**3. Evaluation of PAM-4 modulation format use in WDM-PON systems**

In our research we investigated the 4-channel 10 Gbaud/s (20 Gbit/s) per channel PAM-4modulated WDM-PON access system with minimal allowable channel spacing, which has a direct impact on the utilization of resources like optical spectrum. The research was made with and without fiber chromatic dispersion (CD) fiber Bragg grating compensation module (FBG DCM). We evaluate system performance and found the maximal transmission distance for multichannel PAM-4 modulated WDM-PON transmission system operating at 20 Gbit/s per channel. In OptSim simulation software we created transmission system model to evaluate the performance of 4-channel PAM-4 modulated WDM-PON transmission system operating at 10 Gbaud/s or 20 Gbit/s per channel under the condition with BER threshold of 10<sup>−</sup><sup>3</sup> , by use of Reed Solomon (RS 255,223) forward error correction (FEC) code for 10 Gbit/s PONs [21, 22]. The theoretical FEC relationship restores 1.1 × 10<sup>−</sup><sup>3</sup> pre-FEC BER to a 10<sup>−</sup>12 post-FEC in the PON standards. As it is shown in **Figure 9**, the PAM-4 modulated WDM-PON simulation scheme was created in OptSim simulation software environment. Here the Matlab software was used for BER estimation of received PAM-4 signals. WDM-PON simulation model consists of 4 channels, with central frequency 193.1 THz for second channel and chosen 50 or 100 GHz, according to the previously mentioned ITU G.694.1 rec. According to our previously channel interval research of flexible channel spacing like 37.5 and 25 GHz also was realized. However, the quality of received signal was low, with crosstalk impact and error-free transmission was not possible, performance was above our defined BER threshold 1 × 10<sup>−</sup><sup>3</sup> .

We evaluated the performance of WDM-PON architecture in terms of maximal transmission reach. Optical line terminal (OLT) is located in central office (CO) and consists of four transmitters (OLT\_Tx). Each OLT\_Tx transmitter consists of two pseudo-random bit sequence (PRBS) generators and NRZ drivers, as a result two

#### **Figure 9.**

*Simulation scheme of 4-channel PAM-4 modulated WDM-PON transmission system operating at 10 Gbaud/s per wavelength.*

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

*Comparison of experimental and simulative results: eye diagrams of 40 km 2-channel NRZ modulated optical transmission system with 10 Gbit/s transmission speed per channel with CD post-compensation: (a) 50 GHz channel spacing, (b) 31.25 GHz channel spacing, (c) 25 GHz channel spacing, (d) 50 GHz channel spacing in the environment of OptSim, (e) 31.25 GHz channel spacing in the environment of OptSim, and (f) 25 GHz* 

*Comparison of experimental and simulative results: eye diagrams of 40 km 2-channel NRZ modulated optical transmission system with 10 Gbit/s transmission speed per channel without CD post-compensation: (a) 50 GHz channel spacing, (b) 31.25 GHz channel spacing, (c) 25 GHz channel spacing, (d) 50 GHz channel spacing in the environment of OptSim, (e) 31.25 GHz channel spacing in the environment of OptSim, and (f) 25 GHz* 

*BER dependence on channel interval for a 40 km 2-channel NRZ-OOK modulated optical transmission system* 

**94**

**Figure 8.**

**Figure 6.**

**Figure 7.**

*channel spacing in the environment of OptSim.*

*channel spacing in the environment of OptSim.*

*with 10 Gbit/s transmission speed per channel.*

electrical signals are generated where one of them has twice larger amplitude than other for each particular electrical signal. An electrical coupler is used to couple both electrical signals in such a way generating electrical PAM-4 signal. Afterwards, additional electrical filters were used for ensuring of optimal system performance. Generated PAM-4 signal was send to external MZM with 3 dB insertion loss and 20 dB extinction ratio. Continuous wavelength (CW) laser with linewidth of 50 MHz and output power of +3 dBm is used as the light source [23].

Optical signals from four transmitters are coupled together by using optical coupler with 1 dB insertion loss. Chromatic dispersion pre-compensation by FBG DCM, with additional 3 dB insertion loss is realized for all channels before launching optical signal in ITU-T G.652 single mode fiber (SMF), used for transmission in optical distribution network (ODN). After transmission in ODN, all channels are separated by arrayed waveguide grating (AWG) demultiplexer which insertion loss is 3.5 dB. Here we applied various channel spacings—50 or 100 GHz (3-dB bandwidth is 20 GHz) for research of the crosstalk impact. Each receiver of optical network terminal (ONT) consists of PIN photoreceiver (sensitivity is −19 dBm for BER of 10<sup>−</sup>12). An optimal electrical Bessel low-pass filter (LPF) with bandwidth (3-dB bandwidth is 7.5 GHz), was adopted for more successful system performance. An electrical scope was used for evaluation of received signal bit patterns quality, accordingly, eye diagrams.

As it is shown in **Figure 10(a)** in B2B configuration for first investigated 100 GHz channel spacing, the signal quality is good, eye is open and error-free transmission can be provided. After 59 km transmission which was the maximum transmission distance without use of FBG DCM, the BER of received signal was 7.5 × 10<sup>−</sup><sup>4</sup> , please see **Figure 10(b)**. Dispersion compensation FBG DCM module was implemented to evaluate transmission distance in terms of maximal reach. As it is shown in **Figure 10(c)** by using this technique of FBG DCM, the maximum achievable transmission distance 74 km was reached, where BER of received signal was 9 × 10<sup>−</sup><sup>4</sup> . Extra 15 km or 25.4% of link length was gained.

Therefore, basis on our research data we can conclude that narrower channel spacing for 4-channel PAM-4 10 Gbaud/s WDM-PON system is 50 GHz. As it is shown in **Figure 11(a)** in B2B configuration for second investigated 50 GHz channel spacing, the signal quality is good, eye is open and error-free transmission can be provided. After 58 km transmission, which was the maximum transmission distance without use of FBG DCM, the BER of received signal was 8 × 10<sup>−</sup><sup>4</sup> , shown in **Figure 11(b)**. In our research we show the eye diagrams of received signal for the second channel, the drop in BER performance can be explained by the impact of crosstalk between channels. Dispersion compensation FBG DCM module was

#### **Figure 10.**

*Eye diagrams of received signal (a) after B2B transmission, (b) after 59 km transmission without use of CD pre-compensation, (c) after 74 km transmission with use of CD pre-compensation for 4-channel 20 Gbit/s per channel PAM-4100 GHz spaced WDM-PON transmission system.*

**97**

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

implemented to evaluate transmission distance in terms of maximal reach. As it is shown in **Figure 11(c)**, by using this technique of FBG DCM, the maximum achievable transmission distance was 72 km, with BER of received signal 5.5 × 10<sup>−</sup><sup>4</sup>

*Eye diagrams of received signal (a) after B2B transmission, (b) after 58 km transmission without use of CD pre-compensation (c) after 72 km transmission with use of CD pre-compensation for 4-channel 20 Gbit/s per* 

It was shown, that maximal transmission distance with BER below FEC limit

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

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

by 15 km or 25.4% by use of implemented FBG DCM. In case of 50 GHz channel spacing, maximum transmission system reach can be increased by 14 km or 24% by

**4. Evaluation of PAM-4, NRZ and duobinary modulation formats** 

**performance in WDM-PON system architecture**

capacity by improving the bandwidth utilization [13].

on modulation formats under research [26].

for 100 GHz spaced 4-channel PAM-4 WDM-PON system can be increased

. Extra

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

14 km or 24% of link length was gained.

*channel PAM-4 50 GHz spaced WDM-PON transmission system.*

of 10<sup>−</sup><sup>3</sup>

**Figure 11.**

use of FBG DCM.

*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 11.**

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

50 MHz and output power of +3 dBm is used as the light source [23].

electrical signals are generated where one of them has twice larger amplitude than other for each particular electrical signal. An electrical coupler is used to couple both electrical signals in such a way generating electrical PAM-4 signal. Afterwards, additional electrical filters were used for ensuring of optimal system performance. Generated PAM-4 signal was send to external MZM with 3 dB insertion loss and 20 dB extinction ratio. Continuous wavelength (CW) laser with linewidth of

Optical signals from four transmitters are coupled together by using optical coupler with 1 dB insertion loss. Chromatic dispersion pre-compensation by FBG DCM, with additional 3 dB insertion loss is realized for all channels before launching optical signal in ITU-T G.652 single mode fiber (SMF), used for transmission in optical distribution network (ODN). After transmission in ODN, all channels are separated by arrayed waveguide grating (AWG) demultiplexer which insertion loss is 3.5 dB. Here we applied various channel spacings—50 or 100 GHz (3-dB bandwidth is 20 GHz) for research of the crosstalk impact. Each receiver of optical network terminal (ONT) consists of PIN photoreceiver (sensitivity is −19 dBm for BER of 10<sup>−</sup>12). An optimal electrical Bessel low-pass filter (LPF) with bandwidth (3-dB bandwidth is 7.5 GHz), was adopted for more successful system performance. An electrical scope was used for evaluation of received signal bit patterns quality,

As it is shown in **Figure 10(a)** in B2B configuration for first investigated 100 GHz channel spacing, the signal quality is good, eye is open and error-free transmission can be provided. After 59 km transmission which was the maximum transmission distance without use of FBG DCM, the BER of received signal was

. Extra 15 km or 25.4% of link length was gained.

, please see **Figure 10(b)**. Dispersion compensation FBG DCM module was implemented to evaluate transmission distance in terms of maximal reach. As it is shown in **Figure 10(c)** by using this technique of FBG DCM, the maximum achievable transmission distance 74 km was reached, where BER of received signal

, shown

Therefore, basis on our research data we can conclude that narrower channel spacing for 4-channel PAM-4 10 Gbaud/s WDM-PON system is 50 GHz. As it is shown in **Figure 11(a)** in B2B configuration for second investigated 50 GHz channel spacing, the signal quality is good, eye is open and error-free transmission can be provided. After 58 km transmission, which was the maximum transmission distance without use of FBG DCM, the BER of received signal was 8 × 10<sup>−</sup><sup>4</sup>

in **Figure 11(b)**. In our research we show the eye diagrams of received signal for the second channel, the drop in BER performance can be explained by the impact of crosstalk between channels. Dispersion compensation FBG DCM module was

*Eye diagrams of received signal (a) after B2B transmission, (b) after 59 km transmission without use of CD pre-compensation, (c) after 74 km transmission with use of CD pre-compensation for 4-channel 20 Gbit/s per* 

*channel PAM-4100 GHz spaced WDM-PON transmission system.*

**96**

**Figure 10.**

accordingly, eye diagrams.

7.5 × 10<sup>−</sup><sup>4</sup>

was 9 × 10<sup>−</sup><sup>4</sup>

*Eye diagrams of received signal (a) after B2B transmission, (b) after 58 km transmission without use of CD pre-compensation (c) after 72 km transmission with use of CD pre-compensation for 4-channel 20 Gbit/s per channel PAM-4 50 GHz spaced WDM-PON transmission system.*

implemented to evaluate transmission distance in terms of maximal reach. As it is shown in **Figure 11(c)**, by using this technique of FBG DCM, the maximum achievable transmission distance was 72 km, with BER of received signal 5.5 × 10<sup>−</sup><sup>4</sup> . Extra 14 km or 24% of link length was gained.

It was shown, that maximal transmission distance with BER below FEC limit of 10<sup>−</sup><sup>3</sup> for 100 GHz spaced 4-channel PAM-4 WDM-PON system can be increased by 15 km or 25.4% by use of implemented FBG DCM. In case of 50 GHz channel spacing, maximum transmission system reach can be increased by 14 km or 24% by use of FBG DCM.
