Abstract:
An apparatus for performing a scan procedure according to an orthogonal frequency division multiple access (OFDMA) signal and a mobile station comprising the same are provided. The characteristic of the apparatus is that it comprises two FIT input buffers. In a first period of a downlink subframe, the first FFT input buffer is used for data transmission. In a second period of the downlink subframe, the second FFT input buffer is used for storing more than one OFDMA sample in advance for further processing. In a third period of an uplink subframe, the stored OFDMA samples can be used to generate a refined frame boundary and calculates a CINR value of each of a plurality of neighboring base stations. Therefore, the apparatus and the MS can perform the scan procedure without downgrading the network efficiency.

Description:
This application claims the benefit of priority based on U.S. Ser. No. 60/977,439 filed on Oct. 4, 2007, the disclosures of which is incorporated herein by reference in its entirety. 
    
    
     CROSS-REFERENCES TO RELATED APPLICATIONS 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus, a method for performing a scan procedure, and a mobile station comprising the same. More specifically, the present invention relates to a mobile station, a method for performing a scan procedure according to an orthogonal frequency division multiple access (OFDMA) signal, and a mobile station comprising the same. 
     2. Descriptions of the Related Art 
     For OFDMA application over a wireless channel, a mobile station (MS) has to make a connection with a base station (BS) in order to be served. The BS which serves the MS is called the serving BS (SBS) and the other BSs that the MS is able to listen to are called the neighboring BSs (NBSs). 
     There are some occasions that an MS has to determine whether to switch from its current SBS to one of its NBSs. One of the occasions is when the MS detects that the connection with its SBS is getting worse, and the MS performs a scan procedure to find one of the NBSs to switch to. Another occasion is that the MS keeps monitoring the status of all the NBSs, i.e. the MS periodically performs the scan procedure, and then determines wither to switch from the current SBS to one of its NBSs according to the result of the scan procedure. 
     A scan procedure is used by an MS to determine whether to switch the SBS, which measures the quality of the physical layer connection with each of the NBSs. The most important factor representing the connection quality is the channel interference and noise ratio (CINR). To measure the CINR of each NBS, an MS must have the identity of the targeting NBS, wherein the identity of the NBS is given by a SBS. When the OFDMA application conforms to the WiMAX standard, the identity is the parameter CELL_ID defined in the WiMAX standard. 
     A conventional system for performing a scan procedure utilizes one FFT input buffer, thus the data transmission is often temporary suspended during the transmission. The conventional system may not be able to continue the data transmission until the scan procedure has been done. Thus, the performance of the conventional system is degraded while considering the fact of the delay of the scan procedure. 
     Please refer to  FIG. 1  for better understanding.  FIG. 1  illustrates a timing diagram of the conventional system, wherein the time periods  18   a ,  18   c  correspond to downlink subframes, the time periods  18   b ,  18   d  correspond to uplink subframes, signal BW 0  indicates the buffer read of a single FFT buffer, signal BR 0  indicates buffer write of the single FFT buffer, signal PP 0  indicates the signal of the post-FFT processing, signal CINRM 0  indicates the signal of the CINR measurement, and signal FFTC 0  indicates the signal of the FFT. 
     It is noted that the generations of the refined frame boundaries and the CINR values for NBSs happen at the beginning of each downlink sub-frame, which is shown by the toggles of the signals BR 0 , BW 0 , PP 0 , CINRM 0 , FFTC 0  at the beginning of the downlink subframe  18   a ,  18   c . After the generations of the refined frame boundaries and the calculations of the CINRs for NBSs, the conventional system continues to transmit data. From  FIG. 1 , it is shown that the single FFT buffer, the post-FFT processing, and the CINR measurement are not able to perform effective post-processing during the period correspond to uplink subframe  18   b ,  18   d.    
     Form the viewpoint of the hardware, the scan procedure is no different from a normal receive, so no extra hardware is required. However, before starting the scan procedure, the convention system has to request a period to its SBS, since it will lose track of the SBS during the scan procedure. That is, the data transmission is delayed by the scan procedure. Thus, the request/grant procedure for the scan procedure will downgrade the network efficiency. 
     Other conventional systems are to duplicate CINR measurement for measuring CINR values of different NBSs. The post-FFT processing does not need to generate a refined frame boundary for each NBS. Each of the CINR measurement are measured of the CINR value of the corresponding NBS based on the frame boundary of the SBS but not the refined frame boundary of the NBS. Therefore, the network efficiency is not downgraded when this scan procedure is applied. Regarding to this system, the quality of the CINR measurement suffers from the frame boundary mismatch, especially when it is a large delay spread channel. Moreover, this system raises the cost for utilizing extra CINR measurement modules. 
     According to the aforementioned description, it is desirable to provide a technique that can perform a scan procedure without downgrading the network efficiency. 
     SUMMARY OF THE INVENTION 
     An objective of this invention is to provide an apparatus for performing a scan procedure according to an OFDMA signal. The processing module comprises a first fast Fourier transform (FFT) input buffer, a second FFT input buffer, an FFT core module, a post-FFT processing module, and a channel interference and noise ratio (CINR) measurement module. The first FFT input buffer is configured to contiguously gather a first predetermined number of OFDMA samples related to the OFDMA signal during a first period corresponding to a downlink subframe. The second FFT input buffer is configured to gather a second predetermined number of OFDMA samples related to the OFDMA signal during a second period corresponding to the downlink subframe. The FFT core module is configure to contiguously apply FFT to the first predetermined number of OFDMA samples during the first period and apply FFT to the second predetermined number of OFDMA samples during a third period corresponding to an uplink subframe, the uplink subframe occurs later than the downlink subframe. 
     Another objective of this invention is to provide a mobile station comprising an antenna for receiving an OFDM signal and a processing module for performing a scan procedure according to the OFDMA signal. The processing module comprises an first FFT input buffer, a second FFT input buffer, an FFT core module, a post-FFT processing module, and a CINR measurement module. The first FFT input buffer is configured to contiguously gather a first predetermined number of OFDMA samples related to the OFDMA signal during a first period corresponding to a downlink subframe. The second FFT input buffer is configured to gather a second predetermined number of OFDMA samples related to the OFDMA signal during a second period corresponding to the downlink subframe. The FFT core module is configure to contiguously apply FFT to the first predetermined number of OFDMA samples during the first period and apply FFT to the second predetermined number of OFDMA samples during a third period corresponding to an uplink subframe, the uplink subframe occurs later than the downlink subframe. 
     Another objective of this invention is to provide a method for performing a scan procedure according to an OFDMA signal. The method comprises the steps of: (a) gathering a first predetermined number of OFDMA samples related to the OFDMA signal contiguously during a first period corresponding to a downlink subframe; (b) gathering a second predetermined number of OFDMA samples related to the OFDMA signal during a second period corresponding to the downlink subframe; (c) applying FFT to the first predetermined number of OFDMA samples contiguously during the first period; (d) applying FFT to the second predetermined number of OFDMA samples during a third period corresponding to an uplink subframe occurring later than the downlink subframe; (e) measuring a frame boundary of an SBS during the first period; (f) measuring a refined frame boundary of at least one NBS according to the second predetermined number of OFDMA samples and the frame boundary during the third period; and (g) calculating a CINR value of the at least one NBS based on the refined frame boundary. 
     By adding the second FFT input buffer, the present invention can store more than one OFDMA sample during the second period of the downlink subframe. Then in the third period of uplink subframe, the present invention can retrieve the stored OFDMA samples to generate a CINR values for each of a plurality of NBSs of the apparatus and/or the MS according to the stored sample. In this way, the present invention can achieve the purposes of measuring a CINR value of each of the NBSs without downgrading the network efficiency. With the aforementioned arrangement, the present invention solves the disadvantages of the convention techniques and still has good performance. 
     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 
         FIG. 1  illustrates a timing diagram of a conventional system; 
         FIG. 2  is a block diagram of the mobile station of the first embodiment of the present invention; 
         FIG. 3  is a schematic view of the sub-signals and the capture windows; 
         FIG. 4  is an exemplary timing diagram for the mobile station of the present invention; and 
         FIG. 5  is a flowchart of the second embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the descriptions that follow, the present invention will be described in reference to descriptions and examples about performing a scan procedure over a wireless channel. However, descriptions and examples of the invention are not limited to any particular environment, application, or implementation. Therefore, the descriptions that follow are for the purposes of illustration and not limitation. 
     A first embodiment of the present invention is a mobile station (MS)  3 , whose block diagram is illustrated in  FIG. 2 . The MS  3  is served by a serving base station (SBS). Meantime, there are a plurality of neighboring BSs (NBSs) not serving the MS  3  but being listened by the MS  3 . The MS  3  has the capacity of measuring the channel interference and noise ratio (CINR) value for each of the NBSs so that it can switch from the current SBS to one of its NBSs according to the measured CINR values for some occasions. 
     The MS  3  comprises an antenna  30  and a processing module  31 . The antenna  30  is used for receiving an OFDM signal, while the processing module  31  is used for performing a scan procedure according to the OFDMA signal. The processing module  31  comprises a receiving filter  310 , a pre-Fast Fourier Transform (FFT) processing module  311 , a first FFT input buffer  312 , a second FFT input buffer  313 , an FFT core module  314 , a post-FFT processing module  315 , and a CINR measurement module  316 . 
     First, the antenna  30  receives the OFDMA signal, which comprises a sub-signal from each of the SBS and the NBSs. That is, the OFDMA signal is substantially a mixed signal of the sub-signals from the SBS and the NBSs. The receiving filter  310  filters the OFDMA signal. After filtering, the pre-FFT processing module  311  processes the OFDMA signal. More specifically, the pre-FFT processing module  311  is used for processing the OFDMA signal before the first FFT input buffer  312  gathers a first predetermined number of OFDMA sample and the second FFT input buffer  313  gathers a second predetermined number of OFDMA samples. The roles of the first FFT input buffer  312  and the second FFT input buffer  313  are different, so they are described separately in the following paragraphs. 
     The first FFT input buffer  312  and data flows related to the first FFT input buffer  312  are described first. The size of the first FFT input buffer  312  is equal to the first predetermined number of OFDMA samples. The first FFT input buffer  312  is used for contiguously gathering the first predetermined number of OFDMA samples related to the OFDMA signal during a first period corresponding to a downlink subframe. It means that the first input buffer  312  contiguously gathers the first predetermined number of OFDMA samples from the signal outputted from the pre-FFT processing module  311 . In this embodiment, the first predetermined number is one symbol. Hence, the first FFT input buffer  312  is used for gathering one OFDMA sample for FIT processing. In addition, the first FFT input buffer  312  forward the captured OFDMA preamble to FFT core module  314 , and the preamble is overwritten by the incoming OFDMA samples. Please note that the preamble in the present invention is referred to the first sample of a plurality of OFDMA samples in one frame. 
     The FFT core module  314  is used for contiguously applying FFT to the first predetermined number of OFDMA samples during the first period. The post-FFT processing module  315  is used for measuring a frame boundary of the SBS during the first period. After the frame boundary of the SBS has been measured, the first FFT input buffer  312  gathers the first predetermined number of OFDMA samples (i.e. gathers next OFDMA sample), applies FFT to the first predetermined number of OFDMA samples again during the first period corresponds to the downlink subframe. It means that after the frame boundary of the SBS has been measured, the first FIT input buffer  312  and the FFT core module  314  perform data transmissions during the first period corresponding to the downlink subframe. 
     Then, the second FFT input buffer  313  and data flows related to the second FFT input buffer  313  are described. The size of the second FFT input buffer  313  is equal to the second predetermined number of OFDMA samples. The second FFT input buffer  313  is used for gathering the second predetermined number of OFDMA samples related to the OFDMA signal during a second period corresponding to the downlink subframe. It means that the second FFT input buffer  313  gathers the second predetermined number of OFDMA samples from the signal outputted from the pre-FFT processing module  22 . The second period and the first period may overlap. In this embodiment, the second predetermined number is greater than one. Hence, the second FIT input buffer  313  is used for gathering more than one OFDMA sample (such as 1.5 OFDMA samples) for FIT processing. 
     In the present invention, the second predetermined number should be slightly larger than the first predetermined number because the size of the second FFT input buffer  313  has to be large enough to tolerate the difference of frame boundaries between the SBS and NBSs. The size of the second FFT input buffer  313  needs to cover the frame boundary window of SBS and all NBSs, while the size of the first FFT input buffer  312  may only need to cover the frame boundary window of SBS. Since the MS  3  receives the data from the SBS and NBSs generally within the different arrival times, the size of the second FFT input buffer  313  needs to be large enough to cover all of the SBS and NBSs frame boundaries. 
     Please refers to  FIG. 3 , which is a schematic view of the sub-signals and the capture windows. The MS  3  receives the sub-signal  40  from the SBS and the sub-signals from all the NBSs; specifically, sub-signal  41  from the first NBS and sub-signal  42  from the second NBS. Each of the sub-signals  40 ,  41 ,  42  comprises a plurality of frames, and each of the frames begins with a cyclic prefix CP and comprises a preamble, a downlink sub-frame and an uplink sub-frame. 
     The first predetermined numbers  410  indicates the minimum size of the first input buffer  312 . The second predetermined number  420  indicates the minimum size of the second input buffer  313 . The window  411  for the frame boundary of the sub-signal  40  from the SBS is equal to the size of the first predetermined number  410 , which is the size of first input buffer  312  in the embodiment of the present invention. The second predetermined number  420 , as the size of the second input buffer  313 , needs to cover the windows for the frame boundaries of the sub-signals from the SBS and all NBSs. The frame boundary of the sub-signal  41  from the first NBS occurs in the window  415 , while the frame boundary of the sub-signal  42  from the second NBS occurs in the window  416 . Thus, the second predetermined number  420  needs to be large enough to cover the windows  411 ,  415 , and  416  in the embodiment of the present invention. The second predetermined number  420  may be interpreted as the window from the earliest arriving frame start-point to the latest frame end-point. In this case, the earliest arriving frame start-point is from the second NBS and the latest arriving frame end-point is from the first NBS. 
     The second predetermined number of OFDMA samples comprises a preamble of the OFDMA signal. After the second FFT input buffer  313  has gathered the second predetermined number of OFDMA samples, it stops and does not change its content during a rest period corresponding to the downlink subframe. Comparing to the first input buffer  312 , the preamble data of the OFDMA sample in the first FFT input buffer  312  is contiguously overwritten by the new incoming OFDMA samples. 
     Then, during a third period corresponding to an uplink subframe occurring later than the downlink subframe, the FFT core module  314  applies FFT to the second predetermined number of OFDMA samples. The post-FFT processing module  315  measures a refined frame boundary of each of the NBSs according to the second predetermined number of OFDMA samples, the identities of the NBSs, and the frame boundary during the third period. To be more specific, the sub-signal from the SBS mixed in the OFDMA signal comprises an identity of each of the NBSs; therefore, the MS  3  has the information of the identities of the NBSs. The CINR measurement module  316  calculates a CINR value of each of the NBSs based on the refined frame boundary and according to the corresponding NBS identity. 
     In this embodiment, the MS  3 , the SBS, and the NBSs conform to the WiMAX standard and the identities are CELL_IDs defined in the WiMAX standard. In the other embodiments, the MS  3 , the SBS, and the NBSs may conform to the other wireless network standards. 
     Please refer to  FIG. 4  for better understanding.  FIG. 4  illustrates an exemplary timing diagram of the MS  3 , wherein the time periods  38   a ,  38   c  correspond to downlink subframes, the time periods  38   b ,  38   d  correspond to uplink subframes, signal BW 1  indicates the buffer read of the first FFT input buffer  312 , signal BR 1  indicates buffer write of the first FFT input buffer  312 , signal BW 2  indicates the buffer write of the second FFT input buffer  313 , signal BR 2  indicates the buffer read of the second FFT input buffer  313 , signal PP indicates the signal of the post-FFT processing module  315 , signal CINRM indicates the signal of the CINR measurement module  316 , and signal FFTC indicates the signal of the FFT core module  314 . 
     From the signals BR 1 , BW 1 , it can bee seen that the buffer read and the buffer write of the first FFT input buffer  312  are similar to those of the FFT input buffers  13  in the conventional system  1 . It means that the first input buffer  312  performs both buffer read and buffer write during a period correspond to the downlink subframes  38   a ,  38   c . From the signal FFTC, the operations performs by the FFT core module  314  is similar to those of the FFT core module  14  in the conventional system  1 . On the other hand, it is noted that the signal BW 2  only toggles at the beginning of the downlink subframe  38   a . It means that the second FFT input buffer  313  gathers the second predetermined number of OFDMA samples for future process, and it stops after the second predetermined number of OFDMA samples have been gathered. 
     In the uplink subframe  38   b , the signal BR 2  toggles several times, meaning that the OFDMA samples in the second FFT input buffer  313  are read out to generate a refined frame boundary for each of the NBSs and to calculate the CINR for each of the NBSs. To be more specific, the toggle  301  of the signal BR 2  means that the OFDMA samples in the second FFT input buffer  34  are read out, and the toggle  302  of the signal PP means that the post-FFT processing module  315  generates a refined frame boundary for one of the NBSs according to the frame boundary of the SBS and the identity of the NBS. Then, the toggle  303  of the signal BR 2  means that the OFDMA samples in the second FFT input buffer  34  are read out again, and the toggle  304  of the signal CINRM means that the CINR measurement module  316  generates the CINR value of the selected NBS according to its refined frame boundary. Next, the processing module  31  selects another NBS by indicating the corresponding identity. Then, the toggle  305  of the signal BR 2 , the toggle  306  of the signal PP, the toggle  307  of the signal BR 2 , and the toggle  308  of the signal CINRM are for another NBS, and the toggles  305 ,  306 ,  307 ,  308  play the same roles as the toggles  301 ,  302 ,  303 ,  304 , respectively. After deriving the CINRs of the NBSs, the processing module  31  may determine whether to switch its SBS from the current one to one of its NBSs according to the CINR values. 
     It is noted that an MS may only have one NBS in some other embodiments. For MS only has one NBS, the MS perform the aforementioned operations for that single NBS. In addition, the processing module may operate with other antennas in other embodiments. 
     A second embodiment of the present invention is illustrated in  FIG. 5 , which is a flowchart of a method for performing a scan procedure according to an OFDMA signal. First, step  501  is executed to gather a second predetermined number of OFDMA samples related to the OFDMA signal during a second period corresponding to the downlink subframe. Then, step  502  is executed to contiguously gather a first predetermined number of OFDMA samples related to the OFDMA signal during a first period corresponding to a downlink subframe. In this embodiment, the first predetermined number of OFDMA samples comprise a preamble of the OFDMA signal, and the second predetermined number is greater than the first predetermined number. It is noted, the method may begin the step  501  and step  502  simultaneously. In that case, the step  501  is finished before the step  502 . 
     Step  503  is executed to contiguously apply FFT to the first predetermined number of OFDMA samples during the first period. Then, step  504  is executed to measure a frame boundary of an SBS during the first period. Step  505  is executed to apply FFT to the second predetermined number of OFDMA samples during a third period corresponding to an uplink subframe occurring later than the downlink subframe. Step  506  is executed to measure a refined frame boundary of at least one NBS according to the second predetermined number of OFDMA samples and the frame boundary during the third period. Finally, Step  507  is executed to calculate a CINR value of the at least one NBS based on the refined frame boundary. 
     In addition to the aforesaid steps, the second embodiment can also execute all the operations and functions of the first embodiment. Those skilled in the art will readily know how the second embodiment executes the corresponding operations and functions based on the explanation of the first embodiment, and thus, no further description will be given herein. 
     Comparing with the conventional system, the MS  3  is equipped with the additional second FFT input buffer  313  to store the second predetermined number of OFDMA samples in the second period corresponding to downlink subframe. It is noted that the size of the second FFT input buffer  313  should be large enough to tolerate the difference of frame boundaries between an SBS and an NBS. In addition, every time the MS  3  performs a scan procedure, it calculates the frame boundary of an SBS. Therefore the MS  3  can overcome the disadvantage of the frame boundary mismatch of the large delay spread channel, and the performance is superior to the conventional system. 
     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.