KR102252729B1 - Method and terminal for synchronizing in distributed wireless communication - Google Patents

Abstract

Disclosed are a method for synchronizing in a distributed wireless communication system and a terminal supporting the same. When the terminal receives the synchronization signal in the synchronization slot, the terminal increases its contention window value. In addition, when the terminal does not receive a synchronization signal in a synchronization slot, the terminal decreases its contention window value. The terminal determines whether to transmit a synchronization signal using an increased or decreased contention window value.

Description

Synchronization method in a distributed wireless communication system and a terminal supporting it {METHOD AND TERMINAL FOR SYNCHRONIZING IN DISTRIBUTED WIRELESS COMMUNICATION}

The present invention relates to a method for synchronization in a distributed wireless communication system and a terminal supporting the same.

Wireless communication systems can be broadly classified into a synchronous method and an asynchronous method.

In a synchronous wireless communication system, terminals match a common reference time, and the time domain is divided into frames, which are repeated sections. The frame is further subdivided into slots, which are small sections, and a system that transmits or receives signals within the slot is called a synchronous wireless communication system.

Synchronization methods can be classified into a centralized synchronization method and a distributed synchronization method. A typical example of a centralized synchronization method is a cellular system, and in the cellular system, a base station provides a synchronization signal. The terminal transmits and receives signals using radio resources within a frame structure defined based on a timing reference signal provided by the base station. In the distributed synchronization scheme, synchronization signals are not transmitted by a specific device, but terminals in a network participate in synchronization signal transmission to synchronize. The distributed synchronization method enables synchronization between terminals in the network without a communication infrastructure or a specific device that transmits synchronization signals, so device-to-device communications (D2D communications) networks Or it is suitable for ad hoc networks.

The asynchronous wireless communication system is a system in which terminals send and receive signals without a predetermined reference time. In an asynchronous wireless communication system, since the terminals do not know when a signal is to be received, the terminals always monitor the radio channel to receive the signal. When a radio packet is detected during monitoring, the terminal estimates the start point of the packet using a preamble signal included in the radio packet and then reads the packet information. Since the asynchronous method is simple to implement because there is no set reference time, it is mainly used in a system that does not have to have high resource efficiency, but it has a disadvantage in that it consumes a lot of power because the terminals must always monitor the radio channel.

As one of the distributed synchronization methods, there are the following methods. Each terminal periodically transmits a synchronization signal using its own time synchronization (ie, reference time). In addition, after each terminal receives the synchronization signal transmitted by the neighboring terminals, it synchronizes its own time using the received synchronization. In this case, the synchronization signals used include a pulse signal, a Zadoff-Chu sequence, an m-sequence, and a chirp signal. Since the transmission and reception of the synchronization signal cannot be performed at the same time, the terminals cannot receive the synchronization signal transmitted by the neighboring terminal when transmitting the synchronization signal. However, in this method, when the terminals are dense, synchronization signals transmitted by the terminals may overlap each other to be received, and thus an error may occur in estimating the reception time of the synchronization signal. In addition, when the same radio channel is used by neighboring heterogeneous networks in an unlicensed frequency band, a synchronization signal may not be transmitted or a transmitted synchronization signal may be lost due to collision.

The problem to be solved by the present invention is to provide a distributed synchronization method that improves network performance.

According to an embodiment of the present invention, a method of synchronizing by a first terminal in a distributed wireless communication system is provided. The synchronization method includes, when receiving a synchronization signal in a synchronization slot, increasing a contention window value of the first terminal to a first contention window value, and when no synchronization signal is received in the synchronization slot, the first Decreasing the contention window value of the first terminal to a second contention window value, and determining whether to transmit a synchronization signal using the first contention window value or the second contention window value.

The synchronization signal may include a collision detection field through which a tone signal is transmitted. In the synchronization method, when receiving the synchronization signal in the synchronization slot, the value of the backoff counter of the first terminal Correspondingly, it may further include determining whether to transmit the tone signal at a time when the collision detection field of the synchronization signal is located.

The synchronization method may further include updating an average contention window value for neighboring terminals of the first terminal using a contention window value included in the synchronization signal.

The synchronization method further includes the step of setting an average contention window value for neighboring terminals of the first terminal, and the step of increasing includes the average contention window value and the contention window value of the first terminal, It may include the step of increasing the contention window value of the first terminal.

The reducing may include reducing the contention window value of the first terminal by using the average contention window value and the contention window value of the first terminal.

The increasing may include further increasing the contention window value of the first terminal as the contention window value of the first terminal is smaller than a value obtained by multiplying the average contention window value by a predetermined ratio.

The reducing may include reducing the contention window value of the first terminal less as the contention window value of the first terminal is smaller than a value obtained by multiplying the average contention window value by a predetermined ratio.

The synchronization signal may include a preamble, a time offset, a collision detection field through which a tone signal is transmitted, and a contention window indication field including a contention window value.

According to another embodiment of the present invention, a method of synchronizing by a first terminal in a distributed wireless communication system is provided. The synchronization method includes: setting a contention window value of the first terminal, randomly selecting a predetermined first value as a backoff count from among the contention window values, and determining whether a synchronization signal is received in a synchronization slot. Accordingly, it may include changing the first value, and determining whether to transmit the synchronization signal of the first terminal in response to the first value.

The step of changing the first value may include decreasing the first value when the radio channel is idle for a predetermined period in the synchronization slot.

The determining may include determining to transmit the synchronization signal of the first terminal when the first value is 0.

According to another embodiment of the present invention, a terminal is provided. The terminal changes a contention window value according to an RF module receiving a synchronization signal and whether the synchronization signal is received, and sets a backoff counter using the contention window value, and corresponds to the backoff counter. It may include a processor that determines whether to transmit a synchronization signal.

When receiving the synchronization signal, the processor may increase the contention window value, and when the synchronization signal is not received, the processor may decrease the contention window value.

The synchronization signal may include a collision detection field through which a tone signal is transmitted, and when the processor receives the synchronization signal, a collision detection field of the synchronization signal is located corresponding to the backoff counter. At this point, it is possible to determine whether to transmit the tone signal.

The processor may change the contention window value by using the average contention window value for neighboring terminals of the terminal and the contention window value.

When receiving the synchronization signal, the processor may further increase the contention window value as the contention window value is smaller than a value obtained by multiplying the average contention window value by a predetermined ratio.

When the synchronization signal is not received, the processor may reduce the contention window value less as the contention window value is smaller than a value obtained by multiplying the average contention window value by a predetermined ratio.

The synchronization signal may include a preamble, a time offset, a collision detection field through which a tone signal is transmitted, and a contention window indication field including a contention window value.

According to an embodiment of the present invention, network performance can be improved by synchronizing terminals in a distributed synchronization method.

1 is a diagram illustrating a time resource according to an embodiment of the present invention.
2 is a diagram illustrating a structure of a synchronization slot according to an embodiment of the present invention.
3 and 4 are diagrams each showing an example of a location where a synchronization signal is transmitted.
5 is a flowchart illustrating an operation method for a case in which terminals sense a synchronization signal transmitted from another terminal according to an embodiment of the present invention.
6 is a diagram illustrating a case where the impulse detection fields CDF collide with each other.
7 is a diagram illustrating a terminal according to an embodiment of the present invention.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those of ordinary skill in the art may easily implement the present invention. However, the present invention may be implemented in various different forms and is not limited to the embodiments described herein. In the drawings, parts irrelevant to the description are omitted in order to clearly describe the present invention, and similar reference numerals are attached to similar parts throughout the specification.

Throughout the specification, a terminal is a mobile terminal (MT), a mobile station (MS), an advanced mobile station (AMS), a high reliability mobile station (HR-MS). ), subscriber station (SS), portable subscriber station (PSS), access terminal (AT), user equipment (UE), etc. It may include all or part of functions such as AMS, HR-MS, SS, PSS, AT, and UE.

In addition, the base station (BS) is an advanced base station (ABS), a high reliability base station (HR-BS), a node B (node B), an advanced node B (evolved node B), eNodeB), access point (AP), radio access station (RAS), base transceiver station (BTS), mobile multihop relay (MMR)-BS, relay serving as a base station station, RS), a high reliability relay station (HR-RS) that serves as a base station, etc., and ABS, NodeB, eNodeB, AP, RAS, BTS, MMR-BS, RS, HR It may include all or part of functions such as -RS.

Now, a method for synchronizing in a distributed communication system according to an embodiment of the present invention (ie, a distributed synchronization method) and a terminal supporting the same will be described in detail. Hereinafter, a description will be made on the assumption of a distributed wireless communication system environment, and a detailed description of the distributed wireless communication system will be omitted as it can be understood by those of ordinary skill in the art.

1 is a diagram illustrating a time resource according to an embodiment of the present invention.

As shown in FIG. 1, the time resource according to the embodiment of the present invention includes a frame 100 which is a period that is periodically repeated. In FIG. 1, it is assumed that the length of the frame 100 is predetermined.

The frame 100 includes a plurality of slots, and in FIG. 1, only the synchronization slot 110 is shown among the plurality of slots. That is, FIG. 1 shows that the time resource is divided into a frame 100, and the frame 100 is subdivided into a synchronization slot 110 and the remaining slots 120. The remaining slots 110 may be further divided into data slots, search slots, and the like, but are not shown in FIG. 1 for convenience.

The synchronization slot 110 is a period in which terminals transmit synchronization signals. Terminals can know the positions of the start and end of the frame by using the synchronization signal received in the synchronization slot 110.

The terminals operate in the reception mode for a certain period of time before transmitting the synchronization signal during the initialization process. Through this reception mode operation, the terminals can determine whether there is a synchronization signal transmitted by a neighboring terminal. In this case, when there is a synchronization signal transmitted by the neighboring terminals, the terminals synchronize time using the synchronization signals transmitted by the neighboring terminals. Meanwhile, if the terminals do not receive the synchronization signal transmitted by the neighboring terminal for a certain period of time, the terminals arbitrarily set their own time synchronization. In this case, "to synchronize or set their own time" means that the positions of the start and end of the periodic frame 100 are aligned or set.

When the terminals set their own time synchronization, they transmit or receive a synchronization signal in the synchronization slot 110 of each frame. At this time, transmission of the synchronization signal follows a random access method.

2 is a diagram showing the structure of a synchronization slot 110 according to an embodiment of the present invention.

As shown in FIG. 2, the length of the synchronization slot 110 according to the embodiment of the present invention is the length of the N back off slots 111 and one timing reference signal. It has a length corresponding to the sum of the lengths. That is, the length of the synchronization slot 110 may be expressed as Equation 1 below.

In Equation 1, N is a natural number.

The synchronization signal can be transmitted at any position in the synchronization slot. Meanwhile, transmission of the synchronization signal is completed within the synchronization slot 110. In addition, the transmission start of the synchronization signal is transmitted at the start point of the backoff slot 111.

3 and 4 are diagrams each showing an example of a location where a synchronization signal is transmitted.

3 shows a case in which transmission of a synchronization signal starts at a beginning of 9 ^{th backoff slot.} As shown in Fig. 3, the synchronization signal is transmitted at the starting point of the ninth backoff slot, and the synchronization signal is transmitted during the length of the synchronization signal.

4 shows a case in which transmission of a synchronization signal starts at a beginning of 13 ^{th backoff slot.} As shown in Fig. 4, the synchronization signal is transmitted at the starting point of the 13th backoff slot, and the synchronization signal is transmitted during the length of the synchronization signal.

Meanwhile, terminals store their own contention window values to prevent collision of synchronization signals. In order to prevent synchronization signals transmitted by the terminals from colliding with each other, a contention window is used. A method in which terminals transmit a synchronization signal using a contention window in a synchronization slot (ie, a random access method using a contention window) will be described below.

When the contention window value that the terminals can have is CW, the predetermined terminal is {0, 1, 2, ... , CW-1} randomly selects one of the integers as the contention window value. If, assuming that the integer n is selected, the terminal sets its own backoff counter to n. After setting the backoff counter to n, when the wireless channel is continuously idle for a time corresponding to the length of one backoff slot 111, the terminal sets its own backoff counter by 1 (i.e., n -1) decrease. On the other hand, when the radio channel is not in a continuous idle state at least for a time corresponding to one backoff slot 111, the terminal does not decrease its own backoff counter. After repeating this operation, the terminal transmits a synchronization signal when its backoff counter becomes zero. This procedure is referred to as a'backoff procedure' in the embodiment of the present invention.

On the other hand, when the terminal intends to transmit the synchronization signal but determines that the synchronization signal transmission cannot be completed in the synchronization slot 110, the synchronization signal is transmitted with the start of the next synchronization slot 110. In other words, if the remaining time in the synchronization slot is not sufficient to transmit one synchronization signal, the terminal does not transmit the synchronization signal in the current synchronization slot even if the backoff counter is zero. A synchronization signal is transmitted with the start of.

The synchronization signal transmitted by the terminals in the synchronization slot includes a preamble, a time offset, a collision detection field (CDF), and a contention window indication field (CWIF). The preamble is used to estimate the reception time of the synchronization signal, and the time offset represents the difference between the start of the synchronization slot and the actual transmission time of the synchronization signal. In addition, the collision detection field (CDF) is used to indirectly estimate the number of terminals in the network, and the contention window indication field (CWIF) represents a contention window value for a terminal transmitting a synchronization signal.

Here, the collision detection field (CDF) may consist of one Orthogonal Frequency Division Multiplexing (OFDM) symbol. A terminal to transmit a synchronization signal (hereinafter referred to as a'transmitting terminal') divides M groups of OFDM sub-carriers and randomly selects one subcarrier from each group. . In addition, the transmitting terminal transmits M tone signals by using the M subcarriers selected in each group. If the CDFs transmitted by two different transmitting terminals overlap each other, the terminal receiving the synchronization signal checks the number or pattern of subcarriers in the collision detection field (CDF), and indicates that two or more CDFs overlap. I can judge.

As described above, when the terminal detects a synchronization signal transmitted by another terminal in the synchronization slot 110, the terminal can decrease its own backoff counter by 1 without stopping. For example, if terminal A and terminal B have backoff counters 1 and 5, respectively, and terminal A and terminal B detect a synchronization signal transmitted by another terminal, terminal A and terminal B each have a backoff counter. Decrease it to 0 and 4. At this time, since UE A has its own backoff counter of 0, it may immediately transmit a synchronization signal after waiting for a predetermined time (for example, DIFS (Distributed Inter Frame Space) or EIFS (Extended Inter Frame Space)). When a synchronization signal transmitted by the terminal is detected, the backoff procedure described above is not resumed immediately after the transmission of the detected synchronization signal is completed, but waits for a certain period of time (e.g., DIFS or EIFS) before performing the backoff procedure. For example, the UE may resume the backoff procedure after waiting for a time corresponding to one backoff slot.

In addition, when the remaining time of the synchronization slot is less than the length of the synchronization signal, the terminals stop the backoff procedure and wait for the next synchronization slot.

Referring to FIG. 5, a description will be given of an operation method when terminals detect a synchronization signal transmitted by another terminal.

5 is a flowchart illustrating an operation method for a case in which terminals sense a synchronization signal transmitted from another terminal according to an embodiment of the present invention.

First, the terminal updates the starting point of the frame it has by using the type offset value in the received synchronization signal and the estimated value at the time of reception of the received synchronization signal (S510). As described above, the synchronization signal includes a preamble and a type offset. The terminal may estimate the reception time of the received synchronization signal by using the preamble of the received synchronization signal. Accordingly, the terminal may update the start point of the frame using the type offset value and the estimated timing of receiving the synchronization signal.

When the backoff count value of the terminal is 1, the terminal transmits its own collision detection field (CDF) at a time when the collision detection field (CDF) of the received synchronization signal is located (S520, S530). At this time, the terminal does not transmit other components (preamble, time offset, etc.) of the synchronization signal.

6 is a diagram illustrating a case where the impulse detection fields CDF collide with each other. That is, FIG. 6 shows a case in which terminal A with a backoff counter of zero transmits a synchronization signal, and terminal B with a backoff counter of 1 among neighboring terminals transmits a collision detection field (CDF). At this time, the neighboring terminals excluding the terminal A and the terminal B may determine that a collision has occurred when the two CDFs overlap as described above.

If the backoff count value of the terminal is not 1, the terminal checks whether two collision detection fields (CDF) overlap each other and are received by looking at the collision detection field (CDF) of the received synchronization signal (S540). When two collision detection fields (CDF) overlap each other and are received, the terminal increases its own contention window value (S550).

Meanwhile, the synchronization signal includes a contention window value of the terminal (ie, the transmitting terminal) that transmitted the synchronization signal. The terminal receiving the synchronization signal updates the average contention window value of the neighboring terminals by using the contention window value of the transmitting terminal. The terminal receiving the synchronization signal may update the average contention window value as shown in Equation 2 below.

In Equation 2, CW _other (old) is an average contention window value before the update, and CW _received is a contention window value included in the received synchronization signal. And β is a value representing the ratio and is a constant arbitrarily set.

In step S550, the terminal increases the contention window (CW) value. Hereinafter, a rule for the terminal to increase the contention window (CW) value will be described.

When the terminal has its own contention window value (CW) greater than the _{average contention window value (CW other} ) multiplied by the first ratio (th _{1 ) (th 1} · CW _other <CW, hereinafter referred to as'the first case'), increases the own competition window value (CW) by _rI1 times. Here, the predetermined first ratio th ₁ is a constant value arbitrarily determined by the user.

Terminal is multiplied by a first ratio (th ₁₎ for the second ratio (th ₂₎ value greater than the average contention window value (CW _other), multiplied by their contention window value (CW) is the average contention window value (CW _other) If it is smaller than the value (th ₂ ·CW _other <CW <th ₁ ·CW _other , hereinafter referred to as the'second case'), increase its own competition window value (CW) by r _I2 times. Here, the predetermined second ratio th ₂ is a constant value arbitrarily determined by the user, and the second ratio th ₂ is set lower than the first ratio th _1.

Next, if the terminal's contention window value (CW) is less than the average contention window value (CW _other ) multiplied by the second ratio (th ₂ ) (ie, CW <th ₂ · CW _other , hereinafter 3 case'), increase its own competition window value (CW) by r _I3 times.

_{In the above, r I1} , r _I2 and r _I3 , which are multiples of increasing the contention window value, satisfy the relationship of r _I1 ≤ r _I2 ≤ r _I3. That is, the terminal further increases the contention window value in the second case than in the first case, and further increases the contention window value in the third case than in the second case.

As described above, when the terminals do not receive a synchronization signal in a synchronization slot, their contention window (CW) value is reduced. Hereinafter, a rule for reducing the contention window (CW) value by the terminal will be described.

The terminal has its own contention window value (CW) in the first case ((th ₁ ·CW _other In <CW), reduce its own competition window value (CW) by r _{D1 times.}

The UE decreases its contention window value (CW) by r _D2 times when its contention window value (CW) is the second case (th ₂ · CW _other <CW <th ₁ · CW _{other ).}

Next, when the UE's contention window value (CW) is the third case (CW <th ₁ ·CW _other ), the terminal decreases its contention window value (CW) by r _D3 times.

_{In the above, r D1} , r _D2 and r _D3 , which are multiples for reducing the contention window value, satisfy the relationship of r _D1 ≥r _D2 ≥r _D3. That is, the terminal further reduces the contention window value in the first case than in the second case, and further decreases the contention window value in the second case than in the third case.

Table 1 below summarizes the increase or decrease of the competition window value described above.

Meanwhile, when the terminal detects a synchronization signal transmitted by another terminal in the synchronization slot, the backoff procedure may be stopped until the next synchronization slot. Through this, it is possible to prevent at least two synchronization signals from being received in the same synchronization slot.

7 is a diagram illustrating a terminal according to an embodiment of the present invention.

As shown in FIG. 7, the terminal 700 according to the embodiment of the present invention includes a processor 710, a memory 720, and an RF module 730.

The processor 710 may be configured to implement the procedures, methods, and functions described in FIGS. 1 to 6.

The memory 720 is connected to the processor 710 and stores various information related to the operation of the processor 710.

The RF module 730 is connected to an antenna (not shown) and transmits or receives a radio signal. In addition, the antenna may be implemented as a single antenna or multiple antennas (MIMO antenna).

Although the embodiments of the present invention have been described in detail above, the scope of the present invention is not limited thereto, and various modifications and improvements by those skilled in the art using the basic concept of the present invention defined in the following claims are also provided. It belongs to the scope of rights.

Claims (18)

A method for synchronizing by a first terminal in a distributed wireless communication system,
When receiving a synchronization signal in a synchronization slot, increasing a contention window value of the first terminal to a first contention window value,
If a synchronization signal is not received in the synchronization slot, reducing a contention window value of the first terminal to a second contention window value,
Determining whether to transmit a synchronization signal using the first contention window value or the second contention window value, and
And setting an average contention window value for neighboring terminals of the first terminal,
And the step of reducing, as the contention window value of the first terminal is smaller than a value obtained by multiplying the average contention window value by a predetermined ratio, reducing the contention window value of the first terminal less. The method of claim 1,
The synchronization signal includes a collision detection field through which a tone signal is transmitted,
When receiving the synchronization signal in the synchronization slot, determining whether to transmit the tone signal at a time when a collision detection field of the synchronization signal is located in response to a value of a backoff counter of the first terminal Synchronization method further comprising a. The method of claim 1,
And updating the average contention window value by using the contention window value included in the synchronization signal. The method of claim 1,
The increasing step includes increasing the contention window value of the first terminal by using the average contention window value and the contention window value of the first terminal. delete The method of claim 4,
The increasing step,
And further increasing the contention window value of the first terminal as the contention window value of the first terminal is smaller than a value obtained by multiplying the average contention window value by a predetermined ratio. delete The method of claim 1,
The synchronization signal includes a preamble, a time offset, a collision detection field through which a tone signal is transmitted, and a contention window indication field including a contention window value. A method for synchronizing by a first terminal in a distributed wireless communication system,
Setting a contention window value of the first terminal,
Randomly selecting a predetermined first value from among the contention window values as a backoff count,
Changing the first value according to whether a synchronization signal is received in a synchronization slot,
In response to the first value, determining whether to transmit the synchronization signal of the first terminal, and
And setting an average contention window value for neighboring terminals of the first terminal,
Synchronization method comprising the step of reducing the contention window value of the first terminal less as the contention window value of the first terminal is smaller than a value obtained by multiplying the average contention window value by a predetermined ratio The method of claim 9,
The step of changing the first value includes decreasing the first value when the wireless channel is idle for a predetermined period in the synchronization slot. The method of claim 10,
The determining step includes determining to transmit a synchronization signal of the first terminal when the first value is 0. RF module that receives the synchronization signal, and
And a processor that changes a contention window value according to whether the synchronization signal is received, sets a backoff counter using the contention window value, and determines whether to transmit a synchronization signal in response to the backoff counter, and ,
The processor changes the contention window value by using the average contention window value for neighboring terminals of the terminal and the contention window value,
When the processor does not receive the synchronization signal, the terminal decreases the contention window value less as the contention window value is smaller than a value obtained by multiplying the average contention window value by a predetermined ratio. The method of claim 12,
The processor,
When receiving the synchronization signal, increasing the contention window value,
If the synchronization signal is not received, the terminal decreases the contention window value. The method of claim 12,
The synchronization signal includes a collision detection field through which a tone signal is transmitted,
When receiving the synchronization signal, the processor, in response to the backoff counter, determines whether to transmit the tone signal at a time when a collision detection field of the synchronization signal is located. delete The method of claim 12,
The processor,
When receiving the synchronization signal, the terminal further increases the contention window value as the contention window value is smaller than a value obtained by multiplying the average contention window value by a predetermined ratio. delete The method of claim 12,
The synchronization signal includes a preamble, a time offset, a collision detection field through which a tone signal is transmitted, and a contention window indication field including a contention window value.

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