GB2318481A

GB2318481A - Method of Allocating Time Slots in a Communication System

Info

Publication number: GB2318481A
Application number: GB9621557A
Authority: GB
Inventors: Rafy Carmon; Mark Shahaf; Julian Howard Richardson; Gary Aitkenhead
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 1996-10-16
Filing date: 1996-10-16
Publication date: 1998-04-22
Anticipated expiration: 2016-10-16
Also published as: GB2318481B; GB9621557D0

Abstract

A method of allocating time slots for a communication at a first frequency between a first and a second unit in a communications system comprises the steps of: determining the existence of a current communication on a second frequency different to the first between a third and fourth unit in the communications system, and if there is a current communication, assigning different time slots for the communication between the first and second unit. This reduces the possibility of transmission by one unit operating in close proximity of another blocking reception of signals by the another unit.

Description

A COMMUNICATION METHOD Field of the invention This invention relates to a communication method for use in a communication system and more particularly to a method of allocating resources in the system.

Background of the invention In order to provide the functionality that users demand of radio communications systems the systems are becoming increasingly complex.

Today's systems are not merely capable of point-to-point communication but offer various modes of operation. The modes include simplex, duplex, private, group and trunk call.

In the trans-european trunked radio standard there is provision for duplex private calls between two mobile transceivers and this mode will now be described with reference to Figure 1.

As shown in Fig. 1, a radio system 1 includes a base-station (Bs) 2 and a number of mobile-stations (Ms) 3 to 6. The system operates in accordance with a time division multiple access protocol (TDMA). In this protocol communication between more than one of the system's units takes place on the same frequency with slots of time being allocated to particular units' communication.

As is shown in Fig. 2, in the case of this system,time is repeatedly subdivided into four time slots which are labelled 1 to 4 on one frequency.

The other frequencies available to the system will also be similarly subdivided. For duplex communication there will be a transmission frequency and a reception frequency allocated to form an uplink and a downlink. The uplink is utilised by mobiles transmitting to the base-station and the down-link is utilised by the base-station transmitting to the mobiles.

It should be noted that the slots on the uplink and the downlink coincide temporally.

For private duplex communication between Ms 5 and Ms 6, slot 1 and slot 3 will be allocated typically. Slot 2 and slot 4 will be allocated for communication between other mobiles. Thus communication takes place in the following way.

Ms 5 encodes some speech and transmits the encoded speech on the uplink during slot 1. The base-station 2, acting in effect as a relay, re transmits the coded speech during slot 3 of the downlink. Ms 6 then receives the coded speech from the base-station during slot 3. For the return path Ms 6 transmits on slot 3 of the uplink and base-station repeats on slot 1 and Ms 5 receives on slot 1.

A problem has been identified by the inventors which will impair or even prevent such communication and it is this inventive appreciation of this problem that has lead to the development of the present invention.

Brief Summarv Of The Invention According to the invention there is provided a method of allocating time slots for a proposed communication at a first frequency between a first and a second unit in a communications system comprising the steps of: determining the existence of a current communication on a second frequency different to the first frequency between units in the communications system, and when there is a first current communication, assigning a different time slot for the communication between the first and second unit such that a slot allocated for reception in the proposed communication does not coincide with a slot allocated for transmission for the current communication.

With currently proposed systems, the inventors have identified that the following situation can arise. When in the earlier described duplex call Ms 6 is receiving during slot 1, if a mobile which is close by, in this case Ms 2, is transmitting on a different frequency at the same time during slot 1 then a situation occurs where Ms 6 is unable to receive the desired communication relayed from the base-station because it is saturated by the signal received from the Ms 3. Thus Ms 3 blocks reception of the desired communication.

However, by the use of a method of allocating time-slots in accordance with the invention the blocking effect is alleviated or even eliminated. With reference to the earlier described situation, let us suppose Ms 4 and Ms 3 are already in communication. Ms 5 requests the basestation to set up a duplex private connection call to Ms 6. The base-station 2 will determine that communication is taking place between Ms 4 and Ms 3 and will then allocate different time slots for the new connection. Thus, if Ms 4 and Ms 3 are communicating on slot 1 and slot 3 the base-station 2 will allocate slot 2 and slot 4. This will eliminate the earlier mentioned blocking effect since Ms 6 will be receiving during slot 1 and Ms 3 will transmit at a different time that is to say during slot 2.

Preferably, there will be included a step of determining whether or not units wishing to communicate are sufficiently close to existing communicating units for it to be necessary to allocate different slots. This may be coarsely determined, by for example, a base-station simply determining that a unit is within its range of coverage by reference to the received signal strength or in a more refined way the unit's precise location may be used. The precise location may be determined by utilising a satellite positioning system or in some other way, by for example, signal strength or triangulation techniques or a combination of such techniques.

Brief Description Of the Drawings A specific embodiment of the invention will now be described, by way of example only, with reference to the drawings in which: Figure 1 shows in block diagram form a communications system used in an explanation of the prior art method and a method in accordance with the invention; Figure 2 shows slot allocation used in an explanation of the prior art techniques and a method in accordance with the invention; Figure 3 shows a communications system operating in accordance with the invention; Figure 4 shows a base station present in the system shown in Fig.; Figure 5 shows a mobile station present in the system shown in Figure 3; and Figures 6 and 7 are explanatory diagrams.

Detailed Description Of A Specific Embodiment Of The Invention With reference to Fig. 3, there is shown a communications system generally designated 30 which includes a base-station 31 and eight mobile transceivers 32 to 39 (hereinafter referred to as Ms) lying within the area of coverage of the base-station 31 denoted by the line 40.

The base-station 31, as is shown in Fig. 4, comprises a controller 41, a memory 42, a transmitter 43, a receiver 44 and an antenna 46.

The controller 41 is a microprocessor programmed to control the way in which the base-station 31 operates. The program is held in the memory 42 as a set of instructions. A databus connects the controller 41 to the memory 42.

The memory 42 comprises a set of addressable memory locations provided by semiconductor chips. The memory 42 is subdivided into read only memory (ROM) and random access memory (RAM). The earlier mentioned program is held in the ROM division as a set of hexadecimal numbers. The RAM provides storage for data generated during operation of the base-station 31.

The controller 41 is also connected to the transmitter 43 and the receiver 44. The connection comprises a data-bus and a control bus and thus allows data to flow to and from the controller and the receiver or transmitter as the case may be. The control-bus allows control instructions to be passed from the controller 31 to the receiver 44 or the transmitter 43 to enable the controller 31 to control their function.

The transmitter 43 and the receiver 44 are connected to the antenna 46. It will be noted that the connection is direct and no antenna switch is present because the transmitter 43 operates over a different band of frequencies to the receiver 44, that is to say both units can operate simultaneously using the same antenna.

The controller 41 also performs a communications system management function allocating the frequencies and time-slots to be used by all elements of the system. The commands necessary to accordingly instruct the Ms are transmitted on control channels by means of the transmitter 43 in a manner that will be readily appreciated by the man skilled in the art.

The Ms are all of the same design. As is shown in Fig. 5, the Ms includes a controller 50, a memory 51, a transmitter 52, a receiver 53, an antenna switch 54, an antenna 56, a Geo-Positioning Receiver 57 (GPS), a microphone 58, a pre-amplifier 59, an audio output amplifier 60 and a speaker 61.

The controller 50 controls the operation of the receiver 53 and the transmitter 52 in the same manner as the earlier described controller 41 of the base-station.

The memory 51 includes ROM and RAM and stores a program which the controller executes to control the operation of the Ms. The memory also holds user defined configuration information which typically allocates particular functions to particular keys (not shown) on the mobile. However, the particular user defined configuration is not important for the understanding of the present invention and will thus not be described any further.

The antenna switch 54 is controlled by the controller 50 such that when the transmitter 52 is in transmitting mode the switch connects it to the antenna 56 which radiates the signal. When the transmitter 52 is not transmitting, then the antenna switch 54 connects the receiver 53 to the antenna 56.

In this first embodiment of the invention the GPS receiver 57 receives signals from satellites from which it derives timing information. In effect this provides a system "clock" which enables synchronisation of all the units of the system. The timing information is passed to the controller 50 along a bus.

The microphone 58 is used to input a user's speech to the Ms in conjunction with the pre-amplifier 59 which merely increases the voltage level of the signal derived from the speech by the microphone 58. The amplified signal is then input into the transmitter 52 where it is converted into a digital form by an analogue to digital converter (not shown) and then transmitted.

The receiver 53 is connected to the amplifier 60 and passes received and decoded signals to it. The signals are then amplified and input to the speaker 61.

The operation of the system will now be described with reference to the flow chart shown in Fig. 6.

Let us suppose that mobile 32 is connected by the base station 31 to mobile 38. The type of call involved is a full duplex call. This means that there will in effect be an uplink at a first frequency and a downlink at a different frequency between the respective mobiles and the base station 31.

Mobile 39 is in close proximity to mobile 38 and let us suppose that this is the mobile which is to initiate a direct mode call to mobile 36. Mobile 39 will then issue a request to the base station 31, requesting that a flill duplex call connection. Thus referring now to Fig. 6 in step 61 the base station 31 receives a call connection request from the mobile 39. The base station 31 in step 62 then investigates whether or not there is an existing call connection in progress. In this case there is an existing call connection between mobiles 32 and 38. Thus in the next step 63 the base station 31 determines the slots in the TDMA structure which are being utilised for the existing call connection. Then in step 64 the base station 31 allocates slots for the new call connection which are different to the existing call connection. Then in step 65 the mobiles 39 and 36 establish connection utilising the slots allocated by the base station 31. If in step 62 there is no call connection in progress, then the base station 31 would immediately proceed to the slot allocation step 64.

In Fig. 7 there is shown the downlink between the mobile 32 and mobile 38 and the uplink of the mobile 39 to the mobile 36. It should be appreciated that the two frequencies are different. However, it will be seen that the slots are coincident. Thus, slot 1 of uplink 1 coincides with slot 1 of the downlink and similarly for the other fourteen illustrated slots.

In the sketch of the downlink, there are a number of boxes marked with Xs. The Xs represent the slots during which the mobile 38 is receiving information relayed to it by base station 31 at that particular frequency, frequency 1. Thus, it will be seen that mobile 38 receives during slot 1, slot 5, slot 9, and slot 13. Thus, the base station 31 determines, in the determining step 63, that there is already an existing call connection in progress and that slots 1, 5, 9, and 13 are being utilised. Thus when allocating slots for a new call in step 64 the base station chooses different slots and in the illustrated example it chooses slot 2, slot 6, slot 10 and slot 14 of the uplink. It will be seen that when mobile 39 transmits in a new slot the transmission does not coincide with the reception slot of mobile 38.

Therefore the transmission by mobile 39 does not block the reception by mobile 38.

In the earlier described embodiment of the invention a relatively straight forward method of control was employed. It will be appreciated that should further mobiles wish to establish connection they too will be allocated slots in order to avoid interference with mobile 38. Of course the further mobiles may be so remote relative to mobile 38 that there will be no possibility of interference occurring between them.

Thus, in the preferred embodiment of the invention, the GPS receiver 57 which is present in the mobiles is used to determine the position of the mobile within the cell 40. The location information will be transmitted to the base station periodically or indeed when a call connection request is made to the base station from the mobile station. The base station will then perform an initial step to determine whether or not the mobile station requesting a call connection is located within a predefined distance from another mobile station that already has a call connection. For example, the coverage area of the base station 31 could be subdivided into zones, the location of the mobile stations could then be compared with the zones and channel allocated such that mobiles within a subdivision will be allocated different time slots for their respective connections. Thus, in the already described embodiment the mobile 38 and mobile 39 will be determined as being within the same subdivision and therefore they will be allocated different time slots for their connections.

In the earlier described embodiment, the base-station upon determining that there is a co-located pair of mobiles allocates the slots to ensure that the blocking effect does not occur.

However, a situation may arise where after the allocation of the slots blocking occurs because of movement of the mobiles or a variation in the transmission or reception conditions.

Thus, if the system is to cater for such eventualities a wider radius of interference must be assumed than may be appropriate for most cases. This will reduce the potential efficiency gains possible by implementing the invention.

In a further embodiment of the invention, the mobile experiencing blocking transmits a message to the base-station during a signalling frame.

With reference to figure 3, let us suppose that Ms 36 is in communication with Ms 35 via base-station 31. If Ms 37 establishes communication via the base-station 31 to Ms 32 and the time slots on the uplink co-incide with the downlink of the communication between Ms 36 and 35 then the blocking situation is possible if Ms 37 is within the proximity of Ms 36.

Let us suppose that blocking occurs then Ms 36 will detect interference over a wide range of frequencies. Noting this, and a difficulty in maintaining communication with Ms 35, it will transmit a message indicative of a potential blocking situation to the base-station 31. This transmission may take place on the next slot allocated to the Ms 36 for transmission or during a frame allocated for signalling messaging such as frame 18 of the TETRA standard.

The base-station 31 will then consider if there are any other mobiles in communication wherein the uplink slots coincide with the downlink slots allocated between Ms 36 and Ms 36.

In this case it will determine that Ms 32 and Ms 37 have an uplink slot that coincide and it will instruct Ms 32 and Ms 37 to move slots that is to say it will reallocate the uplink slot.

Whilst in the described embodiments of the invention the base-station allocates slots and governs the communications on the system it would be possible for these functions to be carried out by other units or the "intelligence" for doing so could be distributed throughout the system.

Claims

1. A method of allocating time slots for a proposed communication at a first frequency between a first and a second unit in a communications system comprising the steps of: determining the existence of a current communication on a second frequency different to the first frequency between units in the communications system, and when there is a first current communication, assigning a different time slot for the communication between the first and second unit such that a slot allocated for reception in the proposed communication does not coincide with a slot allocated for transmission for the current communication .

2. A method as claimed in claim 1 including the step of determining whether or not a unit transmitting on a slot is located within the range of another unit receiving on a slot such that blocking will occur and assigning different time slots for the communication if the location is within such a range.

3. A method as claimed in claim 2 wherein the location is determined with reference to a GPS receiver.

4. A method as claimed in claim 2 wherein the location is determined by reference to a received signal strength.

5. A method as claimed in claim 2 wherein the unit is determined as being within range if it is served by a same base-site in the communications system.

6. A method is claimed in any preceding claim wherein a respective pair of frequencies are allocated for each respective communication.

7. A method as claimed in claim 1 wherein the existence of a current communication is determined by at least one of the units in the system and wherein that unit notifies one other unit responsible for the allocation of communication slots.

8. A method of allocating time slots in a communication system substantially as hereinbefore described with reference to an as illustrated by any one of the drawings.