基于FPGA的chirp通信系统的调制与解调技术毕业设计.docx

1、基于FPGA的chirp通信系统的调制与解调技术毕业设计基于FPGA的chirp通信系统的调制与解调技术毕业设计编号： 毕业设计(论文)外文翻译（原文）学 院： 信息与通信学院 专 业： 通信工程 学生姓名： 李忠斌 学 号： 1000210219 指导教师单位： 信息与通信系 姓 名： 樊孝明 职 称： 讲师 2015年 6 月 8 日Investigation of modulator chirp and extinction ratio in different RZ- and NRZ duo-binary transmitter modules for performance opti

2、mizationAbstract - In this work we present a comparative investigation of modulator chirp and extinction ratio in different transmitter modules for 10Gbps, 20Gbps and 40Gbps RZ- and NRZ duo-binary transmission. For comparative analysis three types of transmission modules have been considered viz. pu

3、shpull configuration based on dual arm MZIM, delay-and-add circuit based single arm MZIM and a duo-binary filter followed by single arm MZIM. For each case, the modulator chirp has been optimized with an extinction ratio of 20 dB. Investigation has been carried out to find extinction ratio of single

4、 arm MZIM used for RZ- and NRZ duo-binary transmission that offers system performance comparable to dual arm MZIM at 10, 20 and 40Gbps. The results help choosing the best suitable transmitter module with optimized modulator chirp and extinction ratio.I.INTRODUCTIONIn high speed optical communication

5、 systems duo-binary modulation is an effective solution that provides a better spectral efficiency and reduces the performance degradation owing to dispersion and nonlinear effects 13. In the 1980s and the early 1990s, direct modulation of semiconductor laser was the mostly used technique. However,

6、direct modulation has several limiting factors like it induces unwanted chirps which results in spectra broadening of the signal, hence causing severe dispersion penalties 4. Directly-modulated optical signals experience fluctuations in intensity due to Relative Intensity Noise (RIN) of the semicond

7、uctor laser. Nonzero line width of laser sources introduces laser phase noise thus for high speed transmission direct modulation is usually not preferred. Hence, external modulation has been an essential choice for the high speed long-haul communication 5,6. External modulators remove the large amou

8、nt of wavelength chirping which will otherwise be included if laser diode is directly modulated. Mainly two types of semiconductor external modulators are available viz. Electro-Optic Modulator (EOM) and Electro-Absorption Modulators (EAM) 6. Of these two modulators, EOM that changes the band gap en

9、ergy with applied electric field is mostly preferred because of various advantages like linear response characteristic, high extinction ratio, ability to control phase, frequency or amplitude of the light wave carrier owing to the properties of electro optic material. Even for very low value of driv

10、ing voltage EOM is able to achieve high speed modulation 7,8.II.TheoryIn optical domain, data modulation is achieved using two types of modulators: optical phase modulator and optical intensity modulator. An Electro-Optic Phase Modulator (EOPM) uses only one electrode. When a driving voltage is appl

11、ied to the electrode, the refractive index of the electro-optic waveguide changes accordingly, thus slowing down the light wave and hence inducing a delay on the optical signal. The induced delay corresponds to the phase change, thus an EOPM is able to manipulate the phase of the light wave carrier

12、9.where V is the driving voltage required to create a phase shift, V(t) is a time-varying driving signal voltage and Vbias is dc bias voltage. Optical field Eo at the output of the EOPM is given as:Eo(t) = Ei (t)ej(t) (2)Optical intensity modulator uses two EOPMs in a parallel structure to form a Ma

13、chZehnder interferometer commonly known as the MachZehnder intensity modulator (MZIM) 9. Input optical signal splits equally in the two arms of the MZIM which are actually EOPMs for modulating the phase of the optical carrier. At the output, the two arms are coupled either constructively or destruct

14、ively to provide intensity modulated optical pulses.Fig. 1. Duo binary transmitter module with dual-arm MZIM (T#1).Fig. 2. Duo binary transmitter with single-arm MZIM delay-and-add circuit (T#2).Fig. 3. Duo-binary transmitter with single-arm transmitter and filter circuit (T#3).MZIM can be of two ty

15、pes: single-arm MZIM and dual arm MZIM. In single-arm MZIM only one single driving voltage is applied to the either arm of MZIM 9 and the output transmitted optical field Eo(t) is given as:Existence of the phase term in Eq. (3) shows that the chirping effect is present, thus we can say that the sing

16、le-arm MZIM generated signals are not chirp-free. Particular structure of the MZIM can only minimize the chirping (x-cut MZIM). It has been found that a small amount of chirp is useful for transmission 9. Dual-arm MZIM has pushpull arrangement where the dual drive voltages V1(t) and V2(t) are invers

17、e to each other and thus, able to completely eliminate the chirping effect in the modulation. The transmitted optical field can be written as:Fig. 4. Comparison of 10 Gbps RZ duo-binary transmitter modulesFig. 5. Comparison of 20 Gbps RZ duo-binary transmitter modules.The use of optical duo-binary t

18、ransmitter with the dual-arm MZIM is the usual choice in transmitter design at high data rate, however, dual arm configuration demands more stringent requirement of symmetry to be met 10. Single-arm MZIM with duo-binary filter can also be used where duo-binary filter can be approximated by a low pas

19、s filter with half-power cutoff at approximately one fourth the data rate. At this cutoff frequency, the spectral occupancy of the modulated optical field is restricted to f0 (Bit rate)/2, where f0 is a nominal continuous wave (CW) frequency 11. The frequency spectrum of an ideal duo-binary signal e

20、xhibits the first nulls at f0 (Bit rate)/2. So, we can band-limit the signal at f0 (Bit rate)/2 while preserving at the same time, sufficient information for later reconstruction of the signal 11. The duo-binary filter used for simulation is a fifth order low-pass Bessel filter. Duo-binary signal ge

21、neration can also be achieved by directly applying the RZ/NRZ signal to the delay-and-add circuit followed by band limiting filter of bandwidth (Bit rate)/2.III.Performance measure IV.System description and results A performance measure criterion provides us the platform for investigation and analys

22、is. In this work we have considered the semi-analytic BER evaluation technique for the estimation of the Bit Error Rate (BER) and BER equivalent Q-factor is considered as performance measure criterion. In the present analysis, we are considering three types of duo-binary transmitter modules viz. (1)

23、 duo-binary transmitter based on push-pull configuration of dual-arm MZIM (hereafter denoted by T#1), (2) single-arm MZIM with delay-and-add circuit (here after denoted by T#2) and (3) the single-arm MZIM followed by a duo-binary filter (hereafter denoted by T#3). Simulative analysis has been perfor

24、med using commercial package OptSimTM. We carried out our analysis at three different bit rates of 10, 20 and 40G bps. The simulation setup typically comprises of three sections: duo-binary generation, i.e. transmitter section, optical filter or channel and the receiver section. Receiver section whi

25、ch is common in the link design for all three above-mentioned transmitter modules consists of a PIN photo detector having the responsivity of 0.8 A/W followed by a fourth order electrical Bessel low pass filter of 30 GHz bandwidth centered at 1550 nm which provides filtered data to the BER estimator

26、 for performance measurement. We optimize the modulator chirp C for all three transmitter modules at an Extinction ratio of 20 dB. To find the value of C that optimizes system performance, we vary the C value from -4 to +2. Fig. 1 shows the simulation setup of NRZ duo-binary transmission using T#1 m

27、odule. Optical link consisting of Single Mode Fiber (SMF) of 80 km, 20 km and 4 km lengths have been considered for duo-binary transmission at 10, 20 and 40Gbps respectively. SMF has the following specifications: attenuation loss of 0.2 dB/km, dispersion and dispersion slope of 16 ps/km-nm and 0.07

28、ps/nm2-km at 1550 nm, respectively, effective area of 80 m2 and refractive index of 2.5 10-20 m2/W.SMF is followed by an EDFA that provides a fixed power of 3 dBm and has a noise figure of 4.5 dB. In this simulative work, an optical laser having 10 MHz line width has been considered.To carry out sim

29、ulation, first RZ duo-binary transmission has been considered and then the RZ pulse generator in Figs. 13 has been replaced with NRZ pulse generator to obtain NRZ duo-binary transmission. For a particular data rate optimal value of C has been figured out for each type of transmitter module. Q-factor

30、 of T#1 at optimized C for extinction ratio of 20 dB has been considered as reference to compare with T#2 and T#3. Extinction ratio at which T#2 and T#3 offers comparable performance to T#1 is investigated.Fig. 4 reports that at 10Gbps RZ duo-binary transmissions, T#1 is optimized at C = 0.4 while T

31、#2 and T#3 optimize at 0.8 and 1.4 C values, respectively. It is observed that at 20 dB extinction ratio T#2 performs better than T#1 and T#3. Considering T#1 with 20 dB extinction ratio as reference, it can be interpreted that T#2 achieves the performance of T#1 at a relatively lower value of extin

32、ction ratio.Fig. 5 records optimum C value of 0.2, 1.0 and 1.6 for T#1, T#2, T#3 respectively used in 20Gbps RZ duo-binary data transmission. Comparison of different transmitter modules at 20 dB extinction ratio shows similar qualitative behavior as seen in Fig. 4. Fig. 6 shows that C value of 0.2, 0.6 and 1.4 ensures highest Q factor for the three transmitter modules respectively. At 40Gbps RZ duo-binary transmitted from T#1 provides better performance than T#2 and T#3. The performance of T#1 at 20 dB