Methods and systems of AGC and DC calibration for OFDM/OFDMA systems

Methods and apparatus for automatic gain control (AGC) and DC calibration for orthogonal frequency-division multiplexing (OFDM) or orthogonal frequency division multiple access (OFDMA) systems are provided in an effort to avoid saturation of the analog-to-digital converter (ADC) in a radio frequency (RF) front end of a receiver, to handle dynamic received signal power, or to avoid interruptions in a communication link for DC calibration. For certain embodiments, the quantization error in the RF front end may also be decreased.

TECHNICAL FIELD

Certain embodiments of the present disclosure generally relate to wireless communication and, more particularly, to automatic gain control (AGC) and DC calibration for orthogonal frequency-division multiplexing (OFDM) or orthogonal frequency division multiple access (OFDMA) systems.

BACKGROUND

OFDM and OFDMA wireless communication systems under IEEE 802.16 use a network of base stations to communicate with wireless devices (i.e., mobile stations) registered for services in the systems based on the orthogonality of frequencies of multiple subcarriers and can be implemented to achieve a number of technical advantages for wideband wireless communications, such as resistance to multipath fading and interference. Each base station emits and receives radio frequency (RF) signals that convey data to and from the mobile stations.

A mobile station may include an RF front end with suitable circuitry for receiving the transmitted signals from a base station and processing the received signals in preparation for demodulation and decoding. The signal processing may include automatic gain control (AGC) and DC calibration. Proper AGC and DC calibration are important in order to increase the signal-to-interference-plus-noise ratio (SINR) without saturating the RF front end, thereby possibly leading to invalid data.

SUMMARY

Certain embodiments of the present disclosure generally relate to automatic gain control (AGC) and DC calibration for orthogonal frequency-division multiplexing (OFDM) or orthogonal frequency division multiple access (OFDMA) systems in an effort to avoid saturation of the analog-to-digital converter (ADC) in a radio frequency (RF) front end of a receiver. For certain embodiments, the quantization error in the RF front end may also be decreased.

Certain embodiments of the present disclosure provide a method for AGC in a wireless communication system. The method generally includes estimating power of a received signal from an output of an ADC; adjusting an analog gain based on the estimated received signal power; estimating an adjusted signal power from an output of a digital filter while using the adjusted analog gain, wherein the output of the ADC is input to the digital filter; and adjusting a digital gain based on the estimated adjusted signal power.

Certain embodiments of the present disclosure provide a receiver for wireless communication. The receiver generally includes first estimation logic configured to estimate power of a received signal from an output of an ADC; first adjustment logic configured to adjust an analog gain based on the estimated received signal power; second estimation logic configured to estimate an adjusted signal power from an output of a digital filter while using the adjusted analog gain, wherein the output of the ADC is input to the digital filter; and second adjustment logic configured to adjust a digital gain based on the estimated adjusted signal power.

Certain embodiments of the present disclosure provide an apparatus for AGC in a wireless communication system. The apparatus generally includes means for estimating power of a received signal from an output of an ADC; means for adjusting an analog gain based on the estimated received signal power; means for estimating an adjusted signal power from an output of a digital filter while using the adjusted analog gain, wherein the output of the ADC is input to the digital filter; and means for adjusting a digital gain based on the estimated adjusted signal power.

Certain embodiments of the present disclosure provide a mobile device. The mobile device generally includes a receiver front end for receiving a signal; first estimation logic configured to estimate power of the received signal from an output of an ADC; first adjustment logic configured to adjust an analog gain based on the estimated received signal power; second estimation logic configured to estimate an adjusted signal power from an output of a digital filter while using the adjusted analog gain, wherein the output of the ADC is input to the digital filter; and second adjustment logic configured to adjust a digital gain based on the estimated adjusted signal power.

Certain embodiments of the present disclosure provide a computer-readable medium containing a program for AGC in a wireless communication system, which, when executed by a processor, performs certain operations. The operations generally include estimating power of a received signal from an output of an ADC; adjusting an analog gain based on the estimated received signal power; estimating an adjusted signal power from an output of a digital filter while using the adjusted analog gain, wherein the output of the ADC is input to the digital filter; and adjusting a digital gain based on the estimated adjusted signal power.

Certain embodiments of the present disclosure provide a method for DC calibration in a wireless communication system. The method generally includes setting an analog gain of a received signal to create an amplified signal, wherein the received signal is based on an OFDM or OFDMA frame; estimating a DC offset of the amplified signal during a gap time of the received signal; and applying the estimated DC offset to the amplified signal.

Certain embodiments of the present disclosure provide a receiver for wireless communication. The receiver generally includes gain setting logic configured to set an analog gain for a signal received by the receiver to create an amplified signal, wherein the received signal is based on an OFDM or OFDMA frame; offset estimation logic configured to estimate a DC offset of the amplified signal during a gap time of the received signal; and adjustment logic configured to apply the estimated DC offset to the amplified signal.

Certain embodiments of the present disclosure provide an apparatus for DC calibration in a wireless communication system. The apparatus generally includes means for setting an analog gain of a received signal to create an amplified signal, wherein the received signal is based on an OFDM or OFDMA frame; means for estimating a DC offset of the amplified signal during a gap time of the received signal; and means for applying the estimated DC offset to the amplified signal.

Certain embodiments of the present disclosure provide a mobile device. The mobile device generally includes a receiver front end for receiving a signal based on an OFDM or OFDMA frame; gain setting logic configured to set an analog gain for the received signal to create an amplified signal; estimation logic configured to estimate a DC offset of the amplified signal during a gap time of the received signal; and adjustment logic configured to apply the estimated DC offset to the amplified signal.

Certain embodiments of the present disclosure provide a computer-readable medium containing a program for DC calibration in a wireless communication system, which, when executed by a processor, performs certain operations. The operations generally include setting an analog gain of a received signal to create an amplified signal, wherein the received signal is based on an OFDM or OFDMA frame; estimating a DC offset of the amplified signal during a gap time of the received signal; and applying the estimated DC offset to the amplified signal.

DETAILED DESCRIPTION

Certain embodiments of the present disclosure provide techniques and apparatus for automatic gain control (AGC) and DC calibration for orthogonal frequency-division multiplexing (OFDM) or orthogonal frequency division multiple access (OFDMA) systems in an effort to avoid saturation of the analog-to-digital converter (ADC) in a radio frequency (RF) front end of a receiver, especially in the presence of a large interference signal, to handle quickly changing received signal power, or to avoid interruptions in a communication link for DC calibration. For certain embodiments, the quantization error in the RF front end may also be decreased.

Exemplary Wireless Communication System

The methods and apparatus of the present disclosure may be utilized in a broadband wireless communication system. The term “broadband wireless” refers to technology that provides wireless, voice, Internet, and/or data network access over a given area.

WiMAX, which stands for the Worldwide Interoperability for Microwave Access, is a standards-based broadband wireless technology that provides high-throughput broadband connections over long distances. There are two main applications of WiMAX today: fixed WiMAX and mobile WiMAX. Fixed WiMAX applications are point-to-multipoint, enabling broadband access to homes and businesses, for example. Mobile WiMAX offers the full mobility of cellular networks at broadband speeds.

Mobile WiMAX is based on OFDM and OFDMA technology. OFDM is a digital multi-carrier modulation technique that has recently found wide adoption in a variety of high-data-rate communication systems. With OFDM, a transmit bit stream is divided into multiple lower-rate substreams. Each substream is modulated with one of multiple orthogonal subcarriers and sent over one of a plurality of parallel subchannels. OFDMA is a multiple access technique in which users are assigned subcarriers in different time slots. OFDMA is a flexible multiple-access technique that can accommodate many users with widely varying applications, data rates, and quality of service requirements.

The rapid growth in wireless internets and communications has led to an increasing demand for high data rate in the field of wireless communications services. OFDM/OFDMA systems are today regarded as one of the most promising research areas and as a key technology for the next generation of wireless communications. This is due to the fact that OFDM/OFDMA modulation schemes can provide many advantages such as modulation efficiency, spectrum efficiency, flexibility, and strong multipath immunity over conventional single carrier modulation schemes.

IEEE 802.16x is an emerging standard organization to define an air interface for fixed and mobile broadband wireless access (BWA) systems, such as for fixed BWA systems and for mobile BWA systems. These standards define at least four different physical layers (PHYs) and one media access control (MAC) layer. The OFDM and OFDMA physical layer of the four physical layers are the most popular in the fixed and mobile BWA areas respectively.

FIG. 1illustrates an example of a wireless communication system100. The wireless communication system100may be a broadband wireless communication system. The wireless communication system100may provide communication for a number of cells102, each of which is serviced by a base station104. A base station104may be a fixed station that communicates with user terminals106. The base station104may alternatively be referred to as an access point, a Node B, or some other terminology.

FIG. 1depicts various user terminals106dispersed throughout the system100. The user terminals106may be fixed (i.e., stationary) or mobile. The user terminals106may alternatively be referred to as remote stations, access terminals, terminals, subscriber units, mobile stations, stations, user equipment, etc. The user terminals106may be wireless devices, such as cellular phones, personal digital assistants (PDAs), handheld devices, wireless modems, laptop computers, personal computers (PCs), etc.

A variety of algorithms and methods may be used for transmissions in the wireless communication system100between the base stations104and the user terminals106. For example, signals may be sent and received between the base stations104and the user terminals106in accordance with OFDM/OFDMA techniques. If this is the case, the wireless communication system100may be referred to as an OFDM/OFDMA system.

A communication link that facilitates transmission from a base station104to a user terminal106may be referred to as a downlink108, and a communication link that facilitates transmission from a user terminal106to a base station104may be referred to as an uplink110. Alternatively, a downlink108may be referred to as a forward link or a forward channel, and an uplink110may be referred to as a reverse link or a reverse channel.

A cell102may be divided into multiple sectors112. A sector112is a physical coverage area within a cell102. Base stations104within a wireless communication system100may utilize antennas that concentrate the flow of power within a particular sector112of the cell102. Such antennas may be referred to as directional antennas.

FIG. 2illustrates various components that may be utilized in a wireless device202. The wireless device202is an example of a device that may be configured to implement the various methods described herein. The wireless device202may be a base station104or a user terminal106.

The wireless device202may also include a housing208that may include a transmitter210and a receiver212to allow transmission and reception of data between the wireless device202and a remote location. The transmitter210and receiver212may be combined into a transceiver214. An antenna216may be attached to the housing208and electrically coupled to the transceiver214. The wireless device202may also include (not shown) multiple transmitters, multiple receivers, multiple transceivers, and/or multiple antennas.

The wireless device202may also include a signal detector218that may be used in an effort to detect and quantify the level of signals received by the transceiver214. The signal detector218may detect such signals as total energy, pilot energy from pilot subcarriers or signal energy from the preamble symbol, power spectral density, and other signals. The wireless device202may also include a digital signal processor (DSP)220for use in processing signals.

The various components of the wireless device202may be coupled together by a bus system222, which may include a power bus, a control signal bus, and a status signal bus in addition to a data bus.

FIG. 3illustrates an example of a transmitter302that may be used within a wireless communication system100that utilizes OFDM/OFDMA. Portions of the transmitter302may be implemented in the transmitter210of a wireless device202. The transmitter302may be implemented in a base station104for transmitting data306to a user terminal106on a downlink108. The transmitter302may also be implemented in a user terminal106for transmitting data306to a base station104on an uplink110.

Data306to be transmitted is shown being provided as input to a serial-to-parallel (S/P) converter308. The S/P converter308may split the transmission data into N parallel data streams310.

The N parallel data streams310may then be provided as input to a mapper312. The mapper312may map the N parallel data streams310onto N constellation points. The mapping may be done using some modulation constellation, such as binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), 8 phase-shift keying (8PSK), quadrature amplitude modulation (QAM), etc. Thus, the mapper312may output N parallel symbol streams316, each symbol stream316corresponding to one of the N orthogonal subcarriers of the inverse fast Fourier transform (IFFT)320. These N parallel symbol streams316are represented in the frequency domain and may be converted into N parallel time domain sample streams318by an IFFT component320.

A brief note about terminology will now be provided. N parallel modulations in the frequency domain are equal to N modulation symbols in the frequency domain, which are equal to N mapping and N-point IFFT in the frequency domain, which is equal to one (useful) OFDM symbol in the time domain, which is equal to N samples in the time domain. One OFDM symbol in the time domain, Ns, is equal to Ncp(the number of guard samples per OFDM symbol)+N (the number of useful samples per OFDM symbol).

The N parallel time domain sample streams318may be converted into an OFDM/OFDMA symbol stream322by a parallel-to-serial (P/S) converter324. A guard insertion component326may insert a guard interval between successive OFDM/OFDMA symbols in the OFDM/OFDMA symbol stream322. The output of the guard insertion component326may then be upconverted to a desired transmit frequency band by a radio frequency (RF) front end328. An antenna330may then transmit the resulting signal332.

FIG. 3also illustrates an example of a receiver304that may be used within a wireless communication system100that utilizes OFDM/OFDMA. Portions of the receiver304may be implemented in the receiver212of a wireless device202. The receiver304may be implemented in a user terminal106for receiving data306from a base station104on a downlink108. The receiver304may also be implemented in a base station104for receiving data306from a user terminal106on an uplink110.

The transmitted signal332is shown traveling over a wireless channel334. When a signal332′ is received by an antenna330′, the received signal332′ may be downconverted to a baseband signal by an RF front end328′. A guard removal component326′ may then remove the guard interval that was inserted between OFDM/OFDMA symbols by the guard insertion component326.

The output of the guard removal component326′ may be provided to an S/P converter324′. The S/P converter324′ may divide the OFDM/OFDMA symbol stream322′ into the N parallel time-domain symbol streams318′, each of which corresponds to one of the N orthogonal subcarriers. A fast Fourier transform (FFT) component320′ may convert the N parallel time-domain symbol streams318′ into the frequency domain and output N parallel frequency-domain symbol streams316′.

A demapper312′ may perform the inverse of the symbol mapping operation that was performed by the mapper312, thereby outputting N parallel data streams310′. A P/S converter308′ may combine the N parallel data streams310′ into a single data stream306′. Ideally, this data stream306′ corresponds to the data306that was provided as input to the transmitter302.

Exemplary OFDMA Frame

Referring now toFIG. 4, an OFDMA frame400for a Time-Division Duplex (TDD) implementation is depicted as a typical, but not limiting, example. Other implementations of an OFDMA frame, such as Full and Half-Duplex Frequency-Division Duplex (FDD) may be used, in which case the frame is the same except that both downlink (DL) and uplink (UL) messages are transmitted simultaneously over different carriers. In the TDD implementation, each frame may be divided into a DL subframe402and a UL subframe404, which may be separated by a small guard interval—or, more specifically, by Transmit/Receive and Receive/Transmit Transition Gaps (TTG406and RTG407, respectively)—in an effort to prevent DL and UL transmission collisions. The DL-to-UL-subframe ratio may be varied from 3:1 to 1:1 to support different traffic profiles.

Within the OFDMA frame400, various control information may be included. For example, the first OFDMA symbol of the frame400may be a preamble408, which may contain several pilot signals (pilots) used for synchronization. Fixed pilot sequences inside the preamble408may allow the receiver304to estimate frequency and phase errors and to synchronize to the transmitter302. Moreover, fixed pilot sequences in the preamble408may be utilized to estimate and equalize wireless channels. The preamble408may contain BPSK-modulated carriers and is typically one OFDM symbol long. The carriers of the preamble408may be power boosted and are typically a few decibels (dB) (e.g., 9 dB) higher than the power level in the frequency domain of data portions in the WiMAX signal. The number of preamble carriers used may indicate which of the three segments409of the zone are used. For example, carriers0,3,6, . . . may indicate that segment0(4090) is to be used, carriers1,4,7, . . . may indicate that segment1(4091) is to be used, and carriers2,5,8, . . . may indicate that segment2(4092) is to be used.

A Frame Control Header (FCH)410may follow the preamble408, one FCH410per segment409. The FCH410may provide frame configuration information, such as the usable subchannels, the modulation and coding scheme, and the MAP message length for the current OFDMA frame. A data structure, such as the downlink Frame Prefix (DLFP), outlining the frame configuration information may be mapped to the FCH410. The DLFP for Mobile WiMAX may comprise a used subchannel (SCH) bitmap, a reserved bit set to 0, a repetition coding indication, a coding indication, a MAP message length, and four reserved bits set to 0. Before being mapped to the FCH410, the 24-bit DLFP may be duplicated to form a 48-bit block, which is the minimal forward error correction (FEC) block size.

Following the FCH410in each segment409, a DL-MAP414and a UL-MAP416may specify subchannel allocation and other control information for the DL and UL subframes402,404, respectively. In OFDMA, multiple users may be allocated data regions within the frame400, and these allocations may be specified in the DL and UL-MAP414,416. The MAP messages may include the burst profile for each user, which defines the modulation and coding scheme used in a particular link. Since MAP messages contain critical information that needs to reach all users for that segment409, the DL and UL-MAP414,416may often be sent over a very reliable link, such as BPSK or QPSK with rate ½ coding and repetition coding.

The DL subframe402of the OFDMA frame400may include DL bursts of various bit lengths containing the downlink data being communicated. Thus, the DL-MAP414may describe the location of the bursts contained in the downlink zones and the number of downlink bursts, as well as their offsets and lengths in both the time (i.e., symbol) and the frequency (i.e., subchannel) directions. Altogether, the preamble408, the FCH410, and the DL-MAP414may carry information that enables the receiver304to correctly demodulate the received signal.

Likewise, the UL subframe404may include UL bursts of various bit lengths composed of the uplink data being communicated. Therefore, the UL-MAP416, transmitted as the first DL burst in the DL subframe402, may contain information about the location of the UL burst for different users. The UL subframe404may include additional control information as illustrated inFIG. 4, such as a UL Ranging subchannel422allocated for the mobile station to perform closed-loop time, frequency, and power adjustments during network entry and periodically afterward, as well as bandwidth requests. The UL subframe404may also include a UL ACK (not shown) allocated for the mobile station (MS) to feed back a DL hybrid automatic repeat request acknowledgment (HARQ ACK) and/or a UL CQICH (not shown) allocated for the MS to feed back channel state information on the Channel Quality Indicator channel (CQICH).

Different “modes” may be used for DL and UL transmission in OFDMA. An area in the time domain where a certain mode is used is generally referred to as a zone. One type of zone is called a DL-PUSC (downlink partial usage of subchannels) zone424and may not use all the subchannels available to it (i.e., a DL-PUSC zone424may only use particular subchannels). A DL-PUSC zone424may be divided into a total of six subchannel groups, which can be assigned to up to three segments409. Thus, a segment409may contain one to six subchannel groups (e.g., segment0may contain two subchannel groups0and1, segment1may contain two subchannel groups2and3, and segment2may contain two subchannel groups4and5as illustrated inFIG. 4). Another type of zone is called a DL-FUSC (downlink full usage of subchannels) zone426. Unlike DL-PUSC, DL-FUSC does not use any segments, but can distribute all bursts over the complete frequency range.

FIG. 5illustrates that different OFDM/OFDMA zones within the DL subframe402may vary in signal power. For example, the first zone (ZONE1) may be the first DL-PUSC zone4241and may have a greater signal power than the second zone (ZONE2), which may be the second DL-PUSC zone4242as depicted. As another example, the third zone (ZONE3) may be a DL-FUSC zone426and may also have a greater signal power than ZONE2as shown.

Exemplary Method for Automatic Gain Control (AGC)

FIG. 6Aillustrates a block diagram600of a zero intermediate frequency (ZIF) architecture as one example of an RF front end328′ for a receiver304. In the block diagram600, the antenna330′ may be coupled to a low noise amplifier (LNA)602. The LNA602may be used to provide a high degree of signal gain (e.g., 0, 20, 40, or 60 dB) without introducing significant noise or spurious signal components to the amplified signal. By having a programmable gain, the LNA602may provide a coarse gain adjustment with a resolution of 20 dB, for example, for automatic gain control (AGC).

The LNA602may be coupled to a mixer604in an effort to mix an output of the LNA602with a local oscillator frequency operating at a predetermined frequency (fc). Although not shown inFIG. 6A, those skilled in the art will recognize that the amplified signal may be separated into in-phase (I) and quadrature (Q) signals at the mixer604, and subsequent signal processing may be applied to both the I and Q signals. For simplicity, the diagram600illustrates only one of the signal processing paths following the mixer604.

The mixed signal may be amplified by a programmable gain amplifier (PGA)606in an effort to provide fine gain adjustment compared to the coarse adjustment of the LNA602. For example, the PGA606may provide a fine gain adjustment with a resolution of 1 dB. The PGA606may be coupled to an anti-aliasing filter (AAF)608in an effort to remove out-of-band high frequency components of the amplified signal before they are aliased into the passband by digital sampling. The AAF608may be coupled to a summer610for removing a DC offset before the resulting signal is converted to the digital domain for digital signal processing by an analog-to-digital converter (ADC)612. The ADC612may have a high resolution, such as 16-bits.

The ADC612may be coupled to a digital filter (DF)614in an effort to remove components from out-of-band frequencies. The average power (AP) of the digital output of the DF614may be estimated by the AP block616and sent to an automatic gain control (AGC)/digital automatic gain control (DAGC) block618for processing. The AGC/DAGC block618may send digital control signals to the LNA602and/or the PGA606to adjust the variable gain of these stages based on the estimated signal power from the AP block616. Furthermore, the AGC/DAGC block618may send a digital DC offset to be converted to an analog DC offset by a digital-to-analog converter (DAC)620. The output of the DAC620may be summed with the output of the AAF608by the summer610.

A DC estimator622and a buffer624may also be coupled to the output of the DF614. The DC estimator622may be used to estimate any residual DC offset in the signal output by the DF614, and the buffer624may hold this signal such that the residual DC offset may be subtracted from the buffered signal by a summer626. The same signal power from the AP block616may be used by the AGC/DAGC block618to perform automatic gain control in the digital domain by multiplying the output of the summer626with a control signal from the AGC/DAGC block618using multiplier628. The resulting signal may be further processed in a data demodulator630in an effort to interpret the message in the signal received at the antenna330′.

One problem with the block diagram600ofFIG. 6lies in estimating the signal power with the AP block616after the digital filter614has potentially removed out-of-band interference signals, especially high amplitude interference signals. In this manner, the programmable gain of the LNA602and/or the PGA606may be set too high, and a large interferer may rail one or both of the amplifiers and saturate the output of the ADC612.

One way to solve this might be to estimate the signal power at the output of the ADC612rather than at the output of the digital filter614. However, because the block diagram600ofFIG. 6uses the same signal power estimate for both analog and digital automatic gain control in the AGC/DAGC block618, estimates of the signal power that included the large interference signal would not be correct for automatic gain control in the digital domain after the digital filter614has removed the out-of-band frequencies. Accordingly, what is needed is a new ZIF architecture for automatic gain control.

FIG. 6Billustrates a block diagram650of a ZIF architecture for an RF front end328′ configured to perform AGC without saturating the ADC output and to conduct a separate DAGC in an effort to reduce the quantization error. To accomplish these design goals, the ZIF architecture may contain two separate AP blocks652,658for estimating signal power. The first AP block652may estimate the average power of the output of the ADC612before the output is filtered by the digital filter614, and thus, the power estimate may include out-of-band signal components, such as large interference signals. A separate AGC block654may provide control signals for programming the gain of the LNA602and/or the PGA606based on the estimated signal power from the first AP block652, as well as for DC offset as described above. In this manner, the gain set by the AGC block654may adjust for the amplitude (or, more correctly stated, the power) of signals received in the first few stages of the RF front end328′ in an effort to avoid saturating the ADC612.

For automatic gain control based on signal power, an estimate of the signal power should not include a DC offset error. Therefore, before the second AP block658estimates the signal power from the output of the digital filter614, a DC compensator656, when activated, may remove a DC estimate from an input signal from the digital filter614and send the resulting signal to the second AP block658. The DC estimate may be supplied by the DC estimator622as described above. For some embodiments, the DC compensator656may not be present. In such cases, the DC compensator656as shown may be considered as having a short from its first input from the DF614to its output where the second input from the DC estimator622is ignored.

After the digital filter614has removed out-of-band components from the output of the ADC612(and DC compensation has occurred for some embodiments, courtesy of the DC compensator656), the second AP block658may estimate the average power of the signal. A separate DAGC block660may output a digital control signal or value based on the estimated power, and this control signal may be multiplied with the output of the summer626by the multiplier628in an effort to perform automatic gain control in the digital domain. In this manner, DAGC may be correctly performed based on the signal content in the digital domain, and the quantization error may be reduced.

FIG. 7is a flow chart of example operations700for automatic gain control according to the block diagram650ofFIG. 6B, for example. The operations700may begin, at702, by estimating the signal power of an output of an ADC (e.g., ADC612) with the first AP block652, for example, after a received signal has been signal processed (e.g., amplified, mixed, low-pass filtered, DC compensated, etc.) and input to the ADC. The output of the ADC may most likely not be digitally filtered or otherwise digitally signal processed before estimating the signal power at702. At704, the analog gain may be adjusted based on the estimated received signal power. In this manner, the signal content with potentially large interference signals may be used to automatically adjust the gain of the variable gain amplifiers (e.g., LNA602and/or the PGA606) without running the risk of saturating the ADC.

At706, the signal power of the output of a digital filter (e.g., DF614) may be estimated by the second AP block658, for example, while employing the analog gain set in704. The input of the digital filter may be coupled to the output of the ADC for some embodiments. At708, a digital gain based on the estimated adjusted signal power may be adjusted.

Exemplary Method for Fast AGC using Cyclic Prefixes

Although the block diagram650ofFIG. 6Band the operations700ofFIG. 7may address the problem of saturating the ADC while maintaining analog and digital AGC, there may be other problems associated with AGC. For example, the power of the received signal may be changing quickly in a mobile environment, especially with a user terminal106in a fast-moving vehicle. Existing methods for AGC may use the signal power of the previous OFDM/OFDMA frame to control the receiver gain for the current frame signal. However, this method may be too slow to track the received signal power variation due to fading with a fast-moving user terminal, and the performance may suffer. Accordingly, what is needed is a faster method of AGC.

FIG. 8is a flow chart of example operations800for fast AGC using cyclic prefixes (CPs) of OFDM/OFDMA symbols. The operations800may work in conjunction with the block diagram650ofFIG. 6B, for example. The operations800may begin, at802, by initializing an analog gain for the current OFDM/OFDMA frame. For example, the initial gain of the LNA602and/or the PGA606may be set based on the signal power of the previous OFDM/OFDMA frame. The gain may be set by using control signals from the AGC block654.

At804, the signal power of a received signal amplified with the initial gain may be estimated based on a cyclic prefix (CP) of the signal. To estimate the received signal power, the output of an ADC (e.g., ADC612) may be sent to an AP block (e.g. as the first AP block652) after the received signal has been signal processed (e.g., amplified according to the initial gain, mixed, low-pass filtered, DC compensated, etc.) and input to the ADC, similar to702ofFIG. 7. At806, the analog gain (e.g., the gain of the LNA602and/or the PGA606) may be adjusted based on the estimated received signal power.

FIG. 9illustrates two OFDM/OFDMA symbols902of an OFDM/OFDMA frame, where a latter portion of the data904of each symbol902has been prefixed to the data904to form a cyclic prefix (CP)906, also known as a guard period (GP). With CPs906, a receiver304is able to receive a signal traveling along several different delay paths for a longer time and demodulate the signal without any errors due to intersymbol interference (ISI). A typical OFDM system supports several CP lengths; for example, a WiMAX system supports four different CP lengths: N/4, N/8, N/16, and N/32 where N is FFT size. The CP length may be predetermined for a specific system profile such that a mobile station (MS) can easily determine the CP length by referring to the system profile. In certain systems where the CP length is not predetermined, the CP length may be estimated by the MS during the acquisition process.

When a signal based on an OFDM/OFDMA frame is received, the average power may be estimated during the first part908of the CP906at804, for example. The analog gain may be adjusted during the second part910of the CP906at806, for example. In this manner, by the time the data904is to be read, the analog gain may have been automatically adjusted to a proper level based on the CP906, and the data904of each symbol902may be interpreted with a proper gain.

Referring back toFIG. 8, the signal power of a subsequent OFDM/OFDMA symbol in the current zone may be estimated according to the CP906or the data904within that symbol at808. The received signal for the subsequent symbol may most likely be amplified by the adjusted analog gain from806. The second AP block658may be used to estimate the adjusted signal power. At810, the digital gain may be adjusted based on the estimated adjusted signal power from808. For example, the digital gain may be adjusted by multiplying an output of the DAGC block660based on the estimated adjusted signal power from the second AP block658using the multiplier628.

In this manner, the RF front end328′ may perform fast automatic gain control for the receiver304based on the cyclic prefix of OFDM/OFDMA symbols. Blocks804through810may be repeated to update the AGC and/or DAGC, based on the previous analog gain rather than initializing the analog gain for block804. For some embodiments, AGC and/or DAGC may be updated every CP, while in other embodiments, the analog gain and/or digital gain may be updated every 2 CPs, every 3 CPs, every 4 CPs, etc. The update interval may depend on the speed of the user terminal106: a faster-moving user terminal may suggest a shorter update interval than a fixed user terminal or one that is moving relatively slowly. For some embodiments, the AGC and the DAGC need not be updated at the same CP interval.

For some embodiments, multiple CPs may be evaluated at804and/or at808before an adjustment decision is made at806and/or at810, respectively. For example, the estimated signal power of multiple CPs may be averaged rather than estimating the signal power for a single CP of an OFDM/OFDMA symbol. In the case of averaged signal powers based on multiple CPs, a running average may be performed. For other embodiments, median filtering may be performed on the estimated signal power of multiple CPs, discarding values that are a predetermined statistical difference away from the median sample. Various other types of statistical algorithms may be employed to determine an accurate estimate of signal power based on multiple CPs for fast AGC.

Whatever AGC update interval is selected, different OFDM/OFDMA zones within the DL subframe402may vary in signal power as illustrated inFIG. 5, independent of the velocity of a mobile user terminal. Therefore, fast AGC based on CPs may most likely be conducted at the start of each different zone (e.g., each DL-PUSC zone424or DL-FUSC zone426) within a DL subframe402in an effort to update the analog and digital gains.

Referring back toFIG. 8, for each different zone in the frame at812, the signal power of a received signal for the zone may be estimated based on a CP for that zone, typically on the first CP of the zone, at814. This signal power may be estimated in a similar manner as the power estimation at804. At816, the analog gain (e.g., the gain of the LNA602and/or the PGA606) for the zone may be adjusted based on the estimated received signal power from814, similar to the adjustment at806.

At818, the signal power of a subsequent OFDM/OFDMA symbol in the current zone may be estimated according to the CP906or the data904within that symbol. The received signal for the subsequent symbol may most likely be amplified by the adjusted analog gain from816. This signal power may be estimated in a similar manner as the power estimation at808. At820, the digital gain may be adjusted based on the estimated adjusted signal power from818, similar to the digital gain adjustment at810. For example, the digital gain may be adjusted by multiplying an output of the DAGC block660based on the estimated adjusted signal power from the second AP block658using the multiplier628.

In this manner, the RF front end328′ may perform fast automatic gain control for each zone of an OFDM/OFDMA frame based on the cyclic prefix of OFDM/OFDMA symbols. Blocks814through820may be repeated to update the AGC and/or DAGC within the current zone. As described above, AGC and/or DAGC may be updated based on CPs within the current zone at various intervals.

Exemplary Method for DC Calibration

Not only can large interference signals cause saturation of the ADC in the RF front end328′ of a receiver304as described above, but a DC signal may also be responsible for saturating an ADC, such as the ADC612ofFIGS. 6A and 6B. Therefore, the DC offset may be calibrated in an effort to avoid saturating the ADC, and conventionally, DC calibration has been performed during the initialization stage for the user terminal106. However, because the DC offset may shift due to temperature variation or the Doppler effect for a moving user terminal, DC calibration may need to be performed more frequently, perhaps during communication exchanges between the base station104and the user terminal106. Typically, a receiver304cannot operate during DC calibration, so the communication link is stopped temporarily to update the DC offset, potentially interrupting real-time services such as VoIP (voice over Internet Protocol) and VOD (video on demand). Accordingly, what is needed is a method of updating the DC offset without interrupting a communication link or causing the link to fail.

FIG. 10is a flow chart of example operations1000for DC offset calibration without stopping a communication link. The operations1000may begin, at1002, by calibrating a DC offset during the initialization stage for the user terminal106. This initial DC offset calibration may be performed in any suitable manner known by those of skill in the art.

For each OFDM/OFDMA frame for normal traffic exchange at1003, the analog gain for the RF front end328′ of the receiver304may be set during a gap time, such as the Receive/Transmit Transition Gap (RTG)407before the DL subframe402, at1004. For example, the gain of the LNA602and/or the PGA606may be set during the RTG407to any suitable value in an effort to estimate the DC offset. At1005, the DC offset while amplifying the gap time signal with the gain from1004(i.e., the residual DC) may be estimated and updated. In this manner, the DC offset may be updated during the gap time when no data is being communicated, and therefore, the communication link need not be interrupted for DC offset calibration. Furthermore, by updating the DC offset during the gap time, the ADC is less likely to be saturated when the signal based on the DL subframe402is received.

At1006, the receiver analog gain may be set to the same or a different value from the value in1004in an effort to amplify the received signal based on the DL subframe402without saturating the ADC. The receiver analog gain may be set at1006according to any suitable method for setting the analog gain, including the operations700based on the block diagram650ofFIG. 6Bor the operations800for fast AGC using CPs. Because the analog gain may have changed between the gap time when the DC offset was last updated and the DL subframe402, the DC offset while amplifying the DL subframe402with the gain from1006may be estimated and updated again at1007. The updated DC offset from1007may be used to process additional signals based on subsequent OFDM/OFDMA symbols.

By updating the DC offset during the RTG407before the DL subframe402for every OFDM/OFDMA frame, the DC offset may most likely remain calibrated despite temperature variations and long-term drift of RF front end components, for example. For some embodiments, blocks1004to1005may be repeated at any suitable frame interval, such as once every 2 frames, every 3 frames, every 4 frames, etc, rather than being repeated every frame. Also as described above for fast AGC based on CPs for some embodiments, the analog gain and corresponding DC offset may be updated every symbol according to its CP, while in other embodiments, the analog gain and the corresponding DC offset gain may be updated every 2 CPs, every 3 CPs, every 4 CPs, etc.

Whatever analog gain/DC offset update interval is selected, different OFDM/OFDMA zones within the DL subframe402may vary in signal power as illustrated inFIG. 5, independent of the velocity of a mobile user terminal. Therefore, DC offset calibration may most likely be conducted at the start of each different zone (e.g., each DL-PUSC zone424or DL-FUSC zone426) within a DL subframe402in an effort to update the DC offset according to the potentially adjusted analog gain.

Referring back toFIG. 10, for each different zone in the frame at1008, the receiver analog gain may be set at1010to the same or a different value from the value in1006in an effort to amplify the received signal for the zone without saturating the ADC, similar to the adjustment at1006. Because the analog gain may have changed between the start of the DL subframe402or any DL zone and another subsequent DL zone, the DC offset while amplifying the received signal of the DL zone with the gain from1010may be estimated and updated again at1012. The updated DC offset from1012may be used to process additional signals of the DL zone based on subsequent OFDM/OFDMA symbols in the zone. In other words, the DC offset may be estimated and updated each time the analog gain is or may be modified.

In this manner, the DC offset may most likely remain calibrated despite influences that affect the DC offset (e.g., temperature variations, variations due to the Doppler effect of fast-moving mobile stations, and long-term drift of RF front end components) without saturating the ADC due to DC offset, no matter the gain of the amplifiers in the RF front end328′. Again, the operations1000ofFIG. 10avoid interrupting the communication link for DC offset calibration, thereby permitting real-time services such as VoIP and VOD to operate without disruption or running the risk of saturating the ADC, leading to invalid data.

Exemplary Combination of AGC and DC Calibration

To understand how certain embodiments of the present disclosure may work together,FIGS. 11A-Gillustrate signal power levels and DC offsets in time at various stages in the block diagram650ofFIG. 6Busing the fast AGC based on CPs ofFIG. 8and the DC offset calibration ofFIG. 10, for example.

FIG. 11Aillustrates an example signal power1100as received by the antenna330′ of a receiver304, for example. The signal power1100is based on an OFDM/OFDMA frame and, therefore, has an initial guard interval (during the RTG407) with substantially no power followed by symbols with significant power from the DL subframe402, beginning with the preamble408as described above. As illustrated, the signal power1100is fading with time, perhaps due to the antenna330′ moving further away from a serving base station, such that the last illustrated OFDM symbol1101inFIG. 11Ahas the smallest signal power of the four symbols depicted. The signal power1100may include some interference.

FIG. 11Billustrates an example signal power1102at the output of the ADC612after the signal power1100received by the antenna330′ has been amplified, mixed, filtered, or otherwise processed. During the gap time, the analog gain may be initialized according to blocks802or1004. Once the gain has been initialized, this may create a residual DC offset1103in the signal power1102that should be calibrated out. The residual DC offset1103may be estimated during the gap time according to block1005, for example.

Once the first CP has been received, the average signal power of the first CP in the signal power1102may be estimated according to block804, and the analog gain may be adjusted according to block806or1006, for example. Thus, the signal power1102had an initial gain, but the gain increased during the reception of the first CP such that the signal power1102is significantly greater than the signal power1100. Based on the block diagram650with the two separate AP blocks652,658, the signal power1100may be properly amplified in the presence of large interference signals to the signal power1102without saturating the ADC612, according to embodiments of the present disclosure described above.

If the last illustrated OFDM symbol1101is the first symbol of a new zone, then the average signal power of the first CP of the new zone may be estimated according to block814, for example, as shown for signal power1102. The analog gain for the new zone may be adjusted according to block816or1010, for example. By adjusting the analog gain for the new zone, the signal power1102for the last illustrated OFDM symbol1101may be amplified to a similar signal power as the first OFDM symbol containing the preamble408for the signal power1102at the output of the ADC, in spite of the fading received signal power1100.

FIG. 11Cillustrates an example signal power1104at the output of the digital filter614after the interference has been removed. Because of the removal of the interference, the signal power1104may be significantly smaller than the signal power1102at the output of the ADC612. The DC offset may be estimated by the DC estimator622during the first portion of the OFDM/OFDMA frame (e.g., in the preamble408as the first symbol) according to block1007, for example, and perhaps again in a subsequent symbol as illustrated inFIG. 11C. The DC offset may also be estimated by the DC estimator622during the first symbol of a different DL zone (i.e., the last illustrated symbol1101) according to block1012, for example, as illustrated.

FIG. 11Dillustrates an example signal power1106at the output of the DC compensator656. The signal power1106is similar to the signal power1104, except that DC compensation may have been activated during a data portion of the same symbol with the CP used to estimate the signal power according to block808and again during a data portion of a subsequent symbol according to block808or818, for example. In this manner, the DC offset from the DC estimator622(as measured according to block1007or1010, for example) may be removed from the signal power1104in an effort to correctly estimate the signal power and properly set the digital gain. Note the DC offset deviations1107in the signal power1106.

FIG. 11Eillustrates an example signal power1108at the output of the buffer624, portraying a delayed version of the signal power1104at the output of the digital filter614. The gain and offset of the signal power1108may most likely be the same as the signal power1104.

FIG. 11Fillustrates an example signal power1110at the output of the summer626, depicting a DC-adjusted version of the signal power1108at the output of the buffer624according to the DC estimates made by the DC estimator622on the signal power1104at the output of the digital filter614according to block1007or1010, for example. The signal power1110illustrates the desired removal of the residual DC offset1103if the DC estimates were performed correctly.

FIG. 11Gillustrates an example signal power1112at the output of the multiplier628after digital automatic gain control based on the estimates of the signal power1106have been applied according to block810and again according to block810or820. The signal power1112at the output of the multiplier628is similar to the signal power1110at the output of the summer626with the residual DC offset1103removed, with the exception of the digital gain applied to the OFDM/OFDMA symbols during the symbol interval between estimates of the signal power made by the second AP block658to the signal power1106. The signal power1112may most likely be ready for further processing by the data demodulator630with the DC offset and interference removed, no signal degradation due to ADC saturation, and the effects of fading mitigated for increased signal-to-interference-plus-noise ratio (SINR).

The operations described above may be performed by various hardware and/or software component(s) and/or module(s) corresponding to a number of means-plus-function blocks. For example, the operations700ofFIG. 7described above may be performed by various hardware and/or software component(s) and/or module(s) corresponding to the means-plus-function blocks700A illustrated inFIG. 7A. In other words, blocks702through708illustrated inFIG. 7correspond to means-plus-function blocks702A through708A illustrated inFIG. 7A.

Information and signals may be represented using any of a variety of different technologies and techniques. For example, data, instructions, commands, information, signals and the like that may be referenced throughout the above description may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles or any combination thereof.