JP2000134667A - Mobile communication system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To switch a channel within a synchronizing processing time at the time of hand over short in a passing time. SOLUTION: In this mobile communication system provided with plural base stations, cells formed of the base stations and a mobile station, the base stations (a), (b) and (c) are clock-synchronized with each other, the base stations and the mobile station communicate with each other by using a multicarrier modulation system and at the time of hand over which switches the cells (a), (b) and (c), the channel is switched in a synchronize-holding state without the re- synchronizing processing of a clock.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a handover in a mobile communication system.

[0002]

2. Description of the Related Art In the conventional mobile communication handover, synchronization processing (frequency synchronization, clock synchronization) for a new base station is performed every time the base station is switched. 12 and 13 are diagrams for explaining a conventional handover method.
3, "Channel switching method". A conventional switching method will be described with reference to FIGS. FIG.
As shown in FIG. 2, the mobile station 105 is moving in the direction of the arrow 106 while communicating with the base station 101 by the transmitter 103 and the receiver 104. The mobile station 105 is the base station 10
When approaching the boundary of the first service area, the base station 101 instructs the base station 102 to switch channels.
The mobile station 105 performs processing necessary for switching according to this. This processing includes synchronization processing (frequency synchronization and clock synchronization) between the mobile station 105 and the base station 102 and a continuity test. Hereinafter, the synchronization process will be described.

FIG. 14 is a diagram for explaining frequency asynchronization due to channel switching, and FIG. 15 is a diagram for explaining the positional relationship between two base stations and a mobile station at the moment when a base station is switched in a handover system. FIG. 16 is a diagram for explaining the amount of delay of a received wave due to a difference in transmission path length.
15, a base station 101, a base station 102, and a mobile station 105 are the same as those described in FIG.

The frequency synchronization processing will be described with reference to FIG. Before the channel switching, the carrier oscillator of the mobile station 105 is synchronized with the base station 101. When the base station is switched due to handover, the mobile station 105
Then, the channel (carrier frequency) is switched according to a channel switching instruction from the base station 101. here,
No frequency synchronization between base stations (frequency asynchronous)
In this case, since the absolute accuracy of the oscillator mounted on each base station is different, even if the carrier frequency (fb) of the base station 102 is switched based on the carrier frequency (fa) of the base station 101,
There will be a difference between the carrier frequency of the base station 102 and the carrier frequency of the mobile station 105. Further, even when the frequency is the same, a frequency difference occurs when the base station is switched for the above reason. Therefore, when the base station is switched, frequency synchronization is performed using a pilot signal for carrier frequency synchronization or the like.

[0005] Regarding the clock synchronization processing, FIG.
This will be described with reference to FIG. Here, for the clock,
In some systems, clock synchronization is not established between base stations (each base station uses its own clock oscillator). However, in order to simplify the description, clock synchronization between base stations is not established. Shall be Base station 101, base station 1 at the moment when the base station is switched by handover
02, it is assumed that the positional relationship between the mobile stations 105 is as shown in FIG. In FIG. 15, L1: the distance between the base station a (2) and the mobile station 1 L2: the distance between the base station b (3) and the mobile station 1, and it is assumed that L1> L2. Signals transmitted from the base stations 101 and 102 are delayed due to transmission delay, and a clock in a received signal at the mobile station 105 is a clock synchronized between the base stations (clock cycle: T; hereinafter, master Clock)). Also, L1> L2
In the condition (1), the signal transmitted from the base station 101 has a larger delay amount. The difference between the delay amounts of the received signals between the base station 101 and the base station 102 is represented by t. When t cannot be ignored compared to T, when communication is performed with the base station 102 using a clock synchronized with the base station 101, the clock has an initial phase difference t. For example, in the example of FIG.
/ 2, which is an inverted clock. So, in this case,
A base station 102 using a clock synchronized with the base station 101
It is impossible to communicate with. Therefore, when the base station is switched, the phases of the clocks are synchronized using a clock synchronization signal or the like. However, when t is sufficiently smaller than T, the influence of the clock phase shift is small, and therefore the clock phase synchronization processing is unnecessary.

Conventional example 2. As another conventional example, a method has been proposed in which two demodulators corresponding to two base stations connected before and after channel switching are provided and channel switching time is eliminated. FIG. 17 is a diagram illustrating a configuration example of a receiver having two demodulation units. FIG. 1 shows the operation of the conventional example.
2. Description will be given with reference to FIG. In FIG. 17, 11
0 is a receiving antenna, 111 is a demodulation unit a corresponding to the base station 101, 112 is a demodulation unit b corresponding to the base station 102, 1
A selector 13 compares the demodulated signals output from the demodulators a111 and b112 and selects the one with the better line condition (lower error rate) as the demodulated data.
The decision circuit 112 outputs a selection signal to the demodulation unit a using the decision signal output from the decision circuit 111.
111, a selector for selecting a demodulated signal output from the demodulation unit b112 and outputting it as demodulated data;
Are demodulation unit a111, demodulation unit b112, determination circuit 113,
This is a receiver including a selector 114.

[0007] It is assumed that the demodulation unit a111 is communicating with the base station 101. The demodulation unit b112 performs a synchronization process and a continuity test on the base station 102 where channel switching is expected separately from the demodulation unit a111, establishes communication with the base station 102, and monitors the base station 102. . As the mobile station 105 moves away from the base station 101 and approaches the base station 102, the line state of the line state from the base station 2 becomes better. Accordingly, the determination circuit 113 compares the demodulated signals output from the two demodulation units, and if it is determined that the demodulated signal output from the demodulation unit b112 has a better line condition (lower error rate), the communication partner is determined. The base station 101 is changed to the base station 102. In the selector 114, the determination circuit 11
Demodulation unit a111 and demodulation unit b11 in accordance with the selection signal from
2 is selected and output as demodulated data. The demodulation unit a111 performs synchronization processing on a base station to which a connection is expected next. By repeating this process sequentially, the channel switching time becomes unnecessary.

[0008]

As described above, in the first conventional example, when the mobile station 105 switches base stations, these processes are performed to perform channel switching processing (synchronization processing and continuity test). During this time, there was a drawback that communication was interrupted. In addition, ITS (Intellig
AHS (Au) proposed in ent Transport Systems (Intelligent Transport Systems)
In a tomated Highway System (autonomous driving road system), the vehicle passes through a cell length of 100 m at a maximum speed of 180 km / h, and therefore passes through the cell in 2 seconds. In this case, if the conventional synchronization processing is performed, the synchronization processing time cannot be ignored for 2 seconds,
There is a disadvantage that the communicable time in the cell cannot be sufficiently obtained.

As described above, in a system in which clocks are synchronized between base stations, when t is sufficiently smaller than T (when the transmission speed is slow and T is large, or the cell radius is small). (When t is small), there is no need to perform clock synchronization. Here, T is determined by the transmission speed, but in the current mobile communication, the amount of information has increased from data such as text to images including moving images due to a request from a user. Accordingly, the communication speed has been increasing. For example, 10
When Mbaud transmission is performed, T = 100 ns. Therefore, when high-speed transmission is performed, t cannot be ignored compared to T.

As described above, in the second conventional example, when the mobile station 105 switches the base station, the channel switching time is not required, but since a plurality of demodulation units are required, the circuit scale of the receiver is small. There was a disadvantage that it became larger.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned drawbacks, and it is an object of the present invention to eliminate the need for clock synchronization and to reduce the time during which communication is disabled by using a multi-carrier modulation system for a handover system. Aim. It is another object of the present invention to apply carrier frequency synchronization between base stations, thereby eliminating the need for carrier synchronization and further reducing communication unavailable time. It is another object of the present invention to reduce the circuit scale of a receiver by using a multicarrier modulation scheme to demodulate a plurality of modulation signals using one demodulation unit.

[0012]

According to a first aspect of the present invention, in a mobile communication system having a plurality of base stations, a cell formed by the base stations, and a mobile station, clock synchronization is established between the base stations. Then, communication is performed between the base station and the mobile station using a multi-carrier modulation scheme, and at the time of handover for switching the cell, the channel is switched in a state where synchronization is maintained without resynchronizing the clock.

According to a second aspect of the present invention, in a mobile communication system having a plurality of base stations, a cell formed by the base stations, and a mobile station, clock synchronization and frequency synchronization are established between the base stations, The base station and the mobile station communicate using a multi-carrier modulation scheme, and during handover for switching the cells, hold synchronization without performing clock resynchronization processing, and hold synchronization without performing carrier frequency resynchronization processing. Switch the channel while the switch is on.

According to a third aspect of the present invention, a plurality of base stations modulate and transmit data at a plurality of carrier frequencies, and the mobile station converts a signal modulated at the plurality of carrier frequencies into one demodulation unit. The communication is performed using a multi-carrier modulation method so as to demodulate a plurality of carrier frequencies by discrete Fourier transform.

[0015] According to the fourth aspect, an overlap area is formed by overlapping a part of a cell area between adjacent base stations, and communication is performed using a multicarrier modulation scheme. At the time of handover, the synchronization is maintained without switching the clock again and the channel is switched.

According to the fifth invention, an overlap cell is formed between adjacent base stations so that the entire area of one cell overlaps with a plurality of carrier frequencies used by the adjacent base stations. By performing communication by using the same, it is possible to maintain synchronization and switch channels without performing resynchronization processing of the clock at the time of handover in the overlap cell.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1 FIG. 1 is a diagram illustrating a configuration example of a system in which a multicarrier modulation scheme is applied to a handover system according to the present embodiment. FIG. 2 is a diagram illustrating a difference in transmission path length between a single carrier modulation scheme and a multicarrier modulation scheme. FIG. 3 is a diagram showing the influence of the amount of delay of the received wave on the clock due to the above, and FIG. In FIG. 1, reference numeral 1 denotes a mobile station equipped with a transceiver, and the mobile station is moving in the direction of arrow 5. 2,
Reference numerals 3 and 4 denote a base station a, a base station b, and a base station c, respectively, and it is assumed that the base stations a (2) to c (4) have clock synchronization between base stations. Also, as shown in FIG. 1, base stations a (2) to c (4) communicate with mobile stations within the range of cells a to c, respectively.

First, the multicarrier modulation method will be described. Usually, a transmission signal is transmitted using one carrier (see FIG. 3A). This is called single carrier modulation. On the other hand, a method of dividing a transmission signal into a plurality of parts and transmitting the divided transmission signals by using a plurality of carriers (see FIG. 3B) is called a multi-carrier modulation scheme. As shown in FIG. 3, consider a case where a single-carrier modulated signal having a band W is converted into a multi-carrier modulation. In the case where N-wave multi-carrier modulation is performed, when the band W is fixed, one multi-carrier modulation is performed. Is 1 / N
Becomes Therefore, when considering a modulated signal in one multicarrier wave, the clock frequency is 1 / N. Therefore, when the single carrier modulation scheme and the multi-carrier modulation scheme are used, the master clock cycle is T1,
If T2, T2 is larger than T1. For example, 1
In the case of the 00 wave multicarrier modulation method, T2 = 100 × T1, and the clock cycle in the case of the multicarrier modulation method is 100 times that in the case of the single carrier modulation.

Next, the operation will be described with reference to FIGS. As shown in FIG. 1, it is assumed that the mobile station 1 is communicating with the base station a (2) while moving in the cell a in the direction of arrow 5. When the mobile station 1 approaches the boundary of the cell a, the base station a
There is an instruction from (2) to switch the channel to the channel for base station b (3). In the case of the conventional system using the single carrier modulation system in the same manner as in the conventional case, as described in the conventional example, when the base station is switched, the mobile station 1 performs a synchronization process and establishes a synchronization with a new base station. Start communication. Since clock synchronization is established between base stations,
Even when the single carrier modulation method is used, the clock synchronization is maintained in the case of t << T1, so that there is no need to perform the clock synchronization (even when communicating with the base station b (3), the base station a (Use the clock established when communicating with (2)).

Here, as described above, T1 is determined by the transmission speed, but in the current mobile communication, the communication speed is also increasing, and as shown in the example of FIG. Has become more difficult to ignore than T1. However, if the multi-carrier modulation method is applied, T2 >> T
Since t is 1, as shown in FIG.
Therefore, even if the base station is switched, there is no need to perform clock synchronization. Further, OFDM (Orthogonal) is a multi-carrier modulation method.
Frequency Division Multi
In the case where the lpexing modulation method is used, since the band is limited in each subcarrier unit, it is strong against a timing shift. As described above, in a system in which timing synchronization is established between base stations, by using a multicarrier modulation method, the mobile station holds the clock even when the base station is switched, so that the clock resynchronization processing is performed. Since there is no need to perform the synchronization, the synchronization establishment time can be reduced or eliminated, and thus the time for communication disconnection can be reduced or eliminated.

Embodiment 2 FIG. FIG. 4 is a diagram showing a system in which a multi-carrier modulation scheme is applied to the system for performing handover according to the present embodiment.
Mobile station 1, base station a (2), base station b (3), base station c
(4) Arrow 5 is the same as FIG. This embodiment also employs a multicarrier modulation scheme as the modulation scheme as in the first embodiment, and assumes that clock synchronization is established between base stations.

Next, the operation will be described with reference to FIG. FIG.
It is assumed that the mobile station 1 is communicating with the base station a (2) while moving in the cell a in the direction of arrow 5 as shown in FIG.
When the mobile station 1 approaches the boundary of the cell a, there is an instruction from the base station a (2) to switch the channel to the channel for the base station b (3). In the case of the conventional method, as described in the conventional example, when the base station is switched, the mobile station 1 performs a synchronization process and starts communication with a new base station. However, as described in the first embodiment, since the mobile station 1 maintains the clock synchronization by applying the multi-carrier modulation scheme, there is no need to synchronize the clock. Therefore, the carrier synchronization is considered. In the second embodiment, the frequencies between the base stations are also synchronized. Therefore, even when the base station is switched, the mobile station 1 only needs to change the frequency of the oscillator of the mobile station by a predetermined frequency difference, and the time for frequency synchronization is not required. As described above, by performing frequency synchronization between base stations in a system in which the multicarrier modulation scheme is applied to the system that performs handover, since the mobile station holds the clock even when the base station is switched, frequency synchronization is performed. There is no need to perform this, and the synchronization establishment time can be reduced, thereby shortening the time of communication disconnection.

Embodiment 3 FIG. FIG. 5 is a diagram showing a configuration example of a system in which a multi-carrier modulation scheme is applied to the handover system of the present embodiment. FIG. 6 is a diagram in which the multi-carrier modulation scheme is applied to the handover system of the present embodiment. It is a figure showing the example of composition of the receiver in the system. In FIG. 5, reference numeral 11 denotes a mobile station equipped with a transceiver, and the mobile station 11 moves in the direction of arrow 14. Reference numeral 12 denotes a base station a that performs communication using the carrier frequency fa, and reference numeral 13 denotes a base station b that performs communication using the carrier frequency fb. Also, as shown in FIG. 5, base station a (12), base station b
In (13), it is assumed that communication is performed with respect to mobile stations within the range of cell a and cell b, respectively. In FIG. 6, 15 is an antenna connected to a receiver 19, 16 is a demodulation unit for a multicarrier-modulated reception signal, which is composed of an FFT processing unit and peripheral circuits, and 17 is two demodulation units demodulated by a demodulation unit 16. A decision circuit that compares signals and outputs a selection signal to a selector 18 so as to select the better line condition (lower error rate) as demodulated data. The decision circuit 18 uses the decision signal output from the decision circuit 17. A selector 19 outputs one of the two signals output from the demodulation unit 16 as demodulated data. Reference numeral 19 denotes a receiver including the demodulation unit 16, the determination circuit 17, and the selector 18.

When demodulating a signal transmitted using the multi-carrier modulation scheme, demodulation is performed on each subcarrier. This processing is usually performed by a discrete Fourier transform (hereinafter, referred to as DF
T. Including the inverse discrete Fourier transform. ). In the following, for the sake of simplicity, the FFT,
FT (Fast Fourier Transform) and inverse FFT are also D
It shall be included in FT. In particular, in the case of the OFDM modulation method, FFT and inverse FFT processing are used in modulation and demodulation processing. Therefore, when demodulating a multi-carrier modulated signal, an increase in the band is equivalent to an increase in the number of carriers. Therefore, even if the band increases, the number of carriers to be demodulated increases, but one demodulation unit (FFT processing unit) can cope with the increase.

FIG. 7 is a diagram for explaining an example of channel allocation in each of the single carrier modulation system and the multicarrier modulation system. The operation will be described with reference to FIGS. It is assumed that the base station a12 and the base station b13 are communicating with their own cells at carrier frequencies fa and fb, respectively. In the case of performing communication using the single carrier modulation scheme, it is necessary to arrange channels as shown in FIG. 7A and perform individual demodulation processing for each frequency. In order to demodulate two signals having different frequencies, demodulation is performed using two demodulators so as to correspond to the respective frequencies, or the carrier frequencies are set to fa and f using a synthesizer as time division processing.
The processing for switching to b at high speed was performed. Therefore, the circuit configuration becomes large or a special function such as a high-speed synthesizer is required.

In the case of performing communication using the multi-carrier modulation method (especially in the case of OFDM), as shown in FIG. 7 (b), the bandwidth is doubled as in the case of single carrier modulation, but in demodulation, Only the number of multicarriers is doubled. Now, to simplify the explanation, OF
The case of a DM modulation signal is assumed. In this case, one demodulation unit (FFT processing unit) can simultaneously demodulate received signals for two channels. Specifically, when an M-point FFT is required for the carrier frequency fa and an M-point FFT is required for the carrier frequency fb, demodulation for two channels can be performed by performing a 2M-point FFT in advance.
Therefore, the demodulation unit 16 transmits the current base station a1
Base station b that switches channels while communicating with base station b
13 can perform synchronization processing and the like, and can communicate with the base station b13.

The determination circuit 17 compares the two demodulated signals output from the demodulation unit 16 in the same manner as the determination circuit of the conventional example 2, and the signal from the base station b13 has a better line condition (lower error rate). ), The communication partner is changed from the base station a12 to the base station b13. The selector 18 selects a demodulation signal output from the demodulation unit 16 in accordance with the selection signal from the determination circuit 17 and outputs it as demodulated data. As described above, by using the multi-carrier modulation scheme, the receiver may have only one demodulation unit. Therefore, the circuit configuration can be simplified as compared with the conventional method (configuration having a plurality of receivers). Further, in the above example, channel selection is performed using the determination circuit. However, demodulation processing may be performed on two channels simultaneously, and channel selection may be performed by a handover command or selection processing from an upper layer.

Embodiment 4 FIG. 8 is a diagram illustrating an example of a cell configuration of a system in which a multicarrier modulation scheme is applied to a system that performs handover according to the present embodiment, and FIG. 9 is a diagram illustrating a cell configuration of a conventional system that performs handover. In FIG. 8, reference numeral 21 denotes a base station a that performs communication using a carrier frequency fa, and 22 denotes a carrier frequency fb.
Represents a base station b that performs communication using. As shown in FIG. 8, the base station a21 and the base station b22 communicate with mobile stations within the range of the cell a and the cell b, respectively. Base stations a21 and b22 in FIG. 9 are the same as those described in FIG.

The normal cell configuration is configured such that there is no cell overlap between adjacent base stations as shown in FIG. 9 for efficient transmission. However, in practice, it is impossible to make the overlap area zero due to the performance of the antenna and the surrounding environment.
A radio wave leaked from an adjacent cell is received in addition to a radio wave from the base station currently communicating. Conventionally, a method has been proposed in which a plurality of demodulation units are mounted on a receiver and a synchronization process is performed on the leaked radio waves to eliminate communication interruption due to channel switching. However, when the mobile station is in the vicinity of the communicating base station (near the center of the cell), the power of the leaked radio wave is small, so that even if the demodulation is performed, an error is large. Therefore, there is a disadvantage that stable demodulation cannot be performed unless the mobile station approaches the cell boundary and the power of the leaked radio wave does not increase. In the current mobile communication, the speed of mobiles that need to be responded is increasing. For example, in the above-described AHS, communication is performed without interruption even when a car moving at a maximum speed of 180 km / h switches channels. There must be.

As the moving speed of the moving body increases, the time during which the mobile station can receive the leaked radio wave and perform the channel switching process becomes relatively shorter. Therefore, the present embodiment aims at securing channel switching processing time and intentionally preventing communication interruption due to channel switching by intentionally creating an overlap area. The operation will be described with reference to FIG. The base station a12 and the base station b13 communicate with their own cells at carrier frequencies fa and fb, respectively. The cell configuration determines the size of the overlap area in consideration of the maximum speed of the mobile station that can be supported by the system to which the cell is applied. The base station performs transmission and configures a cell so that a necessary overlap area can be configured.
As a result, the mobile station can secure a sufficient channel switching processing time, so that communication interruption due to channel switching is eliminated. Further, as in the case of the third embodiment, by using a multi-carrier modulation scheme, particularly OFDM, only one demodulator is required in the receiver even when demodulating signals of a plurality of channels. Similar effects can be obtained.

Embodiment 5 In the fourth embodiment, the size of the overlap area is obtained for each system. However, in this case, even if the moving speed of the mobile station that actually performs communication is low,
It is necessary to secure an overlap area corresponding to the maximum speed that can be supported by the system. -A margin is required for the size of the overlap area so that it can cope with changes in the propagation path such as weather conditions (for example, when rainfall occurs, the overlap area becomes smaller due to rain attenuation). For this reason, the transmission efficiency is degraded. Therefore, the transmission power and the antenna pattern may be changed in accordance with changes in the moving speed and propagation path of the mobile station that actually performs communication, and the optimal size of the overlap area may be always secured. by this,
The same effect as in the fourth embodiment can be obtained, and further, the transmission efficiency can be improved by always securing the optimal size of the overlap area in accordance with the movement speed of the mobile station that actually communicates and changes in the propagation path. Be improved.

Embodiment 6 FIG. FIG. 10 is a diagram illustrating a configuration example of an overlap cell in a system in which a multicarrier modulation scheme is applied to the system for performing handover according to the fourth embodiment. In FIG. 10, reference numeral 31 denotes a base station a communicating with a mobile station in a cell a using a carrier frequency f1, and 32 communicates with a mobile station in a cell b using a carrier frequency f1 or f2. Base stations b and 33 communicate with mobile stations in cell c using carrier frequency f2. Base stations c and 34 communicate with carrier frequencies f1 and f2.
Base station d that communicates with mobile stations in cell d using
It is.

As described in the fourth embodiment, considering the cell constituted by the base station, the channel switching processing time is secured by creating the cell overlap region,
It is possible to eliminate communication interruption due to channel switching. In the fourth embodiment, an object similar to that of the third embodiment is intended to be obtained by preparing a cell constituted by the overlap region as a cell in advance.

Next, the operation will be described with reference to FIG. The base station a31 and the base station c33 communicate with their own cells at carrier frequencies f1 and f2, respectively. The base station b32 communicates with its own cell using both the carrier frequencies f1 and f2. The mobile station moves from cell a to cell b to cell c.
In this case, since the cell b is an overlap area of the cell a and the cell b, the mobile station does not need to perform resynchronization processing, and the same effect as in the fourth embodiment is obtained. be able to.

Embodiment 7 FIG. FIG. 11 is a diagram illustrating a configuration example of an overlap cell in a system in which a multi-carrier modulation scheme is applied to the handover system according to the fifth embodiment. In FIG. 11, reference numeral 41 denotes a base station a that communicates with a mobile station in a cell a using a carrier frequency f1, and 42 uses a carrier frequency f2 to communicate with a mobile station in a cell a and a cell b. Base station b, 4
3 is a base station c that communicates with mobile stations in cells b and c using the carrier frequency f1, and 44 is a base station that communicates with mobile stations in cells c and d using the carrier frequency f2. The base station d.

The operation will be described with reference to FIG. When considering a mobile station in a certain cell as a reference, all cells are in the overlap region. Therefore, when the mobile station moves from cell a to cell b to cell c, the mobile station is always in the overlap area. Therefore, the mobile station does not need to perform resynchronization processing, and can obtain the same effect as in the fourth embodiment.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a configuration example of a system in which a multicarrier modulation scheme is applied to a system that performs handover according to a first embodiment.

FIG. 2 is a diagram illustrating an influence of a delay amount of a received wave on a clock due to a difference in transmission path length between a single carrier modulation system and a multicarrier modulation system.

FIG. 3 is a diagram showing a schematic diagram of a modulated wave when a single carrier modulation method and a multi-carrier modulation method are used.

FIG. 4 is a diagram showing a system in which a multi-carrier modulation scheme is applied to a system for performing handover according to a second embodiment.

FIG. 5 is a diagram illustrating a configuration example of a system in which a multi-carrier modulation scheme is applied to a handover system according to a third embodiment.

FIG. 6 is a diagram illustrating a configuration example of a receiver in a system in which a multi-carrier modulation scheme is applied to a handover system according to a third embodiment.

FIG. 7 is a diagram illustrating an example of channel frequency arrangement in each of the single carrier modulation scheme and the multi-carrier modulation scheme.

FIG. 8 is a diagram illustrating a cell configuration example of a system in which a multi-carrier modulation scheme is applied to a system that performs handover according to a fourth embodiment.

FIG. 9 is a diagram showing a cell configuration of a conventional system for performing handover.

FIG. 10 is a diagram illustrating a configuration example of an overlap cell in a system in which a multicarrier modulation scheme is applied to a system that performs handover according to a fifth embodiment.

FIG. 11 is a diagram illustrating a configuration example of an overlap cell in a system in which a multicarrier modulation scheme is applied to a system that performs handover according to a fifth embodiment.

FIG. 12 is a diagram illustrating a conventional handover method.

FIG. 13 is a diagram illustrating a flow of a conventional channel switching operation.

FIG. 14 is a diagram illustrating frequency asynchrony due to channel switching.

FIG. 15 In a system for performing handover,
FIG. 3 is a diagram illustrating a positional relationship between two base stations and a mobile station at the moment when a base station is switched.

FIG. 16 is a diagram illustrating a delay amount of a received wave due to a difference in transmission path length.

FIG. 17 is a diagram illustrating a configuration example of a receiver having two demodulation units.

[Explanation of symbols]

 a base station, code assigned to cell b base station, code assigned to cell c base station, code assigned to cell d base station, code assigned to cell fa carrier frequency fb carrier frequency fc carrier frequency f1 carrier frequency f2 carrier Frequency 1, 11 Mobile station 2, 12, 21, 31, 41 Base station a 3, 13, 22, 32, 42 Base station b 4, 14, 33, 43 Base station c 34, 44 Base station dt, T1, T2 Master clock cycle 15 Antenna 16 Demodulation unit 17 Judgment circuit 18 Selector 19 Receiver

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Claims (5)

[Claims] 1. A mobile communication system having a plurality of base stations, a cell formed by the base stations, and a mobile station, wherein the base stations are clock-synchronized, and the base station and the mobile station are multi-channel. A mobile communication system, wherein communication is performed using a carrier modulation method, and a channel is switched in a state where synchronization is maintained without performing resynchronization processing of a clock at the time of handover for switching the cell. 2. A mobile communication system having a plurality of base stations, a cell formed by the base stations, and a mobile station, wherein the base stations perform clock synchronization and frequency synchronization, and the base station and the mobile station. Communication is performed using a multi-carrier modulation method during the handover when switching the cells, and the synchronization is maintained without resynchronization of the clock during the handover, and the synchronization is maintained without resynchronization of the carrier frequency. A mobile communication system, characterized by switching between: 3. The mobile station modulates and transmits data at a plurality of carrier frequencies from a plurality of base stations, and the mobile station transmits a signal modulated at the plurality of carrier frequencies to a discrete Fourier modulator by a single demodulator. The mobile communication system according to claim 1, wherein communication is performed using a multicarrier modulation scheme so that a plurality of carrier frequencies are demodulated by conversion. 4. An overlap area in which a partial area of a cell is overlapped between adjacent base stations, and communication is performed using a multi-carrier modulation scheme, so that a handover in the overlap area is performed during a handover. 2. The mobile communication system according to claim 1, wherein synchronization is maintained without resynchronization processing to switch channels. 5. An overlapping cell in which the entire area of one cell overlaps with a plurality of carrier frequencies used by adjacent base stations between adjacent base stations, and communication is performed using a multicarrier modulation scheme. 2. The mobile communication system according to claim 1, wherein at the time of handover in the overlap cell, the channel is switched and the channel is switched without performing the resynchronization processing of the clock.