BACKGROUND OF THE INVENTION Mobile telecommunication systems (also called cellular systems) have widely spread in recent years and are quite beneficial for a subscriber to the mobile telecommunication system in that using his terminal device or mobile station MS, he can communicate with nearly any desired other subscriber in his or other mobile telecommunication systems or the public switched telephone network PSTN.

Generally, such mobile telecommunication systems consist of at least one terminal device MS which communicates with one of at least one radio transceiver devices or base stations BS constituting the telecommunication network. The plurality of base stations BS are controlled by a base station controller device BSC in combination with a mobile switching service center MSC. The latter one may also provide an interface to the PSTN network.

With the increasing traffic handled by such networks interference problems between a plurality of ongoing connections arise, which decrease transmission quality.

Moreover, dependent on the location of use of a terminal device (for example in hilly terrain or in a typical urban

area), fading problems due to multi-path propagation caused by reflection and/or shadowing by buildings/mountains may arise, which also contribute to a decrease in transmission quality.

Transmission quality is thus adversely affected in uplink UL (from a respective terminal device MS to a respective radio transceiver device BS) as well as in downlink DL direction (from a respective radio transceiver device BS to a respective terminal device MS).

In order to alleviate such problems, it is known to implement different diversity methods and better receiver devices for uplink transmission in mobile telecommunication systems in order to improve the system performance in uplink direction.

However, since such measures are easier to implement in connection with radio transceiver devices BS than terminal devices, there remains a problem in that the signal quality in downlink mostly limits the performance of the telecommunication system.

However, when implementing diversity methods at the terminal device side, this will require the provision of two or more antennas and two or more receivers.

Additionally, the respective antennas would have to be spaced apart from each other in order to decrease the correlation between antennas.

Such a solution, on one hand increases the dimensions of a terminal device due to spaced plural antennas to be provided, and also increases the required hardware due to the plural antennas as well as plural receivers. This, in turn, will lead to increased costs of the terminal device.

In addition, such an expenditure of hardware resources is not required in all situations, rather it is required only in case of poor or adverse channel conditions. Thus, the additional hardware resources provided for to establish space diversity at the terminal device side would be useless in most times.

Also, instead of locating the diversity at the terminal device side, it could be implemented at the radio transceiver device side. This could be done by providing several antennas at the respective radio transceiver devices BS and to transmit the signals separately in time and phase in order to provide downlink diversity.

However, similarly as explained above, the expenditure in hardware resources at the network side would be unused in most times. Therefore, substantially the same drawbacks as mentioned above are attributable to this solution.

SUMMARY OF THE INVENTION Hence, it is an object of the present invention to provide a method for implementing downlink diversity in a radio telecommunication system operated according to time divisional multiple access (TDMA) and consisting of at least one radio transceiver device and at least one terminal device, which is free from the above drawbacks and which does not require any additional hardware resources.

According to the present invention, this object is achieved by a method for implementing downlink diversity in a radio telecommunication system operated according to time divisional multiple access TDMA and consisting of at least one radio transceiver device and at least one terminal device, the method comprising the steps of transmitting

data from said at least one radio transceiver device to a respective one of said at least one terminal devices using a first downlink channel, and transmitting the same data from said at least one radio transceiver device to said respective one of said at least one terminal devices using a second downlink channel, upon request of said respective terminal device.

Advantageous further developments of the present invention are as set out in the dependent claims.

Additionally, the present invention also proposes accordingly adapted terminal devices and radio transceiver devices.

Accordingly, the proposed method provides an advantage in that a similar diversity gain (as known from uplink) can be obtained in downlink without any additional hardware being required. For example, measurements of the inventor have shown that a diversity gain of about 5 dB can be obtained for a channel established in a typical urban area (TU channel).

Additionally, an interference diversity gain can be obtained. Stated in other words, interference can be reduced, as the interference is also averaged due to the additional channel being activated, thereby providing diversity in frequency and, also possible, in time.

Moreover, the proposed method is adaptive and activates diversity only upon request of a respective terminal device dependent on the condition of the air interface (like transmission quality and available transmission capacity).

This results in an effective use of the transmission capacities via the air interface in order to improve the

connection quality without adding any additional hardware resources neither to a terminal device MS nor to a radio transceiver device BS.

In general, the method provides more flexibility for TDMA systems in order to make maximum use of the air interface.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be more readily understood when referring to the accompanying drawings, in which Fig. 1 shows schematically a part of a radio telecommunication network with downlink diversity being implemented between a radio transceiver device BS and a terminal device MS; Fig. 2A illustrates a flowchart of the method steps as performed on the terminal device MS side; Fig. 2B illustrates a flowchart of the method steps as performed on the radio transceiver device BS side; and Fig. 3 schematically shows details concerning first and second active downlink channels on a TDMA frame level.

DETAILED DESCRIPTION OF THE INVENTION The present invention will subsequently be described in detail with reference to the drawings.

According to the present invention, downlink diversity is realized by using frequency and/or time separation. This means, that at least two different channels (frequencies and/or time slots) are activated for transmitting the same

data contained in a respective burst during a time slot of a TDMA frame to the terminal device or mobile station MS, respectively. Moreover, since downlink diversity is activated upon request, the requesting mobile station MS knows that a second channel (frequency and/or time slot) will be/is activated and waits for the second transmission of the same data, also received via a first channel.

Having received the same data transmitted via the at least two channels, the terminal station MS combines the data contained in the separately transmitted bursts in an equalizer means using either pre-or post-combining methods. This means that signal combining operations can be effected before supply of the signals to an equalizer or after being processed by an equalizer.

In case of pre-combining, only one equalizer is required as the signals are combined before being supplied to the equalizer. That is, when a plurality of two or more (diversified) bursts are successively received, the first one can be stored in a buffer memory while waiting for the reception of the other burst (s). After the subsequent (second) burst has been received, some pre-combining methods could be used for signal combination. Thus, there is only a need for one equalizer.

In case of post-combining, however, (at least) two equalizers are required, dependent on the amount of diversity established. Namely, if a burst is transmitted twice using an implemented diversity feature, two equalizers will be used, while in case the burst is transmitted three times, three equalizers could be used.

This is similar to the processing applied by radio transceiver devices BS in uplink diversity methods, so that according to the present invention, also the same kind of diversity gain as in the uplink case is achieved.

As regards the above mentioned pre-combining methods, these include the following methods used for signal combination in case of diversity: 1) Maximum-Ratio-Combining, according to which the signals to be combined are weighted in terms of their signal quality before being combined; 2) Equal-Gain-Combining, according to which the signals to be combined are equally weighted (irrespective of their signal quality) before being combined; and 3) Selection combining, according to which the signals to be combined are not combined, but the signal with the better quality is selected.

Basically, there are two different implementations in connection with the proposed method: On one hand, using the (at least) second channel (2nd frequency) in downlink, the transmission of the same data as transmitted via the first (reference) channel (lst frequency) is effected at a different frequency at the same time ("pure frequency diversity"). This means that the same time slot TS of a TDMA frame is used on the second channel as on the first channel. Although no additional antenna means are required in this case for the terminal device MS, a second receiving means for the terminal devices is needed, thereby to a certain extent increasing costs.

On the other hand, using the second channel in downlink, transmission of the same data as transmitted via the first channel is effected at a different or the same frequency

but at a different time (i. e. during a different time slot TS of a respective TDMA frame). This leads to"pure time diversity if the same frequency is used and to a mixed frequency/time diversity feature if a different frequency is used.

In the above cases, the data are separately transmitted as regards the used channel (defined by the used frequency as well as the used time slot). The data (or samples) contained in the bursts received first can be stored or buffered in the equalizer at the reception side and equalization is performed after the second burst containing the same data is received. Equalization can be performed after having used any of the above mentioned combining methods. Consequently, the same receiver can receive both bursts as the different data bursts are at least separated in time and possibly also in frequency, so that in this case no additional hardware is required for a respective terminal device to carry out the proposed method.

Only little more digital signal processing (DSP) power is required, but as the digital signal processing power is continuously increasing and terminal devices are adapted to process several time slots of a single frame, this is not a critical point to the present invention.

The time difference between the first and second data burst transmission (when assuming that diversity uses only one additional channel, i. e. an (additional) second channel) on the first and second channel can be expressed in numbers of time slots. For example, the second transmission using the second channel can take place at a fixed time slot before the first (reference) transmission using the first channel.

In this connection it is conceivable to arrange a difference of one time slot or four time slots, for

example. Alternatively, the time slot to be used for the second channel could be configured upon activation of the second channel. The use of a respective time slot for the (diversified) transmission is dependent on the availability of a"free", i. e. unused time slot, which are not occupied by other mobile stations. Also, the timing between received and transmitted time slots could be taken into account so that this does not occur at the same time, because the transmitter (of a terminal station) can disturb the receiver (of the terminal station) if transmission and reception occurs simultaneously.

The receiver at the terminal device used when diversity (two or more signal"branches", i. e. channels) according to the present invention is established is basically the same kind of receiver as for a single signal branch (i. e. when no diversity is established). Therefore, additional processing power is not significant. Moreover, since nowadays terminal devices are adapted to process several time slots of a respective frame, they provide enough processing power for the proposed method. Additionally, since the combining of the two signal branches is effected in the equalizer, the channel decoding processing has to be executed only once as normally and no extra signal processing is required in this connection.

The above outlined general functionality of the present invention will now be set out in greater detail.

Fig. 1 of the drawings shows schematically a part of a radio telecommunication network with downlink diversity being implemented on the air interface between a radio transceiver device BS, denoted by 1 and being provided with an antenna means 1A, and a terminal device MS, denoted by 2 and being provided with an antenna means 2A. In an initial

stage (not shown) a transmission and/or communication is active between the terminal device MS 2 and the radio transceiver device BS 1 in uplink UL direction as well as in downlink direction DL, with the transmission in downlink DL being based on a first (single) downlink channel DL (F1) based on a first frequency F1. In case downlink diversity is required by the terminal device MS 2, a corresponding request is forwarded from the terminal device MS 2 via for example a (logical) control channel CCH in uplink UL direction to the radio transceiver device BS 1. In response thereto and if available, the radio transceiver device BS 1 will allocate and/or activate a second downlink channel DL (F2) for downlink transmission between the radio transceiver device BS 1 and the terminal device MS 2, with the transmission in downlink using said second channel being based on a second frequency F2. (However, as mentioned above, a second frequency is not necessarily required for the second channel, but the second channel can be established using a different time slot activated while still using the same frequency Fl as for the first channel). Then, both downlink channels DL (F1) and DL (F2) (frequencies and/or time slots) are simultaneously active for transmitting the same data bursts thereon, thereby providing downlink diversity. The conditions for issuing the request and allocating said second channel will be described further below.

Fig. 2A illustrates a flowchart of the method steps as performed on the terminal device MS side, with the assumption that a downlink transmission using a first downlink channel DL (F1) based on said first frequency F1 is already active.

Then, the flow starts in a step S10.

In a subsequent step Sll, the terminal device MS measures the transmission quality of the (for the concerned terminal device at present single) active downlink channel DL (Fl) at the frequency F1 (and a time slot within a TDMA frame).

Thereafter, it is checked in a step S12 whether the measured transmission quality is below a transmission quality threshold level.

The transmission quality may be defined as a received signal strength or as a bit error rate of the received signal, or the like. Also, the threshold level may be a suitable one which is predetermined for all terminal devices. Alternatively, the threshold level may be informed to the terminal device by the radio transceiver device BS dependent on a monitored location of the terminal device (e. g. in a hilly terrain or in a typical urban area), so that the threshold level may be set dependent on the environmental conditions.

If, in step S12, the transmission quality is not below said threshold level (NO in step S12), the flow returns to step Sll. However, if the transmission quality is below said threshold level (YES in step S12) the method flow advances to a step S13.

In step S13, the terminal device MS issues a request to the base station BS for downlink diversity to be activated.

Such a request can be forwarded from the terminal station MS to the base station BS via a (logical) uplink control channel such as a dedicated control channel DCCH.

In a following step S14, the processing as effected by the terminal station MS ends.

Now with reference to Fig. 2B, a flowchart of the method steps as performed on the radio transceiver device BS side is illustrated.

The method starts in a step S20.

In a following step S21, the radio transceiver device BS monitors the uplink dedicated control channel DCCH.

Then, in a subsequent step S22, it is checked whether via said dedicated control channel DCCH a request for an additional channel is received from a respective terminal device MS. If no such request is received (NO in step S22), the process returns to step S21 and monitoring the dedicated control channel is continued. If, however, such a request is received at the radio transceiver device from a requesting terminal device (YES in step S22), the method proceeds to step S23.

In step S23, it is then checked, whether there is transmission capacity available on the air interface of the respective radio transceiver device BS serving a corresponding cell of the radio telecommunication system.

Transmission capacity here means for example at least one available frequency at the radio transceiver device. Also, transmission capacity here could mean an available time slot in a TDMA frame transmitted on the carrier frequency of the first (downlink) transmission.

If no further transmission capacity is available (NO in step S23), the flow advances to step S24. In step S24, an other action is initiated. This means that for example a handover is initiated due to the previously measured (S11, S12) insufficient transmission quality, or that the ongoing transmission via the active channel DL (F1) is just dropped.

After step S24, the flow proceeds to step S27 and the method as performed by the radio transceiver device BS ends.

However, if further transmission capacity is available (YES in step S23), the flow advances to step S25. In step S25, the radio transceiver device BS activates an additional channel DL (F2) from the radio transceiver device BS. The additional channel is characterized by the frequency of and/or time slot used in the transmitted TDMA frame, which parameters are set when activating the additional channel.

Namely, in a specific example, in response to the receipt of said request for an additional channel, the radio transceiver device BS activates a requested additional channel at a second frequency DL (F2) and sets a time slot in a TDMA frame of said second channel DL (F2) to be used for transmission of the same data as transmitted on the first channel DL (F1).

Then, upon the parameters of the additional channel being set in step S25, the terminal station MS, in a following step S26, is informed about these parameters (i. e. frequency and/or time slot defining the channel). Thereby, the terminal device is informed on which channel, i. e. on which frequency and/or at which time (time slot) the redundant data, i. e. the same data as the data transmitted via the first channel, can be received. Also, at this time, the transmission capacity information are updated in that an additional channel (frequency and time slot) has been assigned Then, in the following step S27, the method ends as regards the radio transceiver device BS.

Fig. 3 schematically shows details concerning first and second active downlink channels on a TDMA frame level.

As an example only, a TDMA frame according to GSM specification has been chosen which comprises eight time slots TSO,..., TS7 per frame. However, according the American IS-54 system specification, TDMA frames having six time slots only are defined. Apparently, the present invention is also applicable without any problem to systems which have a number of time slots per TDMA frame which is different from eight, and not limited to an implementation to any of the above named specifications.

Fig. 3 a) shows an example of the downlink channel DL (F1) at a first frequency F1. The data DATA are transmitted as a burst in time slot TS5, for example. This initially active connection is defined as a reference for the subsequent explanations.

Fig. 3 b) shows an example in which the second, additionally activated channel DL (F2) at a second frequency F2 is used to establish downlink diversity. In this example, the same data DATA as the data transmitted on the first channel are transmitted at a different frequency (here F2), but at the same time, i. e. during the same time slot as on the first channel (TS5 in the chosen example).

In this case, the implemented diversity is a"pure" frequency diversity.

Furthermore, Fig. 3 c) illustrates an example according to which the diversity is achieved by using a different frequency F2 for the second channel DL (F2) (as compared to the first channel DL (F1)) and also by using a different time. Stated in other words, as shown in Fig. 3 c), on the second channel DL (F2), the same data DATA as those

transmitted on the first channel are transmitted at a different time, i. e. during a different time slot (TS1 in the chosen example). This different time slot may be predetermined and fixed and differs from the time slot of the reference transmission (Fig. 3 a)) by an integer number of time slots. The time slot used on the second channel may precede or follow the time slot used on the first channel (frequency) defined as a reference.

Finally, Fig. 3 d) shows a case in which the time slot TS to be used for the additional channel DL (F2) is configured.

For example, a time slot TS1, TS2, or TS4 for example may be used for transmitting the same data DATA (in Fig. 3d) denoted by" (D)"), if time slot TS5 is used as a reference on the first channel. Such a modification is advantageous in case a frequency like for example F2 is reserved for establishing downlink diversity. Then, the time slots TS available on said frequency are allocated to different diversity requesting terminal devices. However, a time slot to be assigned for implementing diversity would have to be available on said frequency. The criteria for selecting a respective time slot have been specified herein above, so that a repeated explanation thereof is considered to be dispensable.

Note that in case of the situation as depicted in Figs. 3c) and d), the implemented diversity is a"mixed"frequency and time diversity.

Nevertheless, as mentioned earlier above, the present invention may also be implemented also as a"pure"time diversity. Then, not shown in the drawings, Figs. 3c) and d) could be modified such that the second channel is active on the same transmission frequency (i. e. F1) as the reference transmission illustrated in Fig. 3a), with the

difference in channels in downlink transmission direction being manifest due to the different used time slots TS.

As has been described in the foregoing, the present invention proposes a method for implementing downlink diversity in a radio telecommunication system operated according to time divisional multiple access TDMA and consisting of at least one radio transceiver device BS and at least one terminal device MS, the method comprising the steps of transmitting data from said at least one radio transceiver device BS to a respective one of said at least one terminal devices MS using a first downlink channel, and transmitting the same data from said at least one radio transceiver device BS to said respective one of said at least one terminal devices MS using a second downlink channel, upon request of said respective terminal device MS. Thereby, the capacities for connection provided by the air interface can be used to improve connection quality without the need to add hardware resources at the terminal device side or radio transceiver device side.

It should be understood that the above description and accompanying figures are merely intended to illustrate the present invention by way of example only. The preferred embodiments of the present invention may thus vary within the scope of the attached claims.