CN110278560A - A kind of convert communication system based on IEEE 802.11a - Google Patents
A kind of convert communication system based on IEEE 802.11a Download PDFInfo
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- CN110278560A CN110278560A CN201910490891.6A CN201910490891A CN110278560A CN 110278560 A CN110278560 A CN 110278560A CN 201910490891 A CN201910490891 A CN 201910490891A CN 110278560 A CN110278560 A CN 110278560A
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
- H04B1/69—Spread spectrum techniques
- H04B1/707—Spread spectrum techniques using direct sequence modulation
- H04B1/7073—Synchronisation aspects
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/26—Systems using multi-frequency codes
- H04L27/2601—Multicarrier modulation systems
- H04L27/2626—Arrangements specific to the transmitter only
- H04L27/2627—Modulators
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/26—Systems using multi-frequency codes
- H04L27/2601—Multicarrier modulation systems
- H04L27/2647—Arrangements specific to the receiver only
- H04L27/2655—Synchronisation arrangements
- H04L27/2662—Symbol synchronisation
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
- H04L27/00—Modulated-carrier systems
- H04L27/26—Systems using multi-frequency codes
- H04L27/2601—Multicarrier modulation systems
- H04L27/2647—Arrangements specific to the receiver only
- H04L27/2655—Synchronisation arrangements
- H04L27/2689—Link with other circuits, i.e. special connections between synchronisation arrangements and other circuits for achieving synchronisation
- H04L27/2695—Link with other circuits, i.e. special connections between synchronisation arrangements and other circuits for achieving synchronisation with channel estimation, e.g. determination of delay spread, derivative or peak tracking
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04W—WIRELESS COMMUNICATION NETWORKS
- H04W12/00—Security arrangements; Authentication; Protecting privacy or anonymity
- H04W12/02—Protecting privacy or anonymity, e.g. protecting personally identifiable information [PII]
Abstract
A kind of convert communication system based on IEEE 802.11a, the covert communications of confidential information are realized using the null subcarrier in the wireless communication system of IEEE802.11a standard, by secret bit by baseband modulation and after spreading, in the null subcarrier for re-mapping host's transmitter OFDM, transmitted after OFDM is modulated；Null subcarrier modulation symbol is extracted from host's receiver, de-spreading operation is first done, then by hard decision, to recover the secret bit of transmission.The present invention allows the transmission power on these null subcarriers to make an uproar the bottom of lower than, on the one hand reduces the influence to host system, on the other hand reduce the perception probability of listener-in due to having made spread spectrum and despreading processing.Meanwhile spreading code, code length and null subcarrier randomly select, further increase listener-in cracks difficulty.
Description
Technical field
The present invention relates to wireless communication and field of information security technology, and in particular to one kind is embedded into IEEE 802.11a mark
The convert communication system of quasi- agreement.
Background technique
Cryptographic technique is that the cleartext information that will be transmitted is encrypted, and will become in plain text unrecognizable ciphertext,
Encrypted information is set to become difficult to understand.But another problem that brings is done so, encryption technology will be encrypted to one in plain text
Heap messy code is easy the desire that excitation unauthorized person cracks confidential information.In the case where burning desire and demand drive, tasted by various
Examination, it is most likely that the safety of decryption, information transmission can also substantially reduce therewith.
Covert communications are that one kind can carry out secret information hided transmission technology during normal data communications.The present invention
Purpose, in existing, disclosed IEEE 802.11a standard agreement, have little influence on original system communication on the basis of,
Confidential information is transmitted using null subcarrier, to realize a kind of convert communication system.
Summary of the invention
In order to allow legitimate user to realize the hided transmission of confidential information, the invention proposes one kind to be embedded into IEEE
In 802.11a standard, spread spectrum convert communication system, it is a kind of IEEE used by means of disclosed, broad sense
The convert communication system of 802.11a standard, due to its host's communication system be it is disclosed, be not easy to cause the note of malice listener-in
Meaning, and the covert communications scheme proposed has very low transmission power (making an uproar the bottom of lower than), is not easy to be intercepted, is that one kind has
The very covert communications means of Johnson & Johnson's sustainability.
The invention provides the following technical scheme:
A kind of convert communication system based on IEEE 802.11a, utilizes the wireless communication system of IEEE802.11a standard
In null subcarrier come to realize the covert communications of confidential information, the wireless communication system be host's communication system, the place
Main communication system refers to the wireless communication system implemented according to IEEE802.11a standard, and structure is as follows:
1-1: frame structure: IEEE 802.11a standard defines physical layer protocol data unit (Physical Protocol
Data Unit, PPDU) frame structure, this is also the data structure generated required for baseband transmission processor；
1-2: timing synchronization, Nonlinear Transformation in Frequency Offset Estimation and the channel estimation of receiver are completed with two training symbols
, training symbol is by 10 short training symbols (Short Training Symbol, STS) and training symbol (Long 2 long
Training Symbol, LST) composition；
The convert communication system includes:
Hidden transmitter: secret bit by baseband modulation and spread spectrum and then is first mapped to IEEE 802.11a system
It in the null subcarrier of system, is transmitted after OFDM is modulated, since transmission power is well below being used for transmission the other of data
Subcarrier, so these null subcarriers as protection interval, do not interfere with the normal transmitting-receiving of host system；
Lobe-on-receive machine: after host's receiver completes timing synchronization, Nonlinear Transformation in Frequency Offset Estimation and channel estimation function,
These null subcarriers are first done despreading processing, the hidden bit sent is then obtained by hard decision again；Due to made spread spectrum and
Despreading processing, allows the transmission power on these null subcarriers to make an uproar the bottom of lower than, on the one hand reduces the shadow to host system
It rings, on the other hand improves concealment.
Further, host's receiver utilizes these training symbols, to complete timing synchronization, Nonlinear Transformation in Frequency Offset Estimation and letter
The functions such as road estimation are prepared for the transmission of subsequent data；In the OFDM of data frame, 64 subcarriers are shared, wherein 48 sons
Carrier wave is used for transmission normal data.
Further, the OFDM modulation for being used for transmission data uses 64 points of IFFT, the i.e. son that OFDM is used
Carrier number is 64, and corresponding IFFT input terminal number is 0~63.Regulation is used for transmission the son of data in IEEE 802.11a standard
Carrier number is 48；4 pilot tones；The subcarrier that number is 0 is DC component, it is necessary to be 0；Other 11 carry for protection interval
Wave is 0 in a standard.When carrying out IFFT operation, subcarrier -26~-1 is mapped to the IFFT input that number is 38~63
End, subcarrier 1~26 are mapped to the IFFT input terminal of the 1~26 of identical number, and other IFFT input terminals are 0 value.It is such to reflect
The relationship of penetrating is to be able to guarantee that the subcarrier spectrum of 802.11a system is concentrated；It (is not added by the modulated output of OFDM
Cyclic prefix) are as follows:
Here N=64 is subcarrier number, { XkBe k-th of subcarrier data symbol, T be the OFDM symbol time,For subcarrier bandwidth.
The treatment process of the hidden transmitter is as follows:
The IFFT input node that number is 27~37 is null subcarrier, these subcarriers play the role of protection interval,
0 is all set in IEEE 802.11a standard, covert communications transmitter is some or all of to utilize these null subcarriers, comes
Secret bit is transmitted, is by modulated symbol s by secret bit c
In formula, P is the power of symbol, and selection length is that M (M value is even number) spreading code W is
W=1, -1,1, -1 ..., 1, -1
Symbol s is multiplied with W namely Direct-Spread operation, then using M symbol after spread spectrum as M null subcarrier
Symbol, not used null subcarrier, data subcarrier, pilot sub-carrier and direct current subcarrier with host's communication system, one
The input node as IFFT is played, through OFDM modulate emission to channel.
The treatment process of the lobe-on-receive machine is as follows:
M null subcarrier symbol of host's receiver is extracted, the spreading code completely the same with transmitter is multiplied, namely despreading
Operation, the symbol after being de-spreadFor
N is the noise of channel in formula,<,>accorded with for inner product operation, then pass through hard decision, recovers secret bit
EstimationThe rule of hard decision is
In formulaIt is expressed as plural numberPhase.
The length M of spreading code W be it is variable, several adjacent OFDM frames are combined into the null subcarrier of M, for hidden logical
Letter system；The combination of M null subcarrier is chosen, and increase listener-in cracks difficulty, chooses biggish M value, chooses lower
The transmission power level P of null subcarrier to reduce influence of the concealment system to host system, while also reducing stolen hearer's perception
A possibility that.
Spreading code W is optional, the good pseudo-random sequence (such as m-sequence and Gold sequence) of optional autocorrelation,
The good coding (such as Walsh code) of cross correlation, the spreading code as concealment system may be selected；According to the certain data of host system
The bit combination of subcarrier, as the spreading code of concealment system, further increases randomness to select different spreading codes, improves
Listener-in's cracks difficulty.
Technical concept of the invention are as follows: IEEE 802.11a is disclosed, common WLAN physical layer protocol, under the agreement
Communication system use OFDM technology, we analyze the subcarrier situation in OFDM, having little influence on host communication
Under conditions of system, secret bit is first done into spread processing, re-modulation to null subcarrier, other sub- loads with host's communication system
Wave is modulated to time domain together, by OFDM；Lobe-on-receive end handles the null subcarrier of host's receiver by despreading, using
Hard decision estimates secret bit.Due to having used spread spectrum, the power on null subcarrier is made an uproar the bottom of lower than, is eavesdropped
Person perceives less than the presence for having hidden information, to achieve the purpose that covert communications.
The invention has the benefit that utilizing existing, disclosed, standard IEEE 802.11a wireless communication system
Confidential information is passed through the covert communications by being embedded into the convert communication system of host's communication system by (host's communication system)
System transmission, has good concealment.
Detailed description of the invention
Fig. 1 show the frame structure of IEEE 802.11a system.
Fig. 2 show the detailed annotation of PPDU frame structure.
Fig. 3 show subcarrier and IFFT serial number mapping relations.
Fig. 4 show convert communication system entire block diagram.
Fig. 5 show the relationship of 802.11a system bit error rate Yu hidden subcarrier number.
Fig. 6 is shown under conditions of null subcarrier number 8, the relationship of hidden bit error rate and signal-to-noise ratio.
Specific embodiment
The present invention will be further explained below with reference to the attached drawings.
As shown in Figure 1, PDU frame structure includes PLCP header (Header), PSDU, tail bit (Tail) and filling bit
(Pad).Wherein PLCP header includes rate position (Rate), reserved bit (Reserved), length position (Length), parity check bit
(Parity), tail bit (Tail) and service digit (Service).As can be seen from Figure 11 OFDM of signal segment (Signal)
Symbol is made of rate position, reserved bit, length, parity check bit, tail bit and service digit, and data segment (Data) is
It is made of service digit, PSDU, tail bit and filling bit.
As shown in Fig. 2, the frame structure of PPUD is described in detail.It can be seen from the figure that the symbol in receiver section is fixed
When synchronous, Nonlinear Transformation in Frequency Offset Estimation and channel estimation completed with two training symbols.Training symbol is accorded with by 10 Short Trainings
Number (Short Training Symbol, STS) and 2 long training symbol (Long Training Symbol, LST) composition.Its
In each short training symbols time span be 0.8us, be the 1/4 of normal OFDM symbol length, the time of long training symbol is long
Degree is then identical as normal OFDM symbol, and total training time length is 16us.It is the domain Signal after training symbol, length is normal
OFDM symbol length, including important informations such as data modulation type, code rate and data lengths.
64 points of IFFT is used in Fig. 3, in OFDM, i.e. OFDM available subcarrier is 64, corresponding IFFT
Input terminal number is 0~63.The subcarrier that data are transmitted in IEEE 802.11a standard is 52, i.e., in positive frequency section, volume
Number be 1,2 ..., 26 subcarrier and in negative frequency section, number be -1, -2 ..., -26 subcarrier, including 48 numbers
According to subcarrier, 4 pilot tones；The subcarrier that number is 0 is DC component, it is necessary to be 0；Other 11 are protection interval subcarrier,
It is in a standard 0.When carrying out IFFT operation, subcarrier -26~-1 is mapped to the IFFT input terminal that number is 38~63,
Subcarrier 1~26 is mapped to the IFFT input terminal of the 1~26 of identical number, and other IFFT input terminals are 0 value.Such mapping is closed
System is to be able to guarantee that the subcarrier spectrum of 802.11a system is concentrated.(circulation is not added by the modulated output of OFDM
Prefix) are as follows:
Here N=64 is subcarrier number, { XkBe k-th of subcarrier data symbol, T be the OFDM symbol time,For subcarrier bandwidth.
In Fig. 3, by the relationship of analysis 802.11a subcarrier and IFFT serial number it can be found that it can be found that IFFT point
27~37 be sky, i.e. subcarrier -31~-27 and 27~32, these subcarriers play the role of protection interval, in IEEE
802.11a being all set to 0 in standard.Therefore, we are according to this feature, propose it is as shown in Figure 4, be embedded in host system
Convert communication system in system.
As shown in figure 4, being by modulated symbol s by secret bit c in the transmitter of concealment system
In formula, P is the power of symbol.It is assumed that length is that M (M value is even number) spreading code W is
W=1, -1,1, -1 ..., 1, -1
Symbol s is multiplied with W namely Direct-Spread operation passes then M symbol after spread spectrum in conjunction with host system
Transmission of data, together as the input of OFDM modulation.
As shown in figure 4, M null subcarrier symbol is extracted from host's receiver, with transmitting in the receiver of concealment system
The completely the same spreading code of machine is multiplied namely de-spreading operation, the symbol after being de-spreadFor
N is the noise of channel in formula,<,>accorded with for inner product operation.Then pass through hard decision, recover secret bit
EstimationThe rule of hard decision is
In formulaIt is expressed as plural numberPhase.
Herein, spread code length M, and can be described as spreading factor.In order to reduce the influence to host system, and raising
The concealment of concealment system can choose bigger M value.As M > 11, then the gap chosen in adjacent OFDM symbol carries
Wave forms the null subcarrier group that a length is M.For example, M=16, then can choose adjacent 2 OFDM symbols, each OFDM
Symbol takes 8 null subcarriers (IFFT number 27~30,34~37).Further, when M value is larger, lower power can be chosen
Value P to reduce influence of the concealment system to host system, while also reducing a possibility that stolen hearer perceives.
Further, it if listener-in has grasped the structure of the convert communication system proposed, but does not get correctly
Spreading code W, such as obtained the spreading code W orthogonal with transmitting terminal1, then obtained symbol after listener-in's despreadingFor
SymbolIn there is no any information comprising the modulation symbol s after hidden bit modulation, therefore be unable to estimate out hidden
Bit.
Shown in Fig. 5 and Fig. 6, be respectively host system using Matlab software to proposition based on IEEE802.11a and
Bit error rate of concealment system under the conditions of different signal-to-noise ratio.In emulation, convolutional encoding is used as channel coding, coding effect
Rate is 1/2, constraint length 7, and generator matrix is (133,171)；Receiving end carries out convolutional code using Viterbi decoding algorithm
Decoding；The subcarrier of each transmission data all uses BPSK to modulate；The digital modulation mode of concealment system also uses BPSK tune
System, the regulation for following 802.11a agreement of OFDM modulating part.
In Fig. 5, SNR indicates that the ratio of transmitting total power signal and interchannel noise, BER indicate bit error rate.In order to
Influence of the concealment system to host system is evaluated, the sub-carrier power of concealment system and the subcarrier function of host system are specially set
Rate is equally big.N indicates N number of null subcarrier in host system in figure, transmits secret bit for concealment system.It can be with from figure
Find out, when the timing of SNR mono-, the bit error rate of host system can increase with the increase of N, this is because using more hidden
Subcarrier occupies power, and the power of host system is caused to be declined, and signal-to-noise ratio reduces, but general impacts are little.Into one
Step can substantially reduce the influence to host system if reducing the transmission power of concealment system subcarrier.
As shown in fig. 6, the relationship of hidden bit bit error rate and signal-to-noise ratio when hidden sub-carrier number is 8.Here
Signal-to-noise ratio is defined as the power of concealment system subcarrier and the additive white Gaussian noise power ratio of channel.It can from figure
Out, when spreading factor is larger, although transmission power is lower than channel noise power, but still there is sufficiently low bit error rate.Into
One step in order to reduce the transmission power of concealment system to reduce the influence to host system, while enhancing concealment, reduces stolen
A possibility that hearer perceives can realize this purpose by increasing the method for spreading factor.
Claims (9)
1. a kind of convert communication system based on IEEE 802.11a, it is characterised in that: utilize the wireless of IEEE802.11a standard
Null subcarrier in communication system realizes the covert communications of confidential information, and the wireless communication system is host's communication system,
Host's communication system refers to the wireless communication system implemented according to IEEE 802.11a standard, and structure is as follows:
1-1: frame structure: IEEE 802.11a standard defines physical layer protocol data unit (Physical ProtocolData
Unit, PPDU) frame structure, this is also the data structure generated required for baseband transmission processor；
1-2: timing synchronization, Nonlinear Transformation in Frequency Offset Estimation and the channel estimation of receiver are completed with two training symbols,
Training symbol is by 10 short training symbols (Short Training Symbol, STS) and training symbol (Long 2 long
Training Symbol, LST) composition；
The convert communication system includes:
Hidden transmitter: secret bit is passed through into baseband modulation and spread spectrum and then the gap for being mapped to host's transmitter OFDM
In carrier wave, transmitted after OFDM is modulated；
Lobe-on-receive machine: hidden after host's receiver completes the functions such as timing synchronization, Nonlinear Transformation in Frequency Offset Estimation and channel estimation
It covers receiver and extracts null subcarrier modulation symbol in the OFDM frame of transmission data, first do de-spreading operation, then pass through hard decision,
To recover the secret bit of transmission.
2. a kind of convert communication system based on IEEE 802.11a as described in claim 1, it is characterised in that: host receives
Machine utilizes these training symbols, is subsequent data to complete timing synchronization, Nonlinear Transformation in Frequency Offset Estimation and channel estimation function
Transmission is prepared；In the OFDM of data frame, 64 subcarriers are shared, wherein 48 subcarriers are used for transmission normal data.
3. a kind of convert communication system based on IEEE 802.11a as claimed in claim 2, it is characterised in that: the use
64 points of IFFT is used in the OFDM modulation of transmission data, i.e. the sub-carrier number that OFDM is used is 64, corresponding IFFT input
End number is that the sub-carrier number that regulation is used for transmission data in 0~63, IEEE 802.11a standard is 48；4 pilot tones；Number
Subcarrier for 0 is DC component, it is necessary to be 0；Other 11 are protection interval subcarrier, are in a standard 0；Carry out
When IFFT operation, subcarrier -26~-1 is mapped to the IFFT input terminal that number is 38~63, and subcarrier 1~26 is mapped to phase
With 1~26 IFFT input terminal of number, other IFFT input terminals are 0 value, and such mapping relations are to be able to guarantee
802.11a the subcarrier spectrum of system is concentrated；By the modulated output of OFDM are as follows:
Here N=64 is subcarrier number, { XkBe k-th of subcarrier data symbol, T be the OFDM symbol time,
For subcarrier bandwidth.
4. a kind of convert communication system based on IEEE 802.11a as described in one of claims 1 to 3, it is characterised in that:
The treatment process of the hidden transmitter is as follows:
The IFFT input node that number is 27~37 is null subcarrier, these subcarriers play the role of protection interval, in IEEE
0 is all set in 802.11a standard, covert communications transmitter is some or all of to utilize these null subcarriers, carrys out conveyer
Secret bit c is by close bit by modulated symbol s
In formula, P is the power of symbol, chooses the spreading code W that length is M are as follows:
W=1, -1,1, -1 ..., 1-1
M value is even number；Symbol s is multiplied with W namely Direct-Spread operation, then using M symbol after spread spectrum as M
The symbol of null subcarrier, not used null subcarrier, data subcarrier, pilot sub-carrier and direct current with host's communication system
Carrier wave, together as the input node of IFFT, through OFDM modulate emission to channel.
5. a kind of convert communication system based on IEEE 802.11a as described in one of claims 1 to 3, it is characterised in that:
The treatment process of the lobe-on-receive machine is as follows:
M null subcarrier symbol of host's receiver is extracted, the spreading code completely the same with transmitter is multiplied, namely despreading behaviour
Make, the symbol after being de-spreadFor
N is the noise of channel in formula,<,>accorded with for inner product operation, then pass through hard decision, recovers the estimation of secret bitThe rule of hard decision is
In formulaIt is expressed as plural numberPhase.
6. a kind of convert communication system based on IEEE 802.11a as claimed in claim 4, it is characterised in that: spreading code W
Length M be it is variable, several adjacent OFDM frames are combined into the null subcarrier of M, be used for convert communication system；M gap carries
The combination of wave is chosen, and increase listener-in cracks difficulty, chooses biggish M value, chooses the transmitting function of lower null subcarrier
Rate value P.
7. a kind of convert communication system based on IEEE 802.11a as claimed in claim 5, it is characterised in that: spreading code W
Length M be it is variable, several adjacent OFDM frames are combined into the null subcarrier of M, be used for convert communication system；M gap carries
The combination of wave is chosen, and increase listener-in cracks difficulty, chooses biggish M value, chooses the transmitting function of lower null subcarrier
Rate value P.
8. a kind of convert communication system based on IEEE 802.11a as claimed in claim 4, it is characterised in that: spreading code W
It is pseudo-random sequence that is optional, selecting autocorrelation good, also or the good coding of selection cross correlation, as concealment system
Spreading code；According to the bit combination of the certain data subcarriers of host system, to select different spreading codes, as concealment system
Spreading code.
9. a kind of convert communication system based on IEEE 802.11a as claimed in claim 5, it is characterised in that: spreading code W
It is pseudo-random sequence that is optional, selecting autocorrelation good, also or the good coding of selection cross correlation, as concealment system
Spreading code；According to the bit combination of the certain data subcarriers of host system, to select different spreading codes, as concealment system
Spreading code.
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| CN111181597A (en) * | 2019-12-30 | 2020-05-19 | 华南理工大学 | Anti-interference heat hidden channel communication method in multi-core system |
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