SYCHRONISATION APPARATUS AND METHOD FOR TDMA CELLULAR COMMUNICATIONS

Abstract: A communications system (100) comprises a first remote terminal (110) capable of transmitting a first data burst to a first base station (106) within a first cell (102). A monitoring element (105) is arranged to generate a power profile corresponding to the first cell (102) and monitor electromagnetic interference from a second cell (104) and determine an optimal time for transmitting the first data burst in response to the monitored electromagnetic interference.

Description: SYNCHRONISATION APPARATUS AND METHOD FOR TDMA CELLULAR COMMUNICATIONS Field of the Invention The present invention relates to a synchronisation apparatus and a method therefor, of the type used in a time division cellular telephone system, for example, a TDMA (Time Division Multiple Access) system, such as a GSM (Global System for Mobile Communications) system. Background of the Invention In a TDMA based cellular telephone system, signals are transmitted, in bursts, between a given user terminal and a base station. These bursts are known as in the art as TDMA bursts; a desired TDMA burst corresponds to a particular signal transmission between the given user terminal and the base station. Additionally, other TDMA bursts between the base station and other user terminals propagate partially or substantially at the same time as the desired TDMA burst. If signal orthogonality is not achieved, for example, by means of frequency planning, the other TDMA bursts can overlap, fully or partially, with the desired TDMA burst, the result of which being problematic: Firstly, upon receipt of the desired TDMA burst, the midamble of the desired TDMA burst can be subject to high interference levels, caused by the full or partial overlap with the other TDMA bursts. Secondly, in a system employing adaptive antennas, during transmission of the desired burst, the profile of the interference caused by the other TDMA bursts may vary due to partial overlap of the TDMA bursts, thereby requiring different antenna weight settings at the base station. It is thus an object of the present invention to obviate or mitigate the above described problems associated with TDMA bursts. Summarv of the Invention According to a first aspect of the present invention, there is provided a communications system comprising a first remote terminal capable of transmitting a first data burst to a first base station within a first cell, and a monitoring element arranged to monitor within the first cell electromagnetic interference from a second terminal and determine an optimal time for transmitting a training sequence of the first data burst in response to the monitored electromagnetic interference. According to a second aspect of the present invention, there is provided a base station for a first cell capable of receiving a first data burst from a first remote terminal, comprising a monitoring element arranged to monitor within the first cell electromagnetic interference from a second terminal and determine an optimal time for transmitting a training sequence of the first data burst in response to the monitored electromagnetic interference. According to a third aspect of the present invention, there is provided a remote terminal capable of transmitting a first data burst to a first base station in a first cell; a monitoring element arranged to monitor within the first cell electromagnetic interference from a second terminal and determine an optimal time for transmitting a training sequence of the first data burst in response to the monitored electromagnetic interference. According to a fourth aspect of the present invention, there is provided a method of transmitting a first data burst from a first remote terminal to a first base station in a first cell, the method comprising the steps of: monitoring within the first cell electromagnetic interference from a second terminal, and determining an optimal time for transmitting a training sequence of the first data burst in response to the monitored electromagnetic interference. It is thus possible to provide a synchronisation apparatus and method therefor which facilitates the reduction of overlap of data bursts, thereby reducing interference. Other, preferred, features and advantages are set forth in, and will become apparent from, the following description and accompanying dependent claims. 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 a schematic illustration of a pair of adjacent cells; FIG. 2 is a schematic diagram of the structure of a data frame; FIG. 3 is a schematic diagram of a data burst in a time slot; FIG. 4 is a schematic side elevation of FIG. 1; FIG. 5 is a schematic diagram of overlapping data bursts; FIG. 6 is a power v. time graph for data bursts relating to an embodiment of the present invention; FIG. 7 is a flow diagram for the transmission of data bursts of FIG. 5, and FIG. 8 is a schematic diagram of data bursts constituting an embodiment of the present invention. Description of a Preferred Embodiment The following example relates to a GSM system. However, it is not intended to limit the present invention to such a system and other time division systems can be used to implement the invention. A TDMA system 100 (FIG. 1) comprises a first cell 102 and a second cell 104, respectively having a first base station 106 and a second base station 108. A monitoring element 105 is coupled to the first base station 106. A first mobile terminal or handset 110 is located within the first cell 102 and a second mobile terminal or handset 112 is located within the second cell 104. The first and second base stations 106, 108 can be of any type known in the art suitable for the appropriate time division system being used, for example, a set of M-Cell base stations manufactured by Motorola, Limited. The first and second handsets 110, 112 can, similarly, be any known cellular telephone handset, for example a STARTAC ® GSM cellular telephone manufactured by Motorola GmbH. Referring to FIG. 2, the first handset 110 is capable of transmitting a first repeating series of sequential data bursts H^, H^, H^, ..., H X B 7 (hereinafter referred to as "the first series") within a set of respective sequential time slots S 0 , S x , S 2 , ..., S 7 . A first data burst H X B 0 lasts 577μs and comprises a midamble 300 (FIG. 3) of 26 bits, the midamble 300 being located within the centre of the first data burst HJS Q . Data bursts H^, H ] B 2 , ..., also have midambles 300. As is known in the art, the midamble 300 of the first data burst H J B Q comprises a predetermined series of known bits. The second handset 112 is also capable of transmitting a second repeating series of sequential data bursts H 2 B 0 , H 2 B X , H 2 B 2 , ..., H 2 B 7 (hereinafter referred to as "the second series") within a set of respective time slots S 0 , S j , S 2 , ..., S 7 . The set of respective time slots S 0 , S 1} S 2 , ..., S 7 of the first series can be in synchronism with the respective time slots S 0 , S 1? S 2 , ..., S 7 of the second series, although this is not an essential requirement. It should be appreciated that due to propagation delays, the arrival of data bursts at, for example, the first base station 106 will differ in time - even for a system employing time slots which are in synchronism. The data bursts have the same structure as described above in relation to the first data burst H J B Q . Operation of the embodiment of the invention is described below. Referring to Fig. 4, the first handset 110 transmits the first seriesH j Bo, ..., H 1 B 7 to the first base station 106. Similarly, the second handset 112 transmits the second series H 2 B 0 , ..., H 2 B 7 to the second base station 108. Due to the nature of propagation of Radio Frequency (RF) signals., the first base station 106 is able to receive the second series H 2 B 0 , ..., H 2 B 7 transmitted by the second handset 112. Consequently, the receipt of the first series H^, ..., H j B 7 at the first base station 106 is impaired by the additional receipt of the second series H 2 B 0 , ..., H 2 B 7 at the first base station 106, i.e. interference occurs. This example will be further described in the context of a first data burst H ] B 0 transmitted by the first handset 110 and a second data burst H 2 B 0 transmitted by the second handset 112. However, this example is being given for clarity and simplicity of description and it is not intended that the present invention be limited to the interference between the first and second data bursts H X B 0 , H 2 B 0 . Indeed, interference can occur between any one of the data bursts comprising the first series H^ Q , ..., ϋ 1 1 and any one of the data bursts comprising the second series H 2 B 0 , ..., H 2 B 7 . As can be seen from FIG. 5, the first data burst Η.j 0 from the first handset 110 temporally overlaps with the first data burst H 2 B 0 from the second handset 112. Temporal overlap of the first data burst H^ from the first handset 110 can also occur with any one of the data bursts H 2 B 0 , ..., H 2 B 7 from the second handset 112, or any other handset transmitting data bursts from a neighbouring cell (not shown). In order to avoid interference between the first series H]B 0 , ..., H^ and the second series H 2 B 0 , ..., H 2 B 7 , it is sufficient to ensure that the midamble 300 of the first data burst H j Bofrom the first handset 110 does not partially interfere/overlap with the midamble of the first data burst H 2 B 0 from the second handset 112. By avoiding partial overlap of midambles, it is possible to extract the remains of any overlapping data bursts using signal processing techniques known in the art, for example, spatial filtering. In order to avoid partial overlap of midambles, it is necessary to determine an optimal time to transmit the first data burst H J B Q , such that transmission of the first data burst H^ Q from the first handset 110 burst will not cause the midamble of the data burst to partially overlap with the midamble 300 of the first data burst H j B 0 from the second handset 112 and thereby cause interference. Referring to FIGs. 6 and 7, in order to determine the optimal time to transmit the first data burst H j B o , a power profile 600 of interference is generated (step 700) by the monitoring element 105. The power profile can be generated, for example, before a call is set up or during non-transmission periods, such as Discontinuous Transmission (DTX) of voice or data, using amplitude measurement techniques known in the art. Alternatively, the power profile 600 can be generated by cross-correlating the interfering signals transmitted by other terminals with the midamble of the data burst to be transmitted, for example, the first data burst H X B 0 . The result of the above cross-correlation or amplitude measurement is a burst duration profile of the interfering signals. The burst duration profile can be time averaged over several bursts to minimise fluctuations in the burst duration profile caused by multipath fading. The power profile is monitored (step 702) in order to detect (step 704) the optimal time to transmit the first data burst HjB 0 . Although not essential to the present invention, the burst duration profile will also provide sufficient guidance for the minimum required transmit power setting to provide enough processing gain for the successful convergence of antenna weights. The minimum required transmit power setting, Setting,^,.^, is calculated on the following basis: Setting,,^ = InterferLev abs + PathLoss est + Margin^ where: InterferLev abs is the measures absolute interference level during an optimum transmission period, PathLoss est is the estimated path loss based on the distance of the mobile from the base station, and Margin req is the required margin for successful weight convergence. Once the optimal time with least interference and the minimum required transmit power setting minimum requirement for transmission of the first data burst H j B 0 have been determined (step 704), the timing advance associated with the transmission of the first data burst H X B 0 can be varied in order to ensure that the midamble 300 of the first data burst H^ is transmitted at the optimal position as shown in FIG. 8, so that the midamble 300 of the first data burst H 1 B 0 from the first handset 110 does not temporally partially overlap the midambles of other data bursts, thereby obviating or mitigating interference. The first data burst H j B 0 is then transmitted (step 706) and the above process is repeated for a next data burst to be transmitted.

Claims: Claims 1. A communications system comprising a first remote terminal capable of transmitting a first data burst to a first base station within a first cell, and a monitoring element arranged to monitor within the first cell electromagnetic interference from a second terminal and determine an optimal time for transmitting a training sequence of the first data burst in response to the monitored electromagnetic interference. 2. A system as claimed in Claim 1, further comprising a second cell, having a second base station the second terminal being located within the second cell and capable of transmitting a second data burst to the second base station, the electromagnetic interference from the second terminal including electromagnetic energy corresponding to the second data burst. 3. A system as claimed in Claim 1, wherein the training sequence is a midamble and the optimal time for transmitting the first data burst is the optimal time for transmitting the midamble of the first data burst. 4. A system as claimed in Claim 1, wherein the monitoring element is arranged to cross-correlate the midamble of the first data burst with the electromagnetic interference from the second terminal. 5. A system as claimed in Claim 1, wherein the first cell comprises a further remote terminal capable of transmitting a further data burst to the first base station, the electromagnetic interference including electromagnetic energy corresponding to the further data burst. 6. A base station for a first cell capable of receiving a first data burst from a first remote terminal, comprising a monitoring element arranged to monitor within the first cell electromagnetic interference from a second terminal and determine an optimal time for transmitting a training sequence of the first data burst in response to the monitored electromagnetic interference. 7. A base station as claimed in Claim 6, a second cell having a second base station is provided, the second remote terminal capable of transmitting a second data burst to the second base station, the electromagnetic interference from the second terminal including electromagnetic energy corresponding to the second data burst. 8. A base station as claimed in Claim 6, wherein the training sequence is a midamble and the optimal time for transmitting the first data burst is the optimal time for transmitting the midamble of the first data burst. 9. A base station as claimed in Claim 6, wherein the monitoring element is arranged to cross-correlate the midamble of the first data burst with the electromagnetic interference from the second terminal. 10. A base station as claimed in Claim 6, wherein the first cell comprises a further remote terminal capable of transmitting a further data burst to the first base station, the electromagnetic interference including electromagnetic energy corresponding to the further data burst. 11. A remote terminal capable of transmitting a first data burst to a first base station in a first cell, the first base station having a monitoring element arranged to monitor within the first cell electromagnetic interference from a second terminal and determine an optimal time for transmitting a training sequence of the first data burst in response to the monitored electromagnetic interference. 12. A remote terminal as claimed in Claim 11, wherein a second cell having a second base station is provided, the second base station being located within the second cell and capable of transmitting a second data burst to the second base station, the electromagnetic interference from the second cell including electromagnetic energy corresponding to the second data burst. 13. A remote terminal as claimed in Claim 11, wherein the training sequence is a midamble and the optimal time for transmitting the first data burst is the optimal time for transmitting the midamble of the first data burst. 14. A remote terminal as claimed in Claim 11, wherein the monitoring element is arranged to cross-correlate the midamble of the first data burst with the electromagnetic interference from the second terminal. 15. A remote terminal as claimed in Claim 11, wherein the first cell comprises a further remote terminal capable of transmitting a further data burst to the first base station, the electromagnetic interference including electromagnetic energy corresponding to the further data burst. 16. A remote terminal as claimed in Claim 11, further comprising a transmitter arranged to transmit the first data burst in response to the determination by the first base station of the optimal time for transmitting the first data burst. 17. A method of transmitting a first data burst from a first remote terminal to a first base station in a first cell, the method comprising the steps of: monitoring within the first cell electromagnetic interference from a second terminal, and determining an optimal time for transmitting a training sequence of the first data burst in response to the monitored electromagnetic interference. 18. A method as claimed in Claim 17, further comprising providing a second cell having a second base station and transmitting a second data burst from a second terminal to the second base station in the second cell, the electromagnetic interference from the second cell including electromagnetic energy corresponding to the second data burst. 19. A method as claimed in Claim 17, wherein the training sequence is a midamble, and comprising the step of transmitting the first data burst at the optimal time for transmitting the midamble of the first data burst. 20. A method as claimed in Claim 17, further comprising cross-correlating the midamble of the first data burst with the electromagnetic interference from the second terminal. 21. A method as claimed in Claim 17, further comprising transmitting a further data burst from a further remote terminal in the first cell to the first base station, the electromagnetic interference including electromagnetic energy corresponding to the further data burst.

Inventors: MOHEBBI BEHZAD, MOHEBBI, BEHZAD
Applicants: MOHEBBI BEHZAD, MOTOROLA LTD, MOHEBBI, BEHZAD, MOTOROLA LIMITED
Application year: 1998
Main classification: H04B7
IPC codes: H04B7, H04W24