Coherent systems

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As mobile networks evolve towards LTE, smart terminals are widely used, and new services such as FBB users' IPTV, VoD, and cloud computing continue to emerge, the transmission capacity of conventional networks cannot meet requirement. To address the requirements, Huawei introduces transmission systems using the coherent technology. Huawei coherent transmission systems use advanced technologies such as ePDM-QPSK, ePDM-BPSK, and coherent detection to meet the high-speed transmission requirements on OSNR, CD tolerance, PMD tolerance, and nonlinear effects. Huawei provides large-capacity coherent solutions, offering ultra-large bandwidths (100G and 40G).
A system using a coherent board (such as LSC, LTX, TN15LSXL, TN55NS3, and TN54NS4) is a coherent transmission system.

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Coherent systems
As mobile networks evolve towards LTE, smart terminals are widely used, and new services such as FBB users' IPTV, VoD, and cloud computing continue to emerge, the transmission capacity of conventional networks cannot meet requirement. To address the requirements, Huawei introduces transmission systems using the coherent technology. Huawei coherent transmission systems use advanced technologies such as ePDM-QPSK, ePDM-BPSK, and coherent detection to meet the high-speed transmission requirements on OSNR, CD tolerance, PMD tolerance, and nonlinear effects. Huawei provides large-capacity coherent solutions, offering ultra-large bandwidths (100G and 40G). A system using a coherent board (such as LSC, LTX, TN15LSXL, TN55NS3, and TN54NS4) is a coherent transmission system.

Reason why a coherent system uses fewer OA boards than a non-coherent system
OA boards are used to compensate for insertion loss. Only one OA board is required in case that Gmax (maximum gain) �?Fiber loss + DCM loss; otherwise, two OA boards are required. The coherent system does not require DCMs, and a single OA board can compensate for larger link loss. When the link loss is within the permitted range, the coherent and non-coherent systems require the same number of OA boards. When the fiber loss is out of the permitted range, the coherent system, however, requires fewer OA boards. For example, in a system with a 60 km span, 18 dB fiber loss, and 5 dB DCM loss: The gain required by a coherent system is calculated as follows: Gain = Fiber loss + DCM loss = 18 dB + 0 dB (no DCM) = 18 dB < Gmax. Therefore, one OA board (OAU101) is required. The gain required by a non-coherent system is calculated as follows: Gain = Fiber loss + DCM loss = 18 dB + 5 dB = 23 dB < Gmax. Therefore, one OA board (OAU101) is required. In a system with a 100 km span, 28 dB fiber loss, and 9 dB DCM loss: The gain required by a coherent system is calculated as follows: Gain = Fiber loss + DCM loss = 28 dB + 0 dB (no DCM) = 28 dB < Gmax. Therefore, one OA board (OAU101) is required. The gain required by a non-coherent system is calculated as follows: Gain = Fiber loss + DCM loss = 28 dB + 9 dB = 37 dB > Gmax (36 dB for an EDFA board). Therefore, two OA boards (OAU101 and OBU101) are required.

Difference in the CD and PMD processing mechanism between a coherent system and a non-coherent system
100G/40G ePDM-BPSK systems are coherent systems. They use DSP chips for coherent detection, delivering superior performance in mitigating dispersion. Therefore, no DCM is required in these systems for dispersion compensation. For 40G DQPSK systems and other non-coherent systems, DCMs are required for dispersion compensation. The DCU board can also be used on the line.

Method used to measure the OSNR on a coherent system
In the coherent system, the OD solution can be used to measure the OSNR. The OD solution can measure the OSNR by modeling OAs and fibers and accurately measuring the signal power and noise power. E2E OSNR measurement is supported for 10G, 40G, and 100G systems in various scenarios.

Coherent detection technology
The coherent detection technology uses a local oscillator (LO) laser that has the same frequency as the received signal and implements interference between the LO laser signal and the received signal to restore the amplitude, phase, and polarization status information from the received signal. In addition, the technology uses high-speed analog-to-digital converter (ADC) and digital signal processing (DSP) techniques to compensate for CD and PMD on the line. The coherent detection technology improves the CD, PMD, and OSNR tolerance, and system transmission capability, achieving transmission performance equivalent to a 10G network. By using the coherent detection technology, a 40G/100G coherent system does not have to use a fixed dispersion compensation module (DCM) or a tunable DCM (TDCM), which is intended for fine tune the dispersion of a single wavelength in a traditional 40G system. A 40G/100G coherent system requires fewer optical amplifiers, does not require accurate measurement of the fiber length and dispersion, reduces system configuration costs and labor, and improves fiber transmission network performance.

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