10G DWDM Upgrade Without Dispersion Compensation
A regional carrier upgrades a 32-channel, 100 GHz DWDM system from 2.5G to 10G NRZ. The route is four 80 km spans of G.652.D, 320 km total, with an EDFA at each span and no dispersion-compensating modules. Launch power is +1 dBm per channel; measured span loss is 21 dB and received OSNR is 26 dB in 0.1 nm. The 2.5G system was error-free. After the 10G cards are installed, several channels show pre-FEC BER around 1×10^-5 and occasional protection switches. The worst channels are near 1550 nm. The operator has spare 10G transponders but no coherent line cards. The vendor asks whether OSNR is the problem or whether the fiber route itself is unsuitable. The carrier wants to avoid a full re-cable and has limited rack space for DCMs.
1. Calculate the cumulative chromatic dispersion and assess whether it explains the 10G BER.
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At 1550 nm, standard G.652.D has about 17 ps/nm/km. Over 320 km the accumulated dispersion is approximately 17 × 320 = 5,440 ps/nm. A 10G NRZ transponder typically tolerates roughly 800 to 1,000 ps/nm without compensation, depending on the module and chirp. The link is therefore about five to six times beyond the normal uncompensated tolerance.
OSNR is not the limiting factor: 26 dB in 0.1 nm is comfortably above the roughly 15 to 18 dB usually required for 10G NRZ. The BER pattern, worse near 1550 nm and appearing only after the rate upgrade, is consistent with dispersion-induced pulse spreading. The 2.5G system worked because its dispersion tolerance is roughly four times larger.
2. What remedies would you recommend, and what deployment precautions apply?
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The practical fix without coherent cards is to add dispersion compensation. Place DCMs at amplifier mid-stage rather than directly at the receiver, and size them per span: 80 km × 17 ps/nm/km ≈ 1,360 ps/nm per span. Do not overcompensate; leave residual dispersion inside the 10G card tolerance across the C-band. A tunable DCM or per-channel tuning may be needed if the residual varies too much across channels.
DCMs add insertion loss, often several dB each, so recheck amplifier gain, output power, and OSNR after installation. Keep launch power below nonlinear thresholds and verify BER on the worst channels. If rack space or loss margin is too limited, the cleaner long-term option is coherent 100G, which tolerates much higher dispersion and avoids per-span DCMs.