Abstract:
A hand-over control method is disclosed to control the change of connections between a radio communication terminal and a plurality of radio communication base stations for radio communication. The received power of a signal from a radio communication base station in a cell that that is subject to the influence of fading is likely to vibrate within a predetermined strength range. Even if a cell satisfies hand-over conditions, there is a greater likelihood that the cell is still subject to the influence of fading, if the received power is not exceeding this predetermined range The current received quality of a signal from a radio communication terminal in the cell is compared with last received quality of the signal from the radio communication base station when the radio communication terminal finished radio communication with radio communication base station. If the former quality is not higher than the latter quality by a predetermined level or more, the radio communication base station is determined to still be subject to the influence of fading, and hand-over is not performed. If, on the other hand, the former quality is higher than the latter quality by the predetermined level or more, the radio communication base station is determined not to be subject to the influence of fading, and hand-over is not performed.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method of controlling hand-over (change of radio communication base stations to be connected) performed by a radio communication terminal such as a portable telephone or the like. 
   2. Description of Related Art 
     FIG. 1  is a schematic diagram illustrating a radio communication system to explain hand-over performed by radio communication terminal. In  FIG. 1 , cell A indicates an area in which radio communication terminal  31  can perform a radio communication with radio communication base station  32 , and cell B indicates an area in which radio communication terminal  31  can perform a radio communication with radio communication base station  33 . Hand-over performed by radio communication terminal  31  refers to an operation in which radio communication terminal  31  which is communicating with radio communication base station  32  change radio communication with radio communication base station  32  to radio communication with radio communication base station  33  when radio communication terminal  31  moves from cell A to cell B, When performing hand-over from radio communication base station  32  to radio communication base station  33 , radio communication terminal  31  is required to change the frequency used for radio communication from a frequency available in radio communication base station  32  to a frequency available in radio communication base station  33 . 
   Such inconvenience can be solved by using code division multiple access (CDMA) for radio communication. In a CDMA system, radio communication terminal  31  can distinguish between a signal transmitted from radio communication base station  32  and a signal transmitted from radio communication base station  33  and can simultaneously receive both signals. Therefore, with the hand-over performed by radio communication terminal  31  using CDMA, when radio communication terminal  31  moves from cell A to cell B while communicating with radio communication base station  32 , radio communication terminal  31  adds communication with radio communication base station  33  while maintaining the communication with radio communication base station  32 , radio communication terminal  31  being set to have simultaneous reception from both radio communication base stations  32  and  33 . Thereafter, when the received power of signals transmitted from radio communication base station  32  becomes low, radio communication terminal  31  finishes the communication with radio communication base station  32  and communicates only with radio communication base station  33 . 
   A radio communication terminal using such CDMA includes a plurality of receivers called a finger receiver and hence has the capability to simultaneously receive signals transmitted from three radio communication base stations. This feature of simultaneous reception allows hand-over in a radio communication terminal using CDMA to be classified into three general cases: (i) When communicating with one or two radio communication base stations, the radio communication terminal adds communication with another radio communication base station; (ii) When communicating with two or three radio communication base stations, the radio communication terminal finishes the communication with these radio communication base stations; (iii) When communicating with three radio communication base stations, the radio communication terminal changes communication with one of the radio communication base stations to communication with another radio communication base station that is not currently communicating with the radio communication terminal. 
   The processing in case (iii), which has the most problems of the three cases, will be described in detail with reference to  FIG. 2 . 
     FIG. 2  shows that when radio communication terminal  41  is communicating with radio communication base stations  42 ,  43  and  44 , radio communication base station  45  becomes a candidate for hand-over with which radio communication terminal  41  starts communication.  FIG. 3  shows a change in the received power of radio communication terminal  41  over time.  FIG. 4  shows a sequence of messages exchanged between radio communication terminal  41  and radio communication base stations  42  to  45 . 
   As shown in  FIG. 2 , radio communication terminal  41  is communicating with radio communication base stations  42 ,  43  and  44 . Radio communication terminal  41  measures the received power of a signal transmitted from radio communication base station  45  during communication, As shown in  FIG. 3 , the received power of a signal transmitted from radio communication base station  42  is lowered, whereas the received power of a signal transmitted from radio communication base station  45  is increased. As shown in  FIG. 4 , immediately before the received power of a signal from radio communication base station  45  exceeds the received power of a signal from radio communication base station  42 , radio communication terminal  41  transmits message  61  to radio communication base stations  42 ,  43  and  44  to notify the stations that the result of measurement of the received power of a signal transmitted from radio communication base station  45  satisfies the conditions for hand-over. 
   Upon receipt of message  61 , radio communication base station  42  finishes communication with radio communication terminal  41 , and radio communication base station  45  starts communication with radio communication terminal  41 . Each of radio communication base stations  43  and  44  transmits message  62  to radio communication terminal  41  to notify the terminal of conditions to communicate with radio communication base station  45  that is a target for hand-over, such as timing and the like. 
   Upon receipt of message  62 , radio communication terminal  41  sets the notified timing information and the like to the finger receivers thereof, and transmits message  63  to radio communication base stations  43 ,  44  and  45  to notify the base stations that procedure to set the notified communication conditions has been finished. 
   Subsequently, in preparation for the next hand-over, each of radio communication base stations  43 ,  44  and  45  transmits message  64  to radio communication terminal  41  to cause terminal  41  to renew hand-over information such as information about the periphery cells of terminal  41  and the like, thereby finishing the hand-over processing. Radio communication terminal  41  then examines the qualities of signals received from radio communication base stations in the peripheral cells in accordance with received massage  64  to renew hand-over information. 
   In radio communications, fading may occur due to buildings and geographical features, causing fluctuation of the received power from each radio communication base station. This fading exerts a large influence on the processing for the case (iii).  FIG. 5  explains the influence exerted on the processing for the case (iii). 
   As shown in  FIG. 5 , qualities of signals received from radio communication base stations may change greatly over time as in cell C or cell D. Now consider hand-over which is performed under radio signal propagation circumstances as shown in  FIG. 5 . Initially, radio communication terminal  41  communicates with the radio communication base stations in cells A, B and C. Each time the relationship in the qualities of received signals between cells C and D alternates, radio communication terminal  41  performs hand-over to switch between the radio communication base stations in cells C and D. Thereafter, when the qualities of received signals from the radio communication base stations in cells A and B deteriorate and the qualities of received signals from the radio communication base station in cell E is increased, the radio communication base station in cell E becomes a candidate for hand-over. However, because radio communication terminal  41  performs hand-over to switch between the radio communication base stations in cells C and D, it cannot perform hand-over to the radio communication base station in cell E. This causes a problem that a call is undesirably disconnected due to the deterioration of the qualities of received signals from the radio communication base stations in cells A and B. 
   The reason that hand-over to the radio communication base station in cell E can not be performed is that since the sequence of hand-over is being performed between radio communication terminal  41  and the radio communication base stations in order to perform hand-over to switch between the radio communication base stations in cells C and D, as illustrated in  FIG. 5 . message  61  which notifies that cell E has satisfied the hand-over conditions and which is transmitted from radio communication terminal  41  to the radio communication base stations, is disregarded by the radio communication base stations. In this way, in situations where hand-over to switch between two cells C, D is frequently performed, the hand-over that is necessary to maintain communications with radio communication terminal  41  cannot be performed due to unnecessary hand-over that has been frequently performed. 
   Several techniques have been proposed to solve this problem. For example, JP 2001-16932A (hereinafter referred to as Patent document 1) discloses a technique to detect such unnecessary hand-over through a learning means. JP2004-40338A (hereinafter referred to as Patent document 2) discloses a technique wherein the number of hand-over operations between two radio communication base stations is counted, and hand-over between the radio communication base stations is prohibited for a predetermined period of time when the counted value exceeds a predetermined value. 
   However, with the invention of Patent document 1, it is time-consuming to build the learning means, and unnecessary hand-over caused by fading cannot be prevented in a simple manner. With the invention of Patent document 2, the repetition of unnecessary hand-over a predetermined number of times cannot be avoided. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention is to provide a method of preventing unnecessary hand-over caused by fading in an effective and simple manner. 
   To this end, the method according to the present invention is intended to prevent unnecessary hand-over caused by fading in the following procedure. 
   First, when a radio communication terminal executes a first hand-over operation, on the basis of the qualities of signals received from a plurality of radio communication base stations, to change or finish connections between the radio communication terminal and at least part of the radio communication base stations for radio communication, the radio communication terminal stores identifiers of radio communication base station(s), with which the radio communication terminal finished radio communication, and information about the qualities of signals received from said radio communication base station(s) when the radio communication terminal finished radio communication with said radio communication base station(s). 
   Then, when the radio communication terminal executes a second hand-over operation, on the basis of the qualities of signals received from the radio communication base stations, to change connections between the radio communication terminal and at least part of the radio communication base stations for radio communication, the radio communication terminal determines whether or not a particular radio communication base station with which the radio communication terminal is about to start communicating, was previously put in radio communication with the radio communication terminal, based on the presence or absence of the identifier of the particular radio communication base station; 
   If the radio communication terminal has determined that the particular radio communication base station was previously put in radio communication with the radio communication terminal, the radio communication terminal compares the quality of a first signal currently received from the particular radio communication base station with the quality of a second signal received from the particular radio communication base station stored at the first hand-over when the radio communication terminal finished the radio communication with the particular radio communication base station. If the quality of the first signal is higher than the quality of the second signal by a predetermined level or more, the radio communication terminal executes the second hand-over operation to start radio communication with the particular radio communication base station. 
   As exemplified by cells C and D shown in  FIG. 5 , in cells that are subject to the influence of fading, the received power is likely to fluctuate or vibrate within a predetermined strength range. Therefore, even though the conditions for hand-over are satisfied, unless the strength of the received power does not exceed the predetermined strength range, there is a greater likelihood that the cell does not yet get out of the influence of fading. In the present invention, the quality (received power) of a signal currently received from a radio communication base station is compared with the quality (received power) of a signal finally received from the radio communication base station when the radio communication base station finished radio communication with the radio communication terminal. If the former quality does not exceed the latter quality by a predetermined level, it is determined that the cell do not yet get out of the influence of fading, and hand-over is not performed. On the other hand, if the former quality exceeds the latter quality by a predetermined level or more, it is determined that the cell has got out of the influence of fading, and hand-over is performed. 
   Thus, according to the present invention, the use of the algorithm based on the properties of fading that the received power is fluctuated within a predetermined strength range, prevents unnecessary hand-over caused by fading in an effective and simple manner, thereby preventing the failure to perform necessary hand-over and therefore disconnection of communication 
   The above and other objects, features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a view illustrating hand-over performed in a radio communication terminal; 
       FIG. 2  is a view illustrating that when communicating with three radio communication base stations, a radio communication terminal switches its communication with a radio communication base station into a communication with another radio communication base station not currently communicating with the terminal, and more specifically, when radio communication terminal  41  communicates with radio communication base stations  42 ,  43  and  44 , radio communication base station  45  has become a candidate for hand-over; 
       FIG. 3  is a view illustrating that when communicating with three radio communication base stations, a radio communication terminal switches its communication with a radio communication base stations into a communication with another radio communication base station not currently communicating with the terminal, and in particular, a change in received power with time of signals received by radio communication terminal  41 ; 
       FIG. 4  is a view illustrating that when communicating with three radio communication base stations, a radio communication terminal switches its communication with a radio communication base stations into a communication with another radio communication base station not currently communicating with the terminal, and in particular, a sequence of messages exchanged during hand-over between the radio communication terminal and the radio communication base stations; 
       FIG. 5  is a view showing a change in received power over time in a radio communication terminal when it moves to respective cells A to E; 
       FIG. 6  is a view showing an arrangement of a radio communication terminal according to an embodiment of the present invention; and 
       FIG. 7  is a flow chart for explaining procedure of the hand-over control method according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring now to  FIG. 6 , a radio communication terminal according to an embodiment of the present invention comprises antenna  10 , antenna sharing unit  11 , radio receiver  12 , baseband receiver  13 , radio transmitter  14 , and base band transmitter  15 . Baseband receiver  13  includes a plurality of finger receivers  131 ,  132  and  133 , peripheral cell measuring finger receiver  134 , digital signal processor  135 , hand-over controller  136 , and hand-over control memory  137 . 
   Antenna  10  receives and transmits signals for communication with radio communication base stations (not shown). 
   Antenna sharing unit  11  supplies the signal received by antenna  10  to radio receiver  12  and outputs to antenna  10  a transmission signal of the radio communication terminal that is output from radio transmitter  14 . 
   Radio receiver  12  performs radio processing such as frequency conversion and the like on the received signal to supply the resultant signal as a received signal within a baseband to baseband receiver  13 . 
   Baseband receiver  13  performs processing such as despreading, demodulation and the like on the received signal from radio receiver  12  in finger receivers  131 ,  132  and  133  and peripheral cell measuring finger receiver  134 , supplies the processed signal to digital signal processor  135 , and reports the received quality of the received signal to hand-over controller  136 . 
   Hand-over controller  136  controls the timing of signal reception in finger receivers  131 ,  132 ,  133  and peripheral cell measuring finger receiver  134 , and determines from the received qualities of signals transmitted from a radio communication base station currently communicating with the terminal and the received qualities of signals transmitted from radio communication base station(s) in peripheral cell(s) not currently communicating with the terminal whether or not the conditions for hand-over are satisfied in each cell. If hand-over conditions are satisfied in a particular radio communication base station in the peripheral cells, hand-over controller  136  retrieves the results of hand-over for the particular radio communication base station previously performed from hand-over control memory  137  and determines whether or not the particular radio communication base station that has satisfied the hand-over conditions was a target for hand-over to change or finish connections between the radio communication terminal and radio communication base stations for radio communication. If the particular radio communication base station was not a target for hand-over, hand-over controller  136  notifies baseband transmitter  15  that the particular radio communication base station has satisfied the hand-over conditions. The radio communication terminal transmits a message to notify that the particular radio communication base station has satisfied the hand-over conditions, to the radio communication base stations, according to a hand-over sequence shown in  FIG. 4 . Thereafter, hand-over controller  136  returns to the processing to determine, based on the received qualities of signals received from radio communication base station currently communicating with the terminal and the received qualities of signals received from radio communication base stations in the peripheral cells, whether or not each radio communication base station satisfies the hand-over conditions. 
   On the other hand, if the particular radio communication base station was previously a target for hand-over, hand-over controller  136  determines whether or not the particular cell has communicated with the radio communication terminal under circumstances where unnecessary hand-over frequently occurs due to fluctuations in the received quality of signals from the particular radio communication base station that are caused by fading. Specifically, hand-over controller  136  compares the received quality of signals from the particular radio communication base station in the previous hand-over with the received quality of the particular radio communication base station in the current hand-over. The received quality in the previous hand-over was finally received from the particular radio communication base station, when communication with the particular cell was finished in the previous hand-over, and has been stored in hand-over control memory  137 . If the received quality of signals in the current hand-over is almost equal to the received quality in the previous hand-over, that is, if the received quality of signals in the current hand-over is not higher than the received quality of signals in the previous hand-over by a predetermined level, then hand-over controller  136  determines that fading has exerted an influence on communication with the particular radio communication base station, frequently generating an unnecessary hand-over. To reduce this unnecessary hand-over, hand-over controller  136  stops the current hand-over for the particular radio communication base station, and hand-over controller  136  returns to the processing to determine, based on the received qualities of signals from a radio communication base station currently communicating with the terminal and the received qualities of signals from radio communication base station in the peripheral cells, whether or not each radio communication base station satisfies the hand-over conditions. 
   On the other hand, if the received quality in the current hand-over is improved as compared with the received quality in the previous hand-over by a level equal to or higher than a threshold level stored in hand-over control memory  137 , hand-over controller  136  determines that although fluctuations in the received quality of signals from the particular radio communication base station occurs due to fading, the particular radio communication base station has not been put into a condition to frequently generate unnecessary hand-over, and notifies baseband transmitter  15  that the particular radio communication base station has satisfied the hand-over conditions. The radio communication terminal then notifies radio communication base stations that the particular radio communication base station has satisfied the hand-over conditions, according to the hand-over sequence shown in  FIG. 4 , Thereafter, hand-over controller  136  returns to the processing to determine, based on the received qualities of signals from the radio communication base station currently communicating with the terminal and the received qualities of signals from radio communication base stations in the peripheral cells, whether or not each radio communication base station satisfies the hand-over conditions. 
   Digital signal processor  135  performs digital signal processing such as error correction and the like on the signal that was received and inputted, decodes the received signal, and extracts, from the received signal, data such as audio data, image data and the like that were transmitted from the radio communication base stations. 
   Radio transmitter  14  performs radio processing such as frequency conversion and the like on the transmission signal generated in the baseband transmitter  15 . 
   Baseband transmitter  15  generates a transmission signal of the radio communication terminal. 
   Next, procedure of a hand-over control method performed in hand-over controller  136  of the radio communication terminal shown in  FIG. 6  will be described in detail with reference to  FIG. 7 . 
   On receipt of information about the received qualities of signals from finger receivers  131 ,  132  and  133  and peripheral cell measuring finger receiver  134  at step  20 , hand-over controller  136  determines at step  21  whether or not the received qualities of the input signals satisfy the hand-over conditions. If the received qualities do not satisfy the hand-over conditions, that is, in the case of NO at step  21 , hand-over controller  136  waits for the received quality of the next signal. 
   On the other hand, if the received qualities satisfy the hand-over conditions, that is, in case of YES at step  21 , hand-over controller  136  determines at step  22  whether hand-over should be performed to switch communication with a radio communication base station currently communicating with the terminal to communication with a radio communication base station not currently communicating with the terminal (cell changing) or only to disconnect a radio communication base station currently communicating with the terminal from communication with the terminal. For hand-over other than hand-over for cell changing or for disconnecting, that is, hand-over that adds a cell (in case of NO at step  22 ), hand-over controller  136  notifies at step  27 , according to the hand-over sequence shown in  FIG. 4 , radio communication base stations that a cell to be added has satisfied the hand-over conditions, and then performs hand-over processing according to the hand-over sequence shown in  FIG. 4  to start communication between a radio communication base station in the added cell and the terminal. Thereafter, hand-over controller  136  waits for the received qualities of the next signal in order to perform the next hand-over operation. 
   On the other hand, in case of hand-over for cell changing or for disconnecting, that is, in case of YES at step  22 , the procedure proceeds to steps  23  and  24 . 
   At step  24 , hand-over controller  136  stores in hand-over control memory  137  the cell number of a cell whose radio communication base station is to be disconnected from communication with the terminal, and the received quality of signals finally transmitted from the radio communication base station at the completion of the communication with the terminal along with the identifier of the radio communication base station. Data stored in hand-over control memory  137  are periodically renewed. At step  23 , hand-over controller  136  ascertains data stored in hand-over control memory  137 . More particularly, hand-over controller  136  searches hand-over control memory  137  for an identifier and a received quality of the radio communication base station. 
   At step  25 , hand-over controller  136  determines whether or not the particular radio communication base station that is a target for hand-over with which the radio communication terminal is about to start communication, was previously disconnected from communication with the terminal for cell changing or for disconnecting, from the presence or absence of the identifier of the particular radio communication base station. If there is the identifier in hand-over control memory  137 , then the hand-over controller  136  determines that the particular radio communication base station was disconnected in a previous hand-over. In contrast, if there is no identifier of the particular radio communication base station in hand-over control memory  137 , then hand-over controller  136  determines that the particular radio communication base station was not disconnected in the previous hand-over. 
   If the particular radio communication base station was not disconnected in the previous hand-over, that is, in case of NO at step  25 , hand-over controller  136  notifies, according to the hand-over sequence shown in  FIG. 4 , the radio communication base stations that the particular radio communication base station has satisfied the hand-over conditions, and performs hand-over processing shown in  FIG. 4  to start communication between the radio communication terminal and the particular radio communication base station. Thereafter, hand-over controller  136  waits for the received qualities of the next signal in order to perform next hand-over operation. 
   On the other hand, if the particular radio communication base station was disconnected in previous hand-over, that is, in the case of YES at step  25 , hand-over controller  136  compares, at step  26 , the received quality of a signal currently transmitted from the particular radio communication base station with the received quality of a signal finally transmitted from the particular radio communication base station at the competition of the communication with the terminal and stored in hand-over control memory  137 , and determines based on the result of comparison whether or not the current hand-over should be performed to start communication with the particular radio communication base station. 
   Specifically, if the former received quality is almost the same as the latter received quality, hand-over controller  136  can recognize that the received quality of the particular radio communication base station fluctuates due to fading to thereby frequently generate unnecessary hand-over for the particular radio communication base station, to determine that the current hand-over for connection of the particular radio communication base station with the terminal should not be performed. However, if the former received quality is higher than the latter receive quality by a predetermined level, then hand-over controller  136  can recognize that the particular radio communication base station is not in a condition to frequently generate unnecessary hand-over due to fluctuations in the received quality of the particular cell. In order to perform this procedure, hand-over controller  136  stores a threshold value in hand-over control memory  137  to compare the former received quality with the latter received quality, and compares the former received quality with the latter received quality by using the predetermined value to determine whether or not the former received quality is improved as compared with the latter received quality by the predetermined level or more. 
   If the comparison shows that the former received quality is not improved by the predetermined level or more, that is, in case of NO at step  26 , then hand-over controller  136  waits for the received qualities of the next signal at step  20 . However, the comparison shows that the former received quality is improved by the predetermined level or more, that is, in case of YES at step  26 , then hand-over controller  136  notifies, according to the hand-over sequence shown in  FIG. 4 , radio communication base stations that the particular radio communication base station has satisfied the hand-over conditions. Subsequently, at step  27 , hand-over controller  136  performs hand-over processing shown in  FIG. 4  to start communication with the particular radio communication base station. Thereafter, at step  20 , hand-over controller  136  waits for the received qualities of the next signal in order to perform next hand-over operation. 
   According to another embodiment of the present invention, a radio communication terminal has the same basic construction as in the previous embodiment, but differs from the previous embodiment in the processing performed in hand-over controller  136 . That is, hand-over controller  136  in the present embodiment stores a predetermined threshold level for received qualities in hand-over control memory  137 , and even if a particular cell can be a target for hand-over, hand-over controller  136  does not handle the particular cell as a target for hand-over when the received quality of the particular cell is below the predetermined threshold level. This therefore eliminates the need for performing processing after step  20  in  FIG. 7  when the received quality is too low, 
   According to yet another embodiment of the present invention, the processing to determine whether hand-over operation is performed or not, i.e., the processing of steps  25  and  26  in  FIG. 6  is carried out in a radio communication base station. 
   Specifically, after measuring the received quality in each cell, the radio communication terminal transmits to the radio communication base station, a message notifying that a particular cell has satisfied the hand-over conditions, together with information about the received qualities. Based on the message transmitted, the radio communication base station determines whether or not a radio communication base station in the particular cell that is a target for hand-over was previously a target for hand-over for cell changing or for disconnecting from communication and finished the communication with the radio communication terminal. If the radio communication base station in the particular cell was previously disconnected from communication with the terminal to finish the radio communication terminal, then the radio communication base station compares the received quality at the completion of communication with the received quality notified to determine whether the current hand-over should be continued or not. Please note that the radio communication base station includes a memory for storing the received quality that corresponds to hand-over control memory  137 . 
   The radio communication terminal and the radio communication base station described above each may be implemented, other than by a dedicated hardware, by recording a computer program to embody its function on a computer-readable recording medium and executing the program on a computer serving as the radio communication terminal or the radio communication base station. The computer-readable recording medium refers to a flexible disk, a magneto-optical disk, a recording medium such as CD-ROM or the like, a memory such as a hard disk installed in a computer system, or the like. Further, the computer-readable recording medium includes a medium (transmission medium, transmitted waves or carrier waves) for dynamically holding a program in a short period of time such as, for example, in a case where a program is transmitted through the Internet, and a medium for holding a program for a fixed period of time such as a volatile memory installed in a computer system. 
   While preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.