WO2015120894A1

WO2015120894A1 - A method and apparatus for upgrading an optical node in an installed wdm network

Info

Publication number: WO2015120894A1
Application number: PCT/EP2014/052838
Authority: WO
Inventors: Fabio Cavaliere; Luca Giorgi; Roberto Sabella; Luca Poti
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2014-02-13
Filing date: 2014-02-13
Publication date: 2015-08-20

Abstract

The present invention relates to a method and apparatus for upgrading an optical node in an installed WDM optical network comprising a plurality of frequency channels each having a predetermined bandwidth. The optical node is configured to transmit traffic having a first bit rate over each of the plurality of frequency channels. The apparatus comprises a first optical carrier generator and a second optical carrier generator each configured to generate an optical carrier at a respective frequency. The apparatus also comprises first and second narrow-band filters configured to receive and narrow-band filter respective modulation signals, each having a symbol rate and comprising traffic to be transmitted over the WDM optical network, the traffic having a bit rate equal to or greater than the first bit rate. Each of the narrow-band filters has a cut off frequency less than half the symbol rate of its modulation signal. The apparatus further comprises first and second modulators, each configured to modulate a respective one of the optical carriers using QPSK modulation and a respective one of the narrow-band filtered modulation signals. The apparatus further comprises a multiplexer configured to frequency division multiplex the modulated first optical carrier and the modulated second optical carrier to form a combined optical signal, which, when the apparatus is installed in the optical node, is transmitted over a single one of the plurality of frequency channels.

Description

A METHOD AND APPARATUS FOR UPGRADING AN OPTICAL NODE IN AN

INSTALLED WDM NETWORK

Technical Field

The present invention relates to a method and apparatus for upgrading an optical node in an installed WDM (wavelength-division-multiplexed) network, and in particular to a method and apparatus for upgrading an optical node in an installed WDM network to increase the spectral efficiency of the WDM network.

Background

The increase in mobile devices running broadband services requires ever higher capacity in the transport network. Transport networks typically comprise DWDM (dense wavelength- division-multiplexed) networks. Many network operators have recently upgraded their

DWDM networks, such that today these networks can carry 4.8 Tbit/s on a single fibre link. These DWDM networks have 48 wavelength or frequency channels, each with a bandwidth of 50Ghz and spaced according to the ITU-T frequency grid. A 100Gbit/s DP-QPSK carrier is transmitted over each of the frequency channels.

New technologies are being developed to further increase the capacity of these DWDM networks. These technologies include higher speed optical transceivers, which can transmit data at up to 400 Gbit/s, and flexible grid ROADMs (reconfigurable optical add drop multiplexers). Flexible grid ROADMs are able to adjust the bandwidth of each of the frequency channels and the central frequency position of each of the channels with 12.5 GHz and 6.25 GHz granularity, so as to optimise the use of the available spectrum.

However, to introduce a flexi-grid, the existing ROADMs need to be replaced with new flexi- grid nodes, which requires expensive new equipment and a costly manual upgrade. A significant upgrade to the network control system is also required so that the control system can dynamically manage the bandwidth and central frequency position of each of the frequency channels, each time a new optical path is requested.

It has been proposed to improve the spectral efficiency of DWDM networks (i.e. the bit s transmitted per Hz of spectrum) by using larger constellation modulation formats such as 4QAM (Quadrature Amplitude Modulation). These larger constellation modulation formats allow more bits of data to be transmitted per modulation symbol. However, as these modulation formats produce signals with low tolerance to noise and fibre non-linear effects, they can only be satisfactorily transmitted over a short link distance, and therefore these modulation formats are not practical for use in real WDM networks. Lower size modulation formats, such as QPSK, allow transmission over longer link distances but they require more bandwidth. A 400Gbit/s QPSK channel exploiting two orthogonal polarisation states needs a 100GHz channel/slot, which is incompatible with current DWDM networks where the channels are 50GHz. Furthermore, 75GHz bandwidth electronics for the transceiver would be required, which have a much greater bandwidth than the 25GHz electronic devices currently used in existing WDM networks.

Thus, optical network system vendors are proposing to upgrade existing WDM networks to transmit 400Gbit/s signals by using two optical carriers each carrying two orthogonally polarised 16QAM modulated signals. 16QAM has a constellation format lower than 64QAM but greater than QPSK. In this solution, the spectral occupancy of the 400Gbit/s data is still 100GHz (greater than the bandwidth of a channel/slot in current DWDM networks).

However, since each carrier can be allocated to a single 50GHz slot, the solution is compatible with current 100 Gbit s WDM networks. The solution can also reuse existing 25GHz electronic devices. Thus, in this proposal the spectral efficiency of a WDM link can be doubled (although not quadrupled) and the link distance achievable is longer than is possible using higher modulation formats.

Nevertheless, the applicant has appreciated that the maximum link distance possible is still significantly lower than the link distance currently achievable in existing WDM networks. Although a link distance of 1000km has been obtained in the laboratory, 600km is the typical distance achievable in practical network design where unequal span lengths impair the OSNR (optical signal to noise ratio) and margins need to be allocated. Therefore, the optical infrastructure would need to be upgraded, for example to add expensive repeaters/amplifiers or to upgrade the optical fibre, in order to achieve the necessary link distance. This would require a substantial, time consuming and costly network upgrade.

Summary

The applicant has appreciated that it would therefore be desirable to provide an alternative solution for upgrading an installed WDM network to increase the spectral efficiency of the network. According to the present invention there is provided apparatus for upgrading an optical node in an installed WDM optical network to increase the spectral efficiency of the WDM network. The WDM network comprises a plurality of frequency channels each having a predetermined bandwidth. The optical node is configured to transmit traffic having a first bit rate over each of the plurality of frequency channels.

The apparatus comprises a first optical carrier generator configured to generate a first optical carrier at a first frequency. The apparatus further comprises a first narrow-band filter configured to receive a first modulation signal having a symbol rate and comprising traffic to be transmitted over the WDM optical network, the traffic having a bit rate equal to or greater than the first bit rate. The first narrow-band filter is configured to narrow-band filter the first modulation signal. The first narrow-band filter has a cut-off frequency less than half the symbol rate of the first modulation signal. The apparatus further comprises a first modulator configured to modulate the first optical carrier to carry the traffic to be transmitted over the WDM optical network using QPSK modulation and the narrow-band filtered first modulation signal.

The apparatus also comprises a second optical carrier generator configured to generate a second optical carrier at a second frequency different from the first frequency. The apparatus further comprises a second narrow-band filter adapted to receive a second modulation signal having a symbol rate and comprising further traffic to be transmitted over the WDM optical network, the further traffic having a bit rate equal to or greater than the first bit rate. The second narrow-band filter is further configured to narrow-band filter the second modulation signal. The second narrow-band filter has a cut-off frequency less than half the symbol rate of the second modulation signal. The apparatus further comprises a second modulator configured to modulate the second optical carrier to carry the further traffic to be transmitted over the WDM optical network using QPSK modulation and the narrow-band filtered second modulation signal. The apparatus further comprises a multiplexer configured to frequency division multiplex the modulated first optical carrier and the modulated second optical carrier to form a combined optical signal. The apparatus is configured such that, when the apparatus is installed in the optical node, the combined optical signal is transmitted over a single one of the plurality of frequency channels. Thus, the bandwidth of the combined optical signal is equal to or less than the predetermined bandwidth.

Thus, advantageously, embodiments of the present invention enable an optical node in an installed WDM network to be upgraded, to increase the spectral efficiency of the network. The spectral efficiency of the network may be increased by at least two times. In preferred embodiments of the present invention, as will be described below, the spectral efficiency of the network may be increased by four times such that, where the first bit rate is 100 Gbit/s, a 400 Gbit/s may be transmitted per frequency channel. Advantageously, since QPSK modulation is used, a satisfactory link distance may be achieved, without needing to change the underlying network infrastructure, e.g. to add repeaters/amplifiers or to upgrade the optical fibre in the optical links.

The invention utilises a time-frequency packing technique, as disclosed for example in an article by the inventors titled "Casting 1 Tb/s DP-QPSK Communication into 200 GHz

Bandwidth", European Conference and Exhibition on Optical Communications, September 16-20 2012. By narrow band filtering the first and second modulation signals, the transmission bandwidth of the first and second modulated optical carriers can be reduced. The applicant has appreciated that in this way and, by appropriately selecting the first and second frequencies (fi and f₂), the combined signal can be transmitted over a single frequency channel, having a predefined frequency bandwidth, in an installed WDM network.

In an embodiment of the present invention, the first frequency (f-i) and the second frequency (f₂) are selected such that 2-B_T < f₂— f i≤ B_c - 2-B_T, where B_c is the predetermined bandwidth and B_T is the bandwidth of each of the first and second modulated optical carriers.

In most implementations the bandwidth of the first and second modulated optical carriers will be the same. However, it is possible that, in some examples, the bandwidth of the first and second modulated optical carriers may be different.

The predetermined bandwidth may be 50GHz. The first bit rate may be 100Gbit/s.

In preferred embodiments, at least one of or each of the first narrow-band filter and the second narrow-band filter is shaped to filter its received modulation signal so as to at least partially mitigate for inter symbol interference (ISI). Thus, advantageously, as well as narrowing the transmission bandwidth of the modulated carrier signals, the narrow-band filters may mitigate for ISI in the signals.

A number of narrow-band filter configurations are possible. The first and or second narrow- band filter may each comprise one or more analog or digital filters. Advantageously, if the narrow-band filters comprise digital filters, the narrow-band filters may be reconfigurable, whereby the shaping applied to the incoming modulation signals by the filters may be adapted according to the link conditions. In a preferred example, at least one of or each of the first narrow-band filter and the second narrow-band filter is configured or shaped to apply a filter transfer function to its received modulation signal such that the narrow-band filtered modulation signal approximates a prolate spheroidal function or a Gaussian function. In a further preferred embodiment, the apparatus may further comprise a first splitter configured to split the first optical carrier into a first sub optical carrier at a first orthogonal polarisation and a second sub optical carrier at a second orthogonal polarisation.

A first narrow-band filter may be configured to receive a first modulation signal having a symbol rate and comprising traffic to be transmitted over the WDM optical network, the traffic having a bit rate equal to or greater than the first bit rate, and to narrow-band filter the first modulation signal. The first narrow-band filter has a cut off frequency less than half the symbol rate of the first modulation signal. A further first narrow-band filter may be configured to receive a further first modulation signal having a symbol rate and comprising further traffic to be transmitted over the WDM optical network, the further traffic having a bit rate equal to or greater than the first bit rate, and to narrow-band filter the further first modulation signal. The further first narrow-band filter having a cut off frequency less than half the symbol rate of the further first modulation signal. A first modulator may be configured to modulate the first sub optical carrier to carry the traffic to be transmitted over the WDM optical network using QPSK modulation and the narrowband filtered first modulation signal. A further first modulator may be configured to modulate the second sub optical carrier to carry the further traffic to be transmitted over the WDM optical network using QPSK modulation and the narrow-band filtered further first modulation signal. A first combiner may be configured to combine the modulated first sub optical carrier and the modulated second sub optical carrier to form the modulated first optical carrier.

The apparatus may further comprise a second splitter configured to split the second optical carrier into a first sub optical carrier at a first orthogonal polarisation and a second sub optical carrier at a second orthogonal polarisation.

A second narrow-band filter may be configured to receive a second modulation signal having a symbol rate and comprising traffic to be transmitted over the WDM optical network, the traffic having a bit rate equal to or greater than the first bit rate, and to narrow-band filter the second modulation signal. The second narrow-band filter has a cut off frequency less than half the symbol rate of the second modulation signal. A further second narrow-band filter may be configured to receive a further second modulation signal having a symbol rate and comprising further traffic to be transmitted over the WDM optical network, the further traffic having a bit rate equal to or greater than the first bit rate, and to narrow-band filter the further second modulation signal. The further second narrow-band filter has a cut off frequency less than half the symbol rate of the further second modulation signal.

A second modulator may be configured to modulate the first sub optical carrier to carry the traffic to be transmitted over the WDM optical network using QPSK modulation and the narrow-band filtered second modulation signal. A further second modulator may be configured to modulate the second sub optical carrier to carry the further traffic to be transmitted over the WDM optical network using QPSK modulation and the narrow-band filtered further second modulation signal.

A second combiner may be configured to combine the modulated second sub optical carrier and the modulated second sub optical carrier to form the modulated second optical carrier.

Thus, each of the first and second optical carriers may carry two orthogonally polarised QPSK modulated signals which each carry traffic having a bit rate equal to or greater than the first bit rate. Thus, advantageously, in this preferred embodiment of the present invention, the spectral efficiency of the channel may be increased by at least four times (e.g. where the first bit rate is 100 Gbit/s to carry a 400 Gbit s signal). In some embodiments of the present invention, the apparatus comprises only two optical carriers (the first optical carrier and the second optical carrier). However, in alternative embodiments, the apparatus may comprise four optical carriers. In particular, the apparatus may further comprise a third optical carrier generator configured to generate a third optical carrier at a third frequency different from the first frequency and the second frequency. A third narrow-band filter may be configured to receive a third modulation signal having a symbol rate and comprising traffic to be transmitted over the WDM optical network, the traffic having a bit rate equal to or greater than the first bit rate, and to narrow-band filter the third modulation signal. The third narrow-band filter has a cutoff frequency less than half the symbol rate of the third modulation signal. A third modulator may be configured to modulate the third optical carrier to carry the traffic to be transmitted over the WDM optical network using QPSK modulation and the narrow-band filtered third modulation signal.

The apparatus may further comprise a fourth optical carrier generator configured to generate a fourth optical carrier at a fourth frequency different from the first frequency, the second frequency and the third frequency. A fourth narrow-band filter may be adapted to receive a fourth modulation signal having a symbol rate and comprising further traffic to be transmitted over the WDM optical network, the further traffic having a bit rate equal to or greater than the first bit rate, and to narrow-band filter the fourth modulation signal. The fourth narrow-band filter has a cut-off frequency less than half the symbol rate of the fourth modulation signal. A fourth modulator may be configured to modulate the fourth optical carrier to carry the further traffic to be transmitted over the WDM optical network using QPSK modulation and the narrow-band filtered fourth modulation signal.

In this alternative embodiment, the multiplexer is configured to combine or frequency division multiplex the modulated first optical carrier, the modulated second optical carrier, the modulated third optical carrier and the modulated fourth optical carrier to form the combined optical signal.

This embodiment doubles the number of optical carrier generators which are required to produce the combined signal. However, this embodiment may make mapping first bit rate line signals into the combined signal more straightforward, as will be explained in more detail in the Description below. There is also provided (receiver) apparatus for upgrading an optical node in an installed WDM optical network to increase the spectral efficiency of the WDM network. The optical node is configured to receive an optical signal at a first bit rate over each of the plurality of frequency channels.

The apparatus comprises a receiver configured to receive a QPSK modulated optical carrier signal from a single one of the plurality of frequency channels and to split the QPSK modulated optical carrier signal into a first QPSK modulated optical carrier and a second QPSK modulated optical carrier. The first QPSK modulated optical carrier has a first central frequency and carries traffic having a bit rate equal to or greater than the first bit rate. The second QPSK modulated optical carrier has a second central frequency different from the first central frequency and carries further traffic having a bit rate equal to or greater than the first bit rate. The apparatus further comprises a first optical to electrical signal convertor configured to convert the first QPSK modulated optical carrier signal into a corresponding first electrical signal. The apparatus also comprises a first inter symbol interference mitigation processor unit configured to process the first electrical signal to mitigate, at least partially, for inter symbol interference. The apparatus also comprises a first bit sequence estimator adapted to retrieve the traffic carried by the first electrical signal.

The apparatus further comprises a second optical to electrical signal convertor configured to convert the second QPSK modulated optical carrier signal into a corresponding second electrical signal. The apparatus also comprises a second inter symbol interference mitigation processor unit configured to process the second electrical signal to mitigate, at least partially, for inter symbol interference. The apparatus also comprises a second bit sequence estimator adapted to retrieve the traffic carried by the second electrical signal.

In an embodiment, the apparatus may further comprise a first FEC (Forward Error

Correction) unit configured to process FEC data in the traffic retrieved from the first electrical signal. The apparatus may further comprise a second FEC unit configured to process FEC data in the traffic retrieved from the second electrical signal.

In a first embodiment, the first FEC unit may provide a feedback signal via feedback circuitry to the first bit sequence estimator. The second FEC unit may provide a feedback signal via feedback circuitry to the second bit sequence estimator. Thus, feedback may be performed independently for each of the optical carriers.

Alternatively, the apparatus may further comprise a processing unit adapted to process an output from the first FEC unit and an output from the second FEC unit to produce a feedback signal, the feedback signal being provided, via feedback circuitry, to the first bit sequence estimator and or the second bit sequence estimator. This implementation may be more complex. However, the applicant has appreciated that using feedback signals based on outputs from both of the carrier's FEC units may allow better mitigation of ICI (inter-carrier interference) and therefore more accurate and reliable recovery of the transmitted traffic.

The present invention further provides a method of upgrading an optical node in an installed WDM optical network to increase the spectral efficiency of the WDM network. The WDM network comprises a plurality of frequency channels each having a predetermined bandwidth. The method comprises installing one or more apparatus as described above in the optical node.

There is further provided an upgraded optical node in a WDM optical network. The WDM network comprising a plurality of frequency channels each having a predetermined bandwidth. The upgraded optical node comprising one or more apparatus as described above.

Description of the Drawings Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 illustrates two optical nodes in a WDM network; Figure 2 is a schematic diagram showing transmitter apparatus for upgrading an optical node in a WDM network according to an embodiment the present invention;

Figure 3 is a schematic diagram showing transmitter apparatus for upgrading an optical node in a WDM network according to a first preferred embodiment the present invention; Figure 4 is a schematic diagram showing transmitter apparatus for upgrading an optical node in a WDM network according to a second preferred embodiment the present invention;

Figure 5 is a schematic diagram showing receiver apparatus for upgrading an optical node in a WDM network according to an embodiment the present invention; and

Figure 6 is a schematic diagram showing receiver apparatus for upgrading an optical node in a WDM network according to a first preferred embodiment the present invention. Description

Embodiments of the present invention provide a method and apparatus for upgrading an installed WDM network. This WDM network may be a transport network, or any other type of network such as an access network. As indicated above, these WDM networks typically extend over long distances, and comprise a large number of interconnected optical nodes. These optical nodes may be ROADM nodes. Figure 1 shows two optical nodes 10 in an installed WDM network. The optical nodes 10 are connected by an optical transmission link 20. This link 20 may comprise, for example, an optical fibre. Link 20 may be unidirectional or bidirectional and it should be appreciated that, although not shown in Figure 1 , each optical node 10 may be an n-way node, configured to transmit/receive optical signals over a plurality of optical links 20. The WDM network may have a mesh, ring or any other type of configuration.

As explained above, the optical spectrum on each link 20 is divided into a plurality of frequency channels or slots. Each of these frequency channels has a respective central frequency and a predetermined bandwidth. In many existing WDM networks, this predefined bandwidth is 50GHz, and there are 48 channels per link. The channels are spaced according to the ITU-T frequency grid. The optical nodes are further configured to transmit/ receive traffic having a first bit rate of 100 Gbit/s over each of the frequency channels.

However, some existing WDM networks may have a different predefined bandwidth and or a different first bit rate. Note that the term "traffic" is herein used to refer to the payload (i.e. the client signal) transmitted by an optical carrier/signal. In some embodiments, overhead data may be added to the payload, such that the actual bit rate of data transmitted by the optical carrier/signal may be greater than the first bit rate, for example where the first bit rate is 100 Gbit/s, the actual bit rate of data transmitted may be 1 12 or 120Gbit s (including the payload and the overhead).

Figure 2 is a schematic diagram showing transmitter apparatus for upgrading an optical node 10 in an installed WDM network according to an embodiment of the present invention. In this example, the installed WDM network has a plurality of frequency channels each having a predetermined bandwidth of 50 GHz. The optical node is configured to transmit traffic having a first bit rate of 100 Gbit s over each of the frequency channels.

The apparatus comprises a first optical carrier generator 21 and a second optical carrier generator 21 , each configured to generate a respective optical carrier (which will be called a first optical carrier and a second optical carrier respectively). Each of the optical carrier generators 21 may for example comprise a laser, or any other suitable light generating or emitting device. The first optical carrier and the second optical carrier have different frequencies (fi and f₂ respectively).

A first QPSK modulator 22 is arranged to receive the first optical carrier, and a second QPSK modulator 22 is arranged to receive the second optical carrier. The first and second QPSK modulators 22 are also arranged to receive respective modulation signals (first and second modulation signals), which have been narrow-filtered by respective narrow-band filters 23 (first and second narrow-band filters). Each of the narrow-band filters 23 has a cut off frequency less than half the symbol rate of its received modulation signal (i.e. less than the Nyquist frequency).

The modulation signals each have a symbol rate and comprise traffic to be transmitted over the WDM network, the traffic having a bit rate equal to or greater than the first bit rate (which, in this example, is 100 Gbit/s). The modulation signals may further comprise overhead data, which may include FEC (Forward Error Correction) data.

Note that, in practice, a QPSK modulator 22 typically comprises an I branch and a Q branch, and, in this case, the first/second modulation signals may each be made up of two signals each carrying a part of the traffic, which are provided to the I and Q branches of the QPSK modulator 22 respectively.

The first and second narrow-band filters 23 may each comprise one or more digital or analog filters, and preferably the narrow-band filters 23 are configured or shaped so as to at least partially mitigate for inter symbol interference. For example, each of the narrow-band filters may be configured to apply a filter transfer function to its received modulation signal such that the narrow-band filtered modulation signal approximates a prolate spheroidal function or a Gaussian function.

Each of the QPSK modulators 22 is configured to modulate its received optical carrier using QPSK modulation and its received modulation signal such that the output modulated optical carrier carries the traffic comprised in the modulation signal. Thus, in this example, each modulated optical carrier carries traffic at a bit rate of at least 100 Gbit s.

The first and second modulated optical carriers are passed to a multiplexer 24, which frequency division multiplexes the modulated optical carriers to form a combined optical signal to be transmitted over a single one of the frequency channels in the optical link 20, to another optical node 10 in the WDM network.

Thus, in this example, the combined optical signal has a bandwidth equal to or less than 50 GHz, and the combined optical signal carries traffic having a bit rate of at least 200 Gbit s (i.e. at least twice the first bit rate). This is achieved, since by narrow-band filtering the first and second modulation signals, the transmission bandwidth of the modulated optical carriers can be reduced. Thus, by appropriately selecting the first and second frequencies, the two modulated carriers can be fitted in a single frequency channel. In particular, the first and second frequencies are selected such that:

[1 ] 2-BT < f₂ - fi≤ B_c - 2- B_T, , where B_C is the predetermined bandwidth and B_T is the bandwidth of each of the first and second modulated optical carriers. In preferred embodiments of the present invention, an optical node in an installed WDM network may be upgraded by installing a plurality of apparatus embodying the present invention in the optical node, one for each of the frequency channels. However, it is possible that an apparatus embodying the present invention may only be installed in an optical node for one or more of the frequency channels (i.e. not for all of the channels).

Figure 3 illustrates an apparatus for upgrading an optical node in an installed WDM network according to a preferred embodiment of the present invention. In this example, as in the example described above, each of the frequency channels in the WDM network have a predefined bandwidth of 50Ghz, and the first bit rate is 100 Gbit s.

In this preferred embodiment, the first optical carrier is split by a splitter 25, in this example a polarisation beam splitter (PBS), into a first sub optical carrier at a first orthogonal polarisation and a second sub optical carrier at a second orthogonal polarisation different from the first orthogonal polarisation. Similarly, the second optical carrier is split by a splitter 25, in this example a polarisation beam splitter (PBS), into a first sub optical carrier at a first orthogonal polarisation and a second sub optical carrier at a second orthogonal polarisation different from the first orthogonal polarisation. Each of the sub optical carriers is received by a respective QPSK modulator 22. Thus, there are four QPSK modulators 22. However, it is possible that only one of the optical carriers is split by a splitter into a first sub optical carrier at a first orthogonal polarisation and a second sub optical carrier at a second orthogonal polarisation

In this example, each of the QPSK modulators 22 has an I branch and a Q branch.

A modulation signal which carries traffic at a bit rate of, at least, the first bit rate (in this example, 100 Gbit/s) is provided to each of the QPSK modulators 22. Note that in this example, in fact, each modulation signal is made up of two 50 Gbit/s signals (i.e. two signals at half the first bit rate) - which are provided to the I and Q branches of the QPSK modulator 22 respectively.

In this example, four digital client signals at the first bit rate (here 100 Gbit/s) are received by the apparatus. As indicated in the dashed box, these signals may be received in the form of four 100 Gbit/s signals, or each 100 Gbit/s signal may comprise four 25 Gbit/s signals. Thus, to produce the 50 Gbit s signals, if a single 100 Gbit/s client signal is received at the apparatus, the signal is split by a time division multiplexer 27 into two 50 Gbit/s client signals. On the other hand, if 25 Gbit/s signals are received at the apparatus, pairs of the signals are combined, by a time division multiplexer 27, to produce 50 Gbit/s client signals.

In this preferred embodiment, each of these 50 Gbit/s signals is passed to a framer 26, which adds overhead data to the signals, and optionally FEC data, which enables errors in the transmitted data to be corrected at the receiver. The overhead data may for example include a Low Density Parity Check code, and data for synchronisation/channel estimation purposes at the receiver.

The framed signals are then passed to respective narrow-band filters 23, which each have a cut off frequency less than half the symbol rate of their received framed signal. In this example, the narrow-band filters 23 are comprised within a Digital to Analog signal convertor 29. In this example, the narrow-band filters 23 comprise digital filters. However, as mentioned above, the narrow-band filters 23 may alternatively comprise analog filters.

The narrow-band filtered modulation signals are then passed to the QPSK modulators 22, as described above.

The QPSK modulators 23 then modulate the sub optical carriers using QPSK modulation and their received modulation signal. Thus, each of the modulated sub optical carriers carries traffic at, at least, the first bit rate (100 Gbit/s).

The modulated sub optical carriers are then recombined into a modulated first optical carrier and a modulated second optical carrier respectively, by combiners 28, which in this example are polarisation beam combiners (PBC). Thus, in this example, each modulated optical carrier carries traffic having a bit rate of at least 200 Gbit/s (i.e. twice the first bit rate).

The first and second modulated optical carriers are then frequency division multiplexed by multiplexer 24 to produce the combined optical signal. Thus, the combined optical signal, in this example, carries traffic having a bit rate of at least 400 Gbit/s (i.e. four times the first bit rate).

In this preferred embodiment of the present invention, there are only two optical carriers (the first optical carrier and the second optical carrier). However, in an alternative embodiment, as will now be described by way of example with reference to Figure 4, there may be four optical carriers.

The embodiment illustrated in Figure 4 is similar to the embodiment described in Figure 3, except that in the embodiment of Figure 4 there are two additional optical carrier branches.

There is a third optical carrier generator 21 configured to generate a third optical carrier at a third frequency (the third frequency f₃ being different from the first frequency and the second frequency) and a fourth optical carrier generator 21 configured to generate a fourth optical carrier at a fourth frequency (the fourth frequency f₄ being different from the first frequency, the second frequency and the third frequency).

In this embodiment, similarly to the embodiment of Figure 3, each of the optical carriers is split by a respective splitter 25, in this example a polarisation beam splitter (PBS), into a first sub optical carrier at a first orthogonal polarisation and a second sub optical carrier at a second orthogonal polarisation (different from the first sub optical carrier), which are modulated separately by respective QPSK modulators 22. Thus, in this embodiment, there are eight QPSK modulators 22. However, it should be appreciated that in other

embodiments all or some of the optical carriers may not be split into two sub optical carriers having different orthogonal polarisations.

Each of the QPSK modulators 22 receives a modulation signal carrying traffic, which has been narrow-band filtered by a respective narrow-band filter 23. However, in this embodiment, each of the modulation signals comprises traffic at half the first bit rate (i.e. in this example at 50 Gbit/s).

As before, each of the modulation signals is in fact made up of two signals (here each carrying traffic at 25 Gbit/s) which are fed to the I and Q branches of the QPSK modulator 22 respectively.

Thus, in this example, each modulated sub optical carrier has a bit rate of 50 Gbit/s. The modulated sub optical carrier signals are then recombined, by respective combiners 28, in this example polarisation beam combiners (PBS), to produce a first modulated optical carrier, a second modulated optical carrier, a third modulated optical carrier and a fourth modulated optical carrier.

Thus, each of the modulated optical carriers carries traffic having a bit rate of, at least, 100 Gbit/s (the first bit rate). Each of these modulated optical carriers is received by the multiplexer 24, which frequency division multiplexes these modulated carriers to produce the combined signal (which has a bandwidth equal to or less than the predefined bandwidth, in this example 50GHz).

Thus, the combined signal, similarly to the embodiment of Figure 3, carries traffic having a bit rate of 400 Gbit/s (four times the first bit rate).

Note that, in this embodiment, since the bit rate of the traffic carried by the modulated optical carriers is half the bit rate of the traffic carried by the modulated optical carriers in the two carrier example described above with respect to Figure 3, the transmission bandwidth of the modulated optical carriers will be half the transmission bandwidth of the modulated optical carriers in the two carrier example. Thus, according to equation 1 , the spacing between the frequencies (fi , f₂, f3 and f₄) may also be halved, and therefore the four modulated optical carriers can still fit within a single frequency channel in the installed WDM network (which in this example has a predefined bandwidth of 50 GHz).

This embodiment requires additional hardware, including two further optical carrier generators 21. However, advantageously, since in this preferred embodiment, 25 Gbit/s signals need to be fed to the QPSK modulators 23, where 25 Gbit/s digital signals are received by the apparatus, there is no need to time division multiplex / demultiplex these signals as in the embodiment described with respect to Figure 3.

Apparatus for upgrading an optical node arranged to receive the combined optical signal over an optical link in the WDM network will now be described with respect to Figure 5. A preferred embodiment will then be described with respect to Figure 6. The apparatus comprises a receiver 51 configured to split a QPSK modulated optical carrier signal received over a single one of the plurality of frequency channels (the combined signal) into a first QPSK modulated optical carrier having a first central frequency f-i , and a second QPSK modulated optical carrier having a second central frequency f₂. The receiver 51 may comprise a de-multiplexer 50. However, a de-multiplexer 50 is not necessary in all embodiments. For example, if a coherent receiver is used, respective local oscillators can be used to select and split the respective carriers.

It should be appreciated that, in other embodiments, where the combined signal is made up of four modulated optical carriers, receiver 50 may be configured to split the received signal into four modulated optical carrier signals, each having a respective central frequency (f-i , f_2, f₃, ).

Each of the QPSK modulated optical carriers are detected and processed to retrieve the traffic carried by the modulated optical signals, on respective receiver branches.

A first optical to electrical signal convertor 52 is configured to convert the first QPSK modulated optical carrier signal into a corresponding first electrical signal. The first electrical signal is then passed to a first inter symbol interference mitigation processor unit 54, which processes the first electrical signal to mitigate, at least partially, for inter symbol interference. This is necessary, even if the narrow-band filters 22 are configured so as to at least partially mitigate for inter symbol interference, since, because frequency/time packing is used to produce the combined signal, inter symbol interference will occur. The processed electrical signal is then passed to a first bit sequence estimator 56 which is adapted to retrieve the traffic carried by the first electrical signal, and then optionally to a FEC unit 58.

Similarly, a second optical to electrical signal convertor 52 is configured to convert the second QPSK modulated optical carrier signal into a corresponding second electrical signal. The signal is then passed to a second inter symbol interference mitigation processor unit 54 which is configured to process the second electrical signal to mitigate, at least partially, for inter symbol interference. The signal is then passed to a second bit sequence estimator which is adapted to retrieve the traffic carried by the second electrical signal, and then optionally to a FEC unit 58. The ISI mitigation units 54 may use, for example, channel shortening or MIMO techniques. These units 54 may also provide a pre-filtering function to improve SNR (signal to noise ratio) tolerance and the accuracy of the subsequent bit sequence estimation. The bit sequence estimators 56 may use Maximum a Posteriori (MAP) or Maximum

Likelihood Sequence Detection (MLSD). BCJR (Bahl-Cock-Jelink-Raviv) and Viterbi decoders are examples of these types of bit sequence estimators respectively.

In the preferred embodiment illustrated in Figure 6, each of the QPSK modulated optical carrier signals is detected by a respective optical front end 52 which comprises an optical to electrical signal convertor 52. In this example, the optical front ends 52 use a conventional coherent receiver scheme. The optical front ends 52 are fed signals from respective local oscillators (lasers having frequencies fi and f₂ respectively). Each optical front end 52 combines its local oscillator signal with its received carrier signal using a 2x2 90°hybrid coupler. Each optical front end 52 may further comprise two balanced photodiodes and broadband electrical amplifiers.

In this preferred embodiment, there is also provided first and second equalisers 53, which equalise the electrical signals before they are passed to the ISI mitigation units 54.

These equalisers 53 are able to distinguish between the signals transmitted on different orthogonal states and are configured to at least partially compensate for chromatic dispersion caused during transmission of the signals. As will be understood by those skilled in the art, each of the equalisers 53 may comprise a fixed or bulk equaliser part, which has fixed tap values or parameters and a second adaptive part which has adaptive tap values or parameters. This adaptive part may be based on a two dimensional FIR (finite impulse response) architecture.

In preferred embodiments of the present invention, the FEC units 58 provide feedback signals to the bit sequence estimators 56, and or to the equalisers 53 so that the equalisers 53 can adjust their adaptive tap values as appropriate.

In an implementation, there may be separate feedback lines for the two optical carrier branches comprising feedback circuitry (not shown). However, in this preferred embodiment, a processor unit 60 (called a channel estimation unit in Figure 6) is configured to process an output from each of the FEC units 53 to produce the feedback signals, which are provided via feedback circuitry (not shown) to the bit sequence estimators 56 and or the equalisers 53. Processor unit 60 may comprise one or more processors.

Thus, embodiments of the present invention enable an installed or existing WDM network to be upgraded, so as to increase its spectral efficiency. Advantageously, this may be achieved without changing the optical infrastructure of the WDM network. An increased bit rate signal may be transmitted over an existing frequency channel or slot, and over a link distance such that repeaters/amplifiers do not need to be installed along the link / or the optical fibre used in the link upgraded.

Claims

1 . Apparatus for upgrading an optical node in an installed WDM optical network to

increase the spectral efficiency of the WDM network, the WDM network comprising a plurality of frequency channels each having a predetermined bandwidth, the optical node being configured to transmit traffic having a first bit rate over each of the plurality of frequency channels, the apparatus comprising:

a first optical carrier generator configured to generate a first optical carrier at a first frequency;

a first narrow-band filter configured to receive a first modulation signal having a symbol rate and comprising traffic to be transmitted over the WDM optical network, the traffic having a bit rate equal to or greater than the first bit rate, and to narrowband filter the first modulation signal, wherein the first narrow-band filter has a cut-off frequency less than half the symbol rate of the first modulation signal;

a first modulator configured to modulate the first optical carrier to carry the traffic to be transmitted over the WDM optical network using QPSK modulation and the narrow-band filtered first modulation signal;

a second optical carrier generator configured to generate a second optical carrier at a second frequency different from the first frequency;

a second narrow-band filter adapted to receive a second modulation signal having a symbol rate and comprising further traffic to be transmitted over the WDM optical network, the further traffic having a bit rate equal to or greater than the first bit rate, and to narrow-band filter the second modulation signal, wherein the second narrow-band filter has a cut-off frequency less than half the symbol rate of the second modulation signal;

a second modulator configured to modulate the second optical carrier to carry the further traffic to be transmitted over the WDM optical network using QPSK modulation and the narrow-band filtered second modulation signal; and

a multiplexer configured to frequency division multiplex the modulated first optical carrier and the modulated second optical carrier to form a combined optical signal; wherein, when the apparatus is installed in the optical node, the combined optical signal is transmitted over a single one of the plurality of frequency channels.

2. Apparatus according to claim 1 , wherein the first frequency (f-i) and the second

frequency (f₂) are selected such that 2-B_T < f₂- f-i≤ B_c - 2-B_T, , where B_c is the predetermined bandwidth and B_T is the bandwidth of each of the first and second modulated optical carriers.

3. Apparatus according to claim 1 or 2 wherein the predetermined bandwidth is 50GHz.

4. Apparatus according to any preceding claim wherein the first bit rate is 100Gbit/s.

5. Apparatus according to any preceding claim, wherein at least one of or each of the first narrow-band filter and the second narrow-band filter is shaped so as to at least partially mitigate inter symbol interference.

6. Apparatus according to any preceding claim, wherein at least one of or each of the first narrow-band filter and the second narrow-band filter is configured to apply a filter transfer function to its received modulation signal such that the narrow-band filtered modulation signal approximates a prolate spheroidal function or a Gaussian function.

7. Apparatus according to any preceding claim, further comprising:

a first splitting device configured to split the first optical carrier into a first sub optical carrier at a first orthogonal polarisation and a second sub optical carrier at a second orthogonal polarisation;

a first narrow-band filter configured to receive a first modulation signal having a symbol rate and comprising traffic to be transmitted over the WDM optical network, the traffic having a bit rate equal to or greater than the first bit rate, and to narrowband filter the first modulation signal, the first narrow-band filter having a cut off frequency less than half the symbol rate of the first modulation signal;

a further first narrow-band filter configured to receive a further first modulation signal having a symbol rate and comprising further traffic to be transmitted over the WDM optical network, the further traffic having a bit rate equal to or greater than the first bit rate, and to narrow-band filter the further first modulation signal, the further first narrow-band filter having a cut off frequency less than half the symbol rate of the further first modulation signal;

a first modulator configured to modulate the first sub optical carrier to carry the traffic to be transmitted over the WDM optical network using QPSK modulation and the narrow-band filtered first modulation signal; a further first modulator configured to modulate the second sub optical carrier to carry the further traffic to be transmitted over the WDM optical network using QPSK modulation and the narrow-band filtered further first modulation signal; and a first combining device configured to combine the modulated first sub optical carrier and the modulated second sub optical carrier to form the modulated first optical carrier.

Apparatus according to claim 6, further comprising:

a second splitting device configured to split the second optical carrier into a first sub optical carrier at a first orthogonal polarisation and a second sub optical carrier at a second orthogonal polarisation;

a second narrow-band filter configured to receive a second modulation signal having a symbol rate and comprising traffic to be transmitted over the WDM optical network, the traffic having a bit rate equal to or greater than the first bit rate, and to narrow-band filter the second modulation signal, the second narrow-band filter having a cut off frequency less than half the symbol rate of the second modulation signal; a further second narrow-band filter configured to receive a further second modulation signal having a symbol rate and comprising further traffic to be

transmitted over the WDM optical network, the further traffic having a bit rate equal to or greater than the first bit rate, and to narrow-band filter the further second modulation signal, the further second narrow-band filter having a cut off frequency less than half the symbol rate of the further second modulation signal;

a second modulator configured to modulate the first sub optical carrier to carry the traffic to be transmitted over the WDM optical network using QPSK modulation and the narrow-band filtered second modulation signal;

a further second modulator configured to modulate the second sub optical carrier to carry the further traffic to be transmitted over the WDM optical network using QPSK modulation and the narrow-band filtered further second modulation signal; and

a second combining device configured to combine the modulated second sub optical carrier and the modulated second sub optical carrier to form the modulated second optical carrier.

9. Apparatus according to any of claims 1 to 6, further comprising

a third optical carrier generator configured to generate a third optical carrier at a third frequency different from the first frequency and the second frequency;

a third narrow-band filter configured to receive a third modulation signal having a symbol rate and comprising traffic to be transmitted over the WDM optical network, the traffic having a bit rate equal to or greater than the first bit rate, and to narrow-band filter the third modulation signal, wherein the third narrow-band filter has a cut-off frequency less than half the symbol rate of the third modulation signal;

a third modulator configured to modulate the third optical carrier to carry the traffic to be transmitted over the WDM optical network using QPSK modulation and the narrow-band filtered third modulation signal;

a fourth optical carrier generator configured to generate a fourth optical carrier at a fourth frequency different from the first frequency, the second frequency and the third frequency;

a fourth narrow-band filter adapted to receive a fourth modulation signal having a symbol rate and comprising further traffic to be transmitted over the WDM optical network, the further traffic having a bit rate equal to or greater than the first bit rate, and to narrow-band filter the fourth modulation signal, wherein the fourth narrow-band filter has a cut-off frequency less than half the symbol rate of the fourth modulation signal;

a fourth modulator configured to modulate the fourth optical carrier to carry the further traffic to be transmitted over the WDM optical network using QPSK modulation and the narrow-band filtered fourth modulation signal; and

wherein the multiplexer is configured to frequency division multiplex the modulated first optical carrier, the modulated second optical carrier, the modulated third optical carrier and the modulated fourth optical carrier to form the combined optical signal.

10. Apparatus for upgrading an optical node in an installed WDM optical network to

increase the spectral efficiency of the WDM network, the WDM network comprising a plurality of frequency channels each having a predetermined bandwidth, the optical node being configured to receive an optical signal at a first bit rate over each of the plurality of frequency channels, the apparatus comprising:

a receiver configured to receive a QPSK modulated optical carrier signal from a single one of the plurality of frequency channels and to split the QPSK modulated optical carrier signal into a first QPSK modulated optical carrier and a second QPSK modulated optical carrier,

wherein the first QPSK modulated optical carrier has a first central frequency and carries traffic having a bit rate equal to or greater than the first bit rate, and the second QPSK modulated optical carrier has a second central frequency different from the first central frequency and carries further traffic having a bit rate equal to or greater than the first bit rate;

a first optical to electrical signal convertor configured to convert the first QPSK modulated optical carrier signal into a corresponding first electrical signal; a first inter symbol interference mitigation processor unit configured to process the first electrical signal to mitigate, at least partially, for inter symbol interference;

a first bit sequence estimator adapted to retrieve the traffic carried by the first electrical signal;

a second optical to electrical signal convertor configured to convert the second QPSK modulated optical carrier signal into a corresponding second electrical signal;

a second inter symbol interference mitigation processor unit configured to process the second electrical signal to mitigate, at least partially, for inter symbol interference; and

a second bit sequence estimator adapted to retrieve the traffic carried by the second electrical signal.

1 1 . Apparatus according to claim 10, further comprising a first FEC (Forward Error

Correction) unit configured to process FEC data in the traffic retrieved from the first electrical signal; and a second FEC unit configured to process FEC data in the traffic retrieved from the second electrical signal.

12. Apparatus according to claim 1 1 , wherein the first FEC unit provides a feedback signal via feedback circuitry to the first bit sequence estimator; and the second FEC unit provides a feedback signal via feedback circuitry to the second bit sequence estimator.

13. Apparatus according to claim 1 1 , further comprising a processing unit adapted to process an output from the first FEC unit and an output from the second FEC unit to produce a feedback signal, the feedback signal being provided, via feedback circuitry, to the first bit sequence estimator and or the second bit sequence estimator.

14. A method of upgrading an optical node in an installed WDM optical network to

increase the spectral efficiency of the WDM network, the WDM network comprising a plurality of frequency channels each having a predetermined bandwidth, the method comprising installing one or more apparatus according to any preceding claim in the optical node.

15. An upgraded optical node in a WDM optical network, the WDM network comprising a plurality of frequency channels each having a predetermined bandwidth, the upgraded optical node comprising one or more apparatus according to any of claims 1 to 10.