CN106487446B - It is suitable for the light space-time grid code encoding method of intensity modulated/direct-detecting mode - Google Patents
It is suitable for the light space-time grid code encoding method of intensity modulated/direct-detecting mode Download PDFInfo
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- CN106487446B CN106487446B CN201610457471.4A CN201610457471A CN106487446B CN 106487446 B CN106487446 B CN 106487446B CN 201610457471 A CN201610457471 A CN 201610457471A CN 106487446 B CN106487446 B CN 106487446B
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- 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
- H04B10/11—Arrangements specific to free-space transmission, i.e. transmission through air or vacuum
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- 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
- H04B10/516—Details of coding or modulation
- H04B10/54—Intensity modulation
- H04B10/541—Digital intensity or amplitude modulation
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B7/00—Radio transmission systems, i.e. using radiation field
- H04B7/02—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
- H04B7/04—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas
- H04B7/0413—MIMO systems
Abstract
It is suitable for the light space-time grid code encoding method of intensity modulated/direct-detecting mode, using the symmetry of Inverse Fast Fourier Transforms, the PSK/QAM signals of space-time grid code encoder output are converted into be suitable for intensity modulated/direct detection (IM/DD) link transmission signal.Relative to uncoded scheme, the error performance of the program, which has, significantly to be promoted, and improves the bit rate of communication and the information flow-rate of system, reduces influence of the atmosphere random channel to radio optical communication system.
Description
Technical field
The design method of grid coding when wireless light communication multiple-input and multiple-output (MIMO) system is hollow, using luminous intensity tune
System realizes quadrature amplitude modulation (QAM) or the method for phase keying (PSK), belongs to wireless light communication (FSO) technical field.
Background technology
There is high broadband, large capacity, low cost, be not necessarily to frequency license, is the structure integration of three networks, solves " last
Kilometer " bottleneck, the effective means for realizing whole world high speed and real-time Communication for Power.Optical signal can be lost, be declined when transmitting in an atmosphere
Subtract, the various interference such as background radiation and atmospheric turbulance.Wherein, influence of the atmospheric turbulance to optical signal is the most serious, especially exists
The decline more than 30dB can be reached in the case of strong turbulence, cause optical signal heavily disturbed or even miss the target, seriously affect light
The stability and reliability of communication system.
Light MIMO technology can not only effectively inhibit turbulence effect, and can also greatly improve the biography of existing communication system
Defeated rate reduces requirement of the system to tracking accuracy.Space Time Coding is a kind of effective method for realizing MIMO technology,
It is broadly divided into Space-Time Block Coding, hierarchical space-time code, space-time grid code etc..Wherein, space-time grid code can be in not sacrificial system band
Full marks diversity gain and high coding gain are obtained in the case of width, and transmission quality can be improved.It can reach encoding and decoding complexity, property
Optimal compromise between energy and band efficiency, and all there is preferable performance under various channel circumstances, particularly suitable for
Atmospheric turbulence channels.
The thought of Space Time Coding is communicated originating from radio frequency (RF), although there are one between radio communication and wireless laser communication
Fixed similitude, but the two also has larger difference.It is mainly manifested in：By external environment when 1. laser is propagated in an atmosphere
It influences increasingly complex；2. the noise mechanism influenced in two kinds of communication modes is not exactly the same；3. transmitting used tune when signal
Method processed is different；4. the signal form transmitted, meaning and representation method differ.Above-mentioned reason causes to compile when radio frequency is hollow
The method and theory of code cannot be indiscriminately imitated directly in wireless laser communication.Due in radio frequency space-time grid coding exist plural number and
Negative, these values can not be indicated directly using intensity, therefore new derivation and analysis need to be carried out to Space Time Coding theory, be proposed suitable
The light space-time grid encoding scheme of direct detection mode is used using intensity modulated and receiving terminal together in transmitting terminal.In view of quick
Plural number or negative signal can be converted to real number signal by inverse Fourier transform (IFFT) when meeting ell bit symmetry.
Invention content
The object of the present invention is to provide a kind of light space-time grid code coding staffs being suitable for intensity modulated/direct-detecting mode
Method.
The present invention is suitable for the light space-time grid that transmitting terminal uses direct detection mode using intensity modulated and receiving terminal
Code encoding method, target is that the PSK of space-time grid code encoder output or QAM are converted into a positive real number signal, to solve
The problem of complex signal for original space-time grid code encoder output of having determined can not be transmitted directly in optical communication link；
Receiving terminal uses opposite process, by real number to the conversion of plural number, obtains being input to space-time grid code decoder
Signal, be finally completed decoding；Its step is：
(1) binary bit stream is input in space-time grid code coder, and QAM or psk signal are generated after coding；Then will
The signal is sent in the serial-parallel converter that a size is N-1, realizes that serial to parallel conversion, wherein N indicate fast Fourier transforma
The points changed；The signal of serial-parallel converter output is carried out N-1 points again to map to the vector of 2N points, the signal after making it map is full
Sufficient hermitian symmetry；
(2) inverse fast Fourier transform that 2N points are carried out to the signal after mapping, obtains bipolar real number signal.The letter
Number after parallel serial conversion and analog-to-digital conversion, a DC component is added；The direct current component value being added has to be larger than or equal to most
The amplitude of big negative signal, to eliminate negative signal.Then, optical signal is converted the electrical signal to by light intensity modulator, led to
Optical antenna is crossed to launch；
(3) optical signal received is converted to electric signal using photodetector, then removes DC component by receiving terminal
Afterwards, a parallel bipolar signal is obtained after analog-to-digital conversion and 2N point serial to parallel conversion, then by the Fast Fourier Transform (FFT) of 2N points,
It can be changed into a plural number and negative signal.By way of inverse mapping, the 2nd letter in 2N signal of FFT outputs is extracted
Number the signal after inverse mapping is sent in space-time grid code decoder then through parallel serial conversion to n-th signal；
(4) space-time grid code decoding uses the Viterbi decoding algorithm based on maximum likelihood, grid chart when using coding
To calculate path metric.By selected on grid chart unique survivor path from initial time to finish time as it is maximum seemingly
Right path.It is carved into the maximum that the corresponding state of maximum likelihood path that finish time is passed by is exactly encoder output from the outset
Likelihood coding sequence.
Usefulness of the present invention is：Using inverse fast Fourier transform symmetry and DC component is added in the signal
Space-time grid coding method meet the transmission in signal wireless light communication channel.It is effective to resist turbulence effect, improve and lead to
The transmission rate of letter system.
Description of the drawings
Fig. 1 radio optical communication system structures, the principle of the light space-time grid coding of Fig. 2 intensity modulateds, Fig. 3 space-time grids are compiled
Code device, the signal adapter of Fig. 4 plural numbers (negative) to positive real number, Fig. 5 positive real numbers to plural number or the signal adapter of negative, Fig. 6
The error rate of system of the method for the present invention is with signal-to-noise ratio change curve, the S.I.=0.6 4state-QPSK space-time grid code bit error rates,
The error rate of system of Fig. 6 the method for the present invention is with signal-to-noise ratio change curve, S.I.=1 4state-QPSK space-time grid code error codes
Rate.
Specific implementation mode
The present invention is suitable for the light space-time grid that transmitting terminal uses direct detection mode using intensity modulated and receiving terminal
Code encoding method, target is that the PSK of space-time grid code encoder output or QAM are converted into a positive real number signal, to solve
The problem of complex signal for original space-time grid code encoder output of having determined can not be transmitted directly in optical communication link；Receiving terminal
The signal for being input to space-time grid code decoder is obtained, is finally completed by real number to the conversion of plural number using opposite process
Decoding；Its step is：
(1) binary bit stream is input in space-time grid code coder, and QAM or psk signal are generated after coding；Then will
The signal is sent in the serial-parallel converter that a size is N-1, realizes serial to parallel conversion；The signal that serial-parallel converter is exported again
It carries out N-1 points to map to the vector of 2N points, the signal after making it map meets hermitian symmetry；
(2) inverse fast Fourier transform that 2N points are carried out to the signal after mapping, obtains bipolar real number signal.The letter
Number after parallel serial conversion and analog-to-digital conversion, a DC component is added；The direct current component value being added has to be larger than or equal to most
The amplitude of big negative signal, to eliminate negative signal.Then, optical signal is converted the electrical signal to by light intensity modulator, led to
Optical antenna is crossed to launch；
(3) optical signal received is converted to electric signal using photodetector, then removes DC component by receiving terminal
Afterwards, a parallel bipolar signal is obtained after analog-to-digital conversion and 2N point serial to parallel conversion, then by the Fast Fourier Transform (FFT) of 2N points,
It can be changed into a plural number and negative signal.By way of inverse mapping, the 2nd letter in 2N signal of FFT outputs is extracted
Number the signal after inverse mapping is sent in space-time grid code decoder then through parallel serial conversion to n-th signal；
(4) space-time grid code decoding uses the Viterbi decoding algorithm based on maximum likelihood, grid chart when using coding
To calculate path metric.By selected on grid chart unique survivor path from initial time to finish time as it is maximum seemingly
Right path.It is carved into the maximum that the corresponding state of maximum likelihood path that finish time is passed by is exactly encoder output from the outset
Likelihood coding sequence.
The present invention is reached by following technical measures：
1, basic assumption：
Present invention assumes that channel state parameter it is known that and system have reliable synchronization mechanism, which is such system
Typical case, the particular/special requirement of non-present invention.
2, radio optical communication system structure：
Radio optical communication system structure, as shown in Figure 1.The system include optical sender, atmospheric channel, photoreceiver and from
Motion tracking sighting system (ATP).Wherein, optical sender includes signal input processing and modulation circuit, semiconductor laser
Light source and its driving power, optical emission system.Photoreceiver includes receiving optics, photodetector, signal processing, solution
Tune and output circuit.ATP servo-control systems include signal analog/digital conversion with processing, control computer and interface, signal number/
Mould is converted and processing, control corrective network, servo drive unit, feedback control strategies and servo motor group.
3, wireless optical space-time grid code scheme
The present invention using inverse fast Fourier transform symmetry and plus the method for DC component propose one kind and be suitable for
The light space-time grid of intensity modulated encodes, and its principle is as shown in Figure 2.It is passed through in the signal of transmitting terminal, space-time grid encoder output
Plural number becomes positive real number signal after being converted to positive real number.The essence that plural number is converted to positive real number is to utilize inverse fast Fourier transform
Symmetry characteristic space-time grid is encoded in complex signal be converted to real number signal, add a sufficiently large direct current point
Amount, you can negative real number signal is converted into positive real number signal.Transformed positive real number signal completes telecommunications by light intensity modulator
The conversion for number arriving optical signal, then is launched by optical antenna.Receiving terminal is directly received by photoelectric detector through propagation in atmosphere
Light intensity signal afterwards, then it is reduced to plural number and negative signal after positive real number to complex conversion processing, by space-time grid decoder
Complete code.
The radio optical communication system structure of the present invention is as shown in Fig. 2, including transmitting terminal system and receiving terminal system.Hair
Penetrate the transmitting that terminal system mainly completes signal, the device for including successively trellis encoder, plural number (negative) to positive reality at one's leisure
Number converter, modulation and driving circuit array, laser array and optical transmitting antenna array.Receiving terminal system is mainly completed
The reception and recovery of signal include optical receiver antenna array, detector array, positive real number to plural (negative) conversion successively
Device, space-time grid code decoder.
For the space-time grid code system using two emitting antennas, space-time grid code coder is by binary source bit
Group is converted into QAM (PSK) format, and the signal exported at this time is a complex signal, and principle is as shown in Figure 3.
In order to realize that the signal after coding can transmit in IM/DD formula links, need to encode space-time grid code coder
Complex signal afterwards is sent into plural (negative) to positive real number signal adapter, realizes the conversion between plural number and positive signal.The step
Suddenly it is the key that the present invention, transfer principle is as shown in Figure 4.
Plural (negative) to positive real number the signal adapter course of work：First by the QAM (PSK) of space-time grid encoder output
Format signal passes through the serial-parallel converter of a N point, generates parallel signal vector
Wherein, AiIndicate the signal after serial to parallel conversion in i-th branch.The signal phasor is again through N points reflecting to 2N points
Meet Hermitian symmetry after penetrating,
Meet the signal of Hermitian symmetry after Inverse Fast Fourier Transforms (IFFT), it can be achieved that complex signal to
The conversion of real number signal, the signal exported at this time are real number signal sequence si.Later, by the real number signal of IFFT outputs again through simultaneously
It is bipolar signal after string transformation and digital to analog conversion.Then, be added in the signal after digital to analog conversion one it is sufficiently large straight
The bipolar signal can be converted to unipolar signal S by flow component DCi.Finally, optical signal is completed to electricity by light intensity modulator
The conversion of signal, launches finally by optical antenna.
Receiving terminal is directly received the light intensity signal after atmospheric channel transmits by photoelectric detector, then through positive real number to plural number
Complex signal is converted into after conversion process.The essence of positive real number to complex conversion process be the signal that will be received go DC component and
Complex signal is converted into after inverse transformation, transfer principle is as shown in Figure 5.
Positive real number to plural number or negative signal adapter the course of work：First, the signal of transmitting terminal output is believed through air
It is directly received by photodetector after road transmission.Since the optical signal that transmitting terminal is sent out can attenuated, suction in transmission process
It receives and the influences such as atmospheric turbulance, arrival receiving terminal optical signal will appear the random fluctuation of light intensity.Therefore, according to light mimo system
Channel model, the received signal vector that can be obtained on jth root antenna are：
In formula (2),Represent cyclic convolution.yj'Represent the signal that j-th of reception antenna receives；η is imitated for opto-electronic conversion
Rate；SiRepresent the signal of i-th of transmitting antenna of transmitting terminal transmitting；njThe noise vector received for j-th of detector；hj,iIt is
I transmitting antenna is to the channel state information between j-th of reception antenna, under weak turbulent-flow conditions, letter that receiving terminal receives
Number logarithm normal distribution will be obeyed, probability density function is represented by：
Wherein, I represents light intensity,Indicate variance.As E [I]=1, the size of light intensity decline can also use scintillation indexIt indicates, representative value is S.I. ∈ [0.4~1.0].
Then, by being represented by after DC component DC, analog-to-digital conversion, serial to parallel conversion：
Then, by signal yjIt is represented by after the Fourier (FFT) of 2N points transformation：
Wherein, XiFor siSignal vector after 2N point quick Fouriers convert (FFT), Hj,iIt is hj,iFourier become
It changes pair.The signal vector is also referred to asWherein, Yt jIt indicates to receive t-th in signal
Signal is represented by：
Signal after FFT transform can revert to the signal before mapping after inverse mapping is handled again, and mapping relations are
Signal vector after mapping can be write asThe letter
Number again after parallel serial conversion, space-time grid code decoder can be sent into.
Viterbi (Viterbi) decoding algorithm is used when space-time grid code decodes, and utilizes the net of space-time grid code coder
Trrellis diagram calculates path metric.By selecting unique survivor path from initial time to finish time as most on grid chart
Maximum-likelihood path, along maximum likelihood path, it is exactly to compile to be carved into the corresponding state in path that finish time is passed by from the outset
The maximum-likelihood decoding sequence of code device output.
Assuming that a total of P of decoderNA state, at the t=0 moment, the original state of decoder is P1, from the state
The extension for carrying out N paths, calculates separately the branch metric of this N paths, it is possible to emit symbol and it is practical receive symbol it
Between Euclidean distance be likely to be that path caused by last moment state extends since institute is stateful at the t=1 moment, because
This, the moment each state is likely to generate N paths arrival NextState, calculates the branch metric at current time, and will
It is added with the branch metric of last moment, obtains the path metric of t=2 moment each state.And so on, at the t=k moment,
By the branch metric d for calculating the t=k moment：
It after obtaining t=k moment branched measurement values, then is added with the path metric value at t=k-1 moment, obtains the t=k moment
Cumulative path metrics the path of one paths metric minimum is selected as path candidate by continuous iterative step,
Other possible paths are abandoned, determine the state that the path is passed through, coding sequence is obtained by status number.
The wireless optical space-time grid proposed in order to further illustrate the present invention encodes the influence to system performance, special using covering
Carlow emulation (Monte Carlo) analyzes 2 × 1,2 × 2 respectively, shadow of the variation of signal-to-noise ratio to bit error rate when 2 × 3 antenna
It rings.Simulated conditions：(1) space-time grid code coder is modulated using 4state-4psk；(2) 32-IFFT is taken；(3) factor is flickered
S.I. 0.6 and 1 are taken.As shown in Figure 6, Figure 7.
Fig. 6, Fig. 7 are the bit error rate of present system with the change curve of signal-to-noise ratio.As seen from the figure with antenna amount
Increase, the bit error rate of system is significantly improved, and illustrates that this method can effectively overcome the scintillation effect caused by atmospheric turbulance.
By the description of embodiment of above, those skilled in the art can be clearly understood that the present invention can use
Software or common hardware are realized.Understood based on above, technical scheme of the present invention can lead to the contribution part of the prior art
Software or specific hardware are crossed to execute the method described in the embodiment of the present invention.
Claims (1)
1. it is suitable for the light space-time grid code encoding method that transmitting terminal uses direct detection mode using intensity modulated and receiving terminal,
It is characterized in that：Its target is that the PSK of space-time grid code encoder output or QAM are converted into a positive real number signal, to solve
The problem of complex signal for original space-time grid code encoder output of having determined can not be transmitted directly in optical communication link；Receiving terminal
The signal for being input to space-time grid code decoder is obtained, is finally completed by real number to the conversion of plural number using opposite process
Decoding；Its step is：
(1) binary bit stream is input in space-time grid code coder, and QAM or psk signal are generated after coding；Then by the letter
It number is sent in the serial-parallel converter that a size is N-1, realizes that serial to parallel conversion, wherein N indicate inverse fast Fourier transform
Points；The signal of serial-parallel converter output is carried out N-1 points again to map to the vector of 2N points, the signal after making it map meets strategic point
The special symmetry of rice；
(2) inverse fast Fourier transform that 2N points are carried out to the signal after mapping, obtains bipolar real number signal, signal warp
After crossing parallel serial conversion and analog-to-digital conversion, a DC component is added；The direct current component value being added has to be larger than or is born equal to maximum
The amplitude of value signal, to eliminate negative signal；Then, optical signal is converted the electrical signal to by light intensity modulator, passes through light
Antenna is learned to launch；
(3) optical signal received is converted to electric signal by receiving terminal using photodetector, after then removing DC component,
A parallel bipolar signal is obtained after analog-to-digital conversion and 2N point serial to parallel conversion, then by the Fast Fourier Transform (FFT) of 2N points, it can
It is changed into a plural number and negative signal；By way of inverse mapping, the 2nd signal in 2N signal of FFT outputs is extracted
To n-th signal, then through parallel serial conversion, the signal after inverse mapping is sent in space-time grid code decoder；
(4) space-time grid code decoding uses the Viterbi decoding algorithm based on maximum likelihood, is counted using grid chart when encoding
Path metric is calculated, by selecting unique survivor path from initial time to finish time as maximum likelihood road on grid chart
Diameter is carved into the maximum likelihood that the corresponding state of maximum likelihood path that finish time is passed by is exactly encoder output from the outset
Coding sequence.
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| CN108683456A (en) * | 2018-05-03 | 2018-10-19 | 内蒙古科技大学 | Intelligent transportation system based on MIMO and visible light communication and communication means |
| CN109067467B (en) * | 2018-09-26 | 2021-06-01 | 南京信息工程大学 | N-level array forming photoproduction W wave band transmitting method based on internal and external joint coding |
| CN111682900B (en) * | 2020-05-29 | 2021-07-09 | 中山大学 | Decision threshold value design method of marine wireless optical communication MIMO system |
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