Method of route diversity in mobile radio communication systems

Abstract: A method for route diversity in a mobile radio communication using a cellular system formed by a central control station (621) and a plurality of base stations (622-628) comprises an inter-station transmission line (629-635) with which each base station is connected with neighboring base stations and to which the central control station (621) is connected at an end in a signal transmission direction; means for receiving radio waves from a mobile station and measuring a receiving quality at each base station; and means for transmitting a receiving quality information indicating a currently highest receiving quality obtained by the base stations and a transmission signal representing the received radio waves at the highest level, that receiving level information and the transmission signal being updated at each base station (622-628) such that the receiving quality information received by the central control station (621) is the highest receiving quality among the receiving qualities obtained by all the base stations (622-628).

Description: This application is divided from European patent application 91 116 795.5 filed on the 1 st October, 1991. The present invention relates to a method of route diversity operations in a mobile radio communication system such as an automobile telephone system. More particularly, it relates to a method of route diversity in a mobile radio communication using a cellular system formed by a central control station and a plurality of base stations, the method comprising the steps of: providing an inter-station transmission line by which each base station is connected with neighboring base stations and to which the central control station is connected at an end in a signal transmission direction. Such a method is disclosed in GB-A-2138652. In a mobile radio communication system, the efficient utilization of limited frequency spectrum is one of the major technical problems. For this reason, a so called cellular system is adopted in general. In this cellular system, the efficient frequency spectrum utilization is achieved by providing a plurality of mutually distanced base stations within a service area of the systen so as to enable a so called co-channel reuse which is a repetitive use of the same frequency at different base stations, where the base stations are sufficiently distanced from each other to avoid a co-channel interference. In such a cellular system, an area covered by each base station is called a cell. In this cellular system, a higher frequency spectrum utilization efficiency can be achieved by making a radius of each cell smaller, because it becomes possible for the same frequency to be repetitively used at shorter distances. Such a use of smaller size cells is effective in increasing a system capacity regarding a number of subscribers that can be handled by the system. However, as a consequence of an increased number of cells required for covering the entire service area, such a use of smaller size cells also necessitates an increased number of so called handover operations required to be performed when a mobile station moves from one cell to another during a single continuous communication. As a result, the capacities and sizes of each base station and a central control station controlling the base stations will be increased considerably and configurations of the base stations and the central control station will inevitably become quite complicated. Namely, in the cellular system, as a mobile station which is communicating through one particular base station of one particular cell moves from that particular cell to a neighboring cell, the receiving level at that particular base station will gradually decrease, so that in order to continue this communication it is necessary to switch the base station through which the mobile station communicates from that particular base station of that particular cell to the base station of the neighboring cell. Here, in switching the base station, an active land transmission line connecting the central control station with the base station needs to be switched, while the frequency used for the communication by that mobile station needs to be switched from the traffic channel used in an original cell from which the mobile station is moving out to that of an idle traffic channel available in the base station of a destination cell into which the mobile station is moving Such an operation is known as the handover operation (which is sometimes also referred to as a handoff operation). Now, in a mobile radio communication, the radio transmission characteristic is quite adverse because of the large receiving level fluctuation and other causes, such that the occurrence of errors in the transmission signals is unavoidable. For this reason, the mobile radio communication system utilizes various error correction mechanisms. However, these error correction mechanisms are effective only with respect to the instantaneous fluctuation of the receiving level, and not very effective with respect to the relatively gradual variation of the receiving level caused by the shadowing of the transmission path due to the terrain and environmental features. To such a relatively gradual variation of the receiving level, a so called route diversity reception in which the transmitted radio waves received by a plurality of base stations are utilized is known to be effective. For instance, the automobile telephone of NTT (Nippon Telegraph and Telephone) Corporation utilizes the base station route diversity reception for the control channel. More specifically, a conventional route diversity reception is achieved as follows. Namely, a conventional mobile radio communication system utilizing the route diversity reception has a configuration shown in Fig. 1, where the system includes a central control station 21 and a plurality of base stations 22 to 28 each of which is connected with the central control station 21 through land transmission lines 29 to 35, respectively. In this mobile radio communication system, a signal transmitted from each of the base stations 22 to 28 to the central control station 21 has a format shown in Fig. 2 which includes a transmission signal 36 from a mobile station and an encoded receiving level information 37 indicating the receiving level at each base station. In other words, each base station relays the transmission signal 36 from the mobile station to the central control station 21 by attaching the receiving level information 37 to the transmission signal 36. The central control station 21 then selects the base station for which the receiving level is the highest among all the base stations 22 to 28, and uses the transmission signal 36 transmitted from this selected base station as the transmission signal 36 received from the mobile station. Such a conventional mobile radio communication system has drawbacks that the expensive land transmission line must be provided between the central control station 21 and each one of a plurality of the base stations and that the control function of the central control station 21 inevitably becomes complicated as a consequence of utilizing the route diversity reception. It is an object of the present invention to provide a method of route diversity in a mobile radio communication which can be realized less expensively, without requiring additional control function which complicates the central control station. According to the present invention there is provided a method as initially defined, characterized by receiving radio waves from a mobile station and measuring a receiving quality for the radio waves from the mobile station at each base station; and transmitting a receiving quality information indicating a currently highest receiving quality obtained by the base stations and a transmission signal representing the radio waves from the mobile station received at the highest receiving level, the receiving level information and the transmission signal being updated at each base station such that the receiving quality information received by the central control station is the highest receiving quality among the receiving qualities obtained by all the base stations. The invention will be better understood by referring, by way of example, to the accompanying drawings, in which: Fig. 1 is a schematic diagram of a conventional mobile radio communication system utilizing a route diversity; Fig. 2 is a diagram of a format for a signal transmitted from each base station to a central control station in the system of Fig. 1; Fig. 3 is a schematic diagram of a first embodiment of a mobile radio communication system in a form of a cellular system using a method of route diversity according to the present invention; Fig. 4 is a schematic diagram of a configuration of each base station in the system of Fig. 3; and Fig. 5 is a timing diagram for an operation of a base station to achieve the route diversity reception in the system of Fig. 3. Referring now to Fig. 3, a first embodiment of a mobile radio communication system using a method of route diversity according to the present invention will be described in detail. In this first embodiment, the system comprises a central control station 621 and a plurality (seven in Fig. 3) of base stations 622 to 628, which are connected through a inter-station transmission line formed by a transmission line element 629 connecting between the base stations 622 and 623, a transmission line element 630 connecting between the base stations 623 and 624, a transmission line element 631 connecting between the base stations 624 and 625, a transmission line element 632 connecting between the base stations 625 and 626, a transmission line element 633 connecting between the base stations 626 and 627, a transmission line element 634 connecting between the base stations 627 and 628, and a transmission line element 635 connecting between the base station 628 and the central control station 621. Each of the base stations 622 to 628 has a schematic configuration shown in Fig. 4 , where the base station comprises an antenna 636 for receiving the radio waves from a mobile station, a receiver 637 for receiving the transmitted signal from the antenna 626, a control circuit 640 connected with the receiver 637 through a received signal line 638 and a receiving level information line 639, a transmission line element 641 for receiving signals from an upper station through the inter-station transmission line, and a transmission line element 642 for transmitting signals to a lower station through the inter-station transmission line. This system of the first embodiment operates according to a diagram shown in Fig. 5 as follows. The base station 622 is located at a top position of the inter-station transmission line, so that there is no signals from the upper station for this base station 622. Therefore, when this base station 622 receives a signal 643 from a mobile station as a received signal 644, the receiver 637 of the base station 622 measures the receiving level of this received signal 644 at a measurement timing 645, and the measured receiving level is transmitted to the control circuit 640 through the receiving level information line 639 while the received signal 644 is transmitted to the control circuit 640 through the received signal line 638. At the control circuit 640, a signal transmission timing 650 for transmitting the,signal to the lower station is set to be equal to or somewhat longer than the measurement timing 645, and the control circuit 640 outputs an encoded receiving level information 648 indicating the receiving level measured by the receiver 637 and a transmission signal 649 identical to the received signal 644 together to the inter-station transmission line through the transmission line element 642. At each of the remaining base stations 623 to 628, when the signal 643 from a mobile station is received as the received signal 644, the receiver 637 of the base station measures the receiving level of the received signal 644 at the measurement timing 645, and the measured receiving level is transmitted to the control circuit 640 through the receiving level information line 639 while the received signal 644 is transmitted to the control circuit 640 through the received signal line 638. Meanwhile, at the control circuit 640, the signal including the receiving level information 646 and the transmission signal 647 is received from the upper station through the inter-station transmission line and the transmission line element 641. The control circuit 640 then compares the receiving level measured by the receiver 637 and the receiving level indicated by the receiving level information 646. When the receiving level measured by the receiver 637 is higher than the receiving level indicated by the receiving level information 646, the control circuit 640 outputs the encoded receiving level information 648 indicating the receiving level measured by the receiver 637 and the transmission signal 649 identical to the received signal 644 received by the receiver 637 together to the inter-station transmission line through the transmission line element 642 at the signal transmission timing 650. On the other hand, when the receiving level indicated by the receiving level information 646 is higher than the receiving level measured by the receiver 637, the control circuit 640 outputs the encoded receiving level information 648 identical to the receiving level information 646 and the transmission signal 649 identical to the transmission signal 647 together to the inter-station transmission line through the transmission line element 642 at the signal transmission timing 650. Thus, the receiving level information 648 and the transmission signal 649 to be transmitted to the lower station are sequentially updated at the base stations 623 to 628, such that when the central control station 621 receives the receiving level information 646 and the transmission signal 647 from the last base station 628 through the inter-station transmission line, the receiving level indicated by the receiving level information 646 is the highest receiving level obtained among the base stations 622 to 628, and the transmission signal 647 represents the signal received from the mobile station by one of the base stations 622 to 628 at this highest receiving level, so that the route diversity reception is achieved by using the transmission signal 647 received by the central control station 621 as the signal received from the mobile station in the system. Now, the signal transmission timing 650 will be described in further detail. Namely, when the base stations 622 to 628 are sequentially numbered by orders of positions on the inter-station transmission line in a direction of the signal transmission, the signal transmission timing T s,i for the i-th base station is given by: T s,i ≥ T s,i-1 + T m + ΔT r where T m is a time required for receiving the receiving level information 646 transmitted from the upper station, and ΔT r is a sum of a radio propagation delay difference due to the difference of the distances between the mobile station and the base stations 622 to 628 and a signal transmission delay between the neighboring base stations on the inter-station transmission line. The radio propagation delay difference compensates the variation of the receiving time of the radio waves from the mobile station at the base stations due to the difference of the distances between the mobile station and the base stations 622 to 628, and can be considered as a propagation time for a distance between the neighboring base stations on the inter-station transmission line, so that when the distance between the neighboring base stations is 10 km for example, the radio propagation delay difference is equal to 10 × 10 3 /(3 × 10 8 ) = 3 × 10 -5 sec = 0.03 ms. As for the signal transmission delay between the neighboring base stations on the inter-station transmission line, considering the fact that the transmission line is not straight, it is sufficient for this signal transmission delay to take a value equal to several times the radio propagation delay difference, so that ΔT r = 0.2 ms to 0.3 ms is sufficient for the above described exemplary case. Now, the operation of the system of the first embodiment described so far assumes that all the base stations 622 to 628 can receive the signal from the mobile station. However, in reality, only a part of the base stations 622 to 628 will be able to receive the signal from the mobile station in general. In a case only a part of the base stations 622 to 628 can receive the signal from the mobile station, the system of the first embodiment operates as follows. Let the j-th base station be the earliest base station in the order of positions along the inter-station transmission line which can receive the signal from the mobile station. When the signal from the mobile station is received at this j-th base station, the control circuit 640 of this j-th base station will wait for the receiving level information 646 and the transmission signal 647 from the upper station. However, in this case, no upper station can transmit the receiving level information 646 and the transmission signal 647 to the inter-station transmission line because no upper station is capable of receiving the signal from the mobile station. Thus, after waiting for the signal transmission timing 650, the control circuit 640 of the j-th base station outputs the receiving level information 648 indicating the receiving level measured by the receiver 637 and the transmission signal 649 identical to the received signal 644 received by the receiver 637 to the inter-station transmission line through the transmission line element 642. On the other hand, let the k-th base station be the base station whose upper station is capable of receiving the signal from the mobile station but which itself is not capable of receiving the signal from the mobile station. At this k-th base station, the receiving level information 646 and the transmission signal 647 are received from the upper station through the inter-station transmission line and the transmission line element 641 while not receiving the signal from the mobile station at the receiver 637. Thus, after waiting for T m + ΔT r since the reception of the receiving level information 646 and the transmission signal 647 from the upper station, the control circuit 640 of this k-th base station outputs the receiving level information 648 identical to the receiving level information 646 and transmission signal 649 identical to the transmission signal 647 to the inter-station transmission line and the transmission line element 642. In this embodiment, the level fluctuation can be averaged out by using the longer receiving level measurement timing 645 such that the accuracy of the receiving level measurement can be improved. However, the use of the longer receiving level measurement timing also makes the transmission time on the transmission line longer so that the transmission delay may present some problem in such a case. Thus, the receiving level measurement timing 645 should be determined by taking the maximum allowable transmission delay of the system into account. In a case the maximum allowable transmission delay is sufficiently large, the receiving level measurement timing 645 can be set equal to the entire signal duration, whereas in a case the maximum allowable transmission delay is not so large, the receiving level measurement timing 645 should be set equal to only a part of a top portion of the entire signal duration. Also, in this embodiment, the receiving level information 648 and the transmission signal 649 may be transmitted through the inter-station transmission line separately. In such a case, the receiving level informations from a plurality of the base stations may be multiplexed and transmitted by using a channel different from a channel used for transmitting the transmission signals. Thus, according to the first embodiment, it is possible to provide a method of route diversity in a mobile radio communication which can be realized less expensively, without requiring additional control function which complicates the central control station. It is to be noted that in the above embodiment, the central control station and the base stations are provided separately, but the present invention is equally applicable to a system in which one of the base stations plays the role of the central control station as well. Moreover, in the above embodiment, the receiving level is utilized as the key for determining the destination base station, but the other signal reception characteristics such as a number of detected errors or an eye opening in a case of digital signal with error correction encoding, an S/N ratio in a case of analog signal, and their combinations may also be utilized as the key for determining the destination base station.

Claims: A method of route diversity in a mobile radio communication using a cellular system formed by a central control station (621) and a plurality of base stations (622-628), the method comprising the steps of: providing an inter-station transmission line (629-635) by which each base station is connected with neighboring base stations and to which the central control station (621) is connected at an end in a signal transmission direction; characterized by : receiving radio waves from a mobile station and measuring a receiving quality for the radio waves from the mobile station at each base station; and transmitting a receiving quality information indicating a currently highest receiving quality obtained by the base stations and a transmission signal representing the radio waves from the mobile station received at the highest receiving level, the receiving level information and the transmission signal being updated at each base station (622-628) such that the receiving quality information received by the central control station (621) is the highest receiving quality among the receiving qualities obtained by all the base stations (622-628). The method of claim 1, wherein the receiving quality includes at least one of a receiving level, a number of detected errors, an eye pattern opening, and an S/N ratio. The method of claim 1 or 2, wherein at each base station, the receiving quality information is updated by transmitting a larger one of the receiving quality measured at said each base station and the currently highest receiving quality indicated in the receiving quality information transmitted from one of neighboring base stations through the inter-station transmission line as an updated receiving level information to another one of the neighboring base stations through the inter-station transmission line. The method of claim 1, 2 or 3, wherein at each base station, the transmission signal is updated when the receiving quality information is updated by replacing the transmission signal transmitted from one of neighboring base stations through the inter-station transmission line with a transmission signal representing the radio waves from the mobile station received at the currently highest receiving level indicated by an updated receiving quality information. The method of any one of claims 1 to 4, wherein the receiving quality information and the transmission signal are transmitted at a signal transmission timing determined according to a radio propagation delay difference due to a difference of a distance between the mobile station and each of the base stations and a signal transmission delay between neighboring base stations.

Inventors: HATA MASAHARU, SAKAMOTO MASAYUKI, HATA, MASAHARU, SAKAMOTO, MASAYUKI
Applicants: NIPPON TELEGRAPH & TELEPHONE, NIPPON TELEGRAPH AND TELEPHONE CORPORATION, NTT MOBILE COMMUNICATIONS NETWORK INC.
Application year: 1991
Main classification: H04B7
IPC codes: H04W36, H04W92