CN102215484A - Broadband wireless channel encryption method and device based on random time inversion - Google Patents

Abstract

The invention relates to a broadband wireless channel encryption method and device based on random time inversion. A receive-transmit device comprises a transmitter and a receiver, wherein the transmitter comprises a signaling receiving module, a channel estimation module, a random time inverter, a transmitting front end and a transmitting antenna; the receiver comprises a signaling transmitting module, a receiving front end and a receiving antenna; the signaling transmitting module of the receiver sends out a channel detection signal, the channel detection signal is received by the signaling receiving module and then estimated by the channel estimation module and enters the random time inverter; an information source signal is processed by the random time inverter and is transmitted by the transmitting front end, the receiver receives the signal transmitted by the transmitter to carry out demodulation and decoding, then determines the information sent by the transmitter by adopting a maximum-likelihood criterion formula, and finally restores the information source signal. The invention can greatly reduce the probability of the captured signal by illegal users under the circumstance that the authorized users can be ensured to receive the signal normally.

Description

The broadband wireless channel encryption method and the device of random time inverting

(1), technical field: the present invention relates to a kind of encryption method and device, particularly relate to a kind of broadband wireless channel encryption method and device of random time inverting.

(2), background technology: radio communication has solved the defective of the line restriction of traditional wired access, but the more traditional wired system of the opening of himself is compared and had bigger potential safety hazard.At present radio communication still adopts mostly is that information source encryption mechanism in the wire communication ensures communication security, because the opening of wireless channel and the broadcast characteristic of electromagnetic signal propagation, the disabled user is connecting system and then enforcement destruction more easily, threatens the fail safe of communication.

Adopt traditional security strategy to have a series of problems in the radio communication, for example during the radio communication key distribution, can't guarantee this safe transmission in wireless channel of key.Therefore fundamentally do not solve the safety problem that the wireless channel opening causes.Adopt spread spectrum, frequency hopping, the signal of special system design such as ultra broadband possesses the characteristic of low probability of intercept, and making can provide certain encryption performance by the correctly signal of demodulation eavesdropping acquisition of listener-in.Yet specific frequency expansion sequence or frequency hopping pattern in the signal system be in case reveal, and information will be by the correct demodulation of listener-in, thereby lost the purpose of safe transmission.How to ensure that secure wireless communication becomes the key issue that the radio communication development faces.

A kind of new thinking is to utilize the spatial domain characteristic of wireless channel to encrypt.Encryption method shown in Figure 1 at the wireless security model relate generally to the three parts, Alice need be transferred to validated user Bob to information security as the base station end, sends and Eve only carries out passive reception as eavesdropping end and do not do any active.

In the communication process, Bob at first sends the unencrypted solicited message to Alice, and this solicited message comprises training sequence simultaneously; Alice receives solicited message, and estimates that according to the training sequence that receives the channel between them obtains channel information.Because Alice do not send any training sequence, Bob and Eve all do not know the channel information between they and the Alice, therefore are in a kind of total blindness's state.Hypothesis according to reciprocity theorem, can think that the transceiver channel between Alice and the Bob is identical under the situation that channel becomes slowly in TDD radio communication system.Therefore Alice can encrypt the information that is about to send to Bob according to the channel characteristics of estimating.Channel information between Alice and the Bob is the key that Alice uses in this case, finish deciphering automatically behind information via Alice after the encryption and the channel between the Bob, so Bob need not to know how Alice encrypts directly to finish proper communication.And Eve receives is signal after the encryption of a Unknown Channel information, is not knowing to solve the symbol that Alice sends under the situation of key.

At this wireless transmission security model, document " Using antenna array redundancy and channel diversity for secure wireless transmissions " (Journal of Communication, vol.2, pp.24-32, May 2007) a kind of method of carrying out encryption in physical layer based on the array redundancy of proposition.This method is similar to launching beam and forms, and guaranteeing under the constant prerequisite of ideal user end channel, by the weight coefficient of each array element of picked at random, making transmits can directly demodulate the symbol sebolic addressing of being launched through wireless channel stack back at the desired user end.For illegal wiretapping person, the change at random of array element weight coefficient can make signal constellation which that the listener-in receives scramble at random, thereby the method that causes features such as utilizing permanent mould and limited sign indicating number collection to carry out the channel blind equalization can't effectively be implemented final satisfied requirement with low probability of intercept.These class methods at the narrow band signal that sends at transmitting terminal array of designs antenna, implementation complexity height.Therefore in the Wideband Wireless Digital Communication system, adopt the design encipherment scheme of transmitting-receiving single antenna to be necessary.

Time reversal (TR, Time Reversal) is a kind of novel multipath fading inhibition technology of development in recent years.Different with traditional multipath fading technology, TR has an empty focusing function and natural environment self-adaption when unique.TR can adapt to the wireless transmission environment of various complexity automatically, does not need complicated multipath signal to merge and handles and adaptive algorithm, greatly simplified receiver design.What is more important, TR can utilize multipath energy to improve signal to noise ratio, compression pulse expansion fully, reduces intersymbol interference, improve the message capacity of system when suppressing multipath fading.A large amount of theory and experimental studies show that in the environment that multipath enriches, the room and time focusing effect of pure TR technology is apparent in view, and are very remarkable to the effect that improves performance in wireless communication systems.By the TR technology, when making full use of the multipath redundant information, the multipath signals of selecting at random to be used for inverting can also reach the purpose that upsets illegal recipient.

(3), summary of the invention:

The technical problem to be solved in the present invention is: the defective that overcomes prior art, a kind of broadband wireless channel encryption method and device of random time inverting are provided, this method and device greatly reduce the probability of being intercepted and captured by the disabled user under the situation that guarantees the normal received signal of authorized user.

Technical scheme of the present invention:

A kind of R-T unit, contain transmitter and receiver, transmitter contains signaling receiver module, channel estimation module, random time inverter, front end of emission and transmitting antenna, receiver contains signaling transmitter module, receiving front-end and reception antenna, front end of emission contains encoder and modulator, and receiving front-end contains decoder and demodulator; Transmitter and receiver is operated under the tdd mode, up-downgoing has reciprocity, the signaling transmitter module of receiver sends out the channel detection signal, this channel detection signal is after the signaling receiver module of transmitter receives, draw the wireless channel multipath information between the Receiver And Transmitter again after the estimation of channel estimation module, this wireless channel multipath information enters in the random time inverter; After source signal x (t) handles through the random time inverter, after encoded again device coding, the modulators modulate, gone out by transmission antennas transmit, reception antenna receives signal that transmission antennas transmit goes out after restore source signal x (t) after the demodulator demodulation, decoder decode.

The random time inverter contains memory, goes into afterwards (FILO) controller and time domain acoustic convolver at random earlier, what store in the memory is the wireless channel multipath information of channel estimation module output, go into afterwards (FILO) controller at random earlier the channel information in the memory is selected the line time inverting of going forward side by side at random, be input to then in the time domain acoustic convolver, the output signal of time domain acoustic convolver enters in the described front end of emission, and source signal enters the input of time domain acoustic convolver.

The output of front end of emission is connected with the input of signaling receiver module, and the output of signaling transmitter module is connected with the input of receiving front-end.

A kind of broadband wireless channel encryption method of utilizing the random time inverting that described R-T unit realizes contains and has the following steps:

Step 1: at first, the signaling transmitter module send channel detectable signal s (t) of receiver, this channel detection signal s (t) is the raised cosine pulse signal, $s\left(t\right)=\sin c\left(\frac{t}{T}\right)\·\frac{\cos (\mathrm{\π\αt}/T)}{1-{(2\mathrm{\αt}/T)}^2},$ Wherein, T represents the cycle of detectable signal, and α represents rolloff-factor;

Step 2: the signaling receiver module of transmitter is sent the channel detection signal that receives into channel estimation module, and channel estimation module calculates channel condition information h _AB(t) after, again with channel condition information h _AB(t) send in the random time inverter,

Wherein, L represents multipath number, a _iRepresent each footpath amplitude complex coefficient, τ _iRepresent the time delay in each footpath, δ represents the time domain impulse function;

Step 3: in the random time inverter of transmitter, earlier with channel condition information h _AB(t) deposit in the memory, then by going into afterwards (FILO) controller at random earlier with the channel condition information h in the memory _AB(t) upset constructs the time reversal function, and time reversal function note is made h _AB(-t),

Simultaneously, select several footpaths to its amplitude zero setting at random, structure random time inversion procedure function is as the transmission filter factor of random time inverter, and random time inversion procedure function note is made h _ABrand(-t);

Front end of emission by transmitter after the processing of step 4: source signal x (t) process random time inverter sends, and the random time inverter to the processing method of source signal x (t) is:

Y _B(n)＝x(n)H _ABrandH _AB+N _Bob(n)

Wherein, Y _B(n) be the frequency domain transmission vector of n symbol, H _ABBe the Toeplitz matrix form of channel characteristics between transmitter and the receiver, N _Bob(n) be noise vector, H _ABrandBe frequency-domain function after the Fourier transform of random time inversion procedure function, x (n) is the frequency-domain expression after source signal x (t) Fourier transform;

Maximum-likelihood criterion formula below receiver adopts according to the frequency-region signal that receives is judged the information that transmitter sends:

$\tilde{x}1\left(n\right)=\arg \underset{x1\left(n\right)}{\min }{|Y\left(n\right)-x1\left(n\right)|}^2$

Wherein,

Estimated value for the frequency-domain expression after source signal x (t) Fourier transform under the maximum likelihood decision criterion, Y (n) receives vector for the frequency domain of n the symbol that receiver is received, x1 (n) is the exhaustive value of the frequency-domain expression after source signal x (t) Fourier transform.

In the step 4, meet the following relationship formula between the estimated value of the frequency-domain expression after the frequency domain of n the symbol that receiver is received reception vector and source signal x (t) Fourier transform:

$Y\left(n\right)=\tilde{x}1\left(n\right)H_{\mathrm{ABrand}}{H}_{\mathrm{AB}}+N_{\mathrm{Bob}}\left(n\right).$

The signaling transmitter module periodically sends channel detection signal s (t) in order to the detection wireless channel characteristics to transmitter according to pre-set receiving and dispatching mechanism.

In the step 2, measure the channel condition information h of wireless channel _AB(t) time, regard wireless channel as a both-end passive device, channel signal of signaling receiver module output of transmitter is given channel estimation module, and the channel signal note is made s ' (t), by the wireless channel transmission theory, channel estimation module estimates channel condition information according to following relational expression:

$s^{\′}\left(t\right)=s\left(t\right)\⊗h_{\mathrm{AB}}\left(t\right),$

Wherein,

The expression convolution algorithm, s (t) is the channel detection signal, h _AB(t) be channel condition information.

Beneficial effect of the present invention:

1, the present invention utilizes the degree of freedom that time reversal technology and multipath redundancy provide, select to be used for the multipath signals of inverting at random, make validated user need not any processing and can realize optimum reception, and the multipath information of illegally eavesdropping user's received signal is with each symbol change at random, therefore correct restituted signal.The present invention is with a wide range of applications in the Wideband Wireless Digital Communication security fields by making wireless signal have low probability of intercept at encryption in physical layer.

2, the present invention only increases a random time inverter on routine transmitting-receiving single antenna communication system, just can realize the safe transmission of information, has reduced the complexity that system realizes.

3, the present invention is directed to the broadband wireless communications characteristics, the multipath information that space in the complex environment is propagated effectively merges, and has improved the signal to noise ratio of received signal.

(4), description of drawings:

Fig. 1 is existing wireless security mode schematic diagram;

Fig. 2 is the structural representation of R-T unit;

Fig. 3 is the structural representation of random time inverter.

(5), embodiment:

Referring to Fig. 2～Fig. 3, R-T unit contains transmitter and receiver, transmitter contains signaling receiver module, channel estimation module, random time inverter, front end of emission and transmitting antenna, receiver contains signaling transmitter module, receiving front-end and reception antenna, front end of emission contains encoder and modulator, and receiving front-end contains decoder and demodulator; Transmitter and receiver is operated under the tdd mode, up-downgoing has reciprocity, the signaling transmitter module of receiver sends out the channel detection signal, this channel detection signal is after the signaling receiver module of transmitter receives, draw the wireless channel multipath information between the Receiver And Transmitter again after the estimation of channel estimation module, this wireless channel multipath information enters in the random time inverter; After source signal x (t) handles through the random time inverter, after encoded again device coding, the modulators modulate, gone out by transmission antennas transmit, reception antenna receives signal that transmission antennas transmit goes out after restore source signal x (t) after the demodulator demodulation, decoder decode.

The random time inverter contains memory, goes into afterwards (FILO) controller and time domain acoustic convolver at random earlier, what store in the memory is the wireless channel multipath information of channel estimation module output, go into afterwards (FILO) controller at random earlier the channel information in the memory is selected the line time inverting of going forward side by side at random, be input to then in the time domain acoustic convolver, the output signal of time domain acoustic convolver enters in the described front end of emission, and source signal enters the input of time domain acoustic convolver.

The output of front end of emission is connected with the input of signaling receiver module, and the output of signaling transmitter module is connected with the input of receiving front-end.

Utilizing the broadband wireless channel encryption method of the random time inverting that described R-T unit realizes to contain has the following steps:

Step 1: at first, the signaling transmitter module send channel detectable signal s (t) of receiver, this channel detection signal s (t) is the raised cosine pulse signal, $s\left(t\right)=\sin c\left(\frac{t}{T}\right)\·\frac{\cos (\mathrm{\π\αt}/T)}{1-{(2\mathrm{\αt}/T)}^2},$ Wherein, T represents the cycle of detectable signal, and α represents rolloff-factor;

Step 2: the signaling receiver module of transmitter is sent the channel detection signal that receives into channel estimation module, and channel estimation module calculates channel condition information h _AB(t) after, again with channel condition information h _AB(t) send in the random time inverter,

Wherein, L represents multipath number, a _iRepresent each footpath amplitude complex coefficient, τ _iRepresent the time delay in each footpath, δ represents the time domain impulse function;

Step 3: in the random time inverter of transmitter, earlier with channel condition information h _AB(t) deposit in the memory, then by going into afterwards (FILO) controller at random earlier with the channel condition information h in the memory _AB(t) upset constructs the time reversal function, and time reversal function note is made h _AB(-t),

Simultaneously, select several footpaths to its amplitude zero setting at random, structure random time inversion procedure function is as the transmission filter factor of random time inverter, and random time inversion procedure function note is made h _ABrand(-t);

Front end of emission by transmitter after the processing of step 4: source signal x (t) process random time inverter sends, and the random time inverter to the processing method of source signal x (t) is:

Y _B(n)＝x(n)H _ABrandH _AB+N _Bob(n)

Wherein, Y _B(n) be the frequency domain transmission vector of n symbol, H _ABBe the Toeplitz matrix form of channel characteristics between transmitter and the receiver, N _Bob(n) be noise vector, H _ABrandBe frequency-domain function after the Fourier transform of random time inversion procedure function, x (n) is the frequency-domain expression after source signal x (t) Fourier transform;

Maximum-likelihood criterion formula below receiver adopts according to the frequency-region signal that receives is judged the information that transmitter sends:

$\tilde{x}1\left(n\right)=\arg \underset{x1\left(n\right)}{\min }{|Y\left(n\right)-x1\left(n\right)|}^2$

Wherein,

Estimated value for the frequency-domain expression after source signal x (t) Fourier transform under the maximum likelihood decision criterion, Y (n) receives vector for the frequency domain of n the symbol that receiver is received, x1 (n) is the exhaustive value of the frequency-domain expression after source signal x (t) Fourier transform.

In the step 4, meet the following relationship formula between the estimated value of the frequency-domain expression after the frequency domain of n the symbol that receiver is received reception vector and source signal x (t) Fourier transform:

$Y\left(n\right)=\tilde{x}1\left(n\right)H_{\mathrm{ABrand}}{H}_{\mathrm{AB}}+N_{\mathrm{Bob}}\left(n\right).$

The signaling transmitter module periodically sends channel detection signal s (t) in order to the detection wireless channel characteristics to transmitter according to pre-set receiving and dispatching mechanism.

In the step 2, measure the channel condition information h of wireless channel _AB(t) time, regard wireless channel as a both-end passive device, channel signal of signaling receiver module output of transmitter is given channel estimation module, and the channel signal note is made s ' (t), by the wireless channel transmission theory, channel estimation module estimates channel condition information according to following relational expression:

$s^{\′}\left(t\right)=s\left(t\right)\⊗h_{\mathrm{AB}}\left(t\right),$

Wherein,

The expression convolution algorithm, s (t) is the channel detection signal, h _AB(t) be channel condition information.

Further specify technical scheme of the present invention at concrete example below, contain tripartite user: Alice (sending the user), Bob (desired user) and Eve (eavesdropping user) in this example.

Validated user Bob in the wireless security transmission system often is subjected to the influence of various barriers and other moving body, the signal that arrives Bob be from the signal of different propagation paths and, suppose the channel impulse response that receives can by

Expression, wherein, L represents multipath number, a _iRepresent each footpath amplitude complex coefficient, τ _iRepresent the time delay in each footpath, δ represents the time domain impulse function;

Guarantee that at first the signal that Bob receives multipath stack has the time of reaching focusing effect.The training sequence that Alice sends by Bob estimate and Bob between channel information after, send signal by time reversal.Be pre-Rake scheme on the time reversal technological essence, it is to replace sending prefilter by impulse response time reversal.Signal after the time reversal can focus at receiving terminal through the channel between Alice and the Bob, promptly directly is the stack in all footpaths at receiving terminal L.Concerning the channel with rich multipath, strong space-time focuses on can reduce ISI and multi-user interference greatly.

Since the difference of locus, the channel impulse response h between Alice and the Bob _AB(t) and the channel impulse response h between Alice and the Eve _AE(t) be incoherent, so the stack in the not all footpath of signal of Eve reception.Under the opposite extreme situations, the signal that Eve receives is equivalent to noise, has reached certain cipher round results.But Eve can utilize the blind signal processing algorithm correctly to receive the signal of transmission.For this reason, carrying out preliminary treatment to sending signal, is the channel characteristics that has changed between Alice and the Bob in essence, constructs a new generalized channel.Purpose is to become soon in the unauthorized user position after allowing the transmission signal by generalized channel, makes the blind signal processing algorithm can't follow the tracks of the variation of generalized channel.

Broadband signal can produce abundant multipath component in Channel Transmission, the redundancy of utilizing multipath to introduce can increase the encryption performance of system greatly.Utilize the degree of freedom of multipath redundant resource increase demodulation below, produce the transmission signal of change at random in the unauthorized user position.

Suppose that the channel between Alice and the Bob can be expressed as 1 * N n dimensional vector n

h _AB＝[h _AB，1?h _AB，2…h _AB，L] (1)

Wherein, L is the multipath number that produces behind the channel between signal process Alice and the Bob.And the channel between Alice and the Eve can be expressed as equally

h _AE＝[h _AE，1?h _AE，2…h _AE，M] (2)

M is the multipath number that produces behind the channel between signal process Alice and the Eve.Suppose W _Rand(n) be the selection matrix at random of a L * L dimension, wherein W _Rand(n) random distribution r (r≤L) is individual 1, and all the other all elements are zero on the diagonal.Symbol of every transmission, W _Rand(n) select once at random, the coefficient of time reversal device becomes so

Wherein the upset of fliplr () expression vector is used to realize time reversal.Wherein, W _Rand(n) change at random is the amplitude of influence judgement just, and can not influence the position of judgement, so Bob can rule out original symbol.Owing in order to the just part multipath of time reversal, therefore only can influence the peak amplitude of the signal of Bob reception, can not influence the position that Bob carries out symbol judgement.But concerning Eve, the change at random of time reversal device makes it be difficult to the position in main footpath is effectively followed the tracks of, the signal of can't demodulation receiving.

Claims (7)

1. R-T unit, contain transmitter and receiver, it is characterized in that: transmitter contains signaling receiver module, channel estimation module, random time inverter, front end of emission and transmitting antenna, receiver contains signaling transmitter module, receiving front-end and reception antenna, front end of emission contains encoder and modulator, and receiving front-end contains decoder and demodulator; Transmitter and receiver is operated under the tdd mode, up-downgoing has reciprocity, the signaling transmitter module of receiver sends out the channel detection signal, this channel detection signal is after the signaling receiver module of transmitter receives, draw the wireless channel multipath information between the Receiver And Transmitter again after the estimation of channel estimation module, this wireless channel multipath information enters in the random time inverter; After source signal is handled through the random time inverter, after encoded again device coding, the modulators modulate, gone out by transmission antennas transmit, reception antenna receives signal that transmission antennas transmit goes out after restore source signal after the demodulator demodulation, decoder decode.

2. R-T unit according to claim 1, it is characterized in that: described random time inverter contains memory, goes into afterwards to go out controller and time domain acoustic convolver earlier at random, what store in the memory is the wireless channel multipath information of channel estimation module output, go into earlier at random afterwards to go out controller the channel information in the memory is selected the line time inverting of going forward side by side at random, be input to then in the time domain acoustic convolver, the output signal of time domain acoustic convolver enters in the described front end of emission, and source signal enters the input of time domain acoustic convolver.

3. R-T unit according to claim 1 is characterized in that: the output of described front end of emission is connected with the input of signaling receiver module, and the output of signaling transmitter module is connected with the input of receiving front-end.

4. broadband wireless channel encryption method of utilizing the random time inverting that the described R-T unit of claim 1 realizes is characterized in that: contain and have the following steps:

Step 1: at first, the signaling transmitter module send channel detectable signal of receiver, this channel detection signal is the raised cosine pulse signal, channel detection signal note is made s (t), $s\left(t\right)=\sin c\left(\frac{t}{T}\right)\·\frac{\cos (\mathrm{\π\αt}/T)}{1-{(2\mathrm{\αt}/T)}^2},$ Wherein, T represents the cycle of detectable signal, and α represents rolloff-factor;

Step 2: the signaling receiver module of transmitter is sent the channel detection signal that receives into channel estimation module, after channel estimation module calculates channel condition information, channel condition information is sent in the random time inverter again, and the channel condition information note is made h _AB(t),

Wherein, L represents multipath number, a _iRepresent each footpath amplitude complex coefficient, τ _iRepresent the time delay in each footpath, δ represents the time domain impulse function;

Step 3: in the random time inverter of transmitter, earlier channel condition information is deposited in the memory, by going into afterwards to go out controller at random earlier with the upset of the channel condition information in the memory, construct the time reversal function then, time reversal function note is made h _AB(-t),

Simultaneously, select several footpaths to its amplitude zero setting at random, structure random time inversion procedure function is as the transmission filter factor of random time inverter, and random time inversion procedure function note is made h _ABrand(-t);

Step 4: the front end of emission by transmitter after the processing of source signal process random time inverter sends, and the random time inverter to the processing method of source signal is:

Y _B(n)＝x(n)H _ABrandH _AB+N _Bob(n)

Wherein, Y _B(n) be the frequency domain transmission vector of n symbol, H _ABBe the Toeplitz matrix form of channel characteristics between transmitter and the receiver, N _Bob(n) be noise vector, H _ABrandBe frequency-domain function after the Fourier transform of random time inversion procedure function, x (n) is the frequency-domain expression after the source signal Fourier transform;

Maximum-likelihood criterion formula below receiver adopts according to the frequency-region signal that receives is judged the information that transmitter sends:

$\tilde{x}1\left(n\right)=\arg \underset{x1\left(n\right)}{\min }{|Y\left(n\right)-x1\left(n\right)|}^2$

Wherein,

Be the estimated value of the frequency-domain expression after the source signal Fourier transform under the maximum likelihood decision criterion, Y (n) receives vector for the frequency domain of n the symbol that receiver is received, x1 (n) is the exhaustive value of the frequency-domain expression after the source signal Fourier transform.

5. the broadband wireless channel encryption method of random time inverting according to claim 4, it is characterized in that: in the described step 4, meet the following relationship formula between the estimated value of the frequency-domain expression after the frequency domain of n the symbol that receiver is received reception vector and the source signal Fourier transform:

$Y\left(n\right)=\tilde{x}1\left(n\right)H_{\mathrm{ABrand}}{H}_{\mathrm{AB}}+N_{\mathrm{Bob}}\left(n\right).$

6. the broadband wireless channel encryption method of random time inverting according to claim 4 is characterized in that: described signaling transmitter module periodically sends channel detection signal in order to the detection wireless channel characteristics to transmitter according to pre-set receiving and dispatching mechanism.

7. the broadband wireless channel encryption method of random time inverting according to claim 4, it is characterized in that: in the described step 2, channel signal of signaling receiver module output of transmitter is given channel estimation module, the channel signal note is made s ' (t), and channel estimation module estimates channel condition information according to following relational expression:

$s^{\′}\left(t\right)=s\left(t\right)\⊗h_{\mathrm{AB}}\left(t\right),$

Wherein,

The expression convolution algorithm, s (t) is the channel detection signal, h _AB(t) be channel condition information.

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