Patent Application
20230079971
The present disclosure relates to optical communication device and method, and more precisely but not exclusively, to a coherent optical transceiver which can self-optimize the baudrate, modulation format, error-forward correction scheme and shaping factor based on the transmission channel condition for maximizing the system performance and method thereof.
This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
Coherent optical communication is the main high-speed transport transmission technology for undersea, long-haul and metropolitan area networks. In practice, these optical networks contain a variety of optical fiber links which are constructed at different points in time. These optical fiber links have very different characteristics due to the different link lengths, different types of fiber and different amplification schemes. As a result, the capacities of different fiber links are also different. Due to the aging problem of fiber itself any many other network components, the capacities (or the maximum data rates) of all fiber links in the network slowly degrade over time. In addition, the capacities of many fiber links even vary quickly over a short period of time (e.g., over one day) due to their sensitivities with surrounding environment. For example, the fiber links installed along street with busy traffic in the metropolitan area networks are very sensitive to vibration caused by the traffic in rush hours. These links typically provide a much better signal-to-noise ratio (SNR) during the night than during the rush hours with busy traffic. Taking all these issues into account, optical fiber networks are usually designed with plenty of margin to provide a high reliability and prevent network failure. However, with a high network margin, the actual amount of data being transported over an optical fiber network is much smaller than the network capacity, which is very inefficient.
With the ever-increasing demand for connectivity, maximizing the capacity of fiber optical communication networks have become crucial in recent years. Laying out more fiber is extremely expensive and time-consuming, especially in big cities. Maximizing the capacity of fiber optical fiber networks require the development of a new class of flexible optical transceivers with flexible data rates adapted to the channel conditions. Due to the fast-varying nature of many optical fiber channels, the data rate adjustment of optical transceivers must be performed automatically. Providing effective mechanism and hardware designs for optical transceiver to achieve automatic and optimized data rate adjustment based on the optical channel condition is a big challenge that the fiber optical communication industry is facing.
For conventional coherent optical transceivers, the modulation format, symbol rate and coding schemes and thus the data rates can be adjusted based on the link length and optical channel conditions. For systems using probabilistic constellation shaping, the shaping factor (SF) can also be adjusted for maximizing the transmission performance. These tasks are usually done manually when initializing the link. This is due to the lack of effective mechanisms for providing essential feedback information to the transmitter during system's operation.
In view of the foregoing problem, this invention disclosure provides methods for signal generating and processing in the transmitter (Tx) and the receiver (Rx) of a coherent optical transceiver to ensure an effective mechanism for seamless communication about the transmission channel condition between connected optical transceivers without interrupting the data transmission.
According to the first aspect of the invention, a communication method between connected bidirectional coherent optical transceivers for passing information on transmission channel condition is provided. The method comprises the steps of: generating signal with pre-set modulation format, baudrate, forward error correction (FEC) scheme and shaping factor, by transmitter (Tx) of the first optical transceiver, to transmit over a downlink, wherein the downlink is the transmission channel between the transmitter (Tx) of the first optical transceiver and receiver (Rx) of the second optical transceiver; receiving signal transmitted from Tx of the first optical transceiver and calculating the current downlink channel condition, by Rx of the second optical transceiver, by using the information about the modulation format, baudrate, FEC scheme and shaping factor received from Tx of the first optical transceiver; passing, by Rx of the second optical transceiver, the information on the calculated downlink channel condition to Tx within the second optical transceiver; generating signal with pre-set modulation format, baudrate and FEC scheme, shaping factor, by Tx of the second optical transceiver, to transmit this signal together with the information on the downlink channel condition received from Rx of the second optical transceiver to Rx of the first optical transceiver over an uplink, wherein the uplink is the transmission channel between the Tx of the second optical transceiver and Rx of the first optical transceiver; receiving signal transmitted from Tx of the second optical transceiver and calculating the current uplink channel condition, by Rx of the first optical transceiver, by using the information about the modulation format, baudrate, FEC scheme and shaping factor received from Tx of the second optical transceiver; passing, by Rx of the first optical transceiver, the information on the calculated uplink channel condition together with the information on the downlink channel condition received from Tx of the second optical transceiver to Tx within the first optical transceiver; using the information on the downlink channel condition received from the Rx within the first optical transceiver, by Tx of the first optical transceiver, to generate signal with optimized modulation format, baudrate, FEC scheme and shaping factor for maximizing the downlink's capacity; transmitting signal with the optimized modulation format, baudrate, FEC scheme and shaping factor together with the information on the uplink channel condition received from Rx within the first optical transceiver, by Tx of the first optical transceiver, to Rx of the second optical transceiver over the downlink; receiving the signal transmitted from Tx of the first optical transceiver and calculating the current downlink channel condition, by Rx of the second optical transceiver, by using the information about the optimized modulation format, baudrate, FEC scheme and shaping factor generated by Tx of the first optical transceiver; passing, by Rx of the second optical transceiver, the information on the calculated downlink channel condition together with the information on the uplink channel condition received from Tx of the first optical transceiver to Tx within the second optical transceiver; using the information on the uplink channel condition received from the Rx within the second optical transceiver, by Tx of the second optical transceiver, to generate signal with optimized modulation format, baudrate, FEC scheme and shaping factor for maximizing the uplink's capacity; transmitting signal with the optimized modulation format, baudrate, FEC scheme and shaping factor together with the information on the downlink channel condition received from Rx within the second optical transceiver, by Tx of the second optical transceiver, to Rx of the first optical transceiver over the uplink; and continuing the process of passing information on channel condition in an endless loop to update and monitor continuously and/or periodically the condition of downlink and uplink channel for optimizing the operating parameters of connected transceivers.
According to the second aspect of the invention, a processing method for a transmitter (Tx) in a bidirectional coherent optical transceiver to multiplex management message with transmitted data is provided. The method comprises the steps of: de-multiplexing data to be transmitted into data for transmission in x- and y-polarizations; receiving the management message sent by the Rx within the same optical transceiver and de-multiplexing the management message into control message and forwarding message, wherein the control message contains the information on direct transmission channel condition and the forwarding message contains the information on opposite transmission channel condition, wherein the direct transmission channel is the channel on which the Tx of this optical transceiver transmits data to the receiver end on the other side of the transmission channel, and the opposite transmission channel is an opposite one of the direct transmission channel; based on the information on direct transmission channel condition contained in the control message sent by the Rx within the same optical transceiver, choosing baudrate, modulation format, FEC scheme and shaping factor appropriately for maximizing the direct transmission channel's capacity by using a FEC and Modulation format pool; encoding and mapping data to be transmitted in x- and y-polarizations into Quadrature Amplitude Modulation (QAM) symbols by using the information on chosen baudrate, modulation format, FEC scheme and shaping factor; performing pulse-shaping for QAM symbols to generate the transmitted waveforms in x- and y-polarizations; multiplexing the information on chosen baudrate, modulation format, FEC scheme and shaping factor as a new control message with the forwarding message containing the information on the opposite transmission channel condition that the Tx received from the Rx within the same optical transceiver to form a new management message; encoding and mapping the new management message to simple BPSK format; performing pulse-shaping for resulted Binary Phase-Shift Keying (BPSK) symbols to generate a complex waveform, which is denoted as M(t), at baseband carrying the new management message; separating the generated waveform M(t) into two copies, and conjugating one copy of M(t) to obtain the conjugation of M(t) which is denoted as M*(t); multiplying exp(2πjf0t) with M(t) and M*(t) by using a complex digital oscillator to shift these two signals to an intermediate frequency of f0, wherein f0 is the frequency separation between the QAM signal and M(t); splitting each of the received two signals, which are M(t) exp(2πjf0t) and M*(t) exp(2πjf0t), into two copies, one of which is conjugated to generate total four signals: M(t) exp(2πjf0t), M*(t) exp(2πjf0t), M(t) exp(−2πjf0t), and M*(t) exp(−2πjf0t); adding two resulted signals M(t) exp(2πjf0t) and M*(t) exp(−2πjf0t) to the QAM signal for transmission in x-polarization; and adding two remaining signals M(t) exp(−2πjf0t) and M*(t) exp(2πjf0t) to the QAM signal for transmission in y-polarization.
According to the third aspect of the invention, a processing method for a receiver (Rx) of a bidirectional coherent optical transceiver to demultiplex management message from received data is provided. The method comprises the steps of: receiving the incoming signal including information-bearing signal and monitoring signal sent by the Tx on the other side of the transmission channel, wherein information-bearing signal is QAM signal carrying transmitted data and monitoring signal is monitoring subcarriers carrying management message; generating Inphase and Quadrature signal components from the received incoming signal; combining the received Inphase and Quadrature signal components to form complex signals in x- and y-polarizations; performing chromatic dispersion (CD) compensation in x- and y-polarizations; at each polarization, splitting the resulted signal after CD compensation into two copies in which one copy is passed through an LPF (Low pass filter) for filtering out the QAM signal and the other one is passed through an HPF (High pass filter) for filtering out the monitoring subcarriers; feeding the filtered QAM signals in x- and y-polarizations into the first 2×2 MIMO (Multiple Input-Multiple Output) block for polarization de-rotation; feeding the information about the state of polarization obtained from the first 2×2 MIMO block into the second 2×2 MIMO block for performing polarization de-rotation and equalization for the filtered monitoring subcarriers in x- and y-polarizations; obtaining the management message carried in the filtered monitoring subcarriers after the second 2×2 MIMO block; detecting and decoding the received management message to obtain the information about control message and forwarding message included in this management message, wherein: the decoded control message comprises the information on baudrate, QAM format, shaping factor and FEC scheme which the transmitter (Tx) on the other side of the transmission channel chose for encoding data to be transmitted to this receiver over the direct transmission channel, the decoded forwarding message comprises the information on the opposite transmission channel condition which the Tx on the other side of the transmission channel received from Rx within its transceiver to forward to this receiver over the direct transmission channel, wherein the direct transmission channel is the channel on which the Tx on the other side of the transmission channel transmits data to this receiver, and the opposite transmission channel is an opposite one of the direct transmission channel; and using the information on baudrate, QAM format, shaping factor and FEC scheme obtained from the decoded control message to facilitate the carrier recovery, symbol detection, SNR (Signal-to-Noise Ratio) and OSNR (Optical Signal-to-Noise Ratio) estimation of the direct transmission channel and to decode the received data; forwarding the information on the opposite transmission channel condition as a new control message included in a new management message to the Tx within its transceiver; and forwarding the information on the direct transmission channel condition which being calculated after SNR and OSNR estimation as a new forwarding message included in a new management message to the Tx within its transceiver.
Preferably, after carrier recovery, sending the estimated frequency offset information between the local oscillator (LO) of the Rx and carrier frequency of the incoming signal back to the laser controller of the LO as a feedback signal through a low-speed digital-to-analog converter (DAC) to form a phase-locked loop (PLL).
Embodiments of the present invention provide a mechanism for passing information on transmission channel condition between connected coherent optical transceivers to create a flexible and smart optical transceiver which can self-optimize the baudrate, modulation format, error-forward correction scheme and shaping factor by using the feedback information on transmission channel condition for maximizing the transmission performance and system capacity. Further, thanks to appropriately adjusted data rate, the transmission channels are always ensured in operating state.
In addition, by encoding information on the transmission channel's condition on a pair of conjugated narrow-band subcarriers, multiplexing these subcarriers with information-bearing signal to be transmitted, detecting the conjugated pair of subcarriers at receiver end and processing signal appropriately for SNR enhancement, the present invention also provides an effective mechanism to enable the information passing mechanism between two connected optical transceivers without interrupting data transmission.
The effects of the present invention are not limited to the above-mentioned effects, and further effects not described above will be clearly understood by those skilled in the art from the appended claims.
The above and other aspects, features, and advantages of the disclosure will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:
Technical solutions in embodiments of the present disclosure are described below in connection with the accompanying drawings in the embodiments of the present disclosure. Obviously, the described embodiments are part of the embodiments of the present disclosure, but not all the embodiments. Based on the embodiments in the present disclosure, all other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the present disclosure.
It should be understood that in the following description, well known elements, functions, operations, techniques, etc. may not be described or illustrated in detail to avoid obscuring the subject matter of the disclosure.
With reference to
The transmitter of a typical full-coherent optical transceiver may include a Tx DSP (Digital Signal Processor); 4 DACs (Digital to Analog Converter); 4 RF (Radio Frequency) drivers; and a DP-IQ (Dual-Polarization Inphase and Quadrature) modulator with a laser. At the transmitter, the Tx DSP accepts incoming data and generates 4 digital signals for modulating the Inphase and Quadrature components of x- and y-polarizations of an optical carrier. Then, the 4 digital signals are converted into 4 analog waveforms using 4 DACs. These 4 analog signals are then amplified using 4 RF drivers (electrical amplifier). An DP IQ modulator is used then to modulate these 4 signals into the amplitude and phase of the x- and y-polarizations of an optical carrier.
At the receiver side, a conventional coherent receiver is used to convert the optically modulated signal into the electrical domain. The conventional coherent receiver includes a local oscillator (LO); two polarization beam splitters (PBS), two 2×4 90-degree hybrids, 4 balanced photodetectors (PD), 4 transimpedance amplifiers (TIA); 4 analog-to-digital converters (ADC) and a dual-polarization DSP (DP-DSP), which performs channel equalization (polarization demultiplexing, polarization mode dispersion compensation, chromatic dispersion compensation), timing and carrier recovery, detection and decoding. Specifically, incoming optical signal is fed into the first PBS to split into TE (Transverse Electric) and TM (Transverse Magnetic) mode, and the local oscillator (LO) is also split into TE and TM mode using the second PBS. The signals output from the PBSs are then mixed using 2×4 90-degree hybrids. The outputs of these optical hybrids are fed into 4 pairs of balanced PD. The outputs of these balanced PDs are fed into 4 TIAs and then digitized by 4 ADCs. The outputs of 4 ADCs, namely Ix, Qx, Iy, Qy are fed into a Rx DSP. Then, the Rx DSP may perform chromatic dispersion (CD) compensation, polarization de-rotation and polarization mode dispersion (PMD) compensation, timing recovery, carrier recovery, symbol detection and decoding at each polarization to receive the transmitted data.
As shown in
For conventional coherent optical transceivers, the modulation format, symbol rate and coding schemes and thus the data rates can be adjusted based on the link length and optical channel conditions. For systems using probabilistic constellation shaping, the shaping factor (SF) can also be adjusted for maximizing the transmission performance. These tasks are usually done manually when initializing the link. This is due to the lack of effective mechanisms for providing essential feedback information to the transmitter during system's operation. Solving this problem requires novel transceiver design and effective mechanisms for seamless communication between the transmitter end and the receiver end about the link condition without interrupting the data transmission.
The present invention provides methods for signal generation and signal processing, and especially provides an effective mechanism for communication between the Tx and Rx within a coherent optical transceiver and between connected coherent optical transceivers to exchange essential information about the transmission channel that the Tx can use to adaptively choose modulation formats, baudrate, shaping factor and coding scheme for maximizing the transmission performance and system capacity. The general concept of such optical transceiver operating in a bidirectional transmission mode, where the transmitter (Tx) and the receiver (Rx) can pass feedback information about the transmission channel, is illustrated in
Referring to
As shown in
To effectively enable the information passing mechanism between two connected optical transceivers without interrupting data transmission, the invention provides method for encoding information on the transmission channel's condition on a pair of conjugated narrow-band subcarriers, which are called monitoring subcarriers; method for multiplexing these monitoring subcarriers with information-bearing signal to be transmitted; and method for detecting the conjugated pair of subcarriers at receiver end and processing signal appropriately for SNR enhancement.
As shown in
Tx and Rx pass to each other a management message, which contains two parts, namely the control message and the forwarding message. The control message is used to facilitate the data reception at the designated Rx or encoding at Tx end. The forwarding message is the part to be forwarded to the next Tx and Rx in the chain (as shown in
Referring to
As shown in
As shown in
BM<<BU and BM<<BD
The multiplexing scheme of the QAM signal with the channel monitoring subcarriers carrying management message is shown in
Next, each of these two signals is then split into two copies, one of which is conjugated to generate 4 following signals:
M(t) exp(2πjf0t), M*(t) exp(2πjf0t), M(t) exp(−2πjf0t), and M*(t) exp(−2πjf0t)
Then two following signals are added to the QAM signal in x-polarization:
M(t) exp(2πjf0t) and M*(t) exp(−2πjf0t),
Then two following signals are added to the QAM signal in y-polarization:
M(t) exp(−2πjf0t) and M*(t) exp(2πjf0t).
As shown in
Referring to
As shown in
Although the present disclosure has been described with various embodiments, various changes and modifications may be suggested to one skilled in the art. It is intended that the present disclosure encompass such changes and modifications as fall within the scope of the appended claims.
