BACKGROUND OF THE INVENTION In future mobile systems, the proportion of a radio resource to be allocated to different subscribers varies considerably according to the required capacity and the standard of service. The new services offered together with normal speech transmission and the requirements relating to data transmis- sion increase the need for allocating the radio resource more efficiently.

A frequency band reserved for a radio system is allocated to users as radio channels in accordance with the selected multiple access technique (Multiple Access). A radio channel is a frequency band employed in a radio connection or a portion of the frequency band separated for example by means of time or a user-specific code. In analogue systems, radio channels are usually frequency channels, whereby a dedicated frequency band is re- served for each radio connection, the frequency band being a part of the fre- quency resource in the system. This technique is called frequency division multiple access (FDMA). In time division multiple access (TDMA), a time slot is assigned for each radio connection from a common frequency band. Code division multiple access (CDMA) is a multiple access technique implemented by means of a spread spectrum technique wherein radio transmissions em- ploying the same frequency band are coded in such a way that signals for a particular receiver party can be received onlyat particular receivers.

A radio system based telecommunication connection can be a sim- piex or a duplex connection. The simplex connection is a telecommunication connection wherein the user can only either transmit or receive information simultaneously. The duplex connection is a telecommunication connection wherein the user can transmit and receive information simultaneously. A semiduplex connection is a combination of the two aforementioned connec- tions, i.e. a telecommunication connection wherein one party employs the simplex connection and the other party employs the duplex connection.

in radio traffic, the duplex connection is often implemented by em- ploying different transmission and reception frequencies (frequency division duplex, FDD). The difference between the transmission frequency and the re-

ception frequency is called a duplex spacing. Most digital mobile communica- tion systems, such as the GSM (Global System for Mobile Communications) and DCS-1800 (Digital Cellular System for 188 MHz) are based upon time di- vision multiple access (TDMA) implemented by a FDD telecommunication connection. New PCS (Personal Communication System) systems imple- mented in the U.S. will apply a new IS-95 system based upon code division multiple access (CDMA).

Another duplexing method is time division duplex (TDD) wherein the signals are transmitted time-interleaved on the same transmission channel. In systems employing frequency division multiple access (FDMA) implemented by a TDD telecommunication connection, for example in the CT2 (Cordless Telephone, 2nd generation), transmission alternates with reception in a fre- quency band reserved for one subscriber. In systems employing time division multiple access (TDMA) implemented by the TDD connection, such as the DECT (Digital European Cordless Telecommunications), transmission alter- nate with reception in a frequency-band time slot reserved for one subscriber.

In mobile communication systems, the connection between mobile stations and base stations is accomplished through a radio path. A channel from the mobile station to the base station is called an uplink transmission path and, correspondingly, a channel from the base station to the mobile sta- tion is called a downlink transmission path.

A radio connection is based on successive data frames, the data frames being constructed according to the selected multiple access technique of for example adjacent frequency bands and time periods. The part of the frame which can be unambiguously referred to by means of selected parame- ters (such as a frame frequency band, a time slot number or a code) is called a frame address. A radio channel is hereinafter taken to mean a data trans- mission implemented in adjacent frames in one or several determined frame addresses. The properties of the radio channel communication can be affected by allocating the frame addresses. By establishing a radio channel composed of several frame addresses, more data transmission capacity will be obtained for example for the data transmission.

An interactive data transmission is a duplex communication where messages in the uplink and downlink transmission paths interact with each other. An example is given here illustrating a channel allocation between the mobile station and the base station. The uplink transmission path is hereinafter

referred to as the uplink and the downlink transmission path is referred to as the downlink. The base station informs about free addresses in a message transmitted on a Y channel (Yell) in the downlink direction. A mobile station desiring to establish a connection transmits a random access (RA) message in the uplink direction on a channel selected among the channels indicated to be free by the Y channel, the RA message including the desire to establish a connection. The desired standard of service, i.e. the amount of the resource the connection to be established requires, is also informed in the RA message.

The RA message is responded by an access grant (AG) message in the downlink direction, the AG message informing the mobile station about the frame address or frame addresses to be employed in the radio channel to be established.

The above described message transmission is accomplished with- out problems when traffic intensity is low enough, the downlink messages having enough time to react with the previous uplink messages by means of the suitable selection of the frame addresses. The frame becomes full with the increase of traffic, whereby the messages relating to the interactive connection are located within the entire frame matrix and all the message addresses can- not then be selected in a suitable way. Consequently, the downlink messages cannot always be positioned in such frame addresses wherein the messages would have enough time to react with the messages submitted in the previous message in the uplink direction or, on the other hand, where there would be enough time to transmit information useful for all the subsequent frames in the uplink direction. The use of the channel resources in this way is ineffective and radio spectrum is wasted in the implementation of the interactive connection.

A similar problem is faced for example with the data transmission originating from the mobile station in connection with acknowledgement mes- sages. A number of frame addresses is allocated to a subscriber for the data transmission. When the data transmission in the uplink direction is completed, the base station sends an acknowledgement message (for example automatic repeat request, ARQ) to the mobile station, the subscriber using the message for detecting whether the data transmission was successful. If the system dis- connects the connection immediately after the data transmission and, after disconnecting the connection, receives a message of the data transmission failure, the connection must be re-established. If the system maintains the connection until the positive acknowledgement, an extensive channel alloca-

tion must be maintained unnecessarily in connection with the positive ac- knowledgement, i.e. capacity is wasted in the implementation of the interactive message transmission.

BRIEF DESCRIPTION OF THE INVENTION The object of the present invention is to introduce a method which provides a simple way to avoid the above described problem relating to the use of the channel resource in the implementation of an interactive telecom- munication connection.

This object is achieved by the method for the interactive communi- cation in a full-duplex radio band comprising an uplink direction and a downlink direction. The method is characterized by time-duplexing the full-duplex radio band at least into a first sub- band and a second sub-band, communicating in said at least first sub-band and second sub-band simultaneously and yet independently in such a way that the uplink communi- cation in the first sub-band occurs simultaneously with the downlink communi- cation in some other sub-band and the downlink communication in the first sub-band occurs simultaneously with the uplink communication in some other sub-band.

The invention also relates to the methods according to claims 2, 3, or 4 and radio systems according to claims 7, 8 and 9.

The invention is based on the idea that the FDD telecommunication connection is improved by providing the connection with two or more TDD di- mensions. The general approach of the invention is that the full-duplex fre- quency band is time-duplexed into two or more sub-bands with a substantially independent interactive communication. Transmissions in different transmis- sion directions in each sub-band occur at different times but a simultaneous transmission in two different sub-bands can occur in different transmission di- rections. It is an advantage of the invention in the interactive communication that the message is received entirely in one transmission direction before the response is required in a particular sub-band in another transmission direction.

On the other hand, the frequency band is utilized for traffic effectively, since it is possible to transmit simultaneously in different sub-bands in reverse trans- mission directions.

In a preferred embodiment of the invention, one FDD frame, here- inafter referred to as a superframe, is composed of at least two subframes that

are time-duplexed with respect to the corresponding subframes in the reverse transmission direction. Interactive connection messages are formed on the basis of the previous subframe message or subframe messages in the reverse transmission direction.

It is an advantage of the invention that the information needed by the interactive message and transmitted through each subframe can be util- ized as a whole during said subframe. The system has then enough time to react with the information transmitted in the previous subframe. This effect can be improved in some systems by locating the messages wisely in the frame.

Interactive signaliing becomes faster and the utilization of the frequency spec- trum becomes substantially more effective compared with prior art solutions.

For example the allocation of free channels in the downlink direction in association with the channel allocation is based upon the information within the entire previous uplink frame. The frame addresses detected to be free on the basis of the previous uplink subframe are informed in the downlink sub- frame by means of virtual time-duplexing according to the invention. The sys- tem has then enough time to take into account all the random access mes- sages from the previous uplink subframe and to inform about the free chan- nels on the basis of this information in the next corresponding downlink sub- frame. This allows the mobile stations to have the channel allocation informa- tion for use in the next subframe, i.e. they are given access to the channels considerably faster than before. In the prior art solution, a Y channel message in the downlink direction cannot respond to all the uplink frame messages, whereby the corresponding allocation message can be delayed by at least one frame.

The time-dependent separation of'messages transmitted in different transmission directions provides many benefits which are dealt with in more detail in connection with the detailed description of the embodiments of the invention. It is obvious, however, that the invention provides a substantial im- provement in the implementation of the radio connection through the data frames.

LIST OF FIGURES The invention will be described in more detail with reference to the accompanying figures, in which Figure 1 illustrates a frame structure and channels of a radio inter- face,

Figures 2 and 3 illustrate uplink and downlink radio bands, their frame division and two different interactive traffic situations, Figure 4 shows a frame structure in uplink and downlink bands ac- cording to the primary embodiment of the invention, Figure 5 illustrates interactive communication in association with a frame structure as shown in Figure 4, Figure 6 illustrates a data transmission acknowledgement in asso- ciation with a frame structure as shown in Figure 4, and Figure 7 illustrates interactive communication in the case where the superframe of the invention is composed of three subframes.

DETAILED DESCRIPTION OF THE INVENTION In the following, the invention is described in more detail in associa- tion with TDMA multiple access technique, the invention, however, not being restricted to this technique. The application of the invention within the scope of the specification and claims of the invention in other multiple access tech- niques, such as CDMA, is obvious to those skilled in the art. The concepts channel and frame are then applied according to the principles of the multiple access technique to be used.

Figure 1 illustrates a radio interface based on data frames and traf- fic channel allocation in a frame system. In TDMA multiple access technique, a y-direction drawn in Figure 1 shows frequency differentiation, i.e. each over- lapping y level (y1, y2, y3) illustrates one frequency segment. Correspond- ingly, an x-direction drawn in Figure 1 shows time differentiation, i.e. each ad- jacent x level (x1, x2, x3, x4) illustrates one time slot. A radio channel can be unambiguously identified by an address (for example x3, y2) addressing one rectangle in a frame shown in Figure 1. A channel is composed of information transmitted in a determined frame address of one frame or successive frames (F1, F2, F3, F4).

The frame in Figure 1 shows a data frame associated with a sub- stantially one transmission direction. A reverse transmission direction is com- posed of a structurally identical frame in another frequency segment. Figure 2 illustrates channel arrangement for one frequency band (one layer in the y- direction in Figure 1) in the uplink and downlink directions (UL and DL respec- tively). A duplex frequency spacing (dF) exists between the UL band sepa- rated from the uplink frame and the DL band separated from the downlink frame and, in prior art solutions, the downlink frames have been delayed by

some time slots (dT) compared with the uplink frames. For example in the GSM system, the duplex spacing is 45 MHz and the numbering of downlink frame time slots is delayed by three time slots compared with the uplink frames.

An interactive radio connection is a duplex data transmission where a message submitted in one transmission direction affects the corresponding message transmitted in the reverse transmission direction. Signalling of a mo- bile station and a base station during the channel allocation and different types of acknowledgement messages in the data transmission are given as exam- ples of interactive connections.

In future mobile communication systems, the use of interactive con- nections will most likely dramatically increase. In the new systems, a channel is not reserved subscriber-specifically for the whole session (for example a call), but the channel is allocated to the subscriber for the time periods during which there is an actual need for the data transmission (for example speech or data transmission). In that case, channel allocation takes place before each data transmission transaction.

An interactive radio connection is illustrated in Figure 2 by a circle in a UL frame representing an interactive message and by a triangle in a DL frame representing the corresponding message. The interactive data trans- mission is composed of successive circles and triangles, the circle being a re- sponse to the triangle, the triangle being a response to the circle etc. When the system is not heavily loaded, the interactive message transmission is suc- cessful by means of an appropriate selection of frame addresses, and the in- teractive message can be established in both transmission directions on the basis of the received messages. When the frame begins to fill up because of the increasing communication or a greater amount of time slots reserved for the subscriber, the situation becomes more complicated.

Figure 3 illustrates a situation where the interactive message in the downlink direction (marked with a triangle) shown in Figure 2 is composed on the basis of the messages submitted in the entire uplink frame. Such a situa- tion exists for example in connection with an access grant (AG) message given in said DL frame. The base station informs about free addresses in a Y channel message in the downlink direction. A mobile station desiring to estab- lish a connection in the uplink direction selects a channel which has been in- formed to be free and sends a random access (RA) message in the uplink di-

rection on said channel informing in the message about its desire to establish a connection. The random access message is responded by the access grant (AG) message in the downlink direction informing the mobile station about the frame address or the frame addresses to be used on the radio channel to be established.

The frame will become full with the increase of communication, i.e.

messages associated with the interactive connection are located within the entire frame matrix. Consequently, the downlink messages have no time to react with the uplink messages submitted in the previous message, and, on the other hand, they have no time to transmit the information to be utilized in the uplink frame. Using the channel resources in such a way is ineffective and radio spectrum is wasted in the establishment of the interactive connection.

The solution of the invention is described below in association with the channel allocation without, however, restricting it to this embodiment. Fig- ure 4 illustrates the solution of the invention. It is to be noted that only the pa- rameters essential for the invention will be described herein. Thus, although for example timing advance is not dealt with in this connection, all the specifi- cations relating to a normal transmission must be taken into account in the final solution.

Figure 4 illustrates a first embodiment of the invention wherein a ra- dio connection is based upon uplink and downlink superframes. Each super- frame consists of 24 separate frame addresses being divided into two sub- frames f1 and f2 which both comprise 12 frame addresses. The transmission of the UL and DL superframes is timed in such a way that the transmission occurs simultaneously, i.e. the delay between different transmission directions is zero. The subframes ufl/df2 and uf2/dfl in the superframes usf and dfs are time-duplexed in such a way that the subframe transmissions occur at different times.

In the channel allocation, free channels for the subframe uf2 of the UL superframe UF1 are notified on the Y channel of the subframe df1 in the DL superframe DF1 and, correspondingly, free channels for the next subframe ufl of the UL superframe UF2 are notified on the Y channel of the subframe df2 in the DL superframe DF1.

It is assumed that a mobile station listens to the Y channel in the subframe df1 of the DL superframe DF1. The mobile station selects a free channel submitted on the Y channel, such as CH1, and sends a random ac-

cess (RA) message on the free channel CH1 being selected in the subframe uf2 of the UL superframe UF1. A base station responds by an access grant (AG) message in the subframe df1 of the DL superframe DF2. The AG mes- sage informs the mobile station that the channel CH1 (marked in Figure 4 with a black square) has been reserved for it from the next subframe. The mobile station starts transmitting on the channel CH1 in the subframe uf2 of the UL superframe UF2. Since CH1 is now allocated to said mobile station, it is no longer announced through the Y channel in the subframe df1 of the DL super- frame DF2. In the above description, the channel allocation takes place by means of the subframe pair df1 and df2. A simiiar separate channel allocation can occur simultaneously (but in reverse phase) by means of another sub- frame pair df2 and ufl. It is possible that the same mobile station participates in the channel allocation in both subframe pairs.

Interactive acknowledgement messages (Y, AG) occur in each sub- frame of a superframe, preferably in the middle of the subframe, whereby the system has enough time to react with the messages. Traffic channels (such as CH1) occur only once in one superframe, i.e. in the present example only in the subframe uf2. In the present example, two connection layers are formed in the frame structure for the interactive messages, one of which is composed of the subframe uf1 of the UL superframes and the subframe df2 of the DL su- perframes, and correspondingly, the other is composed of the subframe uf2 of the UL superframes and the subframe dfl of the DL superframes. Figure 5 illustrates the connection layers which are formed in the frame structure. In Figure 5, solid arrows illustrate interactive communication at a connection level df1-uf2, and dashed arrows illustrate communication at a connection level df2- uf1. It is possible that the same mobile statiori participates in communication at both connection levels.

By means of the solution of the invention, the allocation situation for each UL subframe is already known in the previous DL subframe and the in- teractive message concerning each frame address can be transmitted before the next transmission of the frame address in the superframe. For example in the example in Figure 4, the AG message relating to the RA message sub- mitted in the channel CHI in the subframe of the superframe UF1 is already submitted in the subframe dfl of the superframe DF2, whereby communication on the channel CH1 can already start in the subframe uf2 of the superframe UF2, the subframe uf2 being the frame wherein the channel CH1 occurs next.

Time-duplexing the connection layer ensures that the information submitted on the Y channel always contains exact information about the free channels in the next subframe. In addition, one AG message in the downlink direction can be employed for responding to all the RA messages at the same connection layer and the AG message can be transmitted to the mobile station before the allocated frame address or allocated frame addresses occur next time in the superframe.

A corresponding example can be presented regarding the afore- mentioned data transmission acknowledgement. The example is illustrated in Figure 6. A channel CH2 composed of two UL-frame frame addresses has been reserved for a subscriber for the data transmission. When the data transmission is completed in the subframe uf2 of the superframe UF1, the connection is maintained and the mobile station waits for an ARQ acknow- ledgement message from the base station. In the case of a successful data transmission, the base station gives the acknowledgement message in the subframe df1 of a DL superframe DF2 and at the same time informs in a Y message that the frame addresses allocated to the channel CH2 are free. In the case of an unsuccessful data transmission, the base station sends a mes- sage informing about a transmission failure in the subframe dfl of the super- frame DF2, whereby the mobile station can continue transmission on the channel CH2. In both cases, interactive information can be transmitted before the actual traffic channels occur next time in the superframe.

Interactive acknowledgement messages are preferably placed in the middle of the frame, whereby the system is given enough time to react with the information submitted in the previous subframe. If more time is needed in the processing of the messages in the UL and DL directions, as is the case in the systems based on CDMA multiple access technique where the duration of the message comprises the entire frame cycle, the superframe can be com- posed of more than one subframes. Figure 7 illustrates the solution of the in- vention in the case of three subframes (connection layers). The first connec- tion layer is composed of subframes uf2 and df1,the second connection layer is composed of subframes uf3 and df2 and the third connection layer is com- posed of subframes uf1 and df3. Reaction time has been increased in the case presented in Figure 7 by delaying the uplink transmission with respect to the downlink transmission by half a frame cycle.

The principles of the invention can also be applied in the conven- tional time division duplex TDD where the signals in different directions are transmitted interleaved in time on the same transmission channel. In accor- dance with the invention, the frame in both directions can be composed of two or more subframes, the subframes in turn forming one or more connection lay- ers.

The drawings and the related description are only intended to illus- trate the inventive idea. The details of the solution and the frame structure of the invention may vary within the scope of the claims. Although the invention has been described in association with the system based on TDMA multiple access technique by using messages relating to channel allocation as an ex- ample, the described solution presented in the invention can also be employed in association with other radio systems using interactive messages.