Disclosure of Invention
In view of this, the present invention provides a robust optimization method for a cognitive radio-powered backscatter communication network, which improves the transmission rate and robustness of a secondary network by combining a backscatter communication mode and a collection-transmission mode.
In order to achieve the purpose, the invention provides the following technical scheme:
a robustness optimization method for a cognitive radio power supply backscattering communication network is provided, wherein the cognitive radio power supply backscattering communication network comprises a main transmitter, a main receiver, N secondary transmitters and N secondary receivers. The method specifically comprises the following steps:
s1: establishing a signal transmission model of a cognitive wireless power supply backscatter communication network based on a lower cushion type;
energy collection is carried out by the secondary transmitter in the first stage; in the second stage, the secondary transmitter reflects signals to the corresponding secondary receiver by using a time division multiple access protocol; in the third phase, the secondary transmitter actively transmits data to the corresponding secondary receiver by using the energy collected in the first two phases.
S2: considering the transmission rate constraint of a secondary receiver, the service quality constraint of a main receiver, the energy collection constraint, the reflection coefficient and the time constraint, and constructing a resource allocation problem of maximizing the throughput of a secondary system;
s3: modeling the robust resource allocation problem by considering parameter uncertainty;
s4: and (4) converting the problem constructed in the step S3 into an equivalent convex optimization form by using a Q function and a variable substitution method, and obtaining an analytic solution of transmission time, transmission power and a reflection coefficient by using a Lagrange dual method.
Further, in step S1, the established signal transmission model specifically includes: definition P0For the transmitting power of the main transmitter, during the energy collection phase t1The energy collected by the nth secondary transmitter is:
wherein eta isn∈[0,1]Represents the energy collection efficiency of the secondary transmitter n; gnIndicating primary to secondary transmittersChannel gain of stage transmitter n;
in the backscatter phase t
2The secondary transmitter n adopts a time division multiple access mode to reflect information to the secondary receiver n; thus in the reflection time slot tau
nReflection rate of internal secondary transmitter n to secondary receiver n
Expressed as:
wherein W represents the bandwidth, β
nRepresenting the reflection coefficient, h, of a secondary transmitter n
nRepresenting the channel gain of the secondary transmitter n to the secondary receiver n,
representing the background noise power of the secondary receiver n; during the energy collection phase t
1And a reflection time slot tau
nTotal collected energy of inner nth secondary transmitter
Comprises the following steps:
in the active transmission phase t3Since a plurality of secondary transmitters transmit information to the secondary receivers using TDMA access, the time slots alpha are usednData transmission rate R from inner secondary transmitter n to secondary receiver nnComprises the following steps:
wherein, PnRepresenting a time slot alphanThe transmit power of the inner secondary transmitter n.
Further, in step S2, the expression of the resource allocation problem with maximized throughput of the constructed secondary system is:
where g is the channel gain from the primary transmitter to the primary receiver,
for the channel gain of the secondary transmitter n to the primary receiver,
representing the noise power of the primary receiver; c
1Indicating the minimum rate constraint of the secondary receiver n during the backscatter information phase
Represents a minimum rate threshold; c
2Indicating the minimum rate constraint of the secondary receiver n,
represents a minimum rate threshold; c
3And C
4Representing quality of service constraints of the primary receiver, ensuring the quality of service of the primary receiver, gamma
minRepresents a minimum quality of service threshold for the primary receiver; c
5The collected energy is larger than the sum of the energy consumed by the self circuit and the energy consumed in the active information transmission phase; c
6~C
8Represents a transmission slot constraint; c
9Representing the reflection coefficient constraint of the secondary transmitter n.
Further, in step S3, modeling the robust resource allocation optimization problem, specifically including: due to channel fading, uncertainty of parameters, etc. in a wireless communication system, it is difficult to obtain perfect channel state information. Therefore, an additive model of the uncertainty parameter is considered and the channel estimation error is assumed to follow a gaussian distribution, i.e.
Wherein the content of the first and second substances,
and
representing a set of uncertainties;
representing the estimated channel gain from the nth secondary transmitter to the nth secondary receiver;
representing the estimated channel gain of the nth secondary transmitter to the primary receiver;
representing the channel estimation error from the nth secondary transmitter to the nth secondary receiver, subject to a mean of zero and a variance of
(ii) a gaussian distribution of;
representing the channel estimation error of the nth secondary transmitter to the primary receiver, subject to a mean of zero and a variance of
(ii) a gaussian distribution of;
the robust resource allocation problem P2 corresponding to P1 can be expressed as
Wherein the content of the first and second substances,
is indicated in the reflection time slot tau
nThe outage probability constraint of the secondary receiver n reflection rate,
representing the n-reflection rate threshold, omega, of the secondary receiver
n∈[0,1]Representing the outage probability threshold of the secondary receiver n;
indicating that in the active transmission time slot alpha
nThe outage probability constraint that the secondary receiver n actively transmits information,
representing an active transmission rate threshold, upsilon
n∈[0,1]Representing the outage probability threshold of the secondary receiver n;
and
representing the outage probability constraint, gamma, of the primary receiver
minRepresents the quality of service threshold of the primary receiver, ζ ∈ [0,1 ]]Representing an outage probability threshold that protects the primary receiver's minimum quality of service requirements; pr {. cndot.) represents the outage probability.
Further, in step S4, converting the problem constructed in step S3 into an equivalent convex optimization form and solving the problem, specifically including the following steps:
s41: converting the interrupt probability constraint into a certainty constraint by using a Q function;
s42: introducing auxiliary variables based on variable substitution
Handling existing coupled variable constraints;
s43: and solving the analytic solution of the convex optimization problem by adopting a Lagrange dual theory.
The invention has the beneficial effects that: the invention improves the robustness and the throughput of the cognitive wireless power supply backscatter communication network system to a certain extent by improving the transmission rate and the robustness of the secondary system.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the means of the instrumentalities and combinations particularly pointed out hereinafter.
Detailed Description
The embodiments of the present invention are described below with reference to specific embodiments, and other advantages and effects of the present invention will be easily understood by those skilled in the art from the disclosure of the present specification. The invention is capable of other and different embodiments and of being practiced or of being carried out in various ways, and its several details are capable of modification in various respects, all without departing from the spirit and scope of the present invention. It should be noted that the drawings provided in the following embodiments are only for illustrating the basic idea of the present invention in a schematic way, and the features in the following embodiments and examples may be combined with each other without conflict.
Referring to fig. 1 to 4, the method for robust optimization of a cognitive radio-powered backscatter communication network provided by the present invention includes a primary transmitter and a primary receiver, and a plurality of secondary transmitters and secondary receivers. The method specifically comprises the following steps:
s1: establishing a signal transmission model of a cognitive wireless power supply backscatter communication network based on a lower cushion type; energy collection is carried out by the secondary transmitter in the first stage; in the second stage, the secondary transmitter reflects signals to the corresponding secondary receiver by using a time division multiple access protocol; in the third phase, the secondary transmitter actively transmits data to the corresponding secondary receiver according to the energy collected in the first two phases.
The invention considers a cognitive wireless power supply backscattering communication network scene based on an underlying type, and a system model is shown as figure 1. The network scene comprises a main network and a secondary network, wherein the main network comprises a main transmitter and a main receiver; the secondary network is composed of N secondary transmitters and N secondary receivers, and the set of the secondary transmitters and the secondary receivers is defined as
The primary transmitter, the primary receiver, the secondary transmitter and the secondary receiver are provided with a single antenna, and all the secondary transmitters are provided with a radio frequency energy collection module, a backscatter communication module and an active data transmission module, so that the secondary transmitter n can communicate with the secondary receiver n by selecting a backscatter mode or an active information transmission mode, but the two modes cannot simultaneously transmit data. It is assumed that the main transmitter is broadcasting the signal all the time, and thus the main channel will be busy for the transmission time frame T. The time frame T is divided into three parts, respectively an energy harvesting phase T
1Phase t of backscatter information
2And an active transmission phase t
3And satisfy
At t
1An energy collection stage, wherein all secondary transmitters collect energy; at t
2In the back scattering information stage, the secondary transmitter n adopts the mode of time division multiple accessThe stage receiver n performs reflection information. In each reflection time slot tau
nIn that only one secondary transmitter n reflects a signal to a secondary receiver n, and that
The remaining secondary transmitters remain silent; at t
3And in the active transmission stage, a plurality of secondary transmitters actively transmit information to a secondary receiver in a time division multiple access mode. In time slot alpha
nIn which a secondary transmitter n actively transmits information to a secondary receiver n, and satisfies
The remaining secondary transmitters remain silent. All channels are assumed to satisfy block fading channels, i.e. remain unchanged for a small time slot, being time-varying over the course of time.
Definition P0For the transmitting power of the main transmitter, during the energy collection phase t1The nth secondary transmitter collects energy of
Wherein eta isn∈[0,1]Represents the energy collection efficiency of the secondary transmitter n; gnRepresenting the channel gain from the primary transmitter to the secondary transmitter n.
In the backscatter phase t2And the secondary transmitter n reflects information to the secondary receiver n by adopting a time division multiple access mode. Thus in the reflection time slot taunThe reflection rate of the inner secondary transmitter n to the secondary receiver n can be expressed as
Wherein W represents a bandwidth; beta is a
nRepresenting the reflection coefficient of the secondary transmitter n; h is
nRepresenting the channel gain of the secondary transmitter n to the secondary receiver n;
representing the background noise power of the secondary receiver n.
During the energy collection phase t1And a reflection time slot taunThe total collected energy of the inner nth secondary transmitter is
In the active transmission phase t3Since a plurality of secondary transmitters transmit information to the secondary receivers using TDMA access, the time slots alpha are usednThe data transmission rate from the inner secondary transmitter n to the secondary receiver n is
Wherein, PnRepresenting a time slot alphanThe transmit power of the inner secondary transmitter n.
S2: and (3) resource allocation problem for maximizing the throughput of the secondary system by considering the transmission rate constraint of the secondary receiver, the service quality constraint of the primary receiver, the energy collection constraint, the reflection coefficient and the time constraint.
Assuming a channel gain g from the primary transmitter to the primary receiver and a channel gain g from the secondary transmitter n to the primary receiver
Under perfect channel state information, the optimization problem can be expressed as:
wherein the content of the first and second substances,
representing the noise power of the primary receiver; c
1At the backscattering information levelSegment, minimum rate constraint of secondary receiver n,
represents a minimum rate threshold; c
2Indicating the minimum rate constraint of the secondary receiver n,
represents a minimum rate threshold; c
3And C
4Quality of service, gamma, of the primary receiver is guaranteed
minRepresents a minimum quality of service threshold for the primary receiver; c
5The collected energy is larger than the sum of the energy consumed by the self circuit and the energy consumed in the active information transmission phase; c
6~C
8Represents a transmission slot constraint; c
9Representing the reflection coefficient constraint of the secondary transmitter n.
S3: and modeling the robust resource allocation optimization problem by considering the uncertainty of the channel parameters.
Due to channel fading, uncertainty of parameters, etc. in a wireless communication system, it is difficult to obtain perfect channel state information. Therefore, an additive model of the uncertainty parameter is considered and the channel estimation error is assumed to follow a gaussian distribution, i.e.
Wherein the content of the first and second substances,
representing the estimated channel gain from the nth secondary transmitter to the nth secondary receiver;
representing the estimated channel gain of the nth secondary transmitter to the primary receiver;
representing channels from nth secondary transmitter to nth secondary receiverError is estimated, and mean is zero and variance is
Represents the channel estimation error of the nth secondary transmitter to the primary receiver, and has a mean value of zero and a variance of
The robust resource allocation problem corresponding to P1 can be expressed as
Wherein the content of the first and second substances,
is indicated in the reflection time slot tau
nThe outage probability constraint of the secondary receiver n reflection rate,
representing the n-reflection rate threshold, omega, of the secondary receiver
n∈[0,1]Representing the outage probability threshold of the secondary receiver n;
indicating that in the active transmission time slot alpha
nThe outage probability constraint that the secondary receiver n actively transmits information,
indicating an active transmission rate threshold, v
n∈[0,1]Representing the outage probability threshold of the secondary receiver n;
and
to representInterruption probability constraint of the primary receiver, gamma
minRepresents the quality of service threshold of the primary receiver, ζ ∈ [0,1 ]]Representing a outage probability threshold, which constraint is used to protect the minimum quality of service requirements of the primary receiver.
Wherein the content of the first and second substances,
is expressed with respect to Δ h
nThe cumulative distribution function of (a). Therefore, we have
Wherein the content of the first and second substances,
Q
-1(. cndot.) represents the inverse of the Q function. According to the formula (8), the
Into a deterministic constraint of
By using a similar transformation method to that of formula (8) and formula (9), the
And
conversion to deterministic constraints
From equations (9) to (12), P2 can be re-expressed as
S4: and converting the original problem into an equivalent convex optimization form by using a Q function and a variable substitution method, and obtaining an analytic solution of transmission time, emission power and a reflection coefficient by using a Lagrange dual theory.
P3 has been transformed into a deterministic non-convex optimization problem, and it is still difficult to solve the solution analytically.
Definition of
P3 may be re-denoted as
According to the variable replacement method, P4 is a convex optimization problem and can be solved through a Lagrangian dual theory. Definition of
The Lagrangian function of P4 is
Wherein, χ
n,ε
n,φ
n,
κ
n,μ,v,θ,
ξ
nRepresenting a non-negative lagrange multiplier. Equation (15) may be re-expressed as
Wherein the content of the first and second substances,
the dual problem of formula (18) is
Wherein the dual function is
According to the Karush-Kuhn-Tucker (KKT) conditions, the following closed solutions can be obtained
Wherein, [ x ]]+=max(0,x);
Based on the gradient descent method, the optimization variables and the Lagrange multiplier update expression are as follows
μl+1=[μl-Δμ×(T-t1-t2-t3)]+ (30)
Wherein l represents the number of iterations; delta tau
n,Δα
n,Δχ
n,Δε
n,Δφ
n,
Δκ
n,Δμ,Δν,Δθ,
Δξ
nA step size greater than zero. According to the method of variable substitution, can be derived
And (3) verification experiment: the application effect of the present invention will be described in detail with reference to the simulation.
1) Simulation conditions
It is assumed that there is one primary transmitter and one primary receiver in the primary network and two secondary transmitters and two secondary receivers in the secondary network. The distance between the main transmitter and the main receiver is 9m, the distance between the two secondary transmitters and the main receiver is 7m and 6m, and the distance between the two secondary transmitters and the two secondary receivers is 4m and 5 m. The antenna gain of the base station and the antenna gain of the secondary transmitter are set to 6 dBi. The channel model is
Wherein d is
nIs the distance between the transmitting end and the receiving end, and χ is 3, the path loss exponent. Other parameters are shown in table 1.
TABLE 1 simulation parameters
2) Simulation result
FIG. 3 depicts the standard deviation e of the actual outage probability and the channel estimation errorRThe relationship (2) of (c). Under different algorithms, the actual outage probability of the primary receiver is related to the channel estimation error of the secondary transmitter n to the primary receiver. With the standard deviation ∈RThe actual outage probability of the algorithm of the present invention is lower than that of the non-robust algorithm and does not exceed the outage probability threshold ζ. The non-robust algorithm ignores the channel estimation error, so that the actual interruption probability is higher than that of the algorithm, and the algorithm has better robustness. Fig. 4 depicts the overall throughput of the secondary system versus channel estimation error under different algorithms. The results show that with σhThe overall throughput of the inventive algorithm, the pure backscatter algorithm and the pure gather-transmit algorithm is reduced. In contrast, the overall throughput remains unchanged, since the non-robust algorithm ignores the uncertainty of the channel. In addition, the total throughput of the algorithm is higher than that of a pure backscattering algorithm and a pure collection-transmission algorithm, and the effectiveness of the algorithm is further explained.
Finally, the above embodiments are only intended to illustrate the technical solutions of the present invention and not to limit the present invention, and although the present invention has been described in detail with reference to the preferred embodiments, it will be understood by those skilled in the art that modifications or equivalent substitutions may be made on the technical solutions of the present invention without departing from the spirit and scope of the technical solutions, and all of them should be covered by the claims of the present invention.