Abstract:
A system including a variable gain amplifier, an automatic gain control module, and a channel estimation module. The variable gain amplifier is configured to amplify an input signal in accordance with a gain. The input signal includes a plurality of orthogonal frequency domain multiplexing symbols. Each of the plurality of orthogonal frequency domain multiplexing symbols is preceded by a respective cyclic prefix. The automatic gain control module is configured to adjust, based on a signal strength of the input signal, the gain of the variable gain amplifier during the respective cyclic prefix preceding each of the plurality of orthogonal frequency domain multiplexing symbols. The channel estimation module is configured to generate a channel estimate for each of the plurality of orthogonal frequency domain multiplexing symbols, and to update the channel estimate in response to the gain of the variable gain amplifier being adjusted.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of U.S. patent application Ser. No. 13/596,513, filed on Aug. 28, 2012, which is a continuation of U.S. patent application Ser. No. 13/169,691 (now U.S. Pat. No. 8,254,503), filed on Jun. 27, 2011, which is a continuation of U.S. patent application Ser. No. 11/963,294 (now U.S. Pat. No. 7,970,066), filed on Dec. 21, 2007, which claims the benefit of U.S. Provisional Application No. 60/882,062, filed on Dec. 27, 2006. The entire disclosures of the above applications are incorporated herein by reference. 
    
    
     FIELD 
     The present disclosure relates to communication systems, and more particularly to tracking automatic gain control (AGC) in systems using orthogonal frequency domain multiplexing (OFDM). 
     BACKGROUND 
     The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
     Referring now to  FIG. 1 , a typical communication system  10  comprises an information source  12 , a transmitter  13 , a communication channel  20 , a receiver  27 , and a destination  28 . The transmitter  13  comprises a source encoder  14 , a channel encoder  16 , and a modulator  18 . The receiver  27  comprises a demodulator  22 , a channel decoder  24 , and a source decoder  26 . 
     The information source  12  may be an analog source such as a sensor that outputs information as continuous waveforms or a digital source such as a computer that outputs information in a digital form. The source encoder  14  converts the output of the information source  12  into a sequence of binary digits (bits) called an information sequence u. The channel encoder  16  converts the information sequence u into a discrete encoded sequence v called a codeword. The modulator  18  transforms the codeword into a waveform of duration T seconds that is suitable for transmission. 
     The waveform output by the modulator  18  is transmitted via the communication channel  20 . Typical examples of the communication channel  20  are telephone lines, wireless communication channels, optical fiber cables, etc. Noise, such as electromagnetic interference, inter-channel crosstalk, etc., may corrupt the waveform. 
     The demodulator  22  receives the waveform. The demodulator  22  processes each waveform and generates a received sequence r that is either a discrete (quantized) or a continuous output. The channel decoder  24  converts the received sequence r into a binary sequence u′ called an estimated information sequence. The source decoder  26  converts u′ into an estimate of the output of the information source  12  and delivers the estimate to the destination  28 . The estimate may be a faithful reproduction of the output of the information source  12  when u′ resembles u despite decoding errors that may be caused by the noise. 
     Communication systems use different modulation schemes to modulate and transmit data. For example, a radio frequency (RF) carrier may be modulated using techniques such as frequency modulation, phase modulation, etc. In wireline communication systems, a transmitted signal generally travels along a path in a transmission line between a transmitter and a receiver. In wireless communication systems, however, a transmitted signal may travel along multiple paths. This is because the transmitted signal may be reflected and deflected by objects such as buildings, towers, airplanes, cars, etc., before the transmitted signal reaches a receiver. Each path may be of different length. Thus, the receiver may receive multiple versions of the transmitted signal. The multiple versions may interfere with each other causing inter symbol interference (ISI). Thus, retrieving original data from the transmitted signal may be difficult. 
     To alleviate this problem, wireless communication systems often use a modulation scheme called orthogonal frequency division multiplexing (OFDM). In OFDM, a wideband carrier signal is converted into a series of independent narrowband sub-carrier signals that are adjacent to each other in frequency domain. Data to be transmitted is split into multiple parallel data streams. Each data stream is modulated using a sub-carrier. A channel over which the modulated data is transmitted comprises a sum of the narrowband sub-carrier signals, which may overlap. 
     When each sub-carrier closely resembles a rectangular pulse, modulation can be easily performed by Inverse Discrete Fourier Transform (IDFT), which can be efficiently implemented as an Inverse Fast Fourier Transform (IFFT). When IFFT is used, the spacing of sub-carriers in the frequency domain is such that when the receiver processes a received signal at a particular frequency, all other signals are nearly zero at that frequency, and ISI is avoided. This property is called orthogonality, and hence the modulation scheme is called orthogonal frequency division multiplexing (OFDM). 
     Referring now to  FIGS. 2A-2C , a wireless communication system  30  may comprise base stations BS1, BS2, and BS3 (collectively BS) and one or more mobile stations (MS). In  FIG. 2A , one MS may communicate with up to three adjacent base stations. Each BS may transmit data that is modulated using an orthogonal frequency division multiplexing access (OFDMA) system. In  FIG. 2B , each BS may comprise a processor  31 , a medium access controller (MAC)  32 , a physical layer (PHY) module  34 , and an antenna  36 . In  FIG. 2C , each MS may comprise a processor  40 , a medium access controller (MAC)  42 , a physical layer (PHY) module  44 , and an antenna  46 . The PHY modules  34  and  44  may comprise radio frequency (RF) transceivers (not shown) that transmit and receive data via antennas  36  and  46 , respectively. Each BS and MS may transmit and receive data while the MS moves relative to the BS. 
     Specifically, each BS may transmit data typically in three segments: SEG1, SEG2, and SEG3. The MS may move relative to each BS and may receive data from one or more base stations depending on the location of the MS relative to each BS. For example, the MS may receive data from SEG 3 of BS1, SEG 2 of BS2, and/or SEG 1 of BS3 when the MS is located as shown in  FIG. 2A . Relative motion between MS and BS may cause Doppler shifts in signals received by the MS. Since systems using OFDMA are inherently sensitive to carrier frequency offsets (CFO), pilot tones are generally used for channel estimation refinement. For example, some of the sub-carriers may be designated as pilot tones for correcting residual frequency offset errors. 
     Additionally, the PHY module  34  of each BS typically adds a preamble to a data frame that is to be transmitted. Specifically, the PHY module  34  modulates and encodes the data frame comprising the preamble at a data rate specified by the MAC  34  and transmits the data frame. When the PHY module  44  of the MS receives the data frame, the PHY module  44  uses the preamble in the data frame to detect a beginning of packet transmission and to synchronize to a transmitter clock of the BS. 
     According to the I.E.E.E. standard 802.16e, which is incorporated herein by reference in its entirety, a first symbol in the data frame transmitted by the BS is a preamble symbol from a preamble sequence. The preamble sequence typically contains an identifier called IDcell, which is a cell ID of the BS, and segment information. The BS selects the preamble sequence based on the IDcell and the segment number of the BS. Each BS may select different preamble sequences. Additionally, each BS may select preamble sequences that are distinct among the segments of that BS. The BS modulates multiple sub-carriers with the selected preamble sequence. Thereafter, the BS performs IFFT, adds a cyclic prefix, and transmits a data frame. The MS uses the cyclic prefix to perform symbol timing and fractional carrier frequency synchronization. 
     When a receiver in the MS is turned on (i.e., when the MS is powered up), the MS may associate with an appropriate segment of a corresponding BS depending on the location of the MS. The MS may detect a preamble sequence in the data frame transmitted by the BS. Then the MS may perform frame synchronization and retrieve an IDcell and a segment number of the BS from the data frame. 
     Specifically, when the receiver in the MS is turned on, the MS initially performs symbol timing and carrier frequency synchronization before the MS can detect a preamble sequence. The MS may perform these tasks using a cyclic prefix in the data frame. Thereafter, the MS determines whether a first symbol in the frame is a preamble symbol. If the first symbol is a preamble symbol, then the MS determines which preamble sequence is present in the frame. Once the MS determines the preamble sequence, the MS can associate with a corresponding segment of an appropriate BS. 
     Base stations and mobile stations may be configured to operate in WiMAX wireless networks. WiMAX is a standards-based technology enabling wireless broadband access as an alternative to wired broadband like cable and DSL. WiMAX provides fixed, nomadic, portable, and mobile wireless connectivity without the need for a direct line-of-sight with a base station. WiMAX technology may be incorporated in portable electronic devices including notebook computers, personal digital assistants (PDAs). 
     Mobile WiMAX supports a full range of smart antenna technologies, including beamforming and spatial multiplexing, to enhance system performance. Mobile WiMAX supports adaptive switching between these options to maximize the benefit of smart antenna technologies under different channel conditions. Smart antenna technologies typically involve complex vector and matrix operations on signals due to multiple antennas. Typically, base stations may have at least two transmit antennas but may transmit preamble symbols via only one transmit antenna. Mobile stations may have at least two receive antennas and may receive signals via more than one receive antenna. 
     SUMMARY 
     A system comprises an input, a variable gain amplifier (VGA), and an automatic gain control (AGC) module. The input receives an input signal comprising frames of N symbols modulated using orthogonal frequency division multiplexing (OFDM), wherein a cyclic prefix (CP) precedes each of the N symbols, and wherein N is an integer greater than 1. The VGA provides a gain when amplifying the input signal. The AGC module selectively adjusts the gain of the VGA during the CP preceding M of the N symbols, wherein M is an integer greater than 1. 
     In another feature, M=N. 
     In another feature, a settling time of the VGA is less than a period of the CP. 
     In another feature, the AGC module changes the gain of the VGA based on signal strength of the input signal. 
     In another feature, the system further comprises a channel estimation module that generates a channel estimate for one of the N symbols based on a plurality of the N symbols. 
     In another feature, the system further comprises a gain adjuster module, wherein the AGC module changes gain of the gain adjuster module in inverse proportion to a change in the gain of the VGA. The channel estimate is independent of the change in the gain of the VGA. 
     In another feature, the channel estimation module updates channel estimates of the N symbols when the AGC module changes the gain of the VGA. 
     In another feature, when signal strength of the input signal changes by a factor of F, the AGC module changes the gain of the VGA by (10 log F) and the channel estimation module updates channel estimates of the N symbols, where F is an integer greater than 2. The channel estimation module updates the channel estimates using a binary shift operation. 
     In another feature, the system further comprises a gain adjuster module, wherein when the AGC module changes the gain of the VGA by G dB, the AGC module changes gain of the gain adjuster module by ((−G)+((Round(G/(10 log F)))*(10 log F)))dB, and the channel estimation module updates channel estimates of the N symbols when signal strength of the input signal changes by a factor of F, where F is an integer greater than 1. 
     In another feature, an OFDM receiver comprises the system and further comprises a decoder module that decodes the N symbols without the CP. 
     In another feature, the decoder module generates noise variance correction based on information generated by the AGC module. 
     In still other features, a method comprises receiving an input signal that includes frames of N symbols modulated using orthogonal frequency division multiplexing (OFDM), wherein a cyclic prefix (CP) precedes each of the N symbols, and wherein N is an integer greater than 1. The method further comprises providing a variable gain amplifier (VGA) having a gain when amplifying the input signal. The method further comprises selectively adjusting the gain of the VGA during the CP preceding M of the N symbols by providing automatic gain control (AGC), wherein M is an integer greater than 1. 
     In another feature, M=N. 
     In another feature, the method further comprises providing the VGA having a settling time less than a period of the CP. 
     In another feature, the method further comprises changing the gain of the VGA based on signal strength of the input signal by using the AGC. 
     In another feature, the method further comprises generating a channel estimate for one of the N symbols based on a plurality of the N symbols. 
     In another feature, the method further comprises providing a gain adjuster module and changing gain of the gain adjuster module in inverse proportion to a change in the gain of the VGA by using the AGC. The channel estimate is independent of the change in the gain of the VGA. 
     In another feature, the method further comprises updating channel estimates of the N symbols when the AGC changes the gain of the VGA. 
     In another feature, the method further comprises changing the gain of the VGA by (10 log F) using the AGC and updating channel estimates of the N symbols when signal strength of the input signal changes by a factor of F, where F is an integer greater than 2. The method further comprises updating the channel estimates using a binary shift operation. 
     In another feature, the method further comprises providing a gain adjuster module, changing gain of the gain adjuster module by ((−G)+((Round(G/(10 log F)))*(10 log F)))dB by using the AGC when the gain of the VGA is changed by G dB, and updating channel estimates of the N symbols when signal strength of the input signal changes by a factor of F, where F is an integer greater than 1. 
     In another feature, the method further comprises decoding the N symbols without the CP. 
     In another feature, the method further comprises generating noise variance correction based on information generated by the AGC. 
     In still other features, a system comprises input means for receiving an input signal that includes frames of N symbols modulated using orthogonal frequency division multiplexing (OFDM), wherein a cyclic prefix (CP) precedes each of the N symbols, and wherein N is an integer greater than 1. The system further comprises variable gain amplifier (VGA) means for providing a gain when amplifying the input signal. The system further comprises automatic gain control (AGC) means for selectively adjusting the gain of the VGA means during the CP preceding M of the N symbols, wherein M is an integer greater than 1. 
     In another feature, M=N. 
     In another feature, a settling time of the VGA means is less than a period of the CP. 
     In another feature, the AGC means changes the gain of the VGA means based on signal strength of the input signal. 
     In another feature, the system further comprises channel estimation means for generating a channel estimate for one of the N symbols based on a plurality of the N symbols. 
     In another feature, the system further comprises gain adjuster means for adjusting gain, wherein the AGC means changes gain of the gain adjuster means in inverse proportion to a change in the gain of the VGA means. The channel estimate is independent of the change in the gain of the VGA means. 
     In another feature, the channel estimation means updates channel estimates of the N symbols when the AGC means changes the gain of the VGA means. 
     In another feature, when signal strength of the input signal changes by a factor of F, the AGC means changes the gain of the VGA means by (10 log F) and the channel estimation means updates channel estimates of the N symbols, where F is an integer greater than 2. The channel estimation means updates the channel estimates using a binary shift operation. 
     In another feature, the system further comprises gain adjuster means for adjusting gain, wherein when the AGC means changes the gain of the VGA means by G dB, the AGC means changes gain of the gain adjuster means by ((−G)+((Round(G/(10 log F)))*(10 log F)))dB, and the channel estimation means updates channel estimates of the N symbols when signal strength of the input signal changes by a factor of F, where F is an integer greater than 1. 
     In another feature, an OFDM receiver comprises the system and further comprises decoder means for decoding the N symbols without the CP. 
     In another feature, the decoder means generates noise variance correction based on information generated by the AGC means. 
     In still other features, a computer program executed by a processor comprises receiving an input signal that includes frames of N symbols modulated using orthogonal frequency division multiplexing (OFDM), wherein a cyclic prefix (CP) precedes each of the N symbols, and wherein N is an integer greater than 1. The computer program further comprises providing a variable gain amplifier (VGA) having a gain when amplifying the input signal. The computer program further comprises selectively adjusting the gain of the VGA during the CP preceding M of the N symbols by providing automatic gain control (AGC), wherein M is an integer greater than 1. 
     In another feature, M=N. 
     In another feature, the computer program further comprises providing the VGA having a settling time less than a period of the CP. 
     In another feature, the computer program further comprises changing the gain of the VGA based on signal strength of the input signal by using the AGC. 
     In another feature, the computer program further comprises generating a channel estimate for one of the N symbols based on a plurality of the N symbols. 
     In another feature, the computer program further comprises providing a gain adjuster module and changing gain of the gain adjuster module in inverse proportion to a change in the gain of the VGA by using the AGC. The channel estimate is independent of the change in the gain of the VGA. 
     In another feature, the computer program further comprises updating channel estimates of the N symbols when the AGC changes the gain of the VGA. 
     In another feature, the computer program further comprises changing the gain of the VGA by (10 log F) using the AGC and updating channel estimates of the N symbols when signal strength of the input signal changes by a factor of F, where F is an integer greater than 2. The computer program further comprises updating the channel estimates using a binary shift operation. 
     In another feature, the computer program further comprises providing a gain adjuster module, changing gain of the gain adjuster module by ((−G)+((Round(G/(10 log F)))*(10 log F)))dB by using the AGC when the gain of the VGA is changed by G dB, and updating channel estimates of the N symbols when signal strength of the input signal changes by a factor of F, where F is an integer greater than 1. 
     In another feature, the computer program further comprises decoding the N symbols without the CP. 
     In another feature, the computer program further comprises generating noise variance correction based on information generated by the AGC. 
     In still other features, the systems and methods described above are implemented by a computer program executed by one or more processors. The computer program can reside on a computer readable medium such as but not limited to memory, non-volatile data storage and/or other suitable tangible storage mediums. 
     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 exemplary communication system according to the prior art; 
         FIG. 2A  is a schematic representation of an exemplary wireless communication system comprising three base stations and a mobile station according to the prior art; 
         FIG. 2B  is a functional block diagram of an exemplary base station utilized in the system of  FIG. 2A ; 
         FIG. 2C  is a functional block diagram of an exemplary mobile station utilized in the system of  FIG. 2A ; 
         FIG. 3  is a functional block diagram of an exemplary receiver of an orthogonal frequency division multiplexing access (OFDMA) system; 
         FIG. 4  depicts a frame of OFDM symbols; 
         FIGS. 5 and 6  are functional block diagrams of OFDM receivers that generate channel estimates for OFDM symbols using multiple OFDM symbols and that utilize AGC tracking systems according to the present disclosure; 
         FIGS. 7 and 8  are functional block diagrams of OFDM receivers that generate channel estimates for OFDM symbols using multiple OFDM symbols and that utilize AGC tracking systems according to the present disclosure; 
         FIG. 9  is a table showing characteristics of different AGC tracking systems according to the present disclosure; 
         FIG. 10-14  are flowcharts of different AGC tracking systems according to the present disclosure; 
         FIG. 15A  is a functional block diagram of a vehicle control system; 
         FIG. 15B  is a functional block diagram of a cellular phone; and 
         FIG. 15C  is a functional block diagram of a mobile device. 
     
    
    
     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. 
     Referring now to  FIG. 3 , a physical layer (PHY) device comprising a receiver  100  that receives input signals modulated using orthogonal frequency domain multiplexing (OFDM) is shown. The receiver  100  comprises an antenna  101 , a variable gain amplifier (VGA)  102 , an analog-to-digital converter (ADC) module, a filter module  106 , an automatic gain control (AGC) module  108 , a synchronization module  110 , a demodulation module  112 , a channel estimation module  114 , and a decoder module  116 . 
     The receiver  100  receives input signals modulated using OFDM via the antenna  101 . The VGA  102  amplifies the input signals. The ADC module  104  converts the output of the VGA  102  from analog to digital format. The filter module  106  filters the output of the ADC module  104 . The gain of the VGA  102  varies based on the strength of the input signals. The AGC module  108  controls the gain of the VGA  102  based on feedback received from the ADC module  104  and/or the filter module  106 . The synchronization module  110  performs symbol timing and carrier frequency synchronization. The demodulation module  112  demodulates the output of the synchronization module  110 . The channel estimation module  114  generates channels estimates of OFDM symbols. The decoder module  116  decodes the OFDM symbols and generates data. 
     Referring now to  FIG. 4 , the input signals comprise frames of OFDM symbols. A frame  118  includes a plurality of OFDM symbols. Each OFDM symbol includes a useful portion preceded by a cyclic prefix (CP). The CP is a copy of the useful portion of the OFDM symbol. The CP is used to perform synchronization and channel estimation. Additionally, the CP guards the useful portion of the OFDM symbol. Generally, the OFDM symbol can be decoded reliably even if the CP of the OFDM symbol is corrupted. Thus, the CP is redundant. 
     Typically, the AGC module  108  tracks the gain of the VGA  102  on a per frame basis. This is because the settling time of the gain of the VGA  102  is generally long since the VGA  102  comprises analog devices with long settling times. The long settling time of the VGA  102  may cause loss of data if the gain of the VGA  102  is changed during the frame. Accordingly, the gain of the AGC module  108  is generally set at the beginning of each frame and is left unchanged for the duration of the frame. 
     The signal strength and channel gain of the input signals, however, may vary during the frame. If the gain of the VGA  102  is not changed based on variations in the signal strength and channel gain of the input signals during a frame, the ADC module  104  may need to have channel fading margin and large dynamic range. Designing the ADC module  104  with channel fading margin and large dynamic range can be challenging. 
     Instead, the gain of the VGA  102  can be changed during the CP if the settling time of the gain of the VGA  102  is less than the duration of the CP. If the gain of the VGA  102  is changed at the beginning of the CP, and if the gain of the VGA  102  settles before the end of the CP, the channel fading margin and the dynamic range of the ADC module  104  can be relaxed. Since the CP is redundant, the data in the OFDM symbol can be decoded even if CP is utilized for changing the gain of the VGA  102 . 
     The AGC module  108  can track the gain of the VGA  102  in different ways depending on whether the OFDM receivers generate channel estimates using every OFDM symbol or using multiple OFDM symbols in the frame. When channel estimation for a given OFDM symbol is performed using current OFDM symbols, the input signals are processed on a symbol-by-symbol basis, and no gain changes are performed during subsequent signal processing if the AGC module  108  changes the gain of the VGA  102 . 
     When channel estimation is performed using multiple OFDM symbols, channel estimates for multiple OFDM symbols may be averaged to improve the accuracy of the channel estimation. Additionally, a digital gain adjuster module may be used to compensate changes in the gain of the VGA  102  so that the channel estimation and subsequent signal processing are unaffected by the changes in the gain of the VGA  102 . 
     Referring now to  FIGS. 5-8 , four AGC tracking systems are shown wherein channel estimation for a given OFDM symbol is performed using multiple OFDM symbols. While the four AGC tracking systems have different channel fading margin, quantization noise, and computational complexity, two elements are common to all four AGC tracking systems: First, the AGC module changes the gain of the VGA during the CP of every OFDM symbol. And second, the channel estimates excludes effects of analog gain variation by digital gain adjustment, which may be performed either explicitly by the digital gain adjuster module or implicitly by channel estimation scaling. For example, when the digital gain adjuster module is used, the AGC module changes the gain of the digital gain adjuster module by an amount that is equal and opposite to the amount by which the gain of the VGA is changed so that the net change in the gain of the OFDM receiver is zero. Additionally, all four AGC tracking systems perform optimally when the decoder module computes noise variance correction based on information passed from the AGC module to the decoder module. 
     In  FIGS. 5 and 6 , two configurations of a first AGC tracking system utilizing a digital gain adjuster module  130  are shown wherein channel estimation for a given OFDM symbol is performed using multiple OFDM symbols. In  FIG. 5 , an OFDM receiver  121 - 1   a  comprises the antenna  101 , the VGA  102 , the ADC module  104 , the filter module  106 , an AGC module  122 , a CP removal module  124 , a carrier frequency correction module  126 , a Fourier Frequency Transform (FFT) module  128 , the channel estimation module  114 , the decoder module  116 , and the digital gain adjuster module  130 . 
     The AGC module  122  changes the gain of the VGA  102  at the beginning of CP of each OFDM symbol. The AGC module  122  comprises a CP detection module  123  that detects the beginning of the CP of each OFDM symbol based on the feedback received from the filter module  106 . Based on the feedback, the AGC module  122  changes the gain of the VGA  102  during the CP. The gain of the VGA  102  settles before the end of the CP. 
     More specifically, the output of the ADC module  104  is input to the digital gain adjuster module  130  instead of the AGC module  122 . Additionally, the digital gain adjuster module  130  receives a control signal from the AGC module  122 . The control signal changes the gain of the digital gain adjuster module  130  by an amount that is equal and opposite to the amount by which the AGC module  122  changes the gain of the VGA  102 . The digital gain adjuster module  130  generates an output that is fed back to the AGC module  122  and input to the filter module  106 . 
     Depending on the variation in the signal strength of the input signal, the AGC module  122  increases or decreases the gain of the VGA  102 . When the AGC module  122  increases the gain of the VGA  102  by an amount G1, the AGC module  122  decreases the gain of the digital gain adjuster module  130  by the amount G1. When the AGC module  122  decreases the gain of the VGA  102  by an amount G2, the AGC module  122  increases the gain of the digital gain adjuster module  130  by the amount G2. Thus, the net change in the gain of the OFDM receiver  121 - 1   a  is zero. The channel estimation module  114  generates channel estimates for every symbol normally and independently of the changes in the gain of the VGA  102 . That is, the channel estimates are unaffected by and are independent of the changes in the gain of the VGA  102 . 
     Generally, OFDM receivers can perform synchronization either using CP or using frequency domain (FD) techniques. Typically, the CP-based synchronization is performed before FFT operation and offers improved data-acquisition relative to FD-based synchronization. The FD-based synchronization is performed after the FFT operation and is more accurate than the CP-based synchronization. 
     In the OFDM receiver  121 - 1   a , since CP is utilized for AGC tracking, the CP removal module  124  discards the CP, and FD-synchronization is performed in the digital domain of the OFDM receiver  121 - 1   a . Specifically, the carrier frequency correction module  126  corrects any carrier frequency offset (CFO) that may be present in the output generated by the filter module  106 . The FFT module  128  performs FFT operation on the output of the frequency correction module  126 . The channel estimation module  114  generates channel estimates for each OFDM symbol. 
     The decoder module  116  decodes each OFDM symbol. Specifically, the decoder module  116  decodes each OFDM symbol without the information that would otherwise be contained in the CP. That is, the decoder module  116  decodes each OFDM symbol independently of the CP. Additionally, the decoder module  116  computes a noise variance correction based on information generated and communicated by the AGC module  122 . 
     In  FIG. 6 , an OFDM receiver  121 - 1   b  is shown wherein the digital gain adjuster module  130  precedes the channel estimation module  114  and the decoder module  116 . In this configuration, the quantization noise is reduced for all modules that precede the digital gain adjuster module  130 . Accordingly, the configuration shown in  FIG. 6  is better than the configuration shown in  FIG. 5 . 
     In  FIG. 7 , a second AGC tracking system is shown wherein channel estimation for a given OFDM symbol is performed using multiple OFDM symbols. In an OFDM receiver  121 - 2 , when the AGC module  122  changes the gain of the VGA  102 , the change in the gain of the VGA  102  is not compensated during subsequent signal processing. Instead, the AGC module  122  generates a control signal that is input to the channel estimation module  114 . The channel estimation module  114  updates the channel estimates for each symbol based on the control signal. While this increases the computational complexity of the channel estimation module  114 , the quantization noise of the OFDM receiver  121 - 2  is reduced compared to the OFDM receiver  121 - 1   a  and  121 - 1   b.    
     In a third AGC tracking system, which is a variation of the second AGC tracking system of  FIG. 7 , the AGC module  122  changes the gain of the VGA  102  only when the signal strength of the input signal changes by more than 3 dB (i.e., by more than a factor of 2 since 10 log 2=3 dB). For example, the AGC module  122  changes the gain of the VGA  102  only when the signal strength more than doubles or halves. Thus, the gain of the VGA  102  is changed only when the change in the gain from a previously set gain value (i.e., a gain adjustment step) is more than 3 dB. Additionally, the AGC module  122  generates a control signal that is input to the channel estimation module  114 . Based on the control signal, the channel estimation module  114  updates the channel estimates. 
     This reduces the computational complexity of the channel estimation module  114  since the channel estimates can be updated by a simple binary shift operation instead of multiplication or division when the gain of the VGA  102  is changed in steps of 3 dB. The ADC module  104 , however, needs to have a 3 dB channel fading margin to withstand the changes in the gain of the VGA  102  in 3 dB steps. 
     In  FIG. 8 , a fourth AGC tracking system is shown wherein channel estimation for a given OFDM symbol is performed using multiple OFDM symbols. The fourth AGC tracking system is a combination of the first AGC tracking system of  FIG. 6  and the third AGC tracking system of  FIG. 7 . Specifically, in an OFDM receiver  121 - 4 , the digital gain adjuster module  130  is used as shown to compensate the changes in the gain of the VGA  102 , and the channel estimation module  114  updates the channel estimates based on feedback received from the AGC module  122 . 
     The AGC module  122  changes the gain of the VGA  102  during the CP of each OFDM symbol by an amount G. The AGC module  122  determines or estimates the amount G based on the signal strength of the input signal, which in turn is determined based on the feedback received from the ADC module  104  and the filter module  106 . 
     When the AGC module  122  changes the gain of the VGA  102  by G dB, the AGC module  122  generates two control signals. A first control signal changes the gain of the digital gain adjuster module  130  by an amount equal to ((−G)+((Round(G/(10 log 2)))*(10 log 2)))dB, where 10 log 2 is approximately equal to 3 dB. This obviates the need for the ADC module  104  to have any channel fading margin. 
     Additionally, based on a second control signal generated by the AGC module  122 , the channel estimation module  114  updates the channel estimates only when the AGC module  122  changes the gain of the VGA  102  by a total of 3 dB. The channel estimation module  114  does not update the channel estimates when G&lt;3 dB. This reduces the computational complexity of the channel estimation module  114  since the channel estimates can be updated by a simple binary shift operation when the gain of the VGA  102  is changed by a total of 3 dB. The OFDM receiver  121 - 4 , however, has a higher quantization noise than the OFDM receivers utilizing the second or third AGC tracking systems. 
     Referring now to  FIG. 9 , a table is shown wherein the four AGC tracking systems where channel estimation is performed using multiple OFDM symbols are compared using three criteria: the channel fading margin rating of the ADC module  104 , the quantization noise of the OFDM receiver, and the computational complexity of the channel estimation module  114 . 
     Although the AGC tracking systems shown in  FIGS. 5-8  change the gain of the VGA  102  during the CP of every symbol in the frame, the AGC module  122  may determine based on the signal strength of the input signal whether to change the gain of the VGA  102  during the CP preceding every symbol, every other symbol, and so on. Accordingly, if the frame comprises N symbols, the AGC module  122  may change the gain of the VGA  102  M times during the frame, where N and M are integers, N&gt;1, and 1&lt;M≦N. 
     Referring now to  FIG. 10 , a method  250  for tracking AGC gain is shown when channel estimates for given OFDM symbols are generated using multiple OFDM symbols. The method  250  begins in step  252 . The CP detection module  123  detects the beginning of the CP for a given OFDM symbol in step  254 . The AGC module  122  determines in step  256  whether to change the gain of the VGA  102  based on the feedback received from the filter module  106  and/or the ADC module  104 . 
     If the result of step  256  is true, the AGC module  122  changes the gain of the VGA  102  at the beginning of the CP in step  258 , and the gain of the VGA  102  settles before the end of the CP in step  260 . The AGC module  122  generates a control signal that changes the gain of the digital gain adjuster module  130  in step  262  to cancel the effect of change in the gain of the VGA  102  during subsequent signal processing. 
     Subsequently, or if the result of step  256  is false, the CP removal module  124  removes the CP in step  264 . The FFT module  128  performs FD synchronization in step  266 . The channel estimation module  114  generates a channel estimate for the OFDM symbol in step  268 . The decoder module  116  decodes the OFDM symbol and computes noise variance correction in step  270 . The method  250  ends in step  272 . 
     Referring now to  FIG. 11 , a method  300  for tracking AGC gain is shown when channel estimates for given OFDM symbols are generated using multiple OFDM symbols. The method  300  begins in step  302 . The CP detection module  123  detects the beginning of the CP for a given OFDM symbol in step  304 . The AGC module  122  determines in step  306  whether to change the gain of the VGA  102  based on the feedback received from the ADC module  104  and/or the filter module  106 . 
     If the result of step  306  is true, the AGC module  122  changes the gain of the VGA  102  at the beginning of the CP in step  308 , and the gain of the VGA  102  settles before the end of the CP in step  310 . Subsequently, or if the result of step  306  is false, the CP removal module  124  removes the CP in step  312 . The FFT module  128  performs FD synchronization in step  314 . 
     In step  316 , the AGC module  122  generates a control signal that changes the gain of the digital gain adjuster module  130  to cancel the effect of change in the gain of the VGA  102  during subsequent signal processing. The channel estimation module  114  generates a channel estimate for the OFDM symbol in step  318 . The decoder module  116  decodes the OFDM symbol and computes noise variance correction in step  320 . The method  300  ends in step  322 . 
     Referring now to  FIG. 12 , a method  350  for tracking AGC gain is shown when channel estimates for given OFDM symbols are generated using multiple OFDM symbols. The method  350  begins in step  352 . The CP detection module  123  detects the beginning of the CP for a given OFDM symbol in step  354 . The AGC module  122  determines in step  356  whether to change the gain of the VGA  102  based on the feedback received from the ADC module  104  and/or the filter module  106 . 
     If the result of step  356  is true, the AGC module  122  changes the gain of the VGA  102  at the beginning of the CP in step  358 . The AGC module  122  generates a control signal based on which the channel estimation module  114  updates the channel estimates in step  360 . The gain of the VGA  102  settles before the end of the CP in step  362 . 
     Subsequently, or if the result of step  356  is false, the CP removal module  124  removes the CP in step  364 . The FFT module  128  performs FD synchronization in step  366 . The channel estimation module  114  generates a channel estimate for the OFDM symbol in step  368 . The decoder module  116  decodes the OFDM symbol and computes noise variance correction in step  370 . The method  350  ends in step  372 . 
     Referring now to  FIG. 13 , a method  400  for tracking AGC gain is shown when channel estimates for given OFDM symbols are generated using multiple OFDM symbols. The method  400  begins in step  402 . The CP detection module  123  detects the beginning of the CP for a given OFDM symbol in step  404 . The AGC module  122  determines in step  406  whether the signal strength of the input signal changed by at least 3 dB or a factor of 2 (e.g., more than doubled or halved) based on the feedback received from the ADC module  104  and/or the filter module  106 . 
     If the result of step  406  is true, the AGC module  122  changes the gain of the VGA  102  at the beginning of the CP in step  408 . The AGC module  122  generates a control signal based on which the channel estimation module  114  updates the channel estimates using a binary shift operation in step  410 . The gain of the VGA  102  settles before the end of the CP in step  412 . 
     Subsequently, or if the result of step  406  is false, the CP removal module  124  removes the CP in step  414 . The FFT module  128  performs FD synchronization in step  416 . The channel estimation module  114  generates a channel estimate for the OFDM symbol in step  418 . The decoder module  116  decodes the OFDM symbol and computes noise variance correction in step  420 . The method  400  ends in step  422 . 
     Referring now to  FIG. 14 , a method  450  for tracking AGC gain is shown when channel estimates for given OFDM symbols are generated using multiple OFDM symbols. The method  450  begins in step  452 . The CP detection module  123  detects the beginning of the CP for a given OFDM symbol in step  454 . The AGC module  122  determines in step  456  whether to change the gain of the VGA  102  based on the feedback received from the ADC module  104  and/or the filter module  106 . 
     If the result of step  456  is true, the AGC module  122  estimates the amount G by which to change the gain of the VGA  102  in step  458 . The AGC module  122  changes the gain of the VGA  102  by G dB in step  460 . The AGC module  122  generates a control signal based on which the gain of the digital gain adjuster module  130  is changed by an amount equal to ((−G)+Round((G/3 dB)*3 dB)) in step  462 . 
     The AGC module  122  determines in step  464  if the total amount by which the gain of the VGA  102  is changed is greater than or equal to 3 dB. If the result of step  464  is true, the AGC module  122  generates a control signal based on which the channel estimation module  114  updates the channel estimates using a binary shift operation in step  366 . If the result of step  464  is false, the method  450  skips step  466 . The gain of the VGA  102  settles before the end of the CP in step  468 . 
     Subsequently, or if the result of step  456  is false, the CP removal module  124  removes the CP in step  470 . The FFT module  128  performs FD synchronization in step  472 . The channel estimation module  114  generates a channel estimate for the OFDM symbol in step  474 . The decoder module  116  decodes the OFDM symbol and computes noise variance correction in step  476 . The method  450  ends in step  478 . 
     Referring now to  FIG. 15A , the teachings of the disclosure may be implemented in a WiMAX interface  552  of a vehicle  546 . The vehicle  546  may include a vehicle control system  547 , a power supply  548 , memory  549 , a storage device  550 , and the WiMAX interface  552  and associated antenna  553 . The vehicle control system  547  may be a powertrain control system, a body control system, an entertainment control system, an anti-lock braking system (ABS), a navigation system, a telematics system, a lane departure system, an adaptive cruise control system, etc. 
     The vehicle control system  547  may communicate with one or more sensors  554  and generate one or more output signals  556 . The sensors  554  may include temperature sensors, acceleration sensors, pressure sensors, rotational sensors, airflow sensors, etc. The output signals  556  may control engine operating parameters, transmission operating parameters, suspension parameters, etc. 
     The power supply  548  provides power to the components of the vehicle  546 . The vehicle control system  547  may store data in memory  549  and/or the storage device  550 . Memory  549  may include random access memory (RAM) and/or nonvolatile memory such as flash memory, phase change memory, or multi-state memory, in which each memory cell has more than two states. The storage device  550  may include an optical storage drive, such as a DVD drive, and/or a hard disk drive (HDD). The vehicle control system  547  may communicate externally using the WiMAX interface  552 . 
     Referring now to  FIG. 15B , the teachings of the disclosure can be implemented in a WiMAX interface  568  of a cellular phone  558 . The cellular phone  558  includes a phone control module  560 , a power supply  562 , memory  564 , a storage device  566 , and a cellular network interface  567 . The cellular phone  558  may include the WiMAX interface  568  and associated antenna  569 , a microphone  570 , an audio output  572  such as a speaker and/or output jack, a display  574 , and a user input device  576  such as a keypad and/or pointing device. 
     The phone control module  560  may receive input signals from the cellular network interface  567 , the WiMAX interface  568 , the microphone  570 , and/or the user input device  576 . The phone control module  560  may process signals, including encoding, decoding, filtering, and/or formatting, and generate output signals. The output signals may be communicated to one or more of memory  564 , the storage device  566 , the cellular network interface  567 , the WiMAX interface  568 , and the audio output  572 . 
     Memory  564  may include random access memory (RAM) and/or nonvolatile memory such as flash memory, phase change memory, or multi-state memory, in which each memory cell has more than two states. The storage device  566  may include an optical storage drive, such as a DVD drive, and/or a hard disk drive (HDD). The power supply  562  provides power to the components of the cellular phone  558 . 
     Referring now to  FIG. 15C , the teachings of the disclosure can be implemented in a network interface  594  of a mobile device  589 . The mobile device  489  may include a mobile device control module  590 , a power supply  591 , memory  592 , a storage device  593 , the network interface  594 , and an external interface  599 . The network interface  594  includes a WiMAX interface and an antenna (not shown). 
     The mobile device control module  590  may receive input signals from the network interface  594  and/or the external interface  599 . The external interface  599  may include USB, infrared, and/or Ethernet. The input signals may include compressed audio and/or video, and may be compliant with the MP3 format. Additionally, the mobile device control module  590  may receive input from a user input  596  such as a keypad, touchpad, or individual buttons. The mobile device control module  590  may process input signals, including encoding, decoding, filtering, and/or formatting, and generate output signals. 
     The mobile device control module  590  may output audio signals to an audio output  597  and video signals to a display  598 . The audio output  597  may include a speaker and/or an output jack. The display  598  may present a graphical user interface, which may include menus, icons, etc. The power supply  591  provides power to the components of the mobile device  589 . Memory  592  may include random access memory (RAM) and/or nonvolatile memory such as flash memory, phase change memory, or multi-state memory, in which each memory cell has more than two states. The storage device  593  may include an optical storage drive, such as a DVD drive, and/or a hard disk drive (HDD). The mobile device may include a personal digital assistant, a media player, a laptop computer, a gaming console, or other mobile computing device. 
     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.