This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2016-037400, filed on Feb. 29, 2016, the entire contents of which are incorporated herein by reference.
The embodiments discussed herein are related to an optical transmitter, and an optical transmission device that performs wavelength multiplexing by using a plurality of optical transmitters, and a transmission method.
With an increase in data traffic, there are demands for higher transmission rates of optical communications networks, and high-speed communication at 40 gigabits per second (Gbps), 100 Gbps, and so on per wavelength is being put into practical use. As a technique for realizing high-speed optical communication, transmission-and-reception of an optical signal through digital signal processing has attracted attention.
In a transmitting end, a signal-processing circuit maps transmission data onto electric-field information and uses the mapped electric-field information to modulate optical waves from a transmission light source, and the transmitting end transmits the modulated optical waves. A plurality of optical transmitters generates an optical signal having different wavelengths or carrier frequencies and multiplexes the optical signal to thereby perform wavelength multiplexing. When the oscillation frequency of the transmission light source is displaced from a desired value owing to a temperature change or aging deterioration, this displacement affects the quality of transmission, thus impeding an increase in the density of wavelength multiplexing.
Accordingly, a method in which a signal-processing circuit pre-corrects displacement of a carrier frequency has been proposed (for example, see Japanese Laid-open Patent Publication No. 2012-120010). Phase rotation in an opposite direction is applied to the electric field phase of mapped electric-field information in accordance with the displacement of the carrier frequency to thereby control the carrier frequency.
A method in which two types of constellation map are prepared and are switched at each transmission timing of bit data is available in order to reduce a peak-to-average power ratio (PAPR) of a multi-value optical signal (for example, see Japanese Laid-open Patent Publication No. 2014-007642). In this method, symbol positions on two types of map are amplitude-limited so as not to exceed the maximum amplitude of outputs of an analog-to-digital converter (ADC).
When phase rotation corresponding to displacement of a carrier frequency is pre-applied to mapped data, high-density wavelength multiplexing is realized, and the efficiency of using a frequency band improves. However, when a signal point exceeds the upper limit of a dynamic range as a result of the phase rotation processing, rounding into the dynamic range occurs. In this case, constellation distortion occurs, the accuracy of detecting symbol positions decreases, and a bit error rate (BER) deteriorates to reduce the transmission distance, thereby reducing the communication performance.
According to an aspect of the invention, an optical transmitter includes a signal-processing circuit configured to perform signal processing on a first transmission signal and output a second transmission signal; an optical modulator configured to modulate input light with the second transmission signal and to output an optical signal; and a control circuit configured to output a control signal for controlling a carrier frequency of the optical signal to the signal-processing circuit, wherein the signal-processing circuit includes a map-adjustment circuit configured to adjust, based on the control signal and a modulation format, a map position of the second transmission signal onto a complex plane, and a phase-rotation circuit configured to apply, on the complex plane, rotation of a phase of the carrier frequency corresponding to the control signal to the second transmission signal at the adjusted map position.
The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed.
A signal-processing circuit in an optical transmitter maps externally input transmission signal onto electric-field information in accordance with a modulation format, such as Quadrature Phase-Shift Keying (QPSK), Quadrature Amplitude Modulation (QAM), or Orthogonal Frequency Division Multiplexing (OFDM). For example, when modulation using a 16-QAM system is performed input data is divided into bit strings each having four bits, and the bit strings are mapped on signal points (symbol points) on a complex plane (IQ plane). This is referred to as “constellation mapping”. Each symbol point on the constellation corresponds to electric-field information determined by an amplitude and a phase.
Owing to variations in the oscillation frequency of a transmission light source and influences of a transmission path, the constellation appears to be rotated at a receiving end. Accordingly, a transmitting end pre-rotates the phase in an opposite direction to perform correction.
In the example illustrated in
As denoted by upper lines in
In contrast, as denoted by lower lines in
Reducing the amplitude so that a trace P of the outermost points during the phase rotation fit in the area of the dynamic range is conceivable in order to ensure that the constellation distortion due to the phase rotation does not occur. However, another problem arises.
As illustrated in the diagram at the middle, when the amplitude is reduced, a distance d2 between the closest symbols becomes smaller than the inter-symbol distance d1 before the amplitude limitation is performed, and as illustrated in the right diagram, even when the phase rotation is applied, constellation distortion due to rounding does not occur. Under a favorable signal-to-noise ratio (S/N ratio) condition, problems of a decrease in the symbol position detection accuracy and a decline in a BER can be solved.
However, under an unfavorable S/N ratio condition, the problems that the BER deteriorates and the transmission distance is not extensible owing to the reduced inter-symbol distance arise. Accordingly, in embodiments, when the carrier-frequency control using phase rotation is performed, mapping adjustment with which the inter-symbol distance is the largest in the area of the dynamic range is performed. This improves the quality of transmission, while maintaining the efficiency of using a frequency band.
Since the trace P of the outermost points during the phase rotation fits in the inscribed circle of the upper limit of the dynamic range, rounding into the dynamic range does not occur, but the quality of transmission deteriorates under an unfavorable S/N ratio condition. In order to address this problem, in the first embodiment, mapping is adjusted so that the smallest one of the distances from one symbol point S1 to the other 15 symbol points is larger than the inter-symbol distance in the amplitude limitation method.
The right diagram in
With mapping pattern 1, even when phase rotation for controlling the carrier frequency is applied, distortion of the constellation is inhibited in a state in which the inter-symbol distance is maintained as large as possible. This makes it possible to improve both the efficiency of using the frequency band and the quality of transmission.
(1) Hexagons H having symbol points at the respective centers thereof are arranged without gaps, and a smallest circle C having at least 16 symbol points therein is determined in the range of the upper limit of the dynamic range. As illustrated in the left diagram in
(2) The symbol positions are adjusted so that 16 symbol points fit in the circle C and the inter-symbol distance is the largest. More specifically, a symbol point So at the center of the circle C is moved to cause three symbol points Si, Sj, and Sk of six symbol points (these are referred to as “outermost points”) that are the farthest from the center of the circle C to lie outside the circle C. In addition, the intervals of all the symbols are increased so that the symbol points at the outermost points inside the circle C lie on the circumference of the circle C or are in the circumference of the circle C and the closest thereto. This makes it possible to maximize the distance between the symbols, while maintaining the inter-symbol distances at equal intervals.
(3) After the mapping adjustment, mapping pattern 1 in which the 16 symbols are arranged at equal intervals and the inter-symbol distances are the largest is generated, as illustrated in
In
The difference between the number of symbol points inside the determined circle and a predefined number of symbols is calculated (S103).
In the case of 16-QAM, a smallest circle that contains at least 16 symbol points includes 19 symbol points, and the difference is three symbols. In the case of 32-QAM, a smallest circle that contains at least 32 symbol points includes 37 symbol points, and the difference is five symbols. The center point of the circle is moved so that a number of symbol points which corresponds to the difference lie outside the circle, and the distances between the symbols are generally extended so that the outermost points inside the circle lie on the circumference or are the closest to the circumference (S104).
When a frequency displacement of a carrier wave occurs during operation, an optical transmitter may execute the flow of this mapping adjustment in real time in accordance with a modulation format. Alternatively, mapping pattern 1 may be generated for each modulation format in advance, and a predefined constellation and mapping pattern 1 may be selectively used depending on whether or not there is a frequency displacement of the carrier wave.
In
In mapping pattern 1 illustrated in
In mapping pattern 1, the symbol points are arranged at equal intervals, and the distance between the symbols is a2. The inter-symbol distance a2 in mapping pattern 1 is larger than the smallest inter-symbol distance a1 in the amplitude limitation method (a2>a1).
Comparison of
In order to overcome this problem, in the second embodiment, mapping is adjusted so that the smallest one of the distances from one symbol point S1 to the other 15 symbol points is larger than the smallest inter-symbol distance d in the amplitude limitation method. The right diagram in
Four symbol points are arranged on the circumference of the innermost shell. Four symbol points are arranged on the circumference of the middle shell. Eight symbol points are arranged on the circumference of the outermost shell. The outermost shell may match the inscribed circle of the upper limit of the dynamic range. The number of power bands (concentric circles), the radii of the power bands, and the number of symbols on each circle are determined using a method (described below) in accordance with the upper limit of the dynamic range and a modulation format.
In
In
When mapping is performed regarding this outermost shell as the inscribed circle of the upper limit of the dynamic range, the symbols can be arranged at equal intervals, and the inter-symbol distance can be maximized.
A description will be given while paying attention to one of four quadrants in an orthogonal coordinate system on an IQ plane. The state of the entire IQ plane can be determined by quadrupling the state in
First, one symbol is arranged in each quadrant in the innermost shell (indicated by “1” in the field “innermost shell” in
Next, a combination of symbol points that can be arranged on the second shell, which has the second smallest radius, is determined. Since one symbol has been arranged in the innermost shell, the remaining number of symbols is three. As the arrangement of the second shell, there are three patterns, that is, a pattern in which one symbol is arranged (“1” is indicated in the field “second shell”), a pattern in which two symbols are arranged (“2” is indicated in the field “second shell”), and a pattern in which three symbols are arranged (“3” is indicated in the field “second shell”).
Next, a combination of symbol points that can be arranged on the third shell, which has the third smallest radius, is determined. When the number of symbols on the second shell is “1”, the remaining number of symbols is two. In this case, there are combination A in which one symbol is arranged on each of the third shell and the outermost shell and combination B in which two symbols are arranged on the third shell.
When the number of symbols on the second shell is “2”, the remaining number of symbol is one, and thus this symbol is arranged on the third shell. As a result, combination C in which “1”, “2”, and “1” are arranged sequentially from the innermost shell is determined. When the number of symbols on the second shell is “3”, a symbol to be arranged does not remain, and thus combination D in which one symbol is arranged on the innermost shell and three symbol points are arranged on the second shell is obtained for each quadrant. A combination in which the amplitude (radius) of the outermost shell fits in the inscribed circle of the upper limit of the dynamic range and the inter-symbol distance is the largest is selected from combinations A to D. When symbols are arranged in a unit circle having a radius normalized to 1 in accordance with the procedure in
This value is the same as the smallest inter-symbol distance in the original mapping in 16-QAM, and the S/N ratio deteriorates. The inter-symbol distance in combination D is 0.518. As a result, combination B with which the inter-symbol distance is the largest is selected. When one quadrant is considered, the numbers of symbol points are 1, 1, and 2 sequentially from the innermost shell, and thus, when four quadrants are considered, the numbers of symbol points to be arranged are 4, 4, and 8.
For example, the number of symbols, i, to be arranged on the second power band is selected from i=1, . . . , n−1 (i is a natural number). The number of symbols, j, to be arranged on the third power band is selected from j=1, . . . , n−1−i (j is a natural number). The smallest value “1” may be first selected as the number of symbols to be arranged on the power band, and then, each time processing is repeated through determinations in steps S204, S205, and S207 described below, the number of symbols may be incremented by 1.
Next, each selected symbol is arranged at a position where the distance to the adjacent symbol is the same as the inter-symbol distance d on the innermost shell and the amplitude (radius) of the power band in interest is minimized (S203). A determination is made as to whether or not the inter-symbol distance when the power band in interest is the outermost shell is larger than the smallest inter-symbol distance in symbol mapping in the original modulation format (S204). If the inter-symbol distance after the adjustment is smaller than or equal to the smallest inter-symbol distance in symbol mapping in the original modulation format (NO in S204), the advantage that the quality of transmission is improved while maintaining the efficiency of using the frequency band is not sufficiently obtained.
For example, combination C in
A determination is made as to whether or not all combinations of symbol arrangements have been determined (S207). If another combination of symbol arrangements remains (NO in S207), S202 to S206 are repeated, and another combination of symbol arrangements is determined. If all combinations of symbol arrangements have been determined (YES in S207), mapping with which the amplitude (radius) of the outermost shell fits in the inscribed circle of the upper limit of the dynamic range is selected from the stored all symbol arrangement combinations (S208), and then the processing ends.
In the example illustrated in
In
These symbols are arranged at equal intervals, and a combination of symbol arrangements in which the inter-symbol distance is the largest is selected. In mapping pattern 2, the distance between the symbols is a2. The inter-symbol distance a2 in mapping pattern 2 is larger than the smallest inter-symbol distance a1 in the amplitude limitation method (a2>a1).
The distance between the closest symbols is a1. In mapping pattern 2 illustrated in
In mapping pattern 2, the distance between the symbols is a2. The inter-symbol distance a2 in mapping pattern 2 is larger than the smallest inter-symbol distance a1 in the amplitude limitation method (a2>a1). Comparison of
<Device Configuration>
Operations of the individual circuits are described later. The DAC 13 converts digital signals output from the signal-processing circuit 12 into analog signals. The driver 14 amplifies the signals from the DAC 13 to generate drive signals and drives the optical modulator 17 by using the drive signals. The optical modulator 17 modulates the output light from the light source 15 by using the drive signals that carry transmission information and outputs the modulated light to the optical transmission path 25 as an optical signal. The carrier-frequency control circuit 11 outputs control signals for controlling the carrier frequency of the optical signal output from the optical modulator 17.
The control signals include a frequency control amount Δf representing displacement of the carrier frequency from a design value. The frequency control amount Δf may be detected by monitoring part of the light output from the optical modulator 17 and observing displacement of the center frequency. Alternatively, the frequency control amount Δf may be determined based on a quality detection result of the BER, the S/N ratio, or the like obtained at the receiver end. The frequency control amount Δf is supplied to both the mapping selection adjustment circuit 121 and the phase rotation circuit 122 in the signal-processing circuit 12.
Referring back to
In mapping pattern 1 or 2, all symbol points are arranged in the inscribed circle of the dynamic range upper limit, with the inter-symbol distance being maintained to be the largest. Thus, even when the phase rotation circuit 122 applies phase rotation, it is possible to inhibit rounding into the dynamic range and it is possible to maintain the quality of transmission. Mapping pattern 1 or 2 may be pre-stored in the memory 123 in association with each modulation format, or mapping adjustment (symbol position adjustment) may be performed in real time by using a computational operation function of the signal-processing circuit 12.
The phase rotation circuit 122 applies phase rotation, given by θ=2πΔft, to the electric field phase of the symbol points. When Δf is zero, the phase rotation is not applied, and electric-field information of signal points determined by the original constellation mapping is output to the DAC 13. When Δf is not zero, the phase rotation circuit 122 applies, at a certain cycle, phase rotation to the electric-field information of symbol points arranged using mapping pattern 1 or 2.
This pre-compensates for displacement of the carrier frequency and phase rotation that occurs on a transmission path. Even when any of mapping patterns 1 and 2 is used, the S/N ratio can be improved, since the inter-symbol distances are the largest and are maintained to be an equal distance. Also, since all symbol points lie in the inscribed circle of the dynamic range upper limit, it is possible to inhibit a decrease in the detection accuracy of symbol points and deterioration of an error rate.
Although the configuration of the optical receiver 20 is not illustrated, a front-end circuit receives an optical signal and converts the optical signal into electrical signals, and an analog-to-digital conversion is performed to convert the electrical signals into digital signals. An error rate detected during error correction in digital signal processing may be transmitted to the optical transmitter 10 through the optical transmission path 25 and be used by the carrier-frequency control circuit 11.
If the frequency control amount Δf is zero (NO in S303), the mapping selection adjustment circuit 121 outputs, to the phase rotation circuit 122, electric-field information of symbol positions resulting from the original mapping. Upon receiving the electric-field information, the phase rotation circuit 122 directly outputs the electric-field information without applying phase rotation thereto (S306). If the frequency control amount Δf is not zero (YES in S303), the mapping selection adjustment circuit 121 adjusts the symbol positions (electric-field information) of the mapped symbol points, as illustrated in mapping pattern 1 or 2, and outputs values after the adjustment to the phase rotation circuit 122 (S304).
In order to compensate for Δf, the phase rotation circuit 122 applies phase rotation to the electric field phase at a certain cycle (S305). In the embodiment, mapping with which influences of the phase rotation are suppressed is used, and thus, even when displacement of the carrier frequency and influences of a transmission path are compensated for in advance, constellation distortion is inhibited while maintaining the inter-symbol distance. It is possible to improve the quality of transmission, while maintaining the efficiency of using a frequency band.
According to the method in Japanese Laid-open Patent Publication No. 2014-007642, the inter-symbol distance on the outer circumference is 0.505, the inter-symbol distance on the inner circumference is 0.475, and the inter-symbol distance between the inner circumference and the outer circumference is 0.527. The smallest inter-symbol distance is 0.475.
In contrast, in the mapping methods in the first and second embodiments, the inter-symbol distances are set to an equal distance, and the inter-symbol distance is larger than the smallest inter-symbol distance in the related scheme. This configuration improves the S/N ratio and can increase the transmission distance.
Each optical transmitter 10 adjusts the mapping positions of transmission signal in accordance with the frequency control amount Δf and outputs an optical signal based on electric-field information obtained by applying phase rotation at the adjusted mapping positions.
The mapping selection adjustment circuit 121 in each optical transmitter 10 performs mapping adjustment that maximizes the inter-symbol distance, and thus, even when phase rotation is applied, it is possible to inhibit constellation distortion and it is possible to maintain the S/N ratio at a favorable value.
The optical signals output from the optical transmitters 10-1 to 10-n are multiplexed by the optical multiplexer 40. In this case, the carrier-frequency control circuits 11 in the individual optical transmitters 10 output different frequency control amounts Δf1 to Δfn, so that wavelength multiplexing by which a plurality of optical signals having different center frequencies is multiplexed at a high density is realized using the same type of light source 15. The carrier-frequency control circuit 11 controls the frequency of each carrier wave, and the mapping selection adjustment circuit 121 adjusts the mapping positions so that constellation distortion does not occur.
Thus, in the case of wavelength multiplexing, it is possible to improve the quality of transmission, while maintaining the efficiency of using the frequency band through reduction of the frequency band occupied by each carrier wave. Although the preferred embodiments of the present disclosure have been described above, the present disclosure is not limited to the above-described embodiments, and various modifications are possible thereto. For example, the present disclosure is also applicable to optical orthogonal frequency division multiplexing (optical OFDM) by which a plurality of sub carriers is densely arranged in one optical signal band.
All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.
Number | Date | Country | Kind |
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2016-037400 | Feb 2016 | JP | national |