EUGENE PARK THE 600GERA
due to the spectral margin gaps across a network can be minimised using adaptive baud rate, thus minimising stranded bandwidth. Advanced techniques of non-integer bit-per-symbol modulation, constellation point manipulation, and adaptive baud rate provide flexible methods to fine-tune line-side coherent modulation allowing network operators to optimise capacity and reach for their network.
of the baud rate over a wide range, giving network operators the ability to reduce regeneration stages and increase network margin. Rather than the line system adjusting to match the optical transmission (e.g., via WSS technology), with an adaptive baud rate, the ability now exists to adjust the optical transmission to match the line system for a new level of network utilisation. Bandwidth fragmentation
achieve the highest capacity, lowest cost-per-bit link between two sites.
FINE-TUNING COHERENT MODULATION Higher modulation modes (higher bits-per-symbol) provide increased capacity at the expense of reach, while lower modulation modes (lower bits- per-symbol) provide reduced capacity with farther reach. Integer (quantised) bits-per-symbol steps such as QPSK (2 bits-per-symbol), 8QAM (3 bits-per- symbol), and 16QAM (4 bits-per symbol) may result in sub-optimal capacity utilisation due to gaps in link margin. 600G era technology introduces the capability to modulate in non-integer steps, supporting many small increments between the integer steps, illustrated by the knobs in Figure 3. These techniques to enable non-integer bits-per-symbol modes include time-hybrid modulation, probabilistic constellation shaping (PCS), and 3D Shaping. These techniques that enable modulation orders in-between traditional integer bit-per-symbol modulation orders allow the link margin to be optimised with greater resolution than prior generation interconnect technology. This in turn minimises capacity gaps as shown in Figure 3 that may result when using an integer bit- per-symbol modulation. In addition to non-integer bit- per-symbol modulation, additional capabilities to fine-tune the coherent optical modulation have been introduced in the 600G era. 3D Shaping and PCS are techniques to manipulate coherent constellation points’ probability and location to optimise reach and capacity. The use of 3D Shaping also provides the ability to fine-tune the transmission baud rate—this enables network operators to turn spectral gaps into usable bandwidth. FINE-TUNING THE BAUD RATE Channel passbands can vary between networks and even between links in the same network. In previous generations, the coherent interconnect baud rate is either fixed or can be selected between a small number of quantised settings which can create spectral gaps in a link. 600G era 3D Shaping technology introduces the capability of an adaptive baud rate where the baud rate can be continuously tuned, enabling the transmission spectrum to expand into the available passband of the channel, filling in any margin gaps (Figure 4). This fine-tune adjustable baud rate capability is very useful in a ROADM- based multi-haul network as shown in Figure 5. By shaping the spectral width using an adaptive baud rate, one can turn the spectral gaps into usable bandwidth. This capability allows for flexible control
Figure 2. Bit error rate curves for three different 400G links, each using different QAM modulations and corresponding baud rates.
Figure 3. Example of capacity utilisation improvement on the orange and green wavelength links when using non-integer bit-per-symbol modulation.
Figure 4. Adaptive baud rate enables the minimisation of spectral gaps within a channel passband.
Figure 5. Adaptive baud rate implementation in a multi-haul network for spectral optimisation.
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ISSUE 14 | Q3 2018
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