February 1, 2019
Consumers and the pressure from 5G wireless are pushing cable TV (CATV) providers to step up their game more than in the past. The CATV industry is now on the fast track to setting new standards for the next-gen cable ecosystem, including Full Duplex (FDX) DOCSIS 3.1, which promises to deliver 10 Gbps data rates upstream and downstream on existing hybrid fiber-coaxial (HFC) systems.
But achieving FDX will require highly linear devices enabled for digital
predistortion (DPD) — including 75-ohm CATV switches. This
blog explores how DPD works in CATV optical nodes, as well as some
considerations for selecting the right switch for your application.
A distributed access architecture (DAA), along with fiber deep and remote PHY/remote MAC PHY, are moving some of the functions out of the head end and to optical nodes closer to the subscriber. However, it’s impossible to achieve 10 Gbps upstream and downstream — and remain competitive with 5G cellular infrastructure — on a DAA without designing for DOCSIS 3.1 FDX architecture.
We covered some of the RF challenges to implementing Full Duplex DOCSIS in a previous blog. Two of the most critical elements for enabling FDX include the following:
Related Blog Post: Enabling 10 Gbps Cable Networks with Full Duplex DOCSIS 3.1
In short, the PA will need to be highly efficient, hit 76.8 dBmV composite
output power and have better ACPR
(linearity) to ultimately meet the MER specifications for FDX. Although the PA
hardware provides most of the linearity improvement, DPD makes a smaller but
At a high level, DPD models and predicts the nonlinear behavior of an amplifier and injects inverse signals at the input of the PA, thereby reducing the nonlinear behavior in the amplifier and overall current consumption. The following figure shows nonlinear characteristics of a PA without and with DPD.
For a CATV optical node, the digital-to-analog converter (DAC) circuit in the node uses software to measure each of the PA outputs through the coupler to determine which PA has the worst linearity. The circuit then calculates a DPD algorithm based on the worst measurement and sends the correction downstream to all the PAs. The worst device is corrected the most, and the end result is multiple devices that run better than without the DPD algorithm.
In a typical 4-port optical node, the RF amplifiers in the chain consume
85 watts, of which 72 watts is from the last power doubler PA.
Using DPD linearizes each PA and reduces overall power consumption as much
Enabling DPD in a cable optical node requires 75‑ohm switches. When should you choose an SPST, SPDT or SP4T switch? It’s all about the geometry — the physical layout — of the node. A node in the field could have outputs on either side of the box, and it may not be able to have traces going into a single SP4T switch.
There’s no single right way to design for DPD. Ultimately, the
design approach is based on the individual customer application, layout and
preference, as well as performance requirements and cost.
Glossary of Terms
The following block diagrams show three different design approaches to implement DPD in a 4-port optical node:
In addition to the number of switches and throws, it’s important to choose the correct type of switch for these DPD node applications. Your choices are an absorptive switch or a reflective switch. Here are some of the key differences:
The choice comes down to cost versus performance. If power handling and
lower insertion loss are key to the design, pick a reflective switch. If
higher isolation is important and the design can handle the extra insertion
loss and higher costs, select an absorptive switch.
Implementing DPD will require more design resources, as well as higher
costs for additional components and switches. However, the advantages outweigh
these downsides because you get adaptive coefficients; self-calibrating,
higher-linearity PA outputs; lower current consumption — and
above all, FDX.
To learn more about our 75-ohm switches to enable DPD in optical nodes,
view the following products or our Cable TV
application information for interactive block diagrams:
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