Coherent detection technology

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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.

Other related questions:
Differences in commissioning a non-coherent system and a coherent system
The main difference in commissioning a non-coherent system and a coherent system is the position of the transmit-end EVOA. In a coherent system, the transmit-end EVOA is always located after the OA board. In a non-coherent system, the transmit-end EVOA is located after the OA board only in the non-standard fiber access mode.

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.

Method used to distinguish coherent boards from non-coherent boards
40G boards have both coherent and non-coherent boards. All 100G boards are coherent boards. The following lists the coherent and non-coherent 40G boards, and 100G boards that are applicable to the OSN 8800: 40G coherent boards: TN15LSXL, TN55NS3, TN56NS3, TN54HUNS3 40G non-coherent boards: TN11LSXL, TN12LSXL, TN11LSXLR, TN12LSXLR, TN11LSQ, TN11LSQR 100G coherent boards: TN12LSC, TN14LSC, TN11LTX, TN12LTX, TN54NS4, TN56NS4

S.M.A.R.T technology concept
Self-monitoring, analysis and reporting technology (S.M.A.R.T) is widely used to disks to protect data security. When disks are working, the monitoring system analyzes the states of motors, circuits, disks, and heads. When an exception occurs, an alarm will be generated and automatic speed-down and data backup may be implemented.

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.

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