1. Field of the Invention
The present invention relates in general to the transmission of data (containing control signals) for handover in mobile telecommunications systems such as a next-generation mobile telecommunications system [international mobile telecommunications-2000 (IMT-2000) system], personal communication service system (PCS) and digital cellular system (DCS), and more particularly to a method for transmitting data for inter-frequency/inter-system handover in an asynchronous mobile telecommunications system of a universal mobile telecommunications system (UMTS) terrestrial radio access (UTRA) type, in which information incapable of being transmitted for an idle period is transmitted over a common channel in the case of the inter-frequency handover or inter-system handover, thereby increasing the quality of speech and more efficiently performing the handover.
2. Description of the Prior Art
FIG. 1 is a view showing an architecture of a conventional asynchronous mobile telecommunications system. As shown in this drawing, the asynchronous mobile telecommunications system comprises an asynchronous terminal 21, a UTRA network (UTRAN) 22 which is an asynchronous radio network including a base station and radio network controller (RNC), and an asynchronous core network 23 connected to the UTRAN 22.
The asynchronous core network 23 includes an asynchronous mobile services switching center (MSC) 24 connected to the UTRAN 22, and a global system for mobile communications-mobile application part (GSM-MAP) network 25 connected to the asynchronous MSC 24.
In the conventional asynchronous mobile telecommunications system with the above-mentioned architecture, the asynchronous terminal 21 receives a system information message from the UTRAN 22 over a broadcast control channel (BCCH) and acquires from the received system information message information necessary to its connection to the asynchronous core network 23, including information related to the asynchronous core network 23 and information about the UTRAN 22.
On the other hand, in the above-mentioned asynchronous UTRA mobile telecommunications system, communication between a mobile station and a base station can be classified into a normal mode and a compressed mode in consideration of inter-frequency handover or inter-system handover. In the normal mode, because there is no possibility that the inter-frequency handover or inter-system handover will occur, a mobile station transmits and receives data to/from a base station to which it is currently connected. FIG. 2 is a view illustrating a state of data transmitted from the base station to the mobile station in the normal mode. As shown in FIG. 2, data are normally transmitted respectively to first and second users User#1 and User#2 in the normal mode.
The compressed mode signifies an interval from an inter-frequency handover (inter-system handover) possibility determination point of time to a point of time where the handover actually occurs. For this interval, while continuously communicating with a currently connected base station, a mobile station monitors an adjacent base station with a different frequency and reports the monitored result to the currently connected base station. At this time, the mobile station must empty a portion of a period for the data transmission to the currently connected base station in order to monitor the adjacent base station with the different frequency. Further, the currently connected base station must transmit information belonging to such a portion, or gap, to the mobile station for a period where it is connected to the mobile station. A separate technique is required to allow the currently connected base station to additionally transmit data belonging to the above portion (gap). Several examples of such a technique are shown in FIGS. 3 to 5.
FIG. 3 is a view illustrating a conventional method where a base station scales a spreading factor down to xc2xd and transmits data to a mobile station according to the scaled-down spreading factor in the compressed mode. In this method, the base station doubles transmission power to maintain the same quality of service (QoS) as that prior to the compressed mode. Further, the base station can vary the position of an idle period within a frame transmission period of 10 ms. For example, the idle period may be placed in the middle of the frame transmission period as shown in FIG. 3.
However, the above-mentioned method has a disadvantage in that the number of codes useable by the base station is reduced, namely, a code shortage problem.
In other words, codes used in the base station are orthogonal variable spreading factor (OVSF) codes, which are tree-structured as shown in FIG. 6.
In order to maintain the property of the OVSF codes, or orthogonality, a given OVSF code and an OVSF code at a lower branch thereof should not be used at the same time, which will hereinafter be mentioned in more detail with reference to FIG. 6.
Assume that a mobile station of SF=4 enters the base station under the condition that an initial mobile station of SF=2 is assigned with an OVSF code Cch,2,0. In this case, an OVSF code assignable to the mobile station of SF=4 is any one of either Cch,4,2 or Cch,4,3, because the use of an OVSF code at a lower branch of the previously used OVSF code Cch,2,0 breaks the orthogonality with other mobile stations.
The above description has been given as an example, and it is prescribed in a proposed standard that spreading factors (SFs) useable for the data transmission from the base station to mobile stations should range from 4 to 512.
Herein, the scale-down of an SF to xc2xd is realized by compressing and transmitting data on the basis of SF=4 for an idle frame period on the assumption that a mobile station of SF=8 enters the compressed mode. Provided that an OVSF code assigned to the mobile station of SF=8 is present at a lower branch of an OVSF code Cch,4,0, an OVSF code useable for an idle frame will be any one of Cch,4,0,1, Cch,4,2 and Cch,4,3. However, with any one OVSF code selected, OVSF codes at lower branches of the selected OVSF code cannot be used while the selected OVSF code is used, resulting in a drain on code resources.
Such a drain on code resources, in turn, leads to a shortage in the number of codes to be assigned to other users on a forward link. In order to solve this code shortage problem resulting from the scale-down of a processing gain to xc2xd, there has been used a non-orthogonal code set of scrambling codes as shown in FIG. 4. Namely, a separate code generator generates non-orthogonal codes, which are then inserted and transmitted to a mobile station for the idle period.
However, the use of the above non-orthogonal code set also encounters problems such as an increase in transmission power resulting from a scale-down of a spreading factor of a slotted frame, an increase in interference with other base station channels due to non-orthogonality and a need for an additional code generator for generating the non-orthogonal code set.
FIG. 5 shows another conventional approach to the code shortage problem resulting from the half scale-down of the processing gain as shown in FIG. 3. This method utilizes parallel scrambling codes. That is, for the process of a slotted frame, an OVSF code identical to that in an existing frame is assigned as a primary scrambling code and a scrambling code generated in FIG. 4 is assigned as a secondary scrambling code.
However, the above-mentioned conventional method is disadvantageous in that a separate code generator must additionally be provided to generate OVSF codes and the QoS is degraded due to an imperfect removal of interference resulting from non-orthogonality.
Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a method for transmitting data for inter-frequency/inter-system handover in an asynchronous UTRA mobile telecommunications system, in which data incapable of being transmitted for an idle period is transmitted over a common channel in the case of the inter-frequency handover or inter-system handover, thereby increasing the quality of speech and more efficiently performing the handover.
In accordance with the present invention, each mobile station belonging to a specific base station transmits and receives data to/from the base station in a normal mode. Each mobile station attempts inter-frequency handover or inter-system handover for connection to a different base station with a different operating frequency or communication system as needed. The specific base station additionally assigns a dynamic common channel to a mobile station performing the inter-frequency (or inter-system) handover.
In the case where a certain mobile station intends to enter a compressed mode in a given area, the specific base station assigns and uses an OVSF code to a common channel corresponding to that mobile station. In the case where a different mobile station intends to enter the compressed mode, the specific base station determines whether the currently used common channel is available for the different mobile station. If it is determined that the currently used common channel is time-divided and available for the different mobile station, then the specific base station assigns and uses a sub-tree code of the previously assigned OVSF code to that common channel. Unless the currently used common channel is available for the different mobile station, then the specific base station produces and uses a new common channel.
Radio resources (such as an OVSF code, etc.) assigned to a given common channel can be released and put to other uses when all mobile stations using that common channel complete the compressed mode.