A space laser communication polarization diversity receiver and method resistant to atmospheric turbulence

Abstract

This invention relates to the field of space laser communication technology, specifically to a space laser communication anti-atmospheric turbulence polarization diversity receiving device and method. The space laser communication anti-atmospheric turbulence polarization diversity receiving device includes an optical receiving unit, a signal sensing unit, and a dynamic adaptive synthesis unit. The optical receiving unit is used to split the received space optical signal into two orthogonally polarized signals and to compensate in real time for random changes in polarization state caused by atmospheric turbulence. The signal sensing unit is used to collect instantaneous power and output real-time quality parameters. The dynamic adaptive synthesis unit is used to send a feedback signal to the optical receiving unit and to maximize the signal-to-noise ratio of the synthesized signal through calculation. This invention solves the problems of polarization state fluctuations and deep signal fading caused by atmospheric turbulence in space laser signals, ensuring high-performance reception, effectively improving communication efficiency, and significantly enhancing the stability of the space laser communication system.

Description

Technical Field

[0001] This invention relates to the field of space laser communication technology, specifically to a space laser communication anti-atmospheric turbulence polarization diversity receiving device and method. Background Technology

[0002] In space laser communication scenarios such as satellite-to-ground links, space laser signals are severely affected by atmospheric turbulence when passing through the atmosphere. Atmospheric turbulence not only causes light intensity flicker and fading but also induces random changes in the signal polarization state, disrupting the spatial coherence of the beam. Traditional single-channel receivers are prone to communication interruptions when facing deep fading. Although polarization diversity techniques can improve reliability by utilizing the complementarity of two orthogonally polarized signals, current solutions often face challenges such as uneven energy distribution due to polarization mismatch and low synthesis efficiency when there are significant differences in signal quality.

[0003] In summary, there is an urgent need in the existing technology for a device that can solve the problems of polarization state fluctuations and deep signal fading caused by atmospheric turbulence, and significantly improve the stability of space laser communication systems. This device can effectively improve communication efficiency and ensure high-performance reception of space laser signals. Summary of the Invention

[0004] Therefore, the technical problem to be solved by the present invention is to overcome the above-mentioned defects in the prior art, thereby providing an anti-atmospheric turbulence polarization diversity receiving device for space laser communication.

[0005] This invention discloses an atmospheric turbulence-resistant polarization diversity receiver for space laser communication, comprising: The optical receiving unit is used to split the received space laser signal into two orthogonally polarized signals and adjust the polarization state of the space laser signal based on the feedback signal in order to compensate for the random changes in polarization state caused by atmospheric turbulence in real time. The signal sensing unit is used to acquire the instantaneous power of two orthogonal polarization signals from the output of the optical receiving unit, calculate the signal-to-noise ratio of the two orthogonal polarization signals through the instantaneous power, and output real-time quality parameters including instantaneous power and signal-to-noise ratio. The dynamic adaptive synthesis unit is used to receive real-time quality parameters output from the signal sensing unit, dynamically calculate and adjust the weighting factor based on the signal-to-noise ratio of the two orthogonally polarized signals, and maximize the output signal-to-noise ratio of the synthesized signal according to the maximum ratio combining algorithm, so that the combining gain is kept at the optimal level under the current conditions during the deep fading caused by turbulence, thereby optimizing the synthesized signal and outputting it. The dynamic adaptive synthesis unit is also used to output a feedback signal based on the instantaneous power processed to the optical receiving unit.

[0006] Furthermore, the optical receiving unit includes an optical antenna and a collimating lens. The optical antenna is used to capture space laser signals, and the collimating lens is used to process the captured space laser signals into a parallel beam.

[0007] Furthermore, the optical receiving unit also includes a polarization controller and a polarization beam splitter. The polarization controller is a piezoelectric ceramic-based polarization controller used to adjust the driving voltage applied to the piezoelectric ceramic according to the feedback signal sent by the dynamic adaptive synthesis unit to change the fiber stress or spatial phase delay, ensuring that the spatial laser signal is always aligned with the optical axis of the polarization beam splitter, and realizing real-time compensation for random changes in polarization state caused by atmospheric turbulence.

[0008] Furthermore, the optical receiving unit also includes two photodetectors, which are used to receive two orthogonally polarized signals respectively and convert the optical signals into electrical signals.

[0009] Furthermore, the signal sensing unit includes a sampling and power detection module and a signal-to-noise ratio (SNR) calculation module. The sampling and power detection module is used to collect the instantaneous power of the two orthogonally polarized signals from the output of the optical receiving unit and send them to the SNR calculation module. The SNR calculation module calculates the SNR of the two orthogonally polarized signals and outputs real-time quality parameters containing instantaneous power and SNR to the dynamic adaptive synthesis unit. The SNR calculation module calculates the SNR of the two orthogonally polarized signals in real time based on the instantaneous power collected by the sampling and power detection module using a blind estimation algorithm.

[0010] Furthermore, the dynamic adaptive synthesis unit includes a digital signal processing module and a weighted synthesizer module. The digital signal processing module receives real-time quality parameters output from the signal-to-noise ratio calculation module, dynamically calculates and adjusts the weighting factors based on the signal-to-noise ratio, and sends them to the weighted synthesizer module.

[0011] Furthermore, the weighted synthesizer module receives the weighting factors calculated by the digital signal processing module, maximizes the signal-to-noise ratio of the synthesized signal according to the maximum ratio combining algorithm, maintains the optimal level of the combining gain under the current conditions during the deep fading caused by turbulence, and outputs the optimized synthesized signal.

[0012] Furthermore, the dynamic adaptive synthesis unit is based on the digital signal processing module to perform low-pass filtering on the received instantaneous power and take the average value to obtain power change information, and then sends the obtained power change information as a feedback signal to the optical receiving unit.

[0013] This invention also discloses a polarization diversity reception method for space laser communication resistant to atmospheric turbulence. The method is based on a polarization diversity reception device for space laser communication resistant to atmospheric turbulence, and includes: S1. The optical antenna captures the space laser signal and processes it into a parallel beam through a collimating lens. The polarization controller adjusts the polarization state of the space laser signal based on the received feedback signal to compensate for random changes in polarization state caused by atmospheric turbulence in real time, ensuring that the space laser signal is always aligned with the optical axis of the polarization beam splitter. The space laser signal is split into two orthogonally polarized signals by the polarization beam splitter and coupled to two photodetectors respectively to convert the optical signal into an electrical signal. S2. The sampling and power detection module collects the instantaneous power of the two orthogonal polarization signals and sends them to the signal-to-noise ratio (SNR) calculation module. The SNR calculation module calculates the SNR using a blind estimation algorithm and sends the SNR and instantaneous power to the digital signal processing module. S3. The digital signal processing module performs low-pass filtering on the received instantaneous power and takes the average value, and sends the power change information obtained after filtering as a feedback signal to the polarization controller. S4. The digital signal processing module determines whether the received signal-to-noise ratio is below the deep fading threshold and dynamically calculates the weighting factor. Then, based on the weighting factor, the weighted synthesizer module uses the maximum ratio combining algorithm to maximize the output signal-to-noise ratio of the synthesized signal, so that the combining gain remains at the optimal level under the current conditions during the deep fading caused by turbulence.

[0014] Furthermore, the method for determining whether the signal-to-noise ratio is lower than the deep fading threshold is as follows: when the signal-to-noise ratios of both channels are not lower than the preset deep fading threshold, the weighting factor is dynamically calculated; when the signal-to-noise ratio of either channel is lower than the preset deep fading threshold, the selection merging mode is switched, the weighting factor corresponding to the channel with the higher signal-to-noise ratio is set to 1, and the weighting factor corresponding to the other channel is set to 0.

[0015] Compared with the prior art, the technical solution of this invention has the following advantages: 1. This invention provides a polarization diversity receiving device and method for space laser communication that is resistant to atmospheric turbulence. It can precisely adjust optical components through a feedback control loop to acquire space laser signals severely affected by atmospheric turbulence, ensuring high-performance reception, effectively improving communication efficiency, and significantly enhancing the stability of the space laser communication system. The device calculates the real-time signal-to-noise ratio (SNR) using a blind estimation algorithm, dynamically calculates and adjusts weighting factors based on real-time quality parameters, and maximizes the SNR of the synthesized signal through a maximum ratio combining algorithm. This solves the problems of polarization state fluctuations and deep signal fading caused by atmospheric turbulence in space laser signals, ensuring that the combining gain remains at the optimal level under current conditions during deep fading caused by turbulence.

[0016] 2. The optical receiving unit provided by the present invention can split the received space laser signal into two orthogonally polarized signals and send them to a photodetector. It can also realize real-time compensation for the random polarization changes caused by atmospheric turbulence based on the feedback signal, thus ensuring high-performance reception of space laser signals.

[0017] 3. The signal sensing unit and dynamic adaptive synthesis unit provided by this invention can accurately acquire front-end signal quality information and feed back real-time quality parameters to the optical receiving unit for adjusting the polarization controller to ensure that the spatial laser signal and the polarization beam splitter maintain optical axis alignment. A blind estimation algorithm is used to calculate the signal-to-noise ratio of the two orthogonally polarized signals, eliminating the need for additional auxiliary training sequences and effectively improving communication efficiency. Furthermore, the maximum ratio combining algorithm and dynamic weighting strategy ensure that the combining gain remains optimal under various turbulence intensities, significantly reducing the bit error rate and ensuring overall system stability. Attached Figure Description

[0018] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0019] Figure 1 This is a schematic diagram of the structure of a space laser communication anti-atmospheric turbulence polarization diversity receiving device and method according to the present invention; Figure 2 This is a flowchart of the blind estimation algorithm for the signal sensing unit of the present invention; Figure 3 This is a flowchart of the maximum ratio merging algorithm for the dynamic adaptive synthesis unit of the present invention. Detailed Implementation

[0020] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0022] Example 1 like Figure 1-3 As shown, this embodiment provides a space laser communication polarization diversity receiving device resistant to atmospheric turbulence, including: an optical receiving unit, a signal sensing unit, and a dynamic adaptive synthesis unit.

[0023] The optical receiving unit, arranged sequentially along the beam propagation direction, comprises an optical antenna, a collimating lens, a piezoelectric ceramic-based polarization controller, a polarization beam splitter, and two photodetectors. The optical components are modularly packaged and pre-aligned using a high-precision adjustment stage. After receiving the space laser signal affected by atmospheric turbulence, the optical antenna collimates the signal through the collimating lens, processing it into a parallel beam. The polarization controller adjusts the driving voltage applied to the piezoelectric ceramic based on feedback signals from the backend to change fiber stress or spatial phase delay, adjusting the polarization axis in real time to ensure the space laser signal remains aligned with the polarization beam splitter's optical axis. The polarization beam splitter splits the space laser signal into two orthogonally polarized signals, which are then coupled to the two photodetectors, ensuring high-performance reception of the space laser signal.

[0024] The signal sensing unit includes a sampling and power detection module and a signal-to-noise ratio (SNR) calculation module. The signal sensing unit is connected to the optical receiving unit. The sampling and power detection module monitors the instantaneous power at the output of the photodetector in real time. This module is also connected to the SNR calculation module, sending the collected data to it. The SNR calculation module uses a blind estimation algorithm to calculate the SNR of the two orthogonally polarized signals in real time and outputs real-time quality parameters, including the instantaneous power of the two orthogonally polarized signals and the calculated SNR of each signal.

[0025] The dynamic adaptive synthesis unit includes a digital signal processing module and a weighted synthesizer module. The dynamic adaptive synthesis unit is connected to the optical receiving unit. It uses the power change information after low-pass filtering by the digital signal processing module as the basis for polarization compensation and sends a feedback signal to the polarization controller through the digital signal processing module.

[0026] The dynamic adaptive synthesis unit is also connected to the signal sensing unit. The digital signal processing module dynamically calculates the signal-to-noise ratio (SNR) of the two orthogonally polarized signals based on the SNR calculation module's blind estimation algorithm and adjusts the corresponding weighting factors. The weighted synthesizer module is connected to the digital signal processing module. Following the maximum ratio combining algorithm, the weighted synthesizer module synthesizes the output signal, ensuring that even when one of the signals encounters deep turbulence fading, the synthesized signal's SNR remains maximized, significantly reducing the bit error rate and ensuring overall system stability.

[0027] In practice, space laser signals are affected by atmospheric turbulence. The space laser signal is captured by an optical antenna, collimated by a collimating lens, and processed into a parallel beam. This parallel beam then enters a piezoelectric ceramic-based polarization controller. The system employs a feedback control loop, using signal power changes fed back from the dynamic adaptive synthesis unit as the basis for polarization compensation to adjust the polarization axis in real time. The driving voltage applied to the piezoelectric ceramic is adjusted to change fiber stress or spatial phase delay, ensuring the space laser signal remains aligned with the optical axis of the polarization beamsplitter. After being split by the polarization beamsplitter, the beams enter two photodetectors, where the optical signals are converted into electrical signals.

[0028] The sampling and power detection module monitors the instantaneous power at the output terminals of the two photodetectors in real time. , And send it to the signal-to-noise ratio calculation module. For example... Figure 2 As shown, the signal-to-noise ratio (SNR) calculation module uses a blind estimation algorithm to calculate the SNR of two orthogonally polarized signals in real time. , The calculation process is as follows: The sampling and power detection module inputs a single sampling signal. The signal-to-noise ratio (SNR) calculation module preprocesses the received signal, removing DC bias and performing bandwidth filtering. After preprocessing, it calculates the statistical moments and second-order moments. and fourth moment The signal power is obtained by decomposing the system of equations. and noise power Determine if the condition is met. If the requirements are not met, the valid value from the previous time step is selected; if the requirements are met, the signal-to-noise ratio is calculated. Smooth the filter and output the result.

[0029] The digital signal processing module senses the signal quality information output by the signal-to-noise ratio calculation module, including instantaneous power. , The signal-to-noise ratio of the two orthogonally polarized signals , The digital signal processing module performs low-pass filtering on the instantaneous power, filtering out high-frequency fluctuations and averaging the results. The obtained power change information is used as the feedback signal to adjust the polarization controller. Simultaneously, as... Figure 3 As shown, the digital signal processing module determines the signal-to-noise ratio based on the signal-to-noise ratio. , Dynamically calculate the corresponding weight factors , The output signal is synthesized according to the maximum ratio merging algorithm. The specific calculation process is as follows: The signal-to-noise ratio calculation module takes two orthogonally polarized signals as input. , . judge , Whether it is below the deep fading threshold is determined based on the signal-to-noise ratio (SNR) of the two orthogonally polarized signals. If the SNR is less than 5 dB, it is considered below the deep fading threshold. , If any channel falls below the deep fading threshold, the system will switch to selective merging mode, which will then merge the instantaneous signals. A relatively high one is defined as the advantage weight factor. A relatively low one is defined as the disadvantage weighting factor. The weighting ratio is .like , If both conditions are met, and the depth fading threshold is not lower than the maximum ratio threshold, then the maximum ratio merging calculation is performed. ; Then based on weighting factors , Calculate the signal. ; Finally, the optimized synthesized signal is output. The calculation process dynamically adjusts the calculation mode to ensure that even when one of the signals encounters deep fading due to turbulence, the signal-to-noise ratio of the synthesized signal can still be maximized, thereby achieving high-performance reception.

[0030] Example 2 This invention also provides a space laser communication polarization diversity receiving method resistant to atmospheric turbulence. The method is based on a space laser communication polarization diversity receiving device resistant to atmospheric turbulence described in Example 1, and includes the following steps: S1. The optical antenna captures the space laser signal and processes it into a parallel beam through a collimating lens. The polarization controller adjusts the polarization state of the space laser signal based on the received feedback signal to compensate for random changes in polarization state caused by atmospheric turbulence in real time, ensuring that the space laser signal is always aligned with the optical axis of the polarization beam splitter. The space laser signal is split into two orthogonally polarized signals by the polarization beam splitter and coupled to two photodetectors respectively to convert the optical signal into an electrical signal. S2. The sampling and power detection module collects the instantaneous power of the two orthogonal polarization signals and sends them to the signal-to-noise ratio (SNR) calculation module. The SNR calculation module calculates the SNR using a blind estimation algorithm and sends the SNR and instantaneous power to the digital signal processing module. S3. The digital signal processing module performs low-pass filtering on the received instantaneous power and takes the average value, and sends the power change information obtained after filtering as a feedback signal to the polarization controller. S4. The digital signal processing module determines whether the signal-to-noise ratios (SNRs) of the two channels are lower than a preset deep fading threshold based on the received SNR. When both SNRs are not lower than the preset deep fading threshold, the weighting factor is dynamically calculated. When either SNR is lower than the preset deep fading threshold, the module switches to the selection combining mode, sets the weighting factor corresponding to the higher SNR of the two channels to 1, and sets the weighting factor corresponding to the other channel to 0. The weighted synthesizer module then combines the weighting factor and uses the maximum ratio combining algorithm to maximize the output SNR of the synthesized signal, ensuring that the combining gain remains at the optimal level under current conditions during deep fading caused by turbulence.

[0031] An embodiment of the space laser communication anti-atmospheric turbulence polarization diversity receiving method of the present invention can be referred to the embodiment section of the space laser communication anti-atmospheric turbulence polarization diversity receiving device. Specific implementation methods can be referred to the descriptions of the corresponding embodiments, which will not be repeated here.

[0032] This invention solves the problems of polarization state fluctuation and deep signal fading caused by atmospheric turbulence in space laser signals, significantly improves the stability of space laser communication systems, effectively enhances communication efficiency, and ensures high-performance reception.

[0033] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A space laser communication polarization diversity receiver resistant to atmospheric turbulence, characterized in that, include: The optical receiving unit is used to split the received space laser signal into two orthogonally polarized signals and adjust the polarization state of the space laser signal based on the feedback signal in order to compensate for the random changes in polarization state caused by atmospheric turbulence in real time. The signal sensing unit is used to acquire the instantaneous power of two orthogonal polarization signals from the output of the optical receiving unit, calculate the signal-to-noise ratio of the two orthogonal polarization signals through the instantaneous power, and output real-time quality parameters including the instantaneous power and the signal-to-noise ratio. The dynamic adaptive synthesis unit is used to receive the real-time quality parameters output from the signal sensing unit, dynamically calculate and adjust the weighting factor based on the signal-to-noise ratio of the two orthogonal polarization signals, and maximize the output signal-to-noise ratio of the synthesized signal according to the maximum ratio combining algorithm, so that the combining gain is kept at the optimal level under the current conditions during the deep fading caused by turbulence, thereby optimizing the synthesized signal and outputting it. The dynamic adaptive synthesis unit is also used to output the feedback signal obtained by processing the instantaneous power to the optical receiving unit.

2. The space laser communication polarization diversity receiving device resistant to atmospheric turbulence according to claim 1, characterized in that, The optical receiving unit includes an optical antenna and a collimating lens. The optical antenna is used to capture space laser signals, and the collimating lens is used to process the captured space laser signals into a parallel beam.

3. The space laser communication polarization diversity receiving device resistant to atmospheric turbulence according to claim 1, characterized in that, The optical receiving unit also includes a polarization controller and a polarization beam splitter. The polarization controller is a piezoelectric ceramic-based polarization controller used to adjust the driving voltage applied to the piezoelectric ceramic according to the feedback signal sent by the dynamic adaptive synthesis unit to change the fiber stress or spatial phase delay, ensuring that the spatial laser signal is always aligned with the optical axis of the polarization beam splitter, and realizing real-time compensation for random changes in polarization state caused by atmospheric turbulence.

4. The space laser communication polarization diversity receiving device resistant to atmospheric turbulence according to claim 1, characterized in that, The optical receiving unit also includes two photodetectors for receiving the two orthogonally polarized signals respectively and converting the optical signals into electrical signals.

5. A space laser communication polarization diversity receiving device resistant to atmospheric turbulence according to claim 1, characterized in that, The signal sensing unit includes a sampling and power detection module and a signal-to-noise ratio (SNR) calculation module. The sampling and power detection module is used to collect the instantaneous power of two orthogonally polarized signals from the output of the optical receiving unit and send them to the SNR calculation module. The SNR calculation module calculates the SNR of the two orthogonally polarized signals and outputs real-time quality parameters containing the instantaneous power and the SNR of the two orthogonally polarized signals to the dynamic adaptive synthesis unit. The SNR calculation module calculates the SNR of the two orthogonally polarized signals in real time based on the instantaneous power collected by the sampling and power detection module using a blind estimation algorithm.

6. The space laser communication polarization diversity receiving device resistant to atmospheric turbulence according to claim 1, characterized in that, The dynamic adaptive synthesis unit includes a digital signal processing module and a weighted synthesizer module. The digital signal processing module is used to receive the real-time quality parameters output from the signal sensing unit, dynamically calculate and adjust the weighting factors based on the signal-to-noise ratio, and send them to the weighted synthesizer module.

7. A space laser communication polarization diversity receiving device resistant to atmospheric turbulence according to claim 6, characterized in that, The weighted synthesizer module receives the weighting factor calculated by the digital signal processing module, maximizes the signal-to-noise ratio of the synthesized signal according to the maximum ratio combining algorithm, maintains the optimal level of the combining gain under the current conditions during the deep fading caused by turbulence, and outputs the optimized synthesized signal.

8. A space laser communication polarization diversity receiving device resistant to atmospheric turbulence according to claim 6, characterized in that, The dynamic adaptive synthesis unit is based on the digital signal processing module performing low-pass filtering on the received instantaneous power and taking the average value to obtain power change information, and then sending the obtained power change information as a feedback signal to the optical receiving unit.

9. A method for anti-atmospheric turbulence polarization diversity reception in space laser communication, said method being implemented based on the anti-atmospheric turbulence polarization diversity reception device for space laser communication according to any one of claims 1-8, characterized in that, include: S1. The optical antenna captures the space laser signal, and the collimating lens processes the space laser signal into a parallel beam. The polarization controller adjusts the polarization state of the space laser signal through the received feedback signal to compensate for random changes in polarization state caused by atmospheric turbulence in real time, ensuring that the space laser signal is always aligned with the optical axis of the polarization beam splitter. The space laser signal is split into two orthogonally polarized signals by the polarization beam splitter and coupled to two photodetectors respectively to convert the optical signal into an electrical signal. S2. The sampling and power detection module collects the instantaneous power of the two orthogonally polarized signals and sends them to the signal-to-noise ratio (SNR) calculation module. The SNR calculation module calculates the SNR using a blind estimation algorithm and sends the SNR and the instantaneous power to the digital signal processing module. S3. The digital signal processing module performs low-pass filtering on the received instantaneous power and takes the average value, and sends the power change information obtained after filtering as a feedback signal to the polarization controller. S4. The digital signal processing module determines whether the signal-to-noise ratios of the two channels are lower than the deep fading threshold based on the received signal-to-noise ratio, dynamically calculates the weighting factor, and then, based on the weighting factor, uses the weighted synthesizer module to maximize the output signal-to-noise ratio of the synthesized signal according to the maximum ratio combining algorithm, so that the combining gain remains at the optimal level under the current conditions during the deep fading caused by turbulence.

10. A space laser communication polarization diversity reception method resistant to atmospheric turbulence according to claim 9, characterized in that, The method for determining whether the signal-to-noise ratio is lower than the deep fading threshold is as follows: when the signal-to-noise ratios of both channels are not lower than the preset deep fading threshold, the weighting factor is dynamically calculated; when the signal-to-noise ratio of any channel is lower than the preset deep fading threshold, the selection merging mode is switched, and the weighting factor corresponding to the channel with the higher signal-to-noise ratio is set to 1, and the weighting factor corresponding to the other channel is set to 0.

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