1. Field of the Invention
The present invention relates to a mobile radio communication system such as a portable radio telephone system or a vehicle telephone system and, more particularly, to a communication system based on a multichannel access scheme.
2. Description of the Related Art
As a system of this type, for example, a cellular portable radio telephone system is available. This system will be described in detail later in comparison with the embodiments of the present invention.
In a system of this type (i.e., a cellular portable radio telephone system), the most important subject is associated with the office service capacity. In a conventional cellular portable radio telephone system, the transmission output of each of base stations BS1 to BSn is set to be as large as several watts to ensure a relatively large wide radio communication zone range. For this reason, it is difficult to take full advantage of the technique of so-called frequency reuse, i.e., a concurrent use of the same frequency within the service area of the system. Therefore, the office service capacity of the system must be set to be relatively small. That is, the utilization efficiency of a radio frequency is low.
Under the circumstances, in recent years, the following two schemes have mainly been studied to increase the office service capacity of a system. In one scheme, the transmission output of each base station is set to be small to increase the number of radio zones. According to this scheme, since the reuse of a radio frequency can be satisfactorily performed, the office service capacity can be increased. In this scheme, however, since the number of radio zones is increased, a large number of base stations must be installed. As a result, the size and cost of the system itself are greatly increased.
In the other scheme, both a control signal and a speech signal are converted into digital signals, and the digital communication speech signal is time-divisionally multiplexed to be transmitted by the TDMA (Time Division Multiple access) method. According to this scheme, the radio frequency is frame-formatted, and the frame is divided into, e.g., six time slots so that these time slots are respectively used as speech channels. If this scheme is employed, the office service capacity can be increased six times that of a conventional system.
In a conventional system using the above-described scheme, however, a control station CS collectively manages not only each radio frequency but also the time slots of each radio frequency. For this reason, the following problems (1) to (3) are posed.
(1) Since a large number of speech channels including the respective time slots are collectively and concentrically managed by the control channel CS, the control station is required to have a large processing capacity.
(2) With an increase in number of base stations or changes of the installation positions of base stations, the management data of each base station in the control station CS must be updated. For this reason, it is not easy to increase the number of base stations or change the installation positions of base stations. That is, flexibility with respect to changes in the system is low.
(3) Even if a small- or medium-sized system having relatively small numbers of base stations and mobile stations is to be constructed, a large control station is required, thus posing a problem in terms of economy.