"Upstream" transport is a critical element of cable systems to support the fast-growing needs of voice and high-speed data applications. Today's return path traffic has evolved to QPSK for limited set-top box interactivity, 16 QAM for DOCSIS^® 1.x, and 64 QAM for DOCSIS 2.0 and 3.0 (with channel bonding techniques in the latter that permit combining of several channels for yet higher speed data applications). Indeed, future expansion to 256 QAM is not unrealistic.

In the late 1990s Aurora developed its digital return path technology as a tool to overcome the shortcomings of analog return path systems. Aurora's digital return technology uses two different digital transport concepts:

1. A single RF return is digitized and combined with several Fast Ethernet channels for optical transport over a single wavelength. This concept supports cascading of nodes in the return path (similar to return path RF summation). This "distributed" return path combining, most popular in Fiber Deep applications, simplifies the return path architecture by saving fiber and/or wavelengths.


2. Two RF returns are independently digitized and time multiplexed for optical transport over a single wavelength. They are digitally demultiplexed at the upstream receiver, resulting in the original two independent return RF signals. However, in this case, with the increased data (two digitized RF channels), there is no payload availability for Ethernet channels.

The advantages of Aurora's digital return when compared to analog return systems include:

1. Improved NPR/BER performance for all distances that is independent of link budget.


2. Supports much higher return link budgets — any system designed for the forward path will always automatically work in the return path (reaches up to 200 km without regeneration, and with optical amplification if required). Additionally, this very high return link budget enables an optical output splitter to be used for return path route redundancy, eliminating the need for a separate transmitter for redundancy.


3. Superior thermal stability, with very stable receiver output levels and NPR/BER performance over the full temperature range.


4. Greater ingress resistance even for full return path loading, providing higher margins against plant performance degradation, improving network availability and reducing maintenance costs. Integrated ingress location/blocking is aided via a wide range attenuator in the digital return receiver.


5. Advanced modulation techniques supported — even up to 256 QAM. (Aurora's "2-fer" technology supports 40 dB CNR for a full 37 MHz load while maintaining a typical 12-13 dB dynamic range.) This provides future proofing to support DOCSIS^® 3.0 with 256 QAM modulation levels and channel bonding even assuming that full return path channel loading becomes the standard rather than the exception.


6. Easy plug-and-play installation with all node return paths set up the same, given the constant gain from the node input to the return receiver output, independent of distance. There is no need for a pilot generator setup of the return path. This results in a lower initial alignment cost and practically no operating alignment costs related to the optical link.


7. Integrated SNMP-compliant monitoring/management without the additional expense of node transponders.


8. Convenience and flexibility of transmit plug-in SFPs, including 1310 nm, 1550 nm, and all CWDM wavelengths.


9. Support for additional revenue streams from the embedded Ethernet service capability.

Aurora is the leading light in digital return technology. As demands placed on the upstream continue to grow, MSOs will need to provide more and more upstream bandwidth. Critical to success will be the ability to fully utilize the upstream, with QPSK, 16 QAM, 64 QAM and, eventually, 256 QAM loading. Digital return brings performance and operational savings to an operator, and the scalability of digital return, from return concatenation to full segmentation, is a compelling fiber-efficient solution.

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