Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/50—Transmitters
  • H04B10/58—Compensation for non-linear transmitter output
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
  • H04B10/40—Transceivers

Definitions

• the present invention relates to the field of wireless communications, and more particularly to an intensity-modulation direct-detection system of an optical transceiver.
• IDD Intensity-modulation and direct-detection
• dispersion compensation module can be used in the IMDD system to compensate for the chromatic dispersion (CD) in the link.
• DCM dispersion compensation module
• CD chromatic dispersion
• Another solution to extend the reach can consist of a CD pre-compensation or a single sideband modulation, which can be carried out by means of a complex signal modulator such as a dual-drive Mach-Zehnder modulator (dual-drive MZM or DDMZM) and a in- phase/quadrature (IQ) modulator (also known as a single sideband modulator).
• a complex signal modulator such as a dual-drive Mach-Zehnder modulator (dual-drive MZM or DDMZM) and a in- phase/quadrature (IQ) modulator (also known as a single sideband modulator).
• Fig. 1 shows a conventional DDMZM in a transmitter (Tx) 100 of an optical transceiver.
• Such a conventional DDMZM consists of two independent phase modulators and each phase modulator will induce a phase shift proportional to the applied voltage.
• the two arms (also designated as upper and lower arms, respectively) of the DDMZM are respectively driven by the two voltage signals di and d 2 (also designated as the drive signals hereafter).
• Each one of the drive signals di and d 2 comprises two components: a direct current (DC) component and a radio frequency (RF) component.
• the DC voltages are bias voltages
• the DDMZM output signal ⁇ which is the electric field at the DDMZM output, is given by the following relationship:
• E TX (t) E in cos ( d iC t )-3 ⁇ 4 ( t )) ⁇ exp ⁇ ( ⁇ 2 M)) (2)
• Ei n is the DDMZM input signal, which is the electric field at the DDMZM input
• ⁇ ⁇ is the voltage needed to induce a ⁇ phase shift between the arms of the two phase modulators. From the transfer function ⁇ / ⁇ ⁇ , it results that the DDMZM can be regarded as an intensity modulator through its cosine part and as a phase modulator through its
• the DDMZM can be used to generate a complex signal.
• the transfer function ⁇ /Ein of the DDMZM can be inverted and the drive signals can be calculated as follows:
• This method of inverting the transfer function has the benefit to fully compensate for the nonlinearity and other distortions of the DDMZM in the case that the bandwidth, the samplin rate and the quantized bits of the digital-to-analog (D/A) converter (DAC) are high enough.
• D/A digital-to-analog
• this method also has several disadvantages. Indeed, the calculated bias point is related to the RF signals being added, which is not acceptable to use in the real product. In addition, it is necessary to sample at a high rate and to quantize bits of the DAC in order to
• the DDMZM works at the quadrature point, i.e., at the operating point that is at the center of the quasi-linear region of the DDMZM characteristic and that thereby offers a maximum signal excursion, and the phase shift between the two arms is ⁇ /2.
• the RF signal is small, namely if the DDMZM is working under small signal condition, then the DDMZM can be considered as a linear modulator and the DDMZM output signal ⁇ can be written as Eout approximated by the following relationship:
• This method based on small signal condition has the benefit to have the DDMZM working at the quadrature bias point.
• this method also has disadvantages. Indeed, the small signal condition leads to a poor performance in terms of optical signal-to-noise ratio (OSNR) and sensitivity. In addition, significant error floor will appear if the drive signal is large.
• OSNR optical signal-to-noise ratio
• the optical carrier signal can be written in a complex form as (a+bj), where the parameters a and b stand for the optical carrier signal in the respective I and Q arms.
• the I and Q arms use the MZM structure in such a manner that the IQ modulator can be referred to as a IQMZM, then the transfer function of each arm is: cos— ⁇ (5) where V is the bias voltage added to the arm, ⁇ ⁇ is the voltage needed to induce a ⁇ phase shift between the arms of the two phase modulators.
• the optical carrier signal i.e., the optical DC carrier signal
• the beating process between the optical carrier signal and the modulated optical signal is a dot multiplication process and the non-zero angle (i.e., ⁇ ) between them will reduce the amplitude by a factor equal to cosP between the optical DC carrier signal and the target signal, which will reduce the detected signal intensity.
• the invention relates to a digital signal processing (DSP) module in a transmitter (Tx) of an optical transceiver.
• the DSP module is adapted to perform a bias- related linear transform (M a ) in order to align, in a vector plane, the vector of an optical carrier signal with the vector of a modulated optical signal that is modulated by a modulator implemented inside the Tx, and to perform a digital pre-distortion (DPD) in order to compensate for the nonlinearity of the modulator.
• M a bias- related linear transform
• DPD digital pre-distortion
• the performance (e.g., the OSNR) of the optical transceiver can be improved thanks to a bias-related linear transform (M a ) allowing to compensate for the non-orthogonality of the modulator and a digital pre-distortion allowing to mitigate the nonlinearity of the modulator.
• M a bias-related linear transform
• the detected signal intensity can increase due to the vector alignment allowing the cosine of the angle between the vector of the optical carrier signal and the vector of the optical signal modulated by the modulator to increase, and in addition, the nonlinearity of the modulator can be compensated.
• an optimal performance of the optical transceiver can be obtained in response to an optimal compensation of the non-orthogonality of the modulator thanks to the perfect alignment between the vector of the optical carrier signal and the vector of the modulated optical signal.
• the bias-related linear transform (M a ) is added to any portion of an existing linear process of the DSP module and is located between the portion of a symbol generation and the portion of the DPD.
• the invention relates to a transmitter (Tx) comprising the DSP module as claimed in the first aspect or any one of the implementations of the first aspect, and the modulator as specified in the first aspect.
• the modulator is a dual-drive Mach-Zehnder modulator (DDMZM) or an in-phase/quadrature (IQ) modulator.
• DDMZM dual-drive Mach-Zehnder modulator
• IQ in-phase/quadrature
• the invention relates to an intensity-modulation (IM) direct- detection (DD) system for transmitting and receiving signals in an optical transceiver.
• the IM DD system comprises the transmitter (Tx) as claimed in the second aspect or the first implementation of the second aspect, and a receiver (Rx).
• the invention relates to a digital pre-distortion (DPD) method for compensating for a nonlinearity of a modulator in a transmitter (Tx) of an optical transceiver.
• the step of deriving the distortion signal (D) is based on decomposing the modulated optical signal (T) in a series of polynomials using a Taylor series.
• the modulator is either an in-phase/quadrature (IQ) modulator or a dual-drive Mach-Zehnder modulator (DDMZM). In the case of the IQ modulator, D* equals D.
• the invention relates to a method for enhancing a system performance of an optical transceiver.
• the method comprises the step of performing a bias- related linear transform (M a ) at a transmitter (Tx) of the optical transceiver in order to align, in a vector plane, the vector of an optical carrier signal with the vector of a modulated optical signal that is modulated by a modulator implemented inside the Tx, and the step of performing a digital pre-distortion (DPD) at the Tx in order to compensate for the nonlinearity of the modulator.
• M a bias- related linear transform
• DPD digital pre-distortion
• the invention relates to a method for selecting a bias-related linear transform (M a ) inside a digital signal processing (DSP) module in a transmitter (Tx) of an optical transceiver.
• the method comprises the step of varying the coefficients of the bias- related linear transform (M a ) through a variation of the bias voltage, the step of measuring a quality performance versus the variation of the coefficients, and the step of selecting the bias- related linear transform (M a ) whose coefficients correspond to the best quality performance.
• the step of measuring the quality performance comprises measuring a signal-to-noise ratio (SNR) or a quality factor (Q-factor) or a bit error rate (BER) or an error vector magnitude (EVM), the SNR and the Q-factor varying in a same direction as the quality performance and the BER and the EVM varying in an opposite direction to the quality performance.
• SNR signal-to-noise ratio
• Q-factor quality factor
• BER bit error rate
• EVM error vector magnitude
• the invention relates to a method for selecting a bias-related linear transform (M a ) inside a digital signal processing (DSP) module in a transmitter (Tx) of an optical transceiver.
• the method comprises the step of adding a respective pilot signal (si, s2) to each arm of both drive signals (dl, d2) of a modulator implemented inside the Tx in order to have a respective target signal (sl+dl, s2+d2) to be modulated through the modulator, the step of detecting through a photo-detector (PD) the respective modulated
• PD photo-detector
• the PD is located at the Tx side or at the receiver (Rx) side of the optical transceiver.
• the invention relates to a computer program comprising a program code for performing the method according to any one of the fourth to seventh aspects and their respective implementations when executed on a computer or with a real time chip.
• the method can be performed in an automatic and repeatable manner.
• the computer program can be run/executed by the above apparatuses. More specifically, it should be noted that the above apparatuses may be implemented based on a discrete hardware circuitry with discrete hardware components, integrated chips or arrangements of chip modules, or based on a signal processing device or chip controlled by a software routine or program stored in a memory, written on a computer-readable medium or downloaded from a network such as the Internet. It shall further be understood that a preferred embodiment of the invention can also be any combination of the dependent claims or above embodiments with the respective independent claim.
• Fig. 1 shows a conventional DDMZM in a transmitter (Tx) of an optical transceiver
• Fig. 2 shows a vector plane representation of an optical carrier signal (a, b) and a modulation optical signal (Si, S 2 );
• Fig. 3 illustrates the modulation theory of the DDMZM biased at the quadrature point under small signal condition;
• Fig. 4 illustrates the modulation theory of the DDMZM biased at a point other than the quadrature point under small signal condition
• Fig. 5 shows a schematic flow diagram describing the scan method for obtaining the appropriate bias-related linear transform, according to an embodiment of the present invention
• Fig. 6 shows a schematic flow diagram describing the pilot tone method for obtaining the appropriate bias-related linear transform, according to an embodiment of the present invention.
• Fig. 7 shows a structure of a digital signal processing (DSP) module 100 implemented in a transmitter (Tx) of an optical transceiver, according to an embodiment of the present invention.
• DSP digital signal processing
• a conventional DDMZM consists of two independent phase modulators. Considering the fact that a phase modulator does not change the intensity of the signal, it is then possible to plot the two modulated optical signals in a same unity circle (i.e., its radius equals unity) and their sum is the output signal of the DDMZM.
• Fig. 3 illustrates the modulation theory of the DDMZM biased at the quadrature point under small signal condition, wherein the first circle represents the case of the modulator biased at any bias point, the second circle represents the case of the modulator biased at the quadrature bias point, the third circle depicts the RF signals when the modulator is biased at the quadrature bias point, and the fourth circle depicts the alignment of the sum vector (denoted by DC in Fig. 3) of the two optical DC carrier signals with the target signal that is the component (denoted by a) of the modulated optical signal (a, b).
• DD direct detection
• only one dimension of the signal i.e., the component a of the target signal in the present example
• the phase shift between its two arms is equal to ⁇ /2 (i.e., the two optical DC carrier signals in the respective I and Q arms are orthogonal to each other) (cf. 32 and 33) and the RF signals are also orthogonal to each other (cf. 33).
• a ⁇ /4 rotation is then needed as the angle between each of them usually equals ⁇ /4 (cf. 34).
• Fig. 4 illustrates the modulation theory of the DDMZM biased at a point other than the quadrature point under small signal condition.
• the target modulated orthogonal signal is given by: a+jb, as disclosed in Fig. 4 on the right, it is then possible to derive the RF signal (denoted by a' and b') from the target modulated orthogonal signal (denoted by a and b) according to the following relationships:
• a first method for selecting an appropriate bias-related linear transform M a can consist, starting from an initialized value of the phase shift a, in varying the coefficients of the bias-related linear transform M a through a variation of the bias voltage, measuring a quality performance versus the variation of the coefficients, and selecting the bias-related linear transform M a whose coefficients correspond to the best quality performance.
• the quality performance can be related, for example, to a signal-to-noise ratio (SNR), a quality factor (Q-factor), a bit error rate (BER), or an error vector magnitude (EVM), the SNR and the Q-factor varying in a same direction as the quality performance and the BER and the EVM varying in an opposite direction to the quality performance.
• SNR signal-to-noise ratio
• Q-factor quality factor
• BER bit error rate
• EVM error vector magnitude
• another method for selecting an appropriate bias-related linear transform M a can consist in adding a respective pilot signal (si, s 2 ) to each arm of both drive signals (di, d 2 ) of a modulator (e.g., the DDMZM), each arm having a respective bias- voltage denoted by bi and b 2 and the modulator being implemented inside the transmitter (Tx) of an optical transceiver (as shown, for example, in Fig. 1), in order to have a respective target signal (si+di, s 2 +d 2 ) to be modulated through the modulator.
• a modulator e.g., the DDMZM
• PD photo-detector
• the bias-related linear transform M a whose coefficients correspond to the maximum value of cross-correlation (xcorr(Rx_si+di, si+di), xcorr(Rx_s 2 +d 2 , s 2 +d 2 )) between the respective modulated target signal (Rx_si+di, Rx_s 2 +d 2 ) and the respective target signal (si+di, s 2 +d 2 ).
• the bias-related linear transform M a can thus be tracked and not just scanned.
• this linear transform M a is suitable not only for the DDMZM but also for the IQ modulator such as the IQMZM. In a general manner, it is suitable for any modulator whose transfer function has a cosine profile versus the bias voltage, and in particular suitable for a LiNb03 DDMZM.
• the distortion can be viewed as a high order signal, which is rather small with respect to the driven signals, and the nonlinearity of the modulator will be mitigated using a digital pre- distortion (DPD).
• DPD digital pre- distortion
• the linear transform M a and the DPD can be implemented in a transmitter (Tx) of an optical transceiver, and in particular inside a digital signal processing (DSP) module 100 corresponding, for example, to the processor module of Fig. 1.
• DSP digital signal processing
• the DSP module 100 is thus adapted to perform the bias-related linear transform (M a ) in order to align, in the vector plane, the vector of the optical carrier signal with the vector of the optical signal that is modulated by the modulator implemented inside the Tx and also adapted to perform the DPD in order to compensate for the nonlinearity of the modulator.
• the bias-related linear transform (M a ) can be added to any portion ((a), (b), (c)) of an existing linear process of the DSP module between the portion of a symbol generation and the portion of the DPD.
• the bias-related linear transform (M a ) can be located either at the rear part (a) of the existing linear process or at the front part (b) thereof or inside (c) thereof.
• the DPD method for compensating for the nonlinearity of the modulator comprises several steps amongst which the step of obtaining an optical signal (T) modulated by the modulator, the modulated optical signal (T) then verifying the relationship:
• T S + D (10) where S denotes a target signal and D denotes a distortion signal, the step of decomposing the modulated optical signal (T), the step of deriving the distortion signal (D) from a comparison between the modulated optical signal (T) and the target signal (S), and the step of converting the target signal (S) into a pre-distorted signal (S') by subtracting a signal (D*) proportional to the distortion signal (D) from the target signal (S) according to the relationship:
• the step of deriving the distortion signal (D) is based on decomposing the modulated optical signal (T) in a series of polynomials using, for example, a Taylor series.
• D* equals D in the case where the modulator is an IQ modulator.
• the target signal S is split into the respective target signals S 1 and S2 corresponding to the two drive signals in each arm of the DDMZM and S' is then decomposed into the equation system:
• S'2 S2 + D/2 (12)
• the present invention relates to a digital signal (DSP) processing module in a transmitter (Tx) of an optical transceiver.
• the DSP module performs a bias-related linear transform (M a ) in order to align, in a vector plane, the vector of an optical carrier signal with the vector of a modulated optical signal that is modulated by a modulator implemented inside the Tx, and also a digital pre-distortion (DPD) in order to compensate for the nonlinearity of the modulator.
• the DPD method mainly consists in converting a target signal into a pre- distorted signal by subtracting a signal proportional to the distortion signal from the target signal.
• the non-orthogonality of the modulator is mitigated by an appropriate bias-related linear transform that is selected as to obtain the best quality performance using a scan method or a maximum value of cross-correlation using a pilot tone method.
• a single processor or other unit may fulfill the functions of several items recited in the claims.
• the mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.
• a computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

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• Physics & Mathematics (AREA)
• Electromagnetism (AREA)
• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Nonlinear Science (AREA)
• Optical Communication System (AREA)
• Optical Modulation, Optical Deflection, Nonlinear Optics, Optical Demodulation, Optical Logic Elements (AREA)

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• 2017
  • 2017-01-30 WO PCT/EP2017/051928 patent/WO2018137782A1/en unknown
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