A power allocation method in an OFDM bidirectional relay system

Abstract

The application discloses a power distribution method in an OFDM bidirectional relay system, comprising the following steps: in the OFDM bidirectional relay system, joint subcarrier suppression-subcarrier pairing method is adopted for data transmission, meanwhile, based on local channel state information, the local suboptimal power distribution strategy obtained in advance is adopted in the multiple access stage, and the optimal power distribution strategy obtained in advance is adopted in the broadcast stage for power distribution; wherein, the local suboptimal power distribution strategy and the optimal power distribution strategy obtaining method comprises the following steps: the bit error rate is solved through the local suboptimal power distribution algorithm and the Lagrange dual method, and the suboptimal power distribution strategy is obtained; the bit error rate is solved through the local optimal power distribution algorithm and the Lagrange dual method, and the optimal power distribution strategy is obtained; under the condition that the terminal power is constant, the power loaded on different subcarriers of each terminal is optimized through the proposed power distribution strategy, and the system bit error rate performance is improved.

Description

Technical Field

[0001] This invention relates to a power allocation method in an OFDM bidirectional relay system, belonging to the field of wireless communication technology. Background Technology

[0002] With the continuous improvement of social informatization, people's demand for the flexibility and diversification of mobile data services is increasing, and mobile data traffic is also increasing exponentially year by year. Orthogonal Frequency Division Multiplexing (OFDM) technology has been widely used in modern communication systems due to its high spectrum utilization and good resistance to multipath fading. However, in OFDM systems, frequency selective fading causes different subcarriers to have different channel gains, and subcarriers with poor channel quality will bring the "barrel effect" to the OFDM system, seriously affecting the overall system performance. In order to solve the impact of channel fading on the performance of OFDM systems and improve the bit error rate (BER) performance of OFDM systems, the literature "BARTOLI G, FANTACCI R, MARABISSI D, et al. Subcarriers Suppression methods for OFDM Systems with Decode-and-Forward Network Coding. IEEE Trans, Wireless Communication, 2013, 12(12): 6034-6042." proposes subcarrier suppression techniques. Subcarrier suppression technology utilizes only subcarriers with good channel quality to transmit information, while suppressing subcarriers with poor channel quality. This technique effectively overcomes the "weakest link" effect caused by subcarriers with poor channel quality, thereby improving the reliability of OFDM system transmission. Therefore, subcarrier suppression technology provides a new approach to further improving the reliability of OFDM system transmission.

[0003] To expand base station coverage and achieve diversity gain, cooperative relay technology, as a key technology in next-generation wireless communication systems, has attracted widespread attention from academia and industry. Its core idea is to use relay nodes to amplify or denoise the information sent by the source node before forwarding it to the destination node. Depending on the signal processing method, relaying can be divided into several types. Among them, Amplify-and-Forward (AF) and Decode-and-Forward (DF) protocols are the most common. The AF protocol means that the relay does not decode the received signal, but only amplifies it; while the DF protocol means that the relay decodes the received signal, recovers the original signal, and then re-encodes it before retransmitting it.

[0004] The introduction of relays can improve the reliability of wireless transmission, but it also brings some drawbacks, such as increased coordination between terminals and increased signal processing complexity. The most significant drawback is reduced bandwidth efficiency. To compensate for this, network coding technology, based on the broadcast characteristics of signal transmission in wireless communication, has been introduced into wireless cooperative communication to improve the bandwidth utilization of wireless cooperative communication systems. A typical application scenario for network coding is a two-way relay network, where a pair of communication terminals exchange information through a relay. Because this network coding method, which involves electromagnetic wave hybrid characteristics, involves physical layer technologies such as modulation / demodulation and encoding / decoding, it is called Physical-Layer Network Coding (PLNC). Depending on the protocol used by the relay (AF or DF), it is also called AF-PLNC and DF-PLNC.

[0005] OFDM relay systems have been extensively studied. Existing literature proposes combining several technologies with OFDM relay technology to improve system data rate and reliability. Besides traditional power allocation and multiple relay selection techniques, these include subcarrier pairing (SP) and subcarrier suppression (SS) techniques.

[0006] Subcarrier pairing technology: It is commonly used in one-way multi-hop OFDM relay systems. Its core idea is to pair different subcarriers of two adjacent hops according to certain criteria, and transmit the same symbol between the paired subcarriers to improve the achievable rate.

[0007] Subcarrier suppression technology is an OFDM technology with great practical application prospects. Its core idea is to select subcarriers with better channel conditions for symbol loading and information transmission based on the instantaneous channel state information of each subcarrier, while suppressing subcarriers with poor channel conditions. This avoids the waste of excessive power by subcarriers with poor channel conditions due to deep fading, thus affecting the overall system performance. Simulation results in the literature "BARTOLI G, FANTACCI R, MARABISSI D, et al. Subcarriers Suppression methods for OFDM Systems with Decode-and-Forward Network Coding[J].IEEE Trans, Wireless Communication, 2013, 12(12): 6034-6042." show that introducing subcarrier suppression technology into OFDM systems can greatly improve the bit error rate performance of the system without affecting the throughput.

[0008] Existing technology proposes a joint subcarrier suppression-subcarrier pairing method for bidirectional OFDM relay systems. This method combines subcarrier suppression and subcarrier pairing techniques, achieving better bit error rate (BER) performance compared to a simple subcarrier suppression scheme. However, this scheme only employs a simple power allocation method and does not consider power optimization for BER performance. Summary of the Invention

[0009] The purpose of this invention is to overcome the shortcomings of the prior art and provide a power allocation method in an OFDM bidirectional relay system, thereby improving the system's bit error rate performance.

[0010] To achieve the above objectives, the present invention is implemented using the following technical solution:

[0011] In a first aspect, the present invention provides a power allocation method in an OFDM bidirectional relay system. In the OFDM bidirectional relay system, two source nodes S1 and S2 exchange information through a relay R. The system adopts a time-division multiple access protocol, and the signal transmission is divided into two stages. The first stage is the multiple access stage, in which S1 and S2 simultaneously send signals to R, and R performs decoding and forwarding processing. The second stage is the broadcast stage, in which R broadcasts the processed signal to S1 and S2, and then S1 and S2 decode to obtain information from each other. The method is characterized by comprising:

[0012] In OFDM bidirectional relay systems, a joint subcarrier suppression-subcarrier pairing method is used for data transmission. Simultaneously, a pre-acquired local suboptimal power allocation strategy is used in the multiple access phase, and a pre-acquired optimal power allocation strategy is used in the broadcast phase. Wherein:

[0013] The local suboptimal power allocation strategy and the optimal power allocation strategy acquisition method include:

[0014] Obtain the first bit error rate on the k-th subcarrier under the condition that the data transmission is performed using the joint subcarrier suppression-subcarrier pairing method during the multiple access phase;

[0015] Under the condition that data transmission is performed using a joint subcarrier suppression-subcarrier pairing method during the broadcast phase, S i The second bit error rate on the k-th subcarrier at (i=1,2);

[0016] The first bit error rate is solved by using a local suboptimal power allocation algorithm and the Lagrange duality method to obtain the suboptimal power allocation strategy.

[0017] The second bit error rate is solved by using the local optimal power allocation algorithm and the Lagrange duality method to obtain the optimal power allocation strategy.

[0018] Furthermore, obtaining the first bit error rate on the k-th subcarrier under the condition of data transmission using the joint subcarrier suppression-subcarrier pairing method during the multiple access phase includes:

[0019] Obtain the first initial bit error rate on the k-th subcarrier under the condition that the joint subcarrier suppression-subcarrier pairing method is not used for data transmission during the multiple access phase;

[0020] The first bit error rate on the k-th subcarrier is calculated based on the first initial bit error rate under the condition that the data transmission is performed using the joint subcarrier suppression-subcarrier pairing method in the multiple access phase.

[0021] Furthermore, obtaining the first initial bit error rate on the k-th subcarrier under the condition that the joint subcarrier suppression-subcarrier pairing method is not used for data transmission during the multiple access phase includes:

[0022] During the multiple access phase, source node S i The baseband signal transmitted on the k-th subcarrier (i = 1, 2) is represented as:

[0023]

[0024] Among them, a i,I [k] and a i,Q [k] represents si The in-phase and quadrature components of [k] are taken with equal probability of ±1;

[0025] Assume the time-domain channel between any two endpoints follows Rayleigh fading along LL paths, with each path having the same variance of 1 / LL, where LL is a positive integer; when LL = K, the frequency-domain signal is modeled as a random vector following a ring-symmetric complex Gaussian distribution with a mean of 0 and a covariance matrix equal to the identity matrix; assume S1, S2, and R have independent power constraints, P1, P2, and P, respectively. R Each endpoint performs power allocation among subcarriers; through power allocation, the received signal of relay R on the k-th subcarrier is represented as:

[0026]

[0027] Where, α i,k S represents i The power allocated to the k-th subcarrier at (i=1,2) is a proportion of the total power of the node, thus a power constraint exists:

[0028]

[0029] H i [k] represents S i The channel gain of the k-th subcarrier between (i=1,2) and R, n R [k] represents the noise on the k-th subcarrier at relay R, and has Assume that all endpoints only know their local channel state information, and that this channel state information remains unchanged throughout the entire data transmission process; since S i (i = 1, 2) Given the channel state information between it and R, phase compensation technology is used to make the phase angle of the information arriving at R 0, i.e., H i [k] can be considered a real number;

[0030] After receiving the mixed signal from S1 and S2, R performs network coding and decoding on each subcarrier, with the decision rule as follows:

[0031]

[0032]

[0033] Given the channel conditions during the multiple access phase, the formula for the first initial bit error rate on the k-th subcarrier is as follows:

[0034]

[0035] in,

[0036] Furthermore, the first bit error rate on the k-th subcarrier is calculated based on the first initial bit error rate under the condition of data transmission using the joint subcarrier suppression-subcarrier pairing method in the multiple access phase, including:

[0037] Based on the first initial bit error rate, the first bit error rate on the k-th subcarrier is further calculated under the condition of data transmission using the joint subcarrier suppression-subcarrier pairing method in the multiple access phase, as shown in the following formula:

[0038]

[0039] Where M represents the number of unsuppressed subcarriers.

[0040] Furthermore, the acquisition of S under the condition that data transmission is performed using a joint subcarrier suppression-subcarrier pairing method during the broadcast phase... i The second bit error rate on the k-th subcarrier at (i=1,2) includes:

[0041] Under the condition that the joint subcarrier suppression-subcarrier pairing method is not used for data transmission during the broadcast phase, S i The second initial bit error rate on the k-th subcarrier at (i=1,2);

[0042] Based on the second initial bit error rate, S is calculated under the condition that data transmission is performed using the joint subcarrier suppression-subcarrier pairing method during the broadcast phase. i The second bit error rate on the k-th subcarrier at (i=1,2).

[0043] Furthermore, the acquisition of S under the condition that the joint subcarrier suppression-subcarrier pairing method is not used for data transmission during the broadcast phase... i The second initial bit error rate on the k-th subcarrier at (i=1,2) includes:

[0044] During the broadcast phase, the relay transmits the estimated symbol. and S i The received signal is represented as:

[0045]

[0046] Where, β k s represents the proportion of the power allocated to the k-th subcarrier at point R to the total power at point R. R [k] represents the transmitted signal on the k-th subcarrier at relay R, given by the following formula:

[0047]

[0048] n i[k] represents S i Noise on the k-th subcarrier at (i=1,2); QPSK decoding is used in broadcast phases S1 and S2, therefore, under given channel conditions, broadcast phase S... i The second initial bit error rate on the k-th subcarrier at (i=1,2) is:

[0049]

[0050] in,

[0051] Furthermore, based on the second initial bit error rate, S is calculated under the condition of data transmission using the joint subcarrier suppression-subcarrier pairing method during the broadcast phase. i The second bit error rate on the k-th subcarrier at (i=1,2) includes:

[0052] Based on the second initial bit error rate, S is further calculated under the condition of data transmission using the joint subcarrier suppression-subcarrier pairing method during the broadcast phase. i The second bit error rate on the k-th subcarrier at (i=1,2) is given by the following formula:

[0053]

[0054] Furthermore, the step of solving for the first bit error rate using a local suboptimal power allocation algorithm and the Lagrange duality method to obtain a suboptimal power allocation strategy includes:

[0055] First, consider the power distribution at S1, assuming that the power at S2 is evenly distributed. The optimization problem then becomes:

[0056]

[0057] Assume the set of subcarriers on both sides of the first type of subcarrier is Ω1, and the set of subcarriers on the right side and the left side of the second type of subcarrier is Ω. 21 The set of subcarriers in the second type, where the left side is suppressed and the right side is not, is Ω. 22 The allocation is performed using local suboptimal power allocation, and the allocation algorithm is as follows:

[0058] Algorithm 1: Local suboptimal power allocation algorithm at point S1:

[0059] S1, Order calculate calculate pass and Compare and determine and

[0060] S2, Through and Sure The expression;

[0061] S3, will As initial values, use optimization algorithms to obtain the values ​​that make... Minimize {α 1,k}, denoted as

[0062] S4, Pass Calculated

[0063] S5, Order

[0064] S6, Pass and Compare and determine and

[0065] S7, if Then output Conversely, let i = i + 1 and return to step 2;

[0066] in,

[0067]

[0068]

[0069] Step 3 in Algorithm 1 can be represented as the following optimization problem:

[0070]

[0071] The solution is obtained using the Lagrange duality method, including:

[0072] (1) Problem Modeling

[0073] The Lagrange function is:

[0074]

[0075] The dual function is:

[0076]

[0077] Note that the dual function is decomposable, and the dual function can be further equivalent to:

[0078]

[0079] in,

[0080]

[0081] At this point, the dual problem is written as:

[0082] maxφ(λ) (20)

[0083] (2) Solving the dual problem

[0084] The dual problem is:

[0085]

[0086] The first-order optimality condition is:

[0087]

[0088] when At that time, there were:

[0089]

[0090] when At that time, there were:

[0091]

[0092] When k∈Ω 21 At that time, there were:

[0093]

[0094] because Each item in it is about Since it is a monotonic function, a one-dimensional search method is used to find the optimal solution to the dual subproblem.

[0095] (3) Solving the dual principal problem

[0096] Will Substituting the value of λ into the dual principal problem, the principal problem is then solved using the subgradient algorithm. The update method for λ is as follows:

[0097]

[0098] Where t (i) This is the step size for each iteration; the subgradient algorithm guarantees convergence to the optimal value, and obtains an approximate optimal value through a sufficient number of iterations.

[0099] The power allocation method at S2 is the same as that at S1. The optimization problem is expressed as follows, with the steps omitted.

[0100]

[0101] Furthermore, the second bit error rate is solved using a local optimal power allocation algorithm and the Lagrange duality method to obtain the optimal power allocation strategy, including:

[0102] Based on the second bit error rate, the optimization problem can now be expressed as:

[0103]

[0104] The solution is obtained using the Lagrange duality method, including:

[0105] (1) Problem Modeling

[0106] The Lagrange function is:

[0107]

[0108] The dual function is:

[0109]

[0110] Note that the dual function is decomposable, and the dual function can be further equivalent to:

[0111]

[0112] in,

[0113]

[0114] At this point, the dual problem is written as:

[0115] maxφ(μ) (33)

[0116] (2) Solving the dual problem

[0117] The dual problem is

[0118]

[0119] The first-order optimality condition is

[0120]

[0121] When k∈Ω1, we have:

[0122]

[0123] Corresponding to β k It can be obtained from the following system of equations:

[0124]

[0125] When k∈Ω 21 At that time, there were:

[0126]

[0127] Find the corresponding

[0128] When k∈Ω 22 At that time, there were:

[0129]

[0130] Find the corresponding

[0131] (3) Solving the dual principal problem

[0132] Will Substituting the value of μ into the dual principal problem, the principal problem is then solved using the subgradient algorithm. The update method for μ is as follows:

[0133]

[0134] Where t (i) It is the step size for each iteration; the subgradient algorithm guarantees convergence to the optimal value, and obtains an approximate optimal value through a sufficient number of iterations.

[0135] Furthermore, computer simulations were used to examine the system BER performance of the joint subcarrier suppression-subcarrier pairing method that uses local suboptimal power allocation strategy and local optimal power allocation strategy for power allocation optimization, and the system BER performance was compared with that of the joint subcarrier suppression-subcarrier pairing method that does not use local suboptimal power allocation strategy and local optimal power allocation strategy for power optimization.

[0136] Compared with the prior art, the beneficial effects achieved by the present invention are as follows:

[0137] This invention provides a power allocation method in an OFDM bidirectional relay system. By analyzing the system's bit error rate (BER) expression, the power optimization problem is divided into two parts. The first part is the multiple access phase, where a locally suboptimal power allocation optimization strategy is proposed, considering known local channel state information, to address the power allocation problem between the two signal sources. The second part is the broadcast phase, where an optimal power allocation optimization strategy is proposed for the power allocation problem at the relay station. Under the condition of fixed terminal power, this strategy reduces the system BER by optimizing the power loaded on different subcarriers of each terminal. Simulation results show that, compared with existing power allocation methods, the proposed power allocation method can improve the system's BER performance. Specifically:

[0138] 1. The power allocation method proposed in this invention improves bit error rate performance by 1-2 dB compared to existing technologies;

[0139] 2. The power allocation method proposed in this invention outperforms existing technologies in terms of bit error rate performance under different conditions. Attached Figure Description

[0140] Figure 1 This is a schematic diagram of a bidirectional OFDM single relay system model provided in an embodiment of the present invention;

[0141] Figure 2 This is a schematic diagram comparing the BER performance of different power allocation methods based on SPSS provided in the embodiments of the present invention;

[0142] Figure 3 These are curves showing the BER of several power allocation methods provided in the embodiments of the present invention as a function of the average number of unsuppressed subcarriers M. Detailed Implementation

[0143] The present invention will be further described below with reference to the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and should not be used to limit the scope of protection of the present invention.

[0144] Example 1

[0145] This embodiment describes a power allocation method in an OFDM bidirectional relay system, including:

[0146] In an OFDM bidirectional relay system, two source nodes S1 and S2 exchange information via relay R. The system employs a Time Division Multiple Access (TDMA) protocol, and signal transmission can be divided into two phases. The first phase is the multiple access phase, where S1 and S2 simultaneously send signals to R, and R performs decoding and forwarding processing. The second phase is the broadcast phase, where R broadcasts the processed signal to S1 and S2, and then S1 and S2 decode to obtain the information from each other. Each node uses OFDM transmission mode, with K subcarriers, and the symbols on each subcarrier use QPSK modulation. The allocation method includes:

[0147] In OFDM bidirectional relay systems, a joint subcarrier suppression-subcarrier pairing method is used for data transmission. Simultaneously, a pre-acquired local suboptimal power allocation strategy is used in the multiple access phase, and a pre-acquired optimal power allocation strategy is used in the broadcast phase. Wherein:

[0148] The local suboptimal power allocation strategy and the optimal power allocation strategy acquisition method include:

[0149] Obtain the first bit error rate on the k-th subcarrier under the condition that the data transmission is performed using the joint subcarrier suppression-subcarrier pairing method during the multiple access phase;

[0150] Under the condition that data transmission is performed using a joint subcarrier suppression-subcarrier pairing method during the broadcast phase, S i The second bit error rate on the k-th subcarrier at (i=1,2);

[0151] The first bit error rate is solved by using a local suboptimal power allocation algorithm and the Lagrange duality method to obtain the suboptimal power allocation strategy.

[0152] The second bit error rate is solved by using the local optimal power allocation algorithm and the Lagrange duality method to obtain the optimal power allocation strategy.

[0153] The power allocation method in the OFDM bidirectional relay system provided in this embodiment involves the following steps in its application process:

[0154] 1. System Model

[0155] In an OFDM bidirectional relay system, two source nodes S1 and S2 exchange information via relay R, such as... Figure 1 As shown. The system employs a Time Division Multiple Access (TDMA) protocol, and signal transmission can be divided into two phases. The first phase is the Multiple Access (MA) phase, where S1 and S2 simultaneously send signals to R, and R performs decoding and forwarding processing. The second phase is the Broadcast (BC) phase, where R broadcasts the processed signal to S1 and S2, and then S1 and S2 decode to obtain the information from each other. Each node uses OFDM transmission mode, with K subcarriers, and the symbols on each subcarrier use QPSK modulation.

[0156] During the multiple access phase, node S i The baseband signal transmitted on the k-th subcarrier (i = 1, 2) can be expressed as:

[0157]

[0158] Where a i,I [k] and a i,Q [k] represents s iThe in-phase and quadrature components of [k] are equally approximated as ±1. It is assumed that the time-domain channel between any two endpoints follows Rayleigh fading along LL (LL is a positive integer) paths, and the variance of each path is equal, being 1 / LL. According to the literature "HAO Z, YUAN L, MEIXIA T. Resource Allocation with Subcarrier Pairing in OFDMA Two-Way Relay Ne tworks[J].IEEE Wireless Communications Letters,2012,1(2):61-64.", when LL=K, the frequency domain signal can be modeled as a random vector following a ring-symmetric complex Gaussian distribution with a mean of 0 and a covariance matrix of identity. It is assumed that S1, S2, and R have independent power constraints, namely P1, P2, and P... R Each endpoint performs power allocation among the subcarriers. Through power allocation, the received signal of relay R on the k-th subcarrier can be expressed as:

[0159]

[0160] Where α i,k S represents i The power allocated to the k-th subcarrier at (i=1,2) is a proportion of the total power of the node, thus a power constraint exists:

[0161]

[0162] H i [k] represents S i The channel gain of the k-th subcarrier between (i=1,2) and R, n R [k] represents the noise on the k-th subcarrier at relay R, and has Assume that all endpoints are only aware of their local Channel State Information (CSI), and that the CSI remains unchanged throughout the entire data transmission process (i.e., the MA and BC phases). Since S i (i = 1, 2) Given the CSI between it and R, phase compensation technology can be used to make the phase angle of the information arriving at R 0, i.e., H i [k] can be considered a real number.

[0163] After receiving the mixed signal from S1 and S2, R performs network coding and decoding on each subcarrier, with the decision rule as follows:

[0164]

[0165]

[0166] At this point, under the given channel conditions during the multiple access phase (MA phase), the bit error rate on the k-th subcarrier can be expressed as:

[0167]

[0168] in,

[0169] During the broadcast phase (BC phase), the relay transmits the estimated symbol. and S i The received signal can be represented as:

[0170]

[0171] Where, β k s represents the proportion of the power allocated to the k-th subcarrier at point R to the total power at point R. R [k] represents the transmitted signal on the k-th subcarrier at relay R, given by the following formula:

[0172]

[0173] n i [k] represents S i Noise on the k-th subcarrier at (i=1,2). Broadcast phases S1 and S2 use QPSK decoding; therefore, under given channel conditions, broadcast phase S... i The bit error rate on the k-th subcarrier at (i=1,2) is:

[0174]

[0175] in,

[0176] 2. Joint Subcarrier Suppression-Subcarrier Pairing Method (SPSS)

[0177] The paper “WANG Jian, MA Wenfeng, XU Youyun, et al. Subcarrier Pairing based Subcarrier Suppression for OFDM systems with Decode-and-Forward Network Coding[C]. 2015 IEEE Wireless Communications and Networking Conference (WCNC), March, 2015: 551-556” proposes a combination of subcarrier pairing and subcarrier suppression techniques. Its implementation is based on the power allocation method used in the paper “BARTOLI G, FANTACCI R, MARABISSI D, et al. Subcarriers Suppression methods for OFDM Systems with Decode-and-Forward Network Coding[J]. IEEE Trans, Wireless Communication, 2013, 12(12): 6034-6042”, which is called channel inversion. Channel inversion means that the power loaded on different subcarriers at the transmitting end is inversely proportional to the square of the CSI modulus corresponding to the subcarrier, thus ensuring that the received level at the receiving end is the same on each subcarrier. At this time, we have:

[0178]

[0179] in, S represents i The subcarrier suppression vector at (i=1,2) is:

[0180]

[0181] Where "0" indicates that the subcarrier is suppressed, and "1" indicates that it is not suppressed; To suppress the threshold, its value is such that The weight is M. Since the number of suppressed subcarriers on both sides of R is the same, three situations can occur: 1) neither subcarrier on either side is suppressed, 2) one side is suppressed and the other side is not suppressed, and 3) both sides are suppressed. In the second situation, the suppressed subcarriers on the left and right sides will not appear in pairs. Without any additional operations, the symbols transmitted on each subcarrier will experience a second hop with poor channel conditions. However, by pairing two subcarriers, the symbols on the paired subcarriers can be exchanged at the relay, so that each pair of symbols can complete the second hop along the first hop path of the other, thus achieving a significant performance gain. Subcarrier pairing can be achieved through... and This is achieved by generating a subcarrier switching matrix.

[0182] During the broadcast phase, relay R uses only a simple average power distribution, i.e.:

[0183]

[0184] in,

[0185] 3. Power Optimization Allocation Based on Joint Subcarrier Pairing and Subcarrier Suppression

[0186] In traditional SPSS schemes, power allocation simply employs channel reversal and average distribution. While simple to implement, this approach doesn't achieve good bit error rate performance. Therefore, we consider optimized power allocation based on the SPSS scheme. Assume the set of subcarriers on both sides of the first type of subcarrier is Ω1, and the set of subcarriers on the right side and the left side of the second type of subcarrier is Ω. 21 The set of subcarriers in the second type, where the left side is suppressed and the right side is not, is Ω. 22 Under these conditions, with the channel conditions fixed, the system's bit error rate can be expressed as:

[0187]

[0188] Where π(·) represents the subcarrier pairing function, the above equation can be simplified to:

[0189]

[0190] As can be seen from the above formula, power allocation optimization in the multiple access phase and the broadcast phase can be performed separately.

[0191] 3.1 Power Allocation in the Multiple Access Phase

[0192] Given the channel conditions, the bit error rate during the multiple access phase of the system can be rewritten as:

[0193]

[0194] Since both S1 and S2 only know the local CSI, a local suboptimal power allocation strategy is given under this condition.

[0195] First, consider the power distribution at S1. Since S1 does not know H2, we assume that the power at S2 is evenly distributed. At this point, the optimization problem becomes:

[0196]

[0197] Since the objective function is in the form of a sum of multiple discontinuous functions, it is difficult to obtain the globally optimal power allocation. Therefore, we consider implementing a locally suboptimal power allocation, and the allocation algorithm is as follows:

[0198]

[0199] in,

[0200]

[0201]

[0202] Step 3 in Algorithm 1 can be represented as the following optimization problem:

[0203]

[0204] The following solution uses the Lagrange duality method, including:

[0205] (1) Problem Modeling

[0206] The Lagrange function is:

[0207]

[0208] The dual function is:

[0209]

[0210] Note that the dual function is decomposable, and the dual function can be further equivalent to:

[0211]

[0212] in,

[0213]

[0214] At this point, the dual problem can be written as:

[0215] maxφ(λ) (24)

[0216] (2) Solving the dual problem

[0217] The dual problem is:

[0218]

[0219] The first-order optimality condition is:

[0220]

[0221] when At that time, there were:

[0222]

[0223] when At that time, there were:

[0224]

[0225] When k∈Ω 21 At that time, there were:

[0226]

[0227] because Each item in it is about Since it is a monotonic function, the optimal solution to the dual subproblem can be found using a one-dimensional search method.

[0228] (3) Solving the dual principal problem

[0229] Will Substituting the value of λ into the dual principal problem allows the principal problem to be solved using the subgradient algorithm. The update method for λ is as follows:

[0230]

[0231] Where t (i) This is the step size for each iteration. The subgradient algorithm can guarantee convergence to the optimal value, and with a sufficient number of iterations, it can obtain an approximate optimal value.

[0232] The power allocation method at S2 is the same as that at S1. The optimization problem can be expressed as follows, with the steps omitted.

[0233]

[0234] 3.2 Power Allocation During Broadcast Phase

[0235] Given channel conditions, the bit error rate during the system broadcast phase can be rewritten as:

[0236]

[0237] The optimization problem can now be expressed as:

[0238]

[0239] The following solution uses the Lagrange duality method, including:

[0240] (1) Problem Modeling

[0241] The Lagrange function is:

[0242]

[0243] The dual function is:

[0244]

[0245] Note that the dual function is decomposable, and the dual function can be further equivalent to:

[0246]

[0247] in,

[0248]

[0249] At this point, the dual problem can be written as:

[0250] maxφ(μ) (38)

[0251] (2) Solving the dual problem

[0252] The dual problem is

[0253]

[0254] The first-order optimality condition is

[0255]

[0256] When k∈Ω1, we have:

[0257]

[0258] Corresponding to β k It can be obtained from the following system of equations:

[0259]

[0260] When k∈Ω 21 At that time, there were:

[0261]

[0262] Find the corresponding

[0263] When k∈Ω 22 At that time, there were:

[0264]

[0265] Find the corresponding

[0266] (3) Solving the dual principal problem

[0267] Will Substituting the value of μ into the dual principal problem allows the principal problem to be solved using the subgradient algorithm. The update method for μ is as follows:

[0268]

[0269] Where t (i) This is the step size for each iteration. The subgradient algorithm can guarantee convergence to the optimal value, and with a sufficient number of iterations, it can obtain an approximate optimal value.

[0270] 4. Computer simulation

[0271] This invention utilizes computer simulation to verify the BER performance of the proposed power allocation method and compares it with the SPSS method without power optimization proposed in the literature "WANG Jian, MA Wenfeng, XU Youyun, et al. Subcarrier Pairing based Subcarrier Suppression for OFDM systems with Decode-and-Forward Network Coding[C]. 2015 IEEE Wireless Communications and Networking Conference (WCNC), March, 2015: 551-556." Method 1 uses average power allocation in both the MA and BC phases; Method 2 uses channel inversion power allocation in the MA phase and average allocation in the BC phase; the proposed method uses local suboptimal power allocation in the MA phase and optimal power allocation in the BC phase. It is assumed that the transmission power of all nodes is set to 1, the number of subcarriers is represented by K, and M represents the number of unsuppressed subcarriers. QPSK modulation is used on each subcarrier.

[0272] Figure 2 The bit error rate (BER) performance of different SPSS-based power allocation methods was examined, with K=32 and M=27. It can be seen that the power allocation method proposed in this invention outperforms existing methods 1 and 2 across the entire signal-to-noise ratio range. Overall, the power allocation method proposed in this invention is approximately 2 dB better than method 1 and approximately 1 dB better than method 2.

[0273] Figure 3 The BER of several power allocation methods as a function of the average number of unsuppressed subcarriers M was compared. Figure 3 In the figure, LL = 16, ρ = 5, 10, 15 dB. M varies between 32 and 24. As can be seen from the figure, as M gradually decreases, the BER of all allocation methods gradually decreases, with a crossover between Method 1 and Method 2. However, the power allocation method proposed in this invention achieves optimal BER performance under different values ​​of M. Therefore, the power allocation optimization method proposed in this invention can significantly improve BER performance under different conditions.

[0274] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A power allocation method in an OFDM bidirectional relay system, wherein in the OFDM bidirectional relay system, two source nodes... and via relay For information exchange, the system uses a time-division multiple access protocol. Signal transmission is divided into two phases: the first phase is the multiple access phase. and At the same time to Send signal, Perform decoding and forwarding processing; The second stage is the broadcast stage. Broadcast the processed signal to and ,Then and Decoding is performed to obtain information from the other party, characterized in that... The allocation method includes: In OFDM bidirectional relay systems, a joint subcarrier suppression-subcarrier pairing method is used for data transmission. Simultaneously, a pre-acquired local suboptimal power allocation strategy is used in the multiple access phase, and a pre-acquired optimal power allocation strategy is used in the broadcast phase. Wherein: The local suboptimal power allocation strategy and the optimal power allocation strategy acquisition method include: Obtain the first bit error rate on the k-th subcarrier under the condition that the data transmission is performed using the joint subcarrier suppression-subcarrier pairing method during the multiple access phase; Under the condition that data transmission is performed using the joint subcarrier suppression-subcarrier pairing method during the broadcast phase, the following conditions are obtained: The second bit error rate on the k-th subcarrier, ; The first bit error rate is solved using a local suboptimal power allocation algorithm and the Lagrange duality method, yielding a suboptimal power allocation strategy. It is assumed that the set of subcarriers on both sides of the first type of subcarrier is... The set of subcarriers in the first type of subcarriers that are suppressed on the right and not suppressed on the left is: The set of subcarriers in the first type of subcarriers that are suppressed on the left and not suppressed on the right is: The set of subcarriers in the second type, where the right side is suppressed and the left side is not, is: The set of subcarriers in the second type of subcarriers that are suppressed on the left and not suppressed on the right is: The allocation is performed using local suboptimal power distribution, and the optimization problem is expressed as follows: (15)； Where M represents the number of unsuppressed subcarriers; S represents i The proportion of the power allocated to the k-th subcarrier to the total power of the node. ; , , ; S represents i The channel gain of the k-th subcarrier between R and R. ;P i express It has independent power constraints. ; The second bit error rate is solved by using the local optimal power allocation algorithm and the Lagrange duality method to obtain the optimal power allocation strategy.

2. The power allocation method in the OFDM bidirectional relay system according to claim 1, characterized in that, The acquisition of the first bit error rate on the k-th subcarrier under the condition of data transmission using the joint subcarrier suppression-subcarrier pairing method in the multiple access phase includes: Obtain the first initial bit error rate on the k-th subcarrier under the condition that the joint subcarrier suppression-subcarrier pairing method is not used for data transmission during the multiple access phase; The first bit error rate on the k-th subcarrier is calculated based on the first initial bit error rate under the condition that the data transmission is performed using the joint subcarrier suppression-subcarrier pairing method in the multiple access phase.

3. The power allocation method in the OFDM bidirectional relay system according to claim 2, characterized in that, The acquisition of the first initial bit error rate on the k-th subcarrier under the condition that the joint subcarrier suppression-subcarrier pairing method is not used for data transmission during the multiple access phase includes: During the multiple access phase, the source node The baseband signal transmitted on the k-th subcarrier is represented as: (1)； in, and They are respectively The in-phase and quadrature components, and take equal approximation. ; Assume that the time-domain channel between any two endpoints follows Rayleigh fading along an LL path, and that the variance of each path is equal. LL is a positive integer; when satisfying At that time, the frequency domain signal is modeled as a random vector following a ring-symmetric complex Gaussian distribution with a mean of 0 and a covariance matrix equal to the identity matrix; assuming , and It has independent power constraints, respectively , and Each endpoint performs power allocation among subcarriers; through power allocation, the relay... In the The received signal on each subcarrier is represented as follows: (2)； in, express The power allocated to the k-th subcarrier is a proportion of the total power of the node, thus a power constraint exists: (3)； express The channel gain of the k-th subcarrier between R and R. Let R represent the noise on the k-th subcarrier at relay R, and have Assume that all endpoints only know their local channel state information, and that this channel state information remains unchanged throughout the entire transmission process; because Given the channel state information between it and R, phase compensation technology is used to ensure that the phase angle of the information arriving at R is 0, i.e. It can be regarded as a real number; When received from and After the mixed signal, R performs network coding and decoding processing on each subcarrier, and the decision rule is as follows: (4)； (5)； Given the channel conditions during the multiple access phase, the formula for the first initial bit error rate on the k-th subcarrier is as follows: (6)； in, , , , , .

4. The power allocation method in the OFDM bidirectional relay system according to claim 3, characterized in that, The first bit error rate on the k-th subcarrier is calculated based on the first initial bit error rate under the condition of data transmission using the joint subcarrier suppression-subcarrier pairing method in the multiple access phase, including: Based on the first initial bit error rate, the first bit error rate on the k-th subcarrier is further calculated under the condition of data transmission using the joint subcarrier suppression-subcarrier pairing method in the multiple access phase, as shown in the following formula: (7)； Where M represents the number of unsuppressed subcarriers, It is the set of subcarriers on both sides of the first type of subcarrier. This refers to the set of subcarriers in the second category whose right side is suppressed and whose left side is not. This is the set of subcarriers in the second type whose left side is suppressed and whose right side is not suppressed.

5. The power allocation method in an OFDM bidirectional relay system according to claim 4, characterized in that, The acquisition is performed under the condition that data transmission is carried out using a joint subcarrier suppression-subcarrier pairing method during the broadcast phase. The second bit error rate on the k-th subcarrier includes: Under the condition that data transmission is not performed using the joint subcarrier suppression-subcarrier pairing method during the broadcast phase, the following conditions can be obtained: The second initial bit error rate on the k-th subcarrier; Based on the second initial bit error rate, the data transmission condition under the combined subcarrier suppression-subcarrier pairing method during the broadcast phase is calculated. The second bit error rate on the k-th subcarrier.

6. The power allocation method in an OFDM bidirectional relay system according to claim 5, characterized in that, The acquisition is performed under the condition that the joint subcarrier suppression-subcarrier pairing method is not used for data transmission during the broadcast phase. The second initial bit error rate on the k-th subcarrier includes: During the broadcast phase, the relay transmits the estimated symbol. and , The received signal is represented as: (8)； in, This represents the proportion of the power allocated to the k-th subcarrier at point R to the total power at point R. The transmitted signal on the k-th subcarrier at relay R is given by the following formula: (9)； express Noise on the k-th subcarrier; broadcast phase and Using QPSK decoding, therefore, under given channel conditions, the broadcast phase... The second initial bit error rate on the k-th subcarrier is: (10)； in, .

7. The power allocation method in an OFDM bidirectional relay system according to claim 6, characterized in that, Based on the second initial bit error rate, the data transmission condition under the combined subcarrier suppression-subcarrier pairing method during the broadcast phase is calculated. The second bit error rate on the k-th subcarrier includes: Based on the second initial bit error rate, the following calculations are performed under the condition of data transmission using the joint subcarrier suppression-subcarrier pairing method during the broadcast phase: The second bit error rate on the k-th subcarrier is given by the following formula: (11)。 8. The power allocation method in an OFDM bidirectional relay system according to claim 7, characterized in that, The step of solving for the first bit error rate using a local suboptimal power allocation algorithm and the Lagrange duality method to obtain a suboptimal power allocation strategy includes: First consideration Power allocation at the location, assuming With power evenly distributed, the optimization problem then becomes: (12)； Assume the set of subcarriers on both sides of the first type of subcarrier is The set of subcarriers in the second type of subcarriers that are suppressed on the right and not suppressed on the left is: The set of subcarriers in the second type of subcarriers that are suppressed on the left and not suppressed on the right is: The allocation is performed using local suboptimal power allocation, and the allocation algorithm is as follows: The local suboptimal power allocation algorithm includes the following steps: S1, Order , ,calculate ,calculate ,pass and Compare and determine and ; S2, Through and Sure The expression; S3, will As initial values, use optimization algorithms to obtain the values ​​that make... Minimized , , recorded as ; S4, Pass Calculated ; S5, Order , ; S6, Pass and Compare and determine and ; S7, if Then output Conversely, and return to step S2; in, (13)； (14)； Step S3 can be represented as the following optimization problem: (15)； The solution is obtained using the Lagrange duality method, including: (1) Problem modeling The Lagrange function is: (16)； The dual function is: (17)； Note that the dual function is decomposable, and the dual function can be further equivalent to: (18)； in, (19)； At this point, the dual problem is written as: (20)； (2) Solving the dual problem The dual problem is: (21)； The first-order optimality condition is: (22)； when At that time, there were: (23)； when At that time, there were: (24)； when At that time, there were: (25)； because Each item in it is about Since it is a monotonic function, a one-dimensional search method is used to find the optimal solution to the dual subproblem. (3) Solving the dual principal problem Will Substituting the value of into the dual principal problem allows the principal problem to be solved using the subgradient algorithm. The update method is as follows: (26)； in This is the step size for each iteration; the subgradient algorithm guarantees convergence to the optimal value, and obtains an approximate optimal value through a sufficient number of iterations. Power distribution method and The optimization problem is the same in the following way, with steps omitted; (27)。 9. The power allocation method in an OFDM bidirectional relay system according to claim 8, characterized in that, The second bit error rate is solved using a local optimal power allocation algorithm and the Lagrange duality method to obtain the optimal power allocation strategy, including: Based on the second bit error rate, the optimization problem can now be expressed as: (28)； The solution is obtained using the Lagrange duality method, including: (1) Problem modeling The Lagrange function is: (29)； The dual function is: (30)； Note that the dual function is decomposable, and the dual function can be further equivalent to: (31)； in, (32)； At this point, the dual problem is written as: (33)； (2) Solving the dual problem The dual problem is (34)； The first-order optimality condition is (35)； when At that time, there were: (36)； correspond It can be obtained from the following system of equations: (37)； when At that time, there were: (38)； Find the corresponding ; when At that time, there were: (39)； Find the corresponding ; (3) Solving the dual principal problem Will Substituting the value of into the dual principal problem allows the principal problem to be solved using the subgradient algorithm. The update method is as follows: (40)； in It is the step size for each iteration; the subgradient algorithm guarantees convergence to the optimal value, and obtains an approximate optimal value through a sufficient number of iterations.

10. The power allocation method in an OFDM bidirectional relay system according to claim 9, characterized in that: Computer simulations were used to examine the system BER performance of the joint subcarrier suppression-subcarrier pairing method that optimizes power allocation using local suboptimal power allocation strategies and local optimal power allocation strategies. The system BER performance was compared with that of the joint subcarrier suppression-subcarrier pairing method that does not use local suboptimal power allocation strategies or local optimal power allocation strategies for power optimization.

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