Channel Equalisation

Channel equalisation is necessary in a ULH DWDM system due to the accumulation of spectral power tilts and ripples across a large number of network nodes leading to variations in performance across many DWDM channels. Channel equalisation is also useful in ROADM based networks where different channels may have travelled on different paths and hence will accumulate different power variations [37].

The primary sources of tilt are: SRS tilt, transmission fibre and DCF attenuation tilt. Fibre and DCF induced tilt is static, but strongly length and fibre type dependent, whilst the SRS contribution is dynamic being dependent on the total signal power (and hence the number of channels) in addition to the fibre type. SRS amplifies longer wavelength channels at the expense of shorter wavelength channels and increases log-linearly with total signal power (and hence channel count). Amplifiers are designed to have a static gain tilt, chosen to minimise the net tilt across a number of channels by opposing the SRS tilt, and fibre / DCF attenuation tilt.

Amplifier gain and noise figure variations are the primary sources of power ripple. The exact shape of the ripple is dependent on the number and types of amplifier in the transmission line.

Pre-Emphasis

Dynamic channel equalisation and signal power pre-emphasis (changing the channel power profile at the input of the link) helps equalise the net gain and hence the power spectrum at the end of the transmission link as shown in Figure 23.

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Figure 23 Principle of pre-emphasis. Left: Input channel powers. Centre: Channel powers with SRS tilt. Right: Output channel powers with combined SRS and EDFA tilt. (a) no pre-emphasis resulting in tilted spectrum after EDFA (b) pre-emphasis results in flattened output (after [19])

At the terminals pre-emphasis involves altering the launch powers of each individual channel according to a spectral profile which either minimises the variation in power across all channels or minimises the variation of a channel performance measure such as OSNR, Q-factor or BER.

Re-Emphasis

Re-emphasis involves repeating the pre-emphasis process periodically along a link to ensure channel power and performance remains within suitable limits. Re-emphasis is performed at dedicated levelling nodes and can also be performed at ROADM nodes since the ROADM can act as a channel specific VOA.

Levelling Sub-System

Figure 24 shows the sub-system architecture of a levelling node. A power monitor unit (PMU) accurately measures the signal power of each channel at the output of the node providing electronic feedback to the channel equalising unit (CEU) which adjusts the power of each channel to achieve the desired spectral profile. The CEU is based on a liquid crystal spatial light modulator capable of independently attenuating individual channels spaced on a 50GHz grid [46]. An SSA-17 EDFA compensates for the loss and channel attenuation due to the CEU. It is assumed that the CEU can correct for channel-to-channel variations to achieve any desired linear tilted channel profile.

P



SRS tilt EDFA tilt

pre-emphasis flat input flattened output (a) (b) Tx

67 DCM _CEU PMU Pre-amp Booster SSA DSA

Figure 24 Levelling sub-system architecture24

Levelling nodes are significantly more expensive than standard amplifier nodes due to the high cost of the CEU, PMU and additional amplification; the cost ratio of a leveller to an average line amplifier node is about 2 to 1. Levelling nodes also impact the OSNR due to the extra EDFA required. It is therefore desirable when designing ULH networks to place as few levelling nodes as possible along the link. Work initially carried out in [25] determined the optimum spacing between levellers to be 6 spans. Following further development of the combined wideband and nonlinear simulation model presented in this thesis, publication [20], and section 2.14 of this thesis show that the trade-off between performance and cost extends the optimum spacing of levelling nodes to 8 spans apart.

2.8.1

OSNRE

P

-

Pre-emphasis is usually performed to equalise either power or OSNR across the spectrum at the end of a link [32] and [33].

To perform OSNR equalisation pre-emphasis, the power in each channel is scaled by a factor inversely proportional to the initial end of line OSNR of that channel. The power of the ith channel _{should be set according to [29]}

            



where OSNRi is the OSNR of the ith channel and Pi is the input power of the ith

channel with no pre-emphasis and is the total signal power; the denominator

ensures a constant total system input power. Multiple iterations of the algorithm are

24

68

normally required, as changing the signal powers will change the saturation conditions of the amplifiers. This pre-emphasis will be re-applied at levelling nodes (re-emphasis) to maintain the required power spectrum for end-to-end OSNR equalisation.

Linear Pre-emphasis Tilt Approximation

Applying the algorithm given by Equation 16 will result in an ideal input power spectrum which can perfectly equalise the OSNR spectrum. An example input power spectrum and the resulting flat OSNR spectrum is shown by the pink lines in Figure 25 and Figure 26 respectively.

In practice, it is impractical to control up to 80 channels in a field-deployed DWDM system with the precision required to achieve this complex pre-emphasis profile due to dynamic control issues such as amplifier transients, forcing system designers to simplify algorithms. The pre-emphasis is therefore approximated with a linear fit to the ideal profile as shown by the green line in Figure 25. The resulting OSNR spectrum shown in green in Figure 26 has an OSNR spread of 1.1dB compared to the 0.0dB with ideal pre-emphasis (pink line) and 1.9dB with no pre-emphasis (blue line). The performance of the worst channel is improved from 15.7dB with no pre- emphasis to 16.3dB with linear pre emphasis and 17.1dB with an ideal pre-emphasis profile.

Equalising the OSNR spectrum does not necessarily result in equal channel powers across the C-band due to the complex balance of the various sources of channel power variation. Figure 27 however does show a small improvement in the spread of output powers from 5.5dB with no pre-emphasis to 3.9dB with the ideal OSNR pre- emphasis and 4.6dB with the linear tilt approximation.

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Figure 25 Input power spectra at start of 3,173km link with: (a) no pre-emphasis (b) ideal OSNR equalisation pre-emphasis (c) linear OSNR equalisation pre-emphasis.

Figure 26 OSNR spectrum at end of 3,173km link with: (a) no pre-emphasis (b) ideal OSNR equalisation pre-emphasis and (c) linear OSNR equalisation pre-emphasis

0.0 0.5 1.0 1.5 2.0 2.5 3.0 1525 1530 1535 1540 1545 1550 1555 1560 1565 wavelength (nm) P o w e r (d B m ) no pre-emphasis (spread 0.0 dBm)

Ideal OSNR equalisation (spread 1.9dBm)

linear OSNR equalisation (spread 1.3 dBm) 15.5 16.0 16.5 17.0 17.5 18.0 1525 1530 1535 1540 1545 1550 1555 1560 1565 wavelength (nm) O S NR ( d B) no pre-emphasis (spread 1.9 dB)

Ideal OSNR equalisation (spread 0 dB)

linear OSNR equalisation (spread 1.1 dB)

70

Figure 27 Received power spectrum at end of 3,173km link with: (a) no pre-emphasis (b) ideal OSNR equalisation pre-emphasis and (c) linear OSNR equalisation pre-emphasis