1. Field of the Invention
The present invention relates to a method of selecting one among many radio zones, in a mobile radio communication system, according to the performance of the zones.
2. Description of the Related Art
FIG. 1 roughly shows a mobile radio communication system. The system includes a mobile network 1. The mobile network 1 includes a radio exchange 2 and is connected to a public network 6.
The exchange 2 manages mobile terminals 5. The exchange 2 handles connection requests from a base station controller 3 as well as calls from calling parties for the mobile terminals 5.
The base station controller 3 controls a base station 4, relays data between the exchange 2 and the base station 4, allocates channels, and measures traffic (channel usage ratio).
The base station 4 communicates with the mobile terminals 5 through radio channels and monitors the quality of the radio channels. The base station 4 forms a radio zone (sector) to transmit radio signals between the base station 4 and the mobile terminals 5 that are used by subscribers.
The mobile radio communication system employs the following techniques:
(a) Channel selection logic
Each mobile terminal 5 monitors, while being in a waiting state, the perch channels of peripheral radio zones according to reports sent from the peripheral radio zones and measures the reception levels of signals transmitted from the peripheral radio zones, to detect a movement of the mobile terminal 5 between radio zones. The mobile terminal 5 sends a report on the reception levels of signals transmitted from the peripheral radio zones when it issues a call request or a reception acknowledgement.
In response to the call request or reception acknowledgement from the mobile terminal 5, the exchange 2 usually selects the radio zone where the mobile terminal 5 exists and finds a channel for the call in the selected radio zone. If the zone in question has no available channel, the exchange 2 finds another radio zone having an available channel according to the reception levels of the peripheral radio zones reported by the mobile terminal. The details of this are described in a document 1 "Digital Automobile Telephone System Standards (R. CR-STD)."
While talking, the mobile terminal 5 detects a move between radio zones by measuring the reception levels of signals transmitted from peripheral radio zones with the use of available slots. The mobile terminal 5 receives reports containing perch channel information from the peripheral radio zones, transmits acknowledgements of the reports, and starts to detect reception levels (Refer to the document 1).
If the channel through which the mobile terminal 5 is talking must be switched to another, the exchange 2 selects an available zone according to the reception levels of the peripheral radio zones reported by the mobile terminal 5 and secures an available channel of the selected radio zone for the mobile terminal 5.
There is selection logic called flexible reuse logic that limits selectable radio zones in the peripheral radio zones according to a reference reception level.
In the selected radio zone, an available channel is specified. The mobile terminal 5 may be a full-rate terminal that makes communication only through a full-rate channel, or a dual-rate terminal that is capable of making communication through either of a full-rate or half-rate channel. To improve the efficiency of use of channels, the exchange 2 examines the type of the mobile terminal 5 when allocating a speech channel to the mobile terminal 5.
(b) Technique of switching talking channel to another
A technique of switching a channel that is being used for speech to another will be explained.
While using a channel, the mobile terminal 5 periodically monitors the level of the channel. If the level of the channel deteriorates below a reference level, the mobile terminal 5 issues a switching request with a radio status report 2 (Refer to the document 1) to the exchange 2.
At the same time, the base station 4 monitors the quality of each channel that is being used, and if it deteriorates below a threshold, the exchange 2 autonomously switches the channel to another of the same or different radio zone.
It is possible to make a call connection through a half-rate channel and then data communication through a full-rate channel. In this case, the exchange 2 autonomously switches the half-rate channel to the full-rate channel.
When a control channel is switched to another, a corresponding speech channel must be switched accordingly.
(c) Active maintenance
When an active radio exchange or base station is subjected to maintenance or is modified, subscribers under the control of the zone of the base station must be purged therefrom. This will be explained.
The base station rejects a new call from a subscriber by radioing prohibitive information.
If there is a busy channel, the base station waits until the busy state ends or until the channel is switched to another.
Alternatively, an operator at the radio exchange may forcibly cut busy channels.
In this way, the prior art relies on the reception levels of signals transmitted from radio zones and received by a given mobile terminal when switching a zone or channel to which the mobile terminal is connected to another. Namely, the prior art assigns, to a mobile terminal, a radio zone that provides a largest reception level for the mobile terminal and selects a channel in the assigned radio zone.
If a radio zone controls many mobile terminals, the channel usage ratio of the radio zone will be high and the number of available channels of the radio zone will be small. In such a radio zone, a call connection ratio (a channel allocation success ratio) for a new call is low, if peripheral radio zones are not available.
Mobile terminals in the same radio zone have the same reception levels. If zone selection for switching is made based on only the reception levels, all of the mobile terminals in the same radio zone will select a specific radio zone. Then, even if there are peripheral radio zones that provide proper reception levels and have available channels, the terminals in the same radio zone will be switched only to channels in the specific radio zone, to cause traffic congestion in the specific radio zone.
The congested radio zone places a high load on resources such as communication frequencies and radio transmission and reception devices. To cope with this problem, a prior art averages the channel usage ratios of the peripheral radio zones. This, however, increases the number of communication frequencies and the number of radio transmission and reception devices.