Engineering-Wise - Get your cable networks ready for higher capacity
It’s a lot of new technology, but the SCTE will be there for you.
The race is on to develop the next-generation access signaling schemes for cable operators to keep up with Nielsen’s Law growth in network capacities and speeds delivered to customers. With the announcement of the development of a DOCSIS 3.1 specification by CableLabs during a special SCTE Standards session at Cable-Tec Expo 2012, CableLabs and the SCTE have partnered in new and deeper ways to accelerate the deployment of DOCSIS 3.1. The SCTE will be there to help you get your networks ready for the next generation of DOCSIS, and even improve your existing DOCSIS deployments in the meantime.
The key technical improvements and benefits of proposed DOCSIS 3.1 technologies are: higher-order modulation for greater spectral efficiency, a new modulation format that is both more robust and more efficient, and a new coding scheme that approaches the Shannon limit for theoretical capacity over a noise-limited communications channel. While these improvements and benefits can be implemented with no change to existing cable networks, obtaining the highest capacities offered by DOCSIS 3.1 will require cable network operations personnel to further tighten plants and eliminate sources of impairments. To achieve the ultimate goal of 10 Gbps downstream and 1 Gbps upstream capacity, these improvements will have to be combined with greater RF spectrum in both the upstream and downstream. But, for now, let’s focus on the key improvements and how to get our current networks ready for them.
How high will the order of modulation go? In Europe, a specification called DVB-C2, which proposed very similar technology to that proposed for DOCSIS 3.1, has been tested in the field to support 4096-QAM on the downstream in current cable networks. Going from 256-QAM on the downstream to 4096-QAM is a 50 percent increase in bits per symbol. But higher orders of modulation do require lower noise levels in the channel so that the modulation error ratio, or MER, is high enough to support the increased order of modulation. Thermal noise can be lowered by reducing the number of actives in the network, or by replacing older actives with newer ones that have lower noise figures. But common sources of broadband noise in the downstream channel, especially with the move to all-digital downstreams, are intermodulation distortion (IMD) products – i.e., our old friends CTB and CSO. When all of the downstream signals are digital, the slight non-linear behavior of normal cable networks causes the QAM pedestals to convolve with one another in the frequency domain and produce a raised noise floor that has dips in it every 6 MHz (U.S. systems). Those dips are the telltale signs of IMD and can be reduced using the same approaches we’ve always used to keep plants clean of IMD products.
But some noise is unavoidable, so we need more efficient coding schemes that reduce the required MER without the overhead of current FEC techniques, such as Reed-Solomon (RS) encoding. The answer is low-density parity check (LDPC) coding, which has been around since the 1960s, but until relatively recently had computing requirements that were beyond practicality. That changed in the late 2000s, when other standards, including Wi-Fi, began using LDPC coding to efficiently increase the order of modulation possible in a noise-limited channel. LDPC coding offers several dB better noise immunity than RS coding at similar channel efficiency, or greater spectral efficiency at the same MER.
Wi-Fi also uses the new modulation scheme being proposed for both the upstream and downstream of DOCSIS 3.1, namely orthogonal frequency division multiplexing, or OFDM. It is a multi-carrier modulation scheme (as opposed to single-carrier, or SC-QAM, in the current DOCSIS specification), but what is special about it is that the sub-carriers in OFDM can be squeezed right up against each other, without the need for guard bands because of their orthogonality. Freedom from 6 MHz channels means that wider RF blocks can be used, thereby eliminating guard bands and getting more bits per Hz of RF spectrum.
Similar to S-CDMA, OFDM is inherently more robust to impulse and burst noise since the sub-carriers of OFDM have much longer symbol durations than SC-QAM. Further, individual sub-carriers can use lower orders of modulation in the presence of upstream ingress, or can even be blanked without having to reduce the efficiency of other sub-carriers in the channel. Think of it as ingress cancellation on steroids, except that it can be implemented in a standardized way so that all chip implementations support it equally well.
It’s a lot of new technology, but the SCTE will be there for you. We’re already working on a new recommended practices document in a Special Working Group of our Engineering Committee that will tell you how to prepare your networks for next-generation access technologies like DOCSIS 3.1. We’re also working on updating our DOCSIS training, including breaking out the DOCSIS component of IPEP into a separate certification. And we’re working very closely with CableLabs to accelerate deployment of DOCSIS 3.1 so you get the benefits as soon as possible to keep your customers happy.
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