Amplification method in a PON access network, computer program product, corresponding optical line termination device

Abstract

The invention relates to an amplification method (10) intended for a PON access network, comprising an OLT with multiple ports and with one bidirectional amplifier per port that is shared for the transmission downlink and uplink, at least one ODN connecting a given port of the OLT to a plurality of ONUs and defining a transmission channel, access to the transmission channel for the uplink being of TDMA type and the transmission channel for the downlink being time-shared. The method comprises, for a given ODN: - receiving (12) bursts from the ONUs, - modulating (13) the gain (G) of the amplifier in synchronicity with the received bursts.

Description

Amplification method in a PON access network, computer program product, corresponding optical line termination equipment Field of invention

[0001] The field of the invention is that of optical telecommunications (optical fiber) and access networks of the "passive optical network" (PON) type. Within this field, the invention relates more particularly to an amplification method intended for such a PON type network. It applies in particular to optical line termination equipment.

[0002] A passive optical network (PON) refers to a fiber optic transport principle used in optical access networks (FTTx, "Fiber To The x"). It is characterized by a passive point-to-multipoint fiber architecture (several users share the same optical fiber and there is no active equipment between the exchange and the subscribers). There are different PON network standards, including GPON (ITU-T G.984 standard), XGS-PON (ITU-T G.9807 standard), NG-PON2 (ITU-T G.989 standard), HS-PON (ITU-T G.9804 standard), etc. These different access networks are called PON networks or PON-type networks. Prior art

[0003] A PON access network, illustrated by the, comprises optical line termination (OLT) equipment which is connected to optical network units (ONUs), via one or more point-to-multipoint optical distribution networks (ODNs).

[0004] The OLT equipment is the network-side termination equipment that provides the interface with the fibers of one or more ODN networks. In France, it is generally located in the optical connection node (NRO) and often located in a so-called central building. It traditionally has a chassis with cards each containing optical ports. An optical port can address several dozen clients, commonly 64, via an ODN network that includes a splitter / combiner (S / C). An OLT equipment can, for example, include more than a hundred optical ports respectively connected to as many ODN networks to address several thousand ONU units.

[0005] Each ONU provides the user interface. It converts optical signals received via a fiber into electrical signals that are then sent to individual subscribers. It is also sometimes referred to as an optical network termination (ONT).

[0006] Each ODN network provides the optical transmission medium for the physical connection of a plurality of ONUs to the OLT equipment, with a range limited to, for example, 20 km. The ODN network provides the optical channel for the transmission of optical signals between the OLT and the ONUs in both the downstream and upstream directions.

[0007] A given ODN network therefore establishes a transmission channel between the given port of the OLT equipment and each of the addressed ONU units, used both in the downstream direction (referenced "D" on the for "downstream", i.e. "OLT to ONU") and in the upstream direction (referenced "U" on the for "upstream", i.e. "ONU to OLT").

[0008] Although an optical fiber is generally dedicated to a single direction, downstream or upstream in the core network, a single optical fiber is used for the downstream and upstream directions in the case of a PON type access network between the OLT and a splitting point also called a sharing point. This sharing point corresponds in the downstream direction to the splitting of the optical fiber into several optical fibers implemented by a splitter and in the upstream direction to the grouping of the optical fibers coming from the different ONU units into a single fiber implemented by the same device called a combiner. This S / C splitter / combiner device is passive, it therefore divides the power of the downstream signal into as many output optical fibers. Depending on deployment constraints, several S / C splitters / combiners can follow one another.

[0009] As illustrated by the for a PON type access network, the channel access in the downstream direction follows a TDM (Time Division Multiplexing) access between the clients addressed according to a time cycle and the optical signal transmitted at a continuous amplitude in time. Indeed, even in the absence of data to transmit and address to a client, the OLT emits known sequences of 0 and 1.

[0010] As illustrated by the for a PON access network, upstream channel access is shared between ONUs using Time Division Multiple Access (TDMA). ONU transmission is performed in bursts. A burst is a sequence of binary data with a header and a data field (payload). A burst has a duration that can vary from a few nanoseconds (ns) to several hundred microseconds (µs).

[0011] The increase in throughput in a PON access network (for example with online throughputs above 10 Gbit / s) increases the constraints and in particular the difficulties in achieving a certain range for the transmitted signals, implied with a certain quality on reception, whether in the downstream direction (from the OLT equipment to the ONU unit) or in the upstream direction (from the ONU unit to the OLT equipment).

[0012] To ensure that the optical signal can be received by ONUs and OLTs with sufficient power, it is known in the field of optical transmission to use an optical amplifier.

[0013] Given that the optical access network generates significant investments, its developments to ensure increased throughput can rely on the use of fibers already deployed, particularly those of the access network, the deployment of which is very costly. Thus, the deployment of developments can be carried out according to specific configurations to limit costs.

[0014] Thus, and as illustrated by the, one of these configurations consists of using a common optical amplifier Amp between all the links processed by an optical port of an OLT to which a given ODN corresponds, both for the upstream and downstream directions.

[0015] To cover the different wavelengths transmitted in the downlink and uplink directions, the amplifier Amp is preferably chosen with a relatively flat gain G over the band comprising these wavelengths, i.e. with a flow variation of at most 3dB.

[0016] SOA (Semiconductor Optical Amplifier) ​​type optical amplifiers are particularly suited to the requirements of such a configuration.

[0017] The illustrates the simplified structure of an SOA type optical amplifier. As shown, an SOA amplifier has an amplification gain that can be controlled by a pump current intensity Ip. The SOA amplifier amplifies the input signal Se to provide the output signal Ss. The represents the gain of an SOA amplifier as a function of the input signal power for different pump current values. This figure illustrates the variation of the gain of the SOA amplifier as a function of the pump current.

[0018] The illustrates the phenomenon of cross-gain modulation that occurs in a PON access network configured with a common optical amplifier Amp for a given optical port of the OLT. The OLT comprises a transmitter Tx (laser transmitter for example) which receives as input a signal to be transmitted. The OLT comprises a receiver Rx of the uplink signal and an optical multiplexer / demultiplexer MUX / DEMUX of the uplink signal and the downlink signal, for example a dichroic filter. Each ONU unit, for example ONU1, comprises a transmitter Tx1, a receiver Rx1 and an optical multiplexer / demultiplexer MUX / DEMUX1 of the downlink signal received and the uplink signal transmitted by the ONU. The ODN network defines an optical channel which connects, in the downlink direction, the transmitter Tx located in the OLT equipment to each receiver Rx located in the ONU units. The optical receiver Rx (photodiode for example) included in the ONU unit receives the signal at the output of the optical channel and generates an output signal to the client.

[0019] The splitter is identified as the PM pooling point. The 1U optical signal uplinked from an ONU is in the form of bursts.

[0020] The point-to-multipoint nature of the PON network and its massive usage (millions of customers) imply a wide variety of OLT / ONU distances and therefore a wide variety of transmission channels. Previous generations of PON networks (notably G-PON, XGS-PON and NG-PON2) have so far provided interoperability for a transmission channel with a distance in the range of 0-20km (i.e. can be between 0 and 20 km, without prior knowledge).

[0021] The uplink signal resulting from the multiplexing of the signals from the active ONUs thus has guard times that can vary between bursts as well as amplitudes that can be different between bursts given that the distances traveled by the uplink signals between the ONUs and the combiner can be different and given that the uplink signals from the ONUs do not come from the same transmitter nor take the same fiber to the combiner.

[0022] When receiving the uplink signal by the OLT, the gain of the optical amplifier is only periodically requested by this uplink signal, i.e. for each burst present in the signal and for different values ​​between the bursts depending on the losses suffered by these bursts from the ONU units.

[0023] In the downlink direction, the envelope of the 2D optical signal is continuous, i.e. it does not appear in the form of bursts. However, this downlink optical signal undergoes temporal gain variations induced by the simultaneous crossings of uplink bursts in the common optical amplifier. These variations are visible on the 3D downlink signal at the amplifier output. This phenomenon, called "cross-gain modulation" (XGM), is similar to an interference phenomenon on the downlink signal which manifests itself by variations in the amplitude of this signal and which particularly impacts the reception performance associated with this signal.

[0024] The invention proposes an amplification method aimed at improving the implementation of a common bidirectional amplifier in the upstream and downstream directions hosted in OLT equipment for a PON type access network.

[0025] The invention relates to an amplification method intended for an access network of the passive optical network type comprising a multi-port optical line termination device and having a bidirectional variable gain amplifier per common port for the downlink and uplink transmission directions, at least one optical distribution network connecting a given port of the optical line termination device to a plurality of optical network units and defining a transmission channel, access to the transmission channel for the uplink being of the multiple time-shared type and the transmission channel for the downlink being time-shared. The method comprises, for a given optical distribution network: receiving bursts from the optical network units, modulating the gain of the amplifier in synchronization with the received bursts.

[0026] The invention further relates to a multi-port optical line termination device intended for a passive optical network type access network comprising at least one optical distribution network connecting a given port of the device to several optical network units and defining a transmission channel, the device having a bidirectional variable gain amplifier per common port for downstream and upstream transmission directions on the channel. The optical line termination device further comprises, for a given optical distribution network: at least one receiver of bursts coming from the optical network units with time-shared multiple access to the transmission channel between the units, a computer for controlling a modulation of the gain of the amplifier in synchronization with the bursts.

[0027] The time modulation of the amplifier gain in synchronization with the reception of a burst ensures an adaptation of the amplifier gain to compensate for the discontinuous nature of the uplink signal and its interference on the downlink signal due to the common amplifier. The inherent energy drop that the uplink signal presents between two consecutive bursts allows the downlink signal to benefit from all the amplification gain between two bursts. However, during the passage of a burst, the amplification gain is shared between the uplink signal and the downlink signal. Consequently, the downlink signal which has a continuous shape at the amplifier input presents breaks in shape at the amplifier output due to this gain sharing.Increasing the amplifier gain during a burst benefits the downlink signal during this passage with the reduction or even elimination of the interference initially caused by the non-continuity of the uplink signal and its discontinuous demand on the gain. The gain is then reduced after the end of the burst.

[0028] According to one embodiment of the invention, the gain modulation is obtained by controlling a supply current of the amplifier. Thus, the modulation is carried out by means of the electric current applied to the amplifier.

[0029] This embodiment is particularly suitable for an SOA type amplifier in which variations in the pumping current cause variations in the gain.

[0030] According to one embodiment of the invention, the modulation of the gain depends on the power of the burst.

[0031] This embodiment is more particularly suited to a deployment of a PON network according to which the optical network units are located at respective distances from the optical line termination device which are different from each other. The differences in distances then lead to differences in the respective powers of the bursts received from different units. Taking into account the received power makes it possible to better adjust the gain between the bursts coming from different ONU units and to combat the cross-gain phenomenon more effectively.

[0032] According to one embodiment of the invention, the access network being associated with a certain bandwidth in the upstream direction, the method further comprises:

[0033] - determination of a bandwidth distribution map between the different optical network units by the optical line termination device with identification of a start and duration of transmission for the optical network units,

[0034] - transmission of the bandwidth distribution map to the different optical network units,

[0035] And the process is such that the synchronization between the modulation and the reception of the bursts takes the map into account.

[0036] Thus, the optical line termination device OLT knows the times at which the bursts are transmitted. If the optical network units ONUs are at identical distances from the OLT then these times correspond to the arrival times to within a time that is easy to determine knowing the distance from an ONU to the OLT. This distance can for example be a parameter of the optical line termination device entered during the deployment of the PON network. If the optical network units ONUs are at different distances from the OLT then known methods for determining these distances can be implemented. Knowing each of these distances, the OLT then easily obtains the arrival times of the bursts by correcting their transmission times with the travel time of the respective distances.Thus, the optical line termination device knows in all cases the arrival times of the bursts and can easily synchronize the adaptation of the amplifier gain with these arrivals.

[0037] According to one embodiment of the invention, the amplifier is an SOA amplifier whose gain is adjustable by controlling a current.

[0038] According to one embodiment of the invention, the calculator is adapted to determine the power of the bursts received and adapt the gain control as a function of this power. List of figures

[0039] Other characteristics and advantages of the invention will appear more clearly on reading the following description of embodiments, given as simple illustrative and non-limiting examples, and the appended drawings, among which:

[0040] already described in relation to the prior art, is a diagram of a PON access network,

[0041] already described in relation to the prior art, illustrates TDM access to the downstream channel in a PON network,

[0042] already described in relation to the prior art, illustrates TDMA access to the upstream channel in a PON network,

[0043] already described in relation to the prior art, illustrates a PON type access network with a bidirectional amplifier common to the OLT,

[0044] already described in relation to the prior art, is a diagram of a bidirectional amplifier of the SOA type,

[0045] already described in relation to the prior art, are curves of the gain as a function of the power of the input signal of a bidirectional amplifier of the SOA type for different values ​​of the pumping current,

[0046] already described in relation to the prior art, is a diagram of a PON type access network to which has been added a temporal representation of the uplink signal transmitted by an ONU unit, of the downlink signal before wavelength multiplexing with the uplink signal, and of the downlink signal after amplification by the bidirectional amplifier,

[0047] is a diagram of a PON type access network comprising a bidirectional amplifier with amplification gain control according to the invention and showing the simplified structure of an OLT according to a particular embodiment of the invention, configured to implement an amplification method according to the invention, to which has been added a temporal representation of the uplink signal transmitted by an ONU, of the downlink signal before wavelength division multiplexing with the uplink signal, and of the downlink signal after amplification by the bidirectional amplifier,

[0048] is a representative curve of the gain as a function of the wavelength of a bidirectional amplifier of the SOA type,

[0049] is a flowchart of an embodiment of an amplification method according to the invention. Description of particular embodiments

[0050] In all figures of this document, identical elements and steps are designated by the same alphabetical or numerical reference.

[0051] The invention is placed in the context of a PON type access network, that is to say according to a passive point-to-multipoint fiber architecture between the OLT and the ONUs. There is therefore no active element in the ODN which makes the link between a port of the OLT and the ONUs receiving this port, particularly for cost reasons. In addition, access to the downstream channel responds to a TDM (Time Division Multiplexing) scheme and access to the upstream channel responds to a TDMA (Time Division Multiple Access) scheme under the control of the OLT.

[0052] In this context, the general principle of the invention is based on controlling the gain of the bidirectional amplifier of the OLT in time synchronization with the reception of an uplink burst and as a function of the energy of this burst.

[0053] This is a diagram of a PON type access network comprising a common bidirectional amplifier in the upstream and downstream directions with amplification gain control according to one embodiment of the invention.

[0054] This PON type access network includes at least one OLT equipment with at least one optical port, several similar ONU units ONU1, ONU2, ONU3 and one ODN network per optical port of the OLT equipment.

[0055] The OLT equipment includes per optical port a Tx transmitter of the downlink signal, an Rx receiver of the uplink signal and an OADM optical multiplexer / demultiplexer of the uplink signal and the downlink signal.

[0056] Each unit, ONU1, comprises at least one transmitter Tx1, one receiver Rx1 and one optical multiplexer / demultiplexer MUX / DEMUX1 of the downlink signal and the uplink signal coming from the unit, ONU1.

[0057] Each transmitter Tx, Tx1 is for example a laser diode. The transmitter Tx of the OLT equipment receives as input a signal comprising data to be transmitted to one or more units ONU1, ONU2, ONU3. This input signal is for example a bit stream in NRZ format and at a rate of several tens of Gb / s.

[0058] The optical multiplexer / demultiplexer MUX / DEMUX1 is for example of the OADM Optical Add and Drop Multiplexer type.

[0059] The ODN network defines an optical channel that connects, in the downstream direction, the Tx transmitter located in the OLT equipment to each Rx1 receiver located in the ONU1, ONU2, ONU3 units and, in the upstream direction, the Tx1 transmitters located in each ONU1, ONU2, ONU3 unit to the Rx receiver located in the OLT equipment.

[0060] The OLT equipment includes for a given optical port, a bidirectional optical amplifier Amp common to the downstream and upstream direction.

[0061] According to one embodiment, the amplifier is an optical amplifier of the SOA (Semiconductor Optical Amplifier) ​​type. The SOA amplifier, the gain of which is shown on the has a bandwidth of between approximately 1300 and 1340nm. This amplifier is particularly suitable for an HS-PON access network.

[0062] According to another embodiment, the amplifier is an optical fiber amplifier (Erbium, Praseodymium).

[0063] The SOA type amplifier has the advantage of being less expensive and more compact than a fiber optic amplifier.

[0064] The OLT equipment comprises a microprocessor µP or equivalent such as a microprogrammed electronic component whose operation is controlled by the execution of a program Pg whose instructions allow the implementation of an amplification method according to the invention. The program is for example stored in a memory MEM.

[0065] The µP microprocessor controls at least the various components via control signals: the bidirectional optical amplifier Amp, the receiver Rx, the transmitter Tx, the optical multiplexer / demultiplexer OADM. The µP microprocessor remotely controls the ONU units via an exchange protocol that is generally transmitted in the transmitted frames (so-called In-band protocol).

[0066] Ladonne provides a flowchart of an amplification method according to the invention.

[0067] When the OLT equipment is initialized, the code instructions of the program Pg are, for example, loaded from the memory MEM into a memory of the microprocessor or into a buffer memory (not shown) before being executed by the microprocessor μP for the implementation of an amplification method 10 according to the invention.

[0068] The OLT equipment knows the PON network architecture. It knows the distances d1, d2 between each ONU1, ONU2 and the OLT. This knowledge can result from a configuration of the OLT equipment or be obtained by implementing a known method (ranging) for determining distances.

[0069] Thus, by executing the instructions, the microprocessor µP establishes 11 a bandwidth distribution map Bwmap (Bandwith map) between the different ONU units.

[0070] This map is periodically re-evaluated to allow dynamic evolution of the bandwidth allocated to the ONUs. The OLT regularly assigns more or less time resources to the ONUs using the DBA (Dynamic Bandwith Assignment) method, which periodically determines the re-evaluated map. This method generally takes into account the distance of each ONU from the OLT, the activity of each ONU (an inactive ONU has little or no bandwidth) and the QoS parameters assigned to the different clients of the ONUs (contractually, a client can benefit from a service with more or less bandwidth).

[0071] The BWmap can be transmitted in a header of each downlink frame and each destination ONU extracts from this header the information concerning it, in particular the time when it can start transmitting T0 in the uplink direction and the authorized transmission duration ∆t.

[0072] The 1U optical signal uplinked from an ONU is in the form of a burst transmitted during the allocated time ∆t for this ONU. The BWmap generated by the OLT thus identifies for each ONU the instant T0 of start of transmission of a burst and the allocated transmission duration ∆t. This allocation of an instant T0 and a duration can possibly be in the form of the identification of allocated time slots. The card can possibly define these allocations for several transmission cycles.

[0073] The bursts from the different ONU units time-multiplexed on the uplink signal can thus be aggregated at the splitter / combiner without there being any collision between the bursts from the different ONU units.

[0074] The OLT thus orders the multiplexing of the uplink signals from the different ONU units and therefore knows the arrival time T1 of a burst since this is determined from the BWmap. The arrival time T1 differs from the transmission time T0 by the channel travel time between the ONU and the OLT by the transmitted burst.

[0075] By executing the instructions, the microprocessor µP controls the receiver Rx to receive, reference 12, a burst and recover, reference 12, the power of this burst. This power can correspond to the RSSI (Received Signal Strength Indicator) or can be determined by any other equivalent means.

[0076] Knowing the time of arrival of a burst and its power, the microprocessor modulates the gain G of the amplifier accordingly.

[0077] Thus, the microprocessor controls the value of the I_SOA current to be applied to a bidirectional amplifier SOA to adapt the gain during the duration of this burst. The increase in gain during the burst is such that it ensures sufficient amplification energy to be shared between the uplink signal and the downlink signal without fluctuation on the downlink signal. At the end of the burst the gain is decreased.

[0078] Thus, with the invention and unlike the state of the art, the 3D downlink optical signal no longer undergoes temporal variation. Adapting the amplifier gain in synchronization with the passage of a burst and in relation to the power of this burst makes it possible to distribute the amplification energy between the uplink and downlink directions while absorbing fluctuations in the uplink signal. The invention makes it possible to effectively combat the phenomenon of "cross-gain modulation" which impacts the downlink signal.

[0079] Accordingly, the invention also applies to one or more computer programs, in particular a computer program on or in an information carrier, adapted to implement the invention. This program may use any programming language, and be in the form of source code, object code, or intermediate code between source code and object code such as in a partially compiled form, or in any other form desirable for implementing a method according to the invention.

[0080] The information carrier may be any entity or device capable of storing the program. For example, the carrier may include a storage medium, such as a ROM, for example a CD-ROM or a microelectronic circuit ROM, or a magnetic recording medium, for example a USB key or a hard disk.

[0081] On the other hand, the information carrier may be a transmissible carrier such as an electrical or optical signal, which may be conveyed via an electrical or optical cable, by radio or by other means. The program according to the invention may in particular be downloaded from a network such as the Internet.

[0082] Alternatively, the information carrier may be an integrated circuit in which the program is incorporated, the circuit being adapted to perform or to be used in the performance of the method in question.

Claims

An amplification method (10) for a passive optical network (PON) access network, comprising a multi-port optical line termination device (OLT) and having a bidirectional variable-gain amplifier (Amp) per common port for the downlink (D) and uplink (U) transmission directions, at least one optical distribution network (ODN) connecting a given port of the optical line termination device (OLT) to a plurality of optical network units (ONUs) and defining a transmission channel, the uplink transmission channel access being of the time-shared multiple (TDMA) type and the downlink transmission channel being time-shared (TDM), characterized in that it comprises, for a given optical distribution network (ODN): reception (12) of uplink bursts from the optical network units (ONUs),modulation (13) of the gain (G) of the bidirectional amplifier (Amp) in synchronization with the received updrafts to reduce interference from updrafts on a downdraft signal. Method (10) according to claim 1, wherein the gain modulation is obtained by controlling a supply current to the bidirectional amplifier. Method (10) according to any one of claims 1 and 2, wherein the gain modulation depends on the power (RSSI) of the rising bursts. Method (10) according to any one of claims 1 to 3, the access network being associated with a certain upstream bandwidth (BW), the method further comprises: determination (11) of a bandwidth allocation map (BWmap) between the different optical network units (ONU) by the optical line termination device (OLT) with identification of a start and duration of transmission for the optical network units (ONU), transmission of the bandwidth allocation map (BWmap) to the different optical network units (ONU), the synchronization between the modulation and the reception of upstream bursts takes into account the map. A multi-port optical line termination (OLT) device for a passive optical network (PON) access network comprising at least one optical distribution network (ODN) linking a given port of the device (OLT) to several optical network units (ONUs) and defining a transmission channel, the device having a bidirectional variable-gain amplifier (Amp) per common port for downlink (D) and uplink (U) transmission on the channel, comprising, for a given optical distribution network (ODN): at least one uplink burst receiver (Rx) from the units (ONUs) with time-shared multiple access (TDMA) to the transmission channel between the units (ONUs), characterized in that it comprises: a computer (µP) for controlling gain modulation (G) of the bidirectional amplifier (Amp) in synchronization with the uplink bursts to reduce uplink burst interference on a downlink signal. Optical line termination (OLT) device according to claim 5 wherein the amplifier is an SOA amplifier whose gain is adjustable by current control. Optical line termination (OLT) device according to any one of claims 5 and 6 wherein the computer is adapted to determine the power of the received upbursts and adapt the gain control according to this power. Computer program on an information carrier, said program comprising program instructions adapted to the implementation of a method according to any one of claims 1 to 4 when said program is loaded and executed in an OLT equipment. Information carrier comprising program instructions adapted to the implementation of a process according to any one of claims 1 to 4, when said program is loaded and executed in an OLT equipment.

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