Abstract:
A method including: receiving, through a wireless channel, a plurality of modulated signals at a plurality of antennas, wherein each antenna receives a corresponding modulated signal; generating a plurality of autocorrelated signals by autocorrelating the plurality of modulated signals; determining whether a signal strength associated with each modulated signal is (i) below a threshold or (ii) above the threshold; for each modulated signal having a signal strength below the threshold, disabling the antenna that received the modulated signal having the signal strength below the threshold; combining the modulated signals having a signal strength above the threshold; generating weighted autocorrelated signals based on (i) the plurality of autocorrelated signals and (ii) the combined modulated signals; generating a combined weighted signal by summing the weighted autocorrelation signals; demodulating the combined weighted signal; and determining a state of the wireless channel based on the demodulation of the combined weighted signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 11/642,442 (now U.S. Pat. No. 7,924,930), filed on Dec. 20, 2006, which claims the benefit of: U.S. Provisional Application No. 60/773,591, filed on Feb. 15, 2006, and U.S. Provisional Application No. 60/776,102, filed on Feb. 23, 2006. The disclosures of the applications referenced above are incorporated herein by reference. 
    
    
     FIELD 
     The present disclosure relates to synchronization and detection mechanisms for orthogonal frequency-division multiplexing (OFDM) receivers in wireless local area network (WLAN) systems. 
     BACKGROUND 
     In OFDM WLAN systems, such as those specified by IEEE Standards 802.11a, 802.11g, 802.11n, and 802.16, performance suffers from the presence of a carrier frequency offset. This is due to the fact that the OFDM subcarriers are spaced closely in frequency. Imperfect frequency synchronization causes a loss in subcarrier orthogonality which severely degrades performance. 
     Referring now to  FIG. 1 , an OFDM receiver system  10  is shown. Antennas  12 - 1 ,  12 - 2 , . . . , and  12 - n  (referred to collectively as antennas  12 ) receive signals and pass the signals through low pass filters  14 - 1 ,  14 - 2 , . . . , and  14 - n  (referred to collectively as low pass filters  14 ). The low pass filters  14  block harmonic emissions which might cause interference with other communications. After being passed through the low pass filters  14 , the signals are sent to autocorrelators  16 - 1 ,  16 - 2 , . . . , and  16 - n  (referred to collectively as autocorrelators  16 ). The autocorrelators  16  find repeating patterns in a signal, such as determining the presence of a periodic signal which has been buried under noise. The signals from the autocorrelators  16  are then combined and sent to a demodulator  18 . The demodulator  18  is used to recover the information content from the carrier waves of the signals. 
     SUMMARY 
     In general, in one aspect this specification describes an orthogonal frequency-division multiplexing (OFDM) receiver system and method. The method includes: receiving, through a wireless channel, a plurality of modulated signals at a plurality of antennas, wherein each antenna receives a corresponding modulated signal; generating a plurality of autocorrelated signals by autocorrelating the plurality of modulated signals received by the plurality of antennas; determining whether a signal strength associated with each modulated signal received by the plurality of antennas is (i) below a threshold or (ii) above the threshold; for each modulated signal having a signal strength below the threshold, disabling the antenna that received the modulated signal having the signal strength below the threshold; combining the modulated signals having a signal strength above the threshold; generating weighted autocorrelated signals based on (i) the plurality of autocorrelated signals and (ii) the combined modulated signals; generating a combined weighted signal by summing the weighted autocorrelation signals; demodulating the combined weighted signal; and determining a state of the wireless channel based on the demodulation of the combined weighted signal. 
     Further areas of applicability of the present disclosure will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the disclosure, are intended for purposes of illustration only and are not intended to limit the scope of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a functional block diagram of an OFDM receiver system. 
         FIG. 2A  is a functional block diagram of an OFDM receiver system incorporating a synchronization module and a first embodiment of an independent assessment module. 
         FIG. 2B  is a functional block diagram of an OFDM receiver system incorporating a synchronization module and a second embodiment of an independent assessment module. 
         FIG. 3A  is a functional block diagram of the synchronization module incorporating signal strength modules, a signal combination module, and a weighted signal generator module. 
         FIG. 3B  is a functional block diagram of the synchronization module according to one aspect of the present invention. 
         FIG. 4  is a functional block diagram of a signal strength module. 
         FIG. 5A  is a functional block diagram of the signal combination module according to one aspect of the present invention. 
         FIG. 5B  is a functional block diagram of the signal combination module according to one aspect of the present invention. 
         FIG. 5C  is a functional block diagram of the signal combination module according to one aspect of the present invention. 
         FIG. 6A  is a functional diagram of the independent assessment module incorporating multiple clear channel assessment modules. 
         FIG. 6B  is a functional diagram of the independent assessment module incorporating a single clear channel assessment module; 
         FIG. 7  is a functional block diagram of a clear channel assessment module. 
         FIG. 8  is a plot illustrating the delay between automatic gain control (AGC) unlock and antenna selection. 
         FIG. 9  is a flow chart illustrating the antenna selection process. 
         FIG. 10A  is a functional block diagram of a high definition television; 
         FIG. 10B  is a functional block diagram of a vehicle control system; 
         FIG. 10C  is a functional block diagram of a cellular phone; 
         FIG. 10D  is a functional block diagram of a set top box; and 
         FIG. 10E  is a functional block diagram of a media player. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is in no way intended to limit the disclosure, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements. As used herein, the term module, circuit and/or device refers to an Application Specific Integrated Circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality. As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A or B or C), using a non-exclusive logical or. It should be understood that steps within a method may be executed in different order without altering the principles of the present disclosure. While embodiments of the present invention are discussed in terms of OFDM WLAN signals such as those specified by IEEE Standards 802.11a, 802.11g, 802.11n, and 802.16, other embodiments employ other signals, including point-to-point signals as well as network signals. 
     Referring now to  FIG. 2A , an OFDM receiver system  20  incorporating a synchronization module  22  and an independent assessment module  24  (e.g. a carrier sense detect module) is shown. Receiver antennas  26 - 1 ,  26 - 2 , . . . , and  26 - n  (referred to collectively as receive antennas  26 ) receive signals and pass those signals through low pass filters  28 - 1 ,  28 - 2 , . . . , and  28 - n  (referred to collectively as low pass filters  28 ). The low pass filters  28  block harmonic emissions which might cause interference with other communications. After being passed through the low pass filters  28 , the filtered signals F 1 , F 2 , . . . , and F n  are sent to autocorrelators  30 - 1 ,  30 - 2 , . . . , and  30 - n  (referred to collectively as autocorrelators  30 ) and to the synchronization module  22 . The autocorrelators  30  find repeating patterns in a signal. For example, the autocorrelators  30  determine the presence of a periodic signal that has been buried under noise. The autocorrelators  30  then send autocorrelated signals A 1 , A 2 , . . . , and A n  to the synchronization module  22  and to the independent assessment module  24 . 
     The synchronization module  22  measures the strength of the signals and combines the signals. The synchronization module  22  outputs weighted autocorrelated signals W 1 , W 2 , . . . , and W n . The weighted signals W 1 , W 2 , . . . , and W n  are summed using a summing module  32  to form a combined weighted signal W C . The combined weighted signal W C  is then sent to the independent assessment module  24 . The independent assessment module  24  demodulates the autocorrelated signals A 1 , A 2 , . . . , and A n  and the combined weighted signal W C . The independent assessment module  24  then outputs an effective signal CCA EFF . The effective signal CCA EFF  is sent to an effective clear channel assessment module  34 . The effective clear channel assessment module  34  then determines what action the system is to take based on the effective signal CCA EFF . 
     Referring now to  FIG. 2B , the independent assessment module  24  receives the combined weighted signal W C  from the summing module  32 . In the present implementation, the independent assessment module  24  does not receive the autocorrelated signals A 1 , A 2 , . . . , and A n  from the autocorrelators  30 . The independent assessment module  24  demodulates the combined weighted signal W C  and outputs the effective signal CCA EFF  accordingly. The effective signal CCA EFF  is sent to the effective clear channel assessment module  34 . In other words, in the present implementation, the independent assessment module determines the effective signal CCA EFF  based only on the combined weighted signal W C . 
     Referring now to  FIG. 3A , a synchronization module  22  incorporating signal strength modules  36 - 1 ,  36 - 2 , . . . , and  36 - n  (referred to collectively as signal strength modules  36 ), a signal combination module  38 , and a weighted signal generator module  40  is shown. Filtered signals F 1 , F 2 , . . . , and F n  are passed through the signal strength modules  36 . The signal strength modules  36  measure the strength of each signal and output strength signals S 1 , S 2 , . . . , and S n . The strength signals S 1 , S 2 , . . . , and S n  are sent to the signal combination module  38 . The signal combination module  38  combines the signals S 1 , S 2 , . . . , and S n . The signal combination module  38  outputs combination signals C 1 , C 2 , and C n , which are used, for example, as combining weights. The combination signals C 1 , C 2 , . . . , and C n  and the autocorrelated signals A 1 , A 2 , . . . , and A n  are sent to the weighted signal generator module  40 . The weighted signal generator module  40  generates weighted autocorrelation signals W 1 , W 2 , . . . , and W n . The outputs of the weighted signal generator module  40  and the outputs of the synchronization module  22  are the weighted signals W 1 , W 2 , . . . , and W n . 
     Referring now to  FIG. 3B , a synchronization module  22  incorporating signal strength modules  36 - 1 ,  36 - 2 , . . . , and  36 - n  (referred to collectively as signal strength modules  36 ), a signal combination module  38 , and a logical AND gates is shown. Filtered signals F 1 , F 2 , . . . , and F n  are passed through the signal strength modules  36 . The signal strength modules  36  measure the strength of each signal and output strength signals S 1 , S 2 , . . . , and S n . The strength signals S 1 , S 2 , . . . , and S n  are sent to the signal combination module  38 . The signal combination module  38  combines the signals S 1 , S 2 , . . . , and S n . The signal combination module  38  outputs combination signals C 1 , C 2 , . . . , and C n . The combination signals C 1 , C 2 , . . . , and C n  and the autocorrelated signals A 1 , A 2 , . . . , and A n  are sent to the logical AND gates. The logical AND gates output weighted autocorrelation signals W 1 , W 2 , . . . , and W n . The outputs of the synchronization module  22  are the weighted signals W 1 , W 2 , . . . , and W n . 
     Referring now to  FIG. 4 , a signal strength module  36  is shown. A filtered signal F x  is input to the signal strength module  36 . The power of filtered signal F x  is determined and passed through an error adjustment feedback loop. The error adjustment feedback loop contains a D flip-flop to store a previous power value. The output of the error adjustment feedback loop (i.e. the output of the signal strength module  36 ) is a strength signal S x . In other words, the signal strength module  36  acts as a low pass filter for filtered signal F x . 
     Referring now to  FIG. 5A , a signal combination module  38  incorporating a weighted combining module  42 , a selection and equal gain control (EGC) module  44 , and a control module  46  is shown. Strength signals S 1 , S 2 , . . . , and S n  are input to the weighted combining module  42  and the selection and EGC module  44 . The weighted combining module  42  relates each of the strength signals S 1 , S 2 , . . . , and S n  to the maximum value of the strength signals S 1 , S 2 , . . . , and S n  to obtain weight signals with values ranging from 0 to 1, where 1 is the maximum weight. The selection and EGC module  44  disables receive antennas  26  (as shown in  FIG. 2A ) that are in deep fade, and equal gain combines the receive antennas  26  that remain enabled. The receive antennas  26  are in deep fade when the corresponding strength signals S 1 , S 2 , . . . , and S n  are below a threshold. Both the weighted combining module  42  and the selection and EGC module  44  output sets of combined signals to the control module  46 . The control module  46  outputs combination signals C 1 , C 2 , . . . , and C n . The combination signals C 1 , C 2 , . . . , and C n  represent the passing of the signals output by either the weighted combining module  42  or the selection and EGC module  44 , or a combination of the signals output by both the weighted combining module  42  and the selection and EGC module  44 . The combination signals C 1 , C 2 , . . . , and C n  are then output by the signal combination module  38 . 
     Referring now to  FIG. 5B , a signal combination module  38  incorporating a weighted combining module  42  is shown. Strength signals S 1 , S 2 , . . . , and S n  are input to the weighted combining module  42 . The weighted combining module  42  relates each of the strength signals S 1 , S 2 , . . . , and S n  to the maximum value of the strength signals S 1 , S 2 , . . . , and S n  to obtain weight signals with values ranging from 0 to 1, where 1 is the maximum weight. The weighted combining module  42  outputs the combination signals C 1 , C 2 , . . . , and C n , which are then output by the signal combination module  38 . 
     Referring now to  FIG. 5C , a signal combination module  38  incorporating a selection and EGC module  44  is shown. Strength signals S 1 , S 2 , . . . , and S n  are input to the selection and EGC module  44 . The selection and EGC module  44  disables receive antennas  26  (as shown in  FIG. 2A ) that are in deep fade, and equal gain combines the receive antennas  26  that remain enabled. The receive antennas  26  are in deep fade when the corresponding strength signals S 1 , S 2 , . . . , and S n  are below a threshold. The selection and EGC module  44  outputs the combination signals C 1 , C 2 , . . . , and C n , which are then output by the signal combination module  38 . 
     Referring now to  FIG. 6A , an independent assessment module  24  as described in  FIG. 2A  includes Clear Channel Assessment (CCA) modules  52 - 1 ,  52 - 2 , . . . ,  52 - n , and  52 -C (referred to collectively as CCA modules  52 ). A combined weight signal W C  and the autocorrelated signals A 1 , A 2 , . . . , and A n  are input to the CCA modules  52 , which determine the states of each channel and accordingly allow or defer data transmission. The CCA modules  52  output signals CCA 1 , CCA 2 , CCA n  and CCA C . The signals CCA 1 , CCA 2 , CCA n  and CCA c  are passed through a logical OR gate  54 . The output of the logical OR gate  54  and the independent assessment module  24  is the effective signal CCA EFF . 
     Referring now to  FIG. 6B , an independent assessment module as described in  FIG. 2B  includes only the CCA module  52 -C. The CCA module  52 -C receives the combined weight signal W C , determines the state of a channel, and allows or defers data transmission accordingly. 
     Referring now to  FIG. 7 , a clear channel assessment (CCA) module  52  is shown. The CCA module determines the state of a channel and accordingly allows or defers data transmission. When the received signal strength is below a specified threshold the channel is declared clear. For example, a media access control (MAC) device (not shown) may receive a channel status signal from the CCA module  52 . When the received signal strength is above the threshold, data transmissions are deferred in accordance with the protocol rules. 
     Referring now to  FIG. 8 , a diagram illustrating the delay between automatic gain control (AGC) unlock and antenna selection is shown and is generally designated  60 . When AGC unlock occurs, a set period of time must expire before antenna selection can occur. This is required to prevent the selection and EGC module  44  (as shown in  FIG. 5 ) from incorrectly disabling one of the receive antennas  26 - 1 ,  26 - 2 , . . . , and  26 - n  (as shown in  FIG. 2A ). For example, the EGC module  44  may include a timer (not shown) to determine when the set period of time has expired. 
     Referring now to  FIG. 9 , steps performed by the selection and EGC module  44  are shown in further detail and are generally designated  70 . Control begins with step  72 . In step  74 , the selection and EGC module  44  determines whether automatic gain control (AGC) is unlocked. If false, control loops back to step  72 . If true, control continues with step  76  where it is determined whether the required delay before antenna selection has expired. If false, control loops back to step  74 . If true, control continues with step  78  where a strongest antenna is determined using a power meter. Control then proceeds to step  80  where the signal-to-noise ratio threshold (SNR TH ) is determined using the signal-to-noise ratio (SNR). Control then proceeds to step  82  where the power threshold (P TH ) is determined using the signal-to-noise threshold (SNR TH ). Control then proceeds to step  84  where the number of effective antennas is set to the number of receive antennas used. Control then proceeds to step  86  where it is determined whether the maximum power is greater than the power of the current antenna. If false, control loops back to step  74 . If true, control continues with step  88  where the current antenna is disabled and the number of effective antennas is reduced by one. Control then loops back to step  74 . 
     Referring now to  FIGS. 10A-10E , various exemplary implementations of the OFDM receiver system are shown. 
     Referring now to  FIG. 10A , the OFDM receiver system can be implemented in a high definition television (HDTV)  420 . For example, the OFDM receiver system could be implemented in a WLAN interface of the HDTV  422 . The HDTV  420  receives HDTV input signals in either a wired or wireless format and generates HDTV output signals for a display  426 . In some implementations, signal processing circuit and/or control circuit  422  and/or other circuits (not shown) of the HDTV  420  may process data, perform coding and/or encryption, perform calculations, format data and/or perform any other type of HDTV processing that may be required. 
     The HDTV  420  may communicate with mass data storage  427  that stores data in a nonvolatile manner such as optical and/or magnetic storage devices for example hard disk drives HDD and/or DVDs. The HDD may be a mini HDD that includes one or more platters having a diameter that is smaller than approximately 1.8″. The HDTV  420  may be connected to memory  428  such as RAM, ROM, low latency nonvolatile memory such as flash memory and/or other suitable electronic data storage. The HDTV  420  also may support connections with a WLAN via a WLAN network interface  429 . 
     Referring now to  FIG. 10B , the OFDM receiver system may implement and/or be implemented in a WLAN interface of a vehicle  430 . In some implementations, the OFDM receiver system implement a powertrain control system  432  that receives inputs from one or more sensors such as temperature sensors, pressure sensors, rotational sensors, airflow sensors and/or any other suitable sensors and/or that generates one or more output control signals such as engine operating parameters, transmission operating parameters, and/or other control signals. 
     The OFDM receiver system may also be implemented in other control systems  440  of the vehicle  430 . The control system  440  may likewise receive signals from input sensors  442  and/or output control signals to one or more output devices  444 . In some implementations, the control system  440  may be part of an anti-lock braking system (ABS), a navigation system, a telematics system, a vehicle telematics system, a lane departure system, an adaptive cruise control system, a vehicle entertainment system such as a stereo, DVD, compact disc and the like. Still other implementations are contemplated. 
     The powertrain control system  432  may communicate with mass data storage  446  that stores data in a nonvolatile manner. The mass data storage  446  may include optical and/or magnetic storage devices for example hard disk drives HDD and/or DVDs. The HDD may be a mini HDD that includes one or more platters having a diameter that is smaller than approximately 1.8″. The powertrain control system  432  may be connected to memory  447  such as RAM, ROM, low latency nonvolatile memory such as flash memory and/or other suitable electronic data storage. The powertrain control system  432  also may support connections with a WLAN via a WLAN network interface  448 . The control system  440  may also include mass data storage, memory and/or a WLAN interface (all not shown). 
     Referring now to  FIG. 10C , the OFDM receiver system can be implemented in a cellular phone  450  that may include a cellular antenna  451 . For example, the OFDM receiver system could be implemented in a WLAN interface of the cellular phone  450 . In some implementations, the cellular phone  450  includes a microphone  456 , an audio output  458  such as a speaker and/or audio output jack, a display  460  and/or an input device  462  such as a keypad, pointing device, voice actuation and/or other input device. The signal processing and/or control circuits  452  and/or other circuits (not shown) in the cellular phone  450  may process data, perform coding and/or encryption, perform calculations, format data and/or perform other cellular phone functions. 
     The cellular phone  450  may communicate with mass data storage  464  that stores data in a nonvolatile manner such as optical and/or magnetic storage devices for example hard disk drives HDD and/or DVDs. The HDD may be a mini HDD that includes one or more platters having a diameter that is smaller than approximately 1.8″. The cellular phone  450  may be connected to memory  466  such as RAM, ROM, low latency nonvolatile memory such as flash memory and/or other suitable electronic data storage. The cellular phone  450  also may support connections with a WLAN via a WLAN network interface  468 . 
     Referring now to  FIG. 10D , the OFDM receiver system can be implemented in a set top box  480 . For example, the OFDM receiver system could be implemented in a WLAN interface of the set top box  480 . The set top box  480  receives signals from a source such as a broadband source and outputs standard and/or high definition audio/video signals suitable for a display  488  such as a television and/or monitor and/or other video and/or audio output devices. The signal processing and/or control circuits  484  and/or other circuits (not shown) of the set top box  480  may process data, perform coding and/or encryption, perform calculations, format data and/or perform any other set top box function. 
     The set top box  480  may communicate with mass data storage  490  that stores data in a nonvolatile manner. The mass data storage  490  may include optical and/or magnetic storage devices for example hard disk drives HDD and/or DVDs. The HDD may be a mini HDD that includes one or more platters having a diameter that is smaller than approximately 1.8″. The set top box  480  may be connected to memory  494  such as RAM, ROM, low latency nonvolatile memory such as flash memory and/or other suitable electronic data storage. The set top box  480  also may support connections with a WLAN via a WLAN network interface  496 . 
     Referring now to  FIG. 10E , the OFDM receiver system can be implemented in a media player  500 . For example, the OFDM receiver system could be implemented in a WLAN interface of the media player  500 . In some implementations, the media player  500  includes a display  507  and/or a user input  508  such as a keypad, touchpad and the like. In some implementations, the media player  500  may employ a graphical user interface (GUI) that typically employs menus, drop down menus, icons and/or a point-and-click interface via the display  507  and/or user input  508 . The media player  500  further includes an audio output  509  such as a speaker and/or audio output jack. The signal processing and/or control circuits  504  and/or other circuits (not shown) of the media player  500  may process data, perform coding and/or encryption, perform calculations, format data and/or perform any other media player function. 
     The media player  500  may communicate with mass data storage  510  that stores data such as compressed audio and/or video content in a nonvolatile manner. In some implementations, the compressed audio files include files that are compliant with MP3 format or other suitable compressed audio and/or video formats. The mass data storage may include optical and/or magnetic storage devices for example hard disk drives HDD and/or DVDs. The HDD may be a mini HDD that includes one or more platters having a diameter that is smaller than approximately 1.8″. The media player  500  may be connected to memory  514  such as RAM, ROM, low latency nonvolatile memory such as flash memory and/or other suitable electronic data storage. The media player  500  also may support connections with a WLAN via a WLAN network interface  516 . Still other implementations in addition to those described above are contemplated. 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.