Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/26—Systems using multi-frequency codes
  • H04L27/2601—Multicarrier modulation systems
  • H04L27/2602—Signal structure
  • H04L27/261—Details of reference signals
  • H04L27/2613—Structure of the reference signals
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/26—Systems using multi-frequency codes
  • H04L27/2601—Multicarrier modulation systems
  • H04L27/2647—Arrangements specific to the receiver only
  • H04L27/2655—Synchronisation arrangements
  • H04L27/2657—Carrier synchronisation
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L25/00—Baseband systems
  • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
  • H04L25/0202—Channel estimation
  • H04L25/0224—Channel estimation using sounding signals
  • H04L25/0228—Channel estimation using sounding signals with direct estimation from sounding signals
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/26—Systems using multi-frequency codes
  • H04L27/2601—Multicarrier modulation systems
  • H04L27/2602—Signal structure
  • H04L27/2605—Symbol extensions, e.g. Zero Tail, Unique Word [UW]
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/26—Systems using multi-frequency codes
  • H04L27/2601—Multicarrier modulation systems
  • H04L27/2602—Signal structure
  • H04L27/261—Details of reference signals
  • H04L27/2613—Structure of the reference signals
  • H04L27/26132—Structure of the reference signals using repetition
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/26—Systems using multi-frequency codes
  • H04L27/2601—Multicarrier modulation systems
  • H04L27/2647—Arrangements specific to the receiver only
  • H04L27/2655—Synchronisation arrangements
  • H04L27/2656—Frame synchronisation, e.g. packet synchronisation, time division duplex [TDD] switching point detection or subframe synchronisation
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/26—Systems using multi-frequency codes
  • H04L27/2601—Multicarrier modulation systems
  • H04L27/2647—Arrangements specific to the receiver only
  • H04L27/2655—Synchronisation arrangements
  • H04L27/2662—Symbol synchronisation
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/26—Systems using multi-frequency codes
  • H04L27/2601—Multicarrier modulation systems
  • H04L27/2647—Arrangements specific to the receiver only
  • H04L27/2655—Synchronisation arrangements
  • H04L27/2668—Details of algorithms
  • H04L27/2673—Details of algorithms characterised by synchronisation parameters
  • H04L27/2675—Pilot or known symbols
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/26—Systems using multi-frequency codes
  • H04L27/2601—Multicarrier modulation systems
  • H04L27/2647—Arrangements specific to the receiver only
  • H04L27/2655—Synchronisation arrangements
  • H04L27/2689—Link with other circuits, i.e. special connections between synchronisation arrangements and other circuits for achieving synchronisation
  • H04L27/2695—Link with other circuits, i.e. special connections between synchronisation arrangements and other circuits for achieving synchronisation with channel estimation, e.g. determination of delay spread, derivative or peak tracking
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L5/00—Arrangements affording multiple use of the transmission path
  • H04L5/0001—Arrangements for dividing the transmission path
  • H04L5/0003—Two-dimensional division
  • H04L5/0005—Time-frequency
  • H04L5/0007—Time-frequency the frequencies being orthogonal, e.g. OFDM(A), DMT
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L5/00—Arrangements affording multiple use of the transmission path
  • H04L5/003—Arrangements for allocating sub-channels of the transmission path
  • H04L5/0048—Allocation of pilot signals, i.e. of signals known to the receiver
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/26—Systems using multi-frequency codes
  • H04L27/2601—Multicarrier modulation systems
  • H04L27/2602—Signal structure
  • H04L27/261—Details of reference signals
  • H04L27/2613—Structure of the reference signals
  • H04L27/26134—Pilot insertion in the transmitter chain, e.g. pilot overlapping with data, insertion in time or frequency domain

Definitions

• the invention relates generally to communications and more particularly toward initial frequency acquisition and synchronization.
• the wireless communication process includes both a sender and a receiver.
• the sender modulates data on a carrier signal and subsequently transmits that carrier signal over a transmission medium (e.g., radio frequency).
• the receiver is then responsible for receiving the carrier signal over the transmission medium. More particularly, the receiver is tasked with synchronizing the received signal to determine the start of a signal, information contained by the signal, and whether or not the signal contains a message.
• synchronization is complicated by noise, interference and other factors. Despite such obstacles, the receiver must still detect or identify the signal and inte ⁇ ret the content to enable communication.
• Communication systems are widely deployed to provide various communication services such as voice, packet data, and so on. These systems may be time, frequency, and or code division multiple-access systems capable of supporting communication with multiple users simultaneously by sharing the available system resources. Examples of such multiple-access systems include Code Division Multiple Access (CDMA) systems, Multiple-Carrier CDMA (MC-CDMA), Wideband CDMA (W- CDMA), High-Speed Downlink Packet Access (HSDPA), Time Division Multiple Access (TDMA) systems, Frequency Division Multiple Access (FDMA) systems, and Orthogonal Frequency Division Multiple Access (OFDMA) systems.
• CDMA Code Division Multiple Access
• MC-CDMA Multiple-Carrier CDMA
• W- CDMA Wideband CDMA
• HSDPA High-Speed Downlink Packet Access
• TDMA Time Division Multiple Access
• FDMA Frequency Division Multiple Access
• OFDMA Orthogonal Frequency Division Multiple Access
• OFDM orthogonal frequency division multiplexing
• OFDM is a parallel transmission communication scheme where a high-rate data stream is split over a large number of lower-rate streams and transmitted simultaneously over multiple sub-carriers spaced apart at particular frequencies or tones. The precise spacing of frequencies provides orthogonality between tones. Orthogonal frequencies minimize or eliminate crosstalk or interference amongst communication signals. In addition to high transmission rates, and resistance to interference, high spectral efficiency can be obtained as frequencies can overlap without mutual interference.
• OFDM systems may be sensitive to receiver synchronization errors. This can cause degradation of system performance. In particular, the system can lose orthogonality amongst subcarriers and thus network users. To preserve orthogonality, the transmitter and the receiver may be synchronized. In sum, receiver synchronization is paramount to successful OFDM communications.
• a method for initial frequency acquisition includes the acts of receiving a stream of input samples from a transmitter, determining an estimate for a frequency offset associated with the transmitter and the receiver based on the received input samples, and compensating for the frequency offset to achieve an initial frequency acquisition.
• Fig. 1 is a block diagram of coarse frame detection system
• Fig. 2a is graph of a correlation curve in an ideal single path environment
• Fig. 2b is a graph of a correlation curve in a real multipath environment
• FIG. 3 is a block diagram of an embodiment of a confirmation component
• FIG. 4 is a block diagram of an embodiment of a trailing edge component
• Fig. 5 is a block diagram of an embodiment of a delayed correlator component
• Fig. 6 is a block diagram of an embodiment of a fine frame detection system
• Fig. 7 is a flow chart diagram of an initial coarse frame detection methodology
• Fig. 8 is a flow chart diagram of a leading edge detection methodology
• Fig. 9 is a flow chart diagram of a leading edge confirmation and flat zone detection methodology
• Fig. 10a is a flow chart diagram of a leading edge confirmation and flat zone detection methodology
• Fig. 10b is a flow chart diagram of a leading edge confirmation and flat zone detection methodology
• Fig. 11 is a flow chart diagram of a trailing edge detection methodology
• Fig. 12 is a flow chart diagram of a frame synchronization methodology
• Fig. 13 is a schematic block diagram of a suitable operating environment for implementing the disclosed embodiments;
• Fig. 14 is a diagram of an embodiment of a super-frame structure for use in an OFDM system;
• Fig. 15 a is diagram of an embodiment of a TDM pilot- 1 ;
• Fig. 15b is diagram of an embodiment of a TDM pilot-2
• Fig. 16 is a block diagram of an embodiment of TX data and pilot processor at an access point;
• Fig. 17 is a block diagram of an embodiment of OFDM modulator at an access point;
• Fig. 18a is a diagram of a time-domain representation of TDM pilot- 1 ;
• Fig. 18b is a diagram of a time-domain representation of TDM pilot-2
• FLL frequency locked-loop
• a component may be, but is not limited to being, a process running on a processor, a processor, an object, an executable, a thread of execution, a program, and/or a computer (e.g.. desktop, portable, mini, palm).
• a component may be, but is not limited to being, a process running on a processor, a processor, an object, an executable, a thread of execution, a program, and/or a computer (e.g.. desktop, portable, mini, palm).
• a computer e.g. desktop, portable, mini, palm
• aspects of the disclosed embodiments may be implemented as a method, apparatus, or article of manufacture using standard programming and/or engineering techniques to produce software, firmware, hardware, or any combination thereof to control a computer to implement the disclosed methods.
• article of manufacture (or alternatively, “computer program product”) as used herein is intended to encompass a computer program accessible from any computer-readable device, carrier, or media.
• computer readable media can include but is not limited to magnetic storage devices (e.g., hard disk, floppy disk, magnetic strips%), optical disks (e.g., compact disk (CD), digital versatile disk (DND)...), smart cards, and flash memory devices (e.g., card, stick).
• a carrier wave can be employed to carry computer-readable electronic data such as those used in transmitting and receiving electronic mail or in accessing a network such as the Internet or a local area network (LAN).
• LAN local area network
• a subscriber station can also be called a system, a subscriber unit, mobile station, mobile, remote station, access point, remote terminal, access terminal, user terminal, user agent, or user equipment.
• a subscriber station may be a cellular telephone, a cordless telephone, a Session Initiation Protocol (SIP) phone, a wireless local loop (WLL) station, a personal digital assistant (PDA), a handheld device having wireless connection capability, or other processing device connected to a wireless modem.
• SIP Session Initiation Protocol
• WLL wireless local loop
• PDA personal digital assistant
• frame detection system 100 is depicted. More specifically, system 100 is a receiver side sub-system associated with synchronization of OFDM transmissions. Synchronization refers generally to the process performed by a receiver to obtain both frame and symbol timing.
• frame detection is based on identification of pilot or training symbols transmitted at the start of a frame or super-frame.
• the pilot symbols are time division multiplexed (TDM) pilots.
• TDM time division multiplexed
• a first pilot symbol can be employed for coarse estimate of frame and OFDM symbol boundary, inter alia, while a second pilot symbol can be utilized for to improve such estimate.
• System 100 is primarily concerned with detection of the first pilot symbol for frame detection, although it can he utilized in conjunction with the detection of other training symbols.
• System 100 includes delayed conelator component 110, leading edge detection component 120, confirmation component 130, and trailing edge detection component 130.
• the delayed conelator component 110 receives a stream of digital input signals from an access terminal receiver (not shown).
• the delayed conelator component 110 processes the input signals and produces detection metrics or conelation outputs (S n ) associated therewith.
• a detection metric or conelation output is indicative of the energy associated with one pilot sequence.
• the computation mechanisms that generate detection metrics from steams of input signals will be presented in detail infra. Detection metrics are provided to a leading edge component 120, a confirmation component 130, and a trailing edge component 140 for further processing.
• Figs. 2a and 2b two exemplary pilot conelation diagrams are illustrated for pmrposes of clarity as well as to facilitate appreciation of one of the problems identified and overcome.
• the conelation diagrams or curves depict a conelator output as captured by the magnitude of the detection metric over time.
• Fig. 2a depicts conelator output in a single path channel without noise.
• the conelator output clearly has a leading edge, a flat portion, and subsequently a trailing edge.
• Fig. 2b illustrates an exemplary conelation curve in a multipath channel with noise. Once can observe that there is a pilot there, however it is obscured by channel noise and multipath delay. Conventionally, a single threshold is employed to detect a pilot symbol.
• the threshold is used to determine the start of a symbol when the conelation values are greater than the set or predetermined threshold.
• the threshold would be set close to the flat zone value and a symbol would be detected when it crosses that value. Subsequently, a count would be initiated to determine the trailing edge. Alternatively, the trailing edge could simply be detected when the curve values dip below the threshold.
• Fig. 2b the leading edge cannot readily be determined from the conelation values as multipath can cause the values to be spread and noise can further obscure the leading edge. This can result in a large number of false positive detections.
• leading edge component 120 can be employed to detect a potential leading edge of a correlation curve.
• a count or counter (e.g., run count) is incremented.
• the counter can be set to zero.
• the counter thereby stores the number of consecutive conelation output values that are above the threshold.
• this value can be modified to optimize detection in a particular system in a specific environment. This technique is advantageous in that it makes it less likely, that a leading edge will be falsely detected as a result of initial noise or spreading, because the samples must consecutively stay above a threshold for a length of time.
• the leading edge component can declare detection of a potential leading edge. Subsequently, a signal can be provided to confirmation component 130 indicating such.
• confirmation component 130 is operable to confirm that a leading edge was indeed detected by the leading edge component 120. Following a leading edge, a lengthy flat period is expected. Hence, if the flat portion is detected then this increases the confidence that the leading edge of the pilot symbol was detected by the leading edge component 120. If not, then a new leading edge will need to be detected.
• the confirmation component 130 can begin to receive and analyze additional detection metric values (S n ).
• Confirmation component 130 can include or be associated with a processor 310, a threshold value 320, an interval count 330, a hit count 340, a run count 350, and a frequency accumulator 360.
• Processor 310 is communicatively coupled with threshold 320, interval counter 330, hit counter 340, run counter 350, and the frequency accumulator 360.
• processor 310 is operable to receive and/or retrieve conelation values S n as well as interact (e.g., receive and transmit signals) with leading edge component 120 (Fig. 1) and trailing edge component 140 (Fig. 1).
• the threshold value 320 can be the same threshold as was employed by the leading edge component 120 (Fig. 1). Furthermore, it should be noted that while the threshold value is illustrated as part of the confirmation component 130 as a hard coded value, for instance, the threshold value 320 can be received and/or retrieved from outside the component to, among other things, facilitate programming of such value.
• interval count 330 can be used in determining when to update a frequency locked loop to determine frequency offset via frequency accumulator 360 as well as detecting the trailing edge.
• Hit count 340 can be utilized to detect the symbol flat zone and run count 350 is used to identify a trailing edge.
• the processor 310 can initialize each of the counters 330, 340, and 350, as well as the frequency accumulator 360 to zero, for example.
• the processor 310 can then receive or retrieve a conelation output S n and the threshold 420.
• the interval count 430 can then be incremented to note that a new sample has been retrieved. Each time a new conelation sample is retrieved the interval count 430 can be incremented.
• the processor 310 can subsequently compare the conelation value to threshold 320. If S n is greater than or equal to the threshold, then the hit count can be incremented. As per the run count, it can be incremented if S n is less then the threshold 320; otherwise, it is set to zero.
• run count can indicate the number of consecutive samples below threshold.
• the count values can be analyzed to determine whether a leading edge has been detected, whether there was a false positive, or whether the leading edge was otherwise missed (e.g., got in to late), among other things.
• the confirmation component 130 can determine that the leading edge component 120 detected a false leading edge by examining the run count and the hit count. Since the confirmation component should be detecting a flat zone of the conelation curve where the values are greater than or equal to the threshold, if the hit count is sufficiently low and the run count is greater than a set value or the hit count and the run count are substantially equal, then it can be determined that noise may have caused inconect detection of a leading edge. In particular, it can be noted that the received conelation values are not consistent with what is expected. According to one embodiment, the determination that a false leading edge can be detected when the run count is greater than or equal to 128 and the hit count is less than 400.
• the values can be optimized or adjusted for a particular frame structure and/or environment.
• the confirmation component 130 can begin to detect the trailing edge of the curve while it is analyzing the flat zone to decide if a proper leading edge was detected. If the trailing edge is detected, the confirmation component can be successfully terminated.
• the interval count and the run count can be employed. As noted above, the interval count includes the number of input samples received and conelated. The length of the flat zone is known to be within a particular count. Hence, if after detecting a potential leading edge and receiving a proper number of flat zone samples there is some evidence of a trailing edge, then the confirmation component can declare detection of the trailing edge.
• the evidence of a trailing edge can be provided by the run count, which counts the number of consecutive times the conelation value is below the threshold.
• the confirmation component 130 can declare detection of the trailing edge when the interval count is greater than or equal to 34 * 128 (4352) and the run count is greater than zero.
• the confirmation component fails to detect any one of the above three conditions then it can simply continue to receive conelation values and update the counters. If one of the conditions is detected, the processor can provide one or more additional checks on the counters to increase the confidence that one of the conditions has actually occuned.
• the processor 310 can insist upon a minimum number of hits in the flat zone as that is what it expected to observe after the leading edge detection. For instance, the processor can test whether the hit count is greater than a set value such as 2000. According to one embodiment of a frame structure disclosed herein, the expected number of hits in the flat zone should be 34 * 128, which over 4,000. However, noise will temper the actual results so the gating value can be set somewhat below 4,000.
• the confirmation component 130 can provide a signal to the trailing edge component alternatively the confirmation component can signal the leading edge component to locate a new leading edge.
• the confirmation component 130 can also provide additional functionality such as saving time instances and updating frequencies.
• the subject frame detection system 100 of Figure 1 is providing course detection of the frame and symbol boundaries. Accordingly, some fine-tuning will need to be performed at a later time to get synchronization that is more precise. Accordingly, at least one time reference should be saved for use later by a fine timing system and or method. According to one embodiment, every time the run count is equal to zero. A time instance can be saved as an estimate of the last time for the conelation curve flat zone or the time just prior to detecting the trailing edge.
• the processor 310 can update a frequency locked loop utilizing the frequency accumulator 360 at particular times such as when the input is periodic.
• the frequency locked loop can be updated every 128 input samples as tracked by the interval counter, for instance.
• trailing edge component 140 can be employed to detect the trailing edge if not detected by the confirmation component 130.
• trailing edge component 140 is operable to detect the trailing edge or simply time out such that another leading edge can be detected by leading edge component 120.
• the trailing edge component 140 can include or be associated with processor 410, a threshold 420, an interval count 430 and a run count 440. Similar to the other detection components, trailing edge component 140 can receive a plurality of conelation values from the delayed conelator component 110 and increment appropriate counts to facilitate detection of a conelation curve trailing edge associated with a first TDM pilot ' symbol. In particular, processor 410 can compare the conelation value with the threshold 420 and populate either or both of the interval count 430 and the run count 440.
• processor 410 can, prior to its first comparison, initialize the interval count 430 and a run count 440 to zero.
• the interval count 430 stores the number of conelation outputs received.
• the processor 410 can increment the interval count 430.
• the run count stores the consecutive number of times the conelation value or output is less than the threshold 420. If the conelation value is less than a threshold then the processor 410 can increment the run count 440, otherwise run count 440 can be set to zero.
• the trailing edge component 140 via processor 410, for example, can test whether an interval count value or a run count value has been satisfied utilizing the interval count 430 and or the run count 440. For instance, if the run count 440 attains a certain value the trailing edge component can declare detection of a trailing edge. If not, the trailing edge component 140 can continue to receive conelation values and update the counts. If, however, the interval count 430 becomes sufficiently large this can indicate that the trailing edge will not be detected and a new leading edge needs to be located. In one embodiment, this value can be 8 * 128 (1024). On the other hand, if the run count 440 hits or exceeds a value this can indicate that a trailing edge has been detected. According to an embodiment, this value can be 32.
• trailing edge component 140 can also save time instances for use in acquisition of fine timing.
• the trailing edge component 140 can save the time instance whenever the run count equals zero thereby providing a time instance just prior to trailing edge detection.
• the saved time instance can conespond to the 256 th sample in the next OFDM symbol (TDM pilot-2).
• TDM pilot-2 next OFDM symbol
• Fig. 5 illustrates a delayed correlator component 110 in further detail in accordance with one embodiment.
• the delayed conelator component 110 exploits the periodic nature of the pilot-1 OFDM symbol for frame detection.
• conelator 110 uses the following detection metric to facilitate frame detection:
• S_ is the detection metric for sample period n
• * denotes a complex conjugate
• I x ⁇ 2 denotes the squared magnitude of x.
• This delayed conelation removes the effect of the communication channel without requiring a channel gain estimate and further coherently combines the energy received via the communication channel.
• Equation (1) then accumulates the conelation results for all Li samples of a pilot-1 sequence to obtain an accumulated conelation result C n , which is a complex value.
• Equation (1) then derives the decision metric or conelation output S n for sample period n as the squared magnitude C n .
• the decision metric S_ is indicative of the energy of one received pilot-1 sequence of length ⁇ , if there is a match between the two sequences used for the delayed conelation.
• a shift register 512 (of length Li) receives, stores, and shifts the input samples ⁇ r n ⁇ and provides input samples ⁇ r n _ L ⁇ that have been delayed by Li sample periods.
• a sample buffer may also be used in place of shift register 512.
• a unit 516 also receives the input samples and provides the complex-conjugated input samples ⁇ r * ⁇ .
• a multiplier 514 multiplies the delayed input sample r n _ t from shift register 512 with the complex- conjugated input sample r * from unit 516 and provides a correlation result c n to a shift register 522 (of length L and a summer 524.
• Lower-case c n denotes the conelation result for one input sample
• upper case C n denotes the accumulated conelation result for Li input samples.
• Shift register 522 receives, stores, and delays the conelation results ⁇ c n ⁇ from multiplier 514 and provides conelation results ⁇ c-_ L ⁇ that have been delayed by Li sample periods.
• summer 524 receives and sums the output C n _ x of a register 426 with the result c n from multiplier 414, further subtracts the delayed result c-_ L from shift register 522, and provides its output C n to register 526.
• Summer 524 and register 426 form an accumulator that performs the summation operation in equation (1).
• Shift register 522 and summer 524 are also configured to perform a running or sliding summation of the I-i most recent conelation results c n through c voyage_ L +1 .
• Fig. 6 depicts a fine frame detection system 600.
• System 650 includes a fine timing component 610 and a data decoder component 620.
• Fine timing component 610 can receive the time instance saved by the coarse frame detection system 100 (Fig. 1). As mentioned above, this time instance can conespond to the 256 th sample of the next OFDM symbol, which can be TDM pilot-2. This is somewhat arbitrary yet optimized for multipath.
• the fine timing component 610 can then utilize the TDM pilot-2 symbol to improve upon this coarse timing estimate (T c ).
• T c coarse timing estimate
• a frequency-locked loop or automatic frequency control loop can be switched from acquisition to tracking mode, which utilizes a different algorithm to compute enor and a different tracking loop bandwidth.
• Data decoder component 620 can attempt to decode one or more data OFDM symbols. This is an e- tra step providing for additional confidence that the synchronization has been accomplished. If the data does not decode, a new leading edge will have to be detected again by the leading edge component 120 (Fig. 1). Further detail concerning fine timing is provided infra.
• a robust method of initial OFDM frame detection is illustrated.
• the method essentially contains three stages.
• the first stage an attempt is made to observe a pilot symbol leading edge is detected.
• the leading edge can be detected by analyzing a plurality of detection metrics or conelation output values produced by a delayed conelator.
• the detection metrics S braid
• some function thereof e.g., S n 2
• Potential detection of the leading edge can then be predicated on the number of times the metric is greater than or equal to the threshold.
• the detected leading edge is confirmed by observing additional conelation values and comparing them to the threshold.
• the conelator output is again compared to the threshold and observations made regarding the number of times the conelator output exceeds the threshold.
• the process can stay in this stage for greater than or equal to a predetermined period of time (conesponding to the flat zone) or upon detection of a consistent trailing edge. It shoixld also be noted that frequency offset can be obtained here be updating a frequency accumulator periodically. If neither of the confirmation conditions is met, then there was a false detection of a leading edge and the procedure can be initialized and started again at 710. At 730, an attempt is made to observe the trailing edge if not previously observed. If the conelator output remains below the threshold for a number of consecutive samples, for example 32, TDM pilot detection can be declared and initial frequency acquisition assumed to be complete.
• the process can be initialized and started again at 710.
• the initial OFDM symbol time estimate is based on the trailing edge.
• the time instance when the conelator output goes below the threshold for the first time during observation of the trailing edge can be views as an index (e.g., 256 th sample) into the next OFDM symbol, here TDM pilot-2.
• Fig. 8 is a flow chart diagram depicting a leading edge detection methodology 800.
• transmitted input symbols are received.
• a delayed conelation is performed, at 820, on the received input and a delayed version thereof.
• a conelation output is then provided to decision block 830.
• the conelation output is compared with a fixed or programmable threshold value. If the conelation value is greater than or equal to the threshold a run count or counter is incremented at 840. If the conelation value is less then the threshold value then the run count is set to zero, at 850.
• the run count is then compared, at 860, with a predetermined value that is optimized for detection of a leading edge in a multipath environment. In one embodiment, the value can be 64 input samples. If the run count is equal to the predetermined value, the process is terminated. If the run count is not equal to the value then the additional input values are received at 810 and the process in repeated.
• Fig. 9 is a flow chart diagram of leading edge confirmation methodology 900.
• Methodology 900 represents the second stage in a coarse or initial frame detection methodology, in which a leading edge detection is confirmed (or rejected) via detection of additional expected results namely a flat zone and/or a trailing edge.
• a leading edge detection is confirmed (or rejected) via detection of additional expected results namely a flat zone and/or a trailing edge.
• one of a myriad of input samples is received.
• a delayed conelation is performed on the input sample and a delayed version thereof, at 920, to produce a conelation output.
• a plurality of conelator outputs are then analyzed with respect to a programmable threshold to make subsequent determinations.
• a determination is made as to whether a false leading edge was detected, which can result from channel noise, among other things.
• This determination can be made if not enough conelation output values are above a threshold.
• a determination is made as to whether a leading edge was detected too late. In other words, the leading edge was not detected until well into the flat zone region of the pilot.
• a determination is made as to whether a trailing edge is being observed. If none of these conditions is true based on the conelation outputs received thus far, the process continues at 910 where more input samples are received. If any one of the conditions is true, the process can continue at 96O, were an additional determination is made concerning whether a long enough flat zone has been observed to provide confidence that it was detected. If yes, the procedure can be terminated. If no, the process can proceed with another method, such as method 800 (Fig.
• Fig. 10 depicts a more detailed method 1000 of detecting the flat zone and confirming detection of the leading edge in accordance with a particular embodiment.
• process three counts or counters are employed: an interval count, a hit count, and a run count.
• counters are all initialized to zero.
• input samples are received.
• the interval count is incremented, at 1014, to indicate receipt of an input sample. It should also be appreciate that although not specifically denoted in the block diagram that a frequency loop can be updated every 128 samples as tracked by the interval count.
• delayed conelation is performed utilizing the input sample and a time-delayed version thereof to produce a conelation output (S n ).
• decision block 1022 is not strictly necessary here but is provided for clarity as well as to highlight further that the order of such method processes does not need to be fixed as shown.
• the method continues to 1028 where the hit count and the run count are scrutinized to determine if a false leading edge was detected. In one embodiment, this can conespond to the run count being greater than or equal to 128 and the hit count being less than 400. If it is decided that a false positive was detected the process proceeds to 1036 where a new leading edge is located. If a false positive was not able to be determined then the process continues at decision block 1030. At 1030, the run count and the hit count are analyzed to determine if the leading edge was detected late.
• this can conespond to when the run count is greater than or equal to 768 and the hit count is greater than or equal to 400. If this is the case, the process can continue at 1034. If the leading edge was not detected late, then the process proceeds to 1032 where the interval count and the run count are analyzed to determine if a trailing edge is being observed. Jh one embodiment this can be where the interval count is greater than or equal to the 4352 (34*128) and the run count is greater than zero. In other words, the full length of the flat zone has been detected and a dip below threshold has just been observed. If no, then all three conditions have failed and the process proceeds to 1012 where more input samples are received.
• the hit count is larger than some programmable value.
• the value can be 2000. However, this is some-what arbitrary. Ideally, the process should see 34 * 128 (4352) samples above threshold, but noise can temper the count. Thus, the programmable value can set to an optimal level that provides a particular level of confidence that the flat zone has been detected. If the hit count is greater than the provided value, then the process terminates. If not, the process proceeds to 1036 where a new edge needs to be detected.
• Fig. 11 illustrates one embodiment of a trailing edge detection methodology
• Trailing edge methodology can be employed to detect the trailing edge of conelation curve associated with a pilot symbol, if not previously detected.
• counters including an interval and a run counters are initialized to zero.
• input samples are received.
• the interval count is incremented conesponding to a received sample, at 1114.
• Each input sample is utilized by a delayed conelator to produce a conelation output S n , at 1116.
• a decision is made at 1118 regarding with the conelation output S n is less than a programmable threshold (T). If S n ⁇ T, then the run count is incremented and the process proceeds to 1126.
• T programmable threshold
• the run counter is set to zero at 1122 and the time instance can be saved at 1124.
• a. determination is made as to whether enough conelation outputs have been observed consecutively to confidently declare successful identification thereof. In one embodiment, this conesponds to a run time greater than or equal to 32. If the run time is large enough, the process can terminate successfully. If the run time is not large enough, the process proceeds to decision block 1128.
• the interval counter can be employed to determine whether the detection method 1100 should be timed out. In one embodiment if the interval count is equal to 8* 128 (1024) the trailing edge detection method 1100 times out.
• Fig. 12 illustrates a frame synchronization methodology 1200.
• the process first waits for automatic gain control to settle. Automatic gain control adjusts the input single to provide a consistent signal strength or level such that the signal can be processed properly.
• a frequency locked loop accumulator is initialized.
• a potential leading edge is detected.
• the leading edge can be confirmed by detection of a flat zone and/or a trailing edge.
• the procedure starts over at 1210. It should be appreciated also that it is at this point, where the frequency locked loop can be updated periodically via the frequency accumulator, for example to acquire the initial frequency offset.
• the trailing edge can be detected if not previously observed. It is here just prior to the initial dip of the trailing edge that the time can be saved to be used later for fine timing. If the trailing edge is not detected at 1222 and was not previously detected then the process proceeds to 1210 where the method begins again. If the trailing edge was detected then the initial coarse detection has been completed. The procedure continues at 1224 where the frequency locked loop is switch to tracking mode. Fine timing is acquired utilizing a second TDM pilot symbol and information provided by the prior coarse estimate.
• the time instance saved can conespond to a particular sample offset within the second pilot symbol.
• the saved time sample can conespond to the 256 th sample in the second pilot symbol.
• Specific algorithms can them be utilized to improve upon that timing estimate as described in later sections.
• one or more data symbols can be retrieved and an attempt made to decode such symbols, at 1228. If, at 1230, the decoding was successful then the process terminates. However, if the process was not successful then the methodology starts over at 1212.
• the synchronization techniques described below and throughout may be used for various multi-carrier systems and for the downlink as well as the uplink.
• the downlink (or forward link) refers to the communication link from the access points to the access terminals
• the uplink (or reverse link) refers to the communication link from the access terminals to the access points.
• these techniques are described below for the downlink in an OFDM system.
• Fig. 13 shows a block diagram of an access point (AP) 1310 and an access terminal (AT) 1350 in an OFDM system 1300.
• Access point 1310 is generally a fixed station and may also be refened to as a base transceiver system (BTS), base station, or some other terminology.
• Access terminal 1350 may be fixed or mobile and may also be refened to as a user terminal, a mobile station, or some other terminology.
• Access terminal 1350 may also be a portable unit such as a cellular phone, a handheld device, a wireless module, a personal digital assistant (PDA), and the like.
• PDA personal digital assistant
• a TX data and pilot processor 1320 receives different types of data (e.g., traffic/packet data and overhead/control data) and processes (e.g., encodes, interleaves, and symbol maps) the received data to generate data symbols.
• a "data symbol” is a modulation symbol for data
• a "pilot symbol” is a modulation symbol for pilot
• a modulation symbol is a complex value for a point in a signal constellation for a modulation scheme (e.g., M-PSK, M-QAM, and so on).
• Processor 1320 also processes pilot data to generate pilot symbols and provides the data and pilot symbols to an OFDM modulator 1330.
• OFDM modulator 1330 multiplexes the data and pilot symbols onto the proper subbands and symbol periods and further performs OFDM modulation on the multiplexed symbols to generate OFDM symbols, as described below.
• a transmitter unit (TMTR) 1332 converts the OFDM symbols into one or more analog signals and -urther conditions (e.g., amplifies, filters, and frequency upconverts) the analog signal(s) to generate a modulated signal.
• Access point 1310 then transmits the modulated signal from an antenna 1334 to access terminals in the system.
• the transmitted signal from access point 1310 is received by an antenna 1352 and provided to a receiver unit (RCNR) 1354.
• RCNR receiver unit
• Receiver unit 1354 conditions (e.g., filters, amplifies, and frequency downconverts) the received signal and digitizes the conditioned signal to obtain a stream of input samples.
• An OFDM demodulator 1360 performs OFDM demodulation on the input samples to obtain received data and pilot symbols.
• OFDM demodulator 1360 also performs detection (e.g., matched filtering) on the received data symbols with a channel estimate (e.g., a frequency response estimate) to obtain detected data symbols, which are estimates of the data symbols sent by access point 1310.
• OFDM demodulator 1360 provides the detected data symbols to a receive (RX) data processor 1370.
• a synchronization channel estimation unit 1380 receives the input samples from receiver unit 1354 and performs synchronization to determine frame and symbol timing, as described above and below. Unit 1380 also derives the channel estimate using received pilot symbols from OFDM demodulator 1360. Unit 1380 provides the symbol timing and channel estimate to OFDM demodulator 1360 and may provide the frame timing to RX data processor 1370 and/or a controller 1390. OFDM demodulator 1360 uses the symbol timing to perform OFDM demodulation and uses the channel estimate to perform detection on the received data symbols.
• RX data processor 1370 processes (e.g., symbol demaps, deinterleaves, and decodes) the detected data symbols from OFDM demodulator 1360 and provides decoded data.
• RX data processor 1370 and/or controller 1390 may use the frame timing to recover different types of data sent by access point 1310.
• the processing by OFDM demodulator 1360 and RX data processor 1370 is complementary to the processing by OFDM modulator 1330 and TX data and pilot processor 1320, respectively, at access point 1310.
• Controllers 1340 and 1390 direct operation at access point 110 and access terminal 1350, respectively.
• Memory units 1342 and 1392 provide storage for program codes and data used by controllers 1340 and 1390, respectively.
• Access point 1310 may send a point-to-point transmission to a single access terminal, a multi-cast transmission to a group of access terminals, a broadcast transmission to all access terminals under its coverage area, or any combination thereof.
• access point 1310 may broadcast pilot and overhead/control data to all access terminals under its coverage area.
• Access point 1310 may further transmit user- specific data to specific access terminals, multi-cast data to a group of access terminals, and/or broadcast data to all access terminals.
• Fig. 14 shows a super-frame structure 1400 that may be used for OFDM system 1300.
• Data and pilot may be transmitted in super-frames, with each super-frame having predetermined time duration (e.g., one second).
• a super-frame may also be refened to as a frame, a time slot, or some other terminology.
• each super-frame includes a field 1412 for a first TDM pilot (or "TDM pilot-1"), a field 1414 for a second TDM pilot (or "TDM pilot-2”), a field 1416 for overhead/control data, and a field 1418 for traffic/packet data.
• the four fields 1412 through 1418 are time division multiplexed in each super- frame such that only one field is transmitted at any given moment.
• the four fields are also ananged in the order shown in Fig. 14 to facilitate synchronization and data recovery. Pilot OFDM symbols in fields 1412 and 1414, which are transmitted first in each super-frame, maybe used for detection of overhead OFDM symbols in field 1416, which is transmitted next in the super-frame. Overhead information obtained from field 1416 may then be used for recovery of traffic/packet data sent in field 1418, which is transmitted last in the super-frame.
• field 1412 carries one OFDM symbol for TDM pilot-1
• field 1414 also carries one OFDM symbol for TDM pilot-2.
• each field may be of any duration, and the fields may be ananged in any order.
• TDM pilot-1 and TDM pilot-2 are broadcast periodically in each frame to facilitate synchronization by the access terminals.
• Overhead field 1416 and/or data field 1418 may also contain pilot symbols that are frequency division multiplexed with data symbols, as described below.
• the OFDM system has an overall system bandwidth of BW MHz, which is partitioned into N orthogonal subbands using OFDM.
• the spacing between adjacent subbands is BW / N MHz.
• M subbands may be used for pilot and data transmission, where M ⁇ N , and the remaining N - M subbands may be unused and serve as guard subbands.
• any OFDM structure with any number of total, usable, and guard subbands may be used for the OFDM system.
• TDM pilots 1 and 2 may be designed to facilitate synchronization by the access terminals in the system.
• An access terminal may use TDM pilot-1 to detect the start of each frame, obtain a coarse estimate of symbol timing, and estimate frequency enor.
• the access terminal may subsequently use TDM pilot-2 to obtain more accurate symbol timing.
• Fig. 15a shows an embodiment of TDM pilot-1 in the frequency domain.
• TDM pilot-1 comprises Li pilot symbols that are transmitted on L ⁇ subbands, one pilot symbol per subband used for TDM pilot-1.
• other values may also be used for N, Li, and Si.
• This structure for TDM pilot-1 can (1) provide good performance for frame detection in various types of channel including a severe multi-path channel, (2) provide a sufficiently accurate frequency enor estimate and coarse symbol timing in a severe multi-path channel, and (3) simplify the processing at the access terminals, as described below.
• Fig. 15b shows an embodiment of TDM pilot-2 in the frequency domain.
• TDM pilot-2 comprises L 2 pilot symbols that are transmitted on L 2 subbands, where L 2 > L j .
• S 2 N / L 2 .
• other values may also be used for N, L 2 , and S 2 .
• This structure for TDM pilot-2 can provide accurate symbol timing in various types of channel including a severe multi-path channel.
• the access terminals may also be able to (1) process TDM pilot-2 in an efficient manner to obtain symbol timing prior to the arrival of the next OFDM symbol, which is can occur immediately after TDM pilot-2, and (2) apply the symbol timing to this next OFDM symbol, as described below.
• a smaller value is used for ⁇ so that a larger frequency enor can be conected with TDM pilot-1.
• a larger value is used for L so that the pilot-2 sequence is longer, which allows an access terminal to obtain a longer channel impulse response estimate from the pilot-2 sequence.
• the Li subbands for TDM pilot-1 are selected such that Si identical pilot-1 sequences are generated for TDM pilot-1.
• the L 2 subbands for TDM pilot-2 are selected such that S 2 identical pilot-2 sequences are generated for TDM pilot-2.
• Fig. 16 shows a block diagram of an embodiment of TX data and pilot processor 1320 at access point 1310.
• a TX data processor 1610 receives, encodes, interleaves, and symbol maps traffic/packet data to generate data symbols.
• a pseudo-random number (PN) generator 1620 is used to generate data for both TDM pilots 1 and 2.
• LFSR linear feedback shift register
• PN generator 1620 includes (1 ) 15 delay elements 1622a through 1622o coupled in series and (2) a summer 1624 coupled between delay elements 1622n and 1622o.
• Delay element 1622o provides pilot data, which is also fed back to the input of delay element 1622a and to one input of summer 1624.
• PN generator 1620 maybe initialized with different initial states for TDM pilots 1 and 2, e.g., to '011010101001110' for TDM pilot-1 and to '010110100011100' for TDM ⁇ ilot-2. In general, any data may be used for TDM pilots 1 and 2.
• the pilot data may be selected to reduce the difference between the peak amplitude and the average amplitude of a pilot OFDM symbol (i.e., to minimize the peak-to-average variation in the time-domain waveform for the TDM pilot).
• the pilot data for TDM pilot-2 may also be generated with the same PN generator used for scrambling data.
• the access terminals have knowledge of the data used for TDM pilot- 2 but do not need to know the data used for TDM pilot-1.
• a bit-to-symbol mapping unit 1630 receives the pilot data from PN generator 1620 and maps the bits of the pilot data to pilot symbols based on a modulation scheme.
• the same or different modulation schemes may be used for TDM pilots 1 and 2.
• QPSK is used for both TDM pilots 1 and 2.
• mapping unit 1630 groups the pilot data into 2-bit binary values and further maps each 2-bit value to a specific pilot modulation symbol.
• Each pilot symbol is a complex value in a signal constellation for QPSK. If QPSK is used for the TDM pilots, then mapping unit 1630 maps 2L j pilot data bits for TDM pilot 1 to i pilot symbols and further maps 2L 2 pilot data bits for TDM pilot 2 to L pilot symbols.
• a multiplexer (Mux) 440 receives the data symbols from TX data processor 1610, the pilot symbols from mapping unit 1630, and a TDM_Ctrl signal from controller 1340. Multiplexer 1640 provides to OFDM modulator 1330 the pilot symbols for the TDM pilot 1 and 2 fields and the data symbols for the overhead and data fields of each frame, as shown in Fig. 14.
• Fig. 17 shows a block diagram of an embodiment of OFDM modulator 1330 at access point 1310.
• a symbol-to-subband mapping unit 1710 receives the data and pilot symbols from TX data and pilot processor 1320 and maps these symbols onto the proper subbands based on a Subband_Mux_Ctrl signal from controller 1340.
• mapping unit 1710 provides one data or pilot symbol on each subband used for data or pilot transmission and a "zero symbol" (which is a signal value of zero) for each unused subband.
• the pilot symbols designated for subbands that are not used are replaced with zero symbols.
• mapping unit 1710 For each OFDM symbol period, mapping unit 1710 provides N "transmit symbols" for the N total subbands, where each transmit symbol may be a data symbol, a pilot symbol, or a zero symbol.
• An inverse discrete Fourier transform (IDFT) unit 1720 receives the N transmit symbols for each OFDM symbol period, transforms the N transmit symbols to the time domain with an N-point IDFT, and provides a "transformed" symbol that contains N time-domain samples. Each sample is a complex value to be sent in one sample period.
• An N-point inverse fast Fourier transform (IFFT) may also be performed in place of an N-point IDFT if N is a power of two, which is typically the case.
• a parallel-to-serial (P/S) converter 1730 serializes the N samples for each transformed symbol.
• a cyclic prefix generator 1740 then repeats a portion (or C samples) of each transformed symbol to form an OFDM symbol that contains N + C samples.
• the cyclic prefix is used to combat inter-symbol interference (ISI) and intercarrier interference (ICI) caused by a long delay spread in the communication channel. Delay spread is the time difference between the earliest arriving signal instance and the latest arriving signal instance at a receiver.
• An OFDM symbol period (or simply, a "symbol period”) is the duration of one OFDM symbol and is equal to N + C sample periods.
• Fig. 18a shows a time-domain representation of TDM pilot-1.
• An OFDM symbol for TDM pilot-1 (or "pilot-1 OFDM symbol") is composed of a transformed symbol of length N and a cyclic prefix of length C. Because the Li pilot symbols for TDM pilot 1 are sent on Li subbands that are evenly spaced apart by Si subbands, and because zero symbols are sent on the remaining subbands, the transformed symbol for TDM pilot 1 contains Si identical pilot-1 sequences, with each pilot-1 sequence containing Li time-domain samples. Each pilot-1 sequence may also be generated by performing an Li -point IDFT on the Li pilot symbols for TDM pilot 1.
• the cyclic prefix for TDM pilot-1 is composed of the C rightmost samples of the transformed symbol and is inserted in front of the transformed symbol.
• Fig. 18b shows a time-domain representation of TDM pilot-2.
• An OFDM symbol for TDM pilot-2 (or "pilot-2 OFDM symbol") is also composed of a transformed symbol of length N and a cyclic prefix of length C.
• the transformed symbol for TDM pilot 2 contains S 2 identical pilot-2 sequences, with each pilot-2 sequence containing L 2 time-domain samples.
• the cyclic prefix for TDM pilot 2 would contain only a portion of the pilot-2 sequence.
• Fig. 19 shows a block diagram of an embodiment of synchronization and channel estimation unit 1380 at access terminal 3150.
• a frame detector 100 receives the input samples from receiver unit 1354, processes the input samples to detect for the start of each frame, and provides the frame timing.
• a symbol-timing detector 1920 receives the input samples and the frame timing, processes the input samples to detect for the start of the received OFDM symbols, and provides the symbol timing.
• a frequency-offset estimator 1912 estimates the frequency offset in the received OFDM symbols.
• a channel estimator 1930 receives an output from symbol timing detector 1920 and derives the channel estimate.
• frame detector 100 performs frame synchronization by detecting for TDM pilot-1 in the input samples from receiver unit 1354.
• the communication channel is an additive white Gaussian noise (AWGN) channel.
• AWGN additive white Gaussian noise
• Frequency offset estimator 1912 estimates the frequency offset in the received pilot-1 OFDM symbol. This frequency offset may be due to various sources such as, for example, a difference in the frequencies of the oscillators at the access point and access terminal, Doppler shift, and so on. Frequency offset estimator 1912 may generate a frequency offset estimate for each pilot-1 sequence (except for the last pilot-1 sequence), as follows:
• r t . is the z ' -tl input sample for the £ -th pilot-1 sequence
• ° f J s samp Af is the frequency offset estimate for the £ -th pilot-1 sequence.
• the range of detectable frequency offset maybe given as:
• Equation (4) indicates that the range of detected frequency offset is dependent on, and inversely related to, the length of the pilot-1 sequence.
• Frequency offset estimator 1912 may also be implemented within the frame detector component 100 and more specifically via the delayed conelator component 110 since the accumulated conelation results are also available from summer 524.
• the frequency-offset estimates may be used in various manners. For example, the frequency-offset estimate for each pilot-1 sequence may be used to update a frequency-tracking loop that attempts to conect for any detected frequency offset at the access terminal.
• the frequency-tracking loop may be a phase-locked loop (PLL) that can adjust the frequency of a carrier signal used for frequency downconversion at the access terminal.
• PLL phase-locked loop
• the frequency-offset estimates may also be averaged to obtain a single frequency offset estimate Af for the pilot-1 OFDM symbol. This Af may then be used for frequency offset conection either prior to or after the N-point DFT within OFDM demodulator 160.
• frequency offset conection which may be used to conect a frequency offset Af that is an integer multiple of the subband spacing
• the input samples may be phase rotated by the frequency offset estimate Af , and the N-point DFT may then be performed on the phase-rotated samples.
• Frame detection and frequency-offset estimation may also be performed in other manners based on the pilot-1 OFDM symbol.
• frame detection may be achieved by performing a direct conelation between the input samples for pilot-1 OFDM symbol with the actual pilot-1 sequence generated at the access point.
• the direct conelation provides a high conelation result for each strong signal instance (or multipath). Since more than one multipath or peak may be obtained for a given access point, an access terminal would perform post-processing on the detected peaks to obtain timing information.
• Frame detection may also be achieved with a combination of delayed conelation and direct conelation.
• the carrier frequency and sampling clock frequency acquisition and/or tracking are achieved in a receiver through a single closed- loop compensator.
• a first-order frequency locked-loop (FLL) is used, where other control schemes, such as linear, nonlinear, adaptive, expert- system, and neural network, of any order of complexity may also be used.
• the carrier frequency and/or sampling clock frequency may be derived from a voltage-controlled local oscillator (NCXO), e.g., in the receiver.
• NCXO voltage-controlled local oscillator
• such local oscillators are very sensitive to environmental factors, such as age, temperature, manufacturer, etc., and do not have a deterministic output (frequency) vs. input (voltage) characteristics.
• a single FLL directly controlling the NCXO may provide both carrier and sampling clock frequency acquisition and tracking.
• the cyclic prefix conelation is used to estimate the frequency offset, e.g., at each OFDM symbol, at each portion of an OFDM frame, or a combination thereof.
• the received signal is denoted by r(t)
• the phase of r*[k Ts] r[(k+N)Ts] provides a measure of the carrier frequency enor associated with the transmitter and receiver, as discussed below.
• n(t) represents the noise signal.
• the cyclic prefix in OFDM symbol defines the periodic structure of the waveform, making it suitable for estimating the frequency offset using the above algorithm.
• FIG. 20 illustrates a block diagram of a frequency locked-loop (FLL), according to one embodiment.
• FLL frequency locked-loop
• the m th estimate of the frequency offset may be obtained by either Equation (8) above or by Equation (4) given previously and repeated below, i.e.,
• m is periodicity index of the duplicate sequences of samples in the first OFDM symbol, for example, 1 to 32 sequences, each of 128 samples.
• the conelated input samples in Equation (8) and/or (9) belong to at least two sequences of input samples received during the first pilot symbol of the OFDM frame.
• the at least two sequences of input samples may be successive sequences of 128 samples each.
• the estimated frequency offset may be updated for a predetermined number of times, which may conespond to the number of duplicate sequences of samples in the first pilot symbol of the OFDM frame, e.g., about 32.
• frequency offset given by either Equation (8) or (9) may be implemented by using a buffer 2002, e.g., of size 512 samples (tracking mode) or 128 samples (acquisition mode), a frequency offset detector 2004 (tracking mode) or 2006 (acquisition mode), and a 2-to-l MUX 2008, which selects the output from one of the detectors 2004, 2006, as the case may be.
• the output of MUX 2008 may be scaled with a gain parameter, e.g., by a multiplier 2010, and then fed into a frequency-offset accumulator 2012.
• the frequency-offset accumulator 2012 generates an actual value of the frequency offset.
• the frequency-offset compensation may be carried out in at least two modes.
• the simultaneous mode of operation of OFDMA with the CDMA where the CDMA portion may digitally control the NCXO, the switch 2014 closes at position "1", and the loop is closed.
• the stand-alone mode where the OFDMA portion may analytically control the NCXO, the switch 2014 opens to position "2" and the loop opens, so that the FLL directly controls the NCXO through the DAC 2016.
• DAC 2016 may be a 1-bit DAC, including a pulse density modulator (PDM) and an RC filter. In this case, the actual value of the frequency offset, Af is converted to a potential difference that is applied to the NCXO, so that the frequency offset is compensated.
• PDM pulse density modulator
• the actual value of the frequency offset is fed, through switch 2014, to a phase accumulator 2018.
• the phase accumulator 2018 generates an actual value of the phase offset, ⁇ .
• sin/cos look up table 2020 generates the complex number "cos ⁇ - j sin ⁇ ", which defines exp(- j ⁇ ), for rotating the phase of the input samples.
• the phase rotator e.g., a complex multiplier, 2024 compensates the phase offset, or equivalently the frequency offset, of the input samples by multiplying the input samples with the complex number "cos ⁇ - j sin ⁇ ".
• a gain of the frequency offset detector 2004, 2006, the NCXO gain, and/or the ratio of NCXO frequency to carrier frequency, etc. may be lumped together in a loop gain parameter ⁇ .
• the parameter may also be quantized to a number which is a power of 2, and the multiplier 2010 may be replaced by a simple programmable shifter.
• ⁇ may be different for the two modes of operations.
• ⁇ is applied to the FLL in increments until the frequency offset converges to a predetermined value, e.g., zero, in a predetermined time.
• the increments are chosen to be small enough, e.g., of 0.2, for maintaining the stability of the FLL, and large enough for the frequency enor to quickly converge to the predetermined level in a predetermined time, e.g., during the first TDM pilot.
• CDMA Code Division Multiple Access
• MC-CDMA Multiple- Carrier CDMA
• W-CDMA Wideband CDMA
• HSDPA High-Speed Downlink Packet Access
• TDMA Time Division Multiple Access
• FDMA Frequency Division Multiple Access
• OFDMA Orthogonal Frequency Division Multiple Access
• the frequency acquisition and synchronization techniques described herein may be implemented by various means. For example, these techniques may be implemented in hardware, software, or a combination thereof.
• the processing units at a access point used to support synchronization e.g., TX data and pilot processor 120
• the processing units at a access point used to support synchronization may be implemented within one or more application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate anays (FPGAs), processors, controllers, micro-controllers, microprocessors, other electronic units designed to perform the functions described herein, or a combination thereof.
• the processing units at an access terminal used to perform synchronization e.g., synchronization and channel estimation unit 180
• the synchronization techniques may be implemented in combination with program modules (e.g., routines, programs, components, procedures, functions, data structures, schemas...) that perform the various functions described herein.
• the software codes may be stored in a memory unit (e.g., memory unit 1392 in Fig. 13) and executed by a processor (e.g., controller 190).
• the memory unit may be implemented within the processor or external to the processor.
• processors e.g., controller 190
• the memory unit may be implemented within the processor or external to the processor.
• the subject inventive methods may be practiced with other computer system configurations, including single-processor or multiprocessor computer systems, mini-computing devices, mainframe computers, as well as personal computers, hand-held computing devices, microprocessor-based or programmable consumer electronics, and the like.

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