CN107026683A - A kind of MIMO FSO system adaptive based on space diversity - Google Patents

Abstract

The invention discloses a MIMO-FSO system based on space diversity self-adaptation, comprising: a transmitting end, a receiving end, an adaptive control unit and a feedback link unit; wherein, the transmitting end uses N transmitting antennas to send information, and the receiving end The terminal adopts M transmitting antennas to receive information; both the receiving terminal and the transmitting terminal are provided with a feedback link and a channel estimation unit, and the feedback link adopts an RF communication link; the adaptive control unit is located at the transmitting terminal, and is used to make the transmitting terminal The transmission power, channel coding length and modulation mode can be adaptively adjusted according to the channel state information obtained from the feedback link. By adopting the technical solution of the invention, the problems of poor performance and low resource utilization of the free space optical communication system in the atmospheric channel are solved.

Description

An Adaptive MIMO-FSO System Based on Space Diversity

technical field

The invention belongs to the field of wireless optical communication (FSO), in particular to an adaptive MIMO-FSO system based on space diversity.

Background technique

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 using laser as light source has become a research hotspot in the field of wireless communication in the new era due to its characteristics of good confidentiality, large capacity, and high flexibility. Although free-space optical communication has been widely concerned by people, there are still some problems that need to be solved urgently. ①The transmission rate is limited and the transmission distance is limited. ②Atmospheric channel interference bit error rate is high. In order to improve the applicability and reliability of FSO, transmission technology that restrains atmospheric effects must be adopted.

1. Adaptive transmission technology

Adaptive transmission technology means that the system adaptively adjusts system transmission parameters according to the currently acquired channel information, and the data transmission rate and spectrum utilization can be improved by adopting adaptive transmission technology; usually adaptive transmission technology includes adaptive modulation And coding technology, power control technology, etc.

The premise of adaptive adjustment technology is that the transmitter needs to obtain channel state information, which can be given through the feedback link. The purpose of self-adaptive adjustment is to maximize the utilization of the existing resources of the system. From the perspective of mathematical model, it is an optimization problem with given optimization objectives and constraints.

2. Multiple-input multiple-output (MIMO) technology

Multiple-input multiple-output (MIMO) literally means a transmission technology with multiple transmitters and multiple receivers, which enables signals to be transmitted and received through multiple antennas at the transmitter and receiver. It can make full use of space resources and achieve Multi-transmission and multi-reception can double the system channel capacity without increasing spectrum resources and antenna transmission power, which has a good advantage in improving communication quality.

MIMO technology can be roughly divided into two categories: space diversity and space multiplexing. Space diversity refers to the use of multiple transmitters to send signals with the same information through different paths, and at the same time obtain multiple independent fading signals of the same data symbol at the receiver, so as to obtain the reception reliability improved by diversity. Spatial multiplexing is different. At the transmitting end, different signals are sent through different paths, and independent fading signals of different data symbols are obtained at the receiving end, so that the frequency utilization can be effectively improved without increasing the bandwidth and transmission power.

In the MIMO-FSO system based on space diversity, the transmission power, the size of the modulation format and the code length are fixed, so in order to ensure the reliability of the system, the MIMO-FSO system must be designed on the basis of the worst channel quality. The size of the transmission power and the size of the modulation format and the encoding method. This will inevitably cause a waste of system resources, because in sunny weather, the channel quality is very good, but the system parameters are designed based on the worst channel quality, so the system performance cannot be optimized.

In MIMO-FSO systems based on space diversity, most of the cases are based on the singular value decomposition (SVD) to design the MIMO system adaptive scheme. In this type of algorithm, the MIMO system needs to obtain accurate channel state information matrix and its Perform SVD decomposition with high complexity to obtain the singular value of each equivalent sub-channel, and then the transmitter adaptively allocates the power of each transmit antenna and selects each antenna coding and modulation method based on this, and performs modulation on the modulation symbol vector of each time slot Linear pre-transformation, and then decompose into each channel of data and send it to each transmitting antenna. Although this algorithm has the best system average spectrum utilization performance, the calculation complexity of SVD decomposition is too high, and the linear pre-transformation will increase the hardware implementation complexity of the transmitter, and this method will be greatly affected by the quality of the feedback link. effect, so it is not suitable for practical application.

Contents of the invention

The technical problem to be solved by the present invention is to provide a space diversity adaptive MIMO-FSO system to solve the problems of poor performance and low resource utilization of free space optical communication systems in atmospheric channels.

In order to achieve the above object, the technical scheme adopted in the present invention:

An adaptive MIMO-FSO system based on space diversity, including: a transmitting end, a receiving end, an adaptive control unit, and a feedback link unit; wherein, the transmitting end uses N transmitting antennas to transmit information, and the receiving end uses M Two transmitting antennas are used to receive information; both the receiving end and the transmitting end are provided with a feedback link and a channel estimation unit, and the feedback link adopts an RF communication link; the adaptive control unit is located at the transmitting end to enable the transmitting end to The channel state information obtained by the link adaptively adjusts the transmission power, channel coding length and modulation mode.

As preferably, the self-adapting unit is used for when the channel parameter h obtained by the feedback link outputs three signal signs respectively through the signal processing unit, which are respectively S ₀ signal, S ₁ signal, and S ₂ signal; The control signal is a function of the channel state information parameter h; when the intensity of atmospheric turbulence is different, the state information h of atmospheric turbulence intensity can be obtained according to the RF feedback channel, and the modulation mode, coding mode and laser optical power can be adaptively adjusted.

Preferably, the adaptive unit modulation process includes the following steps:

1. Preset the performance constraints of the system, that is, the maximum outage probability is PO, and the maximum bit error rate that the system can tolerate is P _th , according to Find the corresponding channel state parameter ε when the system is interrupted;

2. The receiving end digitally processes the signals of multiple receiving antennas according to the maximum power receiving strategy, sends part of the information to the feedback signal processing unit and sends the channel estimation result to the receiving end in the form of RF signals;

3. The decision maker at the receiving end receives the channel state information value h of the feedback link, first compares h with ε, if h<ε, then proceed to step 4, otherwise proceed to step 5;

4. Keep the transmission power unchanged, judge the current turbulence intensity according to the size of h, and select the corresponding modulation format and coding method according to the channel state of different intensity turbulence, and repeat step 3,

5. When the transmission power does not exceed the safe power peak value, increase the transmission power by 20%, without changing the modulation format and coding method, and repeat step 3;

6. The continuous operation of the feedback link enables the system to continuously adjust parameters adaptively.

Description of drawings

Figure 1 is an adaptive transmission system for free space optical communication based on multiple input and multiple output;

Fig. 2 is a schematic diagram of an adaptive transmission control unit;

Fig. 3 is a flow chart of the adaptive transmission algorithm.

detailed description

The invention provides an adaptive MIMO-FSO system based on space diversity, which is composed of four parts: a transmitting end, a receiving end, an adaptive control unit and a feedback link unit. Combined with the structure of the traditional free-space optical communication system, the structure of free-space optical communication based on multiple-input multiple-output adaptive transmission is shown in Figure 1. In this system, the adaptive control unit is located at the transmitting end, and the main control is the transmitting There are three transmission parameters of the transmission power, channel coding length, and modulation format of the terminal. The transmitting end uses N transmitting antennas to send information, and the receiving end uses M transmitting antennas to receive information, and the values of N and M can be the same or different. Both the receiving end and the transmitting end are provided with a feedback link and a channel estimation unit, and the feedback link adopts an RF communication link. In the MIMO-FSO system using space diversity technology, an adaptive control unit is introduced so that the transmitter can adaptively adjust the transmission power, channel coding length and modulation method according to the channel state information obtained from the feedback link, so that the system The reliability is higher, and the spectrum utilization rate and coding rate are increased while the communication quality is guaranteed, and the average transmission power is reduced, thereby making the system more efficient. The schematic diagram of the main structural design of the adaptive control unit is shown in Fig. 2 . When the channel parameter h obtained through the feedback link, the signal processing unit outputs three signals respectively, which are S ₀ signal, S ₁ signal, and S ₂ signal. These three control signals respectively control the modulation mode, coding mode and laser emission power. These three control signals are all functions of the channel state information parameter h. When the intensity of atmospheric turbulence is different, the state information h of atmospheric turbulence intensity can be obtained according to the RF feedback channel, and the modulation mode, encoding mode and laser optical power can be adaptively adjusted. The main method steps of the adaptive modulation strategy are:

1. Carry out conditional constraints on the communication performance index of the system; for example, the system performance constraint parameter selects the outage probability, that is, the probability of the middle end of the system communication when the bit error rate P _b is greater than a specific value P _th . The expression of the outage probability is P _out = P{P _b >P _th }, so the minimum signal-to-noise ratio (SNR) that a system can receive can be determined by the maximum bit error rate P _b ; in order to maintain reliable communication of the system, the outage probability must be less than our The value of the predefined system outage probability.

2. According to the feedback link information, the channel estimation method of least square method (LS) is used to obtain the channel state information h; the receiving part adopts multiple antennas to receive, so the feedback decision unit of the receiving end needs to analyze the received signals of multiple antennas For integration, the principle of maximum gain reception is adopted to obtain the signal-to-noise ratio at the receiving end, so as to estimate the channel state information according to the mathematical model of the atmospheric channel; therefore h can represent the turbulence intensity.

3. Automatically adjust the modulation format, transmission power and channel coding length according to the channel state information obtained from the feedback.

In order to simplify and clearly describe the specific modulation steps, the present invention uses a heuristic reasoning algorithm. When the adaptive modulation scheme is not used for transmission, the fixed transmission power and the fixed modulation format size must satisfy the channel in the worst case parameter. However, when an adaptive modulation scheme is used, the size of the modulation format and the encoding method change according to the channel condition parameters, so we can divide the channel parameters into several segments to express for example ε=h _max ≤h _max-1 ≤...≤h ₂ ≤h ₁ ＝+∞, the corresponding M value can also be selected in sections, for example:

The transmit power _Pt is a function of the channel state parameter h and the modulation format M. Obviously, in order to ensure reliable communication, the signal-to-noise ratio received by the receiving end reaches the minimum value constrained by the system, that is, γhP _t = SNR _min , then the transmission power P _t needs to be as large as possible, and must be lower than the transmission power Maximum limit. Therefore, we define that when the value of h is extremely small, we assume that the transmit power P _t is zero, then we can get

It can be seen here that the transmit power P _t is a function of the channel state parameter h and the modulation format size M, and when h<ε, the system is interrupted, so the expression of the outage probability can be obtained as:

When we stipulate that the highest bit error rate of the system is P _th , and the corresponding maximum outage probability is PO, then we can obtain the corresponding value of ε when the communication is interrupted according to P _out = PO. Furthermore, the relationship between the value of the channel parameter h and ε is determined according to the feedback link, and the size selection of the modulation format M, the selection of the coding method, and the adjustment of the transmission power can be performed according to the value of h, as shown in Table 1.

Table 1 BER relationship diagram of different modulation formats under different turbulence intensities

The table above describes the bit error rate comparison of different modulation modes selected under different turbulence intensities. For the automatic adjustment of the coding method, you need to refer to the following table, which gives the comparison of the bit error rate of different coding methods under different turbulence intensities.

Table 2 The bit error rate relationship diagram of different encoding methods under different turbulence intensities

Encoding without turbulence Turbulence intensity is weak turbulence σ ₀ ² ＝0.1 Turbulence intensity Moderate turbulence σ ₀ ² ＝0.2 no coding < ^10-9 <3.6×10 ^-5 <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 above two tables, we can see that when we get the atmospheric channel parameter h through the channel estimation through the feedback link, we can get the turbulence intensity of the channel, so that we can choose the coding method and modulation format. When M The value of the transmission power is obtained according to the formula M _s /(γP _m )≤ε. Where ε is the maximum value of the bit error rate and the value of the channel parameter when the system is interrupted, SNR _min is the minimum signal-to-noise ratio required to ensure reliable communication, and γ represents the corresponding signal-to-noise ratio coefficient when the MPPM modulation format is used. Therefore, the process of completing the entire adaptive transmission is as follows:

1. Multiple receiving antennas are combined according to the maximum gain, and channel estimation is performed to obtain channel state information h;

2. Determine the size range of the turbulence intensity according to the size of h obtained, so as to select the appropriate coding method and the size of the modulation format.

3. According to the channel state h and the modulation format M, the power value with the smallest average power is selected according to formula 2.

In order to better illustrate the present invention, below with reference to accompanying drawing 3, the concrete implementation of the present invention is described in further detail:

1. Preset the performance constraints of the system, that is, the maximum outage probability is PO, and the maximum bit error rate that the system can tolerate is P _th , according to Find the corresponding channel state parameter ε when the system is interrupted.

2. The receiving end digitally processes the signals of multiple receiving antennas according to the maximum power receiving strategy, sends part of the information to the feedback signal processing unit and sends the channel estimation result to the receiving end in the form of RF signals.

3. The decision maker at the receiving end receives the channel state information value h of the feedback link, first compares h with ε, if h<ε, then proceed to step 4, otherwise, proceed to step 5.

4. Keep the transmit power constant, judge the current turbulence intensity according to the size of h, and select the corresponding modulation format and coding method according to the channel state of different intensity turbulence. Repeat step 3.

5. When the transmission power does not exceed the safe power peak value, increase the transmission power by 20%, without changing the modulation format and coding method, and repeat step 3.

6. The continuous operation of the feedback link can enable the system to continuously adjust parameters adaptively.

The present invention has been described in detail above, but the specific implementation forms of the present invention are not limited thereto. For those skilled in the art, various obvious changes made to the method of the present invention without departing from the spirit of the method and the scope of the claims are within the protection scope of the present invention.

Claims (3)

1. a kind of MIMO-FSO system adaptive based on space diversity, it is characterised in that including：It is transmitting terminal, receiving terminal, adaptive Answer control unit and feedback link unit；Wherein, transmitting terminal enters the transmission of row information using N bar transmitting antennas, and receiving terminal Enter the reception of row information using M bar transmitting antennas；Feedback link and channel estimating unit are both provided with receiving terminal and transmitting terminal, Feedback link uses RF communication links；Self-adaptive controller is located at transmitting terminal, for alloing transmitting terminal according to feedback link Regulation transmission power size, channel coding length and the modulation system of channel condition information adaptively obtained by road.

2. as claimed in claim 1 based on the adaptive MIMO-FSO systems of space diversity, it is characterised in that adaptive single Member, for as the channel parameter h obtained by feedback link, exporting three road signal posts respectively by signal processing unit, respectively For S₀Signal, S₁Signal, S₂Signal；This described three tunnels control signal is all the function on channel state information parameters h；When big When gas turbulent flow is strong and weak different, the status information h of atmospheric turbulence intensity is obtained according to RF feedback channels, can be modulated with automatic adjusument Mode and coded system and laser optical power.

3. as claimed in claim 1 based on the adaptive MIMO-FSO systems of space diversity, it is characterised in that adaptive unit Modulated process comprises the following steps：

1st, the performance constraints of system, i.e. outage probability maximum are preset for PO, the patient maximum bit error rate of system For P_th, according toObtain the channel state parameter ε corresponding when system is interrupted；

2nd, receiving terminal receives strategy according to peak power and carries out digital processing to the signal of multiple reception antennas, by a part of information It is sent to feedback signal processing unit and the result of channel estimation is sent to receiving terminal in the form of RF signals；

3rd, the decision-making device of receiving terminal receives the channel state information values h of feedback link, first contrasts h and ε, if h<ε, then enter The step of row the 4th, on the contrary carry out the 5th step；

4th, keep transmission power constant, the size of current turbulence intensity is judged according to h size, and according to varying strength turbulent flow Channel status selects corresponding modulation format and coded system, repeats the 3rd step,

5th, when keeping transmission power no more than safe power peak, the transmission power of increase by 20% does not change modulation format And coded system, repeat the 3rd step；

6th, the continuous firing of feedback link, makes system constantly carry out automatic adjusument parameter.