Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
2000-02-01
2002-09-17
Urban, Edward F. (Department: 2685)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S434000, C455S501000, C455S513000
Reexamination Certificate
active
06453166
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a traffic channel selecting method and base station equipment preferable for constructing a mobile communications system including a plurality of radio base stations and mobile stations.
More particularly, the present invention relates to a traffic channel selecting method and base station equipment applicable to such systems as a mobile communications system including a plurality of radio base stations and a plurality of mobile stations that carry out communications with them, and in particular to a system whose service area consists of a plurality of radio base stations assuming various cell radii, or to a system in which a plurality of systems, whose mobile stations that receive their services have different characteristics, share the same frequency band.
BACKGROUND ART
A mobile communications system such as mobile phones or car phones which are widely available today carries out its services by dividing its service area into comparatively small radio zones referred to as cells. Such a system comprises as shown in
FIG. 1
a plurality of radio base stations
111
that cover the divided radio zones, and a plurality of mobile stations
112
that conduct communications with the base stations
111
by establishing radio channels.
Radio waves transmitted from the base stations
111
or mobile stations
112
at certain transmission power levels propagate through the space with their powers attenuated, and arrive at their destination terminals. The attenuation the ratio waves undergo increases with the distance between the transmitting terminals and the receiving terminals.
On the other hand, the receiving terminals require a specific level or over of the received power to achieve demodulation of the received radio wave at a desired quality.
Accordingly, increasing cell radii to extend the cover areas of the base stations requires to install in the base stations and mobile stations transmission equipment with greater power. In contrast, reducing the cell radii to decrease the cell areas can reduce the transmission power required by the base stations and mobile stations.
In the mobile communications, a processing what is called handover is carried out to continue communications by successively switching the radio base stations with the roaming of a mobile station. The handover processing, when considered from the standpoint of the cell radius, presents the following problem as the cell radius reduces. That is, the frequency of the handover processing rises, which in turn increases the processing load of the network. When instantaneous interruption takes place because of the handover, it has great influence on the conversation quality a user feels. It is also possible that the handover processing cannot catch up with the roaming of the mobile station, so that the conversation can be disconnected.
It will be understood from the description above that small radius cells are suitable for terminal equipment with low transmission power carried by pedestrians, whereas large radius cells are appropriate for terminal equipment with comparatively large power carried on motor vehicles. Thus, the small and large radius cells have their own advantages and disadvantages. Generally, a comparatively large cell with a radius from one kilometer to a few kilometers is often called a macrocell, and a rather small cell with a radius of a few hundred meters is commonly called a microcell.
Incidentally, users are not uniformly distributed geographically, but concentrate to limited places or roads. In view of this, such a method is taken that the macrocells roughly cover the area, and the microcells cover the places and roads in the area where the users concentrate.
Such a method that forms the area with the macrocells in combination with microcells is referred to as a micro/macro overlay system in the present specification.
The micro/macro overlay system is characterized in its establishing method of the radio channels. In the macrocells, the network conventionally carries out centralized management of the entire channels employed by the base stations.
In contrast with this, since the microcells are located in various propagation environments such as outdoors, underground shopping center and buildings in front of a station or in bustling streets as well as in homes, it is very difficult to assign radio channels considering the interference between the microcells. Since the systems in homes or offices operate as individual systems, it is difficult to apply to them a centralized management style assignment method. Thus, a method is desired in which the individual base stations establish their radio channels through their own autonomous control.
Conventionally, as a method for making a decision as to whether a particular channel is available or not, a radio base station or mobile station measures the received power of interference signals, and decides that the channel is available when the received power (interference power) is less than a fixed level.
However, applying the conventional method to the micro/macro overlay system, in which the microcells and macrocells share a common radio frequency band, has a great problem of being unable to avoid the interference between them.
FIG. 2
shows an example of making a decision as to whether a radio channel is available in a microcell adjacent to a macrocell. In
FIG. 2
, the horizontal axis represents distance, and the vertical axis represents transmission power or received power. Base stations A and C are low power base stations forming a microcell each. The base station A transmits at transmission power Pa through a radio channel f
1
. The base station B is a high power base station forming a macrocell, and transmits at transmission power Pb using a radio channel f
2
.
In
FIG. 2
, the base station C is newly installed in addition to the base stations A and B, assuming that the base station C autonomously determines the radio channel of its own use. As shown in
FIG. 2
, when the base station C receives signals from both the base stations A and B, the received levels of the signals at the base station C are identical at level L. If the level L is lower than the threshold value THR, both the frequencies f
1
and f
2
are available.
Let us consider here the interference given to the base station C from the mobile stations a and b that carry out communications with the base stations A and B, respectively. When the base station C selects the radio channel f
1
, it receives the interference from the mobile station a with low transmission power, which does not presents any problem. However, when the base station C selects the radio channel f
2
, it will receive the interference from the mobile station b with high transmission power. Thus, the base station C cannot use the radio channel f
2
in practice.
Thus, in the conventional decision making as to whether the channel is available, the interference between the microcell and macrocell is unavoidable.
In view of this, therefore, a first object of the present invention is to propose a traffic channel selecting method and station equipment capable of avoiding in advance the interference from a mobile station transmitting at large power by making a decision as to whether the radio base station causing the interference is present in a macrocell or microcell.
A second object of the present invention is to propose a traffic channel selecting method and base station equipment capable of avoiding the occurrence of the interference, and constructing a mobile communications system with high quality and high frequency usage efficiency in a system whose service area consists of a plurality of radio base stations assuming various cell radii, or in a system in which a plurality of systems, whose mobile stations that receive their services have different characteristics, share the same frequency band.
DISCLOSURE OF THE INVENTION
In the first aspect of the present invention, there is provided a traffic channel selecting method in a mobile communications system including a plurality
Hata Masafumi
Ishii Minami
Ishikawa Yoshihiro
Nakano Etsuhiro
Onoe Seizo
Brown Raysman Millstein Felder & Steiner
Davis Temica M.
NTT Mobile Communications Network Inc.
Urban Edward F.
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