JP2003304569A - Hand-off method, and mobile terminal - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a soft hand-off method among base stations utilizing different frequencies in mobile communication wherein the adjacent base stations can utilize the same frequency. <P>SOLUTION: Two transmission antennas are provided to each base station and two reception antennas are provided to each mobile station. When a mobile station moves to a base station utilizing a frequency different from that of a present base station, the mobile station uses one of the two reception antennas to receive a signal from the moving destination base station and the other to receive the signal from the present base station to perform diversity composite, and when the mobile station moves to a base station utilizing the same frequency as that of the present base station, the mobile station uses the two reception antennas to receive the signals from both the base stations to perform diversity composite. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a handoff method in a communication system having a mobile station that communicates using a plurality of antennas and a base station that communicates using a plurality of antennas.

[0002]

2. Description of the Related Art In mobile communication systems such as mobile phones,
It is general to construct a communication network based on a cell system in which a plurality of base stations cover different service areas. A cell-based communication system requires a technique called handoff in which a base station that is a communication partner is switched as the mobile station moves to perform communication.

As one of the handoff techniques, there is a method of establishing communication with a new base station after ending communication with a base station currently in communication. However, in this method, the communication is temporarily interrupted, which may cause a fatal error depending on the application in communication.

Therefore, a technique called soft handoff is used to establish communication with a new base station before ending the communication interruption with the base station currently communicating. As a soft handoff method, for example, Japanese Patent Laid-Open No. 2000-217
The method disclosed in Japanese Patent No. 139 has been proposed. With this method,
The mobile station has one antenna and the base station has two antennas, and normally two communication paths are established with the current base station. Handoff is performed with a new base station that uses the same frequency as the current base station, and the communication path is handed over to the new base station with the same frequency one by one, and finally with the new base station. Two communication paths were established.

[0005]

Even when a communication system such as a CDMA system in which adjacent base stations can use the same frequency is used, the number of users that can be covered by one frequency is limited. In order to cope with the increase in users, it is necessary to install base stations that use different frequencies.

Therefore, in the present invention, the base station adjacent to the base station with which the mobile terminal is currently communicating includes a base station using the same frequency as the current base station and a base station using a different frequency. An object of the present invention is to provide a handoff method that can be used even in an environment and to enable stable wireless communication.

[0007]

In order to solve the above problems, a handoff method of the present invention comprises a plurality of base stations having a plurality of transmitting antennas and a plurality of receiving antennas.
A mobile terminal that receives signals of a plurality of partial channels transmitted from a current base station that is a current communication partner, and a handoff when the mobile terminal moves from the current base station to a target base station In a handoff method used in a wireless communication system having a control station,
On the mobile terminal side, the information on the received power of the signal from the current base station and the information on the received power of the signal from the destination base station are received from the mobile terminal, and these received powers satisfy a predetermined condition. In the case of the first of the current base stations
Controlling the signal of the first partial channel being transmitted by the first transmitting antenna of the destination base station instead of the first transmitting antenna of the current base station. In addition, when the frequency used by the current base station and the movement-destination base station is different, the mobile terminal has the weakest reception power of the signal from the current base station among the plurality of reception antennas. Receiving antenna of the first partial channel from the movement-destination base station, remaining reception antennas of the remaining partial channel from the current base station, and performing diversity combining. And

Also, the handoff method of the present invention comprises a plurality of base stations having a plurality of transmitting antennas and a plurality of receiving antennas, and a plurality of partial channels transmitted from the current base station which is the current communication partner. In a wireless communication system having a mobile terminal for receiving the signal of, the current base station is the mobile terminal side, and information of received power of a signal from the current base station and a communication partner of the destination of the mobile terminal. When the reception power of a signal from a certain movement-destination base station is received from the mobile terminal and these reception powers satisfy a predetermined condition, the first transmission antenna of the current base station is transmitting. The sub-channel signal of the current base station is controlled to be transmitted using the first transmission antenna of the target base station instead of the first transmission antenna of the current base station, and If the frequency used by the current base station and the movement-destination base station is different, the terminal uses the first reception antenna of the plurality of reception antennas having a weak reception power of a signal from the current base station. Diversity combining is performed by receiving the signal of the first partial channel from the first transmitting antenna of the movement-destination base station and receiving the signal of the remaining partial channel from the current base station by the remaining receiving antennas. It is characterized by performing.

[0009]

DETAILED DESCRIPTION OF THE INVENTION Embodiments of the present invention will be described below with reference to the drawings.

(Outline of Communication System) FIG. 1 is a schematic diagram of a communication system according to the present invention, which is a cell type communication system. In the communication system of this communication system, a mobile terminal that can be carried around by a user (hereinafter referred to as a mobile station)
When there are a plurality of mobile stations, each mobile station is identified by code division and wireless communication is performed. In addition, the duplex method of wireless communication between the base station and the mobile station is TDD (Time Div).
Ion Duplication method, uplink (from mobile station to base station) and downlink (from base station to mobile station)
Communication is performed by using the same frequency in time division.

The mobile station 908 is in wireless communication with a base station 905 having a communication range 910 at a frequency f1. The mobile station 909 existing in the boundary area between the communication range 911 and the communication range 912 is the base station 90 having the communication range 911.
6 and the base station 907 having the communication range 912, the wireless communication is performed at the frequency f0.

Base stations 905, 906 and 907 are control stations 9.
It is connected to 03 by wireless or wired. A plurality of control stations such as control stations 903 and 904 are connected to the exchange 901 through communication lines. Exchange 901 is exchange 902
It is connected to a plurality of exchanges including a communication line.

The control stations such as the control stations 903 and 904 control the connected base stations to perform handoff control, diversity control of signals received from the same mobile station among a plurality of base stations, and the like. Exchanges such as the exchanges 901 and 902 are connected to other exchanges and public telephone exchange networks.

(Structure of Mobile Station) FIG. 4 shows the structure of the receiving device portion of the mobile station used in this system. The mobile station
Antennas 401 and 402 for receiving signals from base stations
And a frequency conversion unit 40 that frequency-converts the received signal.
3, 404 and AD converter 4 for digitizing signals
05 and 406, and a demodulation unit 407 that demodulates a signal. The mobile station further includes a combining / decoding unit 409 for performing diversity combining and decoding of two signals, monitoring of the attenuation of received signals, notification of received power to the control stations 903 and 904, switching of base stations, and frequency conversion unit. The control unit 410 includes a control unit 410 that controls the pre-conversion frequency (that is, the reception frequency) handled by 403 and 404 and controls the synthesis ratio of the two signals in the synthesis and decoding unit 409.

The signals received by the antennas 401 and 402 are independently frequency-converted by frequency converters 403 and 404, AD-converted by AD converters 405 and 406, demodulated by demodulator 407, and combined. Then, the decoding unit 409 performs diversity combining / decoding to generate received data.

(Structure of Base Station) FIG. 3 shows the structure of the transmission signal generation unit 800 of the base station used in this system. The base station includes a coding unit 801 that generates coded data from transmission data and modulation units 802 and 80 that modulate the coded data.
3 and DA converters 804 and 805 that convert the modulated coded data into analog data. Further, a frequency conversion unit 80 that frequency-converts the analog coded data.
6 and 807 and antennas 808 and 809 for emitting frequency-converted modulated data as radio waves.

The transmission data encoded by the encoding unit 801 is independently modulated by the modulation units 802 and 803, the DA converters 804 and 805 perform analog conversion, and the frequency conversion units 806 and 807 perform frequency conversion processing. And then transmitted from the antennas 808 and 809 to the mobile station.

(Handoff) FIG. 4 is a diagram for explaining an overview of handoff regarding downlink communication in this embodiment.

Two communication channels, a first channel and a second channel, are established between the base station and the mobile station to perform downlink communication. Uplink communication is performed by one channel (not shown). In the state where handoff is not performed (for example, the mobile station 909a in FIG. 4), one antenna of the base station transmits the signal of the first channel and the other antenna transmits the signal of the second channel to the mobile station. . The mobile station receives the signal transmitted from the base station with two antennas.
The mobile station receives signals of both the first and second channels with one antenna and acquires received data by diversity combining.

In addition to the communication with the mobile station, the base station issues a unique recognition signal for each base station to make the mobile station recognize the existence of the base station (not shown). The mobile station can know this by receiving this signal.

The mobile station uses the idle time of reception by TDD to search for other base stations nearby. And
When a base station that can communicate with the first predetermined reception power or more is found, the reception power of the signal from the base is transmitted to the control station using the upstream channel.

Further, the mobile station always feeds back the received power of the first and second channels to the control station using the upstream channel. Then, the base station notifies the control station of the reception power of the uplink channel from the mobile station.

The control station determines the handoff start based on the received power information from the base station and the mobile station.

The handoff method differs depending on whether the frequency used by the currently communicating base station and the frequency used by the destination base station are the same. And explain.

(Handoff when moving between base stations using different frequencies) The mobile stations 909a and 909 of FIG.
b and 909c are diagrams for explaining an outline of a handoff transition state of downlink communication when moving between service areas 910 and 911 of two base stations using different frequencies. FIG. 5 is a diagram for explaining the transition state in detail.
In FIG. 5, the connection between the control station 903 and the base stations 905 and 906 is omitted.

First, as shown in FIG. 5A, the mobile station 909 exists within the communication range of only the base station 905. Base station 90
5 performs downlink communication with the mobile station 909 using the first and second channels of frequency f1. The mobile station 909 receives signals of the first and second channels having the frequency f1 and performs diversity combining to obtain received data. Meanwhile, the base station 905
Performs uplink communication with the base station 905 using the uplink channel of frequency f1.

The control station 903 determines that the reception power of one of the downlink channels (here, the first channel) received by the mobile station 909 becomes equal to or lower than the second predetermined reception power and the first predetermined reception is performed. When there is a base station (here, the base station 906) capable of communicating with the mobile station 909 with power or more, switching of the transmission source base station of the first channel is started.

First, the control station 903 sends the first data to the mobile station 909.
It informs that the base station of the transmission source of the channel is switched to the base station 906. In response to this, the mobile station 909 selects the antenna having the larger attenuation of the signal from the base station 905 from the two antennas for receiving the downlink communication, and receives the signal from the base station 905 by the antenna. Is stopped (FIG. 5 (b)).

Then, the mobile station 909 enables reception of the signal of the frequency f0 from the base station 906, and then
Notify the control station 903 that preparation is complete. In response to this, the control station 903 transmits the signal of the first channel to the base station 90.
Control is performed so as to transmit to mobile station 909 from both 5 and 906.

As shown in FIG. 5C, the mobile station 909 receives the first channel signal from the base station 906 by using the antenna on the side where communication is stopped. As a result, the mobile station 9
09 will receive signals from both the base station 905 and the base station 906 for the first channel.

Immediately after starting the reception of the signal of the first channel from the base station 906, the communication may not be stable. Therefore, immediately after the start of reception, the combining ratio of the signals from the base station 906 in the diversity combining is set to be low,
Is set to a normal ratio (a ratio according to the signal strength) for a predetermined time, and conversely, the synthesis ratio of the signal from the base station 905 is lowered.

When the signal synthesis ratio from the base station 905 of the first channel becomes 0, the mobile station 909 controls the control station 903.
Notify that the switching of the first channel is completed.
In response to this, the control station 903 receives the first information from the base station 905.
The channel signal transmission is terminated and the uplink channel handoff is started (FIG. 5 (d)).

First, the control station 903 commands the mobile station 909 and the base station 906 to establish communication on the upstream channel of the frequency f0. Then, the control station 903 is the base station 9
05 and the base station 906 receive the uplink channel signals and perform diversity combining.

At this time, the mobile station 909 needs to carry out upstream communication at the frequencies of f0 and f1.
Adjusts the timing of time division of TDD to the mobile station 909.
Eliminates the need to transmit signals of frequencies f0 and f1 at the same time. Further, the control station 903 is the mobile station 90 described above.
As in the case of 9, the diversity combining ratio is gradually changed, the combining ratio of the signal from the base station 906 is reduced first, and finally the combining ratio from the base station 906 is set to 1.
When it is set to 00%, the communication of the uplink channel between the base station 905 and the mobile station 909 is disconnected.

When the base station switching of the upstream channel is completed, the mobile station 909 switches the base station of the second channel in the same procedure as in the case of the first channel (FIG. 5 (e)).
→ Fig. 5 (f).

When the mobile station 909 completes the base station switching of both antennas, the mobile station 909 is in a state of communicating only with the base station 906. This state is shown in FIG. 5 (g). In this state, the base station 906 and the mobile station 909 perform communication on the first and second channels and the upstream channel at the frequency f0.

In this way, in the handoff when moving to the base station using a different frequency, the mobile station 909 side always uses the two antennas to maintain the communication of the downlink 2 channel and the uplink 1 channel. Done.

Here, the control station 903 determines the handoff, but in the case of handoff between different frequencies, the mobile station 909 may determine the necessity of handoff and request the control station 903 for handoff. Absent.

(Handoff when moving between base stations using the same frequency) The mobile stations 909c and 909 in FIG.
d and 909e are diagrams for explaining transition states of handoff when moving between service areas 911 and 912 of two base stations using the same frequency. FIG. 6 is a diagram for explaining the transition state in detail. In FIG. 6, the control station 9
03 and the base stations 906 and 907 are omitted.

First, as shown in FIG. 6A, the mobile station 90
Since 9 exists within the communication range of only the base station 906, the base station 906 and the mobile station 909 perform downlink communication using the first and second channels of the frequency f0. The mobile station 909 receives signals of the first and second channels and performs diversity combining to obtain received data. Base station 906 and mobile station 90
9 performs upstream communication on the upstream channel of frequency f0.

The mobile station 909 moves to the adjacent base station 907.
When the mobile station 909 receives the recognition signal from the base station 907, the control station 903 sends the base station 907 an uplink channel signal from the mobile station 909. To receive. Then, when the reception power of the uplink channel in the base station 907 becomes equal to or higher than the fourth predetermined threshold value, the control station 903 causes the base station 906 and the base station 9
Diversity combining is performed on the uplink channel signals received by both channels.

The control station 903 starts diversity combining of the uplink channel signals received by both the base stations 906 and 907. Immediately after the start of combining, the control station 903 keeps the combining ratio of signals via the base station 907 low. This is because the communication is not stable immediately after the base station 907 starts receiving and the reliability of the signal is low. Then, the composition ratio is gradually increased to the normal ratio over the fifth predetermined time.

The control station 903 receives the first data from the mobile station 909,
Based on the received power information of the second channel, the channel with the weakest received power is switched (here, the first channel). Then, the control station 903 causes the base station 907 to start signal transmission of the first channel, and the mobile station 909
The base station 907 is instructed to receive the signal as well (FIG. 6C).

The mobile station 909 performs diversity combining on the signals of the first channel from the base stations 906 and 907. Immediately after the start of diversity combining in the mobile station 909, the base station 907 is operated similarly to the diversity combining in the control station 903 described above.
The synthesis ratio of the signal of the first channel from 1 is set low, and the normal ratio is set for the sixth predetermined time. The mobile station 909 receives the received power for the first channel for the signals from both base stations 906 and 907, and
Report to 3. The control station 903 determines when the received power from the base station 906 becomes less than or equal to the seventh predetermined threshold value.
Signal transmission from the first channel from the
(D)).

Further, the mobile station 909 moves and the mobile station 9
If the received power of the signal of the second channel originating from the base station 906 at 09 falls below the eighth predetermined threshold value, the control station 9
03 starts switching the base station of the second channel. Similarly to the case of the first channel, the signals of the second channel are transmitted from both the base stations 906 and 907 to the mobile station 909, and the mobile station 909 performs diversity combining of the signals from the two base stations 906 and 907. To receive the received data (FIG. 7 (e)).

Then, when the received power of the second channel originating from the base station 906 becomes equal to or lower than the ninth predetermined threshold value, the control station 903 stops the transmission of the second channel from the base station 906. As a result, the mobile station 909 receives the second channel only from the base station 907 (FIG. 7).
(F)).

The control station 903 also completes the diversity combining of the upstream channels when the switching of the base stations of both the first and second channels is completed, and causes the base station 906 to stop the reception of the upstream channels, and finally the base station 906. Only the station 907 will communicate (FIG. 7 (g)).

In this embodiment, the control station 903 controls the base station 9
05, 906, 907, etc. are separately present, but the control station is eliminated from the communication system of the present embodiment, and each base station is provided with a function equivalent to that of the control station of the present embodiment. The control may be performed by the base station. In this case, which of the source base station and the destination base station controls the handoff by the base station may be freely determined when designing the communication system.

In the present embodiment, the method of realizing the identification of the mobile station by code division has been described as an example, but the present invention can be applied to a general communication system for identifying a communication station by code division and time division. is there.

In this embodiment, the TDD method is used for duplication of communication, but the present invention uses FDD (Freq).
uency Division Duplicatio
It is also applicable to the n) method.

When the identification of the communication station is realized by code division and the duplication of communication is realized by the FDD system, the mobile station has two transmitting antennas so that signals of different frequencies can be transmitted simultaneously. In addition, the mobile station receives at one antenna during normal operation without handoff, and searches for a base station at the other antenna.

On the other hand, when the communication stations are identified by time division, it is possible to search for a base station in an empty time slot regardless of the duplexing method.

[0053]

As described above, according to the present invention, by using two transmitting antennas on the base station side and two receiving antennas on the mobile station side, communication can be performed even if the frequencies between adjacent base stations are different. A soft handoff that is not interrupted can be performed. In addition, in diversity combining during handoff, the effect of communication disturbance immediately after establishment of communication is reduced by decreasing the composition ratio of the communication immediately after establishing communication with the new base station and gradually increasing the composition ratio. Hold down
It is possible to perform stable communication.

[Brief description of drawings]

FIG. 1 is a diagram illustrating an outline of a communication system using a handoff method of the present invention.

FIG. 2 is a diagram illustrating a configuration of a signal receiving unit of a mobile station of a communication system using the handoff method of the present invention.

FIG. 3 is a diagram illustrating a configuration of a transmission signal generation unit of a base station of a communication system using the handoff method of the present invention.

FIG. 4 is a diagram for explaining an outline of transition states of the handoff method of the present invention.

FIG. 5 is a diagram for explaining transition states of the handoff method of the present invention when a mobile station moves between base stations that use different frequencies.

FIG. 6 is a diagram for explaining transition states of the handoff method of the present invention when a mobile station moves between base stations using the same frequency.

[Explanation of symbols]

901, 902 Exchanges 903, 904 Control stations 905, 906, 907 Base stations 908, 908 (a), 908 (b), 908 (c),
908 (d), 908 (e) Mobile stations 910, 911, 912 Communication range of base station 401, 402 Reception antennas 403, 404 Frequency conversion units 405, 406 A / D converters 407, 408 Demodulation unit 409 Combining and decoding unit 410 control unit 801 encoding unit 802, 803 modulation unit 804, 805 D / A converter 806, 807 frequency conversion unit 808, 809 transmission antenna

Claims (8)

[Claims] 1. A mobile terminal comprising a plurality of base stations having a plurality of transmitting antennas and a plurality of receiving antennas, and receiving signals of a plurality of partial channels transmitted from a current base station as a current communication partner. And a control station that controls a handoff when the mobile terminal moves between the current base station and the movement-destination base station, in a handoff method, wherein the control station is the mobile terminal side. In, receiving the information of the received power of the signal from the current base station and the information of the received power of the signal from the movement-destination base station from the mobile terminal, if these received power satisfy a predetermined condition, The first partial channel signal transmitted by the first transmission antenna of the current base station is transmitted to the first transmission antenna of the destination base station instead of the first transmission antenna of the current base station. While controlling to transmit using a mobile terminal, the mobile terminal, if the frequency used by the current base station and the destination base station is different, among the plurality of receiving antennas from the current base station Signal of the first partial channel from the movement-destination base station is received by the first receiving antenna having the weakest received power of the signal, and the remaining partial channel signal from the current base station is received by the remaining receiving antennas. Is received and diversity combining is performed. 2. A mobile terminal having a plurality of base stations having a plurality of transmitting antennas and a plurality of receiving antennas, and receiving signals of a plurality of partial channels transmitted from a current base station which is a current communication partner. In the handoff method for use in a wireless communication system having: the current base station, on the mobile terminal side, information on received power of a signal from the current base station and a destination that is a communication partner of a destination of the mobile terminal. The reception power of the signal from the base station is received from the mobile terminal, and if these reception powers satisfy a predetermined condition, the first partial channel transmitted by the first transmission antenna of the current base station. While controlling the signal of to be transmitted using the first transmission antenna of the movement-destination base station instead of the first transmission antenna of the current base station, the mobile terminal, If the frequencies used by the current base station and the movement-destination base station are different, the movement-destination base station is the first reception antenna of the plurality of reception antennas whose reception power of the signal from the current base station is weak. Receiving the signal of the first partial channel from the first transmitting antenna of the station, receiving the signal of the remaining partial channel from the current base station with the remaining receiving antennas, and performing diversity combining. Characterized handoff method. 3. The control station sets, as the predetermined condition, one signal of a plurality of partial channels from the current base station for one receiving antenna of a plurality of receiving antennas of the mobile terminal. It has been detected that the received power is below a first predetermined threshold and the received power of a signal transmitted from one of the plurality of transmit antennas of the destination base station is above a second predetermined threshold. The handoff method according to claim 1, wherein in this case, the signal of the partial channel is controlled to be transmitted using the transmission antenna of the movement-destination base station. 4. The current base station, as the predetermined condition,
For one receiving antenna of the plurality of receiving antennas of the mobile terminal, the received power of one signal of the plurality of partial channels from the current base station is below a first predetermined threshold, and When it is detected that the received power of one of the plurality of transmission antennas of the destination base station exceeds a second predetermined threshold, the signal of the partial channel is used by the transmission antenna of the movement destination base station. The handoff method according to claim 2, wherein the handoff method is controlled so that the data is transmitted as a transmission. 5. If the frequencies used by the current base station and the destination base station are the same, the mobile terminal uses at least one of the plurality of receiving antennas for the plurality of partial channels. 5. The handoff method according to claim 1, wherein the signals are received and diversity combining is performed. 6. The diversity combining of the mobile terminal comprises:
6. The handoff method according to claim 1, wherein immediately after the start of combining, the combining ratio of the signals from the movement-destination base station is set to be low and the combining ratio is gradually increased. 7. A plurality of receiving antennas, a plurality of frequency converters for frequency-converting received signals of the plurality of receiving antennas, and a plurality of demodulators for demodulating output signals of the plurality of frequency converters. , A combining / decoding unit that obtains received data by diversity combining and decoding the output signals of each of the plurality of demodulation units, and information on the received power of the signal from the current base station that is the current communication partner and the communication partner of the destination. And a control unit for notifying the control station that controls the handoff of the information on the received power of the signal from a certain movement-destination base station,
When the control unit receives the handoff start notification from the control station and the frequencies used by the current base station and the movement-destination base station are different from each other, from the current base station among the plurality of receiving antennas. The first reception antenna having the weakest reception power of the signal receives the signal of the first partial channel from the destination base station, and the remaining reception antennas receive the signal of the remaining partial channel from the current base station. When the frequency used by the current base station and the destination base station is the same, the plurality of receiving antennas from the destination base station are received. A mobile terminal, comprising: receiving a signal of a first partial channel and a signal of the remaining partial channel from the current base station, and causing the combining and decoding unit to perform diversity combining. 8. A plurality of receiving antennas, a plurality of frequency converters for frequency-converting received signals of the plurality of receiving antennas, and a plurality of demodulators for demodulating respective output signals of the plurality of frequency converters. , A combining / decoding unit that obtains received data by diversity combining and decoding the output signals of each of the plurality of demodulation units, and information on the received power of the signal from the current base station that is the current communication partner and the communication partner of the destination. And a control unit for notifying the current base station of information on the received power of a signal from a certain moving base station, the control unit receiving a handoff start notification from the current base station, and the current base station. If the frequency used by the destination base station is different from the destination base station, the first receiving antenna having the weakest reception power of the signal from the current base station among the plurality of receiving antennas is used. The signal of the first partial channel is received, the signal of the remaining partial channel from the current base station is received by the remaining receiving antenna, and the combining / decoding unit performs diversity combining, When the frequencies used by the movement-destination base station are the same, the first partial channel signal from the movement-destination base station and the remaining partial channel signals from the current base station are transmitted to the plurality of receiving antennas. Is received and the combining and decoding section is made to perform diversity combining.