JP5079843B2 - Code division multiple access mobile communication system - Google Patents

Description

  The present invention relates to a code division multiple access (CDMA) type mobile communication system. In particular, the present invention relates to a cell search method using a long code mask symbol in a perch channel.

  In a CDMA mobile communication system, when a mobile terminal starts communication, or when a mobile terminal moves from one base station area (cell) currently in communication to an adjacent cell (handover), a base station (In the case of handover, it is necessary to identify the spreading code used in the adjacent base station) and to identify the frame / slot timing. Such processing is called cell search.

  As an example of the conventional cell search method, only one symbol at the end of the slot is spread using a special short code called a long code mask symbol instead of the usual long code / short code. 96-116, SAT96-111, RCS96-122 (1997-01).

A cell search method using this long code mask symbol will be described.
The cell search uses the perch channel shown in FIG. The perch channel is a control channel that broadcasts an uplink interference power value measured by a base station, a system frame number, and the like. The perch channel is always transmitted with a constant transmission power. Since the control signal for the perch channel is also used as a reference signal for synchronization acquisition between the base station and the mobile terminal, it is spread as follows. In the perch channel, the first perch channel and the second perch channel are multiplexed. CSC (Common Short Code) 104 at the long code mask symbol position 101 of the first perch channel 106 and GISC (Group Identification Short Code) at the long code mask symbol position 101 of the second perch channel 107 105 is mapped. In the data symbol period 102 (the period excluding the long code mask symbol period in one slot period), the control signal transmitted to the mobile terminal is spread by the long code and the short code 103.

  A long code is a long-period spreading code uniquely assigned to a base station, and a short code is a short-period spreading code uniquely assigned to each channel (including a control channel and a transmission channel) of the base station. Since long codes have a long code length and include many types, long codes are classified into a plurality of groups in order to facilitate synchronization acquisition. GISC is a short period code provided corresponding to the classification of long codes. When performing synchronization acquisition of the perch channel, the mobile terminal reduces the load of identifying the long code used by the base station by identifying the GISC and narrowing down the long code to a certain range. CSC is a short-period spreading code defined in common for each base station installed in the mobile communication system.

  Identification of the long code used by the base station using the perch channel and detection of frame / slot timing are performed as follows. (1) The mobile terminal uses CSC to despread the perch channel and identifies a timing with a high correlation value as a slot timing. (2) Despreading is performed on all GISCs in accordance with the identified slot timing, and the GISCs are identified based on the height of the correlation value. (3) Despreading is performed using all long codes belonging to the group associated with the GISC, and the long code is identified based on the height of the correlation value.

  The format and transmission power of the perch channel of the conventional method are shown in FIG. The symbol rate of the perch channel is constant at 16 ksps (256 times spread) in all sections including long code mask symbols. The transmission power of the perch channel after multiplexing is reduced by lowering the transmission power P1 of the first perch channel by the transmission power P2 of the second perch channel in the long code mask symbol period in which the second perch channel is transmitted. The power is constant.

IEICE Technical Report DSP-96-116, SAT96-111, RCS96-122 (1997-01)

  In the conventional method in which the spreading process is performed in the long code mask symbol period at the same symbol rate as the data symbol period, it takes the longest time in the first stage of cell search (slot timing identification). In order to perform timing identification at high speed, a matched filter (MF) that can obtain correlation results at a plurality of timings at a time is often used.

  FIG. 13 shows the time required for each stage of cell search when a cell search is performed on a perch channel with 256-fold spread using 64 stages of MF. The slot timing identification 1301 takes the longest time. In order to speed up cell search, it is essential to reduce the time required for timing identification. In timing identification using MF, the correlation values at all timings in one symbol (256 chip) section are accumulated using CSC of a plurality of slots, and slot timing is identified with high accuracy. For example, the correlation values obtained for the 48-slot CSC are accumulated. In FIG. 13, in one cycle 1301 of timing identification, one accumulated value is obtained for the timing of 64 chips which is the same as the number of MF stages.

  When 64 stages of MF are used, it is necessary to switch the coefficient mode in order to obtain correlation values at all timings, and there is a problem that the time required for timing identification and thus the time required for cell search takes time. On the other hand, if 256 stages of MF are used, it is possible to despread the received signal while setting the coefficient for one symbol to MF, and it is not necessary to switch the coefficient mode. However, both the circuit scale and power consumption of the MF are very large.

In order to perform cell search at high speed while suppressing the circuit scale and power consumption, the spreading ratio of the long code mask symbol is made smaller than the spreading ratio of the other part of the perch channel.
In particular, a symbol rate corresponding to the number of general MF stages used in a mobile terminal is determined. For example, when the mask symbol spreading ratio is 64, timing identification is performed using 64 stages of MFs. In this case, since the symbol length matches the number of stages of the MF, it is possible to perform despreading of the received signal while setting a coefficient for one symbol in the MF, and to search the timing of all 64 chip sections. Thus, high-speed cell search is possible without increasing the circuit scale and power consumption.

  By reducing the spreading ratio of the long code mask symbol, the time required for timing identification can be shortened compared to the conventional method, and the number of MF stages can be shortened to reduce the circuit scale and power consumption. .

It is a figure which shows the channel format of a perch channel. It is a figure which shows the perch channel format and transmission power of a conventional system. It is a figure which shows the perch channel format and transmission power of 1st embodiment. It is a figure which shows the perch channel format and transmission power of 2nd embodiment. It is a figure which shows the perch channel format and transmission power of 3rd embodiment. It is a figure which shows the perch channel format and transmission power of 4th embodiment. It is a figure which shows shortening of search time, reduction of a circuit scale and transmission power. It is a block diagram of a mobile terminal. It is a figure which shows the structural example of the cell search timing identification part of a mobile terminal. It is a figure which shows the structural example of the cell search GISC identification part of a mobile terminal. It is a figure which shows the structural example of the 1st long code identification part of a mobile terminal. It is a figure which shows the structural example of the 2nd long code identification part of a mobile terminal. It is a figure which shows the required time required in each step of a cell search.

  First, the configuration of a mobile terminal used in a CDMA mobile communication system according to the present invention will be described with reference to FIG. The received signal of the carrier frequency received from the antenna is lowered in frequency by the RF unit 801, and the received signal of the baseband is input to the cell search unit 805 and the receiving unit 804 via the RF interface unit 802. The cell search unit 805 performs the cell search described above. The receiving unit 804 performs synchronization acquisition, despreading, error correction, and the like of physical channels other than the perch channel. The decoded transmission signal is output via the user interface unit 807 and used for the subsequent processing. A transmission signal to be transmitted to the base station is input to the transmission unit 803 via the user interface unit 807. The transmission unit 803 performs encoding and spreading of the transmission signal. The control unit 806 performs setting of initial values and timing management for each unit using a DSP.

9 to 12 show configuration examples of each block of the cell search unit. FIG. 9 shows the configuration of the timing identification unit 810. Since the timing identification unit needs to take the correlation value of the timing for one symbol, MF901 that can obtain the correlation results at a plurality of timings at a time is used.
The CSC generated from the CSC generator 902 is used as the coefficient of the MF 901. An accumulator 903 accumulates correlation values output from the MF for a plurality of slots. The maximum value determination unit 904 detects the one having the maximum accumulated correlation value as the slot timing.

  FIG. 10 shows a configuration example of the GISC identification unit 811, FIG. 11 shows a configuration example of the first long code identification unit, and FIG. 12 shows a configuration example of the second long code identification unit. The long code identification unit 812 includes a first long code identification unit and a second long code identification unit. In these circuits, since the frame / slot timing is already known by the timing identification unit, high-speed processing can be efficiently performed by parallelizing the correlator 1001 that performs despreading at one detected timing.

The GISC identification unit 811 (FIG. 10) stores the received signal of the long code mask symbol in the RAM 101, sequentially specifies the GISC from the DSP to the GISC generator 1003, obtains the correlation for each chip, and uses the accumulator 1004 to generate one symbol. Find the correlation value at. These processes can be processed at high speed by appropriately performing parallel processing.
The correlation value results are combined to identify the GISC.

  The first long code identification unit (FIG. 11) calculates a correlation value over approximately 10 symbols, and identifies the long code used by the base station from the long codes belonging to the classification corresponding to the specified GISC. To do. The long code generated by sequentially specifying the long code generator 1102 from the DSP is multiplied by the short code of the perch channel generated from the short code generator 1103, and the correlation for each chip is obtained by the correlator 1001. The correlation value for 10 symbols is accumulated by the calculator 1101. This processing is performed in parallel, and a probable long code is specified based on the accumulated correlation value in about 10 symbols.

  The second long code identification unit (FIG. 12) performs the same processing as the first long code identification unit over one frame period for the long code specified by the first long code identification unit, and performs predetermined accumulation. If the value is obtained, the cell search is complete.

In a CDMA communication system that performs a cell search method using a long code mask symbol, an example in which only a long code mask symbol portion of a perch channel that is normally transmitted at 16 ksps (spreading ratio 256) is set to a spreading ratio 64 will be mainly described.
If not only the diffusion ratio 64 but also the diffusion ratio is 256 or less, the same effect can be obtained.

  As a first embodiment, FIG. 3 shows the channel format and transmission power when the CSC and GISC spreading ratios are both reduced (64 in the example) compared to other symbols in the perch channel and inserted at different timings. The mask symbol section 131 is set to 256 chips as in the conventional case so as not to affect other normal symbol portions. CSC and GISC may be inserted into any of the four positions (133, 134, 135, 136) obtained by dividing the mask symbol section every 64 chips. When the number of GISCs becomes insufficient for the number of long codes to be allocated due to the shortening of the symbol length of GISC, the long code identification group is divided according to where the four insertion positions are located. Is also possible. In the mask symbol section, sections other than CSC and GISC are assumed to have no signal.

  The transmission power needs to be increased in order to obtain the same reception sensitivity because the number of times of accumulation can be reduced if the symbol length is shortened. However, since the perch channel always transmits with constant power, and the long code mask symbol part has poor orthogonality and easily affects other channels as interference power, it is desirable to keep the transmission power as low as possible. Therefore, in the present embodiment, CSC and GISC are not multiplexed, and CSC and GISC are transmitted in a time-sharing manner in the long code mask symbol portion. At this time, even if the spreading ratio is 1/4, the CSC transmission power P3 is twice the transmission power P1 in the case of the prior art, and an equivalent reception sensitivity can be obtained. The same applies to the transmission power P4 of GISC.

  FIG. 4 shows the channel format and transmission power in the second embodiment as the CSC and GISC spreading ratio is sufficiently small (16 in the example) compared to other symbols in the perch channel and transmitted in a multiplexed manner. . The transmission power P5 of CSC and the transmission power P6 of GISC need to be increased corresponding to the spreading ratio. When the symbol rate of channels other than the perch channel is fast, the number of symbols affected by the increase in the power of the perch channel increases. In such a case, as in the present embodiment, the influence of the perch channel on other channels is increased by shortening the section in which the transmission power is increased by multiplexing CSC and GISC. By shortening the symbol section, it is possible to reduce the influence as a whole.

  FIG. 5 shows a third embodiment in which the CSC and GISC spreading ratios are both reduced (64 in the example) and GISC is repeated a plurality of times (3 times in the example) as compared to other symbols in the perch channel. Indicates the format and transmission power. By repeatedly transmitting the GISC n times, the number of accumulation is increased, and the transmission power P8 of one GISC is reduced to 1 / n of the transmission power P7 of the CSC. This suppresses the influence on other channels.

  FIG. 6 shows the channel format and transmission power when the CSC spreading ratio is smaller than the GISC spreading ratio (in the example, the CSC spreading ratio 64 and the GISC spreading ratio 256) as the fourth embodiment. In the above-described three stages of cell search, GISC identification needs to be performed by despreading only at the timing found from the CSC, so a correlator is often used instead of MF (for example, FIG. 10). Therefore, as in this embodiment, the CSC spreading ratio that affects the number of MF stages is reduced, and the GISC spreading ratio is set higher than that to suppress transmission power, thereby suppressing interference with other channels. The speed can be increased.

  FIG. 7 shows a list of required times at each stage of the cell search when the spreading ratio of the long code mask symbol and the number of MF stages to be used are changed.

Claims (5)

In a slot timing identification method in a mobile communication system applying a code division multiple access method,
Transmitting a control signal from a base station through a perch channel, wherein the first section of the control signal is spread with a long period code and a first short period code assigned to the base station, The second long code mask symbol period is spread with the second short period code in the first sub period and with the third short period code in the second sub period, and the spreading ratio of the second short period code and the third short code are spread. The spreading ratio of the periodic code is set to a value smaller than the spreading ratio of the first short period code that spreads the first section, and the transmitting step,
In the mobile terminal, slot timing identification is performed by despreading the second long code mask symbol period of the perch channel with the second short period code and using a correlation value obtained as a result of despreading. The steps of
A method comprising the steps of: The slot timing identification method according to claim 1, comprising:
The base station generates the transmission power of the second long code mask symbol period higher than the transmission power of the first period. The slot timing identification method according to claim 1, comprising:
The third short period code is common to a plurality of base stations in the mobile communication system,
The method of claim 2, wherein the second short period code corresponds to a classification of a long period code that spreads the first interval. The slot timing identification method according to claim 3, wherein
The spreading ratio of the third short period code is set to a value smaller than the spreading ratio of the second short period code. The slot timing identification method according to claim 3, wherein
The second long code mask symbol period is time-divided into a plurality of sub-intervals, the second short period code is spread in the first sub period, and the third short period code is spread in the second sub period. Feature method.

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