MXPA98006734A - Method and system for determining the integrity of a received signal - Google Patents

Abstract

A method for determining the integrity of a received signal in a frequency tracking environment so that a determination can be made whether Automatic Frequency Control (AFC) can be utilized. Several samples of the frequency are taken (205). At least one statistic based on these frequency samples is calculated for use in determining whether the received signal may be used for AFC operation. These statistics may include, for example,the mean, the mean deviation, the standard deviation, and the variance of the measured frequency. A strong signal limit and a weak signal limit are used to determine whether AFC operation should be disabled. If the calculated statistic is less than the strong signal limit (210) then AFC operation is enabled. If the calculated statistic is also greater than the weak signal limit (220) then AFC operation is disabled (225). This allows the receiver to continue AFC operation until the signal level is so weak as to cause erroneous frequency measurements. If the calculated statistic is between the strong signal limit (210) and the weak signal limit (220) then the signal strength is tested (230). If the signal strength is below a minimum value then AFC operation is enabled but if the signal strength is above this minimum value then AFC operation is disabled because a strong interfering signal may be present.

Description

METHOD AND SYSTEM FOR DETERMINING THE INTEGRITY OF A SIGNAL RECEIVED Cross Reference to the Related Request This application claims the priority of the United States Provisional Patent Application Number 60 / 012,198, filed on February 23, 1996. Technical Field refers to a system and method for determining the integrity of a received signal in a frequency tracking environment. In particular, the invention relates to a system and method for determining the integrity of a reference signal in an Automatic Frequency Control tracking environment. Background of the Invention Cell phones are becoming very common in today's world. Cell phones usually include a full-duplex transceiver that can transmit and receive signals at the frequencies authorized for cell phones. Frequency assignments are a limited resource. Therefore, in order to accommodate as many communications as possible, the bandwidth of each communication is limited and each communication must be in a channel of a predefined plurality of channels. There is very little guard space, or free bandwidth, available between the adjacent channels, so it is important that each cell phone operates precisely within its assigned channel and is not diverted to the bandwidth of a channel adjacent. The regulatory authority of each country, such as the Federal Communications Commission (FCC) in the United States of America, sets the frequencies of the channels, the spacing of the channels and the tolerance of the frequency. Frequency tolerance is a measure of how much the cell phone can deviate from its assigned frequency. If the tolerance is too high, then the cell phone can interfere with the communications of an adjacent channel. If the tolerance is too low, then the cell phone will require a very high precision oscillator and the cost of the cell phone will increase. 15 The FCC regulations for cell phones specify that a cell phone must maintain a frequency error of less than ^ < 2.5 parts per million (ppm). To meet this requirement, some cell phones use a crystal oscillator compensating for temperature (TCXO) that has a frequency error of ^ < 2.5 ppm. An alternative for the TCXO is a crystal oscillator controlled by uncompensated voltage (BCXO). The output frequency of the BCXO is compared to the frequency of the received signal transmitted by the Telephone Switching Office Mobile (MTSO) cellular system. The FCC also specifies SS tolerance to the MTSO, a frequency error of less than 0.2 parts per million. In this way, the cell phone adjusts its own frequency to match the frequency of the MTSO. This is commonly referred to as the Automatic Frequency Control (AFC), a well-known method by which one receiver acquires frequency stability from another source by comparing the frequency of the signal received from the source with the frequency of its own oscillator. and then adjusting, as necessary, its own oscillator.

Thus, even if the oscillator in the cell phone is not a high precision oscillator, or is subject to drag due to aging, temperature or battery voltage, the receiver in the cell phone that uses the AFC will track the frequency of the cellular signal received from the MTSO for provide a stable signal with the specified frequency tolerance. ~ Ém However, the AFC should not be performed when the received signal is weak or is subject to strong interference because the receiver can track noise or noise. more interference than the appropriate received signal. A problem encountered is that the power of the received signal decreases as the cell phone moves away from the cellular tower that transmits the cellular signal, in such a way that the relative amount of noise increases. Consider a first case where the received signal is very strong. The measured frequency will be I, iB will mainly be to the received signal, the standard deviation of the measured frequency of the received signal will be small, and there will be a high reliability that the received signal is valid and can be used for operation 5 of the AFC . As the resistance of the received signal decreases the relative amount of noise will increase, so the standard deviation will increase. However, if the standard deviation is still small, then there will be ^ * reliability that the received signal is valid and can be used for the operation of the AFC. As the resistance of the received signal decreases further, the standard deviation continues to increase. However, at this point the center frequency of the IF bandpass filter becomes important. The noise will be Gaussian, but centered on the center frequency of the IF bandpass filter. In this way, if the center frequency of the pass filter If the IF band is, for example, above the frequency of the IF signal resulting from the received signal, then the noise will be distributed irregularly and most will be above the IF frequency of the received signal. Thus, the frequency of the measured average will be above the IF frequency of the received signal, so the AFC will move the frequency of the oscillator in order to center this frequency of the average measured in the IF bandpass. This will cause the IF frequency of the received signal is even lower in the IF bandpass, so the frequency of the measured average will be above the IF frequency of the received signal and the AFC will once again move the frequency of the oscillator with in order to center this frequency of the average measured in step 5 of IF band. This process continues until the AFC has changed the frequency of the oscillator to the point where the IF frequency of the received signal is so far from the frequency of the IF center that it has no effect. In other words, the closing of the frequency with the desired signal. To help combat this problem, a DC voltage measurement corresponding to the power of the received signal, also known as a Receiver Signal Strength Indicator (RSSI), is used to determine if the received signal strength is strong enough, or the signal-to-noise ratio is large enough, to ensure tracking of the exact frequency is carried out. Although the RSSI is useful, it also suffers its own problems. 20 A problem encountered with the RSSI is that it is not useful to represent the quality of the signal over the full sensitivity range of a receiver, especially near the minimum discernible threshold of the receiver. Therefore, the AFC is usually interrupted in signal conditions received weak because there is no valid indication of the quality of the signal. However, interrupting the AFC limits the stability of the transceiver frequency to the frequency stability of its own oscillator. This may be unnecessary because, even when the signal is weak, it may even be strong enough to offer a reference for the operation of the AFC. Therefore, the use of only the RSSI can prematurely deactivate the operation of the AFC. Another problem found with the RSSI is that the RSSI is simply a measure of the strength of the signal received in the IF bandpass of the receiver. The RSSI can not differentiate between a strong interference signal and the desired reference signal if both are in the IF bandpass and can not provide information on whether the interference is strong enough to commit to accurate frequency tracking. For example, consider when the desired received signal is centered on the frequency of the center of the bandpass IF, but there is a strong interference signal above the upper edge of the bandpass.

IF bandpass, or even out of IF bandpass if the interference signal is strong enough. The measured frequency will be between the frequency of the desired received signal and the frequency of the interference signal. Thus, the frequency of the average measurement will be above the The frequency IF of the received signal is such that the AFC will move the frequency of the oscillator in order to center this frequency of the measured mean in the IF bandpass. This will cause the IF frequency of the received signal to be even lower in the IF bandpass and the interference signal will be closer to or more within the IF bandpass, whereby the effect of the interference signal will be even greater , and the AFC will again move the frequency of the oscillator in order to center this frequency of the measured mean on the IF bandpass. This process continues until AFC has changed the frequency of the oscillator to the point where the interference signal controls the AFC. In other words, the frequency closure with the desired signal has been lost. Therefore, even in the presence of a strong received signal, there is a possibility that a signal from The stronger interference can take the transceiver out of the desired frequency in the direction of the frequency of the interfering signal. For the above reasons, and when frequency tolerance requirements are taken into consideration In severe cases, the AFC has not been considered a highly reliable or highly convenient method of cell phone operation. Therefore, there is a need in the art for an improved method for determining the integrity of a signal received in a frequency tracking system.

There is also a need for a method to determine the integrity of a received signal at levels below which the RSSI does not operate accurately. There is a further need for a method for determining the integrity of a received signal when there is the possibility of a strong interference signal. Summary of the Invention The present invention meets the needs described above by offering an improved system and method for determining the integrity of a received signal in a frequency tracking environment. Briefly described, the present invention focuses on a method and system for determining the quality, or integrity, of a received signal in a tracking environment of frequency in such a way that a determination can be made as to whether a Frequency Control can be used Automatic Several consecutive measurements of the R frequency of the limited IF output of a receiver are taken. These frequency measurements are used to calculate the statistics on the frequency of the limited IF output. These statistics are subsequently evaluated to determine if an adjustment in the time base of the oscillator receiver / frequency can be allowed. These statistics can include the mean, the deviation from the mean, the variation and the standard deviation, among other statistics. The deviation of the mean of the measured frequency of the IF output signal * limited increases as the level of the received signal decreases, that is, as the RSSI drops. Finally, as the level of the received signal decreases to 0, the deviation of the mean will be determined by the characteristics of the receiver itself, such as, for example, the IF bandwidth of the receiver. Also, the average (average) frequency of the limited IF output changes as the level of the received signal 10 decreases. Finally, as the level of the received signal decreases to 0, the frequency of the average will be determined by the characteristics of the receiver itself, such as the frequency of the receiver's IF center. A deviation limit of the weak signal is used as the threshold to deactivate AFC operation. If the calculated average deviation is greater than the deviation limit of the weak signal k, then the controller deactivates the operation of the AFC. Therefore, the present invention allows the receiver to continue the operation of the AFC until the level of the signal is so low that the frequency measurements will be erroneous and the AFC will be ineffective. This also offers deactivation of the AFC operation in an environment of a changing signal such as fading. Fading is a condition in the which the resistance of the received signal changes rapidly due to the changing location of the receiver. For example, When the customer of a cell phone is using his cell phone in a car and is on the move, fading may occur, including fading 5 of multiple tracks, and even a complete drop of the signal. As indicated above, if the deviation from the calculated average is greater than the deviation limit of the weak signal, then the control deactivates the operation of the AFC. The present invention proves certain statistics in the frequency measurements and the resistance of the signal to determine if the signal should be used to set the frequency for the operation of the AFC. These statistics may include, for example, the mean, the deviation from the mean, the variation and the standard deviation. It uses a strong signal deviation limit and a weak signal deviation limit to determine whether the AFC operation should be deactivated. If the calculated statistics * is less than the deviation limit of the strong signal then the operation of the AFC is activated. If the calculated statistic is greater than the deviation limit of the strong signal and also greater than the deviation limit of the weak signal then the signal is too weak to be useful and therefore the operation of the AFC is deactivated. This allows the receiver to possibly continue with the operation of the AFC until the The signal level is so weak that it causes erroneous measurements on the frequency. If the calculated statistic is # greater than the deviation limit of the strong signal but less than the deviation limit of the weak signal, then the resistance of the signal (the RSSI) is tested. In this case, if the resistance of the signal is above a minimum value then the AFC operation is deactivated because, with this resistance of the signal, the calculated statistic should have been less than the deviation limit of the strong signal. However, if the resistance of the signal is less than the value At least then the operation of the AFC is deactivated because the calculated statistic is still within the range of acceptable deviation because there is high reliability that a strong interference source is not causing an inaccurate measurement. 15 Described more particularly, the frequency of the received signal is measured several times and the standard deviation of the frequency is determined. If the received signal is strong, and there is no interference signal, then the standard deviation will be less than a first value, referred to in the present as the limit of the strong signal. Therefore, the operation of the AFC can be activated. If the standard deviation is greater than the first value but still less than a second value, referred to herein as the weak signal limit, then the resistance of the signal is tested.

If the increasing standard deviation is due to an increasingly weak signal, then the resistance of the received signal will be less than a value of the resistance of the predetermined signal. In this way, the AFC operation can still be activated. However, if the increasing standard deviation is due to an interference signal, then the resistance of the received signal will be greater than the predetermined value. Thus, the integrity of the received signal is poor and therefore the operation of the AFC will be deactivated. If the standard deviation is greater than the second value, then the integrity of the received signal is poor, indicating a weak and unusable signal. Thus, the operation of the AFC will be deactivated. The present invention offers a method for determining whether the quality of the signal is acceptable. The method includes' the measurement of the frequency of the signal N times, calculating a statistic related to the frequency of the signal, and if the statistic is less than a first predetermined value (the limit of the strong signal) then it is declared that the quality is acceptable. In addition, the method also includes the determination of whether the statistic is greater than a second predetermined value (the limit of the weak signal). If the statistic is greater than the limit of the weak signal then the received signal is considered unacceptable. If the statistic for the signal is between the limit of the strong signal and the limit of the weak signal then the M- method. It also includes the measurement of the resistance of the signal \ r received. If the resistance of the received signal is less than a predetermined value of the resistance of the signal (the minimum value of the RSSI) then the received signal is considered as acceptable. However, if the resistance of the signal is greater than the minimum RSSI value, then there is probably an interference signal, so the received signal is considered unacceptable. The statistics used for the afi > method can be, for example, the deviation from the mean, the standard deviation or the variation of the measured frequency. The present invention also stipulates a method for determining whether the operation of automatic frequency control (AFC) is activated based on the quality of a signal. The method includes measuring the frequency of the N signal times, the calculation of a statistic related to the frequency of the signal, and if the statistic is less than ? First predetermined value (the limit of the strong signal) then the quality is declared as acceptable and the operation of the AFC is activated. In addition, the method also includes the determining whether the statistic is greater than a second predetermined value (the limit of the weak signal). If the statistic is greater than the limit of the weak signal, then the received signal is considered unacceptable and the AFC operation is disabled. If the statistics for the signal is between the limit of the strong signal and the limit of the signal < The weak L then also includes the measurement of the resistance r of the received signal. If the resistance of the received signal is less than a predetermined value of the resistance of the signal (the minimum value of the RSSI) then the received signal is considers as acceptable and activates the operation of the AFC. However, if the resistance of the signal is greater than the minimum value of the RSSI then the probability of an interference signal exists, so that the received signal is considered acceptable and the operation of the AFC is deactivated. The The statistics used for the method can be, for example, the deviation of the mean, the standard deviation or the variation of the measured frequency. The present invention also includes a receiver. The receiver includes a mixer to mix a first Signal, such as for example a received signal, and a second signal, such as for example a mixing frequency of the oscillator flt, to provide a third signal, such as for example a signal of the intermediate frequency (IF). An oscillator supplies the second signal. There is also a frequency control circuit, which responds to the IF signal, to control the frequency of the oscillator in an AFC operation. There is also a circuit to measure the frequency of the third signal and a controller. The controller includes means, such as a program, for To determine a statistic in the frequency of the IF signal, and to activate the frequency control circuit, and therefore the operation of the AFC, if the statistic is less than a predetermined value, such as for example a limit of the strong signal. If the statistic is greater than the limit of the strong signal, the controller determines whether the statistic is greater than a second predetermined value, the limit of the weak signal. If the statistic is greater than the limit of the weak signal then the controller deactivates the frequency control circuit, thus deactivating the operation of the AFC. If the # 10 statistic is between the limit of the strong signal and the limit of the weak signal, the controller measures the resistance of the received signal and determines if the resistance is less than a value of the resistance of the predetermined signal (the minimum value of the RSSI). If the resistance of the received signal is less than the minimum value of the RSSI then the controller activates the frequency control circuit. However, if the resistance of the received signal is greater than the minimum value of the RSSI, then there is a high probability that it is an interference signal. Therefore, in this In this case, the controller deactivates the frequency circuit control. The controller has a program to determine the desired statistic, such as the deviation of the mean, standard deviation or variation. In this way, the present invention offers the evaluation of the quality or integrity of the received signal in such a way that a determination can be made as to whether the received signal can be used as a frequency standard. These and other features, advantages and aspects of the present invention can be understood and appreciated more clearly from a review of the following detailed description of the embodiments presented and by reference to the illustrations and claims appended. Brief Description of the Drawings Figure 1 is a block diagram of a system according to the preferred embodiment of the present invention. Figure 2 is a flow chart of the method for determining the integrity of a signal received in accordance with the present invention. and > Wk Figures 3A-3C are a flow chart of the details of the method for determining the integrity of the received signal according to the preferred embodiment. 20 Figure 4 is a flow diagram of the routine of "Keep AFC." Figure 5 is a flow diagram of the routine of "Find PPM Error". Figure 6 is a flow diagram of the routine of "Adjust PWM".

Jt & Detailed description -? Figure 1 is a block diagram of a system 100 according to the preferred embodiment of the present invention. The apparatus includes a receiver 105, an Analog Application Specific Integrated Circuit (ASIC) 110, a specific digital application integrated circuit 115, a controller 120, a modulation filter 125, a time base (oscillator) 130, a multiplier 135 and a frequency synthesizer 140. As shown below in Figure 1, the receiver 105 includes an antenna 143, a low noise amplifier 145, a first mixer 150, a band pass filter 155 and a second mixer 160. The system 100 is preferably included in a telephone cell phone. However, the system 100 can be part of any device that requires a stable frequency reference and that can receive signals from another and to a radio station that includes a frequency reference of the desired stability. The receiver 105 is preferably a receiver superheterodyne. Superheterodyne receptors are well known in the art. In general, a superheterodyne receiver converts an incoming modulated radio frequency signal into a lower predetermined carrier frequency known as the intermediate frequency. This conversion is usually achieved by using a local JÉ * oscillator that is tuned to the receiver's input stage.

And such that the frequency of the oscillator always differs from the frequency of the incoming signal desired by the intermediate frequency. With a fixed intermediate frequency, a intermediate frequency amplifier can offer much of the amplification and selectivity required by the receiver. After amplification, the intermediate frequency signal can be demodulated to obtain the desired output signal.

Referring still to Figure 1, the operation of the invention in a cellular system will be described. A cellular signal or a received signal is received by the antenna 143 and supplied to a low noise amplifier 145 which is the input stage of the receiver 105. The received signal is amplified by the amplifier 145 and sent to the first mixer 150. The first mixer 150 combines the amplified received signal with a first signal of local oscillator t on line 148 to produce a first frequency signal (IF). The first IF signal subsequently passes through a bandpass filter 155 to eliminate the frequencies out of band unwanted. The first IF signal filtered from the outlet of the filter 155 is subsequently supplied to a second mixer 160. The first filtered IF signal and the second signal of the local oscillator are combined in the second mixer 160 to obtain a second intermediate frequency (IF) signal. In the preferred embodiment, the second IF signal must have a frequency of 450 Khz. The second IF signal subsequently passes through a second bandpass filter 165 to eliminate unwanted frequencies out of band. The second IF signal filtered from the filter 165 is supplied to the analog ASCI 110. As is known to those skilled in the art, an ASIC is a chip that has been constructed for a specific application in such a way that Multiple chips or functions can be combined in a single package to reduce the size of the system board and power consumption. Analog ASIC 110 sends detected Supervisory Audio Tone (SAT) and a limited amplitude version (square wave) of the second IF signal to the digital ASIC 115. The analog ASIC 110 measures the resistance of the received signal and sends the RSSI information to the controller 120. The controller 120 uses the RSSI to determine if the signal strength received is strong enough to ensure that the signal-to-noise ratio is sufficient to provide accurate frequency tracking. The digital ASIC 115 compares the frequency of the second limited amplitude IF signal from the analog ASIC 110 to the frequency of the time base signal from the time base 130 and offers this comparison by means of the signal Inferred from the Serial Port (SPI) to the controller 120. ^ Based on the SPI signal received from the digital ASIC 115 and the RSSI received from the analog ASIC 110, the controller 120 sends a digital pulse width modulation (PWM) signal to 5 digital ASIC 115. The digital ASIC subsequently sends the PWM signal to modulation filter 125. The digital PWM signal could be sent directly from controller 120 to the filter. modulation 125. However, the particular controller used by the inventors only had an 8-bit PWM output port and at least 10-bit output is desirable in order to achieve the desired tuning accuracy. Therefore, in the preferred embodiment, the digital ASIC 115 is designed to offer a 10-bit digital PWM output. The controller 120 sends the 10-bit digital PWM information to the digital ASIC 115 over the serial data link (SPI) and the digital ASIC 115 converts the 10-bit serial data from the SPI link to a parallel data signal of 10. bits for transmission to the modulation filter 125. The modulation filter 125 accepts the digital 10-bit signal, converts it to an analog signal, and filters the analog signal to provide a filtered PWM signal to the oscillator / time base 13Q. The oscillator / time base 130 is a voltage controlled crystal oscillator (VCXO) and provides an output time base signal whose frequency g depends on the output voltage from the modulation filter J 125. This time base signal can also be considered as a signal from the reference oscillator. The reference oscillator signal is sent to the base input of time of the counter of the digital ASIC 115 and also to the multiplier 135 and to the frequency synthesizer 140. The frequency of the signal of the reference oscillator preferably is multiplied by the multiplier 135. The multiplier factor of the multiplier 135 is determined by 10 output frequency of the oscillator / time base 130 and the necessary input base for the second mixer 160 to provide the desired second IF output frequency. The output of the multiplier 135 is the second signal of the local oscillator that is sent to the second mixer 160. The frequency synthesizer 140 preferably supplies two output signals. A first exit sign 148 is the first signal from the local oscillator that is sent to the first mixer 150. A second output signal 149 is supplied to another circuit that needs a frequency of stable reference. For example, the second exit signal 149 can be supplied to the transmitter circuitry in the cell phone. Figure 2 is a flow chart illustrating the method for determining the integrity of a signal received in accordance with the present invention.

Starting at step 205, the resistance of the signal of the received signal is measured. The frequency of the received signal is also measured N times in step 205, where N > 0. In step 205 a measurement statistic of 5 is also calculated, or a statistical parameter, based on the measured frequencies. The measurement statistics of the frequency may be, but is not limited to, the deviation from the average of the measured frequencies, the standard deviation of the measured frequencies or the variation of the measured frequencies. It should be noted that more than one frequency measurement statistic can be calculated in step 205. In decision 210, it is determined whether the frequency measurement statistic is less than or equivalent to a strong signal limit for the measurement statistic from the frequency. For example, if the measurement statistic of the frequency used is the average deviation, then the limit of the strong signal is the deviation from the mean plus-large a received signal can have if the received signal is considered a strong signal. If the measurement statistics of the frequency is less than or equivalent to the limit of the strong signal, then the method continues with step 215 and the received signal is declared as a valid signal. Therefore, the operation of the ASC can be used and, if necessary, the frequency of the oscillator / time base 130 is set to according to this based on the measured frequency of the received signal. After step 215, the method returns to step 205.

# However, if in decision 210 it is determined that the statistics of measurement of the frequency is greater than the limit of the strong signal, then the method continues with decision 220. Subsequently a determination is made in decision 220 of whether the Frequency measurement statistics is less than or equivalent to a weak signal limit. For example, if the measurement statistics of the frequency used is the deviation from the mean, then the * 10 weak signal limit is the deviation from the largest mean that a received signal can have if the received signal is to be used for AFC operation. If, in decision 220, it is determined that the statistics of the measurement of the frequency is greater than the limit of the weak signal, then the received signal is declared as an invalid signal and is not useful for purposes of the ASC. In this way, the ASC is maintained, that is, it is not adjusted in step 225. Subsequently, it is returned to step 205. However, if in decision 220 it is determined that the measurement statistics of the frequency is less than or equivalent to the limit of the weak signal, that is, the measurement statistics of the frequency is between the limit of the strong signal and the limit of the weak signal, then an evaluation is required after the signal and in this way The method proceeds with decision 230. If the error of the higher frequency is due to a weaker signal, then the level of the RSSI will be less than some predetermined value, the minimum value of the RSSI. However, if the error of the higher frequency is due to an interference signal, then the RSSI level will be above the predetermined value. Therefore, decision 230 tests whether the resistance of the signal is less than a minimum dBm signal strength. If the resistance of the signal is weak, then there is no interference signal so the method continues with step 215 where the received signal is declared as a valid signal. In this way, the operation of the AFC can be used and, if necessary, the frequency of the oscillator / time base 130 is adjusted accordingly based on the measured frequency of the received signal. After step 215, The method returns to step 205. However, if in decision 230 it is determined that the signal strength is greater than or equal to the minimum value of the RSSI, then there must be an interference signal. This arises because the level of the RSSI value should indicate a signal that is strong enough to cause the deviation smaller than the limit of the strong signal. Therefore, the largest measured deviation should be caused by a strong interference signal. In this way, the received signal is declared as an invalid signal and is not usable for purposes of the AFC for what the AFC maintains, that is to say, it is not used in step 225. Is it recessed to? He passed 205. FIGS. 3A-3C are a flowchart illustrating the details of the method for determining the integrity of a received signal according to the preferred embodiment. These steps are performed by the controller 120 (Figure 1). The controller 120 has a memory, not shown, that contains a program. The program includes a plurality of steps that function as a means to perform the different operations described herein. The procedure starts at step 300 where the cell phone or other device including the system 100 (Figure 1) is turned on or activated. In step 300, the system is initialized. In this step, the initial operating frequency of the oscillator / base Time 130 is determined by considering factors such as the ambient temperature and the aging factor of the crystal, and the verification that the receiver is tuned to a valid forward control channel (FOCC). The examples of these procedures are shown in the patent of the States U.S. No. 4,992,212, PCT publication numbers WO 88/01810, WO 90-16113, and WO 96/24986 EPO Publication No. 0 483 090, and U.S. Patent Application Publication No. GB 2 205 460. , a variable of "quick acquisition" equivalent to "NOT REALIZED" is adjusted because a rapid acquisition procedure has not yet been carried out. A quick acquisition procedure allows the receiver to make large tuning adjustments in such a way that a suitable frequency can be quickly obtained. A rapid acquisition procedure 5 may be necessary when a receiver tries for the first time to secure a received signal. However, after the closure is obtained, the quick acquisition procedure should not be used because only small adjustments may be necessary to track the frequency of the received signal and a larger step may cause the receiver to break the closure with the received signal. The procedure is then moved to step 305 where a frequency count is read. The frequency count is a measure of the frequency of the second signal IF. In the preferred embodiment, the frequency count is the number of pulses of the reference oscillator signal that occurs during a predetermined number of pulses (N> 1) of the second IF signal. In an alternative embodiment, the signal of the reference oscillator can be divided and the number of pulses of the IF signal can be counted. The frequency counting preferably is read on the SPI output of digital ASIC 115 (Figure 1). Also, it is determined if the frequency count was the frequency count of a suitable signal. For example, a count of frequencies taken can be a measurement of the frequency of & - an interference signal, or noise. These frequency counts should not be considered when adjusting the ASC. Preferably, a determination is made as to whether the value of the Word Synchronizer is equivalent to one 5 for the signal received at the time when each frequency count was made. If the word synchronizer equals 1, then the frequency count is considered valid and marked as such. Otherwise, the frequency counter is considered invalid and is marked as such. The word synchronizer is a binary signal received from the cellular system that is used to verify that the receiver is receiving the proper signal. Preferably, the word synchronizer is equivalent to a binary to indicate that the receiver is receiving an adequate signal. The decision 310 subsequently determines whether a predetermined number of frequency counts has been read in step 305. In the preferred embodiment, 10 readings are taken. Based on the performance test, it is also preferable to use a sample period of 68 milliseconds to count the number of pulses for reading the frequency count. If 10 readings have not been taken then the method returns to step 305 to read another frequency count. If the 10 frequency counts have been taken then the method continues with decision 315. 25 Decision 315 determines whether the frequency counts that were taken in step 305 were valid frequency counts by determining whether at least half of the counts of frequency taken in step 305 are valid, or in other words, if the values of the word synchronizer are equal to one. If a determination is made that less than half of the frequency counts are valid, then the method continues with the "Keep AFC" routine described in Figure 4. However, if it is determined that at least half of the the frequency counts are valid, then the method continues with step 320. In step 320 the statistics are calculated for the frequency counts. These statistics are used to determine if the received signal can be used to make an adjustment to the oscillation frequency. Preferably, these statistics include the deviation of the mean and the frequency of the mean for the frequency counts. The particular controller used by the inventors did not have a floating point processor. Therefore, the deviation from the mean was used. However, if a controller that has a floating-point processor is used, then the standard deviation or variation would be used. However, other statistics can be calculated for the counts, including, but not limited to, the standard deviation and variation. For example, the equations for the deviation of the mean, the standard deviation, the variation and the mean are listed below, * of average media storage = -? I Xi- med? A \ (mean dev) average deviation (mean) mean standard deviation = _ / (?? (mean) media -v Variation O "2 = (S (x" X media) 2 / n 10 (mean) media II frequency of the mean Xmedia = - S Xi (mean) mean? = L 15 Decision 325 determines whether the deviation from the mean is less than or equal to the limit of the strong signal. The limit of the strong signal is preferably the deviation of the largest mean that is acceptable for a signal that is to be considered a strong signal. If the deviation from the mean is less than or equal to the limit of the strong signal, then the counts of the frequencies are sufficiently close to each other that the received signals can considered as strong signals rather than noise, so step 330 is then performed. In step 330, the step size is set to a first value. Preferably, the first value for the step size is selected to allow the oscillator to approach the desired frequency in a single step.

The process then continues with decision 335. If in decision 325, it is determined that the deviation from the mean is greater than the limit of the strong signal, then decision 340 tests whether the deviation from the mean 5 is less than or equal to the limit of the weak signal. The limit of the weak signal is the deviation from the largest acceptable mean for a weak signal. If a received signal has a deviation from the mean larger than the limit of the weak signal it is considered as a signal of poor quality and therefore the ASC operation is not used. That is, no adjustments are made to the oscillator frequency based on the received signal. If the deviation from the mean is greater than the limit of the weak signal, then the method continues with the routine "Keep AFC" described in Figure 4. 15 In decision 340 it is determined that the deviation from the mean is less than or equivalent At the limit of the weak signal, then the method proceeds with the decision 345. At this point it has been determined that the deviation of the mean is between the limit of the strong signal and the limit of the weak signal, by what the deviation from the mean is still within acceptable limits. However, before using this signal for the operation of the AFC, it must be determined whether the deviation from the largest mean is due to a weak signal or an interference signal. If the deviation from the mean more The large signal is due to a weak signal then the received signal can be used for AFC operations. However, if the deviation from the largest mean is due to an interference signal, then the received signal should not be used for AFC operation. Therefore, an additional test is performed. Decision 345 tests whether the resistance of the received signal (RSSI) is lthan a minimum value of the RSSI. Preferably, the minimum RSSI value is set at -110 dBm. If the RSSI is lthan the minimum value of the RSSI, then the received signal is weak but it is strong enough to be used for the operation of the AFC. However, if the RSSI is greater than the minimum value of the RSSI, then the signal received is strong, and therefore the deviation from the mean must be lthan the limit of the strong signal. This can occur when there is a strong interference signal. If the RSSI is greater than or equivalent to -110 dBm, then the received signal must be strong enough to cause the deviation of the mean to be small, that is, lthan or equal to the limit of the strong signal. However, if the RSSI is greater than or equal to -100 dBm and the deviation from the mean is greater than the limit of the strong signal, then there must be an interference signal that is strong enough to affect the frequency count. In this case, it is not possible to tell whether the frequency of the average is due to the desired signal or is caused by the interference signal. Therefore, a deviation from the larger mean coupled with a strong RSSI means that there is an interference signal. Therefore, the received signal is not used for the operation of the AFC. If the RSSI is lthan the minimum value dBm, then the method continues with step 350. In step 350, the step size is the second value. The second value for the step size is selected as a small value so that any change in the frequency of the oscillator is small. The proccontinues later with decision 335. Without # 10 However, if in decision 345 the RSSI is not lthan the minimum dBm value, then the method continues with the "Keep AFC" routine described in Figure 4. At this point a small step size is used because the closure has already been achieved and a step size more large can cause the receiver to lose the closure with the received signal. Also, when the signal is weak, the measured deviation will be due both to the actual frequency difference (between the frequency of the oscillator and the frequency of the received signal) and to the noise. The measures of the deviation from the mean due to noise will have the tendency to average to zero over a period of time. Therefore, if a larger step size is used, the noise will cause the frequency of the oscillator, and therefore the transmitted frequency, to jump randomly. Therefore, a smaller pitch size improves the stability of the frequency. Also, the smaller step size will allow the oscillator to focus more consistently and smoothly on the desired frequency. After the step size is fixed either in step 330 or step 350, then the method continues with decision 335. A determination is made as to whether the Rapid Acquisition is equivalent to "REALIZED" in the decision 335. This is to determine whether or not a rapid acquisition procedure has already been carried out. If it is determined that the rapid acquisition is equivalent to "REALIZED", then the method continues with the routine "Find error PPM" described in Figure 5. However, if in decision 335 it is determined that rapid acquisition is not equivalent to "REALIZED" , then the method continues with step 355 where the parts per million error (PPM) is determined. The PPM error is equivalent to the absolute value of the difference between the average of the counts of the frequency and a reference average. The reference mean is equivalent to the value of the standard frequency of the second signal of the frequency intermediate. For example, the preferred reference mean is equivalent to 45kHz which is the preferred frequency of the second signal of the intermediate frequency. After the error is determined in step 355, decision 360 tests whether the error is greater than the maximum error.

Preferably, the maximum error is set at 2 parts per 4 million. If in decision 360 the error is greater than the maximum error, then the method proceeds to step 370. In step 370 a new PWM value is determined. The PWM value can be determined from the reference with a table, 5 by interpolating between two values for a predetermined range of the error, or through an equation. The PWM is subsequently set in the "Adjust PWM" routine described in Figure 6. The method subsequently returns to step 305. If in decision 360 the error is less than or equivalent to the maximum error, then the method continues with step 365 where the fast acquisition variable is set to "REALIZED" and the step size is used to adjust the PWM value. The method then returns to step 305. Figure 4 is a flow diagram of the routine of "Keep AFC". The fast acquisition variable is set to "NOT REALIZED" in step 405. In step 410, the last valid temperature and the last valid PWM value are read. In decision 415, a determination is made as to whether there has been a change in temperature greater than 10 degrees Celsius from the last valid temperature. Preferably, this determination is made by comparing the last valid temperature with the current temperature. If it is determined that there has not been a change in temperature of more than 10 degrees, then return to step 305. If it is determined that there has been a change in temperature greater than 10 degrees then the cell phone is deactivated. Figure 5 is a flow diagram of the routine of "Find PPM Error". As mentioned above, the PPM error is equivalent to the absolute value of the difference between the average of the counts of the frequency and a reference mean. Decision 505 tests whether the PPM error is less than the maximum error of parts per million. If the PPM error is less than the maximum error then the method continues with the routine of "Maintain AFC" of Figure 4 However, if it is determined that the PPM error is not less than 2 parts per million, then the method continues with step 510. The step size that was set either in step 330 or 350, is used to adjust the PWM value. The adjustment is made either up or down in the direction required to reduce the error. Subsequently, the "Set PWM" routine of Figure 6 is carried out. Then, return to step 305. Figure 6 is a flow diagram of the "Set PWM" routine. In step 605 an upper limit and a lower limit are calculated in plus and minus, respectively, 8 parts per million from the value of the PWM table. Subsequently the magnitude of the difference between the calculated PWM and the value of the PWM table is determined. Decision 610 tests whether the calculated PWM is allowed. A calculated change in the PWM that is too large may cause the receiver to break the closure, or may be due to a strong interference signal. Preferably, this is achieved by determining whether the magnitude of the calculated difference is within the limits calculated above. If the calculated PWM is not allowed, then the method continues with the "Keep AFC" routine of the Figure 4. If the calculated PWM is allowed, then the method • gSiL continues with step 615 where the calculated value of the PWM is writes in the digital ASIC 115, from where it is sent to the modulation filter 125. If the calculated value of the PWM is valid, then the last valid temperature and the last valid PWM are updated. Then you return to the step that requested the "Set PWM" routine. From the foregoing description, it will be apparent to those skilled in the art that the present invention provides a method and system for determining the integrity of a received signal in a frequency tracking environment, whereby a determination of if Automatic Frequency Control can be used. Several frequency samples are taken from the output of a receiver, preferably consecutively. Statistics of these frequency samples are calculated to determine if an adjustment in the frequency of a signal from the reference oscillator. These statistics may include, for example, the mean, the deviation from the mean, the variation and the standard deviation. A weak signal limit is used as a threshold to deactivate the AFC operation. If the deviation of the mean is greater than the limit of the weak signal, then the controller deactivates the operation of the AFC. However, if the standard deviation is less than the limit of the weak signal, but the resistance of the signal is above a specified minimum level, then the controller f also deactivates the operation of the AFC if the deviation standard is greater than the limit of the strong signal. This reduces the likelihood that the oscillator will be attracted to an inadequate frequency by a strong interference signal. Therefore, the present invention allows the receiver to continue the operation of the AFC even with a very weak signal, but deactivates the operation of the AFC in the event that a strong interference signal is present. Jjfl From the above description, it will also be apparent to those skilled in the art that the present invention offers a method to deactivate the operation of the AFC when the signal is too weak or when there is an interference signal. The alternative embodiments will be apparent to those skilled in the art to which the present invention pertains without deviating from its objective and scope. Accordingly, the scope of the present invention is defined by the appended claims.

Claims (21)

1. jfi CLAIMS 1. A method to determine if the quality of a signal is acceptable, which includes the steps of: measuring the frequency of the mentioned signal N 5 times; calculate a statistic related to the mentioned frequency of the mentioned signal; and if the mentioned statistic is less than a predetermined value then the mentioned quality is declared • 10 as acceptable.
2. 2. The method of claim 1 wherein said predetermined value is a first predetermined value, and which further includes the steps of: if the mentioned statistic is greater than the first mentioned predetermined value then it is determined whether said statistic is greater than a second default value ijßß; and if the mentioned statistic is greater than the second predetermined value mentioned then the signal mentioned is declared as unacceptable.
3. 3. The method of claim 2 and further comprising the steps of: if the aforesaid statistic is less than the aforementioned second predetermined value, then the 25 resistance of the mentioned signal and it is determined if this resistance is less than a value of the resistance of the signal # default; if the aforementioned resistance is less than the value of the resistance of the aforementioned predetermined signal 5 then the aforementioned received signal is declared as acceptable.
4. 4. The method of claim 3 further comprising the step of declaring this signal as unacceptable if the »Resistance of the aforementioned signal is greater than the value of the resistance of the aforementioned predetermined signal. The method of claim 1 wherein said step of calculating a statistic includes determining a deviation from the mean for the aforementioned signal. 6. The method of claim 1 wherein said step of computing a statistic includes determining a standard deviation for said signal. 7. The method of claim 1 wherein the step The aforementioned calculation of a statistic includes the determination of a variation for the mentioned signal. 8 A method for determining whether the operation of the automatic frequency control (AFC) should be activated based on the quality of some signal, which includes the steps of: 25 measuring the frequency of the mentioned signal N times; calculate a statistic related to the mentioned frequency of the mentioned signal; Y 5 if the mentioned statistic is less than a predetermined value then the operation of the aforementioned AFC is activated. The method of claim 8 wherein said predetermined value is a first predetermined value, and 10 which further includes the steps of: if the aforesaid statistic is greater than the first predetermined value mentioned above, it is then determined whether said statistic is greater than a second predetermined value; and if the aforementioned statistic is greater than the second predetermined value mentioned, then the aforementioned AFC operation is deactivated. 10. The method of claim 9 and further comprising the steps of: if the aforesaid statistic is less than the aforementioned second predetermined value, then the resistance of the aforementioned signal is measured and it is determined whether this resistance is less than a value of the resistance of the predetermined signal; 25 if the resistance mentioned is less than 4 of the resistance of the aforementioned predetermined signal then the operation of the aforementioned AFC is activated. The method of claim 10 further including the step of deactivating the AFC operation mentioned 5 if the resistance of the mentioned signal is greater than the resistance value of the aforementioned predetermined signal. The method of claim 8 wherein said step of calculating a statistic includes determining a deviation of the mean for the signal 10 mentioned. The method of claim 8 wherein said step of calculating a statistic includes determining a standard deviation for the aforementioned signal. 14. The method of claim 8 wherein said step of computing a statistic includes determining a variation for said signal. 15. A receiver, including: a mixer for mixing a first signal and a second signal for supplying a third signal; an oscillator for supplying the second mentioned signal; a frequency control circuit that responds to the third mentioned signal to control the frequency of said oscillator; a circuit for measuring the frequency of the third mentioned signal; and a controller that includes means for determining a statistic on the aforementioned frequency of the aforementioned third signal, and which activates the control circuit of the mentioned frequency if this statistic is less than a predetermined value. 16. the receiver of claim 15 wherein the predetermined value mentioning is a first predetermined value and said controller further includes: means for determining whether said statistic is greater than a second predetermined value if said statistic is greater than the first predetermined value mentioned; and means for deactivating the mentioned frequency control circuit if this statistic is greater than the second predetermined value mentioned. The receiver of claim 16 wherein said controller further includes: means for measuring the resistance of the aforementioned signal and determining whether said resistance is less than a value of the resistance of the predetermined signal if this statistic is equal to the second value predetermined mentioned; and means for activating said frequency control circuit if this resistance is less than the resistance value of the aforesaid predetermined signal. 18. The receiver of claim 17 wherein said controller further includes means for deactivating 5 the mentioned frequency control circuit if this resistance of the signal is greater than the resistance value of the aforementioned predetermined signal. 19. The receiver of claim 15 wherein said controller further includes means for calculating a deviation from the mean for said signal. 20. The receiver of claim 15 wherein said controller further includes means for calculating a standard deviation for said signal. 21. The receiver of claim 15 wherein said controller further includes means for calculating a variation for said signal. SUMMARY A method for determining the integrity of a signal received in a frequency tracking environment, such that a determination can be made that an Automatic Frequency Control (AFC) can be used. Several samples of the frequency (105) are taken. At least one statistic based on these frequency samples is calculated for use in determining whether a received signal can be used for the AFC operation. These 10 statistics can include, for example, the mean, the deviation from the mean, the standard deviation and the variation of the measured frequency. A strong signal limit and a weak signal limit are used to determine whether the AFC operation should be deactivated. If the statistics 15 calculated is less than the limit of the strong signal (210) then the AFC operation is activated. If the calculated statistic is also greater than the limit of the weak signal (220) then it deactivates the AFC operation (225). This allows the receiver to continue the AFC operation until the signal level is 20 so weak that it causes erroneous frequency measurements. If the calculated statistic is between the limit of the strong signal (210) and the limit of the weak signal (220) then the resistance of the signal (230) is tested. If the resistance of the signal is less than a minimum value then it 25 activates the AFC operation, but if the resistance of the signal is above this minimum value, then the AFC operation is deactivated because a strong interference signal may be present. §