CN107026683B - MIMO-FSO system based on space diversity self-adaption - Google Patents
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- H—ELECTRICITY
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- 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
- H04B7/0417—Feedback systems
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- H—ELECTRICITY
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- 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
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- 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
- H04B10/564—Power control
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- H—ELECTRICITY
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- 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
- H04B7/06—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station
- H04B7/0613—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station using simultaneous transmission
- H04B7/0615—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station using simultaneous transmission of weighted versions of same signal
- H04B7/0619—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas at the transmitting station using simultaneous transmission of weighted versions of same signal using feedback from receiving side
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Abstract
The invention discloses a space diversity self-adaptive MIMO-FSO system, which comprises: the system comprises a transmitting end, a receiving end, a self-adaptive control unit and a feedback link unit; the receiving end adopts M transmitting antennas to receive information; a feedback link and a channel estimation unit are arranged at both a receiving end and a transmitting end, and the feedback link adopts an RF communication link; the adaptive control unit is located at the transmitting end and is used for enabling the transmitting end to adaptively adjust the transmitting power, the channel coding length and the modulation mode according to the channel state information obtained by the feedback link. By adopting the technical scheme of the invention, the problems of poor performance and low resource utilization of the free space optical communication system in an atmospheric channel are solved.
Description
Technical Field
The invention belongs to the field of wireless optical communication (FSO), and particularly relates to a space diversity adaptive MIMO-FSO system.
Background
With the development of science and technology and the progress of society, people have entered the information age, and the demand for high-speed and convenient internet is increasing day by day. The free space optical communication system adopting laser as a light source becomes a hotspot of research in the field of wireless communication of a new era due to the characteristics of good confidentiality, large capacity, high flexibility and the like. Although free space optical communication is widely concerned, there are some problems to be solved urgently (i.e. limited transmission rate and limited transmission distance). And the atmospheric channel interference error rate is high. In order to improve the applicability and reliability of FSO, transmission techniques that combat atmospheric effects must be employed.
1. Adaptive transmission techniques
The self-adaptive transmission technology is that the system self-adaptively adjusts the transmission parameters of the system according to the currently acquired channel information, and the data transmission rate and the spectrum utilization rate can be well improved by adopting the self-adaptive transmission technology; typical adaptive transmission techniques include adaptive modulation and coding techniques, power control techniques, and the like.
The adaptive adjustment technique presupposes that the transmitting end needs to obtain channel state information, which can be given through a feedback link. The purpose of adaptive regulation is to make maximum use of the existing resources of the system, and from the viewpoint of a mathematical model, the adaptive regulation is an optimization problem given optimization targets and constraints.
2. Multiple Input Multiple Output (MIMO) techniques
Multiple Input Multiple Output (MIMO) is a transmission technique literally meaning multiple transmitting ends and multiple receiving ends, which makes signals transmitted and received through multiple antennas of the transmitting ends and the receiving ends, can make full use of space resources, realizes multiple transmission and multiple reception through multiple antennas, can improve system channel capacity by times without increasing spectrum resources and antenna transmitting power, and has a good advantage for improving communication quality.
MIMO techniques can be broadly divided into two categories: spatial diversity and spatial multiplexing. The space diversity is to transmit the signals with the same information through different paths by using multiple transmitting terminals, and obtain multiple independent fading signals of the same data symbol at the receiving terminal, thereby obtaining the receiving reliability improved by the diversity. The spatial multiplexing is different, different signals are sent out through different paths at a transmitting end, and independent fading signals of different data symbols are obtained at a receiving end, so that the frequency utilization rate can be effectively improved under the condition of not increasing the bandwidth and the transmitting power.
In the MIMO-FSO system based on the spatial diversity, the sizes of the transmission power and the modulation format and the coding length are fixed and unchanged, so in order to ensure the reliability of the system, the sizes of the transmission power and the modulation format and the coding mode of the MIMO-FSO system must be designed on the basis of the worst channel quality. This will inevitably cause waste of system resources because the channel quality is very good in a clear weather, but the system parameters are designed based on the worst channel quality, which will cause the system performance to be not optimized.
In the algorithm, the MIMO system needs to obtain an accurate channel state information matrix and carry out SVD with higher complexity to obtain singular values of each equivalent sub-channel, a transmitting end adaptively allocates power of each transmitting antenna and selects coding and modulation modes of each antenna, and carries out linear pre-transformation on a modulation symbol vector of each time slot and then decomposes the modulation symbol vector into each path of data to be sent to each transmitting antenna. Although the algorithm has the optimal system average spectrum utilization performance, the computation complexity of the SVD decomposition is too high, the hardware implementation complexity of the transmitting end is increased by the linear pre-transformation, and the method is greatly affected by the quality of the feedback link, so the algorithm is not suitable for practical application.
Disclosure of Invention
The technical problem to be solved by the invention is to provide a space diversity adaptive MIMO-FSO system, which solves the problems of poor performance and low resource utilization of a free space optical communication system in an atmospheric channel.
In order to achieve the purpose, the invention adopts the technical scheme that:
a spatial diversity adaptive based MIMO-FSO system comprising: the system comprises a transmitting end, a receiving end, a self-adaptive control unit and a feedback link unit; the receiving end adopts M transmitting antennas to receive information; a feedback link and a channel estimation unit are arranged at both a receiving end and a transmitting end, and the feedback link adopts an RF communication link; the adaptive control unit is located at the transmitting end and is used for enabling the transmitting end to adaptively adjust the transmitting power, the channel coding length and the modulation mode according to the channel state information obtained by the feedback link.
Preferably, the adaptive unit is configured to output three signal labels, respectively S, via the signal processing unit when the channel parameter h is obtained through the feedback link0Signal, S1Signal, S2A signal; the three control signals are all functions of a channel state information parameter h; when the strength of the atmospheric turbulence is different, the atmospheric turbulence is obtained according to the RF feedback channelThe intensity state information h can adaptively adjust the modulation mode, the coding mode and the optical power of the laser.
Preferably, the adaptive cell modulation process comprises the steps of:
1. presetting the performance constraint condition of the system, namely setting the maximum interruption probability as PO and the maximum bit error rate tolerable by the system as PthAccording to
Solving the corresponding channel state parameter when the system is interrupted;
2. the receiving end carries out digital processing on signals of a plurality of receiving antennas according to a maximum power receiving strategy, sends part of information to the feedback signal processing unit and sends the result of channel estimation to the receiving end in the form of an RF signal;
3. a decision maker at a receiving end receives a channel state information value h of a feedback link, firstly, h is compared, if h <, the step 4 is carried out, otherwise, the step 5 is carried out;
4. keeping the transmitting power unchanged, judging the intensity of the current turbulent flow according to the intensity of h, selecting a corresponding modulation format and a corresponding coding mode according to the channel states of turbulent flows with different intensities, repeating the step 3,
5. when the transmitting power is kept not to exceed the safe power peak value, increasing 20 percent of the transmitting power without changing the modulation format and the coding mode, and repeating the step 3;
6. the continuous work of the feedback link enables the system to continuously perform self-adaptive parameter adjustment.
Drawings
FIG. 1 is a free space optical communication adaptive transmission system based on multiple inputs and multiple outputs;
FIG. 2 is a diagram of an adaptive transmission control unit;
fig. 3 is a flow chart of an adaptive transmission algorithm.
Detailed Description
The invention provides a MIMO-FSO system based on space diversity self-adaptation, which comprises a transmitting end, a receiving end, a self-adaptive control unit andand a feedback link unit. The structure of the adaptive transmission free-space optical communication based on mimo is shown in fig. 1, in combination with the structural composition of the conventional free-space optical communication system, in the system, an adaptive control unit is located at a transmitting end, and mainly controls three transmitting parameters, namely transmitting power, channel coding length and modulation format, of the transmitting end. The transmitting terminal adopts N transmitting antennas to transmit information, the receiving terminal adopts M transmitting antennas to receive information, and the values of N and M can be the same or different. And a feedback link and a channel estimation unit are arranged at the receiving end and the transmitting end, and the feedback link adopts an RF communication link. In the MIMO-FSO system adopting the space diversity technology, a self-adaptive control unit is introduced, so that a transmitting end can self-adaptively adjust the transmitting power, the channel coding length and the modulation mode according to the channel state information obtained by a feedback link, the reliability of the system is higher, the frequency spectrum utilization rate and the coding rate are increased while the communication quality is ensured, the average transmitting power is reduced, and the system is more efficient. The main structural design diagram of the adaptive control unit is shown in fig. 2. When the channel parameter h is obtained through the feedback link, three signal marks are respectively output through the signal processing unit, and the signal marks are respectively S0Signal, S1Signal, S2A signal. The three control signals respectively control the modulation mode, the coding mode and the emitting power of the laser. These three control signals are all functions of the channel state information parameter h. When the intensity of the atmospheric turbulence is different, the state information h of the intensity of the atmospheric turbulence is obtained according to the RF feedback channel, and the modulation mode, the coding mode and the optical power of the laser can be adjusted in a self-adaptive manner. The main method steps of the adaptive modulation strategy are as follows:
1. carrying out conditional constraint on communication performance indexes of the system; selecting the probability of interruption, i.e. when the bit error rate P isbGreater than a specified value PthThe probability of system communication disruption. The probability of interruption expression is Pout＝P{Pb＞PthThus, the maximum bit error rate P can be passedbDetermining the lowest signal-to-noise ratio (SN) that a system can receiveR; in order to maintain reliable communication of the system, the outage probability must be less than the value of the system outage probability that we have defined in advance.
2. Acquiring channel state information h by adopting a channel estimation method of a least square method (LS) according to the feedback link information; the receiving part adopts a plurality of antennas for receiving, so that received signals of the plurality of antennas need to be integrated in a feedback decision unit of a receiving end, and the signal-to-noise ratio of the receiving end is obtained by adopting a maximum gain receiving principle, so that channel state information estimation is carried out according to a mathematical model of an atmospheric channel; thus h may represent the turbulence intensity magnitude.
3. And automatically adjusting the modulation format mode, the transmitting power and the channel coding length according to the channel state information obtained by feedback.
To simplify and explicitly describe the specific modulation steps, the present invention uses heuristic reasoning algorithms, where both fixed transmit power and fixed modulation format size must meet the worst case channel parameters when not using adaptive modulation scheme transmission. However, when the adaptive modulation scheme is adopted, the size of the modulation format and the coding scheme are varied according to the channel condition parameters, so we can divide the channel parameters into several segments, for example, hmax≤hmax-1≤…≤h2≤h1Then the corresponding M value can also be selected in segments, for example:
transmission power PtIs a function of the channel state parameter h and the modulation format M. It is obvious that if reliable communication is to be ensured, the signal-to-noise ratio received by the receiving end reaches the minimum value of the system constraint, namely gamma hPt＝SNRminThen transmitting power PtIt is desirable to take as large a value as possible and still be below the maximum value of the transmit power limit. Therefore we define that when the value of h is very small, we assume the transmit power PtIs zero, namely the product is obtained
The transmission power P is known heretIs a function of the channel state parameter h and the modulation format size M, and when h < the system is interrupted, so that the interruption probability expression can be obtained as:
when we specify the maximum bit error rate of the system as PthAnd the corresponding maximum interruption probability is PO, we can be according to PoutPO derives the value that it corresponds to when the communication is interrupted. Further, the relationship between the value of the channel parameter h and the magnitude thereof is determined according to the feedback link, so that the selection of the modulation format M, the selection of the coding mode and the adjustment of the transmission power can be performed according to the value h, as shown in table 1.
TABLE 1 graph of bit error rate for different modulation formats under different turbulence intensities
The above table describes the bit error rate magnitude comparison for different modulation schemes chosen at different turbulence intensities. The automatic adjustment for the coding mode needs to give the bit error rate comparison of different coding modes under different turbulence intensities by referring to the following table.
TABLE 2 graph of the relationship between the bit error rate and the coding mode under different turbulence intensity
| Coding method | Without turbulence | The intensity of turbulence is weak turbulence sigma0 2＝0.1 | Moderate turbulence sigma in turbulence intensity0 2＝0.2 |
| Without coding | 10-9 | 3.6×10-6 | ＜4.8×10-4 |
| RS(15，13) | ≤10-9 | ＜10-9 | ＜8×10-6 |
| RS(15，9) | ≤10-9 | ≤10-9 | ＜10-9 |
From the two tables, we can see that when we obtain the atmospheric channel parameter h through channel estimation through the feedback link, the turbulence intensity of the channel can be obtained, so that the selection of the coding mode and the modulation format can be carried out, and when the value of M is determined, the value of M is obtained according to the formula Ms/(γPm) The magnitude of the transmitting power is not more than obtained. Wherein the error rate reaches a maximum value, and the value of the channel parameter at system outage, SNRminIn order to ensure the minimum signal-to-noise ratio required by reliable communication, gamma represents the signal-to-noise ratio coefficient corresponding to the MPPM modulation format. The process of completing the entire adaptive transmission is therefore as follows:
1. combining a plurality of receiving antennas according to the maximum gain, and performing channel estimation to obtain channel state information h;
2. and determining the size range of the turbulence intensity according to the obtained h size, so as to select a proper coding mode and a proper modulation format.
3. And selecting the power value with the minimum average power according to the channel state h and the modulation format M according to the formula 2.
For a better understanding of the present invention, a specific implementation of the present invention is described in further detail below with reference to FIG. 3:
1. presetting the performance constraint condition of the system, namely setting the maximum interruption probability as PO and the maximum bit error rate tolerable by the system as PthAccording to
And solving the corresponding channel state parameter when the system is interrupted.
2. The receiving end carries out digital processing on signals of a plurality of receiving antennas according to a maximum power receiving strategy, sends part of information to the feedback signal processing unit and sends the result of channel estimation to the receiving end in the form of an RF signal.
3. And a decision maker at the receiving end receives the channel state information value h of the feedback link, compares the h with the h, if h <, then the step 4 is carried out, otherwise, the step 5 is carried out.
4. Keeping the transmitting power unchanged, judging the intensity of the current turbulent flow according to the intensity of h, and selecting a corresponding modulation format and a corresponding coding mode according to the channel states of turbulent flows with different intensities. And (5) repeating the step (3).
5. And (3) when the transmitting power is kept not to exceed the safe power peak value, increasing the transmitting power by 20 percent without changing the modulation format and the coding mode, and repeating the step (3).
6. The continuous work of the feedback link can lead the system to continuously carry out the self-adaptive parameter adjustment.
The present invention has been described in detail, but the specific embodiments of the present invention are not limited thereto. It will be apparent to those skilled in the art that various obvious changes can be made therein without departing from the spirit of the process of the invention and the scope of the claims.
Claims (2)
1. A spatial diversity adaptive based MIMO-FSO system, comprising: the system comprises a transmitting end, a receiving end, a self-adaptive control unit and a feedback link unit; the receiving end adopts M transmitting antennas to receive information; a feedback link and a channel estimation unit are arranged at both a receiving end and a transmitting end, and the feedback link adopts an RF communication link; the self-adaptive control unit is positioned at the transmitting end and used for enabling the transmitting end to self-adaptively adjust the transmitting power, the channel coding length and the modulation mode according to the channel state information obtained by the feedback link; the adaptive unit modulation process comprises the following steps:
1. presetting the performance constraint condition of the system, namely setting the maximum interruption probability as PO and the maximum bit error rate tolerable by the system as PthAccording to
Solving the corresponding channel state parameter when the system is interrupted;
2. the receiving end carries out digital processing on signals of a plurality of receiving antennas according to a maximum power receiving strategy, sends part of information to the feedback signal processing unit and sends the result of channel estimation to the receiving end in the form of an RF signal;
3. a decision maker at a receiving end receives a channel state information value h of a feedback link, firstly, h is compared, if h <, the step 4 is carried out, otherwise, the step 5 is carried out;
4. keeping the transmitting power unchanged, judging the intensity of the current turbulent flow according to the intensity of h, selecting a corresponding modulation format and a corresponding coding mode according to the channel states of turbulent flows with different intensities, repeating the step 3,
5. when the transmitting power is kept not to exceed the safe power peak value, increasing 20 percent of the transmitting power without changing the modulation format and the coding mode, and repeating the step 3;
6. the continuous work of the feedback link enables the system to continuously perform self-adaptive parameter adjustment.
2. As claimed in claim 1The MIMO-FSO system based on space diversity adaptation is characterized in that the adaptation unit is used for outputting three paths of control signals respectively, namely S, through the signal processing unit when the channel parameter h is obtained through the feedback link0Signal, S1Signal, S2A signal; the three control signals are all functions of a channel state information parameter h; when the intensity of the atmospheric turbulence is different, the state information h of the intensity of the atmospheric turbulence is obtained according to the RF feedback channel, and the modulation mode, the coding mode and the optical power of the laser can be adjusted in a self-adaptive manner.
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| CN107567068B (en) * | 2017-09-19 | 2020-10-27 | 东北大学 | Method for constructing ultra-dense small base station self-adaptive FSO (frequency selective offset) backhaul network |
| CN111490822B (en) * | 2020-04-17 | 2021-11-19 | 中国人民解放军战略支援部队信息工程大学 | Optical communication method and visible light communication equipment for enhancing one-way transmission |
| CN112787718B (en) * | 2020-12-25 | 2022-03-11 | 武汉邮电科学研究院有限公司 | Atmospheric laser communication rate adjusting method, device and system |
| CN113490267B (en) * | 2021-05-18 | 2023-11-07 | 浙江传媒学院 | Generalized pre-control method for robust stability |
| CN114172573B (en) * | 2021-11-19 | 2023-05-05 | 武汉邮电科学研究院有限公司 | Dynamic compensation method and device for free space optical communication |
| CN114204990B (en) * | 2021-11-30 | 2022-10-21 | 西安理工大学 | Adaptive MIMO-FSO multi-mode transmission method based on channel capacity |
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Granted publication date: 20200814 |