Method and means for allocating time slots in a TDD system

0001The present invention relates to a communication method and unit for communicating in a time division duplex communication system.

0002In a time division duplex communication system, examples of which are the GSM-standard for outdoor mobile communication or the DECT-standard for indoor mobile telephone communication, the receiving and the transmitting channels are separated on a time basis. E.g. in the DECT-standard, the information is transmitted in time frames of a length of 10 ms having 24 time slots. The first 12 time slots are used for the downlink connection, that is the transmission of data from a base station to a mobile station. The last 12 time slots are used for the uplink connection, that is for the transmission of data from a mobile station to the base station. In the GSM-standard, a combination of a frequency division duplex and a time division duplex is used. The information is transmitted in time frames having 8 time slots, whereby the uplink connection is provided in a lower frequency band and the downlink connection is provided in a higher frequency band. Each of the 8 time slots in each frame is assigned to a different mobile station. Thereby, the time slots assigned to a certain mobile station within a downlink time frame are separated from the time slots assigned to the same mobile station in the uplink time frame by two time slots. In other words, if e.g. the first time slot in a downlink time frame is assigned to a certain mobile station, the fourth time slot in an uplink time frame is assigned to the same mobile station. The time basis separation of the uplink and downlink time slots enables the construction of the mobile stations to be made more simple, since the reception and the transmission of data does not take place simultaneously.

0003Since in telecommunication systems as e.g. the GSM-system the data transfer rate is restricted, attempts have been made to increase the data transfer rate by allocating more than one time slot per frame to a certain mobile station. In <patcit id="pcit0001" dnum="JP05153033A"><text>JP 05153033-A</text></patcit> such a TD digital mobile telecommunication system is disclosed, in which the same frequency for transmitting and receiving information is used. 1 to N time slots within one uplink time frame are allocated to one mobile station and 1 to N time slots within one downlink time frame are allocated to one mobile station depending on the information volume to be transferred between the mobile station and a base station.

0004Each frame is allocated either to the uplink transfer of data or the downlink transfer of data. The uplink time frames cannot e used for a downlink transfer of data, so that a strong asymmetric transmission of information with a large difference between the amount of uplink data and the amount of downlink data is not possible.

0005In <patcit id="pcit0002" dnum="JP07107546A"><text>JP 07107546-A</text></patcit>, a TDMA radio communication system is disclosed, in which the ratio between the number of uplink and downlink time slots within one time frame or one super frame consisting of several time frames is changed according to the total amount of traffic between a base station and mobile stations. In case of fast changing data transfer amounts, the switching point within each frame between the uplink time slots and the downlink time slots often changes position. Every change of such a switching point requires a reallocation of several time slots for the different connected mobile stations. This known systems therefore requires a complicated circuitry.

0006The slot allocation method disclosed in <patcit id="pcit0003" dnum="EP654916A2"><text>EP 654916-A2</text></patcit> suffers from the same problems.

0007The document of <nplcit id="ncit0001" npl-type="s"><text>LAN CHEN ET AL: "A dynamic channel assignment algorithm for asymmetric traffic in voice/data integrated TDMA/TDD mobile radio</text></nplcit>" discloses a personal communications system wherein an algorithm moves the TDD boundary adaptively whenever it is required.

0008The document <patcit id="pcit0004" dnum="EP0670640A"><text>EP 0 670 640 A</text></patcit> discloses a radio communications system of the DECT type, whereby said system operates with a TDMA/TDD structure and with time slots for transmission of, inter alia, data. Moreover, two adjacent time slots are combined to form a double time slot.

0009The object of the present invention is therefore to provide a communication method and a communication unit for transmitting and receiving information in a time division duplex communication system, which allow a simple and efficient time slot allocation for varying transfer information amounts.

0010The above object is achieved by a communication method according to claim 1 and a communication unit according to claim 5. Advantageous features of the invention are defined in the respective sub-claims.

0011In the following description, preferred embodiments of the present invention are explained relating to the accompanying drawings, in which <ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">figure 1</figref> shows an example of a fixed block comprising one receiving time slot and one transmitting time slot being located at the beginning of respective time frames,</li><li><figref idref="f0001">figure 2</figref> shows some time frames whith additional transmitting tine slots and additional receiving time slots,</li><li><figref idref="f0001">figure 3</figref> shows some other time frames, wherein one of the time frames is saturated with data to be transmitted or received, so that an additional switcching point is present,</li><li><figref idref="f0001">figure 4</figref> shows an enlarged section of <figref idref="f0001">figure 3</figref> showing a timing advance of an additional transmitting time slot adjacent to a preceding receiving time slot, and</li><li><figref idref="f0001">figure 5</figref> shows a schematic example of a communication unit comprising a means for allocating time slots according to the present invention.</li></ul>

0012In <figref idref="f0001">figure 1</figref>, three time frames F₁, F₂ and F₃ are schematically shown. Each frame contains e.g. eight time slots, as in a GSM-system. Although all time frames shown in <figref idref="f0001">figure 1, 2 and 3</figref> comprise eight time slots, the present invention is not limited to this case and the time frames can comprise any other required number of time slots. In each frame, the first two time slots 1 and 2 build a fixed block comprising a transmitting time slot 1 and a receiving time slot 2. In case that the present invention is implemented e.g. in a mobile station of a telecommunication system, the transmitting time slot is an uplink time slot for transmitting data or information from the mobile station to a base station, and the receiving time slot 2 is a downlink time slot for transmitting data from the base station to the mobile station. The transmitting time slot 1 and the receiving time slot 2 are thus assigned to a certain pair of communication units, e.g. a base station and a mobile station. The base station can thereby be part of the multiple access communication system, in which one frame is assigned to several mobile stations. The fixed block comprising the transmitting time slot 1 and the receiving time slot 2, however, is always on a fixed position.

0013In the first and second frame F₁ and F₂ shown in <figref idref="f0001">figure 1</figref>, the six remaining time slots 3 to 8 in each frame are not used for transferring information. Since the transmitting time slot 1 is placed in advance of the receiving time slot 2, this slot allocation can cope with timing advance as explained above. For a base station, the timing advance is adjusted by adjusting the timing of the time slots transmitted from the base station to the mobile station.

0014In the example shown in <figref idref="f0001">figure 1</figref>, the first frame F₁, the next frame F₂ and the third frame F₃ are not saturated since only the transmitting time slot 1 and the transmitting time slot 2 of the fixed flock are used to transfer information in each of the frames.

0015The last time slot 8 of the third frame F₃ is an additional transmitting time slot of the fixed block of the fourth frame F₄ of <figref idref="f0001">figure 4</figref>.

0016In <figref idref="f0001">figure 2</figref>, information are transmitted in the transmitting time slot 1 and the receiving time slot 2 forming a fixed block in each of the shown frames F₄, F₅ and F₆, as in the first example shown in <figref idref="f0001">figure 1</figref>. However, since there is more information to send and to receive, an additional transmitting time slot 8 is added before the time slot 1 of the fixed block in the time axis direction. Additional receiving time slots 3 and 4 are added behind the receiving time slot 2 in the time axis direction. Thus, an increased amount of information or data can be transferred between a mobile station and a base station or between several mobile stations and one base station. In the later case, the additional transmitting and/or receiving time slots can be allocated to different mobile stations. For example in the fifth time frame F₅, the additional receiving time slots 3, 4, 5 can be allocated to one or more different mobile stations. The position of the basis block, however, remains unchanged, so that the switching point between transmitting and receiving information, which is located between the first time slot 1 and the second time slot 2 in each frame, remains on the same position. This switching point is the only switching point, since the time frames are not saturated with information to be transferred. As can be seen from <figref idref="f0001">figure 2</figref>, according to the present invention, the number of additional transmitting time slots and additional receiving time slots an be increased independently, so that an asymmetric transmission of data is possible. In case of a multiple access communication system, in which one time frame is assigned to several mobile stations, the additional time slots can be allocated to one or more different mobile stations. For a certain mobile station, the time slots to be transmitted or received may not come in a regular interval. However, the pattern of the slot allocation is maintained and continued over at least several frames, e.g. two frames F₁ and F₂ as shown in <figref idref="f0001">figure 1</figref>. In <figref idref="f0001">figure 2</figref>, the slot allocation pattern changes for the succeeding frames F₄, F₅ and F₆. The pattern of the slot allocation is advantageously not changed frame by frame, but is changed only, when the required data amount to be transferred is changed. This is the case for the time frame shown in <figref idref="f0001">figure 2</figref>, in which the data amount to be transferred is reduced to one transmitting time slot from frame F₄ to frame F₅ and the receiving time slot 2 from frame F₅ to frame F₆.

0017In case of a multiple access communication system, in which different time slots are assigned to different mobile stations, the time slots 3 and 4 being used as additional receiving time slots and the time slot 8 being used as additional transmitting time slot in the time frame F₄ can be assigned to a second mobile station, when the transmitting time slot 1 and the receiving time slot 2 or the fixed block are assigned to a first mobile station. The time slots 3 and 4 can also be allocated to a second mobile station and a third mobile station, respectively.

0018In case that the amount of information to be transferred is further increased, the maximum information transfer rate can e achieved by using all the time slots in each time frame for transferring data, as shown in <figref idref="f0001">figure 3</figref> for the time frame F₇. In the shown example, the transmitting time slot 1 and the receiving time slot 2 are still on their fixed position at the beginning of each time frame. The time slots 3 to 7 of the frame F₇ are used as additional receiving time slots. Thereby, the different time slots 3 to 7 can be assigned or allocated to different mobile stations. An additional transmitting time slot 8 is also used in the time frame F₇, so that the last additional receiving time slot 7 and the succeeding additional transmitting time slot 8 are adjacent to each other. If in this situation the base station is located close to the mobile station, so that the propagation delay is small, there is no serious problem. If, however, the base station is located far from the mobile station, e.g. a few kilometre, the mobile station has to transmit the transmitting time slot 8 in advance to compensate for the propagation delay. In other words, a timing advance is necessary. Therefore, the mobile station has less time to receive the last additional receiving time slot 7. This situation is shown in more detail in <figref idref="f0001">figure 4. Figure 4</figref> shows a section of <figref idref="f0001">figure 3</figref> with the last additional receiving time slot 7 and the additional transmitting time slot 8 of the preceding frame F₇ as well as the transmitting time slot 1 of the succeeding time frame F₈. As can be seen from <figref idref="f0001">figure 4</figref>, the last portion of the receiving time slot 7 is emptied and used as a guard period to enable an earlier transmission of the additional transmitting time slot 8. It has to be understood, that the timing advance problem only occurs, when a receiving time slot and a succeeding transmitting time slot are adjacent to each other, which are assigned to the same mobile station. It is therefore advantageous, not to allocate successive transmitting and receiving time slots to one mobile station in this case.

0019In time frame F₈ following time frame F₇ with the maximum information transfer, the amount of information to be transferred is reduced and only the time slots 3, 4 and 5 are allocated as additional receiving time slots. In the following time frame F₉, the amount of information to be transferred is further reduced to the basic block comprising the transmitting time slot 1 and the receiving time slot 2.

0020In <figref idref="f0001">figure 5</figref>, a communication unit 10, in which the present invention is incorporated or implemented, is schematically shown. The communication unit 10 can e.g. be a mobile station or a base station of a mobile telecommunication system.

0021The communication unit 10 comprises an antenna 11, through which information modulated onto respective carrier frequencies can be transmitted and received. The communication unit 10 comprises a receiving means 12, which receives incoming information through the antenna 11 and supplies the received information to a control unit 13, in which the received information are demodulated, decoded, etc. In a known manner. The control unit 13 comprises an allocation means 15, in which the time slots of the predetermined time frames are allocated depending on the amount of information to be transferred as receiving or transmitting time slots according to the method explained above. The control unit 13 can thus also comprise a means for determining the amount of information to be transferred, i.e. received or transmitted to give corresponding information to the allocation means 15, so that the allocation means 15 correspondingly allocates the time slots as receiving or transmitting time slots depending an the amount of transfer information. The allocation means 15 of the control unit 13 allocates the time slots according to the slot allocation method explained above in relation to <figref idref="f0001">figures 1 to 4</figref>. Thereafter, the control unit 13 provides a transmission means 14 with corresponding information to be transmitted within the correspondingly allocated time slots by means of the antenna 11 to another communication unit. The control unit 13 can further comprise a guard period means 16, which, in case that an additional receiving time slot and an additional transmitting time slot become adjacent to each other, e.g. in the case shown in <figref idref="f0001">figures 3 and 4</figref>, provides a guard period in at least one of said adjacent additional time slots. As stated above, this situation becomes only relevant in the case that the preceding receiving time slot and the succeeding transmitting time slot are assigned to the same communication unit 10. In this case it is advantageous, if the guard period means 16 provides said guard period at the end of said additional receiving time slots, e.g. the additional receiving time slot 7 of time frame F₇, shown in <figref idref="f0001">figure 3 and 4</figref>.