AU715950B2 - Domestic cordless phone - Google Patents

Description

P/00/011 28/5/91 Regulation 3.2

AUSTRALIA

Patents Act 1990 a a

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Invention Title: "DOMESTIC CORDLESS PHONE" The following statement is a full description of this invention, including the best method of performing it known to us:- Technical Field This invention relates to the provision of a domestic cordless phone arrangement compatible with a wide area TDMA communication system.

The invention will be described with reference to the GSM/DCS cellular phone system.

Background Art The known cordless phone has a fixed transceiver connected via a line to any telecommunication network, and a portable transceiver tuned to the operating frequencies of the fixed transceiver. This permits a subscriber to use the portable 10 phone anywhere within the range of the fixed transceiver. The range is limited but may extend beyond the subscribers premises. However, the portable phone cannot be used outside the range of the fixed transceiver.

This invention proposes a system in which a portable phone may be used in a similar way to a cordless phone and may also be used as a mobile handset in a 15 mobile phone network. The invention will be described with reference to a TDMA "'*':mobile phone system, and, in particular, with reference to the GSM/DCS systems which are referred to as a cellular mobile phone system. In each cell the carrier frequency is divided into a plurality of time slots.

In the GSM/DCS system each allocated frequency is divided into 8 bi- 20 directional TDM channels, with the up channels (mobile-to-base station) being eo9displaced in frequency by a fixed amount with respect to the corresponding down channels.

A predetermined frequency hopping pattern is often used to mitigate the effects of interference on a specific frequency.

The GSM/DCS system specification prescribes a maximum of 124-374 carrier frequency pairs. Where there are a number of mobile phone service providers, these carrier frequencies are divided into blocks and allocated to the providers so the number of carrier frequencies available to a carrier is limited.

In a domestic base station (DBS) arrangement, a large number of low power base stations are provided in close proximity to each other. Typically the power of a DBS is of the order of 2 to 25 mW. However, even at these low powers, the likelihood of interference with other DBS's on the same frequency is large.

In the GSM/DCS plan each carrier frequency pair is divided into 8 TDM channels, the first of which is used for signalling and control and is referred to as the

BCCH

(Broadcast Control Channel).

An idle mobile in a cell camps on the BCCH and reports its initial entry into the cell to the network control centre via the RACH (Random Access Channel) up-link associated with the BCCH. Once recognised the mobile is allocated an SDCCH (Signalling Channel). The BCCH and SDCCH may time share the same frequency and time slots.

An idle mobile also listens on the BCCH for signals sent to that mobile, e.g. for incoming call messages. Frequency hopping is not applied to the BCCH.

ffWhen a mobile is used to initiate a call it sends a random access request on the f...i up-link, and, after an authentication process, the mobile is allocated one of the O 00 remaining TCH (Traffic Channel) time slots for use in the call.

i' 15 The GSM/DCS system includes a directed retry feature which is used, for t".example, when there are no vacant TCH time slots available for a mobile seeking to establish a call or for a call intended for the mobile.

0000 In these circumstances the control centre to which the base station is connected selects a second base station with which the mobile is able to communicate and re- 20 directs the mobile to tune to a specified time slot of that base station. The control oft. f centre is able to identify the other base stations with which the mobile is able to communicate because the mobile continually reports information on the signals it monitors from the adjacent base stations. Thus the control centre can select a proximate base station with an available time slot and redirect the mobile to that time slot.

Alternatively, the GSM/DCS System includes an intra-cell handover feature which is used, for example, to redirect a mobile from one TCH to another on the same cell where the second TCH is deemed to be better in some respect, for example, it has lower interference.

The control centre to which the umbrella base station is connected may use this method by requiring the mobile to perform an intra-cell handover to another base station, provided the second base station has its timeslots synchronised to the first and a.

S. a a the mobile is in close proximity to the second base station.

Disclosure of the Invention In the system disclosed in this specification, the idle mobiles camp on the BCCH of an umbrella cell (overlay cell) and, where a mobile seeking to establish a call is registered in relation to a DBS within the umbrella cell, the control centre initially redirects the mobile to its own DBS. If the call cannot be established on the DBS, e.g., because the mobile is out of range of the DBS but still within the umbrella cell, the call will be established on a time slot of the umbrella cell if available, or on a time slot of a proximate cell other than a DBS.

10 For an incoming call to the mobile in the umbrella cell, the call establishment information is sent to the mobile on the PCH (Paging Channel) which is associated with the BCCH in the normal manner. When the mobile has been authenticated on an SDCCH of the umbrella cell then a traffic channel is allocated on the DBS with which the mobile is registered using the directed retry mechanism.

If the call cannot be established on the DBS, because the mobile is beyond the coverage of the DBS, the directed retry mechanism will assign a TCH on the umbrella cell base station as a second attempt.

In applying our invention to the GSM/DCS system, either the directed retry mechanism or intra cell transfer mechanism can be adapted to direct a mobile registered in relation to a DBS and within the umbrella cell to first tune to a time slot of its corresponding

DBS.

Thus the operation of the system is modified to cause a call to or from a mobile registered in association with a DBS and within the umbrella cell to be set up on the DBS via the directed retry mechanism as a first attempt, and, if this attempt is unsuccessful, the next attempt by the directed retry mechanism will be on a vacant time slot of the umbrella cell base station or other proximate base station.

This method of operation requires re-programming of the directed retry mechanism to take account of the fact that a mobile is registered in relation to a DBS and that the mobile is within the coverage area of the umbrella cell. A database available to the control centre could be used to record the association of the mobile and DBS, as well as the location of the DBS within the umbrella cell. The presence of the mobile within the umbrella cell is registered in the normal way.

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se Thus according to this invention, the first attempt to set up a call to or from a mobile associated with a DBS and within an umbrella cell is via the DBS, rather than via the umbrella cell base station having the BCCH on which the mobile was camped.

In this arrangement the mobile registered in association with a DBS does not need to monitor its own DBS. A consequence of this is that the DBS does not need to continuously transmit a BCCH.

An extension of our invention takes advantage of this fact by making the DBS time slot normally used for the BCCH functions available as an additional traffic channel (TCH). Thus each DBS has available 8 TCH's, any one of which could be 0 allocated as the TCH.

In a further improvement, the DBS can utilize the copper pair from the exchange to the premises to connect the DBS into the network.

To this end, the DBS is provided with a codec to convert the digital signals received from the mobile to analog signals for transmission over the copper pair to the 5 exchange, and to connect the analog signals received from the exchange via the copper pair to digital format before transmission to the mobile.

Brief Description of the Drawings The invention will be described with reference to the accompanying drawings in which:

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Figure 1 Figure 2 represents an umbrella cell containing a plurality of DBSs; illustrates the timing of channels for the DBSs and the umbrella cell.

Figure 3 is a block diagram showing functions of a DBS embodying the invention.

Figure 1 shows an umbrella base station 101 whose cell includes the coverage areas of each of a plurality of DBS's of which four, 120, 121, 122 and 123 are shown by way of example. The umbrella base station and the DBS's are connected to a remote base station controller BSC1 10. Each DBS has a limited coverage area because of its low power operation. However the DBS's may be located in a compact area so the coverage area of several DBS's may overlap. Because of the limited number of frequencies available, this can cause substantial problems with frequency planning. The complexity of the problems is exacerbated when new DBS subscribers 4 *e t 4

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are added to an existing DBS group.

In the normal GSM/DCS frequency plan, each base station is allocated a specific frequency for its BCCH and no base stations with overlapping coverage areas are allocated the same BCCH frequencies. This ensures that interference between the transmissions in overlapping cells is largely eliminated. The normal GSM/DCS plan makes the 8 channels of one frequency available to one base station.

Thus where there are n frequencies available for the mobile phone system of a particular utility company, this sets the limit on the options available for avoiding frequency overlap using the usual frequency planning methods.

As shown in Figure 1 the coverage of DBS 120 overlaps with that of both 121 and 122. Thus the same frequency/time slot cannot be re-used for these DBS's.

However, the coverage area of 123 does not overlap with 120 or 121 so it would be possible for 123 to occupy the same frequency/time slot as either 120 or 121.

However as there may be in excess of 50 DBS's covered by one umbrella cell, the problems of overlapping DBS coverage areas is substantial. Thus the dynamic allocation of communication channels in accordance with a standard frequency plan in such arrangement is very complex.

According to one embodiment of this invention, the problem of frequency congestion can be substantially mitigated by allocating to each DBS a fixed time slot thus leaving the other time slots of that frequency to be used by other DBS's.

By having a fixed frequency/time slot for each DBS, up to 8 DBS's can use the same frequency while having overlapping coverage areas. This greatly simplifies the process of allocating communication channels dynamically.

In another embodiment, the DBS may monitor idle timeslots those where a mobile is not in direct communication with the DBS) to determine the one with lowest interference and reassign the mobile from the first fixed time slot to another time slot if this time slot is deemed to have lower interference.

The DBS is programmed to monitor the 8 time slots to determine which has the lowest interference, e.g. from neighbouring mobiles, and, via the FACCH, direct the mobile to move to the quietest time slot. This arrangement enables the mobile to achieve the best quality communication.

Preferably, each DBS is synchronized with the umbrella cell. Where each DBS .4 a a a. a is connedted to the base station contact centre via a high speed line a 2 Mbit ISDN line or optical fibre, the synchronization can be achieved via the high speed line.

However, if a DBS is connected only via a copper pair to the exchange an alternative method of synchronization may be required.

According to a further embodiment of this invention a DBS can be synchronized to the umbrella signal transmissions e.g. by synchronizing to the BCCH of the umbrella cell.

The DBS monitors the BCCH of the umbrella cell and synchronizes its timing to that of the umbrella cell. This is necessary to ensure that when the directed retry S10 mechanism allocates the mobile a TCH on the DBS, the mobile (which is synchronized to the umbrella cell) can correctly locate its allocated TCH time slot on the DBS.

An extension of our invention takes advantage of the fact that a mobile should always be in close proximity to the DBS when it is using a TCH of the DBS.

Further, because of its guaranteed proximity to the DBS, no timing advance mechanism is necessary to ensure that the mobile signal arrives at the DBS within the permitted time window.

If the DBS detects in the initial access bursts of the mobile that it is not in close proximity to the DBS (that is, in GSM/DCS a timing advance of at least 1 or -1 is required) then it will refuse the mobile and the mobile will return to the umbrella cell and the cell can be established on a time slot of the umbrella cell if available or on a time slot of a proximate (neighbour) umbrella cell within the reach of the mobile.

In another embodiment, the problem of signalling the disconnection of the mobile is addressed.

In another embodiment the problem of signalling the DBS of the presence of a mobile requiring a traffic channel on the DBS or signalling the DBS that an existing call to a mobile is to be released is addressed.

When a mobile has progressed through authentication on the umbrella cell and is awaiting the assignment of a traffic channel (as a first attempt this will be done on the DBS), the network may notify the DBS to make a channel available by signalling to the DBS.

In one embodiment, this signalling may comprise the normal ring tone used by the network to alert a subscriber to an incoming call. Alternatively, other signalling *C means may be employed.

Thus the DBS may be equipped with a ring detector signal similar to that used with a POT. The DBS may also include a line switch to emulate the OFF-HOOK action of a POT in the event that ring signal is detected.

Additional signalling may be sent to the DBS over the phone line using e.g. FSK or multi-tone signalling or other suitable signalling techniques The DBS would therefore be equipped with an appropriate signalling detector. Signals intended for the mobile would then be encoded for transmission.

If the mobile moves out of coverage of the DBS for a predefined period of time, or the mobile releases the call, then the DBS may signal this event so that the network resources can be released.

In one embodiment, this signalling may comprise the normal on-hook signal used to alert the network that the call has been released. Alternatively, other signalling means may be employed.

Another problem which is addressed by a further embodiment is the handover or warning of the subscriber when the mobile is approaching the edge of the DBS coverage area.

The mobile periodically reports to the DBS the level and quality of the signal it has received from the DBS. Similarly, the DBS monitors the level and quality of the signal received from the mobile. The approximate distance of the mobile from the DBS is known from the timing advance. In one embodiment, the DBS can alert the mobile user to the fact that level and/or quality and/or distance has degraded to an unacceptable point. (That is, the mobile is at the edge of the DBS coverage area) by injecting an audio signal into the speech signal towards the mobile or by some other means disturbing the normal speech signal to the mobile.

In another embodiment, the DBS can alert the system controlling the umbrella cell to the fact that the level and/or quality has degraded to an unacceptable point by injecting a signal DTMF dial tones) into the speech signal towards the network or by some other means disturbing the normal speech signal to the network. The system controlling the umbrella cell can then allocate a TCH on the umbrella cell can then allocate a TCH on the umbrella cell and similarly signal the DBS that a TCH is available. The DBS can then instruct the mobile via a FACCH message to handover to 0

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9 the umbrella where the call can continue in the normal fashion.

Figure 2 is a timing diagram illustrating the umbrella cell 101 channels and exemplary DBS TCH's. In this drawing a) illustrates the time slots of the umbrella cell including BCCH and TCH1 to TCH7. BCCH is usually at a fixed frequency and TCH1 to TCH7 employ a frequency hopping sequence to mitigate the effects of interference at a particular frequency. All TCH's may use the same frequency for one frame and then all step through the same sequence of frequencies for the following frames and return to the original frequency at the end of the cycle. and e) illustrate the preferred first time slot to be used by DBS's 120, 121, 122 and 123. These DBS's may 10 basically follow the same frequency hopping pattern as the umbrella cell 101, but with a different starting point so that they do not ever coincide in frequency with TCH1 to TCH7. Because DBS 120 to DBS 123 have different time slots they can all use the same frequency in one frame. More complex pseudo-random hopping patterns may be used. The GSM/DCS system is sufficiently robust to survive 1 in 4 corrupted frames 15 ofspeech.

Up to 8 DBS's may use the same frequency in a frame because of the time division of the frame. Thus if there are x frequencies available for a carrier, there are 8.x frequency/time slots available. 8 of these are used for the umbrella cell and multiples of 8 are also required for adjacent cells. Thus 8 frequency/time slots 20 are available for use in an umbrella cell without re-use of a frequency/time slot, where is the number of adjacent or overlapping cells. Re-use is possible if DBS's are sufficiently separated to ensure their coverage area does not overlap.

In Figure 2 f) shows the time slots used in the first attempt to allocate a call on TCH 120 to the mobile handset MH 120 associated with DBS 120.

The signalling channel of umbrella cell 101 is used by BSC 110 to instruct

MH

120 to tune to the frequency and time slot of DBS 120. The mobile thus attempts to establish communication with DBS 120 by listening in the time slot marked M which corresponds to TCH 120, and transmitting in the time slot marked -M120 which is delayed by the GSM/DCS standard delay of three channels and offset in frequency by the GSM/DCS standard frequency offset. DBS 120 listens for the response from MH 120 at -TCH 120 which corresponds to -M120. In this way, signalling data bursts are exchanged and the call can be established.

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However, if MH 120 is out of range of DBS 120 the call will not be set up and the mobile reports this failure to the umbrella cell 101 via a signalling

FACCH

message on the up-link when it automatically returns to the umbrella cell 101. This failure is reported to the base station controller BSC 110 which then sends an instruction via BS 101 to cause MH 120 to establish a call on, TCH1, of the umbrella cell BS 101. Thus MH 120 listens on M1 20' and transmits on -M1 20', shown at g) in Figure 2, to exchange signalling data bursts with BS 101 and set up the call at the umbrella cell level.

While Figure 2 shows the BS 101 and DBS 120 to DBS 123 as connected to a 10 geographically remote Base Station controller BSC 110, in an alternative embodiment the functionality (processing and data) to control the DBS's within an umbrella cell could instead be located at the same site as the umbrella cell base station 101 or other convenient location within the umbrella cell. This option may be useful where new housing developments are taking place, in avoiding the need to run a plurality of 15 cables over longer distances to the BSC or telephone network.

Figure 3 is a block diagram illustration functions of a DBS embodying the invention. The DBS has an antenna arrangement for transmitting and receiving, shown here as a single antenna 31 coupled through coupler means 32, e.g. a diplexer, to transmitter 33 and receiver 34.

The receiver 34 feeds into CODEC 35 which can be programmed to detect signalling information such as the FACCH header and can extract instructions intended for the local control of the DBS.

The CODEC 35 passes other received data such as TCH frames to a means for passing the information into the mobile phone system or the PSTN. Where the DBS is connected to the copper pair of the PSTN, the data is e.g. applied to D/A converter 37 for transmission over the two wire line The signal detector 40 can detect signalling from the exchange over line For instance it may detect ring signal and instruct microprocessor 41 to operate a line switch 39 to cause the DBS to go OFF HOOK. This enables communication between the DBS and the exchange.

11 The microprocessor 41 has clock 42, ROM 43 and RAM 44 associated with it.

The microprocessor 41 may also control synchronization of the DBS with the umbrella cell via CODEC 35 which can extract synchronization information from the

BCCH.

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Claims (22)

1. A mobile telephone system including at least one umbrella cell having a first coverage area and, within the first coverage area, a plurality of domestic base stations, wherein at least one mobile phone is associated with each domestic base station, the system including means to recognize when a mobile phone associated with a domestic base station in a first coverage area is within that first coverage area, wherein, when the said mobile phone is within that first coverage area, control means for the system initially attempt to establish calls to or from the mobile phone via 10 the domestic base stations.

2. A system as claimed in claim 2, wherein, if an attempt to establish a call via the domestic base stations is unsuccessful, the control means will attempt to establish the call via the umbrella cell.

3. A system as claimed in claim 1, wherein, if an attempt to establish a call via a domestic base station is unsuccessful, the control means will attempt to establish the call via the umbrella cell or another cell with which the mobile phone is able to communicate.

4. A system as claimed in any one of claims 1 to 3 wherein the system is a TDMA system. 20

5. A system as claimed in any one of claims 1 to 4 wherein the system is a GSM or DCS system.

6. A system as claimed in claim 4 or claim 5 wherein each domestic base station operates on a specific time slot.

7. A system as claimed in claim 6 in which each domestic base station operates on a specific carrier frequency.

8. A system as claimed in any one of claims 5 to 7 wherein the domestic base station does not have a broadcast control channel, and wherein the channel reserved for the broadcast control channel in the GSM/DCS system is used as a traffic channel.

9. A system as claimed in any one of claims 1 to 8 wherein the umbrella cell and each domestic base station within the first coverage area are connected to a control centre by communication links.

A system as claimed in claim 9 wherein one or more of the domestic base a a 0e a stations are connected to the control centre via a public telecommunication network.

11. A system as claimed in claim 4 or claim 5 or any one of claims 6 to 10 as appended directly or indirectly to claim 4 or claim 5, wherein non-traffic information is sent from the mobile phone to the domestic base station by using labelled messages in the traffic channel on which the call is established, and wherein the domestic base stations includes detector means to identify the labelled messages.

12. A system as claimed in claim 11 wherein the domestic base station transmits the contents of labelled messages to the control centre.

13. A system as claimed in claim 11 or 12 wherein the mobile phone transmits a 10 labelled message when the mobile is disconnected.

14. A system as claimed in any one of claims 4 to 13 wherein a warning signal is transmitted to the mobile phone via the domestic base station when it approaches the edge of the domestic base station cell during a call.

A system as claimed in claim 14 as appended to any one of claims 11 to 13 wherein the mobile phone reports the strength of the domestic base station signal via a labelled message and the warning signal is transmitted when the reported signal strength falls below a predetermined level.

16. A system as claimed in any one of claims 11 to 13 wherein a handover message is transmitted to the mobile phone as a mobile approaches the edge of the 20 domestic base station cell.

17. A system as claimed in any one of claims 1 to 16 wherein each domestic base station in an umbrella cell coverage area is synchronized with the umbrella cell.

18. A system as claimed in claim 1 7 wherein one or more of said domestic base stations synchronizes to a control signal from the umbrella cell.

19. A system as claimed in claim 9 or claim 18 or any other claim appended to claim 9 wherein one or more of said domestic base stations is synchronized with the umbrella cell via control signals transmitted over the communication links.

A system as claimed in any one of claims 1 to 19 wherein one or more of the domestic base stations is connected to the public telecommunication network via a telephone line.

21. A system as claimed in claim 20 wherein each domestic base station connected to the public telecommunication network via a telephone line includes an analog-to- 14 digital converter to convert messages for the mobile phone received over the telephone line to digital format, and wherein each said domestic base station includes a digital-to-analog converter to convert messages received from the mobile phone to analog format for transmission via the telephone line.

22. A mobile telephone system substantially as herein described with reference to Figures 1 3 of the accompanying drawings. DATED THIS SEVENTH DAY OF MARCH 1997 ALCATEL AUSTRALIA LIMITED 000 005 363) p. p p p p p *ep pp p