Patent Publication Number: US-2011069775-A1

Title: Signal processing method and apparatus for mimo system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is an application under 35 USC 111(a) and claims priority under 35 USC 119 from Provisional Application Ser. No. 61/244,085 filed Sep. 21, 2009 and Provisional Application Ser. No. 61/244,448 filed Sep. 22, 2009 under 35 USC 111(b), the disclosures of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a wireless system, and more particularly, to a multiple-input-multiple-output (MIMO) wireless system. 
     2. Description of the Related Art 
     In a multiple transmit antenna communication system, such as a MIMO system, a plurality of transmitting streams are transmitted with multiple antennas and received by multiple antennas to achieve spatial diversity effect. However, if the number of the transmitting spatial streams is less than the number of the transmitting antennas, two or more transmitting antennas may transmit highly correlated transmitting streams and cause an unintentional beam forming effect as shown in  FIG. 1 . As shown in  FIG. 1 , the system comprises two inverse fast Fourier transform (IFFT) modules to transform frequency domain MIMO data streams into time domain MIMO data streams. An unintentional beam forming effect is generated when two antennas transmit highly correlated transmitting streams. The unintentional beam forming effect helps the receivers in the direction of the formed beams to receive signals more easily. However, for other receivers, it becomes more difficult to receive signals transmitted by the transmitter. Therefore, the broadcast transmission quality may be degraded due to this unintentional beam forming effect. 
     To overcome the unintentional beam forming effect, a conventional method is to use the cyclic shift delay (CSD) technique to de-correlate the transmitted streams, such as in the system shown in  FIG. 2 , wherein CSD technique is performed after the IFFT computation. For example, in a four transmitting antenna system complying with IEEE 802.11n standard with 20M/40 MHz bandwidth, the CSD is performed in the time domain to avoid the unintentional beam forming effect. Moreover, CSD technique can also be performed before the IFFT computation, such as in the system shown in  FIG. 3 , which means that the CSD is performed in the frequency domain and the streams might have to rotate a certain angle. However, the amount of the CSD may influence the performance of the packet detection and the gain control performance. In IEEE 802.11n standard with 20M/40 MHz bandwidth, which applies four transmitting antennas, the CSD is confined between −200 ns and 0 to get a compromised performance in packet detection and gain control, wherein the cyclic shifts are multiples of 50 ns, which is exactly the sampling interval of the system fundamental sampling rate, i.e. 20 MHz. 
     However, as the number of applied antennas increases, or the transmission bandwidth is extended, the current method to overcome the unintentional beam forming effect is no longer applicable. Therefore, there is a need to design a method or system to solve the unintentional beam forming effect when a more complicated MIMO system is applied. 
     SUMMARY OF THE INVENTION 
     It is therefore an objective of the present invention to provide an architecture and process of the CSD for a wireless system with more than four antennas. 
     It is therefore another objective of the present invention to provide an architecture and process of the CSD for a wireless system with more than four antennas system and support 20/40/80 MHZ bandwidths. 
     The signal processing method for a MIMO system according to one embodiment of the present invention comprises the steps of: arranging a plurality of frequency domain MIMO data streams into a plurality of groups, wherein each group comprises at least a frequency domain MIMO data stream; partitioning sub-carriers of each of the plurality of frequency domain MIMO data streams into a plurality of sub-channels; performing phase rotation for the plurality of frequency domain MIMO data streams, wherein the phases of the sub-carriers in each sub-channel are rotated by the same amount, and different phase rotations are employed on different groups of the plurality of frequency domain MIMO data streams; transforming the plurality of frequency domain MIMO data streams into a plurality of time domain MIMO data streams; and performing CSD for the plurality of time domain MIMO data streams if each group comprises more than one time domain MIMO data stream, wherein the amount of the CSD is different for each time domain MIMO data stream in a group. 
     The signal processing method for a MIMO system according to another embodiment of the present invention comprises the steps of: arranging a plurality of frequency domain MIMO data streams into a plurality of groups, wherein each group comprises at least one frequency domain MIMO data stream; partitioning sub-carriers of each of the plurality of frequency domain MIMO data streams into a plurality of sub-channels; performing phase rotation on the plurality of frequency domain MIMO data streams, wherein the phases of the sub-carriers in a sub-channel are rotated with the same amount, and different phase rotations are performed on different groups of the plurality of frequency domain MIMO data streams; performing cyclic shift delay on the plurality of frequency domain MIMO data streams if each group comprises more than one frequency domain MIMO data streams, wherein the amount of the cyclic shift delay is different for each frequency domain MIMO data stream in a group; and transforming the plurality of frequency domain MIMO data streams into a plurality of time domain MIMO data streams. 
     The signal processing method for a MIMO system according to another embodiment of the present invention comprises the steps of: extending at least one frequency domain MIMO data stream by padding zeroes at the beginning and the end of each of the at least one frequency domain MIMO data stream; transforming the at least one frequency domain MIMO stream into at least one time domain MIMO data stream; and performing CSD for the at least one time domain MIMO data stream to produce a plurality of time domain MIMO data streams, wherein the amount of the CSD is different for each of the time domain MIMO data streams. 
     The signal processing method for a MIMO system according to yet another embodiment of the present invention comprises the steps of: performing cyclic shift delay for at least one frequency domain MIMO data stream to produce a plurality of frequency domain MIMO data streams, wherein the amount of the cyclic shift delay is different for each of the frequency domain MIMO data streams; and transforming the plurality of frequency domain MIMO stream into a plurality of time domain MIMO data stream. 
     The signal processing apparatus for a MIMO system according to one embodiment of the present invention comprises a phase rotation module, an inverse Fourier transform module and a CSD module. The phase rotation module is configured to rotate the phases of the sub-carriers of a frequency domain MIMO data stream, wherein the sub-carriers of the frequency domain MIMO data stream are partitioned into a plurality of sub-channels, and the phases of the sub-carriers in the same sub-channel are rotated the same amount. The inverse Fourier transform module is configured to transform the frequency domain MIMO data stream into a time domain MIMO data stream. The CSD module is configured to perform CSD for the time domain MIMO data stream. 
     The signal processing apparatus for a MIMO system according to another embodiment of the present invention comprises a phase rotation and cyclic shift delay module and an inverse Fourier transform module. The phase rotation and cyclic shift delay module is configured to rotate the phases of the sub-carriers of a frequency domain MIMO data stream and perform cyclic shift delay for the frequency domain MIMO data stream, wherein the sub-carriers of the frequency domain MIMO data stream are partitioned into a plurality of sub-channels, and the phases of the sub-carriers in a sub-channel are rotated the same amount. The inverse Fourier transform module is configured to transform the frequency domain MIMO data stream into a time domain MIMO data stream. 
     The signal processing apparatus for a MIMO system according to another embodiment of the present invention comprises a zero padding module, an inverse Fourier transform module and a CSD module. The zero padding module is configured to extend a frequency domain MIMO data stream by padding zeroes at the beginning and end of the frequency domain MIMO data stream. The inverse Fourier transform module is configured to transform the frequency domain MIMO data stream into a time domain MIMO data stream. The CSD module is configured to perform CSD for the time domain MIMO data stream. 
     The signal processing apparatus for a MIMO system according to yet another embodiment of the present invention comprises a CSD module and an inverse Fourier transform module. The CSD module is configured to perform CSD for a frequency domain MIMO data stream. The inverse Fourier transform module is configured to transform the frequency domain MIMO data stream into a time domain MIMO data stream. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objectives and advantages of the present invention will become apparent upon reading the following description and upon referring to the accompanying drawings of which: 
         FIG. 1  shows an example of an unintentional beam forming; 
         FIG. 2  shows a MIMO system using CSD technique; 
         FIG. 3  shows another MIMO system using CSD technique; 
         FIG. 4  shows a signal processing apparatus for a MIMO system according to an embodiment of the present invention; 
         FIG. 5  shows the flow chart of a signal processing method for a MIMO system according to an embodiment of the present invention; 
         FIG. 6  shows the phase rotation of a frequency domain MIMO data stream according to an embodiment of the present invention; 
         FIG. 7  shows a signal processing apparatus for a MIMO system according to another embodiment of the present invention; 
         FIG. 8  shows a signal processing apparatus for a MIMO system according to another embodiment of the present invention; 
         FIG. 9  shows a signal processing apparatus for a MIMO system according to another embodiment of the present invention; 
         FIG. 10  shows a signal processing apparatus for a MIMO system according to another embodiment of the present invention; 
         FIG. 11  shows the flow chart of a signal processing method for a MIMO system according to another embodiment of the present invention; 
         FIG. 12  shows a signal processing apparatus for a MIMO system according to another embodiment of the present invention; 
         FIG. 13  shows a signal processing apparatus for a MIMO system according to yet another embodiment of the present invention; and 
         FIG. 14  shows the flow chart of a signal processing method for a MIMO system according to yet another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 4  shows a signal processing apparatus for a MIMO system according to an embodiment of the present invention. As shown in  FIG. 4 , the signal processing apparatus  400  comprises eight phase rotation modules  460  to  474 , eight inverse Fourier transform modules  476  to  490 , eight CSD modules  410  to  424 , eight guard interval insertion modules  426  to  440 , eight antennas  442  to  456  and a spatial mapping module  458 . The phase rotation modules  460  to  474  are configured to rotate the phases of the sub-carriers of eight frequency domain MIMO data streams. The spatial mapping module  458  is configured to perform spatial mapping on the eight frequency domain MIMO data streams. The inverse Fourier transform modules  476  to  490  are configured to transform the eight frequency domain MIMO data streams into eight time domain MIMO data streams. The CSD modules  410  to  424  are configured to perform CSD for the eight time domain MIMO data streams. The guard interval insertion modules  426  to  440  are configured to insert guard intervals into the eight time domain MIMO data streams. The antennas  442  to  456  are configured to broadcast the eight time domain MIMO data streams. 
       FIG. 5  shows the flow chart of a signal processing method for a MIMO system according to an embodiment of the present invention. In step  502 , a plurality of frequency domain MIMO data streams are arranged into a plurality of groups, wherein each group comprises at least one frequency domain MIMO data stream, and then step  504  is executed. In step  504 , the sub-carriers of each of the frequency domain MIMO data streams are partitioned into a plurality of sub-channels, and then step  506  is executed. In step  506 , phase rotation procedures are performed for the frequency domain MIMO data streams, wherein the phases of the sub-carriers in one sub-channel are rotated the same amount, and different phase rotations are employed on different groups of the plurality of frequency domain MIMO data streams, and then step  508  is executed. In step  508 , a spatial mapping procedure is performed on the plurality of frequency domain MIMO data streams, and then step  510  is executed. In step  510 , the frequency domain MIMO data streams are transformed into a plurality of time domain MIMO data streams, and then step  512  is executed. In step  512 , the CSD technique is performed for the time domain MIMO data streams if each group comprises more than one time domain MIMO data stream, wherein the amount of the CSD is different for each time domain MIMO data stream in a group. 
     The following illustrates how to apply the signal processing method shown in  FIG. 5  to the signal processing apparatus shown in  FIG. 4 . In a MIMO system compatible with the IEEE 802.11n standard, a plurality of frequency domain MIMO spatial streams are applied to the signal processing apparatus shown in  FIG. 4 . In this example, the frequency domain MIMO spatial streams comprise eight data streams, and the fundamental bandwidth of the MIMO system is 20 MHz, i.e., the fundamental sampling rate of the MIMO system is 50 ns. The MIMO system can be operated under a 20 MHz mode, a 40 MHz mode, an 80 MHz mode, or the mix of the three modes. In the 20 MHz mode, the MIMO data stream comprises 64 sub-carriers. In the 40 MHz mode, the MIMO data stream comprises 128 sub-carriers. In the 80 MHz mode, the MIMO data stream comprises 256 sub-carriers. 
     In step  502 , the eight data streams are arranged into two groups. In this example, the maximum number of MIMO data streams in a group is four. In step  504 , the sub-carriers of each of the frequency domain MIMO data streams are partitioned into a plurality of sub-channels, and in step  506 , phase rotation procedures are performed for the frequency domain MIMO data streams. For the 80 MHz mode frequency domain MIMO data streams, the sub-carriers are partitioned into four sub-channels. For the 40 MHz mode frequency domain MIMO data streams, the sub-carriers are partitioned into two sub-channels. For the 20 MHz mode frequency domain MIMO data streams, the sub-carriers are not partitioned. After the partition, each sub-channel comprises 64 sub-carriers and exhibits a bandwidth of 20 MHz. A first phase rotation is then performed. The phase rotation module  402  transforms the four frequency domain MIMO data streams in the first group, while the phase rotation module  404  transforms the four frequency domain MIMO data streams in the second group, wherein the phases of the sub-carriers in each sub-channel are rotated the same amount, and different phase rotations are performed on different groups of the plurality of frequency domain MIMO data streams. For instance, for the 20 MHz mode frequency domain MIMO data streams, a phase shift of ω 1 =0 can be applied; for the 40 MHz mode frequency domain MIMO data streams, a phase shifts of ω 1 =0 and ω 2 =0.5π can be applied; for the  80 MHz mode frequency domain MIMO data streams, a set of phase shifts of ω 1 , ω 2 , ω 3  and ω 4  can be applied. The following table shows some alternatives for phase shifts for the 80 MHz mode frequency domain MIMO data streams: 
     
       
         
           
               
               
               
               
               
             
               
                   
                   
               
               
                   
                 ω 1   
                 ω 2   
                 ω 3   
                 ω 4   
               
               
                   
                   
               
             
            
               
                   
                 0 
                 0 
                 0 
                 π 
               
               
                   
                 0 
                 0 
                 π 
                 0 
               
               
                   
                 0 
                 0.5 π 
                 0 
                 1.5 π 
               
               
                   
                 0 
                 0.5 π 
                 π 
                 0.5 π 
               
               
                   
                 0 
                 π 
                 0 
                 0 
               
               
                   
                 0 
                 π 
                 π 
                 π 
               
               
                   
                 0 
                 1.5 π 
                 0 
                 0.5 π 
               
               
                   
                 0 
                 1.5 π 
                 π 
                 1.5 π 
               
               
                   
                   
               
            
           
         
       
     
     Accordingly, the phase rotation modules  460  to  466  can perform phase rotation for the frequency domain MIMO data streams in the first group with one set of phase shift in the above table, and the phase rotation modules  468  to  474  can perform phase rotation for the frequency domain MIMO data streams in the second group with another set of phase shift in the above table. For 80 MHz mode frequency domain MIMO data streams, the first phase rotation is sufficient to overcome the unintentional beam forming effect and a peak to average power ratio (PAPR) problem as well. However, for mix mode, i.e. 20 MHz/40 MHz/80 MHz mode, the first phase rotation is mainly to overcome the PAPR problem. Accordingly, a second first phase rotation can be performed to overcome the unintentional beam forming effect. Accordingly, the sub-carriers in each sub-channel can be further partitioned into N parts, and phases of the sub-carriers in each part are rotated by Φ k , wherein k=1, 2 . . . , N. Each MIMO data stream in a group corresponds to a distinct set of Φ 1 , Φ 2 , . . . , Φ N . In this example, the sub-carriers in each sub-channel are further partitioned into two parts. Accordingly, the phase shift set of the first group can be Φ 1 =0 and Φ 2 =0, and the phase shift set of the second group can be Φ 1 =0.5π and Φ 2 =0.  FIG. 6  shows the phase rotation of the first frequency domain MIMO data stream. 
     In step  508 , the spatial mapping module  458  performs spatial mapping procedure on the plurality of frequency domain MIMO data streams. In step  510 , the inverse Fourier transform modules  476  to  482  transforms the frequency domain MIMO data streams in the first group into four time domain MIMO data streams. The inverse Fourier transform modules  484  to  490  transforms the frequency domain MIMO data streams in the second group into another set of four time domain MIMO data streams. In step  510 , the CSD modules  410  to  416  perform CSD for the time domain MIMO data streams in the first group, and the CSD modules  418  to  424  perform CSD for the time domain MIMO data streams in the second group. The following table shows some alternatives for CSD values: 
     
       
         
           
               
               
               
               
               
             
               
                   
               
               
                 Number of MIMO 
                 CSD for 
                 CSD for 
                 CSD for 
                 CSD for 
               
               
                 data streams  
                 the first 
                 the second 
                 the third 
                 the fourth 
               
               
                 in a group 
                 data stream 
                 data stream 
                 data stream 
                 data stream 
               
               
                   
               
             
            
               
                 1 
                 0 
                   
                   
                   
               
               
                 2 
                 0 
                 −200 ns 
                   
                   
               
               
                 3 
                 0 
                 −100 ns 
                 −200 ns 
                   
               
               
                 4 
                 0 
                  −50 ns 
                 −100 ns 
                 −150 ns 
               
               
                   
               
            
           
         
       
     
     It should be noted that the amount of the CSD is different for each time domain MIMO data stream in a group. In some embodiments of the present invention, each group comprises only one time domain MIMO data stream. For such embodiments, step  512  is omitted. Next, the guard interval insertion modules  426  to  432  insert guard intervals into the time domain MIMO data stream in the first group, and the guard interval insertion modules  434  to  440  insert guard intervals into the time domain MIMO data stream in the second group. The antennas  442  to  456  then broadcast the eight time domain MIMO data streams. 
     It should be noted that the number of components of the signal processing apparatus provided by the present invention can be different from that of the signal processing apparatus shown in  FIG. 4 . For instance, the phase rotation modules  460  to  474  can be combined into a single phase rotation module, and the signal processing method shown in  FIG. 5  can still be applied. 
       FIG. 7  shows a signal processing apparatus for a MIMO system according to another embodiment of the present invention. As shown in  FIG. 7 , the signal processing apparatus  700  comprises eight phase rotation modules  760  to  774 , eight inverse Fourier transform modules  776  and  790 , eight CSD modules  710  to  724 , eight guard interval insertion modules  726  to  740 , eight antennas  742  to  756  and a spatial mapping module  758 . The spatial mapping module  758  is configured to perform spatial mapping on a plurality of frequency domain MIMO data streams and produce eight frequency domain MIMO data streams. The phase rotation modules  760  to  774  are configured to rotate the phases of the sub-carriers of the eight frequency domain MIMO data streams. The inverse Fourier transform modules  776  and  790  are configured to transform the eight frequency domain MIMO data streams into eight time domain MIMO data streams. The CSD modules  710  to  724  are configured to perform CSD for the eight time domain MIMO data streams. The guard interval insertion modules  726  to  740  are configured to insert guard intervals into the eight time domain MIMO data streams. The antennas  742  to  456  are configured to broadcast the eight time domain MIMO data streams. 
     As shown in  FIG. 7 , the signal processing apparatus  700  is similar to the signal processing apparatus  400  shown in  FIG. 4  except that the spatial mapping procedure is performed before the phase rotation procedure. Correspondingly, a signal processing method for a MIMO system according to another embodiment of the present invention is similar to the signal processing method shown in  FIG. 5  except that the order of steps  506  and  508  is reversed. 
     In some embodiments of the present invention, the cyclic shift delay is performed in the frequency domain.  FIG. 8  shows a signal processing apparatus for a MIMO system according to one of such embodiments of the present invention. As shown in  FIG. 8 , the signal processing apparatus  800  comprises eight phase rotation and CSD modules  860  to  874 , eight inverse Fourier transform modules  876  to  890 , eight antennas  842  to  856  and a spatial mapping module  858 . The phase rotation and CSD modules  860  to  874  are configured to rotate the phases of the sub-carriers of eight frequency domain MIMO data streams and perform CSD for the eight frequency domain MIMO data streams. The spatial mapping module  858  is configured to perform spatial mapping on the eight frequency domain MIMO data streams. The inverse Fourier transform modules  876  to  890  are configured to transform the eight frequency domain MIMO data streams into eight time domain MIMO data streams. The guard interval insertion modules  826  to  840  are configured to insert guard intervals into the eight time domain MIMO data streams. The antennas  842  to  856  are configured to broadcast the eight time domain MIMO data streams. 
       FIG. 9  shows a signal pressing apparatus for a MIMO system according to another embodiment of the present invention. As shown in  FIG. 9 , the signal processing apparatus  900  comprises eight phase rotation and CSD modules  960  to  974 , eight inverse Fourier transform modules  976  to  990 , eight antennas  942  to  956  and a spatial mapping module  958 . The spatial mapping module  958  is configured to perform spatial mapping on a plurality of frequency domain MIMO data streams and produce eight frequency domain MIMO data streams. The phase rotation and CSD modules  860  to  874  are configured to rotate the phases of the sub-carriers of the eight frequency domain MIMO data streams and perform CSD for the eight frequency domain MIMO data streams. The inverse Fourier transform modules  876  to  890  are configured to transform the eight frequency domain MIMO data streams into eight time domain MIMO data streams. The guard interval insertion modules  826  to  840  are configured to insert guard intervals into the eight time domain MIMO data streams. The antennas  842  to  856  are configured to broadcast the eight time domain MIMO data streams. 
     As shown in  FIG. 9 , the signal processing apparatus  900  is similar to the signal processing apparatus  800  shown in  FIG. 8  except that the spatial mapping procedure is performed before the phase rotation and CSD procedure. Correspondingly, another two signal processing methods for a MIMO system according to some embodiments of the present invention are similar to the signal processing method shown in  FIG. 5  except that the order of steps are rearranged. It can be seen from  FIGS. 4 ,  7 ,  8  and  9  that the number of frequency domain MIMO data stream does not necessary equal to the number of streams after the spatial mapping procedure. However, the number of streams after the spatial mapping procedure equals to the number of time domain MIMO data stream.  FIG. 10  shows a signal processing apparatus for a MIMO system according to another embodiment of the present invention. As shown in  FIG. 10 , the signal processing apparatus  1000  comprises eight zero padding modules  1002  to  1016 , eight inverse Fourier transform modules  1018  to  1032 , eight CSD modules  1034  to  1048 , eight guard interval insertion modules  1050  to  1064  and eight antennas  1066  to  1080 . The zero padding modules  1002  to  1016  are configured to extend eight frequency domain MIMO data streams by padding zeroes at the beginning and the end of each frequency domain MIMO data stream. The inverse Fourier transform modules  1018  to  1032  are configured to transform the eight frequency domain MIMO data streams into eight time domain MIMO data streams. The CSD modules  1034  to  1048  are configured to perform CSD for the eight time domain MIMO data streams. The guard interval insertion modules  1050  to  1064  are configured to insert guard intervals into the eight time domain MIMO data streams. The antennas  1066  to  1080  are configured to broadcast the eight time domain MIMO data streams. 
       FIG. 11  shows the flow chart of a signal processing method for a MIMO system according to another embodiment of the present invention. In step  1102 , at least one frequency domain MIMO data stream is extended by padding zeroes at the beginning and the end of each of the at least one frequency domain MIMO data stream, and step  1104  is executed. In step  1104 , the at least one frequency domain MIMO stream is transformed into at least one time domain MIMO data stream, and step  806  is executed. In step  1106 , CSD process is performed for the at least one time domain MIMO data stream to produce a plurality of time domain MIMO data streams, wherein the amount of CSD is different for each of the time domain MIMO data streams. 
     The following illustrates how to apply the signal processing method shown in  FIG. 11  to the signal processing apparatus shown in  FIG. 10 . In a MIMO system compatible with the IEEE 802.11n standard, a plurality of frequency domain MIMO spatial streams are applied to the signal processing apparatus shown in  FIG. 10 . In this example, the frequency domain MIMO spatial streams comprise eight data streams. The frequency domain MIMO data streams can be categorized into three types: the 20 MHz type frequency domain MIMO data stream, which comprises 64 sub-carriers; the 40 MHz type frequency domain MIMO data stream, which comprises 128 sub-carriers; and the 80 MHz type frequency domain MIMO data stream, which comprises 256 sub-carriers. In this example, the eight frequency domain MIMO data streams are 20 MHz type frequency domain MIMO data streams. 
     In step  1102 , the eight zero padding modules  1002  to  1016  extend the eight frequency domain MIMO data streams by padding zeroes at the beginning and the end of each frequency domain MIMO data stream. In this embodiment, a total of 192 zeroes are padded to each of the frequency domain MIMO data stream, wherein half of them are padded at the beginning of the respective data streams, and the other half are padded at the end of the respective data streams. Accordingly, each frequency domain MIMO data stream is extended so as to have 256 subcarriers. In step  1104 , the inverse Fourier transform modules  1018  to  1032  transform the eight frequency domain MIMO data streams into eight time domain MIMO data streams respectively by performing 256-point inverse fast Fourier transform (IFFT) computations. In step  1106 , the CSD modules  1034  to  1048  perform CSD process for the eight time domain MIMO data streams, respectively. The following table shows some alternatives of CSD values: 
     
       
         
           
               
               
               
               
               
            
               
                   
               
               
                 Number 
                   
                   
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 of MIMO 
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 data 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 streams 
                 CSD1 
                 CSD2 
                 CSD3 
                 CSD4 
                 CSD5 
                 CSD6 
                 CSD7 
                 CSD8 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 1 
                 0 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 2 
                 0 
                 −200 
                 ns 
               
               
                 3 
                 0 
                 −100 
                 ns 
                 −200 
                 ns 
               
               
                 4 
                 0 
                 −50 
                 ns 
                 −100 
                 ns 
                 −200 
                 ns 
               
               
                 5 
                 0 
                 −50 
                 ns 
                 −100 
                 ns 
                 −150 
                 ns 
                 −200 ns 
               
               
                 6 
                 0 
                 −25 
                 ns 
                 −50 
                 ns 
                 −100 
                 ns 
                 −150 ns 
                 −200 ns 
               
               
                 7 
                 0 
                 −25 
                 ns 
                 −50 
                 ns 
                 −100 
                 ns 
                 −125 ns 
                 −150 ns 
                 −200 ns 
               
               
                 8 
                 0 
                 −25 
                 ns 
                 −50 
                 ns 
                 −75 
                 ns 
                 −100 ns 
                 −125 ns 
                 −150 ns 
                 −200 ns 
               
               
                   
               
            
           
         
       
     
     As shown in the above table, the minimum difference of the CSD is 25 ns. Since there are eight MIMO data streams, the last set of CSD is used. However, in some embodiments of the present invention, the number of MIMO data streams to be processed is not eight. In these embodiments, other sets of CSD in the above table may be used. Subsequently, the guard interval insertion modules  1050  to  1064  insert guard intervals into the eight time domain MIMO data streams, respectively. The antennas  1066  to  1080  then broadcast the eight time domain MIMO data streams. 
     It should be noted that in this embodiment, by extending the eight 20 MHz type frequency domain MIMO data streams to have 256 sub-carriers, the bandwidths of these frequency domain MIMO data streams, i.e. 80 MHz, are effectively increased. Accordingly, a CSD process with smaller minimum difference, such as the sampling rate of the MIMO system, 12.5 ns, can be performed. 
     It should be noted that the number of components of the signal processing apparatus provided by the present invention can be different from that of the signal processing apparatus shown in  FIG. 10 . For instance, the zero padding modules  1002  to  1016  can be combined into a single zero padding module, and the signal processing method shown in  FIG. 11  can still be applied. 
     In order to be compatible with the IEEE 802.11n standard, in some embodiments of the present invention, only one frequency domain MIMO data stream needs to be processed.  FIG. 12  shows a signal processing apparatus for a MIMO system according to yet another embodiment of the present invention. As shown in  FIG. 12 , the signal processing apparatus  1200  comprises a zero padding modules  1202 , an inverse Fourier transform modules  1204 , eight CSD modules  1234  to  1248 , eight guard interval insertion modules  1250  to  1264  and eight antennas  1266  to  1280 . The padding module  1202  is configured to a frequency domain MIMO data streams by padding zeroes at the beginning and the end of the frequency domain MIMO data stream. The inverse Fourier transform module  1204  is configured to transform the frequency domain MIMO data stream into a time domain MIMO data streams. The CSD modules  1234  to  1248  are configured to perform CSD for the time domain MIMO data stream with different cyclic shifts. The guard interval insertion modules  1250  to  1264  are configured to insert guard intervals into the eight time domain MIMO data streams. The antennas  1266  to  1280  are configured to broadcast the eight time domain MIMO data streams. In this embodiment, the frequency domain MIMO data stream is padded with zeroes, transformed into a time domain data stream, and then duplicated into a plurality of time domain MIMO data streams. Next, the CSD process can be performed on the plurality of time domain MIMO data streams, wherein the amount of the CSD is different for each of the time domain MIMO spatial streams. 
       FIG. 13  shows a signal processing apparatus for a MIMO system according to yet another embodiment of the present invention. As shown in  FIG. 13 , the signal processing apparatus  1300  comprises eight zero padding modules  1302  to  1316 , eight CSD modules  1318  to  1332 , eight inverse Fourier transform modules  1334  to  1348 , eight guard interval insertion modules  1350  to  1364  and eight antennas  1366  to  1380 . The zero padding modules  1302  to  1316  are configured to extend eight frequency domain MIMO data streams by padding zeroes at the beginning and the end of each frequency domain MIMO data stream. The CSD modules  1318  to  1332  are configured to perform CSD for the eight frequency domain MIMO data streams. The inverse Fourier transform modules  1334  to  1348  are configured to transform the eight frequency domain MIMO data streams into eight time domain MIMO data streams. The guard interval insertion modules  1350  to  1364  are configured to insert guard intervals into the eight time domain MIMO data streams. The antennas  1366  to  1380  are configured to broadcast the eight time domain MIMO data streams. 
     It can be seen from  FIG. 13  that the architecture of the signal processing apparatus  1300  is similar to that of the signal processing apparatus  1000 , except that the CSD procedure is performed in frequency domain in the signal processing apparatus  1000 . Accordingly, the CSD procedure in frequency domain is mainly to rotate the phases of the sub-carriers of the frequency domain MIMO data streams. In this embodiment, however, the CSD modules  1318  to  1332  are required only when better resolutions of the MIMO data streams are preferred. 
       FIG. 14  shows the flow chart of a signal processing method for a MIMO system according to yet another embodiment of the present invention. In step  1402 , at least one frequency domain MIMO data stream is extended by padding zeroes at the beginning and the end of each of the at least one frequency domain MIMO data stream, and step  804  is executed. In step  1404 , CSD process is performed for the at least one frequency domain MIMO data stream to produce a plurality of frequency domain MIMO data streams, and step  1406  is executed, wherein the amount of phase rotation is different for each of the frequency domain MIMO data streams. In step  1406 , the plurality of frequency domain MIMO streams are transformed into a plurality of time domain MIMO data streams. 
     It can be seen from  FIG. 14  that the signal processing method is similar to the signal processing method shown in  FIG. 12  except that the CSD procedure is performed in frequency domain. Accordingly, the CSD procedure in frequency domain is mainly to rotate the phases of the sub-carriers of the frequency domain MIMO data streams. Likewise, the zero padding procedure in step  1402  is performed only when better resolutions of the MIMO data streams are preferred. 
     In conclusion, the signal processing method and apparatus for a MIMO system of the present invention provide a unique solution when the number of applied antennas increases or the transmission bandwidth is extended. By processing the MIMO data streams to be transmitted in the frequency domain, the objective of the present invention is achieved. 
     The above-described embodiments of the present invention are intended to be illustrative only. Those skilled in the art may devise numerous alternative embodiments without departing from the scope of the following claims.