WO2006067657A1

WO2006067657A1 - Method and apparatus for cell search in wireless communication system

Info

Publication number: WO2006067657A1
Application number: PCT/IB2005/054115
Authority: WO
Inventors: Yan Li; Xia Zhu
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-12-24
Filing date: 2005-12-08
Publication date: 2006-06-29

Abstract

The invention provides a method of fast cell search for mobile terminal and the apparatus therefor. The mobile terminal first performs coarse time synchronization on an initial synchronization sequence in a received signal, e.g. the downlink synchronization sequence (SYNC&lowbar;DL) in TD-SCDMA system, to obtain a coarse time parameter of SYNC&lowbar;DL. Next, It performs correlation calculations between signal sequences possibly conforming to the SYNC&lowbar;DL in adjacent subframes based on the coarse time parameter, and determines an accurate time parameter of the SYNC&lowbar;DL from a peak value of the results of correlation calculations. In the end, The mobile terminal determines the SYNC&lowbar;DL sequence in the signal received by the mobile terminal based on the accurate time parameter, thus achieving the SYNC&lowbar;DL synchronization and completing cell search.

Description

METHOD AND APPARATUS FOR CELL SEARCH IN WIRELESS COMMUNICATION SYSTEM

FIELD OF THE INVENTION The present invention relates to a wireless communication system, and more particularly, to a method of cell search for a mobile terminal in a wireless communication system and the apparatus therefor.

BACKGROUND OF THE INVENTION In a wireless communication system adopting code division multiple access

(CDMA), cell search operation needs to be performed when a mobile terminal establishes initial synchronization or a cell handover is being carried out due to the movement of the mobile terminal.

In the system such as wide -band code division multiple access/frequency division duplex (WCDMA/FDD), wide-band code division multiple access/time division duplex

(WCDMA/TDD), time division synchronization code division multiple access (TD- SCDMA) and the like, some special synchronization signals are applied, e.g. the synchronization channel (SCH) in WCDMA and the downlink synchronization signal sequence (SYNC_DL) in TD-SCDMA. These synchronization signals are transmitted on the downlink by a base station, and mobile stations establish and maintain synchronized connection with the base station by searching the synchronization signals.

At present, most cell searching modes make use of the autocorrelation characteristic of synchronization codes. Fig. 1 is a block diagram showing the basic structure of adopting a correlator to implement cell search in a mobile terminal. Synchronization code generator 10 generates local synchronization code, which is sent to correlator 12 together with a received signal. Correlator 12 comprises multiplier 122 and integrator 121. Thanks to the autocorrelation characteristic of the synchronization codes, integrator 121 will output a peak value if the synchronization code generated locally matches the received signal (i.e. having the same signal sequence and the same phase). Otherwise, correlator 12 will output a smaller value. Controller 11 is used for controlling the sequence and phase of the local synchronization code.

To acquire the output peak value of correlator 12, correlator 12 must scan all possible synchronization code sequences and phases. For example, in a TD-SCDMA system, SYNC_DL is a system predetermined PN sequence of 64 chips, having at most 32 possible choices, the SYNC_DL of adjacent cells in the system are different from each other, while the SYNC_DL in non-adjacent cells could be reused. The search of SYNC_DL is performed in a subframe. Fig. 2 shows a standard subframe structure of TD- SCDMA. The length of subframe is 5ms, i.e. 6400 chips. Each subframe is divided into 7 main time slots (TS0-TS6) and 3 special time slots: downlink pilot time slots (DwPTS) of 96 chips, guard period (GP) of 96 chips and uplink pilot time slots (UpPTS). The length of a main time slot is 0.675ms, i.e. 864 chips. The last 16 chips function as a guard period (GP). One subframe has 6400 chips, which means that before the position of SYNC_DL in the subframe is determined, the phase of SYNC_DL may possibly be one of at least 6400 phases. As a result, totally 302800 (6400x32) correlation operations are needed to reach a correct SYNC_DL sequence and phase in a cell search, thus resulting in rather high complexity and long time of cell search.

In traditional techniques, many algorithms and methods have been put forward to reduce the time for cell search, e.g. using a plurality of parallel correlators. For a typical case, please see Fig. 5 in the patent application "cell search apparatus and method in asynchronous communication system" by Samsung Electronics Co., Ltd., which was published on November 9, 2000 under international publication number WO00/67396. However, this type of method requires more hardware resources. Therefore, another solution is disclosed in a CN patent application No. 02160459.2

(hereinafter referred to as No. '459 patent application) filed on December 30, 2002 by the present applicant, the disclosure of which is incorporated herein by reference. According to the solution provided by No. '459 patent application, in a standard subframe structure of TD-SCDMA, DwPTS could be used as the pilot and synchronization channel of a downlink, which is transmitted at full power by a base station and consists of a SYNC_DL of 64 chips and a GP of 32 chips. UpPTS could be used as the pilot and synchronization channel of a uplink, and usually consists of a SYNC_UP of 128 chips and a GP of 32 chips, wherein GP is used as the base station's switching point from transmission to reception, having a time length of 75μs (96 chips). As there is a GP of 48 chips before SYNC_DL, and a GP of 96 chips after it, to overcome the interference of multiple accessing, the transmitters of base stations and mobile terminals will all maintain silent status in these GPs, i.e. do not transmit signals. This shows a power depression on the power pulse of a subframe of TD-SCDMA system during the GPs. Besides, in a TD-SCDMA system, a SYNC_DL is required to be transmitted at full power level, which means that the intensity of SYNC_DL is usually greater than that of noises and thus the SYNC_DL can be detected. Furthermore, in a subframe of TD-SCDMA, the power pulse of the 64 chips SYNC_DL appears only once, so a coarse synchronization could be established by searching this unique power pulse of the 64 chips SYNC_DL.

Based on the above characteristic of the TD-SCDMA standardized subframe, '459 patent application proposes a fast cell search method, which comprises: obtaining a coarse time synchronization first through SYNC_DL power pulse search, then opening a time search window based on this coarse time synchronization, and searching SYNC_DL in this time search window in a traditional correlation manner. Thus, a traditional correlation search will be limited to a very narrow time window instead of the entire time period of a subframe, and thereby reducing correlation calculation.

However, the kind of SYNC_DL sequence is still not determined after the coarse time synchronization. As a result, more accurate synchronization is needed in the above time window, and not only this, to achieve the accurate synchronization, it still needs to perform traditional sliding search operations 32 times to determine the kind of SYNC_DL sequence and achieve synchronization. Therefore, this method is still not efficient enough to accelerate cell search. Therefore, a more efficient method of cell search for a mobile terminal and the apparatus therefor are needed to solve the time consuming problem resulted from the complex calculation caused by the application of existing method to cell search.

OBJECT AND SUMMARY OF THE INVENTION An object of the present invention is to provide a method of fast cell search for a mobile terminal in a wireless communication system and the apparatus therefor.

To achieve the above object, the following method is carried out by a mobile terminal in accordance with the present invention, which comprises: first, performing coarse time synchronization on an initial synchronization sequence in a received signal to obtain a coarse time parameter of the initial synchronization sequence; next, based on the coarse time parameter, correlating, within a corresponding coarse time range, signal sequences in a segment of a specific signal sequence which possibly conform to the initial synchronization sequence respectively with signal sequences at the corresponding positions in another segment of the specific signal sequence to acquire corresponding correlation results; then comparing the correlation results to determine an accurate time parameter of the initial synchronization sequence; in the end, determining the initial synchronization sequence based on the accurate time parameter. Through the use of the present invention, after performing coarse time synchronization on the initial synchronization sequence, it will be enough to perform a sliding correlation operation only once on two adjacent subframes, perform accurate time synchronization and then determine the initial synchronization sequence. Compared with traditional techniques that perform sliding correlation operation 32 times to achieve synchronization, such a synchronization method effectively reduces the calculation amount and the time consumed.

The other objects of the present invention and their implementation, as well as a more comprehensive understanding of the present invention, will become apparent and easier to understand upon review of the following description and claims in conjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is explained in further detail, and by way of example, with reference to the accompanying drawings wherein: Fig. 1 is a block diagram of some modules of a traditional fast cell search apparatus for mobile terminal;

Fig. 2 is the standard subframe structure of TD-SCDMA system;

Fig. 3 is a flow chart of SYNC_DL synchronization of fast cell search for mobile terminal in accordance with the present invention; Fig. 4 is a schematic diagram of performing correlation calculation on SYNC_DL in adjacent subframes;

Fig. 5 is a block diagram of modules of a fast cell search apparatus for mobile terminal in accordance with the present invention;

Fig. 6 is a schematic diagram of adopting dichotomy for determining a SYNC_DL sequence.

In all figures, similar reference signs represent similar or corresponding features or functions. DETAILED DESCRIPTION OF THE INVENTION

Following are further detailed illustrations of the present invention with reference to drawings and specific embodiments.

Referring to Fig. 3 and Fig. 4, according to an embodiment of the present invention, a method of fast cell search for mobile terminal comprises: first receiving signals (step

S 108), then performing coarse time synchronization on a received signal (step SIlO), the method being adopted therein could be the solution provided by No. '459 patent application, or the solution provided by other traditional techniques. After coarse time synchronization, a search range needed for performing SYNC_DL time synchronization is reduced to a rather small coarse range around SYNC_DL. As shown in Fig. 4, supposing tro represents the time position of SYNC_DL chips, then [Ww, t_ro+w] will represent the obtained coarse time range of SYNC_DL after coarse time synchronization.

Next, in each subframe of a received signal x[t) , no matter SYNC_DL is which one of the 32 kinds of possible sequences, the position of SYNC_DL in the subframe is the same. Therefore, in the above range, i.e. between [Ww, t_r0+w], the correlation calculations between signal sequences at the possible time parameter positions of SYNC_DL in the subframe and signal sequences at the same positions in adjacent subframe are performed (step S 120). The specific algorithm is as below:

R₁ = (t)x(t + 6400T_c ), __{w ≤ i ≤ w (1)}

Where, T_c represents code rate, x(t) represents the received signal.

The above procedure is equivalent to respectively selecting, in said two adjacent subframes, two time windows whose lengths are both 64 chips, but there is always a distance of one subframe time length (i.e. 6400T_c) therebetween. The two time windows slide within the range [Ww, W-w], and at the same time, correlation calculations of signal sequences that correspond to the two time windows are respectively performed during the sliding procedure.

The schematic diagram concerning the correlation calculation of SYNC_DL in adjacent subframes is illustrated in Fig. 4, wherein the correlation calculation could be referred to as autocorrelation calculation because the correlation calculation is performed between sequences at the same positions in two adjacent subframes of the received signal. It could be learned from the above algorithm formula (1) that the autocorrelation calculation is performed 2w+l times in total within the range [-w, w].

Thereafter, an accurate time synchronization of SYNC_DL can be obtained based on a peak value acquired by performing correlation calculation within said range (step S 130), the specific algorithm is as below:

R₁, = TaSiX( R₁ ), -W ≤ i ≤ W (2) t_r = t_r0 + r O)

Where, R₁- represents the peak value after correlation calculation, K ⁼ K_o + ^1"' represents the accurate SYNC_DL time parameter. During the performing of step S 120 and step S 130, the sliding correlation operation also needs to be performed only once within the above range because it is only for determining SYNC_DL time parameter, without having to know the accurate sequence of SYNC_DL.

In the end, as the position of SYNC_DL in subframe has been determined, the correlation operations between the signal sequence at said position and 32 kinds of possible SYNC_DL sequences are directly performed, and the accurate SYNC_DL sequence in the received signal can be determined from a peak value of the correlation results (step S 140), thus achieving SYNC_DL synchronization and completing cell search.

Fig. 5 is a block diagram of a fast cell search apparatus for mobile terminal according to an embodiment of the present invention. The search apparatus comprises coarse synchronization module 100, subframe delay module 200, autocorrelation calculation module 300, accurate synchronization module 400, 32 matching filters

501-532 and identification module 600.

First, coarse time synchronization is performed on a received signal by the coarse synchronization module 100.

Next, within the range obtained through coarse synchronization, subframe delay module 200 is used to delay the signal sequence at a possible time parameter position of

SYNC_DL in a subframe by a length of one subframe (i.e. delay by 5ms). The delayed signal sequence and the signal sequence at the same position of next adjacent subframe are sent to autocorrelation calculation module 300 for correlation calculation. Then, accurate synchronization module 400 finds a peak value from the results of correlation calculations outputted from autocorrelation calculation module 300 so as to determine an accurate SYNC_DL time parameter.

Based on the time parameter generated by accurate synchronization module 400, the signal sequence at corresponding position in subframe is inputted to 32 parallel matching filters 501-532, and its correlation operations with 32 kinds of possible SYNC_DL sequences are performed. The correlation results are inputted to identification module 600. If identification module 600 finds a peak value of the correlation results, then the SYNC_DL sequence that is used to generate said peak value is exactly the SYNC_DL sequence in the received signal.

In addition, to identify SYNC_DL sequence after time parameter was determined, a method that uses 32 parallel matching filters to perform correlation operation separately may not be employed. Instead, other optimum algorithms could be adopted. Following is the theory of one of the optimum algorithms explained using dichotomy as an example. Fig. 6 is a schematic diagram of adopting dichotomy for determining SYNC_DL sequence. The signal sequence whose time parameter has been determined (i.e. the unknown SYNC_DL sequence) first is correlated respectively with two groups of sequences: SD_GP1 and SD_GP2, wherein SD_GP1 is the accumulation of the first 16 sequences in the 32 possible SYNC_DL sequences, while SD_GP2 is the accumulation of the last 16 sequences. If the correlation calculation value between SD_GP1 and the unknown SYNC_DL sequence is greater than the correlation calculation value between SD_GP2 and the unknown SYNC_DL sequence, that means the kind of the SYNC_DL sequence in the received signal is one of the first 16 SYNC_DL sequences. Next, the first 16 SYNC_DL sequences are divided into two groups: first 8 sequences and last 8 sequences, then either group is correlated with the unknown SYNC_DL sequence, and then select the group of sequences having greater correlation value to repeat the above operation. After performing such operations 5 times (i.e. performing correlation calculations 10 times), the accurate SYNC_DL sequence in the received signal can be determined, wherein the sequences for correlation in each group are derived from the following formulae:

It could be seen from the above that using dichotomy can reduce the number of correlation calculations.

In the above embodiments of the present invention, after performing coarse time synchronization on SYNC_DL, a sliding correlation operation is first performed on two adjacent subframes to perform accurate time synchronization, and then the accurate SYNC_DL sequence is determined. Such a synchronization method effectively reduces the calculation amount and the time consumed as compared with traditional techniques that perform sliding correlation operations 32 times.

The above embodiments mainly describe the present invention with respect to a TD-SCDMA system, so the cell search procedure is mainly carried out utilizing SYNC_DL. However, the present invention could also be applied to other wireless communication systems as long as the initial synchronization sequences of the wireless communication systems could obtain coarse time synchronization through such manners as power pulse search, while the initial synchronization sequence could either be SYNC_DL, or other types of pilot sequence signals.

Although the present invention has been described with respect to specific embodiments thereof, it is obvious that those skilled in the art could, according to the above illustrations, easily make substitutions, modifications and alternations to the present invention in many aspects. Therefore, the present invention is intended to cover all such substitution, modifications and alternations falling within the conception and scope of the attached claims.

Claims

CLAIMS:

1. A method of cell search for mobile terminal, comprising the steps of:

(a) performing coarse time synchronization on an initial synchronization sequence in a received signal to obtain a coarse time parameter of the initial synchronization sequence;

(b) based on the coarse time parameter, correlating signal sequences in a segment of a specific signal sequence which possibly conform to the initial synchronization sequence respectively with signal sequences at the corresponding position in another segment of the specific signal sequence within a corresponding coarse time range to acquire corresponding correlation results;

(c) comparing the correlation results to determine an accurate time parameter of the initial synchronization sequence; and

(d) determining the initial synchronization sequence based on the accurate time parameter.

2. The method as claimed in claim 1, wherein the coarse time synchronization parameter in step (a) is obtained by searching a power pulse of the signal.

3. The method as claimed in claim 1, wherein in step (c), the accurate time parameter of the initial synchronization sequence is determined based on a peak value of the comparing results.

4. The method as claimed in claim 1, wherein the two segments of the specific signal sequence in step (b) are two adjacent subframes.

5. The method as claimed in claim 1, wherein step (b) comprises steps of:

(i) respectively selecting, within the coarse time range of the two segments of the specific signal sequence, two time windows whose time lengths are both identical with the time length of the initial synchronization sequence ; and (ii) performing correlation calculations between the corresponding signal sequences having said time lengths in the two time windows by sliding the two time windows.

6. The method as claimed in claim 1, wherein step (d) comprises steps of: (i) comparing the signal sequence which corresponds to the accurate time parameter with each predetermined initial synchronization sequence respectively; and

(ii) determining the initial synchronization sequence in the received signal based on the calculation result.

7. The method as claimed in claim 1, wherein step (d) comprises steps of:

(i) using dichotomy to group the predetermined initial synchronization sequences; and (ii) correlating the grouped predetermined initial synchronization sequences with the signal sequence which corresponds to the accurate time parameter, thus determining the initial synchronization sequence in the received signal.

8. The method as stated in any one of claims 1 to 7, wherein the initial synchronization sequence is a downlink synchronization sequence.

9. A mobile terminal, comprising: a coarse synchronization means, for performing coarse time synchronization on an initial synchronization sequence in a signal received by the mobile terminal to obtain a coarse time parameter of the initial synchronization sequence; a correlation means, for based on the coarse time parameter, correlating signal sequences in a segment of a specific signal sequence which possibly conform to the initial synchronization sequence respectively with signal sequences at the corresponding positions in another segment of the specific signal sequence within a corresponding coarse time range to acquire corresponding correlation results; an accurate synchronization means, for comparing the correlation results to determine an accurate time parameter of the initial synchronization sequence; and an identification means, for determining the initial synchronization sequence based on the accurate time parameter.

10. The mobile terminal as claimed in claim 9, wherein the coarse synchronization means obtains the coarse time synchronization parameter by searching a power pulse of the received signal.

11. The mobile terminal as claimed in claim 9, wherein the two segments of the specific signal sequence are two adjacent subframes.

12. The mobile terminal as claimed in claim 11, wherein the correlation means comprises: a subframe delay means, for delaying the first one subframe sequence of the two adjacent subframes by a time length of one subframe; and an autocorrelation means, for correlating the signal sequence that has been delayed by the subframe delay means with the signal sequence at corresponding position in the adjacent subframe.

13. The mobile terminal as claimed in claim 9, wherein the accurate synchronization means comprises an integrator for selecting a peak value of the correlation results outputted from the correlation means to determine the accurate time parameter of the initial synchronization sequence.

14. The mobile terminal as claimed in claim 9, wherein the identification means comprises a plurality of parallel matching filters for correlating the signal sequence which corresponds to the accurate time parameter with each predetermined initial synchronization sequences respectively.

15. The mobile terminal as claimed in claim 9, wherein after using dichotomy to group the predetermined initial synchronization sequences, the identification means correlates the grouped predetermined synchronization sequences with the signal sequence that corresponds to the accurate time parameter.