KR102387854B1 - Method and terminal for synchronization in distributed wireless communication - Google Patents

Abstract

Disclosed are a synchronization method in a distributed wireless communication system and a terminal supporting the same. As a synchronization method, the terminal divides the synchronization period into a plurality of backoff slots and arbitrarily selects a backoff counter using a contention window. In addition, the terminal checks whether the channel is idle during at least one backoff slot among the plurality of backoff slots, and changes the backoff counter according to whether the channel is idle.

Description

A synchronization method in a distributed wireless communication system and a terminal supporting the same

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

A wireless communication system can be broadly classified into a synchronous method and an asynchronous method.

A synchronous wireless communication system refers to a system in which terminals operate according to a common reference time. In a synchronous wireless communication system, since a specific operation is performed at a predetermined time, it is possible to increase the efficiency of use of radio resources and to easily design a system with excellent performance. In addition, power consumption of terminals may be reduced through a power save mode or the like. For example, terminals operating in the power consumption reduction mode may reduce power consumption by operating in the reception mode only for a predetermined time and turning off the receiver during the remaining time. A terminal desiring to transmit data to a terminal operating in the power consumption reduction mode may transmit data or transmit a control message during a predetermined reception mode time to release the power consumption reduction mode.

The synchronous wireless communication system can be further divided into a centralized synchronization method and a distributed synchronization method. A representative example of a centrally controlled synchronization method is a cellular system. In a cellular system, terminals in a cell perform synchronization based on a timing reference signal provided by a base station. In the distributed synchronization method, the synchronization signal is not transmitted by a specific device, but a method in which terminals in a network participate in the transmission of the synchronization signal to synchronize the terminals. This distributed synchronization method is suitable for a device-to-device communications (D2D communications) network or an ad hoc network.

The asynchronous wireless communication system refers to a system in which terminals perform transmission and reception operations without a predetermined reference time. In the case of an asynchronous wireless communication system, terminals always monitor a wireless channel to receive packets from other terminals in the network. When a packet is detected during monitoring, the UEs use a preamble signal included in the packet to estimate the start point of the packet, and then read the packet information. The asynchronous method is simple to implement because there is no set reference time and is mainly used for systems that do not require high resource efficiency. However, since the terminals must always monitor the radio channel, there is a disadvantage in that power consumption is high.

As one of the distributed synchronization methods, there is the following method. Each terminal periodically transmits a synchronization signal using its own time synchronization (ie, a reference time). In addition, each terminal receives the synchronization signal transmitted by the neighboring terminals and then goes out to synchronize its own time using the received synchronization. Through this, all terminals can have a common reference time. In this case, the synchronization signal used includes a pulse signal, a ZC sequence (Zadoff-Chu sequence), an m sequence (m-sequence), a chirp signal, and the like. This method assumes that the terminals transmit the synchronization signal and simultaneously receive the synchronization signal. However, in this method, if the terminals are dense, synchronization signals transmitted by terminals may overlap and be received, which may cause an error in estimating the reception time of the synchronization signal or may make it impossible to estimate the reception time. In addition, when the same radio channel in an unlicensed frequency band is used by a heterogeneous network in the vicinity, there is a problem in that the synchronization signal cannot be transmitted or the transmitted synchronization signal cannot be received due to interference.

An object of the present invention is to provide a distributed synchronization method in a wireless communication system.

According to an embodiment of the present invention, a method for synchronizing a first terminal in a distributed wireless communication system is provided. The synchronization method includes dividing a synchronization period into a plurality of backoff slots, setting a contention window, randomly selecting a backoff counter using the contention window, and at least one of the plurality of backoff slots. checking whether a channel is idle during a backoff slot of , and changing the backoff counter according to whether the channel is idle.

The changing may include decrementing the backoff counter when the channel is idle, and not decrementing the backoff counter when the channel is busy.

The synchronization method may further include transmitting a synchronization signal when the backoff counter is less than a predetermined value.

The selecting may include {0, 1, 2, ... when the contention window is CW. ,CW-1} may include randomly selecting one integer as the backoff counter.

Whether the channel is idle may be checked through Clean Channel Accessment (CCA).

The synchronization method may further include setting a first counter that is a counter different from the backoff counter, and decrementing the first counter when the channel is idle.

The synchronization method may further include updating (updating) the contention window when the first counter is less than a predetermined value.

The synchronization method may further include updating the contention window when the channel is idle for a predetermined time after transmitting the synchronization signal.

The changing may include not decrementing the backoff counter when it is determined that the channel does not complete the synchronization signal transmission during idle or the remaining period of the synchronization period.

The setting of the contention window may include calculating an average reception time difference between first synchronization signals received by the first terminal, and using the first synchronization signals, the average for the first terminal and other terminals Calculating a contention window, and setting the contention window using the average reception time difference and the average contention window.

The synchronization method may further include, when the first terminal receives a first synchronization signal from the second terminal, adjusting a synchronization timer by using the first synchronization signal.

The adjusting of the synchronization timer may include changing the synchronization timer to a predetermined value regardless of the first synchronization signal when the phase of the synchronization timer is between the first value and the second value.

The first synchronization signal may include a backoff indicator indicating a frame start time of the first terminal, and adjusting the synchronization timer includes using a phase value of the synchronization timer corresponding to the backoff indicator. It may include adjusting the phase value of the synchronization timer.

According to another embodiment of the present invention, a method for synchronizing a first terminal in a distributed wireless communication system is provided. The synchronization method includes receiving first synchronization signals from second terminals that are different terminals from the first terminal, calculating an average reception time difference between the synchronization signals, using the first synchronization signals, calculating an average contention window for the second terminals, using the average reception time difference and the average contention window, setting a first contention window that is the contention window of the first terminal, and the first In response to the contention window, the method may include determining whether to transmit a synchronization signal.

The determining may include randomly selecting a backoff counter using the first contention window, and when a channel is idle during a backoff slot in which a synchronization period is divided into a plurality of periods, the backoff counter and, when the backoff counter is zero, transmitting a synchronization signal.

The synchronization method includes the steps of setting the backoff counter to a predetermined value after transmitting the synchronization signal, and updating the first contention window when the set backoff counter is zero The step of (updating) may be further included.

According to another embodiment of the present invention, a terminal is provided. The terminal divides an RF module for transmitting and receiving a synchronization signal, a synchronization period into a plurality of backoff slots, randomly selecting a backoff counter using a contention window, and a first backoff among the plurality of backoff slots. and a processor that decrements the backoff counter when the channel is idle during the off slot.

The processor may transmit the synchronization signal when the backoff counter is zero.

The processor may set the backoff counter to a predetermined value after transmitting the synchronization signal, and update the contention window when the set backoff counter is zero.

The processor calculates an average reception time difference between the received synchronization signals, calculates an average contention window for the terminal and another terminal using the received synchronization signals, and uses the average reception time difference and the average contention window to set the contention window.

According to an embodiment of the present invention, synchronization can be acquired in a distributed manner while reducing signaling overhead between terminals.

According to another embodiment of the present invention, a network can be configured in a distributed synchronous manner not only in a single-hop wireless environment but also in a multi-hop wireless environment.

According to another embodiment of the present invention, since the terminal is synchronized with the neighboring terminals, it is possible to obtain an improvement in the efficiency of radio resource use, a reduction in power consumption through a sleep mode, and an improvement in network performance.

)의 관계를 나타내는 도면이다.
도 18은 본 발명의 실시예에 따른 단말이 동기 신호를 송수신하면서 동기를 맞추는 방법을 나타내는 도면이다.
도 19는 본 발명의 실시예에 따른 동기 타이머 갱신 방법을 나타내는 도면이다.
도 20은 본 발명의 다른 실시예에 따른 동기 타이머 갱신 방법을 나타내는 도면이다.
도 21은 본 발명의 실시예에 따른, 랜덤 백오프로 인해 동기 신호가 프레임의 시작 시점으로부터 시간 지연이 되는 경우, 동기 타이머의 위상을 갱신하는 방법을 나타내는 도면이다.
도 22는 본 발명의 실시예에 따른 초기 동기화 절차를 나타낸다.
도 23은 본 발명의 실시예에 따른 단말을 나타내는 도면이다. 1 is a diagram illustrating a time resource according to an embodiment of the present invention.
2 is a diagram illustrating a case in which three terminals (terminal A, terminal B, and terminal C) are out of synchronization with each other, and FIG. 3 is a case in which three terminals (terminal A, terminal B, terminal C) are synchronized with each other. It is a drawing showing
4 is a flowchart illustrating a synchronization method according to an embodiment of the present invention.
5 is a diagram illustrating a configuration of a synchronization signal according to an embodiment of the present invention.
6 is a diagram illustrating the structure of a synchronization section 110 according to an embodiment of the present invention.
7 is a diagram illustrating an example of a position where a synchronization signal is transmitted.
8 is a diagram illustrating a case in which transmission of a synchronization signal is not completed within a synchronization period.
9A and 9B are flowcharts illustrating a random access scheme according to an embodiment of the present invention.
10 is a flowchart illustrating a random access scheme according to another embodiment of the present invention.
11 is a flowchart illustrating a case where T=0 in the random access method of FIG. 9 .
12 is a flowchart illustrating a case where T=0 in the random access method of FIG. 10 .
13 is a diagram illustrating transmission of a synchronization signal on a time axis when the random access method of FIG. 9 is applied.
14 is a diagram illustrating transmission of a synchronization signal on a time axis when the random access method of FIG. 11 is applied.
15 is a diagram illustrating transmission of a synchronization signal on a time axis when the random access method of FIG. 12 is applied.
16 is a diagram illustrating that UE A transmits a synchronization signal while performing CCA at a discontinuous point in a synchronization period when the random access method of FIG. 12 is used.
17 shows the phase of the frame and the synchronous timer (

) is a diagram showing the relationship between
18 is a diagram illustrating a method in which a terminal performs synchronization while transmitting and receiving a synchronization signal according to an embodiment of the present invention.
19 is a diagram illustrating a method for updating a synchronous timer according to an embodiment of the present invention.
20 is a diagram illustrating a method for updating a synchronous timer according to another embodiment of the present invention.
21 is a diagram illustrating a method of updating a phase of a synchronization timer when a synchronization signal is delayed in time from a start time of a frame due to a random backoff according to an embodiment of the present invention.
22 shows an initial synchronization procedure according to an embodiment of the present invention.
23 is a diagram illustrating a terminal according to an embodiment of the present invention.

Hereinafter, with reference to the accompanying drawings, the embodiments of the present invention will be described in detail so that those of ordinary skill in the art to which the present invention pertains can easily implement them. However, the present invention may be embodied in several different forms and is not limited to the embodiments described herein. And in order to clearly explain the present invention in the drawings, parts irrelevant to the description are omitted, 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) ), a subscriber station (subscriber station, SS), a portable subscriber station (PSS), an access terminal (AT), user equipment (UE), and the like, and may refer to a terminal, MT, It may include all or some functions of AMS, HR-MS, SS, PSS, AT, UE, and the like.

In addition, the base station (base station, BS) is an advanced base station (advanced base station, ABS), a high reliability base station (high reliability base station, HR-BS), a Node B (node B), an advanced node B (evolved node B, eNodeB), an access point (AP), a radio access station (RAS), a base transceiver station (BTS), a mobile multihop relay (MMR)-BS, a relay serving as a base station station, RS), may refer to a high reliability relay station (HR-RS) that serves as a base station, etc., and may refer to 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 (that is, 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 those skilled in the art can understand the present invention. And the distributed synchronization method described below will be described based on the operation of the terminal.

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 an embodiment of the present invention includes a frame 100 that 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 is subdivided into a plurality of sections, and only a synchronization period 110 is shown among the plurality of sections in FIG. 1 . That is, FIG. 1 shows that a time resource is divided into a frame 100 , and the frame 100 is further subdivided into a synchronization period 110 and the remaining period 120 . The remaining slots 110 may be further divided into a data section, a search section, and the like, but is not shown in FIG. 1 for convenience. The synchronization period 110 may be located anywhere in the frame, and for convenience, in FIG. 1 , the synchronization period 110 is shown as being located at the front of the frame. A guard period may be positioned between sections constituting the frame 100 .

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

2 is a diagram illustrating a case in which three terminals (terminal A, terminal B, and terminal C) are out of synchronization with each other, and FIG. 3 is a case in which three terminals (terminal A, terminal B, terminal C) are synchronized with each other. It is a drawing showing

As shown in FIG. 2 , the frame start times of the terminal A, terminal B, and terminal C do not coincide with each other.

The purpose of synchronization is to synchronize terminals that are synchronized with each other. When the frame start time of the two terminals is the same, the two terminals are said to be "synchronized". In addition, in a situation in which a plurality of terminals exist in a predetermined network, when a plurality of terminals in the network are in synchronization, the network is said to be "synchronized".

As shown in FIG. 3 , the frame start times of the terminal A, terminal B, and terminal C coincide with each other. In an actual network, it may be impossible to accurately synchronize the terminals. Therefore, it can be said that synchronization is correct even when the difference between the frame start timings between the terminals is smaller than the error range.

4 is a flowchart illustrating a synchronization method according to an embodiment of the present invention.

As shown in FIG. 4, the synchronization method according to the embodiment of the present invention includes an initial synchronization procedure (S410), a maintaining synchronization procedure (S420), and a re-synchronization procedure (S430, re-synchronization procedure). ) is included.

The non-synchronized terminal synchronizes according to the initial synchronization procedure (S410). For example, when the terminal is powered on, the terminal synchronizes according to the initialization synchronization procedure. The terminal that has successfully performed the initial synchronization performs a synchronization maintenance and management procedure (S420). Then, when it is determined that the terminal, which has been performing the synchronization maintenance and management procedure, has lost synchronization, a resynchronization procedure (S430) or an initial synchronization procedure (S410) is performed.

The initial synchronization procedure ( S410 ), the synchronization maintenance and management procedure ( S420 ), and the resynchronization procedure ( S430 ) are composed of a combination of three operations: transmission of a synchronization signal, reception of a synchronization signal, and updating of a synchronization time.

On the other hand, a synchronization signal (timing reference signal) is a signal transmitted by terminals to synchronize with each other.

5 is a diagram illustrating a configuration of a synchronization signal according to an embodiment of the present invention.

5, the synchronization signal according to the embodiment of the present invention includes a preamble field, a backoff indicator field, and a contention window (CW) indicator field. Here, the competition window indicator field is an optional field. Meanwhile, the synchronization signal may include fields other than the field shown in FIG. 5 .

The preamble field may be used for detection of a synchronization signal, automatic gain control (AGC) of the reception unit, estimation of a reception time of a synchronization signal, and estimation of a radio channel when the synchronization signal arrives at the receiving unit of the terminal. This preamble is located at the beginning of the synchronization signal.

The backoff indicator field is a field indicating a difference between the start point of the synchronization period 110 and the time point at which the synchronization signal is transmitted. As shown in FIG. 1 , when the synchronization period 110 is located at the front of the frame 100 , the start point of the synchronization period and the start time of the frame are the same. Meanwhile, the backoff indicator field is located after the preamble field.

UEs do not transmit synchronization signals in the synchronization period of all frames. If all terminals transmit a synchronization signal in the synchronization period of every frame, in a dense area of terminals, there are too many synchronization signals transmitted in the synchronization period, and collisions between the synchronization signals may occur. Therefore, when the terminals are dense, the probability of the terminals transmitting the synchronization signal in the synchronization period is small, and when the terminals are not dense, the probability of the terminals transmitting the synchronization signal in the synchronization period is increased, so that the frequency of the synchronization signal is constant. can keep The contention window (CW) indicator field includes a contention window value of the terminal that has transmitted the synchronization signal. For example, a method similar to carrier sense multiple access with collision avoidance (CSMA/CA) used in a distributed coordination function (DCF) of IEEE 802.11 may be used as a method of determining the synchronization signal transmission probability of the terminal. When this method is used, the contention window indicator field may include a contention window size value. Meanwhile, the synchronization signal may not include the contention window indicator field. When the contention window field exists, the contention window indicator field is positioned after the preamble field. In addition, the contention window indicator field may be positioned after or in front of the backoff indicator field.

6 is a diagram illustrating the structure of a synchronization section 110 according to an embodiment of the present invention.

As shown in FIG. 6 , the synchronization period 110 according to the embodiment of the present invention includes N backoff slots 111 . The size of the backoff slot may be less than or equal to the size of the synchronization signal.

In order to reduce the probability of collision with a synchronization signal transmitted from another terminal, a random access method is used for transmission of the synchronization signal. Transmission of the synchronization signal is started at the beginning of the backoff slot.

7 is a diagram illustrating an example of a position where a synchronization signal is transmitted.

7 shows a case in which a synchronization signal is transmitted at the beginning of a 9 ^th backoff slot. As shown in FIG. 7 , the synchronization signal 112 is transmitted at the beginning of the ninth backoff slot. Meanwhile, in FIG. 7 , it is assumed that the length of the synchronization signal is equal to the length of three backoff slots.

8 is a diagram illustrating a case in which transmission of a synchronization signal is not completed within a synchronization period.

The transmission of the synchronization signal needs to be completed before the synchronization period 110 ends. As shown in FIG. 8 , when the transmission of the synchronization signal is completed after the synchronization period 110 ends, the synchronization signal is not transmitted. Meanwhile, as described below, when the synchronization signal is not transmitted because the transmission of the synchronization signal is not completed, the backoff counter is not decremented even when the corresponding backoff slot is idle.

A random access method is used for transmission of a synchronization signal according to an embodiment of the present invention, and the rag-dom access method is a contention-based random access method using a contention window (CW). Hereinafter, this will be described in detail.

9A and 9B are flowcharts illustrating a random access scheme according to an embodiment of the present invention. A random access scheme according to an embodiment of the present invention is a random backoff procedure using a contention window (CW).

First, terminals store their contention window size values. Here, the size of the contention window CW is initialized when the random backoff procedure is started (S900).

The terminals select a backoff counter value based on the current contention window size (S910). For example, if the contention window size of the terminal is W _C , the terminal is {0, 1, 2, ... , W _C -1} and randomly selects one of the integers. When the integer n is selected, the terminal sets its own backoff counter value to n.

If n=0, the corresponding terminal transmits a synchronization signal in the next backoff slot (S920, S960). If the value of n is not zero, the following step S930 proceeds.

The terminal having performed the transmission of the synchronization signal performs clear channel assessment (CCA) during the next backoff slot (S961). A method for the UE to perform CCA can be known to those of ordinary skill in the art to which the present invention pertains, so a detailed description thereof will be omitted.

The terminal checks whether the corresponding backoff slot is idle by performing CCA (S962). If the corresponding backoff slot is not idle, the UE repeats the CCA during the next backoff slot until the CCA result becomes idle (S962, S961). And, when the corresponding backoff slot is idle, the terminal additionally performs CCA for a predetermined time (T) (S962, S963). Here, the predetermined time T may be T=0 or T>0.

If T>0 in step S963 and the result of performing CCA is not idle, step S961 is performed (S964, S961). And, when the result of performing CCA in step S963 is idle or T=0, the UE may or may not decrease its backoff counter value by 1 (S964, S965). That is, step S965 is optional.

The terminal that has performed step S965 updates (updates) its contention window size and moves to step S910 (S970, S910).

Meanwhile, if n>0 in step S920, the terminal performs CCA during the next backoff slot (S930).

If the corresponding backoff slot is idle as a result of performing CCA in step S930, the terminal decrements the backoff counter value by 1 (S940 and S950). And the terminal moves to the step S920. As a result of performing CCA in step S930, if the corresponding backoff slot is not idle, the UE performs CCA again in the next backoff slot (S940, S980).

As a result of performing CCA in step S980, if the corresponding backoff slot is not idle, the UE repeats CCA during the next backoff slot until the CCA result becomes idle (S990, S980). And, if the corresponding backoff slot is idle as a result of performing CCA in step S980, the terminal additionally performs CCA for a predetermined time T (S990, S992). Here, the predetermined time T may be T=0 or T>0.

When T>0 in step S992 and the result of performing CCA is not idle, step S980 is performed (S994, S980). In addition, when the result of performing CCA in step S992 is idle or T=0, the UE may or may not decrease its backoff counter value by 1 (S994, S996). That is, step S996 is optional. The terminal having performed step S996 moves to step S920.

10 is a flowchart illustrating a random access scheme according to another embodiment of the present invention. The random access method according to another embodiment of the present invention is a procedure in which fairness between terminals is improved compared to FIG. 9 . In other words, the random access method according to another embodiment of the present invention resets its counter to the Wc-n slot after the terminal transmits a synchronization signal, and updates the contention window size when the reset counter becomes 0.

The terminals store their contention window size values. Here, the size of the contention window CW is initialized when the random backoff procedure is started (S1000).

The terminals select a backoff counter value based on the current contention window (CW) size (S1010). For example, if the contention window size of the terminal is W _C , the terminal is {0, 1, 2, ... , W _C -1} and randomly selects one of the integers. When the integer n is selected, the terminal sets its own backoff counter value to n. And, N is set to the contention window size (W _c ). Here, N is a counter for counting the number of backoff counts after updating the backoff counter value to an arbitrary integer.

If n=0, the corresponding terminal transmits a synchronization signal in the next backoff slot (S1020, S1070). If the value of n is not zero, the following step S1030 proceeds.

The terminal having performed the transmission of the synchronization signal performs clear channel assessment (CCA) during the next backoff slot (S1080).

The terminal checks whether the corresponding backoff slot is idle by performing CCA (S1090). When the corresponding backoff slot is idle, the terminal additionally performs CCA for a predetermined time (T) (S1090, S1092). Here, the predetermined time T may be T=0 or T>0. And, if the corresponding backoff slot is not idle, the terminal moves to step S1080.

When T>0 in step S1092 and the result of performing CCA is not idle, step S1080 is performed (S1094, S1080). And, when the result of performing CCA in step S1092 is idle or T=0, the terminal may or may not decrease its backoff counter value and N by 1, respectively (S1094, S1096). That is, step S1096 is optional.

If N=0, the terminal updates (updates) the size of its contention window, and then moves to step S1010 (S1060, S1062). And when the value of N is not 0, the process moves to step S1020 (S1060, S1020).

If n>0 in step S1020, the UE performs CCA during the next backoff slot (S1030).

If the corresponding backoff slot is idle as a result of performing CCA in step S1020, the terminal decrements the backoff counter value and the N value by 1, respectively (S1040 and S1050). And the terminal moves to the step S1060. As a result of performing CCA in step S1030, if the corresponding backoff slot is not idle, the terminal moves to step S1080.

11 is a flowchart illustrating a case where T=0 in the random access method of FIG. 9 . That is, FIG. 11 simplifies the flowchart by using T=0 and optional steps S965 and S996 in FIG. 9 . Therefore, since the random access method of FIG. 11 is the same as that of FIG. 9, detailed descriptions thereof will be omitted.

12 is a flowchart illustrating a case where T=0 in the random access method of FIG. 10 . That is, FIG. 12 is a simplified flowchart of T=0 in FIG. 10 and an optional step S1096. Therefore, since the random access method of FIG. 12 is the same as that of FIG. 10, a detailed description thereof will be omitted.

13 is a diagram illustrating transmission of a synchronization signal on a time axis when the random access method of FIG. 9 is applied. More specifically, FIG. 13 shows synchronization signals of terminals A, B, and C when T is equal to the length of one backoff slot in the random access scheme of FIG. 9 and the optional steps S965 and S996 are not used. Transmission is represented on the time axis. In FIG. 13 , it is assumed that the length of the synchronization signal is twice that of the backoff slot.

At time t ₁ , it is assumed that terminals A, B, and C have 9, 7, and 3 as backoff counter values, respectively.

At time t ₂ , the backoff counter value of terminal C becomes zero, and terminal C starts transmitting a synchronization signal. In this case, the backoff counter values of terminal A and terminal B are 6 and 4, respectively. That is, since the backoff slot is idle between t1 and t2, _the backoff counter values of terminals _A , B, and C gradually decrease by S920, S930, S940, and S950 of FIG. 9 .

At time t ₃ , UE C completes the synchronization signal transmission.

At time t ₄ , which is after a time corresponding to (one backoff slot + T), UE C arbitrarily sets a backoff counter value. In FIG. 13 , it is assumed that the terminal C sets the backoff counter value to 8. That is, by steps S961, S962, S963, S964, S970, and S910 of FIG. 9 , the terminal C updates the backoff counter value to 8. In this case, the terminal A and the terminal B maintain the backoff counter values of 6 and 4, respectively, by steps S980, S990, S992, and S994 of FIG. 9 .

At time t ₅ , the backoff counter value of UE B becomes zero, and UE B starts transmitting a synchronization signal.

At time t ₆ , 4 is set as the backoff counter value arbitrarily selected by UE B. In this case, the backoff counter values of the terminal A and the terminal C are 2 and 4, respectively.

At time t ₇ , the backoff counter value of terminal A becomes zero, and terminal A starts transmitting a synchronization signal.

And at time t ₈ , 8 is determined as a backoff counter value arbitrarily selected by UE A. In this case, the backoff counter values of the terminal B and the terminal C are 2 and 2, respectively.

At time t ₉ , both the backoff counter values of the terminal B and the terminal C become zero, and the terminal B and the terminal C simultaneously start transmitting a synchronization signal. That is, the synchronization signals of the terminal B and the terminal C collide with each other.

At time t ₁₀ , terminal B sets 5 as a randomly selected backoff counter value, and terminal C sets 0 as a randomly selected backoff counter value. In this case, since the value of the backoff counter of the terminal C is zero, the terminal C starts transmitting the synchronization signal at time t ₁₀ . And, since the CCA result is not idle by the synchronization signal of the terminal C, the terminal B does not decrease the backoff counter value.

And at time t ₁₁ , terminal C determines 9 as a randomly selected backoff counter value. In this case, the backoff counter values of terminal A and terminal B are 6 and 5, respectively.

14 is a diagram illustrating transmission of a synchronization signal on a time axis when the random access method of FIG. 11 is applied. That is, FIG. 14 shows synchronization signal transmission of terminals A, B, and C on a time axis when T=0 and optional steps S965 and S996 are used in the random access method of FIG. 9 . In FIG. 14 , it is assumed that the length of the synchronization signal is twice that of the backoff slot.

At time t ₁ , it is assumed that terminals A, B, and C have 9, 7, and 3 as backoff counter values, respectively.

At time t ₂ , the backoff counter value of terminal C becomes zero, and terminal C starts transmitting a synchronization signal. In this case, the backoff counter values of terminal A and terminal B are 6 and 4, respectively. That is, since the backoff slot is idle between t1 and t2, _the backoff counter values of terminals _A , B, and C gradually decrease by S1120, S1130, S1140, and S1150 of FIG. 11 .

At time t ₃ , UE C completes the synchronization signal transmission.

Since T=0, at time t ₄ after a time corresponding to one backoff slot, UE C arbitrarily selects a backoff counter value. In FIG. 14 , it is assumed that the terminal C sets the backoff counter value to 8. That is, by the steps S11611, S1162, S1165, S1170, and S1110 of FIG. 11 , the terminal C arbitrarily selects a backoff counter value of 8. At this time, terminal A and terminal B decrease the backoff counter values to 5 and 3, respectively, by steps S1120, S1130, S1140, and S1150 of FIG. 11 .

At time t ₅ , the backoff counter value of UE B becomes zero, and UE B starts transmitting a synchronization signal.

At time t ₆ , 4 is set as the backoff counter value arbitrarily selected by UE B. At this time, the backoff counter values of terminal A and terminal C are decreased to 1 and 4, respectively.

At time t ₇ , the backoff counter value of terminal A becomes zero, and terminal A starts transmitting a synchronization signal.

And at time t ₈ , 8 is determined as a backoff counter value arbitrarily selected by UE A. At this time, the backoff counter values of the terminal B and terminal C are decreased to 2 and 2, respectively.

At time t ₉ , both the backoff counter values of the terminal B and the terminal C become zero, and the terminal B and the terminal C simultaneously start transmitting a synchronization signal. That is, the synchronization signals of the terminal B and the terminal C collide with each other.

At time t ₁₀ , terminal B sets 6 as a randomly selected backoff counter value, and terminal C sets 0 as a randomly selected backoff counter value. And at this time, since the value of the backoff counter of the terminal C is zero, the terminal C starts transmission of the synchronization signal at time t ₁₀ . And, the terminal B detects that the CCA result is not idle by the synchronization signal of the terminal C.

At time t ₁₁ , terminal C sets 9 as a randomly selected backoff counter value. At this time, the backoff counter values of the terminal A and the terminal B are decreased to 4 and 3, respectively.

At time t ₁₂ , the backoff counter value of terminal A becomes zero, and terminal A starts transmitting a synchronization signal.

At time t ₁₃ , terminal A sets 7 as a randomly selected backoff counter value. At this time, the backoff counter values of the terminal B and the terminal C decrease to 0 and 4, respectively. Since the value of the backoff counter of the terminal B is zero, the terminal B starts transmitting the synchronization signal at t ₁₃ .

15 is a diagram illustrating transmission of a synchronization signal on a time axis when the random access method of FIG. 12 is applied. That is, FIG. 15 shows synchronization signal transmission of terminals A, B, and C on a time axis when T=0 and an optional step S1096 is used in the random access method of FIG. 10 . In FIG. 15, it is assumed that the length of the synchronization signal is twice that of the backoff slot, and the contention window size is assumed to be 16. Terminals may have different contention window sizes (contention window values) and the contention window size may be changed according to the surrounding environment. assumed that

At time t ₁ , it is assumed that terminals A, B, and C have (9, 12), (7, 13), and (3, 7) as (n, N) values, respectively. Here, n is the backoff counter value, and N is a counter for counting the number of backoffs after updating the backoff counter value to an arbitrary integer.

At time t ₂ , the backoff counter value of terminal C becomes zero, and terminal C starts transmitting a synchronization signal. In this case, terminal C has N=4, and terminal A and terminal B have (n, N)=(6, 9) and (n, N)=(4, 10), respectively. That is, since the backoff slot is idle between t ₁ and t ₂ , (n, N) of terminals A, B, and C gradually decreases by S1220, S1230, S1240, S1250, and S1260 of FIG. 12 .

At time t ₃ , UE C completes the synchronization signal transmission.

Since T=0, at time t ₄ after a time corresponding to one backoff slot, terminals A, B, and C each have (n, N)=(5, 8), (n, N)=(3, 9 ), and (n, N)=(*, 3). That is, terminal A and terminal B reduce (n, N) to (5, 8) and (3, 9) by S1220, S1230, S1240, S1250, and S1260, respectively. Then, the terminal C sets (n, N) to (*, 3) by S1230, S1240, and S1250. Here, the meaning of "*" means that the value of n may have any value except for n=0. That is, at time t ₄ , the terminal C may perform a process of decreasing only the counter N value without setting the backoff counter value to an arbitrary integer value.

At time t ₅ , the backoff counter value of UE B becomes zero, and UE B starts transmitting a synchronization signal. At this time, terminal B has N=6. And, at time t ₅ , the value of N of terminal C becomes zero, and accordingly, terminal C updates the contention window size (refer to S1260, S1262, and S1210). Terminal C arbitrarily sets the value of the backoff counter to 8 using the updated contention window size (contention window value). 15, it is assumed that the updated contention window size is 16. On the other hand, terminal A has (n, N) = (2, 5).

At time t ₆ , terminals A, B, and C set (n, N)=(1, 4), (n, N)=(*, 5), and (n, N)=(7, 15), respectively Have.

At time t ₇ , the backoff counter value of terminal A becomes zero, and terminal A starts transmitting a synchronization signal.

At time t ₈ , terminals A, B, and C set (n, N)=(*, 2), (n, N)=(*, 3), and (n, N)=(5, 13), respectively Have.

At time t ₉ , terminal A has an N value of zero, and terminal A updates (updates) the contention window size. Then, the terminal A arbitrarily sets the backoff counter value to 6 using the updated contention window size.

At time t ₁₀ , terminal B has an N value of zero, and terminal B updates the contention window size (contention window value). Then, the terminal B arbitrarily sets the backoff counter value to 7 using the updated contention window size.

At time t ₁₁ , the value of the backoff counter of the terminal C becomes zero, and the terminal C starts transmitting the synchronization signal.

At time t ₁₂ , terminals A, B, and C set (n, N)=(2, 12), (n, N)=(4, 13), and (n, N)=(*, 7), respectively Have.

At time t ₁₃ , the value of the backoff counter of terminal A becomes 0, so that terminal A starts transmitting a synchronization signal.

At time t ₁₄ , terminals A, B, and C set (n, N)=(*, 9), (n, N)=(1, 10), and (n, N)=(*, 4), respectively Have.

At time t15, the value of the backoff counter of the terminal B becomes zero, and the terminal B starts transmitting the synchronization signal.

13, 14, and 15 illustrate a method in which terminals transmit a synchronization signal using a random access method within a synchronization period. However, discontinuity occurs on the time axis because the synchronization period repeatedly exists at a specific position of the frame.

16 is a diagram illustrating that UE A transmits a synchronization signal while performing CCA at a discontinuous point in a synchronization period when the random access method of FIG. 12 is used. In FIG. 16 , it is assumed that the size of the contention window CW is always 8 after being updated for convenience of explanation, but the size of the contention window may be changed along with the update.

At time t ₁ , it is assumed that terminal A has (4, 6) as a (n, N) value.

At time t ₂ , UE A detects channel busy through CCA, and UE A stops decreasing n and N.

When the channel becomes idle at time t ₃ , UE A starts decreasing n and N again at time t4 .

At time t ₅ , the backoff counter value of terminal A becomes zero, and terminal A starts transmitting a synchronization signal.

At time t ₆ , the terminal A has 1 as a value of N, and restarts the decrease of N, which was stopped with transmission of the synchronization signal. In addition, UE A does not set the backoff counter value to an arbitrary integer value (ie, n=*) and performs a process of decreasing only the counter N value.

At time t ₇ , the value of N of terminal A becomes zero, and terminal A updates the size of the contention window. Then, the terminal A arbitrarily sets the backoff counter value to 3 using the updated contention window size (8). Also, at t ₇ , UE A detects channel busy through CCA, and UE A stops decreasing n and N.

At t ₈ , which is the time when one backoff slot has elapsed as the channel is idle, UE A may decrease the values of n and N again, but transmission of the synchronization signal is completed within the remaining time of the synchronization period. can't be Accordingly, terminal A does not start decreasing n and N at t8.

Time t ₉ is the start point of the synchronization period of the next frame, and at t ₉ , UE A has values of n=2 and N=7. And terminal A starts to decrease n and N again.

At time t ₁₀ , the value of the backoff counter of terminal A becomes zero, and the terminal A starts transmitting a synchronization signal.

At time t11, terminal A has 4 as a value of N, and restarts the decrease of N, which was stopped along with transmission of a synchronization signal. In addition, UE A does not set the backoff counter value to an arbitrary integer value (ie, n=*) and performs a process of decreasing only the counter N value.

At time t ₁₂ , the value of N of terminal A becomes zero, and terminal A updates the size of the contention window. Terminal A arbitrarily sets the backoff counter value to 4 using the updated contention window size (8).

At time t ₁₃ , UE A detects channel busy through CCA and stops decreasing n and N.

Time t ₁₄ is the start point of the synchronization period of the next frame, and at t ₁₄ , UE A has values of n=0 and N=4. Since terminal A is n=0, terminal A starts transmitting a synchronization signal.

At time t ₁₅ , terminal A has a value of N=3, and terminal A restarts the stopped decrement of N with transmission of a synchronization signal.

Meanwhile, the contention window size described above (which is used interchangeably with 'contention window' and 'contention window value' in this specification) may be determined by reflecting the number of peripheral terminals. In order to reduce the probability of collision of the synchronization signal, when there are many terminals in the vicinity, the terminals lower the transmission frequency of the synchronization signal, and when there are not many terminals in the vicinity, the terminals increase the transmission frequency of the synchronization signal. However, it is not easy to determine the number of terminals in the vicinity, and even if the number of terminals is determined, overhead may be large. Therefore, the embodiment of the present invention described below does not directly use the number of neighboring terminals to determine the contention window value. That is, the embodiment of the present invention uses a method of maintaining a constant transmission frequency of a synchronization signal regardless of the number of terminals in a specific area.

A method of maintaining a constant frequency of a synchronization signal transmitted in a specific region is a reception time difference (hereinafter, 'inter-arrival time (IAT)') that is a difference between a time at which a predetermined synchronization signal is received and a time at which the next synchronization signal is received. method) is used. For example, the terminal predetermines the target reception time difference (hereinafter referred to as 'target inter-arrival time (TIAT)') and then the reception time difference (hereinafter referred to as 'MIAT ( measured inter-arrival time)). The UE updates the contention window value in a direction in which the difference between the TIAT value and the MIAT value decreases. Meanwhile, instead of the reception time difference, an idle time, which is a length of time during which the channel is idle from when a predetermined synchronization signal is received until the next synchronization signal is received, may be used instead. At this time, a value obtained by subtracting the length of the synchronization signal from the reception time difference is equal to the idle time. The present invention will be described using the case of using the IAT.

The UEs calculate the MIAT of the region in which they are located by using the IAT measured for a certain period of time. For example, after determining a time window of size W _A , UEs may calculate MIAT as an average value of IATs measured during W _A from the past to the present. As another method, the UE may use a method of updating the MIAT using Equation 1 below whenever they receive a new synchronization signal.

이며, IAT는 마지막에 수신된 동기 신호를 이용한 측정한 수신 시간차를 나타낸다. In Equation 1,

, and IAT indicates a reception time difference measured using the last received synchronization signal.

Meanwhile, the terminals may determine the increase or decrease of their contention window value by using a reception pattern of the received synchronization signals. However, since the reception patterns of synchronization signals for terminals located close to each other are very similar, when the values of contention windows of two terminals are significantly different, the difference value may not be narrowed. In this case, the synchronization signal transmission rates of two terminals located in close proximity may be significantly different. To solve this problem, each terminal estimates the average contention window value (hereinafter referred to as 'CW _oth ') of other nearby terminals, and then uses the estimated CW _oth to update its contention window value. A method for the UE to update its contention window value using the estimated CW _oth is described in Table 1 below.

As described in FIG. 5, the synchronization signal may include a contention window value of the terminal that transmitted the synchronization signal. The terminal receiving the synchronization signal estimates the average contention window value (CW _oth ) of other neighboring terminals by using the contention window value included in the received synchronization signal. For example, the UE may calculate the CW _oth value by averaging the CW received during _WB from the past to the present after determining the time window W _B . As another method, whenever a new synchronization signal is received, the terminal may use a method of updating the CW _oth value using Equation 2 below.

이며, CW는 마지막으로 수신된 동기 신호에 포함된 경쟁 윈도우 값을 나타낸다. In Equation 2,

, and CW indicates a contention window value included in the last received synchronization signal.

As another method of updating CW _oth , it may be updated using Equation 3 below.

이고

이며,

와

는 각각

와

를 만족한다. 그리고 CW는 마지막으로 수신된 동기 신호에 포함된 경쟁 윈도우 값을 나타낸다. In Equation 3,

ego

is,

Wow

is each

Wow

is satisfied with And CW indicates a contention window value included in the last received synchronization signal.

(n=1, 2, 3,….16)은 경쟁 윈도우 값을 갱신할 시 곱해주는 상수이다.

은 갱신 후 경쟁 윈도우의 값이 커져야 할 경우에는 1보다 큰 값을 가지며 갱신 후 경쟁 윈도우의 값이 작아져야 할 경우에는 1보다 작은 값을 갖는다. 만약 단말이 주변 단말의 평균 경쟁 윈도 값의 추정 값(CWoth)을 모르는 경우,

, 그리고

이 만족된다. Table 1 below shows a rule for the UE to update the value of its own contention window when the UE knows the estimated value (CW _oth ) of the average contention window value of the neighboring UE. In Table 1, C ₁ , C ₂ , D ₁ , and D ₂ are real numbers greater than 1.

(n=1, 2, 3,….16) is a constant multiplied when the contention window value is updated.

has a value greater than 1 when the value of the contention window is to be increased after update, and has a value less than 1 when the value of the contention window is to be decreased after update. If the terminal does not know the estimated value (CWoth) of the average contention window value of the neighboring terminals,

, And

This is satisfied.

이고

,

,

,

,

인 경우에 간단하게 정리한 표이다. Table 2 below is from Table 1

ego

,

,

,

,

In the case of , it is a simplified table.

Upon receiving the synchronization signal, the terminal may perform the following three operations. The three operations are the update of the MIAT, the update of the CW _oth value, and its own timing. Here, the timing represents the update of the phase value of the synchronous timer. The update of the MIAT and the update of the CW _oth value have been described above, and the timing will be described below. A method for the terminal to update its own timing uses the reception time of the received synchronization signal.

)이 360도가 되는 순간이 프레임의 끝 또는 시작 시점이다. 도 17은 프레임과 동기 타이머의 위상(

)의 관계를 나타내는 도면이다. 도 11에 나타낸 바와 같이, 동기 타이머의 위상이 360도가 되는 순간이 프레임의 끝 또는 시작 점이 된다. The terminal has a synchronous timer, and the phase (

The moment when ) becomes 360 degrees is the end or start point of the frame. 17 shows the phase of the frame and the synchronous timer (

) is a diagram showing the relationship between As shown in Fig. 11, the moment when the phase of the synchronous timer reaches 360 degrees is the end or start point of the frame.

18 is a diagram illustrating a method in which a terminal performs synchronization while transmitting and receiving a synchronization signal according to an embodiment of the present invention. In FIG. 18 , it is assumed that terminal B is at a distance in which communication is possible with terminal A and terminal C, and that terminal A and terminal C are in a distance where communication is impossible.

As shown in FIG. 18 , terminals A, B, and C are out of synchronization at time 0.

At time t ₁ , the phase of the synchronization timer of terminal A becomes 360 degrees and a synchronization signal is transmitted. In this case, after receiving the synchronization signal transmitted by the terminal A, the terminal B updates its own synchronization timer to match the synchronization timer of the terminal A. However, since the terminal C does not receive the synchronization signal of the terminal A, it does not update its synchronization timer.

At time t ₂ , the phase of the synchronization timer of terminal B becomes 360 degrees and a synchronization signal is transmitted. At this time, after receiving the synchronization signal transmitted by the terminal B, the terminal C updates its own synchronization timer to match the synchronization timer of the terminal B. At this time, although the terminal A receives the synchronization signal transmitted by the terminal B, the synchronization timer is not updated because the synchronization with the terminal B is already matched.

Meanwhile, as shown in FIG. 18 , terminals do not always transmit a synchronization signal when their synchronization timer phase is 360 degrees. Transmission of the synchronization signal is performed through contention-based random access as described above. Accordingly, the transmission of the synchronization signal may be delayed by an arbitrary backoff slot, rather than being performed at a frame start time when the phase of the synchronization timer becomes 360 degrees. In FIG. 18, for convenience of explanation, it is assumed that the synchronization signal is transmitted at the beginning of the frame without indicating the transmission delay of the synchronization signal due to random backoff. The synchronization signal transmission delay due to random backoff will be described in detail with reference to FIG. 21 below.

를 가지고 있는 단말이 동기 신호를 수신하는 경우 아래에서 설명하는 과정을 통해 동기 타이머 위상 값을

로 갱신한다. 19 is a diagram illustrating a method for updating a synchronous timer according to an embodiment of the present invention. As a synchronous timer phase value

When a terminal having a synchronization signal is received, the synchronization timer phase value is

update with

을 가지고 있는 것을 가정한다. In FIG. 19, when the terminal receives the synchronization signal, it is a timer phase value.

Assume that you have

The terminal obtains x ₁ as in Equation 4 below.

는 미리 정해진 (0,0)과 (1.1)을 지나고 0< x <1에서

이고

인 함수이다. In Equation 4,

is passed through predetermined (0,0) and (1.1) and at 0< x <1

ego

is a function

And the terminal obtains x ₂ as in Equation 5 below.

는 미리 정해진 상수이다. In Equation 5,

is a predetermined constant.

를 구한다. And finally, the terminal as shown in Equation 6 below

save

인 경우

로 갱신하다. 그리고,

의 관계가 성립을 알 수 있다. 즉, 동기 타이머의 위상 값의 갱신은 동기 신호를 수신할 시의 동기 타이머 위상 값에 따라 다르게 결정된다. As shown in FIG. 19, in the same way as above, the terminal has its own synchronization timer phase value.

if

update with And,

relationship can be established. That is, the update of the phase value of the synchronization timer is determined differently according to the phase value of the synchronization timer when the synchronization signal is received.

As a method of updating the phase value of the synchronization timer when the terminal receives the synchronization signal, the following method may be used unlike FIG. 19 . Another method for updating the phase value of the synchronization timer when the terminal receives the synchronization signal is to use the method illustrated in FIG. 20 .

20 is a diagram illustrating a method for updating a synchronous timer according to another embodiment of the present invention.

As shown in FIG. 20 , when the terminal receives a synchronization signal, if the phase value of its synchronization timer is between f _m and f _m+1 , the phase value is updated to f _new,m . For example, when the terminal's phase value is between f ₁ and f ₂ , it is updated to a predetermined value f _new,1 regardless of the value of the received synchronization signal.

As a special case of the method illustrated in FIG. 20, when the terminal receives the synchronization signal, if the phase value of its synchronization timer is less than 180 degrees, the current synchronization timer phase value is maintained as it is, and the terminal receives the synchronization signal If the synchronous timer phase value is greater than 180 degrees, there is a method of updating the synchronous timer phase value by 360 degrees.

21 is a diagram illustrating a method of updating a phase of a synchronization timer when a synchronization signal is delayed in time from a start time of a frame due to a random backoff according to an embodiment of the present invention. In FIG. 21 , it is assumed that terminals A, B, C, and D are all in positions capable of communicating with each other.

At time t ₁ , the synchronous timer phase value of terminal A becomes 360 degrees. Since transmission of a synchronization signal (timing reference signal) is performed according to a random backoff procedure, the synchronization signal transmission of UE A occurs at time t ₂ , which is later than time t ₁ . Here, time t ₂ is located between time t ₁ and time t ₃ .

At time t ₂ , terminal B receives a synchronization signal transmitted by terminal A. In this case, the terminal B may know that the frame start time of the terminal A is t ₁ instead of t ₂ by using the backoff indicator included in the synchronization signal received from the terminal A (see arrow 2010). Terminal B uses its own synchronization timer phase value at time t ₁ to update the phase value of the synchronization timer (see arrow 2020). In this case, the method of FIGS. 19 and 20 may be used as a method of updating the phase value of the synchronization timer. And, as indicated by arrow 2030, terminal B calculates the synchronous timer phase value of the current time t ₂ by using the updated synchronous timer phase value at the time t ₁ . Accordingly, after the phase value of the synchronization timer is updated, the terminal B is synchronized with the terminal A.

At time t ₂ , terminal C receives a synchronization signal transmitted by terminal A. In this case, the terminal C can know that the frame start time of the terminal A is t ₁ rather than t ₂ by using the backoff indicator included in the synchronization signal received from terminal A (see arrow 2040). Terminal C uses its own synchronization timer phase value at time t ₁ to update the phase value of the synchronization timer (refer to arrow 2050). In this case, the method of FIGS. 19 and 20 may be used as a method of updating the phase value of the synchronization timer. And, as indicated by arrow 2060, the terminal C calculates the synchronous timer phase value of the current time t ₂ using the updated synchronous timer phase value at the time t ₁ . Accordingly, after the phase value of the synchronization timer is updated, the terminal C is synchronized with the terminal A.

At time t ₂ , terminal D receives a synchronization signal transmitted by terminal A. In this case, the terminal D can know that the frame start time of the terminal A is t ₁ instead of t ₂ by using the backoff indicator included in the synchronization signal received from terminal A (see arrow 2070). Terminal D updates the phase value of the synchronization timer by using its own synchronization timer phase value at time t ₁ . In this case, the method of FIG. 19 and the method of FIG. 20 may be used as a method of updating the phase value of the synchronization timer. As shown in FIG. 21 , the reception of the synchronization signal may occur at any position within the synchronization period 110 . Accordingly, when the terminal updates the phase of its synchronization timer using the received synchronization signal after receiving the synchronization signal, the remaining time in the synchronization period of the corresponding frame may not be sufficient to transmit the synchronization signal. . In this case, the UE may perform a random access procedure for transmission of a synchronization signal in the next frame without performing the random access procedure for transmission of the synchronization signal in the corresponding frame.

The terminal performs an initial synchronization procedure (S410 of FIG. 4) as part of an initialization procedure. Initialization of the terminal may occur when the power of the terminal is turned on or when all communication is stopped and the terminal is in an inactive state and then goes out to an inactive state. Hereinafter, an initial synchronization procedure according to an embodiment of the present invention will be described with reference to FIG. 22 .

22 shows an initial synchronization procedure according to an embodiment of the present invention.

First, the terminal operates in the reception mode for a predetermined time before transmitting the synchronization signal (S2100). The terminal determines whether there is a synchronization signal transmitted by neighboring terminals while operating in the reception mode (S2110).

When the synchronization signal is detected in step S2110, the terminal determines whether the detected synchronization signal is a synchronization signal transmitted from terminals with matching synchronization (eg, terminals constituting a network) (S2120). For example, when a synchronization signal is periodically received and the start time of a frame is continuously and consistently estimated from the synchronization signal, the terminal may determine that synchronization is transmitted from the terminals.

When the synchronization signal is transmitted from the synchronized terminals in step S2120, the terminal adjusts its synchronization timer phase to the synchronization timer phase included in the received synchronization signal, and through the random access procedure described above, the synchronization signal is transmitted. Participate (S2130). At this time, since the terminal has joined the synchronized terminals, the synchronization maintenance and management procedure S420 of FIG. 4 may be performed.

If the synchronization signal is not detected in step S2110, the terminal determines that there is no terminal transmitting the synchronization signal in the vicinity. At this time, the terminal arbitrarily selects a phase value of its own synchronization timer (S2140). In addition, the UE may start transmission of a synchronization signal using the random access procedure described above and perform the synchronization maintenance and management procedure ( S420 ) of FIG. 4 .

If the synchronization signal is not a synchronization signal transmitted by terminals with synchronization in S2120, the terminal randomly selects its own synchronization timer phase value (S2150). And the terminal performs a procedure for synchronizing with neighboring terminals. For example, if the start time of the frame estimated from the received synchronization signal is inconsistent or the estimated frame start time is plural, the terminal may determine that the detected synchronization signal is transmitted from misaligned terminals. . A method for the UE to arbitrarily select its own synchronization timer phase value may be a method of selecting a random real number or a method of randomly selecting one among phase values of a received synchronization signal.

Meanwhile, the terminal transmits a synchronization signal in a synchronization period according to its synchronization timer using the random access method described above. Except for the case of transmitting the synchronization signal, the terminal receives the synchronization signal transmitted by the neighboring terminals while operating in the reception mode. At this time, since synchronization with the neighboring terminals is not matched, the synchronization signal may be received at any position within the frame as well as the synchronization period. When the synchronization signal is received, the terminal updates the above-described MIAT, updates the CW _oth value, and updates the phase value of its own synchronization timer. For updating the phase value of the synchronization timer, the method of FIG. 19 may be used or another method may be used. The terminal, which has updated the synchronization timer phase value, does not transmit a synchronization signal using random access in the current frame (that is, a frame newly determined by updating the synchronization timer phase), but random access for synchronization signal transmission in the synchronization period of the next frame can be performed. After repeating these processes, the terminal may perform the synchronization maintenance and management procedure ( S420 ) of FIG. 4 when it is determined that synchronization with the neighboring terminals is correct. For example, if a synchronization signal is periodically received for a certain time and the start time of a frame estimated from the received synchronization signal is consistently and consistently within an error range, the terminal may determine that synchronization with neighboring terminals is correct. there is.

When the terminal is in synchronization with the neighboring terminals through the initial synchronization procedure, the synchronization maintenance and management procedure (corresponding to S430 of FIG. 4 ) is performed. The synchronization maintenance and management procedure can prevent timing drift caused by errors in oscillators of terminals, and out of synchronization due to the appearance of new terminals out of synchronization or interference in heterogeneous wireless wireless networks. . In addition, through the maintenance and management procedure of the synchronization, it is possible to detect a situation where the synchronization is not correct. Hereinafter, a synchronization maintenance and management procedure according to an embodiment of the present invention will be described.

The UE transmits the synchronization signal in the synchronization period within the frame using the random access method described above. Except for the case of transmitting the synchronization signal, the terminal receives the synchronization signal transmitted by the neighboring terminals while operating in the reception mode. At this time, since the terminal is already synchronized with the neighboring terminals, it receives a synchronization signal in the synchronization period. Accordingly, in order to save power consumption, the terminal may turn off some functions of the terminal in a section in which simultaneous signals are not received.

Upon receiving the synchronization signal, the terminal updates the MIAT, updates the CW _oth value, and updates the phase value of its own synchronization timer. The method of FIGS. 19 and 20 may be used to update the phase value of the synchronous timer. In the synchronization maintenance and management procedure, the synchronization timer phase value after the update does not significantly differ from the synchronization timer phase value before the update. The UE continues the random access for synchronization signal transmission within the current frame even after updating the phase value of the synchronization timer.

The terminal continuously monitors whether synchronization with neighboring terminals is out of an error range while repeating these processes. When the synchronization error is out of a specific error range, the UE may start an initial synchronization procedure or a resynchronization procedure described below. The terminal operating in the power consumption reduction mode may periodically release the power consumption reduction mode and perform a full scan in at least one frame in order to detect whether synchronization has been lost. On the other hand, through such a periodic full scan, when the terminal enters the power consumption detection mode after starting the synchronization signal transmission in S2140 of FIG. 22 , the terminal may detect a terminal having a different synchronization with the terminal.

The initial synchronization procedure and the maintenance and management procedure described above may use frames having different lengths. For example, in order to shorten the synchronization time between terminals performed in the initial synchronization procedure, a frame used in the initial synchronization procedure may be shorter than a frame used in the synchronization maintenance and management procedure.

When the synchronized terminal loses synchronization due to timing drift, interference from a neighboring heterogeneous wireless network, or the like, the terminal performs the resynchronization procedure ( S430 ) of FIG. 4 . This resynchronization procedure is similar to the initial synchronization procedure, but differs in the following points. The frame used in the initial synchronization procedure may have a different length from the frame used in the synchronization maintenance and management procedure. However, the frame used in the resynchronization procedure uses the same length as the frame used in the synchronization maintenance and management procedure. In the initial synchronization procedure, a radio channel scan is performed for a predetermined time to determine whether a synchronization signal exists. However, the scan may not be performed in the resynchronization procedure or may be performed for a shorter period of time than the initial assimilation procedure.

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

As shown in FIG. 23 , the terminal 2200 according to an embodiment of the present invention includes a processor 2210 , a memory 2220 , and an RF module 2230 .

The processor 2210 may be configured to implement the procedures, methods, and functions described with reference to FIGS. 1-21 .

The memory 2220 is connected to the processor 2210 and stores various information related to the operation of the processor 2210 .

The RF module 2230 is connected to an antenna (not shown) and transmits or receives a wireless 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 as defined in the following claims are also provided. is within the scope of the right.

Claims (20)

delete delete delete delete delete delete delete delete delete delete delete delete delete A method for synchronizing a first terminal in a distributed wireless communication system, comprising:
Receiving first synchronization signals from second terminals that are different terminals from the first terminal;
calculating an average reception time difference between the synchronization signals;
calculating an average contention window for the second terminals by using the first synchronization signals;
Using the average reception time difference and the average contention window, setting a first contention window that is the contention window of the first terminal; and
In response to the first contention window, the synchronization method comprising the step of determining whether to transmit a synchronization signal. 15. The method of claim 14,
The determining step is
randomly selecting a backoff counter using the first contention window;
decrementing the backoff counter when a channel is idle during a backoff slot in which a synchronization period is divided into a plurality of periods, and
and transmitting a synchronization signal when the backoff counter is zero. 16. The method of claim 15,
setting the backoff counter to a predetermined value after transmitting the synchronization signal; and
The synchronization method further comprising the step of updating (updating) the first contention window when the set backoff counter is zero. delete delete delete delete

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