Abstract:
Bias introduced by down-sampling may be eliminated, or significantly reduced, by the present embodiments. Methods and apparatuses are described for use in wireless communication systems including LTE and other mobile data systems. The method includes identifying a timing offset estimation bias caused by a misalignment between samples and a zero-offset point of a preamble signature.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates to data communications and more particularly relates to signature detection and timing offset estimation. 
       BACKGROUND OF THE INVENTION 
       [0002]      FIG. 1  illustrates cellular network  100  of the prior art. Each cell  102   a - c  in cellular network  100  is generally defined by an area in which base stations  104   a - c  are able to communicate with User Equipment (UE)  106   a - d . Examples of UEs  106   a - d  include telephones, smartphones, Personal Data Assistants (PDAs), tablet computers, cellular data devices for use with laptop computers, and the like. Generally, each cell  102   a - c  includes one corresponding base station  104   a - c.    
         [0003]    Any number of UEs  106   a - d  may be found in cells  102   a - c , depending on the use habits of the users of cellular network  100 . For example, cell  102   b  includes two UEs  106   c - d . In such an embodiment, both UEs  106   c - d  may communicate with base station  104   b  at the same time. Depending on the protocol used by base station  104   b , UEs  106   c - d  may communicate simultaneously, or substantially simultaneously. Alternatively, UEs  106   c - d  may communicate with base station  104   b  within a time slot. Additionally, UEs  106   a - d  may move between cells as the user travels from one area to another. As shown, UE  106   a  may move from cell  102   a  to cell  102   c . In this sort of case, cell  102   a  includes two UEs  106   a - b  initially, but once UE  106   a  moves to cell  102   c , cell  102   a  only includes UE  106   b . Thus cellular networks  100  are generally dynamic in nature, and changes in the topology of cellular network  100  may be random, based upon the user&#39;s habits. 
         [0004]      FIG. 2  illustrates an example of a topology for any of cells  102   a - c . In addition to base station  104 , cells  102  may include antenna  202  coupled to base station  104 . Antenna  202  receives random access signals from UEs  106  operated by user  204  in a multipath environment and mobile user  206  over one or more Random Access Channels (RACHs) operated by base station  104 . The RACH allows UEs  106  to gain initial access to cellular network  100  and facilitates uplink synchronization. 
         [0005]      FIG. 3  illustrates RACH detection circuit  300  according to the prior art. RACH detection circuit  300  is often included in base station  104 . RACH detection circuit  300  includes CP removal module  302  for removing the Cyclic Prefix (CP) from received symbols. RACH detection circuit  300  also includes downsampling/resampling module  304  for reducing a sample rate to a frequency that is suitable for use by correlator  306 . The reduced sample rate simplifies operations of the correlator  306 , particularly in FFT module  308 . Correlator  306  includes Fast Fourier Transform (FFT) module  308  configured to transform the downsampled symbol and subcarrier demapping module  310 , correlator  306  also includes subcarrier demapping module  310  and multiplier  312 . Multiplier  312  multiplies the demapped subcarrier with a conjugate of a root sequence in the frequency domain. The multiplied carrier is then converted back to time domain by Inverse Discrete Fourier Transform (IDFT) module  314 . In prior systems  300 , signature detection and timing offset estimation module  316  detects a random access signal from UE  106  and determines the timing offset of the detected random access signal. 
         [0006]    In Long Term Evolution (LTE) mobile communication networks, for example, UEs  106   a - d  send random access signals to base stations  104   a - c , when the UEs  106   a - d  are in respective cells  102   a - c , to gain initial access to cellular network  100 . The random access signals may be sampled by the base station at up to 24576 samples (˜1 ms) in the time domain. One problem with the prior art is that it often takes very complex hardware to detect random access signals with high accuracy, particularly with respect to operations performed by FFT module  308 . 
         [0007]    Although some prior methods and devices do use all 24576 samples to perform random access detection, most systems use some form of down-sampling to reduce hardware complexity. Down-sampling typically involves dividing the number of samples by an integer, and then only selecting a reduced number of signal samples. For example, 2-fold down-sampling would use 12288 samples, and 4-fold down-sampling would only use 6144 samples. Although these down-sampling techniques may reduce hardware complexity, the tradeoff is a performance degradation due to a bias introduced at down-sampling for those specific access signals that do not fall directly on the samples. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    Bias introduced by down-sampling may be eliminated, or significantly reduced, by the present embodiments. Methods and apparatuses are described for use in wireless communication systems including LTE and other mobile data systems. The method includes identifying a timing offset estimation bias caused when a zero-offset point of a signature does not fall directly on a first sample of a detection interval. The difference between the first sample point of the detection interval and the zero-offset point may be compensated for in the timing offset estimation. 
         [0009]    More specifically, when downsampling is used by a PRACH device in the base station, there may be a misalignment between the first samples of the detection intervals and the zero-offset points of each interval, because the samples may not fall directly on each zero-offset point, depending upon the rate of downsampling selected. The present embodiments describe methods for calculating the misalignment-induced bias, and then subtracting the misalignment-based bias from the timing offset estimation for each peak. 
         [0010]    In one embodiment, the misalignment-based bias for each signature is calculated in advance of receiving a peak. For example, upon configuring the downsampling rate, the misalignment for each signature may be detected, and then a value representing the misalignment may be stored in a memory device for later use by a timing offset module. The timing offset module may use the value to subtract or otherwise compensate for the bias while estimating the timing offset for a received peak. 
         [0011]    Additionally, the present embodiments describe methods for avoiding missed peak detections due to misalignment between detection interval and the zero-offset point for each preamble signature. To overcome missed peak detections, the present embodiments include methods for ensuring that the zero-offset point is always included in the detection interval. 
         [0012]    The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
           [0014]      FIG. 1  is a schematic diagram illustrating a cellular network of the prior art. 
           [0015]      FIG. 2  is a schematic diagram illustrating a cellular network environment. 
           [0016]      FIG. 3  is a schematic block diagram illustrating a random access channel circuit of the prior art. 
           [0017]      FIG. 4  is a conceptual drawing illustrating an embodiment of a PRACH communication time slot. 
           [0018]      FIG. 5  is a conceptual drawing illustrating one embodiment of PRACH preamble signatures. 
           [0019]      FIG. 6  is a schematic block diagram illustrating one embodiment of a signature detection and timing offset estimation module. 
           [0020]      FIG. 7  is a conceptual diagram illustrating the resolution of timing offset. 
           [0021]      FIG. 8  is a conceptual diagram illustrating misalignment of zero-offset points. 
           [0022]      FIG. 9  is a conceptual drawing illustrating one embodiment of a method for correcting bias from downsampling a preamble signature. 
           [0023]      FIG. 10  is a schematic flowchart diagram illustrating one embodiment of a method for signature detection with timing offset bias compensation. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    The present embodiments relate to improved methods for providing initial network access and uplink synchronization using PRACH.  FIG. 4  is a conceptual drawing illustrating an embodiment of a PRACH communication time slot  402 . PRACH uses sixty-four indexed preamble signatures per cell for initial network access and uplink synchronization between UEs  106  and base stations  104 . For example, UE  106   b  may choose one out of the available preamble signatures and transmit a preamble signal  406   a  to base station  104   a . Time slot  402  may correspond to the selected preamble signature. Base station  104   a  then detects the preamble index (0-63) and the timing offset  408  of preamble  406   a.    
         [0025]    In some embodiments, base station  104   a  may be referred to as eNodeB. If UE  106   a  is physically close to base station  104   a  (eNodeB), then the timing offset  408  may be small. If, on the other hand, UE  106   b  is located far away from base station  104   a , then timing offset  408  may be relatively large. The overall duration  404  of PRACH slot  402  may be determined by the cell size. 
         [0026]      FIG. 5  is a conceptual drawing illustrating one embodiment of a set of PRACH preamble signatures  502 . As described above, LTE networks include sixty-four signatures (0-63). The preamble signatures  502  may be constructed from cyclic shifts of one or more root sequences. For example, a Zadoff-Chu (ZC) sequence may be used. ZC sequences may have Constant Amplitude Zero AutoCorrelation (CAZAC) properties and good cross-correlation. In the embodiment described in  FIG. 5 , the PRACH includes two root sequences, which are used by the correlator  306  to generate signatures 0-31 and 32-63, respectively. Each signature  502  has a zero-offset point  504 , which defines a boundary of the signature  502 . 
         [0027]    When a random access signal is received from UE  106  by base station  104 , correlator  306  may determine the index number of the signature  502 . Additionally, the correlator  306  may estimate the timing offset  508  of the received signal. As shown in  FIG. 5 , the random access signal is represented by a peak  506   a - b . The timing offset  508  is estimated based on the distance of peak  506   a - b  from the zero-offset point  504  for the respective signatures  502 . 
         [0028]      FIG. 6  is a schematic block diagram illustrating one embodiment of a signature detection and timing offset estimation module  316 . In one embodiment, the signature detection and timing offset estimation module  316  includes a peak search module  602 , a decision of signature index module  604  and a timing offset estimation module  606 . In one embodiment, these modules  602 - 606  are configured to perform signal detection and timing offset estimation according to processing rules discussed in further detail with respect to  FIGS. 8-9 . 
         [0029]    Peak search module  602  may use one or more threshold values to filter peaks and identify a set of peaks  506  having predetermined characteristics. For example, peak search module  602  may apply an amplitude threshold to exclude all peaks  506  having an amplitude below a predetermined threshold level. In another embodiment, search windowing may be applied to eliminate power leakage peaks and other peaks deemed to be false according to a set of predetermined criteria. Peak search module  602  may then pass peak  506  to decision of signature index module  604 . 
         [0030]    Decision of signature index module  604  may determine which signature index (e.g., 0-63) corresponds to characteristics of peak  506 . For example, as illustrated in  FIG. 5 , decision of signature index module  604  may determine that peak  506   a  is received on preamble signature 0 and that peak  506   b  is received on preamble signature 34 based upon frequency characteristics or timing characteristics of peaks  506   a - b . As described, peaks  506   a - b  may be assigned to different signature indexes because they may be received from different UEs  106   a - d.    
         [0031]    Once the signature index is decided, then timing offset estimation module  606  may determine a timing offset of peak  506  with respect to zero-offset point  504 . As shown in  FIG. 7 , each signature may be sampled at a predetermined rate by the IFFT  308  and/or IDFT  314  in correlator  306 . The time between each sample  702  may be a predetermined time period  704 . Timing offset estimation module  606  may determine the timing offset based upon the number of samples between zero-offset point  504  and peak  506 . 
         [0032]    Such methods of timing offset estimation may be a source of estimation error when downsampling is used. As illustrated in  FIG. 8 , each detection interval  502   a - d  for each signature (m and m−1) includes multiple samples  702 , each having time period  704  that corresponds to the sampling rate. Where no downsampling is used, time period  704   a  may be smaller than time period  704   b  of corresponding downsampled signature  502   c - d . Depending on the downsampling rate used, there may be some misalignment  802  between first sample  702   c, d  and zero-offset point  504   c, d  of each signature. For example, as shown in signature  502   c  sample  702   c  falls to the left of zero-offset point  504   c . Misalignment  802  may be quantified as the distance between sample  702   c  and zero-offset point  504   c . In another example, first sample  702   d  of signature  502   d  falls to the right of zero-offset point  504   d , so the misalignment  802  may occur on either side of zero-offset point  504 . If the misalignment  802  is large, and it occurs on the right side, peaks  506  which are close to zero correlation point  504  may be missed altogether. Thus, downsampling may cause missed peaks  506  and cause errors in estimating timing offset  508 . 
         [0033]      FIG. 9  is a conceptual drawing illustrating one embodiment of a method for correcting bias from downsampling a preamble signature. Parameters for use in signature detection and timing offset estimation module  316  may be configured according to the embodiments described in  FIG. 9 . In one embodiment, the method includes removing a timing bias for each signature  502  by subtracting misalignment  802  from the estimation of time offset  508 . In one embodiment, misalignment  802  may be subtracted from the estimation of time offset  508  if the sample  702  falls to the left of zero-offset point  504 . Misalignment  802  may be added to the estimation of time offset  508  if sample  702  falls to the right of zero-offset point  504 . 
         [0034]    In one embodiment, signature detection and timing offset estimation module  316  may be configured to always select detection interval  502  that includes zero-offset point  504 . Such an embodiment may help avoid situations where peaks  506  are lost due to misalignment. Additionally, this may simplify the process for correcting bias from misalignment  802 , because the calculation will always include a subtraction. 
         [0035]    In one embodiment, signature detection and timing offset estimation module  316  may be configured to pre-compute timing offset biases caused by misalignment  802 . For example, when the downsampling rate is selected, misalignment  802  for each preamble signature  502  may be determined, and a value representing misalignment  802  may be stored in a memory device associated with base station  104 . 
         [0036]      FIG. 10  is a schematic flowchart diagram illustrating one embodiment of method  1000  for signature detection with timing offset bias compensation. In one embodiment, downsampling module  304  downsamples a random channel access signal received from UE  106  at a predetermined sample period, as shown in block  1002 . Then at block  1004 , correlator  306  may generate one or more signal peaks correlated to a predetermined root sequence. Signature detection and timing offset estimation module  316  may then compensate for a bias in a timing offset estimation, the bias being a result of misalignment  802  between the sample period and a zero-offset point  504  in a random channel signature  502  as shown in block  1006 . For example, signature detection and timing offset estimation module  316  may calculate a value representing misalignment  802  and subtract that value from the estimation of timing offset  508 . In a further embodiment, signature detection and timing offset estimation module  316  may be configured to selected detection interval  502  such that detection interval  502  always includes zero-offset point  504 . 
         [0037]    Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.