JPH11355829A - Mobile communication system, base station device, mobile terminal and hand-over control method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To recover speech quality by conducting hand-over, even when deteriorating the speech quality not only in the case of detecting a strong electric field strength and to reduce call loss rate by suppressing a communication channel resulting in squelch interrupt. SOLUTION: A base station makes a mobile station measure a reception level R of a pilot channel of a peripheral sector during communication (S101) and receives a measured level R (S102). Furthermore, the base station allows the mobile station to measure a speech quality qq of a sector during communication and measures a speech quality Q' (S103) and receives the measured Q (S104). In response to the result of comparing the Q, Q' with a predetermined reference value Q1 respectively, a predetermined reception level R1 which is a reference to set/release a simultaneous communication mode is changed into a smaller level R2, a peripheral sector whose reception level R is higher than the level R2 connects newly to the mobile station, and the simultaneous communication mode is selected (S107). When the Q, Q' of the sector during the simultaneous communication are deteriorated further from a predetermined reference value Q" (Q2<Q1) (S110), a channel corresponding to the sector is disconnected (S114).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a handover control method in a code division multiple access (CDMA) mobile communication system using spread spectrum modulation technology.

[0002]

2. Description of the Related Art The service area of a mobile communication system is:
Generally, a cellular structure in which a plurality of zones are combined is adopted, and a base station is provided in each zone called a cell, and these cells are often provided with a plurality of zones in terms of reducing interference and improving antenna gain. Divided into sectors,
A mobile station located in a certain sector communicates with a base station via a wireless line, and the base station communicates with another mobile station or fixed telephone via another wireless line or a wired line. ing.

In a mobile communication system having such a cellular structure, when a communicating mobile station moves from a serving sector to an adjacent sector, a radio line communicating with the serving base station is set to an adjacent base station. It is necessary to switch to a wireless line for communicating with the network.

[0004] In a CDMA mobile communication system, the same frequency is used in all sectors, and is spread with different spreading codes to perform multiple access communication. There will be no instantaneous interruption during operation (hereinafter referred to as handover), and handover without instantaneous interruption (hereinafter referred to as handover)
Soft handover).

Hereinafter, this soft handover operation will be described with reference to FIG. In FIG. 2, white circles indicate base stations 201 and 202, and black circles indicate mobile station 211. In addition, cells 241 and 24 are located within the service area of each base station.
The area divided by each cell is indicated by sector 22 in FIG.
1, 222. In FIG. 2, the mobile station 211
Is the arrow 21 as shown in FIGS. 2 (a), 2 (b) and 2 (c).
An example in which the robot is moving in the direction 2 will be described. As shown in FIG. 2A, when the mobile station 211 located in the sector 221 of the cell 241 moves in the movement direction 212 while communicating with the base station 201 via a wireless line, the mobile station 21
1 moves, the electric field from the base station 202 in the sector 222 of the cell 242 gradually increases, and the first point 23
1, the electric field becomes sufficient for communication with the base station 202. For this reason, as shown in FIG. 2B, a wireless line with the base station 202 is newly connected to start simultaneous communication, that is, a handover operation. On the other hand, the mobile station 211
As the cell moves, the electric field of the sector 221 of the cell 241 gradually decreases, and as shown in FIG. 2C, it is difficult to continue the communication with the base station 201 at the time when the cell passes the second point 232. Since the electric field is generated, the wireless line with the base station 201 is disconnected to complete the handover operation.

In such a conventional handover operation, the mobile station measures the reception level of the pilot channel of the peripheral sector using the spreading code of the peripheral sector notified from each base station, and the reception level is determined in advance. If the threshold value is exceeded, the base station is notified of the fact and simultaneous communication is performed, and if the reception level of the pilot channel of the simultaneously communicating sector falls below a predetermined threshold value, the base station is notified. The operation of notifying the station and cutting off communication with the sector is performed.

[0007]

Since the service area of the mobile communication system is generally constructed so as to increase the population coverage, the urban area with a large population occupies most of the area. There are very few places that can be considered radio wave propagation, that is, radio wave propagation in free space.

In a mobile communication system, a radio line is used for communication between a mobile station and a base station. Therefore, in a place such as an urban area, a multiple propagation path is formed by reflection and diffraction of a building near the mobile station. The received wave is often a composite wave of multiplexed waves having different phases and amplitudes, and when the mobile station travels, the phase relationship between the multiplexed waves constituting the received wave changes. Fading that fluctuates in a complicated manner, and a transmission error occurs when the amplitude level of the received wave falls close to the thermal noise level of the receiver due to this effect.

Normally, an error rate of 10 ^-3 or less is required for performing digital transmission with sufficient quality. However, in mobile communications, it is a tentative goal to suppress the error rate to about 10 ^-2 or less. Have been.

However, when an error rate of 10 ⁻² is to be ensured in a fading environment, it is necessary to increase the transmission power by 10 dB or more as compared with the case of non-fading. In a mobile communication system, the error rate is reduced by using techniques such as diversity reception, error correction coding, and bit interleaving, but the effect is limited.

In the above-described conventional method, the handover operation is started only by determining the level of the reception level. Therefore, when the reception level of the communicating sector is high and the reception level of the neighboring sector is low, The handover operation is not performed regardless of the communication quality of the communicating sector. For this reason, for example, when the reception level of the communicating sector is high, but the communication quality is degraded, the squelch is disconnected and the communication may be suddenly disconnected.

It is an object of the present invention to provide a mobile station terminal, a base station apparatus, and a handover control method for suppressing loss of a communication channel that causes a squelch break and reducing a call loss rate.

[0013]

To achieve the above object, the present invention relates to a mobile communication system including a mobile terminal and a base station for performing wireless communication by code division multiple access, wherein the base station is in communication. Receiving means for receiving, from the mobile terminal, the communication quality of wireless communication with the base station and the reception level of wireless communication with another base station in the vicinity of the base station; If the quality is lower than a predetermined reference communication quality, a predetermined second predetermined reception level lower than a predetermined first reference reception level.
Changing means for changing to the reference reception level, and a reception level of wireless communication with another base station received by the reception means being higher than the second reference reception level changed by the change means. And a simultaneous communication instructing means for instructing to perform simultaneous communication with the base station, wherein the mobile terminal communicates with a base station that is in communication with the communication quality of wireless communication, and communicates with another base station in the vicinity of the base station. Measuring means for measuring the reception level of the wireless communication, the communication quality of the wireless communication with the base station measured by the measuring means, and the reception level of the wireless communication with another base station near the base station And a communication unit that, when instructed by the simultaneous communication unit, performs simultaneous communication with the instructed base station. Accordingly, even when the reception level of the wireless communication with another base station does not enter the simultaneous communication at the first reference reception level, the communication quality of the wireless communication during the call is reduced to the predetermined reference communication level. If the quality is worse than the above, if the reception level of the wireless communication with another base station is higher than the second reference reception level, the handover can be performed by instructing to perform the simultaneous communication.

[0014] The base station may further include measurement instructing means for instructing the communicating mobile terminal to measure a reception level of radio communication with another base station in the vicinity. Good. Further, the apparatus may further include quality measurement instructing means for instructing the communicating mobile terminal to measure the communication quality of wireless communication with the base station. Further, the changing unit can change the second reference reception level according to the value of the call quality received by the receiving unit. Further, the communication quality can be, for example, a frame error rate and / or a bit error rate.

[0015]

Embodiments of the present invention will be described below.

In the handover control according to the present embodiment, in order to detect the start of the handover process, a pilot channel constantly transmitted by the base station in each sector, that is, the same frequency, is used, and different spreading codes are used. Measuring the reception level R of the pilot channel of the peripheral sector not performing transmission power control,
Further, the communication qualities Q and Q 'in the base station and the mobile station are calculated, and when the communication qualities Q and Q' are worse than the first communication quality Q1 determined in advance, the value of the reference reception level R1 is made smaller. The second reference reception level R2 is changed to the second reference reception level R2, and the handover process is executed when the reception level R is higher than R2. Furthermore, after entering the simultaneous communication mode with the sector selected by the handover process, the line of the unnecessary simultaneous communication sector is disconnected under predetermined conditions, and the handover process is completed. Thus, in addition to the reception level,
The handover process is performed in consideration of the communication quality Q.

The reference speech quality will be described with reference to FIG. 3A shows the positional relationship between the base station and the mobile station, FIG. 3B shows the reception level at the position P of the mobile station shown in FIG. 3A, and FIG. 3A shows the communication quality at the position P of the mobile station shown in FIG. In FIG. 3A, a black circle indicates a mobile station 311;
White circles indicate base stations 301 and 302, black squares indicate building 321 and mobile station 311 moves on line P as an example. In FIG. 3B, R1 indicates a first reference reception level set first, R2 indicates a second reference reception level changed when the communication quality is degraded, and R3 indicates a disconnection time. And the disconnection process is performed when the reception level becomes lower than R3. Further, in FIG. 3C, the communication quality Q can be represented by, for example, an error rate (BER) in units of bits, an error rate (FER) in units of frames, and the like. Indicates that it has deteriorated. FIG.
In (c), Q1 is a reference communication quality for determining a change in reception level, and when the communication quality deteriorates from Q1, the reference reception level is changed to R2. Q2 indicates the reference communication quality at the time of disconnection, and disconnection processing is performed when the communication quality is lower than Q2.

When the mobile station moves to the position P in FIG. 3A, the base station 301 and the base station 302
The reception level R of the radio signal transmitted from the terminal draws a line such as the reception level 331 and the reception level 332 shown in FIG. That is, the reception level R becomes maximum at the position closest to the base station, and decreases as the distance from the base station increases. Accordingly, the reception levels R of the radio waves from the base station 301 and the base station 302 in the mobile station 311 at the position shown in FIG. 3A are the reception levels 341 and 342 shown in FIG.

In the conventional method, when the mobile station 311 shown in FIG.
The reception level 33 from the base station 302 as shown in FIG.
Since 2 gradually increases, the simultaneous communication mode, that is, the handover operation, is started when the reception level becomes higher than the predetermined reference reception level R1. Conversely, FIG.
As long as the mobile station 311 shown in FIG.
Since the reception level 342 from the base station 302 shown in (b) does not become higher than the reference reception level R1, the handover operation is not started.

However, the mobile station 311 shown in FIG.
Although the reception level R from the base station 301 is sufficiently large at the point of, a multiplex propagation path is formed due to the influence of the building 321 and the like near the mobile station 311 and permanent fading occurs. The call quality Q on the line with the station 301 is in a deteriorated state such as the call quality 361 shown in FIG. 3C and is lower than the predetermined call quality Q1, and there is a risk that the squelch will be cut off later. .

In the handover control according to the present embodiment, the communication quality Q on the line with the base station 301 which is currently communicating shown in FIG. 3A is changed to the communication quality 3 shown in FIG.
As in the case of 61, a case where the communication quality has deteriorated from the reference communication quality Q1 is also detected, and the reference reception level serving as a reference for starting the handover operation is changed to R2. As a result, FIG.
(B) a base station capable of receiving a reception wave having a reception level 342 higher than the changed reference reception level R2,
That is, the handover operation starts with the base station 302 shown in FIG.

Next, the configurations of the base station and the mobile station according to the present embodiment will be described with reference to FIG. 4 and FIG. FIG. 4 shows an internal block diagram of base station 401 in the present embodiment, and FIG. 5 shows an internal block diagram of mobile station 431 in the present embodiment.

In FIG. 4, a base station 401 is a mobile station 4
An antenna 402 for communicating with the communication unit 31, a transmission unit 411 for performing processing for transmission, a reception unit 421 for performing processing for reception, a control unit 405 for controlling the transmission unit 411 and the reception unit 421, and a control unit 405. , A memory 425 for storing a program to be executed by the CPU, a duplexer 403, and a synthesizer 404. Further, behind the base station 401, a base station control device 406 connected to the network is connected. Further, transmitting section 411 transmits a signal as a radio wave from signal processor 412 which performs convolutional coding and bit interleaving, spreader 413 which performs spreading using a spreading code, modulator 414 which places a signal wave on a carrier wave, and antenna 402. 415 is provided. Also, the receiving unit 421
Are a reception circuit 422 that demodulates and decodes a received wave received by the antenna 402, a correlation detector 423 that detects correlation of a received signal received by the reception circuit 422, and a speech quality detector 424 that measures speech quality. Is provided.

The mobile station 431 shown in FIG. 5 includes an antenna 432 for communicating with the base station 401, a transmitting unit 441 for performing a transmitting process, a receiving unit 451 for performing a receiving process, a transmitting unit 441, and a receiving unit 451. Control unit 43 that performs control
5. A speech encoder 436 that performs speech encoding / decoding and includes a speech input / output unit, a duplexer 433, and a synthesizer 434. Further, the receiving unit 451 includes a receiving circuit 452 for demodulating a received wave received by the antenna 432, a correlation detector 453 for detecting a correlation of a received signal received by the receiving circuit 452, and a speech quality detection for measuring speech quality. And a level detector 455 for measuring the reception level.

Next, the base station 40 shown in FIGS.
1 and the operation of the mobile station 431 will be described. Base station 401
The transmission data of the pilot channel generated in is subjected to signal processing such as convolutional coding and bit interleaving in a signal processor 412, and spread processing using a pilot channel spread code is performed in a spreader 413. The modulator 414 performs a modulation process for putting the carrier wave on the carrier wave.
Finally, the signal is amplified by the amplifier 415 to become a transmission wave, which is transmitted from the antenna 402 via the duplexer 403.

On the other hand, in the mobile station 431 shown in FIG.
The data is sent to the receiving unit 451 via 33. The reception wave input to the reception circuit 452 inside the reception unit 451 is subjected to demodulation processing by the reception circuit 452, and an output signal is supplied to the correlation detector 453. Correlation detector 453 performs correlation detection using the spreading code of the pilot channel notified from control unit 435, and performs despreading processing using the spreading code of the pilot channel according to the timing obtained by correlation detector 453. Done. The signal obtained by the despreading process is supplied to a speech quality detector 454 and a level detector 455, and the speech quality and the reception level are measured. Then, the communication quality and the reception level obtained by this measurement are supplied to the control unit 435,
It is transmitted to the base station based on a predetermined report timing to the base station.

Also, in the base station 401, the mobile station 4
31 is performed, and the call quality detector 424
The communication quality obtained by the measurement in the control unit 4 and the communication quality and reception level transmitted from the mobile station 431 are the same as those in the control unit 4.
The handover control described below is performed based on these values.

Next, a processing procedure of handover control in the first embodiment will be described with reference to a flowchart shown in FIG. FIG. 1 is a flowchart illustrating a processing procedure of the control unit of the base station when performing the handover control according to the first embodiment. In the mobile communication system of the CDMA system using the spread spectrum modulation technique, the processing shown in FIG. 1 is performed in addition to the usual handover processing required when a mobile station moves from a serving sector to an adjacent sector during communication. The handover process can be performed even when the call quality is significantly degraded due to fading or the like. A program corresponding to this flowchart is stored in the memory 425 shown in FIG.
Is stored in

In FIG. 1, the base station notifies the mobile station in communication of the spreading code of the neighboring sector, and measures the reception level R of the pilot channel from each of the surrounding sectors using the notified spreading code at the mobile station. (S10
1). In the mobile station, based on the instruction from the base station in communication,
The reception level R of the pilot channel from each neighboring sector is measured. Then, the mobile station notifies the base station of the measured reception level R, and the base station receives the measured reception level R from the mobile station (S102). Further, the base station instructs the communicating mobile station to measure the communication quality Q, and also measures the communication quality Q '(S103). The mobile station measures the communication quality Q based on an instruction from the base station in communication or autonomously. Then, the mobile station notifies the base station of the measured communication quality Q, and the base station receives the measured communication quality Q from the mobile station (S104).

When the obtained communication quality Q, Q 'of the communicating sector is lower than the predetermined communication quality Q1, the base station sets the predetermined reference reception level R1 as shown in FIG. 3 (b). Reference reception level R2 (R2 <R
1), notifies the mobile station of the changed reference reception level R2, and holds the reference reception level R2 in the base station (S105). Thereafter, the base station selects a sector in which the reception level R of the neighboring sector received from the mobile station is higher than the reference reception level R2 (S106), and instructs the mobile station to enter the simultaneous communication mode (S107). The mobile station enters the simultaneous communication mode based on an instruction from the base station.

The base station which has entered the simultaneous communication mode instructs the communicating mobile station to measure the communication quality Q of the simultaneously communicating sector, and also measures the communication quality Q '(S1).
08). The mobile station measures the communication quality Q based on an instruction from the base station in communication or autonomously. Then, the mobile station notifies the measured communication quality Q to the base station, and the base station receives the measured communication quality Q from the mobile station (S10).
9).

Thereafter, when the base station detects that the obtained communication quality Q, Q 'of the simultaneously communicating sector has deteriorated from the predetermined communication quality Q2 shown in FIG. 3 (c) (S110). ), The mobile station is instructed to disconnect the line to the sector, and the mobile station disconnects the line to the sector based on the instruction from the base station (S1).
14). If the communication quality is not degraded from Q2 (S110), the base station causes the mobile station to measure the reception level R 'of the pilot channel from the simultaneously communicating sector (S111). The mobile station measures the reception level R 'of the pilot channel from the simultaneously communicating sector based on an instruction from the communicating base station. Then, the mobile station notifies the measured reception level R 'to the base station, and the base station receives the measured reception level R' from the mobile station (S112). The base station determines that the reception level R ′ of the simultaneously communicating sector received from the mobile station is the disconnection reference reception level R3.
When it becomes smaller (S113), an instruction is given to disconnect the line corresponding to the sector in the simultaneous communication (S1).
14).

As described above, according to the present embodiment, the reference reception level serving as the reference for starting the handover is dynamically changed according to the quality of the communication quality of the communicating sector compared with the predetermined communication quality. Then, it is possible to newly connect a sector having a reception level higher than the changed reference reception level and start a handover operation. Also, disconnection is detected when the communication quality of the simultaneously communicating sector is degraded from a predetermined communication quality, or when it is detected that the reception level of the simultaneously communicating sector is lower than the disconnection reference reception level. Then, the line for the sector is disconnected to complete the handover operation. For this reason, while trying to recover the call quality at the time of call quality deterioration that was not possible with the conventional method,
Loss of a communication channel that causes squelch breakage can be suppressed, and the call loss rate can be reduced.

Next, a second embodiment will be described. In the second embodiment, a specific configuration and processing when the communication quality shown in the first embodiment is obtained based on the frame error rate will be described.

In the handover control according to the second embodiment, in order to detect the start of the handover process, a pilot channel constantly transmitted by the base station in each sector, that is, the same frequency is used, and different sectors are used. A reception level R of a pilot channel of a peripheral sector which is not subjected to transmission power control and is spread by a spreading code is measured, and when the reception level R is higher than a predetermined first reference reception level R1, a condition for executing a handover process is determined. In addition, the frame error rates FER and FER 'in the base station and the mobile station are calculated. If the frame error rates FER and FER' are lower than a predetermined first frame error rate FER1, the reference reception level R1 Third, fourth, and fifth reference reception levels R3, R4, R5 with smaller values It was changed to be in either,
When the reception level is higher than the changed reception level, a handover process is executed. Furthermore, after entering the simultaneous communication mode with the sector selected by the handover process,
Under the predetermined conditions, the line of the unnecessary simultaneous communication sector is disconnected, and the handover process is completed. As described above, the handover process is performed in consideration of the frame error rate FER in addition to the reception level.

The reference reception level and the frame error rate will be described with reference to FIG. FIG. 6A shows the positional relationship between the base station and the mobile station, and FIG. 6B shows the reception level at the position P of the mobile station shown in FIG.
(C) shows the frame error rate at the position P of the mobile station shown in FIG. 6 (a). In FIG. 6A, a black circle indicates the mobile station 311, a white circle indicates the base stations 301 and 302, a black square indicates the building 321,
The case where the mobile station 311 moves on the line P will be described as an example. In FIG. 6B, R1 is the first set first.
R3, R4 or R5 indicates a third, fourth or fifth reference reception level changed when the frame error rate is degraded, R2 indicates a reference reception level at disconnection, The disconnection process is performed when the reception level becomes lower than R2. Further, FIG. 6C shows that the communication quality is degraded as the value of the frame error rate FER increases. In FIG. 6C,
FER1 is a reference frame error rate for judging a change in reception level, and includes frame error rates FER and FER ′.
Is lower than FER1, the reference reception level is changed to R3, R4 or R5 (R5 <R4 <R3 <R1).
Change to FER2 indicates a reference frame error rate at the time of disconnection, and performs disconnection processing when the frame error rate has deteriorated from FER2.

Next, the configurations of a base station and a mobile station according to the second embodiment will be described with reference to FIGS. FIG. 7 shows an internal block diagram of base station 401 in the present embodiment, and FIG. 8 shows an internal block diagram of mobile station 431 in the present embodiment.

In FIG. 7, a base station 401 is a mobile station 4
An antenna 402 for communicating with the communication unit 31, a transmission unit 411 for performing processing for transmission, a reception unit 421 for performing processing for reception, a control unit 405 for controlling the transmission unit 411 and the reception unit 421, and a control unit 405. , A memory 425 for storing a program to be executed by the CPU, a duplexer 403, and a synthesizer 404. Further, behind the base station 401, a base station control device 406 connected to the network is connected. Further, the transmission unit 411 transmits a signal as a radio wave from a signal processor 412 that performs convolutional coding and bit interleaving, a spreader 413 that performs spreading using a spreading code, a modulator 414 that places a signal wave on a carrier wave, and an antenna 402. 415 is provided. Also, the receiving unit 421
Is a reception circuit 422 that demodulates and decodes a received wave received by the antenna 402, a correlation detector 423 that detects correlation of a received signal received by the reception circuit 422, and a frame error rate detection that measures a frame error rate Device 440 is provided.

The mobile station 431 shown in FIG. 8 includes an antenna 432 for communicating with the base station 401, a transmitting unit 441 for performing transmission processing, a receiving unit 451 for performing receiving processing, a transmitting unit 441, and a receiving unit 451. Control unit 43 that performs control
5. A speech encoder 436 that performs speech encoding / decoding and includes a speech input / output unit, a duplexer 433, and a synthesizer 434. The receiving unit 451 includes a receiving circuit 452 for demodulating a received wave received by the antenna 432, a correlation detector 453 for detecting a correlation of a received signal received by the receiving circuit 452, and a frame error for measuring a frame error rate. A rate detector 460 and a level detector 455 for measuring a reception level are provided.

Next, the base station 40 shown in FIGS.
1 and the operation of the mobile station 431 will be described. Base station 401
The transmission data of the pilot channel generated in is subjected to signal processing such as convolutional coding and bit interleaving in a signal processor 412, and spread processing using a pilot channel spread code is performed in a spreader 413. The modulator 414 performs a modulation process for putting the carrier wave on the carrier wave.
Finally, the signal is amplified by the amplifier 415 to become a transmission wave, which is transmitted from the antenna 402 via the duplexer 403.

On the other hand, in the mobile station 431 shown in FIG.
The data is sent to the receiving unit 451 via 33. The reception wave input to the reception circuit 452 inside the reception unit 451 is subjected to demodulation processing by the reception circuit 452, and an output signal is supplied to the correlation detector 453. Correlation detector 453 performs correlation detection using the spreading code of the pilot channel notified from control unit 435, and performs despreading processing using the spreading code of the pilot channel according to the timing obtained by correlation detector 453. Done. The signal obtained by the despreading process is supplied to a frame error rate detector 460 and a level detector 455, and the frame error rate and the reception level are measured. Then, the frame error rate and the reception level obtained by this measurement are supplied to the control unit 435, and transmitted to the base station based on a predetermined report timing to the base station.

Here, the frame error rate will be described. The frame error rate indicates a ratio of the number of received bad frames (errored frames) to the total number of received frames. The frame error rate detector 460 of the mobile station counts the number of frames and the number of bad frames in a predetermined measurement period, and sends the number of frames and the number of bad frames to the base station based on a predetermined report timing. Sending. The base station can determine the frame error rate from the number of frames and the number of bad frames. For example, when the number of frames in a predetermined measurement period is set to 320 and the number of bad frames is one, the frame error rate =
1/320 × 100 ≒ 0.3%.

Also, in the base station 401, the mobile station 4
31 is performed, and the frame error rate obtained by the measurement by the frame error rate detector 440 and the frame error rate and the reception level sent from the mobile station 431 are supplied to the control unit 405. The handover control described below is performed based on these values.

Next, a processing procedure of handover control according to the second embodiment will be described with reference to a flowchart shown in FIG. FIG. 9 is a flowchart illustrating a processing procedure of the base station when performing handover control according to the second embodiment. The program corresponding to this flowchart is stored in the memory 425 shown in FIG.

In FIG. 9, the base station notifies the mobile station in communication of the spreading code of the neighboring sector, and measures the reception level R of the pilot channel from each surrounding sector at the mobile station using the notified spreading code. (S90
1). In the mobile station, based on the instruction from the base station in communication,
The reception level R of the pilot channel from each neighboring sector is measured. Then, the mobile station notifies the base station of the measured reception level R, and the base station receives the measured reception level R from the mobile station (S902). Further, the base station instructs the communicating mobile station, and sets the frame error rate FER.
And measures the frame error rate FER ′ by itself (S903). The mobile station measures the frame error rate FER based on an instruction from the base station in communication or autonomously. Then, the mobile station notifies the base station of the measured frame error rate FER, and the base station receives the measured frame error rate FER from the mobile station (S904).

In the base station, when the obtained frame error rates FER, FER 'of the communicating sector are lower than the predetermined frame error rate FER1, or when the reception level R of the peripheral sector is higher than the reference reception level R1. Is detected (S905). The cause is (S906)
If the reception level R of the peripheral sector is higher than the reference reception level R1, the mobile station is instructed to connect the peripheral sector, thereby entering the simultaneous communication mode (S9).
07). Further, the obtained frame error rates FER and FER 'of the communicating sector are determined by a predetermined frame error rate F.
If it is worse than ER1, the reception level R of the surrounding sector
Selects a sector larger than any of the predetermined reference reception levels R3, R4 or R5 (R5 <R4 <R3 <R1) and instructs the mobile station to enter the simultaneous communication mode (S908). The mobile station enters the simultaneous communication mode based on an instruction from the base station.

The base station which has entered the simultaneous communication mode instructs the communicating mobile station to measure the frame error rate FER of the simultaneously communicating sector.
R ′ is measured (S909). The mobile station measures the frame error rate FER based on an instruction from the base station in communication or autonomously. Then, the mobile station notifies the base station of the measured frame error rate FER, and the base station receives the measured frame error rate FER from the mobile station (S91).
0).

Thereafter, at the base station, the obtained frame error rates FER, FER 'of the simultaneously communicating sector are obtained as shown in FIG.
The predetermined frame error rate FER2 shown in (c)
If it is detected that the mobile station has deteriorated further (S911), the mobile station is instructed to disconnect the line for the sector,
The mobile station disconnects the line to the sector based on the instruction from the base station (S915). If the frame error rate has not deteriorated from FER2 (S91)
1), the base station causes the mobile station to measure the reception level R 'of the pilot channel from the simultaneously communicating sector (S912). The mobile station measures the reception level R 'of the pilot channel from the simultaneously communicating sector based on an instruction from the communicating base station. Then, the mobile station notifies the measured reception level R 'to the base station, and the base station receives the measured reception level R' from the mobile station (S91).
3). When the reception level R 'of the neighboring sector received from the mobile station becomes lower than the disconnection reference reception level R2 (S914), the base station instructs to disconnect the line corresponding to the sector performing the simultaneous communication (S915). ).

Next, the case where the value of the reference reception level is changed in accordance with the value of the frame error rate in the above embodiment will be further described by exemplifying the value. FIG. 10 shows a process of changing the value of the reference reception level according to the value of the frame error rate in the process shown in FIG.
FIG. 11 shows the relationship between the reference reception level value and the frame error rate.

The reception level R in the present embodiment is represented by an absolute value whose unit is dBμ or dBm,
Both of those represented by the relative ratios can be applied. In the present embodiment, the ratio of pilot channel power / interference wave power (Ec / I ₀ ) is determined by the reception level R
Treated as The frame error rate is represented by, for example, an average value of the frame error rate as% when the sampling period is 20 ms and the number of samplings per second is 50. In such a case, for example, the reference reception level R1 is set to −12 dB, and the reference frame error rate FER is set.
1 is defined in advance as 1.5%.

In FIG. 10, following the processing of S904 shown in FIG. 9, the obtained frame error rate F of the communicating sector is obtained.
ER, FER 'determined by the frame error rate FE
In the case where it is worse than R1 (1.5%) (S110
1), the obtained frame error rate of the communicating sector FER,
When FER ′ is between (S1102) 1.5% and 2.0%, the mobile station is set so that the reference reception level is −13 dB, and a sector whose reception level of the surrounding sector is higher than −13 dB is selected and connected. (S110)
3). Alternatively, when the obtained frame error rates FER, FER 'of the communicating sector are between 2.0% and 2.5%, the reference reception level is set to -14 dB, and the reception levels of the surrounding sectors are larger than -14 dB. The mobile station is instructed to select and connect a sector (S1104). Alternatively, the obtained frame error rate FER of the communicating sector is obtained.
When FER 'is larger than 2.5%, the reference reception level is set to -15 dB, and the reception levels of the surrounding sectors are set to -15 dB.
The simultaneous communication mode is entered by instructing the mobile station to select and connect to a sector larger than dB (S11).
05). Thereafter, the flow shifts to S909 shown in FIG.

As described above, when the frame error rate is low, the reference reception level may be changed to a smaller value according to the value. Further, the communication quality may be obtained by using the bit error rate instead of the frame error rate. In this case, the bit error rate is indicated by the ratio of the number of error bits to the total number of received bits.

As described above, according to the present embodiment, the reference reception level serving as a reference for starting the handover is adjusted according to the quality of comparison between the frame error rate of the communicating sector and the predetermined frame error rate. The handover operation can be started by newly connecting a sector having a higher reception level than the reference reception level after the change. In addition, disconnection is detected when the frame error rate of the simultaneously communicating sector is degraded from a predetermined frame error rate or when the reception level of the simultaneously communicating sector is lower than the disconnection reference reception level. In this case, the line for the sector is disconnected, and the handover operation can be completed. For this reason, it is possible to suppress the loss of the communication channel which is a squelch break when the frame error rate is deteriorated which cannot be performed by the conventional method, and to reduce the call blocking rate.

[0054]

According to the present invention, the handover is performed not only when a strong electric field is detected but also when the communication quality is degraded, thereby recovering the communication quality and suppressing the loss of the communication channel that causes a squelch break. Thus, the call loss rate can be reduced.

[Brief description of the drawings]

FIG. 1 is a flowchart illustrating a processing procedure of a base station when performing handover control according to a first embodiment.

FIG. 2 is an explanatory diagram showing a soft handover operation which is a feature of a CDMA mobile communication system.

FIG. 3 is an explanatory diagram showing an example of a positional relationship between a base station and a mobile station, a reception level, and communication quality in a mobile communication system.

FIG. 4 is a block diagram showing a configuration of a base station for realizing handover control according to the first embodiment.

FIG. 5 is a block diagram illustrating a configuration of a mobile station for implementing handover control according to the first embodiment.

FIG. 6 is an explanatory diagram showing an example of a positional relationship between a base station and a mobile station, a reception level, and communication quality in the second embodiment.

FIG. 7 is a block diagram showing a configuration of a base station for realizing handover control in a second embodiment.

FIG. 8 is a block diagram illustrating a configuration of a mobile station for implementing handover control according to the second embodiment.

FIG. 9 is a flowchart illustrating a processing procedure of a base station when performing handover control according to the second embodiment.

FIG. 10 is a flowchart illustrating a detailed processing procedure of a base station when performing handover control according to the second embodiment.

FIG. 11 is an explanatory diagram showing a relationship between a reception level threshold and a communication quality threshold when performing handover control according to the second embodiment.

[Explanation of symbols]

 201 ... base station 202 ... base station 211 ... mobile station 212 ... mobile station 211 moving direction 221 ... sector 222 ... sector 231 ... handover start point 232 ... handover completion point 241 ... cell 242 ... cell 301 ... base station 302 ... base station 311 ... mobile station 321 ... building 331 ... base station 301 electric field distribution 332 ... base station 302 electric field distribution 341 ... mobile station 311 reception level (base station 301 electric field) 342 ... mobile station 311 reception level (base station 302 electric field) 361 ... base station 301-mobile station 311 line communication quality 362 ... base station 302-mobile station 311 line communication quality 401 ... base station 402 ... antenna 403 ... duplexer 404 ... synthesizer 405 ... control unit 406 ... base station control device 411 ... transmitting unit 412 ... signal processor 413 ... diffuser 414 ... strange Device 415 Amplifier 421 Receiver 422 Receiver circuit 423 Correlation detector 424 Communication quality detector 431 Mobile station 432 Antenna 433 Duplexer 434 Synthesizer 435 Controller 436 Voice encoder 441 Transmitter 451 receiving section 452 receiving circuit 453 correlation detector 454 speech quality detector 455 reception level detector.

Continued on the front page (72) Inventor Takashi Harashita 216 Totsuka-cho, Totsuka-ku, Yokohama-shi, Kanagawa Prefecture, Hitachi, Ltd. Information and Communication Division

Claims (8)

[Claims] 1. A mobile communication system including a base station and a mobile terminal performing wireless communication by code division multiple access,
The base station, from the mobile terminal in communication, receiving means for receiving the communication quality of wireless communication with the base station and the reception level of wireless communication with other base stations around the base station, Changing means for changing to a predetermined second reference reception level smaller than a predetermined first reference reception level when the communication quality received by the receiving means is lower than a predetermined reference communication quality; Simultaneous communication instruction for instructing to perform simultaneous communication with a base station having a reception level of a radio communication with another base station which is higher than the second reference reception level changed by the changing means and received by the means. Means, wherein the mobile terminal comprises:
Measuring means for measuring the communication quality of wireless communication with the communicating base station and the reception level of wireless communication with another base station in the vicinity of the base station; and the base station measured by the measuring means. Transmission means for transmitting the communication quality of wireless communication with the base station and the reception level of wireless communication with another base station in the vicinity of the base station to the communicating base station; and A mobile communication system for performing simultaneous communication with a designated base station. 2. A base station for performing wireless communication with a mobile terminal through code division multiple access, comprising: a mobile terminal that is communicating with the base station and a communication quality of wireless communication with the base station; Receiving means for receiving a reception level of wireless communication with a station; and a predetermined reception level lower than a predetermined first reference reception level when the communication quality received by the reception means is lower than a predetermined reference communication quality. Changing means for changing to the second reference reception level, and receiving the reception level of wireless communication with another base station received by the reception means which is higher than the second reference reception level changed by the change means. A simultaneous communication instruction means for instructing simultaneous communication with the base station at the level. 3. The base station according to claim 2, further comprising: a measurement instructing unit that instructs the mobile terminal in communication to measure a reception level of wireless communication with another base station in the vicinity. A base station comprising: 4. The base station according to claim 2, further comprising quality measurement instructing means for instructing said communicating mobile terminal to measure the communication quality of wireless communication with said base station. A base station characterized by the above-mentioned. 5. The base station according to claim 2, wherein said changing means changes said second reference reception level in accordance with a value of a communication quality received by said receiving means. Bureau. 6. The base station according to claim 2, wherein the communication quality is a frame error rate and / or a bit error rate. 7. A mobile terminal that performs wireless communication with a base station by code division multiple access, wherein the communication quality of wireless communication with the communicating base station and the wireless communication with other base stations near the base station are controlled. Measuring means for measuring the reception level of the radio communication, the communication quality of radio communication with the base station measured by the measuring means, and the reception level of radio communication with another base station around the base station. A mobile terminal comprising: transmitting means for transmitting to the communicating base station; and communicating means for performing simultaneous communication with the instructed base station when instructed by the communicating base station. 8. A handover control method in a mobile communication system including a base station and a mobile terminal performing wireless communication by code division multiple access, comprising:
Receiving the communication quality of the radio communication with the base station and the reception level of the radio communication with another base station near the base station; and the received communication quality is more than a predetermined reference communication quality. If bad, the first
Changing to a predetermined second reference reception level lower than the reference reception level, and instructing to perform simultaneous communication with a base station having the reception level higher than the second reference reception level. And a step for controlling the handover.