Conventional optical communication networks operate by sending light pulses of a predetermined period, for example such that a pulse represents a ‘1’ and no pulse represents a ‘0’. This technique enables signals to be sent at data rates of up to 10 Gb/s and wavelength division multiplexing (WDM) techniques can be used to send multiple signals over a single fibre. Dense WDM (DWDM) enables around 80 wavelengths to be used such that a single fibre can potentially carry Tb/s of data. In order to enable compatibility between network components from different vendors, the ITU has specified a grid of wavelengths that are used in DWDM systems (see ITU-T G.694.1). One of the transmission phenomena present in optical fibres is chromatic dispersion, which causes the transmitted pulse to spread out, such that it becomes difficult to recover the transmitted signal at the receiver. The effects of dispersion can be mitigated by installing dispersion compensating modules (DCMs) into the network, but this adds to the cost and the complexity of the network.
Coherent optical transmission systems are thought to provide the best option for transmitting data at a rate in excess of 40 Gb/s. Coherent optical transmission systems are similar to the transmission systems used in wireless systems. Rather than only turning an optical transmitter on and off to generate a pulse, an optical signal is modulated, for example in terms of both phase and amplitude, with a data signal. At the receiver end, coherent detection is used with a local laser oscillator to recover the transmitted data, both phase and amplitude components. Dispersion can be compensated for electronically using Digital Signal Processing (DSP) in the receiver.
As data transmission rates increase further, for example beyond 100 Gb/s, then the optical signals required to transmit such data rates may not fit into the 50 GHz grid of wavelengths that are defined in the DWDM specifications. It is preferred, for reasons of flexibility and spectral efficiency, that for such high data rates, network operators are able to determine which regions of the optical transmission window are used to transmit specific signals. A single coherent optical signal, for example one having a data rate of 100 Gb/s or greater, may extend across the 50 GHz spectrum window in a conventional 50 GHz grid-based network. In comparison, flexible grid, or ‘flexgrid’ networks can transmit higher speed (Tb/s) optical signals using a wavelength range that is convenient for the network operator. Flexgrid networks achieve very high spectral efficiency and allow increased utilisation of fibre infrastructure (up to ˜50% more than grid-based DWDM). In flexgrid networks an optical channel is no longer a single wavelength, but is defined as a single capacity entity comprising one or more sub-channels which together form an aggregate optical capacity often referred to as a ‘superchannel’ which can be configured and managed throughout the optical network infrastructure.
Currently deployed optical transponders operate at data rates of 2.5, 10, 40, 100 and 200 Gbit/s, will soon reach rates of 400 Gbit/s and beyond as capacity demands increase. In recent years, there has been significant research and development in the use of both advanced modulation formats (e.g. QPSK, 16-QAM, 64-QAM) and variable baud rates (25, 33.3, 50 Gbaud), in order to increase the fundamental net bit-rates of individual subcarriers generated by the optical transponders. Increasingly, the very latest optical transponders (or optical superchannel generators) are capable of changing their modulation format and/or baud rate configurations under direct software control, which allows a rapid configuration of optical bit-rates. Currently, with state-of-the-art transponders, this takes place on timescales on the order of ˜10 seconds, though it is anticipated that these timescales will decrease significantly in the near future, as the technology becomes more mature and adopted in the field.
For a given capacity requirement, the preferred superchannel format (group of sub-channels) depends on the rate and reach requirements of the optical link in question. Table 1 below shows typical rate-reach characteristics that can be expected for different modulation formats and baud rates that are likely to be deployed in the near future.
TABLE 1Comparison of modulation format propertiesBits per NetTypicalsymbolbaudSubcarrierTypicalOSNRModulation(single/doubleratedata rateReachrequirementFormatpolarisation)(Gbit/s)(Gbit/s)(km)(dB)BPSK1/22550500010QPSK2/425100200012QPSK2/45020010001816-QAM4/8252006002064-QAM 6/123340016026
It should be noted that as data rates are increased, the OSNR requirements increase dramatically, with a corresponding decrease in network reach.
FIG. 1 shows a schematic depiction of three options for creating a superchannel having a capacity of 400 Gbit/s. For long reach applications, the superchannel configuration depicted in FIG. 2(a) of 4×100 Gbit/s (QPSK) sub-channels would be required, giving a total spectral width of ˜150 GHz. Ideally, bearing in mind the need to optimise the efficiency of fibre capacity and optical spectrum, it would be preferable to use more spectrally efficient schemes such as the use of 2×200 Gbit/s (16-QAM) sub-channels (FIG. 2(b)) which takes up a spectral width of ˜75 GHz, but owing to its limited reach (˜600 km) this may not be feasible. For very short distance applications, perhaps on the order of 100-200 km, then the example illustrated in FIG. 2(c) could be used, which illustrates a single carrier variant of 400 Gbit/s (33.3 Gbaud, 64-QAM in this case) with a spectral width of ˜50 GHz. For shorter distances this single carrier option is a highly attractive option.
As an example, consider how a 400 Gbit/s superchannel could be configured for a given optical link. In the calculations, a link OSNR (LOSNR) of 20 dB is assumed and the OSNR requirements of the various transponder formats (TOSNR) are as set out in Table 1 above. Factors included in the TOSNR value are usually back-to-back transponder performance (no transmission link) as well as linear transmission impairments such as dispersion, PMD and optical filtering). A safe operating margin (OOSNR), is assumed to be ˜2 dB, but could be any number that from a long term operational perspective is assumed to be viable to ensure stable error-free operation at end of life specifications. Operational margin usually includes such effects as transponder aging, optical link aging and potentially non-linear degradation.
According to the following simple rules, we can then work out which transponder formats will be allowed for acceptable long term performance:LOSNR−(TOSNR+OOSNR)>0→ALLOWEDLOSNR−(TOSNR+OOSNR)˜0→MARGINALLOSNR−(TOSNR+OOSNR)<0→NOT ALLOWED
The output of these simple rules is shown below in Table 2, along with the modulation format parameters from Table 1, and demonstrates that only two of the five modulation formats meet the optical performance and operational expectations (assuming a link OSNR of ˜19 dB, for example), and that a further modulation format has a marginal performance.
TABLE 2Comparison of modulation format properties with suitability for a particular linkBits per symbolNetSubcarrierTypicalTypical OSNRModulation(single/doublebaud ratedata rateReachrequirementFormatpolarisation)(Gbit/s)(Gbit/s)(km)(dB)OUTCOMEBPSK12550500010ALLOWEDQPSK225100200012ALLOWEDQPSK250200100018MARGINAL16-QAM42520060020NOTALLOWED64-QAM63340016026NOTALLOWED
Thus it can be seen that we have a choice of up to 3 different modulation formats, likely to be either the 25 Gbaud/100 Gbit/s QPSK or 50 Gbaud/200 Gbit/s QPSK formats, as the 25 Gbaud/BPSK is least spectrally efficient and requires the most number of ports/sub-channels. As 50 Gbaud/200 Gbit/s QPSK is viewed as marginal from a longer term operation perspective, then the preferred choice is likely to be 25 Gbaud/100 Gbit/s QPSK.