METHOD FOR ALLOCATING RADIO RESOURCES TO MOBILE STATIONS, TELECOMMUNICATIONS NETWORK AND MOBILE STATION

Abstract: The invention relates to a method for allocating radio resources in a telecommunications system to mobile stations (A, B, C) transmitting data in time slots of TDMA frames in uplink direction and receiving data in time slots of TDMA frames in downlink direction and to such to a telecommunications network and such a mobile station. In order to improve the allocation, the method comprises in the case that a first mobile station A is using a first time slot (1) in a first direction and requires a time slot in the opposite direction and the corresponding fixed duplex distance time slot (1) in the opposite direction is used by a second mobile station (B), the steps of: a) determining a free time slot (4) in the opposite direction; b) moving the connection of the second mobile station (B) to the determined free time slot (4) in the opposite direction and c) allocating the corresponding time slot (1) in the opposite direction to the first mobile station (A).

Description: Method for allocating radio resources to mobile stations, telecommunications network and mobile station FIELD OF THE INVENTION The invention relates to a method for allocating radio resources in a telecommunications system to mobile stations transmitting data in time slots of TDMA (time division multiple access) frames in uplink direction and receiving data in time slots of TDMA frames in downlink direction. The invention equally relates to such a telecommunications network and such a mobile station. BACKGROUND OF THE INVENTION Mobile stations employed in telecommunication systems are usually designed as full-duplex or half duplex mobile stations . Full-duplex mobile stations are able to receive speech in time slots of TDMA frames in a downlink direction and transmit speech in time slots of TDMA frames in an uplink direction at the same time. To this end, they require a receiver and a transmitter with their own hardware and a duplex filter for enabling receiver and transmitter of the mobile station to use the same antenna at the same time. Half-duplex mobile stations are also able to transmit speech in time slots of TDMA frames in an uplink direction and receive speech in time slots of TDMA frames in a downlink direction, but not simultaneously. Accordingly, no duplex filters are needed and some of the hardware of receiver and transmitter can be shared. Therefore, half-duplex mobile stations are less complex and less expensive than full- duplex mobile stations. To support the use of half-duplex mobile stations, in known GSM (Global System for Mobile communication) systems the TDMA frames in uplink and downlink direction have been linked together in order to provide a fixed duplex distance in frequency and in time. Figure 1 illustrates the offset between downlink and uplink TDMA frame structures . Three rows of time slots in the upper part of figure 1 represent the TDMA structures of three different frequencies cO, cl, c2 used in downlink direction from the cell serving a specific half-duplex mobile station. Three rows of time slots in the middle part of figure 1 represent the TDMA structures of three different frequencies cO ' , cl ' , c2 ' used in uplink direction to the serving cell. Frequencies cO and cO ' have a fixed distance, providing the fixed duplex distance in frequency; the same applies to frequencies cl, cl ' and c2, c2 ' . Two rows of time slots at the bottom of figure 1 represent the TDMA structures of two different frequencies do, eO used for downlink connections by adjacent cells . The rows are made up of subsequent TDMA frames, each frame being divided into 8 time slots 0-7. The boundaries between the TDMA frames related to the connections to the serving cell are indicated by thick vertical lines. The boundaries between the time slots are indicated for frequencies cO, c2 , cO ' , c2 ' , dO and eO by thin vertical lines. The numerals 0-7 of the time slots are included for those TDMA frames used by the half-duplex mobile station considered in this example. A timing advance procedure is performed by the mobile station to compensate for the variable propagation delay from the base station to the mobile station and back. Timing advance is realised by slightly advancing the transmission of the mobile station, as indicated in the figure by the black boxes in uplink timeslots 3. As can bee seen in the figure, there is an offset of three time slots between the frames in downlink direction and the frames in uplink direction so that time slot 3 of the TDMA frames in uplink direction always occurs at the same time as time slot 0 of the TDMA frames in downlink direction. This provides for the fixed duplex distance in time, not taking into account the small variation caused by the timing advance procedure. In the example of figure 1, time slot 3 is assigned to the mobile station for transmission and reception of speech signals. The mobile station receives and transmits data alternately, each time slot used being emphasized and the sequence of time slots involved being indicated by arrows. Because of the offset between downlink and uplink TDMA frame structures, the mobile station has a known transition time for shifting from reception to transmission and back and also for carrying out neighbor cell measurements in between. The first action indicated in figure 1 is reception Rx of speech data in time slot 3 of the first completely depicted TDMA frame of the downlink connection with frequency cO, followed by transmission Tx of speech data in time slot 3 of the first completely depicted TDMA frame of the uplink connection with corresponding frequency cO ' . As can be taken from the figure, the mobile station has a transition period of two time slots for switching from reception to transmission. Before switching to the downlink direction for reception Rx again, the mobile station has time to monitor the downlink of an adjacent cell using frequency dO . The transition period available for switching from transmission to monitoring is somewhat more than one time slot. Equally, the transition period available for switching from monitoring to regular reception again is somewhat more than one time slot. The reception Rx following the monitoring is carried out by using time slot 3 of the second completely depicted downlink TDMA frame of frequency cl. Accordingly, the next transmission Tx takes place in time slot 3 of the second completely depicted uplink TDMA frame of corresponding frequency cl ' . After monitoring of the downlink of another adjacent cell transmitting with frequency eO, there is again reception in time slot 3 of the following downlink TDMA frame of a suitable frequency, and so forth. In order to be able to support more users with the same number of radio channels, statistical multiplexing of the mobile stations has been proposed. In statistical multiplexing, the physical radio resource can be given to the user of another mobile station whenever the user of the first mobile station enters a silent period, i.e. stops either talking or receiving speech. The document "GERAN concept proposal", ETSI STC SMG2 # 34, January 2000, Tdoc SMG2 198/00, Lucent Technologies, p. 7, deals with statistical multiplexing of speech. According to this document, the breaking of the association between uplink and downlink channel is necessary in order to maximize the resource pool available for assignments. This way, an independent frequency and slot allocation in the uplink and downlink directions becomes possible and available resources can be allocated dynamically as required. A completely independent allocation of time slots in uplink and downlink direction, however, requires full- duplex mobile stations, since only a linked uplink and downlink ensures the transition period necessary in half- duplex mobile stations for switching between transmission, reception and monitoring . As mentioned above, the implementation of full-duplex mobile stations is very complex. Also the "GERAN concept proposal" points out that the impact on half-duplex mobile stations has to be considered because of their cost advantages. The document refers to another document "Interleaving for Half Rate Channels in EGPRS Phase II", ETSI STC SMG2 # 34, January 2000, Tdoc SMG2 199/00, Lucent Technologies, pp. 1- 2, dealing with the problem of supporting half-duplex mobiles in a manner that makes better use of the available resources . In this latter document, it is proposed to use interleaving of Radio Blocks on half-rate channels over four consecutive bursts or over four alternate bursts and thereby to increase the number of time slots that can be allocated to a mobile station already using a certain time slot in one direction. This enables a statistical multiplexing with some restrictions . The principle of the proposed method is illustrated in figure 2 for half-rate channels over four alternate bursts, meaning that only even or odd TDMA frames are used in the first direction, which is, according to "Interleaving for Half Rate Channels in EGPRS Phase II", the better alternative. Figure 2 shows time slots of the downlink Rx and the uplink Tx connection to the serving cell and the time slots of the downlink connection to some neighboring cell. Time slot 0 of the downlink TDMA frames of the serving cells corresponds in time to time slot 5 of the uplink TDMA frames of the serving cell and of the downlink TDMA frames for monitoring. A mobile station receiving in a certain time slot 10, here time slot 0, of every second TDMA frame in downlink direction can be allocated one of 9 different time slots 11, here time slots 0, 1, ... , 7, 0, of the TDMA frames in uplink direction and the mobile station can use at least one time slot 12, here time slot 2, of the downlink TDMA frames for monitoring, while still having at least the same transition periods as a normal full-rate mobile station. The method proposed in "Interleaving for Half Rate Channels in EGPRS Phase II", however, has the disadvantage that half- rate channels provide a worse speech quality as compared to full-rate channels, since the bit rate is cut in half. SUMMARY OF THE INVENTION It is an object of the invention to provide a method, mobile stations and a telecommunications network allowing for an improved allocation of radio resources for mobile stations. This object is reached by a method for allocating radio resources in a telecommunications system to mobile stations transmitting data in time slots of TDMA frames in uplink direction and receiving data in time slots of TDMA frames in downlink direction, the method comprising in the case that a first mobile station is using a first time slot of TDMA frames in a first direction and requires a time slot of TDMA frames in the opposite direction and the corresponding time slot of TDMA frames in the opposite direction is used by a second mobile station, the steps of: a) determining a free time slot of the TDMA frames in the opposite direction; b) moving the connection of the second mobile station (B) to the determined free time slot of the TDMA frames in the opposite direction and c) allocating the corresponding time slot of the TDMA frames in the opposite direction to the first mobile station. The order of the steps in the method according to the invention can be selected freely, the given order being only the most logical one. The object is equally reached by a telecommunications network and a mobile station comprising means for realizing the method according to the invention. The invention proceeds from an approach which is quite different from the approach known from the state of the art. It is proposed not to determine a free time slot for the mobile station having a connection in one direction and requiring an additional one in the opposite direction. Rather, a free time slot is sought for the second mobile station occupying the time slot corresponding to the time slot used by the first mobile station in the direction already used. Since the second mobile station can only be working in the opposite direction at this time, any free time slot of the TDMA frames in the opposite direction can be chosen without restriction. To the first mobile station, on the other hand, the time slot corresponding to the time slot already used for the first direction is assigned for its connection in the opposite direction. The assignment of this time slot guarantees sufficient transition periods for switching between reception, transmission and a monitoring phase. The invention therefore allows statistical multiplexing to be used on half-duplex mobile stations without reduction of trunking gain. If there are free resources available in the needed direction, it is always possible to move the second mobile station to this free resource. Accordingly, the method according to the invention allows for maximal trunking gain of statistical multiplexing. At the same time, the existing duplex distance can be kept in both, time and frequency domains . Even though the invention is of particular relevance for half-duplex mobile stations, it is suited to be used for full-duplex mobile stations as well. In the method according to the invention the first direction can be the uplink and the opposite direction the downlink direction and equally, the first direction can be the downlink and the opposite direction the uplink direction. Preferred embodiments of the invention become apparent from the subclaims . The method according to the invention can be used in particular for statistical multiplexing of speech transmitted to and from mobile stations. If several frequencies are employed for uplink and downlink, the corresponding time slot is preferably the same time slot of a specified frequency assigned as up/downlink frequency to the used down/uplink frequency. In the example of figure 1 this would be cO and cO ' , cl and cl ' and c2 and c2 ' . The other free time slot determined for use by the second mobile station can be any free time slot of the TDMA frames in the opposite direction of any employed frequency. In the case that a first mobile station uses a first time slot of TDMA frames in a first direction and needs a time slot of TDMA frames in the opposite direction and the corresponding time slot of TDMA frames in the opposite direction is presently not used by a second mobile station, the corresponding time slot is preferably assigned to the first mobile station immediately. In a GSM system, the method according to the invention can be used either with full-rate or half-rate duplex channels, depending on the required quality of transmitted speech. The allocation of the time slots is preferably performed by the network, in particular by the base station systems of the network, by sending assignment messages to the mobile stations . If the first mobile station uses a time slot of TDMA frames in uplink direction and requires a corresponding time slot of TDMA frames in downlink direction, only the transmission of one assignment message is necessary. This assignment message is transmitted to the second mobile station to inform it about the new time slot to be used. The first mobile station can be configured to always listen to the time slot of the downlink TDMA frames corresponding to the used time slot of the uplink TDMA frames and therefore begin reception on the corresponding time slot in downlink direction by itself. Alternatively, a second assignment message can be transmitted to the first mobile station to allow the first mobile station to begin reception on the corresponding time slot of TDMA frames in downlink direction. If the first mobile station uses a time slot of a TDMA frame in downlink direction and requires a corresponding time slot of a TDMA frame in uplink direction, two assignment messages should be used. A first assignment message is transmitted to the second mobile station for moving its transmission to the determined free time slot of the TDMA frames in uplink direction and a second assignment message is transmitted to the first mobile station for allowing it to start transmitting on the corresponding time slot of the TDMA frames in uplink direction. In a preferred embodiment of the invention, the assignment messages are transmitted in the headers of downlink speech frames. Alternatively, the assignments can be transmitted by stealing frames or via special signaling channels. The preferred field of employment of the method according to the invention is to be seen in GERAN (GSM/EDGE RAN; EDGE: Enhanced Data rates for GSM Evolution, RAN: Radio Access Network) or EGPRS (Enhanced General Packet Radio Service) systems. However, it can be employed in any other suitable system. BRIEF DESCRIPTION OF THE FIGURES In the following, the invention is explained in more detail with reference to drawings, of which Fig. 1 shows the switching of a half-duplex mobile station between uplink and downlink direction; Fig. 2 illustrates a known allocation method of time slots for a half-duplex half-rate channel mobile station; Fig. 3a shows a situation where an allocation of a second time slot for one mobile station becomes necessary; Fig. 3b shows an allocation according to the state of the art in a situation of Fig. 3a; and Fig. 3c shows an allocation according to the invention in a situation of Fig. 3a. DETAILED DESCRIPTION OF THE INVENTION Figures 1 and 2 have already been described in the introduction with reference to the state of the art. Figures 3a-3c illustrate the principle of the method according to the invention used in a telecommunications system with TDMA structures in uplink and downlink direction as shown by Figure 1. If a speech connection has to be established only in one direction for the mobile station of a certain user, there are no duplex considerations necessary and any free time slot is assigned to the mobile station. Figures 3a-3c deals with the case that for mobile station A there is speech in one direction and a speech period begins also in the other direction, while the corresponding duplex resource is already used by another mobile station B. Figures 3a-c show four time slots 1-4 of a TDMA frame for each, downlink and uplink. There is an offset between the corresponding TDMS frames so that time slot 4 of the downlink corresponds in time with time slot 1 of the uplink. In Figure 3a, in the uplink direction, mobile station A is in speech mode on time slot 1 and mobile station C is in speech mode on time slot 4. In the downlink direction, mobile station B is in speech mode on time slot 1 and time slot 4 is a free time slot of the TDMA frame. The present allocation of time slots 2 and 3 as well as of the further time slots of the TDMA frame not shown figures 3a-c is of no relevance for the present example. In the described situation, user A with mobile station A wants to start speech mode also in downlink direction. For full-rate channels, existing solutions would assign free time slot 4 or any other free time slot of the downlink to mobile station A, as depicted in Figure 3b. However, with full-rate channels half-duplex operation of the mobile station is only possible, if the same time slot can be used in both directions in order to ensure adequate transition periods. Therefore, the known concept for full-rate channels is only practicable with full-duplex mobile stations. Figure 3c, in contrast, shows the solution according to the invention for half-duplex mobile stations. The network first chooses any free time slot of the TDMA frames in downlink direction, here time slot 4. Then it sends an assignment message via a special signaling channel to mobile station B, informing mobile station B that it has to switch to this free time slot. According to the assignment message, reception of mobile station B is moved from time slot 1 to time slot 4 of the TDMA frames in downlink direction and time slot 1 of the TDMA frames in downlink direction is released for use by mobile station A. In the present embodiment of the invention, all half-duplex mobile stations are continuously listening to the channel corresponding to its uplink channel. An assignment message to mobile station A is therefore not necessary for assigning time slot 1 of the TDMA frames in downlink direction to mobile station A, which starts receiving on time slot 1 of the TDMA frames in downlink direction by itself. It becomes apparent that with the method according to the invention, a mobile station can always be assigned the same time slot of the TDMA frames in uplink and downlink direction. Therefore, a sufficient transition period for switching between transmitting, receiving and monitoring is guaranteed at all times. Summarized, the mobile stations can be simple half-duplex full-rate channel mobile stations, while at the same time, the advantages of statistical multiplexing are assured. When the method according to the invention is used with half- duplex half-rate channels, the known statistical multiplexing for half-rate channel mobile stations is even improved. The allocation of mobile station B is completely independent, any free time slot of any frequency in the downlink can be used. In contrast, with the known half- duplex half-rate channel method only 9 out of 16 (or 10 out of 16 if the monitoring function is neglected) are candidates for selection, as described in the introduction.

Claims: C l a i s 1. Method for allocating radio resources in a telecommunications system to mobile stations (A, B, C) transmitting data in time slots of TDMA frames in uplink direction and receiving data in time slots of TDMA frames in downlink direction, the method comprising in the case that a first mobile station (A) is using a first time slot (1) of TDMA frames in a first direction and requires a time slot of TDMA frames in the opposite direction and the corresponding time slot (1) of TDMA frames in the opposite direction is used by a second mobile station (B) , the steps of: a) determining a free time slot (4) of the TDMA frames in the opposite direction; b) moving the connection of the second mobile station (B) to the determined free time slot (4) of the TDMA frames in the opposite direction and c) allocating the corresponding time slot (1) of the TDMA frames in the opposite direction to the first mobile station (A) . 2. Method according to claim 1, wherein, in the case that a first mobile station uses a first slot in a first direction and requires a time slot of TDMA frames in the opposite direction and the corresponding time slot of TDMA frames in the opposite direction is not used by a second mobile station, the corresponding time slot is assigned to the first mobile station immediately. 3. Method according to one of the preceding claims, wherein, when used in a GSM system, a full-rate duplex channel is used for uplink and downlink connections. 4. Method according to claim 1 or 2 , wherein, when used in a GSM system, a half-rate duplex channel is used for uplink and downlink connections. 5. Method according to one of the preceding claims, wherein the allocation is performed by the network by sending assignment messages to the mobile stations. 6. Method according to one of the preceding claims, wherein, if the first mobile station (A) uses a time slot (1) of TDMA frames in uplink direction and requires a corresponding time slot (1) of TDMA frames in downlink direction, an assignment message is transmitted to the second mobile station (B) causing it to receive via the determined free time slot (4) of TDMA frames in downlink direction, the first mobile station (A) beginning reception on the corresponding time slot (1) of TDMA frames in downlink direction by itself by listening to the time slot (1) of TDMA frames in downlink direction corresponding to the used time slot (1) of TDMA frames in uplink direction. 7. Method according to one of claims 1 to 5 , wherein, if the first mobile station (A) uses a time slot (1) of TDMA frames in uplink direction and requires a corresponding time slot (1) of TDMA frames in downlink direction, a first assignment message is transmitted to the second mobile station (B) causing it to receive via the determined free time slot (4) of TDMA frames in downlink direction, and a second assignment message is transmitted to the first mobile station (A) to allow the first mobile station (A) to beginning reception in the corresponding time slot (1) of TDMA frames in downlink direction. 8. Method according to one of the preceding claims, wherein, if the first mobile station uses a time slot of TDMA frames in downlink direction and requires a corresponding time slot of TDMA frames in uplink direction, a first assignment message is transmitted to the second mobile station for moving its transmission to the determined free time slot of TDMA frames in uplink direction and a second assignment message is transmitted to the first mobile station for allowing the first mobile station to start transmitting in the corresponding time slot of TDMA frames in uplink direction. 9. Method according to one of claims 5 to 8, wherein the assignment messages are transmitted in the headers of TDMA speech frames in downlink direction. 10. Method according to one of claims 5 to 8 , wherein the assignment messages are transmitted by stealing TDMA speech frames in downlink direction. 11. Method according to one of claims 5 to 8 , wherein the assignment messages are transmitted via special signaling channels. 12. Use of a method according to one of the preceding claims for statistical multiplexing of speech transmitted to and from mobile stations. 13. Use of a method according to one of claims 1 to 11 in a GERAN or EGPRS system. 14. Mobile station comprising means for realizing the method according to one of claims 1 to 11. 15. Telecommunications network comprising means for realizing the method according to one of claims 1 to 11.

Inventors: SIPOLA JUSSI, SIPOLA, JUSSI
Applicants: NOKIA CORP, SIPOLA JUSSI, NOKIA CORPORATION, SIPOLA, JUSSI
Application year: 2000
Main classification: H04Q7
IPC codes: H04B7, H04W72