JP4108673B2 - Mobile communication system and frequency band switching method - Google Patents

Description

  The present invention relates to a mobile communication system capable of using a plurality of frequency bands between a mobile terminal station and a radio base station, and in particular, a frequency band switching method for selecting and switching a frequency band to be used from among a plurality of frequency bands. About.

  In a mobile communication system, wireless communication is performed between a radio base station (Base Transceiver Station: BTS) and a plurality of mobile terminal stations (MS: Mobile Station) in the communication area of the radio base station. Calls and data communication services are provided to mobile terminal stations.

  Various communication systems between BTS and MS are known. In a CDMA system, code division multiple access that uses radio waves (carrier waves) of the same frequency and assigns different codes to each MS for communication. It is a method.

  Such a conventional mobile communication system is shown in FIG. The mobile communication system includes BTSs 11 and 14, an MS 12, and a radio network controller (RNC) 13. In FIG. 3, the MS 12 is in the communication area 15 of the BTS 11.

  In the mobile communication system employing this code division multiple access method, as shown in FIG. 3, the RNC 13 performs inter-station data communication with the BTSs 11 and 14, and controls communication between the BTS 11 and the MS 12.

  During communication with the MS 12, the BTS 11 monitors the ratio (Eb / No = SIR) of the communication electric field level (upstream signal = Eb) from the MS 12 and the other electric field level (noise signal = No) and reports it to the RNC 13. . In the RNC 13, a target SIR (optimum value of Eb / No) is set, so that the SIR in the downlink from the BTS 11, 14 to the MS 12 and the uplink from the MS 12 to the BTS 11, 14 becomes this target SIR value. By increasing / decreasing the communication power (downstream signal) level from the BTS 11, 14 to the MS 12 and the communication power (upstream signal) level from the MS 12 to the BTS 11, 14, a plurality of MSs within the communication area of the BTS 11, 14 The communication electric field balance is maintained.

  However, since this communication power control is based on the premise that the SIR that can maintain communication in the BTS 11 is satisfied, the uplink signal level from the MS 12 is lowered for some reason, and the SIR that can decode data in the BTS 11. Is not satisfied, it is determined that communication cannot be maintained, and communication is terminated.

  If the adjacent BTS 14 other than the BTS 11 in communication satisfies the SIR that allows the uplink signal level from the MS 12 to be decoded, switching between BTSs (handoff) is performed as shown in FIG. However, if the adjacent BTS 14 is not satisfied, the communication is terminated without handoff.

  As described above, in the conventional mobile communication system, radio waves of the same frequency are used and communication is performed by assigning different codes to each MS. Therefore, the uplink signal level from the MS decreases for some reason, and the BTS If the SIR that can decode the data cannot be satisfied, it is determined that the communication cannot be maintained and the communication ends.

In order to prevent such an adverse effect, when a plurality of frequency bands are used in communication between the MS and the BTS, and the communication electric field is reduced in one frequency band (for example, 2 GHz band), the other frequency band (for example, , 800 MHz band), and a mobile communication system has been proposed in which operation is switched to maintain communication (see, for example, Patent Documents 1, 2, and 3).
However, since the propagation loss depends on the frequency, it is known that the propagation loss varies depending on the frequency band to be used. For example, when comparing two frequency bands of the 2 GHz band and the 800 MHz band, it can be said that the 2 GHz band is generally more likely to be more difficult to maintain communication.

  Therefore, in a mobile communication system in which a plurality of frequency bands are switched between an MS and a BTS, calls are concentrated on a specific frequency band having a small propagation loss, and the balance between the frequency bands is lost. There is a risk of problems.

When the balance between the frequency bands is lost, problems such as new incoming / outgoing calls cannot be made in the frequency band with a small propagation loss, and switching of the frequency bands cannot be performed smoothly occur. As a result, if a frequency band with small propagation loss is used, communication may be terminated to a mobile terminal station that can maintain a call. This is because a frequency band with a small propagation loss has been allocated to a mobile terminal station that has no problem even if a frequency band with a large propagation loss is used because the communication state is originally good. There is a need for a method for balancing calls between frequency bands.
JP 2004-186923 A JP 10-23502 A Japanese Patent Laid-Open No. 7-23447

  In the above-described conventional mobile communication system, when a plurality of frequency bands are switched between the MS and the BTS, calls are concentrated in a frequency band with a small propagation loss, and the balance between the frequency bands is lost. There was a problem.

  An object of the present invention is to provide a mobile communication system and a frequency band switching method capable of preventing calls from being concentrated on a specific frequency band even when a plurality of frequency bands are switched between an MS and a BTS. Is to provide.

In order to achieve the above object, a mobile communication system of the present invention comprises a plurality of radio base stations and at least one mobile terminal station that moves within a communication area of the plurality of radio base stations, In a mobile communication system that selects and switches a frequency band used between a mobile terminal station and the radio base station from a plurality of frequency bands,
The radio base station preferentially assigns a frequency band with a large propagation loss to a mobile terminal station that newly accepts incoming and outgoing calls.

  According to the present invention, when a radio base station receives a new incoming / outgoing call from a mobile terminal station, the radio base station preferentially assigns a frequency band with a large propagation loss to the mobile terminal station. Concentration of calls in a small frequency band is suppressed, and the balance of calls among a plurality of frequency bands is achieved.

  As described above, according to the present invention, since a frequency band with a large propagation loss is preferentially assigned to a new incoming / outgoing call from a mobile terminal station, calls are concentrated on a frequency band with a small propagation loss. It is possible to obtain the effect that the balance of calls between a plurality of frequency bands is maintained.

  Next, embodiments of the present invention will be described in detail.

  One embodiment of the present invention provides a mobile communication system including a radio base station and a mobile terminal station that can operate a plurality of frequency bands (for example, 2 GHz band and 800 MHz band). When the communication electric field is reduced in the frequency band (for example, 2 GHz band), when switching the operation to the other frequency band (for example, 800 MHz band), calls in the 2 GHz band are reduced due to the operation switching, and calls in the 800 MHz band are performed. In order to compensate for the loss of balance due to the increase, the mobile base station that newly accepts incoming and outgoing calls assigns the 2 GHz band, which has a larger propagation loss than the 800 MHz band, so that the radio base station can communicate. The present invention is characterized in that the balance of the number of mobile terminal stations in a plurality of bands to be performed is maintained.

  Next, the configuration of the embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a diagram illustrating a configuration example of a mobile communication system according to a first embodiment of this invention.

As shown in FIG. 1, the mobile communication system of the present invention is a radio base station (BTS that can operate a plurality of frequency bands (for example, band 1 and band 2, band 1 is 2 GHz band, band 2 is 800 MHz). ) 1 and the mobile terminal station (MS) 2 that can operate a plurality of frequency bands similarly. BTS1 and MS2 enable simultaneous bidirectional communication by individually setting transmission and reception frequencies in each frequency band and simultaneously performing transmission and reception.
[Band 1]
Upstream frequency (BTS1 reception frequency / MS2 transmission frequency) = 1950 MHz
Downlink frequency (BTS1 transmission frequency / MS2 reception frequency) = 2140 MHz
[Band 2]
Upstream frequency (BTS1 reception frequency / MS2 transmission frequency) = 837.5 MHz
Downlink frequency (BTS1 transmission frequency / MS2 reception frequency) = 882.5 MHz
The BTS 1 is connected to a radio network controller (RNC) 3 and exchanges signals between stations (Iub signal) 7. That is, BTS1 reports communication information with MS2 to RNC3, and RNC3 performs inter-station data communication for performing necessary control on BTS1.

  BTS1 uses communication area 5 as a basic communicable range of MS2, uses radio waves (carrier waves) of the same frequency with other MS8s of MS2, and assigns different codes to each MS for communication. Wireless communication is performed by the code division multiple access method (CDMA method).

  The BTS 1 in the present embodiment preferentially assigns a frequency band of 2 GHz band with a large propagation loss to an MS that newly accepts incoming and outgoing calls.

  Next, the operation of the mobile communication system of this embodiment will be described in detail with reference to the drawings.

  In FIG. 1, BTS 1 communicates with MS 2 and a plurality of MSs 8 other than MS 2 in band 1 (2 GHz band) or band 2 (800 MHz band).

  The operation of the present invention will be described with respect to communication between BTS1 and MS2.

  The BTS1 monitors the ratio (Eb / No = SIR) of the communication electric field (upstream signal = Eb) from the MS2 and the other electric field (noise signal = No), and the value for the RNC3 by the Iub signal. Report. Note that communication electric fields from a plurality of MSs 8 other than MS 2 are regarded as noise signals.

  In RNC3, the target SIR (Eb / No optimum value) is set, and the communication power (downstream signal) level from the BTS to the MS and the communication power (upstream signal) level from the MS to the BTS are set for the BTS. By adjusting it, the communication electric field balance with a plurality of MSs in the communication area 5 of the BTS 1 is maintained.

Now, assuming that the RNC 3 has set the target SIR = −10 dB, the operation will be described in each scene.
[Scene 1]
・ Communication electric field from MS2 (upstream signal = Eb) = 10 dBμV
・ Other electric field (noise signal = No) = 50 dBμV
In the case of SIR (scene 1) = Eb / Io = 10 log (10/50) = − 7 dB
That is, the SIR (scene 1) that BTS1 reports to RNC3 is
SIR (Scene 1)> Target SIR
Therefore, the RNC 3 performs downlink signal level control so that the BTS 1 approaches the target SIR.

  The BTS 1 lowers the downlink signal level for the MS 2 and causes the MS 2 to reduce the uplink power (or raises the downlink signal level and increases the uplink power for a plurality of other MSs 8 of the MS 2).

For example, if the upstream power level is lowered by 5 dB for MS2,
・ Communication electric field from MS2 (upstream signal = Eb) = 5 dBμV
・ Other electric field (noise signal = No) = 50 dBμV
And
SIR (scene 1 ′) = Eb / Io = 10 log (5/50) = − 10 dB
As a result, the BTS 1 can maintain a communication electric field balance with a plurality of MSs present in the communication area 5.
[Scene 2]
・ Communication electric field from MS2 (upstream signal = Eb) = 2 dBμV
・ Other electric field (noise signal = No) = 50 dBμV
in the case of,
SIR (scene 2) = Eb / Io = 10 log (2/50) =-14 dB
That is, the SIR (scene 1) that BTS1 reports to RNC3 is
SIR (scene 2) <target SIR
Therefore, the RNC 3 performs downlink signal level control so that the BTS 1 approaches the target SIR.

  The BTS 1 raises the downlink signal level for the MS 2 and causes the MS 2 to raise the uplink power (or lowers the downlink signal level and reduces the uplink power for a plurality of other MSs 8 of the MS 2).

For example, if the upstream power level is increased by 3 dB for MS2,
・ Communication electric field from MS2 (upstream signal = Eb) = 5 dBμV
・ Other electric field (noise signal = No) = 50 dBμV
And
SIR (scene 2 ′) = Eb / Io = 10 log (5/50) = − 10 dB
As a result, the BTS 1 can maintain a communication electric field balance with a plurality of MSs present in the communication area 5.

  However, the operation of the scene 2 is an operation that can be performed only when the MS 2 still has room to increase the uplink power (transmission power of the MS 2) level.

  In FIG. 1, if MS2 is near the end of the communication area 5 of BTS1, or if MS2 is moving and there is an obstacle such as a house or forest between BTS1, When the transmission power of the MS 2 is set to the upper limit in the initial stage, the target SIR is not reached, and it becomes difficult to maintain communication.

  When considering the probability that the communication described above is difficult to maintain in band 1 (2 GHz band) and band 2 (800 MHz band), band 1 (2 GHz band) is generally due to the difference in propagation loss between 2 GHz and 800 MHz. ) Is more likely to be more difficult to maintain communication.

  The operation of the present embodiment contributes to communication maintenance in the case of a failure caused by these reasons.

  That is, when BTS1 and MS2 can operate in a plurality of frequency bands (band 1 (2 GHz band), band 2 (800 MHz band)), by switching the operation from band 1 currently in communication to band 2 Maintain communication.

In general, it is already known that the propagation loss varies depending on the frequency (the propagation loss depends on the frequency).
When propagation loss = L, communication distance = d, wavelength = λ,
L = 20 log (4πd / λ)
When comparing the propagation loss of band 1 and band 2 for the upstream frequency,
L (band 1) -L (band 2)
= 20 log {(4πd / λ (band 1)) / (4πd / λ (band 2)}
= 20 log {(λ (band 2)) / (λ (band 1)}
λ (band 1) = c (propagation speed) / f (band 1 frequency)
λ (band 2) = c (propagation speed) / f (band 2 frequency)
Because
L (band 1) -L (band 2)
= 20 log {f (band 2 frequency) / f (band 1 frequency)}
= 20 log (1950 / 837.5)
= 7.3 (dB)
Therefore, the propagation loss in band 1 is 7.3 dB greater than that in band 2.

  That is, when MS 2 switches the transmission frequency from band 1 to band 2 with the same output value, the upstream electric field level (Eb) received by BTS 1 increases by 7.3 dB.

In the case of scene 2, if the upstream power level is increased by 3 dB relative to MS2,
・ Communication electric field from MS2 (upstream signal = Eb) = 5 dBμV
・ Other electric field (noise signal = No) = 50 dBμV
And
SIR (scene 2 ′) = Eb / Io = 10 log (5/50) = − 10 dB
Therefore, the BTS 1 can respond to the request of the RNC 3 with the increase in the electric field level due to the band switching.

  With the above operation, if the transmission power of MS2 has already been set to the upper limit in the initial stage of scene 2, BTS1 can maintain communication with MS2 by switching operation from band 1 to band 2 Is possible.

  At this time, band 2 is preferentially allocated to an MS that newly accepts incoming / outgoing calls in order to compensate for the disruption of balance due to a decrease in calls in band 1 and an increase in calls in band 2 at the time of operation switching. Thus, the balance of the number of MSs using the band 1 and the band 2 is maintained.

FIG. 2 shows an operation transition of the embodiment regarding the operation switching from the band 1 to the band 2.
(1) Time: T0 to T1
Control from BTS1 to MS2 to lower the upstream power level and converge to the target SIR.
(2) Time: T1-T2
Control from BTS1 to MS2 to increase the upstream power level and converge to target SIR.
(3) Time: T2
Upper power level upper limit of MS2. Band switching control from BTS2 to MS2.
(4) Time: T2 ~
Power level control is performed in band 2 from BTS1 to MS2 and converges to target SIR.

  In the mobile communication system configured as described above, when BTS1 and MS2 can operate in a plurality of frequency bands (band 1 (2 GHz band), band 2 (800 MHz band)), band 1 currently in communication By switching the operation from band 2 to band 2, the propagation loss between BTS1 and MS2 can be reduced, so SIR is improved and the switching of the operation reduces 2 GHz band calls and increases 800 MHz band calls. In order to compensate for the disruption of the balance, a 2 GHz band is preferentially allocated to an MS that newly accepts incoming / outgoing calls so that the number of MSs in a plurality of bands with which the BTS communicates is kept balanced. It becomes possible to do.

  In this embodiment, the case where two frequency bands are used between the MS and the BTS has been described. However, the present invention is not limited to this, and even when three or more frequency bands are used. Similarly, the present invention can be applied.

It is a figure which shows one structure of the mobile communication system of one Embodiment of this invention. It is a figure for demonstrating operation | movement of the mobile communication system of one Embodiment of this invention. It is a figure which shows the conventional mobile communication system. It is a figure which shows the handoff between BTS in the conventional mobile communication system.

Explanation of symbols

1 Radio base station (BTS)
2 Mobile terminal station (MS)
3 Radio network controller (RNC)
4 radio base stations (BTS)
5 BTS1 communication area 6 BTS4 communication area 7 Inter-station signal (Iub signal)
8 Mobile terminal stations (MS)
11 Wireless base station (BTS)
12 Mobile terminal stations (MS)
13 Radio Network Controller (RNC)
14 Wireless base station (BTS)
15 Communication area of BTS11 16 Communication area of BTS14

Claims (4)

A plurality of radio base stations; and at least one mobile terminal station that moves within a communication area of the plurality of radio base stations; and a frequency band used between the mobile terminal station and the radio base station In a mobile communication system that selects and switches from a plurality of frequency bands,
The radio base station preferentially assigns a frequency band with a large propagation loss to a mobile terminal station that newly accepts incoming and outgoing calls.   A plurality of frequency bands that can be used between the radio base station and the mobile terminal station are a first frequency band of 2 GHz band and a second frequency band of 800 MHz band having a propagation loss smaller than that of the first frequency band. The mobile communication system according to claim 1, comprising a frequency band. In a mobile communication system comprising a plurality of radio base stations and at least one or more mobile terminal stations that move within a communication area of the plurality of radio base stations, between the mobile terminal station and the radio base station In the frequency band switching method for selecting and switching the frequency band to be used in the plurality of frequency bands,
The radio base station has a step of preferentially allocating a frequency band having a large propagation loss to a mobile terminal station that newly accepts incoming / outgoing calls.   A plurality of frequency bands that can be used between the radio base station and the mobile terminal station are a first frequency band of 2 GHz band and a second frequency band of 800 MHz band having a propagation loss smaller than that of the first frequency band. The frequency switching method according to claim 3, comprising frequency bands.

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