KR102584562B1 - Handover method in drone-based wireless communication system and apparatus therefor - Google Patents

Abstract

A handover method for a drone-based wireless communication system and a device to which the method is applied are disclosed. In a drone-based wireless communication system, the user terminal determines the power strength of the received signal received from drone base stations, identifies the drone base station subject to handover based on the power strength, generates a random number within a certain range, and then generates a random number within a certain range. Compare the reference value to determine whether to perform handover.

Description

Handover method in drone-based wireless communication system and apparatus to which the method is applied {Handover method in drone-based wireless communication system and apparatus therefor}

Embodiments of the present invention relate to a handover method for a drone-based wireless communication system and a device to which the method is applied.

Drones, also called unmanned aerial vehicles, were developed for military purposes, but are currently attracting attention in various civilian fields such as natural disaster monitoring, search and rescue, and wireless communication. In particular, in order to efficiently cope with the increase in traffic as the spread and demand for smartphones has rapidly increased, various studies are being conducted to utilize drones in wireless communication systems. When using drones in the wireless communication system, the drone's high mobility can have the advantage of not being restricted to locations such as hazardous areas or isolated areas, and can provide users with a more seamless communication service.

When a mobile DBS provides services to UE (User Equipment) in a DBS (Drone Base Station) environment that uses drones as base stations, continuous communication between the DBS and UE through a handover technique is required to ensure service quality for the UE. The connection should proceed. Research on handover techniques has mainly been conducted on base stations fixed on the ground. However, because existing studies are about handover based on base stations fixed on the ground, it is difficult to apply them to the DBS environment that uses drones as base stations.

The technical problem to be achieved by embodiments of the present invention is to provide a handover method between drone base stations that takes into account the distance and small-scale fading between drone base stations and user terminals in a drone-based wireless communication network, and a user terminal to which the method is applied. .

An example of a handover method of a drone-based wireless communication network according to an embodiment of the present invention to achieve the above technical problem is a handover method of a user terminal in a drone-based wireless communication system, from drone base stations. Determining the power intensity of the received signal; Identifying a handover target drone base station based on the power intensity; and generating a random number within a certain range and comparing the random number with a predefined reference value to determine whether to perform a handover.

In order to achieve the above technical problem, an example of a user terminal according to an embodiment of the present invention includes a power detection unit that determines the power strength of a received signal from drone base stations; a candidate identification unit that identifies a drone base station to be handed over based on the power intensity; and a handover performing unit that generates a random number within a certain range and determines whether to perform a handover by comparing the random number with a predefined reference value.

According to an embodiment of the present invention, high frequency efficiency performance can be obtained by performing handover considering not only the distance between the drone base station and the user terminal but also small-scale fading. Additionally, signal overhead can be reduced by preventing frequent handovers between drone base stations that may occur when considering small-scale fading.

1 is a diagram showing an example of a drone-based wireless communication system to which the present invention is applied;
Figure 2 is a flowchart showing an example of a handover method for a drone-based wireless communication system according to an embodiment of the present invention;
Figure 3 is a diagram illustrating an example of a method for determining whether to perform probability-based handover according to an embodiment of the present invention;
Figure 4 is a flowchart showing another example of a handover method for a drone-based wireless communication system according to an embodiment of the present invention;
Figure 5 is a diagram illustrating an example of a handover method of a drone-based wireless communication system implemented with a software algorithm according to an embodiment of the present invention;
Figure 6 is a diagram showing an example configuration of a user terminal operating in a drone-based wireless communication system according to an embodiment of the present invention, and
Figures 7 and 8 are graphs showing the results of simulating the handover method of a drone-based wireless communication system according to an embodiment of the present invention.

Hereinafter, with reference to the attached drawings, a handover method of a drone-based wireless communication system according to an embodiment of the present invention and a device to which the method is applied will be described in detail.

Figure 1 is a diagram showing an example of a drone-based wireless communication system to which the present invention is applied.

Referring to FIG. 1, the drone-based wireless communication system 100 consists of a drone base station 120 (hereinafter referred to as 'DBS') moving in the sky and a user terminal 110 (hereinafter referred to as 'UE') located on the ground. provides network services to For convenience of explanation, this embodiment assumes that the UE 110 is located at the origin in a three-dimensional Cartesian coordinate system, and n DBSs 120 are operated at a height h away from the ground. Additionally, it is assumed that n DBSs 120 are randomly and independently distributed based on a Poisson point process with a density λ at the initial time point, t=0.

The n DBS 120 can move according to time t, and the location of the ith DBS 120 at a specific time t has coordinates of (x _i (t), y _i (t), h). At this time, the distance between the ith DBS 120 and the UE 110 at a specific time t can be expressed as the following equation.

Here, x _i (t) and y _i (t) represent the x and y coordinates of the ith DBS at a specific time t, respectively. T represents the operating time of n DBSs located in the wireless communication system of this embodiment.

The UE 110 may regard signals from DBSs other than the DBS currently providing the service as interference. In order to reduce interference that may occur from a large number of DBSs and at the same time perform efficient communication, a critical distance (d _th ) is set and the space within the critical distance (d _th ) is set as the service space, and the DBS that exists outside the space is set as the service space. It is assumed that interference signals that may arise from the fields are excluded. This can be done by using methods such as separating frequencies according to service sections.

Signals transmitted to the UE 110 by DBSs may experience Rayleigh fading depending on the multipath telephone environment. At this time, the channel gain of the link between the DBS 120 and the UE 110 can be expressed as follows.

Here, K _t represents a set of DBSs that satisfy d _i (t) < d _th at a specific time t. k _t,start and k _t,end are variable values according to time t and represent the smallest and largest index values of DBS that satisfy the condition d _i (t)<d _th, respectively. g _s (t) and g _j (t) refer to random variables following an exponential distribution with a mean of 1, and η refers to the path loss index.

G _s (t) in Equation 2 represents the channel gain for the link between the DBS 120 and the UE 110 that transmits the target signal at a specific time t, where s represents the index value of the corresponding DBS. G _j (t) in Equation 3 represents the channel gain of the link between each DBS that transmits an interference signal in addition to the DBS that transmits the target signal at a specific time t and the UE, where j represents the index value of the corresponding DBS.

Using Equation 2 and Equation 3, SINR (Signal to Interference plus Noise Ratio) at time t can be expressed as the following equation.

Here, P represents the transmission power of the DBS, and it is assumed that the transmission power of each DBS is the same. Additionally, σ ² refers to the power value for additive white Gaussian noise.

According to Equation 4, the frequency efficiency performance at time t in the system of this embodiment can be expressed as follows.

This embodiment presents a handover method for improving the frequency efficiency performance of the UE, and an example of this is shown in FIG. 2 and below.

Figure 2 is a flowchart illustrating an example of a handover method for a drone-based wireless communication system according to an embodiment of the present invention.

Referring to FIG. 2, the UE determines the power strength of the received signal received from a plurality of DBSs (S200). Since the signal transmitted from DBS to UE experiences small-scale fading due to the multipath propagation environment, if handover is performed considering only the distance between DBS and UE, changing channel characteristics cannot be properly reflected, resulting in low average frequency efficiency performance. You can. Accordingly, in this embodiment, the received power intensity received by the UE is considered. The received power intensity of the UE at a specific time t is the value corresponding to the numerator of Equation 4 above and is P _rx,s (t) = PG _s (t).

In another embodiment, the UE may determine the power intensity only for DBS within a predefined threshold distance (d _th ) as shown in FIG. 1. The UE can determine the distance between each DBS and itself using Equation 1 above. For this purpose, it is assumed that the UE can know the location information of each DBS through various existing methods, such as receiving location information from each DBS. The UE can perform the process of identifying the DBS within the threshold distance each time it determines the received power intensity.

The UE identifies the handover target DBS based on the power strength (S210). For example, the UE may select the DBS showing the maximum power strength among the power strengths of signals received from a plurality of DBSs as the handover target DBS. This can be expressed mathematically as follows:

Here, s* is an index value of DBS indicating the maximum power intensity among a plurality of DBS. The UE can select the s*th DBS as the handover target DBS according to Equation 6. Additionally, the UE can calculate the frequency efficiency performance at time t for the s*th DBS using Equation 4.

In one embodiment, the UE may perform handover directly to the handover target DBS. However, in this case, the UE selects the DBS for handover based on power intensity, so higher frequency efficiency performance can be obtained compared to the handover technique that only considers distance. However, power intensity changes frequently as DBSs move, so handover is difficult. The number of times performed may increase. If frequent handovers are performed, overhead for transmission signals may occur and network resources and energy consumption may increase due to the ping-pong effect.

To prevent frequent handover, the UE determines whether to perform handover based on probability (S220). More specifically, the UE does not perform handover if the DBS currently providing the service (hereinafter referred to as current DBS) and the handover target DBS are the same. If the current DBS and the handover target DBS are different, the UE selects whether to perform or not perform a handover based on a predefined probability value. The probability-based selection of whether to perform a handover can use random numbers, and an example of this is shown in FIG. 3. In another embodiment, if the current DBS deviates from the predefined threshold distance (d _th ) shown in FIG. 1, the UE may immediately perform handover to the previously identified service target DBS regardless of the probability value.

Figure 3 is a diagram illustrating an example of a method for determining whether to perform probability-based handover according to an embodiment of the present invention.

Referring to FIG. 3, the UE generates a random number (RN) within a certain range (300). At this time, the UE can generate random numbers using several probability models representing uniform distribution. For example, the random number occurrence range 300 may be between 1 and 100.

The UE predefines a reference value 310 for determining whether to perform a handover. The UE may perform handover if the random number generation value 320 is less than or equal to the reference value 310, and may not perform handover if the random number generation value 320 is greater than or equal to the reference value. For example, if the reference value 310 is set to 70, the probability that the UE will perform a handover is 70%. As another example, if the reference value 310 is set to 100, the probability of handover occurring is 100%, which means that if the DBS for handover is identified, the handover is 100% performed. For example, in the example of FIG. 2, when a handover target DBS that can improve frequency efficiency performance based on power intensity is identified, the UE does not immediately perform handover but generates a random number value 320. Afterwards, if the random number value 320 is less than or equal to the reference value 310, handover is performed.

Figure 4 is a flowchart showing another example of a handover method for a drone-based wireless communication system according to an embodiment of the present invention.

Referring to FIG. 4, the UE can decide whether to perform handover during a predefined operation time. The operating time can be set in various ways depending on the embodiment, such as 300 seconds.

The UE determines whether the elapsed time exceeds the operating time (S440). If the elapsed time does not exceed the operating time, the UE identifies the handover target DBS using the method shown in FIG. 3 and determines whether to perform the handover (S410). If the handover is not performed (S420), the UE repeats the process of determining whether to perform the handover of FIG. 3 again within the operating time (S410).

If the operating time has elapsed (S400) or handover has been performed within the operating time (S420), the UE initializes the operating time and repeats the process of determining whether to perform handover within the operating time again.

Figure 5 is a diagram illustrating an example of a handover method of a drone-based wireless communication system implemented with a software algorithm according to an embodiment of the present invention. In FIG. 5, T means the operating time of FIG. 4, and K _t means a DBS set that determines the power intensity as defined in Equation 6. d _th represents the critical distance. β means the reference value in FIG. 3.

Figure 6 is a diagram showing an example configuration of a user terminal operating in a drone-based wireless communication system according to an embodiment of the present invention.

Referring to FIG. 6, the user terminal 110 (UE) includes a power determination unit 600, a candidate identification unit 610, and a handover performance unit 620. UE 110 refers to all types of terminals that perform wireless communication, such as smartphones. The UE 110 may include a memory, a processor, and an input/output device. In this case, each component may be implemented as software, loaded on the memory, and then performed by the processor.

The power detection unit 600 determines the power intensity of the received signal from the DBS. As an example, the power determination unit 600 may identify DBSs located within a predefined threshold distance (d _th ) and then determine the power intensity received from the DBSs.

The candidate identification unit 610 identifies the handover target DBS based on the power intensity. As an example, the candidate identification unit 610 may identify the DBS with the highest received power intensity as the handover target DBS. If the DBS with the highest power intensity is the same as the current DBS, the candidate identification unit 610 does not select the DBS for handover, and the power identification unit 600 repeats the process of determining the power strengths of the DBSs within the critical distance. It can be done.

The handover performing unit 620 can determine whether to perform a handover by generating a random number within a certain range and comparing the random number with a predefined reference value. An example of a method for determining whether to perform handover using random numbers is shown in FIG. 3.

Figures 7 and 8 are graphs showing the results of simulating the handover method of a drone-based wireless communication system according to an embodiment of the present invention. In the simulation of this embodiment, it is assumed that the DBS moves in a straight line in a uniformly random direction, and the speed of each DBS is set to 45 km/h. In addition, the density (λ) of DBS distributed based on the Poisson point process was set to 1DBS/Km ² and the overall system radius (r) was set to 100Km. The path loss index (η) was set to 3, and the transmission power (P) of DBS was set to 1W. The critical distance (d _th ) was set to 2km, and the noise power (σ ² ) was set to -110dBm.

Referring to FIG. 7, the results of comparing the frequency efficiency performance between the handover method of this embodiment and the existing distance-based handover method in a situation where each DBS moves for t (0≤t≤300) seconds are shown. . The graph shows the results of evaluating the average frequency efficiency performance by applying various probability values (β) to the handover method of this embodiment. It can be seen that the frequency efficiency performance of the handover method of this embodiment is higher than that of the conventional method.

Referring to FIG. 8, the handover probability at time t means the probability that at least one handover will occur during time t. Unlike the conventional method, the handover method of this embodiment performs handover based on received power calculated by considering both distance and small-scale fading, so it can be confirmed that it has a higher handover probability than the conventional method. When β=100, handover is always performed to the s*th DBS, showing the highest handover probability. In the case of β < 100, handover may not be performed even if there is a DBS within the radius that will increase the received power intensity received by the UE more than the DBS currently providing the service. Handover is performed more than in the case of β = 100. This happens a little later. Accordingly, as β becomes smaller, the handover probability appears to decrease. At t=70 seconds, the handover probability in the case of β=100 is 0.9995, the handover probability in the case of β=70 is 0.9988, the handover probability in the case of β=40 is 0.9954, and the handover probability in the case of β=10 is 0.9354. It can be confirmed that this is greater than the handover probability of 0.8274 of the conventional method.

Through the simulation results, it can be confirmed that there is a trade-off between average frequency efficiency and handover probability. Accordingly, the probability value (β) is adjusted according to the conditions required in the system to achieve the performance target required by the system. It can be achieved.

Each embodiment of the present invention can also be implemented as computer-readable code on a computer-readable recording medium. Computer-readable recording media include all types of recording devices that store data that can be read by a computer system. Examples of computer-readable recording media include ROM, RAM, CD-ROM, SSD, and optical data storage devices. Additionally, computer-readable recording media can be distributed across networked computer systems so that computer-readable code can be stored and executed in a distributed manner.

So far, the present invention has been examined focusing on its preferred embodiments. A person skilled in the art to which the present invention pertains will understand that the present invention may be implemented in a modified form without departing from the essential characteristics of the present invention. Therefore, the disclosed embodiments should be considered from an illustrative rather than a restrictive perspective. The scope of the present invention is indicated in the claims rather than the foregoing description, and all differences within the equivalent scope should be construed as being included in the present invention.

Claims (7)

In a DBS (Drone Base Station) environment where multiple drones moving in the sky are used as base stations, in a handover method for a user terminal located on the ground,
Determining the power strength of received signals from drone base stations;
Identifying a handover target drone base station based on the power intensity; and
Generating a random number within a certain range and comparing the random number with a predefined reference value to determine whether to perform a handover,
In the step of determining whether to perform the handover, when a new handover target drone base station is identified based on the power intensity, if the random number is greater than the reference value, handover is not performed, and if the random number is less than or equal to the reference value, the handover is not performed. A handover method for a drone-based wireless communication system comprising: performing handover. The method of claim 1, wherein determining the power intensity comprises:
Identifying drone base stations located within a predefined critical distance; and
A handover method for a drone-based wireless communication system comprising: calculating the power strength of the received signal for the identified drone base stations. According to clause 1,
A drone-based wireless communication system further comprising repeating the steps from determining the power intensity to determining whether to perform the handover until handover is performed during a predefined operating time. handover method. The method of claim 1, wherein the step of determining whether to perform handover comprises:
A handover method for a drone-based wireless communication system comprising: performing a handover when the current drone base station currently providing the service deviates from a predefined threshold distance. The method of claim 1, wherein the step of determining whether to perform handover comprises:
A handover method for a drone-based wireless communication system comprising: determining that handover is not performed if the current drone base station currently providing the service and the drone base station subject to handover are the same. In a user terminal that performs handover in a DBS (Drone Base Station) environment that uses multiple drones moving in the sky as a base station,
A power detection unit that determines the power strength of the received signal received from drone base stations;
a candidate identification unit that identifies a drone base station to be handed over based on the power intensity; and
It includes a handover performing unit that generates a random number within a certain range and determines whether to perform a handover by comparing the random number with a predefined reference value,
The handover performing unit, when a new handover target drone base station is identified based on the power intensity, does not perform a handover if the random number is greater than the reference value, and performs a handover if the random number is less than or equal to the reference value. A user terminal characterized by: A computer-readable recording medium recording a computer program for performing the method according to any one of claims 1 to 5.

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