KR100827064B1 - Method and Apparatus for transmitting synchronization signal in OFDM based cellular communication systems - Google Patents
Method and Apparatus for transmitting synchronization signal in OFDM based cellular communication systems Download PDF
ofdm symbol
subcarriers
KR20070076248A (en
곽용준
이주호
조준영
삼성전자주식회사
2006-01-18 Application filed by 삼성전자주식회사 filed Critical 삼성전자주식회사
2007-07-24 Publication of KR20070076248A publication Critical patent/KR20070076248A/en
230000001413 cellular Effects 0.000 title claims abstract description 13
238000004891 communication Methods 0.000 title claims description 11
230000001360 synchronised Effects 0.000 claims description 19
238000000034 methods Methods 0.000 abstract description 35
230000000875 corresponding Effects 0.000 abstract description 9
230000001702 transmitter Effects 0.000 description 3
241000212893 Chelon labrosus Species 0.000 description 2
206010057190 Respiratory tract infections Diseases 0.000 description 2
229920000970 Repeated sequence (DNA) Polymers 0.000 description 1
230000002596 correlated Effects 0.000 description 1
230000003111 delayed Effects 0.000 description 1
238000010295 mobile communication Methods 0.000 description 1
230000000051 modifying Effects 0.000 description 1
238000010845 search algorithm Methods 0.000 description 1
H04W56/00—Synchronisation arrangements
H04L27/2626—Arrangements specific to the transmitter
H04L5/0014—Three-dimensional division
H04L5/0016—Time-frequency-code
H04L5/0048—Allocation of pilot signals, i.e. of signals known to the receiver
The present invention relates to a method and apparatus for transmitting a synchronization signal for an OFDM-based cellular system. The cell search procedure, which is often applied in an OFDM-based cellular system, is divided into a step of acquiring frame timing synchronization and a step of acquiring a cell-specific scrambling code. In the present invention, in designing a channel for acquiring a frame timing synchronization and a channel for acquiring a cell-specific scrambling code, considering a characteristic of each step of synchronization acquisition, a channel is designed by changing a frequency reuse coefficient applied to a synchronization channel corresponding to each step So that the performance of each step can be improved.
OFDM, cell search, synchronization, P-SCH, S-SCH
BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to an OFDM-based cellular radio communication system,
1 is a diagram showing a frame structure of a physical channel of a WCDMA system
2 is a diagram showing a frame structure in a time domain in an OFDM-based system;
3 is a diagram showing a structure in a frequency and time domain of an OFDM transmission signal
4 is a diagram showing a structure in a time domain of an OFDM symbol
5 is a diagram showing an example of a preamble for downlink synchronization acquisition
6A is a diagram showing a structure in a time domain of a synchronization channel according to the first embodiment of the present invention;
6B is a diagram showing a structure in a frequency and time domain of a synchronization channel according to the first embodiment of the present invention
6C is a diagram showing a structure in a frequency and time domain when a synchronization channel according to the first embodiment of the present invention is transmitted only in a part of system bands
7 is a view showing an example of a multi-cell model according to the first embodiment of the present invention
8A is a diagram illustrating an example in which S-SCHs of respective cells are transmitted with different distributed frequency resources with a frequency reuse factor of 3 between cells belonging to the same Node B according to the first embodiment of the present invention
FIG. 8B illustrates an example in which the S-SCH of each cell is transmitted to a different sub-band with a frequency reuse factor of 3 between cells belonging to the same Node B according to the first embodiment of the present invention.
9 is a diagram illustrating an example in which terminals having different bandwidths exist in a system band in a system supporting scalable bandwidth
FIG. 10 is a diagram illustrating an example of mapping an S-SCH to a frequency resource among cells belonging to another Node B according to the first embodiment of the present invention
11 is a diagram illustrating a cell search procedure according to the synchronization channel structure of the first embodiment of the present invention
12 is a diagram showing an S-SCH code acquisition procedure in the cell search procedure of FIG.
13 is a diagram illustrating a structure of a terminal cell search receiver according to a synchronization channel structure of a first embodiment of the present invention
14A is a diagram showing a structure in a time domain of a synchronization channel according to the second embodiment of the present invention
14B is a diagram showing a structure in a frequency and time domain of a synchronization channel according to a second embodiment of the present invention;
15 is a diagram illustrating a cell search procedure according to the synchronization channel structure of the second embodiment of the present invention
16 is a diagram showing the structure of a cell search receiver according to the synchronization channel structure of the second embodiment of the present invention
17 is a diagram showing a structure in a frequency and time domain of a synchronization channel according to a third embodiment of the present invention;
18 is a diagram illustrating a cell search procedure according to the sync channel structure of the third embodiment of the present invention
19 is a diagram illustrating a structure of a transmitter for generating a synchronization channel according to an embodiment of the present invention;
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structure of a downlink synchronization channel and a method of acquiring a synchronous acquisition cell search in a cellular wireless communication system based on Orthogonal Frequency Division Multiplex (OFDM) .
Recently, OFDM technology has been widely applied to the technologies of broadcast and mobile communication systems. OFDM technology eliminates interference between multipath signal components present in a wireless communication channel, ensures orthogonality among multiple access users, and enables efficient use of frequency resources. Therefore, it is a technique used for high-speed data transmission and broadband system as compared with the existing code division multiple access (CDMA) technology.
In order to demodulate received data and control information in a cellular wireless communication system, synchronization and cell search must be performed first between the transmitter and the receiver.
1 shows a structure of a dedicated physical channel (DPCH) for data transmission applied in a WCDMA (Wideband CDMA) cellular radio communication standard downlink.
As shown in FIG. 1, the radio frame 102 having a length of 10 ms is formed of 15 slots, and one slot is composed of 2,560 chips of 3.84 Mcps.
The downlink synchronization and cell search process refers to a process of finding a frame start point of physical channels transmitted in a cell to which a user belongs and finding a cell-specific scrambling code applied at the time of transmitting the physical channel. Hereinafter, the above process is referred to simply as a cell search.
In the WCDMA system, a mobile station acquires synchronization of slot timing as a first step of cell search. The second step is to obtain a frame timing synchronization and a group of cell specific scrambling codes applied to the cell. In a final step, a cell-specific scrambling code applied to the base station is obtained by searching for cell-specific scrambling codes belonging to the code group based on the obtained cell group code.
Thus, the UE can demodulate the received data and control channels by acquiring the frame timing synchronization and the scrambling code information of the cell to which the UE belongs, and can recognize the cell ID by demodulating the BCH (Broadcasting Channel) . In the asynchronous system, since the number of possible cell-specific scrambling codes is considerably large, the scrambling code group detection and the cell-specific scrambling code detection are separately performed as described above.
Meanwhile, FIG. 2 shows a frame structure in a time domain in an OFDM-based system.
Referring to FIG. 2, one radio frame 202 is composed of OFDM symbols 200 of L (# 0, # 1, # 2, ..., # (L-1)). In addition, the frame structure of the OFDM system should be shown not only in the time domain but also in the frequency domain as shown in Fig. The OFDM technique is a multi-carrier transmission technique in which data to be transmitted and control channel information can be divided into a plurality of subcarriers for parallel transmission.
3 shows a structure in the frequency and time domain of an OFDM transmission signal.
3, an OFDM symbol 300 is composed of subcarriers 302 of N (# 0, # 1, # 2, ..., # (N-1) have. Modulation symbols 304 of transmission information are carried on the respective sub-carriers 302 and transmitted simultaneously in parallel.
In addition, the OFDM symbol structure can be represented as shown in FIG. 4 in detail in the time domain.
In the N subcarrier symbols 302 shown in FIG. 3, The IFFT (Inverse Fast Fourier Transform) of FIG. 4 is applied, s 0 , s 1 , ... , and N samples (408) of s N-1 can be obtained. The OFDM symbol 404 is generated by copying and attaching the M samples (s NM , ..., s N-2 , s N-1 ) 406 of the rear part of the N samples to the beginning of the OFDM symbol interval do. The M sample parts copied and attached at the beginning as described above are referred to as a CP (Cyclic Prefix) 400, and the original N sample parts are referred to as useful symbols 402.
Considering the above-described frame structure of the OFDM system, a cell search method similar to that applied in the WCDMA system can be applied in the OFDM-based cellular wireless communication system. For example, IEICE Trans. Commun.Vol. E86-B, No. 1, January 2003 (M. Tanno, H. Atarashi, K. Higuchi, M. Sawahashi, "Three-step cell search algorithm exploiting common pilot channel for OFCDM broadband wireless access" The cell search procedure in the OFDM based system is also composed of three steps. That is, cell search is performed in the same procedure as that applied in WCDMA, except that OFDM symbol timing synchronization is performed instead of slot timing synchronization in the first step. This is because, as shown in FIG. 2, in the OFDM system, a basic unit constituting a frame becomes an OFDM symbol. Considering the OFDM symbol structure of FIG. 4, the OFDM symbol timing can be detected using the property that the samples constituting the CP 400 are the same as the M samples 406 after the effective OFDM symbol interval. In the second step, the frame timing synchronization and the scrambling code group are obtained, and in the third step, the cell specific scrambling code is finally found in the same manner as in the case of the WCDMA system.
However, since the synchronization acquisition procedure simultaneously performs the frame synchronization acquisition and the cell group code search in the second step, there is a problem that the complexity of the second step is high and the synchronization acquisition time is long. This problem makes it impossible to quickly perform the handover due to the delay of the cell search time, especially when the terminal must perform handover to another cell. The problem of the second stage of cell search is closely related to the structure of the downlink synchronization channel. In the synchronous channel structure proposed in the reference document, a cell group code is placed in the sub-carriers belonging to the first OFDM symbol in the frame, and a cell-specific scrambling code is carried in the other OFDM symbols. Due to the above-described frame structure, the terminal can detect the start point of a frame only by detecting a group to which the scrambling code applied to the cell belongs.
The above-mentioned frame synchronization acquisition performance problem can be improved by repeating the same sequence in the time domain as in the synchronous channel structure applied to the IEEE 802.16 standard shown in FIG. The synchronous preamble OFDM symbol 500 of FIG. 5 is located in front of the frame and is composed of three repeated sequences 502, 504, and 506. More precisely, the three sequences 502, 504 and 506 can be expressed as products of any complex number e jq of size 1 with each other. When the terminal detects the preamble 500, the start point of the frame can be known. The terminal uses the property that the sequences 502, 504, and 506 are identical even if they do not know the precise pattern of the sequences 502, 504, and 506 in the preamble, and the OFDM symbol timing It is possible to detect the preamble by searching and obtain frame synchronization. Since the MS does not need to know the sequence applied to the preamble, the MS can obtain excellent frame synchronization performance as compared with the case of applying the synchronous channel structure proposed in the reference document.
Since the IEEE 802.16 system is a synchronous system in which synchronization of transmission signals is synchronized between base stations, as shown in FIG. 5, a mobile station can successfully perform cell search by configuring a synchronization channel using only one OFDM symbol 500. However, in an asynchronous system or a synchronous system having a large number of possible cell scrambling codes, it is difficult to ensure the performance and complexity to smoothly acquire downlink frame synchronization and cell specific scrambling codes using only one OFDM symbol preamble. Therefore, it is common to construct a synchronization channel with two or more OFDM symbols and to divide the synchronization channel into several stages like the above-described WCDMA system to obtain frame timing synchronization and cell specific codes. At this time, the performance and the complexity of the cell search step vary depending on how the synchronization channel is configured.
SUMMARY OF THE INVENTION The present invention provides a downlink synchronization channel structure for enabling fast cell search in a system constituting a synchronization channel using at least two OFDM symbols.
In addition, the present invention provides a synchronous channel structure capable of smoothly performing cell search on a neighboring cell so that a terminal located at a cell boundary can quickly handover to an adjacent cell.
Also, the present invention provides a synchronous channel structure capable of performing cell search smoothly by minimizing interference from neighboring cells even when a terminal is located at a cell boundary in a system supporting scalability of a system and a terminal bandwidth .
The present invention also provides a method for improving frame synchronization acquisition performance by combining synchronization channels from neighboring cells when a terminal is located at a cell boundary.
According to an embodiment of the present invention, there is provided a method for transmitting a synchronization signal for cell search in an orthogonal frequency division multiplexing (OFDM) -based cellular wireless communication system, the method comprising: at a first at least one first OFDM symbol interval, Transmitting a sequence constituting a primary-SCH through first subcarrier symbols mapped to N subcarriers by applying a frequency reuse factor 1; and transmitting, in at least one second OFDM symbol interval, Transmitting a sequence constituting a second synchronization channel through second subcarrier symbols mapped to M subcarriers of the N subcarriers by applying a frequency reuse factor greater than 1, and transmitting the second subcarrier symbols except for the first and second OFDM symbol intervals And transmitting a sequence of channels other than the first and second synchronization channels in the OFDM symbol intervals of the first and second synchronization channels.
According to another aspect of the present invention, there is provided an apparatus for transmitting a synchronization signal for cell search in an orthogonal frequency division multiplexing (OFDM) based cellular radio communication system, the apparatus comprising: A second synchronization channel sequence generator for generating a sequence constituting a second synchronization channel; a third channel sequence generator for generating a sequence constituting a channel other than the first and second synchronization channels; In a first OFDM symbol interval, first sub-carrier symbols mapped to N sub-carriers by applying a frequency reuse factor of 1 to the sequence constituting the first synchronization channel, and in at least one second OFDM symbol interval, The sequence constituting the second synchronization channel is mapped to M subcarriers among the N subcarriers by applying a frequency reuse factor greater than 1 And transmitting a second subcarrier symbol sequence other than the first and second subcarrier symbols in the OFDM symbol intervals other than the first and second subcarrier symbols, And a subcarrier mapping unit for subcarrier mapping.
The operation principle of the preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings. In the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear. The following terms are defined in consideration of the functions of the present invention, and these may be changed according to the intention of the user, the operator, or the like. Therefore, the definition should be based on the contents throughout this specification.
The present invention relates to a synchronization channel structure for downlink cell search in an OFDM-based cellular system. The synchronous channel structure proposed in the present invention is based on performing cell search in two steps as follows.
* Step 1: Obtain frame timing synchronization using P-SCH (Primary Synchronization Channel)
Step 2: Obtain cell-specific scrambling code using S-SCH (Secondary Synchronization Channel)
If the cell search is performed by separating the frame timing synchronization acquisition and the cell group scrambling code acquisition procedure as described above, the frame timing acquisition performance can be improved and the cell search complexity can be improved. The present invention proposes a synchronous channel structure suitable for each step so as to maximize the synchronization acquisition performance in each cell search step. Depending on the system, the cell group code acquisition may be required before acquiring the cell specific scrambling code. In this case, it is assumed that the cell group code acquisition procedure is performed after the first frame timing synchronization acquisition.
The basic concept of the synchronous channel structure proposed by the present invention is that all the cells can apply the same code to the P-SCH for frame synchronization acquisition in the cell search step 1, and in the case of acquiring the 2-step cell specific scrambling code It is based on the fact that each cell applies a different code to the S-SCH. Therefore, in the case of the P-SCH, by setting the frequency reuse factor (FRF) to 1 so that all the cells transmit the P-SCH using the same frequency resource, the UE combines the P- So that the frame timing synchronization can be smoothly obtained and the frequency error between the base station and the terminal can be accurately estimated. On the other hand, since the cell ID is generally used for the S-SCH in the case of the S-SCH, the frequency reuse factor (FRF) is set to be larger than 1 so that different frequency resources are used for S-SCH transmission Thereby preventing interference between S-SCHs of neighboring cells. That is, the performance of acquiring the cell-specific scrambling code using the S-SCH is improved.
≪ Embodiment 1 >
6A shows a structure of a synchronization channel in a time domain according to the first embodiment of the present invention.
6A, the P-SCH 600 is transmitted over two OFDM symbol periods and the S-SCH 602 is transmitted over one OFDM symbol period. P-SCH 600 is used for DL frame timing synchronization acquisition and S-SCH 602 is used for cell specific scrambling code acquisition. The sequences in the time domain transmitted to the two OFDM symbols constituting the P-SCH 600 are the same. 6A, the CP 604 of the second OFDM symbol is located behind the effective symbol period. When the terminal estimates the frequency error between the base station and the terminal using the P-SCH 600, the sub- the frequency error of an integral multiple of the spacing is estimated accurately. The location or existence of the CP 604 in the P-SCH 600 is not directly related to the technology proposed in the present invention, and thus a detailed description thereof will be omitted.
FIG. 6B shows a synchronous channel structure according to the first embodiment of the present invention in a two-dimensional region of time and frequency.
As can be seen in FIG. 6B, the sequence {p n , n = 1, 2, ...) applied to the P-SCH 610 , N} are repeatedly transmitted during two OFDM symbol periods, and symbol p n constituting the sequence of P-SCH 610 of length N is transmitted on each subcarrier. Since the P-SCH 610 sequence is the same in the first 614 and second 616 OFDM symbol intervals, the OFDM sample sequences in the time domain obtained by IFFTing the P-SCH 610 sequence are also the same . In FIG. 6B, the P-SCH 610 and the S-SCH 612 are transmitted in the OFDM symbol interval adjacent to the start point of the frame. Even if they are not adjacent to each other or are transmitted in another OFDM symbol interval, If only the index of the OFDM symbol to be transmitted is promised, there is no problem in successfully performing the downlink cell search. Therefore, in the present invention, there is no restriction on the positions of the synchronization channels in the frame.
In FIG. 6B, the P-SCH 610 sequence {p n , n = 1, 2, ... , N} are transmitted using all subcarriers, whereas the S-SCH 612 sequence {s i , m , m = 1, 2, ... , M} are mapped to subcarriers at three subcarrier intervals and no signal is transmitted to the null subcarrier 618 between the subcarriers to which the S-SCH 612 sequence is transmitted. Where i represents the index of the cell unique sequence. In the first embodiment of the present invention, it is assumed that the P-SCH 610 applies the same sequence between cells and the S-SCH 612 applies a cell-specific sequence.
6B shows a case where the synchronization channel is transmitted to the entire system band 611 while FIG. 6C shows a case where the synchronization channel is transmitted only to some band 620 of the entire system band 628 Respectively.
In FIG. 6C, a channel other than the sync channel may be transmitted in the dummy frequency band 622 that is not used for the synchronization channel transmission. In such a case, the sequence carried on the P-SCH 624 and the S-SCH 626 is shorter (Q <N) than the case of FIG. 6B using the entire system band. The technique of applying the frequency reuse factor to be described below to the P-SCH and the S-SCH may be applied to both the cases of FIGS. 6B and 6C.
FIG. 7 illustrates an example of a system composed of three cells for each Node B to illustrate mapping of an inter-cell S-SCH according to the first embodiment of the present invention.
7, the Node B # 1 706 is composed of three cells: a cell A 1 700, a cell A 2 702, and a cell A 3 704, and the rest of the Node Bs B # 2, Node B # 4) are also each composed of three cells. In the cell structure as described above, for example, a terminal located at a boundary between cell A1 700 and cell A3 704 receives signals of similar power from the two cells 700 and 704. In this situation, since all the cells apply the same sequence to the P-SCH and the cells under the same Node B generally apply the same transmission timing, the terminal transmits the P (P) transmitted from the cell A1 700 and the cell A3 704 -SCH are soft-combined to receive relatively higher power than individual signals. Therefore, the P-SCH can be detected well. On the other hand, since different codes are applied to cells for S-SCH, the S-SCH is not soft combined unlike the P-SCH, and interference occurs between S-SCHs of other cells transmitted using the same frequency resource.
8A and 8B illustrate inter-cell frequency resource mapping of the S-SCH according to the first embodiment of the present invention.
In the S-SCH structure of FIG. 8A, since the cell re- ceiver 800, cell A2 802, and cell A3 804 under the same Node B transmit the S-SCH using different subcarriers with a frequency reuse factor of 3, There is no interference between the S-SCHs of the three cells (800, 802, and 804). 8A and 8B, symbols of the S-SCH sequence of the corresponding cell are transmitted to the S-SCH subcarriers 808 and 818, and nothing is transmitted to the null subcarriers 806 and 816. 8A and 8B, {s i , m}, {s j , m} and {s l , m} are S-SCH sequences transmitted to data subcarriers 808 and 818 of cell A 1, , And the subscripts i, j, l represent the index of the corresponding S-SCH sequence, respectively.
As described above, in the first embodiment of the present invention, the frequency reuse factor of the S-SCH is set to 3. However, even when another frequency reuse factor is applied, the S- Frequency resources can be allocated.
8A, subcarriers are used at intervals of three subcarriers so that the cells do not overlap each other, and the S-SCH codes of the respective cells are allocated to all sub-bands or S-SCH allocated subcarriers ) And transmitted. On the other hand, in the S-SCH structure of FIG. 8B, the sub-bands allocated for the entire band or S-SCH transmission are divided into three bands of consecutive sub-carriers. Each cell is divided into sub- -SCH is transmitted.
As shown in FIG. 8A, the frequency resources used for S-SCH transmission are interleaved and spread among the cells to be transmitted, which has the following advantages over the structure of FIG. 8B.
First, in the structure of FIG. 8A, since the S-SCH is spread and transmitted in a wide band in each cell, a higher frequency diversity gain can be obtained as compared with the structure of FIG. 8B. Second, in a system supporting scalable bandwidth, if the S-SCH spreads to the entire system band and is transmitted, a terminal supporting a bandwidth smaller than the bandwidth of the base station receives the S-SCH of the adjacent cell even in a handover situation The cell search for the neighboring cell can be performed.
9 shows a situation in which three terminals, UE (User Equipment) A 902, UE B 904, and UE C 906, are receiving services in system 900 in the 20 MHz bandwidth 908.
The UE A 902 is capable of transmitting and receiving signals up to a 5 MHz bandwidth and is receiving signals in the 5 MHz band 910 on the left hand side of the entire 20 MHz band and the UE B 904 is also capable of transmitting and receiving signals up to 5 MHz bandwidth And receives service in the middle portion 912 of the 20 MHz band. And UE C 906 is receiving service from the 20 MHz total band 914.
In this situation, the UE A 902 that has received the data in the band 910 shown in FIG. 9 receives the S-SCH from the cell A2 812 and the cell A3 814 under the S-SCH structure of FIG. 8B Can not. This is because the S-SCH from the cell A2 812 and the cell A3 814 in FIG. 8B is transmitted in a band different from that in which the UE A 902 is located. Therefore, in order for the UE A 902 to receive the S-SCH from the cell A2 812 and the cell A3 814, the cell search execution time for the neighboring cell must be agreed between the system and the UE A 902 . That is, at the agreed search execution time, the UE A 902 moves to the frequency band in which the S-SCH from the cell A2 812 or the cell A3 814 is transmitted and transmits the cell A2 812 or the cell A3 814, SCH, and performs cell search using the received S-SCH. However, when the mobile station can not always receive the synchronization channel of the neighboring cell and only receives the synchronization channel in a specific period, there is a restriction on performing a fast handover and the synchronization acquisition procedure becomes complicated.
In the case of the P-SCH, as shown in FIG. 6B, since it is transmitted in the entire band, some or all of the P-SCH sequences can always be received irrespective of the band in which the UE A 902 is located.
8A, since the S-SCHs from the cell A1 800, the cell A2 802, and the cell A3 804 are spread over the entire band and transmitted, the UE A 902 or UE A terminal having a bandwidth less than the system bandwidth, such as B 904, can also receive the S-SCH from the neighboring cell regardless of the band in which the data is being received. In more detail, among the S-SCH sequences transmitted from the cell A1 800, the cell A2 802, and the cell A3 804 transmitted in the full band in FIG. 8A, the UE A 902 is transmitted in the 5 MHz band to which it belongs It is possible to perform cell search for the S-SCH by receiving only the sequence part.
On the other hand, in the case of a synchronous system in which transmission timings coincide with those of other Node Bs, as shown in FIGS. 8A and 8B, S-SCHs are transmitted using different frequency resources among neighboring cells belonging to different Node Bs, It is possible to prevent interference between the S-SCHs of the cells belonging to the S-SCH.
For example, when the S-SCH is mapped as shown in FIG. 10 under the cell structure of FIG. 7, different frequency resources are used for S-SCH transmission between adjacent cells, thereby preventing mutual interference.
More specifically, since cell B2 708 in FIG. 7 is adjacent to cell A1 700 and cell A3 704 and is not adjacent to cell A2 702, cell A2 702, as shown in FIG. 10, And transmits the S-SCH using the same frequency resource as that of the S-SCH (1000). Since the cell C1 710 is adjacent to the cell B2 708 and the cell A3 704, a frequency resource not used for the S-SCH transmission is used in the cell B2 708 and the cell A3 704 as shown in FIG. 10 S-SCH is transmitted (1001). In these cases as well, the P-SCH is transmitted in all cells using the same sequence and frequency resources so that the terminal can receive the soft combined P-SCH signal.
FIG. 11 shows a procedure for performing a cell search to be applied to a mobile station under a synchronization channel structure based on FIG. 6B.
In step 1100, the terminal first determines the start point of the P-SCH using the property that the same sequence is repeatedly transmitted in the P-SCH as shown in FIG. 6A to obtain frame timing synchronization. This is done by finding a received signal sample timing at which the correlation value becomes maximum between the current received signal and the received signal component delayed by the sample sequence length of the P-SCH. After obtaining the frame timing synchronization, the terminal estimates and removes a frequency error between the BS and the MS in order to obtain the cell-specific scrambling code from the S-SCH in step 1102. [ The frequency error estimation is possible by correlating the P-SCH sequence with the received P-SCH sequence in the time domain and the frequency domain. If the frequency error is corrected through the above procedure, the UE obtains the cell specific scrambling code from the received S-SCH in step 1104. The terminal performs final verification in step 1106 with respect to the synchronization information acquired in the above steps. If the acquired frame timing and the cell specific code pass the verification in step 1108, the cell search is terminated and the verification fails The cell search process starts again from the initial stage (step 1100). If the cell specific code verification fails in the verification step 1106 and the synchronization verification for the P-SCH has succeeded, the UE can perform the S-SCH synchronization acquisition in step 1104.
FIG. 12 shows a detailed procedure of step 1104 of obtaining the cell-specific scrambling code through the S-SCH in FIG. In FIG. 12, N represents the number of possible frequency mapping patterns of the S-SCH and n represents the index of the frequency mapping pattern under test of the synchronization of the current S-SCH code. As shown in FIG. 8A, when the frequency reuse factor is 3, N = 3.
The terminal sets n = 0 in step 1200 and increases the value of n by 1 in step 1202. [ In step 1204, a frequency mapping pattern of the S-SCH corresponding to n is selected. In step 1206, correlation values between available S-SCH codes and received S-SCH codes are obtained. The above steps 1202 to 1206 are repeated until n = N. That is, steps 1202 through 1206 are repeated for all the frequency mapping patterns. If n = N in step 1208, the terminal selects a code and a frequency mapping pattern corresponding to the largest correlation value among the frequency mapping patterns and code combinations obtained in step 1206, SCH synchronization acquisition process.
FIG. 13 illustrates a structure of a receiver for performing the above-described cell search procedure.
Referring to FIG. 13, the A / D-converted digital reception signal 1300 is correlated in the time domain with respect to the P-SCH in the P-SCH time domain correlator 1302, Timing synchronization is obtained. The obtained frame timing information is transmitted to a frequency error estimator 1308 and a cell search controller 1316 (1318), and a frequency error estimator 1308 estimates a frequency error between the base station and the terminal through the time domain correlation . Under the P-SCH structure of FIG. 6A, the frequency error value that can be estimated by the frequency error estimator 1308 is limited to a value equal to or smaller than the subcarrier interval. After the frequency error corrector 1312 corrects the frequency error estimated by the frequency error estimator 1308, the P-SCH code correlator 1304 detects a detailed frequency error value through correlation in the frequency domain of the P-SCH code . In more detail, in the detailed frequency error estimator 1310, the P-SCH code received in the P-SCH structure shown in FIG. 6B is the P-SCH code {p n , n = 1, 2, ... , N} and estimates the detailed frequency error by observing how many subcarriers are shifted on the frequency axis. The detailed frequency error is further corrected in the detailed frequency error corrector 1314 and the received signal is input to the S-SCH code searcher 1306. The S-SCH code searcher 1306 acquires the cell specific scrambling code through the procedures shown in FIG. 12 and transmits the information to the cell searcher controller 1316 (1318).
In the first embodiment of the present invention described above, the case where the cell group code is not transmitted is considered. However, in the case of an asynchronous system such as a WCDMA system, the number of required cell-specific scrambling codes may be considerably large. In such a case, when only the cell specific scrambling code is transmitted without cell group code as in the first embodiment of the present invention, the number of cell specific scrambling codes to be searched is large, so that the cell search performance is degraded and the cell search complexity can be considerably increased. Therefore, in the second embodiment of the present invention, a cell group code and a cell-specific scrambling code are transmitted through the S-SCH.
14A and 14B illustrate a structure of a synchronous channel according to a second embodiment of the present invention.
Comparing the structure of FIG. 14A with the first embodiment of the present invention shown in FIG. 6A, the synchronization channel according to the second embodiment of the present invention is characterized in that the S-SCH 1400 occupies two OFDM symbols 1 embodiment. This is because the OFDM symbol 1420 of the S-SCH for transmitting the cell group code is transmitted before the OFDM symbol 1422 for transmitting the cell specific scrambling code as shown in the S-SCH 1424 of FIG. 14B . The P-SCH structures 1402 and 1426 are the same as the P-SCH structures 600 and 610 of the first embodiment of the present invention shown in FIG. 6A.
Referring to FIG. 14B, the frequency reuse factor 3 is also applied to the S-SCH 1420 that transmits the cell group code. Since the cell group code is also different depending on the cell, the cell group code sequence {g k , m} is different from the cell-specific scrambling sequence {s k , m} It is for mapping.
FIG. 15 illustrates a cell search procedure of a synchronous channel structure according to a second embodiment of the present invention.
The difference from the cell search procedure according to the first embodiment of the present invention shown in FIG. 11 is that a cell group code is acquired from the S-SCH in step 1504. That is, in the second embodiment of the present invention, the cell group code is acquired between step 1502 of estimating and compensating for the frequency error from the P-SCH and step 1506 of obtaining the cell-specific scrambling code from the S-SCH, do.
Unlike the cell-specific scrambling code acquisition procedure in step 1206 of FIG. 12, the cell-specific scrambling code is obtained in step 1508. In step 1508, Only the synchronization acquisition test is performed.
FIG. 16 shows the structure of a cell searcher according to the synchronous channel structure of the second embodiment of the present invention.
A difference from the receiver structure according to the first embodiment shown in FIG. 13 is that an S-SCH cell group code searcher 1602 for obtaining a cell group code from the S-SCH is added. That is, after the cell group code acquisition is performed in the S-SCH cell group code searcher 1602 for the reception signal in which the detailed frequency error is corrected in the detailed frequency error corrector 1600, S (k) is calculated based on the scrambling code belonging to the group -SCH cell specific code searcher 1604 performs cell specific scrambling code search.
&Lt; Third Embodiment &gt;
In the third embodiment of the present invention, only the cell group code is transmitted through the S-SCH and the cell-specific scrambling code for cell ID acquisition is transmitted through the pilot channel. Since the acquisition of the cell ID is performed through acquisition of the cell-specific scrambling code used in the pilot, the cell search process includes the process of acquiring the cell unique code via the pilot channel.
17 shows a synchronization channel structure according to the third embodiment of the present invention.
The structure of the synchronization channel according to the third embodiment of the present invention is basically the same as the structure shown in FIG. 6B. In FIG. 6B, a cell-specific scrambling sequence {s k , m } is transmitted to the S-SCH 612 while a cell group code sequence {g k , m } There is a difference.
18 shows a cell search procedure according to the third embodiment of the present invention.
The cell search procedure according to the third embodiment of the present invention is substantially the same as the cell search procedure according to the second embodiment shown in FIG. 15, except that after obtaining the cell group code through the S-SCH in step 1800, The process of acquiring the cell specific scrambling code is performed through the pilot channel.
The structure of the cell search receiver for performing the process of FIG. 18 is the same as that of FIG. 16 except that the S-SCH cell unique code searcher 1604 of FIG. 16 is replaced with a cell unique code searcher through a pilot channel. Is the same as the structure of the cell searcher of the second embodiment.
19 is a structural diagram of a transmitter for generating a synchronization channel according to the first to third embodiments of the present invention.
19, the P-SCH sequence generator 1900 and the S-SCH sequence generator 1902 generate a sequence of P-SCH and S-SCH, respectively, and the other channel data generator 1904 generates a sequence of P- Lt; / RTI &gt; The channels are input to a multiplexer 1906 and then input to a subcarrier mapping unit 1908 at a corresponding transmission timing by a controller 1912. The subcarrier mapping unit 1908 maps a multiplexer output signal to an input terminal of the IFFT unit 1910 according to a subcarrier mapping rule of the channels. For example, each of the chips s i , m of the S-SCH 612 shown in FIG. 6B is controlled by the controller 1912 and transmitted to the corresponding IFFT 1910 through the sub-carrier mapping unit 1908 according to the corresponding frequency mapping pattern. ). &Lt; / RTI &gt; When all the chips of the sequence of the S-SCH 612 are mapped to the input terminal of the IFFT 1910, a transmission OFDM signal 1914 corresponding to the S-SCH 612 is finally output.
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.
For example, although the P-SCH is transmitted over two OFDM symbol periods in the exemplary embodiment of the present invention, when the P-SCH is transmitted in only one OFDM symbol period or in three or more OFDM symbol periods The present invention is equally applicable.
In the present invention that operates as described in detail above, the effects obtained by the representative ones of the disclosed inventions will be briefly described as follows.
In the present invention, a P-SCH and an S-SCH constituting a synchronization channel for downlink cell search have different characteristics in terms of interference between adjacent cells, and apply different frequency reuse factor values to each other, . As a result, in the case of P-SCH, P-SCHs from a plurality of cells are combined to improve reception performance, and in the case of S-SCH, interference between adjacent cells is prevented, thereby improving overall cell search performance. Also, in applying a frequency reuse factor greater than 1 to the S-SCH, each adjacent cell interleaves the subcarriers used for the S-SCH so that the synchronization channel transmitted by each cell is spread over a wide band and transmitted smoothly Thereby enabling neighbor cell state measurement and cell search.
A method for transmitting a synchronization signal for cell search in an orthogonal frequency division multiplexing (OFDM) based cellular radio communication system,
In a first OFDM symbol interval, at least one first sub-carrier symbol mapped to N subcarriers by applying a frequency reuse factor 1 to a sequence constituting a first synchronization channel (Primary-SCH) Transmitting,
Transmitting a sequence constituting a second synchronization channel through second subcarrier symbols mapped to M subcarriers of the N subcarriers by applying a frequency reuse factor greater than 1 in at least one second OFDM symbol interval;
And transmitting a sequence of channels other than the first and second synchronization channels in OFDM symbol intervals other than the first and second OFDM symbol intervals.
The method according to claim 1,
And the second subcarrier symbols are mapped to subcarriers spaced apart in a frequency band.
Wherein the second subcarrier symbols are mapped to adjacent subcarriers.
And transmitting the sequence constituting the second synchronization channel,
A cell group code sequence is transmitted in a third OFDM symbol interval among the at least one second OFDM symbol interval and a cell specific scrambling code sequence is transmitted in a fourth OFDM symbol interval among the at least one second OFDM symbol interval. To be synchronized.
5. The method of claim 4,
Wherein the step of transmitting the sequence configuring the second synchronization channel comprises transmitting a cell group code sequence through the second subcarrier symbols in the second OFDM symbol interval,
Further comprising the step of transmitting a cell specific scrambling sequence in a third OFDM symbol interval allocated as a pilot channel.
An apparatus for transmitting a synchronization signal for cell search in an orthogonal frequency division multiplexing (OFDM) based cellular radio communication system,
A first synchronization channel sequence generator for generating a sequence constituting a first synchronization channel,
A second synchronization channel sequence generator for generating a sequence constituting a second synchronization channel,
A third channel sequence generator for generating a sequence constituting a channel other than the first and second sync channels,
And transmitting a sequence constituting the first synchronization channel in at least one first OFDM symbol interval through first subcarrier symbols mapped to N subcarriers by applying a frequency reuse factor of 1, , And transmits the sequence constituting the second synchronization channel through second subcarrier symbols mapped to M subcarriers of the N subcarriers by applying a frequency reuse factor greater than 1, and transmits the second subcarrier symbols except for the first and second OFDM symbol intervals And a subcarrier mapping unit for transmitting a sequence constituting a channel other than the first and second sync channels in the OFDM symbol intervals of the first and second subcarrier symbols except for the first and second subcarrier symbols.
11. The method of claim 10,
The subcarrier mapping unit,
And transmits the sequence constituting the second synchronization channel through the second subcarrier symbols mapped to subcarriers spaced apart in a frequency band.
And the sequence configuring the second synchronization channel is transmitted through the second subcarrier symbols mapped to subcarriers adjacent to each other with the subcarrier symbols.
Wherein the second synchronization channel sequence generator generates a cell group code sequence and a cell specific scrambling code sequence,
Wherein the subcarrier mapping unit transmits the cell group code sequence in a third OFDM symbol interval of the at least one second OFDM symbol interval and performs the cell specific scrambling in a fourth OFDM symbol interval of the at least one second OFDM symbol interval. And transmits the code sequence.
And transmits the sequences constituting the second synchronization channel through the second subcarrier symbols mapped to subcarriers spaced apart in a frequency band.
And transmits the sequences constituting the second synchronization channel through the second subcarrier symbols mapped to adjacent subcarriers.
Wherein the second synchronization channel sequence generator generates a cell group code sequence,
Wherein the third channel sequence generator generates a cell specific scrambling sequence,
Wherein the subcarrier mapping unit transmits the cell-specific scrambling sequence in a third OFDM symbol interval allocated as a pilot channel.
And transmits the sequence constituting the second synchronization channel through the second subcarrier symbols mapped to adjacent subcarriers.
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