Description Apparatus and method for operation of adaptive modulation and coding in wireless broadband communication system Technical Field

[1] The present invention relates generally to an apparatus and method for operation of

Adaptive Modulation and Coding (AMC), and in particular, to an AMC operation apparatus and method for enabling modulation and coding in various channel environments by setting an offset value in a Carrier to Interference and Noise Ratio (CINR) received from a terminal in a wireless broadband communication system. Background Art

[2] Resources in wireless communication systems correspond to frequency bands. The methodology that efficiently allocates the limited frequency bands among users is multiple access, and the connection methodology that distinguishes a connection between an uplink and a downlink in bidirectional communications is multiplexing. The wireless multiple access and multiplexing scheme is a platform technology that becomes the basis of a wireless transmission technology for efficiently using the limited frequency resources, and is determined according to allocated frequency bands, the number of users, transfer rate, mobility, cell configuration, wireless environments, etc.

[3] Orthogonal Frequency Division Multiplexing (OFDM), which is one of such wireless transmission schemes, is a kind of a Multi-Carrier Transmission/Modulation (MCM) scheme using multiple carriers, that converts input data into parallel data according to the number of carriers and transmits the parallel data on each of the carriers. OFDM can be classified into OFDM-FDMA, OFDM-TDMA and OFDM-CDMA according to multiple access schemes for the users.

[4] Among them, OFDM-FDMA (OFDMA), which is a scheme suitable for the 4 generation macro/micro-cellular infrastructure, has no intra-cell interference and high frequency reuse efficiency and is superior in adaptive modulation. To make up for the faults of OFDMA, it is also possible to increase diversity and reduce an inter-cell interference effect by using a distributed frequency hopping technique, a multi-antenna technique, a strong coding technique, and the like. In particular, OFDMA is efficiently applied to a wireless communication system having a cell in a broad area where a time delay spread is relatively greater, since it is useful when a large number of sub-carriers are used.

[5] Generally, OFDMA employs AMC that adjusts a Modulation and Coding Scheme

(MCS) level of bursts allocated in a frame using measured CINR information from a

terminal. AMC can be effectively applied when the wireless environment is stabilized, but it may not support the real wireless environments, including the case where the measured CINR is distorted or leaves for the limited range, and the case where fast channel fading occurs. Such improper application of AMC may cause serious problems, including incomplete transmission of data and signal interruption with a terminal.

Disclosure of Invention Technical Problem

[6] The present invention is proposed to satisfy above described requests, accordingly, an aspect of the present invention is to provide an AMC operation apparatus and method which is effective for a change in wireless environments by compensating a CINR, when the CINR is distorted or leaves for a limited range (i.e. the CINR is saturated) during its transmission from terminal in a wireless broadband communication system.

[7] Another aspect of the present invention is to provide an AMC operation apparatus and method for operating AMC in consideration of a CINR received from a terminal and a Packet Error Rate (PER) of data bursts in a wireless broadband communication system.

[8] Further another aspect of the present invention is to provide an AMC operation apparatus and method for estimating a PER of an uplink using data packet information received from a terminal, and operating AMC according to a CINR which is adjusted using the estimated PER in a wireless broadband communication system.

[9] Yet another aspect of the present invention is to provide an AMC operation apparatus and method for estimating a PER of a downlink using ACK/NAK information received from a terminal, and operating AMC according to a CINR which is adjusted using the estimated PER in a wireless broadband communication system. Technical Solution

[10] According to one aspect of an embodiment of the present invention, there is provided a method for operation of Adaptive Modulation and Coding (AMC) in a wireless broadband communication system. The method includes (a) measuring a Carrier to Interference and Noise Ratio (CINR) of an accessed terminal; (b) computing a packet error for the terminal, and adjusting the CINR by applying an offset value to the measured CINR according to the packet error computation result; and (c) determining a Modulation and Coding Scheme (MCS) level corresponding to the adjusted CINR as an MCS level for the terminal.

[11] According to another aspect of an embodiment of the present invention, there is provided an apparatus for operation of AMC in a wireless broadband communication

system. The apparatus includes a Carrier to Interference and Noise Ratio (CINR) receiver for being reported a measured CINR from a terminal; a packet error calculator for computing a packet error for the terminal; an offset applicator for applying a positive offset or a negative offset to the measured CINR according to the packet error computation result; and a Modulation and Coding Scheme (MCS) selector for selecting an MCS level for the terminal based on an output of the offset applicator. [12] According to further another aspect of an embodiment of the present invention, there is provided a wireless broadband communication system an AMC determiner for computing at least one of an uplink packet error and a downlink packet error for an accessed terminal, and determining a Modulation and Coding Scheme (MCS) level for the terminal by correcting a measured Carrier to Interference and Noise Ratio (CINR) from the terminal based on the computed packet error.

Advantageous Effects

[13] According to the present invention, an AMC operation apparatus and method can determine an MCS level suitable for the real wireless environment by compensating a CINR in consideration of channel fading and a change of the measured and reported CINR, making it possible to apply an MCS level adaptive to a change in the wireless environment.

[14] In addition, the present invention adjusts a reported or measured CINR in consideration of a PER of data bursts from a terminal, and selects an MCS level using the adjusted CINR, thus facilitating efficient operation of AMC.

[15] Moreover, the present invention estimates PERs of an uplink and a downlink separately, and adjusts distortion of a CINR according thereto, making it possible to apply AMC based on the substantial CINR information. Brief Description of Drawings

[16] 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:

[17] FIG. 1 is a diagram illustrating a structure of a base station provided to describe an

AMC operation apparatus according to an embodiment of the present invention;

[18] FIG. 2 is a diagram illustrating a structure of the AMC operator in FIG. 1;

[19] FIG. 3 is a diagram illustrating a structure of the packet error calculator in FIG. 2;

[20] FIG. 4 is a diagram illustrating a frame structure in a wireless broadband communication system;

[21] FIG. 5 is a flowchart illustrating an AMC operation method according to an embodiment of the present invention;

[22] FIG. 6 is a flowchart illustrating a detailed CINR adjustment method for an uplink in

FIG. 5; and

[23] FIG. 7 is a flowchart illustrating a detailed CINR adjustment method for a downlink in FIG. 5. Mode for the Invention

[24] 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.

[25] FIG. 1 is a diagram illustrating a structure of a base station provided to describe an

AMC operation apparatus according to an embodiment of the present invention, in which the base station includes a base station supporting OFDMA.

[26] As illustrated in FIG. 1, a base station includes an interface 100, a band signal processing module 200, a transmission module 300, a reception module 600, a scheduler 500, an AMC operator 700, and an antenna 400. This base station, which is for supporting Time Division Duplexing (TDD) scheme, can be divided into a reception path and a transmission path.

[27] In the reception path, the reception module 600 receives one or more radio signals transmitted by terminals via the antenna 400, and converts the received radio signals into baseband signals. For instance, for data reception of the base station, the reception module 600 removes noises from the received signals, amplifies the noise-removed signals, down-converts the amplified signals into baseband signals, and digitalizes the down-converted baseband signals. The band signal processing module 200 extracts information or data bits from the digitalized signals and performs demodulation, decoding and error correction processes thereon. The received information is delivered to an adjacent wire/wireless network via the interface 100, or transmitted to other terminals being serviced by the base station, back through the transmission path.

[28] In the transmission path, the interface 100 receives voice, data or control information from a base station controller or a wireless network, and the band signal processing module 200 encodes the voice, data or control information, and then outputs them to the transmission module 300. The transmission module 300 modulates the encoded vo ice, data or control information with a carrier signal having desired transmission frequency or frequencies, amplifies the modulated carrier signal to a level proper for transmission, and propagates it over the air via the antenna 400.

[29] The AMC operator 700 determines an MCS level to be applied in the scheduler 500 using DownLink (DL) or UpLink (UL) CINR information reported or measured by the reception module 600. The AMC operator 700 compensates the reported or measured

CINR, and determines an MCS level corresponding to the compensated CINR as an MCS level for each terminal. The AMC operator 700 selects an MCS level corresponding to the compensated CINR based on an AMC table in which an MCS level corresponding to each CINR is determined.

[30] Meanwhile, the scheduler 500 controls operations and respective components of the reception path and the transmission path. Particularly, in regard to the present invention, the scheduler 500 creates a frame to be transmitted to respective terminals in the transmission path depending on the MCS level determined by the AMC operator 700, and performs mapping on corresponding bursts in the frame.

[31] FIG. 2 is a diagram illustrating a structure of the AMC operator in FIG. 1, FIG. 3 is a diagram illustrating a structure of the packet error calculator in FIG. 2, and FIG. 4 illustrates a frame structure in a wireless broadband communication system. With reference to FIGs. 2 through 4, a detailed description will be made of an AMC operation apparatus according to the present invention.

[32] Referring first to FIG. 2, the AMC operator 700 according to the present invention includes a CINR receiver 710, a packet error calculator 720, an offset applicator 730, and an MCS selector 740.

[33] A terminal must report a downlink CINR which is measured in a period that a base station previously determined. The CINR report can be accomplished through a Channel Quality Information (CQI) channel in an uplink, and the CINR receiver 710 receives an estimating value of CINR.

[34] The packet error calculator 720 computes an error of packets transmitted/received by the base station. In the present invention using TDD scheme, the packet error calculator 720 computes packet errors separately for the downlink and the uplink, since PERs of the downlink and the uplink may become different.

[35] The packet error calculator 720 will be described in more detail with reference to

FIG. 3. The packet error calculator 720 includes a CRC (Cyclic Redundancy Check) checker 722 for checking CRC information included in an uplink packet transmitted by a terminal, an ACK/NAK (Acknowledgement/Non- Acknowledgement) checker 724 for checking ACK/NAK information which is a response to a downlink packet that the base station transmitted to the terminal, an error calculator 726 for calculating a packet error based on the information provided from the CRC checker 722 or the ACK/NAK checker 724, and an error determiner 728 for determining whether there is an error according to the result of the error calculation.

[36] When transmitting a packet to the base station in an uplink interval, the terminal adds

CRC information to the transmission packet, and the CRC checker 722 checks the CRC information for each packet to determine whether a CRC error occurs in the corresponding packet. Therefore, the CRC checker 722 determines that the uplink packet

is normal when no CRC error is checked, and determines that an uplink packet error has occurred when a CRC error is checked.

[37] As for a downlink to which Hybrid Automatic Repeat reQuest (HARQ) is applied, when the base station transmits a downlink packet to a terminal, the terminal responds with ACK information when it normally receives the downlink packet, and the terminal responds with NAK information when it fails to normally receive the downlink packet or an error occurs in the downlink packet.

[38] The ACK/NAK response will be described below. As shown in FIG. 4, in a frame for a wireless broadband communication system to which the present invention is applied, packet transmission units in frequency and time domains are represented by a subchannel and a symbol, respectively, and the vertical axis represents indexes of subchannels which are frequency resource allocation units, while the horizontal axis represents indexes of OFDM symbols which are time resource allocation units. Further, a Transmit/receive Transition Gap (TTG), which is a guard region, exists between the downlink and the uplink.

[39] Meanwhile, the above frame is divided into a downlink subframe constructed to transmit a packet from a base station to a terminal, and an uplink subframe configured to transmit a packet from the terminal to the base station, and the uplink subframe is composed of control symbols (Ranging, ACK and CQI) and uplink bursts.

[40] The control symbols in the uplink subframe are used for a ranging channel, a CQI channel and an ACK channel, and the ACK channel provides a feedback for downlink HARQ. For example, when a downlink packet is received without error, the ACK channel transmits a feedback with ACK information, and when an error occurs in the downlink packet, the ACK channel transmits a feedback with NAK information.

[41] Accordingly, the ACK/NAK checker 724 checks the ACK or NAK information.

After the check, the ACK/NAK checker 724 determines that the downlink packet transmission is normal when it receives ACK information, and determines that a packet error has occurred in the downlink when it receives NAK information.

[42] The error calculator 726 calculates an uplink packet error based on CRC check information provided from the CRC checker 722, and calculates a downlink packet error based on ACK/NAK check information provided from the ACK/NAK checker 724.

[43] To be more specific, the CRC checker 722 checks whether a CRC error occurs separately for each uplink packet transmitted from the terminal. The CRC checker 722 counts an u ^r plink error number N ERRl when a CRC error occurs, and counts an u ^r plink non-error number N NONl when a CRC error dose not occur. On the other hand, the

ACK/NAK checker 724 checks ACK or NAK information indicating information about whether a downlink packet error occurs. The ACK/NAK checker 724 counts a downlink error number N ERR2 when it detects NAK information, and counts a downlink

non-error number N when it detects ACK information.

N0N2

[44] In addition, the error calculator 726 calculates an uplink's packet number N by

PACl adding the counted uplink error number N and uplink non-error number N , or

ERRl NONl calculates a downlink's packet number N by adding the counted downlink error r PAC2 ^{J &} number N and downlink non-error number N

ERR2 N0N2

[45] When it is determined that there is an uplink packet error, the error determiner 728 compares the uplink error number N with a first threshold number N , and

ERRl THRl compares the uplink's packet number N with a first maximum packet number MAX . Further, when it is determined that there is a downlink packet error, the error

PACl determiner 728 compares the downlink error number N with a second threshold

ERR2 number N , and compares the downlink's packet number N with a second

THR2 PAC2 maximum packet number MAX

PAC2

[46] The first threshold number N indicates a threshold value preset to determine an

THRl uplink packet error, and the second threshold number N THR2 indicates a threshold value preset to determine a downlink packet error. The first threshold number N THRl and the second threshold number N THR2 can be set different from or equal to each other according to the environment of the base station. [47] In addition, the first maximum ^r packet number MAX PACl indicates the maximum number of packets, used to determine an uplink packet error, and the second maximum packet number MAX PAC2 indicates the maximum number of packets, used to determine a downlink packet error. Also, the first maximum packet number MAX and the

PACl second maximum packet number MAX can be set different from or equal to each

PAC2 other according to the environment of the base station. [48] The error determiner 728 compares the uplink error number N with the first

ERRl threshold number N , and determines to apply a negative offset -δoffset when the

THRl uplink error number N exceeds the first threshold number N . However, when

ERRl THRl the uplink error number N is less than or equal to the first threshold number N ,

ERRl THRl the error determiner 728 compares the uplink's packet number N with the first maximum packet number MAX , and determines to apply a positive offset +δoffset

PACl when the uplink's packet number N exceeds the first maximum packet number

PACl

MAX .

PACl

[49] Further, the error determiner 728 compares the downlink error number N with the

ERR2 second threshold number N , and determines to apply a negative offset -δoffset

THR2 ^{rr J &} when the downlink error number N ERR2 exceeds the second threshold number N THR2

However, when the downlink error number N ERR2 is less than or equal to the second threshold number N , the error determiner 728 compares the downlink's packet number N PAC2 with the second maximum packet number MAX PAC2 , and determines to apply a positive offset +δoffset when the downlink's packet number N exceeds the

second maximum packet number MAX

PAC2

[50] Referring back to FIG. 2, the offset applicator 730 applies a positive offset +δoffset or a negative offset -δoffset to a measured CINR depending on the packet error information calculated by the packet error calculator 720. The offset applicator 730 applies the negative offset -δoffset when a packet error PER is higher than the allowable maximum packet error PERupper, and applies the positive offset +δoffset when the packet error PER is lower than the allowable minimum packet error PERlower.

[51] More specifically, the offset applicator 730 applies the negative offset -δoffset to the measured CINR when the uplink error number N exceeds the first threshold

ERRl number N THRl in the packet error calculator 720, and a xp-xp-lies the x p-ositive offset

+δoffset to the measured CINR when the uplink error number N is less than or

ERRl equal to the first threshold number N and the uplink's packet number N exceeds

THRl PACl the first maximum ^r packet number MAX PACl . However, when the u ^r plink error number

N ERRl is less than or equal to the first threshold number N THRl and the uplink's packet number N PACl is less than or equal to the first maximum packet number MAX PACl , the offset applicator 730 keeps the measured CINR without applying the offset value. [52] Moreover, the offset applicator 730 applies the negative offset -δoffset to the measured CINR when the downlink error number N ERR2 exceeds the second threshold number N THR2 in the packet error calculator 720, and applies the positive offset

-fδoffset to the measured CINR when the downlink error number N is less than or

ERR2 equal to the second threshold number N and the downlink's packet number N

THR2 PAC2 exceeds the second maximum packet number MAX . In addition, when the r PAC2 downlink error number N is less than or equal to the second threshold number N

ERR2 and the downlink's packet number N is less than or equal to the second

THR2 PAC2 maximum packet number MAX , the offset applicator 730 keeps the measured

PAC2

CINR without applying the offset value.

[53] The MCS selector 740 selects an MCS level for a corresponding terminal based on the CINR which is adjusted by applying the offset value thereto in the offset applicator 730. Further, the MCS selector 740 refers to the AMC table, extracts an MCS level corresponding to the adjusted CINR from the AMC table and provides the extracted MCS level to the scheduler 500. To reduce its design complexity, the AMC table is constructed to store a reference CINR for AMC of bursts allocated in the frame so that reference can be made to a reference CINR corresponding to a particular MCS level.

[54] FIG. 5 is a flowchart illustrating an AMC operation method according to an embodiment of the present invention, FIG. 6 is a flowchart illustrating a detailed CINR adjustment method for an uplink in FIG. 5, and FIG. 7 is a flowchart illustrating a detailed CINR adjustment method for a downlink in FIG. 5. With reference to FIGs. 5

to 7, a detailed description will now be made of an AMC operation method according to an embodiment of the present invention.

[55] Referring first to FIG. 5, it sets an offset applied to a measured CINR to zero (0) on initial condition in step SlOOO, and measures a CINR from a terminal in step S 1002. The CINR measurement is achieved through a CQI channel on the uplink.

[56] Thereafter, it computes packet errors of the downlink and the uplink, and calculates an adjusted CINR by applying a negative offset -δoffset or a positive offset +δoffset to the measured CINR based on the packet error computation result. A detailed description of steps S 1004 and S 1006 will be given below.

[57] Next, it extracts an MCS level corresponding to the adjusted CINR from an AMC table, and selects an MCS level in step S 1008.

[58] A description of steps S 1004 and S 1006 will now be made with reference to FIGs. 6 and 7. In a CINR adjustment method for an uplink in FIG. 6, on initial condition, it sets an uplink error number N ERRl , an uplink non-error number N NONl , and an uplink's r packet number N PACl determined by J adding b the u fplink error number N _ERR1 and the u ^r plink non-error number N NONl to zero (0) in ste ^r p Sl 100. To be sure, when the u ^γ plink error number N ERRl and the uplink non-error number N NONl are set to zero, the uplink's packet number N is also automatically set to zero (0). The uplink error number N

ERRl indicates the number of occurrences of a CRC error, and the uplink non-error number N NONl indicates the number of non-occurrences of a CRC error.

[59] Subsequently, it receives an uplink packet transmitted from the terminal in step

Sl 102, and checks CRC information included in the received packet to determine whether a CRC error has occurred in step Sl 104. When it is determined in step Sl 104 that no CRC error has occurred, it counts the uplink non-error number N in step

NONl

S 1106, and when a CRC error has occurred, counts the uplink error number N in

ERRl step S 1108.

[60] After that, it calculates an uplink's packet number N by adding the counted uplink error number N and uplink non-error number N in step S1110.

ERRl NONl

[61] Thereafter, it determines whether the uplink error number N exceeds the first

ERRl threshold number N in step Sl 112. When the uplink error number N exceeds

THRl ERRl the first threshold number N in step S 1112, it applies the negative offset -δoffset to

THRl the measured CINR in step Sl 114. it turns back to the beginning, sets the uplink error number N , the uplink non-error number N , and the uplink's packet number N

ERRl NONl

PACi to zero ( ^w 0), and then checks whether a CRC error exists for a new u rplink r packet.

[62] When the u ^r plink error number N ERRl is less than or eq ⁿ ual to the first threshold number N THRl in ste ^r p S 1112, it determines whether the u ^r plink's ^r packet number N PACl exceeds a first maximum packet number MAX PACl in step S 1116. When the uplink's packet number N PACl exceeds the first maximum packet number MAX PACl in step

Sl 116, it applies the positive offset +δoffset to the measured CINR in step Sl 118. it turns back to the beginning, sets the uplink error number N , the uplink non-error

ERRl number N , and the uplink's packet number N to zero (0), and then checks

NONl PACl whether a CRC error exists for a new uplink packet. [63] However, when the uplink's packet number N is less than or equal to the first

PACl maximum packet number MAX in step S 1116, it checks whether a CRC error

PACl exists for a new uplink packet after turning back, with the measured CINR being kept without applying the offset value. Though step Sl 108 is followed by step Sl 110 in the foregoing description, step Sl 110 should not necessarily be performed immediately after step Sl 108. It can be performed anytime after step Sl 108 and before step SI l 16. [64] In a CINR adjustment method for a downlink in FIG. 7, as in the uplink, it sets a downlink error number N and a downlink non-error number N to zero on initial

ERR2 N0N2 condition in step S 1200. When the downlink error number N and the downlink

ERR2 non-error number N N0N2 are set to zero, a downlink's packet number N PAC2 is also auto- matically ^J set to zero ( ^V 0) '. The downlink error number N ERR2 indicates the number of receptions of ACK information from the terminal, and the downlink non-error number N N0N2 indicates the number of receptions of NAK information from the terminal.

[65] Thereafter, it receives ACK or NAK information transmitted from the terminal in step S 1202, and it determines whether the received information is ACK information or NAK information in step S 1204. When the received information is ACK information in step S 1204 that, it counts the downlink non-error number N in step S 1206, and

N0N2 when the received information is NAK information, counts the downlink error number N ERR2 in step S 1208.

[66] After that, it calculates the downlink's packet number N by adding the counted

PAC2 downlink error number N and downlink non-error number N in step S 1210.

ERR2 N0N2

[67] Subsequently, it determines whether the downlink error number N exceeds the

^ ^J ERR2 second threshold number N in step S 1212. When the downlink error number N

THR2 ERR2 exceeds the second threshold number N THR2 in step S 1212, it a rpxp-lies the negσative offset

-δoffset to the measured CINR in step S 1214. it turns back to the beginning, sets the downlink error number N , the downlink non-error number N , and the

ERR2 N0N2 downlink's packet number N to zero (0), and then checks new ACK/NAK in-

PAC2 formation. [68] When the downlink error number N is less than or equal to the second threshold

ERR2 ^ number N THR2 in ste ^r p S 1212, it determines whether the downlink's ^r packet number N PAC2 exceeds the second maximum packet number MAX PAC2 in step S 1216. When the downlink's ^r packet number N PAC2 exceeds the second maximum ^r packet number MAX in step S 1216, it applies the positive offset +δoffset to the measured CINR in step S 1218. it turns back to the beginning, sets the downlink error number N ERR2 , the

downlink non-error number N , and the downlink's packet number N to zero (0),

N0N2 PAC2 and then checks new ACK/NAK information. [69] When it determines the downlink's packet number N is less than or equal to the

PAC2 second maximum packet number MAX in step S 1216, checks new ACK/NAK in- r PAC2 ^r formation after turning back, with the measured CINR being kept without applying the offset value.

[70] Though step S 1208 is followed by step S 1210 in the foregoing description, step

S 1210 should not necessarily be performed just after step S 1208. It can be performed anytime after step S1208 and before step S1216.

[71] Meanwhile, functions used in an apparatus and a method disclosed in the present specification can be embodied in storage media that a computer can read as codes that the computer can read. The storage media that the computer can read, include all sorts of record devices in which data that can be read by a computer system is stored. Examples of the storage media that the computer can read, include ROMs, RAMs, CD- ROMs, magnetic tape, floppy discs, optic data storage devices, etc., and also, include things embodied in the form of carrier wave (e.g., transmission through the internet). Furthermore, the storage media that the computer can read is distributed in a computer system connected with networks. Then, the codes that the computer can read, are stored in the distributed storage media in a distribution scheme, and the codes can be executed in the distribution scheme.

[72] While the invention has been shown and described with reference to certain exemplary embodiments 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. Therefore, the spirit and scope of the present invention must be defined not by described embodiments thereof but by the appended claims and equivalents of the appended claims.