WO1998000995A1

WO1998000995A1 - Congestion relief for cellular radio communication systems

Info

Publication number: WO1998000995A1
Application number: PCT/EP1997/002911
Authority: WO
Inventors: Graham Eastwick
Original assignee: Motorola Limited
Priority date: 1996-06-27
Filing date: 1997-06-05
Publication date: 1998-01-08
Also published as: GB9613530D0; AU3032297A; GB2314732A

Abstract

A method of reducing traffic congestion in cells of a cellular radio communication system in which the handover margin to be satisfied to enable handover of a call from one base station to another to be initiated is reduced, so facilitating the handover of mobile stations in the periphery of a congested cell to neighbouring cells so relieving the congestion in the congested cell.

Description

Congestion Relief for Cellular Radio Communication

Systems

Field of the Invention

The present invention relates to methods for reducing traffic congestion in cells of cellular radio communication systems .

Background of the Invention

In principle, cellular radio communication systems consist of a number of base stations in an area delineated by the ranges of transceivers forming part of the base stations . The area covered by each base station of the system constitutes a cell of the system. Mobile stations are dispersed throughout each cell of the system and they have to be linked to one or other of the base stations as they move from one cell to another. The base stations radiate beacon signals of a standard strength and mobile stations monitor the relative strengths of beacon signals received from a number of base stations in their vicinity. This data is relayed to the base station to which they are linked at any given moment or a control centre which then determines which base station beacon signal exceeds the others by a pre- determined amount, known as the handover margin and initiates the transfer of the mobile station to that base station. Alternatively, the mobile station may carry out the base station selection process .

Ideally, to make the most efficient use of the system, each cell of the system should carry the same amount of traffic. Of course, in practice this does not occur because mobile stations enter or leave given cells and the number of calls made to and from each mobile station varies continuously. As a result, the traffic channels of one base station may be saturated while neighbouring base stations have traffic capacity unused. Various ways to overcome this problem have been proposed. For example, in a paper entitled "Traffic Sharing Scheme for Distributed Dynamic Channel

Allocation" by Matsuma Serizawa et al, published in Mobile and Personal Communications 13-15 pp 131-135 Dec. 93 there is disclosed a method of diverting overload traffic in a cellular radio communication system by reducing the power of the beacon signals generated by busy base stations in the system. As mobile stations select base stations on the basis of the received strength of beacon signals, fewer mobile stations select busy base stations, so relieving the load upon them. In effect, the size of a cell varies according to the traffic within it. However, the traffic level is maintained near the saturation level and also, because a given busy base station covers a smaller area, the problem of traffic overload could well be exacerbated rather than relieved.

Summary of the Invention

The present invention provides a method of reducing traffic congestion within a cell of a cellular radio communication system, comprising the operations of determining when congestion of the traffic channels associated with a base station of a cell of a cellular radio- communication system occurs, selecting which of the base stations of cells neighbouring that in which traffic congestion occurs are appropriate for the handover of mobile stations from the congested base station and effecting the handover of the mobile stations from the congested base station to selected neighbouring base stations which have unused traffic channels. Preferably, the appropriate base stations for handover are selected by reducing the handover margin from its normal value to a reduced value which is sufficient to allow handover of mobile stations on the periphery of the congested cell to other base stations, but insufficient to allow handover to base stations which are at a distance from the congested cell such that communication between them and the to be handed over mobile stations would be unsatisfactory.

A suitable criterion for determining when a base station is becoming congested is the number of free traffic channels remaining at the base station for example, 1.

Alternatively, the criterion of congestion may be related to the number of requests for traffic channels queued at the base station.

The initial handover margin may be 5dB and a suitable lower value may be 3dB.

Unlike the method of relieving traffic congestion referred to above, the beacon signals transmitted by the base stations, and hence the sizes and distribution of the cells of the system, are unchanged, allowing a more constant coverage to be maintained with a consequent rapid response to changing traffic conditions within the cells of the system.

Brief Description of the Drawings

The invention will now be described and explained, by way of example, with reference to the accompanying drawings, in which Figure 1 is a schematic representation of an arrangement forming part of a cellular radio communication system, and

Figure 2 is a flow diagram of a traffic congestion relief process embodying the invention.

Description of a Preferred Embodiment

Referring to Figure 1, a portion of a cellular radio communication system comprises a number of abutting regions of space 101 each of which surrounds a fixed transceiver unit which constitutes a base station 102 of the cellular communication system. Each of the regions of space 101 and associated base station 102 constitutes a cell 103 of the radio communication system, the size of each cell 103 being determined by the power of beacon signals transmitted by its base station 102. A group of cells 103 form a cluster which are under the overall control of a mobile switching centre 104. Within each cell 103 and free to move at will from one cell 103 to another are a number of mobile stations 105.

Each base station 102 has associated with it a fixed number of traffic channels by means of which the mobile stations 105 can be linked to it, and via the mobile switching centre 104 and the base stations 102 of other cells 103, to mobile stations 105 in the cells 103 associated with those other base stations 102. As explained previously, in addition to communication signals relayed from one mobile station 105 to another, each base station 102 radiates beacon signals which are used to determine to which base station 102 a mobile station 105 shall be linked as it moves about within the region covered by the cellular radio communication system. As already explained, the mobile stations 105 are handed over from one base station 102 to another when the strength of the beacon signal received from the second base station 102 exceeds that received from the first base station by a predetermined amount, usually 5dB although the value of this parameter can be chosen in the range -24dB to 31dB.

As each base station 102 has only a limited number of communication, or traffic, channels associated with it, at times more mobile stations 105 may be trying to establish a link with a given base station 102 than there are traffic channels available. When this happens, channel requests are queued until a channel becomes available. A traffic congestion parameter can be established based either on the number of free traffic channels available at the base station, or on the number of traffic channel requests in the queue . In either case, when the traffic congestion criterion is exceeded, the base station 102 control centre reduces the handover margin for mobile stations 105 in the congested cell 103 so as to facilitate the handover of mobile stations 105 on the periphery of the congested cell 103 to base stations 102 in other cells 103 to which they would not normally be handed over because the remoteness of these base stations 103 causes the strengths of their beacon signals, as received by mobile stations 105 in the congested cell 103, to be below the normal handover margin criteria, and effects handover of the peripheral mobile stations 105 in the congested cell 103 to base stations 102 in neighbouring cells 103 which have free traffic channels.

The situation is monitored continuously and when the original base station 102 is no longer considered to be congested, the original handover margin is restored and any appropriate mobile stations 105 are handed over to the original base station 102.

Figure 2 is a flow chart of the decision process involved. Referring to Figure 2, an incoming request 201 for a traffic channel is made to a base station 102 of a cell

103, a decision 202 is made as to whether the base station

102 is or is not congested. If it is not, normal operation procedure 203 is instituted by the mobile service centre 104.

If the base station 102 is considered to be congested, the mobile service centre 104 scans the base stations 102 surrounding the congested base station 102 to locate any with free traffic channels 204 if any are found, then the lower handover margin is substituted for the normal one 205 and the normal handover procedure 206 is carried out between mobile stations 105 in the congested cell 103 and appropriate base stations 102 in uncongested cells 103 neighbouring that in which the congested base station occurs.

When traffic congestion in the first cell 103 no longer exists, the system reverts to normal operating procedure.

In practice, because of the time it takes for a mobile station to synchronise with, and report on, the situation of the base stations 102 of neighbouring cells 103, existing calls are handed over rather than new requests for traffic channels .

Claims

1. A method of reducing traffic congestion within a cell of a cellular radio communication system, comprising the operations of determining when congestion of the traffic channels associated with a base station of a cell of a cellular radio-communication system occurs, selecting which of the base stations of cells neighbouring that in which traffic congestion occurs are appropriate for the handover of mobile stations from the congested base station and effecting the handover of the mobile stations from the congested base station to selected neighbouring base stations which have unused traffic channels.

2. A method according to Claim 1 wherein a base station is deemed to be congested if less than a specified number of free traffic channels exist at the base station.

3. A method according to Claim 1 wherein a base station is deemed to be congested if more than a specified number of traffic channel requests are queued at the base station concerned.

4. A method according to any of Claims 1 to 3 wherein the appropriate base stations are selected by determining which of the base stations neighbouring the base station concerned satisfy a reduced handover margin, as hereinbefore described, criterion.

5. A method according to Claim 4 wherein the reduced handover margin criterion constitutes a received beacon signal, as hereinbefore described, strength differential of 2dB compared with the normal value.

6. A method according to any of Claims 3 to 5 wherein sufficient handover procedures are carried out to reduce the queue of traffic channel requests to zero.

7. A method according to Claim 4 or Claim 5 wherein handover procedures are carried to every base station which has unused traffic channels and satisfies the reduced handover margin criterion .

8. A method according to any of Claims 1 to 7 wherein established calls are handed over to the selected base stations in preference to new requests tor traffic channels.

9. A method of reducing traffic congestion in cells of a cellular radio communication system substantially as hereinbefore described and with reference to the accompanying drawings .