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while also taking up valuable space in the data centre. Another downside of DMT is it has been engineered with optical dispersion compensation and is only point-to-point, though more advanced versions of DMT such as single-sideband (SSB) DMT may not require optical dispersion compensation.


Oclaro’s CFP2-ACO coherent module


generally used, with each sub-carrier occupying about 100MHz of bandwidth. Tis results in an aggregate signal bandwidth of 25–30GHz on each wavelength. DMT can transmit the data in such a way


that the capacity of every single sub-channel is maximised. Figure 1 shows DMT transmission measurements at 112Gb/s using 256 sub- channels. In 1(a) we see the expected high signal-to-noise ratio (SNR) at low frequency, and lower SNRs as the sub-channel frequency increases. Te way in which DMT uses bandwidth efficiently can be seen in Figures 1(b) and 1(c): many bits are packed into the low-frequency sub-channels, such as sub- channel 5 (in this case, five bits per symbol with a 32 quadrature amplitude modulation (32QAM) constellation), whereas the high- frequency sub-channels are still utilised, but


The industry is now starting to think that coherent is too expensive from a dollar per bit-kilometre perspective


with lower bit-packing (two bits per symbol with a quadrature phase-shiſt keying (QPSK) constellation) due to the lower SNR. Tis format offers high spectral efficiency, and fits in the QSFP28 form factor. Te downside of this scheme is it requires


high optical signal-to-noise ratio (OSNR) while simultaneously meeting the challenging low power consumption required by data centre operators. Data centres are very averse to anything that is not low power because if it gets too hot, they need to add equipment to cool it all down. Tis requires significant investment, adding to the cost of operation and equipment,


PAM4 modulation Pulse-amplitude modulation (PAM) is a transmission scheme that features multi-level amplitude signalling. We are all familiar with PAM2, typically known as non-return to zero (NRZ) or on-off keying (OOK), which has been used for many years in standard lower-speed optical transmission. Its successor, PAM4, is now being considered as a possible alternative to coherent technology for shorter distances. PAM4 uses four levels to signal one of four


possible symbols, carrying two bits of data per symbol. Figure 2 shows a typical four-level eye, for both 112Gb/s optical PAM4 simulations (a) and measurements (b). Te advantage of the PAM4 modulation scheme for optical suppliers is that it can send double the amount of data at


the same rate required by NRZ signalling because it transmits two bits per symbol, instead of the one bit in NRZ. PAM4 is ideal for shorter distances, such as


intra-data centre (from 0.5km to 2km) and inter-data centre (less than 80km) links that require optical transceivers with much lower cost and power consumption than those provided by long-haul coherent optical technologies. Because PAM4 is a relatively simple modulation scheme, all functionality can be implemented in CMOS, with lower power than the more complex DMT modulation scheme. Tis also enables the PAM4 to be put into smaller form factors. In the case of 400G, for example, only four lasers are required instead of the eight that would be needed by NRZ to achieve the same capacity. Tis helps reduce cost and power dissipation in the transceiver module. Te downside of PAM4 is it places the most stringent requirements on the optics bandwidth, requires an engineered link with dispersion compensation, is only point to point and may have difficulty scaling to single-channel 400G.


Table 1: High-speed modulation formats compared


Format DMT


Pros


High spectral efficiency Adaptive


Less sensitive to bandwidth of optics


Fits in QSFP28 form factor


Proven technology in low-speed ASDL


PAM4


Simplest higher-order modulation format


Lowest complexity chip


Best required OSNR for a direct detect scheme


Lowest power consumption Fits in QSFP28 form factor


Coherent-Lite Under standardisation in OIF Simple line system Lowest power per bit 400G single wavelength


FlexCoherent


Flexible modulation format-baud rate grid spacing to maximise capacity on any system


Multi-haul application, from DCI to submarine system


Simple line systems


100-600G under development and scales to even higher data rates


Issue 15 • Spring 2017 FIBRE SYSTEMS 17 Cons


Required OSNR Power consumption Chip complexity


Sensitive to system nonlinearities


Engineered link with dispersion compensation


Only point to point operation


Most stringent requirements on optics bandwidth


Engineered link with dispersion compensation


Only point to point operation Difficult to scale to 400G


Expensive 7nm DSP Significant technical risk


Very application specific (sub 120km distances only)


Most complex DSP Highest power consumption


Designed for flexibility not for short reach cost-effectiveness


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