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Oct 07, Accepted Date: Jan 05, Published Date: J Laser Opt Photonics 3: This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Optical links using traditional modulation formats are reaching a plateau in terms of capacity, mainly due to bandwidth limitations in the devices employed at the transmitter and receivers.
Advanced modulation formats, which boost the spectral efficiency, provide a smooth migration path towards effectively increase the available capacity. Advanced modulation formats however require digitalization of the signals and digital signal processing blocks to both generate and recover the data. There is therefore a trade-off in terms of efficiency gain vs complexity. Polybinary modulation, a generalized form of partial response modulation, employs simple codification and filtering at the transmitter to drastically increase the spectral efficiency.
At the receiver side, polybinary modulation requires low complexity direct detection and very little digital signal processing. This paper provides an overview of the current research status of the key building blocks in polybinary systems.
The results clearly show how polybinary modulation effectively reduces the bandwidth requirements on optical links while providing high spectral efficiency. Optical communications ; Advanced modulation formats; Microwave filter; Polybinary modulation. In order to transmit bits of information between two points, it is necessary for both the transmitter and the receiver to agree on the way to carry the information.
In communication systems, this is called modulation format. The current boost on traffic demands in optical systems has forced researchers to look into better modulation formats; better in the sense of efficiency in transmitting bits using fewer resources. The most common measure for efficiency is the spectral efficiency, and these formats are called advanced modulation formats.
By studying advanced modulation format schemes capable of sustaining the traffic demand, the energy required to convey data will be reduced; a green internet providing affordable bandwidth for services and applications that society demands can then reach the citizenry.
If we focus on advanced modulation formats that can graciously scale up to G regimes, we found several approaches experimentally demonstrated, including: These demonstrations however required fairly complex transmitters and receivers, both in terms of hardware and software in the form of digital signal processing algorithms.
Our work focuses on polybinary modulation formats, which in principle overcome these two obstacles. Polybinary modulation was proposed originally to overcome the bandwidth limitation of copper-based transmission media. This modulation provides an increment of the number of levels of the original binary signal by introducing controlled amounts of inter symbol interference ISI [ 5 ]. The primary consequence of this modulation is the redistribution of the spectral density of the original signal into energy compressed low frequencies; this compression has a price in terms of noise sensitivity [ 6 ].
However, it offers the opportunity to filter out the high-frequencies, as they no longer carry information, thereby effectively increasing the spectral efficiency. The simplest polybinary modulation, known as duobinary, correlates a symbol with its predecessor; if the original signal is binary two levels , the duobinary modulated signal will present three levels.
In particular, high bit rates such as and 56 Gbps polybinary systems have been achieved combining a 4-PAM signal with doubinary for 5 and 10 km transmissions, resulting in a 7-levels signal [ 9 , 10 ]. This paper presents the challenges and state-of-the-art of the different identified building blocks to develop future polybinary modulation systems and identifies the main obstacles to be overcome. Partial response signaling or polybinary modulation was proposed to increase the spectral efficiency by constructively utilizing ISI, which is in general an impairment which drastically reduces the performance of signals when transmitted.
Polybinary signals are generated by following the next bit relations:. Where ak is the original bit sequence, b k a precoded binary sequence, and c k the polybinary signal. By simply sampling c k and conducting the modulo n operation, being n the number of levels of the polybinary signal, the original stream can be recovered.
Given the method to generate and recover polybinary signals, we can visualize the whole system as indicated in Figure 1. System overview, highlighting in orange the building blocks under study. Figure 1 shows the two building blocks of a polybinary transmitter, the data coding block and the tunable microwave filter. Furthermore, it also highlights the digital signal processing blocks for signal recovery. The laser, optical channel and optical frontend remain normally the same regardless of the modulation format.
The following sections describe our current efforts in each of these challenges. In polybinary modulation , the simplicity of the transmitter is outstanding: The coding can be implemented employing either digital signal processing or by a hardware implementation with a delay-andadd block.
Either way, the filter removing the excess bandwidth is key; the following section explains the challenges on the design and fabrication of reconfigurable microwave filters. However, here we focus on the requirements of the filters in the process of polybinary generation. Table 1 shows a comparison table on the spectral efficiency, number of amplitude thresholds and minimum signal to noise ratio SNR requirements of polybinary modulation for different Bessel filter orders compared to 4-PAM and 8-PAM.
Summary of polybinary generation features. The challenge in generating the optical polybinary signal, beyond the filter adequacy, arises in the need of large extinction ratios. In short-range systems, vertical-cavity surface-emitting lasers VCSELs are the most common building block due to their manufacturability; however, VCSELs have two main drawbacks in terms of performance: Extinction ratio remains a challenge, as current values are in the vicinity of dB [ 14 ], which provide a limited budget for multilevel formats, which in turn, degrades the performance and the maximum transmission reach.
As it can be observed, the distribution of symbols increase from an even to an odd number, which effectively reduces the threshold spacing to effectively discern the bit probability [ 15 ].
This limitation, along with the previously mentioned challenge with the extinction ratio, highlights how the performance in polybinary suffers from a penalty compared to regular amplitude modulation formats.
However, Figure 2 has a different reading: This drastically reduces the complexity of the digital signal processing blocks at the receiver side, and enable analog hardware implementations, which effectively reduce to a bare minimum the latency at the receiver side.
The simplest method to generate a polybinary signal is filtering the high frequency components of the original electrical signal with a Bessel low-pass filter LPF [ 11 ]. A Bessel filter is a filter derived using Bessel functions with a goal of linear phase, i. The drawback of Bessel filters is their shallow slope compared to other types such as Butterworth or Chebyshev.
Current efforts are focused on implementing electrical Bessels LPFs which maximized the performance of the generated polybinary signals. There have been some attempts already: This filter is a combination of microstrips and suspended striplines.
An initial study on what are the optimal parameters of the LPF to generate polybinary signals, including the order, type and cut-off and stop frequency have been conducted, and a first generation of polybinary filters designed to maximize the performance of polybinary signals fabricated.
Figure 6 shows the fabricated filters once connectorized. First generation polybinary filters designed to maximize the performance of polybinary signals. Although these Bessel LPFs provide an accurate frequency response, phase response and tunability remains critical factors. Tunability is key as future transceivers are expected to be reconfigurable, and therefore, able to operate at different bitrates depending on the network requirements. Different technologies have been proposed addressing this field: The central frequency is GHz and the bandwidth is 15 GHz; the central frequency can be tuned between and GHz.
Another monolithic integrated tunable LPF at 4. However MMCI solutions change significantly the phase characteristic when the filter is reconfigured. Tunable LPFs using micro-electromechanical systems MEMs to implement reconfigurable series inductors and capacitive shunt switches have been used to demonstrate operation up to the millimeter-wave frequency range. A tunable active elliptic LPF is presented in [ 26 ].
This filter concept is based on a high order LPF at which an active capacitor circuit has been added; the capacitor allows to compensate the overall losses of the filter and makes it tunable.
This filter has a tunable cut-off frequency that ranges between MHz. A tunable Bessel LPF for duobinary signals generation has been patented [ 27 ]. This filter uses a controller circuit to tune variable capacitors and inductors; the technology used for the variables components is not specified, but MEMS is reported to be the most suitable.
This paper had presented polybinary modulation as modulation of choice for bandwidth limited optical links. Polybinary modulation effectively reduces the necessary bandwidth to transmit a data stream by introducing a controlled ISI; although bandwidth reduction positively affects the strength of the signal in front of optical transmission impairments such as chromatic dispersion and Brillouin backscattering, the trade-off comes in the form of a degraded sensitivity when compared to traditional on-off keying signaling.
However, this drawback may be assumable in short-range systems, where power is relatively abundant as transmission losses are low, and complexity, simplicity and power efficiency are key metrics that need to be maximized. At the transmitter side, polybinary modulation presents a channel in selecting the specifications of the filter conducting the strong filtering leading to a reduce bandwidth signal.
The order, profile, and phase and amplitude response are critical to obtain an electrical polybinary signal able to drive directly modulated laser sources, which are common building block in short-range systems. Integrated Bessel LPF filters preserving linear phase are readily available and can be designed to maximize the performance of polybinary signals at almost any desired bitrate.
Reconfigurable Bessel LPF filters are under heavy research and promise us to enable multibitrate transceivers. Polybinary modulation suffers from lower sensitivity, as the bit distribution reduces the threshold spacing; in presence of amplitude noise, polybinary performs worst. However, the bit distribution is phase independent, and simple point-and-shot detection algorithms can be put in place.
Furthermore, as polybinary modulation requires codification at the transmitter, error correction gain can be anticipated. The modulation also permits to implement analog receivers based on filter banks. Home Publications Conferences Register Contact. Research Article Open Access. Jan 15, Citation: Select your language of interest to view the total content in your interested language.
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