WO2007114638A2

WO2007114638A2 - Tdm based cell search method in ofdm cellular system, frame transmission method thereof and system thereof

Info

Publication number: WO2007114638A2
Application number: PCT/KR2007/001627
Authority: WO
Inventors: Il-Gyu Kim; Hyeong-Geun Park; Kap-Seok Chang; Young-Jo Ko; Hyo-Seok Yi; Young-Hoon Kim; Seung-Chan Bang; Seung-Ku Hwang
Original assignee: Electronics And Telecommunications Research Institute
Priority date: 2006-04-03
Filing date: 2007-04-03
Publication date: 2007-10-11
Also published as: WO2007114638A3

Abstract

A cell searching method in a time-division multiplexing (TDM)-based Orthogonal Frequency-Division Multiplexing (OFDM) cellular system having a primary synchronization channel and secondary synchronization channel, and a frame transmission method and system thereof are provided. In the cell searching method in an OFDM cellular system including a plurality of cells allocated with scrambling codes unique to the cells, wherein each base station transmits a frame of each cell and a terminal searches for a target cell by using the frame received from the base station, the cell searching method includes: acquiring Sync-block synchronization corresponding to a period of a primary synchronization channel symbol included in the frame received by the terminal and a primary synchronization channel sequence number by using the primary synchronization channel symbol; and acquiring a frame boundary of the frame and the target cell of the terminal based on the Sync-block synchronization and the primary synchronization channel sequence number by using a secondary synchronization channel symbol included in the frame received by the terminal. Accordingly, the synchronization can be acquired with low complexity.

Description

TDM BASED CELL SEARCH METHOD IN OFDM CELLULARSYSTEM, FRAME TRANSMISSION METHOD THEREOFAND SYSTEM THEREOF

TECHNICAL FIELD The present invention relates to a cell searching method in an Orthogonal

Frequency-Division Multiplexing (OFDM) cellular system and a frame transmitting method therefor and, more particularly, to a method of searching an initial cell and an adjacent cell in an OFDM cellular system and a mobile station, a base station, a system, and a frame structure using the same.

BACKGROUND ART

In a wideband code division multiple access (WCDMA) of the Third-Generation Partnership Project (3GPP), in order to identify a base station, a system uses 512 long pseudo noise (PN) scrambling codes, and neighboring base stations use different long PN scrambling codes as scrambling codes of forward-link channels.

When a mobile station is powered on, the mobile station performs an operation of acquiring a system timing of a base station having the highest-level received signal, that is, a base station in which the mobile station is included and has a long PN scrambling code ID which the current base station uses. These operations are collectively called a cell searching procedure of a mobile station.

In the WCDMA, in order to easily perform the cell searching procedure, 512 long PN scrambling codes are divided into 64 groups, and a primary synchronization channel and a secondary synchronization channel are provided in the forward-link. The primary synchronization channel allows the mobile station to acquire slot synchronization, and the secondary synchronization channel allows the mobile station to acquire a 10 msec frame boundary and the long PN scrambling code group ID.

The WCDMA-based cell searching procedure is mainly performed by using three operations. In a first operation, the mobile station acquires the slot synchronization by using a primary synchronization channel code (PSC). In the WCDMA system, the same PSC is transmitted to all the base stations in units of a slot in a time period of 10 msec. More specifically, the slot synchronization (slot timing information) is acquired by using a matching filter for the PSC.

In a second operation, long PN scrambling code group information and a 10 i msec frame boundary is acquired by using the slot synchronization (slot timing information) acquired in the first operation and a secondary synchronization channel code which is sometimes referred to as a secondary scrambling code (SCC).

In a third operation, a long PN scrambling code ID of the current base station is acquired based on the long PN scrambling code group information and the 10 msec frame boundary acquired in the second operation by using a common pilot channel code correlator.

Eight scrambling codes are mapped to one code group. Therefore, in the third operation, the mobile station compares outputs of eight PN scrambling code correlators in order to detect a long PN scrambling code ID of the current cell.

In the WCDMA system, the synchronization channel includes the primary synchronization channel and the secondary synchronization channel. The primary synchronization channel, the secondary synchronization channel, the common pilot channel, and other data channels are multiplexed in the CDMA method which is based on time domain direct sequence band spreading.

Recently, in the 3GPP, an Orthogonal Frequency-Division Multiplexing (OFDM)-based wireless communication standard has been actively discussed as a 3G long term evolution (3G-LTE) in order to solve the shortcomings of the WCDMA system. The synchronization channel structure, the common pilot channel structure, and the cell searching procedure of the mobile station used in the WCDMA system is suitable for a direct sequence (DS)-CDMA system, but is not applied to an OFDM forward-link.

For this reason, there is a need for a synchronization channel structure, a common pilot channel structure, an initial cell searching procedure of a mobile station, and an adjacent cell searching procedure for handover, which can be used in the forward-link of the OFDM cellular system.

DETAILED DESCRIPTION OF THE INVENTION

TECHNICAL PROBLEM

The present invention provides a cell searching apparatus and method including initial cell searching and adjacent cell searching for handover in an Orthogonal Frequency-Division Multiplexing (OFDM) cellular system.

The present invention also provides a forward-link frame transmitting apparatus and method supporting the cell searching method. The present invention also provides an OFDM cellular system employing the cell searching method.

The present invention also provides a forward-link frame structure used for the cell searching method. The present invention also provides a computer-readable medium having embodied thereon a computer program for executing the cell searching method.

TECHNICAL SOLUTION According to an aspect of the present invention, there is provided a cell searching method in an Orthogonal Frequency-Division Multiplexing (OFDM) cellular system including a plurality of cells allocated with scrambling codes unique to the cells, wherein each base station transmits a frame of each cell and a terminal searches for a target cell by using the frame received from the base station, the cell searching method comprising: (a) acquiring the synchronization of Sync-block which corresponding to a period of a primary synchronization channel symbol and a primary synchronization channel sequence number by using primary synchronization channel; and (b) acquiring frame boundary of a frame and the cell identity (cell ID) or cell ID group by using secondary synchronization channel based on the Sync-block timing and the primary synchronization channel sequence number obtained in (a). In (a), a primary synchronization channel symbol which occupies several subcarriers in a frequency domain may be used and a primary synchronization channel symbol is transmitted in every Sync-block.

In (a), the Sync-block synchronization may be acquired by calculating a differential correlation value using a repetition pattern in a time domain of the primary synchronization channel symbol or by calculating a correlation value using a time domain waveform which is Fourier transform pair of the frequency domain primary synchronization channel sequence when the primary synchronization channel symbol occupies all the subcarriers applied to the primary synchronization channel in a frequency domain. (a) may comprise: (a1) calculating correlation values between all the primary synchronization channel sequences used in the OFDM cellular system and the received signal which contains primary synchronization channel transmitted from the target cell; and (a2) detecting a Sync block timing corresponding to a maximum value of the correlation values.

(a1 ) may include: (a1-1) calculating correlation values between time domain waveforms of all the primary synchronization channel sequences used in the OFDM cellular system and the received signal which contains primary synchronization channel transmitted from the target cell; and (a1-2) accumulating the correlation values of different Sync blocks.

(a1) may include: in a case where the terminal receives signals through a plurality of receiving antennas thereof, (a1-1) calculating correlation values between all the primary synchronization channel sequences used in the OFDM cellular system and received signals through the receiving antennas of the terminal; and (a1-2) adding the correlation values for the receiving antennas.

In (b), the frame boundary and the target cell ID (or cell ID group) may be acquired based on the acquired Sync-block timing and the primary synchronization channel sequence number by performing cyclic shift correlations on the secondary synchronization channel symbol which corresponds to a cell ID or a cell ID group of a target cell and are mapped so that a length of the secondary synchronization channel sequence is equal to a total length of the subcarriers occupied by the secondary synchronization channel symbols included in the frame in a frequency domain.

The frame boundary and the target cell ID (or group ID) may be acquired by using only one secondary synchronization channel symbol since the secondary synchronization channel symbols are different for each Sync-blocks wihitn a radio frame and the secondary synchronization symbols are different between neighboring cells.

(b) may comprise: (b1 ) estimating a frequency offset by using the primary synchronization channel symbol existing in each Sync block section with reference to the Sync block timing; (b2) correcting a frequency offset of the synchronization channel symbol based on the estimated frequency offset; and (b3) detecting a frame timing and a target cell ID of the terminal by using the corrected synchronization channel symbol.

(b3) may include: (b3-1 ) performing Fourier transformation on the frequency offset corrected synchronization channel symbol into a frequency domain signal and calculating correlation values between the frequency domain signal and the all the possible cyclic-shifted secondary synchronization channel sequences used in the OFDM cellular system; (b3-2) detecting an index value λ corresponding to a maximum correlation value among the correlation values of the all cyclic-shifted secondary synchronization channel sequences; and (b3-3) detecting a frame timing and a cell ID (or cell group ID) corresponding to the index value Λ .

(b3-1) may include: performing Fourier transformation on time domain signals corresponding to the synchronization channel symbols to acquire frequency domain converted signals; acquiring the primary synchronization channel portion and the secondary synchronization channel portion among all the acquired frequency domain converted signal; and performing correlation between the frequency domain signal corresponding to the secondary synchronization channel symbol and the all the candidate cyclic-shifted secondary synchronization channel sequences used in the system.

The cell searching method may further include estimating a position of the synchronization channel using the primary synchronization channel symbols, wherein the correlation of the secondary synchronization channel symbol at the position of the estimated synchronization channel is calculated

In (b3-3), the target cell ID is detected by performing an L-modulo operation on the index value Λ , wherein L is the number of all the synchronization channel symbols used in the OFDM system.

In (b3-3), a maximum value among natural numbers which are equal to or less than a value obtained by dividing the index value Λ with the number L of all the synchronization channel symbols used in the OFDM system is detected as the frame boundary identifier.

(b) may include, in a case where the terminal receives frames through a plurality of receiving antennas thereof, (b1) estimating frequency offsets by using the primary synchronization channel symbol included in the frame received through the receiving antennas over the Sync block length sections with reference to the Sync block synchronization; (b2) correcting the frequency offsets of the synchronization channel symbols included in the frame received through receiving antennas based on the estimated frequency offsets; and (b3) detecting a frame timing and the target cell ID by using a combination of the correlation values of the secondary synchronization channel symbols included in the frame received through the receiving antennas based on corrected synchronization channel symbols.

A frame which is obtained by filtering only the band occupied by synchronization channel among whole signal band of the OFDM cellular system may be used.

The frame may be a forward-link frame which has a time duration of 10msec and includes 20 sub-frames.

The sub-frame may have a time duration of 0.5msec and includes 7 OFDM symbols

Two primary synchronization channel symbols and two secondary synchronization channel symbols may exist in the frame.

When one primary synchronization channel sequence is used in the OFDM cellular system, the same primary synchronization channel sequence may be used among the cells, or when multiple primary synchronization channel sequences are used in the OFDM cellular system, different primary synchronization channel sequences of the primary synchronization channel symbols may be used among the adjacent cells.

Primary synchronization symbols in a frame of a cell use the same primary synchronization sequence. The primary synchronization channel symbol may occupy a half of the allocated subcarriers or all the allocated subcarriers.

The two secondary synchronization channel symbols may use the same sequence between frames, but use different sequences between Sync blocks wihin a frame, and the two secondary synchronization channel symbols may use different sequences among cells.

The secondary synchronization channel symbol may include information which is mapped to a cell ID or a cell ID group of a cell which transmits a frame including the secondary synchronization channel symbol.

The repetition period of the primary synchronization channel symbol may be one Sync block in one frame, and the repetition period of the secondary synchronization channel symbol may be one frame.

The synchronization channel symbol in the frame may occupy a portion of the bands used in the OFDM cellular system with reference to a center of the frequency domain. The primary synchronization channel symbol and the secondary synchronization channel symbol may be constructed based on TDM (time-division multiplexing) and are disposed to be adjacent to each other.

According to another aspect of the present invention, there is provided a frame transmitting method in an OFDM cellular system including a plurality of cells allocated with scrambling codes unique to the cells, wherein a base station in an arbitrary cell transmits a frame, the frame transmitting method comprising: (a) generating a primary synchronization channel sequence including Sync-block timing information of the frame and a secondary synchronization channel sequence including a frame boundary information, the cell ID of the cell, or the cell ID group in which the cell ID is included; and (b) generating a frame including the primary and the secondary synchronization channel symbols and transmitting the frame.

A forward-link frame which has a time duration of 10msec and includes 20 sub-frames may be generated and transmitted as the frame.

The sub-frame may have a time duration of 0.5msec and include 7 OFDM symbols.

The frame may be generated and transmitted so as to have two primary synchronization channel symbols and two secondary synchronization channel symbols. When one primary synchronization channel sequence is used in the OFDM cellular system, the same primary synchronization channel sequence of the primary synchronization channel symbols may be used among the cells, or when multiple primary synchronization channel sequences are used in the OFDM cellular system, different primary synchronization channel sequences of the primary synchronization channel symbols may be used among the adjacent cells.

The primary synchronization channel symbol may occupy a half of the allocated subcarriers or all the allocated subcarriers.

The two secondary synchronization channel symbols may use the same sequence among the frame, the two secondary synchronization channel symbols use different sequences among Sync blocks in the frame, and the two secondary synchronization channel symbols may use different sequences among base stations.

The secondary synchronization channel symbol includes information which may be mapped to a cell ID or a cell ID group of a cell which transmits a frame including the secondary synchronization channel symbol. The repetition period of the primary synchronization channel symbol may be one

Sync block in one frame, and the repetition period of the secondary synchronization channel symbol is one frame.

The synchronization channel symbol in the frame may occupy a portion of the bands used in the OFDM cellular system with reference to a center of the frequency domain.

The primary synchronization channel symbol and the secondary synchronization channel symbol may be constructed based on the TDM and be disposed to be adjacent to each other.

In (b), the synchronization channel symbols may be transmitted by using spatial diversity, time switching transmit diversity, or pre-coding vector switching transmit diversity.

According to another aspect of the present invention, there is provided an neighboring cell searching method in an OFDM cellular system including a plurality of cells allocated with scrambling codes unique to the cells, wherein each base station transmits a frame of each cell and a terminal searches for a neighboring cell by using the frame received from the base station, wherein, when a home cell and the neighboring cell operate in a base-station synchronization mode, the terminal can search the neighboring cell based on the timing previously acquired from a frame received from the home cell by using a secondary synchronization channel symbol included in the frame received from the adjacent cell.

The adjacent cell searching method may include: detecting a cell ID of the adjacent cells by calculating correlation value of a O-shifted secondary synchronization channel sequences among all the available cyclic-shifted secondary synchronization channel sequences used in the OFDM cellular system; and detecting a frame timing of the frame received from the adjacent cell and an OFDM symbol timing by using a time domain signal corresponding to the secondary synchronization channel symbol of the adjacent cell. The adjacent cell searching method may further include receiving a control channel of the home cell and determining whether or not the home cell and the neighboring cell operate in the base-station synchronization mode. The control channel from the home cell includes the information whether or not the home cell and the neighboring cell operate in the base-station synchronization mode. In the neighboring cell search case with inter-base station synchronized mode,

(b1 ) and (b2) may not be performed, and only (b3) may be performed.

According to another aspect of the present invention, there is provided a cell searching apparatus in an OFDM cellular system including a plurality of cells allocated with scrambling codes unique to the cells, wherein each base station transmits a frame of each cell and a terminal searches for a target cell by using the frame received from the base station, the cell searching apparatus comprising: a synchronization detection unit which acquires a Sync-block synchronization corresponding to a period of a primary synchronization channel symbol included in the frame received by the terminal and a primary synchronization channel sequence number by using the primary synchronization channel symbol; and a cell detection unit which acquires a frame boundary of the frame and the target cell ID or cell group ID based on the Sync-block synchronization and the primary synchronization channel sequence number by using a secondary synchronization channel symbol included in the frame received by the terminal.

According to another aspect of the present invention, there is provided a frame transmitting apparatus in an OFDM cellular system including a plurality of cells allocated with scrambling codes unique to the cells, wherein a base station in an arbitrary cell transmits a frame, the frame transmitting apparatus comprising: a synchronization channel generating unit which generates a primary synchronization channel sequence and a secondary synchronization channel sequence including a frame boundary information, a cell ID of the cell, or a cell ID group in which the cell ID is included; and a frame transmitting unit which generates a frame including the synchronization channel symbols and transmits the frame.

The frame transmitting unit may include an OFDM symbol mapping unit which maps data values of the channels included in the frame to positions in a frequency domain and a time domain.

The frame transmitting unit may include a scrambling unit which multiplies cell-unique scrambling codes in the frequency domain with remaining OFDM symbols among the mapped results excluding the synchronization channel symbols included in the frame.

The frame transmitting unit may include a CP(cyclic prefix) inserting unit which inserts cyclic prefixes in order to enable modulation of the OFDM signal even in a multi-path delay state of the channel included in the frame.

According to another aspect of the present invention, there is provided an adjacent cell searching apparatus in an OFDM cellular system including a plurality of cells allocated with scrambling codes unique to the cells, wherein each base station transmits a frame of each cell and a terminal searches for a neighboring cell by using the frame received from the base station, wherein, when a home cell and the adjacent cell operate in a base-station synchronization mode, the terminal acquires the adjacent cell based on a Sync-block synchronization previously acquired from a frame received from the home cell by using a secondary synchronization channel symbol included in the frame received from the adjacent cell.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view illustrating a structure of a forward-link frame according to an embodiment of the present invention.

FIG. 2 is a view illustrating a sub-frame including synchronization channel symbols according to an embodiment of the present invention.

FIG. 3 is a view illustrating a structure of a primary synchronization channel symbol signal in a time domain according to an embodiment of the present invention. FIG. 4 is a view illustrating a symbol-mapped synchronization channel sequence according to an embodiment of the present invention.

FIG. 5 is a block diagram illustrating a construction of a base station according to an embodiment of the present invention.

FIG. 6 is a block diagram illustrating a construction of a mobile station receiver according to an embodiment of the present invention.

FIG. 7 is a block diagram illustrating a construction of a synchronization detection unit illustrated in FIG. 6 according to an embodiment of the present invention. FIG. 8 is a graph illustrating correlation values at sample positions calculated by a correlation value calculating unit illustrated in FIG. 7 according to an embodiment of the present invention.

FIG. 9 is a view illustrating a structure of input signals S3 and S4 applied to a cell detection unit illustrated in FIG. 6 based on a Sync block timing obtained by the synchronization detection unit illustrated in FIG. 6 according to an embodiment of the present invention. FIG. 10 is a block diagram illustrating a construction of the cell detection unit illustrated in FIG. 6 according to an embodiment of the present invention.

FIG. 11 is a block diagram illustrating a construction of a boundary and cell ID detection unit illustrated in FIG. 10 according to an embodiment of the present invention.

FIG. 12 is a view illustrating a process of performing correlation on all available cyclic-shifted secondary synchronization channel sequences in order to acquire a frame boundary and a cell ID according to an embodiment of the present invention. FIG. 13 is a flowchart of a cell searching procedure of a mobile station according to an embodiment of the present invention.

FIG. 14 is a flowchart of an adjacent cell searching procedure in a case where a home cell and an adjacent cell operate in synchronization with a base station according to an embodiment of the present invention. FIG. 15 is a flowchart of an adjacent cell searching procedure using a process of determining whether or not a home cell and an adjacent cell operates in synchronization with a base station according to an embodiment of the present invention.

BEST MODE

Hereinafter, exemplary embodiments of the present invention are described in detail with reference to the accompanying drawings.

In general, in an Orthogonal Frequency-Division Multiplexing (OFDM) cellular system, the base stations scramble OFDM symbols by a long pseudo noise (PN) scrambling code. However, instead of the long PN scrambling code, other types of scrambling codes may be used. Therefore, hereinafter, the PN scrambling code is referred to as a scrambling code.

In an embodiment of the present invention, the base station may have a plurality of transmitting antennas for performing transmit diversity such as a time switching diversity (TSTD), a pre-coding vector switching transmit diversity, and a frequency switching transmit diversity (FSTD). For convenience of description, the base station has two transmitting antennas as shown in the accompanying drawings.

In addition, in the embodiment of the present invention, the mobile station may have a plurality of receiving antennas for performing reception diversity. For convenience of description, the mobile station has two receiving antennas as shown in the accompanying drawings. In the mobile station having such a structure, it is necessary to combine data of data paths corresponding to the reception diversity. In the present invention, a simple addition method is used for the combining of the data. However, the present invention is not limited to this method, but other methods may be used as is well known to those of ordinary skilled in the related art.

The present invention relates to a cell searching method including synchronization acquisition, frame boundary detection, and cell ID detection (sometimes, referred to as scrambling code detection) in the OFDM cellular system.

In the present invention, a term "Sync block detection" means Sync block boundary detection, and "to detect a Sync block" includes "to detect OFDM symbol synchronization", "to detect a position of a primary synchronization channel in the Sync block", and "to detect a position of a secondary synchronization channel in the Sync block".

In the present invention, the term " frame boundary detection" means detecting timing 140 of the frame boundary, and the term " frame boundary information" means information on the timing of the frame boundary.

In the present invention, the term " scrambling code detection" collectively means scrambling code identifier detection and scrambling code detection, and a term " scrambling code information" collectively means scrambling code identifier and scrambling code.

In the present invention, the term "primary synchronization channel sequence" means a set of primary synchronization channel chips which are mapped to subcarriers occupied by primary synchronization channel symbols in a frequency domain.

In the present invention, the term "secondary synchronization channel sequence" means a set of secondary synchronization channel chips which are mapped to subcarriers occupied by secondary synchronization channel symbols in a frequency domain. In the present invention, the term "Fourier transform" collectively means discrete

Fourier transform and fast Fourier transform.

FIG. 1 is a view illustrating a structure of a forward-link frame according to an embodiment of the present invention. Referring to FIG. 1 , each forward-link frame has a time duration of 10 msec and includes 20 sub-frames 110. In FIG. 1 , the horizontal axis is a time axis, and the vertical axis is a frequency

(OFDM subcarrier) axis.

Each sub-frame has a length of 0.5 msec and includes 7 OFDM symbols 120. In the embodiment shown in FIG. 1 , one primary synchronization channel OFDM symbol 100-A and one secondary synchronization channel OFDM symbol 101 -A exist in the interval of 10 sub-frames. Therefore, two primary synchronization channel symbols and two secondary synchronization channel symbols exist in one frame (10 msec).

In this case, a repetition period 130 of the synchronization channel symbols is equal to a sum of lengths of the 10 sub-frames. Therefore, the number of repetition periods of the synchronization channel symbols in one frame becomes 2. The repetition period of the synchronization channel symbols is referred to as a Sync block. Namely, FIG. 1 exemplifies a case where the number of Sync blocks in one frame (10 msec) is 2. In a case where the primary synchronization channel and the secondary synchronization channel are combined in a time-division multiplexing (TDM) scheme as shown in FIG. 1 , the secondary synchronization channel needs to be disposed so as to be adjacent to the primary synchronization channel in order to use a channel estimation value of the primary synchronization channel for coherent modulation.

Remaining OFDM symbols excluding the synchronization channel symbols are multiplied with cell-unique scrambling codes in the frequency domain in order to distinguish each cell. FIG. 2 is a view illustrating a sub-frame including a synchronization channel symbol according to an embodiment of the present invention. The sub-frame illustrated in FIG. 2 is the ninth sub-frame of the first Sync block illustrated in FIG. 1.

In the sub-frame illustrated in FIG. 2, the OFDM symbol section 100-A for transmitting the primary synchronization channel symbol includes a traffic data subcarrier 230, a primary synchronization channel subcarrier 240, and a null carrier 260.

The OFDM symbol section 101 -A for transmitting the secondary synchronization channel symbol includes the traffic data subcarrier 230, a secondary synchronization channel subcarrier 250, and the null carrier 260. Other OFDM symbol sections 200 include the traffic data subcarrier 230, a pilot subcarrier, or others.

In this manner, the last two OFDM symbols in the sub-frame are the primary synchronization channel symbol and the secondary synchronization channel symbol. The present invention is not limited to the example illustrated in FIG. 2, but the synchronization channel symbols may be located to other positions in the Sync block 130.

It should be noted that the positions of the synchronization channel symbols are the same in all the Sync blocks where the synchronization channel symbols exist.

In a method of allocating the synchronization channel acquisition band, the synchronization channel is allowed to occupy the remaining bands excluding a guard band. In an alternative method, the synchronization channel may be allowed to occupy some of the remaining bands. As an example of a system using the latter method, there is a third generation long term evolution (3G-LTE) system where a scalable bandwidth needs to be supported.

Namely, in order to allow a mobile station using only a band of 1.25 MHz, a mobile station using a band of 2.5 MHz, and all the mobile stations using bands of 5MHz, 10 MHz, 15MHz, and 20 MHz to occupy the base station system synchronization, each of the synchronization channel symbols is designed to occupy some of the system band width 220 as shown in FIG. 2.

For example, when the system bandwidth is 10 MHz, only a central band of 1.25MHz excluding the DC subcarrier may be used. As described later below, a cell searching unit of the mobile station performs filtering for passing only the synchronization channel acquisition band 210 so as to improve performance of cell searching.

Referring to FIG. 2, the primary synchronization channel and the secondary synchronization channel occupy some bands of the entire system band 220 as described above.

As shown in FIG. 2, the primary synchronization channel may use one of the adjacent two subcarriers without use of the other.

Alternatively, the primary synchronization channel may use all the subcarriers in the synchronization channel acquisition band excluding the guard band. In the present invention, as a specific example of allocation of the subcarriers to the primary synchronization channel symbols, one of the adjacent two subcarriers is used, and the other is not used. In this case, the unused subcarrier is allocated with a predetermined value, for example, 0. The value of 0 is called a null symbol. In the case of using the latter method, a time domain signal (hereinafter, referred to as a synchronization channel symbol signal) of the synchronization channel symbol excluding a cyclic prefix has a pattern repeating in the time domain as shown in FIG. 3.

FIG. 3 is a view illustrating a structure of a primary synchronization channel symbol signal in a time domain according to an embodiment of the present invention.

FIG. 4 is a view illustrating a symbol-mapped synchronization channel sequence according to an embodiment of the present invention.

Referring to FIG. 3, N_τ denotes the number of samples in an entire OFDM symbol section; Ncp denotes the number of samples in a cyclic prefix section 300; and Ns denotes the number of samples in a symbol section excluding the cyclic prefix (CP) section 300.

In the case of using the structure shown in FIG. 3, a differential correlator may be used in the first operation of the cell searching procedure as described later below. On the other hand, in a case where the primary synchronization channel uses all the subcarriers in the synchronization channel acquisition band excluding the guard band, the differential correlator cannot be used, but a correlator which performs correlation after generating a time domain waveform of the primary synchronization channel at a receiving stage in advance is used.

The secondary synchronization channel uses all the subcarriers excluding DC subcarriers in the synchronization channel acquisition band excluding the guard band.

For example, in the 3G-LTE, the synchronization channel acquisition band is defined to be 1.25 MHz, and 128 subcarriers exist in the synchronization channel acquisition band. 75 subcarriers excluding the guard band and the DC subcarriers may be allocated to the secondary synchronization channel. In the forward-link frame according to an embodiment of the present invention, the primary synchronization channel sequence and the secondary synchronization channel sequence allocated to the base station are mapped to the subcarriers of the primary synchronization channel symbol and the secondary synchronization channel symbol as shown in FIG. 4. An element mapped to a subcarrier of a synchronization channel is defined as a chip.

The length of the primary synchronization channel sequence is equal to the number (37 in the example illustrated in FIG. 4) of subcarriers allocated to one primary synchronization channel symbol and has a characteristic of repeating in units of a primary synchronization channel symbol section. The length of the secondary synchronization channel sequence is equal to the total number (150 in the example illustrated in FIG. 4) of subcarriers in the frequency domain allocated to a plurality of the secondary synchronization channel symbols in the frame. As a result, the period of the primary synchronization channel sequence is equal to the Sync block 130, and the period of the secondary synchronization channel sequence is equal to one frame.

Namely, the primary synchronization channel sequence, which an arbitrary cell transmits in units of a primary synchronization channel symbol, can be expressed by Equation 1.

[Equation 1]

Each element in the primary synchronization channel sequence expressed by

Equation 1 is defined as a "chip" of the primary synchronization channel sequence.

In Equation 1 , Ni denotes the number (37 in the example illustrated in FIG. 4) of subcarriers allocated to one primary synchronization channel symbol.

In the case of the primary synchronization channel, the same primary synchronization channel sequence is transmitted in units of a symbol.

In this manner, if the same primary synchronization channel sequence is transmitted in units of a primary synchronization channel symbol, one correlator using the time domain waveform of the primary synchronization channel sequence in the first operation of the cell searching procedure at the receiving stage can be used to acquire the Sync block boundary.

Basically, all the cells in the system use the same primary synchronization channel sequence. In some cases, the cells may use a plurality of primary synchronization channel sequences (for example, 8 or less primary synchronization channel sequences). In these cases, the adjacent cells use different ones among a plurality of primary synchronization channel sequences. In addition, a plurality of time domain signal correlators need to be used at the first operation in the cell searching procedure.

In the present invention, a case where one primary synchronization channel sequence is used in the system is exemplified.

An arbitrary code sequence having a good correlation characteristic may be used as the primary synchronization channel sequence. For example, a Generalized-Chirp-Like (GCL) sequence may be used. The secondary synchronization channel sequences can be in one-to-one correspondence with cell IDs or cell ID groups. In the present invention, a case where the secondary synchronization channel sequence is in one-to-one correspondence with the cell IDs is exemplified.

The secondary synchronization channel sequence in one-to-one correspondence with the cell ID provides information on the frame boundary to the mobile station. Namely, the mobile station that detects the Sync block boundary 141 by using the primary synchronization channel can detect the cell ID and the frame boundary 140 by using the secondary synchronization channel.

For this reason, the length of the secondary synchronization channel sequence is designed to be equal to the total number (150 in the example illustrated in FIG. 4) of subcarriers allocated to the secondary synchronization channel in the frame.

As a result, the secondary synchronization channel sequence can be expressed by Equation 2.

[Equation 2]

The S_n ^{k) is an n-th chip of a secondary synchronization channel sequence having a sequence number of k (or a cell ID number of k). In Equation 2, P denotes the number (2 in the examples illustrated in FIGS. 1 and 4) of secondary synchronization channel symbols in the frame, and N₂ denotes the number (75 in the example illustrated in FIG. 4) of subcarriers allocated to the secondary synchronization channel excluding the DC subcarriers and the subcarriers for the guard band in the secondary synchronization channel symbols 101-A and 101-B. As a result, the length of the secondary synchronization channel sequence becomes P x N₂.

An arbitrary sequence having a good correlation characteristic may be used as the secondary synchronization channel sequence. For example, an extended GCL sequence may be used in order to extend the number of sequences.

The same secondary synchronization channel sequence for the signals which the base stations in the same cell transmit is transmitted in units of a frame. In addition, different secondary synchronization channel sequences are used for different cells.

Namely, according to the present invention, the cell base station is allocated with the secondary synchronization channel sequence which is mapped to the cell ID or the cell ID group unique to the cell, and the chips of the allocated synchronization channel sequences are carried on the respective subcarriers in the synchronization channel acquisition band.

With respect to the partial sequences of the secondary synchronization channel symbols of the secondary synchronization channel sequence, for example, the partial sequences of the first secondary synchronization channel symbol in the frame in the example illustrated in FIG. 4 are expressed by (S_o ^(k) ,S^^k) ,S₂ ^(k) ,...,S^) , and the partial sequences of the second secondary synchronization channel symbol are expressed by

• These partial sequences are different from each other.

Therefore, the mobile station can detect the frame boundary and the cell ID at the second operation by using only one among P secondary synchronization channel symbols in the frame.

The partial sequences of the secondary synchronization channel sequence can be generated by using various methods, for example, by modulating a sequence having a length of N₂ into a value corresponding to the Sync slot number.

As one example, assuming that the basic sequence is defined as

, the first sequence

can be defined as

and the second sequence

can be defined as

.

Here, the "a" is a modulation symbol value (for example, 1 ) corresponding to the first Sync block, and the "b" is a modulation symbol value (for example, -1 ) corresponding to the second Sync block.

As an alternative method, a secondary synchronization channel sequence having a total length of P x N₂ may be generated by mapping a short sequence having N₂ subcarriers allocated to the secondary synchronization channel symbols to a modulated symbol value corresponding to the Sync block number in each secondary synchronization channel symbol region.

According to the present invention, in the cell searching unit used for the forward-link frame structure shown in FIGS. 1 and 4, at the first operation of the cell searching procedure, a differential correlator or a correlator using a time domain waveform of the primary synchronization channel sequence is used to acquire one of the Sync block boundaries 141 -A and 141 -B, and at the second operation of the cell searching procedure, a secondary synchronization channel sequence correlator is used to acquire the synchronization channel sequence number, that is, a cell ID and the 10 msec frame boundaries 140-A and 140-B.

In order to improve the performance of the secondary synchronization channel correlation, coherent correlation may be performed by using a channel estimation value obtained from the primary synchronization channel, which will be described later in detail.

The number of secondary synchronization channel sequences used in the system is equal to or less than the number of scrambling codes (cell IDs) used in the system.

If the number of secondary synchronization channel sequences used in the system is equal to the number of scrambling codes (cell IDs) used in the system, the secondary synchronization channel sequence numbers are in one-to-one correspondence with the scrambling code numbers (or cell IDs).

If the number of secondary synchronization channel sequences used in the system is less than the number of scrambling codes (cell IDs) used in the system, the secondary synchronization channel sequence numbers are in correspondence with the scrambling code group numbers (cell ID group numbers). In this case, a third operation of the cell searching procedure is needed.

Namely, at the second operation of the cell searching procedure, the frame boundary and the scrambling code group information are acquired, and at the third operation, one of the scrambling code numbers is searched in the group. The third operation is performed on the common pilot signal in the forward-link by using a parallel correlator in the frequency domain.

According to another embodiment, the number of scrambling codes (the number of cell IDs) may be allocated so as to be a product of the number of primary synchronization channel sequences and the number of secondary synchronization channel sequences (the number of cell ID groups). For example, when the number of primary synchronization channel sequences and the number of scrambling codes (cell IDs) are 8 and 512, respectively, the 512 scrambling codes (cell IDs) may be allocated so that the number of secondary synchronization channel sequences is 64 (512 = 8 x 64). In this case, all the scrambling codes (cell IDs) are classified into 64 cell ID groups. Each cell ID group includes the 8 scrambling codes. In addition, the scrambling codes in each group are allocated so as to be identified by different primary synchronization channel sequences (or to be in one-to-one correspondence with the primary synchronization channel sequences).

In this case, the third operation of the cell searching procedure is not needed. More specifically, at the first operation of the cell searching procedure, time domain correlation is performed on a plurality of the primary synchronization channel sequences in order to acquire the Sync-block synchronization and the primary synchronization channel sequence information, so that one of the 8 scrambling codes in each cell ID group can be identified.

Next, at the second operation, the frame boundary and the secondary synchronization channel sequence number (cell ID group number) are acquired, so that one of the 64 cell ID groups can be identified. Accordingly, one scrambling code (cell ID) can be identified and detected. As a result, the cell ID information can be acquired as a combination of the primary synchronization channel sequence number and the secondary synchronization channel sequence number.

In the present invention, the cell searching procedure including the first and second operations is exemplified.

FIG. 5 is a block diagram illustrating a construction of a base station according to an embodiment of the present invention. Referring to FIG. 5, the base station includes a synchronization channel generating unit 500, a traffic channel and pilot generating unit 512, a diversity control unit 513, OFDM symbol mapping units 514-A and 514-B, scrambling units 515-A and 515-B, inverse-Fourier transform units 516-A and 516-B, CP inserting units 517-A and 517-B, IF/RF units 518-A and 518-B), and transmitting antennas 519-A and 519-B.

The traffic channel and pilot channel generating unit 512 generates the traffic data 230 or pilot data, which is shown in FIG. 2. The synchronization channel generating unit 500 generates the primary synchronization channel sequence 240 and the secondary synchronization channel sequence 250, which are shown in FIGS. 2 and 4 and defined by Equations 1 and 2. The OFDM symbol mapping units 514-A and 514-B map data values of the channels to positions in the frequency and time domains as shown in FIG. 2.

The scrambling units 515-A and 515-B multiply the scrambling code unique to the base station in the frequency domain with the OFDM symbols excluding the synchronization channel symbols obtained from the outputs of the OFDM symbol mapping units 514-A and 514-B, that is, the mapping result.

The inverse-Fourier transform units 516-A and 516-B perform inverse-Fourier transformation on the outputs of the scrambling units 515-A and 515-B to generate time domain signals.

The CP inserting units 517-A and 517-B insert the cyclic prefixes into the outputs of the inverse-Fourier transform units 516-A and 516-B in order to enable modulation of the OFDM signal even in a multi-path delay state of the channel.

The IF/RF units 518-A and 518-B perform up-conversion of the base band signals, that is, the outputs of the CP inserting units 517-A and 517-B and amplify the up-converted signals. The transmitting antennas 519-A and 519-B transmit the amplified signals.

In the embodiment shown in FIG. 5, two transmitting antennas 519-A and 519-B are used. If the base station according to the current embodiment has only the transmitting antenna 519-A without the transmitting antenna 519-B, the corresponding component such as the OFDM symbol mapping units 514-B, the scrambling unit 515-B, the inverse-Fourier transform unit 516-B, the CP inserting unit 517-B, the IF/RF unit 518-B, and the diversity control unit 513 can be omitted.

As shown in FIG. 5, the two transmitting antennas at the transmitting stage of the base station system are used to transmit the synchronization channel symbols with the transmit diversity. Now, the transmit diversity controlled by the diversity control unit 513 shown in

FIG. 5 will be described. In order to obtain the spatial diversity, the synchronization channel symbols in the adjacent Sync blocks are transmitted through different antennas. For example, the primary synchronization channel symbol and the secondary synchronization channel symbol in the first Sync block are transmitted through the first transmitting antenna 519-A, and the primary synchronization channel symbol and the secondary synchronization channel symbol in the second Sync block are transmitted through the second transmitting antenna 519-B.

The diversity control unit 513 performs switching for the diversity. More specifically, in a method of applying the time switching transmit diversity (TSTD) to the synchronization channel, the diversity control unit 513 switches the output of the synchronization channel generating unit 500 in order to provide the output to the OFDM symbol mapping unit 514-A or the OFDM symbol mapping unit 514-B.

Alternatively, instead of the spatial diversity or the TSTD diversity, a pre-coding vector switching transmit diversity may be applied.

In the method of using the pre-coding vector switching, pre-coding vectors for the two transmitting antennas are set to pre-coding vectors, for example, defined by Equation 3. Next, the primary synchronization channel symbol and the secondary synchronization channel symbol in the first Sync block are transmitted by using the first pre-coding vector, and the primary synchronization channel symbol and the secondary synchronization channel symbol in the second Sync block are transmitted by using the second pre-coding vector.

[Equation 3]

In Equation 3, the first and second elements of the pre-coding vector are weighting values of the first and second antennas, respectively.

In the method of applying the pre-coding vector switching diversity, the diversity control unit 513 performs the pre-coding vector switching in order to provide it to the OFDM symbol mapping unit 514-A and the OFDM symbol mapping unit 514-B. Alternatively, instead of the TSTD and the pre-coding vector switching diversity, a frequency switching transmit diversity (FSTD) may be applied.

In the method of applying the FSTD, sequence elements mapped to even-numbered subcarriers of the subcarrier allocated to the primary synchronization channel symbol are transmitted to the first antenna, and sequence elements mapped to odd-numbered subcarriers are transmitted to the second antenna. Similarly, sequence elements mapped to even-numbered subcarriers of the subcarrier allocated to the secondary synchronization channel symbol are transmitted to the first antenna, and sequence elements mapped to odd-numbered subcarriers are transmitted to the second antenna.

In the method of applying the FSTD, the diversity control unit 513 is also used.

FIG. 6 is a block diagram illustrating a construction of a mobile station receiver according to an embodiment of the present invention. The mobile station may include at least one receiving antenna. In the example illustrated in FIG. 6, the mobile station includes two receiving antennas.

Referring to FIG. 6, the mobile station receiver includes two receiving antennas 600-A and 600-B, two down-converting units 610-A and 610-B), a cell searching unit 620, a data channel modulating unit 630, a control unit 640, and a clock generator 650.

The frames that are transmitted as RF signals from the base stations are received through the receiving antennas 600-A and 600-B and converted into base band signals S1 and S3 by the down-converting units 610-A and 610-B.

The cell searching unit 620 searches for the target cell by using synchronization channel symbols contained in the down-converted signals S1 and S2.

As a result of the cell searching procedure, for example, a synchronization channel symbol timing, a frame boundary, and a cell ID of the target cell are detected. As an example of the to-be-searched target cell, there are an initial cell for which the mobile station initially searches and a neighbor cell for which the mobile station searches for handover.

The control unit 640 controls the cell searching unit 620 and the data channel modulating unit 630.

The control unit 640 controls timings of the data channel modulator 630 and descrambling based on the result of the cell searching procedure performed by the cell searching unit 620.

The control unit 640 may include a synchronization mode determining unit which receives a control channel of the home cell and determines whether or not the home cell and the adjacent cell operate in a base-station synchronization mode. Under the control of the control unit 640, the data channel modulating unit 630 demodulates the traffic channel data 230 (see FIG. 2) included in the down-converted signal.

All the hardware components of the mobile station operate in synchronization with a clock generated by the clock generator 650.

The cell searching unit 620 includes synchronization channel band filters 621 -A and 621 -B, a synchronization detection unit 622, and a cell detection unit 623.

The synchronization channel band filters 621 -A and 621 -B perform band-pass filtering of the down-converted signals S1 and S2 in order to pass only the synchronization channel acquisition band 210 from among the entire OFDM signal band 220 as described with reference to FIG. 2.

The synchronization detection unit 622 acquires a Sync block timing S5 by using the primary synchronization channel symbol included in the filtered signals S3 and S4.

The cell detection unit 623 detects the cell ID and the 10 msec frame timing from the received signal by using the Sync block timing S5.

In order to improve detection performance, the cell detection unit 623 may perform frequency offset estimation and correction before the detection of the cell ID and the frame timing.

FIG. 7 is a block diagram illustrating a construction of the synchronization detection unit 622 illustrated in FIG. 6 according to an embodiment of the present invention. Referring to FIG. 7, the synchronization detection unit 622 includes two correlation value calculating units 701 -A and 701 -B, a signal combining unit 702, a accumulating unit 703, and a timing detection unit 710.

If the even-numbered or odd-numbered subcarriers among the subcarriers in the synchronization channel acquisition band are allocated to the synchronization channel symbols as shown in FIG. 2, a differential correlator using the repetition pattern in the time domain illustrated in FIG. 3 can be used for the example illustrated in FIG. 7. Alternatively, the time domain waveform of the primary synchronization channel sequence expressed by Equation 1 may be stored in the mobile station receiver in advance, and a matching filter may be used to perform time domain correlation.

The outputs of the correlation value calculating units 701 -A and 701 -B are combined in the signal combining unit 720 and accumulated in the accumulating unit 730. According to the structure of the frame illustrated in FIG. 1 , the correlation value calculating units 701 -A and 701 -B generate 9,600 outputs per Sync block length, and the timing detection unit 710 detects a position of a sample corresponding to the peak value of the correlation values to determine the position of the sample as the synchronization channel symbol timing.

In addition, according to the embodiment of the present invention, the synchronization detection unit 622 may further include the accumulating unit 703 in order to improve the performance of the symbol synchronization detection.

The accumulating unit 703 adds the correlation values at the 9,600 sample positions to the correlation value of samples at the positions separated by the Sync block length from the sample positions.

In the current embodiment where the synchronization detection unit 622 further includes the accumulating unit 703, the timing detection unit 710 detects a maximum value of the 9,600 values stored in the accumulating unit 703 and outputs the sample position corresponding to the maximum value as the timing information S5.

FIG. 8 is a graph illustrating correlation values at sample positions calculated by the correlation value calculating unit illustrated in FIG. 7 according to an embodiment of the present invention

In the embodiment, it is assumed that there is no padding or noise in the channel between the base station transmitting stage and the mobile station receiving stage.

In FIG. 8, the horizontal axis is a time axis indicating sample indices, and the vertical axis indicates correlation values at sample positions.

Reference numeral 800 denotes a position of a first sample where the first correlation value calculating units 701 -A and 701 -B start performing correlation. The first correlation value calculating units 701 -A and 701 -B calculate 9,600 correlation values from the position of the first sample and provide the correlation values to the accumulating unit 703. Next, the first correlation value calculating units 701 -A and 701 -B calculate 9,600 correlation values from the position of the sample next to the first sample and provide the correlation values to the accumulating unit 703. The first correlation value calculating units 701 -A and 701 -B repeat the process on all the samples.

As shown in FIG. 8, there are peak positions in the M samples, which are obtained as a result of the repetition pattern of the synchronization channel symbols. FIG. 9 is a view illustrating a structure of input signals S3 and S4 applied to the cell detection unit illustrated in FIG. 6 based on the Sync block timing obtained by the synchronization detection unit illustrated in FIG. 6 according to an embodiment of the present invention. The cyclic prefixes of the region corresponding to the primary synchronization channel symbol and the region corresponding to the secondary synchronization channel symbol are removed based on the synchronization channel symbol timing 900 acquired by the synchronization detection unit 622, and the sample values corresponding to the primary synchronization channel positions and the secondary synchronization channel positions estimated in units of Sync block are input to the cell detection unit 623.

Reference numerals 910-A and 910-B denote the positions of the primary synchronization channel symbol obtained based on the synchronization channel symbol timing 900. Reference numerals 920-A and 920-B denote the positions of the secondary synchronization channel symbol obtained based on the synchronization channel symbol timing 900.

The sample values of the primary synchronization channel are used for coherent correlation of the secondary synchronization channel. The coherent correlation of the secondary synchronization channel will be described later in detail. FIG. 10 is a block diagram illustrating a construction of the cell detection unit 623 illustrated in FIG. 6 according to an embodiment of the present invention. Referring to FIG. 10, the cell detection unit 623 includes a frequency offset correlation unit 1000 and a boundary and cell ID detection unit 1010.

The frequency offset correlator 1000 sets the synchronization channel symbol timing 900 based on the output signal S5 of the synchronization detection unit 622. The frequency offset correlator 1000 stores the received signal samples 910-A and 910B at the 2 x Ns primary synchronization channel estimated positions provided from the synchronization channel band filters 621-A and 621-B over the Sync block length sections with reference to the synchronization channel symbol timing 900. The frequency offset correlator 1000 estimates the frequency offsets based on the received signal samples 910-A and 910B.

The frequency offset correlator 1000 corrects the frequency offsets of the 4 x Ns received signal samples 910-A, 920-A, 910-B, and 920-B based on the estimated frequency offsets. The frequency offset correlator 1000 provides the corrected 4 x Ns received signal samples S9 and S10 to the boundary and cell ID detection unit 1010.

The boundary and cell ID detection unit 1010 detects the scrambling code identifier and the 10 msec frame timing by using the frequency offset corrected samples S9 and S10 and transfers the scrambling code identifier and the 10 msec frame timing to the control block.

The boundary and cell ID detection unit 1010 performs Fourier transformation on the Ns received signal sample values at each position of the synchronization channel symbols 910-A, 920-A, 910-B, and 920-B to obtain the frequency domain signals. The boundary and cell ID detection unit 1010 performs correlation on all the available secondary synchronization channel sequences and all the available cyclic-shifted secondary synchronization channel sequences and selects a maximum correlation component.

The boundary and cell ID detection unit 1010 obtains the cyclic shift index values and the identifiers of the secondary synchronization channel sequences. As a result, the frame timing of the target cell as well as the cell ID of the target cell can be simultaneously acquired.

The primary synchronization channel components 910-A and 910-B are used to perform channel estimation for the coherent correlation of the secondary synchronization channel sequence.

FIG. 11 is a block diagram illustrating a construction of the boundary and cell ID detection unit 1010 illustrated in FIG. 10 according to an embodiment of the present invention. Referring to FIG. 11 , the boundary and cell ID detection unit 1010 includes second correlation value calculating units 1100-A and 1100-B, a combining unit 1110, a shifted sequence detection unit 1120, and an index searching unit 1130 having a frame boundary detection unit 1132 and a cell ID detection unit 1131.

The mobile station has no information on the cyclic shift index values and the identifiers of the secondary synchronization channel sequences which are mapped to the received secondary synchronization channel symbols 920-A and 920-B). The boundary and cell ID detection unit 1010 needs to perform Fourier transformation on the Ns received signal sample values at each position of the secondary synchronization channel symbols 920-A and 920-B to obtain the frequency domain signals. In addition, the boundary and cell ID detection unit 1010 needs to perform correlation on all the available secondary synchronization channel sequences and on all the available cyclic-shifted secondary synchronization channel sequences.

The second correlation value calculating units 1100-A and 1100-B perform Fourier transformation on the frequency-corrected synchronization channel symbols S9 and S10 transferred from the frequency offset correlation unit 1000 to obtained frequency domain signals. The second correlation value calculating units 1100-A and 1100-B perform correlation on all the available secondary synchronization channel sequences and on all the available cyclic-shifted secondary synchronization channel sequences.

The combining unit 1110 combines the outputs of the second correlation value calculating units 1100-A and 1100-B and provides P x L combined correlation values to the shifted sequence detection unit 1120. L indicates the number of secondary synchronization channel sequences (or the number of cell IDs). P indicates the number of shifts of the available secondary synchronization channel sequences (or the number of Sync blocks per frame), which is 2 in the examples illustrated in FIGS. 1 and 4.

The shifted sequence detection unit 1120 selects the maximum value from among the correlation values of the P x L shifted sequences. The shifted sequence detection unit 1120 provides the index value λ of the maximum value to a scrambling code (cell) identifier detection unit 1131 and a frame boundary detection unit 1132.

The scrambling code (cell) identifier detection unit 1131 performs an L-modulo operation on the output transferred from the shifted sequence detection unit 1120 in order to detect the scrambling code identifier (or cell ID) of the target base station. The L-modulo operation is expressed by Equation 4.

[Equation 4] scrambling code identifier = Λ mod L

λ is an output of the shifted sequence detection unit 1120. L indicates the total number of synchronization channel sequences used in the system. The frame boundary detection unit 1132 performs an operation defined by

Equation 5 on the output transferred from the shifted sequence detection unit 1120 to acquire the frame boundary information.

[Equation 5] frame boundary identifier = [ λ ÷ L]

The operator [x] is an operator for obtaining a maximum natural number from among natural numbers equal to or less than x.

The frame boundary identifier indicates by how many Sync blocks the 10 msec frame boundary is separated from the first position 930-A from among the P secondary synchronization channel symbol sections 920-A to 920-B used in the code and boundary detector 650. P may equal 2 in the example illustrated in FIG. 9 corresponding to FIGS. 1 and 4.

If the frame boundary identifier is 0, the 10 msec frame boundary is located at the position 930-A of the first secondary synchronization channel. If the frame boundary identifier is 1 , the 10 msec frame boundary is located at the position 930-B of the second secondary synchronization channel.

The second correlation value calculating unit 1100-A and 1100-B includes Fourier transform units 1101 -A and 1101 -B, de-mapping units 1102-A and 1102-B, channel estimating units 1103-A and 1103-B, and secondary synchronization channel correlation value calculating units 1104-A and 1104-B, respectively.

The Fourier transform units 1101 -A and 1101 -B perform Fourier transformation on the time domain samples 910-A, 920-A, 910-B, and 920-B corresponding to the synchronization channel symbol regions in order to acquire Ns frequency-converted values for the symbols.

The de-mapping units 1102-A and 1102-B acquires, from among the frequency-converted values P x N-i, values corresponding to the subcarriers of the primary synchronization channel sequences and P x N₂ values corresponding to the subcarriers of the secondary synchronization channel sequences (see FIG. 4).

The channel estimating units 1103-A and 1103-B perform channel estimation on the subcarriers based on the P x Ni primary synchronization channel frequency domain received sample values transferred from the de-mapping unit by using the previously stored primary synchronization channel sequences expressed by Equation 1. The secondary synchronization channel code correlation unit performs correlation on the P x N₂ secondary synchronization channel frequency domain received sample values transferred from the de-mapping unit and the available P x L shifted secondary synchronization channel sequences. In order to increase detection probability, the secondary synchronization channel code correlation unit corrects channel distortion for each subcarrier by using the channel estimated values transferred from the channel estimating units 1103-A and 1103-B and, after that, performs the correlation.

The correlation of the P x N₂ secondary synchronization channel frequency domain received sample values and the available P x L shifted secondary synchronization channel sequences is described with reference to FIG. 12.

FIG. 12 is a view illustrating a process of performing correlation on all available cyclic-shifted secondary synchronization channel sequences in order to acquire a frame boundary and a cell ID according to an embodiment of the present invention. When the outputs 1210 of the de-mapping unit for the positions of the first and second secondary synchronization channel symbols are given, it cannot be explicitly determined where the frame boundary is. Therefore, the correlation is performed on all the available sequences of Equation 6 (0-cyclic shift) and Equation 7 (1 -cyclic shift)

[Equation 6]

/ c(*) c(t) c(*) c (*) c(*) c(t) C-W \ lr - f\ λ T 1

Wo J °l > °2 >-)°JV_rl ' ^ύN₂ > ⁰JV₂H-I v "^2XN₂-I J ^Λ - U,l,..., ^ - 1

[Equation 7]

/ cW <?⁽*⁾ <?(*) c⁽*⁾ C-W C- (*) C ⁽*) \ lr - (\ Λ T 1

\^ύN₂ )°W₂+l V")^ύ2rf_rl »^O0 > °1₂ > °2 > — ^N₁-I J K - V,l,..., L. 1

As a result, the total number of outputs of the secondary synchronization channel correlation value calculating unit becomes 2 x L (P=2).

The P x L outputs of the code correlation unit according to each component are combined in the combining unit 1110 and transferred to the shifted sequence detection unit 1120.

The shifted sequence detection unit 1120 detects the maximum value of the P x L outputs of the combining unit 1110 and transfers the index value Λ corresponding to the maximum value to the scrambling code (cell) identifier detection unit 1131 and the frame boundary detection unit 1132.

The scrambling code (cell) identifier detection unit 1131 performs an L-modulo operation (expressed by Equation 4) on the output λ transferred from the shifted sequence detection unit 1120 in order to detect the scrambling code identifier(or cell ID) of the target base station and transfers the scrambling code identifier(or cell ID) to the control unit 640.

The frame boundary detection unit 1132 performs an operation defined by Equation 5 on the output transferred from the shifted sequence detection unit 1120 in order to acquire the frame boundary information and transfers the frame boundary information to the control unit 640.

FIG. 13 is a flowchart of a cell searching procedure of a mobile station according to an embodiment of the present invention.

As described above, in the first operation S 1300, the Sync-block synchronization is acquired. In the second operation S1310, the frequency offset is corrected, and the frame boundary and the scrambling code identifier are detected.

In operation S1320, the maximum value from among the correlation values of the synchronization channel sequences detected in the second operation and the shifted sequences is compared with a predetermined threshold value. If the maximum value is larger than the threshold value, the cell searching procedure is completed. If the maximum value is smaller than the threshold value, the cell searching procedure is repeated from the first operation.

FIG. 14 is a flowchart of an adjacent cell searching procedure in a case where a home cell and an adjacent cell operate in synchronization with a base station according to an embodiment of the present invention. FIG. 15 is a flowchart of an adjacent cell searching procedure using a process of determining whether or not a home cell and an adjacent cell operates in synchronization with a base station according to an embodiment of the present invention. In the above embodiments, the initial cell searching procedure, which is performed with the mobile station initially powered on, is described. As in other embodiments, the handover cell searching procedure may be performed when the mobile station is in an idle state or a busy state. During the handover cell searching procedure, the mobile station has already been in the state in which the mobile station corrects the frequency from the home cell signal. Therefore, the frequency offset correction operation shown in FIG. 10 may be by-passed (S 1520).

When the home cell and the target base station, which the mobile station searches for, are in the base-station non-synchronization mode, in the first operation similarly to the initial synchronization acquiring operation, the Sync-block synchronization of the target base station is acquired by using the primary synchronization channel received from the target base station (S1510). Next, in the second operation, the cyclic shift correlation of the secondary synchronization channel is performed in order to acquire the cell ID of the target cell and the frame boundary (S1400 and S1530).

The adjacent cell searching operations for the handover are the same as those illustrated in FIG. 13 (S1410 and S1540).

When the home cell and the target base station, which the mobile station searches for, are in the base-station synchronization mode, since the frame boundaries of the home cell and the adjacent cell match each other, the first operation of the cell searching procedure may be omitted (S1500).

In this case, only the second operation of the cell searching procedure is needed. Since the frame boundary is known, the correlation operation is performed on only the O-shifted sequence, that is, the secondary synchronization channel sequence corresponding to the Hypothesis-0 operation illustrated in FIG. 2.

In this case, the delays of the signals received from the home cell and the adjacent cell may be different from each other. Therefore, after the cell ID of the target cell is detected, the accurate frame timing and OFDM symbol timing of the received signal from the adjacent cell can be acquired based on the time domain signal of the secondary synchronization channel sequence of the adjacent cell.

The invention can also be embodied as computer readable codes on a computer readable recording medium. The computer readable recording medium is any data storage device that can store data which can be thereafter read by a computer system. Examples of the computer readable recording medium include read-only memory (ROM), random-access memory (RAM), CD-ROMs, magnetic tapes, floppy disks, optical data storage devices, and carrier waves (such as data transmission through the Internet). The computer readable recording medium can also be distributed over network coupled computer systems so that the computer readable code is stored and executed in a distributed fashion.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The exemplary embodiments should be considered in descriptive sense only and not for purposes of limitation. Therefore, the scope of the invention is defined not by the detailed description of the invention but by the appended claims, and all differences within the scope will be construed as being included in the present invention.

INDUSTRIAL APPLICABILITY

In an Orthogonal Frequency-Division Multiplexing (OFDM) cellular system according to the present invention, a time taken to perform a cell searching procedure of a mobile station can be reduced, and a cell searching unit having low complexity can be implemented.

In addition, according to a synchronization acquiring method of the present invention, the synchronization can be acquired with low complexity.

Claims

1. A cell searching method in an OFDM (Orthogonal Frequency-Division Multiplexing) cellular system including a plurality of cells allocated with scrambling codes unique to the cells, wherein each base station transmits a frame of each cell and a terminal searches for a target cell by using the frame received from the base station, the cell searching method comprising:

(a) acquiring the synchronization of Sync-block which corresponding to a period of a primary synchronization channel symbol included in the frame received by the terminal and a primary synchronization channel sequence number by using the primary synchronization channel symbol; and

(b) acquiring a frame boundary and the cell ID by using a secondary synchronization channel symbol included in the frame received by the terminal based on the Sync-block synchronization and the primary synchronization channel sequence number.

2. The cell searching method of claim 1 , wherein in (a), a primary synchronization channel symbol which is mapped to a subcarrier occupying each Sync block in a frequency domain is used.

3. The cell searching method of claim 1 , wherein in (a), the Sync-block synchronization is acquired by calculating a differential correlation value using a repetition pattern in a time domain of the primary synchronization channel symbol or by calculating a correlation value using a signal in time domain corresponding to the primary synchronization channel sequence when the primary synchronization channel symbol occupies all the subcarriers applied to the primary synchronization channel in a frequency domain.

4. The cell searching method of claim 1 , wherein in (b), the frame boundary and the target cell of the terminal are acquired based on the acquired Sync-block synchronization and the primary synchronization channel sequence number by performing cyclic shift correlation on the secondary synchronization channel symbol which corresponds to cell IDs or cell ID groups of cells and are mapped so that a length of the secondary synchronization channel sequence is equal to a total length of the subcarriers occupied by the secondary synchronization channel symbols included in the frame in a frequency domain.

5. The cell searching method of claim 4, wherein the frame boundary and the target cell ID are acquired based on the difference of the secondary synchronization channel sequences in the cells and in a frame

6. The cell searching method of claim 1 , wherein (b) comprising: (b1 ) estimating a frequency offset by using the primary synchronization channel symbol existing in each Sync block section with reference to the Sync block synchronization;

(b2) correcting a frequency offset of the synchronization channel symbol based on the estimated frequency offset; and (b3) detecting a frame timing and a target cell ID of the terminal by using the corrected synchronization channel symbol.

7. The cell searching method of claim 1 , wherein a frame which is obtained by passing only a synchronization channel acquisition band from signal bands of the OFDM cellular system is used.

8. The cell searching method of any one of claims 1 to 7, wherein two primary synchronization channel symbols and two secondary synchronization channel symbols exist in the frame.

9. The cell searching method of claim 8, wherein, when one primary synchronization channel sequence is used in the OFDM cellular system, the same primary synchronization channel sequence of the primary synchronization channel symbols are used among the cells, or when multiple primary synchronization channel sequences are used in the OFDM cellular system, different primary synchronization channel sequences of the primary synchronization channel symbols are used among the neighboring cells.

10. The cell searching method of claim 8, wherein the two secondary synchronization channel symbols uses the same sequence among the frame, the two secondary synchronization channel symbols use different sequences among Sync blocks in the frame, and the two secondary synchronization channel symbols use different sequences among base stations.

11. The cell searching method of claim 10₁ wherein the secondary synchronization channel symbol includes information which is mapped to a cell ID or a cell ID group of a cell which transmits a frame including the secondary synchronization channel symbol.

12. The cell searching method of claim 8, wherein the repetition period of the primary synchronization channel sequence is one Sync block in one frame, and the repetition period of the secondary synchronization channel sequence is one frame.

13. The cell searching method of any one of claims 1 to 7, wherein the synchronization channel symbol in the frame occupies a portion of the bands used in the OFDM cellular system with reference to a center of the frequency domain.

14. The cell searching method of any one of claims 1 to 7, wherein the primary synchronization channel symbol and the secondary synchronization channel symbol are constructed based on the TDM (time-division multiplexing) and are disposed to be adjacent to each other.

15. A frame transmitting method in an OFDM cellular system including a plurality of cells allocated with scrambling codes unique to the cells, wherein a base station in an arbitrary cell transmits a frame, the frame transmitting method comprising:

(a) generating a primary synchronization channel sequence including Sync-block timing information and a secondary synchronization channel sequence including a frame boundary information, a cell ID of the cell, or a cell ID group in which the cell ID is included; and

(b) generating a frame including the primary synchronization channel symbols and the secondary synchronization symbol and transmitting the frame.

16. The frame transmitting method of claim 15, wherein the frame is generated and transmitted so as to have two primary synchronization channel symbols and two secondary synchronization channel symbols.

17. The frame transmitting method of claim 16, wherein, when one primary synchronization channel sequence is used in the OFDM cellular system, the same primary synchronization channel sequence of the primary synchronization channel symbols is used among the cells, or when the one or more primary synchronization channel sequences are used in the OFDM cellular system, different primary synchronization channel sequences of the primary synchronization channel symbols are used among the adjacent cells.

18. The frame transmitting method of claim 16, wherein the primary synchronization channel symbol occupies a half of the allocated subcarriers or all the allocated subcarriers.

19. The frame transmitting method of claim 16, wherein the secondary synchronization channel symbol includes information which is mapped to a cell ID or a cell ID group of a cell which transmits a frame including the secondary synchronization channel symbol.

20. The frame transmitting method of claim 16, wherein the repetition period of the primary synchronization channel symbol is one Sync block in one frame, and the repetition period of the secondary synchronization channel symbol is one frame.

21. The frame transmitting method of claim 15, wherein the synchronization channel symbol in the frame occupies a portion of the bands used in the OFDM cellular system with reference to a center of the frequency domain.

22. The frame transmitting method of claim 15, wherein the primary synchronization channel symbol and the secondary synchronization channel symbol are constructed based on the TDM and are disposed to be adjacent to each other.

23. The frame transmitting method of claim 15, wherein in (b), the synchronization channel symbols are transmitted by using spatial diversity, time switching transmit diversity, or pre-coding vector switching transmit diversity.