WO2007051843A1

WO2007051843A1 - Method for controlling power levels in a point-to-multi point network

Info

Publication number: WO2007051843A1
Application number: PCT/EP2006/068082
Authority: WO
Inventors: Samir Satish Sheth
Original assignee: Nokia Siemens Networks & Co. Kg
Priority date: 2005-11-04
Filing date: 2006-11-03
Publication date: 2007-05-10
Also published as: EP1946463A1

Abstract

The invention relates to a method for power levelling in an optical point to multipoint network, whereby a central optical line termination unit transmits a downstream signal to a plurality of optical network terminals. According to the invention the power level of the received downstream signal is measured at at least one network terminal and the power level of an upstream signal to be transmitted from at least one network terminal to said central unit is determined in dependence of the results of the measuring. This way the power level of the upstream signal is advantageously determined independent of the central optical line termination unit.

Description

METHOD FOR CONTROLLING POWER LEVELS IN A POINT-TO-MULTIPOINT NETWORK

This application claims priority of U.S. Provisional Patent Application No. 60/733,985 filed November, 04, 2005, titled "Upstream Power Levelling Method for Passive Optical Network- ing Systems", the disclosure of which is incorporated fully herein by reference.

The present invention relates to a method for power levelling in an optical point to multipoint network, an optical network terminal usable in an optical point to multipoint network and an optical point to multipoint network.

Modern access networks can be realized as passive optical networks, which comprise a point-to-multipoint network archi- tecture in which unpowered optical splitters are used to en^¬ able a connection between a central unit and multiple network user units. A passive optical network consists of an optical line termination (OLT) at the service provider's office and a number N of optical network units or terminals (ONUl, ..., ONUi, ... ONUN) near the end users. The optical line termina^¬ tion (OLT) provides the network-side interface of the optical access network and is connected to one or more optical dis^¬ tribution networks. An optical terminal (ONUi) provides the user-side interface of the access network. In downstream di- rection the signals are broadcast from the OLT to the multi^¬ ple terminals in time multiplex broadcast mode in a continu^¬ ous manner. In upstream direction the network terminals commonly share the link in a time-multiplexed way. This means that different network terminals transmit information to the central optical line termination OLT in different timeslots. Each network terminal thus sends upstream information in short bursts to the OLT. Details concerning the transmission in PONs are described in the ITU_T Recommendation G.984.3: Gigabit-capable passive op^¬ tical networks (G-PON) : Transmission convergence layer speci^¬ fication .

The losses in the described access network are not uniform because the distances between single users and the OLT may vary. Furthermore the ONU transmitter output power per user may vary. Due to such differences in the losses for different ONUs, the OLT receiver must provide a high sensitivity and a large dynamic range for reception at high bit rates. In order to relax the dynamic range of the OLT receiver, the transmit^¬ ter power level of the optical terminals experiencing a low network loss could be reduced in order to avoid overload of the OLT receiver. Similarly, in case of a high network loss, the transmitter power level of the ONUs could be increased. Power Levelling is the process whereby the ONU changes (in^¬ creases or decreases) its transmit power in order to improve the signal-to-noise ratio at the OLT.

According to chapter 10.8 of the Amendment 1 (from 07/2005) of the ITU-T recommendation G.984.3 there are two methods for initiating the process of power levelling: ONU-activated and OLT-activated. ONU-activated Power Levelling is initiated when the ONU responds to a specified number of S/N Requests without receiving an Assign _0NU-ID message from the OLT. This specified number of S/N Requests is called the S/N_Request_Threshold and the recommended value is 10. Initially, the ONU uses the Power Level mode specified in the Upstream Overhead message. When the S/N_Response_Threshold is crossed, the ONU increments its operating Power Level using modulo 3( ,0, 1, 2, 0, 1, 2, ) and resumes responding to S/N Requests. If the ONU again responds to a specified number of S/N Requests without response, it increments again its Power Level using modulo 3. This cycle is repeated until it receives either an Assign_ONU-ID message or a Disable ONU message . OLT-activated Power Levelling is initiated when the OLT de^¬ termines the ONU needs to change its power level. This deter^¬ mination could occur when the ONU is in Ranging State or Operation State and is indicated by an unacceptable BER for a particular ONU. In this case, the OLT sends a directed Change Power Level message to the specific ONU to increase and/or decrease power level as needed.

In both methods the OLT controls the process of the power leveling. For both methods software interfaces are required at both the OLT and ONUs for implementation. Interoperability with other OLT vendors is difficult. Furthermore finer power levelling than 6 dB with 3 dB steps as proposed in the ITU recommendation also over a wider range is not possible.

Therefore it is the object of the present invention to pro^¬ vide a more simple method for power levelling the upstream signal in a point-to multipoint network independent of the central OLT. It is a further object of the invention to pro- vide an optical network unit usable in a point-to multipoint network for supporting power levelling independent of the OLT.

In accordance with the foregoing objectives, there is pro- vided by the invention a method for power levelling in an optical point to multipoint network, whereby a central unit transmits a downstream signal to a plurality of optical net^¬ work terminals. According to the invention the power level of the received downstream signal is measured at at least one network terminal and the power level of an upstream signal to be transmitted from at least one network terminal to said central unit is determined in dependence of the results of the measuring. This way the power level of the upstream sig^¬ nal is advantageously determined independent of the OLT. The terminal controls the adjustment of the upstream power level only in dependence of the power level of the downstream sig^¬ nal. Thus there is no communication between the ONU and OLT necessary and the software complexity is reduced. In addition a continuous tunig of the transmitted power level at the ONU is possible.

In a preferred embodiment of the invention the total loss be- tween an ONU and the OLT is determined, then the operating power range of a receiver element of said central unit is de^¬ termined and after a comparison of the total loss with a maximum receivable power value of the receiver element of said central unit the power level of the upstream signal is set. In this embodiment the method uses received signal strength of the downstream signal in order to approximate up^¬ stream loss. In addition the OLT received power is maintained within a small window thus reducing OLT receiver complexity and error rates. If the link loss it large, the transmitter power at the ONU will be set at maximum. If the link loss is small, the transmitter power at the ONU will be clipped to ensure that the power at the OLT receiver does not exceed a pre-defined value.

In another preferred embodiment a correction factor is considered in the total loss calculations considering different wavelengths of the upstream and downstream signals and dif^¬ ferent fiber types. Here it is the advantage that the knowl^¬ edge of the link characteristics of the downstream direction is used to approximate the loss for the upstream direction. This way a more flexible operation of the link is possible. If there are changes in the loss because for example a dif^¬ ferent fiber is inserted, the power levelling can be adjusted easily .

In another preferred embodiment power level of the transmit^¬ ter element of said central unit is communicated to the net^¬ work terminal. This way the accuracy of the inventive may be improved.

The invention will now be described with reference to the figure . Figure 1 shows a flowchart diagram to point out the single steps of the power levelling method in accordance with the present invention.

With reference to Fig. 1 the power levelling methodology is described. In the described embodiment of the invention a passive optical network is considered. The downstream signal is transmitted at a wavelength of 1490 nm whereas multiple users transmit the upstream signal at the same wavelength which is here chosen to be 1310 nm. The transmission from an ONU to an OLT takes place in a fiber of a special fiber type. Corning NexCor SMF-28e fiber is predominantly used in PON systems. The fiber has the characteristics that it enables double the launch power compared to other ITU-T G.652.D fi- bers and therefore provides improved reach. It is a full spectrum fiber thus providing 50 % more usable spectrum. The fiber is ideal for PON because it supports transmission at 1310 nm, 1490 nm, and 1550 nm.

In a first step the power of the received downstream signal at the receiver element of a network terminal ONU is meas^¬ ured. This is indicated in box 1 of Fig.l. Here the received signal wavelength is 1490 nm. In this example the received power Rx at the ONU is R = -20 dBm. An uncertainty of +/- 1 dB is assumed.

In a second step the total loss of the link between the OLT and ONU is calculated. This is done in several steps, because there are numerous factors to be considered such as differ- ence in loss for the different wavelengths of the downstream and upstream signal or fixed and distributed losses of the link .

To determine the loss the transmitted downstream power from the OLT has to be known. Therefore, as indicated in box 2, a mid value of the transmitted downstream signal at 1490 nm from the OLT is determined. For example if the power range of the transmit power of the OLT is between + 2 and -2 dBm, then a value of Td = 0 dBm is assumed for the OLT transmitter. Thus Td corresponds to the mid range or tolerance of the transmitted power of the OLT. The transmitted downstream power from the OLT can be either estimated or communicated from the OLT to the ONU. In the latter case a higher accuracy will be achieved.

Generally the total loss of the link is calculated by sub^¬ tracting the received power at the ONU from the transmitted power of the OLT, i.e. in our example Td-R. The uncertainty is now the uncertainty of the transmitter power of the OLT and the uncertainty of the receiver power of the ONU measure^¬ ment. The total loss is here estimated to be +/- 3 dB . If the transmitted power of the OLT at 1490 nm is communicated to the ONU, this uncertainty can be further reduced.

In order to get the correct loss, the difference of the at^¬ tenuation constant of the fiber for different wavelengths has to be taken into account. Therefore a correction factor C is introduced to account for the loss difference between the up^¬ stream wavelength of 1310 nm and the downstream wavelength of 1490 nm. This is indicated in box 3 of Fig.l. The correction factor C can be determined by knowing the fixed and distrib^¬ uted losses in the system. There is a large fixed loss due to the 32 way splitter. Additionally, there are connectors and/or splice losses. Assuming two connectors and 0.2 dB per connection, we have 0.4 dB loss. Fibers are spliced every two kilometers. The average splice loss is 0.05 dB or less per splice. If the fibers are connectorized, the losses could in- crease to 0.1 dB per km. Therefore, this can be normalized to 0.025 dB per km or 0.1 dB per km depending on connection method. The total distributed loss is then the fiber attenua^¬ tion plus the distributed loss. A Method to determine the correction factor C listed below:

a) determination of fixed losses:

The splitter loss for a 32 way splitter (assuming the same loss for all wavelengths) is calculated as 10*log(32)+l = 16 dB . Other losses of connectors and splices are estimated to be 1 dB . The sum of splitter losses and other losses yields a value of 17 dB for the fixed losses.

b) determination of distributed losses:

The loss per km for a Corning NexCor SMF -28e fiber at a wavelength of 1490 nm equals to 0.21 dB/km. The loss per km for a Corning NexCor SMF -28e fiber at a wavelength of 1310 nm equals to 0.35 dB/km. The distributed fiber loss per km at 1490 nm assuming dis^¬ tributed splices is 0.21 dB/km +0.025 dB/km =0.235 dB/km. (The distributed fiber loss per km at 1490 nm assuming dis^¬ tributed connectors is 0.21 dB/km +0.1 dB/km =0.31 dB/km.) The distributed fiber loss per km at 1310 nm assuming dis- tributed splices is 0.35 dB/km +0.025 dB/km =0.375 dB/km.

c) computation of the correction factor C using the fiber length

The fiber length can be calculated by the following formula: Fiber length = (Measured 1490 nm loss- (fixed loss))/ (dis^¬ tributed fiber loss per km @1490 nm) . If for example a loss of 20 dB is measured, the fiber length is determined to be: (20 dB-17 dB) /0,235 dB/km = 12,76 km. The correction factor C is calculated using the folowing for- mula:

C = Fiber length* (distributed fiber loss per km @1310 km- distributed fiber loss per km @1490 km) = difference of the attenuation constants of both wavelengths In the example considered above C equals to 12,76 km* (0,375- 0,235)=l,78 dB

As indicated in box 4 of Fig. 1, the total loss is calculated as idfference between the mid-range of the transmitted power of the OLT and the mid-range of the received power of the ONU plus the correction factor C: Loss=Td-R+C

Thus The method uses the 1490 nm received signal strength in^¬ dicator to approximate upstream loss. In a next step, as indicated in box 5 of Fig. 1, the operat^¬ ing range of the receiver of the OLT is determined. For exam^¬ ple, if the sensitivity S of the receiver is -30 dBm, then the dynamic range should start at -30 dBm but the maximum power level Pmax cannot exceed -30 dBm plus the extinction ratio ER. This ensures that the decision threshold is below - 30 dBm. In practice, a margin M of at least 3 dB should be applied. For the maximum power level Pmax of the OLT receiver the following formula is yielded: Pmax = S + ER -M

The extinction ratio ER here is defined as the ratio of the power in ones to the power in the zeros. When the upper limit P is set above the sensitivity by the extinction ratio, this ensures that the power in the zeroes will be at the sensitiv^¬ ity of the receiver. The margin M moves that power in the ze^¬ roes below the sensitivity by M dB .

Summarizing the operating range of the receiver of the OLT results in Pmin = S and Pmax = S + ER -M

The OLT received power is maintained within a small window thus reducing OLT receiver complexity and error rates. The window is defined by the transmitter extinction ratio to ensure that the power in the zeroes is below receiver sensitiv- ity.

As example a sensitivity of -30 dBm is assumed. The extinc^¬ tion ratio is 10 dB and the margin shall be 3 dB . Therefore the operating range of the receiver of the OLT lies between - 30 dBm and -23 dBm.

In order to set the power level of upstream signal at the transmitter at the ONU, the total loss of the link is com^¬ pared to the difference between the maximum power of the ONU transmitter and the OLT receiver (Tumax-Pmax) . This means that the ONU transmitter output power is determined with re^¬ spect to the maximum value of power Pmax desired at the OLT receiver. If the total link loss is smaller than the difference between the maximum power of the ONU transmitter and the maximum power of OLT receiver then the transmission power of the ONU is set to the maximum value of power Pmax desired at the OLT plus the link loss (compare box 6 in Fig.l) . Other^¬ wise in the case of large losses the transmission power of the ONU is set to the maximum value of the ONU transmitter

(compare box 7 in Fig.l) . This keeps the transmit power equal to Pmax for lower insertion loss and at higher insertion loss the transmit power is at its maximum. If the link loss it large, the ONU transmitter power will be at maximum Tumax . If the link loss is small, the transmitter power will clipped to ensure that the power at the OLT receiver does not exceed the pre-defined value Pmax.

Because a margin value is defined to account for the uncer^¬ tainty of the loss measurement, the power window is narrowed by the margin M to ensure that the maximum value is less than the sensitivity plus the extinction ratio.

Claims

Patent Claims

1. A method for power levelling in an optical point to multi^¬ point network, whereby a central unit (OLT) transmits a downstream signal to a plurality of optical network terminals (ONUl, ..., ONUN), the method comprising the steps of measuring the power level of a received downstream signal at at least one network terminal (ONUi) , determining the power level of an upstream signal to be transmitted from at least one network terminal (ONUi) to said central unit (OLT) in dependence of the results of the meas^¬ uring .

2. The method of claim 1,

Wherein the determination of the power level of the upstream signal comprises the steps of determining the transmitted power level of a transmitter element of said central unit (OLT) , determining the total loss for the link between the said central unit (OLT) and said terminal (ONUi) , determining the operating power range of a receiver element of said central unit (OLT) comparing the total loss with a maximum receivable power value of the receiver element of said central unit (OLT) setting the power level of the upstream signal to a maximum power value of the transmitter element of said terminal (ONU) for a large total loss value, setting the power level of the upstream signal to the maximum receivable power value of the receiver element of said cen^¬ tral unit (OLT) plus the loss value for a small total loss value .

3. The method of claim 2, wherein a correction factor is considered in the total loss calculations considering different wavelengths of the up^¬ stream and downstream signals and different fiber types.

4. The method of claim 2, wherein the transmitted power level of the transmitter ele^¬ ment of said central unit (OLT) is determined by sending it to the network terminal (ONU) .

5. The method according to any of the preceding claims wherein the point to multipoint network comprises of an ac^¬ cess passive optical network.

6. An optical network terminal (ONUi) usable in an optical point to multipoint network, the network comprising a central optical line termination unit (OLT) coupled to a plurality of optical network terminals (ONUl, ..., ONUN), the network terminal (ONUi) comprising a receiver element to receive a downstream signal from a transmitter element of said central unit (OLT) and characterized in that the network terminal (ONUi) comprises means for measuring the power level of the received downstream sig^¬ nal and means for determining a power level of an upstream signal to be transmitted from a transmitter element of the terminal (ONUi) to a receiver element of the optical line termination unit (OLT) in dependence of the results of the measuring.

7. The optical network terminal (ONUi) of claim 6, further comprising means for determining the transmitted power level of the transmitter element of said central unit (OLT) , determining the total loss for the link between the said central unit and said terminal determining the operating power range of the receiver element of said central unit (OLT) comparing the total loss with a maximum receivable power value of the receiver element of said central unit (OLT) setting the power level of the upstream signal to a maximum power value of the transmitter element of said terminal (ONU) for a large total loss value setting the power level of the upstream signal to the maximum power value of the receiver element of said central unit (OLT) plus the loss value for a small total loss value

8. An optical point to multipoint network, comprising a cen^¬ tral optical line termination unit (OLT) coupled to a plural- ity of optical network terminals (ONUl, ..., ONUN), said net^¬ work terminals being designed according to anyone of claims 6 and 7.