Updated August 27, 2019
Originally published in October 2015, this technical guide is designed to equip network professionals with the information they need to fully understand DOCSIS 3.1, and to leverage the standard’s capabilities to provision next-generation cable services.
Fast forward to 2019! Incognito is pleased to provide a number of updates on our DOCSIS 3.1 technical guide. Part 3 builds on DOCSIS 3.1 capacity improvements detailed in part 1 and part 2 of the series.
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Orthogonal Frequency Division Multiplexing (OFDM), the new multi-carrier transmission enabled by DOCSIS 3.1, yields a limitation termed ‘clipping’ due to non-linear amplified waveform high peaks. Digital Video Broadcasting – Terrestrial (DVB-T) employs two techniques addressing this issue: Active Constellation Extension (ACE) in the QAM carriers, and Tone Reservation (TR) reducing the peak-to-average power ratio (PAPR). TR directly cancels out peaks in the time domain using simulated impulse kernels from reserved sub-carriers in the OFDM signal. ACE reduces PAPR by extending outer constellation points in the frequency domain. As observed with forward error correction (FEC), this comes at a cost of a small average power increase and at most 1% served sub-carriers. However, these approaches complement one another, as TR is used in low-order modulation, while ACE is used in the high-order modulations available in DOCSIS 3.1. Together TR and ACE provide an approximate 2dB gain.
The next major OFDM limitation is the channel bandwidth must be greater than the modulation rate in samples per second in order to prevent intersymbol interference. Termed the orthogonal condition of spectral efficiency improvements gained solely by higher-order modulation at the cost of lower noise and nonlinearity tolerance.
Improving the DOCSIS 3.1 Standard
You can augment spectral efficiency with SEFDM (spectrally efficient frequency division multiplexing) by compressing sub-carrier spacing while maintaining the modulation rate and system performance.
Consider SEFDM as an OFDM improvement – SEFDM adopts an equivalent sub-carrier approach, while productively shrinking the spacing between adjacent sub-carriers as demonstrated in industry research where 20-25% spectral efficiency improvement is achieved with negligible SNR impact. With increasing receiver and transmitter complexity, approximately every two years the computation power per unit price doubles according to Moore’s law, so operators should plan for the necessary computing power supporting SEFDM digital multi-carrier modulation to ensure spectral efficiency optimization.
An alternative spectral efficiency approach transitions from traditional analog optics to digital optics in the HFC network fiber segment. Analog optics, utilizing optical signal amplitude modulation, has its roots in the HFC network from initial DOCSIS deployments. By moving a subset of, or the total DOCSIS physical layer (PHY) functionality to the fiber node as employed by R-PHY (remote PHY), MAC+PHY (remote MAC + PHY), D-CCAP (distributed converged cable access platform), and DAA (distributed access architecture), the optical signal then utilizes digital modulation thus eliminating:
- Impairments plaguing analog optics
- High operational costs from maintaining a well-balanced analog HFC plant
- Dependence on distance
- Noise and signal strength preventing higher-order modulation
Digital optics support higher-order modulation levels with superior noise performance, enabling cable operators to further leverage DOCSIS 3.1 capabilities. CMTS, CCAP, and DAA vendors continue to demonstrate DOCSIS 3.1 technology utilizes an even greater amount of the HFC network RF spectrum availability, well beyond what the current standards support. One approach to achieving higher throughput is R-PHY or R-MAC+PHY architecture where an operator pushes the fiber node deep enough into the HFC network to eliminate amplifiers altogether (N+0 configuration) where the RF co-axial segment becomes the last-mile drop to the home.
Brownfield vs. Greenfield
Fiber introduction is becoming the norm in cable networks as operators are facing increasing bandwidth demands, gigabit pressures, aging infrastructure, increasing OPEX, and decreasing ARPU.
Optical fiber is the most obvious choice in greenfield deployments. And for operators with both pure HFC and pure fiber networks, several technologies combine DOCSIS with fiber. One widely deployed operator approach is RF over Glass (RFoG), to leverage existing OSS provisioning systems, headend, and subscriber equipment investments. This is an extremely attractive approach while transitioning the plant to fiber. Beware – unless RFoG is combined with DOCSIS 3.1 higher modulation capabilities and larger spectrum utilization, or if leveraged for future optical node splits, the customer would essentially see the same bandwidth. RFoG solely does not provide any additional enhancements besides lower operating costs, reliability, QoS, and potential growth capacity.
Operators can rollout additional capacity using either DPoG or DPoE (DOCSIS Provisioning over GPON/EPON). Both DPoG and DPoE afford operators similar training cost savings while leveraging the existing DOCSIS provisioning OSS stack to deploy PON access technology, however, new equipment is required at both the headend and subscriber. DPoG provides approximately a 2.5Gbps downstream and 1.25Gbps upstream service, while DPoE (using 10G-EPON) can support 10Gbps downstream and 1Gbps upstream. As observed with the DOCSIS standard, PON standards continually evolve. With time wavelength division multiplexing passive optical networking, TWDM PON (also referred to NG-PON2), operators can reach 40Gbps via wavelengths that specifically avoid interference on the same fiber from GPON, 10G-PON, and RFoG. By introducing a DOCSIS overlay, the ONTs (optical network terminals) appear as cable modems in terms of provisioning and service setup.
Recent fiber advancements are shifting cable operator deployment and architecture focus. Rather than a completely new physical infrastructure and logical architecture, operators benefit from fiber launches using their existing infrastructure. In particular, R-PHY and D-CCAP enable operators to feed a remote node with fiber to distribute the signal over the existing HFC infrastructure to the home. When combined with DOCSIS 3.1, increased spectrum utilization, spectral efficiency improvements, and high order modulation, digital R-PHY provides a cost-effective means to dramatically increase network capacity while investing in a fiber backbone.
With DOCSIS continually improving reliability and bandwidth capacity in the access network, DOCSIS’s future remains bright to deliver highly immersive subscriber services and power revenues for communication service providers.
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