GB2330484A

GB2330484A - Mobile initiated handover during periods of communication inactivity

Info

Publication number: GB2330484A
Application number: GB9721881A
Authority: GB
Inventors: Svend Frandsen
Original assignee: Motorola AS
Current assignee: Motorola Solutions Danmark AS
Priority date: 1997-10-15
Filing date: 1997-10-15
Publication date: 1999-04-21
Also published as: GB9721881D0

Abstract

In a radio communication system having a plurality of base stations, a mobile station 38 communicating to a first base station initiates handoff to a second base station during periods of communication inactivity. The operation may be performed in silent or idle periods during a call or in periods between calls. The invention is applicable to semi-duplex calls which are divided into several transactions separated by an 'over' period, or duplex calls in which Discontinuous Transmission (DTX) is employed. A Push to Talk (PTT) feature provides inactivity periods. During periods of inactivity the mobile monitors the first and second base station downlink signal quality. When a threshold is exceeded for the second base station the mobile performs a call handover. The mobile is then registered with the second base station and waits for a call to be initiated.

Description

COMMUNICATIONS SYSTEM, COMMUNICATIONS UNIT AND METHOD OF OPERATION Field of the Invention This invention relates to a radio communications system. The invention is applicable to, but not limited to, a multi-site radio communications system having a number of cells and the hand-over of communications between cells when radio units travel therebetween.

Background of the Invention In current multi-site radio communication systems, a need exists for providing a smooth and seamless hand-over of communications calls between each site in the radio communications system when radio units roam between sites.

Typically, on trunked radio or mobile cellular communications systems, a fixed base station provides the conduit for communications between mobile and portable radio communication units and other communication units within the system, or indeed, to alternative communications systems. Each base station effectively provides for communications within a particular geographic area, often termed "a cell", the coverage range being determined by its rated transmit output power and receiver sensitivity. In this manner, mobile and/or portable communications units moving within the cell can receive transmissions of sufficient power from the base station to interpret the messages being sent to the mobile or portable communications unit and reply with transmissions of sufficient power that the base station receiver 5 sensitivity can interpret the response.

Once a mobile or portable radio communications unit moves outside of the coverage range (cell) of the base station, an alternative base station has to be used for the call. Should the mobile or portable communications unit move outside of the coverage range, whilst participating in a communications call, a hand-over of the communications call needs to be performed by the radio communications system, from the base station (or cell) the mobile or portable communications unit is leaving, to the base station (or cell) that the mobile or portable communications unit is entering.

It is highly desirable for the user, and in some cases imperative, for example with emergency services calls, to continue with an on-going radio communication call after travelling out of a particular area.

When a mobile station in a digital cellular system relinquishes the current serving cell during a call, i.e. during vocoded voice transmission, the quality of the voice will degrade as a drop-out in the voice transmission will occur. The drop-outs are results of mobile station controlled hand-overs, where the current serving cell link quality degrades to a level where the mobile station is forced to charlge cell, thus dropping the current call and reconnecting on the adjacent cell. In current digital cellular communications systems, two mechanisms exist for providing such a hand-over operation.

The first mechanism is a "base station-controlled hand-over", where the base station in the cell is provided with enough information to perform the call handsover when required. The base station controlled mechanism minimises the drop-out, as a lot of preparation can be done in advance in terms of preparing the adjacent cell for hand-over and when the link quality degrades to a certain level, the base station can smoothly hand the mobile station over to the next adjacent cell. However, this approach has a number of drawbacks in terms air interface signalling required to perform the hand-over operation, thereby using a significant amount of usable bandwidth.

The second mechanism is a mobile station controlled hand-over", where the mobile station performs the call hand-over when required. With this approach the bandwidth utilisation is less of a problem, as there is less signalling information required to be transmitted. However, a problem will still exist particularly in areas of substantially equal signal levels, received at the mobile or portable units from at least two base stations, where the optimum cell/base station will continuously change. In this scenario, the mobile or portable unit may initiate a call hand-over one minute to a second cell offering better signal quality and find the next minute that the first cell is subsequently a better cell.

This invention seeks to provide a communications system and a call hand-over and registration method of operation that mitigates at least some of the above mentioned disadvantages.

Summarv of the Invention According to a first aspect of the present invention there is provided a multi-site communications system. The multi-site communications system has a plurality of base stations for communicating with a plurality of remote communication units. A first remote communications unit communicates to a first base station of the plurality of base stations and wishes to hand over the communication to a second base station of the plurality of base stations. The multi-site communications system is arranged such that the remote communications unit initiates the communication hand-over during periods of inactivity of the communication between the first remote unit and the first base station.

In a preferred embodiment of the invention, the remote communications unit monitors a signal quality of transmissions from at least one of the first base station and the second base station and initiates the communication hand-over during periods of inactivity of the communication in response to said monitoring. Furthermore, the signal quality of transmissions preferably includes at least one of: bit error rate, message error rate, received signal strength and/or voice quality and the remote communications unit initiates the call hand over based on monitoring said signal quality of transmissions in its allocated communications resource.

Advantageously, the multi-site communications system does not require that the remote communications unit is capable of scanning alternative communications resources and/or communications systems in order to initiate the call hand over. Additionally, no signalling is used to initiate the call hand-over, thereby enabling the scarce communication resource to be better optimised.

According to a second aspect of the present invention there is provided a method of hand-off of communication calls in a multi-site communications system. The multi-site communications system has a plurality of base stations for communicating with a plurality of remote communication units. The method includes the steps of communicating from a first remote communications unit to a first base station of the plurality of base stations; and initiating communications from the first remote communications unit to a second base station of the plurality of base stations during periods of inactivity of the communication.

In the preferred embodiment of the invention, the method further includes the step of monitoring by the remote communications unit of a signal quality of transmissions from at least one of the first base station and the second base station, wherein the step of initiating communications from the first remote communications unit to the second base station is performed during periods of inactivity of the communication in response to said monitoring. Preferably, the communications between the remote communications unit and the second base station is initiated substantially before the remote communications unit loses communications with the first base station.

According to a second aspect of the present invention, a mobile communications unit for operation in a radio communications system is provided. The radio communications system includes a plurality of base stations to facilitate communications from the mobile communications unit to at least one of the plurality of base stations. The mobile communications unit includes means for initiating hand over of the communication from a first base station of the plurality of base stations to a second base station of the plurality of base stations, during periods of communication inactivity between the mobile communications unit and the first base station.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the drawings.

Brief Descrintion of the Drawings FIG. 1 shows a block diagram of a multi-site radio communications system according to a preferred embodiment of the invention; FIG. 2 is a pictorial representation showing the operation of the multi-site radio communications system according to the preferred embodiment of the invention; and FIG. 3 shows a flow chart of a method of call hand-over and registration in a communications system according to a preferred embodiment of the invention.

Detailed Description of the Drawings Referring first to FIG. 1, a block diagram of a multi-site radio communications system, according to the preferred embodiment of the invention, is shown. The multi-site radio communications system 10 is preferably a trunked radio communications, but it is within the contemplation of the invention that it is equally applicable to other radio communication systems, for example, cellular communications systems. The multi-site radio communications system 10 has a master controller 11 operably coupled to both a first site controller 12 and a second site controller 14. The master controller 11, together with first site controller 12 and second site controller 14 controls communications within the multi-site radio communications system 10. The first site controller 12 is operably coupled to at least one base station, base station 24, and the second site controller 14 operably coupled to at least a second base station 26.

In this manner communications channels are allocated by the master controller 10 to a plurality of communicating units 37, 38 and 40 over a plurality of communication channels 30, 31 and 32 on a number of communication sites.

In operation, a first communications unit 37 is in communication with the first site controller 12 via the base station 24 on a communications channel 30 and a second communications unit 40 is in communication with the second site controller 14 via the base station 26 on communications channel 32. A third communications unit 38 is communicating on communications channel 31 to base station 26 but is within range of both base station 24 and base station 26. When the third communications unit 38 wishes to transfer its communication over to the base station 24, it does so during periods of communications inactivity, i.e. when it is not transmitting or receiving information to/from base station 26. In this manner, call-drop-outs are minimised.

The preferred operation of the hand-over process is further described in FIG. 2 and FIG. 3 below.

Referring now to FIG. 2, a pictorial diagram of the multi-site communications system according to the preferred embodiment of the invention is shown. Two adjacent cells 50, 52 having first and secondary boundaries are shown. The extreme (relinquishable) boundary "3" on the circumference 56, 60 of each cell 50, 52 respectively indicates the geographical region where communications (either voice and/or data communications), from the particular cell base station to a remote mobile unit, would be unusable. A signal quality vs. position representation is shown for the scenario of the mobile communications unit 38 travelling along the line A-A'. The signal quality representation shows the variation in signal quality as the mobile communications unit 38 travels through the present cell 50, as indicated by line 62, as it starts to enter the coverage range of the second cell 52, as indicated by line 64, and as it eventually leaves the effective coverage area of the first present cell 50. The improvable boundary "1", as indicated by cross-over point 66, is where the mobile communications unit 38 may well judge that the adjacent cell 52 has a better link quality than the present cell 50 and that the voice quality service can be improved if hand-over is performed. It is worth noting that the present cell does not necessarily provide a bad voice quality after the improvable boundary is crossed.

Another interesting boundary is the boundary "2" at which the mobile station crosses a threshold at which its present cell starts to degrade so much that it severely impacts the voice quality. The boundary "2" indicates to the mobile station that it shall prepare itself for hand-over very soon and before the edge of coverage is met by the relinquishable boundary "3".

The invention is applicable not least to the cases where the call is set-up as a semi-duplex call, in which case the call is effectively already call-divided into several call transactions, each of them separated with an "over" period. In this situation the involved mobile station continuously enters a hang-time and no voice traffic is exchanged.

Alternatively, the call may be set-up as a duplex call where a mechanism such as Discontinuous Transmission (DTX) may be employed. In such a situation, a Push To Talk (PTT) feature may be used to provide discontinuous transmissions, thereby providing for inactivity periods resulting in the same situation as the "over" period for a semi-duplex call.

It is within the contemplation of the invention that any access scheme employed in the multi-site communications system, such as frequency division multiple access (FDMA) or code division multiple access (CDMA) would benefit from this invention.

Referring now to FIG. 3, a method of hand-over of communications between a mobile communications unit 38 and a first base station 26 to a second base station 24 in the multi-site communications system 10 is provided. The method includes the mobile communications unit 38 communicating with the first base station 26 substantially within the coverage range of the first base station 26, as shown in step 100. During periods of communication inactivity the mobile communications unit 38 monitors the signal quality of transmissions on the downlink from the first base station 26, either to the mobile communications unit 38 or to other mobile communications units in the coverage area, as in step 102. The mobile communications unit 38 continues to do this whilst it is not in active communication with the first base station 26, as shown in step 104. Once the signal quality has degraded to a particular threshold, either pre-determined or variable, and the mobile communications unit 38 is also monitoring signal transmissions from a second base station 24, the mobile communications unit 38 monitors the signal quality of transmissions on the downlink from either the first base station 26 and/or the second base station 24, as in steps 106, 108. When the signal quality of transmissions from the second base station 24 exceeds a second threshold, for example the "over" state, as in step 110, the mobile communications unit 38 performs a call hand-over, as shown in step 114. For the period of time when the signal quality of transmissions from the second base station 24 does not exceed the second threshold, the mobile communications unit 38 continues to monitor the signal quality of transmissions on the downlink from either the first base station 26 and/or the second base station 24, as in steps 112, 108. The mobile communications unit 38 is then registered with the second base station 24 and waits for a call to be initiated, as in step 116.

In this manner, the mobile communications unit 38 initiates the hand-over of communications to a preferred base station, without impacting the signalling performance of the multi-site communications system. Although, the hand-over operation is described with reference to a mobile communications unit being between calls, it is equally applicable, and indeed desirable, that the mobile communications unit 38 can perform the same operation during a call, with the hand-over operation being performed during periods of communication inactivity, i.e. silent/quiet periods.

A yet further advantage of the aforementioned present invention occurs with systems such as the TErrestrial Trunked RAdio system (TETRA). TETRA is a system designed for inter-operability between radios of various manufacturers. As such, a number of difficulties can arise in ensuring that the system is compatible with all such manufacturers. Having the ability in the mobile communications unit to dictate when and where to hand-over to alternative base stations or communications systems eases a significant burden on the complex inter-operability functions.

Thus a multi-site communication system, communications unit for operation in such a communications system and a method of operation therefor are provided that mitigate at least some of the aforementioned disadvantages with call hand-over in mobile communications systems.

Claims

Claims 1. A multi-site communications system having a plurality of base stations for communicating with a plurality of remote communication units, wherein a first remote communications unit communicating to a first base station of the plurality of base stations wishes to hand over the communication to a second base station of the plurality of base stations, the multi-site communications system arranged such that the remote communications unit initiates the communication hand-over during periods of inactivity of the communication.
2. A multi-site communications system according to claim 1, wherein the remote communications unit monitors a signal quality of transmissions from at least one of the first base station and the second base station and initiates the communication hand-over during periods of inactivity of the communication in response to said monitoring.
3. A multi-site communications system according to claim 2, wherein the signal quality of transmissions includes at least one of: bit error rate, message error rate, received signal strength and/or voice quality and the remote communications unit initiates the call hand over based on monitoring said signal quality of transmissions in its allocated communications resource.
4. A multi-site communications system according to claims 1, 2 or 3, wherein the remote communications unit is not capable of scanning alternative communications resources and/or communications systems in order to initiate the call hand over.
5. A multi-site communications system according to any one of the preceding claims, wherein the call hand over operation is performed substantially before the remote communications unit loses communications with the first base station.
6. A multi-site communications system according to any one of the preceding claims, wherein the multi-site communications system supports semi-duplex communications for providing the periods of communication inactivity.
7. A multi-site communications system according to any one of claims 1 to 5, wherein the multi-site communications system supports duplex communications and discontinuous transmissions for use in the duplex communications thereby providing the periods of communication inactivity.
8. A method of hand-off of communication calls in a multi-site communications system having a plurality of base stations for communicating with a plurality of remote communication units, the method comprising the steps of: communicating from a first remote communications unit to a first base station of the plurality of base stations; and initiating communications from the first remote communications unit to a second base station of the plurality of base stations during periods of inactivity of the communication.
9. A method of hand-over of communication calls in a multi-site communications system according to.claim 8, further comprising the step of: monitoring by the remote communications unit of a signal quality of transmissions from at least one of the first base station and the second base station; wherein the step of initiating communications from the first remote communications unit to the second base station is performed during periods of inactivity of the communication in response to said monitoring.
10. A method of hand-over of communication calls in a multi-site communications system according to claim 9, wherein the signal quality of transmissions includes at least one of: bit error rate, message error rate, received signal strength and/or voice quality.
11. A method of hand-over of communication calls in a multi-site communications system according to any one of preceding claims 8 to 10, wherein communications between the remote communications unit and the second base station is initiated substantially before the remote communications unit loses communications with the first base station.
12. A mobile communications unit for operation in a radio communications system having a plurality of base stations to facilitate communications from the mobile communications unit to at least one of the plurality of base stations, wherein the mobile communications unit comprises means for initiating hand over of the communication from a first base station of the plurality of base stations to a second base station of the plurality of base stations, during periods of communication inactivity between the mobile communications unit and the first base station.
13. A mobile communications unit according to claim 12, wherein the remote communications unit further comprises monitoring means for monitoring a signal quality of transmissions from at least one of the first base station and the second base station and the mobile communications unit initiates the communication hand-over during periods of inactivity of the communication in response to said monitoring.
14. A mobile communications unit according to claim 12 or 13, wherein the signal quality of transmissions includes at least one of: bit error rate, message error rate, received signal strength and/or voice quality and the remote communications unit initiates the call hand over based on monitoring said signal quality of transmissions in its allocated communications resource.
15. A mobile communications unit according to claims 12, 13 or 14, wherein the remote communications unit is incapable of scanning alternative communications resources and/or communications systems in order to initiate the call hand over.
16. A multi-site communications system substantially as hereinbefore described with reference to, and/or as illustrated by, FIG. 1 or FIG. 2 of the drawings.
17. A method of hand-over of communication calls in a multi-site communications system substantially as hereinbefore described with reference to, and/or as illustrated by, FIG. 3 of the drawings.