JPH088706B2 - Handover method for mobile communication system - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a handover method of a mobile communication system used for a microcell type mobile phone system or a business cordless phone system installed in a business office or the like.

Conventional technology The simple type of office cordless telephone system that has been used so far supports the handover function that switches the call to a better wireless zone when the call quality deteriorates due to moving in the area during a call. Not many systems.

In general, when performing a handover, as a method for searching for a wireless zone to be switched next, a base station-initiated method in which a base station in the vicinity of the base station in the call monitors the transmitted radio waves of the mobile station in the current call Then, there are two mobile station-led methods for searching for a candidate for a base station to be handed over by a mobile station by monitoring a radio wave transmitted from a base station in a peripheral zone when the call quality deteriorates.

The base station-led search method requires a field-forced measurement receiver (SSR) that monitors the radio waves of mobile stations in addition to the radio for communication in the base station. It tends to be expensive.

On the other hand, the mobile station-led search method has the advantage that the mobile station monitors the electric field strength of the surrounding base stations with its own receiver, so it does not require a special receiver and can provide a handover function easily and inexpensively. is there.

In particular, when the wireless method is the digital TDMA (Time Division Multiple Access) method, which is the mainstream these days, it is possible to monitor other channels without interrupting the line being communicated in slots other than the time slot in which the self transmits and receives. So
This mobile station initiative type handover base station search method is suitable.

However, in a car telephone system, etc., even if all channels for communication are free, each base station always transmits the control channel, so the mobile station should always measure the electric field strength of the peripheral zone base station. However, in a system that performs dynamic channel allocation such as a microcell type mobile phone system, there is a problem of coexistence with other systems, and it is prohibited to constantly transmit radio waves from the base station. Often. In this case, since radio waves are not always transmitted from the neighboring base stations, it is impossible to search for a base station candidate for handover under the initiative of the mobile station.

FIG. 2 is a transmission / reception timing chart showing an example of the operating conditions of the control / call channel of the base station in the microcell type mobile telephone system which performs communication by the TDMA / TDD system.

In Figure 2, FS (Fixed Station) is the base station and PS
(Portble Stotion) is a mobile station.

Here, 1 TDMA frame is divided into two groups: base station transmission / mobile station reception time slot group (FS → PS) and mobile station transmission / base station reception time slot group (PS → FS).
Allocating time slots.

The two lines on the horizontal axis represent the timing at which the base station FS operates at the frequencies of the control channel (hereinafter abbreviated as Cch) and the S channel (hereinafter abbreviated as Sch), respectively.
In h, it controls incoming and outgoing calls, and in Sch, individual calls are made with mobile stations. Further, the upper side of the horizontal axis shows the transmission of the base station FS, and the lower side shows the reception timing.

The CCH, BCCH, is a broadcast time slot, and the CCCH, is a control time slot. As shown in this figure, the base station FS only transmits intermittently on Cch, and in this example, one base station FS is CC (CC
After transmitting in two transmission slots of CH) and BC (BCCH), it does not transmit in the subsequent transmission slots but only receives.

For example, as disclosed in Japanese Patent Application No. 2-180814, mutual interference between FSs is mutually detected by monitoring the transmission of other FSs, and the frequency used in the system is dynamically and autonomously. This is done to realize a mechanism for allocating data.

Also, the time slots in Sch from 2 to 8 are shown as transmitting and receiving by using all the time slots in this figure, but transmission and reception are not performed in slots that are not actually used for communication. Not done

Next, Fig. 3 shows a multi-frame configuration that allows each base station FS to monitor the radio waves transmitted to each other in order to dynamically allocate frequencies in this system, and Cch
In the figure, the time slots to be transmitted at are arranged in a staggered manner so as not to overlap.

Here, after the FSI transmits CCCH and BCCH, the transmission timing is assigned to FS2, then the transmission timing is sequentially shifted to FS3, and it is devised to form a multiframe with n FSs.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention However, in the above system, each base station FS adopts a relatively long cycle TDMA multi-frame structure so as not to disturb other communication when there is no call, and intermittently. The notification signal is sent to Cch. The frequency of this intermittent transmission (duty ratio) cannot coarse more than a certain limit in order to coexist with other systems.

Therefore, if the mobile station PS measures the electric field strength of the peripheral zone by monitoring the radio wave emission of the Cch of the base station FS, it needs to take the longest time of this multiframe period.

However, for a mobile station whose electric field state has deteriorated during a call, the call may be disconnected during a search for a handover candidate for such a long time, which is not practical.

The present invention solves such a conventional problem, and even in a business cordless system for a premises,
Providing a low-cost handover function that accompanies movement during a call,
It is an object of the present invention to provide a handover method for a mobile communication system that spreads more advanced microcell type mobile phones.

Means for Solving the Problem In order to achieve the above-mentioned object, the present invention, when a mobile station starts a search for a base station of a handover candidate, receives a base station in a call from a neighboring base station by the mobile station. A signal requesting to emit a radio wave for measurement is emitted at such a timing that the system controller of the center notified from the base station that receives this signal measures the electric field to the neighboring base stations that are candidates for handover. It controls to emit a radio wave for use.

Operation Therefore, according to the present invention, the mobile station of the TDMA / TDD system to be used enables the electric field measurement for the handover at a timing other than the transmission and reception time slots used for the normal communication.

In this case, it is necessary to switch the reception frequency on the mobile station side, because the peripheral base station that is the target of handover often communicates using a frequency other than the radio carrier used for the mobile station's call. There is.

Generally, when a wireless device switches the transmission / reception frequency, a response time is required to change the oscillation frequency of the synthesizer. Therefore, the base station which emits the electric field measuring radio wave needs to control the timings in consideration of these timings.

Embodiment FIG. 1 is a timing diagram showing the method of one embodiment of the present invention, which enables a mobile station PS to monitor an electric field. FIG. 1 shows that the mobile station PS currently in communication is communicating with the base station FS (described as old FS in the figure) using the time slot with the slot number T4 at the carrier frequency f1. There is.

Here, it is assumed that the mobile station PS needs time for three TDMA time slots in order to switch the reception frequency. In FIG. 1, the base station FS monitored as a handover candidate is described as a new FS, but it is assumed that this new FS is communicating at a carrier frequency f2 different from the old FS.

Therefore, after the mobile station PS receives the radio wave from the old FS in the time slot of slot number T4,
If the frequency is switched from f1 to the carrier frequency f2, the radio wave of the new FS can be received only at the time of slot number T8 after the time of slot numbers T5 to T7 has elapsed. In the figure, this timing is expressed as if the mobile station PS is receiving a radio wave in the slot of the monitor M.

After performing the electric field measurement of the new FS in the time slot of slot number T8, the corresponding base station FS again performs frequency switching, and transmits from the mobile station PS at this time to slot number T4. By doing so, the mobile station PS can monitor the electric field strength of another base station FS without interrupting communication.

Of course, if the new FS is already performing other communication at the timing of this slot number T4 in Sch, the base station FS transmitting to the mobile station PS in this call without performing special control. The electric field can be measured by receiving the radio wave.

However, since there is no call, if the new FS has stopped emitting radio waves in this time slot, the time slot that can be received by the mobile station PS that is going to perform the handover destination base station FS candidate search will be used. It is specially controlled to emit radio waves.

In the example of FIG. 1, the base station FS does not distinguish between Cch and Sch for transmission, but there is no particular problem even if the timing of slot number T8 is Sch or Cch.

However, if this is Sch and the mobile station PS does not know that the carrier frequency f2 should be measured,
After requesting a search for a handover candidate base station from the mobile station PS, the system is transferred to the mobile station PS through the base station PS in the call.
Need to be informed of frequency and other information about the new FS.

Signals such as a base station search request and new FS frequency notification are transmitted at a low speed Cch (C) that is superimposed on the call so that the call between the base station FS and the mobile station PS during communication can be performed without interruption.
The FACCH) specified in CITT Recommendation Q.1063 can be used.

Note that the above description is an example for performing the mobile station-initiated handover of the TDMA / TDD system, but the same method can be applied to the case of a business cordless system that uses the FM system cordless telephone system.

In these cordless systems, the base station (connecting device) with no call normally stops emitting radio waves, so if you want to perform a handover with a system led by a mobile station,
This is because it is necessary to control to emit radio waves for measuring electric field strength from the peripheral base stations in the same manner.

EFFECTS OF THE INVENTION As is apparent from the above embodiments, the present invention enables a mobile station to receive radio waves from one or more base stations that are candidates for switching destinations in a mobile communication system in which the base station does not always emit radio waves. By doing so, it becomes possible to perform a handover led by a mobile station, and it is possible to provide at a low cost an advanced personal communication system in which an individual can communicate using a mobile terminal anytime and anywhere. Some of them have great utility value.

[Brief description of drawings]

FIG. 1 is a timing chart showing the method of one embodiment of the present invention, in which charges can be monitored by a mobile station, and FIG. 2 is TDMA / TDD.
Figure 3 shows TDMA / TDD transmission / reception timing diagram of base station
FIG. 6 is a distribution diagram of transmission timing between base stations of the method. FS ... Base station, PS ... Mobile station, f1, f2 ... Carrier frequency, T1-T8 ... Slot number.

Claims (3)

[Claims] 1. A mobile station sends a signal to a base station currently in a call requesting that a base station in a wireless zone, which is a candidate for switching, emits a radio wave for electric field strength measurement.
This signal causes the base station in the corresponding wireless zone to control the timing so that the electric field strength can be measured by the corresponding mobile station for a certain period of time, emits radio waves, and the electric field strength information measured by this mobile station is currently called. A handover method for a mobile communication system, comprising: collecting and analyzing via a local base station, identifying a base station of a handover switching destination, and switching a communication line. 2. A base station in a wireless zone which is a candidate for a handover switching destination receives a signal from a mobile station requesting to emit a radio wave for electric field strength measurement, through a base station in a current call, The mobile station transmits a signal for notifying the carrier frequency of a radio wave emitted by the base station of the switching destination candidate for measuring the electric field strength, or the slot number and the carrier frequency, to the mobile station. Handover method for mobile communication system. 3. When a mobile station requires a certain switching time to switch from a carrier frequency during a call to a carrier frequency for measuring electric field strength, a wireless zone base station that is a handover destination candidate is a mobile station. The TD the call is using
The mobile communication system according to claim 1, wherein the timing is controlled so as to emit a radio wave so that the electric field strength can be measured after a time required for frequency switching has passed from the MA time slot. Handover method.