**1. Introduction**

The exponential growth of Internet data traffic and progress of Information and Communication Technology (ICT) sector pushes hard the telecommunication infrastructure for upgrading the transmission data rate [1]. Power and cost-efficient fiber optical access networks, like passive optical network (PON) and short-range fiber optical links are one of the key technologies enabling bandwidth hungry services like video on demand (VoD), high definition TV, and cloud computing supported by large scale high-performance computers and data centers. Such optical links typically use direct detection and on-off keying modulation (OOK) with NRZ line code. Today's challenge for optical access networks and data centers is to increase the serial line rate of a NRZ link meeting the requirements to the physical bandwidth of the photonic and electronic components like optical signal modulators and photodiodes [2].

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

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[33] Malina L, Munster P, Hajny J, Horvath T. Towards secure gigabit passive optical networks: Signal propagation based key establishment. In: Proceedings of SECRYPT 2015. Colmar, Francie: IEEE; 2015.

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Solution for telecommunication infrastructure upgrade and alternative solution for increase of the serial line rate of the NRZ link is to use multi-level signaling formats such as pulse-amplitude modulation (PAM), abbreviated as PAM-M or M-PAM, where multiple digital bits per symbol are encoded into M different signal amplitude levels. The four-level PAM modulation format is receiving significant attention because of its relative ease of implementation in comparison to higher-order modulation formats like quadrature phase-shift keying (QPSK), and m-ary quadrature amplitude modulation (m-QAM). It is clear that M-PAM offers a good trade-off between performance and complexity. Usage of PAM-4 format is effective way to double the data rate of NRZ link. Previously PAM-4 modulation formats have been investigated for application with traditional electrical networks [3, 4], but now researchers are focused on investigation of PAM-4 and M-PAM modulation formats for utilization in optical access networks as well as data center interconnections [5]. Also, there are very limited number of studies which are focused on spectrum slicing and stitching back method, which deals with bandwidth bottleneck problem by slicing the broadband signal in lower-bandwidth signal slices. This spectrum slicing and stitching back method or technique allows transmission of wide bandwidth signals from the service provider to the end user over an optical distribution network via low bandwidth equipment [6, 7]. It is ideally suited for cost sensitive fiber optical access networks where variable bandwidth and scalability as well as flexibility are important. It must be noted that this method is investigated for intensity modulated direct detection NRZ-OOK and duobinary systems, but there are no investigations on its usage together with M-PAM systems [8, 9]. It must be noted that multi-level signaling also changes some rules, which were used in NRZ coded transmission systems. For M-PAM systems it is important to implement more complex and precise level threshold detection for signal inputs, also signal-to-noise (SNR) requirements are higher than in case of NRZ. Eye time skew, amplitude compression in lower eye diagram eyes, intersymbol interference for M-PAM systems also is an issue which must be investigated. So, we can say that PAM-4 links are new science—still learning what impairments create errors in receivers [10, 11]. Significant efforts have been put on investigation of PAM-4 format in fiber optical transmission networks, however there are following aspects, which have not been studied or have been studied insufficiently. High-level PAM modulation techniques, like PAM-4, can dramatically improve the spectral efficiency and available bitrate by using the bandwidth of already existing optical, electro-optical or electrical devices. Minimal available channel spacing (which has direct impact on the utilization of resources like optical spectrum), maximal available number of channels, by wavelength division multiplexing (WDM) technique, maximal transmission distance (network reach) in dispersion compensated and non-compensated M-PAM modulated WDM-PON optical access systems.

Another way to improve capacity of limited bandwidth is by using duobinary modulation format. Transmission capacity will be increased in comparison with NRZ, utilization of DB will increase the transmission capacity by improving the bandwidth efficiency and reducing channel spacing with this modulation format [12]. Duobinary modulation format is type of proficient pseudo-multilevel modulation format, and therefore is the area of interest due to its increased spectral efficiency. It has been already used to increase the channel capacity by improving the bandwidth utilization in commercial links. The most important feature of duobinary modulation format is its usage for longer transmission distances where it has high tolerance to the influence of chromatic dispersion (CD) [13].

At first, in the paper we investigate the performance and minimal channel interval of 10 Gbit/s per channel NRZ-OOK (which is basically PAM-2) modulated transmission system, then we investigate PAM-4 and raise the transmission speed up to 20 Gbit/s per wavelength and in the end compare it to NRZ and duobinary modulation formats.

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**Table 1.**

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

**2. Evaluation of various channel spacings for increasing spectral** 

At the moment passive optical networks have been standardized to nextgeneration NG-PON2 accordingly to ITU-T G.989.2 recommendation standards and are widely investigated. Operators are widely deploying time-division multiplexing (TDM) based passive optical networks in urban areas with bitrates up to 10 Gbit/s,

The ITU-T G.694.1 recommendation provides a frequency grid for (WDM) transmission systems and specifies inter-channel intervals. The same frequency grid or channel spacing is used for spectral effectiveness improvement of PON system in our research. Anchored to 193.1 THz (central channel frequency), it supports a variety of inter-mediate channel spacings ranging from narrowed 12.5 GHz to 100 GHz and wider. Depending on the selected step of the inter-channel interval

There are two types of inter-channel interval definitions in (WDM) systems:

According to ITU-T G.694.1 rec. the minimum step of a fixed channel interval is 12.5 GHz (please see **Table 1**). The flexible channel step is half of the 12.5 GHz, that can be used for the inter-channel interval like 6.25 GHz. Reducing the inter-channel interval leads to increase of crosstalk and non-linear effects (NOE) of transmitted

For research of spectral efficiency increasing, the experimental 2-channel NRZ-OOK modulated 10 Gbit/s bit rate per channel transmission system model was created for Next-generation WDM-PON systems based on tunable wavelength transmitters, please see in **Figure 1**. First step of the research is based on various channel spacing impact on the end user transmitted signal with following fixed

As one can see in **Figure 1**. transmitter (Tx) part of our investigated transmission system model consists of two continuous wave (CW) laser sources—Agilent 81949A, with fixed central frequency 193.1 THz or 1552.524 nm in wavelength, and COBRITE DX-1 laser with tunable central frequency, which can be set the necessary channel spacing. Agilent 81949A continuous wave laser source was connected

**efficiency of WDM-PON transmission system**

but WDM-PON's still are in stage of research [14, 15].

are defined the following abbreviations and acronyms:

CWDM—coarse wavelength division multiplexing. DWDM—dense wavelength division multiplexing.

WDM—wavelength division multiplexing.

Fixed inter-channel interval (fixed grid). Flexible inter-channel interval (flexible grid).

10 Gbit/s transmission speed per channel.

*Nominal central frequencies grid of the DWDM grid [17].*

optical signal [16–18].

*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*
