Method for PAPR suppression in MIMO-OFDM system

Abstract

The application discloses a method for restraining PAPR of MIMO-OFDM system, which constructs M phase rotation matrices composed of different phase rotation symbols, and carries out point multiplication with frequency domain sending signals of the MIMO-OFDM system to obtain M signals which are independent of each other and contain the same information; then the signals are subjected to IDFT processing, the results are linearly combined, and the combined signals are subjected to cosine operation; the PAPR of the generated signals is calculated, and the signal with the minimum PAPR is taken as effective information and is emitted through an antenna. The MIMO-OFDM system constructed by the method can effectively reduce the PAPR of the signal without introducing distortion, and will not affect the working efficiency of devices in the system, so that the MIMO-OFDM system can achieve better communication performance.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of wireless communication, and relates to a digital signal processing method in the field of communication, in particular to a method for suppressing PAPR of a MIMO-OFDM system. BACKGROUND

[0002] Multiple Input Multiple Output (MIMO) technology is a key technology to obtain high data rate and high spectrum efficiency. Based on the multipath fading characteristics of a wireless channel, the spectrum efficiency is improved by configuring multiple antennas at the transmitting end and the receiving end, and the gain is obtained by using diversity and multiplexing technology to improve the link reliability and expand the coverage range. The MIMO-OFDM transmission technology combines the advantages of MIMO technology and OFDM technology, has very high spectrum utilization and transmission rate, and on the basis of not increasing the scarce frequency bandwidth, uses space, time and frequency domain resources and corresponding diversity technology to increase the system design degree of freedom, improve the system performance and solve the problem of frequency selective fading of the channel.

[0003] A multi-domain index modulation system constructs a sparse signal in multiple dimensions to enhance system performance and obtain higher transmission rate and spectrum efficiency, and has higher design freedom. A generalized space-frequency index modulation (GSFIM) system uses multiple antennas and subcarriers for joint indexing to obtain a sparse signal in the spatial and frequency domains, and has higher transmission rate. However, in the process of transmitting the signal, a large number of silent subcarriers do not transmit data. The existence of these subcarriers reduces the interference between subcarriers caused by Doppler shift, reduces the sensitivity of the MIMO-OFDM system to frequency offset, and has a higher peak to average power ratio (PAPR), which causes the power amplifier to work at the signal peak, increases the possibility of nonlinear distortion, and seriously affects the stability of the system. The existing technology mainly reduces the PAPR of the MIMO-OFDM system through TR, PTS, precoding and other methods, but these technologies have the disadvantages of complex algorithm and affecting the communication performance of the system. SUMMARY

[0004] In view of the deficiencies of the prior art, the present application provides a method for suppressing PAPR of a MIMO-OFDM system. By constructing a phase rotation symbol to form a phase rotation matrix, the signal is linearly combined, the number of selectable transmitting signals is increased without increasing the hardware cost, the PAPR of the signal is effectively reduced, the possibility of nonlinear distortion of the power amplifier is reduced, and the stability of the system is enhanced.

[0005] A method for PAPR suppression in a MIMO-OFDM system, comprising the following steps:

[0006] Step 1: Define a variable with N t A MIMO-OFDM system with one transmit antenna and one receive antenna, assuming that the OFDM symbols transmitted on each antenna include N c After performing spatial index modulation, frequency index modulation, and QAM mapping on the source bit sequence S using n subcarriers, the signal of the g-th antenna is represented as X. g =[X g [1], X g [2], ...X g [N C ]], where g = 1, 2, ..., N t .

[0007] Step 2: Send signal X in the frequency domain g Construct the following conjugate sequence:

[0008]

[0009] The superscript * indicates the conjugate of the matrix. Indicates a length of N zp The zero vector. Based on the number of zero elements in the conjugate sequence, the oversampling rate is set to N. DFT / (N DFT -N zp ), where N DFT This represents the computation length of the IDFT and DFT.

[0010] N t The frequency domain transmission signal is obtained by cascading the signals from the transmitting antennas.

[0011] Step 3: Randomly construct M phase rotation matrices, where the i-th matrix P i The size is N t ×N DFT Let i = 1, 2, 3, ..., M. In the phase rotation matrix, each element represents a phase rotation symbol:

[0012]

[0013] Matrix P i The element in the p-th row and q-th column is represented as:

[0014]

[0015] p=1,2,3,...,Nt, q=1,2,3,...,N DFT ,and Uniformly distributed in [0, 2π].

[0016] Step 4, the frequency domain signal X is respectively multiplied with M phase rotation matrixes to obtain M phase-rotated signals X i = X x P i , which are independent of each other but contain the same information.

[0017] Step 5, IDFT calculation is performed on the phase-rotated signal X i to obtain M time domain candidate signals x i .

[0018] Step 6, the M time domain candidate signals are randomly divided into two groups, each group including M / 2 time domain candidate signals, and the two groups of signals are respectively multiplied with sinα and cosα to obtain two groups of signals x sin and x cos , wherein α∈(0, 2π) is a constant modulation index.

[0019] Step 7, random linear combination is performed on the signals in x sin and x cos , and then M 2 / 4 combined signals x sincos (d) can be obtained:

[0020] x sincos (d) = x sin (a) + x cos (b) (7)

[0021] , wherein x sin (a) represents the a-th signal in x sin , x cos (b) represents the b-th signal in x cos , a = 1, 2, …, M / 2, b = 1, 2, …, M / 2, and d = 1, 2, …, M 2 / 4.

[0022] Step 8, assuming that the total transmission power of the MIMO-OFDM system is 1, the transmission power of each transmission antenna is γ = 1 / N t , and the combined signal x sincos (d) is added with a positive revision coefficient β and then cosine transformation is performed to obtain a signal S NSC (d):

[0023] S NSC (d) = γcos(Ax sincos (d) + β)) (8)

[0024] , wherein A is a modulation coefficient. The positive revision coefficient β is used to make the numerical value of the signal before the cosine transformation positive.

[0025] Step 9, calculating the signal S obtained from step 8 NSC PAPR value of (d):

[0026]

[0027] PAPR(d) is the PAPR value of signal S NSC (d). M 2 PAPR values of the four signals S NSC (d) are compared, and the signal with the minimum PAPR value is selected The signal S is sent through the transmitting antenna.

[0028]

[0029] The present application has the following beneficial effects:

[0030] The present method uses a phase rotation matrix composed of phase rotation symbols to multiply the frequency domain sending signal, performs IFFT operation on the obtained signal, groups the results, and linearly combines any signal in the group. Without increasing the IDFT operation, the number of alternative sending signals is increased, the system calculation complexity is reduced, the algorithm performance is further improved, the non-linear cosine calculation is increased, the signal amplitude is limited to change between -1 and 1, the dynamic amplitude range of the signal is determined, and the peak-to-average ratio of the signal is reduced. By reasonably setting the constant coefficient, the modulation index, and the number of phase rotation matrices, the PAPR in the MIMO-OFDM system is greatly reduced, the system obtains better performance, the working efficiency of the device is improved, and the MIMO-OFDM system achieves better communication performance. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 is a traditional GSFIM system sending end block diagram;

[0032] Figure 2 is a system sending end block diagram of the present method;

[0033] Figure 3 is a PAPR simulation curve diagram of different systems in the embodiment. DETAILED DESCRIPTION

[0034] The present application will be further explained and described below in conjunction with the accompanying drawings;

[0035] Figure 1 is a traditional GSFIM system sending end system block diagram, Figure 2 is a system sending end block diagram of the present method.

[0036] As Figure 2As shown, for the source bit sequence S of the MIMO-OFDM system, the sequence is subjected to spatial index modulation, frequency index modulation, and QAM mapping in sequence, a conjugate sequence is constructed for the obtained signal, the OFDM signal generated after IDFT calculation is grouped and multiplied by different trigonometric functions, the obtained signal is subjected to random linear combination, the linearly combined signal is subjected to cosine transformation, and finally the signal with the minimum PAPR is selected to be transmitted through the radio frequency transmission chain.

[0037] In the embodiment, the system is provided with N t = 4 transmitting antennas and 1 receiving antenna, the OFDM symbol transmitted on each antenna includes N c = 63 subcarriers. A 0 vector with N zp = 384 is set, the oversampling rate is 4, N DFT = 512 is set as the calculation length of IDFT and DFT, M = 8 phase rotation matrices are randomly constructed, the modulation coefficient A = 0.5 is set, and a = 11p / 8 is set.

[0038] The complementary cumulative distribution function (CCDF) curves of the PAPRs of different systems are drawn, as shown in Figure 3 In the SLM system, the PAPR of the system reaches about 8.1 dB at the complementary cumulative distribution function of 10 -3 , which is reduced by about 2.7 dB relative to the original OFDM signal; in the G-SLM system, the PAPR of the system reaches about 7.8 dB, which is reduced by about 3 dB relative to the original OFDM signal; in the system of the method, the PAPR of the system reaches about 0.15 dB at the complementary cumulative distribution function of 10 -3 , which is reduced by about 10.65 dB relative to the original OFDM signal, and the effect of reducing the PAPR is relatively obvious.

Claims

1. A method for PAPR reduction in a MIMO-OFDM system, characterized in that: Specifically comprising the following steps: Step 1, randomly construct M phase rotation matrices P of size N x N t Step 2, compute the average phase rotation matrix Pavg DFT Step 3, compute the average phase rotation matrix Pavg i : i = 1, 2, 3,..., M, p = 1, 2, 3,..., N t q = 1, 2, 3,..., N DFT , and is uniformly distributed within [0, 2π], where N t represents the number of transmit antennas in the MIMO-OFDM system, N DFT is the length of IDFT and DFT computation for source bit data; Step 2, the frequency domain sending signal X of the system is respectively multiplied with M phase rotation matrices, to obtain M phase-rotated signals X i = X x P i , and then IDFT calculation is performed, to obtain M time domain alternative signals x i ; Step 3, divide the M time domain candidate signals into two groups randomly, each group includes M / 2 time domain candidate signals, multiply the two groups of signals by sinα and cosα respectively to obtain two groups of signals x sin and x cos wherein, α∈(0, 2π), represents a constant modulation index; Step 4, for x sin and x cos sincos (d):​ x sincos (d) = x sin (a) + x cos (b) (3) where x sin (a) denotes the a-th signal in x sin , x cos (b) denotes the b-th signal in x cos , a = 1, 2,..., M / 2, b = 1, 2,..., M / 2, d = 1, 2,..., M / 4 2 ; Step 5, cosine transform of the combined signal x sincos (d) cosine transform after adding the positive revision coefficient β to obtain signal s NSC (d): s NSC (d) = γ cos(Ax sincos (d) + β) (4) Wherein, gamma indicates the transmitting power of each transmitting antenna, and A is a modulation coefficient; The computed cosine-transformed signal s NSC The PAPR value PAPR(d) of (d): Step 6, from M 2 selecting a signal with the minimum PAPR value from the 4 signals transmitting the signal S through the transmitting antenna.

2. The method of claim 1, wherein the PAPR reduction is performed for a MIMO-OFDM system. After spatial index modulation, frequency index modulation and QAM mapping on the source bit sequence S, the signal of the gth antenna is represented as X g = [X g [1], X g [2], … X g [N C ]], where g = 1, 2, …, N t ; N c represents the number of subcarriers of the OFDM symbol transmitted on each transmitting antenna.

3. The method of claim 2, wherein the PAPR reduction is performed for a MIMO-OFDM system. The frequency domain transmission signal X g The conjugate sequence is constructed as follows: where the superscript * denotes the conjugate of a matrix, denotes a 0 vector of length N zp ; according to the number of 0 elements in the conjugate sequence, the oversampling rate is set as N DFT / (N DFT -N zp ), where N DFT is the calculation length of IDFT and DFT; and the signals of N t root transmitting antennas are concatenated to obtain a frequency domain sending signal

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