GB2328583A

GB2328583A - Frequency hopping in a TDMA communications system

Info

Publication number: GB2328583A
Application number: GB9717837A
Authority: GB
Inventors: Behzad Mohebbi; Howard Benn
Original assignee: Motorola Ltd
Current assignee: Motorola Solutions UK Ltd
Priority date: 1997-08-23
Filing date: 1997-08-23
Publication date: 1999-02-24
Also published as: GB9717837D0; AU8979398A; WO1999011084A1

Abstract

In a TDMA communications system, each time slot in a time frame 200 is analysed to determine whether it is to contain traffic channel data or control channel data. If the time slot is for traffic channel data then frequency hopping is permitted for the time slot. A GSM compatible indoor picocell system is described having BCCH and SDCCH control channel time slots. A base station receives time slots from a BSC and transmits the control channel time slots on a single predetermined frequency with no hopping. Traffic channel time slots are transmitted using a frequency hopping algorithm such that each time slot is transmitted on a frequency selected according to a sequence. For empty time slots, dummy BCCH data is inserted and transmitted on the single predetermined frequency. The above described functions may take place in the BSC.

Description

APPARATUS AND METHOD FOR TRANSMITTING DATA Field of the Invention The present invention relates to a method and apparatus for transmitting data in a picocell or microcell of a time division communications system, for example, a Time Division Multiple Access (TDMA) system, such a Global System for Mobile communications (GSM).

Background of the Invention It is known in the art to locate a plurality of base stations in a GSM picocell or microcell in a GSM cellular telephone network in order to provide radio frequency coverage (RF) for the picocell or microcell in a geographic area, such a room. In the picocell, a Broadcast Control Channel (BCCH) message is transmitted in a first time slot by each of the plurality of base stations using a first frequency. The BCCH message transmitted is the same for each of the plurality of base stations. A plurality of predetermined sets of frequencies are used for the transmission of traffic channel (TCH) messages, each set of predetermined frequencies being allocated to a respective one of the plurality of base station. It is therefore necessary to plan the frequency use of each base station in order to avoid interference between the plurality of base stations.

It is thus an object of the present invention to obviate the need to frequency plan a picocell or microcell.

Summary of the Invention According to a first aspect of the present invention, there is provided a method of transmitting data in a time slot between a base station and a terminal using a single carrier unit, the method comprising the steps of: determining whether the time slot is to contain data relating to a traffic channel or data relating to a control channel; selecting, in response to the time slot containing data relating to the traffic channel, a frequency for transmission of the time slot in accordance with a predetermined sequence of frequencies, and transmitting the data in the time slot at the frequency.

It is thus possible to provide a picocell or microcell system which obviates the need for frequency planning, whilst maintaining resource efficiency by using a single carrier system of transmission. The system is also filly backwards compatible with GSM cellular networks.

In accordance with a second aspect of the present invention, there is provided a base station for transmission of data in a time slot to a terminal, the base station comprising a single carrier unit and a monitoring element for determining whether the time slot is to contain data relating to a traffic channel or data relating to a control channel, the base station being arranged to select, in response to the time slot containing data relating to the traffic channel, a frequency for transmission of the data in the time slot in accordance with a predetermined sequence of frequencies.

Other, preferred, features and advantages are set forth in the following description and the appended dependent claims 2 to 14.

Brief Description of the Drawings At least one embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: FIG. 1 is schematic diagram of a picocell system constituting an embodiment of the present invention; FIG. 2 is a schematic representation of a data frame used with the system of FIG. 1, and FIG. 3 is a flow diagram constituting an operation of the embodiment of FIG.

1.

Description of a Preferred Embodiment A picocell system 100 (FIG. 1), for use with a GSM system is installed in an indoor environment 101, for example, a room. A first base station 102, a second base station 104 and a third base station 106 are located in the indoor environment 101 and generate a respective first, second and third picocell coverage area 108, 110, 112. An example of the first, second and third base stations 102, 104, 106 is a single carrier M-CELLbase station unit, manufactured by Motorola Limited, and having appropriate software and hardware modifications in order to facilitate operation in accordance with the description below.

The first, second and third base stations 102, 104, 106 are connected to a Base Station Controller (BSC) 116, the BSC 116 being connected to a Mobile Switching Centre (MSC) 120 which is connected to a Public Switching Telecommunications Network (PSTN) 122.

The BSC 116 includes a Broadcast Control Channel (BCCH)/ Stand-alone Dedicated Control Channel (SDCCH) data generator 118 capable of generation out-of-band control data for transmission by the first, second and third base stations 102, 104, 106 to a mobile terminal 114, for example, a cellular telephone handset, located within the indoor environment 101. The mobile terminal 114 can be a StarTacs GSM cellular telephone manufactured by Motorola GmbH. Out-of-band control data is control data which is not sent using a Traffic Channel (TCH).

In accordance with any technique known in the art, the BCCH(SDCCH data generator 118 is arranged to transmit the same BCCH/SDCCH data to each of the first, second and third base stations 102, 104, 106, thereby effecting simultaneous broadcast of the same BCCH/SDCCH data within the first, second and third picocell coverage areas 108, 110, 112.

Referring to FIG. 2, the first, second and third base stations 102, 104, 106 include a single carrier unit (not shown). The single carrier unit is arranged to transmit and/or receive a data frame structure 200 using a single transmitter, receiver, transceiver or transmitter/receiver chain.

The data frame structure 200 comprises a series of eight sequential time slots TS1,...., TS8. Such a structure is known in GSM systems, but it is conceivable to employ a greater or fewer number of time slots in the data frame structure 200 for a different time division system.

In accordance with the GSM system specification, a first time slot TS1 contains BCCH data. Similarly, a third and a fifth time slot TS3, TS5 contain SDCCH data. However, it is not essential to provide SDCCH data in the fifth time slot TS5, which can be used for transmitting other data, for example, application data, i.e. data relating to information that a user wishes to transfer across the infrastructure, such as voice, fax or video data.

The second, fourth, sixth, seventh and eighth time slots TS2, TS4, TS6, TS7, TS8 are used for transmitting application data, the nature of which is described above, or alternatively in-band control data. In-band control data is control data transmitted using the TCH.

The first, second and third base stations 102, 104, 106 are capable of transmitting the TCH data assigned to the second, fourth, sixth, seventh and eighth time slots TS2, TS4, TS6, TS7, TS8 over a number of predetermined frequencies f,,..., fn. The first, second and third base stations 102, 104, 106 have a frequency hopping algorithm for selecting one of the number of predetermined frequencies f,,... fn. The frequency hopping algorithm can be any frequency hopping algorithm known in the art. Parameters relating to the frequency hopping algorithm are assigned at call set up to each of the second, fourth, sixth, seventh and eighth time slots TS2, TS4, TS6, TS7, TS8 for frequency hopping.

For the purpose of simplicity and clarity, operation 300 of the above described apparatus will be described below with the reference to the first base station 102.

A first data frame 103, having the data frame structure 200, is generated by the BSC 116 and transmitted to the first base station 102. Similarly, a second and a third data frame 105, 107 is generated and transmitted to the second and third base stations 104, 106, respectively.

Referring to FIG. 3, the first base station 102 awaits receipt, from the BSC 116 of a given time slot, such as the first time slot TS1, in the series of time slots TS1,... ,TS8 of the first data frame 103. The first base station 102 analyses the given time slot and determines whether the given time slot contains BCCH data (step 302). If the given time slot contains BCCH data (as in the case of the first time slot TS1), the first base station 102 transmits (step 304) the first time slot TS1 at a first single predetermined frequency f, (which is fixed by the GSM standard) and then continues to await receipt of a next time slot in the series of time slots TS1, ..., TS8 from the BSC 116. No frequency hopping is performed in relation to the transmission of the BCCH data.

If the given time slot received does not contain BCCH data, the first base station 102 determines whether the time slot received contains SDCCH data (step 306). If SDCCH data is detected (such as in the case of the third time slot TS3), the first base station 102 transmits (step 308) the third time slot TS3 on the first single predetermined frequency fO, as described above in relation to the BCCH data. The first base station 102 then continues to await receipt of the next time slot in the series of time slots TS1, ..., TS8 from the BSC 116. Similarly, no frequency hopping is performed in relation to the transmission of the SDCCH data.

If the time slot received contains neither BCCH nor SDCCH data, the first base station 102 determines whether the given time slot contains TCH data or is an empty time slot (as in the case of the second, fourth, sixth, seventh and eighth time slots TS2, TS4, TS6, TS7, TS8). If the TCH data relates to an empty time slot, "dummy" BCCH data is inserted into the given time slot and transmitted (step 312) on the first single predetermined frequency f0 as described above in relation to the BCCH data. The first base station 102 then awaits the next time slot from the BSC 116. No frequency hopping is performed in relation to the transmission of the "dummy" BCCH data.

If the TCH data relates to application data of the type described above, the first base station 102 initiates a frequency hopping algorithm based on parameters assigned to the time slot by the BSC 116 at call set up. Different parameters relating to the frequency hopping algorithm are assigned to different time slots. Each time the first base station 102 determines that TCH data comprising application data is to be transmitted, the next frequency dictated by the frequency hopping algorithm is selected.

Thus, the first base station 102 selects a frequency in accordance with the frequency hopping algorithm for the time slot and transmits (step 314) the time slot on the frequency selected. The first base station 102 then awaits the next time slot from the BSC 116.

Once each of the series of time slots TS1, ..., TS8 of the first data frame 103 have been transmitted by the first base station 102, the first base station 102 awaits receipt of a subsequent data frame having the data frame structure 200 for transmission. The above process continues (in relation to the first base station 102) until it is no longer necessary for the first base station 102 to transmit data frames.

Although the above operation has been described in relation to the first base station 102, the second and third base stations 104, 106 operate in a substantially similar manner. Additionally, the above example has been described in the context of a picocell system, but it is conceivable to apply the above example to a microcell system, or any other simulcast BCCH system.

Although not described in the above example, it is conceivable to transfer the above described functionality to the BSC 116.

It is thus possible to provide frequency hopping for a single carrier picocell system. Consequently, the probability of traffic channels hopping to a same frequency is reduced sufficiently to maintain a desirable quality of service, thereby obviating the need for frequency planning.

Claims

1. A method of transmitting data in a time slot between a base station and a terminal using a single carrier unit, the method comprising the steps of: determining whether the time slot is to contain data relating to a traffic channel or data relating to a control channel, selecting, in response to the time slot containing data relating to the traffic channel, a frequency for transmission of the time slot in accordance with a predetermined sequence of frequencies, and transmitting the data in the time slot at the frequency.

2. A method as claimed in Claim 1, wherein the time slot is one of a plurality of time slots in a data frame, and further comprising selecting the time slot from the plurality of time slots before determining whether the time slot is to contain data relating to a traffic channel or data relating to a control channel.

3. A method as claimed in any one of the preceding claims, wherein the data relating to the traffic channel is application data.

4. A method as claimed in any one of the preceding claims, wherein the data relating to the traffic channel is in-band control data.

5. A method as claimed in any one of the preceding claims, wherein the data relating to the control channel is out-of-band control data.

6. A method as claimed in Claim 5, wherein the out-of-band control data is BCCH data.

7. A method as claimed in Claim 5, wherein the out-of-band control data is SDCCH data.

8. A method as claimed in any one of the preceding claims, wherein the predetermined sequence of frequencies is a frequency hopping algorithm.

9. A method as claimed in any one of the Claims 5 to 7, further comprising transmitting the out-of-band control data on a single predetermined frequency.

10. A method as claimed in any one of the Claims 5 to 9, further comprising providing a further base station and transmitting the out-of-band data to both the base station and the further base station.

11. A method as claimed in any one of the preceding claims, wherein the single carrier unit is a transmitter.

12. A method as claimed in any one of Claims 1 to 10, wherein the single carrier unit is a receiver.

13. A method as claimed in any one of Claims 1 to 10, wherein the single carrier unit is a transceiver.

14. A method as claimed in any one of Claims 1 to 10, wherein the single carrier unit is a transmitter/receiver chain.

15. A base station apparatus for transmission of data in a time slot to a terminal, the base station comprising a single carrier unit and a monitoring element for determining whether the time slot is to contain data relating to a traffic channel or data relating to a control channel, the base station being arranged to select, in response to the time slot containing data relating to the traffic channel, a frequency for transmission of the data in the time slot in accordance with a predetermined sequence of frequencies.

16. A picocell system comprising the base station of Claim 15.

17. A microcell system comprising the base station of Claim 15.

18. A communication system substantially as hereinbefore described with reference to FIG. 1.

19. A method of transmitting a time slot substantially as hereinbefore described with reference to FIGs. 2 and 3.