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Soft optics, hard photons

Geoff Bennett examines the evolution of coherent super- channels and how the use of software programmable modulation formats gives operators new options for network optimisation

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ince products based on coherent super-channels hit the market in mid-2012, this technology has transformed the way that service

providers deploy long-haul capacity. A super- channel is an evolution in DWDM technology in which multiple optical carriers, implemented on a single line card, are seen as a single unit of capacity, and are brought into service in one operational cycle. Super-channels take the spectral efficiency that is delivered by a coherent transponder and make it operationally scalable beyond 100Gb/s. As this technology advances, there have been a series of enhancements that roughly follow the

progression: l

Te move from hard-decision to soſt-decision forward error correction (SD-FEC), delivering almost a doubling in the reach for a given type of modulation.

Te move from fixed grid to flexible grid. Flexible grid forms the foundation of future super- channel capabilities, as I will explain below.

Te addition of a ‘fluid’, or ‘instant’ bandwidth capability, in which a full super-channel can be installed, but service providers activate 100G chunks of it using a license system.


Most recently the proposal is to take the capacity from a super-channel line card and allow it to be ‘sliced’ so that it can be optically directed to different end points. Tis is particularly useful in metro aggregation scenarios to maintain capacity efficiency as the network scales.

From the very beginning of commercial shipments, super-channel line cards have used ‘soſt optics’ – the ability to program in a ‘flexible coherent’ modulation technique to allow network designers to optimise the reach and capacity on a given optical link. Last year I was lucky enough to be involved in a field trial of a prototype terabit super-channel line card in which we showcased a full range of these modulations types: PM-BPSK, PM-3QAM, PM-QPSK, PM-8QAM and PM-16QAM modulation, on the same line card, over a link

between Budapest in Hungary and Bratislava in the Slovak republic (see explanation of terms). In the first-generation line cards switching, for

example, from PM-QPSK to PM-BPSK had the effect of massively increasing the potential reach (from about 4,500km to more than 10,000km). But in doing this the line card capacity dropped from 500Gb/s to 250Gb/s, and the maximum fibre capacity was also be halved. Note that this is the reason why PM-3QAM is so valuable, because it offers an intermediate reach, intermediate card capacity and intermediate fibre capacity between PM-BPSK and PM-QPSK. Te next generation of super-channel technology

adds another variable – the ability to switch the baud rate of the signal to create another dimension of programmability. So why would a programmable baud rate be so

important? Before I answer that let me lay out two disclaimers. First, all of the examples I will give are just that – examples. Tey are not intended to be specific future product specifications and have been kept as generic as possible. Second is that the spectral widths are approximate; I’ve avoided a direct statement of spectral widths. Te actual spectrum used by individual carriers will vary depending on the SD-FEC overhead for a given manufacturer, as well as other details of the implementation. In Figure 1a I have shown a typical pulse shape and width for a widely used 100Gb/s

implementation, which is 32Gbaud PM-QPSK. Tis has a central ‘bump’ and two side lobes. In fact there are an infinite number of these lobes, but they decay rapidly, and so we typically just draw the first order lobes. Tis type of signal just about fits into a classic 50GHz fixed slot defined in ITU-T G.694.1, and the small amount of ‘spillage’ (known as inter-symbol interference, ISI) to neighbouring channels is one of the factors that limits reach in these implementations. Te latest coherent technology uses transmitter-

based pulse shaping to move the energy from the side lobes into the central pulse, to create the signals shown in Figure 1b. Tese fit easily into a 50GHz slot, so much so that there’s a significant amount of wasted spectrum, shown as shaded areas between the peaks. Te fixed, 50GHz ITU grid would be an inhibitor to efficient use of spectrum for this type of implementation. So Figure 1c shows what the peak would look like

if we simply ramp up the baud rate from 32Gbaud to 64Gbaud. If we stay with PM-QPSK modulation this would double the data rate on this carrier to 200Gb/s, but the peak would be twice as wide. Tis is a key point, because increasing the baud rate on its own has no effect on the spectral efficiency (i.e. total fibre capacity) in this system. If we wanted to increase the spectral efficiency here we’d need to move to a higher order modulation technique, such as PM-8QAM or PM-16QAM. A 64Gbaud PM-8QAM format would deliver 300Gb/s, and a


Geoff Bennett

Figure 1: Modulation broadening and pulse shaping of coherent WDM signals Issue 9 • Autumn 2015 FIBRE SYSTEMS 13

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