KR101302462B1 - Apparatus and method for detecting cell id in mobile communication system - Google Patents

Abstract

The present invention relates to an apparatus and a method for detecting a cell ID in a mobile communication system. In particular, by placing codes that are orthogonal to each other in a group, the detection performance is improved by minimizing the influence of each other, and energy detection of the scrambling code of the downlink channel is performed. This reduces the coherent time and bandwidth required.

Cell ID, LTE, Sync Channel, Cell Search, Orthogonal Code, Scrambling Code

Description

Apparatus and method for detecting cell ID in mobile communication system {APPARATUS AND METHOD FOR DETECTING CELL ID IN MOBILE COMMUNICATION SYSTEM}

1 is a structure diagram of a synchronization channel used for cell search in a WCDMA system;

2 is a circuit diagram of generation of a synchronization channel on a base station;

3 is a flowchart illustrating a cell search process in a WCDMA system;

4 illustrates an example of implementing a reference channel in an OFDM communication system;

5 illustrates a scrambling code structure of a reference signal according to an embodiment of the present invention;

6 is a structural diagram of a receiver for cell ID detection in a mobile communication system according to an embodiment of the present invention;

7 is a flowchart illustrating a cell ID detection method in a mobile communication system according to an embodiment of the present invention.

The present invention relates to a mobile communication system, and more particularly, to an apparatus and method for detecting a cell ID in a mobile communication system.

In general, in a mobile communication system system, a terminal accesses a communication network through a base station in a cell to which the terminal belongs. The terminal goes through a sequence of time synchronization and cell information acquisition for the communication with the base station at the moment of power on. This is called cell discovery or initial synchronization.

In addition, in the mobile communication system, a distinction between adjacent base stations uses a method of allocating different scramble codes. In the case of a wideband code division multiple access (WCDMA) system of asynchronous type, which is a current generation mobile communication, 512 different scramble codes are allocated to distinguish a base station.
The terminal finds the base station that transmits the strongest signal through cell search and then accesses the network through the base station to make a call or data transmission and reception. However, if the cell search, ie, the phase of all possible codes (512 in the case of WCDMA), is checked in an asynchronous base station system to find out the base station scramble code, this method is not preferable because the cell search takes considerable time. For this reason, in the current WCDMA system, a multi-stage cell search algorithm that searches cells by dividing 512 cell codes into 64 groups of 8 by 8 is used.

1 illustrates a synchronization channel structure used for cell search in an asynchronous WCDMA system.

The synchronization channel includes a first synchronization channel 110 and a second synchronization channel 120. In order to finally determine the cell number, a common pilot channel is used.
For example, the basic unit for transmission and reception in the physical layer of the WCDMA system is a radio frame. One radio frame consists of 15 slots, and the length of each slot corresponds to 2560 chips. For example, in an asynchronous WCDMA system, one frame is 10ms long. This coincides with one period of the downlink scrambling code or pseudo PN sequence.

The base station transmits the first sync channel 110 and the second sync channel 120 by 256 chips at the beginning of every slot. Since the two synchronization channels maintain orthogonality with different codes, they can be overlapped and transmitted simultaneously.
In this case, the first synchronization channel 110 includes a first synchronization code (PSC) of 256 chips long. The PSC is the same for all cells. The terminal detects the slot timing by using a matched filter corresponding to the known PSC. The second synchronization channel 120 is composed of a code sequence consisting of fifteen second synchronization codes (SSCs). In this case, the code sequence indicates a group number of the scrambling code used by the corresponding base station.
Adjacent base stations are assigned to use different group numbers. Each SSC in the code sequence is 256 chips long and may have one of 16 different codes. By using the second synchronization channel 120 transmitted from the base station, the terminal finds out the radio frame in addition to the group number of the scramble of the base station.

2 shows a circuit for generating a synchronization channel on a base station.

Referring to FIG. 2, the first S / P converter 211 converts a signal of a common pilot channel to be transmitted to a received transmission antenna into parallel I and Q channel data.

The first and second multipliers 212 and 213 spread common pilot data separated into I and Q channels, respectively, using the channel spreading code C _ch .

The first phase shifter 214 phase shifts the spread data of the Q channel by 90 degrees.

The first adder 215 adds the outputs of the first and second multipliers 212 and the first phase shifter 214 to generate a complex spread add signal I + jQ.

In addition, the second serial-to-parallel converter 221 converts data of a primary synchronization channel (P_sch) to be transmitted in parallel and converts the data into I and Q channel data.

The third and fourth multipliers 222 and 223 spread the first sync channel data separated into the I and Q channels, respectively, using the channel spreading code Cp.

The second phase shifter 224 phase shifts the spread data of the Q channel by 90 degrees.

The second adder 225 adds the outputs of the third multiplier 222 and the second phase shifter 224 to generate a complex spread add signal I + jQ.

The third serial / parallel converter 231 converts the data of the second synchronization channel (S_sch) to be transmitted in parallel and converts the data into I and Q channel data.

The fifth and sixth multipliers 232 and 233 spread the second sync channel data separated into the I and Q channels using the channel spreading code Cs.

The third phase shifter 234 phase shifts the spread data of the Q channel by 90 degrees.

The third adder 235 adds the outputs of the fifth multiplier 232 and the third phase shifter 234 to generate a complex spread add signal I + jQ.

In addition, the structure of the channel transmitter may further include other common channels or dedicated channels in addition to the common pilot channel, the first and the second synchronization channels. The forward channel transmitters that may be further provided herein may be transmitters of other forward common channels and forward dedicated channels, not shown.

The gain controller 200 generates and transmits a gain control signal for controlling the transmission power of the signal transmitted through each channel and controlling the presence or absence of the intermittence of the channel to the gain adjusters 216, 226, and 236.

The gain adjusters 216, 226, and 236 receive gain control signals output from the gain controller 200 to gain at the outputs of the first adder 215, the second adder 225, and the third adder 235. Adjust it.

The fourth adder 260 adds and outputs each channel signal whose gains are adjusted by the gain adjusters 216, 226, and 236.

Baseband filters 261 and 263 filter baseband signals among the signals output from the fourth adder 260.

The seventh and eighth multipliers 262 and 264 multiply the outputs of the corresponding baseband filters 261 and 263 by the outputs of the corresponding baseband filters 261 and 263 and output corresponding carriers, respectively. At this time, the output of the seventh multiplier 262 and the output of the eighth multiplier 264 are added by the fifth adder 265 and transmitted to the antenna.

3 is a flowchart illustrating a cell search process in a WCDMA system.

Through the cell search process, the UE finds the downlink scramble code and the frame synchronization of the cell.

First, in step 301, the terminal detects slot synchronization of a corresponding cell by using a first synchronization signal. In general, slot synchronization is detected using a matched filter corresponding to the same PSC in all cells.

Thereafter, the second synchronization signal received in step 303 is compared with all possible second sync code strings to detect the frame number and the group number of the scramble code of the cell detected in step 301. The second synchronization signal is encoded and transmitted according to a frame boundary in units of 10ms. The receiver attempts to demodulate the second synchronization signal every slot to determine a time point at which decoding is properly performed as a frame boundary.

In step 305, the correct scramble code is detected through the correlation of the received common pilot channel with all the codes in the scramble code group detected in step 303 by symbol unit.
For example, in a WCDMA system, 512 different scrambling codes may be used as a scrambling code of a pilot channel, and the scrambling codes are divided into 64 groups and eight scrambling codes are allocated to each group. The scrambling code in one group performs descrambling with eight possible scrambling codes in one group on a pilot channel for a predetermined length with different PN sequences, thereby obtaining the maximum BCH (Broadcast CHannel) information. In WCDMA and the pilot channel are always transmitted to detect the cell ID of the cell that received the scrambling code of the cell with the value.

Once the scramble code of the cell is detected, primary CCPCH (Primary Common Control Physical Channel) is detected to perform channel synchronization and scrambling code identification through this associated with the system or cell to perform synchronization.
However, in a communication system of an orthogonal frequency division multiplexing (OFDM) scheme, a reference signal is transmitted instead of a pilot signal. The reference signal is not always transmitted but is transmitted at a specific time and frequency.
The terminal performs channel estimation using the reference signal. In addition, by using a different scrambling code for each cell in the reference signal, scrambling can cause a cell ID to be detected as in WCDMA.

However, in an OFDM communication system, since the number of reference signals transmitted within the coherent bandwidth and the coherent time is limited, there is a limit in the performance of detecting the cell ID by descrambling different scrambling codes.

Accordingly, an object of the present invention is to provide an apparatus and method for detecting a cell ID in a mobile communication system for obtaining efficient initial synchronization in an OFDM communication system.

Another object of the present invention is to provide an apparatus and method for detecting a cell ID in a mobile communication system for acquiring synchronization with low overhead of downlink.

It is still another object of the present invention to provide an apparatus and method for detecting a cell ID in a mobile communication system capable of efficiently detecting a cell ID after a terminal initially acquires frame time and information of a cell or a group to which a cell belongs through a synchronization channel. In providing.

Another object of the present invention is to provide an apparatus and method for detecting a cell ID in a mobile communication system that groups a scrambling code of reference signals of multiple cells so that a terminal can efficiently detect a cell ID.

In a mobile communication system based on an orthogonal frequency division multiplexing scheme according to an embodiment of the present invention for achieving the above object, a terminal apparatus searches for a cell by initial synchronization using a reference signal transmitted at a specific time and frequency. The method may include acquiring slot timing using a first synchronization channel, acquiring a frame number and a group number to which a cell belongs by using a second synchronization channel, and at least one included in the obtained group number. Combination used for at least one reference signal of a reference signal transmitted by a predetermined pattern within a frame transmitted by a base station based on the orthogonal frequency division multiplexing scheme among combinations of a scrambling code and a plurality of orthogonal codes Searching for the cell that is currently located.

Terminal apparatus for searching for a cell by initial synchronization using a reference signal transmitted at a specific time and frequency in a mobile communication system based on an orthogonal frequency division multiplexing method according to an embodiment of the present invention for achieving the above object In the frame transmitted by the base station based on the orthogonal frequency division multiplexing scheme based on the frame synchronization obtained from the second synchronization channel and the group number to which the cell belongs using the slot timing obtained using the first synchronization channel. A reference signal selector for selecting at least one reference signal from among reference signals transmitted by a predetermined pattern at a second signal, and an inverse scrambling of the selected reference signal by at least one scrambling code included in the obtained group number 1 multiplier and output by the first multiplier A second multiplier that inversely scrambles the inverse scrambled reference signal by each of a plurality of orthogonal codes included in the obtained group number, and energy values detected by the reference signal that is inversely scrambled by each of the plurality of orthogonal codes And an energy detector for searching for a cell presently located by a combination of an orthogonal code and a scrambling code corresponding to the largest energy value.

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 may be changed according to the intentions or customs of the user, the operator, and the like. Therefore, the definition should be based on the contents throughout this specification.

The present invention relates to a synchronization method for cell search. In the present invention, it is assumed that a terminal acquires sync for a frame and a group ID of a cell through a sync channel. According to an embodiment of the present invention, the terminal obtains slot timing using the first sync channel and transmits a frame time and a group number to which the cell belongs using the second sync channel. However, the present invention is not limited to the detailed description of obtaining the frame sync and the group ID to which the cell belongs. The present invention provides a method for efficiently detecting a cell ID in the same group in a state where a terminal acquires a frame sync and a group ID to which a cell belongs.

One group consists of several cells and is distinguished by different scrambling codes. In the case of WCDMA, 512 different cell IDs (or scrambling codes) are divided into 64 groups, and 8 different scrambling codes are configured in each group.

In a WCDMA system, a pilot channel is continuously transmitted, and the pilot channel is spread with different scrambling codes. The scrambling code used here is a different gold sequence. The terminal descrambles different scrambling codes belonging to the detected group ID to detect the cell ID mapped to the scrambling code in which the descrambling energy is largely detected.

 On the other hand, in an OFDM communication system, there is no pilot channel continuously transmitted, and a reference sequence is transmitted discontinuously at a specific time and frequency. The pattern of the reference signal considered in the Long-Term Evolution (LTE) system is shown in FIG. 4. 4 illustrates an example of implementing a reference channel in an OFDM communication system.

으로 표시된 위치는 기지국의 제 1 안테나에서 전송되는 레퍼런스 심볼의 위치이다. 또한

로 표시된 위치는 기지국의 제 2 안테나에서 전송되는 레퍼런스 심볼의 위치이다.

The position indicated by denotes the position of the reference symbol transmitted by the first antenna of the base station. Also

The position indicated by denotes the position of the reference symbol transmitted by the second antenna of the base station.

In an OFDM communication system, channel estimation is performed using a reference signal. In addition, by using a different scrambling code for each cell in the reference signal, scrambling can cause a cell ID to be detected as in WCDMA.
However, in an OFDM communication system, since the number of reference signals transmitted within a coherent bandwidth and a coherent time is limited, there is a limit in performance in detecting a cell ID by descrambling different scrambling codes. Therefore, there is a performance limitation in detecting cell ID through different scrambling codes as in WCDMA. In particular, when the number of reference signals that can be accumulated or filtered within the coherent bandwidth and the coherent time is limited, the correlation value between codes is an important measure for the cell ID detection performance.

Therefore, the correlation value between the scrambling codes assigned in one group should be assigned low, and the period for calculating the correlation value should be short.

One approach being considered in LTE is to use sequences that are orthogonal to one another between different sectors of a base station. Since a signal transmitted from one base station can be transmitted in time synchronization, the reference signal between the sectors can be transmitted with a small amount of interference when transmitted using a sequence orthogonal to each other. In this case, Walsh codes may be used as possible orthogonal sequences. An example of a Walsh code having a length of 4 may be represented by Equation 1 below.

You can also generate orthogonal sequences with the COMPLEX signal. An example of an orthogonal code of length 3 is as follows.

The codes of W and C satisfy orthogonality to each other. This sequence is further multiplied by a longer length sequence, resulting in a scrambling code of the finally transmitted reference signal. In the present invention, it is assumed that the additional scrambling code is the same gold sequence as used in WCDMA. However, it is not limited to the additional scrambling code. That is, the scrambling code for distinguishing each cell is generated by multiplying codes that are orthogonal to each other when converted into different codes such as gold sequences.

5 is a diagram illustrating a scrambling code structure of a reference signal according to an embodiment of the present invention.

In this way, the scrambling codes used for the downlink reference signals are mixed with codes having no orthogonal relation to codes having orthogonal relation to each other. The performance of cell ID detection depends on how to group them. In the present invention, a sequence of orthogonal relations with each other is allocated to the same group to improve performance of a cell search process. That is, sequences having orthogonal relations with each other have a low correlation value, which facilitates cell ID detection.

Table 1 shows an example of allocating sequences having orthogonal relations to the same group. In Table 1 below, one group includes scrambling codes for six different reference signals. In practice, however, there are only two first scrambling codes used in one group. Additional cell division is done through orthogonal code. Three different orthogonal codes are used as the downlink scrambling codes, and all 2 (number of first scrambling codes) X 3 (number of orthogonal codes) = 6 different scrambling codes exist in a group. do.

Table 2 below shows another example of allocating sequences that are orthogonal to each other to the same group. In the implementation example of Table 2, one group includes scrambling codes for three different reference signals. In practice, however, there is only one number of first scrambling codes used in one group. Cell division is done through orthogonal code.
For example, three different orthogonal codes are used as downlink scrambling codes, and all 1 (number of first scrambling codes) X 3 (number of orthogonal codes) = three different scrambling codes exist in a group. Done. Thus, if the scrambling code existing in each group is designed to be orthogonal to each other, the detection performance can be improved compared to the implementation of Table 1 in which orthogonal codes and non-orthogonal codes are mixed. However, the number of scrambling codes that can be assigned to one group is reduced.

 In the embodiment of Table 2, one group for initial synchronization is orthogonal to each other. Since the correlation length is short, reception can be performed within a coherent band and a coherent time. Therefore, it is possible to easily distinguish the accumulated time in determining the scrambling code within a group by taking the integer multiple of the orthogonal code, and the interference between the terminals is small, so that the cell ID in the group can be easily detected.

6 illustrates a receiver structure in which a terminal determines a cell ID in a group for a group having the above structure. That is, FIG. 6 illustrates a receiver structure for cell ID detection in a mobile communication system according to an embodiment of the present invention.

Referring to FIG. 6, the FFT 602 converts a signal demodulated by OFDM into a signal on a frequency axis and outputs the signal to the reference signal selector 604. The signal converted to the frequency axis is a mixture of several signals.

The reference signal selector 604 selects only the reference signal from the signals converted into the frequency axis. The selected reference signal is descrambled by the first scrambling code 510 and the multiplier 608 and multiplied by the orthogonal code 520 and the multiplier 612. The orthogonal sequences should be assigned to the same group. The signal multiplied by the scrambling code is output to the low pass filter (LPF) 614, filtered by the low pass filter (LPF) 614 to filter only the components of the low frequency and then output to the energy detector 616. . In this case, the low pass filter 614 may be implemented to accumulate for a predetermined period of time and frequency. That is, they accumulate reference signals at specific times and frequencies with the same gain.

The energy detector 616 detects energy at the output of the low pass filter 614 and detects a cell ID used in the group based on the energy. That is, after descrambling all scrambling codes (a combination of the first scrambling code and an orthogonal code) belonging to a group, the energy is compared and the specific cell ID in the group is based on the energy of the largest signal and the reliability of the signal. Will be detected.

The important thing in this process is that a group contains orthogonal codes. In the cell group of Table 2, only one scrambling code is orthogonal to each other in one group. Therefore, while the first scrambling code is fixed, the descrambling energy can be calculated and compared while only changing the orthogonal code. Therefore, coherent time and frequency are maintained in a relatively short orthogonal code, thereby improving detection performance. In addition, since the correlation value between each orthogonal code is zero or close to zero, detection performance is improved. Since the first group of scrambling codes is included in the cell grouping of Table 2, the UE is assigned to a scrambling code corresponding to a combination of (number of first scrambling codes) X (number of orthogonal codes) within a group. Reverse scrambling is performed to detect the cell ID of the signal having the greatest energy among them.

7 is a flowchart illustrating a cell ID detection method in a mobile communication system according to an embodiment of the present invention.

Referring to FIG. 7, when the demodulated signal is input in OFDM in step 701, the receiver converts the signal into a signal on a frequency axis and outputs the converted signal.

Thereafter, the receiver selects only the reference signal from the signals converted to the frequency axis in step 703.

In operation 705, the selected reference signal is descrambled by the first scrambling code 510 and the multiplier 608 and multiplied by the orthogonal code 520 and the multiplier 612. The orthogonal sequences should be assigned to the same group. In addition, descrambling of all scrambling codes (a combination of the first scrambling code and the orthogonal code) belonging to one group is performed.

In step 707, the receiver filters the signal multiplied by the scrambling code and outputs only the low frequency components.

The receiver calculates energy at the output of the filtering at step 709.

The receiver determines whether it is the largest energy among the energy calculated in step 711. In this case, the energy is compared after descrambling all scrambling codes of a group.

If not the greatest energy, the receiver terminates. However, in the case of the greatest energy, the receiver detects the cell ID used in one group in step 713.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the scope of the present invention.

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.

According to the present invention, by placing codes that are orthogonal to each other in one group, the detection performance can be improved by minimizing the influence of each other, and the coherent time and bandwidth required for energy detection of the downlink scrambling code can be reduced.

In addition, the present invention can optimize the detection performance if only one group consists of codes that are orthogonal to each other.

Claims (10)

In a mobile communication system based on an orthogonal frequency division multiplexing scheme, a terminal apparatus searches for cells by initial synchronization using a reference signal transmitted at a specific time and frequency. Acquiring slot timing using a first synchronization channel, acquiring a frame synchronization and a group number to which a cell belongs by using a second synchronization channel; A reference signal transmitted by a predetermined pattern within a frame transmitted by a base station based on the orthogonal frequency division multiplexing scheme among at least one combination of at least one scrambling code and a plurality of orthogonal codes included in the obtained group number group And searching for a cell that is currently located by a combination used for at least one of the reference signals. The method of claim 1, And the combinations of the at least one scrambling code and the plurality of orthogonal codes are orthogonal to each other, and the plurality of orthogonal codes are designed to be orthogonal to each other. The method of claim 1, wherein the searching of the cell comprises: Based on the orthogonal frequency division multiplexing scheme, at least one scrambling code included in the obtained group number of at least one reference signal of a reference signal transmitted by a predetermined pattern within a frame transmitted by a base station Reverse scrambling process, Inverse scrambling the reference signal inversely scrambled by the at least one scrambling code by each of the plurality of orthogonal codes; Searching for a cell currently located by a combination of an orthogonal code and a scrambling code corresponding to the largest energy value among the energy values detected by the inverse scrambled reference signal by each of the plurality of orthogonal codes. . The method of claim 3, Wherein the detected energy value is an energy value obtained by accumulating energy values of each of a plurality of reference signals received on a specific frequency and frequency. delete In a mobile communication system based on an orthogonal frequency division multiplexing scheme, a terminal apparatus searching for a cell by initial synchronization using a reference signal transmitted at a specific time and frequency, Predetermined within a frame transmitted by a base station based on the orthogonal frequency division multiplexing scheme based on the frame synchronization obtained from the second synchronization channel using the slot timing obtained using the first synchronization channel and the group number to which the cell belongs. A reference signal selector for selecting at least one reference signal among reference signals transmitted by a pattern of A first multiplier for descrambling the selected reference signal by at least one scrambling code included in the obtained group number; A second multiplier for descrambling the inverse scrambled reference signal output by the first multiplier by each of a plurality of orthogonal codes included in the obtained group number; Terminal device including an energy detector for searching for a cell currently located by a combination of an orthogonal code and a scrambling code corresponding to the largest energy value among the energy values detected by the descrambling reference signal by each of the plurality of orthogonal codes . The method according to claim 6, And the combinations of the at least one scrambling code and the plurality of orthogonal codes are orthogonal to each other, and the plurality of orthogonal codes are designed to be orthogonal to each other. The method of claim 7, wherein And a low pass filter for accumulating energy values of each of the plurality of reference signals received on a specific frequency and the frequency and outputting the energy values as the detected energy values. delete 9. The method of claim 8, The orthogonal code has a shorter length than the scrambling code.

Images