JP3512593B2 - Base station switching method for mobile communication system - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mobile communication system in which a plurality of base stations are provided and communication is continued by switching a base station to be a communication partner in the mobile station. Related to the system,
In particular, it relates to a method of selecting a base station to be a next switching candidate. 2. Description of the Related Art In a mobile communication system, it is necessary to reliably perform communication under a complicated radio wave propagation condition. One method for this purpose is to provide a plurality of base stations, 2. Description of the Related Art A mobile communication system of a type in which a base station serving as a communication partner is switched as needed in accordance with a change in electric field strength due to a position or the like and communication is continued is known. FIG. 3 shows this conventional mobile communication system. As shown in the figure, base stations 2 to 4 and a control station 5 are arranged on the assumption that the mobile station 1 moves in advance. Station 1
Are communication areas A ₂ , A ₃ , A by these base stations 2 to 4.
_4, the communication is performed with the corresponding base station, and the communication area is switched from one communication area to the other communication area at a portion where each communication area is overlapped and at an adjacent portion. Is to be continued. [0004] In FIG. 3, the mobile station 1 reaches the outer periphery of the zone of the communication area A ₂ while performing communication via the base station 2 within the communication area A ₂ of the base station 2. In this case, it is necessary to switch the communication partner to the adjacent base station 3 or base station 4 in order to continue the communication. Therefore, for this purpose, the mobile station 1 switches the channel and switches the base station as the communication partner as follows. Namely, the mobile station 1 in communication is adapted to receive electric field strength therein so as to constantly measured as RSSI, for example, as shown, received signal strength RSSI reach the zone outer periphery of the communication area A ₂ And this is
Becomes below a predetermined switching determination level L _th that is set in advance, and its state, when this was also continues for a predetermined fixed time T1 or more set in advance, first, so as to determine the level degradation ing. Next, the mobile station 1 executes the following processing as the peripheral zone monitoring processing upon determining that the level has deteriorated. That is, based on the monitoring control channel numbers F1, F2,..., Fn (n = the number of base stations) notified by the system information transmitted from each base station,
Similarly, the level of received signals is measured and averaged a predetermined number of times m, and the base station in the communication area showing the highest received signal level is selected as the next switching candidate base station. The signal level L _max is compared with the received signal level L of the base station that is currently communicating. When the received signal level L _max is higher than the received signal level L, that is, when L _max > L Activate channel switching for the base station in the communication area. The above prior art does not take into account that it takes a considerable amount of time to select the next base station to be a candidate for switching. There has been a problem that the signal level may sometimes fluctuate. Further, according to the related art, there is no guarantee that the selection of a base station as a next switching candidate is necessarily obtained appropriately, and it is necessary to switch the base station again in a short time depending on the moving direction of the mobile station. There was a problem.
The following is a more detailed description of these problems. The detection of the received signal level in the above-described peripheral zone monitoring process is, specifically, as shown in FIG.
This is performed using a frame S1 set on a superframe composed of a plurality of frames. Then, this level detection process is performed over m times of measurement specified by the system information, and an average value is obtained. In FIG. 2, for example, in a digital mobile communication system for public service, the transmission rate is 40 mS / frame, and one superframe is 18
Since it consists of frames, the time for one superframe is 7
At 20 ms, the time of frame S1 is 40 ms. Therefore, if the start-up time in hardware, such as the time until lock-in of the frequency synthesizer circuit included in the communication circuit, and the time required for level detection are at least 10 ms, then one superframe In the period of the frame S1 in the above, detection of four base stations is limited. Therefore, the number of monitoring control channels n (=
When the number of base stations is 4 or less, as shown in FIG.
Although the level detection of all base stations can be performed within the period of frame S1 in one superframe, if the number of monitoring control channels n (= the number of base stations) exceeds 4, one superframe period Requires a detection time of n / 4 times the time detectable by the above, that is, a detection time of (n / 4 × 720) mS. This is measured as one measurement period, and is measured and averaged. There must be. For this reason, if the number n of base stations is 5 or more, one measurement period increases by one superframe every time n exceeds 4 and further exceeds a multiple of 4, and n = 5 ~
8, one measurement period is two superframe periods, and when n = 9 to 12, one measurement period is three superframe periods.
It becomes a super frame period. Therefore, for example, in the case of the above-mentioned public service, the maximum value of the number n of base stations in one system is determined to be 20, so that n = 20 and the number of measurements m is
If m = 3, the time required for the selection of the next candidate base station is 20/4 × 720 mS × 3 = 10.8 seconds. Therefore, in the related art, the reception signal level is as long as 10.8 seconds. Therefore, the time until the switching to the next candidate base station is delayed and the communication quality is deteriorated, for example, the received signal level fluctuates. [0013] At this time, as shown in FIG. 3, the mobile station 1 via the peripheral zone of the communication area A ₃ from the communication area A ₂ and moved in a short period of time the communication area A _4, in this case Assuming that the received signal level in the communication area A ₃ is slightly higher than the received signal level in the communication area A ₄ , in the background art, the communication area A ₃
After switching from ₂ to the communication area A ₃ once, the communication area is switched to the communication area A ₄ in a short time. Therefore, in the related art, it is necessary to switch the base station again in a short time. The problem is that this will happen. [0014] It is an object of the present invention to provide a base station switching method for a mobile communication system in which the frequency of base station switching accompanying the movement of a mobile station is suppressed. SUMMARY OF THE INVENTION An object of the present invention is to provide a mobile station having a plurality of base stations and measuring the electric field strength of all the base stations in the mobile station a predetermined number of times as one measurement period. ,
In a base station switching method in a mobile communication system in which a base station serving as a communication partner is switched according to the measurement result and communication of the mobile station is continued, as a result of the measurement processing in the plurality of measurement periods, This is achieved by selecting the base station having the highest electric field strength among the base stations whose levels are simply increasing as the next switching candidate base station. Hereinafter, a base station switching method for a mobile communication system according to the present invention will be described in detail with reference to the illustrated embodiments. First, an example of a mobile communication system to which an embodiment of the present invention is applied is described with reference to FIG.
The system is the same. Therefore, the mobile station 1 includes the base stations 2 to 4 and the control station 5, and when the mobile station 1 is in any one of the communication areas A ₂ , A ₃ , and A ₄ by the base stations 2 to ₄ , it communicates with the corresponding base station. The point that the communication area is changed from one communication area to the other communication area at the part where the communication areas overlap and the part where the communication areas are adjacent Same as technology. Further, for this reason, the mobile station 1 during communication is
They become a reception electric field strength therein so as to constantly measured as RSSI, continuing the received signal strength RSSI is, become less than a predetermined switching determination level L _th that is set in advance, and its state, the time T1 or more Then, it is determined that the level is degraded, thereby proceeding to the peripheral zone monitoring process, thereby selecting a base station to be the next switching candidate and activating channel switching.
Same as the prior art. Thus, in the embodiment of the present invention described here, the process of selecting a base station to be the next switching candidate is performed.
The content of the received signal level detection process in the peripheral zone monitoring process is significantly different from that of the related art. Therefore, the received signal level detection process according to the embodiment of the present invention will be described below with reference to FIG. First, in FIG. 1, the processing in the first one superframe period is as shown in FIG.
In the same manner as in the case of the prior art described in the above section, each received signal level by all base stations of the monitoring control channel numbers F1, F2,..., Fn is measured. Therefore, when the number n of base stations is 5 or more, processing in a plurality of superframe periods according to the numerical value of n becomes one measurement period. By the way, in the prior art, the processing in the second and subsequent measurement periods is the same as the processing in the first measurement period, and the control channel numbers for monitoring F1, F2,.
.., Fn, all the received signal levels of each base station are measured. However, in the embodiment of the present invention, first, each reception signal level Li (i = 1) by all the base stations of the monitoring control channel numbers F1, F2,..., Fn measured in the first measurement period. ,..., N) are compared with the signal level L received by the base station currently communicating. Then, only the monitoring control channel numbers that satisfy the condition of "L <Li" are subjected to measurement processing of the respective received signal levels. In FIG. 1, the processing of measuring the received signal level only for the monitoring control channel number that satisfies “L <Li” is described as “F equal to or higher than a specified level” in FIG. Therefore, according to this embodiment, the number of monitoring control channels to be measured for the received signal level in the second and subsequent measurement periods can be made smaller than the number n of base stations. As a result, the time required to select the next base station to be the switching candidate can be reliably reduced, and the fluctuation of the signal level accompanying the switching of the base station can be sufficiently suppressed. In general, in such a mobile communication system, among the signal levels received from n base stations, a mobile station in a certain communication area is higher than a received signal level L of a base station currently communicating. As is clear from FIG. 3, the number of base stations indicating the reception signal level is only a few stations adjacent to the communication area of the base station currently communicating. As a result, according to the embodiment of the present invention, when the number n of base stations is 20, for example, as in the communication system for public service described above, in the second and subsequent measurement periods, The number of monitoring control channels is changed from 20 to 3 at most, so that it is possible to sufficiently reduce the time required for selecting the next base station to be switched. Next, another embodiment of the present invention will be described. First, also in the other embodiment of the present invention, the mobile station moves to the surrounding zone, for example, due to the movement of the mobile station during communication, thereby selecting the next switching candidate base station and activating the channel switching. Up to this point, it is the same as the prior art described with reference to FIG. Then
The other embodiment is different from the above-mentioned prior art in that the content of the process of selecting the next switching candidate base station from the received signal level of each base station obtained by averaging by the process in the measurement period It is in. First, in the prior art, as described above, the received signal level of each base station obtained by averaging by processing during the measurement period is checked, and the base station in the communication area showing the highest received signal level is obtained. Is selected as the next switching candidate base station. On the other hand, in another embodiment of the present invention, only the base station whose electric field strength level, that is, the received signal level is simply increased, is selected from the measurement processing results in a plurality of measurement periods. Among them, the base station having the highest electric field strength is selected as the next switching candidate base station. [0028] In such a mobile communication system, as already described in FIG. 3, the mobile station 1 via the peripheral zone of the communication area A ₃ from the communication area A ₂ move in a short time the communication area A ₄ in the history of this time, towards the received signal level in the communication area a ₃ are, in some cases higher than the received signal level in the communication area a _4. In this case, in the prior art, since only the magnitude of the received signal level is viewed, once from the communication area A ₂ ,
After switching to communication area A ₃ , communication area A ₄
Will be switched to
It is necessary to switch the base station again in a short time, and the base station may be frequently switched with the movement of the mobile station. In FIG. 3, for example, it is assumed that the mobile station 1 is moving in the direction of the arrow P, and the history of the change of the received signal level in the communication area A ₃ and the communication area A The history of the change in the received signal level in ₄ is as follows. First, in the communication area A _3,
The moving direction of the mobile station 1, since it in a direction deviating from the communication area A _3, the reception signal level due to it should illustrating a decreasing trend. On the other hand, in the communication area A ₄ , the moving direction of the mobile station 1 is a direction in which the mobile station 1 moves into the mobile station 1, so that the received signal level should show an increasing tendency. Therefore, at this time, when picked out by the base station the received signal level is simply increased, the base station of the communication area A ₄ in this case, i.e., only the base station 4 is singled out. Further, if a base station having the highest received signal level is selected from the base stations whose received signal levels are simply increasing, the base station 4 is used in this case. Here, in another embodiment of the present invention,
Only the base stations whose received signal levels are simply increasing are selected, and the base station having the highest electric field strength among them is selected as the next switching candidate base station. For example, in the case of FIG. , once the communication area a _2, without Setsu換Ru the communication area a _3, will be switched from the communication area a ₂ immediately communication area a _4, therefore, switching of useless base station with the movement of the mobile station Can be sufficiently suppressed. According to the present invention, the time required for selecting a mobile station to be the next switching candidate can be shortened, so that the switching of the base station can be completed in a short time. Level fluctuation is small, and stable communication can be easily obtained. Further, according to the present invention, the selection of a mobile station as a next switching candidate can always be appropriately obtained, so that the frequency of base station switching accompanying the movement of a mobile station can be suppressed, and stable communication can be easily obtained. it can.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a timing chart for explaining an operation of an embodiment of a base station switching method for a mobile communication system according to the present invention. FIG. 2 is a timing chart for explaining a base station switching operation according to the related art. FIG. 3 is an explanatory diagram illustrating an example of a mobile communication system to which an embodiment of the present invention is applied. [Description of Signs] 1 Mobile stations 2 to 4 Base station 5 Control stations A _{1 to} A ₃ Communication area of each base station

Claims (1)

(57) [Claims 1] A plurality of base stations are provided, and the measurement of the electric field strength of all the base stations in the mobile station is performed a predetermined number of times as one measurement period, and the measurement result is obtained. A base station switching method in a mobile communication system in which a base station serving as a communication partner is switched in accordance with the base station and communication of the mobile station is continued, wherein the electric field strength level is simple as a result of the measurement processing in the plurality of measurement periods. A base station switching method for a mobile communication system, wherein a base station having the largest electric field strength among the increasing base stations is selected as a next switching candidate base station.