Abstract:
A multiple input multiple output (MIMO) antenna system, a signal transmission method, a signal transmission apparatus and a computer program product for the MIMO antenna system are provided. The signal transmission method comprises the following steps of transmitting a signal with a first signal transmission mode and a first transmission power via a signal transmission channel; receiving a signal to noise ratio (SNR) of the signal; receiving an interference value of the signal transmission channel; obtaining a power weight value according to the interference value; determining a system threshold of the signal transmission channel to the SNR of the signal; determining a second signal transmission mode of the signal transmission channel based on the system threshold; and determining a second transmission power of the signal transmission channel according to the power weight value.

Description:
[0001]    This application claims the benefit of priority based on Taiwan Patent Application No. 097130589, filed on Aug. 11, 2008, the contents of which are incorporated herein by reference in their entirety. 
       CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0002]    Not applicable. 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The present invention is related to a multiple input multiple output (MIMO) antenna system, a signal transmission method, a signal transmission apparatus and a computer program product for the MIMO antenna system. More particularly, the present invention is related to an MIMO antenna system, a signal transmission method, a signal transmission apparatus and a computer program product for the MIMO antenna system that are capable of adjusting a transmission power and a transmission mode of a signal. 
         [0005]    2. Descriptions of the Related Art 
         [0006]    As mobile communication systems have evolved from the third generation (3G) to later generations, such as the beyond third generation or the fourth generation, limited spectrum resources have hindered the development of the wireless communication technology. To increase the data rate and more efficiently utilize the spectrum, some technologies and schemes have been used to improve the spectrum utilization factor, such as optimization modulation schemes, program code multiplexing systems or MIMO technologies. Over recent years, MIMO technology has been widely applied in the industry. For example, the emerging Worldwide Interoperability for Microwave Access (WiMAX) standard and the new generation of Wireless Local Area Network (WLAN) are both incorporated with the MIMO technology. 
         [0007]    MIMO systems indicate that signals are transmitted and received through multiple antennas synchronously. The MIMO system adopts a plurality of antennas both at the transmitting end and the receiving end, so that data can be transmitted via a plurality of signal transmission channels to increase the data rate. More specifically, a signal is divided into multiple portions at the transmitting end for synchronous transmission through a plurality of antennas. Because individual portions of the signal are transmitted through different signal transmission channels, they may arrive at the receiving end at different times. To prevent failure of recombination due to the different received times of individual portions, the receiving end has a plurality of antennas to receive these signals simultaneously. By digital signal processing and re-computation, the separate signal portions are then recombined into the original signal rapidly and properly. 
         [0008]    By dividing the signal, the traffic in the single signal transmission channel can be decreased so that the signal transmission distance may be increased. Accordingly, MIMO technology has been able to speed up the signal transmission and avoid the use of additional spectrum, as well as increase the signal transmission distance. Therefore, many wireless network apparatuses have adopted MIMO technology to meet the increased requirements of signal transmission speed and distance. Thus, MIMO technology has become a key technology that must be adopted in the new generation of mobile communication systems. 
         [0009]    Compared to a single-antenna system, an MIMO antenna system is capable of receiving a larger data amount and consequently, has a higher data rate. In the MIMO antenna system, a signal transmission channel may be established between each of the transmitting antennas at the transmitting end and a corresponding receiving antenna at the receiving end. Since channel conditions may not be all the same among the signal transmission channels, wireless network apparatus manufacturers usually use adaptive modulation coding (AMC) technology to transmit the divided signals. With AMC technology, when a signal transmission channel is in good condition, i.e., a low signal error rate or a high signal to noise ratio (SNR), the signal is transmitted in a transmission mode featuring a higher transmission speed. Conversely, the signal is transmitted in a transmission mode featuring a lower transmission speed. In this way, a better data throughput is achieved in the MIMO antenna system. Here, the data throughput is defined as the number of correct signals received by the receiving end within a unit time. 
         [0010]    To implement the AMC technology, the conventional practice is to acquire characteristics of the individual signal transmission channels by simulation or practical measurements, and to predetermine a system threshold value related to the signal transmission channels. Then, during the operation of the MIMO antenna system, a signal transmission mode will be determined for the signal transmission channels according to the predetermined system threshold value. Table 1 (below) shows the signal transmission modes versus predetermined system threshold values in a conventional MIMO antenna system. 
         [0000]    
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Data rate 
                   
               
               
                 Modulation scheme, Code rate 
                 (kbits/sec) 
                 Range of SNR 
               
               
                   
               
             
             
               
                 QPSK, ½ 
                 228 
                 SNR &lt; 10 dB 
               
               
                 16QAM, ½ 
                 462 
                 10 dB ≦ SNR &lt; 32 dB 
               
               
                 64QAM, ⅔ 
                 942 
                 SNR ≧ 32 dB 
               
               
                   
               
             
          
         
       
     
         [0011]    In table 1, the signal transmission modes (modulation scheme, code rate) are (QPSK, ½), (16 QAM, ½) and (64 QAM, ⅔), while the predetermined system threshold values are set to be 10 dB and 32 dB. In the conventional MIMO antenna system, if the receiving end measures an SNR of the signal transmitted via a first signal transmission channel to be 20 dB, which falls within an SNR range of 10 dB≦SNR&lt;32 dB, the transmitting end may set the transmission mode of the first signal transmission channel to be (16 QAM, ½), which corresponds to a data rate of 462 kbits/sec. If an SNR of the signal transmitted via a second signal transmission channel in the MIMO antenna system is measured to be 60 dB, which falls within an SNR range of SNR≧32 dB, the transmitting end will set the transmission mode of the second signal transmission channel to be (64 QAM, ⅔), which corresponds to a data rate of 942 kbits/sec. The SNR of the signal transmitted via the second signal transmission channel is much higher than that of the signal transmitted via the first signal transmission channel, which implies that the second signal transmission channel experiences a better channel condition than the first signal transmission channel. The second signal transmission channel may use a transmission mode with a higher data rate to transmit a signal. In this way, the MIMO antenna system using AMC technology can maximize the data throughput. 
         [0012]    However, in a realistic environment, parameters related to each signal transmission channels of an MIMO antenna system vary with time. If the related parameters of a signal transmission channel changed after a period of time due to environmental changes while the corresponding system threshold value of the signal transmission channel remains the same, the overall data throughput of the MIMO antenna system will be adversely affected because of setting the wrong transmission mode. 
         [0013]    Accordingly, it is important to decrease the error rate of data transmissions in an MIMO antenna system when the related parameters of the signal transmission channel vary with time. 
       SUMMARY OF THE INVENTION 
       [0014]    One objective of this invention is to provide an MIMO antenna system, a signal transmission method and a computer program product for the MIMO antenna system. The MIMO antenna system has a first signal transmission channel and at least one second signal transmission channel. The MIMO antenna system is capable of adjusting a system threshold value related to a signal transmission mode, and a power weight value related to a signal transmission power according to channel conditions of the signal transmission channels to mitigate interference among the signal transmission channels and increase the data throughput of the MIMO antenna system. 
         [0015]    The MIMO antenna system of this invention comprises a first signal transmission apparatus and a second signal transmission apparatus. The first signal transmission apparatus is configured to transmit a signal with a first transmission mode and a first transmission power via the first signal transmission channel. The second signal transmission apparatus is configured to calculate and transmit an interference value of the first signal transmission channel and to calculate and transmit an SNR of the signal after receiving the signal. The interference value is related to a transmission power of the at least one second signal transmission channel. The first signal transmission apparatus calculates a power weight value according to the interference value, sets a system threshold value of the first signal transmission channel according to the SNR of the signal, determines a second transmission power of the first signal transmission channel according to the power weight value, and determines a second transmission mode of the first signal transmission channel according to the system threshold value. 
         [0016]    The signal transmission method for the MIMO antenna system of this invention comprises the following steps of: enabling a first signal transmission apparatus to transmit a signal with a first transmission mode and a first transmission power via the first signal transmission channel; enabling a second signal transmission apparatus to receive the signal; enabling the second signal transmission apparatus to calculate and transmit an interference value of the first signal transmission channel, wherein the interference value is related to a transmission power of the at least one second signal transmission channel; enabling the first signal transmission apparatus to calculate a power weight value according to the interference value; enabling the second signal transmission apparatus to calculate and transmit an SNR of the signal; enabling the first signal transmission apparatus to set a system threshold value of the first signal transmission channel according to the SNR of the signal; enabling the first signal transmission apparatus to determine a second transmission power of the first signal transmission channel according to the power weight value; and enabling the first signal transmission apparatus to determine a second transmission mode of the first signal transmission channel according to the system threshold value. 
         [0017]    This invention further provides a computer program product comprising a plurality of instructions stored in a computer readable medium for the MIMO antenna system of this invention to perform the signal transmission method described above. 
         [0018]    Another objective of this invention is to provide a signal transmission apparatus, a signal transmission method and a computer program product for the signal transmission apparatus. The signal transmission apparatus is used for an MIMO antenna system having a first signal transmission channel and at least one second signal transmission channel. The signal transmission apparatus is capable of adjusting a system threshold value related to a signal transmission mode, and a power weight value related to a signal transmission power according to channel conditions of the signal transmission channels. As a result, the interference among the signal transmission channels is mitigated, thereby, increasing the data throughput of the MIMO antenna system. 
         [0019]    Another signal transmission apparatus of this invention comprises at least one transmission unit, a calculation module, a microprocessor and an adjustment module. The at least one transmission unit is configured to transmit a signal with a first transmission mode and a first transmission power via the first signal transmission channel, and to receive an SNR of the signal and an interference value of the first signal transmission channel. The calculation module is configured to calculate a power weight value according to the interference value of the first transmission channel. The microprocessor is configured to set a system threshold value of the first signal transmission channel according to the SNR of the signal. The adjustment module is configured to determine a second transmission mode of the first signal transmission channel according to the system threshold value, and determine a second transmission power of the first signal transmission channel according to the power weight value. The interference value is related to a transmission power of the at least one second signal transmission channel. 
         [0020]    Also, another signal transmission method of this invention comprises the following steps of: enabling at least one transmission unit to transmit a signal with a first transmission mode and a first transmission power via the first signal transmission channel; enabling the at least one transmission unit to receive an SNR of the signal and an interference value of the first signal transmission channel, wherein the interference value is related to a transmission power of the at least one second signal transmission channel; enabling a calculation module to calculate a power weight value according to the interference value; enabling a microprocessor to set a system threshold value of the first signal transmission channel according to the SNR of the signal; and enabling an adjustment module to determine a second transmission mode of the first signal transmission channel according to the system threshold value, and determine a second transmission power of the first signal transmission channel according to the power weight value. 
         [0021]    Further, another computer program product stored in a computer readable medium for the signal transmission apparatus of this invention to perform the signal transmission method described above. 
         [0022]    In summary, the MIMO antenna system, the signal transmission method, the signal transmission apparatus and the computer program product for the MIMO antenna system of this invention are adapted to dynamically adjust the system threshold value according to the SNR of the signal transmitted via the signal transmission channel, and switch among transmission modes of the signal transmission channels accordingly. This invention may further calculate the interference caused by other signal transmission channels and, based on the calculated interference, allocate a transmission power for the signal transmission channels to mitigate data transmission errors caused by interference among the signal transmission channels and increase the data throughput of the MIMO antenna system. In this way, the shortcomings of the prior art are overcome. 
         [0023]    The detailed technology and preferred embodiments implemented for the subject invention are described in the following paragraphs accompanying the appended drawings for people skilled in this field to well appreciate the features of the claimed invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  is a schematic view illustrating a first embodiment of this invention; 
           [0025]      FIG. 2  is a schematic view illustrating related parameters in an MIMO antenna system of the first embodiment; 
           [0026]      FIG. 3  is a schematic view illustrating updated related parameters in the MIMO antenna system of the first embodiment; 
           [0027]      FIG. 4  is another schematic view illustrating updated related parameters in the MIMO antenna system of the first embodiment; 
           [0028]      FIG. 5  is a flow chart illustrating a second embodiment of this invention; and 
           [0029]      FIG. 6  is a flow chart illustrating a third embodiment of this invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0030]    This invention provides an MIMO antenna system, a signal transmission method, a signal transmission apparatus and a computer program product for the MIMO antenna system. The following description of embodiments of this invention is only for purposes of illustration, not limitation. In the following embodiments and the attached drawings, elements unrelated to this invention are omitted from depiction. 
         [0031]      FIG. 1  depicts a first embodiment of this invention, which is an MIMO antenna system  1 . In this embodiment the MIMO antenna system  1  is a system with a hybrid automatic retransmission request (HARQ) structure. More specifically, when receiving a signal transmitted from a transmitting end of the MIMO antenna system  1 , a receiving end of the MIMO antenna system  1  determines whether the signal is correct. If the signal is determined to be correct, the receiving end feedbacks an acknowledgment (ACK) to the transmitting end to inform that the signal has been received correctly and no retransmission of the signal is needed. On the other hand, if the signal is determined to be incorrect, the receiving end feedbacks a negative acknowledgement (NACK) to inform the transmitting end to retransmit the signal. In other examples, the MIMO antenna system  1  may be any system with any kind of feedback mechanism structure. The type of feedback mechanism structure that the MIMO antenna system  1  adopts is not limited in this invention. 
         [0032]    The MIMO antenna system  1  comprises a first signal transmission apparatus  11  and a second signal transmission apparatus  13 . In this embodiment, the first signal transmission apparatus  11  is a signal transmitting end, while the second signal transmission apparatus  13  is a signal receiving end. The first signal transmission apparatus  11  comprises a plurality of transmission units  111 ,  113 ,  115 , a calculation module  117 , a microprocessor  119 , an adjustment module  121  and a register  123 . The second signal transmission apparatus  13  also has a plurality of transmission units  131 ,  133 ,  135 . Thus, a plurality of signal transmission channels  151 ,  153 ,  155  are respectively established in the MIMO antenna system  1  by the transmission units  111 ,  113 ,  115  of the first signal transmission apparatus  11  and the transmission units  131 ,  133 ,  135  of the second signal transmission apparatus  13 . 
         [0033]    For purposes of simplicity, in this embodiment, the first, second and third transmission units  111 ,  113 ,  115  will be taken as an example to describe the transmission units of the first signal transmission apparatus  11 . The first, second and third transmission units  131 ,  133 ,  135  will be taken as an example to describe the transmission units of the second signal transmission apparatus  13 . The first, second and third signal transmission channels  151 ,  153 ,  155  will be taken as an example to describe the signal transmission channels of the MIMO antenna system  1 . 
         [0034]    Although only three signal transmission channels  151 ,  153 ,  155  in the MIMO antenna system  1  are depicted in  FIG. 1 , the number of signal transmission channels of the MIMO antenna system is not limited in this invention. In other words, those of ordinary skill in the art may readily implement more than three signal transmission channels in the MIMO antenna system based on the above description, and thus no further description will be made herein. 
         [0035]    In this embodiment, each of the signal transmission channels  151 ,  153 ,  155  of the MIMO antenna system  1  has three transmission modes, i.e., a transmission mode (QPSK, ½) with a data rate of 228 kbits/sec, a transmission mode (16 QAM, ½) with a data rate of 469 kbits/sec and a transmission mode (64 QAM, ⅔) with a data rate of 924 kbits/sec. The register  123  of the first signal transmission apparatus  11  stores a first predetermined threshold value μ 1 i, a second predetermined threshold value μ 2 , a first predetermined threshold range D 1  and a second predetermined threshold range D 2 . The first predetermined threshold value μ 1  is 18 dB, the second predetermined threshold value μ 2  is 28 dB, and both the first predetermined threshold range D 1  and the second predetermined threshold range D 2  are 6 dB. As can be seen from  FIG. 2  and the above description, three SNR ranges can be formed by the first predetermined threshold value μ 1  and the second predetermined threshold value μ 2 , i.e., 0 dB˜18 dB, 18 dB˜28 dB and from 28 dB above. Depending on the SNR range that falls within, the MIMO antenna system  1  transmits a signal in one of the three transmission modes. Meanwhile, the first predetermined threshold range D 1  and the second predetermined threshold range D 2  defined according to the first predetermined threshold value μ 1  and the second predetermined threshold value μ 2  are 15 dB˜21 dB and 25 dB˜31 dB respectively. 
         [0036]    More specifically, the number, kinds and values of the transmission modes and the predetermined threshold values are not limited in the MIMO antenna system  1  of this invention, and those of ordinary skill in the art may define the number of transmission modes and values of the predetermined thresholds in the MIMO antenna system according to the existing MIMO technologies, and thus no further description will be made herein. 
         [0037]    Hereinafter, operations and functions of the MIMO antenna system  1  of this invention will be described in detail. Initially, the first signal transmission apparatus  11  transmits a first signal  150  from the first transmission unit  111  to the second signal transmission apparatus  13  with a first transmission mode (e.g., 16 QAM, ½) and a first transmission power via the first signal transmission channel  151 . At the same time, the first signal apparatus  11  transmits a second signal  152  from the second transmission unit  113  to the second signal transmission apparatus  13  via the second signal transmission channel  153 . Meanwhile, the first signal apparatus  11  transmits a third signal  154  from the third transmission unit  115  to the second signal transmission apparatus  13  via the third signal transmission channel  155 . 
         [0038]    When the first, second and third signals  150 ,  152 ,  154  are transmitted, the first transmission unit  131  of the second signal transmission apparatus  13  receives the first signal  150  via the first transmission channel  151 . However, the first transmission unit  131  may further receive interference  152   a  from the second signal  152  and interference  154   a  from the third signal  154 . For example, if the second signal  152  is transmitted with an overly high transmission power and/or the first signal transmission channel  151  is too close in distance to the second signal transmission channel  153 , the second signal  152  will have an impact on the first signal  150  transmitted over the first signal transmission channel  151 , thus causing interference  152   a.  Similarly, if the third signal  154  is transmitted with an overly high transmission power and/or the first signal transmission channel  151  is too close in distance to the third signal transmission channel  155 , the third signal  154  will have an impact on the first signal  150  transmitted over the first signal transmission channel  151 , thus causing interference  154   a.    
         [0039]    Likewise, expect for the second signal  152  transmitted via the second signal transmission channel  153 , the second transmission unit  133  of the second signal transmission apparatus  13  may also receive interference (not shown) from the first signal  150  and the third signal  154 . The third transmission unit  135  of the second signal transmission apparatus  13  may also receive interference (not shown) from the first signal  150  and the second signal  152  expect for the third signal  154  transmitted via the third signal transmission channel  155 . 
         [0040]    Upon receiving the first signal  150  via the first transmission unit  131 , the second signal transmission apparatus  13  calculates an SNR  150   b  of the first signal  150  and an interference value  151   b  of the first signal transmission channel  151 . Briefly speaking, the second signal transmission apparatus  13  calculates and quantizes the interference  152   a  from the second signal  152  and the interference  154   a  from the third signal  154  in a physical way to derive the interference value  151   b.  Then, the second signal transmission apparatus  13  transmits the SNR  150   b  of the first signal  150  and the interference value  151   b  of the first signal transmission channel  151  from the first transmission unit  131  to the first signal transmission apparatus  11 . Meanwhile, if the second signal transmission apparatus  13  determines that the first signal  150  is correct, the first transmission unit  131  feedbacks an ACK to the first signal transmission apparatus  11  to inform that no retransmission of the first signal  150  is needed. Conversely, if the second signal transmission apparatus  13  determines that the first signal  150  is incorrect, the first transmission unit  131  feedbacks an NACK to the first signal transmission apparatus  11  to require the first signal transmission apparatus  11  a retransmission of the first signal  150 . 
         [0041]    After the first transmission unit  111  of the first signal transmission apparatus  11  receives the SNR  150   b  of the first signal  150  and the interference value  151   b  of the first signal transmission channel  151 , the calculation module  117  calculates a power weight value  110  according to the interference value  151   b  of the first signal transmission channel  151  by an iterative method. According to the power weight value  110 , the adjustment module  121  allocates a second transmission power to the first signal transmission channel  151 . 
         [0042]    More specifically, the calculation module  117  uses a water-filling algorithm to calculate the power weight value  110  according to the interference value  151   b  of the first signal transmission channel  151 . According to the water-filling algorithm, without increasing the total transmission power of the first, second and third signal transmission channels  151 ,  153 ,  155  of the MIMO antenna system  1 , signal transmission channels with lower interference values are allocated a larger transmission power, while those with higher interference values are allocated a smaller transmission power. 
         [0043]    For example, if the interference value  151   b  of the first signal transmission channel  151  is smaller than the interference value (not shown) of the second signal transmission channel  153 , the calculation module  117  will increase the power weight value  110  of the first signal transmission channel  151 , and decrease the power weight value (not shown) of the second signal transmission channel  153 , so that a balance in transmission power can be achieved among the first, second and third signal transmission channels  151 ,  153 ,  155  through continuous calculations with the water-filling algorithm. Meanwhile, through the aforesaid calculation process, the interferences of the first, second and third signal transmission channels  151 ,  153 ,  155  from each other are converged respectively. The adjustment of power weight values of a corresponding signal transmission channel may be made by those of ordinary skill in the art based on the above description of the water-filling algorithm and the existing MIMO technology, thus will not be further described herein. 
         [0044]    Upon completing the adjustment of the power weight values of the corresponding signal transmission channels, the adjustment module  121  determines a second transmission power of the first signal transmission channel  151  according to the power weight value  110  of the first signal transmission  151 . The second transmission power of the first signal transmission channel  151  is higher than the previous first transmission power, so that the first transmission unit  111  of the first signal transmission apparatus  11  can transmit the next signal with the second transmission power. 
         [0045]    The microprocessor  119  of the first signal transmission apparatus  11  sets a system threshold value  112  of the first signal transmission channel  151  according to the SNR  150   b  of the first signal  150 . More specifically, if the transmission unit  111  of the first signal transmission apparatus  11  receives an ACK transmitted by the first transmission unit  131  of the second signal transmission apparatus  13 , the microprocessor  119  determines whether the SNR  150   b  of the first signal  150  falls within the first predetermined threshold range D 1  or the second predetermined threshold range D 2  (i.e., 15 dB˜21 dB and 25 dB˜31 dB respectively). If the SNR  150   b  of the first signal  150  does not fall within either of the predetermined threshold ranges, the adjustment module  121  determines a second transmission mode of the first signal transmission channel  151  according to the AMC technology of the prior art for, so that the first transmission unit  111  of the first signal transmission apparatus  11  can transmit the next signal with the second transmission mode. 
         [0046]    On the other hand, if the SNR  150   b  of the first signal  150  falls within either of the predetermined threshold ranges, the microprocessor  119  then determines whether the SNR  150   b  is less than the first predetermined threshold value μ 1  (i.e., 18 dB) or the second predetermined threshold value μ 2  (i.e., 28 dB). 
         [0047]    For example, if the SNR  150   b  of the first signal  150  is 27 dB, the microprocessor  119  determines that the SNR  150   b  of the first signal  150  is less than the second predetermined threshold value μ 2  (i.e., 28 dB). In this case, the calculation module  117  of the first signal transmission apparatus  11  retrieves the second predetermined threshold value μ 2  from the register  123  and subtracts a correction value from the second predetermined threshold value μ 2 . Here, the correction value may be designed depending on different conditions and is not limited in this invention. In this embodiment the correction value is set to be 2 dB. The microprocessor  119  sets the system threshold value  112  of the first signal transmission channel  151  to be a result of subtracting the correction value from the second predetermined threshold value μ 2 , i.e., 26 dB. Next, the calculation module  117  updates the second predetermined threshold value μ 2  stored in the register  123  to 26 dB. Simultaneously, the second predetermined threshold range D 2  is updated to 23 dB˜29 dB. These updated parameters of the MIMO antenna system  1  are shown in  FIG. 3 . 
         [0048]    Here, the result of subtracting the correction value from the second predetermined threshold value μ 2  (i.e., 26 dB) still falls within the original second predetermined threshold range D 2  (i.e., 25 dB˜31 dB). Subsequently, according to the system threshold value  112 , the adjustment module  121  switches the transmission mode of the first signal transmission channel  151  from the first transmission mode (16 QAM, ½) with the data rate of 462 kbits/sec to the second transmission mode (64 QAM, ⅔) with a data rate of 924 kbits/sec, so that the first signal transmission unit  111  will transmit the next signal with the second transmission mode featuring a higher data rate and the determined second transmission power. 
         [0049]    On the other hand, if the SNR  150   b  of the first signal  150  is 20 dB, the microprocessor  119  determines that the SNR  150   b  of the first signal  150  is higher than the first predetermined threshold value pi (i.e., 18 dB). In this case, the calculation module  117  of the first signal transmission apparatus  11  retrieves the first predetermined threshold value μ 1  from the register  123  and adds a correction value to the first predetermined threshold value μ 1 . Here, the correction value may be designed depending on different conditions and is not limited in this invention. In this embodiment, the correction value is defined to be 2 dB. The microprocessor  119  sets the system threshold value  112  of the first signal transmission channel  151  to be a result of adding the correction value to the first predetermined threshold value μ 1 , i.e., 20 dB. Next, the calculation module  117  updates the first predetermined threshold value μ 1  stored in the register  123  to 20 dB. Simultaneously, the first predetermined threshold range D 1  is updated to 17 dB˜23 dB. These updated parameters of the MIMO antenna system  1  are shown in  FIG. 4 . 
         [0050]    Here, after adding the correction value to the first predetermined threshold value μ 1  (i.e., 20 dB), the added result still falls within the original first predetermined threshold range D 1  (i.e., 15 dB˜21 dB). Subsequently, according to the system threshold value  112 , the adjustment module  121  switches the transmission mode of the first signal transmission channel  151  from the first transmission mode (16 QAM, ½), with a data rate of 462 kbits/sec, to the second transmission mode (QPSK, ½) with a data rate of 228 kbits/sec, so that the first signal transmission unit  111  will transmit the next signal with the second transmission mode featuring a lower data rate and the determined second transmission power. 
         [0051]    Generally, the original first predetermined threshold value μ 1  (18 dB) and the original second predetermined value μ 2  (28 dB) may be adjusted dynamically by the calculation module  117  according to the SNR  150   b  of the first signal  150  each time, and the aforesaid determination made by the microprocessor  119  to reduce data transmission errors caused by setting the wrong transmission mode and increase the data throughput of the MIMO antenna system  1 . 
         [0052]      FIG. 5  depicts a second embodiment of this invention, which is a signal transmission method adapted for an MIMO antenna system, e.g., the MIMO antenna system  1  described in the first embodiment. The MIMO antenna system has a first signal transmission channel and at least one second signal transmission channel. More specifically, the signal transmission method of the second embodiment may be implemented by a computer program product. When the computer program product is loaded in a computer and a plurality of instructions contained therein is executed, the signal transmission method of the second embodiment will be accomplished. This computer program product may be stored in a tangible machine-readable medium, such as a read only memory (ROM), a flash memory, a floppy disk, a hard disk, a compact disk, a mobile disk, a magnetic tape, a database accessible to networks, or any other storage media with the same function and well known to those skilled in the art. 
         [0053]    The signal transmission method of the second embodiment comprises the following steps. Initially in step  501 , a first signal transmission apparatus transmits a signal with a first transmission mode and a first transmission power via the first signal transmission channel. Then, in step  503 , a second signal transmission apparatus receives the signal. Next, in step  505 , the second signal transmission apparatus calculates and transmits an interference value of the first signal transmission channel. The interference value is related to a transmission power of the at least one second signal transmission channel. In step  507 , a power weight value is calculated using an iterative method according to the interference value. Subsequently in step  509 , the second signal transmission apparatus calculates and transmits an SNR of the signal. Next, in step  511 , it is determined whether the first signal transmission apparatus has received an ACK of the signal from the second signal transmission apparatus. If not, the process returns back to step  501  for the first signal transmission apparatus to continue transmitting a signal with a first transmission mode and a first transmission power via the first signal transmission channel. 
         [0054]    If the first signal transmission apparatus receives an ACK of the signal from the second signal transmission apparatus in step  511 , it is determined in step  513  whether the SNR of the signal falls within a predetermined threshold range. If not, the process proceeds to step  515  without adjusting the system threshold value. Conversely, if it is determined in step  513  that the SNR of the signal falls within the predetermined threshold range, the first signal transmission apparatus determines whether the SNR of the signal is less than a predetermined threshold value in step  517 . If so, then in step  519 , the first signal transmission apparatus subtracts a correction value from the predetermined threshold value and sets the system threshold value to be the subtracting result. Then, in step  521 , the first signal transmission apparatus updates the predetermined threshold value into the subtracting result. Here, the subtracting result falls within the predetermined threshold range. Finally, in step  523 , a second transmission mode is determined for the first signal transmission channel according to the system threshold value, while a second transmission power is determined for the first signal transmission channel according to the power weight value. Here, a data rate of the first transmission mode is lower than that of the second transmission mode. 
         [0055]    If it is determined in step  517  that the SNR of the signal is no less than the predetermined threshold value, the first signal transmission apparatus adds a correction value to the predetermined threshold value and sets the system threshold value to be a result of adding the correction value to the predetermined threshold value in step  525 . Then, in step  527 , the first signal transmission apparatus updates the predetermined threshold value into the adding result. Here, the adding result falls within the predetermined threshold range. Finally, in step  529 , a second transmission mode is determined for the first signal transmission channel according to the system threshold value, while a second transmission power is determined for the first signal transmission channel according to the power weight value. Here, a data rate of the first transmission mode is higher than that of the second transmission mode. 
         [0056]    In addition to the aforesaid steps, the second embodiment can also execute all the operations and functions set forth in the first embodiment. The methods in which the second embodiment executes these operations and functions will be readily appreciated by those of ordinary skill in the art based on the explanation of the first embodiment, and thus will not be further described herein. 
         [0057]      FIG. 6  depicts a third embodiment of this invention, which is a signal transmission method adapted for use in a signal transmission apparatus, e.g., the first signal transmission apparatus  11  of the MIMO antenna system  1  described in the first embodiment. More specifically, the signal transmission method of the third embodiment may be implemented by a computer program product. When the computer program product is loaded in a computer and a plurality of instructions contained therein is executed, the signal transmission method of the third embodiment will be accomplished. This computer program product may be stored in a tangible machine-readable medium, such as an ROM, a flash memory, a floppy disk, a hard disk, a compact disk, a mobile disk, a magnetic tape, a database accessible to networks, or any other storage media with the same function and well known to those skilled in the art. 
         [0058]    The signal transmission method of the third embodiment comprises the following steps. Initially in step  601 , a transmission unit transmits a signal with a first transmission mode and a first transmission power via a first signal transmission channel. Then, in step  603 , the transmission unit receives an interference value of the first signal transmission channel and an SNR of the signal, in which the interference value is related to a transmission power of at least one second signal transmission channel. Next, in step  605 , a calculation module calculates a power weight value using an iterative method according to the interference value. In step  607 , it is determined whether an ACK of the signal is received. If not, the process returns back to step  601 , where the transmission unit continues to transmit the signal with the first transmission mode and the first transmission power via the first signal transmission channel. 
         [0059]    If the transmission unit receives an ACK of the signal in step  607 , then, in step  609  a microprocessor determines whether the SNR of the signal falls within a predetermined threshold range. If not, the process proceeds to step  611  without adjusting the system threshold value. Conversely, if it is determined in step  609  that the SNR of the signal falls within the predetermined threshold range, then, in step  613  the microprocessor determines whether the SNR of the signal is less than a predetermined threshold value. If so, in step  615 , a calculation module subtracts a correction value from the predetermined threshold value, and the microprocessor sets the system threshold value to be the result of subtracting the correction value from the predetermined threshold value. Then, in step  617 , a register updates the predetermined threshold value into the subtracting result. Here, the subtracting result falls within the predetermined threshold range. Finally, in step  619 , an adjustment module determines a second transmission mode of the first signal transmission channel according to the system threshold value, and determines a second transmission power of the first signal transmission channel according to the power weight value. Here, a data rate of the first transmission mode is lower than that of the second transmission mode. 
         [0060]    If the microprocessor determines in step  613  that the SNR of the signal is no less than the predetermined threshold value, then in step  621 , the calculation module adds a correction value to the predetermined threshold value and the microprocessor sets the system threshold value to be a result of adding the correction value to the predetermined threshold value. Next, in step  623 , the register updates the predetermined threshold value into the adding result. Here, the adding result falls within the predetermined threshold range. Finally, in step  625 , a second transmission mode is determined for the first signal transmission channel according to the system threshold value, and a second transmission power is determined for the first signal transmission channel according to the power weight value. Here, a data rate of the first transmission mode is higher than that of the second transmission mode. 
         [0061]    In addition to the aforesaid steps, the third embodiment can also execute all the operations and functions set forth in the first embodiment. The method in which the third embodiment executes these operations and functions will be readily appreciated by those of ordinary skill in the art based on the explanation of the first embodiment, and thus will not be further described herein. 
         [0062]    According to the MIMO antenna system, and the signal transmission method, the signal transmission apparatus and the computer program product for the MIMO antenna system of this invention, the system threshold value can be adjusted dynamically according to an SNR of a signal and finally approach a proper system threshold value. Furthermore, by using an iterative method to derive a power weight value, a better balance in interference is achieved among the signal transmission channels. As a result a better data throughput is obtained in the MIMO antenna system. 
         [0063]    The above disclosure is related to the detailed technical contents and inventive features thereof. People skilled in this field may proceed with a variety of modifications and replacements based on the disclosures and suggestions of the invention as described without departing from the characteristics thereof. Nevertheless, although such modifications and replacements are not fully disclosed in the above descriptions, they have substantially been covered in the following claims as appended.