SR: Why Cable Operators need a

SR: Why Cable
Operators need a
new return path
solution today
Originally Authored by Peter Wolff, Sep 2012;
Amended by Charlie Chen, Jun 2013
Introduction
Cable operator’s today face a growing threat from
competitors expanding their return path speeds.
Generally, today’s North American operators are
limited to 5 MHz to 42 MHz in the return path radio
frequency spectrum (RF) and a capacity of 100
Mbps using DOCSIS 3.0. Verizon FiOS, recently
announced a higher tier data speed of 300 Mbps
downstream with 65 Mbps in the upstream.
Competitions know that the CATV network is
limited to 42 MHz and are starting to offer data
rates closer to the 100 Mbps maximum. In a paper
by tier one vendors - Cisco, ARRIS, Motorola and
Intel recommended the Cable operators moving the
return path to DOCSIS 3.1 specifications and
perform one node segmentation as a phase one
approach. This is key to offering higher tier return
rates up to 300 Mbps. With a total capacity of 8
Gbps per 4-way segmented node in the upstream
return path, Cable operators now have a 10 year
plan that would support the rapid data rate growth.
single channel, 5 MHz to 42 MHz where Digital
Return are either 2 channels 5 MHz to 42 MHz or 2
channels of 5 MHz to 85 MHz. While Analog Return
may easily adapt to a 250 MHz return path, it is not
as easy for a Digital Return as its performance is
limited by how fast it can sample twice the highest
frequency to satisfy Nyquist rates. Higher Digital
Return frequency splits mean more costly optics to
support the higher data transmission rate over
fiber. The approximate calculation for the Optical
Transport requirements for a 245 MHz return would
be (10 bits per sample * 490 Million Samples /
second) * 2 Channels = 9.8 Gbps! Digital Return
transmitters would therefore require more costly
10G optics and ADC (analog digital converters). To
support the recommended segmentation typically
one Analog return transmitter per return segment
required and one Digital return path transmitter per
two return segments. A second Digital return path
transmitter is added to the node if further
segmentation is required then.
To show how this evolves, Figure 1 depicts a single
Analog Return Transmitter used to provide return
service. If further segmentation is required, then
another return transmitter is needed for each phase
disrupting service and adding operational costs.
Technical Comparison
Cable operators today have had analog and digital
solutions for return path transport. Analog
transport remains cost effective while Digital Return
costs are approaching analog return. However,
extending Digital Return to DOCSIS 3.1 at band split
of 5-250 MHz would require new, more costly
electronics and optics. Today’s Analog return is a
Titan Photonics, Inc
Figure 1. 1 x 4 Node Segmentation with Analog
Return Transmitter
White Paper SR Return Path
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A Digital Return Path solution in Figure 2, whereby a
single Return Path Transmitter is used for 2 x 4node segmentation. If further segmentation is
required, an additional transmitter is needed or if a
band split change is necessary for next generation
DOCSIS services then a removal of the transmitter
and replacement is necessary again disrupting
service and adding operational costs.

increases distance in the return path over
analog and avoids many non-linear fiber
effects
In addition to the aforementioned benefits, this
unique method would support 4 x 42 MHz, 65 MHz,
85 MHz and 250 MHz spectrum modulated at 256
QAM (quadrature amplitude modulation) levels and
segmentation for costs comparable to Analog
Return Path Transmission links offered today. This
method so-called Sub-Octave Radio Frequency (SR)
is offered by Titan Photonics to provide advanced
segmentation capability for tomorrow’s networks
today. A return path transmitter solution by Titan
Photonics depicted in Figure 3 shows a full 4 x4node segmentation using only a single SR Return
Path Transmitter.
Figure 2. 2 x 4 Node Segmentation with Digital
Return Transmitter
SR Technology
Let us consider a new and advanced approach to
the return path that lowers OpEx by:





increasing the return channel capacity per
wavelength avoiding the use of additional
scarce fiber resources
lowering power requirements using fewer
modules and improved density in the head
end
lowers per link costs using advanced optical
transmission techniques
compatible and immediate cost saving to
existing DOCSIS 3.0 upgrades
provides for future residential or business
class DOCSIS 3.1 services using quad 250
MHz return capacity
Titan Photonics, Inc
Figure 3. SR Return Path Transmitter
SR technology offers one optical return path
wavelength for four, RF return channels saving fiber
and its associated costs. Today demonstrated at 4
times of RF frequency between 5-250MHz, SR is
deployed by using an SR Return Path Transmitter
and a corresponding SR Return Path Receiver. The
Receiver has four RF output ports each providing up
to 250 MHz of RF return path spectrum. SR
technology employs a patented method to avoid
the effects of Chromatic Dispersion normally
generating Composite Second Order Distortions
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called (CSO) that impact analog transmission.
Typically, a single 1 GHz of RF spectrum transported
over fiber is composed of multiple octaves of RF
spectrum. The SR technique sub-divides the
transported spectrum during up-conversion to
higher order carrier level and down coverts the
transported spectrum back to its original format
and content. This technique allows for the rejection
of the signals affected by CSO. Costs reduced by
avoiding the use of expensive A/D and D/A
converters and normally used distributed feedback
lasers. Titan’s SR technology provides high
performance while overcoming the limitations of
the use of lower cost laser diodes. Today SR has
demonstrated a 35 dB noise power ratio (NPR) over
a 20 dB RF signal range at 256 QAM and a 5-250
MHz upstream spectrum window. See Figure 4 to
review the NPR and Bit Error Rate curves. If plant
conditions grow to use 1024 QAM, Titan’s SR has
capability to achieve that modulation level as well.
If operators decide to use an erbium doped fiber
amplifier in conjunction with transmissions in the
ITU C band (1550 nanometer light wave window),
SR Return Path products are plausible at long links
beyond the 60km distance.
The table below highlights the superior value
proposition for the SR Return path technology.
Lowest cost for SR is a significant standout if one
calculates a normalized cost ratio based on $x per
link for an equivalent Analog solution and then
divides by the available spectrum the technology is
capable of transporting. While link distance is a
stand out for Digital return, SR is certainly capable
of meeting the needs of the majority of networks
deployed today with a typical link distance of 60
km. Analog return transmitters are capable of
transmitting beyond 250 MHz, but are bandwidth
limited at the existing receiver to 200 MHz. Digital
return will continue to be limited by the rate at
which manufacturers can increase the sampling
rates of A/D and D/A converters. It is likely that it
may be years before it is possible to transport one
or two channels of 250 MHz spectrum. SR Return
path technology is available today providing the
means to offer advanced service capabilities to
operators as they drive to increase new revenue
streams.
Analog
Digital
Titan SR
Equipment
(Cap-Ex)
Cost/Spectrum
(MHz)
Medium
x
High
1.25x
Very Low
0.2x
Link Distance
(km)
Low
20km
Very High
100km >
High
100km
Operating Pass
band (MHz)
Medium
5-42, 250
Low
2 x 5-42
High
4 x 5-250
Future Proof
No
No
Yes
Figure 4. QUAD CH1 BER Test under 250 MHz noise
loading, +8dBm, 20 km
Titan Photonics, Inc
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Summary
References
Next generation DOCSIS 3.1 will provide
tremendous opportunities for operators to continue
competing with Telcos in offering new services.
This is going to require some advances in the return
path spectrum and solutions that keep CapEx and
OpEx costs lower. Operators segmenting nodes and
considering new return path solutions should
evaluate their future needs as they make purchase
decisions today. SR Return Path solutions by Titan
Photonics provide a unique strategy to continue to
leverage the existing DOCSIS infrastructure while
offering increased data rates, lower costs, low
power and fiber savings over alternatives in the
market today.
http://www.cedmagazine.com/articles/2012/07/an
-evolutionary-approach-to-gigabit-class-docsis
Titan Photonics, Inc
http://www22.verizon.com/home/fios-fastestinternet/fastest-internet-plans/
Contact Us
Titan Photonics, Inc.
48501 Warm Springs Blvd #106,
Fremont, CA 94539, USA
Phone: (510) 687-0488
Email: sales@titanphotonics.com
Website: www.titanphotonics.com
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