CN107171999B

CN107171999B - A method of for ofdm system feature suppression

Info

Publication number: CN107171999B
Application number: CN201710441198.0A
Authority: CN
Inventors: 林楷佳; 金芳利; 刘皓
Original assignee: University of Electronic Science and Technology of China
Current assignee: University of Electronic Science and Technology of China
Priority date: 2017-06-13
Filing date: 2017-06-13
Publication date: 2019-08-02
Anticipated expiration: 2037-06-13
Also published as: CN107171999A

Abstract

The invention belongs to fields of communication technology, are related to a kind of method for ofdm system feature suppression.Common cyclic prefix both brought unrivaled advantage to system in ofdm system, also inevitably exposes the performance parameters of system, becomes security breaches, is highly prone to malicious attack.The present invention changes the periodicity of cyclic prefix so that symbol lengths change at random by the redundancy subcarrier that random number is added to each OFDM symbol, to conceal its cyclostationary characteristic being exposed.In addition, but also the gap size between subcarrier equally shows the characteristic changed at random, this also plays protective effect for the Higher Order Cumulants blind estimate of frequency domain character such as sub-carrier number and subcarrier spacing since sub-carrier number purpose changes at random.Meanwhile the algorithm idea can be applied in the similar SC-FDE system of feature by the present invention.Simulation result shows that the algorithm has little influence on the transmission performance of system.

Description

A method of for ofdm system feature suppression

Technical field

The invention belongs to fields of communication technology, are related to a kind of method for ofdm system feature suppression.

Background technique

Common cyclic prefix both brought unrivaled advantage to system in ofdm system, also inevitably exposed The performance parameters of system.Specifically, the periodicity that cyclic prefix is stabilized due to it, so that the time domain of CP-OFDM is special Sign such as significant character length is exposed with circulating prefix-length information.In addition, in addition to cyclic prefix bring feature, OFDM Signal also has other higher order cumulants measure features, the sub-carrier number and the frequency where each subcarrier that can expose ofdm system Rate position.Both features make system vulnerable.

In order to eliminate apparent time domain cyclophysis, existing algorithm is mainly set about in terms of two.One is by disappearing Except the correlation of front and back, but since the inborn characteristic of cyclic prefix makes so, eliminating correlation means not use cyclic prefix, this The algorithm of sample often brings the dual compromise of Receiver Complexity and performance.Second it is the cyclic prefix for hiding OFDM symbol Periodic feature.Mainly there is selection region to move back method at random and is randomized method with symbol lengths.

This two methods has its respective superiority and inferiority point, and selection region moves back method since it is by the partial data of latter symbol Be copied among previous symbol, actually introduce the interference of intercarrier so that the partial carrier wave of previous symbol lose it is orthogonal Property, and then influence whether the communication performance of system；Although symbol lengths are randomized the shortcomings that method and do not show on system performance, And mainly in resource using upper, but it inherently eradicates cyclic prefix bring cyclostationary characteristic, so that Algorithm validity is poor.

Summary of the invention

To be solved by this invention to be: in an ofdm system, the strong correlation of cyclic prefix and symbol end, which exposes, is System parameter, there are security breaches, lead to the problem that system is vulnerable.

The technical scheme is that a kind of method for ofdm system feature suppression, the ofdm system feature is extremely It less include one of the frequency location where significant character length, circulating prefix-length, sub-carrier number and each subcarrier, It is characterised by comprising:

In transmitter terminal: after sending signal progress IFFT, respectively a redundancy being added to carry before and after the orthogonal sub-carriers of acquisition Wave, is inserted into cyclic prefix, output OFDM signal, and the redundancy carrier wave is known in receiver end；

In receiver end: after the ofdm signal received is carried out cyclic prefix and FFT, to the frequency domain symbol of acquisition into Row de-redundancy carrier processing removes corresponding length thereof before and after frequency domain symbol specially according to the length of redundancy carrier wave.

The total technical solution of the present invention, relative to traditional ofdm system signal transmitting and receiving process, the present invention it is existing just Respectively add a redundancy carrier wave before and after orthogonal sub-carrier, the length of the OFDM time-domain symbol generated is made also correspondingly to change, changes The periodicity of cyclic prefix to conceal its cyclostationary characteristic being exposed solves conventional OFDM systems feature Caused a series of problems.

It is further, described respectively to add a redundancy carrier wave before and after orthogonal sub-carriers method particularly includes:

Random value constitutes redundancy carrier wave in certain numberical range, and it is inserted into before and after orthogonal sub-carriers.

The purpose of above scheme is under the premise of keeping effective subcarrier, front and back redundancy sub-carrier number certain Random value and it is inserted into system in numberical range, to obtain the OFDM symbol string of symbol lengths randomization.

Further, in the numberical range of [0,100] in certain numberical range.

Above scheme is a concrete scheme of supplement, it is therefore an objective to so that the redundancy carrier wave in the present invention program embodies, It is easy to understand and realizes.

Beneficial effects of the present invention are the random variation of valid data length, symbol rate also corresponding change, then its circulation from The periodic peaks of correlation function will not exist, and can not be extracted useful parameter information, therefore the present invention is to second-order cyclic Stationary nature significantly inhibits effect；In addition, the sub-carrier number of ofdm system is changed in the present invention at random, so that from frequency domain It sees that the sub-carrier positions of system are also changing, peak value discrimination can be made to decline to a great extent after mutually superimposed, the also table on frequency domain Reveal preferable concealment effect.

Detailed description of the invention

Fig. 1 is OFDM transmitter subcarrier schematic diagram (4 subcarriers)；

Fig. 2 is that OFDM increases redundancy subcarrier-wise schematic diagram；

Fig. 3 is insertion redundancy subcarrier schematic diagram before and after effective subcarrier.

Fig. 4 is the ofdm signal Cyclic Autocorrelation Function being added after redundancy subcarrier；

Fig. 5 is the ofdm signal subcarrier feature being added after redundancy subcarrier；

Fig. 6 is the transmission performance simulation curve figure of system.

Specific embodiment

With reference to the accompanying drawings and examples, the technical schemes of the invention are described in detail:

Embodiment

The present embodiment using the receiving-transmitting chain of an ofdm system as example for state the process of ofdm system feature suppression. Ofdm system is a series of system for transmitting signal with mutually orthogonal carrier waves, and the orthogonality of intercarrier ensure that system will not Inter-carrier interference is generated, the signal of transmitter section functional block diagram such as Fig. 1, X [k] are frequency domain symbol, f_kFor orthogonal sub-carriers, show It is 4 subcarrier ofdm systems in intention.Redundancy subcarrier is added on this basis, as shown in Fig. 2, A therein and B can be to appoint Meaning reasonable value, the then length of the OFDM time-domain symbol generated also can correspondingly change, and corresponding symbol rate is variation, so that its The periodic peaks of Cyclic Autocorrelation Function no longer occur.

The present embodiment specific implementation process is as follows:

The ofdm system of emulation shares 2048 subcarriers, and is modulated using QPSK.Simulated environment is set as five diameter Rayleighs Channel and awgn channel, maximum doppler frequency 50Hz.Respectively transmitter and receiver carry out accordingly plus redundancy carrier wave, Symbol manipulation is gone, and the relevant parameter after addition algorithm is emulated.

Transmitter terminal: in the modern OFDM system realized by Fast Fourier Transform (FFT), before keeping effective subcarrier It puts, it random value and is inserted into system by front and back redundancy sub-carrier number in the numberical range of [0,100].Redundancy carries Wave is using fixed position inserted mode, as shown in Figure 3.So we will obtain the OFDM symbol string of symbol lengths Ts randomization.

Cyclic Autocorrelation Function is calculated to the symbol string after change, as shown in figure 4, it will be evident that being added after algorithm Ofdm system signal only remains signal autocorrelation peak value at α=0, and cyclic prefix bring can expose the peak value of signal parameter It all disappears, illustrates that the time domain cyclic stationary nature of CP-OFDM signal is thoroughly inhibited.In addition, changing at random in the present invention Become the sub-carrier number of ofdm system, so that the sub-carrier positions of system are also changing from frequency domain, mutually superimposed meeting later So that peak value discrimination declines to a great extent, as shown in Figure 5.

Receiver: in the case where accurately knowing transmitting terminal random sequence information, only FFT need to being exported result, (frequency domain is accorded with Number) front and back corresponding length part removes the information that can be obtained script.

The transmission performance simulation curve of system is as shown in Figure 6.

Obviously, those skilled in the art should be understood that each module of the invention or each step can use general calculating Device realizes that they can be concentrated on a single computing device, or be distributed in network constituted by multiple computing devices On, optionally, they can be realized with the program code that computing device can perform, so as to be stored in storage dress It is performed by computing device in setting, perhaps they are fabricated to each integrated circuit modules or will be multiple in them Module or step are fabricated to single integrated circuit module to realize.In this way, the present invention is not limited to any specific hardware and soft Part combines.In addition, the present embodiment is for ofdm communication link only to tell, but invention thinking not only limits in ofdm system.Similar The thinking that random change system sends symbol lengths is also applied in the SC-FDE system of only single carrier wave, can be effective Inhibit the second-order cyclostationary feature of SC-FDE system, implementation complexity is reasonable, and has substantially no effect on the transmission performance of system.

Claims

1. a kind of method for ofdm system feature suppression, the ofdm system feature includes at least significant character length, follows One of frequency location where ring prefix length, sub-carrier number and each subcarrier characterized by comprising

In transmitter terminal: after sending signal progress IFFT, a redundancy carrier wave is respectively added before and after the orthogonal sub-carriers of acquisition, so After be inserted into cyclic prefix, output OFDM signal, the redundancy carrier wave is known in receiver end；It is described before orthogonal sub-carriers Afterwards respectively plus redundancy carrier wave method particularly includes: front and back redundancy sub-carrier number in certain numberical range random value structure It is inserted into before and after orthogonal sub-carriers at redundancy carrier wave, and by it；

In receiver end: after the ofdm signal received is carried out cyclic prefix and FFT, being gone to the frequency domain symbol of acquisition Redundancy carrier processing removes corresponding length thereof before and after frequency domain symbol specially according to the length of redundancy carrier wave.

2. a kind of method for ofdm system feature suppression according to claim 1, which is characterized in that described is certain Numberical range in for [0,100] numberical range in.