CN110445737B

CN110445737B - OFDM peak-to-average power ratio reduction method and system based on two-stage index modulation

Info

Publication number: CN110445737B
Application number: CN201910613902.5A
Authority: CN
Inventors: 陈振兴; 张煜蓉; 张惠捷; 王勇; 王家豪
Original assignee: China University of Geosciences
Current assignee: CERTUSNET CORP
Priority date: 2019-07-09
Filing date: 2019-07-09
Publication date: 2020-09-04
Anticipated expiration: 2039-07-09
Also published as: CN110445737A

Abstract

The invention discloses an OFDM peak-to-average power ratio reduction method and system based on two-stage index modulation. In the first stage index modulation, each OFDM signal subframe activates part of subcarriers for transmitting data according to index bit information, in the second stage index modulation, part of subcarriers are selected again from the subcarriers which are not activated in the first stage according to more index bit information for activation and data transmission, the main purpose of the second stage index modulation is to reduce the peak-to-average power ratio of the two-stage index modulation OFDM system and improve the spectrum efficiency, and meanwhile, a simple search algorithm for reducing the peak-to-average power ratio is adopted. The receiving end adopts maximum likelihood detection, which comprehensively considers the index pattern and the mapping signal for detection and bit information recovery. The method and the system can effectively reduce the peak-to-average power ratio performance of the two-stage index modulation OFDM, and the spectrum efficiency, the power efficiency and the bit error rate performance of the system are improved.

Description

OFDM peak-to-average power ratio reduction method and system based on two-stage index modulation

Technical Field

The invention relates to the technical field of wireless communication, in particular to a method and a system for reducing peak-to-average power ratio (PAPR) of Orthogonal Frequency Division Multiplexing (OFDM) based on two-stage index modulation.

Background

In the field of wireless communication, the index modulation-based OFDM technology is being widely researched and applied due to its advantages of high spectrum utilization, effective resistance to multipath, and resistance to frequency selective fading, and is a favorable technical candidate even for fifth generation (5G) mobile communication. The index modulation OFDM technology can effectively improve the system frequency band utilization rate and the power efficiency of a transmitting end signal, because only part of subcarriers of an OFDM signal based on index modulation are activated for transmitting information, other subcarriers are 0, and more transmitting bit information is hidden in the index information, namely the transmitting bit information is utilized to determine the activation pattern of the subcarriers. In order to further improve the spectral efficiency of the system, a dual-mode index modulation OFDM technique is proposed, which uses two constellations for mapping data, wherein the active sub-carriers selected by the index information implement the mapping process by using the constellation a, and the rest of the sub-carriers implement the mapping process by using the constellation B, and the signal points in the two constellations are mutually disjoint.

However, the OFDM system based on index modulation inherits the disadvantage of high PAPR of the conventional OFDM system, and the high PAPR requires a power amplifier at the transmitting end of the system to have a large linear range, so as to avoid signal distortion caused by the transmitted OFDM signal exceeding the maximum linear range, and increase the linear range of the power amplifier too high cost. Some commonly used PAPR reduction methods for OFDM systems include clipping techniques, selective mapping, partial transmission sequence and virtual sequence insertion techniques, etc. A PAPR reduction method suitable for an index modulation based OFDM system is yet to be developed.

Disclosure of Invention

The invention aims to solve the technical problems of reducing the PAPR of an index modulation OFDM system, improving the frequency band utilization rate of the traditional index modulation OFDM system, improving the power efficiency of a dual-mode index modulation OFDM system and improving the bit error rate performance of the index modulation OFDM system, and provides an OFDM based on two-stage index modulation and a PAPR reduction method and a system thereof.

According to an aspect of the technical problem to be solved by the present invention, an OFDM peak-to-average power ratio reduction method based on two-stage index modulation includes the steps of:

(1) serial-to-parallel conversion and bit grouping: serial-to-parallel conversion of a binary sequence to be transmitted, converting a serial bit data stream into a parallel data stream, where each p bits is a group of sub-frames input to a frame of OFDM signals, where a frame of OFDM signals comprising N sub-carriers is divided into G-N/N sub-frames, G_g＝[X_g,1X_g,2…X_g,n]And G is more than or equal to 1 and less than or equal to G, n is the number of subcarriers contained in each subframe, and X represents the subcarriers in one frame of OFDM signal.

(2) First stageIndex modulation: the first p in each group of p bits in the step (1) is₁The bits are input to a first stage index selector which selects the active sub-carriers, i.e. p, in each sub-frame of the OFDM signal₁Individual bit decision G_gActivating the sub-carrier, and then combining p in each group of p bits in the step (1)₂The bits are input to the mapper a for mapping.

(3) Second-stage index modulation: p in each group of p bits in the step (1)₃Bit input to second stage index selector from G of step (2)_gThe sub-carriers are selected again from the sub-carriers which are not activated to be activated, and then p in each group of p bits in the step (1) is selected₄The bits are input to the mapper B for mapping.

(4) Generating a frequency domain OFDM signal, and according to the subcarrier activation patterns in the steps (2) and (3), carrying out p in each group of p bits in the step (1)₂+p₄The bits are mapped to corresponding active subcarriers by a mapper a and a mapper B.

(5) And (4) converting the frequency domain OFDM signal obtained in the step (4) into a time domain through N-point Inverse Discrete Fourier Transform (IDFT).

(6) And (4) inputting the time domain OFDM signal obtained in the step (5) into a peak-to-average power ratio comparator, and finding the optimal combination of the second-stage signal mapping through a search algorithm to minimize the PAPR.

(7) And (4) sending the time domain OFDM signal with the minimized PAPR obtained in the step (6) into a channel for transmission after parallel-serial conversion, cyclic prefix addition, digital-to-analog conversion and up-conversion processing.

(8) At a receiving end, the received OFDM signal is processed by down-conversion, analog-to-digital conversion, cyclic prefix removal and serial-to-parallel conversion.

(9) And (4) performing Discrete Fourier Transform (DFT) on the output signal of the step (8) to convert the time domain OFDM signal into the frequency domain.

(10) And (4) carrying out maximum likelihood detection, de-indexing and de-mapping processing on the output signal of the step (9) to restore the output signal into binary information.

(11) And (4) performing parallel-serial conversion on the output signal of the step (10) to obtain an originally sent binary sequence.

Further, the method can be used for preparing a novel materialIn the method for reducing a peak-to-average power ratio of OFDM based on two-stage index modulation according to the present invention, in step (1), the number of bits p ═ p included in each subframe of one frame of OFDM signal₁+p₂+p₃+p₄Wherein the front p₁A sum of bits p₃The bits are respectively input into a first stage index selector and a second stage index selector for selecting the activation pattern, p, of the subcarrier in each sub-frame of the OFDM signal₂A sum of bits p₄The bits are input to a mapper A and a mapper B respectively for signal mapping, p₁、p₂、p₃And p₄The corresponding bit information is arranged in sequence.

Further, in the method for reducing the peak-to-average power ratio of the OFDM based on the two-stage index modulation of the present invention, the index modulation of the first stage in the step (2) selects k subcarriers from n subcarriers of each subframe for mapping bit information, so that:

p₂＝klog₂M_A

in the formula

Expressing a plateau function, i.e. rounding down, C (n, k) expressing a binomial coefficient, i.e. taking the number of combinations of k from n, while satisfying k<n,M_ARepresenting the size of the constellation employed in the mapper a. From p₁The index information determined by the bit information is I_A＝[I_A,1I_A,2…I_A,k]From p₂The signal determined by the bit information is S_A＝[S(I_A,1)S(I_A,2)…S(I_A,k)]。

Further, in the method for reducing the peak-to-average power ratio of the OFDM based on the two-stage index modulation of the present invention, the index modulation in the second stage in step (3) is to select k' subcarriers from the n-k inactivated subcarriers in step (2) for mapping the bit information, so that:

p₄＝k'log₂(M_B/u)

in the formula M_BThe size of the constellation diagram adopted in the mapper B is represented, and each u signal points in the constellation diagram B represent the same bit information and are used for reducing the PAPR of the system, and random selection is carried out during first mapping. The signal points in the constellation diagram B and the constellation diagram A are mutually intersected and simultaneously satisfy k + k'<n, ensuring that 0 carrier, namely the inactivated subcarrier exists so as to improve the power efficiency of the system. From p₃Index information I determined by bit information_B＝[I_B,1I_B,2…I_B,k’]From p₄Signal S determined by bit information_B＝[S(I_B,1)S(I_B,2)…S(I_B,k’)]。

Further, in the method for reducing the peak-to-average power ratio of the OFDM based on the two-stage index modulation according to the present invention, the frequency domain OFDM signal generator of step (4) is configured to activate the subcarriers in steps (2) and (3) according to the subcarrier activation pattern of steps (2) and (3), and then the S in steps (2) and (3) is transmitted to the OFDM receiver_AAnd S_BMapping to corresponding active sub-carriers I by mapper A and mapper B_AAnd I_BThe above.

Further, in the OFDM peak-to-average power ratio reduction method based on two-stage index modulation according to the present invention, the time domain OFDM signal output in step (5) may be represented as:

x_T＝[x₀x₁... x_N-1]＝IDFT{X_T}＝IDFT{[X₀X₁... X_N-1]}

in the formula, IDFT { } represents inverse discrete Fourier transform operation, X_T＝[X₀X₁…X_N-1]Representing the transmitted frequency domain OFDM signal, including signal points from constellation a and constellation B and 0 carriers.

Further, in the method for reducing the peak-to-average power ratio of the OFDM based on the two-stage index modulation of the present invention, the step (6) is a PAPR minimization process of the transmitted time domain OFDM signal, and includes the following steps:

(61) calculating an initial PAPR value PAPR, setting a variable i-1, ii-1, and g-1;

(62) with S_i,g(I_B,ii) Replacing the original signal on the corresponding sub-carrier, recalculating new PAPR value, and if PAPR is reduced, using S_i,g(I_B,ii) Replacing the original signal point and assigning the new PAPR value to the PAPR, otherwise, the signal point is unchanged, S_i,g(I_B,ii) Represents one of the selectable signals mapped by the mapper B in the second stage index modulation of each subframe;

(63) i +1, returning to the step (62) until i is equal to u, and i is equal to or less than 1 and equal to or less than u, which represents the selectable signal index mapped on the active subcarrier of each subframe in the second stage;

(64) ii +1, returning to step (62) until ii is over, 1 ≦ ii ≦ k', indicating the active subcarrier index for each subframe of the second phase;

(65) and returning to the step (62) until the G is equal to G, and 1 ≦ G ≦ G, which represents the subframe index of one frame of OFDM signal.

Further, in the method for reducing the peak-to-average power ratio of the OFDM based on the two-stage index modulation of the present invention, the calculation formula of the PAPR value in step (61) is:

in the formula, E { } represents expectation, and max { } represents maximum value.

Further, in the method for reducing the peak-to-average power ratio of the OFDM based on the two-stage index modulation of the present invention, the maximum likelihood detection is adopted in step (10), according to step (2) and step (3), the detection process comprehensively considers all possible subcarrier activation patterns and mapped signals of each frame of OFDM signal subframe, and the specific detection process can be expressed as:

in the formula X_RRepresenting the received frequency domain subcarrier signal, H represents the channel in the frequency domainAttenuation coefficient, superscript g denotes the g-th sub-frame of each OFDM signal,

respectively representing the index information of each subframe of the receiving end and the estimated value of the mapping signal, and then performing indexing and demapping by a table look-up method according to the detected index information and signal to recover binary information.

Further, in the method for reducing the peak-to-average power ratio of the OFDM based on the two-stage index modulation according to the present invention, through the two-stage index modulation process from step (1) to step (7), the proposed OFDM band utilization ratio can be expressed as:

in the formula L_CPDenotes the length of the cyclic prefix added, and G denotes the number of subframes per OFDM signal.

According to another aspect of the technical problem to be solved by the present invention, there is also provided an OFDM peak-to-average power ratio reduction system based on two-stage index modulation, mainly comprising the following modules and functions:

a serial-to-parallel conversion and bit grouping module at the transmitting end for converting the binary sequence to be transmitted into a serial bit data stream into a parallel data stream by serial-to-parallel conversion, wherein each p bits is a group of sub-frames input into a frame of OFDM signals, and the frame of OFDM signals containing N sub-carriers is divided into G-N/N sub-frames_g＝[X_g,1X_g,2…X_g,n]And G is more than or equal to 1 and less than or equal to G, n is the number of subcarriers contained in each subframe, and X represents the subcarriers in one frame of OFDM signal.

A first stage index selector module for selecting the first p bits from each group₁The bits select the subcarrier activation pattern in each subframe of the OFDM signal.

Mapper A module for mapping p of each group of p bits by A mapper₂The individual bits are mapped to signal points in constellation a.

Second stageA segment index selector module for selecting a segment index according to p in each group of p bits₃The bits again select the active pattern of subcarriers from among the inactive subcarriers in each subframe in the first stage.

Mapper B module for mapping p in each group of p bits by B mapper₄The bits are mapped to signal points in a constellation diagram B, and the signal points in the constellation diagram B and the signal points in the constellation diagram A are not intersected with each other.

And the frequency domain OFDM signal generator module is used for mapping the signals to the active subcarriers of the OFDM signals through a mapper A and a mapper B respectively according to the active subcarriers selected by the first-stage index and the second-stage index.

And the N-point IDFT module is used for converting the frequency domain OFDM signals obtained by the two-stage index modulation into a time domain through the IDFT module.

And the peak-to-average power ratio comparator module is used for selecting the signal point mapped by the mapper B through a search algorithm so as to minimize the PAPR of the OFDM signal.

And the sending end parallel-to-serial conversion, cyclic prefix adding, digital-to-analog conversion and up-conversion module is used for performing parallel-to-serial conversion, cyclic prefix adding, digital-to-analog signal conversion and up-conversion on the time domain OFDM signal generated by the sending end.

And the receiving end down-conversion module, the analog-to-digital conversion module, the cyclic prefix removal module and the serial-to-parallel conversion module are used for performing down-conversion, analog-to-digital signal conversion, cyclic prefix removal and serial-to-parallel conversion processing on the received time domain OFDM signal.

And the N-point DFT module is used for converting the received time domain OFDM signal into a frequency domain.

And the maximum likelihood detection, de-indexing and de-mapping module is used for comprehensively considering all possible subcarrier activation patterns and mapped signals in each frame of OFDM signal subframe by adopting maximum likelihood detection, searching all possible conditions so as to detect the subcarrier activation patterns and the signals mapped by each activated subcarrier, and recovering binary information through de-indexing and de-mapping processing.

And the receiving end parallel-serial conversion module is used for performing parallel-serial conversion on the recovered binary information and recovering the originally sent binary sequence.

The invention provides an OFDM peak-to-average power ratio reduction method and system based on two-stage index modulation, wherein the first stage is consistent with the traditional index modulation OFDM, through the index modulation of the second stage, the optimal signal mapping is found by utilizing a search algorithm, the PAPR value of an OFDM signal is sent to be minimized, and simultaneously, transmission bit information including index bits and signal bits of the second stage is increased, so that the frequency spectrum frequency of the system is improved under the condition of not changing the number of subcarriers of the OFDM signal. Meanwhile, in the second stage, all the sub-carriers of each sub-frame of the OFDM signal are not activated for transmitting data, but some sub-carriers are kept to be 0, so that the power efficiency of transmitting the OFDM signal is improved.

Drawings

Fig. 1 is a block diagram of a transmitting end of an OFDM peak-to-average power ratio reduction system based on two-stage index modulation according to the present invention.

Fig. 2 is a block diagram of a receiving end of an OFDM peak-to-average power ratio reduction system based on two-stage index modulation according to the present invention.

Fig. 3 is a constellation a and a constellation B in the embodiment of the present invention.

Fig. 4 is a schematic diagram of a performance curve of a two-stage index modulation OFDM peak-to-average power ratio reduction system bit error rate according to an embodiment of the present invention.

Fig. 5 is a schematic diagram of a performance curve of a PAPR reduction system for a two-stage index modulation OFDM according to an embodiment of the present invention.

Detailed Description

So that the technical features, objects, and effects of the present invention can be more clearly understood and appreciated, specific embodiments of the present invention will be described in detail with reference to the accompanying drawings.

Referring to fig. 1 and 2, fig. 1 is a block diagram of a transmitting end of an OFDM papr reduction system based on two-stage index modulation according to the present invention, which includes a serial-to-parallel conversion and bit grouping module, a first-stage index selector module, a second-stage index selector module, a mapper a module, a mapper B module, a frequency domain OFDM signal generator module, an N-point IDFT module, a papr comparator module, a parallel-to-serial conversion, a cyclic prefix adding module, a digital-to-analog conversion module, and an up-conversion module. Fig. 2 is a block diagram of a receiving end of an OFDM peak-to-average power ratio reduction system based on two-stage index modulation according to the present invention, which includes a down-conversion module, an analog-to-digital conversion module, a cyclic prefix removal module, a serial-to-parallel conversion module, an N-point DFT module, a maximum likelihood detection module, an index removal module, a demapping module, and a parallel-to-serial conversion module.

Setting the number of subcarriers of an OFDM signal to N, such a transmitted frequency domain OFDM signal can be represented as X_T＝[X₀X₁… X_N-1]Each OFDM signal sub-carrier is divided into G-N/N sub-groups, each sub-group is an OFDM signal sub-frame, N is the number of sub-carriers contained in each sub-frame, and each OFDM signal sub-frame carries p-p₁+p₂+p₃+p₄Information of one bit, p₁、p₂、p₃And p₄The corresponding bit information is arranged in sequence, wherein each OFDM signal contains m ═ pG bit information, p₁、p₂、p₃And p₄Are all positive integers.

The OFDM peak-to-average power ratio reduction method based on two-stage index modulation comprises the following steps:

(1) serial-to-parallel conversion and bit grouping: serial-to-parallel conversion of a binary sequence to be transmitted, converting a serial bit data stream into a parallel data stream, wherein each p bits is a group of one sub-frame input to an OFDM signal, and a frame of the OFDM signal is divided into G sub-frames, G_g＝[X_g,1X_g,2… X_g,n](1 ≦ G ≦ G) represents the G-th subframe of the OFDM signal, X represents the subcarriers in one frame of the OFDM signal, and n is the number of subcarriers contained in the subframe. For p bits of information contained in each subframe, where p is the first₁A sum of bits p₃The bits are respectively input into a first stage index selector and a second stage index selector for selecting the activation pattern, p, of the subcarrier in each sub-frame of the OFDM signal₂A sum of bits p₄The bits are input to the mapper a and the mapper B, respectively, for signal mapping.

(2) First-stage index modulation: in each group of p bits in the step (1)Front p of (A)₁The bits are input to a first stage index selector which selects the active sub-carriers, i.e. p, in each sub-frame of the OFDM signal₁Individual bit decision G_gActivation pattern of sub-carriers, index of activated sub-carriers I_ATo express, p in each group of p bits in step (1) is added₂The bit input mapper A maps the signal used for mapping with S_ATo indicate. The first stage index modulation selects k subcarriers from n subcarriers of each subframe for mapping bit information, p₁And p₂Can be calculated as:

p₂＝k log₂M_A

in the formula

Expressing a plateau function, i.e. rounding down, C (n, k) expressing a binomial coefficient, i.e. taking the number of combinations of k from n, while satisfying k<n,M_AThe size of the constellation employed in mapper a is shown, where mapper a may employ a BPSK constellation, a QPSK constellation, etc. From p₁The index information determined by the bit information is I_A＝[I_A,1I_A,2…I_A,k]From p₂The signal determined by the bit information is S_A＝[S(I_A,1)S(I_A,2)…S(I_A,k)]. When n is 4, k is 1, p₁When 2, the correspondence between the index bits and the active subcarriers in the subframe can be represented by table 1.

TABLE 1

Index bit	Indexing	Sub-frame
				0 0	1	[S _A 0 0 0]
0 1	2	[0 S _A 0 0]
			1 0	3	[0 0 S_A 0]
1 1	4	[0 0 0 S_A]

(3) Second-stage index modulation: p in each group of p bits in the step (1)₃Bit input to second stage index selector from G of step (2)_gThe sub-carriers are selected again from the non-activated sub-carriers to be activated, and the activated sub-carriers refer to I_BTo express, p in each group of p bits in step (1) is added₄The bit input mapper B maps the signal used for mapping with S_BTo indicate. The second stage index modulation is to select k' sub-carriers from the n-k non-activated sub-carriers in step (2) for mapping bit information, p₃And p₄Can be calculated as:

p₄＝k'log₂(M_B/u)

in the formula M_BRepresents the size of the constellation employed in the mapper B, each u signals in the constellation BThe dots represent the same bit information, are used for reducing the PAPR of the system, and are randomly selected when mapping is carried out for the first time. The signal points in the constellation diagram B and the constellation diagram A are mutually intersected and simultaneously satisfy k + k'<n, ensuring that 0 carrier, namely the inactivated subcarrier exists so as to improve the power efficiency of the system. From p₃Index information I determined by bit information_B＝[I_B,1I_B,2…I_B,k’]From p₄Signal S determined by bit information_B＝[S(I_B,1)S(I_B,2)…S(I_B,k’)]. The mapper B may adopt a QPSK constellation, an 8PSK constellation, etc., and the size of the constellation B may be larger than that of the constellation a in order to not reduce the spectral efficiency of the system and reduce the PAPR. On the basis of the first-stage index modulation, when k' is 1, p₃When 1, the correspondence between the index bits and the active subcarriers in the subframe can be represented by table 2.

TABLE 2

If the QPSK constellation is used for data mapping in the mapper B, the signal points (d, d) and (-d, -d) represent the same bit information, and the signal points (-d, d) and (d, -d) represent the same bit information for PAPR reduction of the index modulated OFDM system, as shown in fig. 3, where the empty signal represents constellation a and the solid signal represents constellation B.

(4) Generating a frequency domain OFDM signal, wherein the frequency domain OFDM signal generator generates S in the steps (2) and (3) according to the subcarrier activation pattern in the steps (2) and (3)_AAnd S_BMapping to corresponding active sub-carriers I by mapper A and mapper B_AAnd I_BThe above.

(5) Converting the frequency domain OFDM signal obtained in step (4) into a time domain through an N-point Inverse Discrete Fourier Transform (IDFT), and outputting the time domain OFDM signal as:

x_T＝[x₀x₁... x_N-1]＝IDFT{X_T}＝IDFT{[X₀X₁... X_N-1]}

in the formula, IDFT { } represents inverse discrete Fourier transform operation, X_TIncluding signal points from constellation a and constellation B and the 0 carrier.

(6) And (4) inputting the time domain OFDM signal obtained in the step (5) into a peak-to-average power ratio comparator, and finding the optimal combination of the second-stage signal mapping through a search algorithm to minimize the PAPR. A PAPR minimization procedure for transmitting an OFDM signal, comprising the steps of:

(61) calculating an initial PAPR value PAPR by the following formula:

in the formula, E { } represents expectation, max { } represents maximum, and a variable i is set to 1, ii is set to 1, and g is set to 1;

(7) And (4) sending the time domain OFDM signal with the minimized PAPR obtained in the step (6) into a channel for transmission after parallel-serial conversion, cyclic prefix addition, digital-to-analog conversion and up-conversion processing. Through the index modulation process of the two stages of step (1) to step (7), the proposed OFDM band utilization can be expressed as:

(10) And (4) carrying out maximum likelihood detection, de-indexing and de-mapping processing on the output signal of the step (9) to restore the output signal into binary information. According to the step (2) and the step (3), the maximum likelihood detection process comprehensively considers all possible subcarrier activation patterns and mapped signals of each frame of OFDM signal subframe, and the specific detection process can be expressed as:

in the formula X_RRepresenting the received frequency domain subcarrier signal, H representing the channel attenuation coefficient in the frequency domain, superscript g representing the g-th subframe of each OFDM signal,

Example (b):

the specific parameter scheme is as follows: the number N of subcarriers of one OFDM signal is 128, the number G of subframes of each OFDM signal is 32, the number N of subcarriers in each subframe is 4, and the activated subcarriers in the first stageThe number k of the carriers is 1, the number k' of the activated subcarriers in the second stage is 1, and the mapper A adopts a BPSK constellation, namely M_AWith 2, the mapper B uses the QPSK constellation, i.e. M _B4, u-2 means that two signal points on the diagonal of the QPSK constellation are taken to represent the same bit information, as shown in fig. 3, for reducing PAPR of the OFDM signal by the search algorithm, the distance between adjacent signal points in the constellation B and the constellation a is d, and the length L of the cyclic prefix is L_CP16. Each subframe may contain 6 bits of information and the spectral efficiency of the system may be calculated as 1.3333 bits/sec/hz, without taking into account PAPR reduction, and 5 bits of information and the spectral efficiency of the system may be calculated as 1.1111 bits/sec/hz, with PAPR reduction taken into account. The channel employs a frequency selective rayleigh fading channel, wherein the rayleigh channel has a channel impulse response length of 10.

The simulation results are shown in fig. 4 and 5, where the horizontal axis of fig. 4 represents the signal-to-noise ratio, i.e., the power per bit of the signal to the noise power, and the vertical axis represents the bit error rate. In order to prove the advantages of the invention, under the same spectrum efficiency condition, fig. 4 also provides simulation results of the conventional index modulation OFDM and the dual-mode index modulation OFDM, the number of subcarriers of each subframe is 4, the conventional index modulation OFDM system adopts 16QAM for signal mapping, and k is 1 subcarrier in each subframe is activated; in the dual-mode index modulation OFDM system, a BPSK constellation is adopted in a mode A, a BPSK constellation which is perpendicular to the mode A is adopted in a mode B, the signal power of the BPSK constellation is larger than that of a signal in the mode A, wherein k is 2 subcarriers, the data are mapped by the mode A, and the data are mapped by the mode B in the other two subcarriers. According to simulation results, the bit error rate performance of the OFDM system based on the two-stage index modulation is better than that of the traditional OFDM system based on the index modulation under the same spectrum efficiency condition. Under the condition of considering the PAPR reduction algorithm, the OFDM peak-to-average power ratio reduction system of the two-stage index modulation provided by the invention can also provide better bit error rate performance, but the spectrum efficiency of the system is slightly reduced, but the PAPR performance can be improved by about 5dB compared with the classical OFDM system.

Compared with the prior art, the invention has the following advantages: in the aspect of reducing the peak-to-average power ratio, the OFDM peak-to-average power ratio reducing system based on two-stage index modulation can effectively improve the PAPR performance of the system; in the aspect of improving the utilization rate of frequency bands, the OFDM peak-to-average power ratio reduction system based on two-stage index modulation provided by the invention is consistent with the traditional index modulation OFDM in the first stage, and the transmission bit information is increased through the index modulation in the second stage, wherein the transmission bit information comprises the index bits and the signal bits in the second stage, so that the frequency spectrum frequency of the system is improved under the condition of not changing the number of OFDM signal subcarriers, and meanwhile, the function of improving the PAPR of the system is achieved through a search algorithm; in the aspect of the transmission power efficiency of a transmitting end, the OFDM peak-to-average power ratio reduction system based on two-stage index modulation provided by the invention does not activate all the inactivated subcarriers of each subframe of an OFDM signal in the first stage in the second stage for transmitting data, but reserves some subcarriers as 0, and compared with the traditional dual-mode index modulation OFDM technology, the power efficiency of transmitting the OFDM signal can be improved. Meanwhile, under the condition of the same spectrum efficiency or under the condition of considering PAPR reduction (at the cost of slightly reducing the spectrum efficiency), the system provided by the invention has better bit error rate performance than the traditional index modulation OFDM and dual-mode index modulation OFDM systems.

While the present invention has been described with reference to the embodiments shown in the drawings, the present invention is not limited to the embodiments, which are illustrative and not restrictive, and it will be apparent to those skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

1. A method for reducing the peak-to-average power ratio of OFDM based on two-stage index modulation is characterized by comprising the following steps:

(1) serial-to-parallel conversion and bit grouping: serial-to-parallel conversion is carried out on binary sequences to be transmitted, serial bit data streams are converted into parallel data streams, wherein each p bits is a group which is input into a frame of OFDM signalsOne sub-frame of the symbol, where one frame of an OFDM signal containing N sub-carriers is divided into G ═ N/N sub-frames, G_g＝[X_g,1X_g,2…X_g,n]Representing the G-th subframe, wherein G is more than or equal to 1 and less than or equal to G, n is the number of subcarriers contained in each subframe, and X represents the subcarriers in one frame of OFDM signals;

(2) first-stage index modulation: the first p in each group of p bits in the step (1) is₁The bits are input to a first stage index selector which selects the active sub-carriers, i.e. p, in each sub-frame of the OFDM signal₁Individual bit decision G_gActivating the sub-carrier, and then combining p in each group of p bits in the step (1)₂The bit input mapper A carries out mapping;

(3) second-stage index modulation: p in each group of p bits in the step (1)₃Bit input to second stage index selector from G of step (2)_gThe sub-carriers are selected again from the sub-carriers which are not activated to be activated, and then p in each group of p bits in the step (1) is selected₄The bit input mapper B carries out mapping;

(4) generating a frequency domain OFDM signal, and according to the subcarrier activation patterns in the steps (2) and (3), carrying out p in each group of p bits in the step (1)₂+p₄Mapping the bits to corresponding active subcarriers through a mapper A and a mapper B;

(5) converting the frequency domain OFDM signal obtained in the step (4) into a time domain through N-point inverse discrete Fourier transform;

(6) inputting the time domain OFDM signal obtained in the step (5) into a peak-to-average power ratio comparator, and finding the optimal combination of the second-stage signal mapping through a search algorithm to minimize the PAPR;

(7) sending the time domain OFDM signal with the minimized PAPR obtained in the step (6) into a channel for transmission after parallel-serial conversion, cyclic prefix addition, digital-to-analog conversion and up-conversion processing;

(8) at a receiving end, carrying out down-conversion, analog-to-digital conversion, cyclic prefix removal and serial-parallel conversion processing on a received OFDM signal;

(9) performing discrete Fourier transform on the output signal of the step (8) to convert the time domain OFDM signal into a frequency domain;

(10) carrying out maximum likelihood detection, indexing solution and demapping processing on the output signal of the step (9) to restore the output signal into binary information;

2. The method according to claim 1, wherein the number of bits p-p contained in each sub-frame of the one-frame OFDM signal in step (1) is p₁+p₂+p₃+p₄Wherein the front p₁A sum of bits p₃The bits are respectively input into a first stage index selector and a second stage index selector for selecting the activation pattern, p, of the subcarrier in each sub-frame of the OFDM signal₂A sum of bits p₄The bits are input to a mapper A and a mapper B respectively for signal mapping, p₁、p₂、p₃And p₄The corresponding bit information is arranged in sequence.

3. The method of claim 1, wherein the first-stage index modulation of step (2) selects k subcarriers from n subcarriers of each subframe for mapping bit information, so that:

p₂＝k log₂M_A

in the formula

Expressing a plateau function, i.e. rounding down, C (n, k) expressing a binomial coefficient, i.e. taking the number of combinations of k from n, while satisfying k<n,M_ARepresents the size of the constellation employed in mapper A, denoted by p₁The index information determined by the bit information is I_A＝[I_A,1I_A,2…I_A,k]From p₂The signal determined by the bit information is S_A＝[S(I_A,1) S(I_A,2)…S(I_A,k)]。

4. The method according to claim 3, wherein the second-stage index modulation in step (3) is performed by selecting k' subcarriers from n-k unactivated subcarriers in step (2) for mapping bit information, so that:

p₄＝k'log₂(M_B/u)

in the formula, C (n-k, k ') represents a binomial coefficient, i.e., the number of combinations of k' from n-k, M_BThe size of a constellation diagram adopted in a mapper B is represented, each u signal points in the constellation diagram B represent the same bit information and are used for reducing the PAPR of the system, random selection is carried out during first mapping, the signal points in the constellation diagram B and the constellation diagram A are not intersected with each other and meet k + k'<n, ensuring the existence of 0 carrier, i.e. non-activated sub-carrier, to improve the power efficiency of the system, p₃Index information I determined by bit information_B＝[I_B,1I_B,2…I_B,k’]From p₄Signal S determined by bit information_B＝[S(I_B,1) S(I_B,2)…S(I_B,k’)]。

5. The OFDM PAPR reduction method based on two-stage index modulation according to claim 4, wherein the frequency domain OFDM signal generator of step (4) is configured to apply S in steps (2) and (3) according to the subcarrier activation pattern in steps (2) and (3)_AAnd S_BMapping to corresponding active sub-carriers I by mapper A and mapper B_AAnd I_BThe above.

6. The method of claim 5, wherein the time-domain OFDM signal outputted from step (5) is:

x_T＝[x₀x₁...x_N-1]＝IDFT{X_T}＝IDFT{[X₀X₁...X_N-1]}

7. The OFDM PAPR reduction method according to claim 6, wherein the step (6) is a PAPR minimization procedure for transmitting the OFDM signal, comprising the steps of:

(61) an initial PAPR value PAPR is calculated, and a variable i is set to 1, ii is set to 1, and iii is set to 1, and the PAPR value is calculated by the following formula:

in the formula, E { } represents expectation, and max { } represents maximum value;

(62) with S_i,iii(I_B,ii) Replacing the original signal on the corresponding sub-carrier, recalculating new PAPR value, and if PAPR is reduced, using S_i,iii(I_B,ii) Replacing the original signal point and assigning the new PAPR value to the PAPR, otherwise, the signal point is unchanged, S_i,iii(I_B,ii) Represents one of the selectable signals mapped by the mapper B in the second stage index modulation of each subframe;

(65) and returning to the step (62) until the end of the period of iii & ltg & gt, 1 & ltiii & ltg & gt and represents the subframe index of one frame of the OFDM signal.

8. The method according to claim 7, wherein maximum likelihood detection is adopted in step (10), and according to step (2) and step (3), the detection process comprehensively considers all possible subcarrier activation patterns and mapped signals of each frame of OFDM signal subframe, and the specific detection process is expressed as:

9. The method of claim 8, wherein the proposed OFDM band utilization ratio is as follows through the two-stage index modulation process from step (1) to step (7):

10. An OFDM peak-to-average power ratio reduction system based on two-stage index modulation is characterized by comprising the following modules:

a serial-to-parallel conversion and bit grouping module at the transmitting end for performing serial-to-parallel conversion on the binary sequence to be transmitted and converting the serial binary sequenceThe bit stream is converted into a parallel data stream, wherein each p bits is a group of sub-frames input to a frame of OFDM signals, wherein a frame of OFDM signals containing N sub-carriers is divided into G-N/N sub-frames, G_g＝[X_g,1X_g,2…X_g,n]Representing the G-th subframe, wherein G is more than or equal to 1 and less than or equal to G, n is the number of subcarriers contained in each subframe, and X represents the subcarriers in one frame of OFDM signals;

a first stage index selector module for selecting the first p bits from each group₁Selecting a subcarrier activation pattern in each subframe of the OFDM signal by a bit;

mapper A module for mapping p of each group of p bits by A mapper₂Each bit is mapped to a signal point in the constellation diagram A;

a second stage index selector module for selecting a p-bit index according to p in each group of p bits₃The bits select the active pattern of the sub-carriers again from the inactive sub-carriers of each sub-frame in the first stage;

mapper B module for mapping p in each group of p bits by B mapper₄Mapping the bits to signal points in a constellation diagram B, wherein the signal points in the constellation diagram B and the signal points in the constellation diagram A are not intersected with each other;

a frequency domain OFDM signal generator module, which is used for mapping the signal to the active sub-carrier of the OFDM signal through a mapper A and a mapper B respectively according to the active sub-carrier selected by the first stage index and the second stage index;

the N-point IDFT module is used for converting the frequency domain OFDM signals obtained through two-stage index modulation into a time domain through the IDFT module;

the peak-to-average power ratio comparator module is used for selecting the signal point mapped by the mapper B through a search algorithm to minimize the PAPR of the OFDM signal;

the sending end parallel-to-serial conversion, cyclic prefix adding, digital-to-analog conversion and up-conversion module is used for performing parallel-to-serial conversion, cyclic prefix adding, digital-to-analog signal conversion and up-conversion processing on a time domain OFDM signal generated by the sending end;

the receiving end down-conversion module, the analog-to-digital conversion module, the cyclic prefix removal module and the serial-to-parallel conversion module are used for performing down-conversion, analog-to-digital signal conversion, cyclic prefix removal and serial-to-parallel conversion processing on the received time domain OFDM signal;

the N-point DFT module is used for converting the received time domain OFDM signal into a frequency domain;

the maximum likelihood detection, de-indexing and de-mapping module is used for comprehensively considering all possible subcarrier activation patterns and mapped signals in each frame of OFDM signal subframe by adopting maximum likelihood detection, searching all possible conditions so as to detect the subcarrier activation patterns and the signals mapped by each activated subcarrier, and recovering binary information through de-indexing and de-mapping processing;