A new transmission paradigm in optical networks: Evolution from fixed grid to gridless

1. A new transmission paradigm in optical networks: Evolution from fixed grid to gridless Dr. CicekCavdar cavdar@kth.se Next Generation Optical Networks (NEGONET) group The Royal Institute of Technology (KTH) Sweden

2. Outline • Introduction • Motivation • Enabling Technologies • From RWA to RSA • Network DesignShift • Conclusions

3. FixedGrid • Signalsmultiplexedintoone fiber with a method of “grid” • Fixedintervalsbetweencenterfrequencies • Problem: GapbetweenneighboringWswhendifferentlinerates of Wsaremultiplexed.  Bandwidthwasted

4. FlexibleGrid (orGridless?) • Centralfrequenciesare not fixed. Enablessqueezing! • Underlimitedtuningresolutions mini grid is morepractical. Gridless is possibleiffine-tuningmechanismsaredevelopedfortunableopticalcomponents.

5. ITU-T DWDM frequencygrid • Singleslot on thegridapproach • Doublesidedhalfslotapproach

6. Enabling Technologies • The ongoing advances in photonic technologiesmake it possible to treat optical fiber as a sharable continuous resource pool. • Optical multilevel modulation, optical orthogonal frequency-division multiplexing (O-OFDM), and • Seamlessly bandwidth-variable wavelength selective switch (WSS) .

7. Optical-transport networksin 2015 • In September 2006, about five years after the telecom bubble burst, Nippon Telegraph and Telephone Corporation (NTT) conducted an experiment that set a new world record for optical-fibre communication, by sending 14 Tbit s- 1 over 160 km of optical fibre. This experiment, which involved simultaneously transmitting 140 wavelength-division and polarization-division multiplexed (WDM/PDM) signals each with a bit rate of 111 Gbit s- 1, delivers two important statements about the probable direction of optical-fibre communication technologies. A wavelength in a link is no longer a static path between two specific connection end-points, but, now, part of a pool of flexible wavelengths or bandwidths that can be reconfigured on request MasahikoJinno, YutakaMiyamoto& YoshinoriHibino, NaturePhotonics, 2007

8. State of the Art: Worst case design inModulation Format • For example, once the modulation format to transmit the 40 Gb/s client data streams is determined to be, say, differential quadrature phase shift keying (DQPSK), the format is applied to every 40 Gb/s optical path. In such a case every path occupies the same spectral width, regardless of each path’s distance or the number of transmitted hop nodes. • As a result, most optical paths whose path lengths are far less than that of the worst case have large transport margins at the receiving end. • If, instead, an adaptation mechanism for various physical impairments were introduced into the optical networks, the utilization efficiency could further be enhanced.

9. FlexibleSpectrumAllocation • Distance-adaptive spectrum resource allocation • Minimum necessary spectral resource is adaptively allocated according to the end-to-end physical condition of an optical path • Two important parameters to determine the necessary spectral resources to be allocated for an optical path • Modulation format and • optical filter width. MasahikoJinno, BartlomiejKozicki, Hidehiko Takara, AtsushiWatanabe, Yoshiaki Sone, TakafumiTanaka, andAkiraHirano, NTT Corporation, “Distance-AdaptiveSpectrumResourceAllocation in Spectrum-SlicedElasticOpticalPath Network”, Com. Mag. Aug. 2010.

10. Link Adaptation Technologies • Link adaptation is a widely used key technology that increases the spectral efficiency of broadband wireless data networks and digital subscriber lines. • The basic idea behind link adaptation technologies is to adjust the transmission parameters, such as modulation and coding levels, to take advantage of prevailing channel conditions. • The most efficient set of transmission parameters under a certain channel condition is selected based on criteria such as maximizing data rate or minimizing transmit power

11. Link adaptation • Under good channel conditions an efficient set of parameters optimized for spectral efficiency (i.e., more bit loading per symbol using multilevel modulation schemes and less redundant error correction) is used to increase throughput. • In contrast, under poor channel conditions a set of parameters optimized for robustness (i.e., less bit loading and more redundant data adding) is used to ensure connectivity.

12. Modulation Format Adaptation • In good channel conditions: 16-ary quadrature-amplitude modulation (QAM), despite having worse receiver sensitivity than quadrature phase shift keying (QPSK), provides • better spectral efficiency and • able to transmit double the data rate under good channel conditions. • Some attempts to introduce link adaptation technologies into optical networks [1] Q. Yang, W. Shieh, and Y. Ma, “Bit and Power Loading for Coherent Optical OFDM,” IEEE Photonics Tech. Lett., vol. 20, no. 15, 2008, pp. 1305–7. [2] O. Rival, A. Morea, and J. Antona, “Optical Network Planning with Rate Tunable NRZ Transponders,” Proc. ECOC ‘09, 2009.

13. Rate tunable planning [1][2] • Data rate dynamic adjustment according to the quality of channels was experimentally demonstrated by using bit and power loading of optical OFDM subcarriers without modifying the channel bandwidth and launch power. [1] • Network performance of symbol rate tuning in non-return-to-zero (NRZ) modulation formats in an opaque optical network was also studied [2] • It was shown that reach-dependent link capacity adjustment can benefit from the added available capacity for short-distance demands and from the saved optoelectronic interfaces on lowrate long-distance demands . • Such studies are based on the idea of increasing data rate for shorter links with large signal-to-noise ratio (SNR) and nonlinear effect (NLE) margins in opaque optical networks based on the fixed International Telecommunication Union — Telecommunication Standardization Sector (ITU-T) frequency grid.

14. Distance-Adaptive Spectral Resource Allocation • 1. Bits per symbol adjustment for spectrum resource saving • 2. Filter Bandwidth adjustment for spectrum resource saving

15. 1. Bits per symbol adjustment for spectrum resource saving • The traditional link adaptation technologies that maximize channel data rates may also be considered in the following manner: the unused SNR and NLE margins for shorter connections can be used for spectrum resource saving, while ensuring a constant data rate. • For the same data rate 16-QAM carries twice the number of bits per symbol of QPSK, therefore requiring half the symbol rate and, consequently, half the spectral bandwidth. • Similarly, 64-QAM carries three times the number of bits per symbol of QPSK, and requires 1/3 the spectral bandwidth. • Thus, spectral bandwidth can be saved by reducing the symbol rate and increasing the number of bits per symbol to transmit the same data rate. Since higher-level bit loading decreases the distance between the two closest constellation points, 16-QAM and 64-QAM suffer from SNR penalty per bit of 4 dB and 8.5 dB, respectively, when compared with QPSK [3]. • [3] K.-P. Ho, Phase-Modulated Optical Communication Systems, Springer, 2005.

16. Single Carrier and Multi Carrier Difference • In the single-carrier modulation approach, the symbol rate is reduced to obtain a narrower spectral width while increasing the number of bits per symbol to keep the data rate constant. Conversely, • In the multicarrier approach the number of subcarriers, with uniform symbol rate and bits per symbol, is changed to adjust the spectral bandwidth

17. Parameters in tunable spectral width modulation format

18. 2. Filter Bandwidth adjustment for spectrum resource saving • When the scope of discussion is extended from point-to-point to wavelength-routed transparent optical networks, it is necessary to consider the effect of waveform distortion due to cascaded optical filters in ROADMs and WXCs. • Cascaded optical filters introduce significant narrowing of the passband, the filtered optical signal suffers distortion due to unwanted spectral clipping. • The effect is stronger for optical paths experiencing larger numbers of node hops. The current design of filters in optical nodes allows for the accumulated filter narrowing effect by assuming wide filter bandwidth to accommodate the worst case (i.e., the paths with the largest numbers of node hops). As a result, most optical paths have large spectral clipping margins and are assigned redundant spectral resources. • Spectral resource saving is achieved by adaptively choosing filter bandwidth according to the numbers of node hops. The necessary minimum bandwidth of optical filters for an optical path is determined to ensure that the effective passband of cascaded filters measured at the end of the optical path maintains acceptable performance. • This functionality can be realized using bandwidth-variable optical filters employing a spatial light modulator, such as liquid crystal on silicon (LCoS), configured with a dispersive element to separate WDM signals.[4] [4] G. Baxter et al., “Highly Programmable Wavelength Selective Switch Based on Liquid Crystal on Silicon Switching Elements,” Proc. OFC/NFOEC ‘06, 2006, paper no. OTuF2.

19. Transmission parameters • Modulation format and coding levels • Adjusting the number of bits per symbol = Choosing the right modulation format • Adjusting the filter bandwidth • --> To ensure constant data rate

20. Available spectral resources in an optial fiber • Restricted by the gain bandwidth of the optical amplifier used. • When Erbium-doped fiber amplifiers for C-band or L-band are used, the available spectral width ranges from about 4 to 5 THz.

21. Spectrum resource allocation in distance adaptive SLICE • In distance-adaptive SLICE the most efficient set of transport parameters is chosen to minimize the allocated spectrum resources under a certain optical path condition while keeping the data rate unchanged. The parameters to be adapted include • modulation level and • optical filter bandwidth. • Path A: 16-QAM and filter width of 37.5 GHz) is selected. • For path C or D having a larger number of node hops, a more robust set of parameters (e.g., QPSK and 50 GHz) is utilized. • Since the filter narrowing effect is most crucial for the longest path, B, the broadest filter bandwidth (e.g., 62.5 GHz) should be assigned to ensure an acceptable passband at the egress optical node. • From the viewpoint of practical implementations: • Spectral frequency resource in optical fibers may be quantized to an appropriate unit, which we refer to as a frequency slot. • If a 12.5 GHz slot width is assumed, the 37.5, 50, and 62.5 GHz spectra correspond to 3, 4, and 5 slots, respectively. • The channel spacing is standardized by the ITUT frequency grid with granularities of 12.5, 25, 50, or 100 GHz

22. New Network Problems • Wavelengthassignment SpectrumAssignment • WavelengthContinuity  SpectrumContinuity

23. WavelengthandSpectrumAssignment • RWA + SpectrumContinuityConstraint [1] • Given: Fixedroutesfrom s to d. • Allfrequencyslotsarenumbered. • (1) When a connectionrequestarrives, selectroutefromthelist. • (2) Searchforcontiguousfrequencyslotsalongthepath/route. • (3) Tryallroutesuntilyoufindone.

24. Flexibleoptical WDM network • In a FWDM network, the control plane must follow • (1) the wavelength continuity constraint, whichis defined as the allocation of the same wavelength on each fiber link along the route of a channel, • (2) the spectralcontinuity constraint, which is defined as allocation of the same continuous spectrum on each fiber along the route ofa channel, and • (3) the spectral conflict constraint, which is defined as non-overlapping spectrum allocation to differentchannels on the same fiber.

25. Defragmentation ofTransparent Flexible Optical WDM (FWDM) Networks • Network defragmentation problem for FWDM networks is formulated,and heuristicsareproposed minimizing the number of interrupted connections. • Given: set of existingconnectionsoperating at a specificline rate (a spectrum) and a wavelength. Ankitkumar N. Patel, Philip N. Ji, Jason P. Jue, TingWang, OFC 2011

26. Defragmentation ofTransparent Flexible Optical WDM (FWDM) Networks • D) ShiftedtolowerWswithoutchangingtheirroutes • C) request G can be rerouted on path B-C-A since both fiber link (B, C) and (C, A) have sufficient continuousspectrum (75 GHz) available at the same wavelength 191.65 GHz. However, the same connection cannot be routedon the same path at wavelength 191.65 GHz due to the spectral conflict constraint. Ankitkumar N. Patel, Philip N. Ji, Jason P. Jue, TingWang, OFC 2011

27. BandwidthAllocation in Flexible OFDM basednetworks • Dynamiccase: OFC 2011 • Staticcase: ECOC 2010 • Physicallayerimpairmentsareneglected • ILP forstatic: Candidatepathsandroutingsaregiven. • Objective: Min.utilizedspectrum I. ThomkosandVarvarigos, OFC 2011

28. BandwidthAllocation in Flexible OFDM basednetworks • The spectrum is divided insubcarrier slots of F GHz, and each subcarrier ismapped to an integer number. To route the pathsthrough the WXC a guardband of G subcarriershas to separate adjacent spectrum paths. Servinga connection i that requires Ti subcarriers istranslated to finding a starting subcarrierfrequency fi after which it can use Ti contiguoussubcarriers (in addition to the guardbands). • Spectrumtrafficmatrix is given. (number of subcarriersrequired, corresp. to tr. rate) • Spectrum continuity constraint is translated tonon-overlapping spectrum allocation. Thus, thestarting frequencies of the connections that utilizea common link are ordered so that their allocatedspectrums do not overlap.

29. DynamicBandwidthAllocation in Flexible OFDM basednetworks (1) No distanceconsideration [5] • Foreachsource-destinationpair, traffic model,thebiggestandsmallestfluctuationratesareknowntogetherwithcurrent traffic rate. • Two types of subcarriers in the network: • (a) those that are pre-reserved by the connections (“guaranteed” orreserved subcarriers) • (b) shared on a demand basis, and can be allocated/deallocatedto the connections according to their time-varying requirements (“best effort” or shared subcarriers). (1) Distanceconsideration [6] • Onlybatcharrivalsarecoveredandsameapproach as in [1] [5] I. Thomkos et. al., “DynamicBandwidthAllocation in Felxible OFDM basednetworks”, OFC 2011 [6] T. Takagi et. al, “DynamicRoutingandFrequencySlotAssignmentforElasticOpticalPath Networks”, OFC 2011

30. TrafficGrooming • Inordertoreducetheoverhead of filterguardband • Gigherspectrumefficiencywith TG [7] Y.. Zhang et. al., “TrafficGrooming in SpectrumElasticOpticalPath Networks”, OFC 2011

31. WhatweneedforFlex-Grid? • ControlPlanewhich can do periodic “Garbagecollection” byregroomingchannelallocationstomaximumcontiguousfreespectrum. • Software definedopticaltransceivers • Abilitytodynamicallychangechannelassignmentsforcircuits in service • Anyroutingandchannelassignmentalgorithmmust be rathersophisticatedtotakenon-linearalitiesintoaccount. • Weneed WSS switch 1x20-ishports in aboutthefootprint of today’s 1x5 ports. • It is very hard tocalculatenon-linearimpairments in such a mixedline-rate, mixed-modulation format channel. No waytoguarantee. (?) Maybeyou start, monitor, andbackoffandtryagain (??)

32. Flexibilityandadaptiveness • Whatchanges? • Modulation Format • Symbol rate • Data rate ( Ch. Capacity) • SpectralWidth • Reach • Why? • Adaptto • Traffic • Impairments • Fiber types • Adapt rate totrafficdemand • Adaptmodulation format toimpairments

33. OpticalfollowsRadioCommunication (?) • Can welearnfromradiocommunication? • Emergence of smallcellotherthenbuildinghugecelltowers: MovefromMacrocellto metro cell-picocell-femtocell.. • SDR : Software definedradio (similaritieswith software definedtransceivers) • Flexibility in • Channelcoding, • Modulation, • Multiplecarriers, • SpatialDiversity

34. Thank You! Questions?