A transmission method of a non-orthogonal multiple access system based on double-frequency index modulation

Abstract

The application discloses a kind of non-orthogonal multiple access system transmission methods based on dual-frequency index modulation.The method is first in the signal processing end of base station to the input multi-user bit stream signal respectively carries out specific M-QAM modulation and dual-frequency index modulation, then generates OFDM-DFIM (Dual Frequency Index Modulation) modulation signal using orthogonal frequency division multiplexing technology, finally compares the channel conditions of each user, and allocates power for each user, and is superimposed after being sent to time domain.Thus, the signal of multiple users is transmitted simultaneously in the same frequency domain in the power domain.In the signal receiving end, signal separation is carried out using serial interference cancellation technology, and then the bit stream signal of each user is demodulated and restored in turn by maximum likelihood estimation.The method improves the proportion of index information in the total information, thereby improving the spectral efficiency and reducing the bit error rate, thereby further improving the system performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of wireless communications, and relates to a method for processing index modulation data in the communication field, and in particular to a non-orthogonal multiple access system transmission method based on dual-frequency index modulation. Background Art

[0002] Non-orthogonal Multiple Access (NOMA), as an alternative to orthogonal multiple access (OMA), is a key technology in 5G wireless communications and has great application prospects. NOMA introduces a new dimension - the power domain - through multiplexing in resource blocks. It can allocate different users in the power domain and enable multiple users to transmit in the same channel, using the same frequency and time domain resources, resulting in significant gains in spectrum efficiency. In addition, in applications, the NOMA system can allocate different transmission powers to different user signals based on the detected channel conditions of each user. Reasonable power allocation between multiplexed users can effectively improve the system transmission performance. When receiving signals, the NOMA system uses SIC (Serial Interference Cancellation) technology to detect superimposed signals, treating the user signals with low power as noise, directly decoding the high-power user signals, and then subtracting the high-power signals to achieve user signal separation.

[0003] Index Modulation OFDM (OFDM-IM) is a new multi-carrier transmission technology. The principle of IM is to encode data into a combination of active / silent sub-elements in many different domains, such as frequency domain, spatial domain, channel domain, and time domain, so as to achieve additional transmission bit information without consuming energy. More specifically, it is achieved by activating only a small part of certain index resource entities, such as subcarriers, antennas, time slots, or channel states for data transmission, while the rest remain unused, and other information bits are implicitly conveyed through index usage or activation pattern.

[0004] However, IM index modulation technology uses a large number of silent elements during signal transmission, where no data is transmitted. The implicitly transmitted index information accounts for a small proportion of the total information, reducing spectral efficiency and increasing demodulation complexity at the receiver. NOMA systems have certain limitations. While NOMA can achieve user fairness through appropriate power allocation, improving performance for weak users can degrade performance for users with strong channel conditions. Summary of the Invention

[0005] In view of the deficiencies of the prior art, the present application provides a non-orthogonal multiple access system transmission method based on double-frequency index modulation, which divides a subcarrier block into two parts on average, divides modulation symbols into in-phase components and quadrature components, so that input bit information and indexes jointly confirm the positions of activated subcarriers, thereby implicitly transmitting additional signal bits, improving spectral efficiency, reducing the bit error rate by increasing the index proportion in user signals, and further improving the performance of the IM-NOMA system.

[0006] The non-orthogonal multiple access system transmission method based on double-frequency index modulation is implemented based on a downlink non-orthogonal multiple access system NOMA-IM system, and the bit stream signals of near users and far users are subjected to M-QAM modulation of a specific alphabet and double-frequency index modulation at a signal processing end of a base station, each user is allocated different proportions of signal power at the base station, and the signals are superimposed and transmitted to multiple users. At a signal receiving end, after the far user allocated with high-power signals ignores the influence of low-power near user signals, the far user directly uses ML (Maximum Likelihood Detection) to detect the signals. The near user signal uses SIC (Serial Interference Cancellation) technology to separate the direct signal from the superimposed signal and then uses ML to detect the signal. The method specifically comprises the following steps:

[0007] Step 1: Convert a user input Q-bit signal into a serial-parallel conversion according to the M-QAM symbol of modulation, determine the number of bits q needed by each group according to the number of activated subcarriers and the modulation order M, divide the Q-bit signal into k groups, k = Q / q, and when Q < k*q, add 0-bit signals to the end of the original signal. , , wherein the above-mentioned k and q are integers, and the above-mentioned Q is an integer. The above-mentioned k and q are integers, and the above-mentioned Q is an integer. The length of the signal is an integer. Divide N all subcarriers into g equal small blocks, and each small block has a fixed number of subcarriers, 2N, g = N all / 2N.

[0008] Step 2: Map the information of the k groups to the subcarriers of the g small blocks in order. The bit signals in each group are divided into q1, q2 and q3 parts, wherein the q1 part selects the M-QAM modulated signal to be mapped to the corresponding activated subcarriers, including the in-phase component index and the quadrature component index:

[0009] (1)

[0010] N0 is the number of activated subcarriers in the first N subcarriers of each small block.

[0011] The q2 part is the M-QAM modulated bit signal carried on the corresponding activated subcarrier:

[0012] (2)

[0013] M is the base number of the quadrature amplitude modulation (M-QAM) symbol.

[0014] The q3 part is the dual-frequency compensation index signal bit. The number of bits required for q3 is 2N0. The activated subcarrier that needs to be changed in the q1 part is determined based on the q3 part. The subcarrier corresponding to 1 in the q3 part is selected. The Nth subcarrier after the subcarrier is selected as the changed activated subcarrier to complete the dual-frequency compensation index.

[0015] The in-phase vector I that needs to be transmitted on the βth subcarrier block i and the orthogonal vector I q Respectively expressed as:

[0016] (3)

[0017] (4)

[0018] where i i (r) and i q (r) denotes the in-phase and quadrature transmission symbols of the rth subcarrier in the βth subcarrier block, r = 1, ..., 2N, the superscript T denotes transposition, β = 1, ..., g. The complete signal transmitted on the βth subcarrier block is generated by modulating a set of bit signal indices, and the transmitted symbol vector I is:

[0019] (5)

[0020] i(r) represents the data symbol generated by index modulation transmitted on the rth subcarrier in the βth subcarrier block. The g subcarrier blocks are combined to generate an OFDM-DFIM modulation symbol X:

[0021] (6)

[0022] X(β) represents the modulation symbol generated by modulating a set of bit signals indexed by the βth subcarrier block. Therefore, a single user message needs to be transmitted using W = k / g OFDM symbols. When W is not an integer, a zero bit signal is appended to the end of the last OFDM symbol to complete it.

[0023] Step 3: Generate OFDM-DFIM modulation symbols X1, X2, .....X for each user in sequence according to the method of steps 1 and 2. i, i is the number of users. Then power is allocated according to the channel status information of each user, and then the signals are superimposed:

[0024] (7)

[0025] Among them, Y represents the frequency domain transmission signal after multi-user superposition, P BS is the total transmission power of the base station, α1, α2, .....α i is the power allocation coefficient of the user, and α1+α2+.....+α i =1,n~CN(0,σ 2 ) has a mean of 0 and a variance of σ 2 Complex baseband additive white Gaussian noise.

[0026] Construct a conjugate symmetric vector matrix for the superimposed frequency domain transmitted signal Y:

[0027] (8)

[0028] Where Y(·)* represents the conjugate matrix of the corresponding matrix, N zp Indicates a matrix size of 1×N zp An all-zero matrix is ​​generated. Then, an inverse Fourier transform is performed to map the frequency domain signal Y into a time domain signal S and transmit it. This enables simultaneous superposition and transmission of multi-user signals in the power domain at the same frequency.

[0029] Step 4. When the near user receives the transmission signal S, it first subtracts the cyclic prefix to eliminate multipath interference, then performs channel equalization and inverse Fourier transform according to the pilot, and then divides the signal into in-phase signal and orthogonal signal. According to the index set, the q3 part index bit information in each group signal is obtained, and the q1 part index bit information is determined according to the position of the activated subcarrier in the frequency domain before dual-frequency index modulation. Finally, the q2 part bit information after M-QAM modulation is restored by combining the in-phase component and the orthogonal component to restore the signal before dual-frequency index modulation.

[0030] Step 5: For the signal restored in step 4, separate the superimposed signals using serial interference cancellation technology, and then demodulate the multi-user modulated signal using maximum likelihood estimation. The specific steps are as follows:

[0031] s5.1. Based on the signal index set and the power allocation size, use the following ML detection expression to detect the signal of the user with the highest power: :

[0032] (9)

[0033] Where Y1 and H1 are the received signal and channel gain of the detected near user, b1 is the M-QAM modulation symbol vector of the far user, and B1 is the M-QAM modulation symbol set of the far user.

[0034] s5.2、The high power signal of the first demodulation After reconstructing the signal X', the timing signal Y of the low-power near-user received signal is obtained through SIC (Serial Interference Cancellation) 1-2 :

[0035] (10)

[0036] s5.3, for Y 1-2 Perform ML detection and modulate the bit stream signal of user 2 :

[0037] (11)

[0038] b2 is the near-user M-QAM modulation symbol vector, and B2 is the near-user M-QAM modulation symbol set.

[0039] s5.4. Consider the signal of low-power user 2 as noise and use the following ML detection expression to directly detect the received signal of the distant user:

[0040] (12)

[0041] Y2 and H2 are the received signal and channel gain detected at the far user, respectively. is the high-power far-user signal obtained after ML detection at the far-user location.

[0042] The present invention has the following beneficial effects:

[0043] By evenly dividing the subcarrier block into two parts and the modulation symbol into an in-phase component and a quadrature component, the input bit information and index jointly determine the activated subcarrier position, thereby increasing the proportion of index information in the total information and spectral efficiency. Increasing the index ratio in the user signal reduces the bit error rate, further improving the performance of the IM-NOMA system. Compared with traditional NOMA systems, this reduces the bit error rate of nearby users, ensures a certain level of user communication fairness, and reduces the complexity of ML detection required in the receiving stage. The user signal modulation order can also be adjusted based on different channel state information, improving signal transmission speed while maintaining a certain bit error rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 This is the block diagram of the system sending end of this method;

[0045] Figure 2 It is a simulation curve diagram of the system of the present method and the NOMA system in the embodiment; DETAILED DESCRIPTION

[0046] The present invention will be further explained below with reference to the accompanying drawings;

[0047] A non-orthogonal multiple access system transmission method based on dual-frequency index modulation, such as Figure 1 The specific steps are as follows:

[0048] Step 1: In this embodiment, it is assumed that the signal processing end of the base station first inputs the signals of the near user and the far user, each including Q=2000 random bits, and the number of subcarriers N all = 80. All subcarriers are divided into g = 10 subcarrier blocks, and the number of subcarriers in each block is 2N = 8.

[0049] The signal is modulated with 4-ary QAM and the input signal is grouped. The number of bits in each group is q=16, so it can be divided into k=Q / q=125 groups.

[0050] Step 2: Map the information of k groups to g small blocks of subcarriers in sequence. The g subcarrier groups are combined into one OFDM symbol. Then the user information that needs to be sent is W = k / g = 12.5 OFDM symbols. Since W is not an integer, a 0-bit signal is added to the end of the last OFDM symbol to complete it into one OFDM symbol. For one user, a total of 13 OFDM symbols need to be sent.

[0051] In this embodiment, the number of subcarriers selected for activation is 3 among the 4 subcarriers. Therefore, the number of bits that can be carried by the q1 part in each bit signal group is 4 bits. Assuming that the q1 part is {0111}, the in-phase signal activates 1, 3, and 4 of the first 4 subcarriers, and the orthogonal signal activates 1, 2, and 3 of the first 4 subcarriers. The q2 part has a total of 6 bits of signal for QAM modulation, and the generated constellation symbol is: i i (1)+i q (1), i i (2)+i q (2), i i (3)+i q (3). Therefore, in this subcarrier block, the signal before dual-frequency index modulation is:

[0052] {i i (1)+i q (1), 0+i q (2), i i (2)+i q(3), i i (3) +0, 0, 0, 0, 0

[0053] Because the q1 part only selects 3 subcarriers to be activated for 4-QAM modulation of the in-phase component and the quadrature component of the symbol on the first 4 subcarriers in each subcarrier block, and the q3 part only selects the changed positions in the last 4 subcarriers in each subcarrier block, there is a bit signal for each subcarrier to select whether to change the position, and there are 6 input random conditions for whether to change the subcarrier position, so the q3 can carry 6 bits. Assuming that the q3 part is {010110}, the position of the 2nd subcarrier in the in-phase signal is changed to 7, and the positions of the 1st and 2nd subcarriers in the quadrature signal are changed to 5 and 6, so the signal after the dual-frequency index modulation is:

[0054] {i i (1), 0, i q (3), i i (3), i q (1), i q (2), i i (2), 0

[0055] The index mapping relationship between the in-phase and quadrature components is shown in Table 1:

[0056] Table 1

[0057]

[0058] Step 3, after the data of the far user and the near user are index-modulated according to the method in steps 1 and 2, power allocation is performed according to the channel state information, the signals after the power allocation are superimposed to generate a total transmission signal, and the total transmission signal is conjugated and inverse Fourier transformed to obtain a time-domain signal for transmission.

[0059] Step 4, after receiving the signal, the far user receiver performs channel equalization, and then regards the superimposed low-power user signal as noise, directly demodulates the high-power signal according to the corresponding bit index set to obtain the original signal. After receiving the signal, the near user receiver performs channel equalization according to the channel state information, demodulates the high-power signal, and then reconstructs the high-power signal according to the SIC principle. After the total power at the receiver is reduced by the reconstructed signal, the low-power signal is left to demodulate the low-power signal by ML.

[0060] The bit error rate of the above method under different signal-to-noise ratios in a Rayleigh fading channel with multipath interference is calculated, and the performance of the theoretical NOMA system is compared, and the results are as follows Figure 2As shown in the figure. Within a signal-to-noise ratio of 20-25dB, this method achieves a reduction in the bit error rate for near-user signals. Due to the reduction in the number of signals that need to be sent and the improvement in system spectrum utilization, the overall system power consumption of this index-modulated NOMA system is significantly lower than that of a NOMA system without index modulation. Furthermore, the system maintains good communication quality in multipath environments.

Claims

1. A non-orthogonal multiple access system transmission method based on dual-frequency index modulation, implemented based on a downlink non-orthogonal multiple access system NOMA-IM system, characterized by: The specific steps include: Step 1: Perform serial-to-parallel conversion on the Q-bit signal input by a user according to the modulated M-QAM symbol. The number of bits required for each group is determined according to the number of subcarriers to be activated and the modulation order M. The signal is divided into k groups with q-bit signals. all The subcarriers are divided into g equal small blocks; Step 2: Map the information of the k groups to the g small blocks of subcarriers in sequence; the bit signal in each group is divided into three parts: q1, q2, and q3. The q1 part selects the signal after M-QAM modulation and maps it to the corresponding activated subcarrier: N represents half of the number of subcarriers in each small block, and N0 is the number of activated subcarriers in the first N subcarriers of each small block; The q2 part is the M-QAM modulated bit signal carried on the corresponding activated subcarrier: q2=N0log2M (2) M is the base number of the quadrature amplitude modulation M-QAM symbol; The q3 part is used to determine the activated subcarrier that needs to be changed in the q1 part. For the subcarrier corresponding to 1 in the q3 part, the Nth subcarrier after the subcarrier is selected as the activated subcarrier after the change to complete the dual-frequency compensation index; Combine the information transmitted by g subcarrier blocks to generate an OFDM-DFIM modulation symbol X. After completing the information mapping of k groups, all OFDM-DFIM modulation symbols of a user are obtained. Step 3: Generate OFDM-DFIM modulation symbols X1, X2, .....X for each user in sequence according to the method of steps 1 and 2. i , i is the number of users; power is allocated according to the channel state information of each user and then the signals are superimposed: Among them, Y represents the frequency domain transmission signal after multi-user superposition, P BS is the total transmission power of the base station, α1, α2, .....α i is the power allocation coefficient of the user, and α1+α2+.....+α i =1, n~CN(0,σ 2 ) has a mean of 0 and a variance of σ 2 The complex baseband additive white Gaussian noise is generated; the frequency domain signal Y is mapped into a time domain signal S and sent; thereby achieving simultaneous superposition and transmission of signals of multiple users in the power domain at the same frequency; Step 4: When the near user receives the transmitted signal S, it first subtracts the cyclic prefix to eliminate multipath interference, then performs channel equalization and inverse Fourier transform according to the pilot, and then divides the signal into an in-phase signal and an orthogonal signal. The q3 part of the index bit information in each group signal is obtained according to the index set, and the q1 part of the index bit information is determined according to the position of the activated subcarrier in the frequency domain before dual-frequency index modulation. Finally, the q2 part of the bit information after M-QAM modulation is restored by combining the in-phase component and the orthogonal component to obtain the signal before dual-frequency index modulation; Step 5: For the signal restored in step 4, separate the superimposed signals using serial interference cancellation technology, and then demodulate them in sequence through maximum likelihood estimation to obtain the modulated signals of multiple users.

2. The non-orthogonal multiple access system transmission method based on dual-frequency index modulation according to claim 1, characterized in that: Indicates rounding up. When Q < k*q, Q / q is made an integer by adding a 0-bit signal at the end of the original signal.

3. The non-orthogonal multiple access system transmission method based on dual-frequency index modulation according to claim 1, characterized in that: The in-phase vector I that needs to be transmitted on the βth subcarrier block i and the orthogonal vector I q Respectively expressed as: I i =[i i (1),i i (2),…,i i (r),…i i (2N)] T (3) I q =[i q (1),i q (2),…,i q (r),…i q (2N)] T (4) where i i (r) and i q (r) denotes the in-phase and quadrature transmission symbols of the r-th subcarrier in the β-th subcarrier block, r = 1, ..., 2N, the superscript T denotes transposition, β = 1, ..., g; the complete signal transmitted on the β-th subcarrier block is generated by modulating a set of bit signal indices, and the transmitted symbol vector I is: I=I i +jI q =[i(1),i(2),…,i(r),…i(2N)] T (5) i(r) represents a data symbol generated after index modulation and transmitted on the r-th subcarrier in the β-th subcarrier block.

4. The non-orthogonal multiple access system transmission method based on dual-frequency index modulation according to claim 1 or 3, characterized in that: Combine g subcarrier blocks to generate an OFDM-DFIM modulation symbol X: X=[X(1),X(2),…X(β),…X(g)] T (6) X(β) represents the modulation symbol carried by the βth subcarrier block and generated by modulating a group of bit signal indices.

5. The non-orthogonal multiple access system transmission method based on dual-frequency index modulation according to claim 1, characterized in that: One user information needs to be sent through W OFDM symbols. When W=k / g is not an integer, a 0-bit signal needs to be added to the end of the last OFDM symbol to make it a complete OFDM symbol.

6. The non-orthogonal multiple access system transmission method based on dual-frequency index modulation according to claim 1, characterized in that: Construct a conjugate symmetric vector matrix for the superimposed frequency domain transmitted signal Y: [Y(1),Y(2),…Y(g),N zp ,Y(g) * ,…And(2) * ,Y(1) * ] (8) Where Y(·)* represents the conjugate matrix of the corresponding matrix, N zp Indicates a matrix size of 1×N zp All-zero matrix; Then perform inverse Fourier transform to convert the frequency domain signal into time domain signal.

7. The non-orthogonal multiple access system transmission method based on dual-frequency index modulation according to claim 1, characterized in that: The specific steps of obtaining the multi-user modulated signal through maximum likelihood estimation demodulation are as follows: s5.

1. Based on the signal index set and the power allocation size, use the following ML detection expression to detect the signal of the user with the highest power: Where Y1 and H1 are the received signal and channel gain of the detected near user, respectively; b1 is the M-QAM modulation symbol vector of the far user, and B1 is the M-QAM modulation symbol set of the far user; s5.2、The high power signal of the first demodulation After reconstructing the signal X', the timing signal Y of the low-power near-user received signal is obtained through SIC 1-2 : s5.3, for Y 1-2 Perform ML detection and demodulate the bit stream signal of user 2 b2 is the near-user M-QAM modulation symbol vector, and B2 is the near-user M-QAM modulation symbol set; s5.

4. Consider the signal of low-power user 2 as noise and use the following ML detection expression to directly detect the received signal of the distant user: Y2 and H2 are the received signal and channel gain detected at the far user, respectively. is the high-power far-user signal obtained after ML detection at the far-user location.

8. The non-orthogonal multiple access system transmission method based on dual-frequency index modulation according to claim 7, characterized in that: The signal index set is:

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