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Phase Noise Cancellation in Coherent Communication Systems Using a Radio Frequency Pilot Tone

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Phase Noise Cancellation in Coherent Communication Systems Using a Radio Frequency Pilot Tone applied sciences Article Phase Noise Cancellation in Coherent Communication Systems Using a Radio Frequency Pilot Tone Tianhua Xu 1,2,3,* , Cenqin Jin 2, Shuqing Zhang 4,*, Gunnar Jacobsen 5,6, Sergei Popov 6, Mark Leeson 2 and Tiegen Liu 1 1 Key Laboratory of Opto-Electronic Information Technology (MoE), School of Precision Instruments and Opto-Electronics Engineering, Tianjin University, Tianjin 300072, China; [email protected] 2 School of Engineering, University of Warwick, Coventry CV4 7AL, UK; [email protected] (C.J.); [email protected] (M.L.) 3 Optical Networks Group, University College London, London WC1E 7JE, UK 4 Department of Optical Engineering, Harbin Institute of Technology, Harbin 150001, China 5 NETLAB, RISE Research Institutes of Sweden, SE-16440 Stockholm, Sweden; [email protected] 6 Optics and Photonics Group, KTH Royal Institute of Technology, SE-16440 Stockholm, Sweden; [email protected] * Correspondence: [email protected] (T.X.); [email protected] (S.Z.) Received: 26 September 2019; Accepted: 3 November 2019; Published: 5 November 2019 Featured Application: This work is performed for the compensation of the laser phase noise (LPN) and the equalization enhanced phase noise (EEPN) in high-capacity, long-haul, coherent optical fiber networks. Abstract: Long-haul optical fiber communication employing digital signal processing (DSP)-based dispersion compensation can be distorted by the phenomenon of equalization-enhanced phase noise (EEPN), due to the reciprocities between the dispersion compensation unit and the local oscillator (LO) laser phase noise (LPN). The impact of EEPN scales increases with the increase of the fiber dispersion, laser linewidths, symbol rates, signal bandwidths, and the order of modulation formats. In this work, the phase noise cancellation (PNC) employing a radio frequency (RF) pilot tone in coherent optical transmission systems has been investigated. A 28-Gsym/s QPSK optical transmission system with a significant EEPN has been implemented, where the carrier phase recovery (CPR) was realized using the one-tap normalized least-mean-square (NLMS) estimation and the differential phase detection (DPD), respectively. It is shown that the RF pilot tone can entirely eliminate the LPN and efficiently suppress the EEPN when it is applied prior to the CPR. Keywords: coherent optical fiber communication; laser phase noise (LPN); carrier phase recovery (CPR); phase noise cancellation (PNC); equalization enhanced phase noise (EEPN); radio frequency (RF) pilot tone 1. Introduction Long-haul optical communication is seriously deteriorated by transmission impairments, e.g., chromatic dispersion (CD), polarization mode dispersion (PMD), laser phase noise (LPN), and Kerr fiber nonlinearities [1,2]. The combination of coherent detection, digital signal processing (DSP), and advanced modulation formats offers a very promising solution for long-haul, high-capacity optical transmission, to offer great capabilities and flexibilities in the design, deployment, and operation of core telecommunication networks [3–5]. In the reported phase noise cancellation (PNC) methods, the radio frequency (RF) pilot tone scheme was verified to be an efficient approach to remove laser phase Appl. Sci. 2019, 9, 4717; doi:10.3390/app9214717 www.mdpi.com/journal/applsci Appl. Sci. 20192019,, 99,, x 4717 FOR PEER REVIEW 22 of of 9 radio frequency (RF) pilot tone scheme was verified to be an efficient approach to remove laser phase fluctuationsfluctuations [6–9]. [6–9]. However, However, these these works only studied the the behaviors of RF pilot tones in short short-reach‐reach systems, where the enhancement effect effect of fiber fiber dispersion on the the LPN LPN was was neglected neglected [10,11]. [10,11]. Actually, Actually, due toto thethe interplay interplay between between the the electronic electronic dispersion dispersion compensation compensation (EDC) (EDC) module module and theand LPN the fromLPN fromthe local the local oscillator oscillator (LO), (LO), a phenomenon a phenomenon of equalization of equalization enhanced enhanced phase phase noise noise (EEPN) (EEPN) is induced is induced and andplays plays a significant a significant role in therole carrier in the phase carrier recovery phase (CPR) recovery in high-capacity (CPR) in opticalhigh‐capacity communication optical communicationsystems [10–16]. However,systems [10–16]. so far, no However, DSP-based so CPR far, has no been DSP developed‐based CPR to ehasffectively been compensatedeveloped forto effectivelyEEPN [16– 18compensate]. Therefore, for it EEPN will be [16–18]. of great Therefore, significance it to will study be theof great suppression significance of LPN to andstudy EEPN the suppressionusing an RF pilotof LPN tone and scheme. EEPN using an RF pilot tone scheme. In this work, the performance of an orthogonally polarized RF pilot tone scheme is investigated for eliminating both the LPN and the EEPN in long-haullong‐haul optical transmission systems. A 28 28-Gsym‐Gsym/s/s quadrature phase shift keying (QPSK) system is numerically implemented, where the CPR is realized realized using a one one-tap‐tap normalized least mean square (NLMS) estimation and a differential differential phase detection (DPD) scheme,scheme, respectively respectively [ 19[19–21].–21]. The The results results show show that that the the LPN LPN can becan fully be fully removed removed and the and EEPN the EEPNcan also can be also effectively be effectively suppressed, suppressed, when the when RF the pilot RF tone pilot scheme tone scheme is applied is applied prior to prior the CPR. to the CPR. 2. EEPN EEPN in in Optical Optical Communication Communication Systems Figure 11 describesdescribes thethe blockblock diagramdiagram ofof a a long-haul, long‐haul, coherent coherent optical optical communication communication systemsystem with EDC EDC and and CPR. CPR. The The LPN LPN from from the the transmitter transmitter (Tx) (Tx) laser laser passes passes through through the the optical optical fiber fiber and and the dispersionthe dispersion compensation compensation unit, unit, and andthus thus the net the CD net CDexperienced experienced by the by Tx the LPN Tx LPN approaches approaches zero. zero. By contrast,By contrast, the the LO LO LPN LPN goes goes only only through through the the dispersion dispersion compensation compensation unit. unit. Consequently, Consequently, the the LO LPN interplaysinterplays with with the the dispersion dispersion compensation compensation component component in EDC in andEDC the and interactions the interactions will degrade will degradethe performance the performance of the optical of the communication optical communication system. Thissystem. effect This is called effect EEPNis called [10 EEPN–13]. [10–13]. Figure 1. BlockBlock diagram diagram of coherent transmission system and equalization-enhancedequalization‐enhanced phase noise (EEPN). ϕφTx:: Tx Tx laser laser phase phase noise noise (LPN), (LPN), ϕφLOLO: LO: LO LPN, LPN, ADCs: ADCs: analogue analogue-to-digital‐to‐digital converters. converters. The variance of the EEPN distortion will increase with fiber fiber dispersion, local oscillator laser linewidth, and signal symbol rate. The noise variance of EEPN can be written as follows [10,19]: [10,19]: 2 2 2 πλ DD LLD ffLO 2 LO , (1) σEEPNEEPN= · · , (1) 2c2c· TTSS where λ is the central wavelength of the optical carrier, c is the speed of the light in vacuum, L is the length, D is the CD coecoefficientfficient of the fiber,fiber, TTSS isis thethe symbolsymbol periodperiod ofof thethe signal,signal, andand ΔDffLOLO isis the 3 3-dB‐dB linewidth of the LO laser. It is noted that the EEPN evaluation in Equation (1) only works for the static time-domaintime‐domain and frequencyfrequency-domain‐domain EDCs, which involve no phase noise compensation functionsfunctions [[22,23].22,23]. Appl. Sci. 2019,, 9,, x 4717 FOR PEER REVIEW 33 of of 9 3. CPR Using One‐Tap NLMS 3. CPR Using One-Tap NLMS A one‐tap NLMS filter could be effectively applied in the CPR [20], and its tap weight w(k) is expressedA one-tap using NLMS the following filter could equations: be effectively applied in the CPR [20], and its tap weight w(k) is expressed using the following equations: wk1 wk x* k ek, µ 2 (2) w(k + 1) = w(k) + xk x (k)e(k), (2) 2 ∗ x(k) ek dk wk xk , e(k) = d(k) w(k) x(k), (3) − · where x((k)) is the input signal, k is the symbol index, d(k) is is the the desired desired symbol, e(k) is is the the error error between between the output and the desired symbols, and µμ represents aa parameterparameter ofof thethe stepstep size.size. The schematic ofof the the NLMS NLMS CPR CPR is is illustrated illustrated in in Figure Figure2, which 2, which can can actually actually be implemented be implemented in a infeed-forward a feed‐forward structure structure and canand be can realized be realized in parallel in parallel in the fieldin the programmable field programmable gate array gate (FPGA) array (FPGA)circuits forcircuits a real-time for a real operation‐time operation [4]. [4]. Figure 2. BlockBlock diagram diagram of of the the one one-tap‐tap normalized normalized least least-mean-square‐mean‐square (NLMS) carrier phase recovery (CPR). Similar to the definition of the Tx and the LO, a concept of effective linewidth Df is employed Similar to the definition of the Tx and the LO, a concept of effective linewidth ΔfEEffff is employed here to describe the total phase noise in coherent transmissiontransmission systemssystems consideringconsidering EEPNEEPN [[17,19]:17,19]: σ222+σ2 + σ 22 f TxTx LO EEPNEEPN D fE fEff f (4)(4) ≈ 22πTTSS 2 2 σTx = 2πD fTx TS (5) Tx 2fTx ·TS (5) 2 σLO = 2πD fLO TS (6) 2 · LO 2f LO TS (6) 2 2 where σTx and σLO are the LPN variance of the Tx and LO lasers, respectively, and DfTx is the 3-dB 2 2 wherelinewidth Tx of and the Tx LO laser.
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