CDMA mobile communications apparatus and base station detecting method used therefor

To provide a CDMA mobile communications apparatus which can enhance a detection probability of a base station received with low power. A first threshold determination unit selects a peak correlation value output from a slot synchronizing unit based on a predetermined first threshold T1. A second threshold determination unit selects the peak correlation value selected by the first threshold determination unit based on the predetermined second threshold T2. A first storage unit and a second storage unit hold slot timing corresponding to the peak correlation value selected by the second threshold determination unit. The slot timing selection unit appropriately selects the slot timing held in the first storage unit and the second storage unit, and sequentially transmits the slot timing to a frame synchronization/code group identifying unit and a Scrambling Code identifying unit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a CDMA mobile communications apparatus and a base station detecting method used for the apparatus, and more specifically to a procedure (cell search) for use by the CDMA (Code Division Multiple Access) mobile communications apparatus detecting a base station which is a direct connection partner.

2. Description of the Related Art

Conventionally, in the CDMA mobile communications, in order to synchronize the CDMA mobile communications apparatus (hereinafter referred to as a mobile unit) with the base station, there are a system and a sub-system of using three physical channels for constant transmission from the base station, that is, pSCH (Primary Synchronization Channel) and the sSCH (Secondary Synchronization Channel), and the CPICH (Common Pilot Channel).

These systems are described in the Standard Specifications TS25.211, TS25.213, and TS25.214 by the 3GPP (3rd Generation Partnership Project) which is a standard specification association of the CDMA mobile communications.

The structure of the physical channel shown in the standard specification TS25.211 has a radio frame of 10 ms as a basic unit, and one frame is formed by 15 slots as shown inFIG. 5. The symbol rate of the physical channels pSCH, sSCH, and CPICH is 10 per slot, but the pSCH and sSCH are assigned only to the leading symbol period of a slot.

The pSCH is spread by a spread code PSC (Primary Synchronization Code) commonly used in all base stations. The sSCH is spread by 16 types of SSC (Secondary Synchronization Code) assigned to each slot according to the unique sequence corresponding to 64 code groups to which the Scrambling Code assigned to each base station described later belongs.

The CPICH is spread by the Scrambling Code assigned to each base station. The other physical channels excluding the pSCH and sSCH transmitted from a base station are spread by the same Scrambling Code, and these physical channels are spread by a chanelization code assigned to each physical channel. However, the explanation is omitted here.

A common base station detecting method using the above mentioned three physical channels is described in the standard specification TS25.214. A base station detecting process is performed by detecting a frame transmission timing of a base station with a chip precision which is a spread unit, and by further detecting a Scrambling Code which is used at the base station. The process is performed in three steps of ‘slot synchronization’, ‘frame synchronization/code group identification’, and ‘Scrambling Code identification’.

These steps will be briefly explained below. First, in the ‘slot synchronization’ step (hereinafter referred to as step #1), in the timing of all chips in 1 slot period, a correlation operation corresponding to the spread code PSC is performed, and the profile of the correlation power value for the timing is obtained. When the pSCH transmission timing of the base station matches the timing of the correlation operation of the mobile unit, a large correlation value appears in the profile.

Commonly, a digital matched filter having the spread code PSC as a filter coefficient is used in the correlation operation. When the reception base band signal of the mobile unit is input into the matched filter, a sharp peak is output at the timing of the pSCH matching the filter coefficient. By detecting the peak, the synchronization is obtained with the transmission timing of the pSCH, that is, the slot timing.

In the ‘frame synchronization/code group identification’ step (hereinafter referred to as step #2), at the synchronous slot timing in step #1, correlation operations of 16 types are performed. Each SSC correlation value is obtained over a plurality of slots, the correlation values are accumulated according to 64 SSC sequences of the SSC, and the code group is identified in order from the highest accumulation value. Furthermore, a frame can be synchronized from the sequence of the SSC.

In the ‘Scrambling Code identification’ step (hereinafter referred to as step #3), at the frame timing synchronizing in step #2, a correlation operation for8Scrambling Codes belonging to the code group identified in step #2 is performed, and the Scrambling Code is identified in order from the highest correlation value.

When transmission signals of a plurality of base stations are received by the mobile unit at different timing, the correlation value profile obtained in step #1 shows a plurality of peaks corresponding to the timing and the received power. Therefore, on each of the plurality of peaks obtained in step #1, the processes in steps #2 and #3 are repeatedly performed, thereby detecting a plurality of base stations.

If a plurality of base stations are detected by this method, either the processing method shown inFIG. 6or the processing method shown inFIG. 8can be used to select a plurality of peak values from the correlation value profile obtained in step #1.

In the case of the processing method shown inFIG. 6, the peak value in the correlation value profile having a larger correlation power value is selected (steps S21to S22inFIG. 6), and the processes in and after step #2 are sequentially performed. For example, when the correlation value profile as shown in profile as shown inFIG. 7is obtained, then the processes in and after step #2are performed in the order of (a)→(b) →(c)→(d)→(e)→(f).

When the processing method shown inFIG. 8is performed, a threshold T1is set for the power value of the correlation value profile, the correlation power value equal to or smaller than the threshold T1is processed as noise (steps S31and S32shown inFIG. 8), the correlation power value larger than the threshold T1is selected in the order of the occurrence of a peak value (steps S31, S33, and S34shown in step S8), and the processes in and after step #2 are sequentially performed. For example, when the correlation value profile as shown in FIG.9is obtained, then the processes in and after step #2 are performed in the order of (a)→(b)→(c)→(d)→(e)→(f).

In the above mentioned conventional CDMA mobile communications, the environment as shown inFIG. 10is assumed. If a mobile unit (mobile object provided with a mobile unit)201is moved in the right direction inFIG. 10, and if a shelter301exists between a base station103and the mobile unit201, then the electric wave from base stations101and102can be predicted and therefore transmitted or received by the mobile unit201with sufficiently strong power. However, the waves from the base station103are reduced by the shelter301and received by the mobile unit201with lower power than the base stations101and102.

When the mobile unit201starts detecting the base stations101to103in the above mentioned method in the state shown inFIG. 10, then the order in which the base stations101to103are detected is not certain in the processing method shown inFIG. 8. However, in the processing method shown inFIG. 6, the probability that the base station103can be detected is clearly lower.

Furthermore, in the state shown inFIG. 10, if the mobile unit201starts communications with the base station101, some time has passed, and if the state shown inFIG. 11is entered, no influence of the shelter301is detected any more. When the most appropriate base station to the mobile unit201is the base station103as a connection partner, the mobile unit201performs hand-over from the base station101to the base station103.

However, if the mobile unit201cannot detect the base station103in the order of detecting base stations, it cannot perform the hand-over, and the electric wave from the base station101becomes weaker, thereby disconnecting the communications between the mobile unit201and the base station101.

SUMMARY OF THE INVENTION

The present invention aims at solving the above mentioned problems, and providing a CDMA mobile communications apparatus capable of enhancing the probability of detecting a base station with low power, and a base station detecting method used for the apparatus.

The CDMA mobile communications apparatus according to the present invention has the function of detecting a base station which is a direct connection partner by synchronizing the base station at slot timing and frame timing in this order by stepwise detecting a physical channel for constant transmission from the base station, and by identifying a Scrambling Code used in the base station, and includes: first selection means for selecting noise and significant slot timing for a peak correlation value at which the slot timing synchronization is performed; and second selection means for selecting a relatively large correlation value and a relatively small correlation value for the selected significant slot timing, in which the relatively large correlation value and the relatively small correlation value are appropriately selected at an optional ratio to synchronize the frame timing and identify the Scrambling Code.

The base station detecting method for the CDMA mobile communications apparatus according to the present invention has the function of detecting a base station which is a direct connection partner by synchronizing the base station at slot timing and frame timing in this order by stepwise detecting a physical channel for constant transmission from the base station, and by identifying a Scrambling Code used in the base station, and includes: a first step of selecting noise and significant slot timing for a peak correlation value at which the slot timing synchronization is performed; and a second step of selecting a relatively large correlation value and a relatively small correlation value for the selected significant slot timing, in which the relatively large correlation value and the relatively small correlation value are appropriately selected at an optional ratio to synchronize the frame timing and identify the Scrambling Code.

That is, the CDMA mobile communications apparatus according to the present invention intentionally selects a peak having a small correlation power value at a certain rate when a peak value is selected in the process in and after the step of ‘frame synchronization/code group identification’ from the correlation value profile obtained in the step of ‘slot synchronization’ in the 3-stage process of detecting a base station having three steps of ‘slot synchronization’, ‘frame synchronization/code group identification’, and ‘Scrambling Code identification’.

To be more practical, when a peak value in performing the processes in and after the step of ‘frame synchronization/code group identification’ in the 3-stage process of detecting a base station is to be selected, the CDMA mobile communications apparatus according to the present invention selects noise and a significant correlation value based on the first threshold T1, selects a peak value larger than a second threshold T2and a peak correlation value equal to or smaller than the second threshold T2based on the second threshold T2set between the largest peak correlation value and the first threshold T1for the selected significant correlation value, holds the slot timing corresponding to each peak correlation value in the first and second storage units, and selects probable slot timing alternately from the first and second storage units.

Thereby, it is possible to intentionally select a peak having a correlation power value smaller than other peaks at a certain rate, and the probability of detecting a base station received with low power can be enhanced by performing at an earlier stage the processes in and after the step of ‘frame synchronization/code group identification’ on the probable slot timing detected with low correlation power value when a correlation value profile is obtained.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will be described below by referring to the attached drawings.FIG. 1is a block diagram of a configuration of a base station detection unit of a CDMA mobile communications apparatus according to an embodiment of the present invention. InFIG. 1, a base station detection unit1comprises an input unit10, a slot synchronizing unit11, a first threshold determination unit12, a second threshold determination unit13, a first storage unit14, a second storage unit15, a slot timing selection unit16, a frame synchronization/code group identifying unit17, a Scrambling Code identifying unit18, and an output unit19.

The slot synchronizing unit11is a digital matched filter having a spread code PSC (Primary Synchronization Code) as a filter coefficient, and performs a correlation operation at a chip rate, or an integral multiple of the chip rate on a reception base band signal input from the input unit10in the CDMA mobile communications apparatus (hereinafter referred to as a mobile unit) (not shown in the attached drawings).

The first threshold determination unit12selects a peak correlation value output from the slot synchronizing unit11based on the predetermined first threshold T1. The second threshold determination unit13selects a peak correlation value selected by the first threshold determination unit12based on the predetermined first threshold T1. The first storage unit14and the second storage unit15hold the pSCH (Primary Synchronization Channel) timing, that is, the slot timing, corresponding to the peak correlation value selected by the second threshold determination unit13.

The slot timing selection unit16appropriately selects the slot timing held in the first storage unit14and the second storage unit15, and sequentially transmits it to the frame synchronization/code group identifying unit17and the Scrambling Code identifying unit18. The frame synchronization/code group identifying unit17performs a frame synchronization/code group identifying process on the slot timing and received base band signal sequentially transmitted from the slot timing selection unit16, and the Scrambling Code identifying unit18performs a Scrambling Code identifying process on a process result of the frame synchronization/code group identifying unit17and the received baseband signal.

FIG. 2is a flowchart of the base station detecting process in the base station detection unit1shown inFIG. 1. The base station detecting process according to an embodiment of the present invention is described below by referring toFIGS. 1 and 2.

In the present embodiment, the CDMA mobile communications apparatus detects a base station not shown in the attached drawings in the base station detecting method (described in the standard specification TS25.214) using the three physical channels of the pSCH, the sSCH (Secondary Synchronization Channel), and the CPICH (Common Pilot Channel).

In the base station detection, the frame transmission timing of a base station is detected with the chip precision, that is, a spread unit, and the Scrambling Code used in the base station is detected in the three steps of ‘slot synchronization’, ‘frame synchronization/code group identification’, and ‘Scrambling Code identification’.

First, in the ‘slot synchronization’ step (hereinafter referred to as step #1), the slot synchronizing unit11performs a correlation operation on a spread code PSC at the entire chip timing in one slot period, and obtains the profile of a correlation power value for the timing (step S1shown inFIG. 2).

When the pSCH transmission timing of the base station matches the timing of the correlation operation on the mobile unit side, a large correlation value appears in the profile. In general, a digital matched filter having a spread code PSC as a filter coefficient is used in the correlation operation.

When a received baseband signal of the mobile unit is input into the matched filter, a sharp peak is output at the timing of the pSCH matching the filter coefficient. By detecting the peak, the transmission timing of the pSCH, that is, the slot timing, can be synchronized.

Successively, the peak value (slot timing synchronized in step51) output from the slot synchronizing unit11is selected by the first threshold determination unit12, the second threshold determination unit13, the first storage unit14, second storage unit15, and the slot timing selection unit16(step S2shown inFIG. 2).

In the step of ‘frame synchronization/code group identification’ (hereinafter referred to as step #2), the frame synchronization/code group identifying unit17performs the correlation operation on the 16 types of SSC (Secondary Synchronization Code) at the slot timing selected in the prior step.

The frame synchronization/code group identifying unit17obtains each SSC correlation value over a plurality of slots, accumulates the correlation values in the 64 SSC sequence, and identifies code groups in order from the highest accumulation value (step S3shown inFIG. 2). Furthermore, the frame synchronization can be realized in the SSC sequence.

In the step of ‘Scrambling Code identification’(hereinafter referred to as step #3), the Scrambling Code identifying unit18performs correlation operations on the eight Scrambling Codes belonging to the code groups identified in step #2 at the frame timing synchronized in step #2, and identifies Scrambling Codes in order from the highest correlation value (step S4inFIG. 2).

In this state, when transmission signals of a plurality of base stations are received by a mobile unit at different timing, a plurality of peaks appear depending on the timing and reception power in the correlation value profile obtained in step #1. Therefore, a plurality of base stations can be detected by repeating the processes in steps #2 and #3 (steps S3to S5shown inFIG. 2) on the plurality of peaks obtained in step #1.

FIG. 3is a flowchart of the peak value selecting process according to an embodiment of the present invention.FIG. 4shows the order of selecting peak values according to an embodiment of the present invention. The base station detecting process according to an embodiment of the present invention will be practically described below by referring toFIGS. 1,3, and4.

The slot synchronizing unit11is a digital matched filter having a spread code PSC as a filter coefficient for the slot synchronizing process, and performs a correlation operation at a chip rate, or an integral multiple of the chip rate on a reception base band signal in the mobile unit input from the input unit10. A correlation power value as shown inFIG. 4is output in time series from the slot synchronizing unit11.

On the other hand, the first threshold determination unit12selects noise and a significant correlation value using the first threshold T1(step S11shown inFIG. 3), and inputs the significant correlation value and the corresponding pSCH timing, that is, the slot timing into the second threshold determination unit13. At this time, the first threshold determination unit12processes a correlation value equal to or smaller than the first threshold T1as noise (step S12shown inFIG. 3), and processes a correlation value larger than the first threshold T1as a significant correlation value.

The second threshold determination selects a peak correlation value larger than or equal to the second threshold T2and a peak correlation value smaller than the second threshold T2based on the second threshold T2set between the largest peak correlation value and the first threshold T1(step S13shown inFIG. 3), and holds the slot timing for each peak correlation value in the first and second storage units14and15(steps S14and S15shown inFIG. 3).

In the example shown inFIG. 4, the slot timing corresponding to the peak correlation values (a), (c), and (e) is held in the first storage unit14, and the slot timing corresponding to the peak correlation values (b), (d), and (f) is held in the second storage unit15.

Then, the following slot timing selection unit16selects probable slot timing alternately from the first and second storage units14and15(steps S16and S17shown inFIG. 3), and a base station can be detected through the processes in the frame synchronization/code group identifying unit17and the Scrambling Code identifying unit18.

In the peak value selecting process according to an embodiment of the present invention, the order of detecting a base station in an example shown inFIG. 4is (a)→(b)→(c)→(d)→(e)→(f).

Thus, when a peak value in performing the processes in and after step #2is selected from the correlation value profile obtained in step#1in the 3-stage base station detecting process, a peak having a correlation power value smaller than other peaks is intentionally selected at a certain rate in the above mentioned method, the processes in steps #2and #3are performed at an early stage on the slot timing detected with a small correlation power value when the correlation value profile is obtained in step #1, thereby enhancing the detection probability of a base station received with low power.

In the present embodiment, the slot timing selection unit16selects probable slot timing alternately from the first and second storage units14and15, but it is also possible to select the correlation values held in the first and second storage units14and15in order from the largest or smallest value. In this case, the order of detecting a base station in an example shown inFIG. 4is (a)→(d)→(e)→(f)→(c)→(b).

As described above, according to the present invention, the CDMA mobile communications apparatus having the function of detecting a base station which is a direct connection partner by synchronizing the base station at slot timing and frame timing in this order by stepwise detecting a physical channel for constant transmission from the base station, and by identifying a Scrambling Code used in the base station, selects noise and significant slot timing for a peak correlation value at which the slot timing synchronization is performed, and selects a relatively large correlation value and a relatively small correlation value for the selected significant slot timing, wherein the relatively large correlation value and the relatively small correlation value are appropriately selected at an optional ratio to synchronize the frame timing and identify the Scrambling Code, thereby enhancing the detection probability of a base station received with low power.