Serial interference cancellation multi-user detection method for underwater wireless optical code division multiple access communication system

Abstract

The invention discloses a multi-user detection method for an underwater wireless optical code division multiple access communication system, and belongs to the technical field of wireless optical communication. Comprising the following steps: aiming at a multi-user underwater wireless optical code division multiple access communication system, constructing a spreading code by using a pseudo-random sequence to carry out multi-user communication, and carrying out multi-user detection through an interference elimination method, so that the detection performance of weak users under the condition that power control is not ideal or near-far effects exist is effectively improved. Compared with using the optical orthogonal code as the spreading code, the method provided by the invention can improve the system user capacity under the condition of the same channel bandwidth.

Description

Technical Field

[0001] The invention belongs to the technical field of underwater wireless optical communication (UWOC), and relates to a multi-user signal detection method of an underwater wireless optical code division multiple access (OCDMA) communication system. Background Art

[0002] Underwater wireless optical communication (UWOC) has the advantages of high speed, low latency and high confidentiality, and has become the research focus of underwater communication. In recent years, researchers have studied the key technologies in UWOC systems, including channel modeling, coding modulation technology and optical power control technology. With the development of underwater wireless optical communication technology, the research focus of underwater wireless optical communication has gradually shifted from a single point-to-point communication link to a networked multi-user multiple communication link.

[0003] Among various multiple access schemes supporting multi-user communication, such as time division multiple access (TDMA), wavelength division multiple access (WDMA), space division multiple access (SDMA) and code division multiple access (CDMA), CDMA has the advantages of high bandwidth utilization, asynchronous access and strong anti-multipath fading ability, so it is suitable for underwater communication environment. In addition, at the cost of a certain signal bandwidth, spread spectrum technology can improve the sensitivity of the receiving end of the UWOC system, extend the transmission distance of the system, and improve the anti-interference ability of the system. CDMA using spread spectrum technology has more advantages than other multiple access schemes. However, due to the random access of each user, the non-strict orthogonality of the spread spectrum code and the asynchronous transmission in the actual channel, the optical signals between users of the CDMA system have serious mutual interference, namely multiple access interference (MAI). With the increase of the number of access users, the multiple access interference will increase rapidly, seriously affecting the performance of the system.

[0004] At present, the selection of spread spectrum codes in the research on multi-user underwater wireless OCDMA communication networks is mainly focused on optical orthogonal codes. However, the structural characteristics of optical orthogonal codes determine that the number of codewords available for users is limited, and the number of users allowed to access the system at the same time is limited. It is necessary to consider using other spread spectrum codes with more codewords to meet the communication needs of more users. In addition, by adopting a good multi-user detection method, the impact of multiple access interference can be reduced, and the receiving performance of the system can be effectively improved, thereby improving the quality of multi-user communication. Summary of the invention

[0005] In view of the above problems in the prior art, the purpose of the present invention is to provide a multi-user detection method, which adopts a pseudo-random sequence to construct a spread spectrum code to support multi-user communication and performs multi-user detection by an interference elimination method, so as to better meet the needs of multi-user communication.

[0006] The present invention is achieved through the following technical solutions:

[0007] A multi-user detection method for an underwater wireless OCDMA communication system comprises the following steps:

[0008] 1) Use pseudo-random sequences to construct the user's spreading code and despreading code;

[0009] 2) According to the underwater channel conditions, establish the communication model of multi-user underwater wireless OCDMA system;

[0010] 3) Set the signal decision mechanism at the receiving end;

[0011] 4) Using synchronization information, user power and other information, use interference elimination methods to restore the target user signal.

[0012] In the above technical solution, the present invention also has a further limited solution:

[0013] Further, in step 1), the user's spreading code and despreading code are constructed using a pseudo-random sequence and obtained by the following steps:

[0014] K pseudo-random sequences g of length L are k = {g k，1 , …, g k，l , ..., g k，L}, g k，l ∈{0, 1} After bitwise inversion and concatenation, we get the spreading code when user sends bit i and the spreading code when user k sends bit 1. Spreading code when sending bit 0 in Indicates g k，l (l∈[1, L]) is negated, so the spreading codes used by user k are orthogonal, satisfying (c k，0， c k，1 )=0.

[0015] The pseudo-random sequence g k The first position g k.l Take the opposite value The result of the subtraction is taken as the new sequence g′ k The bipolar sequence is used as the user's despreading code, i.e., g′ k = {g′ k,1 ,…,g′ k.i ,...,g′ k,L},in

[0016] Further, in step 2), the communication model of the multi-user underwater wireless OCDMA system is established by the following steps:

[0017] 2a) Establish a user signal transmission model in a multi-user underwater wireless OCDMA system:

[0018] For uplink transmission, OOK signal modulation is adopted, and the signal sent by the kth user can be expressed as:

[0019]

[0020] in, is the average power of the signal sent by the kth user, is the i-th binary bit data sent by the k-th user, is the jth chip of the spreading code used by the kth user to send the i-th bit of data, L is the length of the pseudo-random sequence (2L is the spreading code length), q rect (t) is a chip time T c The rectangular pulse inside, T b =2LT c is the duration of a bit;

[0021] 2b) Assuming a channel model affected by seawater absorption and scattering, the path loss in line-of-sight communication can be expressed as:

[0022] h sd (t)=(a1exp(-c1d sd )+a2exp(-c2d sd ))δ(t-t0) (2)

[0023] Among them, d sd represents the distance that the light wave propagates in the water body. a1, c1, a2 and c2 are the parameters of the double exponential function, and their values ​​are obtained by fitting the channel response curve obtained by Monte Carlo simulation of the selected wavelength and seawater turbidity. The time t0 spent by the signal in the channel can be calculated as t0 = d sd / v, where v is the speed of light in water;

[0024] 2c) Establish the receiving signal model of the multi-user underwater wireless OCDMA system:

[0025] The signals transmitted by the K users as signal sources in step 2a) reach the receiving end after passing through the underwater channel set in step 2b). The uplink optical signal received at the input aperture of the photoelectric converter can be expressed as:

[0026]

[0027] Where * represents the convolution operation, K is the number of users, and n(t) represents the noise term at the receiving end.

[0028] Further, in step 3), the decision mechanism of the received signal is obtained by the following steps:

[0029] The number of photoelectrons generated by the photodetector within one chip time follows the Poisson distribution, and the corresponding probability density function expression is The average number of photoelectrons Γ(*) is the gamma function. For all users, the statistics of the number of photons determine the decision of two hypotheses H0 and H1 for sending bits, where H0 assumes that the user sends a symbol bit 0, and H i It is assumed that the transmitted sign bit is 1. Z = (z (1) , ..., z (j) ,…,z (2L) ) is the photon number statistic in one bit symbol at the receiving end, z (j) is the photon count at the jth code position, and the photon count z at the jth code position (j) Subtract the photon count z at the j+Lth symbol position (j+L) Obtain the statistic Z′=(z′ (l) ,…,z′ (l) ,...,z′ (L) ), where z′ (l) =z (l) -z (l+L) , l∈[1,L] are independently distributed, so the decision statistic of the receiver for the signal sent by user k can be expressed as

[0030]

[0031] Through the above decision mechanism, the bit sequence actually sent by user k can be recovered from the received signal.

[0032] Further, in step 4), the interference elimination signal detection scheme for the target user is obtained by the following steps:

[0033] 4a) Use a matched filter bank to obtain the power strength relationship of all users except the target user, and obtain the order of eliminating interfering users according to the power strength order p = {p1, ..., p m , ..., p K-1}, m∈[1,K-1], where p m represents the user number of the mth interfering user to be eliminated;

[0034] 4b) Handling interfering users p m In the process, the matched filter is used to obtain the interference user p m After the test statistic T is obtained, the estimation of the actual bit sequence sent by the user is obtained through the decision mechanism in step 3). m The estimated bit sequence is re-spread using the spreading code, and the interfering user p is reconstructed according to its signal power and channel m The optical signal at the receiving end;

[0035] 4c) Based on the interfering user p m The reconstructed signal of the target user is used to eliminate the interference caused to the target user, that is, the reconstructed user p is subtracted from the received signal according to the synchronization information. m signal, that is, to a certain extent, the interference user p is eliminated m The impact of

[0036] 4d) Select the next interfering user and repeat steps 4b) to 4c) until the influence of all reconstructed interfering user signals on the target user signal detection is removed from the received signal, and finally the target user's transmission signal is restored.

[0037] The channel information used in the above process may be ideal channel information or channel information obtained by using a channel estimation method.

[0038] The present invention has the following advantages:

[0039] The present invention proposes a multi-user detection method for alleviating multi-user interference in an underwater wireless OCDMA communication system. The method realizes multi-user communication by constructing a spread spectrum code based on a pseudo-random sequence. Compared with using an optical orthogonal code as a spread spectrum code, the method improves the user capacity under the same bandwidth. At the same time, after multi-user interference elimination, the interference of a high-power user to a low-power user can be effectively reduced, and the adverse effect of a strong user on a weak user can be overcome to a certain extent, which is conducive to weak users to obtain better signal detection effects when power control is not ideal or there is a near-far effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 It is a block diagram of a multi-user UWOC system;

[0041] Figure 2 This is a flow chart of the multi-user detection algorithm proposed in the present invention. DETAILED DESCRIPTION

[0042] To make the purpose and technical solution of the present invention clearer, the following is a further detailed description in conjunction with the accompanying drawings and specific implementation methods. This embodiment only represents the principle description of the present invention and does not represent any limitation to the present invention.

[0043] The present invention is a multi-user detection method based on an underwater wireless optical code division multiple access (OCDMA) communication system. The specific implementation method and application examples are described as follows:

[0044] Step 1), using a pseudo-random sequence to construct a user's spreading code and despreading code is obtained by the following steps:

[0045] K pseudo-random sequences g of length L are k = {gk，l , …, g k，l , …, g k，L}, g k，l ∈{0, 1} After bitwise inversion and concatenation, we get the spreading code when user sends bit i and the spreading code when user k sends bit 1. Spreading code when sending bit 0 in Indicates g k.l (l∈[1, L]) is negated, so the spreading codes used by user k are orthogonal, satisfying (c k，0 , c k,l )=0.

[0046] The pseudo-random sequence g k The first position g k,l Take the opposite value The result of the subtraction is taken as the new sequence g′ k The first bit of this bipolar sequence is used as the user's despreading code, that is, g′ k = {g′ k,1 ,...,g′ k,l ,…,g′ k,L},in

[0047] Step 2), establish the communication model of the multi-user underwater wireless OCDMA system through the following steps:

[0048] 2a) Establish a user signal transmission model in a multi-user underwater wireless OCDMA system:

[0049] For uplink transmission, OOK signal modulation is adopted, and the signal sent by the kth user can be expressed as:

[0050]

[0051] in, is the average power of the signal sent by the kth user, is the i-th binary bit data sent by the k-th user, is the jth chip of the spreading code used by the kth user to send the i-th bit of data, L is the length of the pseudo-random sequence (2L is the spreading code length), q rect (t) is a chip time T c The rectangular pulse inside, T b =2LT c is the duration of a bit;

[0052] 2b) Assuming a channel model affected by seawater absorption and scattering, the path loss in line-of-sight communication can be expressed as:

[0053] h sd (t)=(a1exp(-c1d sd )+a2exp(-c2d sd ))δ(t-t0) (2)

[0054] Among them, d sd represents the distance that the light wave propagates in the water body. a1, c1, a2 and c2 are the parameters of the double exponential function, and their values ​​are obtained by fitting the channel response curve obtained by Monte Carlo simulation of the selected wavelength and seawater turbidity. The time t0 spent by the signal in the channel can be calculated as t0 = d sd / v, where v is the speed of light in water;

[0055] 2c) Establish the receiving signal model of the multi-user underwater wireless OCDMA system:

[0056] The signals transmitted by the K users as signal sources in step 2a) reach the receiving end after passing through the underwater channel set in step 2b). The uplink optical signal received at the input aperture of the photoelectric converter can be expressed as:

[0057]

[0058] Where * represents the convolution operation, K is the number of users, and n(t) represents the noise term at the receiving end.

[0059] The noise of underwater optical communication system is mainly composed of background noise, thermal noise and dark current noise. The noise variance of background radiation can be calculated as Where q is the electron charge, B is the electron bandwidth, represents the response coefficient of the photodetector, η is the quantum efficiency of the photodetector, h is Planck's constant, f is the frequency of the light wave in the seawater, and the solar radiation power P b It can be calculated as P b =A r F ov F ac ΔλT F L sol , where A r =πD 2 / 4, D is the diameter of the receiving aperture, F ov F ac =π(FOV) 2 , FOV is the field of view, Δλ is the filter bandwidth, T F is the transmittance of the filter, solar radiation L sol It can be calculated as Where E is the submerged irradiance, R is the reflectivity of the underwater irradiance, and L fac is a factor describing the directional dependence of underwater irradiance, K is the diffusion attenuation coefficient, Dw is the underwater depth; the dark current noise variance can be calculated as Among them I dc is the photodetector dark current; the thermal noise variance can be calculated as Where T e represents the equivalent temperature, F represents the system noise index, R L represents the load resistance. Each noise is independent of each other and can be regarded as additive Gaussian white noise. Therefore, the total noise can be modeled as having a mean of 0 and a variance of Gaussian white noise.

[0060] Step 3), the decision mechanism of the received signal is obtained by the following steps:

[0061] Using OOK modulation and a PIN photodiode at the receiving end, the photoelectron count rate generated by background noise, thermal noise, and dark current noise can be calculated as The thermal noise power P th It can be calculated as The photoelectron count rate generated by the transmitted signal of user k can be calculated as In the formula is the average signal power of user k at the receiving end, which can be calculated as in is the average power of the signal sent by user k.

[0062] The number of photoelectrons generated by the photoelectric conversion device at the jth code chip position of the received signal follows a Poisson distribution, and the mean can be calculated as where Λ j represents the set of users that send pulse signals at the jth chip position, T c is the chip time. (1) , ..., z (j) ,…,z (2L) ) is the photon number statistic in one bit symbol at the receiving end, z (j) is the photon count at the jth code position, and the photon count z at the jth code position (j) Subtract the photon count z at the j+Lth symbol position (j+L) Obtain the statistic Z′=(z′ (l) ,…,z′ (l) ,…,z′ (L) ), where z′ (l) =z (l) -z (l+L) , l∈[1,L] are independently distributed, so the decision statistic of the receiver for the signal sent by user k can be expressed as

[0063]

[0064] Through the above decision mechanism, the bit sequence actually sent by user k can be recovered from the received signal.

[0065] In step 4), the target user signal is restored using the interference elimination method by the following steps:

[0066] 4a) When the user power control is not ideal or there is a near-far effect, the strong user will cause great interference to the detection of the weak user. Before eliminating the interference effect of the strong user in the received signal, there will be a large error in the detection of the weak user. Therefore, a matched filter group is used to perform regular sampling and integration on all users except the target user. The power strength relationship between the users is obtained according to the integration result, and the order of eliminating interfering users is obtained according to the power strength order p = {p1, ..., p m , ..., p K-1}, m∈[1,K-1], where p m represents the user number of the mth interfering user to be eliminated;

[0067] 4b) Handling interfering users p m In the process, the matched filter is used to obtain the interference user p m After the test statistic T is obtained, the estimation of the actual bit sequence sent by the user is obtained through the decision mechanism in step 3). Then, use the interference user p m The estimated bit sequence is re-spread using the spreading code, and the interfering user p is reconstructed according to its signal power and channel m The optical signal at the receiving end

[0068] 4c) Based on the interfering user p m The reconstructed signal of the target user is used to eliminate the interference caused to the target user, that is, the reconstructed user p is subtracted from the received signal according to the synchronization information. m signal, if y is used m represents the received signal after eliminating the first m interfering users, then the interfering user p is processed m+1 The received signal at time can be expressed as That is to say, the interference of user p is eliminated to a certain extent. m The impact of

[0069] 4d) Select the next interfering user and repeat steps 4b) to 4c) until the influence of all reconstructed interfering user signals on the target user signal detection is removed from the received signal, and the signal obtained after removing all interfering user interference estimation from the received signal is Finally, the target user's transmitted signal is restored through a matched filter.

[0070] The present invention is not limited to the above-mentioned embodiments. On the basis of the technical solution disclosed in the present invention, technicians in this field can make some substitutions and deformations to some technical features therein according to the disclosed technical content without creative labor, and these substitutions and deformations are all within the protection scope of the present invention.

Claims

1. A serial interference elimination multi-user detection method for an underwater wireless optical code division multiple access (OCDMA) communication system, characterized in that: The method comprises the following steps: 1) Use pseudo-random sequences to construct the user's spreading code and despreading code; 2) According to the underwater channel conditions, a communication model of a multi-user underwater wireless OCDMA system is established; 3) Set the signal decision mechanism at the receiving end; 4) Using synchronization information, user power and other information, use interference elimination methods to restore the target user signal.

2. The serial interference elimination multi-user detection method for underwater wireless OCDMA communication system according to claim 1, characterized in that: In the step 1), the spreading code and despreading code of the user are constructed using a pseudo-random sequence by the following steps: K pseudo-random sequences g of length L are k = {g k,1 , …, g k,l , ..., g k，L }, g k，l ∈{0, 1} After bitwise inversion and concatenation, we get the spreading code when user sends bit i and the spreading code when user k sends bit 1. Spreading code when sending bit 0 in Indicates g k,l (l∈[1, L]) is negated, so the spreading codes used by user k are orthogonal, satisfying (c k,0 , c k，1 )=0. The pseudo-random sequence g k The first position g k，l Take the opposite value The result of the subtraction is taken as the new sequence g′ k The first bit of this bipolar sequence is used as the user's despreading code, that is, g′ k = {g′ k,1 ,…,g′ k，l , ..., g′ k,L },in 3. The serial interference elimination multi-user detection method for underwater wireless OCDMA communication system according to claim 1, characterized in that: In the step 2), the communication model of the multi-user underwater wireless OCDMA system is established by the following steps: 2a) Establish a user signal transmission model in a multi-user underwater wireless OCDMA system: For uplink transmission, OOK signal modulation is adopted, and the signal sent by the kth user can be expressed as: in, is the average power of the signal sent by the kth user, is the i-th binary bit data sent by the k-th user, is the jth chip of the spreading code used by the kth user to send the i-th bit of data, L is the length of the pseudo-random sequence (2L is the spreading code length), q rect (t) is a chip time T c The rectangular pulse inside, T b =2LT c is the duration of a bit; 2b) Assuming a channel model affected by seawater absorption and scattering, the path loss in line-of-sight communication can be expressed as: h sd (t)=(a1exp(-c1d sd )+a2exp(-c2d sd ))δ(t-t0) (2) Among them, d sd represents the distance that the light wave propagates in the water body. a1, c1, a2 and c2 are the parameters of the double exponential function, and their values ​​are obtained by fitting the channel response curve obtained by Monte Carlo simulation of the selected wavelength and seawater turbidity. The time t0 spent by the signal in the channel can be calculated as t0 = d sd / v, where v is the speed of light in water; 2c) Establish the receiving signal model of the multi-user underwater wireless OCDMA system: The signals transmitted by the K users as signal sources in step 2a) reach the receiving end after passing through the underwater channel set in step 2b). The uplink optical signal received at the input aperture of the photoelectric converter can be expressed as: Where * represents the convolution operation, K is the number of users, and n(t) represents the noise term at the receiving end.

4. The serial interference elimination multi-user detection method for underwater wireless OCDMA communication system according to claim 1, characterized in that: In step 3), the decision mechanism of the received signal is obtained by the following steps: The number of photoelectrons generated by the photodetector within one chip time follows the Poisson distribution, and the corresponding probability density function expression is The average number of photoelectrons Γ(*) is the gamma function. For all users, the statistics of the number of photons determine the decision of two hypotheses H0 and H1 for the transmitted bit, where H0 assumes that the user sends a symbol bit of 0, and H1 assumes that the transmitted symbol bit is 1. Z = (z (1) ,…,z (j) ,…,z (2L) ) is the photon number statistic in one bit symbol at the receiving end, z (j) is the photon count at the jth code position, and the photon count z at the jth code position (j) Subtract the photon count z at the j+Lth symbol position (j+L) Obtain the statistic Z′=(z′ (1) ,...,z′ (l) ,...,z′ (L) ), where z′ (l) =z (l) -z (l+L) , l∈[1,L] are independently distributed, so the decision statistic of the receiver for the signal sent by user k can be expressed as Through the above decision mechanism, the bit sequence actually sent by user k can be recovered from the received signal.

5. The serial interference elimination multi-user detection method for underwater wireless OCDMA communication system according to claim 1, characterized in that: In step 4), the interference elimination signal detection scheme for the target user is obtained by the following steps: 4a) Use a matched filter bank to obtain the power strength relationship of all users except the target user, and obtain the order of eliminating interfering users according to the power strength order p = {p1, ..., p m , ..., p K-1 }, m∈[1,K-1], where p m represents the user number of the mth interfering user to be eliminated; 4b) Handling interfering users p m In the process, the matched filter is used to obtain the interference user p m After the test statistic T is obtained, the estimation of the actual bit sequence sent by the user is obtained through the decision mechanism in step 3). m The estimated bit sequence is re-spread using the spreading code, and the interfering user p is reconstructed according to its signal power and channel m The optical signal at the receiving end; 4c) Based on the interfering user p m The reconstructed signal of the target user is used to eliminate the interference caused to the target user, that is, the reconstructed user p is subtracted from the received signal according to the synchronization information. m signal, that is, to a certain extent, the interference user p is eliminated m The impact of 4d) Select the next interfering user and repeat steps 4b) to 4c) until the influence of all reconstructed interfering user signals on the target user signal detection is removed from the received signal, and finally the transmission signal of the target user is restored.

6. The serial interference elimination multi-user detection method for underwater wireless OCDMA communication system according to claim 1 is characterized in that The channel information used by the receiving end in the process may be assumed ideal channel information or channel information obtained by using a channel estimation method.

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