Cross-Ref erence to Related Applications
[0001] This application claims priority to U.S. provisional
patent application serial number 61/557,719 entitled METHOD
AND APPARATUS FOR RAMAN CROSS-TALK MITIGATION INTO VIDEO,
filed November 9 , 2011, the entire contents of which are
herein incorporated by reference.
Technical Field
[0002] Example embodiments provide a method and apparatus
of reducing the amount of Raman cross-talk occurring on
data content channels, such as video channels in optical
networking systems .
Background
[0003] Currently, passive optical network (PON) systems
continue to deliver content to homes and offices across the
world. Increasing bandwidth and data content demands have
caused newer signaling protocols and corresponding data
speeds to emerge. Interference signaling and signal
degradation remains a known concern in PONs and next
generation gigabit (XG) PON-type systems. In one example,
Raman cross-talk is believed to occur from lower
wavelengths into higher wavelengths. For instance, a GPON
operating at 1490 nm may cause Raman cross-talk into a 1550
nm video overlay service.
[0004] One known implementation may include the use of GPON
payload scrambling and using a lower GPON transmit power
(approximately +5 dBm) to achieve acceptable performance at
the optical network termination units (ONTs) . Raman crosstalk
may also occur at higher wavelengths that traverse
into lower wavelengths, such a s from 1577 nm into 1550 ran.
Though the wavelength spacing is close, which in turn,
results in a lower Raman coupling coefficient, the XGPON-1
power spectral density may b e reduced since the data rate
i s 10 Gbps, which implies less power on a per-Hz basis. A s
a result, the +12.5 dBm optical transmitter power level
still results in video service degradation when following
transmission over the ODN (i.e., 10 — 20 km o f fiber and
splitter loss) . The optical input level to an ONT is on
the order o f -12 dBm. Under these conditions the Raman
cross-talk is a significant factor in the recovered
carrier-to-noise ratio (CNR) , signal-to-noise ratio (SNR) ,
and modulation error ratio (MER) for the first few
recovered video channels (55 MHz - 120 MHz) .
[0005] The above-noted performance criteria may b e reduced
to levels incompatible with network deployment guidelines.
In the case o f digital video (256 QAM) the bit error rate
(BER) may b e reduced to unacceptable levels. Unacceptable
performance levels impact video customer service b y placing
impairments o r complete loss o f recovered video service on
some channels . Some known ways to mitigate the Raman
cross-talk impact upon the video data include significantly
reducing the XGPON-1 overall transmit power level, and
using pre-emphasis on the lower video channel modulation
applied to the 1550 nm head-end video transmitter.
[0006] Reducing the power transmission results in the
inability o f the XGPON-1 service to have the desired link
budget o r service distance. Modifying the transmitters
requires modifications to existing deployed PON systems and
re-configuring thousands of 1550 nm optical video
transmitters. As a result, the existing options for
reducing Raman cross-talk include unfeasible service
restrictions and/or expense and complex upgrades which are
commercially unacceptable and may also lead to backwards
compatibility issues with existing deployments.
Summary :
[0007] One example embodiment may include a method of
receiving a data signal at an optical line termination
(OLT) . The method may further provide identifying, by a
processor, signal interference in the data signal,
modifying, by a processor, a shape of the data signal in
the electrical domain, and transmitting, via a transmitter,
the modified data signal to at least one optical
termination unit (ONT) .
[0008] Another example embodiment may include an apparatus
including a receiver configured to receive a data signal
and a processor configured to identify signal interference
in the data signal and modify a shape of the data signal in
the electrical domain. The apparatus may also include a
transmitter configured to transmit the modified data signal
to at least one optical termination unit (ONT) .
Brief Description of the Drawings :
[0009] FIG. 1 illustrates an example PON network system
according to example embodiments .
[0010] FIG. 2A illustrates an example power spectral
density comparison according to example embodiments .
[0011] FIG. 2B illustrates an example power spectral
density comparison emphasizing a lower frequency range
according to example embodiments .
[0012] FIG. 2C illustrates a filter response according to
example embodiments .
[0013] FIG. 3 illustrates an example network entity
configured to perform certain operations according to
example embodiments .
[0014] FIG. 4 is a flow diagram of an example method of
operation according to an example embodiment.
Detailed Description:
[0015] It will be readily understood that the components of
the present embodiments as generally described and
illustrated in the figures herein, may be arranged and
designed in a wide variety of different configurations.
Thus, the following detailed description of the embodiments
of a method, apparatus, and system, as represented in the
attached figures, is not intended to limit the scope of the
embodiments as claimed, but is merely representative of
selected embodiments .
[0016] The features, structures, or characteristics of the
described throughout this specification may be combined in
any suitable manner in one or more embodiments. For
example, the usage of the phrases "example embodiments",
"some embodiments", or other similar language, throughout
this specification refers to the fact that a particular
feature, structure, or characteristic described in
connection with the embodiment may be included in at least
one embodiment. Thus, appearances of the phrases "example
embodiments", "in some embodiments", "in other
embodiments", or other similar language, throughout this
specification do not necessarily all refer to the same
group of embodiments, and the described features,
structures, or characteristics may be combined in anysuitable
manner in one or more embodiments .
[0017] In addition, while the term "message" has been used
in the description of the example embodiments of the
present disclosure, the embodiments may be applied to many
types of network data, such as, packet, frame, datagram,
etc. For example purposes, the term "message" also
includes packet, frame, datagram, and any equivalents
thereof. Furthermore, while certain types of messages and
signaling are depicted in exemplary embodiments, which are
not limited to a certain type o f message, and the
embodiments are not limited to a certain type of signaling.
[0018] FIG. 1 illustrates an example PON configuration
according to example embodiments. Referring to FIG. 1 , a
passive optical network (PON) 100 implementation may
include algorithms, hardware and software aimed at offering
efficient and optimized PON and next generation gigabit
(XG) PONl-type systems. PON technology provides a pointto-
multipoint implementation (central office point to
multiple termination points) to provide bandwidth, content
and other telecommunications services. A PON may include
an optical line termination (OLT) 110 located at the
central office and multiple units o f optical network units
(ONTs) 132, 134 and 136 at the customer sites (i.e.,
offices, homes, etc.) . The PON may also include a splitter
or optical distribution network (ODN) node 120 that
provides data content to the various ONTs.
[0019] The International Telecommunications Union (ITU) and
the Institute of Electrical and Electronics Engineers
(IEEE) have proposed various solutions with respect to PONs
in years past . A gigabit-capable PON (GPON) standard
provides a sizable amount o f total bandwidth and bandwidth
efficiency, with a fundamental bandwidth size of 2.488 Gbps
o f downstream bandwidth and 1.244 Gbps o f upstream
bandwidth. The GPON standard is widely deployed in many
geographical regions.
[0020] Example embodiments provide a XGPON-1 type N2B
(+12.5 dBm (wavelength 1577 run) downstream transmit power
level) device and/or algorithm which utilizes GPON with a
1550 n video overlay. Raman cross-talk is known to occur
from lower wavelengths into higher wavelengths . For
example, in a GPON at 1490 nm Raman cross-talk may b e
generated into the 1550 nm video overlay service. Example
embodiments may provide adding XGPON-1 service on a
deployed ODN without impacting the recovered video
performance level at the subscriber (ONTs) . This
implementation would allow a network operator to reduce
installation costs and migrate customers to higher
bandwidth services with less capital investment.
[0021] According to one example embodiment, Raman cro ss
talk may cause incompatibility between existing and new
services at specific frequency spectrum locations and at
predictable power levels. B y applying de-emphasis to the
electrical modulation signal o f a 1577 nm transmit laser,
it is possible to shape the downstream output optical data
signal to reduce the Raman cross-talk level at the critical
video frequencies. The shaping o f the downstream output
optical data signal reduction procedure can b e performed so
the previously degraded video channels can b e acceptably
recovered .
[0022] According to another example embodiment, the shaping
of the downstream output optical data signal may be
performed in the digital domain. For example, a binary
input data stream may be digitally filtered to create a
required pulse shape. The desired output waveform would be
converted to an analog waveform by way of a D/A (digital to
analog) converter. Another implementation would be to
include an analog filter in-between the input data stream
and an optical modulator. The ideal filter function would
be a limited high-pass function which would reduce the
baseband non-return-to-zero (NRZ) spectrum of a XGPON-1
configuration just enough to drop the Raman cross-talk low
frequency content to an acceptable recovered video CNR,
SNR, and AMR levels. These criteria would be balanced
against the reduction in XGPON-1 recovered BER performance
and increased jitter. The shaping would mildly impact the
1577 n XGPON-1 BER while resulting in a significant
increase in the 1550 nm recovered video performance.
[0023] According to one example method of operation, an
optical receiver may be configured to receive a video
signal via the optical distribution network node (ODN) from
an optical line termination (OLT) . The OLT may include a
processor configured to identify signal interference in the
data signal and modify a shape of the data signal in the
electrical domain. The OLT may be configured to transmit
the modified data signal to at least one optical
termination unit (ONT) . In the procedure, the OLT may also
be configured to remove Raman cross-talk interference into
the video signal such that the data signal is modified and
subsequently provided to a transmitter which includes a
1577 nanometer (nm) laser transmitter. The OLT may also be
configured to digitally filter the data signal to create a
desired pulse shaped signal and convert the pulse shaped
signal to an analog waveform via a digital to analog (D/A)
converter. Alternatively, the OLT may also filter the data
signal by an analog filter set between the input data
stream and an optical modulator to remove a low frequency
Raman cross-talk interference component and to obtain a
limited high pass function with a reduced baseband non
return to zero spectrum of the data signal. The resulting
data signal may be a pulse-shaped video signal with a
removed low frequency Raman cross-talk interference
component in a 1550 nm video channel range.
[0024] FIG. 2A illustrates an example of a power spectral
density graph (PSD) according to example embodiments. The
dotted line represents an unshaped or unaltered non-return
to zero (NRZ) PSD of a XGPON-1. The solid line, which is
above the dotted line except from 0 to about 1 x 10 9 Hz,
indicates an example of a high pass filtered signal, which
in operation would reduce the first few video channel Raman
cross-talk levels. This example graph 200 illustrates how
the shaped or filtered signal provides a larger power
spectral density over the various frequency ranges. Other
implementations are possible and results may vary from one
implementation to another. The only visible difference
between the unshaped and the shaped signals is in the lower
frequency range 202 .
[0025] FIG. 2B illustrates an example of a power
spectral density graph (PSD) zoomed into the lower
frequency range according to example embodiments.
Referring to FIG. 2B, the lower frequency range 204
demonstrates a large amount of signal shaping in the lower
frequencies while the unshaped signal (dotted line) is kept
even without any variations. FIG. 2C illustrates the
filter responses for the high-pass filter represented by an
alternating dotted and dashed line, the low-pass filter
represented by a solid line and the composite/summation of
the high-pass filter and the low-pass filter which is the
dotted line. The composite band reject shaping approach
may reduce the Raman crosstalk by 3 dB in a first video
channel .
[0026] Raman cross-talk interference with video overlay
can occur in various different situations. For example, if
the interfering wavelength is smaller than the wavelength
of the video content (i.e., 1550-1560 nm) , then the
offending or interfering wavelength creates or "pumps"
interfering signals into the video content frequency range
causing distortion of the target signal received at the
ONTs . If the interfering wavelength (s) are larger than the
video content then by contrast the video signals will
"pump" interfering signals into the interfering wavelength
range. This scenario may seem negligible, however, the
pumping may deplete the video signals power level. The
depletion may appear as noise injected into the video
signal .
[0027] The strength of the signals and fiber distance
greatly influence the Raman cross-talk interaction and
system/video degradation. Generally, worst case long
distance GPON transfers occur in the 9 - 10 km fiber
distance range. Worst case XG-PON1 transfers occur in the
18 - 20 km fiber distance range. Typically the biggest
impacts from Raman cross-talk occur on the 1s handful of
video channels. The Raman transfer signals are much
stronger with GPON data of the video content. The impact
from video into XG-PON1 data is approximately 500% less
than the previous scenario. Digital data has a lesser SNR
than video. The cross-talk from XG-PON1 into video is also
less, however video is more susceptible to cross-talk.
[0028] There are various different approaches to Raman
cross-talk reduction in video. For example, by using a 2nd
feeder fiber for video delivery and implementing 2:n
splitters. Also, by applying video signal pre-emphasis and
compensating low channel degradation. 10G-TX de-emphasis
may also be used to shape or move the energy spectrum.
For orthogonal frequency-division multiplexing (OFDM) ,
interfering low-f requency subcarriers can be blanked.
[0029] Raman cross-talk could limit co-existence of
XGPON-1 in existing deployed PON networks with video
overlay, when high performance (i.e., low internal noise)
video ONTs are deployed, resulting in degradation of CNR,
SNR and MER. An implementation may include altering the
data pulse shaping in the electrical domain and applying
the pulse into a 157 7 nm laser transmitter, which reduces
the resulting Raman cross-talk in specific 1550 nm video
channels. This implementation may allow coexistence of
XGPON-1 and GPON video equipped services in the same ODN
node, and ensures that acceptable video CNR, SNR, and MER
levels result for the standardized XGPON-1 transmit power
levels (up to +12.5 dBm) . The implementation allows
control of the power penalty impact upon the downstream
1577 signals of the XGPON-1 10 Gbps data path. A s a
result, significant cost and capital savings result by
applying this for the network operator.
[0030] The operations of a method or algorithm described
in connection with the embodiments disclosed herein may be
embodied directly in hardware, in a computer program
executed by a processor, or in a combination of the two. A
computer program may be embodied on a non-transitory
computer readable storage medium. For example, a computer
program may reside in random access memory ("RAM"), flash
memory, read-only memory ("ROM") , erasable programmable
read-only memory ("EPROM") , electrically erasable
programmable read-only memory ("EEPROM") , registers, hard
disk, a removable disk, a compact disk read-only memory
("CD-ROM"), or any other form of storage medium known in
the art .
[0031] An exemplary storage medium may be coupled to the
processor such that the processor may read information
from, and write information to, the storage medium. In the
alternative, the storage medium may be integral to the
processor. The processor and the storage medium may reside
in an application specific integrated circuit ("ASIC") . In
the alternative, the processor and the storage medium may
reside as discrete components. For example, FIG. 3
illustrates an example network element 300, which may
represent any of the above-described components of the
previous drawings .
[0032] A s illustrated in FIG. 3 , a memory 310 and a
processor 320 may be discrete components of the network
entity 300 that are used to execute an application or set
of operations. The application may be coded in software in
a computer language understood by the processor 320, and
stored in a computer readable medium, such as, the memory
310. The computer readable medium may be a non-transitory
computer readable medium that includes tangible hardware
components in addition to software stored in memory.
Furthermore, a software module 330 may be another discrete
entity that is part of the network entity 300, and which
contains software instructions that may be executed by the
processor 320. In addition to the above noted components
of the network entity 300, the network entity 300 may also
have a transmitter and receiver pair configured to receive
and transmit communication signals (not shown) .
[0033] FIG. 4 illustrates an example flow diagram
according to an example embodiment. Referring to FIG. 4 ,
the method may include receiving a data signal at an
optical line termination (OLT) , at operation 402 and
identifying, by a processor, signal interference in the
data signal, at operation 404. The method may also include
modifying, by a processor, a shape of the data signal in
the electrical domain at operation 406 and transmitting,
via a transmitter, the modified data signal to at least one
optical termination unit (ONT) at operation 408.
[0034] Although an exemplary embodiment of the system,
method, and computer readable medium of the present
embodiments has been illustrated in the accompanied
drawings and described in the foregoing detailed
description, it will be understood that the embodiments are
not limited to the embodiments disclosed, but is capable of
numerous rearrangements, modifications, and substitutions
without departing from the spirit or scope of the
embodiments as set forth and defined by the following
claims. For example, the capabilities of the system of FIG.
1 can be performed by one or more of the modules or
components described herein or in a distributed
architecture. For example, all or part of the functionality
performed by the individual modules, may be performed by
one or more of these modules. Further, the functionality
described herein may be performed at various times and in
relation to various events, internal or external to the
modules or components. Also, the information sent between
various modules can be sent between the modules via at
least one of: a data network, the Internet, a voice
network, an Internet Protocol network, a wireless device, a
wired device and/or via plurality of protocols. Also, the
messages sent or received by any of the modules may be sent
or received directly and/or via one or more of the other
modules .
[0035] While preferred embodiments of the present
embodiments have been described, it is to be understood
that the embodiments described are illustrative only and
the scope of the embodiments is to be defined solely by the
appended claims when considered with a full range of
equivalents and modifications (e.g., protocols, hardware
devices, software platforms etc.) thereto.
Claims

WHAT IS CLAIMED IS:
1 . An apparatus comprising:
a receiver configured to receive a data signal;
a processor configured to identify signal interference
in the data signal and modify a shape of the data signal in
an electrical domain; and
a transmitter configured to transmit the modified data
signal to at least one optical termination unit.
2 . The apparatus of claim 1 , wherein the processor
is further configured to remove Raman cross-talk
interference from the received data signal.
3 . The apparatus of claim 1 , wherein the modified
data signal is provided to a 1577 nanometer (nm) laser
transmitter .
4 . The apparatus of claim 1 , wherein the processor
is further configured to filter the data signal to create a
desired pulse shaped signal.
4 . The apparatus of claim 4 , wherein the processor
is further configured to convert the pulse shaped signal to
an analog waveform via a digital to analog converter.
5 . The apparatus of claim 1 , wherein the processor
is further configured to filter the data signal by an
analog filter.
6 . The apparatus of claim 5 , wherein the analog filter
is set between the input data stream and an optical
modulator of an optical distribution network.
7 . The apparatus of claim 6 , wherein the analog filter
is configured to remove a low frequency Raman cross-talk
interference component.
8 . The apparatus of claim 7 , wherein the removed low
frequency Raman cross-talk interference component is in a
1550 nm video channel.
9 . The apparatus of claim 1 , wherein the processor
is further configured to obtain a limited high pass
function with a reduced baseband non-return to zero
spectrum of the data signal.
10. The apparatus of claim 1 , wherein the data signal
is a video signal.