GB2381699A

GB2381699A - Alternative mode monitoring by a transceiver which switches to half rate traffic channels when it monitors other communication modes

Info

Publication number: GB2381699A
Application number: GB0126046A
Authority: GB
Inventors: Russell John Haines
Original assignee: Toshiba Research Europe Ltd
Current assignee: Toshiba Europe Ltd
Priority date: 2001-10-30
Filing date: 2001-10-30
Publication date: 2003-05-07
Anticipated expiration: 2021-10-30
Also published as: GB0126046D0; GB2381699B

Abstract

The present invention provides a means for alternative mode monitoring within the framework of the existing GSM standard. The invention provides for switching communication from full-rate communication to half-rate communication during an active call. By doing so, a time slot in each frame is shared with another subscriber. In this way alternative frames are allocated to each subscriber. The invention utilises the fact that alternate frames are not used by a subscriber to switch to monitoring other modes during this interval. Consequently the transceiver/terminal switches to half rate traffic channels when it monitors other communication modes.

Description

ALTERNATIVE MODE MONITORING This invention relates to a wireless communication system which can operate in multimode.

Current mobile communications such as cellular telephone systems generally operate in single mode, i. e. they only operate using a single communication protocol or standard, over a single radio access technology. An example of such a system is the widely used GSM (Global System for Mobile communications) system. Single mode terminals are only able to operate on a single system, e. g. the GSM system, even if there are other systems available.

With the development of new systems it is intended that terminals (i. e. the descendants of today's mobile phone) will be multi-mode. This means that such terminals will not be restricted to operation in a single mode but will be able to offer the user service on one or more other modes, such as UMTS (Universal Mobile Telecommunications System), HiperLAN-2 or IEEE802. 11. Thus. a user of a terminal may be operating in GSM mode but where another service is available e. g. UMTS, the terminal will be able to reconfigure itself to operate using that service if needed. In this way, the terminal provides improved functionality by allowing the ability to select which system it is using according to availability, bandwidth requirement, cost and so on. In addition, in areas where there is more than one mode available, load can be spread between systems but in areas where there are only one system available, the ability to use more than one mode allows greater coverage to be provided.

However, in order to provide multi-mode functionality, it is necessary to monitor the availability of other systems and frequencies beyond the current operational mode.

When the terminal is idle, i. e. in standby mode, the terminal does not need to be communicating on one system for a significant portion of time. Thus contact may be

maintained with more than one system relatively easily by switching between systems consecutively.

However, when the terminal is engaged in an active operation (e. g. during a call) resources are at a premium and it becomes difficult to find an appropriate opportunity to switch to monitoring an alternative mode without disrupting the active call. The problem is particularly acute where the call in progress is a voice call where interruption of the transfer of data on the active call can lead to disruption of the transmitted voice signal which is clearly disadvantageous for the users of the communications link.

New systems that are being introduced are able to be designed with alternative mode monitoring considerations in mind. In this way, provision can be made to provide periods of inactivity to enable the terminal to monitor alternative modes while still maintaining the flow of user data for the active call. However, such provision was not provided in older systems. When these systems were defined and the standards were determined, there was no perceived need for alternative mode monitoring and so no provision was made. GSM is a good example of a well established system that has a well defined standard and which is extensively deployed around the world but which has no provision for alternative mode monitoring.

GSM does provide opportunities whilst in a call for monitoring neighbouring cells within the GSM system. Current proposals within the 3GPP systems are that these opportunities should be used for supporting the monitoring of UMTS systems for providing dual mode operation. However, this method of alternative mode monitoring is at the expense of internal GSM monitoring. The time that would be allocated to monitoring UMTS would deprive the terminal of the opportunity of monitoring neighbouring cells within the GSM system. Therefore whilst this may provide a way of implementing alternative mode monitoring, this would be at the expense of reliable monitoring of the primary (i. e. GSM) system potentially leading to poorer service quality within the primary system. Furthermore, the time available for these opportunities is very limited. Therefore, whilst there may be just sufficient time to monitor UMTS systems there may not be sufficient time to monitor more esoteric or

complex systems that are envisaged for the future. Therefore this method only provides a limited solution and at the expense of internal GSM monitoring.

One solution is to provide a completely separate receiver in the terminal for monitoring the alternative mode independently of the receiver monitoring the main mode.

However, such repetition of parts in a terminal is undesirable because it leads to increased size and cost as well as reduced power efficiency. In addition, a separate receiver would be needed for each mode (unless one of the receivers was reconfigurable for each different mode to be monitored). It is much more efficient to have a single receiver which is capable of operating in a group of different modes or even reconfigurable to operate in any number of different systems.

Therefore according to the present invention there is provided a terminal for a communication network comprising: a transceiver capable of communication with a base station using a first mode; control means for controlling the transceiver to operate using one of at least full-rate traffic channels and half-rate traffic channels, wherein said control means is arranged to control the transceiver to operate with half-rate traffic channels for monitoring one or more other communication modes.

The present invention provides a means for allowing alternative mode monitoring within the existing framework of the GSM protocol/standard. In addition, the invention provides the ability to allow alternative mode monitoring in other non-GSM systems which use TDMA. By allowing substantial periods of time for alternative mode monitoring within the framework of the existing GSM standard, integration of new multi-mode systems is made possible in a practical manner. This avoids the need for complicated dual receiver terminals or poor service due to interruptions caused by alternative mode monitoring at the expense of other GSM functionality.

The present invention makes use of the already implemented half-rate traffic channel functionality. This allows normal traffic to be fitted into half the normal number of traffic time slots. This normally allows the other half to be used for a second subscriber who shares the TDMA time slot. However, whilst operating using half-rate traffic channels, a terminal is only actively communication during half (or thereabouts) the

available time slots. In the intervening periods, the terminal is effectively idle. It is this idle period which is utilised in the present invention to allow the terminal to be configured to operate in accordance with a different mode to monitor that mode to determine availability and quality of service, etc. without interfering with normal GSM operation.

The terminal can be switched to half-rate mode whenever alternative mode monitoring is required. This may be: all the time, only when sufficiently high quality of speech channel can be maintained on the GSM channel, at selected intervals determined by the base station or by the terminal; on demand by the user; when service quality or availability requires alternative modes; and so on. For regular monitoring, it may be desirable to schedule the periods of monitoring under the control of the base station to minimise the control traffic needed to negotiate a changeover from full to half rate mode operation.

The present invention further provides a method of controlling a terminal for use in a communications network operating in a first mode comprising: switching the terminal from operating using full-rate traffic channels to using half-rate traffic channels; controlling the terminal to operate using a second different mode; and monitoring the second mode.

The present invention also provides a method for monitoring alternative modes in a terminal for a first mode capable of operating using full-rate traffic channels and second, lower rate traffic channels, comprising: switching to communicating using the second lower rate traffic channels when monitoring of an alternative communication mode is required; consecutively switching the terminal between monitoring the first mode and an alternative mode to be monitored such that monitoring of the alternative mode occurs during intervals when no communication is being carried out in the first mode.

A specific embodiment of the present invention will now be described by way of example with reference to the accompanying drawings in which:

Figure 1A shows the layout of a multiframe in the GSM system; Figure 1B shows the structure of a frame of the GSM system ; Figure 2A shows the structure of a multiframe using half-rate communication in the GSM system; Figure 2A shows the structure of a frame using half-rate GSM; Figure 2B shows the structure of a different frame using half-rate GSM; Figure 3 shows a sequence of frames of a system operating in half-rate mode ; and Figure 4 shows the timing for the mode monitoring operation.

The following description relates to a terminal operating using the GSM system (including sibling systems like PCS-1900 and DCS-1800) although it is not intended that the present invention should be limited to such a system. The operation of the GSM system is well known in the art and so the entirety of the system is not described in detail here.

In the GSM system, the available frequency spectrum is divided into two bands of 25 MHz. One band is used for communications from a terminal to a base station whilst the other band is used for communications from a base station to a terminal. The available forward and reverse frequency bands are divided into 200 kHz wide channels identified by the absolute radio frequency channel number (ARFCN). This theoretically provides 125 channels although a guard band is typically provided at each end of the GSM spectrum.

Each of these channels is shared between up to eight subscribers using TDMA. Each of these eight subscribers uses the same frequency but occupies their own unique slot within a frame. Each of the other subscribers are allocated a different time slot within each frame such that each frame includes a total of eight time slots. Again, in practice, not all the time slots will be allocated to subscriber traffic but may be allocated to other uses such as for control and management of the system.

Figure 1A shows the arrangement of frames in the GSM system. Each group of twentysix consecutive TDMA frames is called a multiframe. Included in each of these multiframes are two frames, the thirteenth and twenty-sixth frame which are provided

for Slow Associated Control Channel (SACCH) data although they are not always used.

The remaining twenty-four frames contain Traffic Channel (TCH) data, i. e. carrying digitally encoded speech or user data. Figure IB shows the arrangement of a single frame. From this figure, it can be seen that each frame is divided up into eight time slots shown as TSO-TS7. Each of these time slots corresponds to one TDMA channel.

In the above described arrangement, 125 frequency channels are available, with each comprising a maximum of eight time slots. This limits the total number of traffic channels to 1000. In practice, this may be less due to the provision of guard bands around the available spectrum and the use of traffic channels for carrying control and management data. Thus in areas where there are a large number of subscribers, it is possible that the available capacity may be exceeded.

GSM provides for a system where speech can be coded into a data rate which is half of the usual full-rate speech channel of 22. 8 kbps. By using this reduced bit rate voice encoding system, the speech signal only requires half as much of the available capacity.

To make use of this, GSM provides a half-rate speech protocol. In such a protocol, the single time slot which would be allocated to one user under normal full rate channel coding is shared between two users. This is achieved by transferring data from a single subscriber in alternate frames.

As shown in Figure 2A, the frames T carry the data for one subscriber in a time slot.

Correspondingly, the frames t carry data for a different subscriber in the same time slot, the data for both subscribers being carried at half-rate. Figure 3 shows schematically how the frames of a given multiframe are arranged. As can be seen in Figure 3, subscribers X and Y have been allocated time slot 2. Whereas subscribers A and B have been allocated time slot 3. In frame TO data from subscribers X and A are sent whereas in the subsequent frame, tl, the data from subscribers Y and B is transmitted.

This continues in alternating frames throughout the multiframe except where the SACCH channels (S and s) are provided in the thirteenth and twenty-sixth slots.

This arrangement allows more subscribers to be fitted into the same available bandwidth. However, the reduced data rate does result in a slightly reduced quality of

speech being transmitted. It also reduces the amount of data being transmitted in a nonspeech call.

In the present invention, operation using a half-rate traffic channel is used to provide an opportunity to carry out alternative mode monitoring. During half-rate mode operation, only one channel is communicating during any one time slot. This provides the opportunity for a period between two frames when a terminal is transmitting where the channel is not being used by the terminal. Thus, in the example shown in Figure 3, as the two subscribers X and Y are sharing a time slot, a time slot is still occupied by only one subscriber e. g. X. Consequently when subscriber Y is using the time slot, subscriber X's terminal is not using the channel and is effectively idle.

Each frame is 4.615ms long. This means that subscriber X is only communicating with a base station for 4. 615ms out of every 9.230ms, i. e. every other frame (other than where a SACCH and a TCH are adjacent). During the 4.615ms interval when the mobile terminal is not involved in communication with the base station, it can switch to an alternative mode and monitor the availability, quality, etc. of any alternative modes during that period. The terminal can then reconfigure itself to operate according to the GSM standard ready for the next 4.615ms period to continue communication with a base station. Figure 4 shows how the terminal is actively transmitting during the same time slot in each frame in full-rate mode. Figure 4 also shows that in half-rate mode, a

terminal is only actively transmitting during every other frame and so the intervening CD frames are available for mode monitoring.

The terminal is only transmitting for 1/8 of a frame (i. e. in one time slot out of eight) and is only receiving for 1/8 of the frame (in a different time slot of a frame on the receive channel). During the rest of the time for each frame, the terminal is occupied carrying out other internal tasks. These include assembling the burst to be transmitted,

tuning the frequency synthesiser, transmitting a burst (for 577pus), retuning the frequency synthesiser, receiving a burst (for 577rus) and decoding the received burst.

An additional benefit of this system is that when there is an adjacent SACCH/TCH combination, once in every multiframe, then there is a duration of two frames lengths,

i. e. 9.23ms within which the receiver is not active and so is able to carry out alternative mode monitoring. This particular period could be reserved for monitoring modes which require time consuming monitoring to obtain the required system status information. As indicated above, in each multiframe, the thirteenth and twenty-sixth frames (S12, s25) are for carrying control data. In half-rate mode, frame S 12 is used for carrying the SACCH data for a first subscriber, labelled T in Figure 2A, and the second SACCH channel s25 is used for carrying the SACCH data for the second subscriber, labelled t in Figure 2A. This is in contrast to full-rate mode where the second SACCH channel is idle and the first SACCH channel carries the control data for the voice data carried within the multiframe. Therefore, during half mode operation the first subscriber, T can carry out alternative mode monitoring during both of frames t24 and s25 to provide the 9.230ms gap. Similarly the second subscriber, t, can carry out alternative mode monitoring during frames S12 and T13, similarly providing a 9.230ms gap. This is of course in addition to the single frame alternative mode monitoring opportunities.

A further advantage of the present invention is that this alternative mode monitoring activity does not affect the ability of the terminal to carry out"native"mode measurements to the same level that it would in single mode operation. Thus, the terminal can still carry out monitoring of neighbouring cells to maximise the efficiency and quality of the GSM communication. This is in contrast to the proposals in the 3GPP system where native mode measurements are sacrificed. The present invention therefore provides the ability to carry alternative mode monitoring without affecting the ability of the terminal to carry out those single mode type operations carried out in the GSM system.

Whilst operating in half-rate mode, the amount of data available to carry a speech channel and also a data channel is halved and this can result in a reduction in the quality of the speech transmitted. In order to avoid this degradation of quality overshadowing the advantage of alternative mode monitoring, the terminal may only allow alternative mode monitoring when the aggregate quality of service available over the current GSM link and in the current environment, is maintained within a predefined limit. This ensures that where signal quality is not good, a full-rate channel is allocated to the existing GSM communication at the expense of not carrying out alternative mode

monitoring. However, when the quality of the GSM channel is sufficiently high, alternative mode monitoring can be carried out allowing the terminal to determine what alternative modes are available without disadvantageously affecting the quality of service provided over the existing channel.

The present invention further provides the possibility of scheduling periods of alternative mode monitoring. In this way, the GSM channel would be nominally at full rate. But at scheduled intervals, the communication can be switched to half-rate to allow alternative mode monitoring to be carried out. In this way, the terminal can carry out regular monitoring of alternative modes albeit at a reduced regularity whilst still maintaining good speech transmission quality for the rest of the time.

In addition to the above, because the invention allows intermittent monitoring of alternative modes, control of when alternative mode monitoring is carried out can be left partially or completely to the user. In this way, the user could control whether the terminal operates in a single mode manner until the user wishes to enquire as to the availability of an alternative mode whereupon they could select that the device switches into alternative mode monitoring. The device could then either automatically return to single mode monitoring having completed scanning for alternative mode or continue to monitor alternative modes until the user controls the device to operate otherwise.

The above description assumes that full-rate traffic signals will continue to be preferred due to their higher quality. As such, GSM will continue to operate predominately with full-rate traffic channels, with some channels configured for half-rate solely for the purpose of alternative mode monitoring. The network could either actively support alternative mode monitoring and therefore help to schedule when it is carried out or it could simply offer a half-rate service that terminals can request on an ad hoc basis.

Whichever way the system is operated, the terminal negotiates with the network via the base station for a half-rate resource. The terminal would then undergo a (presumably intra-cell) handover onto a half-rate traffic channel. The terminal would then remain allocated to that channel for the duration of the period required to perform the necessary alternative mode measurements. Once it has completed those measurements, the terminal can request a hand-over back to a full rate traffic channel.

Where the network provides active support for alternative mode monitoring, it could automatically schedule the terminals to switch from full-rate to half-rate at appropriate intervals. Alternatively, where the network does not actively schedule the switching, the terminal can simply request a transfer to a half-rate channel. However, because there is an inherent signalling overhead involved in requesting and negotiating a handover with the network, it would be more efficient if the network were to schedule handovers.

Whilst the above described invention is particularly applicable to the above described GSM system (including sibling systems like DCS-1800 and PCS-1900), the invention can be applied to other similar systems particularly TDMA systems, such as DECT, where the rate of speech or data coding can be reduced in order to provide time slots where data is transmitted at the normal rate interspersed with periods when no communication needs to be taking place with a terminal such that alternative mode monitoring can be carried out.

In the present invention, reference to half-rate or half-rate mode indicates the operation of a communication link in which data is only communicated between a terminal and a base station during one out of two traffic channel frames. In contrast, full-rate and fullrate mode refers to normal communication operation in which data is transmitted in all traffic channels.

The present invention provides a system for monitoring other communication systems or modes. The term mode refers to a specific protocol/standard and/or frequency. The alternative modes referred to are intended to include all manner of wireless communications including existing and future radio access technologies (RATs).

Claims

CLAIMS: 1. A terminal for a communication network comprising : a transceiver capable of communication with a base station using a first mode; control means for controlling the transceiver to operate using one of at least fullrate traffic channels and half-rate traffic channels, wherein said control means is arranged to control the transceiver to operate with half-rate traffic channels for monitoring one or more other communication modes.
2. A terminal according to claim I wherein the first mode is in accordance with the Global System for Mobiles (GSM).
3. A terminal according to claim 1 or 2 wherein said control means is arranged to control the transceiver to return to full-rate traffic channels when monitoring is completed.
4. A terminal according to claim 1,2 or 3 wherein the control means is arranged to control the transceiver to switch to operating in the mode to be monitored during periods when no communication is taking place in the first mode using half-rate traffic channels.
5. A terminal according to any one of claims I to 4 wherein the control means is arranged to control the transceiver to operate using full-rate traffic channels if the transmission quality is below a predetermined level.
6. A terminal according to any one of claims 1 to 5 wherein the control means is arranged to control the transceiver to operate with half-rate traffic channels for monitoring one or more other communication modes intermittently and to control the transceiver to operate with full-rate traffic channels otherwise.

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7. A terminal according to claim 6 wherein the control means causes the transceiver to operate with half-rate traffic channels for monitoring in response to control signals from a base station with which it is communicating in said first mode.
8. A communication system comprising: a base station for communication with one or more terminals using a first mode; the base station being arranged to communicate with a terminal using one of at least full-rate traffic channels and half-rate traffic channels, wherein the base station switches to communicating with the terminal using half-rate traffic channels when the terminal is to carry out monitoring of other communication systems.
9. A communication system according to claim 8 wherein the first mode is in accordance with the Global System for Mobiles (GSM).
10. A communication system according to claim 8 or 9 wherein the base station switches to communicating with the terminal using half-rate traffic channels in response to control signals received from the terminal.
11. A communication system according to claim 8 or 9 wherein the base station switches to communicating with the terminal using half-rate traffic channels at predetermined intervals.
12. A method of controlling a terminal for use in a communications network operating in a first mode comprising: switching the terminal from operating using full-rate traffic channels to using half-rate traffic channels; controlling the terminal to operate using a second different mode; and monitoring the second mode.
13. A method of controlling a terminal according to claim 12 wherein the first mode is in accordance with the Global System for Mobiles (GSM).
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] 4. A method of controlling a terminal according to claim 12 or 13 further comprising controlling the terminal to operate using the first mode after monitoring the second mode.
15. A method of controlling a terminal according to claim 12, 13 or 14 wherein monitoring of the second mode is carried out during time slots when no communication is carried out in the first mode operating using half-rate traffic channels.
16. A method of controlling a terminal according to any one of claims 12 to 15 further comprising switching the terminal from operating using half-rate traffic channels to using full-rate traffic channels after monitoring the second mode.
17. A method of controlling a terminal according to any one of claims 12 to 16 further comprising switching the terminal from operating using half-rate traffic channels to using full-rate traffic channels if the transmission quality is below a predetermined level.
18. A method of controlling a terminal according to any one of claims 12 to 17 further comprising switching the terminal from operating using full-rate traffic channels to using half-rate traffic channels intermittently and switching the terminal to operating using full-rate traffic otherwise.
19. A method of controlling a terminal according to any one of claims 12 to 18 further comprising switching the terminal from operating using full-rate traffic channels to using half-rate traffic channels in response to control signals received from a base station with which the terminal is communicating using said first mode.
20. A method of controlling a communication system capable of operating using at least half-rate traffic channels and full-rate traffic channels in a first mode, comprising: switching a communication link between a terminal and a base station to operate using half-rate traffic channels; and controlling the terminal to monitor one or more other communication modes.

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21. A method according to claim 20 wherein the first mode operates in accordance with the Global System for Mobiles (GSM).
22. A method of controlling a terminal operating using a TDMA mode to allow monitoring of alternative modes comprising: switching to half-rate traffic channels whilst communicating in the TDMA mode; reconfiguring the terminal for monitoring one or more alternative modes during those frames when communication is not carried between the terminal and a base station; and reconfiguring the terminal for operation in the first mode when communication is to be carried out.
23. A method according to claim 22 wherein the TDMA mode operates in accordance with the Global System for Mobiles (GSM).
24. A method for monitoring alternative modes in a terminal for a first mode capable of operating using full-rate traffic channels and second, lower rate traffic channels, comprising: switching to communicating using the second lower rate traffic channels when monitoring of an alternative communication mode is required; consecutively switching the terminal between monitoring the first mode and an alternative mode to be monitored such that monitoring of the alternative mode occurs during intervals when no communication is being carried out in the first mode.
25. A method according to claim 24 wherein the first mode operates in accordance with the Global System for Mobiles (GSM).
26. A method of controlling a terminal substantially as described herein with reference to the attached drawings.
27. A communication terminal substantially as described herein with reference to the attached drawings.

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28. A method of controlling a communication system substantially as described herein with reference to the attached drawings.
29. A communication system substantially as described herein with reference to the attached drawings.