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Benefits and Limits of Modulation Formats for Optical Communications OPTICS AND OPTOELECTRONICS VOLUME: 12 j NUMBER: 2 j 2014 j JUNE Benefits and Limits of Modulation Formats for Optical Communications Rajdi AGALLIU, Michal LUCKI Department of Telecommunications Engineering, Faculty of Electrical Engineering, Czech Technical University in Prague, Technicka 2, 166 27 Prague, Czech Republic [email protected], [email protected] Abstract. This paper is focused on benefits and lim- ification of shortcomings to be solved while proposing its of intensity and phase modulation formats used in new solutions. optical communications. The simulation results are ob- This paper investigates modulation formats in Opt- tained using OptSim software environment, employing Sim software environment (version 5.2) from the per- Time Domain Split Step method. Non-Return to Zero, spective of the Bit Error Rate (BER), Q-factor and Return to Zero, Chirped Return to Zero and Carrier- physical reach to find their main advantages, and the Suppressed Return to Zero formats are compared in performance limits. The transmission schemes for terms of Bit Error Rate and spectral efficiency to find high-density optical systems operating at 40 and 100 the limits for selected transmission network topologies. Gb·s−1 wavelength channels can use phase modula- It is shown that phase modulation formats offer many tion combined with Polarization Division Multiplexing advantages compared to intensity formats. Differential (PDM), coherent detection and digital signal process- Phase-Shift Keying and mainly Differential Quadrature ing [1], [2]. PDM halves the symbol rate, which en- Phase-Shift Keying improve the Bit Error Rate and ables usage of higher bit rates, cheaper components transmission reach, among others. A promising solu- and fitting into a proper channel grid at the cost of tion is the application of Polarization Division Multi- an increased transceiver complexity [1], [3]. It has plexing Quadrature Phase-Shift Keying, which primar- been shown that PDM Quadrature Phase-Shift Key- ily benefits in spectral efficiency, estimated reach, opti- ing (PDM-QPSK) format is very promising for high cal signal to noise ratio and chromatic dispersion tol- fiber reaches and huge data flows. For this reason, the erances. model of this modulation format has been developed. Keywords 2. State of the Art BER, CRZ, CSRZ, DPSK, DQPSK, Duobi- 2.1. Intensity Modulation Formats nary, eye diagram, modulation formats, NRZ, OptSim, PDM-QPSK, Q-factor, RZ. This paper, among others, deals with binary inten- sity formats due to the significant back-to-back re- ceiver sensitivity penalty of multilevel intensity formats 1. Introduction [4], [5]. Although a certain combination of Amplitude Shift Keying (ASK) and phase modulations (e.g. RZ- DPSK-3ASK format) [1] have advantages, the limited The upgrade of fiber optic telecommunication systems extinction ratios of the ASK modulated levels limits to higher bit rates very often requires solving the im- the Optical Signal-to-Noise Ratio (OSNR) tolerance of pact of polarization mode dispersion and nonlinear ef- the format. fects, such as Four Wave Mixing (FWM), that can sig- nificantly affect transmission at 10 Gb·s−1 speeds and The two most common intensity modulations are higher. For transmission rates higher than 40 Gb·s−1 Non Return to Zero (NRZ) and Return to Zero (RZ). per channel, the use of more advanced formats is nec- Conventional NRZ format has been widely imple- essary and the design of new modulations is expected. mented, mainly because of its signal bandwidth and This requires detailed knowledge on performance effi- its relatively easy generation. We compare and dis- ciency of modulation formats, as well as the clear spec- cuss features of these formats together with other bi- c 2014 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 160 OPTICS AND OPTOELECTRONICS VOLUME: 12 j NUMBER: 2 j 2014 j JUNE nary intensity formats such as: Chirped Return to 2.3. Advanced Modulation Formats Zero (CRZ) and the most widespread pseudo-multilevel format: Carrier-Suppressed Return to Zero (CSRZ), PDM-QPSK has been widely differently denoted either which could be an optimal solution for high speed by polarization division multiplexing, polarization mul- transmission systems [4]. tiplexing, dual polarization or orthogonal polarization Duobinary (DB) format represents correlative cod- [1]. Its transmitter is the same as in PDM-DQPSK. In- ing, a subclass of which is known as partial-response novation in PDM-QPSK stands for the employment of signaling. The main benefit of the DB format is its high a coherent receiver. The use of digital signal process- tolerance to Chromatic Dispersion (CD) and narrow- ing simplifies the receiver design although a large num- band optical filtering [4]. The main goal of using this ber of components is required, as well as low-linewidth format at 10 Gb·s−1 is to increase the dispersion toler- lasers [7]. Despite the fact that other formats have ance, whereas at 40 Gb·s−1 it is to achieve high spectral been designed and some of them are already com- mercially available, such as PM-OFDM-QPSK; PDM- efficiency in Wavelength Division Multiplexing (WDM) −1 systems. Nevertheless, the immunity of DB to nonlin- QPSK proves to perform better at 100 Gb·s and at ear effects at 40 Gb·s−1 does not differ much from simi- greater reaches, with respect to estimated reach, spec- lar duty cycle On/Off Keying (OOK). In section 3.3, we tral efficiency, OSNR, CD and differential group delay compare DB to OOK to find which of the formats per- tolerances [1]. forms better for a selected network topology. A further In PDM, two optical signals are coupled to two or- discussion and comparison of DB and Phase-Shaped thogonal polarizations being mutually delayed by a Binary Transmission with respect to transmission im- symbol period to improve OSNR. The two delayed pairments at 40 Gb·s−1 can be found in reference [6]. lines: a coupled resonator and a photonic crystal waveguide are compared by using PDM transmission in the study by F. Morichetti et al [8]. PDM can also 2.2. Phase Modulation Formats double transmission capacity of other modulation for- mats. The PDM has been applied experimentally by L. Cheng et al. to increase 8 DQPSK channels with Phase-based modulation formats provide higher spec- 200 GHz DWDM grid from 100 Gb·s−1 to 200 Gb·s−1 tral efficiency and better OSNR tolerances meanwhile [9]. Data were transmitted through a 1200 km long increasing the complexity of a transceiver. The main link with completely compensated chromatic disper- advantage of Differential Binary Phase-Shift Keying sion. However, in their experiment, an automatic po- (DBPSK or simply DPSK) over OOK is a 3 dB re- larization control was not implemented and proper po- ceiver sensitivity improvement [4]. Although the resis- larization should be set every ten minutes manually. tance of DPSK and CSRZ formats to fiber nonlineari- In section 3.5, we investigate this modulation format ties may be similar, the improved sensitivity of DPSK at 100 Gb·s−1 in a 2400 km long transmission system. receivers generally results in a better overall system performance. Detectors for DPSK signals are also more complex as they must convert the phase difference into an intensity signal which can be converted into an elec- 3. Methods trical signal by photo detectors. In section 3.4, we com- pare the NRZ and RZ variants of DPSK to find which of them offers better results in terms of BER and Q- Simulations are performed in OptSim environment us- factor for a selected topology. ing the Time Domain Split Step (TDSS) method. Sim- ulation results are performed on the created models of Knowing how to eliminate shortcomings of two-level modulation formats, incorporated into a model of an formats, it is suitable to investigate models of mul- optical transmission system, with respect to the eye tilevel formats. Differential Quadrature Phase-Shift diagram, BER, OSNR and Q-factor. Keying (DQPSK) is a multilevel format that has re- ceived appreciable attention. Leaving aside aspects of the transceiver design, DQPSK is an appropriate solu- 3.1. Time Domain Split Step tion to achieve narrow signal spectra. At the same bit rate, DQPSK is more robust to Polarization Mode Dis- Method persion due to its longer symbol duration, while com- paring it with binary formats [4]. Its spectrum shape OptSim employs the TDSS method to realize the signal is similar to that of DPSK; however its compression in distribution equation in a fiber. The method is based frequency enabled DQPSK to achieve higher spectral on the following formula [10]: efficiency and increased tolerance to CD [4]. Similarly as for DPSK, we compare the NRZ and RZ variants of @A(t; z) = (L + N) · A(t; z); (1) DQPSK, again in terms of BER and Q-factor. @z c 2014 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 161 OPTICS AND OPTOELECTRONICS VOLUME: 12 j NUMBER: 2 j 2014 j JUNE where A(t,z) is the complex envelope, L is the opera- 3) Optical Signal to Noise Ratio and Q tor which describes linear effects and N describes the Factor impact of non-linear phenomena on the signal prop- agation. The Split-Step algorithm applies L and N OSNR is obtained as the ratio of the net signal power operators to calculate A(t; z) over small fiber spans @z to the net noise power. The predominant source for its separately. The TDSS algorithm calculates L in the degradation is noise inserted by optical amplifiers. Q- time domain by applying convolution in sampled time factor is another important parameter, which is used [10]: in this paper for the evaluation of simulations. It can be expressed, as follows [12]: T DSS ! AL [n] = A [n] ∗ h [n] = P1 = k=−∞ A [k] · h [n − k] ; (2) µ − µ Q [−] = 1 0 ; (3) where h is the impulse response of a linear operator L.
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