WO2009075546A1

WO2009075546A1 - System and method for checking connection in a communication system

Info

Publication number: WO2009075546A1
Application number: PCT/KR2008/007372
Authority: WO
Inventors: Hyung-Joon Jeon; Jae-Hyeong Kim; Kuk-Jin Song; Jung-Nam Yun; Hee-Hwan Kwak; Dong-Hee Kwon; Jae-Myung Jang; Il-Nyun Kim; Shin-Ha Kang
Original assignee: Posdata Co., Ltd.
Priority date: 2007-12-13
Filing date: 2008-12-12
Publication date: 2009-06-18
Also published as: KR20090062696A

Abstract

The present invention is provided a method and system for checking a connection with a Mobile Station (MS) at a Base Station (BS) in a communication system including the BS and the MS being provided with a service by the BS. The present invention checks whether data is received from the MS operating in an active mode, checks whether at least one message of a bandwidth allocation request message, an UpLink (UL) transmission power report message, and a UL transmission power control request message is received from the MS, if the BS fails to receive the data from the MS for a predetermined time, and ascertains that a connection with the MS is kept, if the at least one message is received.

Description

Description System and method for checking connection in a communication system Technical Field

[1] The present invention relates generally to a communication system, and in particular, to a system and method for checking a connection state between a Mobile Station (MS) and a Base Station (BS) in a Broadband Wireless Access (BWA) communication system. Background Art

[2] Research on the next-generation communication system is being conducted to provide users with various Quality of Service (QoS) services at a high data rate. Particularly, study of the next-generation communication system is made to support highspeed services guaranteeing mobility and QoS for a BWA communication system such as Wireless Local Area Network (WLAN) system and Wireless Metropolitan Area Network (WMAN) system. Typical next-generation communication systems can include an Institute of Electrical and Electronics Engineers (IEEE) 802.16a/d communication system and an IEEE 802.16e communication system.

[3] The IEEE 802.16a/d communication system and the IEEE 802.16e communication system, which are the BWA communication systems, employ Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) to support a broadband transmission network for physical channels of the WMAN system. The IEEE 802.16a/d communication system presently considers only the state where a Subscriber Station (SS) is stationary, i.e., the state where mobility of the SS is never taken into account, and also considers only the single-cell configuration. Unlike the IEEE 802.16a communication system, the IEEE 802.16e communication system considers mobility of the SS. Herein, the SS having mobility will be referred to as an MS.

[4] When the IEEE 802.16e communication system considers mobility of an MS, power consumption of the MS may act as an important factor in determining the system efficiency. Therefore, a sleep mode operation and an active mode operation corresponding to the sleep mode operation have been proposed between an MS and a BS, to minimize the power consumption of the MS, and there is a need for a scheme that checks a connection state between the BS and the MS in the sleep mode and the active mode.

[5] As the scheme for checking a connection state between the BS and the MS, there is a known 'Keep-alive check in sleep mode' scheme in which in the sleep mode, the BS checks the connection state by periodically performing ranging with the MS, or checks the connection state using an UpLink (UL) data grant message. However, there is no proposed scheme for checking the connection state between the BS and the MS in the active mode. Thus, in the active mode, the BS checks the connection state merely through reception of UL data from the MS, but has no way to check the connection state, if it fails to receive the UL data for a predetermined time.

[6] More specifically, an operation in the sleep mode, an operation provided for an occasion where there is no data exchange between the BS and the MS, is to minimize power consumption of the MS or to delete information on the MS by checking the connection state between the BS and the MS. In other words, after checking the connection state, if it is checked that the connection between the BS and the MS is kept in the sleep mode, the BS increases a sleep interval for the MS to reduce power consumption of the MS, and if it is checked that the connection between the BS and the MS is cut off or broken, the BS deletes information on the MS, stored in the BS, for example, deletes a Connection Identifier (CID) allocated to the MS, preventing a waste of CID and BS's memory.

[7] Accordingly, the BS checks the connection state in the sleep mode through periodical ranging between the BS and the MS, i.e., through periodic exchange of a Ranging Request (RNG-REQ) /Ranging Response (RNG-RSP) message between the BS and the MS, or the BS checks the connection state in the sleep mode through a process of transmitting a UL data grant message to the MS and receiving a UL resource allocation request message from the MS. Herein, when the MS does not need its UL resource allocation, the MS transmits a UL resource allocation request message being set to '0' to the BS. However, when the MS needs UL resource allocation, the MS transmits a UL resource allocation request message including allocation request information to the BS, and the BS checks the connection state with the MS by identifying whether the UL resource allocation request message is received.

[8] Meanwhile, in the active mode operation, which is an operation provided for data exchange between the BS and the MS, the BS checks the connection state between the BS and the MS through data transmission/reception as the data transmission/reception is achieved in the state where the connection between the BS and the MS is kept. However, if there is no data exchange between the BS and the MS in the active mode for a predetermined time, especially if the BS fails to receive data from the MS for a time longer than or equal to a data reception interval even though it transmitted data to the MS, the BS has no way to check the connection state of the MS.

[9] As the BS fails to check the connection state between the BS and the MS in the active mode, the BS may fail to perform an operation corresponding to the connection state between the BS and the MS. In other words, in the state where the connection between the BS and the MS is kept, the BS may fail to perform a normal operation for the MS, for example, to perform such operation as resource allocation, transmission power control, and Modulation and Coding Scheme (MCS) level control for the MS, and in the state where the connection between the BS and the MS is cut off, the BS may fail to perform an operation of deleting information on the MS as described in the sleep mode. Particularly, when the BS mistakenly checks the connection cut-off state as the state where the connection with the MS is kept, due to the failure to check its connection state with the MS, the CID allocated to the MS and the BS's memory are wasted. In addition, when the BS mistakenly checks the connection is kept state as the state where the connection with the MS is cut off, the BS may cut off the connection with the MS, causing a decrease in reliability of the communication. Disclosure of Invention Technical Problem

[10] An aspect of the present invention is to address at least the problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of the present invention is to provide a system and method for checking a connection state between a BS and an MS by monitoring whether a predetermined message is received from the MS in a communication system.

[11] Further, an aspect of the present invention is to provide a system and method for checking a connection state between a BS and an MS by monitoring whether a predetermined message is received from the MS in an active mode in a communication system.

[12] In addition, an aspect of the present invention is to provide a system and method for checking a connection state between a BS and an MS by monitoring whether a predetermined message is received from the MS, if there is no data exchange between the BS and the MS in an active mode for a predetermined time in a communication system. Technical Solution

[13] According to one aspect of the present invention, there is provided a method for checking a connection with a Mobile Station (MS) at a Base Station (BS) in a communication system including the BS and the MS being provided with a service by the BS. The method includes checking whether data is received from the MS operating in an active mode; checking whether at least one message of a bandwidth allocation request message, an UpLink (UL) transmission power report message, and a UL transmission power control request message is received from the MS, if the BS fails to receive the data from the MS for a predetermined time; and ascertaining that a connection with the MS is kept, if the at least one message is received. [14] According to another aspect of the present invention, there is provided a method for checking a connection with a Mobile Station (MS) at a Base Station (BS) in a communication system including the BS and the MS being provided with a service by the BS. The method includes checking whether data is received from the MS operating in an active mode; checking whether a report response message is received from the MS, or monitoring whether channel quality information is received from the MS through a feedback channel, if the BS fails to receive the data from the MS for a predetermined time; and ascertaining that a connection with the MS is kept, if the report response message or the channel quality information is received.

[15] According to further another aspect of the present invention, there is provided a method for checking a connection with a Mobile Station (MS) at a Base Station (BS) in a communication system including the BS and the MS being provided with a service by the BS. The method includes checking whether data is received from the MS in an active mode; and determining whether a connection with the MS is kept according to whether a response message to an UpLink (UL) data grant message or a channel quality information message indicating channel quality is received, if the BS fails to receive the data from the MS for a predetermined time.

[16] According to yet another aspect of the present invention, there is provided a system for checking a connection. The system includes a Base Station (BS); and a Mobile Station (MS) which is provided with a service by the BS and operates in an active mode; wherein the BS checks whether data is received from the MS, checks whether at least one message of a bandwidth allocation request message, an UpLink (UL) transmission power report message, and a UL transmission power control request message is received from the MS, if the BS fails to receive the data for a predetermined time, and ascertains that a connection with the MS is kept, if the at least one message is received.

[17] According to further another aspect of the present invention, there is provided a system for checking a connection. The system includes a Base Station (BS); and a Mobile Station (MS) which is provided with a service by the BS and operates in an active mode; wherein the BS checks whether data is received from the MS, checks whether a report response message is received from the MS, or checks whether channel quality information is received from the MS through a feedback channel, if the BS fails to receive the data from the MS for a predetermined time, and ascertatins that a connection with the MS is kept, if the report response message or the channel quality information is received.

[18] According to still another aspect of the present invention, there is provided a system for checking a connection. The system includes a Base Station (BS); and a Mobile Station (MS) which is provided with a service by the BS and operates in an active mode; wherein the BS checks whether data is received from the MS, and determines whether a connection with the MS is kept according as a response message to an UpLink (UL) data grant message or a channel quality information message indicating channel quality is received, if the BS fails to receive the data from the MS for a predetermined time.

Advantageous Effects

[19] In a communication system according to the present invention, even though a BS fails to receive data from an MS in an active mode for a predetermined time, the BS can check a connection state with the MS by monitoring whether a bandwidth allocation request message, a UL transmission power report message, a UL transmission power control request message, and/or a DL CQI report response message is received from the MS, or whether CQI is received through a dedicated channel allocated to the MS.

[20] Further, according to the present invention, even though a BS fails to receive a bandwidth allocation request message, a UL transmission power report message, a UL transmission power control request message, a DL CQI report response message, and/ or CQI in an active mode, the BS can check a connection state with an MS by converting an operation mode of the MS to a sleep mode and checking a bandwidth allocation request message responsive to a UL data grant message. Brief Description of the Drawings

[21] The above and other aspects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:

[22] FIG. 1 is a diagram schematically illustrating a configuration of a communication system according to an embodiment of the present invention;

[23] FIG. 2 is a flowchart illustrating an operating process of a BS in a communication system according to an embodiment of the present invention;

[24] FIG. 3 is a flowchart illustrating an operating process of a BS in a communication system according to another embodiment of the present invention;

[25] FIG. 4 is a flowchart illustrating an operating process of a BS in a communication system according to another embodiment of the present invention;

[26] FIG. 5 is a flowchart illustrating an operating process of a BS in a communication system according to another embodiment of the present invention;

[27] FIG. 6 is a flowchart illustrating an operating process of a BS in a communication system according to further another embodiment of the present invention; and

[28] FIG. 7 is a flowchart illustrating an operating process of a BS in a communication system according to further another embodiment of the present invention. Mode for the Invention

[29] Preferred embodiments of the present invention will now be described in detail with reference to the annexed drawings. In the drawings, the same or similar elements are denoted by the same reference numerals even though they are depicted in different drawings. In the following description, a detailed description of known functions and configurations incorporated herein has been omitted for clarity and conciseness.

[30] The present invention provides a system and method for checking a connection state in a communication system, for example, in an Institute of Electrical and Electronics Engineers (IEEE) 802.16 communication system, which is a Broadband Wireless Access (BWA) communication system. Although a description of the present invention will be made herein with reference to an IEEE 802.16 communication system employing Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA), by way of example, the connection check system and method proposed by the present invention can also be applied to other communication systems.

[31] Further, the present invention provides a system and method for checking a connection state between a Base Station (BS) that manages a predetermined cell, and a Mobile Station (MS), which is located in the predetermined cell and receives a communication service provided from the BS, in a communication system. An embodiment of the present invention provides a system and method for checking a connection state between a BS and an MS operating in a sleep mode and an active mode, especially checking a connection state between the BS and the MS in the active mode, in a communication system employing OFDM/OFDMA.

[32] According to the present invention, if there is no data exchange between a BS and an

MS for a predetermined time in the active mode in which data is exchanged between the BS and the MS, especially if the BS fails to receive data from the MS for a time longer than or equal to a data reception interval even though it transmitted data to the MS, the BS checks a connection state with the MS by transmitting an UpLink (UL) data grant message, or unicast data grant message, to the MS, and by monitoring whether UL data, or unicast data, is received from the MS in response to the UL data grant message.

[33] Further, according to the present invention, if the BS fails to receive data from the

MS for a time longer than or equal to a data reception interval even though it transmitted data to the MS in the active mode as stated above, the BS checks a connection state with the MS by monitoring whether control messages, for example, bandwidth allocation request message (or resource allocation request), UL transmission power report message and UL transmission power control request message, are received from the MS through a dedicated channel that the BS allocated to the MS.

[34] In addition, according to the present invention, if the BS fails to receive data from the

MS for a time longer than or equal to a data reception interval in the active mode, the BS checks a connection state with the MS by monitoring whether a Channel Quality Information (CQI) message, for example, DownLink (DL) CQI Report Request (REP-REQ)/Report Response (REP-RSP) message, is received from the MS through a dedicated channel that the BS allocated to the MS, or by monitoring whether CQI is received from the MS through a feedback channel(or Channel Quality Information Channel (CQICH)), which is a dedicated channel that the BS allocated to the MS so that the MS can transmit CQI to the BS.

[35] Moreover, according to the present invention, if the BS fails to receive the above- stated messages and information from the MS, the BS converts an operation mode of the MS to the sleep mode by transmitting a Unsolicited Mobile Sleep Response (Unsolicited MOB_SLP-RSP) message to the MS in the active mode, and checks a connection state between the BS and the MS by means of a UL data grant message that the BS uses to check the connection state between the BS and the MS in the sleep mode. In other words, in order to check a connection state between the BS and the MS in the active mode, if the BS fails to receive the above-stated grant message, control message, CQI message and CQI, the BS converts an operation mode of the MS to the sleep mode by transmitting the Unsolicited MOB_SLP-RSP message to the MS in the active mode, and checks a connection state with the MS by transmitting to the MS the UL data grant message that the BS transmits to check the connection in the sleep mode, and by monitoring whether a UL bandwidth request message is received from the MS in response to the transmitted UL data grant message. Herein, the MS transmits a UL bandwidth request message being set to '0' to the BS when it does not need its UL resource allocation, and transmits a UL bandwidth allocation request message including resource allocation request information to the BS when it needs UL resource allocation. Then the BS checks a connection state with the MS by monitoring whether the UL bandwidth request message is received.

[36] Here, according to the present invention, the BS checks an MS, a connection state of which it intends to check, and checks the connection state with the MS, by monitoring the bandwidth allocation request message, UL transmission power report message, UL transmission power control request message and REP-REQ/REP-RSP message received from the MS, the CQI received through CQICH, or the UL bandwidth request message.

[37] After checking the connection state between the BS and the MS, if it is checked that the connection between the BS and the MS is kept, the BS performs a normal operation for the MS in the active mode, for example, performs such operation as resource allocation, transmission power control, and Modulation and Coding Scheme (MCS) level control for the MS, thereby supporting data transmission/reception through the normal operation for the data transmission/reception between the BS and the MS and thus improving the data transmission/reception efficiency. Even though no data is received from the MS for a predetermined time, the BS can check whether the connection with the MS is kept, preventing the BS from deleting information on the MS, i.e., cutting off the connection with the MS, thus causing a reduction in reliability of communication as it mistakenly checks the connection is kept state as the state where the connection with the MS is cut off.

[38] In addition, if it is checked that the connection between the BS and the MS is cut off, the BS deletes information on the MS, for example, deletes a Connection Identifier (CID) that it allocated to the MS in an initial connection procedure for connection of the MS, thereby preventing a waste of the CID allocated to the MS and the BS's memory. If the BS re-receives a connection request from the checked MS in the connection cut-off state, the BS re -performs the initial connection procedure with the MS. With reference to FIG. 1, a description will now be made of power control in a communication system according to an embodiment of the present invention.

[39] FIG. 1 is a diagram schematically illustrating a configuration of a communication system according to an embodiment of the present invention.

[40] Referring to FIG. 1, the communication system includes a BS 110 managing a specific cell 100, and an MSl 120 and MS2 130 which are located in the cell 100 and are provided with communication services from the BS 110. Herein, both of the MSs, i.e., MSl 120 and MS2 130, have mobility and fixity. For convenience, it will be assumed herein that the MSl 120 and the MS2 130 operate in the active mode, and signal exchange between the BS 110 and the MSs 120 and 130 is achieved using OFDM/OFDMA.

[41] The BS 110, as described above, checks a connection state with each of the MSs 120 and 130 by monitoring whether data is received from the MSs 120 and 130 in the active mode. If the BS I lO fails to receive data from the MSs 120 and 130 for a predetermined time, the BS 110 determines connection states with the MSs 120 and 130 by monitoring whether the MSs 120 and 130 have received a unicast data grant message, or a grant message including information indicating its data transmission. Alternatively, the BS 110 checks/determines connection states of the MSs 120 and 130 by monitoring whether control messages, for example, bandwidth allocation request message, UL transmission power report message and UL transmission power control request message, are received from the MSs 120 and 130; whether CQI report messages, or REP-REQ/REP-RSP messages, are exchanged with the MSs 120 and 130; or whether CQI is received from the MSs 120 and 130 through CQICH.

[42] After monitoring whether the above-stated messages and information, for example, grant message, control message, report message and CQI, are received in order to check the connection states with the MSs 120 and 130, if it is determined that the BS 110 has failed to receive the messages and information from the MSs 120 and 130, the BS 110 converts an operation mode of the MSs 120 and 130 from the active mode to the sleep mode by transmitting an Unsolicited MOB_SLP-RSP message to the MSs 120 and 130 in the active mode, and transmits to the MSs 120 and 130 a UL data grant message that it uses to check a connection state between the BS and the MS in the sleep mode. Thereafter, the BS 110 checks the connection state with the MSs 120 and 130 by monitoring whether a UL bandwidth request message is received from the MSs 120 and 130 in response to the UL data grant message. As described above, the MSs 120 and 130 transmit a UL bandwidth request message being set to '0' to the BS 110 when they do not need their UL resource allocation as there is no data to transmit to the BS 110, and the MSs 120 and 130 transmit a UL bandwidth request message including UL resource allocation request information to the BS I lO when they need UL resource allocation as there is data to transmit to the BS 110. Then the BS I lO checks/determines connection states of the MSs 120 and 130 by monitoring whether the UL bandwidth request message is received.

[43] In addition, the BS 110 identifies the MSl 120 and the MS2 130, connection states of which it intends to check, and checks connection states of the MSl 120 and the MS2 130, by monitoring whether the control messages, for example, bandwidth allocation request message, UL transmission power report message and UL transmission power control request message, are received from the MSs 120 and 130; whether the REP- REQ/REP-RSP messages are exchanged with the MSs 120 and 130; whether the CQI is received from the MSs 120 and 130 through CQICH; or whether the UL bandwidth request message is received from the MSs 120 and 130.

[44] After checking the connection states of the MSs 120 and 130, the BS 110 deletes information on the MSs 120 and 130 and CIDs allocated to the MSs 120 and 130, acquired during an initial connection procedure with the MSs 120 and 130, or performs a normal operation for the MSs 120 and 130, according to the connection states of the MSs 120 and 130. With reference to FIG. 2, a detailed description will be made of an operation of a BS in a communication system according to an embodiment of the present invention.

[45] FIG. 2 is a flowchart illustrating an operating process of a BS in a communication system according to an embodiment of the present invention.

[46] Referring to FIG. 2, in step S210, the BS performs an operation with an MS in an active mode, i.e., performs a data transmission/reception operation with the MS. Thereafter, in step S220, the BS checks a connection state with the MS by monitoring whether data is received from the MS through resources allocated to the MS, for a predetermined time, or a data reception interval, i.e., monitoring whether data is received from the MS in the active mode. If it is monitored in step S220 that the BS has failed to receive data from the MS, i.e., if no data is received from the MS in the active mode for a time longer than or equal to the data reception interval, the BS proceeds to step S230, by checking that the connection with the MS is cut off. In step S230, the BS transmits a UL data grant message to the MS to check a connection state with the MS.

[47] Herein, the BS performs scheduling for data exchange with the MS, and then transmits a UL data grant message including MAP information to the MS, so that the MS can transmit UL data, i.e., unicast data, to the BS. In other words, the BS allocates resources and transmits the UL data grant message so that the MS can transmit unicast data through the allocated resources. And the MS receives the UL data grant message, so the MS transmits unicast data to the BS, and in step S240, the BS checks again the connection state with the MS by monitoring whether unicast data is received from the MS. At this point, if there is unicast data to transmit to the BS, the MS transmits the unicast data to the BS in response to the UL data grant message, and if there is no unicast data to transmit to the BS, the MS transmits a padding data unit to the BS in response to the UL data grant message. Then the BS checks a connection state with the MS by monitoring whether the unicast data is received.

[48] If the MS transmits a padding data unit as there is no unicast data to transmit to the

BS, the BS may fail to check reception of the unicast data, i.e., padding data unit, as the padding data unit is not transmitted from an upper layer(e.g., form physical (PHY) layer to a Medium Access Control (MAC) layer).

[49] That is, though the BS may incorrectly check the connection state with the MS because it fails to monitor in step S240 whether the unicast data is received, the BS according to the present invention can check a connection state with the MS by monitoring whether control message such as a bandwidth allocation request message, a UL transmission power report message, or a UL transmission power control request message is received, and the BS can check an MS, a connection state of which it desires to check, and correctly check a connection state with the MS, by monitoring whether REP-REQ/REP-RSP messages are exchanged as CQI messages, or CQI is received through CQICH.

[50] If it is determined in step S240 that the BS has failed to receive the unicast data, the

BS proceeds to step S250, by determining that the connection with the MS is cut off. In step S250, the BS, as described above, converts an operation mode of the MS from the active mode to the sleep mode by transmitting an Unsolicited MOB_SLP-RSP message to the MS, and transmits a UL data grant message to the MS to check a connection state between the BS and the MS in the sleep mode.

[51] Next, in step S260, the BS checks again the connection state with the MS by monitoring whether a UL bandwidth request message is received from the MS in response to the UL data grant message that it transmitted in step S250. The UL bandwidth request message that the MS transmits is, as described above, a message through which the MS transmits a request for UL resource allocation to the BS according to existence/nonexistence of data to be transmitted to the BS. That is, the MS transmits a UL bandwidth request message being set to '0' to the BS when it does not need UL resource allocation as there is no data to transmit to the BS, and the MS transmits a UL bandwidth allocation request message including allocation request information to the BS when it needs UL resource allocation as there is data to transmit. Herein, when the MS does not need allocation of UL resources, since the MS transmits the UL bandwidth request message being set to '0' instead of transmitting the padding data unit, the BS determines a connection state with the MS by monitoring whether the message is received.

[52] If it is identified in step S260 that the BS has failed to receive the UL bandwidth request message from the MS, the BS proceeds to step S270, by checking that the connection with the MS is cut off. In step S270, the BS deletes information on an MS with a connection cut off (hereinafter referred to as a 'connection cut-off MS'), i.e., deletes the information on an MS, which was acquired during initial connection with the MS due to an MS's MAC reinitialization error, thereby preventing a waste of its memory due to the storage of the MS information and a waste of the CID allocated to the MS. In this case, if it is received a connection request from the connection cut-off MS, the BS performs an initial connection procedure with the MS.

[53] However, if it is identified in step S220 that the BS has received data from the MS, the BS proceeds to step S280, since its connection with the MS is kept in the active mode. In step S280, the BS performs a normal operation for the MS in the active mode, for example, performs such operation as resource allocation, transmission power control and MCS level control for data exchange with the MS. If it is monitored in step S240 that the BS has received unicast data from the MS, the BS proceeds to step S280, by checking/determining that the connection with the MS is kept in the active mode. In step S280, the BS performs a normal operation for the MS in the active mode as described above. Also, if it is monitored in step S260 that the BS has received the UL bandwidth request message from the MS, i.e., if the connection with the MS is kept, the BS proceeds to step S280, by ascertaining that the connection with the MS is kept in the active mode as described above. In step S280, the BS performs a normal operation for the MS.

[54] To sum up, in the communication system according to an embodiment of the present invention, even though the BS fails to check the connection state through data reception as it has failed to receive data from the MS in the active mode for a predetermined time, the BS may check the connection state with the MS not only in the sleep mode but also in the active mode by checking the connection state with the MS by monitoring whether it has received the unicast data that the MS transmitted. In addition, even though the BS fails to receive the unicast data in checking the connection with the MS in the active mode, the BS converts an operation mode of the MS from the active mode to the sleep mode by transmitting an Unsolicited MOB_SLP-RSP message to the MS, and checks a connection state with the MS by transmitting a UL data grant message in the sleep mode and then monitoring whether a UL bandwidth request message is received in response to the UL data grant message. As a result, the present invention prevents a waste of BS's memory due to the storage of the MS information and a waste of the CID allocated to the MS, thus improving reliability of communication. With reference to FIG. 3, a detailed description will be made of an operation of a BS in a communication system according to another embodiment of the present invention.

[55] FIG. 3 is a flowchart illustrating an operating process of a BS in a communication system according to another embodiment of the present invention. [56] Referring to FIG. 3, in step S305, the BS performs an operation with an MS in the active mode, i.e., performs a data transmission/reception with the MS. Thereafter, in step S310, the BS transmits an Uplink Channel Descriptor (UCD) message including a transmission power report (Tx Power Report) Type-Length- Value (TLV), to the MS, and monitors whether the MS supports a MAC header and an extended subheader, by monitoring a MAC header and extended subheader support TLV using a TLV in Registration Request (REG-REQ)/Registration Response (REG-RSP) messages exchanged with the MS. Herein, the Tx Power Report TLV is defined as Table 1, and the MAC header and extended subheader support TLV is defined as Table 2.

[57] [Table 1] [58]

[59] [Table 2] [60]

[61] Thereafter, in step S315, the BS checks a connection state with the MS by monitoring whether data is received from the MS through resources allocated to the MS for a predetermined time, or a data reception interval, i.e., monitoring whether data is received from the MS in the active mode. If it is monitored in step S315 that the BS has failed to receive data from the MS, i.e., if no data is received from the MS in the active mode for a time longer than or equal to the data reception interval, the BS proceeds to step S320, checking that the connection with the MS is cut off. In step S320, the BS transmits a UL data grant message to the MS to check a connection state with the MS.

[62] Next, in step S325, the BS checks again a connection state with the MS by monitoring whether control messages, for example, bandwidth allocation request message, UL transmission power report message and UL transmission power control request message, are received in response to the UL data grant message. Herein, the bandwidth allocation request message is a message through which the MS transmits a request for bandwidth allocation to the BS to exchange data with the BS in the active mode. The UL transmission power report message is a message through which the MS reports, to the BS, information on its transmission power determined by the MS itself if its transmission power control scheme is an open-loop power control scheme, and the UL transmission power control request message is a message through which the MS transmits its transmission power control request to the BS if its transmission power control scheme is a closed-loop power control scheme.

[63] That is, the BS performs scheduling for data exchange with the MS, and then notifies the scheduling result to the MS through MAP information so that the MS can request resource allocation, report UL transmission power, and/or request control of UL transmission power, and the MS checks the MAP information, and then transmits the bandwidth allocation request message, UL transmission power report message and UL power control request message to the BS through a channel allocated thereto. The bandwidth allocation request message, UL transmission power report message and UL power control request message include therein identification information of the MS that transmitted the above messages. Then the BS checks a connection state with the MS by monitoring whether the bandwidth allocation request message, UL transmission power report message and UL power control request message are received, i.e., by monitoring a bandwidth request header, a UL transmission power report header, a PHY channel report header, or UL transmission power report extended subheader.

[64] If it is monitored in step S325 that the BS has failed to receive the control message(s), the BS proceeds to step S330, by determining that the connection with the MS is cut off. In step S330, the BS, as described above, converts an operation mode of the MS from the active mode to the sleep mode by transmitting an Unsolicited MOB_SLP-RSP message to the MS, and transmits a UL data grant message to the MS to check a connection state between the BS and the MS in the sleep mode. Next, in step S335, the BS checks again the connection state with the MS by monitoring whether a UL bandwidth request message is received from the MS in response to the UL data grant message transmitted in step S330. The UL bandwidth request message that the MS transmits has been described in detail above.

[65] If it is monitored in step S335 that the BS has failed to receive the UL bandwidth request message from the MS, the BS proceeds to step S340, by ascertaining that the connection with the MS is cut off. In step S340, the BS deletes information on the connection cut-off MS, i.e., deletes MS information which was acquired during initial connection with the MS due to an MS's MAC reinitialization error, thereby preventing a waste of its memory due to the storage of the MS information and a waste of the CID allocated to the MS. In this case, if it is received a connection request from the connection cut-off MS, the BS performs an initial connection procedure with the MS.

[66] However, if it is monitored in step S315 that the BS has received data from the MS, the BS proceeds to step S350, since the connection with the MS is kept in the active mode. In step S350, the BS performs a normal operation for the MS in the active mode, for example, performs such operation as resource allocation, transmission power control and MCS level control for data exchange with the MS.

[67] Further, if it is monitored in step S325 that the BS has received the control message, the BS monitors in step S345 whether the received control message is a control message it received from the MS that exchanges data with the BS itself in the active mode in step S305, i.e., a control message received from the MS, a connection state of which the BS intends to check, by monitoring a header of the received control message, i.e., monitoring the bandwidth request header, UL transmission power report header, PHY channel report header or UL transmission power report extended subheader as described above. In other words, in step S345, the BS monitors whether the MS that transmitted the bandwidth allocation request message, UL transmission power report message and/or UL transmission power control request message, is an MS, a connection state of which it intends to check, by monitoring TLV of the bandwidth allocation request message, UL transmission power report message and/or UL transmission power control request message header.

[68] If it is monitored in step S345 that the received control message is a control message that the BS has received from the MS, a connection state of which it intends to check, the BS proceeds to step S350 since the connection with the MS is kept in the active mode. In step S350, the BS performs a normal operation for the MS in the active mode as described above. However, if it is monitored in step S345 that the received control message is not a control message that the BS has received from the MS, a connection state of which it intends to check, since the control message received in step S325 is a message transmitted by an MS other than the MS, a connection state of which the BS intends to check, the BS proceeds to step S330, by monitoring that the BS has failed to receive a control message from the MS, a connection state of which it intends to check. As described above, in step S330, the BS converts an operation mode of the MS from the active mode to the sleep mode by transmitting an Unsolicited MOB_SLP-RSP message to the MS, and transmits a UL data grant message to the MS in the sleep mode.

[69] However, if it is monitored in step S335 that the BS has received the UL bandwidth request message from the MS, i.e., if the connection with the MS is kept, the BS proceeds to step S350 since the connection with the MS is kept in the active move as stated above. In step S350, the BS performs a normal operation for the MS.

[70] To sum up, in the communication system according to another embodiment of the present invention, even though the BS fails to check the connection state through data reception as it has failed to receive data from the MS in the active mode for a predetermined time, the BS monitors an MS that transmitted the control message, and checks a connection state with the MS, by monitoring whether the control message transmitted by the MS, i.e., bandwidth allocation request message, UL transmission power report message and/or UL transmission power control request message, is received, and monitoring whether the received control message is a message received from an MS, a connection state of which the BS intends to check, using a header of the received control message. In addition, even though the BS fails to receive the control message in the active mode, the BS converts an operation mode of the MS from the active mode to the sleep mode by transmitting an Unsolicited MOB_SLP-RSP message to the MS, and checks a connection state with the MS by transmitting a UL data grant message in the sleep mode, and then monitoring whether a UL bandwidth request message is received in response to the UL data grant message. As a result, the present invention prevents a waste of BS's memory due to the storage of the MS information and a waste of the CID allocated to the MS, thus improving reliability of communication. With reference to FIG. 4, a detailed description will now be made of an operation of a BS in a communication system according to another embodiment of the present invention.

[71] FIG. 4 is a flowchart illustrating an operating process of a BS in a communication system according to another embodiment of the present invention.

[72] Referring to FIG. 4, in step S405, the BS performs an operation with an MS in the active mode, i.e., performs a data transmission/reception with the MS. Next, in step S410, the BS transmits to the MS a UCD/MAP message in which feedback region information or feedback allocation Information Element (IE) is included. Thereafter, in step S415, the BS checks a connection state with the MS by monitoring whether data is received from the MS through resources allocated to the MS for a predetermined time, or a data reception interval, i.e., by monitoring whether data is received from the MS in the active mode. If it is monitored in step S415 that the BS has failed to receive data from the MS, i.e., if no data is received from the MS in the active mode for a time longer than or equal to the data reception interval, the BS proceeds to step S420, by determining that the connection with the MS is cut off. In step S420, the BS monitors whether it is possible to use CQICH which is a feedback channel that the BS allocated to the MS on a dedicated basis after an initial connection procedure with the MS. The CQICH is a channel, or a feedback channel, that the BS allocated on a dedicated basis to receive CQI of the downlink (DL) from the MS in performing scheduling for data exchange with the BS, and the process in which the BS allocates CQICH to the MS is not directly related to the present invention, so a detailed description thereof will be omitted herein.

[73] If it is monitored in step S420 that it is not possible to use the CQICH allocated to the

MS, the BS proceeds to step S425 where it transmits a REP-REQ message to the MS so that the MS can transmit CQI of the DL to the BS. And the MS receives the REP- REQ message, so the MS generates CQI by measuring a DL channel state between the MS itself and the BS, for example, by measuring Received Signal Strength Indication (RSSI), Signal to Interference and Noise Ratio (SINR), and/or Carrier to Interference and Noise Ratio (CINR) on the DL from the BS, and then transmits a REP-RSP message including the CQI to the BS. In step S430, the BS receives the REP-RSP message that the MS transmitted, and then proceeds to step S435.

[74] In step S435, the BS monitors whether the REP-REQ message is transmitted in step S425 and the REP-RSP message is received in step S430 are messages that have been normally exchanged without error, by performing Cyclic Redundancy Check (CRC) between the REP-REQ message that the BS transmitted to the MS and the REP-RSP message that the BS received from the MS and monitoring whether the CRC result is equal to a reference CRC, i.e., by monitoring whether there is no error in the REP-RSP message the BS received from the MS in response to the REP-REQ message. If it is monitored in step S435 that the REP-REQ message and the REP-RSP message have been normally exchanged, the BS proceeds to step S440. The BS monitors in step S440 whether an MS that received the REP-REQ message is equal to an MS that transmitted the REP-RSP message, by identifying a CID of the MS that received the REP-REQ message and a CID of the MS that transmitted the REP-RSP message.

[75] If it is monitored in step S440 that the MS that received the REP-REQ message is equal to the MS that transmitted the REP-RSP message, i.e., if the MS transmits the REP-RSP message to the BS in response to the REP-REQ message received from the BS, and the BS receives the REP-RSP message, then the BS proceeds to step S445 since the connection with the MS is kept in the active mode. In step S445, the BS performs a normal operation for the MS in the active mode as described above.

[76] However, if it is monitored in step S435 that the REP-REQ message transmitted in step S425 and the REP-RSP message received in step S430 are not the messages that have been normally exchanged without error, in other words, if the CRC result for the REP-RSP message received in step S430 indicates impossibility of decoding CQI included in the REP-RSP message, then the BS, regarding the received REP-RSP message as an interference message, proceeds to step S455, by determining that the connection with the MS is cut off since the BS has failed to receive the REP-RSP message from the MS, a connection state of which it intends to check. In step S455, the BS, as described above, converts an operation mode of the MS from the active mode to the sleep mode by transmitting an Unsolicited MOB_SLP-RSP message to the MS, and transmits a UL data grant message to the MS to check a connection state between the BS and the MS in the sleep mode.

[77] Next, in step S460, the BS checks again the connection state with the MS by monitoring whether a UL bandwidth request message is received from the MS in response to the UL data grant message transmitted in step S455. The UL bandwidth request message that the MS transmits has been described in detail above.

[78] If it is monitored in step S460 that the BS has failed to receive the UL bandwidth request message from the MS, the BS proceeds to step S465, by checking that the connection with the MS is cut off. In step S465, the BS deletes information on the connection cut-off MS, i.e., deletes the MS information which was acquired during initial connection with the MS due to an MS's MAC reinitialization error, thereby preventing a waste of its memory due to the storage of the MS information and a waste of the CID allocated to the MS. In this case, if it is received a connection request from the connection cut-off MS, the BS performs an initial connection procedure with the MS. However, if it is monitored in step S460 that the BS has received the UL bandwidth request message from the MS, i.e., if the connection with the MS is kept, the BS proceeds to step S445 since the connection with the MS is kept in the sleep mode. In step S445, the BS performs a normal operation for the MS, for example, performs a normal operation for the MS in the sleep mode.

[79] However, if it is monitored in step S440 that the MS that received the REP-REQ message is different from the MS that transmitted the REP-RSP message, i.e., if the BS has failed to receive the REP-RSP message from the MS in response to the REP-REQ message, the BS proceeds to step S455, by ascertaining that the connection with the MS is cut off since the REP-RSP message received in step S430 is not a REP-RSP message received from the MS, a connection state of which the BS intends to check, i.e., the received REP-RSP message is an interference message transmitted by an MS other than the MS, a connection state of which the BS intends to check, and the BS has failed to receive the REP-RSP message from the MS, a connection state of which it intends to check. In step S455, the BS, as described above, converts an operation mode of the MS from the active mode to the sleep mode by transmitting an Unsolicited MOB_SLP-RSP message to the MS, and transmits the UL data grant message to the MS.

[80] Further, if it is monitored in step S420 that use of the CQICH allocated to the MS is possible, the BS proceeds to step S450 where it monitors whether CQI is received from the MS through CQICH which is allocated dedicatedly to the MS. Herein, the BS allocates CQICH to the MS during scheduling as described above, and allocation information for the CQICH is transmitted to the MS along with the MAP message transmitted in step S410, especially with a UL-MAP message. Thus, the MS checks its CQICH allocation information included in the MAP-message, and then transmits the generated CQI to the BS through the CQICH. Then, in step S450, the BS monitors whether the CQI is received from the MS through the CQICH.

[81] If it is monitored in step S450 that the CQI is received through the CQICH, the BS proceeds to step S445, by monitoring that the CQI received through the CQICH is a CQI transmitted by the MS, a connection state of which the BS desires to check, i.e., by ascertaining that the connection with the MS is kept in the active mode, since the CQICH is a dedicated channel for the MS, a connection state of which the BS intends to check. In step S445, the BS performs a normal operation for the MS in the active mode as described above. However, if it is monitored in step S450 that no CQI is received through the CQICH, the BS proceeds to step S455, by ascertaining that the connection with the MS is cut off. In step S455, the BS converts an operation mode of the MS from the active mode to the sleep mode by transmitting an Unsolicited MOB_SLP-RSP message to the MS, and transmits the UL data grant message to the MS.

[82] To sum up, in the communication system according to further another embodiment of the present invention, even though the BS fails to check the connection state through data reception as it has failed to receive data from the MS in the active mode for a predetermined time, the BS checks an MS, a connection state of which it intends to check, and checks a connection state with the MS, by monitoring whether REP- REQ/REP-RSP messages are received that the BS exchanges with the MS to receive CQI from the MS, or by monitoring whether CQI is received through CQICH allocated dedicatedly to the MS. Further, even though the BS fails to receive the control message in the active mode, the BS converts an operation mode of the MS from the active mode to the sleep mode by transmitting an Unsolicited MOB_SLP-RSP message to the MS, and checks a connection state with the MS by transmitting a UL data grant message in the sleep mode and then monitoring whether a UL bandwidth request message is received in response to the UL data grant message. As a result, the present invention prevents a waste of BS's memory due to the storage of the MS information and a waste of the CID allocated to the MS, thus improving reliability of communication. With reference to FIG. 5, a description will now be made of an operation of a BS in a communication system according to another embodiment of the present invention.

[83] FIG. 5 is a flowchart illustrating an operating process of a BS in a communication system according to another embodiment of the present invention.

[84] Referring to FIG. 5, the BS monitors in step S505 whether it operates with an MS in an active mode. If it is monitored in step S505 that the BS operates with the MS in the active mode, the BS checks in step S510 a connection state with the MS by monitoring whether UL data is received, i.e., if data is received from the MS from the MS through resources allocated to the MS for a predetermined time, or a data reception interval. Alternatively, the BS checks in step S515 a connection state with the MS by monitoring whether a UL transmission power report is received from the MS, or the BS monitors in step S520 whether a CQI report is received from the MS.

[85] Herein, as to BS's monitoring whether UL data is received, whether a UL transmission power report is received, and whether a CQI report is received, their priority is determined according to reception order in the UL. For convenience, although the present invention will be described herein on the assumption that the BS monitors whether a UL transmission power report is received or whether a CQI report is received, after monitoring whether UL data is received, the present invention can also be applied to the case where the BS monitors whether UL data is received after monitoring whether a UL transmission power report is received, or whether a CQI report is received. If it is monitored in step S510 that the BS has failed to receive UL data, the BS proceeds to step S515 or S520 where it monitors whether a UL transmission power report or a CQI report is received. If it is monitored in step S515 that the BS has failed to receive the UL transmission power report, the BS proceeds to step S520 where it monitors whether a CQI report is received. For convenience, while the present invention has been described herein on the assumption that if the BS fails to receive the UL data, the BS monitors whether the CQI report is received, after monitoring whether the UL transmission power report is received, the present invention can also be applied to the case where the BS monitors whether the UL transmission power report is received, after monitoring whether the CQI report is received.

[86] If it is monitors in step S510 that the BS has received the UL data, the BS proceeds to step S525 where it identifies whether a CID of the MS that transmitted the UL data is equal to Basic CID (BCID), Primary Management CID (PMCID), or Transport CID (TCID) of an MS, a connection state of which the BS intends to check. If it is identified in step S525 that the CID of the MS that transmitted the UL data is different from the BCID, PMCID or TCID of the MS, a connection state of which the BS intends to check, the BS proceeds to step S515 or S520 where it monitors whether a UL transmission power report or a CQI report is received. If it is monitored in step S515 that the BS has failed to receive the UL transmission power report, the BS monitors in step S520 whether a CQI report is received.

[87] However, if it is identified in step S525 that the CID of the MS that transmitted the

UL data is equal to the BCID, PMCID or TCID of the MS, a connection state of which the BS intends to check, the BS proceeds to step S530, by ascertaining that the connection with the MS is kept. In step S530, the BS monitors UL transmission power and CQI of the MS, a connection state of which the BS intends to check. Thereafter, in step S535, the BS performs a normal operation for the MS since the connection state with the MS is kept.

[88] Further, if it is monitored in step S520 that the BS fails to receive the CQI report, the

BS proceeds to step S540 where it monitors whether a Power Saving Class (PSC) type for reducing power consumption of the MS is determined, i.e., whether it is possible to convert an operation mode of the MS from the active mode to the sleep mode by transmitting an Unsolicited MOB_SLP-RSP message to the MS.

[89] If it is monitored in step S540 that the PSC type is not determined, the BS proceeds to step S545 where it transmits a UL data grant message to the MS. As described in step S230 of FIG. 2, the BS transmits the UL data grant message to the MS so that the MS can transmit UL data, or unicast data, to the BS. Thereafter, in step S550, the BS receives the UL data, or unicast data, from the MS. If there is unicast data to transmit to the BS, the MS transmits the unicast data to the BS in response to the UL data grant message, and if there is no unicast data to transmit to the BS, the MS transmits a padding data unit to the BS in response to the UL data grant message.

[90] Next, in step S555, the BS identifies whether a CID of the MS that transmitted the unicast data is equal to BCID, PMCID or TCID of an MS, a connection state of which the BS intends to check. If it is identified in step S555 that the CID of the MS that transmitted the unicast data is different from the BCID, PMCID or TCID of the MS, a connection state of which the BS intends to check, the BS proceeds to step S560, by checking that the connection with the MS is cut off. In step S560, the BS, as described above, deletes information on the connection cut-off MS, i.e., deletes the MS information which was acquired during initial connection with the MS due to an MS's MAC reinitialization error, thereby preventing a waste of its memory due to the storage of the MS information and a waste of the CID allocated to the MS. In this case, if it is received a connection request from the connection cut-off MS, the BS performs an initial connection procedure with the MS.

[91] Meanwhile, if it is monitored in step S515 that a UL transmission power report is received, if it is monitored in step S520 that a CQI report is received, or if it is identified in step S555 that the CID of the MS that transmitted the unicast data is equal to the BCID, PMCID or TCID of the MS, a connection state of which the BS intends to check, the BS proceeds to step S530, by ascertaining that the connection with the MS is kept. In step S530, the BS, as described above, monitors the UL transmission power and CQI of the MS, a connection state of which it intends to check, and then proceeds to step S535.

[92] Further, if it is monitored in step S540 that a PSC type is determined, the BS proceeds to step S565 where it converts an operation mode of the MS from the active mode to the sleep mode by transmitting an Unsolicited MOB_SLP-RSP message to the MS. Thereafter, in step S570, the BS monitors a receivable interval of the MS for the UL data grant message that the BS transmits to check a connection state with the MS in the sleep mode, and transmits the UL data grant message in the monitored receivable interval. In step S575, the BS receives UL data in response to the UL data grant message. In response to the UL data grant message, the BS receives a UL bandwidth request message from the MS, receives unicast data instead of the padding data unit, i.e., unicast data including a data unit that the MS should actually transmit to the BS, or receives unicast data which is set to '0' as there is no data unit that the MS should actually transmit.

[93] Next, in step S580, the BS identifies wheter a CID of the MS that transmitted UL data is equal to BCID, PMCID or TCID of the MS, a connection state of which the BS intends to check. If it is identified in step S580 that the CID of the MS that transmitted the UL data is different from the BCID, PMCID or TCID of the MS, a connection state of which the BS intends to check, the BS proceeds to step S560, by checking that the connection with the MS is cut off. In step S560, the BS deletes information on the connection cut-off MS as described above.

[94] However, if it is monitored in step S505 that the BS operates with the MS in the sleep mode, the BS, as described above, monitors a receivable interval of the MS for the UL data grant message that the BS transmits to check a connection state with the MS in the sleep mode, and transmits the UL data grant message in the monitored receivable interval in step S570. Thereafter, in step S575, the BS receives UL data in response to the UL data grant message. In this case, the BS receives a UL bandwidth request message, or receives the actually transmitted data unit, or unicast data being set to '0', and then proceeds to step S580.

[95] In step S580, the BS, as described above, identifies if a CID of the MS that transmitted UL data is equal to BCID, PMCID or TCID of the MS, a connection state of which the BS intends to check. If it is identified in step S580 that the CID is different from the BCID, PMCID or TCID, the BS proceeds to step S560, by checking that the connection with the MS is cut off. In step S560, the BS deletes information on the MS. However, if it is identified in step S580 that the CID of the MS that transmitted UL data is equal to BCID, PMCID or TCID of the MS, a connection state of which the BS intends to check, the BS proceeds to step S585, by ascertaining that the connection with the MS is kept. In step S585, the BS monitors the UL transmission power and CQI of the MS, a connection state of which it intends to check, and then proceeds to step S570 where the BS transmits a UL data grant message in the identified receivable interval in which the UL data grant message for the MS can be received.

[96] To sum up, in the communication system according to another embodiment of the present invention, the BS checks a connection state with the MS by monitoring whether UL data is received from the MS in the active mode, whether a UL transmission power report is received, or whether a CQI report is received. Even though the BS fails to check the connection state with the MS by monitoring whether the UL data is received, whether the UL transmission power report is received, or whether the CQI report is received, the BS checks the connection state with the MS by monitoring whether the UL data is received from the MS after converting an operation mode of the MS from the active mode to the sleep mode. As a result, the present invention prevents a waste of BS's memory due to the storage of the MS information and a waste of the CID allocated to the MS, thus improving reliability of communication. With reference to FIGs. 6 and 7, a detailed description will be made of an operation of a BS in a communication system according to further another embodiment of the present invention. Further another embodiment described in FIGs. 6 and 7, unlike the embodiments described in FIGs. 2 to 5, provides how the BS can check the connection state with the MS, if the conversion of an operation mode is not supported in checking the connection state with the MS, i.e., if the BS cannot convert an operation mode of the MS from the active mode to the sleep mode by transmitting an Unsolicited MOB_SLP-RSP message to the MS.

[97] FIG. 6 is a flowchart illustrating an operating process of a BS in a communication system according to further another embodiment of the present invention.

[98] Referring to FIG. 6, the BS monitors in step S605 whether it operates with an MS in an active mode. If it is monitored in step S605 that the BS operates with the MS in the active mode, the BS checks in step S610 a connection state with the MS by monitoring whether UL data is received, i.e., if data is received from the MS through resources allocated to the MS for a predetermined time, or a data reception interval. Further, in step S615, the BS monitors whether a CQI report is received from the MS. Herein, as to BS's monitoring whether UL data is received and whether a CQI report is received, their priority is determined according to reception order in the UL. For convenience, while the present invention will be described herein on the assumption that the BS monitors whether the CQI report is received, after monitoring whether the UL data is received, the present invention can also be applied to the case where the BS monitors whether the UL data is received, after monitoring whether the CQI report is received.

[99] If it is monitored in step S610 that the BS receives UL data, the BS proceeds to step

S620 where it identifies whether a CID of the MS that transmitted the UL data is equal to BCID, PMCID or TCID of an MS, a connection state of which the BS intends to check. If it is identified in step S620 that the CID of the MS that transmitted the UL data is different from the BCID, PMCID or TCID of the MS, a connection state of which the BS intends to check, the BS proceeds to step S615 where it monitors whether a CQI report is received.

[100] However, if it is identified in step S620 that the CID of the MS that transmitted the UL data is equal to the BCID, PMCID or TCID of the MS, a connection state of which the BS intends to check, the BS proceeds to step S625, by ascertaining that the connection with the MS is kept. In step S625, the BS monitors a CQI from the MS, a connection state of which it desires to check. Thereafter, in step S630, the BS performs a normal operation for the MS as described above, since the connection with the MS is kept.

[101] If it is monitored in step S615 that the BS has failed to receive the CQI report, the BS proceeds to step S635 where it transmits a UL data grant message to the MS. As described in step S545 of FIG. 5, the BS transmits the UL data grant message to the MS so that the MS can transmit UL data, or unicast data, to the BS. Thereafter, in step S640, the BS receives UL data, or unicast data, from the MS. If there is unicast data to transmit to the BS, the MS transmits the unicast data to the BS in response to the UL data grant message, and if there is no unicast data to transmit to the BS, the MS transmits a padding data unit to the BS in response to the UL data grant message.

[102] Next, in step S645, the BS identifies whether a CID of the MS that transmitted the unicast data is equal to BCID, PMCID or TCID of the MS, a connection state of which the BS intends to check. If it is identified in step S645 that the CID of the MS that transmitted the unicast data is different from the BCID, PMCID or TCID of the MS, a connection state of which the BS intends to check, the BS proceeds to step S650, by checking that the connection with the MS is cut off. In step S650, the BS, as described above, deletes information on the connection cut-off MS, i.e., deletes the MS information which was acquired during initial connection with the MS due to an MS's MAC reinitialization error, thereby preventing a waste of its memory due to the storage of the MS information and a waste of the CID allocated to the MS. In this case, upon receipt of a connection request from the connection cut-off MS, the BS performs an initial connection procedure with the MS.

[103] However, if it is monitored in step S615 that the BS receives the CQI report from the MS, or if it is identified in step S645 that the CID of the MS that transmitted the unicast data is equal to the BCID, PMCID or TCID of the MS, a connection state of which the BS intends to check, the BS proceeds to step S625, by ascertaining that the connection with the MS is kept. In step S625, the BS, as described above, monitors a CQI from the MS, a connection state of which it desires to check, and then proceeds to step S630.

[104] Further, if it is monitored in step S605 that the BS operates with the MS in the sleep mode, the BS monitors in step S655 a receivable interval of the MS for a UL data grant message that the BS transmits to check a connection state of the MS in the sleep mode, and then transmits the UL data grant message in the monitored receivable interval. In step S660, the BS receives UL data in response to the UL data grant message. In response to the UL data grant message, the BS receives a UL bandwidth request message from the MS, receives unicast data instead of a padding data unit, i.e., unicast data including the data unit that the MS should actually transmit to the BS, or receives unicast data which is set to '0' as there is no data unit that the MS should actually transmit.

[105] Next, in step S665, the BS identifies whether a CID of the MS that transmitted UL data is equal to BCID, PMCID or TCID of an MS, a connection state of which the BS intends to check. If it is identified in step S665 that the CID of the MS that transmitted UL data is different from the BCID, PMCID or TCID of the MS, a connection state of which the BS intends to check, the BS proceeds to step S650, by checking that the connection with the MS is cut off. In step S650, the BS deletes information on the connection cut-off MS as described above.

[106] However, if it is identified in step S665 that the CID of the MS that transmitted UL data is equal to the BCID, PMCID or TCID of the MS, a connection state of which the BS intends to check, the BS proceeds to step S670, by ascertaining that the connection with the MS is kept. In step S670, the BS monitors a CQI from the MS, a connection state of which it intends to check, and then proceeds to step S655 where it transmits a UL data grant message in a receivable interval in which the UL data grant message for the MS can be received.

[107] FIG. 7 is a flowchart illustrating an operating process of a BS in a communication system according to further another embodiment of the present invention.

[108] Referring to FIG. 7, the BS determines in step S705 whether it operates with an MS in an active mode. If it is monitored in step S705 that the BS operates with the MS in the active mode, the BS checks in step S710 a connection state with the MS by monitoring whether UL data is received, i.e., whether data is received from the MS through resources allocated to the MS for a predetermined time, or a data reception interval. Further, the BS checks in step S715 the connection state with the MS by monitoring whether a UL transmission power report is received from the MS, or the BS monitors in step S720 whether a CQI report is received from the MS. If it is monitored in step S710 that the BS has failed to receive the UL data, the BS proceeds to step S715 or S720 where it monitors whether the UL transmission power report is received, or the CQI report is received. If it is monitored in step S715 that the BS has failed to receive the UL transmission power report, the BS proceeds to step S720 where it monitors whether a CQI report is received.

[109] Herein, as to BS's monitoring whether UL data is received, wheter a UL transmission power report is received, and whether a CQI report is received, their priority is determined according to reception order in the UL as described above. According to the present invention, after performing any one of monitoring whether UL data is received, monitoring whether a UL transmission power report is received, and monitoring whether a CQI report is received, the BS checks the connection state by performing any one of the remaining identifications, if it fails to receive the UL data or the reports, and performing the remaining last identification, if it fails to receive the UL data or the report.

[110] If it is monitored in step S710 that the BS has received the UL data, the BS proceeds to step S725 where it identifies whether a CID of the MS that transmitted the UL data is equal to BCID, PMCID or TCID of an MS, a connection state of which the BS intends to check. If it is identified in step S625 that the CID of the MS that transmitted the UL data is different from the BCID, PMCID or TCID of the MS, a connection state of which the BS intends to check, the BS proceeds to step S715 or S720 where it monitors whether the UL transmission power report is received, or the CQI report is received. If it is monitored in step S715 that the BS has failed to receive the UL transmission power report, the BS proceeds to step S720 where it monitors a CQI report is received.

[I l l] However, if it is identified in step S725 that the CID of the MS that transmitted the UL data is equal to the BCID, PMCID or TCID of the MS, a connection state of which the BS intends to check, the BS proceeds to step S730, by ascertaining that the connection with the MS is kept. In step S730, the BS monitors the UL transmission power and CQI of the MS, a connection state of which it desires to check. Thereafter, in step S735, the BS performs a normal operation for the MS as described above since its connection with the MS is kept.

[112] If it is monitored in step S720 that the BS has failed to receive the CQI report, the BS proceeds to step S740 where it transmits a UL data grant message to the MS. As described in step S545 of FIG. 5, the BS transmits the UL data grant message to the MS so that the MS can transmit UL data, or unicast data, to the BS. Thereafter, in step S745, the BS receives UL data, or unicast data, from the MS. If there is unicast data to transmit to the BS, the MS transmits the unicast data to the BS in response to the UL data grant message, and if there is no unicast data to transmit to the BS, the MS transmits a padding data unit to the BS in response to the UL data grant message.

[113] Next, in step S750, the BS identifies whether a CID of the MS that transmitted the unicast data is equal to BCID, PMCID or TCID of the MS, a connection state of which the BS intends to check. If it is identified in step S750 that the CID of the MS that transmitted the unicast data is different from the BCID, PMCID or TCID of the MS, a connection state of which the BS intends to check, the BS proceeds to step S755, by checking that the connection with the MS is cut off. In step S755, the BS, as described above, deletes information on the connection cut-off MS, i.e., deletes the MS information which was acquired during initial connection with the MS due to an MS's MAC reinitialization error, thereby preventing a waste of its memory due to the storage of the MS information and a waste of the CID allocated to the MS. In this case, upon receipt of a connection request from the connection cut-off MS, the BS performs an initial connection procedure with the MS.

[114] Meanwhile, if it is monitored in step S715 that the BS has received the UL transmission power report from the MS, when it is monitored in step S720 that the BS has received the CQI report from the MS, or when it is identified in step S750 that the CID of the MS that transmitted the unicast data is equal to the BCID, PMCID or TCID of the MS, a connection state of which the BS intends to check, the BS proceeds to step S730, by ascertaining that the connection with the MS is kept. In step S730, the BS, as described above, checks the UL transmission power and CQI of the MS, a connection state of which it intends to check, and then proceeds to step S735.

[115] However, when it is monitored in step S705 that the BS operates with the MS in a sleep mode, the BS identifies in step S760 a receivable interval of the MS for a UL data grant message that the BS transmits to check a connection state with the MS in the sleep mode, and transmits the UL data grant message in the monitored receivable interval. In step S765, the BS receives UL data in response to the UL data grant message. In response to the UL data grant message, the BS receives a UL bandwidth request message from the MS, receives unicast data instead of a padding data unit, i.e., unicast data including the data unit that the MS should actually transmit to the BS, or receives unicast data which is set to '0' as there is no data unit that the MS should actually transmit.

[116] Next, in step S770, the BS identifies whether a CID of the MS that transmitted UL data is equal to BCID, PMCID or TCID of an MS, a connection state of which it intends to check. When it is identified in step S770 that the CID of the MS that transmitted UL data is different from the BCID, PMCID or TCID of the MS, a connection state of which the BS intends to check, the BS proceeds to step S755, by checking that the connection with the MS is cut off. In step S755, the BS deletes information on the connection cut-off MS as described above.

[117] However, when it is identified in step S770 that the CID of the MS that transmitted UL data is equal to the BCID, PMCID or TCID of the MS, a connection state of which the BS intends to check, the BS proceeds to step S785, by ascertaining that the connection with the MS is kept. In step S785, the BS monitors the UL transmission power and CQI of the MS, a connection state of which it intends to check, and then proceeds to step S760 where it transmits a UL data grant message in the monitored receivable interval in which the UL data grant message for the MS can be received.

[118] While the invention has been shown and described with reference to a certain preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

[1] A method for checking a connection with a Mobile Station (MS) at a Base

Station (BS) in a communication system including the BS and the MS being provided with a service provided by the BS, the method comprising: checking whether data is received from the MS operating in an active mode; checking whether at least one message of a bandwidth allocation request message, an UpLink (UL) transmission power report message, and a UL transmission power control request message is received from the MS, if the BS fails to receive the data from the MS for a predetermined time; and ascertaining that a connection with the MS is kept, if the at least one message is received.

[2] The method of claim 1, wherein the step of checking whether at least one message is received comprises: monitoring a header of the at least one message to identify whether an MS which transmitted the at least one message is equal to an MS, a connection state of which the BS intends to check.

[3] The method of claim 1, further comprising: transmitting a UL data grant message to the MS, if the BS fails to receive the data from the MS; monitoring whether UL data is received in response to the UL data grant message; and checking that a connection with the MS is kept, if the UL data is received.

[4] The method of claim 3, wherein the step of monitoring whether UL data is received comprises: identifying whether a Connection Identifier (CID) of an MS which transmitted the UL data is equal to a CID of an MS, a connection state of which the BS intends to check.

[5] The method of claim 1, further comprising: converting an operation mode of the MS to a sleep mode by transmitting an unsolicited sleep response message to the MS, if the BS fails to receive the at least one message; transmitting the UL data grant message; and monitoring whether an UL bandwidth allocation request message is received in response to the UL data grant message.

[6] The method of claim 1, wherein the step of checking whether at least one message is received comprises: determining that a connection with the MS is kept, if UL bandwidth allocation request information or a UL bandwidth request message being set to zero (0) is received from the MS.

[7] A method for checking a connection with a Mobile Station (MS) at a Base

Station (BS) in a communication system including the BS and the MS being provided with a service by the BS, the method comprising: checking whether data is received from the MS operating in an active mode; checking whether a report response message is received from the MS, or identifying whether channel quality information is received from the MS through a feedback channel, if the BS fails to receive the data from the MS for a predetermined time; and ascertaining that a connection with the MS is kept, if the report response message or the channel quality information is received.

[8] The method of claim 7, wherein the step of checking whether a report response message is received comprises: performing Cyclic Redundancy Check (CRC) of the report response message.

[9] The method of claim 7, wherein the step of checking whether a report response message is received comprises: identifying a Connection Identifier (CID) of an MS which receives a report request message and a CID of an MS which transmits the report response message; and monitoring whether an MS which is transmitted the report response message is equal to an MS, a connection state of which the BS intends to check.

[10] The method of claim 7, wherein the step of checking whether a report response message comprises: monitoring whether channel quality information included in the report response message is normally decoded.

[11] The method of claim 7, wherein the step of checking whether channel quality information is received comprises: monitoring whether an MS which transmitted the channel quality information through the feedback channel is equal to an MS, a connection state of which the BS intends to check.

[12] A method for checking a connection with a Mobile Station (MS) at a Base

Station (BS) in a communication system including the BS and the MS being provided with a service by the BS, the method comprising: checking whether data is received from the MS in an active mode; and determining whether a connection with the MS is kept according to whether a response message to an UpLink (UL) data grant message or a channel quality information message indicating channel quality is received, if the BS fails to receive the data from the MS for a predetermined time.

[13] The method of claim 12, further comprising: transmitting a request message for mode conversion to a sleep mode to the MS, if the BS fails to receive the response message or the channel quality information message; and monitoring whether a UL bandwidth request message is received in response to the UL data grant message.

[14] The method of claim 13, wherein the step of checking whether a UL bandwidth request message is received comprises: determining that a connection with the MS is kept, if UL bandwidth request information or the UL bandwidth request message being set to zero (0) is received from the MS.

[15] A system for checking a connection, the system comprising: a Base Station (BS); and a Mobile Station (MS) which is provided with a service by the BS and operates in an active mode; wherein the BS checks whether data is received from the MS, checks whether at least one message of a bandwidth allocation request message, an UpLink (UL) transmission power report message, and a UL transmission power control request message is received from the MS, if the BS fails to receive the data for a predetermined time, and ascertains that a connection with the MS is kept, if the at least one message is received.

[16] The system of claim 15, wherein the BS monitors a header of the at least one message to monitor whether an MS which transmitted the at least one message is equal to an MS, a connection state of which the BS intends to check.

[17] The system of claim 15, wherein the BS transmits a UL data grant message to the

MS, if the BS fails to receive the data from the MS, monitors whether UL data is received in response to the UL data grant message, and determines that a connection with the MS is kept, if the UL data is received.

[18] The system of claim 17, wherein the BS identifies whether a Connection

Identifier (CID) of an MS which transmitted the UL data is equal to a CID of an MS, a connection state of which the BS intends to check.

[19] The system of claim 15, wherein the BS converts an operation mode of the MS to a sleep mode by transmitting an unsolicited sleep response message to the MS, if the BS fails to receive at least one message, transmits the UL data grant message, and monitors whether a UL bandwidth request message is received in response to the UL data grant message.

[20] The system of claim 15, wherein if UL bandwidth allocation request information or a UL bandwidth request message being set to zero (0) is received from the MS, the BS checks whether a connection with the MS is kept according to whether the at least one message is received.

[21] A system for checking a connection, the system comprising: a Base Station (BS); and a Mobile Station (MS) which is provided with a service by the BS and operates in an active mode; wherein the BS checks whether data is received from the MS, checks whether a report response message is received from the MS, or checks whether channel quality information is received from the MS through a feedback channel, if the BS fails to receive the data from the MS for a predetermined time, and as- certatins that a connection with the MS is kept, if the report response message or the channel quality information is received.

[22] The system of claim 21, wherein the BS monitors whether the report response message is received, by performing Cyclic Redundancy Check (CRC) of the report response message.

[23] The system of claim 21, wherein the BS monitors whether an MS which is transmitted the report response message is equal to an MS, a connection state of which the BS intends to check, by identifying a Connection Identifier (CID) of an MS which receives a report request message and a CID of an MS which transmits the report response message.

[24] The system of claim 21, wherein the BS monitors whether channel quality information included in the report response message is normally decoded.

[25] The system of claim 21, wherein the BS monitors whether an MS which transmitted the channel quality information through the feedback channel is equal to an MS, a connection state of which the BS intends to check.

[26] A system for checking a connection, the system comprising: a Base Station (BS); and a Mobile Station (MS) which is provided with a service by the BS and operates in an active mode; wherein the BS checks whether data is received from the MS, and determines whether a connection with the MS is kept according as a response message to an UpLink (UL) data grant message or a channel quality information message indicating channel quality is received, if the BS fails to receive the data from the MS for a predetermined time.

[27] The system of claim 26, wherein the BS transmits a request message for mode conversion to a sleep mode to the MS, if the BS fails to receive the response message or the channel quality information message, and monitors whether a UL bandwidth request message is received in response to the UL data grant message. [28] The system of claim 27, wherein the BS determines that a connection with the

MS is kept, if UL bandwidth request information or the UL bandwidth request message being set to zero (0) is received from the MS.