CN105472753B - A kind of dual resource allocation of cognition cellulor and interference management method based on LTE - Google Patents
A kind of dual resource allocation of cognition cellulor and interference management method based on LTE Download PDFInfo
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Abstract
The resource allocation and interference management problem that the present invention relates to cognitive users in authorized spectrum band and unauthorized frequency range, especially authorized spectrum band and authorized user and unauthorized frequency range and WiFi user coexist and resource allocation problem.The frequency spectrum distribution policy of existing network causes limited frequency spectrum resource wait exhaust, and communication channel, which becomes congestion, can't bear.The present invention considers the cognition small cell network with spectrum detection function, it can detecte the spectrum interposition in authorization and unauthorized frequency range, and existing LTE technology is combined to transmit information using these spectrum interpositions, cognitive user and authorized user share frequency spectrum in authorized spectrum band, cognitive user and WiFi user's competition use frequency spectrum in unauthorized frequency range, it is proposed a kind of simple and effective interference management and resource allocation mechanism, realize the reasonable distribution of authorization and the unauthorized resource of frequency range, to maximize the transmission rate of whole network, improve user satisfaction and systematic entirety energy.
Description
Technical Field
The invention relates to the technical field of mobile communication, in particular to a coexistence method and a resource allocation problem of authorized users and unauthorized users in a frequency band and WiFi users.
Background
The current method for improving transmission rate includes ① allocating more frequency spectrum to the network, ② network densification through small cell configuration, ③ space processing technique and other techniques, wherein the use of more frequency spectrum is the simplest and effective method, the current method for improving transmission rate adopts a fixed allocation policy to finish allocation of most available frequency spectrum, especially the low frequency band resource with higher communication quality is not only limited and expensive, but also is rapidly consumed by the rapidly increasing data traffic demand, the fixed spectrum allocation makes the usage rate of authorized frequency band resources very low, according to the investigation result of the U.S. Federal Communications Commission (FCC), the usage rate of authorized frequency spectrum resources is only 15% -85%, the usage rate of intelligent broadband data traffic generated according to the situation, the usage rate of the intelligent broadband data traffic is a very low-rate Radio (PU) system, the User is detected to be a secondary spectrum allocation, and the User is detected to be a User who is not authorized by a private subscriber (PU) and the User is detected to be a secondary spectrum detection.
However, due to the limited licensed band resources, the cognitive radio technology can only alleviate the resource shortage. The exploration and use of other unlicensed band resources is therefore an effective way to increase network capacity. In the 5GHz band, the globally allocated unlicensed band resources have been in excess of 500 MHz. To date, WiFi (Wireless-Fidelity, Wireless local area network) is one of the most commonly used Wireless Access methods in the 5GHz band, and it accesses to the channel in a random manner, detects whether there is a user using the channel by CSMA/CA (Carrier Sense Multiple Access with Collision) before using the channel, and transmits data if it detects that the channel is idle in a DIFS (distributed inter-frame Spacing) (according to 802.11b standard) time period; and if the channel is detected to be occupied, randomly selecting a backoff count within a contention window range to reduce the collision probability among WiFi users. When there are many wireless users competing for the use of the frequency band, the collision avoidance mechanism of WiFi makes the spectrum usage not high.
In summary, the existing methods for increasing network capacity do not fully utilize spectrum resources, the invention provides a novel Cognitive small cell concept, and a small cell with spectrum detection capability is called a Cognitive small cell (Cognitive small cell), on one hand, the method has the characteristics of low small cell transmitting power and small coverage area, and can adopt frequency reuse to improve the utilization rate of a spectrum and increase the network capacity; on the other hand, the method has the characteristic of dynamically detecting the spectrum resources by cognitive radio, can detect other authorized and unauthorized frequency bands while using the authorized frequency band of the user, and solves the problem of low utilization rate of the spectrum resources of the traditional network by repeatedly using the spectrum. The cognitive user shares the authorized frequency band with the authorized user by using the frequency spectrum hole on the authorized frequency band to improve the transmission rate; a channel is used by a cognitive user and WiFi competitively in an unauthorized frequency band, and a simple and effective interference management and channel access scheme is provided, so that the cognitive user and the WiFi user coexist in the unauthorized frequency band. The cognitive small cell with the dual spectrum access function has very important significance for improving the spectrum utilization rate of authorized and unauthorized frequency bands and increasing the overall performance of a network.
Disclosure of Invention
Aiming at the problems in the prior art, the invention provides a novel cognitive small cell which can detect and use idle spectrum resources on other authorized and unauthorized frequency bands while using the authorized frequency band of the cognitive small cell. Because the transmission rate generated by the self-authorized frequency band is certain, the invention does not consider the transmission rate generated by the cognitive small cell using the self-frequency band. The method comprises the steps that users in a cognitive small cell service range are cognitive users, the cognitive users and authorized users share a frequency spectrum on an authorized frequency band, and an interference management scheme is provided for protecting the service quality of the authorized users from being influenced by the cognitive users. The method includes that a cognitive user and a WiFi user compete for using a channel on an unauthorized frequency band, and a simple and effective interference management and channel access scheme is provided for coexistence of the cognitive user and the WiFi user in order to improve the spectrum utilization rate of the unauthorized frequency band. The dual resource allocation and interference management method of the cognitive small cell fully utilizes authorized and unauthorized frequency band resources, maximizes the transmission rate of the whole network, and improves the user satisfaction and the system performance.
The technical scheme provided by the invention for solving the technical problems is as follows: the small cognitive cell simultaneously uses authorized and unauthorized frequency bands, and the cognitive user and the authorized user both transmit data through an LTE interface according to an LTE (Long Term Evolution) frame structure. The specific technology is as follows:
a cognitive small cell dual resource allocation and interference management method based on LTE comprises the following steps: the cognitive small cell simultaneously uses an authorized frequency band and an unauthorized frequency band, a cognitive user dynamically accesses the authorized frequency band according to frequency spectrum detection, the cognitive user opportunistically accesses the authorized frequency band, when the interference on the authorized user is lower than a threshold value, the cognitive user and the authorized user share the frequency spectrum, and simultaneously, the optimal cognitive small cell base station is selected for data transmission; and the cognitive user and the WiFi user compete to use the unauthorized frequency band, and the detection time interval of the cognitive user is dynamically adjusted according to the transmission probability of the WiFi user.
The method for dynamically accessing and using the authorized frequency band by the cognitive user according to the frequency spectrum detection specifically comprises the following steps: the cognitive small cell base station acquires an energy detection value of an authorized user, if the energy detection value is lower than a detection threshold, the cognitive user dynamically accesses the authorized frequency band to transmit data, and the cognitive user timely exits the authorized frequency band in use when detecting that the authorized user appears again.
The method for the cognitive user and the WiFi user to compete for use of the unlicensed frequency band further comprises the following steps: the cognitive user transmits information at intervals of preset time T before using the unauthorized frequency band0And detecting a channel once, if the channel is busy, waiting for a preset time period again and then detecting the channel again until the channel is idle, and accessing and using the channel by the cognitive user according to an LTE frame structure.
CSBS for ith cognitive small cell base stationiAt sampling time T (T is more than or equal to 0 and less than or equal to T)s) When receiving signal yi(t) is fullFoot formula:medium condition H0When the authorized frequency band is not occupied, the cognitive user can access the use licenseA weight band; satisfies the condition H1Then, the authorized frequency band is occupied, where n (t) represents a mean value of 0 and a variance of δ2White gaussian noiseSound, TiIs a macro cellular base station MBS and CSBSiThe channel matrix in between is used to determine,
after information transmission of the cognitive user is finished, at least 5% of occupied time of a channel is reserved as idle time, WiFi users access the channel in a random mode, before the channel is accessed, whether users use the channel is detected through a CSMA/CA collision avoidance mechanism, if the users use the channel is detected, a random back-off time is generated by a back-off counter to avoid collision with other users, and if the channel is detected to be idle, the WiFi users send information.
When a user uses an unauthorized frequency band channel, the back-off counter stops counting, and the state is not transferred; when detecting that the unauthorized frequency band channel is idle, subtracting 1 from the backoff time, and transferring the state, when the backoff counter time is 0, if the unauthorized frequency band channel is still idle, sending information, otherwise, adding 1 to the state, and generating a backoff time again randomly to continue detecting the channel state.
Status of states (t) ═ 1, b (t) ═ 0 indicates that there is no data transmission request for WiFi user, when there is data transmission for WiFi userThen the state will transition, according to the formula: the transmission probability of any timeslot WiFi user is calculated,wherein,lambda is the arrival rate of WiFi users, p is the collision probability at any moment, WIs the initial backoff window size; according to the formula:calculating the idle probability P of any time slot channelidle，There is a probability P of successful information transmission for WiFi usersucAnd probability of transmission collision PcolWherein α represents cognitive userProbability of successful access to the channel, according to the formulaCalculating a time profile for WiFi to occupy a channelRate, TdFor DIFS duration, TidleFor free time slot, TsucFor a successful transmission duration, TcolIs the length of the conflict.
Dynamically adjusting the detection time interval of the cognitive user according to the transmission probability of the WiFi user further includes, if the WiFi transmission information increases, the cognitive small cell detection time interval increases, otherwise the detection time interval decreases, i.e. according to the formula:calculating cognitive small cell detection time interval T0Wherein η is a correlation factor, PidleChannel idle probability of any time slot; according to the formula:determining the time probability, T, of a cognitive user occupying an unlicensed frequency band channelcFor cognitive user transmission duration, TgapAnd reserving an idle interval for WiFi transmission after information is transmitted for the cognitive user.
The invention shares the authorized frequency band for the authorized frequency band, the cognitive user and the authorized user, improves the utilization rate of the frequency spectrum, dynamically selects the small cellular base station which is most suitable for communication, and improves the transmission rate of the whole network. The cognitive user uses the authorized frequency band in an opportunistic access mode, the cognitive small cell base station dynamically detects the state of the authorized user based on energy detection, if the energy detection value of the authorized user is lower than the detection threshold, the authorized user is judged not to use the authorized frequency band, the cognitive user accesses the authorized frequency band to transmit data, and when the energy detection value of the authorized user is higher than the detection threshold, the cognitive user timely quits the authorized frequency band which is being used when the authorized user is detected to appear again.
For the unlicensed band: the cognitive user and the WiFi user compete for using the Unlicensed frequency band through the LTE interface at the same time, and the mode of using the Unlicensed frequency band is LTE-U (LTE-Unlicensed, LTE using the Unlicensed frequency band).
However, due to different user performances, the access modes of each user to the unlicensed frequency band are also different:
1) the cognitive user transmits information at intervals of preset time T before using the unauthorized frequency band0And detecting a primary channel, and judging whether a user uses the unauthorized frequency band or not through energy detection.
① if the energy detection value of the channel is higher than the detection threshold, the channel is busy, that is, the user uses the unauthorized frequency band, and the cognitive user waits for the next predetermined time T0Carrying out channel detection again;
② if the energy detection value of the channel is lower than the detection threshold, the channel is idle, the cognitive user immediately accesses the channel to use the unauthorized frequency band, and transmits information according to the LTE frame structure;
③ A predetermined idle time interval T is reserved after the cognitive user finishes transmitting informationgapAnd then carrying out channel detection.
For example, the duration of using the channel by the cognitive user is generally 1ms to 10ms, which ensures that all users can access to the unlicensed frequency band, and an idle time is reserved after the cognitive user data transmission is finished, where the idle time interval is usually at least 5% of the occupied time of the channel.
2) WiFi users access to a channel in a random mode, and in order to reduce collision with other users, the WiFi users detect whether any user uses an unauthorized frequency band of the channel through a collision avoidance mechanism CSMA/CA before accessing the channel.
① if it is detected that the channel is free, it continues to detect DIFS period, if the channel is free in this period, the WiFi user sends data successfully, ② if it is detected that the channel is busy, it generates a random back-off count and continues to detect the channel state, if the channel is busy, the back-off count is not changed, if it is detected that the channel is free, it decreases 1, ③ until the back-off count equals 0, it continues to detect DIFS period, if the channel is free in this period, the WiFi user sends data.
The increase of the collision probability and the reduction of the WiFi performance caused by the improper use and allocation of the cognitive user and the WiFi user on the unauthorized frequency band; the resource allocation of the cognitive user and the WiFi user in the unauthorized frequency band not only can ensure the transmission performance of the WiFi user, but also can make full use of the residual frequency spectrum resources and improve the transmission rate of the cognitive small cell. The sending and detecting time intervals of the cognitive users can be adjusted through the information probability prediction of WiFi user transmission, and the purpose of improving the overall performance of the network is achieved. The method adopts a Markov prediction model, predicts the transmission probability of the WiFi user in any time slot according to the Markov chain theory, and dynamically adjusts the detection time interval T of the cognitive user according to the transmission probability of the WiFi user by combining with the historical record0That is, when the WiFi user transmits more information, the cognitive small cell base station detection time interval becomes larger, otherwise, the detection time interval decreases. By the method, the conflict between the cognitive user and the WiFi user is reduced, and the unauthorized frequency band can be used fairly and reasonably.
In order to ensure that each user can access the network, the user satisfaction is used as an evaluation index of the whole network, namely,
U(R)＝∑ln(R)
wherein ln (·) is a natural logarithm, R represents the rate of each user, and is determined by the power allocation of the cognitive user in the authorized frequency band and the unauthorized frequency band and the channel allocation time of the cognitive user and the WiFi user in the unauthorized frequency band, respectively; u (R) represents satisfaction.
User satisfaction increases with increasing network transmission rates, and increases rapidly when transmission rates are lower, and slows as transmission rates are higher.
The method simultaneously considers the use conditions of the cognitive small cells on authorized and unauthorized frequency bands, the cognitive user accesses the authorized frequency band and the authorized user to share resources based on energy detection on the authorized frequency band, reasonably distributes power resources through interference control among users and dynamically adjusts the selection of the cognitive small cell base station by the cognitive user; the method comprises the steps that a cognitive user and a WiFi user compete to use a channel on an unauthorized frequency band, in order to ensure that the cognitive user and the WiFi user fairly use the channel, the invention provides a simple and effective interference management and channel access scheme, the cognitive user and the WiFi user compete to use the unauthorized frequency band is comprehensively considered, the probability of using the channel by the WiFi user is predicted through Markov, and then the transmission and detection time of the cognitive user is dynamically adjusted. By the method, the use efficiency of the authorized frequency band and the unauthorized frequency band can be improved simultaneously, the transmission rate of the network is improved, the performance of the whole network is optimized, and better user satisfaction is obtained.
Drawings
FIG. 1 is a diagram of a system network scene model of the present invention;
FIG. 2 is a diagram of a licensed band cognitive radio spectrum sensing frame structure used in the present invention;
fig. 3 shows an unlicensed band channel access scheme used by the present invention;
figure 4 a WiFi user behavior prediction markov chain model used by the present invention.
Detailed Description
The cognitive small cell can detect and use idle resources on other authorized and unauthorized frequency bands while using the self authorized frequency band. Because the transmission rate generated by the self-authorized frequency band is certain, the communication rate generated by the self-authorized frequency band used by the cognitive small cell is not considered in the invention. The cognitive small cell users, namely the cognitive users, share the frequency spectrum with the authorized users on the authorized frequency band, so that the communication quality of the authorized users is protected from being influenced by the cognitive users, the cognitive users and the WiFi users compete to use the frequency spectrum on the unauthorized frequency band, and a simple and effective interference management and channel access scheme is provided for the coexistence of the cognitive users and the WiFi users for improving the frequency spectrum utilization rate of the unauthorized frequency band. The dual resource allocation and interference management method of the cognitive small cell fully utilizes authorized and unauthorized frequency band resources, maximizes the transmission rate of the whole network, and improves the user satisfaction and the system performance. The cognitive small cell can use the authorized frequency band and the unauthorized frequency band simultaneously, and two or more authorized or unauthorized carriers are aggregated together through LTE carrier aggregation to support larger transmission bandwidth and improve uplink and downlink transmission rate. In the authorized frequency band, the cognitive user is accessed to the authorized frequency band at random, when the interference of the authorized user is lower than a threshold value, the cognitive user and the authorized user share the frequency spectrum, and simultaneously, the best cognitive small cell base station is dynamically selected for data transmission; in the unlicensed frequency band, the cognitive user and the WiFi user compete to use the unlicensed frequency band.
However, due to the lack of coordination and management of mutual interference among various wireless devices, although most of the technologies can deal with interference among systems of the same type when being designed, cognitive small cell and WiFi users belong to different systems respectively and have different slot scheduling modes, so that interference handling among them has certain difficulty, and the coexistence problem of them will be a great challenge. In an unlicensed frequency band, WiFi devices have been deployed widely, and in order to ensure that the performance of a WiFi user is not affected, a new resource allocation scheme is required to enable a cognitive small cell and the WiFi user to coordinate and coexist in the unlicensed frequency band.
The cognitive user uses the authorized frequency band in a dynamic access mode of frequency spectrum detection, namely, the cognitive small cell base station obtains the state information of the authorized user based on energy detection, if the energy detection value is lower than the detection threshold, the cognitive user judges that the authorized user does not use the frequency band, the cognitive user dynamically accesses the authorized frequency band to transmit data, and the cognitive user can timely quit the authorized frequency band being used when detecting that the authorized user appears again. The cognitive user and the authorized user share the authorized frequency band, the frequency spectrum utilization rate is improved, meanwhile, the cognitive small cellular base station which is most suitable for communication is dynamically selected, and the performance of the whole network is improved.
The cognitive user and the WiFi user compete for using the unlicensed frequency band. Cognitive user transmits information at intervals of time T before using unauthorized frequency band0Detecting a channel once, judging whether a user uses the channel, if the channel is busy, namely a WiFi user uses the channel, recognizing a duration T for the user to wait for one time0And then, detecting again, and if the channel is idle, immediately accessing and using the channel by the cognitive user according to the LTE frame structure. In order to ensure that various wireless users can fairly access to the unauthorized frequency band, an idle time is reserved after the information transmission of the cognitive user is finished, and the idle time is at least 5% of the occupied time of a channel. The WiFi user accesses to a channel in a random mode, in order to reduce collision with other users, before accessing to the channel, whether a user uses the channel is detected through a CSMA/CA collision avoidance mechanism, if the channel is detected to be idle, the WiFi user successfully sends information, otherwise, a random backoff count is generated to avoid collision with other users.
The communication network is a heterogeneous network formed by authorized users, cognitive users and WiFi users, wherein the cognitive users can use not only authorized frequency bands but also unauthorized frequency bands to transmit information, and therefore the transmission rate of the communication network is formed by the authorized frequency bands and the unauthorized frequency bands. Considering the fairness problem of the whole network, the user satisfaction serves as an evaluation index of the whole network, and the user satisfaction serves as an objective function of u (R) ═ ln (R), wherein R represents the rate available to each user.
The following detailed description of the embodiments of the invention is provided in connection with the accompanying drawings and the specific examples.
The following considerations are given by way of example to a mobile communication heterogeneous network system comprising a macro cell Base Station (MBS), I Cognitive Small-cell Base stations (CSBS), and a WiFi access point, where the other network structure is implemented in a similar manner, where MBS is associated with a macro cell user, I CSBS is associated with J Cognitive users, and WiFi access point is associated with n WiFi users. The macro cellular user is an authorized user and only transmits on an authorized frequency band, the WiFi user only can use a 5GHz unauthorized frequency band, the CSBS can detect and use spectrum holes on the authorized frequency band and the 5GHz unauthorized frequency band while using the self frequency band, and the transmission rate generated by the self authorized frequency band of the cognitive user is certain, so that the cognitive user is only considered to transmit information by using the detected frequency band. Only downlink transmission links of authorized users and cognitive users are considered in the authorized frequency band, and downlink transmission links of the cognitive users are considered in the unauthorized frequency band. And the uplink and downlink transmission of WiFi are interactive, so the uplink and downlink transmission links are considered. An FDD (Frequency Division duplex) LTE mode is adopted by both the authorized user and the cognitive user in authorized and unauthorized Frequency bands, and the 802.11b standard is adopted by the WiFi user. Assume MBS is equipped with MtRoot antenna, authorized user equipped with MrRoot antenna, each CSBS being provided with NtRoot antenna, each cognitive user is provided with NrA root antenna. The specific network model structure diagram is shown in fig. 1.
Representing the relation between the CSBS and the cognitive user by a binary incidence matrix C, wherein the element C in the matrix Cij(I-1, 2, …, I, J-1, 2, …, J) represents the I-th cognitive microcellBase station CSBSiAnd cognitive user j. If C is presentij1 CSBSiAnd the cognitive user j is transmitting information, otherwise CijNo information is transmitted when 0, so Cij∈{0,1}。
Since each cognitive user can only belong to one CSBS, ΣiCij1. Meanwhile, the number of cognitive users which can be accommodated under each CSBS is limited, namely the maximum number of cognitive users which can be accommodated under each CSBS does not exceed D-Nt/NrWhen is, i.e. ΣjCijLess than or equal to D. At this time, there is no interference among cognitive users in the same CSBS coverage, but there is interference among cognitive users in different CSBS coverage.
Multi-antenna spectrum sensing based on energy detection. To improve the transmission rate and the spectrum sensing capability of the cognitive user, a MIMO transmission system is used as an example for explanation, a frame structure of the MIMO cognitive user is shown in fig. 2, and a total frame length is T + Ts，TsFor the sensing duration, T is the data transmission duration.
The energy detection unit detects the energy of the transmission signal within a period of time, and then compares the energy with a preset judgment threshold value to make a judgment to obtain a detection result. If the energy value is lower than the judgment threshold and the detection result is that the channel is idle, the cognitive user accesses the channel; otherwise, if the energy value is higher than the judgment threshold, the authorized user occupies the channel, and the cognitive user cannot access the channel. The energy detection can quickly and accurately calculate the false alarm probability and the detection probability, and the interference of the cognitive user to the authorized user is reduced.
Thus, for any one CSBSiAt sampling time T (T is more than or equal to 0 and less than or equal to T)s) Time of day, received signal yi(t) can be expressed as:
H0:yi(t)＝n(t)
H1:yi(t)＝TisM(t)+n(t)
wherein，H0Indicating that the authorized user channel is unoccupied, H1Indicating that the authorized user channel is occupied, n (t) indicating that the mean is 0 and the variance is δ2Additive white gaussian noise, TiIs MBS and CSBSiChannel gain matrix s betweenM(t) denotes a transmission signal of the MBS.
According to the formula:calculating the detection probability Pd(i) And false alarm probability Pfa(i) Which isIn (1),
wherein, tauiIs a detection threshold, Q (·) denotes the Q function, S ═ E { S ═M(sM)HIs the transmission covariance matrix of MBS, (T)i)HIs TiConjugate transpose matrix of (1), NrIs the number of the receiving antennas,and respectively, the mean and variance of the detected energy in the absence (presence) of an authorized user.
And in the authorized frequency band, based on the energy detection, if the channel is idle, namely the authorized user does not use the channel, the cognitive user accesses and uses the channel to transmit information. Missed detection may occur due to energy detection, i.e. authorized user is usingThe channel is detected by the cognitive user wrongly, and the cognitive user interferes with the authorized user. In order to protect authorized users from harmful interference, interference power constraint needs to be considered, namely, transmission power of cognitive users is limited to ensure that noise interference of cognitive users to authorized users is less than interference threshold, namely (1-P)d(i))Tr(GiQi(Gi)H) Gamma is less than or equal to gamma, wherein GiIndicating CSBSiAnd the channel matrix between the authorized users, Γ is the maximum interference power that the authorized users can tolerate,indicating CSBSiDue to Q, transmission poweriIs a semi-positive definite matrix, therefore QiIs greater than 0. Meanwhile, in order to guarantee the persistence of the cognitive user equipment, each CSBS needs to consider a transmission power constraint, namely Tr (Q)i)≤Qmax。
In the unlicensed frequency band, the cognitive users and the WiFi users compete to use the frequency spectrum resources. Fig. 3 shows an unlicensed band channel access scheme used in the present invention. The cognitive user does not transmit information and every other preset time length T0Detecting a primary channel, TLBTFor channel detection time, if T isLBTIf the channel is busy when the duration is detected, namely a WiFi user uses the channel, the waiting duration T of the cognitive user is determined0Then sensing again; and if the channel is idle, the channel is accessed immediately, and information transmission is carried out according to a standard LTE frame structure. WiFi users access to the channel in a random mode, before using the channel, the channel is detected through CSMA/CA, if the channel is detected to be idle for a DIFS (binary offset system) duration, the channel is used for transmitting data, otherwise, a back-off time count is randomly generated to reduce collision with other WiFi users. Because the cognitive users are transmitted according to the LTE frame structure, the time length of the cognitive users occupying the channel is integral multiple of 1ms and is 1 ms-10 ms, in order to ensure that various wireless users can fairly access to the unauthorized frequency band, an idle interval T is reserved after each cognitive user information transmissiongapThis idle interval is at least 5% of the channel occupancy time.
If the cognitive user detects the time interval T0The method has the advantages that the method is fixed, when WiFi information is more, the conflict probability between the cognitive user and the WiFi user is larger, when the WiFi user information is less, the channel utilization rate is low, therefore, the behaviors of the WiFi need to be predicted, and the detection time interval T of the cognitive user is dynamically adjusted0。
In order to predict the behavior of a WiFi user, a two-dimensional Markov chain is adopted to represent WiFi state transition, s (t) represents the backoff state of a base station at the time t, b (t) represents randomly generated backoff time count, and t +1 correspond to two continuous times.
As shown in fig. 4, a markov chain model for predicting WiFi user behavior is used, where p represents the probability of busy channel and conditional collision, and λ represents the probability of data transmission for WiFi users. The condition for causing state transition is that an idle time slot exists, when the unauthorized frequency band channel is detected to be busy or conflicted, the back-off time stops counting, and the state does not transition; and when the unauthorized frequency band channel is idle, the backoff time is reduced by 1, and the state is transferred. When the back-off count is 0, if the unlicensed frequency band channel is still idle, the information is sent, otherwise, the state s (t) is added with 1, and a back-off time count is randomly generated again to continue detecting the channel state.
Let W be the initial backoff window size, and the backoff window size W in the kth backoff statek＝2kW, where K is 0,1,2, …, K denotes the backoff state, and K is the maximum backoff state. Special state s (t) ═ 1, b (t) ═ 0 indicates that there is no data transmission request by the WiFi user, at which time W-11. When the WiFi user has data transmission, the state will be transferred. The one-step transition probability can be obtained from fig. 4.
The smooth distribution of the markov chain can be expressed as:all bk,wAre all reacted with b0,0Are related to and satisfyWhereinThen
Then the transmission probability of the WiFi user at any time slot is expressed as:
Pidle，Psucand PcolRespectively indicate the probability that no WiFi user transmits information in any time slot, the probability that information transmission of one WiFi user succeeds and the probability of transmission collision, namely:
Pidle＝(1-τ)n
Psuc＝nτ(1-τ)n-1(1-α)
Pcol＝1-Pidle-Psuc
α represents the probability of successful channel access of cognitive user, respectively recording DIFS duration, idle time slot, successful transmission duration and conflict duration as Td，Tidle，TsucAnd TcolThen the time probability that WiFi occupies the channel can be expressed as:
the cognitive user sends information by using the channel when detecting that the channel is idle within the sensing time, so the probability α of the cognitive user successfully accessing the channel can be expressed as:
wherein upsilon represents the number of continuous idle time slots upsilon0Representing the minimum number of free slots required for successful detection by the cognitive user.
In order to reasonably distribute the unauthorized frequency band resources, the detection time interval of the cognitive user is dynamically adjusted according to the transmission probability of the WiFi user, namely, if the WiFi transmission information is more (the idle time slots are fewer), the detection time interval of the cognitive small cell is increased, otherwise, the detection time interval is reduced. According to the formula:calculating the detection time interval T0Wherein η is a correlation factor between cognitive user detection time interval and WiFi transmission probability, PidleIs the probability that any slot associated with lambda is free. Can be determined according to the formula:and determining the time probability of the cognitive user occupying the unlicensed frequency band channel. Wherein, TcIndicating the duration of cognitive user transmission, TgapAnd reserving an idle interval for WiFi transmission after information is transmitted for the cognitive user. T since different users do not use the unlicensed band for communication at the same timeC+tW≤tmax. Wherein t ismaxIs the maximum available time probability of the unlicensed band, if tmaxIf 1 channel is always used, the cognitive user and the WiFi user detect that the channel is always busy and cannot access the channel, so tmax＜1。
Cognitive user detection time interval T0The detection time interval is smaller when the arrival rate is lower, and the cognitive user has more chances to access the unauthorized frequency band, so that the utilization rate of the frequency band is improved; otherwise the detection time intervalThe higher the probability that the cognitive user accesses the unauthorized frequency band is, the smaller the probability is, and the conflict with the WiFi user is avoided.
The overall network performance is determined. The communication network is composed of three users, namely an authorized user, a cognitive user and a WiFi user, so that the communication performance of the network is determined by the three users. The cognitive user can detect the use of other authorized frequency bands and also can detect the use of unauthorized frequency band transmission information while using the inherent authorized frequency band of the cognitive user. Since the transmission rate generated by the self authorized frequency band of the cognitive user is certain, the transmission rate of the self authorized frequency band is not considered in the invention, so the transmission rate comprises two parts of the authorized frequency band and the unauthorized frequency band.
In the licensed band, the transmission rate of the licensed user can be expressed as:
wherein, BLIndicating the licensed band bandwidth, P0Indicating the probability that the licensed band is not occupied by the licensed user,indicating the channel gain matrix to authorized users in the authorized band MBS,indicating the transmit power of the MBS. The total transmission rate of the cognitive user can be expressed as the sum of the transmission rates of all CSBSs, that is:
wherein, the ith CSBSiTransmission rate of
Wherein P is0(1-Pfa(i) Denotes CSBSiThe probability of correctly detecting that the spectrum of the authorized user is free,indicating in a licensed band CSBSiGain matrix to cognitive user j.
In unlicensed band CSBSiThe transmission rate of (d) may be expressed as:
wherein,indicating in unlicensed band CSBSiGain matrix to cognitive user j.
Since the WiFi user adopts the CSMA/CA collision avoidance mechanism when transmitting information, once information collision exists on a channel, the information is not transmitted, so that the external interference received when the WiFi user transmits the information is considered to be small and only background noise is considered, and the transmission rate of the WiFi is considered to be a constant RW。
The total transmission rate of the communication network includes the traffic generated by all users operating in both licensed and unlicensed frequency bands, i.e.: r ═ RM,L+RC,L+tCRC,U+tWRW
User satisfaction can be expressed as:
in the above formula, the first term is the satisfaction of the authorized user on the authorized frequency band, which increases with the increase of the transmission power of the macrocell base station; the second item is the satisfaction degree of the cognitive user on authorized and unauthorized frequency bands, the cognitive user transmits data by sharing the authorized frequency band with the authorized user on the authorized frequency band and under the condition that the interference of the transmitting power of the CSBS to the authorized user is lower than a certain threshold value, the cognitive user and the WiFi user compete to use the unauthorized frequency band on the unauthorized frequency band, and the detection time interval of the cognitive user is dynamically adjusted according to the transmitting probability of the WiFi so as to adjust the use time probability of the cognitive user in using the unauthorized frequency band; the third item is the satisfaction degree of the WiFi user on the authorized frequency band, which is influenced by the cognitive user to a certain extent. In conclusion, the method improves the spectrum utilization rate of the authorized and unauthorized frequency bands, and has great advantages in improving the overall performance of the network.
Claims (8)
1. A cognitive small cell dual resource allocation and interference management method based on LTE is characterized in that a cognitive small cell simultaneously uses an authorized frequency band and an unauthorized frequency band, a cognitive user dynamically accesses to use the authorized frequency band according to frequency spectrum detection, the cognitive user opportunistically accesses to the authorized frequency band, when the interference to the authorized user is lower than a threshold value, the cognitive user and the authorized user share the frequency spectrum, and simultaneously an optimal cognitive small cell base station is selected for data transmission; and the cognitive user and the WiFi user compete to use the unauthorized frequency band, and the detection time interval of the cognitive user is dynamically adjusted according to the transmission probability of the WiFi user.
2. The method according to claim 1, wherein the dynamically accessing the licensed band by the cognitive user according to the spectrum sensing specifically comprises: the cognitive small cell base station acquires an energy detection value of an authorized user, if the energy detection value is lower than a detection threshold, the cognitive user dynamically accesses the authorized frequency band to transmit data, and the cognitive user timely exits the authorized frequency band in use when detecting that the authorized user appears again.
3. The method of claim 1, wherein the cognitive user and the WiFi user contend for use of the unlicensed frequency band further comprises: the cognitive user transmits information at intervals of preset time T before using the unauthorized frequency band0And detecting a channel once, if the channel is busy, waiting for a preset time period again and then detecting the channel again until the channel is idle, and accessing and using the channel by the cognitive user according to an LTE frame structure.
4. The method of claim 1, wherein CSBS is configured for ith cognitive small cell base stationiAt sampling time T (T is more than or equal to 0 and less than or equal to T)s) When, TsFor sensing duration, e.g. of received signal yi(t) satisfies the formula:medium condition H0When the authorized frequency band is not occupied, the cognitive user can access the authorized frequency band; satisfies the condition H1Then, the authorized frequency band is occupied, where n (t) represents a mean value of 0 and a variance of δ2Additive white gaussian noise, TiIs a macro cellular base station MBS and CSBSiS (t) represents the transmission signal of the macrocell base station.
5. The method as claimed in claim 3, wherein after the end of the information transmission of the cognitive users, at least 5% of the occupied time of the channel is reserved as idle time, the WiFi users access the channel in a random manner, before accessing the channel, the CSMA/CA collision avoidance mechanism is used to detect whether there are users using the channel, if it is detected that there are users using the channel, the backoff counter generates a random backoff time to avoid collision with other users, and if it is detected that the channel is idle, the WiFi users send information.
6. The method of claim 5, wherein when there is a user using the unlicensed band channel, the back-off counter stops counting and no transition occurs in the state; when detecting that the unauthorized frequency band channel is idle, subtracting 1 from the backoff time, and transferring the state, when the backoff counter time is 0, if the unauthorized frequency band channel is still idle, sending information, otherwise, adding 1 to the state, and generating a backoff time again randomly to continue detecting the channel state.
7. The method of claim 6, wherein the state transition occurs when the WiFi user has data to transmit according to the formula:calculating the transmission probability of any time slot WiFi user, wherein K is 0,1,2, …, K represents the backspacing state, K is the maximum backspacing state,lambda is the arrival rate of the WiFi user, p is the collision probability at any moment, and W is the size of an initial backoff window; according to the formula:calculating the idle probability P of any time slot channelidleThere is a probability P of successful information transmission of WiFi usersucAnd probability of transmission collision PcolWherein α represents the probability of successful channel access of the cognitive user according to the formulaCalculating the time probability, T, of the WiFi occupying the channeldFor DIFS duration, TidleFor free time slot, TsucFor a successful transmission duration, TcolIs the length of the conflict.
8. The method of any of claims 1-6, wherein dynamically adjusting the detection time interval for the cognitive user based on the transmission probability of the WiFi user further comprises increasing the cognitive cell detection time interval if the WiFi transmission information increases, and decreasing the detection time interval otherwise, according to the formula:calculating cognitive small cell detection time interval T0Wherein η is a correlation factor, PidleChannel idle probability of any time slot; according to the formula:determining the time probability, T, of a cognitive user occupying an unlicensed frequency band channelcIndicating the duration of cognitive user transmission, TgapAnd reserving an idle interval for WiFi transmission after information is transmitted for the cognitive user.
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| CN106358316B (en) * | 2016-11-22 | 2019-11-08 | 重庆邮电大学 | A kind of channel access method and device |
| CN108233995A (en) * | 2016-12-13 | 2018-06-29 | 浙江工商大学 | A kind of method for spacial multiplex based on beam forming in unauthorized frequency range LTE system |
| CN106792779B (en) * | 2016-12-30 | 2019-09-10 | 浙江大学 | It is a kind of permit and exempt from licensed band work cellular network connection control method |
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