Abstract:
The present invention provides a method and apparatus reporting a channel quality indicator of a communication system, including: detecting a first measurement reflecting a first communication quality of the communication system; providing a series of thresholds and mapping functions, each mapping functions in association with a bin defined by two adjacent thresholds, so the first measurement is mapped to the channel quality indicator by the mapping function in association with the bin which matches the first measurement; and updating at least one of the thresholds according to a second measurement which reflects a second communication quality of the communication system.

Description:
FIELD OF THE INVENTION 
     The present invention relates to method and apparatus reporting channel quality indicator of communication system, and more particularly, to method and apparatus dynamically adapting mapping relation which maps a channel measurement to the channel quality indicator. 
     BACKGROUND OF THE INVENTION 
     Communication systems, especially wireless communication systems, have become an important portion of modern society. Generally speaking, in a wireless communication system, a base station (e.g., NodeB) establishes radio coverage over a cell, and a terminal (e.g., UE, user equipment) can therefore communicates with the base station by signal transmission through a wireless communication channel between the base station and the terminal. By different communication parameter combinations such as combinations of different modulation schemes and/or coding schemes, the communication channel, involving medium and environment where wireless signals transmit, can be separated to a plurality of physical channels for multiple-access. Some of the physical channels implement downlink channels for transmission from the base station to the terminal; others are allocated as uplink channels for transmission from the terminal to the base station. From another aspect, some of the physical channels are used for data transmission, and others are used for transmission of control information which is used for initiating, managing, handover and/or ending of the communication channel. 
     As a wireless communication system becomes more popular, the demand of higher throughput becomes more important. To fulfill throughput demand, the base station needs to provide the data service in an efficient way to accommodate various applications, e.g. voice service, data download, streaming, gaming, browsing, and etc, under the limited channel bandwidth. An efficient way to maximize the cell capacity is to allocate higher data rate to terminals with better channel qualities. In modern wireless communication systems, this concept is realized by adaptive modulation and coding scheme (adaptive MCS). The terminal shall monitor the downlink channel quality and report the quality metric, often referred to as channel quality indicator (CQI), to the base station. Then the base station can schedule proper data transmission for the terminals to fit channel capacity of each terminal, and maximize the cell capacity by adjusting the coding rate of channel decoder to control the capability of error protection, and by selecting the suitable modulation scheme to achieve the spectral efficiency. 
     For example, in communication systems following the third generation (3G) wideband code division multiple access (WCDMA) standard, the CQI reporting is a mandatory feature in the evolution version of the standard, which is named as high speed packet access (HSPA), for the procedure of high-speed downlink shared channel (HS-DSCH) reception. In response to CQI reporting of a terminal, the base station reports information about a transport format resource combination (TFRC) to the terminal; with TFRC, the terminal can reliably receive data from the base station under the experienced or to-be-experienced channel conditions. The TFRC means a communication parameter combination allocating the physical channel resources, including modulation and number of physical channels, and the size of transport block transmitted in the downlink data channel(s). The terminal shall determine the supportable TFRC as the CQI reporting value, and this reporting must be independent of channel variations due to, e.g., the Doppler shift, delay spread and so on. In other words, same values of CQI reporting mean same block error rate (BLER) or same throughput that can be achieved if the base station follows CQI reporting of the terminal. 
     As a terminal equips a receiver which includes an inner receiver and an outer receiver (a channel decoder), a common quality metric (e.g., SIR, Signal to Interference Ratio) reflecting a quality of the communication channel is estimated after the inner receiver and before the outer receiver; however, this kind of quality metric cannot directly reflect the BLER quality and throughput. Moreover, the quality metric measurement depends on pilot part in one physical channel, not the data part in another physical channel; therefore it leads to differences under different channel variations. That is, a fixed mapping relation which directly maps the quality metric to CQI does not generate proper CQI reporting against channel variations. 
     SUMMARY OF THE INVENTION 
     Therefore, the present invention adapts the mapping relation between measured channel quality and CQI to generate a universal quality reporting value that reflects BLER and/or throughput directly. 
     One objective of the invention is to provide a method reporting a channel quality indicator CQI of a communication system; the method can be applied to a terminal of the communication system, and includes: detecting a first measurement, e.g., SIR, reflecting a first communication quality (e.g., signal to interference ratio) of the communication system; providing a series of a plurality thresholds (TH(i−1), TH(i) and TH(i+1), etc.) and a plurality of mapping functions (g(i,.) and g(i+1,.), etc); the plurality of thresholds corresponding to a plurality of bins (B(i) and B(i+1), etc) with each bin B(i) defined by two adjacent thresholds TH(i−1) and TH(i); each mapping function g(i,.) in association with a bin B(i); wherein the first measurement SIR is matched into one of the bins, such as a bin B(i_m), and the mapping function g(i_m,.) in association with the matched bin B(i_m) maps the first measurement SIR to the channel quality indicator CQI by CQI=g(i_m, SIR); and updating at least one of the plurality of thresholds according to a second measurement e.g., BLER reflecting a second communication quality (e.g., a block error rate which represents CRC error rate of a transport block) of the communication system or the receiving throughput of the terminal. 
     In an embodiment, the communication system adopts one of a plurality of communication parameter combinations for communication, the communication parameter combinations are categorized to a plurality of combination schemes MCS(i−1), MCS(i) and MCS(i+1), etc.; each combination schemes MCS(i) corresponds to two adjacent thresholds TH(i−1) and TH(i) respectively as a bottom threshold and an upper threshold. 
     One embodiment of the invention implements a target BLER criterion. When the communication system adopts a communication parameter combination which is categorized to an operating combination scheme MCS(i_op) with an operating bottom threshold TH(i_op−1), if the first measurement SIR falls in a predetermined neighborhood of the operating bottom threshold TH(i_op−1), update the operating bottom threshold TH(i_op−1) by lowering the operating bottom threshold TH(i_op−1) if the second measurement BLER is lower than a target value, or by increasing the operating bottom threshold TH(i_op−1) if the second measurement BLER is higher than the target value. 
     One embodiment of the invention implements an optimum throughput criterion, including: collecting a plurality of the second measurements U(i−1), U(i), and U(i+1) etc., each second measurement U(i) being the throughput corresponding to the combination schemes MCS(i); and collecting a plurality of third measurements BLER(i−1), BLER(i) and BLER(i+1) etc., each third measurement being BLER(i) corresponding to the combination schemes MCS(i). When the communication system adopts a communication parameter combination which is categorized to an operating combination scheme MCS(i_op) with an operating bottom threshold TH(i_op−1) and an operating upper threshold TH(i_op), if the first measurement SIR falls in a predetermined neighborhood of the operating bottom threshold TH(i_op−1) and if a third measurement BLER(i_op−1) corresponding to a lower combination scheme MCS(i_op−1) falls out of two predetermined ranges, update the operating bottom threshold TH(i_op−1) by lowering the operating bottom threshold TH(i_op−1) if a second measurement U(i_op) corresponding to the operating combination scheme MSC(i_op) is higher than a second measurement U(i_op−1) corresponding to the lower combination scheme MCS(i_op−1), or by increasing the operating bottom threshold TH(i_op−1) if the second measurement U(i_op−1) is lower than the second measurement U(i_op). 
     The two predetermined ranges are respect proximities of a bottom bound and an upper bound of BLER. If the first measurement SIR falls in the predetermined neighborhood of the operating bottom threshold TH(i_op−1) and if the third measurement BLER(i_op−1) falls in the predetermined range of the upper bound, increase the operating bottom threshold TH(i_op−1). If the first measurement SIR falls in the predetermined neighborhood of the operating bottom threshold TH(i_op−1) and if the third measurement BLER(i_op−1) corresponding to the lower operating combination scheme falls in the predetermined range of the lower bound, update the operating bottom threshold TH(i_op−1) according to a comparison between a target value and the third measurement BLER(i_op). 
     Furthermore, if the first measurement SIR falls in a predetermined neighborhood of the operating upper threshold TH(i_op) and if the third measurement BLER(i_op) falls out of the two predetermined ranges, update the operating upper threshold TH(i_op) by increasing the operating upper threshold TH(i_op) if the second measurement U(i_op) is higher than a second measurement U(i_op+1) corresponding to a higher operating combination scheme MCS(i_op+1), or by lowering the operating upper threshold TH(i_op) if the second measurement U(i_op) is lower than the second measurement U(i_op−1). If the first measurement SIR falls in the predetermined neighborhood of the operating upper threshold TH(i_op) and if the third measurement BLER(i_op) falls in the predetermined range of the upper bound, increase the operating upper threshold TH(i_op). 
     Another objective of the invention is to provide an apparatus reporting a channel quality indicator CQI of a communication system; the apparatus can be applied to a terminal of the communication system, and includes a first estimation unit detecting a first measurement SIR of the communication system, a mapping module providing the thresholds TH(i), the bins B(i) and the mapping functions g(i,.), a second estimation unit collecting the measurements BLER(i) and/or throughput U(i), and an adaptation module implements the target BLER criterion and/or the optimum throughput criterion. If the first measurement SIR matches a bin B(i_m), the mapping module maps the first measurement SIR to the channel quality indicator ICQ by the mapping function g(i_m, SIR). 
     Numerous objects, features and advantages of the present invention will be readily apparent upon a reading of the following detailed description of embodiments of the present invention when taken in conjunction with the accompanying drawings. However, the drawings employed herein are for the purpose of descriptions and should not be regarded as limiting. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above objects and advantages of the present invention will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings, in which: 
         FIG. 1  illustrates a communication system according to an embodiment of the invention; 
         FIG. 2  illustrates adaptation of SIR to CQI mapping relation according to en embodiment of the invention; 
         FIG. 3  illustrates a threshold decision criterion according to an embodiment of the invention; 
         FIG. 4  illustrates threshold updating based on the criterion of  FIG. 3  according to an embodiment of the invention; 
         FIG. 5  illustrates a flow implementing the threshold decision of  FIG. 3  according to an embodiment of the invention; 
         FIG. 6  illustrates a threshold decision criterion according to another embodiment of the invention; 
         FIG. 7  and  FIG. 8  illustrate threshold updating based the threshold decision of  FIG. 6  according to an embodiment of the invention; and 
         FIG. 9  and  FIG. 10  illustrate flows implementing the threshold decision of  FIG. 6  according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Please refer to  FIG. 1  illustrating a terminal  14  communicating to a base station  12  in a communication system  10 , e.g., a 3G wireless mobile communication system. The invention can be applied to the terminal  14 , which includes an inner receiver  16 , an outer receiver  18 , an SIR estimation unit  20 , a mapping module  22 , a second estimation unit  24  and an adaptation module  26 . 
     For downlink communication, data to be transmitted to the terminal  14  are arranged into data blocks (transport blocks) in the base station  12 , and the base station  12  transmits to the terminal  14  with data blocks carried in the high speed downlink shared channel (HS-DSCH) and with control information carried in the high-speed shared control channel (HS-SCCHs), including TFRC. The terminal  14  decodes the data and control information by the inner receiver  16  and the outer receiver  18 . The inner receiver  16  behaves like an inverse function of the fading channel effect, or called the equalization, to get the estimation of transmitted symbols from the base station  12 . These symbols are further decoded and transformed into the information bits by the outer receiver  18 . The inner receiver  16  performs the functionalities of filtering, frequency and timing synchronization, removal of channel effects, etc. The outer receiver  18  executes the operation of physical channel and constellation de-mapping, de-interleaving, de-rate-matching, HARQ combining, channel decoding, bit descrambling, and CRC (Cycle Redundancy Check) de-attachment, etc. According to signals received by the inner receiver  16 , the SIR estimation unit  20  provides SIR to reflect a signal to interference quality of the communication channel. 
     For CQI reporting, a common way is to determine the received quality by estimating SIR. For example, in the 3GPP technical specification 25.214, a CQI mapping table defines 30 CQIs (CQI1 to CQI30); their required SIRs for supporting a BLER of 0.1 are monotonically increasing by a step of 1 dB SIR differences in the static channel condition. Among the CQI1 to CQI30, CQI1 and CQI30 respectively represent the TFRC with the lowest and highest required channel quality for reliable reception with BLER=0.1. Hence, the mapping of reported CQI and SIR is also a linear relation under a static channel. However, when the communication channel from the base station  12  to the terminal  14  acts like a fading channel instead of a static channel, the linear relation is no longer valid, and proper CQI reporting can not be accomplished. 
     Please refer to  FIG. 2  which illustrates SIR to CQI mapping relation according to an embodiment of the invention. The SIR to CQI mapping relation works with a plurality of thresholds TH(i−2), TH(i−1), TH(i), TH(i+1), TH(i+2) etc, and a plurality of piecewise mapping functions such as g(i−1,.) and g(i,). Every two adjacent thresholds form a bin, such as the bin B(i) has an upper threshold TH(i) and a bottom threshold TH(i−1), and the bin B(i−1) has thresholds TH(i−2) and TH(i−1) as its bottom and upper thresholds, respectively. Each of the bin corresponds to a mapping function; for example, the bin B(i) corresponds to the mapping function g(i,.), and the bin(i−1) is in association with the mapping function g(i−1,.). While mapping a given SIR of value x to a corresponding CQI, the SIR value x is first matched to a bin. For example, if the SIR value x is less than the threshold TH(i) but greater than the threshold TH(i−1), it is matched to the bin B(i); and therefore the mapping function g(i,.) which corresponds to the bin B(i) is used to map the SIR value x to a mapped CQI value y by y=g(i,x). The CQI value y can be further quantized to one of the CQI1 to CQI30 if necessary. In an embodiment, each of the mapping function g(i,.) is a linear function defined over thresholds TH(i−1) to TH(i), e.g., g(i,x)=CQI(i−1)+(x−TH(i−1))*(CQI(i)−CQI(i−1))/(TH(i)−TH(i−1)) with CQI(i−1) and CQI(i) being two constants. 
     As previously discussed, communication channel has its own characteristics dependent on many factors, such as propagation delay spread, Doppler and/or multiple-path fading. There is a mutual correlation between SIR, BLER, throughput supported by the channel, channel characteristics, and communication parameter combination adopted to establish the channel. For example, with given (fixed) BLER and channel characteristics, a communication parameter combination delivering higher throughput needs higher SIR. With given BLER and throughput, a fading channel demands better SIR than a static channel. 
     As the channel characteristics vary, a fixed SIR to CQI mapping relation cannot reflect the mutual correlation. To address the issue, the invention provides an adaptation technique for updating the SIR to CQI mapping according to channel characteristics. As shown in  FIG. 2 , the adaptation is achieved by adjustment of the thresholds. In an embodiment, each of the thresholds can be individually updated with distinct adjustment. 
     With adjusted thresholds, bins and corresponding mapping functions work differently to meet nature of channel characteristics. For example, the SIR value x originally matched to the bin B(i) now falls in the bin B(i−1) between the updated threshold TH(i−2) and TH(i−1), and it will be mapped to a new lowered CQI value by the mapping function g(i−1,.). The threshold TH(i) for a fading channel can be greater than that for a static channel; it reflects the correlation: with given SIR, the fading channel suffers from lower throughput of lower CQI; or equivalently, if the fading channel and the static channel adopt the same CQI (and therefore the same throughput), the fading channel demands better SIR than the static channel. 
     In two embodiments of the invention, two criterions are provided to update the thresholds. Please refer to  FIG. 3  which illustrates a threshold decision according to one embodiment of the invention. For a given communication parameter combination and a given channel characteristics, BLER increases as SIR decreases. This correlative relation is illustrated by two curves cv(i) and cv(i+1) respectively corresponding to two different communication parameter combinations. The two communication parameter combinations are respectively categorized to combination schemes MCS(i) and NCS(i+1), each combination scheme generally refers to a collection of communication parameter combinations which have similar SIR/BLER/throughput performances. For example, the combination scheme MCS(i) associated with the curve cv(i−1) can correspond to a lower CQI, so the combination scheme MCS(i) delivers lower throughput but gains better (lower) BLER with a given SIR. 
     The SIR to BLER curves of different MCSs can be used to decide the thresholds of  FIG. 2 . By setting a target value target_BLER for BLER performance, intersection of the target_BLER and each curve cv(i) can be used to define corresponding threshold TH(i−1). That is, the threshold TH(i−1) acts as a bottom threshold for combination scheme MCS(i) to reflect whether the combination scheme MCS(i) can properly work under a given SIR; if SIR of the communication channel is lower than the threshold TH(i−1), the combination scheme MCS(i) suffers BLER higher than the target value target_BLER, and therefore the CQI corresponding to the combination scheme MCS(i) is not preferred; instead, the mapping function defined between thresholds TH(i−1) and TH(i−2) can be applied to decide a suitable (lower) CQI for the given SIR. 
     Different channel characteristics lead to different curve cv(i) and different threshold TH(i−1). To approach the ideal bottom threshold TH(i−1) at intersection of the curve cv(i) and the target value target_BLER, measured SIR and measured BLER are referred to track actual behavior of the curve cv(i). Following discussion of  FIG. 3 , please refer to  FIG. 4  illustrating threshold updating according to an embodiment of the invention. When the terminal  14  is working with a given combination scheme MCS(i_op) (an operating combination scheme), a measured SIR and a measured BLER are used for adjustment of the corresponding bottom threshold TH(i_op−1). To update a current threshold TH(i_op−1) toward the ideal threshold TH(i_op−1), a neighborhood around the current threshold TH(i_op−1) is defined. If the measured SIR falls in the neighborhood, the curve cv(i_op) corresponding to the ideal threshold TH(i_op−1) can be well tracked; and the pair of the measured SIR and the measured BLER will effectively indicate a point on the curve cv(i_op−1). If the measured BLER is higher than the target value target_BLER like the scenario shown in  FIG. 4 , it is implied that the current threshold TH(i_op−1) is less than the ideal threshold TH(i_op−1); so the current threshold TH(i_op−1) is adjusted by increasing its value. On the contrary, if the measured BLER is lower than the target value target_BLER, the current threshold TH(i_op−1) is too high and it is adjusted by lowering its value. 
     Following the discussion of  FIG. 4 , please refer to  FIG. 5  illustrating a flow  100  for adjust the thresholds according to an embodiment of the invention. The flow  100  includes the following steps. 
     Step  102 : While a measured SIR is obtained, the flow  100  can start. First, quantize (categorize) the currently adopted communication parameter combination, e.g., TFRC, by finding which combination scheme the currently adopted communication parameter combination belongs to. The found combination scheme is identified as the operating combination scheme MCS(i_op). Corresponding to the operating combination scheme MCS(i_op), adjustment for the current bottom threshold TH(i_op−1) (as an operating bottom threshold) is considered. 
     Step  104 : if the measured SIR is in the neighborhood of the current threshold TH(i_op−1), go to step  106 ; otherwise go to step  114 . 
     Step  106 : update measured BLER for the operating combination scheme MCS(i_op). In an embodiment, when the terminal  14  works under a given combination scheme MCS(i), the estimation unit  24  of  FIG. 1  can measure a short-term BLER by CRC information for the combination scheme MCS(i), and then collect and accumulate a long-term measured BLER(i) for the combination scheme MCS(i) according to short-term measured BLER of the combination scheme MCS(i). As the terminal  14  works with different combination schemes MCS(i 1 ), MCS(i 2 ), . . . etc over time, it collects corresponding long-term measured BLER(i 1 ), BLER(i 2 ), . . . etc. When the terminal  14  again works with the combination scheme MCS(i 1 ) and obtains a new short-term measured BLER, the long-term measured BLER(i 1 ) of the combination scheme MCS(i 1 ) is updated. In another embodiment, the measured BLER used in step  106  is a short-term measurement. 
     Step  108 : if the measured BLER is lower than the target value target_BLER, go to step  110 ; otherwise go to step  112 . 
     Step  110 : update the current threshold TH(i_op−1) by lowering its value. For example, the current threshold TH(i_op−1) can be lowered by a predetermined decrement. Then the flow  100  can proceed to step  114 . 
     Step  112 : update the current threshold TH(i_op−1) by increasing its value. For example, the current threshold TH(i_op−1) can be increased by a predetermined increment. Then the flow  100  can proceed to step  114 . The increment can be equal to or different from the decrement of step  110 . The increment and/or the decrement can be constant, or can be dynamically set. 
     Step  114 : finish the flow  100 . 
     The flow  100  can be regularly or periodically executed based on either short or long intervals, and/or it can be executed whenever necessary. For the first execution, the flow  100  can start with the thresholds set to predetermined initial values (e.g., thresholds designed for channel of predetermined characteristics, such as thresholds for a static channel) as initial guess. As the terminal  14  communicates with different combination schemes at different times, different thresholds respectively corresponding to the adopted combination schemes can be respectively adjusted toward their ideal values which adapt actual channel characteristics. Because the flow  100  works with a target value of BLER, it implements a target BLER criterion for threshold adjustment. 
     Please refer to  FIG. 6  illustrating another threshold decision criterion. For a given communication parameter combination and a given channel characteristics, throughput increases as SIR increases. This correlative relation is illustrated by curves tp(i−1), tp(i) and tp(i+1) respectively corresponding to combinations schemes MCS(i−1), MCS(i) and MCS(i+1). The curves tp(i−1), tp(i) and tp(i+1) respectively have maximum throughputs TPmax(i−1), TPmax(i) and TPmax(i+1), as well as a minimum throughput TPmin. The maximum throughput TPmax(i) is achieved when BLER is 0, i.e., a perfect transmission without any error; on the other hand, the minimum throughput TPmin corresponds to BLER of 0, i.e., transmitted data are all incorrect. The combination scheme MSC(i+1) corresponds to a higher CQI, so it has a higher maximum throughput TPmax(i+1). However, the higher maximum throughput TPmax(i+1) demands higher SIR. Therefore, intersections of the curves tp(i−1), tp(i) and tp(i+1) can be utilized to indicate ideal thresholds: intersection of the curves tp(i) and tp(i+1) defines an ideal value for the threshold TH(i), and the intersection of the curves tp(i−1) and tp(i) defines an ideal value for the threshold TH(i−1). With SIR lower than the threshold TH(i), throughput by communication adopting the combination scheme MCS(i+1) becomes lower than that of the combination scheme MCS(i), so the CQI corresponding to the combination scheme MCS(i+1) is not preferred; instead, the mapping function defined between the thresholds TH(i−1) and TH(i) is used for proper SIR to CQI mapping. 
     For a inside understanding, notice that a long-term overall throughput of correct data, T(TH( 0 ), TH( 1 ), . . . , TH(i), . . . , TH(N−1)), can be expressed as: 
               T   ⁡     (       TH   ⁡     (   0   )       ,     TH   ⁡     (   1   )       ,   …   ⁢           ,     TH   ⁡     (     N   -   1     )         )       =       ∑     i   =   0     N     ⁢       R   ⁡     (   i   )       ·       ∫     TH   ⁡     (     i   -   1     )         TH   ⁡     (   i   )         ⁢         [     1   -     e   ⁡     (     i   ,   z     )         ]     ·     f   ⁡     (   z   )         ⁢       ⅆ   z     .                   
Where −∞=TH(−1)=TH( 0 )=−∞≦TH( 1 )≦TH( 2 )≦ . . . ≦TH(N−1)≦TH(N)=∞, R(i) is a nominal throughput (e.g., throughput regardless whether data are correct or not) corresponding to the combination scheme MCS(i), and e(i,z) is an error rate (e.g., BLER) corresponding to the combination scheme MCS(i) under SIR of value z. Along with R(i) and (1−e(i,z)), throughput of correct data while communicating by the combination scheme MCS(i) is obtained by integration over SIR valued from the thresholds TH(i−1) to TH(i). To optimize the overall throughput T(TH( 0 ), TH(N−1)), the optimization condition R(i)*[1−e(i, TH(i))]=R(i+1)*[1−e(i+1, TH(i))] has to be satisfied for i=1 to (N−1). That is, throughput of correct data during the combination scheme MCS(i) under SIR of value TH(i) must equal that during the combination scheme MCS(i+1) under SIR of value TH(i) to fulfill the optimization condition. Since threshold decision of  FIG. 6  sets ideal value of the threshold TH(i) to the SIR value corresponding to the intersection of the curves tp(i) and tp(i+1), the optimization condition can be satisfied.
 
     Different channel characteristics lead to different curve tp(i) and different threshold TH(i−1). To approach the ideal threshold TH(i−1) at intersection of the curves tp(i−1) and tp(i) as well as the ideal threshold TH(i) at intersection of the curves tp(i) and tp(i+1), measured SIR, measured BLER and measured throughput of the combination schemes MCS(i−1), MCS(i) and MCS(i+1) are referred to follow actual behavior of the curves tp(i−1), tp(i) and tp(i+1). To implement the adjustment, the estimation unit  24  ( FIG. 1 ) collects measured BLER(i) and measured throughput U(i) (of long-term or short-term) for different scheme combination MSC(i). 
     Following discussion of  FIG. 6 , please refer to  FIG. 7  and  FIG. 8  respectively illustrating threshold updating according to embodiments of the invention. As shown in  FIG. 7 , when the terminal  14  communicates with a given operating combination scheme MCS(i_op) and a measured SIR is obtained, if the measured SIR fall into a neighborhood around the current threshold TH(i_op−1), measured throughput U(i_op) of the operating combination scheme MCS(i_op) can be updated, and adjustment of the threshold TH(i_op−1) can be considered; if the measured BLER(i_op) is neither close to 0 nor close to 1, the current threshold TH(i_op−1) can be updated by increasing its value if the measured throughput U(i_op) of the operating combination scheme MCS(i_op) is lower than the measured throughput U(i_op−1) corresponding to the combination scheme MCS(i_op−1), like the scenario shown in  FIG. 7 . On the contrary, if the measured throughput U(i_op) is higher than the measured throughput U(i_op−1), the current threshold TH(i_op−1) is higher than the ideal threshold TH(i_op−1), so the current threshold TH(i_op−1) is adjusted by lowering its value. 
     On the other hand, if the measured BLER(i_op) is close to 1, i.e., falls in a predetermined proximity of the upper bound of BLER, the current threshold TH(i_op−1) is too small; the current threshold TH(i_op−1) intersects the curve tp(i_op) or tp(i_op−1) at minimum throughput TPmin. Then the current value of the threshold TH(i_op−1) can be adjusted by increasing. If the measured BLER(i_op) is close to 0, a lower bound of BLER, the adjustment can proceed following the target BLER criterion. Notice when BLER(i_op) is 0, the optimization condition becomes R(i_op−1)=R(i_op)*[1−e(i_op, TH(i_op−1))]; or equivalently, e(i_op, TH(i_op−1))=1−R(i_op−1)/R(i_op). That is, the optimization condition suggests a target BLER of value (1−R(i_op−1)/R(i_op)) for adjusting the threshold TH(i_op−1) with the target BLER criterion. 
     Since the threshold TH(i_op) can be considered as an upper threshold of the operating combination scheme MCS(i_op), adjustment for the threshold TH(i_op) can be considered if the measured SIR fall into a neighborhood around the current threshold TH(i_op), as shown in  FIG. 8 . If the measured BLER(i_op) is neither close to 0 nor close to 1, the current threshold TH(i_op) can be updated by lowering its value if the measured throughput U(i_op) of the operating combination scheme MCS(i_op) is lower than the measured throughput U(i_op+1) corresponding to the combination scheme MCS(i_op+1), like the scenario shown in  FIG. 8 . On the contrary, if the measured throughput U(i_op) is higher than the measured throughput U(i_op+1), the current threshold TH(i_op−1) is lower than the ideal threshold TH(i_op−1), so the current threshold TH(i_op−1) is adjusted by increasing its value. 
     Following the discussion of  FIG. 7  and  FIG. 8 , please refer to  FIG. 9  illustrating a flow  200  for adjust the thresholds according to an embodiment of the invention. The flow  200  includes the following steps. 
     Step  202 : While a measured SIR is obtained, the flow  200  starts. First, quantize (categorize) the currently adopted communication parameter combination, e.g., TFRC, by finding which combination scheme the currently adopted communication parameter combination belongs to. The found combination scheme is identified as the operating combination scheme MCS(i_op). Corresponding to the operating combination scheme MCS(i_op), adjustment for the current bottom threshold TH(i_op−1) (as an operating bottom threshold) and the top threshold TH(i_op) can be considered in the following steps. 
     Step  204 : if the measured SIR is in the neighborhood of the current threshold TH(i_op−1), go to step  206 ; otherwise go to step  218 . 
     Step  206 : update the measured throughput U(i_op) for the operating combination scheme MCS(i_op). In an embodiment, when the terminal  14  works under a given combination scheme MCS(i), the estimation unit  24  of  FIG. 1  can measure a short-term throughput for the combination scheme MCS(i), and then collect and accumulate a long-term throughput U(i) for the combination scheme MCS(i) according to the short-term measured throughput of the combination scheme MCS(i). As the terminal  14  works with different combination schemes MCS(i 1 ), MCS(i 2 ), . . . etc, it collects corresponding long-term throughput U(i 1 ), U(i 2 ), . . . etc. When the terminal  14  again works the combination scheme MCS(i 1 ) and obtains a new short term measured throughput, the long term measured throughput U(i 1 ) of the combination scheme MCS(i 1 ) is updated. In another embodiment, the measured throughput used in step  206  is a short-term measurement. 
     Step  208 : if the measured BLER(i_op−1) is close to 0 or 1, go to step  216 , otherwise proceed to step  210 . 
     Step  210 : if the measured throughput U(i_op) is higher than the measured throughput U(i_op−1), proceed to step  212 , otherwise proceed to step  214 . 
     Step  212 : lower the threshold TH(i_op−1) for adjustment and then proceed to step  234 . For example, the threshold TH(i_op−1) can be decreased by subtracting a decrement from its current value. 
     Step  214 : increase the threshold TH(i_op−1) and the proceed to step  234 . For example, the threshold TH(i_op−1) can be increased by adding a increment to its current value. 
     Step  216 : execute an exception processing. The detail will be discussed in  FIG. 10 . 
     Step  218 : if the measured SIR is in the neighborhood of the threshold TH(i_op), go to step  220 ; otherwise go to step  234 . Notice that the range covered by the neighborhood of the threshold TH(i_op) does not have to overlap that of the threshold TH(i_op−1). 
     Step  220 : update the measured throughput U(i_op) for the operating combination scheme MCS(i_op). 
     Step  222 : if the measured BLER(i_op) is close to 0, proceed to step  234 , otherwise proceed to step  224 . 
     Step  224 : if the measured BLER(i_op) is close to 1, proceed to step  232 , otherwise proceed to step  226 . 
     Step  226 : if the measured throughput U(i_op) is higher than the measured throughput U(i_op+1), proceed to step  228 , otherwise proceed to step  230 . 
     Step  228 : increase the threshold TH(i_op), then proceed to step  234 . 
     Step  230 : decrease the threshold TH(i_op), then proceed to step  234 . 
     Step  232 : execute an exception procedure which will be discussed with  FIG. 10 . 
     Step  234 : finish the flow  200 . 
     Please refer to  FIG. 10  illustrating the exception processing of the step  216 , which includes the following steps. 
     Step  300 : if the measured BLER(i_op−1) is close to 0, proceed to step  302 , otherwise go to step  304 . 
     Step  302 : follow the target BLER criterion of  FIG. 5  for adjustment of the threshold TH(i_op−1), i.e., update the operating bottom threshold TH(i_op−1) according to a comparison between a target value (target BLER) and the measured BLER(i_op). As discussed above, the target BLER can be set to (1−R(i_op−1)/R(i_op)). 
     Step  304 : if the measured BLER(i_op−1) is close to 0, go to step  306 ; otherwise proceed to step  234 . 
     Step  306 : adjust the threshold TH(i_op−1) by increasing its value. The value of the threshold TH(i_op−1) can be boosted by an increment larger than that used in step  214  and/or step  228  of  FIG. 9 . 
     The exception procedure of step  232  ( FIG. 9 ) is similar to step  306  of  FIG. 10 ; the threshold TH(i_op) can be boosted if the measured BLER(i_op) is close to 1. 
     The flow  200  can be regularly or periodically executed based on either short or long intervals; and/or it can be executed whenever necessary. As the terminal  14  communicates with different combination schemes at different times, different thresholds respectively corresponding to the adopted combination schemes can be respectively adjusted toward their ideal values. Because the flow  200  works based on maximizing throughput, it implements an optimum (maximum) throughput criterion for threshold adjustment. 
     For implementation of the threshold setting according to  FIG. 3  and/or  FIG. 6 , the adaptation module  26  of  FIG. 1  executes the flow  100  of  FIG. 5  and/or the flow  200  of  FIG. 9 . The adaptation module  26  can be implemented by hardware, firmware and/or software. For example, the terminal  14  can include a memory (volatile or nonvolatile) which records codes, and a processor which executes the codes to implement the flow  100  and/or  200 . 
     To sum up, for adaptation of channel characteristics, the invention provides CQI reporting which dynamically updates SIR to CQI mapping relation by adjusting thresholds of the piecewise mapping functions. Comparing arts with constant thresholds which are vulnerable to variation of channel characteristics, the thresholds decision of the invention not only tracks actual channel characteristics, but also achieves target BLER and/or optimum throughput. Though some technique terms used in discussion are similar to those used in 3GPP standards/specifications, the invention can be generalize to communication systems which need channel quality reporting for setting of communication parameters. 
     While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.