Unique Challenges of Submarine Systems
Component Health Monitoring: In submarine networks, assessing service performance involves monitoring the health status of essential components such as Basic Units (BUs) and intermediate repeaters. This data is gathered by coherent transponders placed at cable ends, presenting a more complex monitoring scenario compared to terrestrial networks.
Total Output Power Constraint: Unlike terrestrial systems, submarine systems face Total Output Power (TOP) constraints, altering the calculation of Signal-to-Noise Ratio (SNR). TOP constraints lead to signal depletion and noise accumulation, necessitating adjustments in performance evaluation methodologies.
Evolution of Coherent Modems: Advancements in coherent modems, particularly on D+ submarine optical cables, redefine system capacity parameters. Consequently, the ITU updated commissioning processes to accommodate these advancements, ensuring accurate performance assessments.
Performance Metrics for Submarine Systems
Optical Signal-to-Noise Ratio (OSNR): OSNR quantifies the ratio of service signal power to noise power within a specified bandwidth. However, comparing OSNR between systems with different baud rates requires careful consideration to ensure accurate performance evaluation.
Signal-to-Noise Ratio ASE (SNRASE): SNRASE, akin to OSNR, accounts for noise within the signal bandwidth, enabling comparisons across systems with varying baud rates.
Effective SNR (ESNR): ESNR offers a line rate-independent measurement by considering both linear and non-linear noises. Unlike traditional metrics like Q factor, ESNR remains consistent across different line rates, providing valuable insights into future system capacities.
Generalized OSNR (GOSNR): GOSNR combines linear and non-linear noise contributions, offering a comprehensive assessment of system performance. Its baud-independent version, GSNR, provides a holistic view of performance, accounting for effects like guided acoustic-optic wave Brillouin scattering (GAWBS) and signal droopSubmarine cable systems are more challenging compared to terrestrial ones. As a con- sequence, they have major differences in the ways information about service performance can be determined and measured.
Some major differences are the following:
In submarine networks the service performance can be determined by information de- scribing the health status of basic network components (BUs, intermediate repeaters). This information is obtained by coherent transponders which are placed at the ends of a submarine cable. Their terrestrial counterparts are by far more easy to monitor. Terrestrial networks can process more data with regard to each unit’s contribution to the whole system’s performance.
Total output power (TOP) constraint is another key difference between terrestrial and submarine systems as it changes the way that total SNR is calculated. The TOP constraint in submarine amplifiers results in signal depletion whereas amplifier noise is accumulated because the total channel power (S+N) remains fixed with distance.
New coherent modems used on D+ submarine optical cables change the parameters that define total system capacity and so ITU updated the commissioning process (thus the final tests before going commercial) in a G.977.1 recommendation.
Optical signal-to-noise ratio (OSNR) refers to the ratio of service signal power to noise power for a valid bandwidth (0.1 nm, so ~12.5 GHz at 1550 nm). OSNR is used to quantify the linear noise from amplified spontaneous emission (ASE). However, if two systems are using different baud rates, the OSNR of the higher baud rate system will be reported as higher (although the two systems may have the same noise level). So, OSNR has to be defined without reference to channel spacing and symbol rate.
Signal-to-noise ratio ASE (SNRASE) is similar to OSNR except that the noise bandwidth is equivalent to the signal bandwidth. This leads to a measurement not dependent on symbol rate and which accounts for all noise detected on the receiver side. Therefore, by implementing this approach it is easier to compare the SNR metric for signals with different baud rates.
Similar to the baud independence of SNR, a metric that provides a line rate-independent measurement of performance would be useful for evaluating potential system capacities. Effective SNR (ESNR) measures linear and non-linear noises and reports their impact to the signal performance. As a result, ESNR will not be changed for the same noise levels, no matter the signal’s line rate. This is an improvement over Q factor, which varies for signals experiencing the same total noise on different line rates. In this way, ESNR can be used as a future teller (i.e., if measured at one line rate, it can then be used to predict performance of higher data transmission rates for the same cable system).
As performance of upcoming systems depends also on optical nonlinearity (SNRNL), it would be convenient to measure both linear and non-linear performance. Generalized OSNR (GOSNR) sums the non-linear and the linear noise of the wet plant optical systems. GOSNR’s updated baud-independent version is GSNR. Other effects are both guided acoustic–optic wave Brillouin scattering (GAWBS) and signal droop. GAWBS is an effect which leads to a penalty for a given wet plant design. This effect is caused by the interaction between light and the acoustic modes that occur in the optical fiber.
GSNR is evaluated directly through simulations or analytic models and indirectly through experiments. Figure below summarizes all existing and new metrics and indicates at which exact point each of them is measured.
Figure: Summation of existing and new cable performance metrics and indication of the exact location where each one of them is measured.
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Cited: Papapavlou, C., Paximadis, K., Uzunidis, D., & Tomkos, I. (2022). Toward SDM-Based Submarine Optical Networks: A Review of Their Evolution and Upcoming Trends. Telecom.
Future Perspectives and Conclusion
The performance of upcoming submarine systems hinges on the accurate measurement and evaluation of both linear and non-linear factors. As technology advances, metrics like ESNR and GSNR will play pivotal roles in predicting and optimizing system capacities.
In conclusion, submarine systems present unique challenges that demand specialized performance metrics and evaluation methodologies. By embracing advancements in coherent modems and refining performance metrics, we pave the way for more efficient, reliable, and high-capacity submarine networks, ensuring seamless global connectivity for generations to come.
