Abstract:
A method for frame synchronization of a receiver in a wireless communication system wherein data is transmitted in frame units in a multipath environment begins by extracting data samples for a predetermined window size. A training sequence corresponding to a given cell parameter is generated. The data is correlated with the training sequence over different lags to locate the position of the first significant path, which defines the beginning of the frame. The correlated data is accumulated N times for each lag position to produce at least one accumulation vector. A most significant path value and position is determined that is the largest value among the accumulation vectors. A frame synchronization correction value is calculated based on the difference between the first significant path position and a constant called frame offset. The frame synchronization is adjusted based upon the frame synchronization correction value.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. patent application Ser. No. 60/507,848 filed on Sep. 30, 2003, which is incorporated by reference as if fully set forth herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to channel estimation in wireless communications. More particularly, the invention relates to tracking channel impulse response drifts in time for controlling frame synchronization of received significant paths in a multipath environment by mobile receivers. 
     BACKGROUND OF THE INVENTION 
     Hereafter, a wireless transmit/receive unit (WTRU) includes, but is not limited to, a user equipment, a mobile station, a fixed or mobile subscriber unit, a pager, or any other type of device capable of operating in a wireless environment. When referred to hereafter, a base station includes, but is not limited to, a Node B, a site controller, an access point, or other interfacing device in a wireless environment. 
     Full downlink (DL) synchronization between a base station and a WTRU is obtained when the frame synchronization, code timing, and code locations are synchronized. Frame synchronization defines the beginning of a frame as seen by the WTRU receiver. Code timing is an integer multiple of the sampling period of the received signal in the WTRU receiver front end. Code location is the position of a path or multipath in time as observed by the WTRU receiver. Full synchronization is completed in three stages with three different algorithms: cell search (CS), automatic frequency control (AFC), and frame tracking (FT). 
     In the first stage, the CS algorithm finds the cell on which the WTRU is camped and performs frame synchronization based on the location of the first significant path (FSP) in the delay spread of a multipath channel. After the CS is completed, AFC commences. The AFC algorithm adjusts the code timing by adjusting the control voltage of the voltage controlled oscillator (VCO). Code timing is initially adjusted and also maintained by the AFC. When AFC is in its converged state (when the VCO operating frequency is adjusted), code locations are found by channel estimation. The output of channel estimation is code locations for every DL slot for the WTRU receiver. 
     Although CS performs the initial frame synchronization, there is still a need for maintenance of frame synchronization. Frame tracking is one way to maintain the DL frame synchronization of the WTRU. Since the frame synchronization is based on the location of the FSP, the frame tracking procedure is responsible for updating the FSP. The frame tracking procedure will run periodically after the initial frame,synchronization. 
     If the frame tracking procedure does not run periodically, some paths at either end of the channel estimation vector of the WTRU may disappear, resulting in degraded performance due to loss of these paths. There are three main cases that may cause this to happen: WTRU motion, shadowing, and a fading multipath channel. 
     WTRU motion will result in a time shift of the channel estimation vector to either side depending on the initial and current positions of the WTRU. When the WTRU moves closer to the base station after being initially synchronized to it based on a particular distance, the propagation delay decreases. The FSP then appears earlier in time compared to the initial position. The paths will drift to the left of the channel estimation vector and will eventually disappear. The paths will drift in the opposite direction (i.e., towards the right edge of the channel estimation sequence vector), if the WTRU moves away from the base station. As long as frame synchronization with respect to the FSP position is updated, the channel estimation vector will show the FSP at or near the beginning of the channel estimation vector and all the paths in the delay spread will appear throughout the vector. As an example, for a WTRU radial velocity of 120 km/h, the drift of the FSP will happen very slowly, approximately a one-chip drift in 260 frames at 3.84 Mcps as used for 3GPP W-CDMA. 
     Multipath channel shadowing is another case where a frame synchronization update is required. During the initial synchronization of the WTRU, an object may block the direct path from the base station to the WTRU. When the blocking object or the WTRU changes position, the direct path may appear earlier than the currently known FSP and even earlier than the channel estimation window. To use this path, an FSP location update is required to provide frame synchronization. 
     Fading multipath channels are yet another challenge for frame synchronization. CS may not be able to detect the FSP successfully under multipath fading channel conditions. This situation may be avoided by using a longer accumulation period during the initial CS. However, due to a non-synchronized VCO and a limited time budget for initial frame synchronization, a number of frames of accumulations are performed that are insufficient in all cases to successfully find the position of the FSP. 
     For a multipath fading channel, the channel estimation must find the time locations and complex magnitudes of each path. A channel estimation algorithm should be able to follow the relatively slow and fast varying characteristics of the channel. One example of slowly varying channel characteristics is motion of a WTRU. Also, the difference of the frequencies of the WTRU and the base station local oscillators may result in a drift in the channel impulse response. When these effects are combined, they result in a drift in time in the channel impulse response. 
     Faster channel characteristic changes are due to the well-known multipath fading phenomena, which quickly makes dramatic changes in the magnitudes of the paths. The motion of the WTRU receiver affects all the paths in a similar manner. However, the multipath fading affects the paths in a unique manner by changing their power levels independently. Conventional channel estimation algorithms do not make use of these differences efficiently. This may result in excessive computations or lack of accuracy. 
     Multipath fading requires more frequent updates with higher resolution, where the frame of the data is fixed. For example, the commonly used RAKE receiver locates the position of the paths and tracks them individually by assigning a code tracker for each path. Meanwhile, coping with WTRU motion requires signal processing with less frequent updates and with lesser time resolution. These differences in update frequencies and resolutions are a challenge for channel estimation. 
     SUMMARY OF THE INVENTION 
     A method for frame tracking of a wireless communication channel is provided which allows signal processing at over-sampled transmission rates in a multipath environment. The frame tracking method operates in conjunction with another algorithm to provide the final channel estimates. These channel estimates are useful for a RAKE, an equalizer, or a multi-user detection (MUD) receiver for channel equalization. Frame tracking of a mobile receiver is maintained to counteract motion of the receiver and local oscillator drift. The frame tracking operates on the first significant signal path and updates its position at a relatively slow speed. In response to this, channel estimation of the received multipath signal is enhanced by operating with a fixed short window size, which in turn allows frequent updates of channel estimates. The frame tracking allows all moving paths to remain inside the channel estimation window. As a result, fast changing profiles of the multipath propagation channel are estimated with improved accuracy. Tracking each path is avoided by separating the frame tracking and channel estimation algorithms. 
     A method for frame synchronization of a receiver in a wireless communication system wherein data is transmitted in frame units in a multipath environment begins by extracting data samples for a predetermined window size. A training sequence corresponding to a given cell parameter is generated. The training sequence may be a pilot, as in FDD CDMA, or a midamble, as in TDD W-CDMA. The data is correlated with the training sequence over different lags to locate the position of the first significant path, which defines the beginning of the frame. The correlated data is accumulated N times for each lag position to produce at least one accumulation vector. A most significant path value and position is determined that is the largest value among the accumulation vectors. A frame synchronization correction value is calculated based on the difference between the first significant path position and a constant called frame offset. The frame synchronization is adjusted based upon the frame synchronization correction value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more detailed understanding of the invention may be had from the following description of preferred embodiments, given by way of example, and to be understood in conjunction with the accompanying drawings, wherein: 
         FIG. 1  shows a block diagram of inputs and outputs for a frame tracking (FT) procedure; 
         FIG. 2  shows a block diagram of the FT procedure of  FIG. 1  in greater detail; 
         FIG. 3  shows a block diagram of FT procedure elements; 
         FIG. 4  shows a block diagram of a midamble generator and correlator; 
         FIG. 5  shows generation of shifted midamble training sequences; 
         FIG. 6  shows a processing timeline of the FT procedure in accordance with the present invention; and 
         FIGS. 7A-7C  are flowcharts showing operation of the FT procedure in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Although the embodiments are described hereinafter in conjunction with a third generation partnership program (3GPP) wideband code division multiple access (W-CDMA) system utilizing the 3.84 Mcps time division duplex mode (TDD), the embodiments are applicable to any hybrid code division multiple access (CDMA)/time division multiple access (TDMA) communication system, such as TD-SCDMA. Additionally, the embodiments are applicable to CDMA systems, in general, such as the proposed frequency division duplex (FDD) mode of 3GPP W-CDMA or 3GPP2 CDMA 2000. 
     Frame tracking (FT) is a process to maintain the downlink (DL) frame synchronization of the wireless transmit/receive unit (WTRU). Since frame synchronization is based on the location of the first significant path (FSP), the FT process is responsible for updating the FSP. The FT process will commence after initial cell search (CS) is completed and automatic frequency control (AFC) has converged. Afterwards, it runs periodically. 
       FIG. 1  shows a block diagram of input and output signals of a FT process. The data input includes a twice over-sampled broadcast channel (BCH) in time slot intervals  102 . Three control signals are also provided as inputs: the system frame number (SFN) even/odd indicator  104 , the BCH transmitter time diversity indicator  106 , and the identification of the resident base station cell  108  (step  702 ). A frame synchronization correction signal  110  is the output of the FT process. 
       FIG. 2  shows a block diagram of elements used to implement the FT process. A FT processor  200  outputs a location for the FSP  202  and an indicator  204  whether the most significant path (MSP) is valid. An FSP processing portion  206  analyzes the FSP location  202  and the MSP valid indicator  204  to produce the frame synchronization correct signal  110 . A timing manager  208  uses the frame synchronization correct signal  110  to adjust the timing of the system. 
     For example, in a  3 GPP TDD system, data extraction from the input data is preferably taken from the midamble position in the BCH timeslot, which is 976 chips or 1952 half-chip complex samples from the beginning of the slot. This midamble section contains the training sequence encoded onto the data, which is useful for channel estimation. Preferably, FT is performed using a window sampling technique. Accordingly, the BCH data input further comprises 2×MWS complex half-chip samples before and after the nominal midamble location, where MWS is the multipath window size. For convenience, these samples may be split into separate even and odd sequences. 
       FIG. 3  shows a block diagram of the FT process elements, including an extract and split unit  302 ; two midamble correlators  304 ,  306  for even sample processing; two midamble correlators  308 ,  310  for odd sample processing; an even sample delay unit  312 ; an odd sample delay unit  314 ; correlator adders  316 ,  318 ; an even sample accumulator  320 ; an odd sample accumulator  322 ; and the frame tracker processor  200 . Each midamble correlator  304 ,  306 ,  308 ,  310  is fed by a midamble generator  400  as shown in FIG.  4 . It is noted that one midamble generator  400  could be used, provided that it is time shared among the correlators  304 ,  306 ,  308 ,  310 . The midamble generator  400  produces a 512 chip long midamble m (1) , and if BCH_tx_diversity is ON, a midamble m (2) , based on the basic midamble sequence corresponding to Cell ID, where m (k)  represents a basic midamble sequence shifted by k chips. 
     The position of the FSP is found by performing correlations of the received BCH midamble(s) over different lags. If control signal BCH_tx_diversity is OFF, only midamble m (1)  is used; otherwise, both midambles m (1)  and m (2)  are used. The part of the BCH timeslot from 2×(976−MWS) to 2×(976+MWS) half-chip complex samples is extracted, i.e., the midamble plus 50 earlier and 50 later chips. These samples are split into even and odd sequences for separate processing. 
     For midamble m (1) , there are two correlators  304 ,  308 , one for the even sequence  304 , and the other for the odd sequence  308 . If midamble m (2)  is processed (BCH_tx_diversity is ON), there is an additional pair of correlators  306 ,  310 , as shown in FIG.  3 . In each of the two or four correlators, for each of 101 lag positions n, where n varies as −50, −49, . . . +49, +50 chip positions, the correlation is computed as: 
                   p   c     ⁡     (   n   )       =            ∑     i   =   0     511     ⁢           ⁢       r   ⁡     (     i   +   n   +   50     )       ⁢       m   *     ⁡     (   i   )                  ,           Equation   ⁢           ⁢   1             
 
where p c (n) is the output of correlator c, r(i) is the extracted data of length 612, where 0≦i≦611, and m(i) is the midamble. Note that the output of the correlator is a magnitude, so that further processing of correlator outputs is non-coherent.
 
     The delay units  312 ,  314  provide a preferable 57 chip delay between midamble m (1)  and midamble m (2) . The delay corresponds to the sequence shift between the two midambles.  FIG. 5  shows midamble sequences m (1)    502  and m (2)    504  in relation to the basic midamble sequence  506  of length  456 . Using a 57 chip delay as an example, with BCH_tx_diversity ON, elements −50 thru −7 of the output vectors of the even and odd m (2)  correlators  306 ,  310  are added to elements 7 thru 50 of the output vectors of the even and odd m (1)  correlators  304 ,  308 , respectively. This is equivalent to adding a 57-element delayed version of m (2)  correlator output vector to an m (1)  correlator output vector. 
     The two correlator outputs, or summed output vectors for the case of BCH_tx_diversity ON, are computed four times, each time five frames apart. These outputs are accumulated over this period by accumulators  320 ,  322  as follows: 
                   A   c     ⁡     (   n   )       =       ∑     i   =   1     4     ⁢           ⁢       p   c     (   i   )       ⁡     (   n   )           ,           Equation   ⁢           ⁢   2             
 
where p (i)   c  (n) is the output vector of correlator c for the i th  computation, for the n range −50≦n≦+50. The accumulators  320 ,  322  are reset after frame tracking value Frame_Sync_Correction  110  is computed. The elements of  FIGS. 1-4  can be implemented on a single integrated circuit (IC), multiple ICs, as discrete components, or as a combination of these.
 
       FIG. 6  shows a timing diagram for the FT processing. The FT process begins working after initial cell search is completed and AFC comes to steady state (point A). Afterwards, FT reads and performs correlations of the BCH timeslot every five frames (point B), which equals every 50 ms for preferable frame size of 10 ms. After the fourth BCH is processed (point C), the frame tracking correction value  110  is computed and applied for the frame of the next read BCH timeslot. 
       FIGS. 7A and 7B  show a flowchart of a frame tracking procedure  700  in accordance with the present invention. The procedure  700  begins by gathering the BCH and the other signal inputs (step  702 ). A data window is extracted around the midamble of the BCH (step  704 ) and a midamble sequence is generated (step  706 ). The BCH midamble is correlated over different lags to find the position of the FSP (step  708 ). The correlator output is accumulated four times, spaced five frames apart (step  710 ). Steps  702 - 710  have been described in greater detail above in connection with  FIGS. 1-6 . 
     After accumulating four times, the FT processor performs peak detection, in which the process finds the element containing the largest value among the two accumulator vectors. The identified largest value is the most significant path (MSP) value (step  712 ). The position of the MsP value is the MSP position and the accumulator that contains the MSP is the MSP accumulator. 
     The validity of the MSP, i.e., when the SNR of the accumulator output is strong enough to assume that a valid path has been identified, (step  714 ) is determined as follows and as shown in FIG.  7 C. An estimate of the noise power is obtained, for example, using the following two step procedure. First, the preliminary noise estimate (PNE) is the average of all elements of the MSP accumulator (step  730 ). The preliminary noise threshold is C 1 ×PNE (step  732 ); a preferred value for C 1  is 1.5. Second, the final noise estimate (FNE) is the average of all elements in the MSP accumulator below the preliminary threshold (step  734 ). 
     The final noise threshold is determined by C 2 ×FNE if control signal BCH_tx_diversity is OFF or C 3 ×FNE if it is ON (step  736 ); a preferred value for C 2  is 1.83 and a preferred value for C 3  is 2.2. If the MSP value is above the final noise threshold (step  738 ), then control signal MSP_valid is ON (step  740 ) and the method terminates (step  742 ). If the MSP value is below the final noise threshold (step  738 ), then the MSP_valid control signal is OFF (step  744 ) and the method terminates (step  742 ). 
     Referring back to  FIGS. 7A and 7B , detection of the FSP involves examining the status of control signal MSP_valid (step  714 ). If control signal MSP_valid is ON, starting from the earliest (−50) position, elements of both even and odd accumulators  320 ,  322  are checked against the final noise threshold. The first position that has a value above the final noise threshold is selected as the FSP position (step  716 ). If control signal MSP_valid is OFF, then this step is skipped. 
     Next, FSP processing is performed as follows. If control signal MSP_valid is ON, frame tracking value Frame_Sync_Correction  110  is computed (step  718 ) as:
 
Frame_Sync_Correction=FSP position−Frame Offset  Equation 3
 
     If the magnitude of Frame_Sync_Correction is greater than a predetermined maximum frame sync correction value, Smax, it is limited to ±Smax, which is preferably in a range from six to ten chips (step  720 ). A positive value of Frame_Sync_Correction indicates that frame synchronization should be delayed, i.e., the FSP was detected later than expected (step  722 ). The timing is adjusted (step  724 ) and the method terminates (step  725 ). A negative value of Frame_Sync_Correction indicates that frame timing should be advanced, i.e., the FSP was detected earlier than expected (step  726 ). The timing is adjusted (step  724 ) and the method terminates (step  725 ). If the signal MSP_valid is OFF (step  714 ), then the value of Frame_Sync_Correction is set to zero and all accumulators are reset to zero (step  728 ). 
     Following the computation of correlations for relative frame  15  of the BCH, for purposes of this process, the value of Frame_Sync_Correction can be computed and applied anywhere in the interval of relative frames  16 - 20 . However, to synchronize with receive signal code power (RSCP) measurements, it is preferable to apply the Frame_Sync_Correction value at the start of relative frame  20 . 
     Although the invention has been described as having to perform accumulations preferably four times according to Equation 3, reading the frame numbers in preferred increments of five, and using a preferred frame offset of five, these values are not intended to be limiting, and can be adjusted to suit the needs of the operating system. 
     While specific embodiments of the present invention have been shown and described, many modifications and variations could be made by one skilled in the art without departing from the scope of the invention. The above description serves to illustrate and not limit the particular invention in any way.