Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B7/00—Radio transmission systems, i.e. using radiation field
  • H04B7/24—Radio transmission systems, i.e. using radiation field for communication between two or more posts
  • H04B7/26—Radio transmission systems, i.e. using radiation field for communication between two or more posts at least one of which is mobile
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W56/00—Synchronisation arrangements
  • H04W56/0035—Synchronisation arrangements detecting errors in frequency or phase
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B7/00—Radio transmission systems, i.e. using radiation field
  • H04B7/24—Radio transmission systems, i.e. using radiation field for communication between two or more posts
  • H04B7/26—Radio transmission systems, i.e. using radiation field for communication between two or more posts at least one of which is mobile
  • H04B7/2662—Arrangements for Wireless System Synchronisation
  • H04B7/2671—Arrangements for Wireless Time-Division Multiple Access [TDMA] System Synchronisation
  • H04B7/2675—Frequency synchronisation
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/18—Phase-modulated carrier systems, i.e. using phase-shift keying
  • H04L27/22—Demodulator circuits; Receiver circuits
  • H04L27/233—Demodulator circuits; Receiver circuits using non-coherent demodulation
  • H04L27/2332—Demodulator circuits; Receiver circuits using non-coherent demodulation using a non-coherent carrier
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L7/00—Arrangements for synchronising receiver with transmitter
  • H04L7/04—Speed or phase control by synchronisation signals
  • H04L7/041—Speed or phase control by synchronisation signals using special codes as synchronising signal
  • H04L7/042—Detectors therefor, e.g. correlators, state machines
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/0014—Carrier regulation
  • H04L2027/0044—Control loops for carrier regulation
  • H04L2027/0063—Elements of loops
  • H04L2027/0065—Frequency error detectors

Definitions

• This invention relates generally to a method, apparatus, and system for determining a location of a frequency synchronization signal. More particularly, this invention relates to a method, apparatus, and system for determining a location of a frequency synchronization signal among data transmitted from a transmitter and received by a receiver in a communication system.
• a receiver In any communication system, it is important for a receiver to be synchronized with a transmitter so that messages can be successfully exchanged between the transmitter and the receiver. In a radio communication system, in particular, it is important that a receiver be tuned to the frequency of the transmitter for optimal reception.
• remote stations communicate with one or more base stations via a radio air interface.
• Various approaches have been employed to prevent transmissions between the various base stations and remote stations from interfering with each other.
• neighboring base stations are each assigned a different carrier frequency with which to communicate with remote stations so that transmissions from one base station do not interfere with transmissions from a neighboring base station.
• FDMA Frequency Division Multiple Access
• TDMA Time Division Multiple Access
• a base station may allocate a particular time slot or slots within a frame on a carrier to each remote station. Some remote stations can use the same carrier frequency but different time slots to communicate with the base station.
• CDMA Code Division Multiple Access
• each remote station is assigned a particular digital code word(s) that is orthogonal to code words assigned to other stations.
• Neighboring base stations can exchange messages with remote stations using the same frequency but different digital orthogonal code words to indicate w ich remote station the messages are designated for.
• a radio communication system employs FDMA, TDMA, CDMA, a combination of these approaches, or some other approach, it is important for a remote station to be time and frequency synchronized to the base station serving the area from which it desires to communicate.
• the frequency reference of the remote station must be tuned to the carrier frequency of the base station, and the time reference of the remote station must be synchronized to the time reference of the base station.
• a periodic synchronization signal is typically transmitted from the base station to the remote station for this purpose.
• GSM Global System for Mobile Communications
• NB Normal Burst
• the carrier of the base station is also modulated from time to time with a Frequency Correction Burst (FCB) and a Synchronization Burst (SB) to form a frequency synchronization signal.
• FCB Frequency Correction Burst
• SB Synchronization Burst
• the carrier of the base station is typically modulated with the FCB using Gaussian Minimum Shift Keying (GMSK).
• GMSK Gaussian Minimum Shift Keying
• a FCB is a sequence of 148 symbols, each symbol a zero, that transforms into a pure sinusoidal signal after modulation.
• the frequency of the resulting frequency synchronization signal is thus equal to 1/4T Hz, where T represents a symbol duration.
• T is typically 48/13 microseconds ( ⁇ s), so that the frequency synchronization signal has a frequency of approximately 67.7 KHz.
• the FCB is repeated every tenth frame for the first four times, and then for the fifth time, the FCB is repeated on the eleventh frame. This frame sequence is then repeated indefinitely, to maintain synchronization between the remote station and the base station.
• the remote station From the information in the FCB, the remote station is able to roughly synchronize itself with the time slot(s) allocated to it. This rough time synchronization is then sufficient to locate the SB, which is typically located eight bursts after the FCB, and to decode the information it carries. The information obtained by decoding the SB is then used to finely tune the frequency reference of the remote station to the carrier frequency of the base station and to adjust the remote station's time reference to the time slot(s) allocated to it by the base station.
• each base station transmits a frequency synchronization signal in the form of, for example, a pilot sequence on each of the frequencies assigned to that particular base station as well as possibly on some or all of the frequencies that are not assigned to that particular base station. If the frequency has been assigned to the base station, the corresponding pilot sequence may be transmitted with slightly more power than the other frequencies used by the base station.
• Each remote station receiving the carrier modulated by the pilot sequence demodulates the signal. As a result, each remote station can receive signals designated for it and simultaneously measure the signal strengths of neighboring base stations using different pilots or carriers. This information is used by the remote station to determine which received pilot sequence has the strongest signal strength, and the frequency reference of the remote station is adjusted to the appropriate carrier frequency, accordingly.
• any frequency difference between the frequency reference of the remote station and the carrier frequency of the base station is readily detected in the demodulated frequency synchronization signal.
• the difference between the frequency of the modulated frequency synchronization signal, which is known to be 67.7 KHz, and the frequency of the received frequency synchronization signal, demodulated to the baseband is a direct measure of the error in the frequency reference of the remote station.
• the difference between the known frequency of the strongest transmitted pilot sequence and the frequency of the demodulated pilot sequence is used by the remote station as a measure of the error in the frequency reference of the remote station.
• the remote station To accurately estimate the frequency offset and thus tune the remote station to the carrier frequency of the base station, it is important to know the actual location of the frequency synchronization signal among data received by the remote station, e.g., where the FCB occurs in a frame. Otherwise, the signal used in the frequency offset estimation may not correspond entirely to the frequency synchronization signal, which may result in less than optimal tuning. Thus, there is a need to determine the location of a detected frequency synchronization signal among data received by a receiver.
• this and other objects are met by a method, apparatus, and system for determining the location of the frequency synchronization signal among data transmitted from a transmitter and received by a receiver.
• a peak value representmg a signal detected by the receiver is calculated, a frequency offset between the frequency reference of the receiver and the carrier frequency of the transmitter is estimated, and a quality factor indicating the accuracy of the estimated frequency offset is estimated.
• the location of the frequency synchronization signal is determined by comparing the calculated peak value and the estimated quality factor with predetermined peak and quality thresholds, e.g., determining whether the peak value is greater than or equal to a peak threshold and whether the quality factor is less than or equal to a quality threshold.
• predetermined peak and quality thresholds e.g., determining whether the peak value is greater than or equal to a peak threshold and whether the quality factor is less than or equal to a quality threshold.
• the peak value, frequency offset, and quality factor are stored.
• the location of the frequency synchronization signal corresponds to the location of a stored maximum peak value and a stored minimum quality factor.
• FIG. 1 illustrates a communication system in which the present invention can be implemented
• FIG. 2 illustrates an exemplary apparatus for detecting a frequency synchronization signal
• FIG. 3 illustrates an exemplary method for detecting a frequency synchronization signal
• FIG. 4 illustrates an exemplary apparatus for estimating a frequency offset and a quality factor
• FIG. 5A illustrates an exemplary method for estimating a frequency offset
• FIG. 5B illustrates an exemplary method for estimating a quality factor
• FIG. 6A illustrates an exemplary apparatus for determining a location of a frequency synchronization signal
• FIG. 6B is a graphical representation of peak values, quality factor values, and frequency offsets over time.
• FIG. 7 illustrates an exemplary method for determining a location of a frequency synchronization signal.
• FIG. 1 illustrates an exemplary communication system in which the present invention can be implemented.
• the system includes at least one transmitter 100 and at least one receiver 150.
• the transmitter 100 and the receiver 150 are depicted in FIG. 1 as a base station and a mobile station, respectively, it will be appreciated that the transmitter can be implemented in many ways, e.g., as a terrestrial or satellite repeater, and the receiver can be implemented in many ways, e.g., as a fixed cellular terminal (wireless local loop).
• a base station and a mobile station are depicted in FIG. 1 and described in the following for illustrative purposes only.
• the base station 100 and the mobile station 150 communicate via a radio air interface 125.
• Each neighboring base station 100 is assigned a particular carrier frequency, and each base station 100 allocates specific time slots for each mobile station 150.
• a mobile station 150 To communicate with a base station 100, a mobile station 150 must be time and frequency synchronized to the base station 100. In other words, the frequency reference and time reference of the mobile station 150 must be synchronized with the carrier frequency assigned to the base station 100 and the time slot(s) allocated by the base station, respectively. In a CDMA system, the mobile station 150 must be synchronized with the base station's carrier frequency and the code words transmitted.
• the base station 100 transmits a frequency synchronization signal to the mobile station.
• the base station 100 modulates its carrier frequency with a FCB to form a frequency synchronization signal.
• the mobile station 150 receives and demodulates signals transmitted from the base station 100, including the frequency synchronization signal.
• the frequency synchronization signal may be detected by any of various methods, several of which are disclosed in the aforementioned U.S. Patent Application entitled "Method and Apparatus for Detecting a Frequency Synchronization Signal". For illustrative purposes, one of the methods disclosed in this patent application will be described.
• the similarity of the in-phase and quadrature components of the received frequency synchronization signal can be used to detect the frequency synchronization signal, e.g., the FCB.
• the in-phase and the quadrature components of the received synchronization signal y(n) can be represented as:
• -fP, ⁇ F, v n), and v Q (n) denote the carrier amplitude, the frequency offset between the frequency reference and the carrier frequency F s , the in-phase noise component, and the quadrature noise component, respectively.
• every period of the sinusoid of the FCB contains four samples.
• Hasy,(n) and y Q (n) components of the FCB are phase shifted by ⁇ /2 and thus differ from each other by one sample.
• y Q (n) can be obtained by delaying v 7 (n) by one time index.
• cross-correlating y n - 1) and y Q (n) produces the following cross-correlation value r IQ (l):
• cross correlating y, (n-1) and ⁇ (n) results in a peak whenever the received signal corresponds to an FCB.
• the magnitude of the peak is dependent on the carrier amplitude and the frequency offset ⁇ F. As ⁇ F grows, the magnitude of the peak decreases. If a signal corresponding to, for example, an NB or noise is received, there is no correlation between , (n-1) and y Q (n).
• the cross-conelation value has a peak which is at least as great as a predeteimined detection threshold, it can be determined whether or not the signal transmitted from the base station corresponds to an FCB.
• the incoming ⁇ aXa y(n) can be normalized by converting the received signal y(n) from the Cartesian domain to the Polar domain and then reconverting the signal back to the Cartesian domain.
• This can be implemented with two tables, one for the conversion from the Cartesian domain to the Polar domain and the other for conversion from the Polar domain to the Cartesian domain.
• the normalization is performed by using the first table to obtain the signal phase corresponding to the in-phase and quadrature components of the received signal and the second table to obtain the normalized in-phase and quadrature components from the signal phase and unity amplitude.
• Equation 4 the quantity on the right side of Equation 4 must first be determined.
• One way to determine this value is to estimate the cross-correlation value r, Q (l) as follows:
• Equation 4 e ⁇ r IQ (l) ⁇ denotes an estimated cross-correlation value
• length ( ⁇ ) corresponds to the length of the FCB, i.e., the number of symbols in the FCB.
• Equation 5 the method of estimation in Equation 5 can be modeled as a Moving Average (MA) process with the transfer function:
• This MA process can be implemented with a filter with a 148 sample long memory.
• the MA process can be rewritten as an Auto Regressive (AR) process having the transfer function:
• Equation 7 can be approximated as follows:
• FIG. 2 illustrates an exemplary apparatus for detecting a frequency synchronization signal.
• the apparatus includes a Normalizer 210 into which in-phase and quadrature components y/n) and y Q (n) of a signal y(n) received from the base station are input at a given time n.
• These components can be obtained according to any suitable technique, such as that disclosed in U.S. Patent No. 5,276,706 to Critchlow.
• the Normalizer 210 normalizes the components y/n) and y Q (n), thus reducing effects of, e.g., fading.
• the Normalizer 210 can be implemented with conversion tables.
• the normalized in-phase component is passed through a Delay 220 and delayed by one sample.
• the delayed in-phase component and the normalized quadrature component are derotated, i.e., shifted to the baseband, in a Derotator 222, low pass filtered in a LP Filter 225 to remove surrounding noise, and rotated, i.e., shifted back to the center frequency, in the Rotator 227.
• the Averager 245 can be implemented with, for example, a Moving Averager having the transfer function: 128
• the Moving Averager can be implemented with a LP filter, which makes this method less complicated than averaging with an FIR filter. If the estimated cross-correlation value has a peak which is at least as great as a predetermined detection threshold, then the signal transmitted from the base station corresponds to an FCB.
• the frequency selective filter shown in FIG. 2 is arranged between the Delay 220 and the Multiplier 230, it should be understood that the frequency selective filter can be arranged in any other suitable place, e.g., in front of the Normalizer 210.
• the estimated cross-correlation value e ⁇ r IQ (l) ⁇ can be used in place of the actual cross-correlation value r, Q (l) to determine the frequency offset between the carrier frequency of the base station and the frequency reference of the mobile station.
• FIG. 3 illustrates an exemplary method for detecting a frequency synchronization signal.
• the method begins at step 310 at which in-phase (I) and quadrature (Q) components of a received signal are normalized.
• the normalized I and Q components are filtered to remove surrounding noise.
• the filtered and normalized I component is delayed.
• the order of the steps 320 and 330 can be reversed, i.e., the normalized I and Q components can be filtered after the I component is delayed.
• the normalized and filtered Q component is multiplied by the delayed, filtered, and normalized I components.
• the multiplication products are smoothed, e.g., averaged, at step 360 to produce an estimated cross-correlation value.
• the detected signal corresponds to a frequency synchronization signal, and the detection process ends at step 380.
• the detected frequency synchronization signal can be used to estimate the frequency offset in the received frequency synchronization signal, and the mobile station can be synchronized to the base station based on the estimated frequency offset. Once the mobile station is synchronized to the base station, the method shown in FIG. 3 can be repeated to maintain synchronization.
• the frequency offset between the frequency reference of the mobile station and the carrier frequency of the base station can be estimated by any of various methods, including those disclosed in the afore-mentioned U.S. Patent Application No. 08/971,666. For illustrative purposes, one of the methods disclosed in this application will be described.
• phase of the actual received synchronization signal y(n), with the initial phase ⁇ set to zero can be represented as:
• v (n) denotes the zero-mean white Gaussian phase noise. Assuming there is a frequency offset ⁇ F in the actual received signal y(n) ,
• Equation 10 can be rewritten as:
• the frequency offset can be estimated as:
• N 0 samples of the FCB can be grouped into N groups of M phase differences each, and then the sum of each group can be computed.
• block sums of phase differences between successively collected samples of the detected frequency synchronization signal can be used to estimate the frequency offset, rather than individual phase differences between successively collected samples. This reduces the number of calculations required for frequency offset estimation.
• Equation 13 is expressed as follows:
• Equation 14 can be approximated by setting j - - in
• the method of linear regression can be modified to compensate for the phase variations without having to shift each sample and keep track of the phase shift in a memory.
• the phase difference between successively collected samples of the FCB can be used for this purpose.
• ⁇ A/* ⁇ * ⁇ * - ! + ⁇ ( ⁇ (* + J ) ⁇ ⁇ (* M + J ⁇ *» ⁇ 17 )
• fy M (k) is related to its initial value kg) and the sum ⁇ tjr of the phase differences between successively collected samples of the FCB as follows:
• Equation 19 the frequency offset ef Fj can be estimated as:
• U.S. Patent Application No. 08/971,666 discloses a technique for estimating a quality factor ⁇ which indicates accuracy of the estimated frequency offset.
• the estimated quality factor e ⁇ b ⁇ can be computed by, for example, adding the absolute values of the differences between each of the N sums of M phase differences and the estimated frequency offset e ⁇ F, , as follows:
• the resulting estimated quality factor e ⁇ should be zero. Any deviation indicates the presence of noise and/or an estimation error. Thus, the lower the value of estimated quality factor e ⁇ , the more accurate the estimated frequency offset.
• FIG. 4 illustrates an exemplary apparatus for estimating a frequency offset and a quality factor.
• a detected frequency synchronization signal y(n) is normalized in a Normalizer 400 which limits the dynamic range of the detected signal.
• the Normalizer 400 may be implemented with tables in the same manner as the Normalizer 210 shown in FIG. 2. To obtain an optimal frequency offset estimation, ideally only samples of the FCB should be used. Thus, the received signal can be prefiltered in a frequency selective filter to select the frequency band corresponding to the FCB to discriminate the FCB from noise and thus enhance the signal to noise ratio.
• the received signal can be shifted from the center frequency of 67.7 KHz to the baseband by a Shifter 410 and then the shifted signal can be lowpass filtered in an LP Filter 420.
• the Shifter 410 can be implemented as a Derotator 222 as shown in FIG. 2.
• the phase ⁇ yLP (n) of the filtered signal y LP (n) is then measured in the Phase Measuring Circuit 430.
• the phase difference ⁇ y (n) between successively collected samples of the received and demodulated frequency synchronization signal is computed in a known manner in the Differentiator 440.
• the Differentiator 440 can be conveniently implemented by, for example, a high pass filter.
• Each phase difference is added to accumulated previous phase differences by the Sum & Dump Circuit 450.
• the Sum & Dump Circuit 450 can be implemented by an adder and a filter, such as a FIR filter, with M coefficients set to unity. Alternately, the phase differences can be added by other devices, e.g., an integrate and dump circuit, a resettable integrator, or a low pass filter. When phase differences have been added, the sum is "dumped", i.e., output by the Sum & Dump Circuit 450 to a Frequency Offset Estimation Circuit 460.
• the Frequency Estimation Circuit 460 computes a weighted sum of groups of phase differences between successively collected samples of the detected frequency synchronization signal to estimate the frequency offset, thus compensating for phase variations between successively collected samples of the frequency synchronization signal, i.e., performing phase unwrapping, without requiring a memory to keep track of phase shifts.
• the Frequency Estimation Circuit 460 can be implemented with a FIR filter with delay elements, accumulators, and coefficients c k where:
• a Quality Factor Estimation Circuit 470 computes the quality factor estimate e ⁇ h) according to Equation 22 above. If the estimated quality factor e ⁇ 6) indicates that the estimated frequency offset is not sufficiently accurate, i.e., the estimated quality factor is greater than a predetermined threshold, the estimated frequency offset can be adjusted, e.g., computed again.
• the Quality Factor Estimation Circuit 470 can be implemented with N subtractors for computing N differences between each of the N groups of M phase differences and the estimated frequency offset, N absolute value circuits for computing the absolute values of the N differences, and an adder for adding the N absolute values to produce the estimated quality factor e ⁇ b ⁇ . Although shown as being separate from the Frequency Offset Estimation Circuit 460 for ease of illustration and explanation, it should be understood that the Quality Factor Estimation Circuit 470 and the Frequency Offset Estimation Circuit 460 can be combined in a single device.
• FIG. 5A illustrates an exemplary method for estimating a frequency offset.
• the method begins at step 500, at which a received signal is prefiltered to improve detection of the frequency synchronization signal.
• a sample of the detected frequency synchronization signal is collected.
• a successive sample is collected.
• the phase difference between the successively collected samples is computed.
• the phase difference is added to other accumulated phase differences.
• a determination is made whether Mphase differences have been added. If not, the process returns to step 520. If Mphase differences have been added, the sum of the Mphase differences is dumped at step 560.
• a determination is made whether N sums of Mphase differences have been dumped.
• this step can be performed by computing a weighted average of the ⁇ sums using, for example, linear regression, and the result is the estimated frequency offset.
• FIG. 5B illustrates an exemplary method for a estimating quality factor.
• the process starts after the frequency offset has been estimated.
• the estimated frequency offset, weighted by M is subtracted from each of the N sums of Mphase differences to produce N differences.
• the absolute values of the N differences are computed.
• the N absolute values are added to produce the estimated quality factor.
• Techniques such as that described above use the detected frequency synchronization signal to estimate a frequency offset between the carrier frequency of the base station and the frequency reference of the mobile station. It is important to know the location of the frequency synchronization signal among data received at the mobile station, so that the estimation is performed using the actual frequency synchronization signal and not other data.
• the location of the frequency synchronization signal can be determined based on a detected frequency synchronization signal, synchronized with an estimated frequency offset and quality factor.
• This process can be performed in an apparatus such as that shown in FIG. 6A, which may be included in a receiver, e.g., a mobile station.
• the apparatus includes state memories 620 and 630.
• the state memory 620 receives a signal representing the detected frequency synchronization signal, e.g., the cross-correlation peak value output by the Moving Averager 245 (shown in FIG. 2), and the estimated frequency offset and quality factor values output, e.g., by the Frequency Offset Estimator 460 and the Quality Factor Estimator 470 (shown in FIG. 4), respectively. These values are written into the state memory 630.
• a frequency offset estimation method such as that disclosed in U.S. Patent
• the apparatus includes a Filter 610 for expanding the rates of the estimated frequency offset and quality factor by M.
• a frequency synchronization signal detection technique such as that described above, delays the in-phase component, resulting in a delay in the calculated cross- correlation peak value.
• the estimated frequency offset and quality factor are delayed by time n 0 in Delay unit 600 prior to expansion so that they are synchronized to the cross-correlation peak value.
• the Delay unit 600 can be implemented with, e.g., a buffer.
• FIG. 6B is a graphical representation of peak values, quality factors and frequency offsets over time.
• the peak value is at a maximum greater than or equal to the peak threshold
• the quality factor is at a minimum less than or equal to the quality threshold at a time n,.
• This time n corresponds to the location of a frequency synchronization signal.
• the corresponding frequency offset is shown at time n ⁇ .
• the peak and quality threshold conditions will not be met, i.e., the peak value will be less than the peak threshold, and the quality factor will be greater than the quality threshold.
• the maximum peak value and the minimum quality factor at time n can be used to indicate the location of frequency synchronization signal.
• local extrema including local peak, frequency, and quality values are stored in the state memory 620.
• the local peak, frequency, and quality values correspond to the calculated cross-correlation peak value, estimated frequency offset, and estimated quality factor.
• the local peak, frequency, and quality values which are stored in the state memory 620 are written into the state memory 630, e.g., the Delay unit 635, and then read from the state memory 630.
• the cross-correlation peak value and the quality factor value are compared with predetermined peak and quality thresholds, respectively, e.g., in the Comparison Circuit 625.
• a determination is made, e.g., in the Comparison Circuit 625, whether the cross-correlation peak value read from the Delay unit 635 is less than a successive calculated peak value stored in the state memory 620.
• the local peak, frequency and quality values are updated with corresponding successive extreme values.
• the locations of the peak values are also estimated relative to each other by using a Counter 627 in the state memory 620.
• the Counter 627 counts the number of incoming values, e.g., from the Moving Averager 245 (shown in FIG. 2), and from the Frequency Offset Estimator 460 and the Quality Factor Estimator 470 (shown in FIG. 4), and is reset when a frequency synchronization signal is detected.
• the count value is stored in the state memory 630, e.g., in the Delay unit 635.
• the local extreme values are written as global extreme values, e.g., to a register, and a frequency synchronization signal having the global peak and quality is assumed to be detected. The local extreme values are thereafter reset to zero.
• the corresponding estimated frequency offset is also considered to be the frequency offset between the frequency reference of the mobile station and the carrier frequency of the base station. This frequency offset is used to tune the frequency of the mobile station to the carrier frequency of the base station.
• FIG. 7 illustrates an exemplary method for determining a location of a frequency synchronization signal.
• the method begins at step 700 at which the peak, frequency offset, and quality values are read from the state memory 620.
• the local peak, frequency offset, and quality values are read from the state memory 630, e.g., from the Delay unit 635.
• the counter value is read from the state memory 630, e.g., from the Delay unit 635.
• the counter value is updated by adding one to the counter value in the Counter circuit 627.
• the global peak, frequency offset, and quality values are set equal to zero.
• a method, apparatus, and system are provided for determining the location of a frequency synchronization signal among data transmitted by a transmitter and received by a receiver. This ensures optimal tuning of the receiver to the carrier frequency of the transmitter.

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• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Mobile Radio Communication Systems (AREA)
• Synchronisation In Digital Transmission Systems (AREA)
• Digital Transmission Methods That Use Modulated Carrier Waves (AREA)

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