Wireless optical communication method based on cooperative non-orthogonal multiple access and multi-user diversity

Abstract

The application discloses a wireless optical communication method based on cooperative non-orthogonal multiple access and multi-user diversity, and mainly solves the problems of few accessible users and low user resource utilization in the prior art. The implementation scheme is as follows: a cooperative non-orthogonal multiple access underwater multi-user diversity wireless optical communication system is constructed; a source node selects far users and near users with the best channel according to a multi-user scheduling algorithm; the source node superimposes far user signals and near user signals in a power domain, and simultaneously sends the superimposed signals to the far users and the near users; the near users demodulate their own signals through serial interference cancellation; the near users send signals to the far users as relays by using a decode-and-forward protocol or an amplify-and-forward protocol; and the far users demodulate their own signals by selecting and combining two signals sent by the source node and the relay node. The application improves the number of user access, the source node can serve multiple user groups at the same time, and the system interruption performance is improved, and the application can be used in a multi-user underwater wireless optical communication system.

Description

Technical Field

[0001] This invention belongs to the field of optical communication technology, and specifically relates to a wireless optical communication method based on cooperative nonorthogonal multiple access and multi-user diversity, which can be used in multi-user underwater wireless optical communication systems. Background Technology

[0002] Underwater wireless optical communication is a novel line-of-sight transmission communication technology that uses blue-green light as the carrier and seawater as the transmission medium to achieve bidirectional communication. Due to its advantages of wide bandwidth, high transmission speed, strong anti-interference capability, strong confidentiality, and small size, it has the potential to achieve high-speed, high-capacity communication. However, the propagation of light in water is affected by path loss caused by seawater turbulence and particles and impurities distributed in the water. To improve system performance, researchers have proposed several technologies to suppress the influence of these factors, including cooperative non-orthogonal multiple access technology and multi-user diversity technology, among which:

[0003] Multi-user diversity technology utilizes the randomness of channel fading caused by turbulence to optimize resource allocation and improve the overall system performance. In 2019, Pang et al. published a paper titled "Performance Study of Multi-User Diversity Scheduling Scheme in Underwater Wireless Optical Communication System" in *Optics Communications*, proposing to introduce multi-user diversity technology into underwater wireless optical communication systems and analyzing the performance of different user scheduling algorithms. However, this method, in each transmission time slot, only allows the central node to select one user for communication based on the channel state information of all users in the current system. It cannot serve multiple users simultaneously in the same time slot, failing to meet the growing demand for communication between more and more underwater devices as human exploration of the ocean increases.

[0004] Non-orthogonal multiple access (NOMA) technology, a core 5G technology, differs from traditional orthogonal multiple access methods. It employs superposition coding technology, allocating different power factors to ensure that superimposed signals are transmitted in the same time, frequency band, and spatial domain, effectively improving the system's spectral efficiency. In 2021, M. Elamassi et al. published "Capacity analysis of NOMA-enabled underwater VLC networks" in *IEEE Access*, analyzing the introduction of NOMA into underwater optical communication. The results showed that compared to orthogonal frequency division multiple access (OFDMA), NOMA systems can achieve greater capacity and allocate more power to distant users with poor channel conditions, but the performance improvement for distant users remains limited.

[0005] Cooperative non-orthogonal multiple access (COMA) technology treats users with good channel conditions as relays, forwarding information to users with poor channel conditions, thus further improving the performance of distant users with poor channel conditions. In 2021, KWS Palitharathna et al. published "Average rate analysis of cooperative NOMA aided underwater optical wireless systems" in the *IEEE Open Journal of the Communications Society*, introducing COMA into underwater optical communication and studying the average transmission rate of the system. However, because this COMA technology is designed for scenarios involving only a single near or far user, it does not utilize other idle users in the system, thus wasting resources. Summary of the Invention

[0006] The purpose of this invention is to address the shortcomings of the prior art by proposing a wireless optical communication method based on cooperative nonorthogonal multiple access and multi-user diversity, so as to improve the number of users accessing and the utilization rate of network resources in underwater wireless optical communication.

[0007] To achieve the above objectives, the implementation steps of the present invention include the following:

[0008] 1) Assuming the source node knows the channel state information of all near and far users, select the far user with the best channel state based on the multi-user scheduling algorithm. and a close user Perform data transmission;

[0009] 2) Based on the electrical power P sent by the source node S The power allocation factor α for remote users F Power allocation factor α for users near the user N The source node superimposes the distant user signal x through the power domain. F and the user signal x N Simultaneously send superimposed signal x to users near and far:

[0010]

[0011] 3) Proximity to users Heyuan users Receive the signals sent by the source node and obtain the received signals respectively. and

[0012]

[0013]

[0014] Where ρ is the photoelectric conversion coefficient, υ is the electro-optic conversion coefficient, and n N The mean is 0 and the variance is Gaussian white noise, n F The mean is 0 and the variance is Gaussian white noise, and These are the ocean channel fading coefficients from the source node to the distant user and from the source node to the near user, respectively.

[0015] 4) Near the user, use serial interference to remove the signal; first demodulate the high-power signal x. F From the received signal Subtract x from the middle F To obtain intermediate signals From again Demodulate its own signal x N ;

[0016] 5) The nearby user acts as a relay, sending signals to the distant user via a decoding-forwarding protocol or an amplification-forwarding protocol. The distant user receives the signal... or

[0017]

[0018]

[0019] in P represents the channel fading coefficient from near user to far user. N It is the relay transmission power. It is the magnification factor;

[0020] 6) The remote user receives two signals from the source node and the relay node respectively. or By selecting and merging, the decoded signal x is obtained. F This completes wireless optical communication.

[0021] Compared with the prior art, the present invention has the following advantages:

[0022] First, this invention combines cooperative nonorthogonal multiple access technology with multi-user diversity technology, that is, it considers multiple near users and multiple far users. The source node can serve both the far user group and the near user group at the same time, which expands the number of users that can be accessed by the underwater wireless optical system, makes full use of all users in the system, improves the system resource utilization rate, and improves the spectrum efficiency and throughput of the entire system.

[0023] Secondly, this invention takes into account the channel characteristics caused by the combined effects of path loss and ocean turbulence in underwater environments, making it more suitable for practical applications; at the same time, by considering both decoding-forwarding and amplification-forwarding relay schemes, its application is more extensive and comprehensive. Attached Figure Description

[0024] Figure 1 This is a flowchart illustrating the implementation of the present invention;

[0025] Figure 2 A schematic diagram of a system model according to an embodiment of the present invention:

[0026] Figure 3 This is a graph showing the interruption probability curves for near and far users under different numbers of users according to the present invention;

[0027] Figure 4 This is a comparison chart of the overall outage probability curves of the present invention and existing systems based on orthogonal multiple access and non-orthogonal multiple access methods.

[0028] Figure 5 This invention only considers ocean turbulence, and includes outage probability curves under different receiving apertures and different relay forwarding protocols;

[0029] Figure 6 This is a graph showing the interruption probability curves for users near and far under different seawater environments.

[0030] Figure 7 This is a graph showing the interruption probability curves for near and far users under different target transmission rates according to the present invention;

[0031] Figure 8 This is a graph showing the interruption probability curves for near and far users under different power allocation coefficients according to the present invention; Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this invention clearer, the following detailed description is provided in conjunction with the accompanying drawings and specific embodiments. This embodiment is merely a illustrative description of the principles of this invention and does not represent any limitation thereof.

[0033] Reference Figure 1 The implementation steps for this example are as follows.

[0034] Step 1: Construct a cooperative non-orthogonal multiple access underwater multi-user diversity wireless optical communication system.

[0035] Reference Figure 2 The system model constructed for the embodiment includes an underwater source node, M distant users, and N near users, denoted as F1,...,F1,... M ,N1,...,N N ;

[0036] Optical signals experience energy loss during transmission through seawater channels. The channel fading coefficients between the source node and each distant user are denoted as... The channel fading coefficients between the source node and each nearby user are denoted as...

[0037]

[0038] Step 2: The source node selects the far user and near user with the best channel according to the multi-user scheduling algorithm.

[0039] Assuming the source node knows the channel state information of all distant and near users, select the maximum value from the M distant user fading coefficients. in Select the maximum value from N near-user fading coefficients. in Thus, the m-th * The nth remote user and the nth * Nearest users are far users with the best channel conditions. and close to users

[0040] Among them, the channel fading coefficient of the underwater wireless optical communication system and The following is confirmed:

[0041] In ocean channels, optical signal propagation is affected by factors such as absorption, scattering, and ocean turbulence, leading to attenuation and fluctuations in the intensity of the received optical signal. Path loss I caused by absorption and scattering effects and beam diffusion is a significant factor. l Its expression is:

[0042]

[0043] In the formula, D is the receiving aperture, θ is the divergence angle of the emitted beam, L is the link distance, c is the extinction coefficient, and T is the correction coefficient.

[0044] Under weak turbulence conditions, ocean turbulence can be modeled using the LN distribution model, and the fading I caused by ocean turbulence... c The probability density function is:

[0045]

[0046] In the formula, Let be the logarithmic variance of the light intensity. The light intensity flicker index.

[0047] Considering the combined effects of path loss and ocean turbulence, the channel fading coefficient of an underwater wireless optical communication system is g = I. l I c probability density function f g (g) is:

[0048]

[0049] Assuming that all underwater links are independent and identically distributed, the channel fading coefficients from the source node to the M distant users are obtained. probability density function Channel fading coefficients from the source node to N nearby users probability density function They are respectively:

[0050]

[0051]

[0052] By generating random numbers that follow the above probability distribution model through simulation, the channel fading coefficients from the source node to each user are obtained. and

[0053] From respectively and The channel fading coefficient with the largest value was selected as the ocean channel fading coefficient from the source node to the remote user. and the fading coefficient of the ocean channel from the source node to the user

[0054] Step 3: The source node transmits information with the selected user at the optimal distance from the selected channel.

[0055] 3.1) The source node transmits the signal x to the distant user through the power domain. F and the user signal x N By superimposing the signals, we obtain the superimposed signal x:

[0056]

[0057] Among them, P S α represents the electrical power transmitted by the source node. F For the power allocation factor of remote users, α N For the power allocation factor near the user, α F >α N , and α F +α N =1;

[0058] 3.2) The laser modulates the electrical signal x onto a semi-collimated Gaussian beam, and transmits the optical signal through an optical antenna into an underwater wireless optical channel with absorption, scattering and turbulence effects for propagation;

[0059] 3.3) Proximity to Users Heyuan users Optical signals are received using a photodetector, and the received electrical signals are obtained accordingly. and

[0060]

[0061]

[0062] Where ρ is the photoelectric conversion coefficient, υ is the electro-optic conversion coefficient, and n N The mean is 0 and the variance is Gaussian white noise, n F The mean is 0 and the variance is Gaussian white noise, and These are the ocean channel fading coefficients from the source node to the distant user and from the source node to the near user, respectively.

[0063] Step 4: The user resolves their signal x using a serial interference cancellation method. N .

[0064] Near-user signal reception is achieved through a serial interference cancellation algorithm. First, select the low-power signal near the user (x). N Treat it as interference, and solve for the high-power distant user signal x. F Signal-to-interference-to-noise ratio for:

[0065]

[0066] Then from the received signal Subtract x from the middle F To obtain intermediate signals Then from the intermediate signal Decipher its own signal x N Signal-to-interference-to-noise ratio for:

[0067]

[0068] Step 5: The nearby user acts as a relay to send signals to the distant user.

[0069] Nearby users act as relays to send signals to distant users. The transmission method can employ either decoding-and-forwarding or amplification-and-forwarding, where:

[0070] The specific implementation of the near user acting as a relay through the decoding and forwarding protocol is as follows: the near user processes the far user signal x decoded in step 4. F After re-encoding, the signal is forwarded to the remote user, who then receives the signal.

[0071]

[0072] in P represents the channel fading coefficient from near user to far user. N It is the relay transmission power.

[0073] The specific implementation of the near user as a relay through the amplification and forwarding protocol is as follows: the near user relays the signals it receives. The signal is amplified and forwarded directly to the remote user.

[0074]

[0075] in It is a variable amplification gain, which is related to the channel state between the source node and the nearest user. related.

[0076] Step 6: The remote user extracts its own signal by selecting and merging the two signals sent from the source node and the relay node.

[0077] Received signals from the source node by the remote user Solve the signal x in F Signal-to-interference-to-noise ratio

[0078]

[0079] The remote user receives the signal from the relay node via the decoding and forwarding protocol. Solve the signal x in F Signal-to-interference-to-noise ratio

[0080]

[0081] The remote user receives the signal from the relay node via the amplification and forwarding protocol. Solve the signal x in F Signal-to-interference-to-noise ratio for:

[0082]

[0083] Remote users compare the received signal-to-interference-plus-noise ratio (SIR) of the two links. and or Select the output signal from the link with the higher signal-to-noise ratio as the decoded signal x. F .

[0084] Interruption probability is a parameter used to measure the frequency of interruption events in a communication system. To more accurately analyze the effectiveness of this invention, further proof can be obtained through the following interruption probability simulation.

[0085] I. Simulation Conditions

[0086] The simulation software used is Matlab;

[0087] Simulation parameters are shown in Tables 1 and 2;

[0088] Table 1. Values ​​of extinction coefficient and correction factor for different aquatic environments.

[0089] Types of water Extinction coefficient c Correction factor T Pure seawater 0.056 0 clear seawater 0.15 0.05 Coastal seawater 0.305 0.13

[0090] Table 2 Parameters of a Cooperative Non-Orthogonal Multiple Access Underwater Multi-User Diversity Wireless Optical Communication System

[0091]

[0092] Based on the LN ocean turbulence distribution, and considering the impact of path loss, the probability of outage near the user. Disruption probability of remote users under the decode-forward protocol And the probability of outage for remote users under the amplified forwarding protocol The calculation formula is as follows:

[0093]

[0094]

[0095]

[0096] In the formula, and R1 and R2 are the signal-to-noise ratio thresholds for near and far users, respectively, and the target transmission rates for near and far users are the target transmission rates for near and far users, respectively. and It is the average signal-to-noise ratio of the source node and the relay node, x k It is a Laguerre polynomial The k-th root, denoted by , where is the weight in the Laguerre formula, and n is the total number of terms in the Gaussian Laguerre polynomial.

[0097] If μ2 > θ, the interruption probability near the user Disruption probability of remote users under the decode-forward protocol And the probability of outage for remote users under the amplified forwarding protocol All are 1;

[0098] The theoretical value of the above interruption probability was calculated analytically using MATLAB, and then verified by Monte Carlo simulation.

[0099] II. Simulation Content and Results

[0100] Under the above simulation conditions, the interruption performance of the present invention for both near and far users is simulated.

[0101] Simulation 1: By varying the number of near and far users, the simulation project simulates the change curve of the near and far user interruption probability with respect to the average signal-to-noise ratio in clear seawater, and compares it with the theoretical value calculated using the above formula. and The comparison yielded the following results: Figure 3 As shown. From Figure 3 It can be seen that, compared to a single-user system (N=M=1), multi-user diversity technology can improve the performance of both near and far users. It can also be seen that the more near and far users there are, the better the interrupt performance for both.

[0102] Simulation 2, in a coastal seawater environment, simulates the change in system outage probability with respect to the average signal-to-noise ratio for the present invention and existing methods based on orthogonal multiple access and non-orthogonal multiple access. The results are as follows: Figure 4 As shown. From Figure 4 It can be seen that, compared with existing orthogonal multiple access and non-orthogonal multiple access methods, the present invention has the lowest interruption probability under the same average signal-to-noise ratio, thus proving that the present invention has the best performance.

[0103] Simulation 3: Changing the receiving aperture, the simulation examines the changes in the interruption probability of near and far users of the present invention with respect to the average signal-to-noise ratio under both decoding-forwarding and amplification-forwarding protocols, considering only ocean turbulence conditions. These changes are then compared with the theoretical values ​​calculated using the aforementioned formula. and The comparison yielded the following results: Figure 5 As shown. From Figure 5 It can be seen that the theoretical and simulated values ​​fit well, proving the correctness of the invention. Furthermore, it can be seen that the performance of the decode-forward protocol is superior to that of the amplify-forward protocol, and the larger the receiving aperture, the better the interruption performance for both near and far users.

[0104] Simulation 4: The seawater environment was altered, and the simulation curves showing the intermittent probability of the present invention for near and far users as a function of the average signal-to-noise ratio were compared with the theoretical value calculated using the above formula. and The comparison yielded the following results: Figure 6 As shown. From Figure 6 It can be seen that the theoretical and simulated values ​​fit well, proving the correctness of the invention. At the same time, it can be seen that as seawater quality deteriorates, the interruption performance for both near and far users worsens.

[0105] Simulation 5: By changing the target transmission rate, the simulation shows the change curve of the interruption probability of near and far users in clear seawater as a function of the average signal-to-noise ratio, and compares it with the theoretical value calculated by the above formula. and The comparison yielded the following results: Figure 7 As shown. From Figure 7 It can be seen that the higher the target transmission rate, the worse the interruption performance for users at different distances.

[0106] Simulation 6 simulates the interruption probability of near and far users in coastal seawater under different signal-to-noise ratio conditions, with respect to the power allocation factor α. N The curve of change is compared with the theoretical value calculated using the above formula. and The comparison yielded the following results: Figure 8 As shown. Among them, Figure 8 (a) represents the probability of near-user outage as a function of the power allocation factor α. N The curve of change, Figure 8 (b) represents the remote user outage probability as a function of the power allocation factor α. N The curve showing the change.

[0107] from Figure 8 (a) It can be seen that the interruption probability of the user closer to the power allocation factor α increases with the power allocation factor α. N The increase first decreases and then increases.

[0108] from Figure 8 (b) It can be seen that the interruption probability of remote users varies with the power allocation factor α. N It increases with the increase of .

Claims

1. A wireless optical communication method based on cooperative non-orthogonal multiple access and multi-user diversity, characterized in that, Includes the following steps: 1) Assuming the source node knows the channel state information of all near and far users, select the far user with the best channel state based on the multi-user scheduling algorithm. and a close user Perform data transmission; 2) Based on the electrical power P sent by the source node S The power allocation factor α for remote users F Power allocation factor α for users near the user N The source node superimposes the distant user signal x through the power domain. F and the user signal x N Simultaneously send superimposed signal x to users near and far: 3) Proximity to users Heyuan users Receive the signals sent by the source node and obtain the received signals respectively. and Where ρ is the photoelectric conversion coefficient, υ is the electro-optic conversion coefficient, and n N The mean is 0 and the variance is Gaussian white noise, n F The mean is 0 and the variance is Gaussian white noise, and These are the ocean channel fading coefficients from the source node to the distant user and from the source node to the near user, respectively. 4) Near the user, use serial interference to remove the signal; first demodulate the high-power signal x. F From the received signal Subtract x from the middle F To obtain intermediate signals From again Demodulate its own signal x N ; 5) The nearby user acts as a relay to send signals to the distant user via a decoding-forwarding protocol or an amplification-forwarding protocol. The distant user receives the signal... or in P represents the channel fading coefficient from near user to far user. N It is the relay transmission power. It is the magnification factor; 6) The remote user receives two signals from the source node and the relay node respectively. or By selecting and merging, the decoded signal x is obtained. F This completes wireless optical communication.

2. The method according to claim 1, characterized in that, In step (1), a far user with the best channel state is selected based on a multi-user scheduling algorithm. and a close user The following are determined respectively: Assume the source node knows the state information of all users, and there are M distant users and N near users, denoted as F1, ..., F2. M ,N1,...,N N ; Based on the characteristic that the underwater environment causes energy loss in the transmission of optical signals from the source node to each user, the channel fading coefficient between the source node and each remote user is denoted as... The channel fading coefficients between the source node and each nearby user are denoted as... Find the largest value from M fading coefficients and N fading coefficients respectively. and in Then select the far user with the best channel condition. and close to users 3. The method according to claim 1, characterized in that, The power allocation coefficient α near the user in step (2) formula N Power allocation factor α for remote users F The value α is determined according to the principle of allocating more power to distant users with poor channel conditions. F >α N , and α F +α N =1.

4. The method according to claim 1, characterized in that, In step (3), the ocean channel fading coefficient from the source node to the distant user is calculated. and the fading coefficient of the ocean channel from the source node to the user The following are determined respectively; Assuming that all underwater links are independent and identically distributed, the channel fading coefficients from the source node to the M distant users are obtained. probability density function Channel fading coefficients from the source node to N nearby users probability density function They are respectively: By generating random numbers that follow the above probability distribution model through simulation, the channel fading coefficients from the source node to each user are obtained. and From respectively and The channel fading coefficient with the largest value was selected as the ocean channel fading coefficient from the source node to the remote user. and the fading coefficient of the ocean channel from the source node to the user 5. The method according to claim 1, characterized in that, In step (5), the nearby user acts as a relay to send a signal to the distant user via a decoding and forwarding protocol. This is achieved by the nearby user demodulating the distant user signal x obtained in step (4). F After re-encoding, it is forwarded to the remote user.

6. The method according to claim 1, characterized in that, In step (5), the nearby user acts as a relay to send signals to the distant user via the amplification and forwarding protocol. The nearby user then transmits the received signals. The amplification gain is directly amplified and forwarded to the remote user. This gain is related to the channel state between the source node and the nearest user. related.

7. The method according to claim 1, characterized in that, In step (6), the remote user will forward the two received signals from the source node and the relay node respectively through decoding. In the middle, the signal x is decoded by selection and merging. F The following can be achieved; Received signals from the source node by the remote user Demodulated signal x F Signal-to-interference-to-noise ratio Received signals from relay nodes by remote users Demodulated signal x F Signal-to-interference-to-noise ratio Remote users compare the received signal-to-interference-plus-noise ratio (SIR) of the two links. and Select the output signal from the link with the higher signal-to-noise ratio as the decoded signal x. F .

8. The method according to claim 1, characterized in that, In step (6), the remote user receives two signals from the source node and the relay node respectively through amplification and forwarding. The signal x is decoded by selection and merging. F The following can be achieved; Received signals from the source node by the remote user Demodulated signal x F Signal-to-interference-to-noise ratio for Received signals from relay nodes by remote users Demodulated signal x F Signal-to-interference-to-noise ratio for: Remote users compare the received signal-to-interference-plus-noise ratio (SIR) of the two links. and Select the output signal from the link with the higher signal-to-noise ratio as the decoded signal x. F .

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