This patent document relates to digital communication, and, in one aspect, optical communication systems.
There is an ever-growing demand for data communication in application areas such as wireless communication, fiber optic communication and so on. The demand on core networks is especially higher because not only are user devices such as smartphones and computers using more and more bandwidth due to multimedia applications, but also the total number of devices for which data is carried over core networks is increasing. Equipment manufacturers and network operators are continually looking for ways to meet the demand for ultra-high data rate transmission.
The present document discloses techniques for optical communication. In particular, methods and systems for optical signal transmission with carrier-less amplitude and phase (CAP) modulation and direct detection are disclosed.
In one exemplary aspect, a method of optical signal transmission is disclosed. The method includes receiving information bits at an input interface; mapping the information bits to a plurality of modulation symbols; separating in-phase (I) and quadrature (Q) components of the plurality of modulation symbols such that the I and Q components form a Hilbert pair in a resulting signal; pre-dispersing the resulting signal with an inverse of a phase delay of an expected chromatic dispersion to obtain a pre-dispersed signal; converting the pre-dispersed signal from digital domain to analog domain using a digital to analog conversion circuit; performing modulation of an output of the digital to analog conversion circuit to generate an output signal; and transmitting, over an optical transmission medium, the output signal from the modulation.
In another exemplary aspect, a method of optical signal reception is disclosed. The method includes receiving a carrier-less amplitude and phase (CAP) modulated optical signal over an optical transmission medium, wherein the optical signal comprises I and Q components forming a Hilbert pair, the digital signal pre-dispersed with an inverse of a phase delay of chromatic dispersion; extracting symbol estimates from the optical signal using decision-directed least mean squares (DD-LMS); and de-mapping the symbol estimates to obtain information bits modulated in the CAP-modulated optical signal.
In another example aspect, an optical communication apparatus that includes a processor and an optical transceiver are disclosed. The processor is configured to implement one of the method described above.
The above and other aspects and their implementations are described in greater detail in the drawings, the description and the claims.
Recently, the demand for ultra-high data rate optical transmission has been continuously growing in optical transport networks, metro networks, and access networks. Wavelength Division Multiplexing (WDM) and even Ultra Dense WDM (UDWDM) with advanced modulation formats are widely used in coherent systems to realize the most promising solutions for 400 Gb/s and 1 Tb/s transmission. A metro network, as a medium distance transmission system, poses a special challenge of transmission capacity and cost. In particular, for metro networks, both transmission distance and cost should be considered in the architecture to achieve 100 Gb/s per lane. Compared with coherent receivers, direct-detection (DD) optical transmission is considered as a more attractive and feasible solution in terms of system construction cost, computation complexity, and power consumption.
One advanced single carrier modulation format that uses low-cost and bandwidth limited optical components is carrier-less amplitude and phase modulation (CAP). Although many researchers have investigated advanced modulation formats for metro networks, there has been no 100 Gb CAP transmission reported over 400 km standard-single-mode-fiber (SSMF) using low-cost direct detection. The major reason is that long-haul transmission suffers chromatic dispersion (CD) penalties.
There are three main ways to compensate chromatic dispersion: pre-CD method, single sideband (SSB) method, and dispersion compensating fiber (DCF). Applying SSB or vestigial side band (VSB) is one way to overcome the CD limitation in systems with direct detection. For example, 100 Gb/s SSB DMT over 80 km fiber and 110.3 Gb/s VSB discrete multi-core (DMT) over 100 km fiber have been achieved. Pre-CD compensation is another way to suppress CD distortion. For example, 336 Gb/s PDM-64 QAM have been experimentally demonstrated with in-phase and quadrant (IQ) modulator over 40 km SSW. In another implementation, 56 Gb/s DMT over 320 km SSMF and 100 Gb/s DMT over 80 km SSMF with dual-drive Mach-Zehnder modulator (DDMZM) have been realized.
This patent document describes a transmission method using low-cost CAP modulation with direct detection. The method is capable of achieving a single-wavelength 100 G transmission over a long distance in metro networks. In one embodiment, a single-wave 100 G transmission was achieved over 480 km SSMF. A bit rate of 112 Gb/s/λ is achieved by utilizing CAP with commercial optical components (λ represents wavelength). This patent document also includes comparison of system performance between dispersion compensating fiber (DCF) and pre-CD compensation over 80 km SSMF, and evaluation of transmission performance of SSB and pre-CD signal with DDMZM and IQ modulator.
CAP-16 Format
Pre-CD Method
The main factor that limits the transmission distance for DSB signal is the power fading issue caused by chromatic dispersion. The general frequency domain channel response of CD is:
D is the dispersion parameter, L is the fiber length, λ is the carrier wavelength, and c is the speed of light. Eq. (1)'s corresponding time domain expression is:
According to Eq. (2) and the square law detection, the final formula is presented as:
When the phase sum of signal is
where N is an integer, the signal will suffer the destructive power fading. So the bandwidth of the first lobe is expressed as:
In order to compensate for the serious power fading, the modulated signal may be pre-distorted by the inverse of CD channel response. The pre-CD method, however, introduces the phase information to the signals: the signals now carries the phase information at the same time. This make the pre-CD method particularly suitable for DDMZM and/or IQ modulators.
Generation of SSB Signal
SSB signal is another way to avoid power fading caused by CD. A DDMZM consists of two parallel phase modulators (PMs) and they are driven with a bias difference of Vπ/2. The output of the DDMZM can be expressed as:
From Eq. (5), it is observed that the electrical signal I(t)+j*Q(t) is linearly converted to the optical domain.
For the generation of SSB signal, the electrical signal I(t) is as a real signal x and the signal Q(t) is set as the corresponding Hilbert pair x. The output of x+j*{circumflex over (x)} is the analytic signal of x and is a single-band signal. Then, the optical domain expression could be:
E
out
=E
in*+(x+j*{circumflex over (x)}) Eq. (6)
The output of Eq. (6) then becomes an optical single-band signal.
Example Setup and Results
In particular,
Comparison Between Pre-CD and DCF with DDMZM
Firstly, the BER performance of CAP-16 in back-to-back (BTB) and 80 km SSMF cases is investigated. Dispersion compensating fiber (DCF) is used to compensate for the CD caused by 80 km fiber as shown in
To study the effect of Peak to Average Power Ratio (PAPR), the PAPR of CAP is evaluated with different DSP processes.
The BER performance between applying pre-CD and DCF fiber is then compared.
Comparison Between Pre-CD and SSB with DDMZM
SSB is another way to overcome the CD limitation in the systems with direct detection. Comparison of the performance between applying pre-CD and SSB for CAP modulation is also conducted. The results are firstly obtained over 240 km SSMF transmission as shown in
Comparison Between DDMZM and IQ Modulator
As DDMZM is roughly similar as the IQ function, the performance of the IQ modulator is also investigated.
It has been observed that, among the three main types of CD compensation methods including pre-CD, SSB, and DCF, DCF has the worst performance but introduces no extra DSP. Therefore, it is suitable for low-cost single-drive MZM. Utilizing pre-CD method, for example, can get about 2 dB receiver sensitivity gain at the HD-FEC threshold compared with DCF. Pre-CD signal shows consistent improvement over SSB signal for IQ modulator. However, when coupled with DDMZM, pre-CD signal shows performance variation due to the nonlinearity of the DDMZMs. In particular, when the transmission length is short (about less than 240 km), pre-CD signal is better than SSB signal when coupled with DDMZM. When the transmission distance increases, SSB shows a better performance. Therefore, among the methods and combinations examined herein, pre-CD method with an IQ modulator shows the best performance in the medium distance transmission system with direct detection.
In some embodiments, the method 1500 may further include techniques described with respect to
Some of the embodiments described herein are described in the general context of methods or processes, which may be implemented in one embodiment by a computer program product, embodied in a computer-readable medium, including computer-executable instructions, such as program code, executed by computers in networked environments. A computer-readable medium may include removable and non-removable storage devices including, but not limited to, Read Only Memory (ROM), Random Access Memory (RAM), compact discs (CDs), digital versatile discs (DVD), etc. Therefore, the computer-readable media can include a non-transitory storage media. Generally, program modules may include routines, programs, objects, components, data structures, etc. that perform particular tasks or implement particular abstract data types. Computer- or processor-executable instructions, associated data structures, and program modules represent examples of program code for executing steps of the methods disclosed herein. The particular sequence of such executable instructions or associated data structures represents examples of corresponding acts for implementing the functions described in such steps or processes.
Some of the disclosed embodiments can be implemented as devices or modules using hardware circuits, software, or combinations thereof. For example, a hardware circuit implementation can include discrete analog and/or digital components that are, for example, integrated as part of a printed circuit board. Alternatively, or additionally, the disclosed components or modules can be implemented as an Application Specific Integrated Circuit (ASIC) and/or as a Field Programmable Gate Array (FPGA) device. Some implementations may additionally or alternatively include a digital signal processor (DSP) that is a specialized microprocessor with an architecture optimized for the operational needs of digital signal processing associated with the disclosed functionalities of this application. Similarly, the various components or sub-components within each module may be implemented in software, hardware or firmware. The connectivity between the modules and/or components within the modules may be provided using any one of the connectivity methods and media that is known in the art, including, but not limited to, communications over the Internet, wired, or wireless networks using the appropriate protocols.
While this patent document contains many specifics, these should not be construed as limitations on the scope of any invention or of what may be claimed, but rather as descriptions of features that may be specific to particular embodiments of particular inventions. Certain features that are described in this patent document in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable sub-combination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a sub-combination or variation of a sub-combination.
Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. Moreover, the separation of various system components in the embodiments described in this patent document should not be understood as requiring such separation in all embodiments.
Only a few implementations and examples are described and other implementations, enhancements and variations can be made based on what is described and illustrated in this patent document.