Patent Publication Number: US-2010119020-A1

Title: Blanking Techniques in Receivers

Description:
RELATED APPLICATION(S) 
     The present application claims the benefit of co-pending India provisional application serial number: 2763/CHE/2008, entitled: “An automatic Blanking Strategy for GPS Receivers”, filed on Nov. 11, 2008, naming Texas Instruments, Inc. (the intended assignee of this U.S. application) as the Applicant, and naming the same inventors as in the present application as inventors, attorney docket number: TXN-243, and is incorporated in its entirety herewith. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     Embodiments of the present disclosure relate generally to receivers used in communication devices, and more specifically to blanking techniques in receivers. 
     2. Related Art 
     A receiver refers to a component which receives a modulated signal bearing information, as an input signal, extracts the information (by demodulation) from the modulated signal and provides the information for further processing. For example, a GPS (global positioning system) receiver receives modulated signals from satellites, extracts the information contained in the received input signal, and provides the extracted information for determination of the position (geographical coordinates, for example, in terms of longitude and latitude) of the receiver. 
     Blanking techniques are often employed in receivers. As is well known in the relevant arts, blanking refers to blocking the input signal (containing potentially both of the modulated signal and interference/jamming signal) from being provided to one or more internal components of the receiver upon the occurrence of an undesirable state in the input signal. For example, blanking is implemented in GPS receivers to force signal/data inputs (provided to various components) to ground (or provide binary zeros as inputs) when a transmitter indicates a time duration in which a signal is being transmitted. The receiver is blanked in view of the possibility that the transmitted signal may contain interference (jamming signals). Well known situations in which such jamming signals may be produced include durations when GSM, WLAN, CDMA types of transmitters transmit corresponding signals. 
     By blanking the receiver, the jamming signal is prevented from causing various undesirable effects in receivers. For example, the a GPS receiver may be prevented from interpreting a jamming signal (received in addition to satellite signals in the input signal) as having valid content, and thereby from potentially computing a wrong position and/or from failing to detect a satellite signal that is present in the input signal. As an illustrative example, some GPS receivers are designed to extrapolate a present position based on previously computed positions in the absence of input signal, and by forcing the input signal to ground, the GPS receiver treats the situation as absence of input signal. Accordingly, an extrapolated position is provided as the present position until valid input data is received on the input signal. In some other GPS receivers, saturation of components such as amplifiers, may be prevented by blanking the input signal. 
     It is generally desirable that receivers not be blanked when not necessary such that the information in the input signal can be effectively used. 
     SUMMARY 
     This Summary is provided to comply with 37 C.F.R. §1.73, requiring a summary of the invention briefly indicating the nature and substance of the invention. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. 
     An aspect of the present invention detects the presence of interference by examining an input signal provided to the receiver, and blanks the receiver if interference is detected. Information contained in the input signal may be recovered otherwise. By detecting the presence of interference based on the content of the input signal within the receiver itself, the need for external indicators of interference (e.g., by a separate path) are avoided. 
     In an embodiment in which the interference is periodic (e.g., as in GSM based interference), the period of the interference, the start and end time points (of the interference) in each period/cycle are also determined by examining the input signal. The information may be used to blank the receiver only during ON-intervals of each cycle thereafter. 
     According to another aspect, the duty cycle of an interference is determined by examining the input signal, and a threshold strength having a positive correlation with the duty cycle is determined. If the amplitude of the jamming signal during the on-interval (of the duty cycle) is greater than the threshold strength, only then is the receiver blanked, in addition to reduction in gain of an amplifier in the path from the input path to a baseband processor. Otherwise, only the gain of an amplifier in the path from the input path to a baseband processor is reduced. 
     Thus, in such an embodiment, a baseband processor may receive samples (with such reduced amplitude) and still be able to recover the information in the input signal when only low level of interference is present. 
     Several aspects of the invention are described below with reference to examples for illustration. It should be understood that numerous specific details, relationships, and methods are set forth to provide a full understanding of the invention. One skilled in the relevant art, however, will readily recognize that the invention can be practiced without one or more of the specific details, or with other methods, etc. In other instances, well-known structures or operations are not shown in detail to avoid obscuring the features of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE VIEWS OF DRAWINGS 
       Example embodiments of the present invention will be described with reference to the accompanying drawings briefly described below. 
         FIG. 1  is a block diagram of an example device in which several aspects of the present invention can be implemented. 
         FIG. 2  is a block diagram of a receiver in an embodiment of the present invention. 
         FIG. 3  is a flowchart illustrating the manner in which a receiver is blanked in an embodiment of the present invention. 
         FIG. 4  is a diagram used to illustrate jamming interference and measurement detection windows in a receiver, in an embodiment of the present invention. 
         FIG. 5  contains a table showing the relationship between duty cycle of a jamming signal and corresponding threshold strengths to be used in providing blanking, in an embodiment of the present invention. 
         FIGS. 6A and 6B  are example waveforms illustrating interference mitigation provided in an embodiment of the present invention. 
     
    
    
     The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number. 
     DETAILED DESCRIPTION  
     Various embodiments are described below with several examples for illustration. 
     1. Example Device 
       FIG. 1  is a block diagram of an example device in which several aspects of the present invention can be implemented. The block diagram shows mobile phone  100 , which is in turn shown containing GPS receiver  150 , GSM (Global System for Mobile Communication) block  151 , WLAN (wireless local area network) block  152 , blue tooth (BT) block  153 , WCDMA block  154 , application block  160 , display  170 , input/output (I/O) block  180 , and memory  190 . The components/blocks of mobile phone  100  in  FIG. 1  are shown merely by way of illustration. However, mobile phone  100  may contain more or fewer components/blocks. Further, while in the examples below, blanking techniques are described with respect to GPS receivers, the techniques can be applied in the context of other types of receivers (whether wireless or wireline), and in other environments as well. 
     GPS receiver  150  is shown containing GPS processing block  110 , filter  115  and GPS antenna  105 . GSM block  151  is shown containing GSM transceiver  120  and transmit antenna  106 . WLAN (Wireless LAN) block  152  is shown containing WLAN transceiver  130  and transmit antenna  107 . Blue tooth (BT) block  153  is shown containing BT transceiver  140  and transmit antenna  108 . WCDMA block  154  is shown containing WCDMA (Wideband Code Division multiple Access) transceiver  145  and transmit antenna  109 . GSM block  151 , WLAN block  152 , BT block  153  and WCDMA block  154  may contain respective receive antennas and filters as well, but are not shown in  FIG. 1 . 
     Blocks  110 ,  120 ,  130 ,  140  and  145  may be implemented, for example, as separate integrated circuits (IC), or implemented on a single IC. Typically, the components (ICs, filter and antennas) of  FIG. 1  are mounted on a printed circuit board (PCB), with corresponding tracks providing the electrical connectivity represented by paths  111 ,  101 ,  126 ,  137 ,  148  and  149 . 
     GSM block  151  operates to provide wireless telephone operations, with GSM transceiver  120  containing receiver and transmitter sections to perform the corresponding receive and transmit functions. WCDMA block  154  operates to provide wireless telephone features according to WCDMA techniques. Similarly, each of WLAN block  152 , and BT block  153  provides wireless (data and/or voice) communication (receive as well as transmit) according to the corresponding techniques/protocols. 
     GPS antenna  105  receives satellite signals from GPS satellites, and provides the satellite signals to filter  115  via path  101 . Filter  115 , which may be implemented as a surface acoustic wave (SAW) filter, provides band-pass filtering to the satellite signals received on path  101 , and provides filtered signals to GPS processing block  110  via path  111 . Either of paths  111  or  101  may be viewed (and referred to below) as an input path of GPS receiver  150 . GPS processing block  110  processes the received GPS signals, decodes/demodulates the signals, extracts data contained in the signals, and computes the position of GPS receiver  150 . 
     Application block  160  may contain corresponding hardware circuitry (e.g., processors), and operates to provide various user applications provided by mobile phone  100 . The user applications may include voice call operations, data transfers, providing positioning information, etc. Application block  160  may operate in conjunction with blocks  150 - 154  to provide such features, and communicates with the respective blocks  150 - 154  via paths  161 - 165 . 
     Display  170  displays image frames in response to the corresponding display signals received from application block  160  on path  176 . The images may be generated by a camera provided in mobile phone  100 , but not shown in  FIG. 1 . Display  170  may contain memory (frame buffer) internally for temporary storage of pixel values for image refresh purposes, and may be implemented, for example, as a liquid crystal display screen with associated control circuits. I/O block  180  provides a user with the facility to provide inputs via path  186 , for example, to dial numbers. In addition I/O block  180  may provide outputs (on path  186  that may be received via application block  160 . Such outputs may include position, data, images etc. 
     Memory  190  stores program (instructions) and/or data (provided via path  196 ) used by applications block  160 , and may be implemented as RAM, ROM, flash, etc, and thus contains volatile as well as non-volatile storage elements. 
     Transmissions by GSM block  151 , WLAN block  152 , BT block  153  or WCDMA block  154  that fall within the receive band of GPS receiver  150  (and propagated on paths  111  or  101 ) may interfere with the normal operations of GPS receiver  150  (or GPS processing block  110 ), thereby potentially degrading the sensitivity of the receiver or causing complete disruption of normal operations. As an example, the bandwidth of interest of GPS receiver  150  (assuming an LI C/A band receiver) is 1575.42±1 MHz. 
     Transmissions (for example, spurious missions due to insufficient transmit filtering from one or more of blocks  160 ,  170  and  180  falling within the bandwidth of interest) may be received by GPS processing block  110  via antenna  105 , filter  115  and input path  111 ). Such transmissions represent jamming interference with respect to GPS receiver  150 . In this document, the terms ‘jamming signal’, ‘interference’ and ‘jamming interference’ are used interchangeably, and all have the same meaning(s). 
     As another example, transmit signals from WLAN block  152  and GSM block  151  on respective paths  137  and  126  may be coupled (e.g., by electromagnetic induction, due to proximity of the paths) into path  101  due to the manner in which the paths are physically provided on a PCB. Assuming GSM signals on path  126  are in the band 824-849 MHz, and BT signals on path  148  are in the band 2402-2480 MHz, non-linear characteristics of filter  115  (implemented, for example, as a SAW filter) may cause the generation of interference falling in the bandwidth of interest of GPS receiver  150 . For example, non-linearity of filter  115  may cause the generation of an inter-modulation product of frequency of approximately 1575 MHz, which is within the bandwidth of interest of GPS receiver  150 , and thus constitutes interference. It is noted such interference due to inter-modulation products may be particularly strong (and hence disruptive) when transmissions from WCDMA block  154  mix with WLAN signals in filter  115 . Other sources, such as devices external to mobile phone  100 , may also be potential sources of interference. 
     Unless measures are taken, the interference may cause undesirable effects in receivers. For example, a GPS receiver may not be able to continue to compute positions accurately since the GPS signal itself is weak, compared to possible strength of interference signals in several situations. As noted above, blanking is often used to minimize the adverse effects of jamming interference, and is illustrated next. First, however, the details of a receiver in an embodiment of the present invention are provided. 
     2. Receiver 
       FIG. 2  is a block diagram of a receiver in an embodiment of the present invention. GPS receiver  150  is shown containing GPS antenna  105 , filter  115 , low-noise amplifier (LNA)  220 , mixers  225 A and  225 B, filters  230 A and  230 B, programmable gain amplifiers (PGA)  240 A and  240 B, analog to digital converters (ADC)  250 A and  250 B, multiplexers (MUX)  260 A and  260 B, baseband processor  270 , automatic gain control block (AGC)  280 , and storage  290 . Again, the details are provided merely by way of illustration, and other designs/implementations are also possible. 
     Filter  115  and GPS antenna  105  operate as described above with respect to  FIG. 1 . The band-pass characteristics of filter  115  (e.g., implemented as a SAW filter) are selected corresponding to the desired bandwidth of interest of GPS receiver  150 . Filter  115  provides band-pass filtered signals on path  111  to LNA  220 . Either of paths  101  and  111  is referred to below as ‘input path’ of GPS receiver  150 . 
     LNA  220  amplifies signals on input path  111  with minimal addition of noise, and provides the amplified signals (on path  222 ) to each of mixers  225 A and  225 B. Mixers  225 A and  225 B operate to down-convert signals on path  222  to a lower frequency (e.g., an intermediate frequency (IF), or a final baseband frequency depending on the specific implementation). 
     Mixer  225 A receives a local oscillator (LO) signal (I-component) on path  229 A, and multiplies the LO signal with the signal  222  to generate an output that is provided to filter  230 A. Mixer  225 B receives a local oscillator (LO) signal (Q component,  90  degrees phase shifted from the I-component) on path  229 A, and multiplies the LO signal with the signal  222  to generate an output that is provided to filter  230 B. Filters  230 A and  230 B remove (filter) the undesired side-band outputs generated by the respective mixers, and provide corresponding down-converted I and Q signals to respective PGAs  240 A and  240 B. 
     PGAs  240 A and  240 B receive corresponding gain values on respective paths  284 B and  284 A, and amplify the respective inputs received from respective filters  230 A and  230 B by the corresponding received gain values. PGAs  240 A and  240 B provide the respective gain signals to respective ADCs  250 A and  250 B. In alternative embodiments, PGAs  240 A and  240 B may not be implemented separately, and the programmable gain may instead be integrated into the basic operation/function of ADCs  250 A and  250 B. 
     ADCs  250 A and  250 B sample the respective inputs received from the PGAs, and generate corresponding digital codes representing the strengths of the sampled input. ADCs  250 A and  250 B provide the respective sequences of digital codes on respective paths  256 A and  256 B. 
     Multiplexer (MUX)  260 A forwards one of the inputs on paths  256 A and  281  on path  267 A, based on the binary value of select signal  286 . Similarly, MUX  260 B forwards one of the inputs on paths  256 B and  281  on path  267 B, based on the binary value of select signal  286 . 
     Baseband processor  270  processes the inputs received on paths  267 A and  267 B, to determine the position of GPS receiver  150 . Baseband processor  270  may contain hardware acceleration blocks such as hardware correlators, in addition to general purpose processing blocks. Baseband processor  270  provides on path  161  (to application block  160 ) outputs such as position, time, and any other desired parameters (for example, parameters generated during internal processing, or raw data received from GPS satellites). The outputs may be used by various user applications noted above. 
     Storage  290  stores data values (received via path  297 ) extracted by baseband processor  270  from corresponding GPS signals. Various intermediate data values generated during processing of signals by baseband processor  270  are also stored in storage  290 . In addition, assuming baseband processor  270  operates using instructions (rather than as a hardwired component such as FPGA, ASIC, etc.) the corresponding program (instructions) and/or data may also be stored in storage  290 . Storage  290  may be implemented as RAM, ROM, flash, etc, and thus contains volatile as well as non-volatile storage elements. 
     AGC block  280  receives digital codes on paths  256 A and  256 B, and determines the amplitude of the signal represented by the codes. Based on the amplitude determined, AGC  280  may increase or decrease the gain of PGAs  240 A and  240 B. When no interference is determined to be present, AGC block  280  typically operates to adjust the gain of the PGAs such that a substantially constant amplitude level is maintained in the signal represented by the digital codes. 
     Thus, in the absence of interference, the gain values provided to the PGAs is greater for a smaller amplitude of the ‘desired’ signal (GPS signal in the receiver of  FIG. 2 ) in the input path, and smaller for a larger amplitude of the desired signal. The amplitude of the desired signal may be determined by baseband processor  270 . In the context of GPS signals, such amplitude is determined by correlation operations, as is well known in the relevant arts, and indicated by baseband processor  270  to AGC block  280  via path  278 . 
     However, in other implementations, the gain values for PGAs  240 A and  240 B even in the absence of interference may be determined by AGC block  280  based on the sample values received on paths  256 A and  256 B. It is noted here (and also well-known in the relevant arts) that the desired signal strength of a GPS signal is typically much weaker than thermal noise generated by GPS receiver  150 . Hence, gain values provided to PGAs  240 A and  240 B are determined according to the amplitude of the thermal noise (or any other dominant noise source) of GPS receiver  150  rather than the “strength” of the GPS signal transmitted from satellites. 
     However, when AGC block  280  detects interference on the input path of GPS receiver  150 , AGC block  280  may either provide a blanking signal (e.g., digital codes with a predetermined value, for example, binary zero) on path  281 , or reduce the gain of PGAs  240 A and  240 B, as described below with respect to  FIG. 3 . 
     While AGC block  280  is shown as a block separate from baseband processor  270 , in other embodiments of the present invention, the operations of AGC block  280  may be integrated within (provided by) baseband processor  270 . Further, alternative embodiments of the present invention can be implemented with greater or lesser level of integration and/or different techniques and blocks from those shown in  FIG. 2 , as will be apparent to one skilled in the relevant arts. 
     In an embodiment of the present invention, AGC block  280  operates (in addition to the functions noted above) to determine if jamming interference is present or not, and to operate to mitigate the effects of such interference, as described next with respect to a flowchart. 
     3. Interference Mitigation 
       FIG. 3  is a flowchart illustrating the manner in which interference mitigation is provided in a receiver, in an embodiment of the present invention. The flowchart is described with respect to the device and components of  FIGS. 1 and 2 , and in relation to AGC block of a GPS receiver, merely for illustration. However, various features described herein can be implemented in other environments and using other components, as will be apparent to one skilled in the relevant arts by reading the disclosure provided herein. 
     Furthermore, the steps in the flowchart are described in a specific sequence merely for illustration. Alternative embodiments using a different sequence of steps can also be implemented without departing from the scope and spirit of several aspects of the present invention, as will be apparent to one skilled in the relevant arts by reading the disclosure provided herein. The flowchart starts in step  301 , in which control passes immediately to step  320 . 
     In step  320 , AGC block  280  monitors the input signal for the presence of a jamming signal (interference). Monitoring implies that the strength of the input signal is examined. The input signal is shown received on path  101 , filtered in filter  230 A and shown provided as digital samples to baseband processor  270 . AGC block  280  determines/detects the presence of jamming signal(s), for example, by checking if sample values in the input path have energy (in theory, defined as amplitudes summed/integrated over an interval of time) greater than that expected if no jamming signals were present. 
     In an embodiment, AGC block  280  adds received signal values contained in one or more of corresponding time windows to determine the corresponding energy level, and compares the sum with a threshold. With respect to  FIG. 2 , the threshold is typically equal to a value (offset+noise floor of GPS receiver  150 ), the offset being pre-computed and stored in GPS receiver  150 . (Example offset are noted in column two of the table of  FIG. 5 ) A sum greater than the (threshold) value is deemed to indicate the presence of a jamming signal in the input path. In an embodiment, AGC block  280  computes the average of squared values of signal samples received in a time interval. AGC block  280  then compares the computed average with the threshold to determine whether jamming has occurred (is present) or not. 
     In an alternative embodiment, AGC block  280  computes the average of the sample values in a time window. AGC block  280  then compares the average with the threshold, and if the average is greater than the threshold, AGC block  280  determines that a jamming signal is present. Control then passes to step  330 . 
     In step  330 , AGC block  280  determines if a jamming signal is present. If a jamming signal is deemed to be present (e.g., using techniques noted above), control passes to step  340 , otherwise control passes to step  320 . 
     In step  340 , AGC block  280  measures the duty cycle as well as a strength (amplitude level) of the jamming signal (detected or deemed to be present as noted above). As is well known, the duty cycle is generally computed as the ratio of ON time and the sum of ON time and OFF time. It may be appreciated that determination of the duty cycle of the jamming signal may require several occurrences of the jamming signal. AGC block  280  may therefore record the times of occurrences (e.g., start and end) of the jamming signal over a period of time (e.g., several seconds), and then compute the time difference between the end instance of the jamming signal and the immediately next start instance of the jamming signal. Based on the above computations, AGC  280  may then compute the duty cycle of the jamming signal. During the initial duration of operation of GPS receiver  150 , (e.g., immediately after power-ON) AGC block  280  may select a pre-determined value for the duty cycle. The pre-determined value may be based on past history (past occurrences of jamming signals), or may be pre-computed and stored in GPS receiver  150 . 
     In an embodiment, AGC block  280  determines the strength of the jamming signal as the average power level of the jamming signal, obtained by averaging the squared values of the samples contained during an interval (ON interval) in which the jamming signal is present. However, in alternative embodiments, AGC block  280  may use other indicators of the strength (measured strength) a jamming signal, such as, for example, summed sample magnitudes over a time window. In general, the measured strength represents the amount by which the noise floor of the receiver has increased from a value corresponding to when the jamming signal is absent. Control then passes to  345 . 
     In step  345 , AGC block  280  reduces the gain of an amplifier in a path from the input path to a baseband processor. To illustrate with respect to  FIG. 2 , AGC block  280  may reduce the gain provided by PGAs  240 A and/or  240 B (or the gain of ADCs  250 A and  250 B) for the duration of the jamming signal. In an embodiment, AGC block  280  reduces the gain of PGAs  240 A and  240 B by a value equal (in decibels) to the measured strength of the jamming signal. Control then passes to step  350 . 
     In step  350 , AGC block  280  obtains a threshold strength corresponding to the duty cycle of the jamming signal determined in step  340 . The threshold strength, which may be determined a priori as a function of duty cycle, ensures that optimal receiver performance is obtained for all jammer power levels and all duty cycles. For example, if actual duty cycle of the jammer (jamming signal) is 70%, but is assumed to be 30%, then, suboptimal receiver performance is obtained if jammer is 8 dB stronger than receiver noise floor. In an embodiment, the threshold strengths are designed to have a positive correlation (i.e., a higher threshold is generally selected for correspondingly higher duty cycle value) with respect to duty cycles. Control then passes to  360 . 
     In step  360 , AGC block  280  determines if the measured strength of the jamming signal is less than the threshold strength. If the measured strength is less than the threshold strength, control passes to step  320 , otherwise control passes to step  380 . 
     In step  380 , AGC block  280  blanks GPS receiver  150 . With respect to  FIG. 2 , blanking (also referred to as receiver blanking) is performed by blocking the input signal to baseband processor  270 . The ‘normal’ input (e.g., digital codes on paths  256 A and  256 B) is blocked from being propagated further, and a (constant) digital value of zero (i.e., representing ground or no input signal) is instead provided. Control then passes to step  320 , and the flow chart may be repeated with respect to subsequent segments (portions) of the input signal received on path  101 . 
     It may be appreciated that the technique described above does not require an external indication of a possible presence of interference, as may be employed in some prior approaches. Such prior approaches may lead to less-than-optimal solutions at least for the reason that the interference indication may be provided even when the actual interference falls outside of the receivers&#39; band of interest (receiver input band). For example, GSM signals may be broadcast in any of several frequency bands, not all of which may fall (at least partially) within a receivers input band. Further, such prior approaches may not take into account the strength of the interference, thereby causing the receiver not to use information in the modulated signal. 
     Thus, according to an aspect of the present invention, interference detection is performed within the receiver itself based on the signal levels (represented by amplitudes of the samples) of the input signal. Further, both the strength as well as the duty cycle of the interference are determined, and the specific interference mitigation approach is based on both the strength as well as the duty cycle of the interference signal. Thus, for example, when the measured strength (step  360 ) is less than the corresponding threshold strength (set based on the duty cycle of the interference), the receiver may not be blanked, but only a signal gain reduced (step  345 ). 
     Therefore, the receiver may continue to operate normally to recover information contained in the input signal (GPS raw data in the example of  FIG. 2 ) even in the presence of interference if such interference is weak enough not to cause significant degradation in the sensitivity of the receiver in extracting and processing the signal of interest (GPS signals in example of  FIG. 2 ). The receiver may be blanked only when the measured strength exceeds the corresponding threshold strength. 
     In several operating scenarios (such as, for example, in the presence of GSM transmissions), the interference may be periodic/cyclical. In an embodiment, prior to blanking the receiver, AGC block  280  may determine the start and end time instances of the interference, as well as the time period of the interference. The manner in which such determination is done is described next with respect to an example waveform. 
     4. Detection of Start and End of Interference 
       FIG. 4  is a diagram used to illustrate the manner in which the start and end points of a periodic interference is detected, in an embodiment of the present invention. The diagram is shown containing waveform  411 , which represents levels of the input signal of GPS receiver  150 . Interference signal  420  is shown as being present (ON) in intervals (time windows) t 0 -t 1  and t 2 -t 3 . Time interval t 0 -t 2  represents one cycle of the interference, which is assumed to repeat with the same period starting from time instance t 2 . 
     A portion of the next cycle (starting from time instance t 2 ) is also shown. Merely for illustration, both ON intervals (t 0 -t 1 ) as well as (t 2 -t 3 ) are shown as having the same strength  410 . However, in general, the strength of the jamming signal may vary from cycle to cycle. Further, portion/segment t 1 -t 2  of input signal  411  is shown as representing the receivers noise floor (NF) even though GPS signals (modulated signals) may be present in the interval t 1 -t 2 , since GPS signals typically have very low signal strengths, and are buried in noise, as is well known in the relevant arts. 
     It may be appreciated that the start of the jamming signal may first need to be determined. In an embodiment, AGC block  280  adds digital values of samples provided on each of paths  256 A and  256 B for the duration of short time intervals (detection windows). AGC block  280  adds digital values on paths  256 A and  256 B for the duration of a detection window, and then adds the individual sums (obtained for each of paths  256 A and  256 B) to obtain a final sum. AGC block  280  compares the final sum with a threshold (noise floor of GPS receiver  150  in the example of  FIG. 2 ). If the final sum corresponding to a ‘current’ detection window is greater than the threshold, but a final sum for an immediately previous detection window is not greater than the threshold, AGC block  280  concludes that the start of the ON duration of the jamming signal has occurred in the ‘current’ detection window. 
     As an illustration, three short detection windows represented by corresponding time intervals t 40 -t 41 , t 41 -t 42  and t 42 -t 43  are shown in  FIG. 4 . It may be observed that the sums of sample values in each of windows t 41 -t 42  and t 42 -t 43  would be greater than a threshold represented by the receiver noise floor power level (NF), while sum in window t 0 -t 1  would (approximately) equal the NF. Assuming detection window t 41 -t 42  to corresponds to a currently received set of sample values, since the sum of the sample values in detection window t 41 -t 42  is greater than the threshold, while the sum in the previous detection window is equal to the threshold, AGC block  280  concludes that the start of the jamming signal has occurred in the ‘current’ detection window t 41 -t 42 . 
     In an embodiment, the width (time duration) of the detection windows is selected to be much shorter than an ‘expected duty cycle’ (which may range for example, from 2% to 85%) or ON duration of a potential jamming signal. It is noted here that, at least in some operating scenarios, the ‘expected duty cycle’ or the ON duration of a potential jamming signal may be reasonably assumed or estimated. For example, transmit durations (ON times) as well as transmit repetition intervals of GSM transmitters (e.g., contained in GSM transceiver  120 ) are often fixed by the corresponding standard, and may therefore be reasonably well predicted. 
     In an embodiment, the sum of sample values in a detection window is computed according to the following equation: 
     
       
         
           
             
               
                 
                   
                     Q 
                     n 
                   
                   = 
                   
                     
                       ∑ 
                       
                         k 
                         = 
                         1 
                       
                       N 
                     
                      
                     
                       
                         W 
                          
                         
                           ( 
                           k 
                           ) 
                         
                       
                        
                       
                         x 
                         k 
                         2 
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   1 
                 
               
             
           
         
       
     
     wherein, 
     x k  represents the value of the k th  sample, for values of k from 1 to N, N being the total number of samples in a detection window,
 
W(k) is a windowing or weighting function used for the detection window, with the weights specified by W(k) having a greater value for greater values of index ‘k’.
 
     In an embodiment, windowing function W(k) has the following form/expression: 
     
       
         
           
             
               W 
                
               
                 ( 
                 k 
                 ) 
               
             
             = 
             
               { 
               
                 
                   
                     
                       1 
                       , 
                       
                         k 
                         &lt; 
                         
                           N 
                           2 
                         
                       
                     
                   
                 
                 
                   
                     
                       2 
                       , 
                       
                         
                           N 
                           2 
                         
                         ≤ 
                         k 
                         &lt; 
                         
                           
                             3 
                              
                             N 
                           
                           4 
                         
                       
                     
                   
                 
                 
                   
                     
                       4 
                       , 
                       
                         
                           
                             3 
                              
                             N 
                           
                           4 
                         
                         ≤ 
                         k 
                         &lt; 
                         
                           
                             7 
                              
                             N 
                           
                           8 
                         
                       
                     
                   
                 
                 
                   
                     
                       8 
                       , 
                       
                         k 
                         ≥ 
                         
                           
                             7 
                              
                             N 
                           
                           8 
                         
                       
                     
                   
                 
               
             
           
         
       
     
     In yet another embodiment, windowing function W(k) has the following form/expression: 
         W ( k )= k   2    
     Since the values of windowing function W(k) increase as the index ‘k’ increases, samples closer to the end the detection window are provided greater weight. Such a windowing technique enables better detection of a jamming signal that begins late in the integration cycle. The detection windows employed in the embodiment may be non-overlapping. However, in other embodiments, each detection window may be selected to overlap with previous and next detection windows, to enable quicker detection of a potential jamming signal. While windowing function W(k) is described above as being used for detection of a jamming signal (i.e., when receiver has not yet detected a jammer), it can also be used for detection of “end of jamming signal” after the jamming signal has been detected. 
     By similarly summing sample values in corresponding detection windows (not shown) in other portions of waveform  411 , AGC block  280  determines the end of the ON interval (e.g., time instance t 1  in  FIG. 4 ). As will be readily appreciated, the sum of samples would need to be lower than a threshold, for the end of the ON interval to be detected. In a similar manner AGC block  280  also determines the end of a cycle and thus the time period (time interval t 0 -t 2  in  FIG. 4 ) of jamming signal  420 . As will be appreciated, the end of a cycle may be determined in a manner similar to determination of start of a cycle described above. 
     Having thus determined the ON interval and the time period of jamming signal  420 , AGC block  280  computes the duty cycle as the ratio of the ON interval (t 0 -t 1 ) and the time period (t 0 -t 2 ) of the cycle of the jamming signal. In an embodiment, AGC block  280  computes the average value of the duty cycles (computed as described above) over an interval of time, and uses the average value in further operations. AGC block  280  may then determine the strength of the jamming signal, as described next. 
     5. Measuring the Strength of Interference 
     In an embodiment, AGC block  280  determines the strength of the jamming signal by averaging the squared values (average power) of the samples contained during ON interval (t 0 -t 1 ), as noted above, though alternative approaches can be used to measure the strength of the jamming signal. The value of the average thus obtained is subtracted from the noise floor (NF) of the receiver, and the difference represents the strength of the jamming signal. In  FIG. 4 , the average power of jamming signal  420  is denoted by marker  410 . 
     In an embodiment, to determine the NF of the receiver, AGC block  280  computes the average value of the square of the sample values on either of paths  256 A and  256 B for the duration of a time interval of several seconds (e.g., 5 seconds). AGC block  280  then subtracts a value representing the current gain setting in the corresponding PGA (i.e., PGA  240 A if the average value is computed based on samples on path  256 A, and PGA  240 B if the average value is computed based on samples on path  256 B). AGC block  280  may repeat the operation of above for multiple (5-second) time intervals, and selects the least difference obtained as the NF, which is then used, as noted above, as the reference for measurements of jamming signal strengths. It may be appreciated that the NF computed as described above represents the noise floor as would be measured at output  222  of LNA  220 . 
     It is noted that noise floor (NF) of GPS receiver  150  may vary with respect to time. Further, noise floor may also be different for different GPS receivers. It may be appreciated from  FIG. 4 , that since jamming signal strengths (e.g., as indicated by  410  in  FIG. 4 ) are measured with respect to the NF, it may be required to compute (or recalibrate) the NF at regular intervals. According to an aspect of the present invention, AGC block  280  computes the NF at regular intervals (e.g., every few seconds throughout the duration of operation of GPS receiver  150 ). It may be appreciated that the regular recalibration of the NF enables an accurate determination of the strength of jamming signals, since jamming signal strength is measured relative to the NF. 
     Having thus, determined the strength of the jamming signal, AGC block  280  determines the specific type of interference mitigation (gain reduction alone, or blanking in addition to gain reduction), as described next. 
     6. Applying Interference Mitigation 
     As noted above with respect to the flowchart of  FIG. 3 , AGC block  280  compares the measured strength of a jamming signal with a threshold strength corresponding to the duty cycle of the jamming signal. In an embodiment, values of duty cycles and corresponding threshold strengths are determined with the goal of recovering information from GPS signals (modulated signal in the example of  FIG. 2 ) in as many situations as possible. However, alternative approaches can be used to determine the threshold strengths. The duty cycles and corresponding threshold values (shown in table  500  of  FIG. 5 ) may thus be pre-computed and stored (for example, in the form of a table, in storage  290 ) for quick access.  FIG. 5  shows an example table ( 500 ) containing as entries, duty cycle values and corresponding threshold strengths. Table  500  is shown merely by way of illustration, and typically more table entries (more number of duty cycle values and corresponding threshold strengths) may be used. 
     Column  1  of table  500  contains the duty cycle entries, with the duty cycles expressed as a percentage. Column  2  of table  500  contains corresponding threshold strength entries, which are specified in decibels above the noise floor of GPS receiver  150 . It may also be observed from table  500  that the threshold strengths have a positive correlation with the duty cycle of the interference. 
     As noted above with respect to step  320  of the flowchart of  FIG. 3 , AGC block  280  compares the sum of the value in column  2  of table  500  (assuming duty cycle is determined to equal the corresponding value in column  1 ) and the current average value of the receiver NF computed with the received signal values contained in corresponding time windows. For example if the receiver NF is − 40  dBm, and the ‘threshold strength’ corresponding to the duty cycle (e.g., 10%) of the jamming signal is 6.5 dB, AGC block  280  compares the received signal values with −33.5 dB (i.e., −40+6.5 dBm). 
     7. Example Operation 
       FIGS. 6A and 6B  are example waveforms used to illustrate the operation of AGC block  280  in mitigating interference. It is assumed in the description below that AGC block  280  computes the duty cycle and strength of the jamming signal, and determines whether blanking is to be applied or not prior to jamming signal occurrences in interval t 62 -t 63  ( FIG. 6A ) and t 66  onwards ( FIG. 6B ). Further, duty cycle is noted below as being computed based on measurements of the immediately preceding cycle of the jamming signal (interval t 60 -t 62  in  FIG. 6A , and t 64  to t 66  in  FIG. 6B ) merely for illustration. It is noted again that in practice, AGC block  280  uses an average value of duty cycles determined over a much longer duration (e.g., several seconds). 
     Waveform  611  of  FIG. 6A  represents example signal strengths of the input signal of GPS receiver  150  with respect to time, when interference is present. The strength of jamming signal  620  in ON interval (t 60 -t 61 ) is represented by marker  610 . The duty cycle of the jamming signal equals [(t 61 -t 60 )/(t 62 -t 60 )]. 
     Assuming that the duty cycle is 20%, and if jamming signal strength  610  is greater than the corresponding threshold strength (6.6 dB in table  500 , corresponding to the 20% duty cycle), AGC block  280  blanks GPS receiver  150  (corresponding to step  380  of  FIG. 3 ) for the duration of the ON interval of future time periods (cycles) of the jamming signal. ON interval t 62 -t 63  of a next cycle is shown in  FIG. 6A . Assuming that the duty cycle and strength of jamming signal  620  is determined prior to time instance t 62  (as noted above), the receiver is blanked starting from time instance t 62  when processing the signal segment starting from t 62 . Such blanking is continued for all future ON durations of the jamming signal. 
     To blank GPS receiver  150 , AGC block  280  provides select signal  286  (shown in  FIG. 6A  as a logic high in time interval t 62 -t 63 ) to cause MUX  260 A and  260 B ( FIG. 2 ) to provide input  281  on each of paths  267 A and  267 B. AGC block  280  may provide a constant value (e.g., binary zero) on path  281 , thereby providing baseband processor  270  with a known constant (or zero-valued) input signal. It is assumed in  FIG. 6A , that AGC block  280  utilizes at least time interval t 60 -t 62  to measure the strength and duty cycle of the interference. Blanking pulse is shown as being applied in the next ON interval t 62 -t 63 , in which select signal  286  is indicated as being logic high to provide constant input  281  on paths  267 A and  267 B. In addition, AGC block  280  also reduces the gain of PGAs  240 A and  240 B (or ADCs  250 A and  250 B), as reflected by the reduced strength  625  of jamming signal  620  in the interval t 62 -t 63 . 
     Waveform  612  of  FIG. 6B  illustrates another example of signal levels with respect to time on the receiver&#39;s input path when interference is present. In the Figure, the strength of jamming signal  630  in ON interval (t 60 -t 61 ) is represented by marker  640 . The duty cycle of jamming signal  630  equals [(t 65 -t 64 )/(t 66 -t 64 )]. Assuming that the duty cycle is 80%, and if jamming signal strength  640  is less than the corresponding threshold strength (10.3 dB in table  500 , corresponding to the 80% duty cycle), AGC block  280  only reduces the gain of PGAs  240 A and  240 B (or ADCs  250 A and  250 B) for the ON duration of jamming signal  630  in future cycles. 
     ON interval of a next cycle starting at t 66  is partially shown in  FIG. 6B , with the gain shown reduced to a level  650 . Although jamming signal  630  is shown as having a reduced strength ( 650 ) greater than the receiver noise floor, in some implementations, the gain is reduced such that the signal level equals the noise floor, i.e., gain is reduced by strength  640  (in dB). Alternatively, the gain reduction may be provided such that the NF is itself reduced, (for example, to level denoted by dotted line  651 ). It is noted that ideally it may be desirable to apply a gain reduction of 2X dB for a jamming signal strength of X dB. However, in practical scenarios it may not be feasible to provide such a large change in gain (e.g., assuming the jamming signal strength is 10 dB, the ideal gain reduction may be 20 dB, which is large value). Therefore, in practical situations, the gain reduction may be limited to X dB or less. 
     To reduce gain, AGC block  280  provides (or maintains) select signal  286  (shown in  FIG. 6B  as a logic low continuously), to cause MUX  260 A and  260 B to provide signals on paths  256 A and  256 B to baseband processor  270  via respective paths  267 A and  267 B. AGC block  280  does not apply any blanking pulse(s). In general, gain reduction may be provided to reduce the amplitude of the jamming signal by a power level equal to the determined strength (6.6 dB in the example). However, smaller or greater gain reductions may also be provided based, for example, on the specific operating environments, sensitivity of the receiver, etc. 
     AGC block  280  similarly measures duty cycles and strengths of the jamming signal for future cycles of a jamming signal. AGC block  280  performs the corresponding interference mitigation operation based on the measured strength, duty cycle and threshold strength corresponding to the measured duty cycle, as noted above. However, assuming the jamming signal&#39;s strength and duty cycle are substantially constant over several cycles (as may be the scenario for transmissions from GSM transceiver  120 ), the strength and duty cycle of the jamming signal may need to be measured only once, with the corresponding action (gain reduction or blanking) being applied for future cycles of the jamming signal till a change in the duty cycle and/or jamming signal strength is determined. 
     Thus, baseband processor  270  may not receive data representing the input signal  111  in durations jamming signal is detected. However, it may be appreciated that the jam signal pulses usually span a small fraction of the total duration required for extracting the GPS data and thus the operation (recovery of the data) may not be impacted due to the blanking during jam durations, described above. Detectability of the satellite signal may accordingly be enhanced. 
     References throughout this specification to “one embodiment”, “an embodiment”, or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment”, “in an embodiment” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment. 
     While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the present invention should not be limited by any of the above-described embodiments, but should be defined only in accordance with the following claims and their equivalents.