JP5871031B2 - Receiver - Google Patents

Description

  The present invention relates to a receiver that performs coherent reception in an optical access network, for example.

  In recent years, communication demand has been rapidly increasing due to the spread of the Internet and the like. Correspondingly, a high-speed and large-capacity optical access network using an optical fiber or the like is being developed. In such an optical access network, a multiplex transmission technique is indispensable for realizing high speed and large capacity. As multiplex transmission technologies, optical time division multiplexing (OTDM) technology and wavelength division multiplexing (WDM) technology are being put into practical use, and optical code division multiplexing (OCDM) optical code division multiplexing (OCDM). ) Technology research is also actively conducted.

  Services have been diversified in response to the development and expansion of optical access networks. Since the conditions required for the optical access network differ for each service, the optical access network is often constructed for each service. For this reason, existing and new optical access networks providing different services are mixed. As a result, the introduction cost and management cost of the optical access network for dealing with the increased service increase.

  Under such circumstances, there is a need for an optical access network that can easily add and delete services and that can efficiently aggregate requests to optical access networks that differ from service to service.

  As a technique for realizing such an optical access network, coherent optical OFDM (CO-OFDM: Coherent Optical) in which Orthogonal Frequency Division Multiplexing (OFDM), which is widely used in wireless communication, is applied to optical fiber transmission. -OFDM) is drawing attention.

  OFDM is a multicarrier transmission technique in which a plurality of orthogonal carriers are digitally modulated and multiplexed. As a method of digital modulation, for example, a limited bandwidth can be maximized by combining with multi-level modulation techniques such as quadrature phase shift keying (QPSK) and quadrature amplitude modulation (QAM). It can be used as much as possible. In other words, the bandwidth required to transmit the same amount of information can be minimized, and services can be easily added or deleted. Furthermore, since transmission capacity and bandwidth can be allocated for each service, different services can be efficiently aggregated.

  Now, a conventional transmitter and receiver used in the CO-OFDM system will be described with reference to FIG.

  In the transmitter 150, the transmission signal S49 is digitally modulated by a digital signal processor (DSP) 52 and then converted into an analog signal by a digital-to-analog converter (DAC) 54. The Transmission light generated by a laser diode (LD) that is the transmission light source 60 is modulated by the modulator 56 based on this analog signal, and is transmitted to the transmission line as OFDM signal light S63. The OFDM signal light S69 received by the receiver 170 interferes with the local oscillation (LO) light from the local oscillation (LO) light source 80 in the coherent reception unit 76, and then the analog-digital converter (ADC). An analog-to-digital converter 74 converts the digital signal. A received signal S73 is obtained by demodulating the digital signal by the DSP 72. Control signals S51 and S71 are input to the DSP 52 provided in the transmitter 150 and the DSP 72 provided in the receiver 170, respectively.

JP 2010-109847 A

  In the ADC, the reception characteristics are greatly affected by how much the digitally received signal can reproduce the analog signal before digital conversion. The main characteristic parameters of ADC that contribute to digital conversion include resolution and sampling rate (SR). The resolution contributes to the strength of the signal, and the SR contributes to the time interval (or frequency interval) of the signal.

  Among these ADC characteristic parameters, SR has a greater influence on reception characteristics. In general, an ADC is required to have an SR that is twice or more the frequency of an analog signal to be converted. That is, when the frequency Fsig of the analog signal to be converted is 1 GHz, SR of 2 Gsps (samples per second) or higher is required, and when Fsig is 2 GHz, SR of 4 Gsps or higher is required.

  As described above, in order to realize a large capacity / high-speed transmission, a higher-speed signal is a conversion target, and thus an ADC having a higher-speed SR is required.

  However, as an ADC that is currently on the market and is relatively easily available, the 3.6 Gsps SR is the fastest as far as the inventors know. There are ADCs having SRs of several tens of Gsps, but since they are large in size and very expensive, it is not practical to incorporate them into a receiver of an optical access network.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a receiver that enables digital conversion of a higher-speed signal without increasing the device size and cost. There is.

  In order to achieve the above-described object, a receiver used in an optical access network includes a local oscillation light source, a coherent reception unit that interferes with signal light and local oscillation light, and generates an electrical signal. And an AD conversion unit for digital conversion. The AD conversion unit includes a first ADC and a second ADC having different sampling rates in parallel. The analog signal input to the AD conversion unit is branched into two, and analog-to-digital conversion is performed by the first ADC and the second ADC, respectively. Thereafter, they are added and output as a digital signal.

  According to the receiver of the present invention, a signal having an arbitrary frequency can be converted into a digital signal that does not affect the reception characteristics even when an ADC having a sampling rate lower than that normally required is used. As a result, higher-speed AD conversion can be realized without causing an increase in cost such as the use of an ADC having a high-speed sampling rate.

It is a schematic diagram for demonstrating a receiver. It is a figure which shows the measurement data in this Example. It is a figure which shows the correlation coefficient and the relationship between EVM and OSR. It is a schematic diagram for demonstrating the conventional transmitter and receiver used with a CO-OFDM system.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the shape, size, and arrangement relationship of each component are merely schematically shown to the extent that the present invention can be understood. In the following, a preferred configuration example of the present invention will be described. However, the material and numerical conditions of each component are merely preferred examples. Therefore, the present invention is not limited to the following embodiments, and many changes or modifications that can achieve the effects of the present invention can be made without departing from the scope of the configuration of the present invention.

  A CO-OFDM system receiver will be described with reference to FIG. FIG. 1 is a schematic diagram of a receiver. Usually, a terminal that performs communication such as a station side device (OLT: Optical Line Terminal) and a subscriber side device (ONU: Optical Network Unit) is provided with both a transmitter and a receiver. Since the transmitter can have the same configuration as that of the prior art described with reference to FIG. 4, the description thereof is omitted here.

  The receiver 70 includes a DSP 72, an AD conversion unit 73, an LO light source 80, and a coherent reception unit 76. The LO light generated by the LO light source 80 is sent to the coherent receiver 76. The coherent receiving unit 76 causes the OFDM signal light S69 received by the receiver 70 to interfere with the LO light. The OFDM signal light S69 interferes with the LO light and is then converted into a digital signal by the AD conversion unit 73. A received signal S73 is obtained by demodulating the digital signal by the DSP 72.

  The control signal S71 is input to the DSP 72 included in the receiver 70.

  The AD conversion unit 73 includes a first ADC 75-1 and a second ADC 75-2 in parallel. The analog signal input to the AD conversion unit 73 is branched into two and one is sent to the first ADC 75-1, and the other is sent to the second ADC 75-2. The first digital signal obtained by AD conversion by the first ADC 75-1 and the second digital signal obtained by AD conversion by the second ADC 75-2 are added and output from the AD conversion unit 73.

  Thus, in this configuration example, two ADCs of the first ADC 75-1 and the second ADC 75-2 are used. If the sampling timings of the first ADC 75-1 and the second ADC 75-2 are shifted, the added digital signal has more sampling points than when one ADC is used. This corresponds to a pseudo increase in SR.

  Here, when the SRs of the first ADC and the second ADC are equal, the position of the sampling point does not change relatively. For this reason, it is necessary to shift the positions of the sampling points of the first ADC and the second ADC by ½ of the respective sampling periods.

  On the other hand, in this configuration example, the SRs of the first ADC 75-1 and the second ADC 75-2 are different from each other. For this reason, the relative position of the sampling point in the second ADC 75-2 with respect to the sampling point in the first ADC 75-1 is not constant. Therefore, it is possible to increase the sampling points without adjusting the time position between the first ADC 75-1 and the second ADC 75-2.

  For example, assuming that SR of the first ADC 75-1 is SR1, and SR of the second ADC 75-2 is SR2, the relative SR of the AD conversion unit 73 is given by SR = (SR1 + SR2) / 2.

  FIG. 2 is a diagram showing measurement data in this example. It is a figure which shows the conversion data when SR (SR1) of 1st ADC is 5 Gsps, and SR (SR2) of 2nd ADC is 4 sps, when Fsig is 2.8 GHz. Data at this time is indicated by a black line, and as a comparative example, data obtained by high-speed SR (= 80 Gsps) is indicated by a white line.

  Comparing the two, the correlation coefficient is about 0.9, and even when an ADC whose SR is twice or less than Fsig is used, SR is twice that of Fsig by using two ADCs in parallel. A result close to the case of using the above ADC is obtained.

  FIG. 3 is a diagram illustrating a correlation coefficient, a relationship between an EVM (Error Vector Magnet) and an OSR (Over Sampling rate).

  FIG. 3A shows the OSR on the horizontal axis and the correlation coefficient on the vertical axis. OSR is given by SR / Fsig. When the first ADC and the second ADC are used in parallel as in this embodiment, SR is given by the average of SR1 and SR2 (= (SR1 + SR2) / 2). In FIG. 3A, the data when there is one ADC is indicated by a diamond, and the data when there are two ADCs is indicated by a triangle.

  Here, the SR of the ADC indicates six types of 1, 1.25, 2, 2.5, 4, and 5 Gsps when there is one ADC. The data in the case of two ADCs shows 15 kinds of data obtained by selecting two different from these six SRs.

  As shown in FIG. 3A, when using two ADCs, the value of the correlation coefficient for OSR is improved as compared with the case of using one ADC.

  In FIG. 3B, the horizontal axis indicates OSR, and the vertical axis indicates EVM. The EVM is used as a scale indicating the performance of the demodulator, and is given by the difference between the ideal signal and the actually measured signal vector.

  In FIG. 3B, similarly to FIG. 3A, the data in the case of one ADC is indicated by a diamond, and the data in the case of two ADCs is indicated by a triangle.

  As shown in FIG. 3B, when two ADCs are used, the EVM for OSR tends to improve as compared to the case where one ADC is used.

52, 72 Digital signal processor (DSP)
54 Digital-to-analog converter (DAC)
56 Modulator 60 Transmitting light source (LD)
70, 170 Receiver 73 AD converter 74, 75 Analog-digital converter (ADC)
76 Coherent receiver 80 Local oscillation (LO) light source 150 Transmitter

Claims (2)

A receiver used in an optical access network,
A local oscillation light source;
A coherent receiver for generating an electric signal by causing the signal light and the local oscillation light to interfere with each other;
An AD converter for analog-digital conversion of the electrical signal;
The AD converter includes a first ADC and a second ADC having different sampling rates in parallel,
The receiver is characterized in that the analog signal input to the AD conversion unit is branched into two, is subjected to analog-to-digital conversion by the first ADC and the second ADC, respectively, is added, and is output as a digital signal. The receiver according to claim 1, wherein a sampling rate of at least one of the first ADC and the second ADC is equal to or higher than a frequency of the analog signal.

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