High Directivity Couplers Isolate Upstream/Downstream Paths in Full Duplex DOCSIS® 3.1 Systems
WeiPing Zheng, Steven Scheinkopf, Jeremy Cortez, and Brandon Kaplan, Mini-Circuits
Background: The Push toward Symmetric, Multi-Gigabit Broadband Service
Continuous growth in demand for consumer and enterprise broadband data service is driving the efforts of cable industry researchers, operators, and hardware suppliers to extend the DOCSIS 3.1 standard to offer symmetric, multi-gigabit service over existing hybrid-fiber-coax (HFC) networks. The current version of the standard enables downstream capacity of up to 10 Gbps, but upstream capacity is still essentially limited to under 1 Gbps. DOCSIS 3.1 uses Frequency Division Duplexing (FDD) to partition the allocated spectrum (5 to 1220 MHz) for upstream and downstream signal traffic. The upstream bandwidth, typically from 5 to 85 MHz places inherent limitations on data capacity for the return path, posing a barrier for emerging applications requiring higher upstream speed.
The industry has responded to this looming barrier by exploring the application of full duplex communication to the DOCSIS 3.1 standard, which would allow upstream and downstream signal traffic to utilize the same portion of spectrum at the same time, doubling the efficiency of spectrum use. Full Duplex DOCSIS 3.1 technology, once deployed over 1 GHz HFC networks, could ultimately achieve 10 Gbps capacity in both upstream and downstream directions. Since the announcement by CableLabs in 2016 of the commencement of Full Duplex DOCSIS 3.1 to an R&D phase project, CableLabs members and key suppliers have entered into a major collaboration to define the specification and develop new system architectures to achieve symmetric multi-gigabit performance over broadband networks.
The Challenge: Isolating Forward and Reverse Signal in Full Duplex DOCSIS 3.1 Systems
Because network nodes are transmitting at high signal power and receiving at low signal power over the same frequencies, one of the unique challenges to realizing working Full Duplex DOCSIS 3.1 systems is the tendency of transmit signal to leak into the receive path. If upstream signal leaks into the downstream signal chain, for example, it can get fed back through the CATV amplifier resulting in intermodulation products and other kinds of interference. The system therefore requires circuit elements that allow forward and return signal to pass in close proximity through network nodes with enough isolation to prevent leakage between the paths.
To address this problem, Mini-Circuits has partnered with broadband operators and hardware suppliers to develop broadband, high-isolation 75Ω directional couplers to prevent leakage between forward and return signal within network nodes. In the forward direction, downstream signal is injected through the coupled port onto the mainline toward subscriber premises. In the return direction, upstream signal is injected onto the mainline toward the head end. The difficulty is designing a wideband coupler with sufficient isolation between the coupled port and the in/out ports to prevent leakage of the injected signal into the opposing path.
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Figure 1: Simplified schematic of high isolation directional couplers (SYDC-8-122-75+) isolating upstream and downstream signal traffic in Full-Duplex DOCSIS 3.1 system.
System requirements call for input/output operating frequency from 5 to 1218 MHz, coupled frequency range from 40 to 780 MHz and typical isolation (In/Out – Coupled Port) of roughly 30 to 35 dB or better up to 780 MHz. Achieving this level of isolation over a frequency range this wide is challenging, but Mini-Circuits leveraged our in-house design expertise to develop model SYDC-8-122-75+, a surface-mount, transformer-type 75Ω directional coupler to support the requirements for Full Duplex DOCSIS 3.1 systems.
Test data for this coupler exhibits isolation greater than 40 dB up to 1000 MHz and greater than 30 dB up to 1220 MHz at the upper limit of the DOCSIS 3.1 application band. This design has a coupling ratio of 8.9 dB with ±0.5 dB flatness over the 5 to 750 MHz range and ±0.9 dB flatness over the 5 to 1218 MHz range. It provides 1W RF input power handling, 1.4 dB typical mainline loss, and 18 dB typical return loss (input/output/coupling). It comes housed in a miniature plastic package (0.38 x 0.50 x 0.25”) mounted on printed wiring laminate base with a wraparound terminations for excellent solderability.
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Figure 2: SYDC-8-122-75+ case style.
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Figure 3: Curves of measured test data for SYDC-8-122-75+ for Coupling, Isolation and Directivity, swept over DC to 1500 MHz.
Its exceptional isolation of over wide frequency range, flat coupling and combination of other performance characteristics, make the SYDC-8-122-75+ an ideal candidate for isolating upstream and downstream paths in Full Duplex DOCSIS 3.1 systems.
Conclusion: One Piece of a Complex Puzzle
Full Duplex DOCSIS 3.1 presents a number of new challenges that cable operators and suppliers are working diligently to overcome. Isolating the forward and return signal paths at the node is one of the key challenges unique to full duplex communication, requiring innovative problem solving at the component level. Mini-Circuits’ SYDC-8-122-75+ enables system architects to effectively isolate forward and return signals at the same frequency and prevent signal leakage that would otherwise lead to systemic problems.
The successful implementation of Full Duplex DOCSIS 3.1 ultimately depends on a wide and complex variety of factors. Mini-Circuits is proud to contribute one piece in the puzzle and play a role in the collaborative efforts of the broadband industry to bring the market faster connectivity. The SYDC-8-122-75+ high-directivity directional coupler is an example of our ability to respond to challenging, application-specific system requirements with technical expertise, flexibility and fast turnaround.
 Hamzeh, B. (2016, February 16). Full Duplex DOCSIS® 3.1 Technology: Raising the Ante with Symmetric Gigabit Service. From http://www.cablelabs.com/full-duplex-docsis-3-1-technology-raising-the-ante-with-symmetric-gigabit-service/
 Hamzeh, B. (2016, September 20). Full Duplex DOCSIS® 3.1 Specification Effort Launches. From http://www.cablelabs.com/full-duplex-docsis-3-1-specification-effort-launches
 Shulman, S. and Kurtz, B. (2017). Full Duplex DOCSIS® 3.1 Evolves Networks with Faster Speeds. From https://www.intel.com/content/www/us/en/smart-home/connected-home/full-duplex-docsis-cable-paper.html