Patent Application
20190319712
This document relates to digital communications, and in one aspect, optical communication systems that use pulse amplitude modulation.
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 and access networks are all growing 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 the whole network is increasing. For profitability and to meet increasing demand, equipment manufacturers and network operators are continually looking for ways to support high-speed and high-capacity communication links.
This document relates to methods, systems, and devices for phase insensitive amplitude coherent detection for pulse amplitude modulation (PAM) signals. In some examples, equally-spaced amplitude levels are implemented for intensity modulation at the transmitter.
In one exemplary aspect, a digital communication method is disclosed. The method, which may be implemented at an optical transmitter, includes modulating an input signal based on pulse amplitude modulation, performing a pre-distortion operation on an output of the modulating, filtering an output of the pre-distortion operation using a pulse shaping filter, and transmitting an output of the filtering using intensity modulation, where the intensity modulation includes an equally-spaced amplitude distribution.
In another exemplary aspect, a digital communication method is disclosed. The method, which may be implemented at an optical receiver, includes receiving an optical signal, performing a coherent detection operation on the optical signal, where the coherent detection comprises an equally-spaced amplitude distribution, converting an output of the coherent detection operation from an optical domain to an electrical domain by digitizing the output, filtering an output of the digitizing using a pulse shaping filter, and demodulating an output of the filtering using a pulse amplitude modulation demodulator.
In yet another exemplary aspect, the above-described methods are embodied in the form of processor-executable code and stored in a computer-readable program medium.
In yet another exemplary embodiment, a device that is configured or operable to perform the above-described methods is disclosed.
The above and other aspects and their implementations are described in greater detail in the drawings, the descriptions, and the claims.
The advance of high-speed passive optical networks (PONs) has been propelled by new business and technology drivers, such as cloud services, 5G (Fifth Generation) wireless transport, and high bandwidth 4K/8K video applications. To meet these increased capacity needs, a passive optical network for access is moving from the classic spectrally inefficient non-return to zero (NRZ), to more advanced modulation formats with digital signal processing (DSP). This is further evidenced by the progress in standards bodies developing next generation high speed PON standards. Example implementations include the standardization of 25/50G Ethernet PON (EPON) based on wavelength multiplexing of 25 Gbps single channel, with 29 dB power budget by using low-density parity-check code (LDPC).
Despite these advances, an implementation of a 100G (100 Gigabit) PON via wavelength multiplexing of four 25 Gb/s channels is still currently an active area of industry and academic research and development. An initial challenge is that the wavelength multiplexer and demultiplexer typically add an additional ˜2.5 dB loss, which makes meeting the power budget even more difficult. Also, channel bonding of four 25 Gb/s channels may require a significant amount of 0-band wavelength resources.
A possible solution to achieving higher speed in PONs is to increase the data rate per wavelength, and may result in a 100G TDM-PON (time-division multiplexed PON) on a single wavelength, which not only reduces the number of optical components and the associated cost, but also conserves the wavelength resources. However, limited sensitivity is an issue for 100 Gb/s/λ, TDM-PON (where λ is the wavelength) to support a high power budget using direct detection.
Some existing implementations rely on coherent detection as an effective method to increase the receiver sensitivity. One example includes Ultra-Dense Wavelength Division Multiplexing (UDWDM) coherent PON with 16-QAM modulation of 10 Gb/s per wavelength, and another example includes a 100G PON based on 32 Gbaud dual-polarization QPSK signals. However, both these examples require complicated I/Q modulators as well as an integrated coherent receiver based on four 90° optical hybrids (where an optical hybrid is a four-port optical device with two inputs and two outputs) designed for long-haul transmissions. Furthermore, carrier phase recovery with considerable complexity is also required, which significantly increases the total power consumption of coherent receiver. Yet another example includes coherent detection of intensity modulated NRZ/OOK (on-off keying) based on low cost coherent receiver with simplified DSP, but requires a 3×3 fiber coupler to achieve a 25 Gb/s data rate.
The present application describes techniques that can be implemented to meet the above-discussed bitrates with none or insignificant increase in receiver complexity. Some embodiments of the disclosed technology include a single wavelength 100 Gb/s PAM-4 TDM-PON transmission in the C-band using simplified and phase insensitive heterodyne coherent detection. In some embodiments, 50-GBaud PAM-4 intensity (amplitude) modulation is performed without using any I/Q modulators. Compared to homodyne detection, heterodyne detection reduces the number of optical hybrids, photodetectors (PDs) and analog-to-digital converters (ADCs) of coherent receiver by half. As only the intensity of the carrier is modulated, carrier phase recovery can be avoided, which further reduces the DSP complexity and power consumption. In some embodiments, optimized Nyquist pulse shaping is also performed on PAM-4 signal to reduce the required transceiver bandwidth. For example, a receiver sensitivity of −26 dB is achieved for 100-Gb/s PAM-4 at BER threshold of 1×10−2, which corresponds to a greater than 32-dB power budget being enabled after 20/40 km fiber transmission.
Experimental results have confirmed the benefits of the disclosed technology.
Section headings are used in the present document to improve readability of the description and do not in any way limit the discussion or the embodiments to the respective sections only.
Example Experimental Embodiment of an Optical Communication System
The method 100 includes, at step 120, a pre-distortion for equally-spaced amplitudes. In some embodiments, the pre-distortion is required to achieve an equally-spaced amplitude distribution after coherent detection by using intensity modulators. In other embodiments, the pre-distortion is based on the modulation response of the transmitter, including the linearization of any device nonlinearity, and the conversion of the equally-spaced amplitude signal to the corresponding intensity drive signal.
The method 100 includes, at step 130, optimized Nyquist pulse shaping, which is used to reduce the bandwidth of the signal, and consequently, to reduce the bandwidth required by the transceiver. In some embodiments, the optimization of the Nyquist pulse shape may be based on a strength of a clock signal and the bandwidth requirements of the system.
The method 100 includes, at step 140, optical intensity modulation, which may be implemented by direct modulation laser (DML), external modulation laser (EML) based on an electro-absorption modulated laser, or a Mach-Zehnder Modulator (MZM).
The features described in
In some embodiments, and at the transmitter-side (Tx), the DFB laser at 1538.51 nm is externally intensity-modulated by a 50-GBaud PAM-4 signal, using a 30-GHz Mach-Zehnder Modulator (MZM). The 50-GBaud PAM-4 signal is generated from a 80-Gsa/s digital-to-analog converter (DAC) with a 3-dB analog bandwidth of 18 GHz, and then amplified by a 30 GHz linear electrical driver before signal modulation.
For PAM-4 signal generation, nonlinear pre-distortion and linear pre-equalization is used after PAM-4 symbol mapping at the transmitter-side. Nyquist pulse shaping is then applied to further reduce the signal bandwidth. Since the amplitude of optical signal can be linearly extracted by coherent detection, the PAM-4 signal is modulated on the amplitude dimension with equal spacing. To achieve equally-spaced amplitudes, the PAM-4 signal is pre-distorted according to the modulation curve of the MZM. The principle of PAM-4 signal pre-distortion for MZM-based modulation is shown in
The Mach-Zehnder Modulator (MZM) typically has a cos2(x) function as the intensity modulation curve, and an amplitude modulation curve of cos(x), as shown in
At the receiver-side (Rx), and as shown in
The sampled signal is down-converted to baseband with low-pass filtering and then re-sampled to 2 samples per symbol. Since coherent detection advantageously enables the availability of full electrical field information, digital chromatic dispersion (CD) compensation is enabled before clock recovery. This is followed by the use of a T/2-spaced cascaded multi-modulus algorithm (CMMA) for channel equalization.
Since no information is modulated in the phase of the optical signal, carrier phase recovery is not needed, and each amplitude level of the PAM-4 signal becomes a ring instead of a point in the I-Q plane due to the residual frequency offset and laser phase noise, as shown in
In an example, the launch power is kept at −6 dBm, and the 32 dBm loss budget is achieved, which is enough to meet the PR-30 power budget (defined as having an upstream and downstream line rate of 10.3125 Gbit/s and a channel insertion loss of 29 dB).
Example Embodiments of Optical Receiver Structures
In some embodiments, the signal may then be equalized for signal recovery. In an example, the cascaded multi-modulus algorithm (CMMA) may be used. The PAM-N signal may then be demodulated based on a decision on the modulus of each sample.
Example Embodiments for Nyquist Shape Filtering
As discussed in this document, using a pulse shaping filter may reduce the bandwidth requirements of the transceiver, especially the receiver. For example, the pulse shaping filter may be a Nyquist filter. In some embodiments, the roll-off factor of the Nyquist pulse shaping filter may need to be optimized to improve performance of the optical communication system.
In some embodiments, a family of spectra that satisfy the Nyquist theorem is the raised cosine family whose spectra are
where the parameter roll-off factor β is a real number in the interval 0≤β≤1 that determines the bandwidth of the spectrum.
Since the spectrum is zero for |f|>(1+β)/2Ts, the bandwidth of the baseband pulse is (1+β)/2 Ts. For bandpass QAM modulation, the bandwidth is twice that:
where Rs is the transmitted symbol rate.
The ideal low-pass rectangular spectrum is the special case where which has a passband equal to the symbol rate. The corresponding time-domain signal is
Note z(t) has zero-crossings at t=±Ts, ±2 Ts, . . . , and the time series corresponding to the special case β=0 (the ideal low-pass rectangular spectrum) is sin(πt/Ts)/(πt/Ts). The spectra and corresponding time series for various values of the roll-off factor (β) are plotted in the upper and lower plots of
In some embodiments, another family of spectra that satisfy the Nyquist theorem is the square-root raised cosine pulse shape p(t) and its Fourier transform P(f) are given by
The spectra corresponding time series for various values of the roll-off factor (β8) for the square-root raised cosine pulse shape are plotted in the upper and lower plots of
The square-root raised cosine pulse shape is advantageously characterized by the corresponding matched filter output having no inter-symbol interference (ISI), but results in the pulse shape having an infinite support in the time-domain. Since a pulse cannot last indefinitely in a practical system, the pulse shape is truncated. The result of the truncation is the presence of non-zero side lobes in the frequency domain. In other words, the spectrum is no longer zero for |f|>(1β)/2Ts.
Typical systems need a stop band attenuation of about 40 dB, and an approximation of the number of filter symbol needed is given by
L
symbol=−44β+33, (5)
for 0.2<β≤0.75 and Lsymbol is the length of the pulse shaping filter measured in symbols.
The resulting filter characteristics may be verified using the Discrete Fourier Transform (DFT) prior to their inclusion in embodiments of the disclosed technology.
Example Methods of Coherent Detection for PAM Signals
The method 1100 includes, at step 1120, performing a pre-distortion operation on an output of the modulating. In some embodiments, the pre-distortion operation is based on amplitude levels of the pulse amplitude modulation. In other words, the definition of the pre-distortion operation is based on the values of the PAM amplitude levels. Different amplitude levels would result in a different pre-distortion operation. In some embodiments, the pre-distortion operation may be implemented by a processor, and is based on either a hardware look-up table or real-time computations.
The method 1100 includes, at step 1130, filtering an output of the pre-distortion operation using a pulse shaping filter. In some embodiments, the pulse shaping filter is a Nyquist filter, and the roll-off factor of the Nyquist pulse shaping filter is based on a quality of a clock information and a bandwidth requirement, as described in the “Example Embodiments for Nyquist Shape Filtering” section.
The method 1100 includes, at step 1140, transmitting an output of the filtering using an intensity modulation, which may include an equally-spaced amplitude distribution. In some embodiments, the intensity modulation is performed using one of a direct modulation laser (DML), an external modulation laser (EML), or a Mach-Zehnder Modulator (MZM), as described in the “Example Experimental Embodiment of an Optical Communication System” section. In some embodiments, polarization division multiplexing may be used to transmit multiple similarly-generated polarized optical signals.
The method 1200 includes, at step 1220, performing a coherent detection operation, which includes an equally-spaced amplitude distribution, on the optical signal. In some embodiments, the coherent detection operation is performed by a heterodyne coherent receiver.
In some embodiments, the optical signal may include multiple polarizations. The method 1200 may further include generating a reference optical signal, coupling one of the multiple polarizations with the reference optical signal, and processing the coupled signal using a balanced photodiode, as part of the coherent detection operation.
The method 1200 includes, at step 1230, converting an output of the coherent detection operation from an optical domain to an electrical domain by digitizing the output. In some embodiments, step 1230 may be performed by an analog-to-digital converter.
The method 1200 includes, at step 1240, filtering an output of the digitizing using a pulse shaping filter. In some embodiments, the pulse shaping filter is a Nyquist filter, and the roll-off factor of the Nyquist pulse shaping filter is based on a quality of a clock information and a bandwidth requirement, as described in the “Example Embodiments for Nyquist Shape Filtering” section.
The method 1200 includes, at step 1250, demodulating an output of the filtering using a pulse amplitude modulation demodulator. In some embodiments, the method 1200 may further include equalizing the filtered output prior to demodulation. For example, the equalization is based on a cascaded multi-modulus algorithm (CMMA). Upon performing the equalization, further output-side processing may be performed to demap PAM symbols into data bits, and then recovering data bits by performing error correction decoding, if needed. As can be seen, the PAM technique enables recovery of information bits that have been modulated and transmitted by the transmission apparatus.
It is intended that the specification, together with the drawings, be considered exemplary only, where exemplary means an example and, unless otherwise stated, does not imply an ideal or a preferred embodiment. As used herein, “or” is intended to include “and/or”, unless the context clearly indicates otherwise.
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 document contains many specifics, these should not be construed as limitations on the scope of an invention that is claimed or of what may be claimed, but rather as descriptions of features specific to particular embodiments. Certain features that are described in this 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 a 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.
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 disclosure.
