KR20160094864A - Method and apparatus for time synchronization in wireless communication system of unlicensed frequency band - Google Patents

Abstract

The present invention provides a method and an apparatus for time synchronization in a wireless communication system of an unlicensed frequency band. When an unlicensed device to be operated in an unlicensed frequency band performs time synchronization with a licensed frequency band, it determines whether a channel state is a busy state or an idle state. If the channel state is the idle state, time synchronization with the licensed frequency band is performed by transmitting a variable length of a preamble signal after adjusting the preamble signal to a sub-frame of the licensed frequency band.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a time synchronization method and apparatus for a wireless communication system of a license-

The present invention relates to a method and an apparatus for time synchronization in a wireless communication system in a license-exempt frequency band.

Long term evolution (LTE) cellular networks have only been operating in the licensed band. Despite the development of technology for continuous capacity increase, as the demand for higher and higher capacity data service is increasing, research on increasing the capacity by accepting the license-exempted band, not limited to the existing license band, .

However, the license-exempt zone must resolve the coexistence issue with devices operating in other license-exempt bands, unlike licensed bands that are not subject to interference from other operators or other devices. That is, there is a need for a channel access / occupancy mode that can be used for a limited time when opportunities are given, without significantly lowering the performance of other devices on the same license-exempt channel.

To solve this coexistence problem, a method known as " listen before talk " (LBT) is widely used. In this way, channel access is first achieved by channel monitoring. Specifically, it senses the activity of the license-exempt channel that is being shared, suspends transmission of the radio signal when the energy of the channel is measured, and uses the channel when the energy of the channel is not measured. When a channel idle state is detected and transmitted, other devices judge that the channel is busy and measure the energy on the channel and suspend the transmission.

In principle, the frame of the license-exempted band must be synchronized with the frame operated in the licensed band. However, there is a problem that the frame synchronization must be performed at the same time as occupying the channel.

There may also be a carrier frequency error in the license-exempt band of the transmitting end and the receiving end. In the existing license band, the frequency error can be relatively easily handled using the carrier aggregation technique. In the licensed band operation, the base station always transmits a reference signal such as a common reference signal (CRS), so that the terminal can relatively easily and accurately adjust the correction of the frequency error that can occur for each frequency. However, in the case of the license-exempt band, in the case where there is no data signal to be transmitted in the license-exempt band as well as the non-continuous characteristics of the transmission signal, since the base station does not transmit CRS and does not transmit any other signal, Can perform frequency correction discontinuously only when there is a received signal. However, when frequency correction is performed upon receipt of a data signal in the LTE subframe unit of the current standard, deterioration of reception demodulation performance occurs and many hardware complexity is required to overcome this.

SUMMARY OF THE INVENTION The present invention provides a method and apparatus for performing time synchronization with a licensed band in a license-exempt band, characterized by discontinuous signal transmission.

The time synchronization method according to an aspect of the present invention is a method for determining whether a channel state is in a busy state or an idle state in a method of performing time synchronization with a licensed band in a license-exemption device operating in a license- step; And performing time synchronization with the license band by transmitting a preamble signal of a variable length in accordance with a subframe of the license band when the state of the channel is idle.

The step of performing time synchronization may include performing time synchronization by transmitting the variable length preamble signal to a start point of a subframe period of the license band.

The step of performing time synchronization includes performing time synchronization by transmitting the variable-length preamble signal up to the end of a subframe period of the license band.

The variable length preamble signal may include a first region having a fixed length and a second region having a variable length.

The step of performing time synchronization may further include transmitting a sub-frame including a payload after transmitting the variable-length preamble signal.

The first area may be located before the boundary of the subframe including the payload and the second area may be located starting from the boundary of the subframe including the payload. The second area may include preset digital information. In addition, the digital information may include an identifier of the license-exempt device. In addition, the digital information may include system broadcast information.

The variable length preamble signal may be configured in a sequence unit having a predetermined pattern.

According to another aspect of the present invention, there is provided a signal transmission apparatus in which a license-exempt device operating in an unlicensed band transmits a signal by performing time synchronization with a license band, a radio frequency converter for transmitting and receiving a signal through an antenna, A processor coupled to the RF transceiver and performing signal transmission processing, the processor comprising: a channel determination unit determining whether a channel state is busy or idle; A preamble signal generator for generating a variable length preamble signal when the channel state is an idle state; And a transmission processor for performing time synchronization with the license band by transmitting in accordance with a subframe of the license band.

The transmission processing unit may perform time synchronization by transmitting the variable length preamble signal to a start point of a subframe period of the license band.

The transmission processor may perform time synchronization by transmitting the variable-length preamble signal until a subframe period of the license band ends.

The variable length preamble signal may include a first region having a fixed length and a second region having a variable length.

The first area is located before a boundary of a sub-frame including a payload transmitted after the variable-length preamble signal, and the second area is positioned at a boundary of a sub- .

The second area may include predetermined digital information, and the digital information may include at least one of an identifier of the license-exempt device and system broadcasting information.

According to an embodiment of the present invention, time synchronization with a licensed band in a license-exempt band can be performed to maintain synchronization and synchronization with a frame of the license band.

In addition, in the license-exempted band, the signal transmission and the preamble transmission in accordance with the channel activity detection can maintain synchronization with the license band without significantly changing the existing physical layer standard. Therefore, the LTE system can be operated in the license-exempt band and the received signal can be synchronized or maintained.

Also, by transmitting a preamble signal including a sequence generated in a predetermined pattern, the receiver can utilize the preamble signal as a signal section for estimating a carrier frequency error. In addition, since digital information promised to the preamble can be included and transmitted, various functions can be performed at once using the preamble.

1 is a diagram illustrating a synchronization process according to an embodiment of the present invention.
2 is a diagram illustrating a structure of a preamble according to an embodiment of the present invention.
FIG. 3 and FIG. 4 are diagrams illustrating time synchronization with a license band using a preamble according to an embodiment of the present invention.
5 is a flowchart of a time synchronization method according to an embodiment of the present invention.
6 is a structural diagram of a signal transmission apparatus according to an embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art can easily carry out the present invention. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. In order to clearly illustrate the present invention, parts not related to the description are omitted, and similar parts are denoted by like reference characters throughout the specification.

Throughout the specification, when an element is referred to as "comprising ", it means that it can include other elements as well, without excluding other elements unless specifically stated otherwise.

Hereinafter, a time synchronization method and apparatus according to an embodiment of the present invention will be described with reference to the drawings.

1 is a diagram illustrating a synchronization process according to an embodiment of the present invention.

(LTE) licensed-access (LAA) device) operating in a license-exempted band may not be able to access the wireless local area network (WLAN) devices (WLAN STA1, WLAN STA2) In the case of using a 5 GHz frequency band, synchronization is achieved with the coexistence and licensed bands as in the attached Figure 1. The STA is an abbreviation for the station, and CCA is an acronym for clear channel assessment, LBT is an abbreviation of listen before talk and performs the same function as CCA. Busy indicates the occupied state of the channel, idle indicates that any device uses the channel .

As shown in FIG. 1, when the WLAN STA1 is transmitting in time, the WLAN STA2 and the base station (referred to as LTE-LAA base station) judge that the channel of the license-exempt band is vacant and hold the transmission. When the transmission of the WLAN STA1 is finished, the WLAN STA2 and the LTE-LAA base station detect that the channel is in the idle state.

After detecting the idle state of the channel according to the CCA check function, the WLAN STA2 prepares for transmission. According to the established communication standard, a time delay interval called a DCF Inter Frame Space (DIFS) and a random back-off And then transmit it. And is included in the function of a channel access scheme called WLAN distributed coordinated function (DCF). Likewise, the LTE-LAA base station also performs LBT (Listen Before Talk) including CCA and optional delay functions to detect the idle state of the channel, and after random random delays, prepare for transmission.

At this time, the WLAN STA2 and the LTE-LAA base station are contention for using the license-exempt band. In order to transmit a signal by winning the competition, an arbitrary delay time "q" must first be passed. The device can transmit. Here, q is a temporal concept, and can be, for example, a counter of 1 us unit. Thus, both the WLAN STA2 and the LTE-LAA base stations can transmit only after a total delay time "q" including a certain delay and a random backoff, respectively.

For WLAN STA2, q includes a DIFS time (e.g., 34us) and a random backoff (a multiple of 9us including 0, i.e. (0 to N) x 9us time, where N is in accordance with the IEEE 802.11 standard) do. In case of LTE-LAA base station, q by LBT function is similar to DIFS-like xIFS value and random backoff (N × 20us, where N is basically random and ETSI (European Telecommunications Standards Institute) 0.0 > 24). ≪ / RTI >

In FIG. 1, when the WLAN STA2 is undergoing a random backoff interval (DIFS + CCA check), the LTE-LAA base station likewise goes through an arbitrary q interval (extended CCA check) first. Here, the LTE-LAA base station may first transmit a preamble through a q interval, and then transmit a sub-frame with a payload. According to this preamble transmission, the WLAN STA2 or the WLAN STA1 judges that the channel is occupied and does not transmit the signal. In FIG. 1, it should be noted that the channel continues to idle until the LTE-LAA base station starts transmission. Also, in the licensed band, interference does not occur with the signal of the license-exempt band, so the base station of the license band is transmitting a continuous signal, and the signal is transmitted based on the format of a certain subframe. The length of the LTE subframe is 1 ms (1000 us) and is defined in the standard specification. Here, it is necessary to understand that the LBT includes a function of avoiding collision between WLAN and other LTE-LAA providers as well.

On the other hand, when the LTE-LAA base station completes the transmission of one subframe, the WLAN STA1 and the WLAN STA2 detect that the channel is idle and start a competition to occupy the channel. In FIG. 1, since WLAN STA1 has first passed an arbitrary delay time q first, WLAN STA1 first transmits a signal. After the transmission of the WLAN STA1 is completed, the LTE-LAA base station again occupies the channel and transmits two subframes. In the remaining period, the WLAN STA1 and the WLAN STA2 occupy the channels and transmit signals. There is no restriction on the duration itself in which the LTE-LAA base station or the WLAN is transmitting.

The LBT and the preamble may be used to provide the function of transmitting the physical layer subframe used in the existing license band without modification, even in the unlicensed band. The current LBT is defined in ETSI, and in the embodiment of the present invention, the preamble is utilized in the license-exempt band.

The preamble can be of variable length, and the preamble is equal to or shorter than the subframe period. The license-exempt LTE-LAA device can use the preamble to coexist with other devices such as a wireless LAN and occupy the channel without causing or receiving interference, so that it can be used for a certain period of time. The preamble is transmitted to the start point (or end point) of the subframe section of the license band. Therefore, the time synchronization between the signal transmission period of the license band and the signal transmission period of the license-unlicensed band can be realized. When the subframes of the license-exempted band and the far-end band are synchronized in time, there is an advantage in terms of implementation or scheduling.

2 is a diagram illustrating a structure of a preamble according to an embodiment of the present invention.

The preamble according to the embodiment of the present invention is divided into two areas, the first area is represented by s (n) and the second area is represented by v (n). The signal s (n) in the first region exists before the boundary with reference to the boundary of the subframe, and the signal v (n) in the second region exists with the boundary of the subframe as the starting point. The length of v (n) may be fixed to 2208, for example, on a 30.72 MHz sampling basis. s (n) has a variable length, for example, a length of about 0.521 us, which is the minimum signal unit transmission interval.

The basic unit of the preamble according to the embodiment of the present invention is a sequence, and the sequence is, for example, 16 samples and has a length of 16 as shown in FIG.

When the digital sample rate is, for example, 30.72 MHz, the time Ts required to transmit one sample is 1 / (30.72e6) = 0.326us. Since the sequence according to the embodiment of the present invention has a length of 16, the time taken to transmit this sequence is 16 / (30.72e6) = 0.521us. For reference, the time taken to transmit orthogonal frequency division multiplexing (OFDM) symbols is 2048 / (30.72e6) = 66.67us.

The length of the CP (cyclic prefix) may be 144 or 160, so the time taken to transmit the CP is 144 / (30.72e6) = 4.69us or 160 / (30.72e6) = 5.2083us. The length of one LTE subframe is 30720, and thus the time required to transmit one subframe is 30720 / (30.72e6) = 1ms. That is, when 1920 consecutive sequences, which are the basic unit of the preamble, having a length of 16, are transmitted in succession, 1 ms is required. One LTE subframe can be divided into 1920 intervals.

On the other hand, a sequence s (n) of a time domain having a length of 16 can be generated according to the following equation.

Where p is a constant for normalizing the signal. z (k) denotes a sequence in the frequency domain, and k is an index. z (k) and the index can be defined as follows.

This expression (2) represents? F = (30.72 MHz) / 16. In Equation (2)

from

Is a complex number and can be defined as follows by a binary bit.

Binary bit

from

Is a physical cell ID of the base station.

and

And is mapped.

here

It is a binary operator that converts to binary. E.g

ego

, The binary number

Is determined to be 110000110. therefore

The

.

When p is 4,

Can be represented by the following s (n) sequence by transforming into a time domain using Equation (1).

The signal v (n) in the second region of the preamble is a signal corresponding to a primary synchronization signal (PSS) region in FIG. 2, and can be generated as follows.

first,

Is calculated based on the following equation (6).

here

Is an arbitrary number from 3 to 61, and m is an index indicating the frequency subcarrier of the next frequency. m can be remapped to k as follows.

here,

Represents the bandwidth. For example, if the sampling rate is 30.72 MHz,

Lt; / RTI >

Can be remapped to w (k) as follows.

w (k) is finally transformed into time domain to obtain e (n) as follows.

If CP is added to e (n), the following v (n) is generated.

The generated v (n) may have a length 2208.

Since the preamble having variable length according to the embodiment of the present invention has a granularity of about 0.5us, it is possible to occupy the coexistence channel and synchronize time with the license band with high degree of freedom.

FIG. 3 and FIG. 4 are diagrams illustrating time synchronization with a license band using a preamble according to an embodiment of the present invention.

The preamble according to an embodiment of the present invention is used when occupying or reserving a license-exempted bandwidth channel, as shown in FIGS. 3 and 4, where the length of the preamble is variably adjusted as necessary. That is, as shown in FIG. 3, the preamble having the first length can be used to occupy the channel near the end of the LTE subframe of the license band. Alternatively, as shown in FIG. 4, the preamble having the second length may be used to occupy the channel at the beginning of the subframe after passing the boundary of the LTE subframe in the license band.

In this way, the variable length preamble according to the embodiment of the present invention is a field for performing an automatic gain control (AGC) function of the receiver in addition to adjusting the length of the preamble and occupying the channel to synchronize the time with the license band Can also be utilized. In addition, since the variable length preamble according to the embodiment of the present invention always has the same repetitive pattern regardless of the length, it is possible to provide an estimation function for compensating impairment occurring in an analog stage such as a carrier frequency offset Can be performed.

Concretely, in the variable-length preamble period, if the result obtained by auto-correlation processing the input signal y (n) of the terminal is successively acquired and an average of the accumulated results is taken,

(radian) can be estimated as follows.

Time synchronization can be estimated using the PSS signal corresponding to the signal v (n) of the second region of the preamble in the variable-length preamble section as follows.

The correlation power obtained by cross-correlation processing v (n) on the terminal's input signal y (n)

And the time synchronization with the license band of the transmission signal can be estimated.

The variable length preamble has a function of including information of PSS or SSS (secondary synchronization sequence) defined in LTE. And the binary number of z (k)

Not the physical cell identifier of the base station, but also the system broadcast information for notifying the 8-bit message.

Thus, by transmitting a preamble signal including a sequence generated in a predetermined pattern, the receiving end can utilize the preamble signal as a signal section for estimating a carrier frequency error. In addition, since digital information promised to the preamble can be included and transmitted, various functions can be performed at once using the preamble.

5 is a flowchart of a time synchronization method according to an embodiment of the present invention.

The license-exempt device operating in the license-exempted band performs time synchronization with the licensed band while using the wireless LAN device and the license-exempt band. The license-exempt device is also referred to as an LTE-LAA device, and may be, for example, an LTE-LAA base station.

The license-exempt device determines whether the channel state is busy or idle (S100). Here, if the energy of the channel is measured or the measured energy is equal to or higher than the set value, it is determined that the channel state is busy. If the energy of the channel is not measured or the measured energy is smaller than the set value, have.

The channel state is in a busy state, and the license-exemption device does not perform signal transmission (S110, S120). On the other hand, when the channel state is the idle state, signal transmission is performed after the set delay time has elapsed (S130).

Upon signal transmission, the license-exempt device first transmits a preamble having a variable length (S140), and then transmits a subframe including the payload (S150). At this time, the license-exempt device transmits the preamble having the variable length to the start point of the subframe section of the license band or the end of the subframe section. In particular, the preamble having a variable length includes a first area having a fixed length and a second area having a variable length, the first area being before the boundary of the subframe, and the second area being the boundary of the subframe It exists as a starting point. The length of the second area may be adjusted to match the subframe of the license band. This preamble transmits a sequence generated in a certain pattern.

The preamble may also include an identifier of the license-exempt device (e.g., a physical identifier of the base station) or may include separate information (e.g., system broadcast information, etc.).

According to the embodiment of the present invention, synchronization with the license band can be maintained without significantly changing the existing physical layer standard due to signal transmission and preamble transmission according to channel activity detection.

6 is a structural diagram of a signal transmission apparatus according to an embodiment of the present invention.

6, a signal transmission apparatus 100 according to an embodiment of the present invention includes a processor 110, a memory 120, and a radio frequency (RF) converter 130. The processor 110 may be configured to implement the method described above based on Figs. 1-6. Here, a detailed description of parts similar to the above method will be omitted.

For this, the processor 110 includes a channel determination unit 111, a preamble signal generation unit 112, and a transmission processing unit 113.

The channel determination unit 111 determines whether the channel state is a busy state or an idle state.

When the channel state is the idle state, the preamble signal generating unit 112 generates a variable length preamble including a first region and a second region. The variable length preamble may include an identifier or other information.

The transmission processing unit 113 transmits the preamble signal, and in particular, transmits the preamble signal until the start point of the subframe period or the end of the subframe period of the license band. Then, a preamble signal having a variable length is transmitted first, and then a subframe including a payload is transmitted.

The memory 120 is coupled to the processor 110 and stores various information related to the operation of the processor 110. [ The memory 120 may be located inside or outside the processor, and the memory may be coupled to the processor through various means already known. The memory may be any type of volatile or nonvolatile storage medium, e.g., the memory may include read-only memory (ROM) or random access memory (RAM).

The RF converter 130 is connected to the processor 110 and transmits or receives radio signals. The RF converter 130 transmits a feedback signal transmitted from the processor 110.

The embodiments of the present invention are not limited to the above-described apparatuses and / or methods, but may be implemented through a program for realizing functions corresponding to the configuration of the embodiment of the present invention, a recording medium on which the program is recorded And such an embodiment can be easily implemented by those skilled in the art from the description of the embodiments described above.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, It belongs to the scope of right.

Claims (16)

In the way that license-exempt devices operating in the license-exempt zone perform time synchronization with the licensed band,
Determining whether a state of the channel is busy or idle; And
When the state of the channel is idle, performing a time synchronization with the license band by transmitting a preamble signal of a variable length according to a subframe of the license band
/ RTI > The method according to claim 1,
The step of performing time synchronization
Performing time synchronization by transmitting the variable length preamble signal to a start point of a subframe period of the license band
/ RTI > The method according to claim 1,
The step of performing time synchronization
Performing time synchronization by transmitting the preamble signal of the variable length up to the end of a subframe period of the license band
/ RTI > The method according to claim 1,
Wherein the variable length preamble signal includes a first region having a fixed length and a second region having a variable length. 5. The method of claim 4,
The step of performing time synchronization
Transmitting a variable length preamble signal and then transmitting a subframe including a payload
Further comprising the steps of: 6. The method of claim 5,
Wherein the first area is located before a boundary of a subframe including the payload and the second area is located starting from a boundary of a subframe including the payload. The method according to claim 6,
And the second area includes preset digital information. 8. The method of claim 7,
Wherein the digital information includes an identifier of the license-exempt device. 8. The method of claim 7,
Wherein the digital information comprises system broadcast information. The method according to claim 1,
Wherein the variable length preamble signal is configured in a sequence unit having a predetermined pattern. In a signal transmission device in which a license-exempt device operating in a license-exempt band transmits a signal by performing time synchronization with the license band,
A radio frequency converter for transmitting and receiving signals through an antenna, and
A processor coupled to the RF transceiver and performing signal transmission processing,
The processor comprising:
A channel determination unit determining whether the channel status is busy or idle;
A preamble signal generator for generating a variable length preamble signal when the channel state is an idle state; And
And a transmission processing unit for performing time synchronization with the license band by transmitting the sub-
The signal transmission apparatus comprising: 12. The method of claim 11,
Wherein the transmission processing unit performs time synchronization by transmitting the variable length preamble signal to a start point of a subframe period of the license band. 12. The method of claim 11,
Wherein the transmission processing unit performs time synchronization by transmitting the variable length preamble signal until a subframe period of the license band ends. 12. The method of claim 11,
Wherein the variable length preamble signal includes a first region having a fixed length and a second region having a variable length. 15. The method of claim 14,
Wherein the first area is located before a boundary of a subframe including a payload transmitted after the variable length preamble signal and the second area is located at a boundary of a subframe including the payload, Signal transmission device. 16. The method of claim 15,
Wherein the second area includes preset digital information, and the digital information includes at least one of an identifier of the license-exempt device and system broadcast information.

Images