Note: Descriptions are shown in the official language in which they were submitted.
CA 02315869 2000-08-14
1
Method of Transmitting an Information Signal
This invention relates to a method of transmitting an
information signal over the upstream channel of a
communications network with a downstream and an
upstream channel, and to a transmitting/receiving
facility of the communications network.
Communications networks are designed as point-to-
multipoint networks, for example. Information such as
voice, data, and video is transmitted from a center
over a downstream channel to a plurality of customer
premises, partly via intermediate facilities. From the
customer premises, information such as voice, data, and
video is transmitted over an upstream channel to the
center and from there, if necessary, to other customer
premises or another center. Applications are, for
example, telephony, Internet, video-on-demand, video
telephony, cable television, mobile radio. The topology
of the communications network is, for example, a hybrid
fiber/coax (HFC) or hybrid-fiber/radio (HFR) network.
In HFC and HFR, transmission from the center to at
least one intermediate facility and to directly
connected customer premises takes place over optical
lines. The intermediate facilities in HFC are called
CA 02315869 2000-08-14
2
optical network units (ONUS) or broadband optical
network terminations (BONTs), and those in HFR are
called base stations (BSs). To each ONU or each BONT
and to each BS, a plurality of customer premises are
usually connected, e.g., by coaxial cable and radio,
respectively.
To transmit information from the center to the
intermediate facilities, use is made of one wavelength,
for example. This necessitates a light source, e.g., a
laser or a laser diode, at the center. From the
intermediate facilities to the center, information is
transmitted over the upstream channel using a further
wavelength. Each intermediate facility therefore
requires a light source. Particularly if dense
wavelength-division multiplexing (DWDM) is used, high
wavelength stability is needed, because the individual
wavelengths have a small frequency spacing. The
intermediate facilities are generally located outdoors,
i.e., not in a building that is kept at a moderate
temperature. Hence, they are subject to temperature
variations. Particularly the wavelengths of laser
sources, however, are highly temperature-sensitive,
e.g., 12 GHz/°C, so that the stringent requirements
placed on wavelength stability frequently cannot be
met. In addition, because of the large number of
intermediate facilities in a communications network, a
correspondingly large number of laser sources are
needed. Laser sources. are comparatively expensive
components, so that the implementation of a
communications network is very costly.
Approaches that have been followed to solve this
problem include removing the laser sources from the
w CA 02315869 2000-08-14
3
intermediate facilities in order to save costs and
eliminate the temperature sensitivity.
EP 0 461 380 discloses a radiocommunications system in
which an intermediate facility is equipped with a
reflective optical modulator that modulates an
unmodulated carrier frequency, which is additionally
transmitted by the center, with the information signal
to be transmitted, thus transmitting information over
the downstream channel to the center without having a
separate light source. For the transmission of
information from the center to the intermediate
facilities, use is made of a spectrum of carrier
frequencies that are transmitted at a single
wavelength. One carrier frequency is transmitted
unmodulated. The other carrier frequencies are
modulated with the respective information signals. At
the receiving end, the output frequency for the radio
signals is generated by mixing the modulated carrier
frequencies with the unmodulated carrier frequency.
Various intermediate facilities, such as base stations,
which must transmit different information on the same
frequency, cannot be served directly by this method.
Each corresponding intermediate facility would have to
be supplied over a separate optical line, which is
costly.
In EP 0 809 372, a part of a modulated wavelength
transmitted by the center is fed in an intermediate
3p facility to a modulator in which individual time slots
are modulated in a TDM mode with the information signal
to be transmitted, which is encoded in a subsequent
CDMA encoder and then transmitted over the upstream
channel to the center (TDM = time-division
multiplexing, CDMA = code division multiple access).
' CA 02315869 2000-08-14
4
The invention provides three coherent solutions which
permit temperature-independent operation of a
communications network.
The method according to claim 1 is characterized in
that in the downstream channel, optical signals are
transmitted using wavelength-division multiplexing, the
optical signals consisting of pairs of optical signals
to permit optical heterodyne detection, each pair
containing a modulated and an unmodulated optical
signal, that a portion of the unmodulated optical
signal of a given pair is extracted from the optical
signals received over the downstream channel, and that
the extracted optical signal is modulated with the
information signal to be transmitted, and sent out over
the upstream channel. In this method, individual
wavelengths are utilized doubly. An unmodulated optical
signal of a given wavelength, on the one hand, is mixed
with a given modulated optical signal to generate the
desired transmit frequency (for use in HFR), and, on
the other hand, is modulated with an information signal
to be transmitted. This represents an optimum
utilization of existing wavelength capacities and
allows more information signals to be transmitted with
unchanged transmission capacity, particularly if
optical heterodyne detection is used.
The transmitting/receiving facility for carrying out
this method, which is claimed in claim 2, is
characterized in that a receiving unit is provided for
receiving optical signals transmitted in the downstream
channel using wavelength-division multiplexing and
containing at least one modulated and at least one
unmodulated optical signal, and for extracting a given
' CA 02315869 2000-08-14
unmodulated optical signal from the optical signals,
that an extraction unit is provided for extracting a
portion of the extracted optical signal and for feeding
said portion to a modulator, and that the modulator is
adapted to modulate the extracted optical signal with
an information signal to be transmitted, and to
transmit the modulated signal over the upstream channel
of the communications network.
The.method according to claim 3 is characterized in
that in the downstream channel, optical signals are
transmitted using wavelength-division multiplexing, the
optical signals containing at least one modulated and
at least one unmodulated optical signal, that. the
unmodulated signal is transmitted as Cw light, that a
given unmodulated optical signal is extracted from the
optical signals received over the downstream channel,
and that the extracted optical signal is modulated with
the information signal to be transmitted, and
subsequently sent out over the upstream channel. This
method features the exclusive reservation of individual
wavelengths for the transmission of CW light. Through
the use of CW light, arbitrary types of modulation of
the information signal to be transmitted can be carried
out in the upstream channel. For instance, time
division multiple access (TDMA), CDMA, or frequency
division multiple access (FDMA) can be used. The price
to be paid for this advantage is that individual
wavelengths on which no information can be transmitted
from the center to the intermediate facilities have to
be reserved. Furthermore, if CW light is used, no
synchronization problems will arise.
The method according to claim 4 is characterized in
that in the downstream channel, optical signals are
~
CA 02315869 2000-08-14
6
transmitted using wavelength-division multiplexing, the
optical signals containing at least one modulated and
at least two unmodulated optical signals on different
wavelengths, that at least two given unmodulated
optical signals are extracted from the optical signals
received over the downstream channel, and that the
extracted optical signals are each modulated with an
information signal to be transmitted, and then combined
onto the upstream channel. The key feature of this
method is that for the first time, wavelength-division
multiplexing is performed in an intermediate facility.
The introduction of wavelength-division multiplexing
allows more information to be transmitted over the same
optical line. The network can be extended simply by
inserting modulators and multiplexers in the
intermediate facilities instead of providing new
intermediate facilities, a new network topology, etc.
This particularly saves manufacturing, installation,
and maintenance costs.
A transmitting/receiving facility for carrying out this
method, which is claimed in claim 5, is characterized
in that that a receiving unit is provided for receiving
optical signals transmitted in the downstream channel
using wavelength-division multiplexing, the optical
signals containing at least one modulated and at least
two unmodulated optical signals on different
wavelengths, and for extracting at least two given
unmodulated optical signals from the optical signals,
that at least two modulators are provided for
modulating the two extracted optical signals with a
respective information signal to be transmitted, and
that a multiplexer is provided for combining the
modulated signals onto the upstream channel of the
CA 02315869 2000-08-14
7
communications network using wavelength-division
multiplexing.
The method according to claim 5, in particular, can be
combined with either of the methods claimed in claims 1
and 3, as will be apparent from the following
description.
Three embodiments of the invention will now be
explained- with reference to the accompanying drawings,
in which:
Fig. 1 is a schematic block diagram of a first
communications network in accordance with the
invention;
Fig. 2 is a schematic block diagram o.f a second
communications network in accordance with. the
invention; and
Fig. 3 is a schematic block diagram of a third
communications network in accordance with the
invention.
The first embodiment will now be explained with
reference to Fig. 1. Fig. 1 shows a first
communications network in accordance with the
invention. The communications network is designed as a
point-to-multipoint network comprising a center that is
connected by optical lines to a number of intermediate
facilities, of which one is shown: For the sake of
clarity, the plurality of customer premises that are
connected to the intermediate facilities or directly to
the center are not shown. The communications network is
designed as an HFC or HFR network, for example.
~
CA 02315869 2000-08-14
8
Alternatively, the communications network may consist
of optical lines between the center and the
intermediate facilities and of copper lines between the
intermediate facilities and the customer premises, over
which the information signals are transmitted in, e.g.,
the ADSL format or the like (ADSL = Asynchronous
Digital Subscriber Line). The communications network
may also consist of hybrids of the above network
structures. For the implementation of the downstream
channel and the upstream channel, two separate optical
lines, as shown, or a common optical line can be used.
In the simplest case, the communications network can
also be a point-to-point connection between two
transmitting/receiving facilities.
Information signals are transmitted from the center to
the intermediate facilities using dense wavelength-
division multiplexing (DWDM). By assigning four
wavelengths to one intermediate facility, a base
station, for example, can be supplied over the
downstream channel with the fourfold amount of data.
For each 90° sector of a base station, one wavelength
is used. In DWDM networks, a channel spacing of 50,
100, 200 GHz is possible. High wavelength stability is
necessary. Laser sources exhibit high temperature
sensitivity: about 12 GHz/°C, as opposed to other DWDM
components, such as optical filters, about 700 MHz/°C,
and arrayed waveguides (AWGs), about 1.5 GHz/°C.
Outside of buildings, temperatures may vary between
-40°C and +85°C. Then, particularly with a 50-GHz
channel spacing, wavelength stability is no longer
ensured. Therefore, provision is made to move the laser
sources from the intermediate facilities to the center,
CA 02315869 2000-08-14
9
which is located inside a building and is not subject
to the above temperature variations.
The center contains eight optical transmitters TX1,
TX2, TX3, TX4, TX5, TX6, TX7, TX8, a multiplexer 10, a
demultiplexer 12, and four optical receivers RX5, RX6,
RX7, RX8.
The intermediate facility shown contains a receiving
unit 11, four optical receivers RX1, RX2, RX3, RX4,
four optical modulators M1, M2, M3, M4, and a
multiplexer 13.
Optical transmitter TX1 contains a light source, e.g.,
a laser or a laser diode. The light source is operated
in the CW mode, i.e., it emits CW light. This light is
modulated directly or indirectly with the information
signal to be transmitted, the indirect modulation being
effected by an electro-optic modulator. The modulated
output signal is applied to multiplexer 10. Optical
transmitter TX1 generates modulated light of a first
wavelength ~,1.
Optical transmitters TX2, TX3, TX4 operate on the same
principle as optical transmitters TX1. They also
generate an optically modulated signal that is applied
to multiplexer 10. The difference is that firstly,
information signals with other contents are used.
Secondly, each optical transmitter TX1, TX2, TX3, TX4
generates light of a different wavelength. Optical
transmitter TX2 generates light of wavelength ~,2.
Optical transmitter TX3 generates light of wavelength
CA 02315869 2000-08-14
~,3. Optical transmitter TX4 generates light of
wavelength ~,4.
Optical transmitter TX5 contains a light source, e.g.,
a laser or a laser diode. The light source is operated
in the CW mode, i.e., it emits CW light. This light is
transmitted directly to multiplexer 10. The generated
light is not modulated. It is transmitted over the
downstream channel to the intermediate facilities,
modulated there with an information signal to be
transmitted, and then transmitted over the upstream
channel back to the center, where the transmitted
information signals are evaluated.
Optical transmitters TX6, TX7, TX8 operate on the same
principle as transmitter TX5. They also generate an
unmodulated optical signal that is applied to
multiplexer 10. Each of the optical transmitters TX1,
TX2, TX3, TX4, TXS, TX6, TX7, TX8 generates light of a
different wavelength.
Multiplexer 10 is designed as a wavelength-division
multiplexer, for example. It contains, for example,
DWDM components, such as optical splitters, optical
combiners, or the like.
Receiving unit 11 receives the optical signals
transmitted by the center over the downstream channel.
Receiving unit 11 contains, for example, a wavelength-
division demultiplexer including DWDM components for
separating the received optical signals into different-
wavelength signals. At the output of receiving unit 11~,
eight different signals with eight different
wavelengths, for example, are then available. Four of
CA 02315869 2000-08-14
11
these signals are applied to the four optical receivers
RX1, RX2, RX3, RX4. In each receiver RX1, RX2, RX3,
RX4, the received signals are demodulated, so that the
transmitted information signals can be detected and
analyzed or further processed. Alternatively, only a
conversion of the received signals is performed, e.g.,
in order to transmit them in the correct format to
customer premises by radio. The four signals at the
output of receiving unit 11, with wavelengths ~,5, ~,6,
~ ~~~ ~,g~ are unmodulated. They are applied to modulators
M1, M2, M3, M4. Modulators M1, M2, M3, M4 are electro-
optic modulators that modulate the signals of
wavelengths ~,5, ~,6, ~.7, ~,8 with respective information
signals to be transmitted. The signals to be
transmitted originate, for example, from customer
premises that are connected to the intermediate
facility. The information signals contain, for example,
voice, data, and/or video for telephony, Internet, E-
mail, video telephony, etc. Modulation is effected
using TDMA, CDMA, or FDMA. The signals so modulated are
applied to multiplexer 13. Multiplexer 13 is designed
as a wavelength-division multiplexer, for example. It
contains DWDM components, for example, such as optical
splitters, optical combiners, optical couplers, or the
like. The combined optical signals are then transmitted
simultaneously over the upstream channel to the center.
Demultiplexer 12, constructed from DWDM components,
receives the optical signals of wavelengths ~,5, ~,6, ~,7,
~,8, separates them into four signals with one
wavelength each, and feeds these four signals to the
four optical receivers RX5, RX6, RX7, RX8. In each
optical receiver RX5, RX6, RX7, RX8, the received
optical signal is demodulated and then fed to a
CA 02315869 2000-08-14
12
processing device for analysis. Alternatively, only a
conversion is performed in order to retransmit a
corresponding information signal directly over the
downstream channel. The center then serves as a relay
station or an exchange, e.g. in the case of a telephone
connection between two customer premises connected to
the center.
The second embodiment will now be explained with
reference to Fig. 2. Fig. 2 shows a second
communications network in accordance with the
invention. The communications network serves to
optically transmit millimeter-wave signals using
wavelength-division multiplexing. The carrier
frequencies are in the range of 20 GHz to 60 GHz, for
example. Applications are particularly in mobile radio
systems with optical feeders to base stations that
serve cells with a radius of 10 m to 2 km. This
embodiment uses optical heterodyne detection, employing
heterodyne sources. The heterodyne sources are light
sources that generate two carrier frequencies, one of
which is transmitted unmodulated, and the other is
transmitted after being modulated with the information
signal to be transmitted. At the receiving end, the
desired output frequency for the transmission of radio
signals containing the signal to be transmitted is
generated by mixing the two carrier frequencies. At
least two unmodulated carrier frequencies and at least
two carrier frequencies modulated with information
signals are transmitted on at least two different
wavelengths. In this way, different intermediate
facilities, such as base stations, can transmit
different information on the same frequencies. A first
pair of carrier frequencies consisting of an
unmodulated carrier and a carrier modulated with a
~
CA 02315869 2000-08-14
13
first information signal, whose frequency separation is
equal to that of a second pair of carrier frequencies
consisting of an unmodulated carrier and a carrier
modulated with a second information signal, is
transmitted at a first wavelength, while the second
pair is transmitted at a second wavelength.
Alternatively, each carrier frequency can be
transmitted at a separate wavelength, so that four
wavelengths are needed to transmit the four carrier
frequencies. It is also possible to transmit two
different information signals using three carrier
frequencies that are transmitted at at least two
different wavelengths. To accomplish this, one carrier
frequency is transmitted unmodulated and serves as a
reference, and the other two carrier frequencies are
modulated with the information signals and are
separated from the unmodulated carrier frequency by the
same distance. Alternatively, two or more unmodulated
reference carrier frequencies can be used at one
wavelength in order to be able to generate identical
output frequencies for different information signals in
different intermediate facilities. To do this, in an
ascending frequency sequence, for example, one
reference frequency, three modulated carrier
frequencies, one reference frequency, three modulated
carrier frequencies, etc. are transmitted.
The communications network is designed as a point-to-
multipoint HFC or HFR network, for example. It
comprises a center from which information is
transmitted over a downstream channel to at least one
intermediate facility, of which only which one is shown
for simplicity. The link between the center and the
intermediate facilities consists of optical lines, such
as glass optical fibers. An intermediate facility is
CA 02315869 2000-08-14
14
designed as a base station or ONU, for example. The
base station serves a cell of a mobile radio system,
and the ONU serves a number of customer premises in
which a set-top box, for example, is used as a
communications unit. Transmission from the intermediate
facilities to the center takes place over the upstream
channel, which consists of optical lines. The optical
lines may correspond to those for the downstream
channel, in which case they are operated
bidirectionally. Alternatively, different types of
unidirectional lines can be used.
The center contains four optical transmitters TX10,
TX20, TX30, TX40, a multiplexer 14, a demultiplexer 16,
and four optical receivers RX50, RX60, RX70, RX80.
The intermediate facility shown contains a receiving
unit 15, four extraction units 18, 19, 20, 21, four
coupling units 22, 23, 24, 25, four optical receivers
RX10, RX20, RX30, RX40, four electro-optic modulators
M10, M20, M30, M40, and a multiplexer 17. As a minimum
configuration of an intermediate facility, which is
designed only for unidirectional operation, for
example, only the receiving unit 15, one coupling unit,
e.g., 22, and one optical receiver, e.g., RX10, are
necessary. Another intermediate facility could then
contain, for example, receiving unit 15, coupling unit
23, and optical receiver RX20. To permit bidirectional
operation, it suffices to insert one extraction unit,
e.g., 18 or 19, and one electro-optic modulator, e.g.,
M10 or M20.
Optical transmitter TX10 contains two light sources,
such as lasers or laser diodes, for generating two
different wavelengths X10 and X20. On wavelength X10,
CA 02315869 2000-08-14
an unmodulated carrier frequency is transmitted, and on
wavelength X20, a carrier frequency modulated with an
information signal is transmitted. Wavelengths X10 and
X20 are applied to multiplexer 14. The multiplexer is
designed as a wavelength-division multiplexes to
transmit all wavelengths over a common optical line.
Optical transmitters TX20, TX30, TX40 operate on the
10 same principle as optical transmitter TX10 except that
other information signals are transmitted.
All optical transmitters TX10, TX20, TX30, TX40 can be
upgraded to transmit more than one carrier frequency on
one wavelength. Then, two or more wavelengths are
transmitted from the center to the intermediate
facilities, with each wavelength transmitting one or
more carrier frequencies. In this way, the capacity of
the optical lines is utilized in an optimum manner. A
defined assignment of the carrier frequencies to
intermediate facilities must then be provided. Each
intermediate facility then includes, for example,
filters, e.g., bandpass filters, for selecting the
carrier frequencies assigned to it.
Receiving unit 15 contains a demultiplexer that
separates the received optical signals according to
wavelengths. Wavelengths X10 and X20 are applied
through extraction unit 18 and coupling unit 22 to
optical receiver RX10. Wavelengths X30 and X40 are
applied through extraction unit l9 and coupling unit 23
to optical receiver RX20. Wavelengths X50 and X60 are
applied through extraction unit 20 and coupling unit 24
to optical receiver RX30. Wavelengths X70 and X80 are
CA 02315869 2000-08-14
16
applied through extraction unit 21 and coupling unit 25
to optical receiver RX40.
All extraction units 18, 19, 20, 21 are designed as
asymmetrical optical couplers, for example: They
extract a portion of the received optical signal of a
given wavelength and feed it to one of electro-optic
modulators M10, M20, M30, M40. The extracted portion,
which amounts to, e.g., 30~ of the received signal and
contains an unmodulated carrier frequency, is used to
transmit an information signal over the upstream
channel to the center. The unmodulated carrier
frequency is modulated in the respective electro-optic
modulator M10, M20, M30, M40 and then applied to
multiplexes 17. Multiplexes 17 is designed as a
wavelength-division multiplexes to combine the signals
to be transmitted onto a single optical line. If
necessary, multiplexes 17 is followed by an optical
amplifier.
All coupling units 22, 23, 24, 25 are designed, for
example, as optical couplers to combine the received
optical signals that belong together into one. The
outputs of the couplers are fed to the associated
optical receivers RX10, RX20, RX30, RX40, in which
processing or retransmission of the signal as, e.g., a
radio signal takes place.
An extraction unit with associated coupling unit can
also be designed as a single component by using an
3~ asymmetric, wavelength-dependent optical coupler with
two input ports and two output ports.
The optical signals transmitted over the upstream
channel are separated according to wavelengths at the
CA 02315869 2000-08-14
17
center in demultiplexer 16, and each wavelength is
applied to an optical receiver RX50, RX60, RX70, RX80
for evaluation.
Multiplexers 14 and 17, receiving unit 15, and
demultiplexer 16 may also be constructed from arrayed
waveguides (AWGs). This reduces the manufacturing
costs.
The third embodiment will now be explained with
reference to Fig. 3. Fig. 3 shows a third
communications network in accordance with the
invention. The communications network is designed as a
point-to-multipoint network comprising a center that is
connected by optical lines to a number of intermediate
facilities, of which one is shown. For the sake of
clarity, the plurality of customer premises that are
connected to the intermediate facilities or directly to
the center are not shown. The communications network is
designed as an HFC or HFR network, for example.
Alternatively, the communications network may consist
of optical lines between the center and the
intermediate facilities and of copper lines between the
intermediate facilities and the customer premises, over
which the information signals are transmitted in, e.g.,
ADSL format or the like. The communications network may
also consist of hybrids of the above network
structures. For the implementation of the downstream
channel and the upstream channel, two separate optical
00 lines, as shown, or a common optical line can be used.
In the simplest case, the communications network can
also be a point-to-point connection between two
transmitting/receiving facilities.
CA 02315869 2000-08-14
18
Information signals are transmitted from the center to
the intermediate facilities using dense wavelength-
division multiplexing (DWDM). By assigning four
wavelengths to one intermediate facility, a base
station, for example, can be supplied over the
downstream channel with the fourfold amount of data.
For each 90° sector of a base station, one wavelength
is used. In DWDM networks, a channel spacing of 50,
100, 200 GHz is possible. High wavelength stability is
necessary. Laser sources exhibit high temperature
sensitivity: about 12 GHz/°C, as opposed to other DWDM
components, such as optical filters, about 700 MHz/°C,
and arrayed waveguides (AWGs), about 1.5 GHz/°C.
Outside of buildings, temperatures may vary between
-40°C and +85°C. Then, particularly with a 50-GHz
channel spacing, wavelength stability is no longer
ensured. Therefore, according to the invention, the
laser sources are moved from the intermediate
facilities to the center, which is located inside a
building and is not subject to the above temperature
variations.
The center contains four optical transmitters TX100,
TX200, TX300, TX400, a multiplexer 28, a demultiplexer
26, and four optical receivers RX500, RX600, RX700,
RX800.
The intermediate facility shown contains a receiving
unit 29, four optical receivers RX100, RX200, RX300,
RX400, four extraction units 30, 31, 32, 33, four
optical modulators M100, M200, M300, M400, and a
multiplexer 27.
CA 02315869 2000-08-14
19
Optical transmitter TX100 contains a light source,
e.g., a laser or a laser diode. The light source is
operated in a CW mode, i.e_, it emits CW light. This
light is modulated directly or indirectly with the
information signal to be transmitted, the indirect
modulation being effected by an electro-optic
modulator. The modulated output signal is applied to
multiplexes 28. Optical transmitter TX100 generates
modulated light of a first wavelength x,100. The
modulation technique used is TDM, for example. Not all
existing time slots are used for transmitting
information from the center to the intermediate
facilities. The time slots not used for transmission
are used to transmit information from the intermediate
facilities to the center without employing optical
transmitters in the intermediate facilities. In the
assigned time slots, the intermediate facilities can
use TDMA, FDMA, or CDMA. Instead of TDM, frequency-
division multiplexing (FDM) or code-division
multiplexing (CDM) can be used as the modulation
technique. In FDM, part of the available frequencies
are used for the transmission of information from the
center to the intermediate facilities, and the
remainder are used for the transmission of information
from the intermediate facilities to the center. In CDM,
part of the available codes are used for the
transmission of information from the center to
intermediate facilities, and the remainder are used for
the transmission of information from the intermediate
facilities to the center.
Optical transmitters TX200, TX300, TX400 operate on the
same principle as optical transmitters TX100. They also
generate an optically modulated signal that is applied
to multiplexes 28. The difference is that firstly,
CA 02315869 2000-08-14
information signals with other contents are used.
Secondly, each optical transmitter TX100, TX200, TX300,
TX400 generates light of a different wavelength.
Optical transmitter TX200 generates light of wavelength
X200. Optical transmitter TX300 generates light of
wavelength X300. Optical transmitter TX400 generates
light of wavelength X400.
10 - Multiplexer 28 is designed as a wavelength-division
multiplexer, for example. It contains, for example,
DWDM components, such as optical splitters, optical
combiners, or the like.
Receiving unit 29 receives the optical signals
transmitted by the center over the downstream channel.
Receiving unit 29 contains, for example, a wavelength-
division demultiplexer including DWDM components for
separating the received optical signals into different-
wavelength signals. At the output of receiving unit 29,
eight different signals with eight different
wavelengths, for example, are then available. Four of
these signals are applied to the four optical receivers
RX100, RX200, RX300, RX400. In each receiver RX100,
RX200, RX300, RX400, the received signals are
demodulated, so that the transmitted information
signals can be detected and analyzed or further
processed. Alternatively, only a conversion of the
received signals is performed, e.g., in order to
transmit them in the correct format to customer
premises by radio.
All extraction units 30, 31, 32, 33 are designed as
asymmetrical optical couplers, for example. They
extract a portion of the received optical signal of a
CA 02315869 2000-08-14
21
given wavelength and feed it to one of electro-optic
modulators M100, M200, M300, M400. The extracted
portion, e.g., 30~ of the received signal, is used to
transmit an information signal over the upstream
channel to the center.
Modulators M100, M200, M300, M400 are electro-optic
modulators that modulate the signals of wavelengths
7.100, x,200, x,300, 7,400 with respective information
signals to be transmitted. The signals to be
transmitted originate, for example, from customer
premises that are connected to the intermediate
facility. The information signals contain, for example,
voice, data, and/or video for telephony, Internet, E-
mail, video telephony, etc. Modulation is effected
using TDMA, CDMA, or FDMA. The signals so modulated are
applied to multiplexer 27. Multiplexer 27 is designed
as a wavelength-division multiplexer, for example. It
contains DWDM components, for example, such as.optical
splatters, optical combiners, optical couplers, or the
like. The combined optical signals are then transmitted
simultaneously over the upstream channel to the center.
Demultiplexer 26, constructed from DWDM components,
receives the optical signals of wavelengths x,100, x,200,
x,300, x,400, separates them into four signals with one
wavelength each, and feeds these four signals to the
four optical receivers RX500, RX600, RX700, RX800. In
each optical receiver RX500, RX600, RX700, RX800, the
received optical signal is demodulated and then fed to
a processing device for analysis. Alternatively, only a
conversion is performed in order to retransmit a
corresponding information signal directly over the
downstream channel. The center then serves as a relay
CA 02315869 2000-08-14
22
station or an exchange, e.g. in the case of a telephone
connection between two customer premises connected to
the center.
