**4. Evaluation of PAM-4, NRZ and duobinary modulation formats performance in WDM-PON system architecture**

In case of research we improve our previously made 4-channel PAM-4 WDM-PON system simulation model capacity by increasing number of multilevel channels and implement the use of different modulation formats in terms of system performance by maximal achievable reach. Several modulation formats have been proposed in the past and have become standards. In this research are investigated several modulation formats for use in WDM-PON architecture-based system, like NRZ, PAM-4 and duobinary (DB). Alternative solution instead widely used direct detection on-off keying modulation format NRZ-OOK with physical bandwidth limitations is to use more spectrally efficient multi-level formats such as PAM-4 [24, 25]. Another way to improve the bandwidth efficiency and reduce channel spacing is by using duobinary modulation format [12]. The most important feature of this multi-level modulation format duobinary is a viability of usage for longer transmission distances without regeneration with high tolerance to chromatic dispersion CD influence. As we know duobinary is used to increase the channel capacity by improving the bandwidth utilization [13].

The goal of our created 8-channel 20 Gbit/s per channel WDM-PON simulation model evaluate maximum transmission reach using different modulation formats, discussed previously in this paper like NRZ, PAM-4 and perspective duobinary modulation format. As it is shown in **Figure 12** the 8-channel WDM-PON simulation scheme with different optical transmitters (Tx) located in CO Optical Line Terminal (OLT\_Tx) part for each modulation format realization are shown. According to ITU-T G.694.1 rec. Frequency with grid central frequency of 193.1 THz and channel spacing of 50 and 100 GHz are chosen for research of crosstalk impact on modulation formats under research [26].

**Figure 12.**

*Simulation scheme of 8-channel 20 Gbit/s transmission speed per channel PAM-4, DB and NRZ modulated WDM-PON optical transmission system.*

In first simulation model PAM-4 transmitter is designed like previously, from two 10 Gbit/s NRZ coded electrical data signals (where one of them has twice larger amplitude), by coupled together with electrical coupler. Coupled PAM-4 electrical signal filtered with electrical Bessel low-pass filter (3-dB bandwidth is 10 GHz) and send to external MZM [21].

Second simulation model duobinary transmitter was realized with 20 Gbit/s bit rate per channel. Data source element with pseudo random bit sequence (PRBS) has only one logical output, where the output signal is divided in two signals. One of those signals is inverted by logical NOT element. Afterwards each data signal sent to NRZ drivers and filtered by Bessel low-pass filters (3-dB bandwidth is 5 GHz). Each NRZ coded electrical signal is passed to inputs of dual-arm MZM, at the end forming the DB transmitter [27].

Third simulation model NRZ transmitter consists of one NRZ driver with electrical signal input of data source with PRBS sequence. Afterwards NRZ coded data signal are directly connected to MZM RF signal input.

Following fixed parameters of optical and electrical elements was used: continuous wavelength (CW) laser output power + 6 dBm, extinction ratio 20 dB and 3 dB insertion loss of MZM, ITU-T G.652 SSMF with dispersion coefficient 17 ps/ (nm × km), dispersion slope 0.056 ps/nm2 × km and 0.2 dB/km attenuation coefficient [28]. Bandwidth of electrical LPF filters has been adjusted for optimal performance of each modulation format and have not been changed during research.

Each receiver consists of 40 GHz PIN photodiode with sensitivity equal to −19 dBm at 10 Gbit/s reference bit rate, dark current of 10 nA and responsivity of 0.8 A/W [29]. An electrical LPF filter bandwidth was adopted at receiver side for more successful system performance depending on the used modulation format. During the simulations LPF bandwidth of 15 GHz was chosen for PAM-4 modulated signals, and 10 and 17 GHz for DB and NRZ modulated electrical signals.

**99**

The BER threshold of 10<sup>−</sup><sup>3</sup>

channel spacing.

3.7 × 10<sup>−</sup><sup>4</sup>

**Figure 13.**

3.1 × 10<sup>−</sup><sup>4</sup>

reach of 62 km.

*Research of M-PAM and Duobinary Modulation Formats for Use in High-Speed WDM-PON…*

with additional FEC was used for our investigated

and

WDM-PON transmission system to compare performance in terms of maximal network reach for PAM-4, DB, NRZ modulated optical signals. During the simulations it was observed that maximal achievable distance has minimal crosstalk impact on BER for all modulation formats, which was negligible, depending on our chosen

*Eye diagrams of received (a) PAM4, (c) DB and (e) NRZ signals after B2B transmission, and after maximal reached transmission distance: (b) 50 km with PAM-4, (d) 62 km with DB, (f) 27 km with NRZ modulated* 

*signals for 8-channel 20 Gbit/s per channel WDM -PON transmission system.*

As it is shown in **Figure 13(a, c** and **e)** in B2B configuration for narrowest investigated 50 GHz channel spacing, the signal quality is good, eye is open and error-free transmission can be provided. After transmission the BER of received DB modulated signal with maximum reached distance of 62 km was

below defined threshold, was provided by DB modulation format, extending the

reached transmission distance, where BER of received signal was 5.8 × 10<sup>−</sup><sup>4</sup>

. PAM-4 and NRZ modulated signals shows 50 km and 27 km maximal

, please see **Figure 13(b, d** and **f )**. The largest network reach with BER

*DOI: http://dx.doi.org/10.5772/intechopen.84600*

*Research of M-PAM and Duobinary Modulation Formats for Use in High-Speed WDM-PON… DOI: http://dx.doi.org/10.5772/intechopen.84600*

**Figure 13.**

*Telecommunication Systems – Principles and Applications of Wireless-Optical Technologies*

In first simulation model PAM-4 transmitter is designed like previously, from two 10 Gbit/s NRZ coded electrical data signals (where one of them has twice larger amplitude), by coupled together with electrical coupler. Coupled PAM-4 electrical signal filtered with electrical Bessel low-pass filter (3-dB bandwidth is 10 GHz) and

*Simulation scheme of 8-channel 20 Gbit/s transmission speed per channel PAM-4, DB and NRZ modulated* 

Second simulation model duobinary transmitter was realized with 20 Gbit/s bit rate per channel. Data source element with pseudo random bit sequence (PRBS) has only one logical output, where the output signal is divided in two signals. One of those signals is inverted by logical NOT element. Afterwards each data signal sent to NRZ drivers and filtered by Bessel low-pass filters (3-dB bandwidth is 5 GHz). Each NRZ coded electrical signal is passed to inputs of dual-arm MZM, at the end

Third simulation model NRZ transmitter consists of one NRZ driver with electrical signal input of data source with PRBS sequence. Afterwards NRZ coded

Following fixed parameters of optical and electrical elements was used: continuous wavelength (CW) laser output power + 6 dBm, extinction ratio 20 dB and 3 dB insertion loss of MZM, ITU-T G.652 SSMF with dispersion coefficient 17 ps/

cient [28]. Bandwidth of electrical LPF filters has been adjusted for optimal performance of each modulation format and have not been changed during research. Each receiver consists of 40 GHz PIN photodiode with sensitivity equal to −19 dBm at 10 Gbit/s reference bit rate, dark current of 10 nA and responsivity of 0.8 A/W [29]. An electrical LPF filter bandwidth was adopted at receiver side for more successful system performance depending on the used modulation format. During the simulations LPF bandwidth of 15 GHz was chosen for PAM-4 modulated

signals, and 10 and 17 GHz for DB and NRZ modulated electrical signals.

× km and 0.2 dB/km attenuation coeffi-

data signal are directly connected to MZM RF signal input.

**98**

send to external MZM [21].

*WDM-PON optical transmission system.*

**Figure 12.**

forming the DB transmitter [27].

(nm × km), dispersion slope 0.056 ps/nm2

*Eye diagrams of received (a) PAM4, (c) DB and (e) NRZ signals after B2B transmission, and after maximal reached transmission distance: (b) 50 km with PAM-4, (d) 62 km with DB, (f) 27 km with NRZ modulated signals for 8-channel 20 Gbit/s per channel WDM -PON transmission system.*

The BER threshold of 10<sup>−</sup><sup>3</sup> with additional FEC was used for our investigated WDM-PON transmission system to compare performance in terms of maximal network reach for PAM-4, DB, NRZ modulated optical signals. During the simulations it was observed that maximal achievable distance has minimal crosstalk impact on BER for all modulation formats, which was negligible, depending on our chosen channel spacing.

As it is shown in **Figure 13(a, c** and **e)** in B2B configuration for narrowest investigated 50 GHz channel spacing, the signal quality is good, eye is open and error-free transmission can be provided. After transmission the BER of received DB modulated signal with maximum reached distance of 62 km was 3.7 × 10<sup>−</sup><sup>4</sup> . PAM-4 and NRZ modulated signals shows 50 km and 27 km maximal reached transmission distance, where BER of received signal was 5.8 × 10<sup>−</sup><sup>4</sup> and 3.1 × 10<sup>−</sup><sup>4</sup> , please see **Figure 13(b, d** and **f )**. The largest network reach with BER below defined threshold, was provided by DB modulation format, extending the reach of 62 km.
