Multiplex communications – Communication over free space – Combining or distributing information via time channels
Reexamination Certificate
1998-08-24
2002-11-12
Kizou, Hassan (Department: 2662)
Multiplex communications
Communication over free space
Combining or distributing information via time channels
C370S310000, C370S324000, C370S328000, C370S498000, C370S503000, C370S507000, C375S356000
Reexamination Certificate
active
06480483
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a mobile communication system for performing wireless communication by connecting a base station and a mobile station to each other using a TDD (Time Division Duplex) system such as a digital personal handyphone system and a simplified type personal handyphone system. More particularly, the present invention relates to a system for effectively utilizing wireless frequency by improving an inter-base-station frame synchronous system.
BACKGROUND ART
Recently, a simplified type personal handyphone system (hereinafter, briefly called a PHS) using a micro cell has been developed as one type of digital mobile communication systems. This simplified type personal handyphone system will be practically used in the near future.
FIG. 1
is a view showing the basic construction of a conventional PHS. Plural PHS base stations CS
1
to CSm are dispersively arranged in a service area. Wireless zones Z
1
to Zm called cells each having a radius from 100 to 500 meters are formed by these PHS base stations CS
1
to CSm. Each of the PHS base stations CS
1
to CSm is connected to an integrated service digital network (hereinafter, briefly called an ISDN) INW having a PHS connecting device PM.
Each of mobile stations PS
1
to PSn is selectively connected to each of the PHS base stations CS
1
to CSm through a wireless channel within the wireless zones Z
1
to Zm formed by the above PHS base stations CS
1
to CSm. Each of the mobile stations PS
1
to PSn is connected to each of wire telephones TEL
1
to TELk through the above ISDN or this ISDN and a subscription phone network SNW from these PHS base stations CS
1
to CSm. Direct communication can be also performed between the mobile stations PS
1
to PS
2
by performing connecting control in the PHS base stations CS
1
.
The PHS has a control center CC having a database, a customer information management database, etc. Information relative to the above mobile stations PS
1
to PSn and the PHS base stations CS
1
to CSm are collected into this control center CC through the ISDN and a packet network PNW. Service management and control such as authentication, accounting, network management, etc. are performed on the basis of this information.
The PHS adopts a time division multiple access (TDMA) system as an access system between the PHS base stations CS
1
to CSm and the mobile stations PS
1
to PSn. Further, the PHS adopts a time division bidirectional multiplex (TDD: Time Division Duplex) system as a transmission system.
For example, as shown in
FIG. 2
, a frame of the TDMA-TDD system is constructed by a forward link and a reverse link. In the forward link, four transmitting slots T
1
to T
4
are time-divided and multiplexed. In the reverse link, four receiving slots R
1
to R
4
are time-divided and multiplexed. One frame length is set to 5 ms and a transmission speed in transmission and reception is set to 384 kbps. One slot length is set to about 625 &mgr;sec and a transmission speed of information per one slot is set to 32 kbps except for an error correction code, etc.
In calling-out of each of the mobile stations PS
1
to PSn, each of the PHS base stations CS
1
to CSm transmits timing information of the above TDMA-TDD frame to each of the mobile stations existing within the wireless zones Z
1
to Zm of the PHS base stations CS
1
to CSm, and synchronizes transmission/reception timing of each of the mobile stations with transmission/reception timing of its own station. In this state, an idle slot within the frame and an idle wireless frequency are allocated to each of the mobile stations as a channel for communication. Thereafter, wireless communication is performed between each PHS base station and each mobile station by using this channel for communication. Accordingly, with respect to each of the PHS base stations CS
1
to CSm, the wireless communication can be performed such that the plural mobile stations located within the wireless zones Z
1
to Zm of the PHS base stations CS
1
to CSm are operated in synchronization with each other and do not come into collision with each other.
However, in the PHS, each of the PHS base stations CS
1
to CSm generally generates the TDMA frame independently and performs the communication. Accordingly, no synchronization of the TDMA frame is performed between the PHS base stations CS
1
to CSm. Therefore, there is a case in which an interference is caused between the plural base stations of which the wireless zones are adjacent to each other. For example, a transmitting signal of the transmitting slot T
1
of an adjacent base station CSa is received in the receiving slot R
4
of a base station CSb between the base stations CSa and CSb performing communication in timing shown in FIG.
3
. Therefore, an interference is caused in the slot R
4
of the base station CSb. To avoid this interference, another slot must be used instead of the slot causing the interference, or a wireless frequency used in the slot causing the above interference must be changed to another frequency.
However, the number of slots of one frame is set to only four in each of transmission and reception. Therefore, interference cannot be avoided by changing the above slots in many cases so that the frequency change must be used. Namely, since no frames are synchronized with each other between the PHS base stations, the channels for communication which were originally usable, become unusable. As a result, utilization efficiency of the communication channel is reduced. This reduction is not very preferable since this reduction causes an increase in lost-call rate when the number of subscribers is increased.
Therefore, there are conventionally several proposals for synchronizing frames with each other between the PHS base stations. Frame synchronous systems provided by these proposals are first classified into a fixed connecting system and an autonomous connecting system in view of synchronous connection.
(A) Fixed Connecting System
This system is a system for predetermining from which base station a certain base station obtains a timing signal for synchronization. Since a position of each base station is known, a propagation delay time between the base stations can be approximately calculated so that the delay time can be simply corrected. However, different data must be inputted and set to each base station. Further, when a new base station is arranged and an arranging position of the base station is changed, a drawback exists in that it is necessary to re-input and reset data for every change.
(B) Autonomous Connecting System
This system is a system for selecting a base station obtaining a timing signal for synchronization on the basis of a certain constant rule. For example, four base stations are selected in the order of larger receiving levels and its average timing is selected. In this system, it is not necessary to input different data for every base station and it is possible to easily cope with situations in which a new base station is arranged and its arranging position is changed. However, it is necessary to repeatedly make calculations for determining timing until the synchronization of an entire system is converged. Further, it is difficult to correctly know a delay time.
Next, the frame synchronous systems are divided into a mutual synchronous system and a master-slave synchronous or master-slave and mutual hybrid synchronous system in view of a synchronous hierarchy.
(1) Mutual Synchronous System
This system is a system for determining frame timing of its own station based on a receiving signal from a mutually adjacent station without arranging a master station as a synchronous source. In this system, problems exist in that it takes much time to converge synchronization since the base stations are mutually influenced, and a converging state is influenced by a delay time.
(2) Master-slave Synchronous or Master-slave and Mutual Hybrid Synchronous System
This system is a system in which a master station as a synchronous source is arranged and all other base stations (slave stations
Mishima Katsuhiko
Tsujimura Satoru
Yahata Haruki
Banner & Witcoff , Ltd.
Kabushiki Kaisha Toshiba
Kizou Hassan
Logsdon Joe
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