rate passed through the cable
- Reduce the number of users connected to the cable
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