Apparatus and method for performing initial cell search in wireless communication systems
The system and method of the present invention establishes a communication link between a user equipment (UE) and a base station in a communication system having a plurality of base stations which each transmit a common primary synchronization code (PSC) in a primary synchronization channel in conjunction with a base station specific secondary synchronization code (SSC) within a system frame, which receives with the UE an input signal including the PSC and SSC from at least one of the base stations. The UE analyzes the input signal to detect any received PSCs within a selected time period which has duration corresponding to the length of a system frame and determining a relative location of a strongest PSC within the selected time period. The input signal is then processed to remove the PSC from at least the determined PSC location. A secondary synchronization code (SSC) is then detected for the determined location from the processed signal. The communication link is then established using the detected SSCs.
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This application is a continuation of U.S. patent application Ser. No. 09/998,885, filed on Oct. 31, 2001, which claims the benefit of U.S. Provisional Application Ser. No. 60/300,412, filed on Jun. 22, 2001, which are incorporated by reference as if fully set forth.
BACKGROUNDThe present invention relates to user equipment (UE) synchronization to a base station. More specifically, the present invention relates to an improved initial cell search method and system.
When a UE 4 is first activated, it is unaware of its location and which base station 2 (or cell 6) to communicate. The process where the UE 4 determines the cell 6 to communicate with is referred to as “cell search.”
In typical code division multiple access (CDMA) communication systems, a multi-step process is used for cell search. For step one, each base station 2 transmits the same primary synchronization code (PSC) in a primary synchronization channel (PSCH). In a time division duplex (TDD) communication system using CDMA, the PSCH is one timeslot out of fifteen for case 1 cell search (as shown in
The UE 14 determines the base station 12 to be synchronized to by searching the PSCH for received PSCs, such as using a matched filter. An example of the results of such a search are shown in
Referring back to
A drawback of the initial cell search system described above is that the performance of the second step (SSC detection) is governed by the quality of the received signal which could result in false detections if this signal is of poor quality. In past systems, the second step, receives no benefit from successful execution of step I.
Accordingly, there is a need for an initial cell search system wherein the second step's performance is not solely governed by the received input signal, providing more accurate SSC detection.
SUMMARYThe system and method of the present invention establishes a communication link between a user equipment (UE) and a base station in a communication system having a plurality of base stations which each transmit a common primary synchronization code (PSC) in a primary synchronization channel in conjunction with a base station specific secondary synchronization code (SSC) within a system frame, which receives with the UE an input signal including the PSC and SSC from at least one of the base stations. The UE analyzes the input signal to detect any received PSCs within a selected time period which has duration corresponding to the length of a system frame and determining a relative location of a strongest PSC within the selected time period. The input signal is then processed to remove the PSC from at least the determined PSC location. A secondary synchronization code (SSC) is then detected for the determined location from the processed signal. The communication link is then established using the detected SSCs.
BRIEF DESCRIPTION OF THE DRAWINGS
The preferred embodiments will be described with reference to the drawing figures where like numerals represent like elements throughout.
The initial cell search system 10 in accordance with the preferred embodiment of the present invention is illustrated in
Step 1 of the initial cell search algorithm is accomplished using the step 1 processor 12.
The PSC decision device 22, coupled to the HGC 21, receives the correlation values output by the HGC 21 for each chip in a frame worth of chips. A frame's worth of chips is preferably equivalent to the system frame, which by way of example, is equivalent to 38,400 chips. As those having skill in the art know, the system frame can be more or less than that which is used in this disclosure.
The decision device 22 stores each chip correlation value from the HGC 21 over a predetermined number of frames N and averages each chip's correlation values. As an example, a system frame is 4 chips long, and N=2. The HGC 21 outputs the correlation values A1, B1, C1, and D1, respectively for each of the four chips. The decision device 22 stores these values and receives the output of the next frame's correlation values for each chip from the HGC 21, which are A2, B2, C2, D2. Each chip''correlation values are then averaged, (i.e., A1,+A2/2; B1+B2/2; C1+C2/2; D1+D2/2).
Once the decision device 22 finds the average correlation value for each average correlation chip in a frame, the position of the maximum average of the frames is determined and its value compared with a determined threshold. The threshold is based on the noise level (i.e., interference plus thermal noise) at the receiver. The noise estimator 24 has an auxiliary HGC (not shown) that is based on a code which has very low cross correlation with the PSC and the SSCs. The noise estimator 24 HGC calculates a noise estimate for every chip in the system frame. The noise estimator 24 iterates over the same number of frames as the HGC 21 and averages several of the noise estimates in a window around the estimated PSCH location. The window size is preferably about 128, i.e., 64 chips on both sides of the PSCH location. As those having skill in the art know, the window size may be larger or smaller than 128.
If the maximum average is greater than the threshold, the decision device 22 determines whether the transmission pattern of the base station associated with the maximum average location is case 1 or case 2. This determination is made by comparing the correlation value of the chip at the maximum location +(8*2560) or maximum location +(7*2560). If this value is greater than the threshold, then the transmission pattern is case 2. Otherwise, the transmission is case 1.
If the maximum location value is less than the threshold, the step 1 processor 12 continues processing the input signal I until a correlation value greater than the threshold is found or a failed condition is met. As those skilled in the art know, the decision processor 22 may utilize any of a number of methods for determining the location of the strongest PSC code. Once the maximum location value is found, the decision processor 22 forwards the location and the PSC to the cancellation device 18 and the step 2 processor 14.
The cancellation device 18, coupled to the step 1 processor 12 and the step 2 processor 14, takes the maximum location, the PSC and the input signal I and subtracts the PSC from the input signal I. This subtraction eliminates the PSC from the chip at the maximum location in the input signal I. The subtraction of the PSC from the input signal I can be done by one of several cancellation methods, such as interference cancellation. Using interference cancellation, the PSC is converted, using an interference construction device (not shown), into an estimate of its contribution to the input signal I. The received PSC's contribution is subtracted, such as by a subtractor. The resulting signal has the PSC's contribution removed from the input signal I at the maximum location. In code multiplexing systems, one code appears as noise to other codes. Accordingly, the PSC is essentially noise to the SSC. As a result, when the PSC is cancelled from the input signal I, the step 2 processor 14 is able to locate the SSC and slot offset with greater accuracy and speed.
The step 2 processor 14, coupled to the cancellation device 18, the step 1 processor 12 and the step 3 processor 16, receives the modified input signal from the cancellation device 18 and the location of the PSC from the step 1 processor 22.
One example of a step 2 processor 14 is illustrated in
cr(i)=c1(i)*z(i),i=0, . . . ,255 Equation 1
The 16 complex correlation values, Rc (K) are obtained using the above code. Rc (K) is obtained by the following Equation 2:
where tcp is the PSC position obtained from the step 1 processor 12 and N is the maximum number of PSCH time slots used for averaging.
The correlation values obtained at the output of the correlator 31 are applied to the FHT 33. The FHT 33, coupled to the correlator 31 and a derotate device 34, obtains 16 complex correlation values that correspond to the correlation of 16 SSCs and the received signal. That is:
As those skilled in the art know, taking FHT of Rc(K)'s is equivalent to the correlation of unmasked SSCs with the received signal. This is possible due to the special structure of the 16 SSCs. Please note that a case 1 signal uses six (6) SSCs and a case 2 signal uses twelve (12) SSCs. Four (4) SSCs are unused.
The phase estimator 37 receives the modified chip sampled received signal, as well as the PSC position from the step 1 processor 12. The output of the step 1 HGC 21 at the PSC position corresponds to the correlation of the PSC with the received signal at the PSC position. This complex correlation value is the input to the phase estimator 37. In this phase estimator 37, the complex correlation value is normalized and then conjugated. The phase estimation is necessary for the derotation of the SSCs.
The derotate device 34, coupled to the phase estimator 37 and the FHT 33, receives the 16 SSCs from the FHT 33 and the phase estimation from the phase estimator 37. The derotate device 34 derotates the output of the FHT 33. The derotation phase is the phase of the PSC. The complex correlation values are complex multiplied with the phase.
The derotated correlation values are then forwarded to the accumulator 36. The accumulator 36 is coupled to the derotate device 34 and the step 2 decision device 39. The derotated correlation values are added coherently with a period of two (for case 1) or four (for case 2), for N iterations in accordance with equation 4:
ra1(k,n)=ra1(k,n−1)+rd(k,n)δ(1−n mod L), k=0, . . . , K; n=0, . . . , N; 1=0, . . . L Equation 4
where N is the maximum number of iterations to obtain a reliable signal value, K is the number SSCs used (K=6 for case 1 and K=12 for case 2) and L is periodicity of the PSCH (L=2 for case 1 and L=4 for case 2). These correlation values are initially set to zero. The decision variables are formed from the correlation values according to the SSC transmission patterns.
The decision variables obtained in the accumulator 36 are forwarded to the decision device 39. There are 64 decision variables for case 1, 32 code groups and 2 frames indices. For case 2, there are 128 decision variables, 32 code groups, 2 frame indices and 2 slots (K or K+8). The decision device 39 compares all the decision variables sequentially (one by one). This scheme is efficient since the number of decision variables is not large and the scheme can be implemented without much complexity. The transmission pattern that the maximum decision variable belongs to indicates the code group number of case 1 and case 2 and PSCH slot index for case 2.
The toffset, scrambling code group number, SSCs, and the location of the PSC are then forwarded to the step 3 processor 16. The step 3 processor 16, coupled to the step 2 processor 14, retrieves the midambles and primary scrambling code that are used by the UE. The code group number retrieved by the step 2 processor 14 is associated with four cell parameters. Therefore, identification of the code group number identifies the midamble codes used by the cell. The four cell parameters associated with the code group are cycled through System Frame Numbers (SFNs) as depicted in Table 1.
The basic midamble code toggles with the SFN (odd/even). If the SFN is even, the correlation device 41 correlates against the basic midamble code. If the SFN is odd, the correlation device 41 correlates against the cycled midamble code. For example, in the case of code group 0, the correlation device 41 correlates against midamble codes 0, 1, 2 and 3 on even SFN, and the correlation device 41 correlates against midamble codes 1, 0, 3 and 2 on odd SFN. It should be noted that cell search does not know the SFN, but it does know whether the SFN is even or odd based on the frame index (1 or 2) found by the step 2 processor 14.
The correlation device 41 calculates 4×WS3 correlations. The periodic window allows the correlation device 41 to find the maximum correlation. The purpose of the multipath window is to adjust the PSCH position to include the maximum amount of multipath. This may be necessary if the strongest multipath component is not the first significant multipath component.
The correlation values output from the correlation device 41, are forwarded to the accumulation device 42 which is coupled to the correlation device 41 and the decision device 43. The accumulation device 42 accumulates the correlation values over a predetermined number of frames N3. It should be noted that initial cell search does not know frame boundaries so the initial cell search system typically uses blocks of 38400 chips (2560 chips×15 slots) in lieu of frames. The accumulation device 42 forms the decision variables by adding the absolute value of the real and imaginary parts of the complex number that represents the correlation value. A decision variable is the magnitude measure of the corresponding correlation value. In order to have a more reliable decision, these decision variables can be accumulated for N3 iterations, where N3 is the maximum number of iterations for a reliable signal to noise ratio level.
The decision variables generated by the accumulation device 42 are forwarded to the decision device 43. The decision device 43, coupled to the accumulation device 42, determines the maximum decision variable by simple sequential comparison. The maximum decision variable corresponds to the basic midamble used for the cell. The scrambling code number associated with the identified midamble is the scrambling code of the cell. The scrambling code is then utilized by the UE for broadcast channel processing.
The flow diagram for the initial cell search system is illustrated in
Since the second step of the initial cell search is the weakest, the cancellation of the PSC from the signal input to the step 2 processor 14 provides a cleaner signal and results in a better estimation of the SSCs time. This results in a more accurate slot offset and code group number determination. Ultimately, this procedure reduces the number of false detections by the UE.
A second embodiment is illustrated in
A third embodiment is illustrated in
Claims
1. A user equipment (UE) configured to perform initial cell search in a wireless communication system having a plurality of base stations, each base station transmitting a common primary synchronization code (PSC) in a primary synchronization channel in conjunction with a base station specific secondary synchronization code (SSC) at a different time within a system frame, said UE receiving an input signal including the PSC and SSC from at least one of the base stations;
- said UE comprising: a step one processor for detecting PSCs within a selected time frame in said received input signal and determining a maximum PSC location, which corresponds to the maximum PSC value detected within the frame, and producing an output complex correlation value; a cancellation device for subtracting the PSC value at the maximum PSC location within the frame to enhance detection of the SSC; a step two processor, coupled to the step one processor and the cancellation device, configured to determine a scrambling code group number for the cell and a frame timing offset value for locating the frame boundary; and a step three processor, coupled to the step two processor, configured to determine the scrambling code of the cell.
2. The UE of claim 1 wherein said cancellation device uses interference cancellation to remove said PSC from said input signal.
3. The UE of claim 1 wherein the step two processor comprises:
- a correlator configured to correlate the input signal with a code sequence corresponding to the maximum PSC location.
4. The UE of claim 3 wherein the step two processor further comprises:
- a device configured to correlate the input signal with N SSCs outputting a set of N complex correlation values t, where N is an integer greater than one.
5. The UE of claim 4 wherein the step two processor further comprises:
- a phase estimator device configured to determine a phase estimate of the complex correlation value from the step one processor; and
- a derotation device configured to derotate the set of N complex correlation values using the phase estimate.
6. The UE of claim 5 wherein the step two processor further comprises:
- an accumulator configured to coherently add the derotated set of N correlation values at a predetermined period for a predetermined number of iterations, producing a set of decision variables; and
- a decision device configured to select a maximum decision variable, from which the scrambling code group number for the cell and the frame timing offset value are determined.
7. The UE of claim 1 wherein the step three processor comprises:
- a step three correlation device configured to produce step three correlation values that are the maximum correlation of the input signal correlated with midambles associated with the code group;
- a step three accumulation device configured to accumulate the step three correlation values over a predetermined number of system frames producing step three decision variables; and
- a step three decision device configured to determine the maximum step three decision variable from which the basic midamble for the cell and the scrambling code of the cell is determined.
8. The UE of claim 7 further comprising a second cancellation device coupled between the step two and step three processors configured to subtract the SSCs from the input signal to enhance detection of the code group number and midambles associated with the code group.
Type: Application
Filed: May 16, 2005
Publication Date: Oct 6, 2005
Applicant: InterDigital Technology Corporation (Wilmington, DE)
Inventors: Prabhakar Chitrapu (Blue Bell, PA), Louis Guccione (East Chester, NY), Andrew Bednarz (Garden City, NY)
Application Number: 11/129,789