KR20080040524A - Apparatus and method for detecting cell id in mobile communication system - Google Patents

Abstract

An apparatus and a method for detecting a cell ID(Identification) in a mobile communication system are provided to improve a detection performance by arranging codes having an orthogonal relation in one group and minimizing an influence from each other. An apparatus for detecting a cell ID(Identification) in a mobile communication system includes an FFT(Fast Fourier Transform)(602), a reference signal selector(604), an inverse scrambler, and an energy detection unit(616). The FFT transforms a demodulated input signal to a signal of a frequency axis. The reference signal selector selects a reference signal from the signal which is transformed to the frequency axis. The inverse scrambler performs and outputs an inverse scrambling for all scrambling codes included in one group. The energy detection unit calculates an inverse scrambled energy, and detects a cell ID in case of a maximum energy value.

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. Asynchronous broadband code division multiple access (WCDMA) system, which is a current generation mobile communication, is allocated 512 different scramble codes to distinguish base stations. 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 phase of all possible codes (512 in the case of WCDMA) is checked to find the cell search, that is, the base station scramble code in an asynchronous base station system, this method cannot be used. In the current WCDMA system, a multi-step cell search algorithm is used. For this method, 512 cell codes are divided into 64 groups of 8 each.

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

The synchronization channel is composed of a primary synchronization channel 110 and a secondary synchronization channel 120, in which a common pilot channel is finally determined to determine a cell number. This is used. The basic unit of transmission and reception in the physical layer of the WCDMA system is a radio frame. One radio frame consists of 15 slots, each slot having a length of 2560 chips. In an asynchronous WCDMA system, the length of one frame is 10 ms, which corresponds to one period of the scrambling code or pseudo PN sequence of the downlink channel.

 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. The first synchronization channel 110 sent by the base station consists of a first synchronization code (PSC) of 256 chips in length. The PSC is the same for all cells, and the terminal detects the slot timing using a matching filter corresponding to the known PSC. The second synchronization channel 120 consists 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 a code sequence is 256 chips long and can 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 diagram of generation of a synchronization channel on a base station.

Referring to FIG. 2, a first S / P converter 211 converts a signal of a common pilot channel to be transmitted to a received transmission antenna in parallel and is converted into 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-to-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 downlink scramble code and 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 sync 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. In a WCDMA system, 512 different scrambling codes can be used as scrambling codes of a pilot channel. The scrambling codes are divided into 64 groups, and eight scrambling codes are allocated to each group. The scrambling code in one group performs inverse scrambling with eight possible scrambling codes in one group in a pilot channel for a predetermined length with different PN sequences to obtain 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 having the value.

Once the scramble code of the cell is detected, the primary primary control physical channel (CCPCH) is detected and the channel estimation and scrambling codes are identified through this associated with the system or the cell to perform synchronization. However, in an orthogonal frequency division multiple access (OFDM) communication system, 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.

The method according to an embodiment of the present invention for achieving the above object is a method for detecting a cell ID in a mobile communication system, the process of converting the signal demodulated by OFDM into a signal of the frequency axis, and converting to the frequency axis Selecting a reference signal from the decoded signals, descrambling and outputting the scrambling code for all the scrambling codes belonging to one group, calculating the descrambling energy, and, in the case of the largest energy value, a cell ID. And detecting the process.

An apparatus according to an embodiment of the present invention for achieving the above object, the FFT for converting the signal demodulated by OFDM into a signal on the frequency axis, and the reference for selecting a reference signal from the signal converted on the frequency axis And a signal selector, an inverse scrambler for descrambling and outputting all scrambling codes belonging to one group, and an energy detector for calculating the descrambling energy and detecting a cell ID if the energy value is the largest. It is done.

In the following description of the present invention, detailed descriptions of well-known functions or configurations will be omitted if it is determined that the detailed description of the present invention may unnecessarily obscure the subject matter of the present invention. Terms to be described later are terms defined in consideration of functions in the present invention, and may be changed according to intentions or customs of users or operators. Therefore, the definition should be made based on the contents throughout the 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 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. 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 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 may be transmitted in time synchronization, when the transmission is performed using orthogonal sequences, reference signals between sectors may have a small amount of interference. 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. It is assumed in the present invention that the additional scrambling code is the same gold sequence as used in WCDMA. The present invention is not limited to this 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, so that cell ID detection is easy.

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) = six 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. However, in practice, only one number of first scrambling codes is used in one group. Cell division is done through orthogonal code. 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. do. 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 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. In addition, since the length for obtaining the correlation value is short, reception can be performed in a coherent band and a coherent time. Therefore, the terminal can easily distinguish the accumulated time in determining the scrambling code in one 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.

FIG. 6 illustrates a structure of a receiver in which a terminal determines a cell ID in a group for the group having the above structure. That is, FIG. 6 illustrates a structure diagram of a receiver 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 largest energy.

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.

Meanwhile, in the detailed description of the present invention, specific embodiments have been described, but it will be apparent to those skilled in the art that various modifications are possible without departing from the scope of the present invention.

In the present invention operating 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 (4)

In the cell ID detection method in a mobile communication system, Converting the demodulated signal into the OFDM signal on the frequency axis; Selecting a reference signal from the signals converted into the frequency axis; Descrambling and outputting all scrambling codes of one group; Calculating the descrambling energy; And detecting the cell ID when the largest energy value is obtained. The method of claim 1, All scrambling codes belonging to the one group are A cell ID detection method in a mobile communication system, characterized in that a combination of a scrambling code and an orthogonal code. An apparatus for detecting a cell ID in a mobile communication system, An FFT for converting a signal demodulated by OFDM into a signal on a frequency axis; A reference signal selector for selecting a reference signal from the signals converted into the frequency axis; An inverse scrambler that descrambles and outputs all scrambling codes of one group, And an energy detector for calculating the descrambling energy and detecting a cell ID if the energy is the largest energy value. The method of claim 3, All scrambling codes belonging to the one group are Cell ID detection apparatus in a mobile communication system, characterized in that the combination of a scrambling code and an orthogonal code.

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