JP2004336635A - Mobile communication system and its transmission control method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a CDMA (code division multiple access) mobile communication system capable of reducing transmitting power from a base station to a down link without lowering the service quality of a communication line. <P>SOLUTION: A mobile station finds the number of paths whose receiving quality is equal to or greater than a first threshold in a down link as the number of excellent paths among simultaneously set paths (step 202) and finds the path with the best receiving quality as the best path (step 203). Then, the mobile station transmits downstream transmission information including the number of excellent paths and the best path to an up link (step 204). A base station controls the transmission of a signal to the down link on the basis of the information of the number of excellent paths and the best bus (step 205). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to downlink transmission power control of a CDMA mobile communication system.
[0002]
[Prior art]
The CDMA communication system is a system in which a transmitting side spreads a user signal with a spreading code and transmits the same, and a receiving side performs despreading using the same spreading code to obtain an original user signal.
[0003]
In the CDMA mobile communication system, a plurality of transmitting sides perform spreading by using different spreading codes having orthogonality to each other, and the receiving side selects a spreading code to be used when performing despreading to thereby obtain a desired spread code. Identify communications. This allows a plurality of communications to use the same frequency band simultaneously.
[0004]
However, it is difficult to make all of the used spreading codes have perfect orthogonality to each other.In practice, each spreading code does not become completely orthogonal and has a correlation component with other codes. It becomes. These correlation components become interference components for the own communication, and cause deterioration of communication quality.
[0005]
Since interference components occur due to such factors, the interference components increase as the number of communications increases. If there is a certain level of interference, normal communication cannot be maintained. Therefore, if the interference is reduced by reducing the transmission power of each communication multiplexed in a certain frequency band, the number of communication lines that can be multiplexed in one frequency band increases. On the other hand, even if the transmission power is excessively reduced, normal communication cannot be maintained.
[0006]
Therefore, the base station in the CDMA mobile communication system sets the communication quality of each communication line set with each mobile station to the minimum Eb / I0 (desired received wave power (Eb) vs. interference wave power (I0)). Ratio) to control the transmission power of each mobile station.
[0007]
Specifically, the downlink transmitted from the base station to the mobile station includes TPC (Transmission Power Control) information for instructing the mobile station to increase / decrease the transmission power. , The base station instructs each mobile station to increase or decrease the transmission power based on the TPC information.
[0008]
Similarly, up / down control is performed for downlink transmission power control. However, in a downlink from a base station to a terminal, when SHO (Soft Handover: soft handover) is performed, a plurality of base stations transmit the same information addressed to the same mobile station to the downlink. As a result, the transmission power of the downlink per communication increases, and as a result, the mutual interference power increases for each communication, and the number of communication (communication capacity) that can be accommodated per frequency band decreases.
[0009]
In addition, as for the uplink from the mobile station to the base station, one transmission is always performed per communication from the mobile station, and since the plurality of base stations receive it, the effect of SHO is obtained. Even if executed, there is no problem that interference increases.
[0010]
Conventionally, in order to improve the restriction on the communication capacity generated in the downlink, the transmission to the downlink during SHO is limited to only one base station having the least propagation loss. A method for reducing the transmission power of a line has been proposed (for example, see Non-Patent Document 1).
[0011]
FIG. 5 is a diagram for explaining an operation of a conventional CDMA mobile communication system. Referring to FIG. 5, the conventional CDMA mobile communication system includes an exchange 140, a base station controller 130, and base stations 120 ₁ and 120 ₂ . The mobile station 110 controls establishment and release of communication and performs actual communication by transmitting and receiving signals to and from the base station by radio waves. A base station controller 130 is connected to a switching station 140 constituting a mobile communication network, and base stations 120 ₁ and 120 ₂ are connected to the base station controller 130. Each of the base stations 120 ₁ and 120 ₂ forms a cell 150 that covers a predetermined area by radio waves.
[0012]
In this example, mobile station 110 is executing SHO, and the uplink signal transmitted from mobile station 110 is received by both base stations 120 ₁ and 120 _2. only it has been sent from a lower base station 120 ₁ of the propagation loss. Thereby, the CDMA mobile communication system of FIG. 5 reduces the transmission power of the downlink per communication.
[0013]
[Non-patent document 1]
Furukawa [NEC] "Base station selective power transmission control in downlink of DS-CDMA cellular system" IEICE Communications Society Conference B-5-118 March 1998 [0014]
[Problems to be solved by the invention]
In Non-Patent Document 1 conventional downlink transmission power control method as described in, since only always one base station 120 ₁ is to transmit a downlink signal to the mobile station 110 during SHO, the communication line If the fading is large, communication quality cannot be ensured.
[0015]
In multimedia communication, it is predicted that the proportion of communication forms that send large amounts of data to mobile stations using downlinks from databases on the network side will be higher than in conventional voice call-oriented communication. . In the future, if the specific gravity of multimedia communication increases, the downlink will require a larger radio channel capacity than the uplink. Therefore, it is strongly desired to reduce downlink transmission power in a CDMA mobile communication system.
[0016]
An object of the present invention is to provide a CDMA mobile communication system capable of reducing transmission power from a base station to a downlink without reducing service quality of the communication channel.
[0017]
[Means for Solving the Problems]
In order to achieve the above object, a mobile communication system according to the present invention is a mobile communication system capable of simultaneously setting a plurality of paths for one communication. The number of paths whose reception quality is equal to or greater than the first threshold is determined as the number of good paths, the path with the best reception quality is determined as the best path, and the downlink transmission information including the information on the number of good paths and the best path is transmitted to the uplink. Receiving at least one mobile station to transmit, and the downlink transmission information transmitted by the mobile station to the uplink, and controlling transmission of a signal to the downlink based on the number of good paths and the information of the best path; And at least one base station.
[0018]
Further, when the number of good paths is larger than a second threshold, the base station turns on transmission of a signal to a downlink when a path between itself and the mobile station corresponds to the best path, If not, the signal transmission to the downlink is turned off.
[0019]
Therefore, according to the present invention, the signal is transmitted from the base station only to the best path having the best reception quality in the mobile station, and the signal is not transmitted to the other paths. Interference with the mobile station's communication can be reduced.
[0020]
In addition, a path with low reception quality at the mobile station that is the original destination is likely to be a large interference component to other mobile stations. However, by turning off transmission of a signal to that path, efficient interference is obtained. Can be reduced.
[0021]
Further, according to the present invention, highly reliable control is possible because the transmission of a signal from the base station to the downlink is controlled based on information on the reception quality of the downlink signal in the mobile station.
[0022]
In the mobile communication system of the present invention, the mobile station further obtains the propagation loss of the downlink of all the paths set at the same time, transmits the information to the uplink including the downlink transmission information,
The base station may control transmission of a signal to a downlink based on the information on the propagation loss in addition to the number of good paths and the best path.
[0023]
Further, if the propagation loss of all the paths is larger than a third threshold, the base station turns on signal transmission to the downlink, and the propagation loss of any of the paths is equal to or less than the third threshold. If so, the transmission of a signal to the downlink may be controlled based on the number of good paths and the information on the best path.
[0024]
Therefore, if there is a path with a small propagation loss due to fading or the like, only the base station for which the path with the highest reception quality at the mobile station is set among the base stations for which a communication line is set up with the mobile station However, the transmission of the signal to the downlink is continued, and the other base stations stop transmitting the signal. If the propagation loss of all the paths is large, the transmission of the signal to the downlink is not turned off, and the reception quality is maintained by multipath combining. Therefore, since an optimal method is selected according to the propagation loss due to fading or the like, it is possible to maintain both reception quality and suppress interference under various environments.
[0025]
BEST MODE FOR CARRYING OUT THE INVENTION
An embodiment of the present invention will be described in detail with reference to the drawings.
[0026]
FIG. 1 is a block diagram illustrating a configuration of a CDMA mobile communication system according to an embodiment of the present invention. Referring to FIG. 1, CDMA mobile communication system includes mobile switching center 40, the base station controller 30, the base station ₂₀ 1, 20 _2, and the mobile station 10.
[0027]
The exchange 40 is an exchange device constituting a mobile communication network, and usually includes a plurality of exchanges 40 in the system. Some of the plurality of exchanges 40 serve as a gateway to another network.
[0028]
The base station controller 30 is connected to the switching center 40 and the base station ₂₀ 1, 20 _2, and controls the base station ₂₀ 1, 20 _2, establish or release the communication with the moving body 10 and the communication partner And relay the communication. Usually, one base station control device 30 accommodates a plurality of base stations.
[0029]
The base station 20 _1, 20 _2, and send and receive signals in the mobile station 10 and the radio waves, relays communication between the mobile station 10 and the communication partner. Each base station 20 _1, 20 ₂ forms a cell 50 that covers a predetermined area with radio waves. The cells 50 formed by the base stations 20 ₁ and 20 ₂ partially overlap each other, enabling soft handover (SHO).
[0030]
The mobile station 10 establishes and releases communication and performs actual communication by transmitting and receiving signals to and from the base station by radio waves. The mobile station 10 in the cell 50 can transmit and receive radio waves to and from the base station forming the cell.
[0031]
The mobile station 10 establishes a communication line with the base station and performs communication, but may move out of the cell of the base station while moving. In that case, the mobile station 10 sets up a new communication line with another base station constituting the destination cell and performs handover. In a CDMA mobile communication system, SHO is performed between base stations to maintain service quality by avoiding instantaneous interruption or the like due to handover.
[0032]
SHO, as shown in FIG. 1, one mobile station 10 is intended to perform a plurality of base stations 20 _1, 20 ₂ at the same time communication, the spread spectrum communication in which a plurality of communication lines can coexist on the same radio frequency This is a unique function. At the time of SHO, the CDMA mobile communication system of the present embodiment operates as described below for controlling signal transmission to the downlink.
[0033]
FIG. 2 is a flowchart showing an operation in the CDMA mobile communication system of the present embodiment. Referring to FIG. 2, first, mobile station 10 obtains a downlink reception level of a path set between each base station in SHO (step 201).
[0034]
Next, the mobile station 10 is out of the path between each base station 20 _1, 20 _2, the number of the reception level is above a predetermined threshold level path (hereinafter, good pass number hereinafter) seek (step 202). The fact that the reception level is equal to or higher than the threshold level indicates that the reception quality of the path is equal to or higher than a predetermined level.
[0035]
Next, the mobile station 10 is in the path reception level is equal to or greater than a predetermined threshold level from each base station 20 _1, 20 _2, the reception level is maximum path (hereinafter, referred to as the best path) is determined (step 203).
[0036]
Next, the mobile station 10 to each base station 20 _1, 20 ₂ using an uplink, and notifies the downlink transmission information including information of good number of paths and the best path (step 204).
[0037]
When receiving the downlink transmission information from the mobile station 10, the base stations 20 ₁ and 20 ₂ compare the number of good paths with a predetermined threshold number, and if the number of good paths is equal to or more than the threshold number, the base station Signal transmission is turned on, and signal transmission to the downlink of another path is turned off (step 205). Since there is only one best path, one of the base stations will continue the signal to the downlink and the other base stations will stop transmitting.
[0038]
Here, the quality of the reception of each path is determined based on the reception level, but the present invention is not limited to this. In addition, reception of a path using power of CPICH (Common Pilot Channel: common pilot channel), signal-to-noise ratio SIR, received signal energy-to-noise ratio Ec / N0, propagation loss (for example, bit error rate or frame error rate) and the like. The quality may be determined, or multiple combinations thereof may be used.
[0039]
There are various examples of the timing at which the mobile station 10 transmits the information on the number of good paths and the best path as downlink transmission information to the uplink, for example, when the number of good paths changes periodically, or This is when the best paths are exchanged.
[0040]
FIG. 3 is a flowchart showing the operation of the base station in FIG. Referring to FIG. 3, first, the base station detects downlink transmission information from mobile station 10 from the uplink (step 301). Next, the base station compares the number of good paths (P) with a predetermined threshold number (X) (step 302). If P is equal to or less than X, the base station does not perform on / off control of downlink transmission, and waits for detection of the next downlink transmission information while transmitting a signal to the mobile station 10 according to a normal SHO operation.
[0041]
If P is larger than X, the base station determines from the information on the best path (B) whether the path set between itself and the mobile station 10 corresponds to B (step 303).
[0042]
If the own path corresponds to the best path (B), the base station turns on transmission of a signal to the downlink of the path (step 305). If the own path does not correspond to the best path, the base station turns off the transmission of the signal to the downlink of the path (step 304). As a result, of the plurality of base stations for which a communication line is set up with the mobile station 10, only the base station for which a communication line with the highest reception quality at the mobile station is set is used for signal transmission to the downlink. , And the other base stations stop transmitting signals.
[0043]
As described above, according to the present embodiment, as a result of the operations in FIGS. 2 and 3, only the base station on which the best path having the highest reception level in the mobile station 10 is set among the SHO multipaths has a signal. Is transmitted, and the base station on which the other path is set stops transmitting the signal. Therefore, while maintaining the reception quality by the best path, by stopping the transmission on the other path, the communication with the other mobile station is stopped. Interference can be reduced.
[0044]
In addition, a path having low reception quality in the mobile station 10, which is the original destination, is likely to be a large interference component with respect to other mobile stations 10. However, by turning off signal transmission to that path, the efficiency is reduced. The interference can be reduced well.
[0045]
Further, according to the present embodiment, highly reliable control is possible because the mobile station 10 controls the transmission of the signal to the downlink based on the information on the reception quality of the signal of the downlink.
[0046]
Further, according to this embodiment, the number of paths to be RAKE-combined in the mobile station can be equalized.
[0047]
According to the operation shown in FIG. 3, in an environment where there is little fading such as being stationary, or in a mobile station under an environment of low-speed fading, a signal is transmitted only to the best path and its transmission power is increased / decreased by TPC information. The reception quality can be maintained by bringing down. Note that fading is a phenomenon in which, when a plurality of signals arriving at a time difference due to a multipath phenomenon due to reflection or diffraction of a radio wave or movement of a mobile station are received, the reception level of the combined signal is changed due to a phase shift between the signals. This is a phenomenon in which the strength is increased or weakened.
[0048]
However, when the propagation loss of each path is large, such as when fading is large, multipath combining may have a greater effect on maintaining the reception quality than increasing / decreasing the transmission power of the best path.
[0049]
A CDMA mobile communication system according to another embodiment of the present invention controls a transmission signal in consideration of the magnitude of fading. The mobile station according to the present embodiment measures the downlink transmission loss of each path, and transmits the downlink transmission information including the transmission loss information of each path.
[0050]
FIG. 4 is a flowchart showing an operation of the base station according to another embodiment of the present invention. Referring to FIG. 4, first, the base station detects downlink transmission information including information on the number of good paths, the best path, and the propagation loss of the downlink of each path (step 401).
[0051]
Next, the base station determines whether the propagation loss of all paths is greater than a predetermined threshold (Step 402).
[0052]
If the propagation loss of at least one of the paths is equal to or less than the threshold, the base station operates similarly to steps 302 to 305 in FIG. 3 (steps 403 to 406). That is, the base station compares the number of good paths (P) with a predetermined threshold number (X) (step 403), and does not perform downlink transmission on / off control if P is less than or equal to X. If P is greater than X, the base station determines from the information on the best path (B) whether or not the path set between itself and the mobile station 10 corresponds to B (step 404). For example, the transmission of the signal to the downlink of the path is turned on (step 406), and if not, the transmission of the signal to the downlink of the path is turned off (step 405).
[0053]
If the propagation loss of all paths is greater than the threshold, the base station proceeds to the process of step 406, and turns on signal transmission to the downlink of that path.
[0054]
As a result, if there is a path having a small propagation loss due to fading or the like, a path having the highest reception quality at the mobile station is set among a plurality of base stations for which a communication line is set up with the mobile station 10. Only the current base station continues to transmit signals on the downlink, and the other base stations stop transmitting signals. If the propagation loss of all the paths is large, the transmission of the signal to the downlink is not turned off, and the reception quality is maintained by multipath combining. Therefore, since an optimal method is selected according to the propagation loss due to fading or the like, it is possible to maintain both reception quality and suppress interference under various environments.
[0055]
【The invention's effect】
According to the present invention, a signal is transmitted from the base station only to the best path having the best reception quality in the mobile station, and no signal is transmitted to the other paths. Interference with station communication can be reduced.
[0056]
In addition, a path with low reception quality at the mobile station that is the original destination is likely to be a large interference component to other mobile stations. However, by turning off transmission of a signal to that path, efficient interference is obtained. Can be reduced.
[0057]
Further, according to the present invention, highly reliable control is possible because the transmission of a signal from the base station to the downlink is controlled based on information on the reception quality of the downlink signal in the mobile station.
[0058]
Also, if there is a path with a small propagation loss due to fading or the like, only the base station for which the path with the highest reception quality at the mobile station is set among the base stations for which a communication line is set up with the mobile station. However, the transmission of the signal to the downlink is continued, and the other base stations stop transmitting the signal. If the propagation loss of all the paths is large, the signal transmission to the downlink is not turned off, and the reception quality is maintained by multipath combining. Therefore, since an optimal method is selected according to the propagation loss due to fading or the like, it is possible to maintain both reception quality and suppress interference under various environments.
[Brief description of the drawings]
FIG. 1 is a block diagram illustrating a configuration of a CDMA mobile communication system according to an embodiment of the present invention.
FIG. 2 is a block diagram illustrating a configuration of a CDMA mobile communication system according to an embodiment of the present invention.
FIG. 3 is a flowchart illustrating an operation of the base station in FIG. 2;
FIG. 4 is a flowchart illustrating an operation of a base station according to another embodiment of the present invention.
FIG. 5 is a diagram for explaining an operation of a conventional CDMA mobile communication system.
[Explanation of symbols]
10 mobile station ₂₀ 1, 20 ₂ base station 30 the base station controller 40 switching centers 50 cells 201～205,301～305,401～406 step

Claims (10)

A mobile communication system capable of simultaneously setting a plurality of paths for one communication,
From the paths set at the same time, the number of paths having reception quality equal to or greater than the first threshold in the downlink is obtained as the number of good paths, the path having the best reception quality is obtained as the best path, and the number of good paths and the number of the best paths are obtained. At least one mobile station transmitting downlink transmission information including information to the uplink,
A mobile station that receives the downlink transmission information transmitted by the mobile station to the uplink and controls at least one base station that controls transmission of signals to the downlink based on the number of good paths and the information of the best path; Communications system. When the number of good paths is larger than a second threshold, the base station turns on signal transmission to the downlink if a path between itself and the mobile station corresponds to the best path, and must not apply. 2. The mobile communication system according to claim 1, wherein transmission of a signal to a downlink is turned off. The mobile station further determines the propagation loss of the downlink of all the paths set at the same time, transmits the information to the uplink including the downlink transmission information,
The mobile communication system according to claim 1, wherein the base station controls transmission of a signal to a downlink based on the information on the propagation loss in addition to the number of good paths and the best path. The base station turns on transmission of a signal to the downlink if the propagation loss of all the paths is greater than a third threshold, and if the propagation loss of any of the paths is equal to or less than the third threshold. 4. The mobile communication system according to claim 3, wherein transmission of a signal to a downlink is controlled based on the number of good paths and the information on the best path. The mobile communication system according to any one of claims 1 to 4, wherein the mobile station sets a plurality of paths during soft handover. The mobile communication system according to claim 1, wherein the reception quality is determined by at least one of a reception level, a signal-to-noise ratio, an energy-to-noise ratio, a propagation loss, and power of a common pilot channel. A transmission control method for controlling transmission of a signal of a base station in a mobile communication system capable of simultaneously setting a path between the mobile station and a plurality of base stations for communication of one mobile station. hand,
A first step of obtaining, as the number of good paths, the number of paths whose reception quality is equal to or more than a first threshold in the downlink from the paths set simultaneously in the mobile station;
A second step of determining a path having the best reception quality as the best path;
A third step of transmitting downlink transmission information including the number of good paths and the information of the best path to an uplink,
In the base station, a fourth step of receiving the downlink transmission information transmitted by the mobile station to the uplink,
And a fifth step of controlling transmission of a signal to a downlink based on the number of good paths and the information on the best path. In the fifth step, when the number of good paths is larger than a second threshold, if the path between itself and the mobile station corresponds to the best path, the base station transmits a signal to a downlink. 8. The transmission control method according to claim 7, wherein the transmission control signal is turned on, and if not applicable, the transmission of the signal to the downlink is turned off. In the mobile station, further determine the propagation loss of the downlink of all the paths set at the same time, in the third step, the information is included in the downlink transmission information and transmitted to the uplink,
8. The transmission control method according to claim 7, wherein in the fifth step, the base station controls transmission of a signal to a downlink based on the information on the propagation loss in addition to the number of good paths and the best path. . In the fifth step, if the propagation loss of all the paths is larger than a third threshold, the base station turns on transmission of a signal to a downlink, and the propagation loss of any of the paths is reduced to 10. The transmission control method according to claim 9, wherein if the number is equal to or smaller than a third threshold, transmission of a signal to a downlink is controlled based on the number of good paths and information on the best path.

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