Method for operating a radio-based communication system

A method for operating a radio-based communication system is described. During a first time slot (UL) the sending of data from a first mobile station (UE 2 ) to a base station (node B) is provided. During a second time slot (DL) the receiving of data from the base station (node B) in the first mobile station (UE 2 ) is provided. The first and second time slots (DL, UL) are provided in succession for an exchange of data. Normal codes (C 2 U, C 2 D) are allocated to the data exchanged in this way between the first mobile station (UE 2 ) and the base station (node B). A second mobile station (UE 2 ) sends data with a special code (C 2 *) in the first time slot (UL) and the first mobile station (UE 1 ) receives the data with the special code (C 2 *) in the first time slot (UL).

In a CDMA communication system (CDMA&equals;code division multiple access) a base station is present, also designated as node B. This base station covers a specific local area in which it can communicate with a plurality of mobile stations. The mobile stations are also designated as UEs (UE&equals;user equipment). Communication between the base station and the mobile stations takes place by the time division duplex method (TDD&equals;time division duplex). This means that the frequency at which information is transmitted from the base station to the mobile stations (so-called downlink&equals;DL) is identical to the frequency at which information is transmitted from the mobile stations to the base station (so-called uplink&equals;UL). The distinction between the two transmitting devices, in other words between the uplink and the downlink, takes place with the aid of various time slots, made available by the time division duplex method. Successive time slots are therein provided for the uplink and the downlink. The base station may therefore transmit information to the mobile stations only during the time slots for the downlink. During these downlink time slots the mobile stations may not send any information to the base station, but here they may only receive the information transmitted from the base station. The reverse applies correspondingly to the time slots for the uplink. In these time slots only the mobile stations may transmit information, which is then received by the base station. The base station may not, however, send any information in this time slot. With the described method the information to be transmitted is transmitted with specific codes. These codes therein differ as a function of the respective mobile station and the direction of transmission. For example, in the uplink every mobile station which sends information to the base station uses its own code which has been allocated in advance, e.g. by the base station. The aforementioned codes are subsequently designated as normal codes. These methods and allocations are schematically illustrated in the figure. In a mode characterised as NORMAL a first mobile station UE 1 and a second mobile station UE 2 communicate with a common base station node B. During the uplink UL the first mobile station UE 1 sends (S&equals;to send) data with a normal code C 1 U to the base station node B. Simultaneously the second mobile station UE 2 sends data with a normal code C 2 U to the same base station node B. During the downlink DL the first mobile station UE 1 receives (R&equals;to receive) data from the base station node B with the normal code C 1 D and the second mobile station UE 2 receives data from the base station with the normal code C 2 D. In this way, with the aid of the normal codes C 1 U, C 2 U, C 1 D, C 2 D and with the aid of the distinction between uplink and downlink, mutually independent exchange of data between the first mobile station UE 1 and the base station node B and between the second mobile station UE 2 and the base station node B takes place. It goes without saying that this communication can also be extended to further mobile stations in a corresponding manner. In a mode designated as INTERCOM direct communication between the two mobile stations UE 1 and UE 2 is possible without intermediate switching of the base station node B. It is assumed hereinafter that the second mobile station UE 2 wishes to begin direct communication of this kind with the first mobile station UE 1 . It is furthermore assumed that the first mobile station UE 1 is set to a so-called Intercom-Receive-Mode, which will be explained in greater detail later. To start direct communication between two mobile stations, the second mobile station UE 2 transmits data during the uplink (see figure: UE 2 ->UE 1 : S during UL). To send these data a code C 2 * is used, which is a special code differing from the already described normal codes C 1 U, C 2 U, etc. Moreover, the data with the code C 2 * are transmitted by the mobile station UE 2 at a substantially lower transmitting power than would be the case per se in the mode designated as NORMAL, so the base station is disturbed as little as possible. As the first mobile station UE 1 is set to Intercom-Receive-Mode, it is also able to receive data during the uplink, contrary to the mode designated as NORMAL. The first mobile station UE 1 therefore receives—as long as it is located in the spatial vicinity of the second mobile station UE 2 —the data transmitted by the second mobile station UE 2 (see figure: UE 2 ->UE 1 : R during UL). Owing to the special code C 2 * the first mobile station UE 1 recognises that the data are of a kind allocated to a direct communication between two mobile stations and not of a kind intended only for the base station. The base station node B on its part has not configured a receiver adapted to the special code C 2 *. The base station node B therefore ignores the associated data. As these data, in addition, are emitted by the second mobile station UE 2 at only a very low transmitting power, they also hardly disturb the remaining receipt of data in the base station node B. During the next uplink, as response, the mobile station UE 1 transmits data with a special code C 1 * and at a substantially reduced transmitting power (see figure: UE 1 ->UE 2 : S during UL). The base station node B is set to receiving data from the mobile stations UE 1 , UE 2 during the uplink and could therefore if necessary receive the data transmitted by the first mobile station UE 1 in spite of their low transmitting power. The base station node B, however, has not configured a receiver adapted to the special code C 1 *, so the data are again ignored by the base station node B. The second mobile station UE 2 , which initiated the direct communication as described and which is therefore expecting a response from the first mobile station UE 1 , now switches —contrary to the mode designated as NORMAL—to receipt of data during the uplink (see figure: UE 1 ->UE 2 : R during UL). Thus far the second mobile station UE 2 also switches to Intercom-Receive-Mode, this taking place automatically owing to the direct communication initiated by it. As long as the second mobile station UE 2 is located in spatial proximity to the first mobile station UE 1 , the second mobile station UE 2 receives the data transmitted by the first mobile station UE 1 during the uplink in spite of its low transmitting power. Using the special code C 1 * the second mobile station UE 2 recognises that these are data which belong to the direct communication with the first mobile station UE 1 it initiated itself. After this the second mobile station UE 2 can send further data together with the special code C 2 * to the first mobile station UE 1 , as has already been explained in connection with the starting of direct communication. Direct communication can then in this way be continued as long as wished. A pre-condition for direct communication between the two mobile stations UE 1 and UE 2 is that they are located in spatial proximity to one another, so the data transmitted by one of the two mobile stations at the reduced transmitting power can be received by the other mobile station. Furthermore, for direct communication a special code has to be used, which is different from the otherwise used code. This can be achieved in that a set of special codes is present in the communication system, which is provided only for this direct communication and which is not otherwise used. This can also be achieved in that the base station, for example via a so-called broadcast channel, continually communicates to all mobile stations present those codes which can be used by the mobile stations as special codes for direct communication. As has been explained, the first mobile station UE 1 must already be in an Intercom-Receive-Mode when the second mobile station UE 2 starts direct communication between the two mobile stations. This can be done in various ways. It is possible for the user of the first mobile station UE 1 to set the Intercom-Receive-Mode on his mobile station manually, e.g. by appropriate programming or by means of a switch provided for this purpose. It is also possible for the second mobile station UE 2 to transmit a special access code CA* in the preceding downlink time slot DL before starting the actual direct communication. As the first mobile station UE 1 in the downlink time slot DL in the mode characterised as NORMAL is set to receiving data, it receives this special access code CA*. Using the special access code CA* the first mobile station UE 1 recognises that another mobile station wishes to start direct communication. The first mobile station UE 1 thereupon switches automatically into Intercom-Receive-Mode, so then the second mobile station UE 2 can start direct communication, as described. The use of the access code CA* is also possible when this is used in the uplink time slot UL. The first mobile station UE 1 then admittedly has to be set manually to Intercom-Receive-Mode, but it does not have to “listen” or correlate to all conceivable codes C 2 * in order to find out whether the second mobile station UE 2 wishes to communicate with it. Instead, the first mobile station UE 1 “listens” only on a limited set of access codes CA* 1 , CA* 2 , . . . , CA*n and then responds also in an uplink slot UL with a code CR*, which depends on the code CA* received. As a special case it can therein emerge that the codes C 1 * and C 2 * are identical.